Stephany Gilbert

Philippe

Vero, Didier, Melanie, Herbé Nancy Kevin

To Sabrina

Anil, Nour, Lynda, Quarentin, Alexandre Mathilde Stephan

Emilian Catherine Sandrine

Jackob, Athul, Lorena, Dirk Tanja Mattias

Anabel Yanik

Luan

Ameer Gerardo Manos

Bertille

René

Alvaro Bruno

Juan D Andrea

Dyn, Tang, Jaime, Ivan Lucia Lizeth

Jairo Pablo Hugo

Porter, Diana Anne-Claire Diana

To Marine Desage-El

Résumé

Cette thèse de doctorat s'inscrit dans le domaine de la chimie organométallique, visant à développer des complexes photoactifs à base de fer en vue de leur utilisation dans des applications de conversion énergétique. L'un des objectifs actuels de l'humanité est la recherche d'alternatives aux combustibles fossiles comme principale source d'énergie, en raison de la production de déchets liée aux problèmes environnementaux (changement climatique, pollution de l'eau, du sol et de l'air). Les sources d'énergie renouvelables comme l'énergie éolienne, hydroélectrique et solaire, entre autres, constituent une nouvelle alternative. En particulier, l'énergie solaire est une alternative verte, durable et renouvelable. On calcule que le soleil fournit à la Terre l'énergie nécessaire pour une année entière en une heure environ (voir §I, Figure 1). Cette quantité d'énergie pourrait répondre aux besoins en électricité de l'humanité d'aujourd'hui et de demain. En fait, ce type d'énergie peut être transformé en électricité au moyen de cellules solaires, ou être utilisé comme énergie chimique dans des réactions photocatalytiques, par exemple. Une façon d'exploiter ce type d'énergie est d'utiliser des matériaux moléculaires, parmi lesquels les métaux nobles (Ru, Ir, Pt, etc.) et les terres rares (lanthanides) ont joué un rôle clé dans ce domaine en raison de leurs performances exceptionnelles et des propriétés photophysiques et électrochimiques de leurs complexes.

Certaines de leurs caractéristiques englobent une forte absorption de la lumière visible, un comportement redox réversible, des états excités pertinents à longue durée de vie et une stabilité thermique.

Les métaux nobles ont été largement utilisés pour la conversion énergétique des marsouins avec différentes applications telles que la photothérapie dynamique, les diodes électroluminescentes organiques, les cellules solaires à colorant ou la catalyse par exemple.

Pourtant, leur rareté, l'impact environnemental de leur extraction, de leur exploitation et de leur séparation, combinés à leur prix élevé, rendent leur application à grande échelle impossible.

Pour ces raisons, on s'intéresse de plus en plus aux métaux de transition de la première rangée (fer, manganèse, chrome), car ils sont plus abondants, moins chers et leur processus d'extraction est moins agressif pour l'environnement (voir §1, Figure 3). Parmi les métaux de transition de la première rangée, le fer se distingue par le fait qu'il est le métal de transition le plus abondant sur la croûte terrestre et qu'il est environ 25 000 fois moins cher que le Ru, largement utilisé.

Par conséquent, le développement de matériaux moléculaires photoactifs robustes et efficaces basés sur des métaux abondants sur Terre constitue un véritable jalon pour remplacer les processus nuisibles à l'environnement par la durabilité. Cependant, le développement d'un photosensibilisateur basé sur un métal de transition 3d n'est pas trivial, le problème principal est que les métaux 3d font face à des champs de ligands plus faibles que les espèces 4d ou 5d ; par exemple, si nous comparons le [Ru(bpy)3] 2+ largement utilisé avec le 3d homologue [Fe(bpy)3] 2+ , le temps de vie de l'état excité est de 1μs et 150 fs respectivement (voir §I.1). De plus, les complexes 3d présentent des propriétés photophysiques qui ne sont pas appropriées à leur application en tant que composés photoactifs, ce qui constitue un défi auquel s'attaque la communauté scientifique, surtout au cours de la dernière décennie. Afin d'augmenter la force du champ du ligand, la stratégie consistant à utiliser des ligands fortement donneurs de C comme les carbènes N-hétérocycliques (l'un des plus étudiés, voir §I.2.3) a été appliquée.

Comme l'ont rapporté Wärnmark et ses collègues dans leurs travaux précurseurs, où ils ont décrit le premier complexe de fer avec un MLCT (transfert de charge métal-ligand, voir §I.1) de 9 ps (voir §I.2.3, Figure 14) (presque deux ordres de grandeur plus long que [Fe(bpy)3] 2+ ).

Ce résultat exceptionnel a clairement mis en évidence l'importance primordiale d'une sphère de coordination conçue de manière rationnelle et a ouvert la porte des applications de conversion énergétique aux complexes à base de fer. Depuis lors, des résultats impressionnants ont été obtenus, y compris les premiers exemples de complexes de fer luminescents (voir §I.2.4, Figure 20). Cependant, si des concepts originaux ont été explorés, des développements supplémentaires sont encore nécessaires pour que les complexes à base de fer soient réellement compétitifs par rapport aux autres composés photoactifs.

Dans ce travail de doctorat, nous avons travaillé à deux niveaux différents. D'un point de vue fondamental, nous avons étudié le contrôle de la géométrie moléculaire dans les complexes azine-NHC Fe(II) bidentés par modification du ligand, ainsi que l'exploration d'unités de coordination (formellement) anioniques. En outre, nous nous sommes lancés dans une chimie plus appliquée, comme pour la synthèse de complexes Fe(II) hétéroleptiques en vue de la production de H2 dans des réactions de fractionnement de l'eau.

En ce qui concerne la synthèse stéréosélective de complexes azine-NHC Fe(II) bidentés, nous avons développé une méthodologie à température ambiante pour accéder sélectivement aux complexes à géométrie faciale ou méridienne. Pour réaliser ce contrôle sélectif fac/mer, nous avons conçu plusieurs ligands avec une substitution modifiée de l'azine par des groupes de différents degrés d'encombrement tels que Mes et CH3) (Voir §III.4, Schéma 9). Nous avons découvert que la sélectivité est étroitement liée à l'encombrement à proximité de l'atome coordinateur N de l'azine. La meilleure sélectivité a été obtenue lorsque le CH3 (encombrement stérique plus faible) était en position alpha de l'azote coordonné au centre du fer (voir §III.4, Tableau 4). Il est intéressant de noter qu'en augmentant l'encombrement stérique dans cette position (série de dérivés 6-substitués), avec des substituants tels que des groupes isopropyle ou chlore, la même sélectivité a été obtenue (voir §III.4.3, tableau 6). En fait, on a souvent observé que des séquences de substitution comparables de ligands bidentés conduisaient à l'isomère mer, compte tenu de l'orientation commune de ces substituants. Dans le cas d'un groupe plus exigeant stériquement (Mes), une sélectivité mer n'a été obtenue qu'avec Mes en position 5 de l'azine (voir §III.4, Tableau 4). Pour évaluer la portée de notre méthodologie sélective fac/mer, nous avons utilisé des carbènes de nature électronique variée, allant de groupements plus acceptants (bIm) à des groupements plus donneurs (4MeIm). Nous avons observé que, quelle que soit la nature électronique du NHC, la sélectivité était maintenue, ce qui a permis d'obtenir un ensemble de complexes faciaux et méridiens avec différents carbènes (voir §III.4.1, tableau 5). Pour améliorer les propriétés optiques des complexes à base de pyridine, des ligands à base de quinoléine et de quinoxaline ont été préparés. De manière intéressante, la complexation avec ces deux ligands a permis d'obtenir la stéréosélectivité faciale exclusivement (voir §III.4.3, schémas 12 et 13). Comme dans la série substituée par la pyridine, nous avons étudié plus avant l'effet du NHC. La synthèse de deux ligands supplémentaires à base de quinoxaline avec bIm et 4MeIm comme NHC a été réalisée. Il est intéressant de noter que la complexation s'est déroulée avec une fac stéréosélectivité, ce qui est cohérent avec les résultats précédents (voir §III.4.3, schéma 14).

Malgré les calculs préliminaires effectués par le Dr. Antonio Francés Monerris (Valencia), la compréhension de la sélectivité fac/mer nécessite une étude plus approfondie du mécanisme de complexation, qui est actuellement en cours. Jusqu'à présent, la caractérisation structurelle a montré que les isomères fac présentent des sphères de coordination plus distordues (voir §III.4.4, Figure 42, Tableau 7) que les isomères mer, ce qui pourrait être délétère pour la dynamique de leur EE. Cependant, il est également concevable que la configuration faciale entraîne des interactions interligands qui ne sont pas présentes dans la configuration méridienne (voir §III.4.5, Figure 44, Tableau 8). Ces interactions dans la conformation faciale pourraient empêcher l'élongation de la liaison Fe-N, qui a été démontrée comme étant le facteur principal causant la désintégration des états excités de la MLCT. D'autre part, la nature de la NHC et de la π-conjugaison semble être le paramètre clé contrôlant leurs propriétés optiques et redox, les deux isomères présentant des comportements distincts (voir §III.4.6). L'utilisation d'azines π-extendues est particulièrement pertinente en raison de leur collecteur MLCT de faible hauteur, ce qui entraîne une meilleure absorbance dans le spectre UV-vis (voir §III.4.6, Figures 46 et 47, Tableau 9). En outre, la réduction possible du couplage électronique avec les états MC pourrait contribuer à une durée de vie plus longue de la MLCT.

A cet égard, une collaboration avec le Prof. Dr. Stefan Haacke à Strasbourg est en cours pour étudier l'influence sur la cinétique de l'EE. Cette partie de la thèse met en évidence le grand potentiel de ces complexes, qui peuvent profiter de l'effet synergique du couplage d'un motif de substitution spécifique à l'unité azine avec le NHC, est ainsi mis en évidence. Il en résulte une synthèse de complexes non labiles du fer(II) avec une stéréosélectivité vraisemblablement donnée par les interactions inter-ligands. De plus, les caractéristiques opto-électroniques qui peuvent être facilement modifiées en fonction de l'unité azine et de la partie NHC.

En ce qui concerne les complexes de Fe(III) portant des ligands anioniques tridentés, nous avons cherché à obtenir des complexes de fer ayant une durée de vie accrue des états excités à longue période. La stratégie consistait à utiliser des ligands donneurs forts tels que des amides et des unités de cyclométallisation afin d'induire un fort champ de ligand et de déstabiliser les états MC (voir §IV). Nous avons donc synthétisé et caractérisé différents ligands anioniques qui portent des unités NHC avec des donneurs N-amido (série carbazolate) (voir §IV.2.1, Schéma 15) ou avec des donneurs C-anioniques (série cyclométallisée) (voir §IV.4.2, Schéma 20). Concernant la série N-amido, nous avons rapporté la synthèse de trois complexes, un homoleptique (voir §IV.2.2, Schéma 16) et deux hétéroleptiques (voir §IV.3, Schéma 17) avec des interactions métal-ligand maximisées grâce à la formation de chélates à 6 membres. La force de la sphère de coordination a été modulée en fonction du nombre de ligands N-amido coordonnés, bien que dans tous les cas, des espèces Fe(III) aient été obtenues.

Il est intéressant de noter que les composés homoleptiques et hétéroleptiques semblent avoir un mélange d'orbitales basé sur le métal et sur le ligand, comme le montre leur comportement en oxydation (voir §IV.4.4, Figure 57 et Tableau 12), ce qui suggère que ce type de ligands pourrait être utile pour obtenir une " inversion HOMO " complète telle que définie par Jakubikova. Des propriétés plus distinctes sont apparues dans leur comportement de réduction, avec les complexes hétéroleptiques présentant un premier potentiel de réduction remarquablement décalé anodiquement qui pourrait refléter l'ion métallique moins enrichi par rapport au complexe homoleptique. En outre, la réduction des fragments tpy a également été observée en raison de leur fort caractère d'accepteur. Il est intéressant de noter que les deux séries de complexes ont montré des bandes de transfert de charge de nature différente presque sur toute la gamme visible et s'étendant jusqu'à la région IR.

Quant à la série C-anionique, lors d'un stage de 2 mois avec le Prof. Dr. Bauer à Paderborn dans le cadre du programme DrEAM de l'Université d'Excellence de Lorraine (LUE), nous avons développé une méthodologie qui nous a permis d'obtenir de nouveaux complexes Fe-CCC bis-cyclométallisés (Voir §IV. 4.3, Schéma 22), sans utiliser de boîte à gants, de réactif transmétallant ou réducteur. Les propriétés de l'état fondamental des complexes homoleptiques obtenus ont été étudiées. Ces complexes présentent une unité pyrimidine cyclométallisante qui permet une stabilisation orbitale induite par le squelette pyrimidine et les substituants en position 2 (-Ph, -CF3), lorsque le caractère attracteur d'électrons augmente, un décalage anodique est observé dans leurs ondes de potentiel de réduction. Les deux complexes présentent trois bandes d'absorption distinctes correspondant aux transitions π-π*, MLCT et LMCT qui se trouvent à une énergie plus faible par rapport à celle rapportée par Bauer avec un squelette phényle au lieu d'une pyrimidine. La présence du substituant phényle sur la pyrimidine conduit à une augmentation de l'intensité d'absorption (voir §IV.4.4, Figure 60, Tableau 14). Des calculs sont en cours en collaboration avec le Dr.

Antonio Francés Monerris afin de mieux comprendre la nature des absorptions UV-vis observées pour les complexes de fer N-amido et cyclométallés. Des mesures complémentaires sont également effectuées en collaboration avec le Prof. Dr. Haacke à Strasbourg pour mesurer la dynamique de la relaxation de l'état excité (expériences TAS). Ensuite, une série de tests de réduction sera effectuée sur les complexes cyclométallisés afin d'obtenir des complexes Fe(II)-CCC, puis les propriétés de l'état fondamental et de l'état excité des complexes seront étudiées.

Enfin, la synthèse de complexes hétéroleptiques avec des ligands push-pull sera tentée pour obtenir des complexes Fe(II)-CCC avec un flux électronique vectoriel.

Enfin, et c'est un aspect important de ce travail de doctorat, nous nous intéressons à la tendance croissante de la production d'hydrogène vert, en raison de son énorme densité énergétique (120 MJ kg-1) et de ses nombreuses applications industrielles (pharmacie, raffinage, agriculture, etc.), la poursuite de la production d'hydrogène vert sans empreinte carbone devient de plus en plus importante ces dernières années. Afin d'atteindre cet objectif, nous avons planifié la synthèse d'un ensemble de ligands afin de concevoir des dyades bimétalliques hétéroleptiques ditopiques Fe-Co qui seront testées comme photosensibilisateurs dans la réaction de séparation de l'eau dans la réduction des protons (2𝐻 + + 2𝑒 -⇌ 𝐻 2 𝐸 0 = 0 𝑉 𝑣𝑠 𝑅𝐻𝐸). Cette partie de la synthèse des ligands a été réalisée dans notre laboratoire (L2CM) tandis que la synthèse hétérolytique du complexe a été faite dans le cadre du programme DrEAM en Allemagne avec le Prof. Dr. Bauer.

Après différentes approches pour obtenir les ligands ciblés (voir §V.2.1, schémas 26, 27 et 29), nous avons réussi à obtenir deux des trois ligands : un qui se coordonnera de manière monodentée au Co, donnant une dyade (voir §V.2, Figure 68), et un qui se coordonnera de manière bidentée au cobalt, permettant l'accès à une dyade et une triade (voir §V.2, Figure 68). -La stabilité de la triade, la caractérisation de l'état fondamental et l'ESL.

-Leurs performances dans l'évolution photocatalytique de H2 et comparer les activités entre la dyade et les triades. L bi = 2,5-bis (3,5-di-tert-butyl-2-isocyanophenyl)thiophene LLCT = Ligand-to-ligand charge-transfer LMCT = Ligand-to-metal charge-transfer L tri = 5,5'- (2-isocyano-5-methyl-1,3-phenylene)bis (2-(3,5-di-tert-butyl-2-isocyanophenyl)

Table of Contents

I. General Introduction

I. General Introduction

Already since several decades, the modern societies are in their quest for stopping their dependence on non-renewable fossil fuels as a principal energy source, that arises up to 90% of energy consumed by humankind, 1 which is around 18 TW and it is predicted to reach 43 TW in 2100. The use of fossil fuels produces a large quantity of pollutants and greenhouse gases related to serious environmental problems (climate change, water, soil and air pollution). 2 As an alternative to avoid fossil fuels, the scientific community has searched for alternative renewable energy sources like solar, wind power and hydroelectric among others. (Figure 1). The solar energy is a green, sustainable, and renewable option. Actually, the sun delivers to the Earth the energy needed for a whole year in about one hour. Thus, such an extraordinary and practically limitless power could easily supply the electricity needs of our society, nowadays and in the future. 4 In fact, sunlight can be easily transformed into electricity by means of solar cells. [START_REF] Nazeeruddin | Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO 2 -Based Solar Cells[END_REF] Furthermore, it can be also converted into chemical energy through, for instance, photocatalytic reactions for storage reasons due to its intermittent and diffuse character. 6 In both cases, when molecular materials are considered, noble (Ru, Ir, Pt, Au) and rare-Earth metals (lanthanides) have played a key role due to their outstanding performances stemming mainly from the ideal photophysical and electrochemical properties of their complexes. These properties encompass strong visible light absorption, reversible redox behaviour, long-lived relevant excited states (ES) and thermal stability.

Noble and rare metals have been used in photo dynamic therapy (PDT), 7 as light emitters organic light-emitting diodes (OLEDs), 8 phosphorescent organic light-emitting diodes (PhOLEDs), 9 dye-sensitizer solar cells (DSSCs), [START_REF] Grätzel | Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells[END_REF] catalysis and energy conversion reactions. 6,[START_REF] Arias-Rotondo | The Photophysics of Photoredox Catalysis: A Roadmap for Catalyst Design[END_REF] However, the scarcity and high cost of these elements are detrimental to their sustainable application on a large scale. In addition to this, geopolitical issues are to be considered since there are often only few countries that mine and produce them. For instance, in Figure 2 it is shown the global production for rare earth oxides in the second half of the last century, being controlled by the United States of America and then by China. [START_REF]Rare Earth Elements-Critical Resources for High Technology[END_REF] In addition, the environmental impact for mining, extraction, separation and earth remediation produces a huge amount of toxic waste and consumes enormous quantities of water. In recent years, an increasing interest in switching from rare metals to first-row transition (3d) metals like iron, manganese, vanadium or chromium instead of noble metals, since they are remarkably more abundant [START_REF] Goldschmidt | The Principles of Distribution of Chemical Elements in Minerals and Rocks. The Seventh Hugo Müller Lecture[END_REF] and generally cheaper (Figure 3). [START_REF]Rare Earth Elements-Critical Resources for High Technology[END_REF] Actually, significant and successful strategies have been reported for 3d transition metals. In this regard iron takes a special role since it is the most abundant transition metal in Earth-crust and cheaper in comparison to noble metals. In comparison, iron is approximately 25000 times cheaper than Ru, [START_REF]Ruthenium today | Precious Metals | SMM -China Metal Market[END_REF] one of the most widely used for photocatalytic applications. Moreover, its extraction process is more sustainable and efficient, since the remediation and by-products are less toxic. 17 However, developing photosensitizers based on 3d transition metal complexes (TMC) that can contend with those based on noble metals is a real challenge. [START_REF] Mccusker | Electronic Structure in the Transition Metal Block and Its Implications for Light Harvesting[END_REF][START_REF] Wegeberg | Luminescent First-Row Transition Metal Complexes[END_REF] The main issue is that 3d-metals face weaker ligand fields than 4d-or 5d-species, since 3d orbitals are more compact than those of 4d-or 5d-metals. In consequence, the overlap between ligand and metal orbitals is weaker in the former case and, as a result, the corresponding ligands tend to be more labile than those of 4d-or 5d-complexes (Fig 4a). Moreover, 3d-complexes present a large number of metal-centered (MC) states of similar energy close to the electronic ground state (GS) energy level. Access to these excited MC states results typically in more distorted molecular structures respect to that of the GS as a consequence of the elongation of the metal-ligand bonds, 21 favoring a nonradiative relaxation process from the MC states to the GS (Fig 4b). Concerning 4d-and 5d-metal complexes, they present stronger ligand fields leading to larger energy gaps between the MC states. In some cases, it is worth noting that the energy gap of the lowest MC and the GS can be large enough to avoid a non-radiative relaxation. In a such scenario, the lowest MC state can become luminescent and photochemically active, 21 as well there are reports of 3d complexes with MC state emitters. 20 Notwithstanding their inherent small ligand fields, important advances for photoactive 3d TMC has been made in recent years achieving stable compounds with sufficiently longlived relevant excited states and even presenting luminesce. [START_REF] Wenger | Photoactive Complexes with Earth-Abundant Metals[END_REF][START_REF] Wegeberg | Luminescent First-Row Transition Metal Complexes[END_REF] As mentioned before, among the transition metals, iron is the most abundant and the fourth most common element in the Earth's crust. Besides the concomitant advantages regarding its low price and accessibility, iron is placed in the same column as ruthenium in the periodic table and thus, similar properties could have been expected for iron complexes while offering a greener and cheaper alternative.

Unfortunately, their photophysical behavior is rather different, spurring the scientific community in the last decade to contribute to this very important field of chemistry. In the following sections we will first describe and compare the photophysical properties of Fe(II)

MC -1 MC -2 GS MC -3 MC -1 GS a) b) c)
and Ru(II) complexes that will allow for a better understanding of the subsequently explained design of ferrous compounds explored so far to address the issues regarding light conversion applications.

I.1 Photophysical properties of Fe(II) and Ru(II) complexes.

In order to rationalize the design of photoactive complexes, it is imperative to understand the fundamentals of their photophysics and photochemistry. To begin with, it is helpful to identify the possible low-lying electronic ES arising from one-electron excitations. [START_REF] Crosby | Spectroscopic Investigations of Excited States of Transition-Metal Complexes[END_REF] In the case of metal complexes bearing π-conjugated ligands, four main electronic states can be identified: i) Metal-centred (MC) states, involving electronic transitions within the d orbitals (d-d) and whose energy directly depends on the ligand-field splitting parameter

 (Figure 5, MC).
ii) Ligand-centred (LC) states, which result from transitions between occupied and empty π orbitals (π-π*) of the ligands (Figure 5, LC). complexes, Tanabe-Sugano diagrams can be an important tool (Figure 6). These diagrams provide information on the relative order of the different spectroscopic states as a function of the field splitting of the complex. Moreover, in the case of d 6 complexes, a drastic change in the state ordering is observed as a consequence of the spin configuration. The Fe-complex is found very close to the high/low spin barrier, consequently presenting metal-centred states ( 5 T2, 3 T1) lower in energy than the MLCT states. In contrast, the Ru analogue has a higher Δ/B value, hence the MC states are more destabilized, leaving the MLCT states as the lowest-lying.

For sake of explanation, a representation of ligand field splitting in [Ru(bpy)3] 2+ and [Fe(bpy)3] 2+ complexes is depicted in Figure 6b and 6c respectively. With an assumed octahedral symmetry and a simplified 4d 6 electronic structure, upon [Ru(bpy)3] 2+ complex formation, the metal-centred d orbitals split in three low-energy orbitals with t2g symmetry and two high-energy orbitals with eg symmetry. As a consequence of the high Δ0 value for this 4d complex, a (t2g) 6 low spin ground-state configuration is privileged. The highest occupied molecular orbital (HOMO) being localized on the t2g system and the lowest unoccupied molecular orbital (LUMO) on the empty π* orbitals. Upon light absorption (λ = 452 nm)

[Ru(bpy)3] 2+ complex lead to the population of 1 MLCT states, followed by fast intersystem crossing (ISC) to the energetically lower 3 MLCT state (Figure 7, left). The weakly distorted 3 MLCT state is long enough (τMLCT = 1μs) to perform an oxidizing/reducing single electron transfer (SET) or to allow a phosphorescent decay. [START_REF] Prier | Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis[END_REF] The small distortion favours the radiative excited-state relaxation against the nonradiative deactivation. In sharp contrast, although having the same type of orbitals, the MLCT lifetime of [Fe(bpy)3] 2+ is several orders on magnitude shorter. The weaker ligand field exerted by Fe(II) implies that the eg orbitals are less destabilized and lie consequently at lower energies than the π* ligand orbitals (Figure 6b). After light absorption, [Fe(bpy)3] 2+ populates the 1 MLCT state, followed by ISC into 3 MLCT state. Due to the small ligand field splitting, MC states are energetically lower to MLCT states leading to an ultrafast relaxation (~ 150 fs) from the 3 MLCT into 3/5 MC states, followed by vibrational relaxation on a time scale of few ps into the 1 GS (Figure 7, right). [START_REF] Cannizzo | Light-Induced Spin Crossover in Fe(II)-Based Complexes: The Full Photocycle Unraveled by Ultrafast Optical and X-Ray Spectroscopies[END_REF][START_REF] Zhang | Mechanistic Studies of Photoinduced Spin Crossover and Electron Transfer in Inorganic Complexes[END_REF][START_REF] Auböck | Sub-50-Fs Photoinduced Spin Crossover in [Fe(Bpy)3]2+[END_REF] As a result, Fe(II) complexes as [Fe(bpy)3] 2+ are unsuitable for applications seeking to exploit the MLCT state, having been mostly studied for spin-crossover applications. The strategies to avoid this detrimental non-radiative deactivation, and thus to increase luminescent or photocatalytic properties, rely on preventing (or slowing down) the MC states population upon their destabilization together with a further stabilization of the MLCT states.

Increase of the ligand field strength can be attained by using strong ligands such as Nheterocyclic carbenes (NHC), 28 cyanides 29 or cyclometalating units [START_REF] Steube | Excited-State Kinetics of an Air-Stable Cyclometalated Iron(II) Complex[END_REF] which has proven the most successful way to increase the MLCT excited-state lifetimes in Fe(II) complexes.

Additionally, improving metal-ligand interaction further enhances the effects exerted by a certain ligand.

Indeed, when the frontier orbitals are spatially separated, an notable stabilization of the 1/3 MLCT states can be induced upon introduction of electron-withdrawing groups [START_REF] Liu | A New Record Excited State 3 MLCT Lifetime for Metalorganic Iron(II) Complexes[END_REF] or increase of the π-conjugation at the LUMO (π* orbital) [START_REF] Carrillo | Substituent-Induced Control of Fac / Mer Isomerism in Azine-NHC Fe(II) Complexes[END_REF] . In the case of heteroleptic complexes with push/pull ligands, the MLCT manifold can be lowered, with the concomitant appearance of even lower ligand-to-ligand charge-transfer (LLCT) states. [START_REF] Mengel | A Heteroleptic Push-Pull Substituted Iron(II) Bis(Tridentate) Complex with Low-Energy Charge-Transfer States[END_REF] However, although this way of reasoning is very helpful when designing a complex, it is important to bear in mind that most ligand modifications will likely affect both the MC and MLCT manifolds.

I.2 Strategies towards Iron(II)-based complexes with long-lived

MLCT

Several synthetic approaches have been explored to increase the MLCT lifetimes in ferrous complexes, including modifying the nature of the coordinating units, improving the geometry depending on the type of ligand chelation or strain of the coordination sphere. In the following sub-sections, the main strategies will be reviewed in detail, where the examples have been selected so as to better illustrate the parameter being discussed.

I.2.1 Maximizing M-L interaction: ligand chelation

Maximizing the overlap between metal and ligand orbitals is key for enhancing the ligand field. In the case of an octahedral coordination, the optimal interaction is found between the axially aligned eg metal system (orbitals dx 2 -y 2 , dz 2 over the axes x,y,z, Figure 8) with the sigma ligand orbitals due to symmetry, achieving maximal values when bond angles approach those of the ideal octahedron (90° for L-M-L cis angles and 180° for L-M-L trans angles). This interaction will impact the eg * level, being more destabilized as the interaction upon complex formation gets stronger. This can be nicely illustrated when comparing bidentate and tridentate coordination as in [Ru(bpy)3] 2+ and [Ru(tpy)2] 2+ (tpy= 2,2':6',2''-terpyridine), respectively.

While [Ru(bpy)3] 2+ is emissive at room temperature, [Ru(tpy)2] 2+ is rather non-emissive. [START_REF] Medlycott | Designing Tridentate Ligands for Ruthenium(Ii) Complexes with Prolonged Room Temperature Luminescence Lifetimes[END_REF] This is a consequence of the poor interaction between the tpy and metal d orbitals, due to a smaller N-Ru-N angles as shown in Figure 9. This strategy has also been applied to iron complexes, of which some examples will be described in Chapter 2. complexes.

Apart from the ligand denticity, changing the ring chelation also allows for tuning the metal-ligand orbital overlap. McCusker and co-workers reported [Fe(dcpp)2] 2+ (dcpp=2,6bis(2-carboxypyridinyl)-pyridine), [START_REF] Jamula | Synthesis and Characterization of a High-Symmetry Ferrous Polypyridyl Complex: Approaching the 5 T 2 / 3 T 1 Crossing Point for Fe II[END_REF] a ferrous complex bearing a tridentate ligand with a N-Fe-N trans angle of 178.3° (Figure 10a). The improvement of the bite angle was accomplished by expanding the tpy ligand with carbonyl bridges between the pyridine units. The electron withdrawing character of these groups contributed to a concomitant stabilization of the π* orbitals that allows for a better interaction with the t2g system, thereby further increasing .

These modifications of the metal-ligand interaction can be nicely observed in the ESL that increases up to 280 ps at room temperature. However, in spite of the improvement, it was found that the excited state corresponds to a 3 MC state instead of MLCT state. [START_REF] Britz | Using Ultrafast X-Ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(Dcpp)2] 2+[END_REF] Following this train of thought, Heinze and co-workers applied this concept to obtain the heteroleptic complex

[Fe(dcpp)(ddpd)] 2+ (ddpd= N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine) with N-
Fe-N trans angles 178.4° (Figure 10b). [START_REF] Mengel | A Heteroleptic Push-Pull Substituted Iron(II) Bis(Tridentate) Complex with Low-Energy Charge-Transfer States[END_REF][START_REF] Förster | Ddpd as Expanded Terpyridine: Dramatic Effects of Symmetry and Electronic Properties in First Row Transition Metal Complexes[END_REF] Apart from the more appropriate coordinating geometry of the six-membered chelating rings, the push-pull character afforded by the electronwithdrawing dcpp with the electron-rich ddpd ligand resulted in lower MLCT states with a 3 MC of 548 ps -double in comparison with that of McCusker-and no participation of 5 MC state due to higher destabilization attributed to ligand push-pull design. It is worth to note the concomitant appearance of an even lower-lying ligand-to-ligand charge-transfer (LLCT) states.

Gros and co-workers reported the synthesis of [Fe(dqp)2] 2+ (dqp=2,6-di(quinoline-8yl)pyridine) complex (Figure 10c). [START_REF] Darari | Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties[END_REF] This pincer-type ligand combined an improved coordination of the metal centre (N-Fe-N bite angle of 178.4°) with the lower π* orbitals of the quinoline moieties, together with a reduced flexibility respect to the aforementioned expanded tpy ligands. As a result, an improvement of the photophysical properties was expected. Yet, the complex presented a 3 MLCT lifetime of 0.45 ps, which is just slightly longer than that observed for [Fe(tpy)2] 2+ . This result can be ascribed to the weaker ligand field created by the six Fe-N bonds that stabilizes the 5 MC, which participates actively in the relaxation process unlike [Fe(dcpp)2] 2+ and [Fe(dcpp)(ddpd)] 2+ complexes. From these results it is evident that improving the coordinating symmetry at the metallic center is not enough to efficiently destabilize the MC states. In addition to this, the increase of flexibility results in non-radiative vibrational relaxations that appear to rule the ES dynamics.

I.2.2 Rigidity increase

An

I.2.3 Strong field ligands

The increase of the ligand field strength can be alternatively be reached by using strong C-donor ligands, [START_REF] Bowman | Tuning the Electronic Structure of Fe(II) Polypyridines via Donor Atom and Ligand Scaffold Modifications: A Computational Study[END_REF] which proved to be the most successful way to alter the order of MC and MLCT levels to increase the MLCT excited-state lifetimes in Fe(II) complexes. Graffney and co-workers have reported the effect of strong donor ligands in the [Fe(bpy)(CN)4] 2-complex (Figure 12). [START_REF] Zhang | Manipulating Charge Transfer Excited State Relaxation and Spin Crossover in Iron Coordination Complexes with Ligand Substitution[END_REF] They measured a 20 ps MLCT lifetime which, in comparison to that of [Fe(bpy)3] 2+ , is two orders of magnitude longer. Furthermore, no experimental evidence of 3 MC and 5 MC excited states was found, suggesting a significant destabilization of the MC excited states relative to the 3 MLCT manifold. Nevertheless, the most studied C-donor ligands are certainly N-heterocyclic carbenes (NHC). A carbene is a neutral carbon species with only six electrons in its valence shell. [START_REF] Bourissou | Stable Carbenes[END_REF] Carbenes are classified as singlet (spin paired) (Figure 13a) or triplet (unpaired spin) (Figure 13b) as a function of their electronic structure. In general, the carbon possesses a sp 2 hybridization with an empty orbital p (pπ) and one full σ orbital (according to different authors is a px with strong σ character). Most carbenes are bent but in extreme case carbenes with linear geometry can be found (Figure 13c). [START_REF] Bourissou | Stable Carbenes[END_REF] Another sub-type of carbenes, the so-called persistent carbenes (also known as stable carbenes) is a type of carbene which demonstrate particular stability, it can either be of triplet or singlet nature as well. A widely studied type of persistent carbenes are the NHCs, relevant in coordination chemistry for their air stability [START_REF] Arduengo | An Air Stable Carbene and Mixed Carbene "Dimers[END_REF] and their easily tunable electronic and steric properties. 45 In particular, NHC have an out-of-plane empty p orbital and a full σ-orbital (Figure 13d). Thus, they are neutral ligands, with a strong σ-donor character and a varying π-acceptor character as a function of the π-donation exerted by the nitrogen atoms adjacent to the carbenic carbon. Although first described in 1969, [START_REF] Öfele | Nucleophilic Cyclic Carbenes as Complex Ligands on Transition Metals[END_REF] iron NHC complexes have received increasing interest since the 2000s, especially for their successful application in catalysis. [START_REF] Riener | Chemistry of Iron N -Heterocyclic Carbene Complexes: Syntheses, Structures, Reactivities, and Catalytic Applications[END_REF] However, it was not until 2013 that Wärnmark and co-workers investigated he photophysics of this type of complexes. [START_REF] Liu | Towards Longer-Lived Metal-to-Ligand Charge Transfer States of Iron(Ii) Complexes: An N-Heterocyclic Carbene Approach[END_REF] 2+ , where methyl groups were introduced. This result was attributed to the higher steric demand of t Bu, that impedes the metal coordination with longer Fe-L bond distances, which results in a photophysical behavior resembling that of the classic Fe-polypyridine complexes. [START_REF] Fredin | Exceptional Excited-State Lifetime of an Iron(II)-N -Heterocyclic Carbene Complex Explained[END_REF][START_REF] Lindh | Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis[END_REF] Electronic modification within the tridentate scaffold was thoroughly investigated by Gros, Haacke and coworkers. [START_REF] Darari | Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties[END_REF][START_REF] Duchanois | NHC-Based Iron Sensitizers for DSSCs[END_REF] These authors prepared [Fe(cpbmi)2] 2+

(cpbmi=(carboxypyridine-2,6-diyl)bis(1-methyl-imidazol-2-ylidene)), a complex bearing a carboxylic acid group in the pyridine seeking to stabilize the MLCT states, as well to examine the possibility of engaging it in electron transfer processes (Figure 15a). [START_REF] Duchanois | An Iron-Based Photosensitizer with Extended Excited-State Lifetime: Photophysical and Photovoltaic Properties[END_REF] Nicely, the introduction of these electron withdrawing groups led to an increase of the 3 MLCT lifetime to 16.5 ps due to the stabilization of the π* ligand orbitals. More interestingly, sensibilization of a TiO2 semiconductor surface with this COOH-based complex led for electron injection, [START_REF] Harlang | Iron Sensitizer Converts Light to Electrons with 92% Yield[END_REF][START_REF] Marri | A Series of Iron(II)-NHC Sensitizers with Remarkable Power Conversion Efficiency in Photoelectrochemical Cells*[END_REF] allowing for the generation of a photocurrent in a dye-sensitized solar cell (DSSC) device. This result was of paramount importance since it opened the door to the development of materials for solar energy conversion based on Earth-abundant elements.

On the other hand, replacement of the imidazole-based NHC by a more π-conjugated benzimidazole afforded [Fe(pbmbi)2] 2+ (pbmbi=(pyridine-2,6-diyl)bis(1-methylbenzimidazol-2-ylidene) (Figure 15b), which also resulted in an increased 3 MLCT lifetime of 16 ps. The increase in the MLCT lifetime is attributed to an enhanced charge transfer character as well as a destabilization of the MC level due to the more π -accepting character of the BIm units. The same group reported the complex [Fe(cpbmbi)2] 2+ (cpbmi=(carboxypyridine-2,6diyl)bis(1-methyl-benzimidazol-2-ylidene)) by mixing -COOH and BIm units affording an even more increased 3 MLCT lifetime of 26 ps (Figure 15b). Furthermore, Gros and Haacke attempted both MLCT stabilization and MC destabilization within a series of azine-modified ferrous complexes. [START_REF] Darari | Iron( II ) Complexes with Diazinyl-NHC Ligands: Impact of π-Deficiency of the Azine Core on Photophysical Properties[END_REF] As a result of a judicious design combining pyrazine and benzimidazole units, [Fe(pzbhbi)2] 2+ (pzbhbi=(pyrazine-2,6diyl)bis(1-hexyl-benzimidazol-2-ylidene)) attained a 3 MLCT lifetime of 32 ps, which is the actual record for NHC-based Fe(II) complexes (Figure 15c). Apart from the electron accepting character of the tridentate ligand, an essential role was attributed to the hexyl chains since it has a double action; at first the hexyl chains can interact with the diazine rings of the other ligand, this interaction may hinder the puckering motion and thus structural relaxation. At second, it can shield the complex to reduce the ligand-solvent interaction, diminishing the dynamic solvation and preventing a faster relaxation.

Bauer and coworkers showed that increasing the number of NHC groups coordinating the Fe center leads to a destabilization of the MC states (HOMO), as well this leads to a destabilization of the π* orbitals (LUMO) due to the lower number of pyridines coordinated. [START_REF] Zimmer | The Connection between NHC Ligand Count and Photophysical Properties in Fe(II) Photosensitizers: An Experimental Study[END_REF] Moreover, a clear correlation between the 3 MLCT lifetime and the number of Fe-NHC bonds was found. While one and two NHC donors led to 3 MLCT lifetimes below 100 fs, the 3 MLCT reached 3.6 ps and 8.1 ps with three and four NCH units, respectively. However, Bauer and Heinze later probed that only 2 NHC are enough to achieve comparable lifetimes (9.2 ps) on improving the M-L interaction (Figure 16). [START_REF] Reuter | Higher MLCT Lifetime of Carbene Iron( II ) Complexes by Chelate Ring Expansion[END_REF] They used a tripodal CNN ligand dpmi (dpmi=(di(pyridine-2-yl)(3-methylimidazol-2-yl)methane)) with 6-membered chelated rings with in the complex [Fe(dpmi)2] 2+ leading to close to 90° bite angles enhancing iron-ligand orbital overlap. Other attempts to increase even more the ligand field splitting with other types of carbenes have been made. For instance, mesoionic carbenes (MICs) have a stronger σ-donor character and better π-accepting capability thanks to their structure and increased number of N atoms in its structure. [START_REF] Schweinfurth | Metal Complexes of Click-Derived Triazoles and Mesoionic Carbenes: Electron Transfer, Photochemistry, Magnetic Bistability, and Catalysis: Metal Complexes of Click-Derived Triazoles and Mesoionic Carbenes: Electron Transfer, Photochemistry, Magnetic Bistability, and Catalysis[END_REF][START_REF] Brown | Tridentate) Ruthenium-Terpyridine Complexes Featuring Microsecond Excited-State Lifetimes[END_REF][START_REF] Araki | Synthesis and Chemical Transformations of 1,3-Diaryltetrazolium Salts. Preparation of Mercury(II) and Palladium(II) Complexes of 1,3-Diaryltetrazolylene and Reactions of 5-Substituted 1,3-Diphenyltetrazolium Salts with Nucleophiles[END_REF] In this regard, the groups of Sundström, Wärnmark and co-workers coordinated the mesoionic ligand btz (btz=3,3'-dimethyl-1,1'-di-p-tolyl [4,4'bis-(1,2,3-triazol-5-ylidene) to Fe(bpy)Cl2, affording the heteroleptic complex [Fe(btz)2bpy] 2+ with a 3 MLCT lifetime of 13 ps (Fig 17a). [START_REF] Liu | A Heteroleptic Ferrous Complex with Mesoionic Bis(1,2,3-Triazol-5-Ylidene) Ligands: Taming the MLCT Excited State of Iron(II)[END_REF] These authors described as well the corresponding homoleptic complex [Fe(btz)3] 2+ (Fig 17b). As a result of the very strong coordination sphere due to the 6 carbene units, an impressive ESL of 528 ps was obtained for the 3 MLCT state, being still one of the longest achieved so far for a Fe(II) complex. [START_REF] Chábera | Fe II Hexa N -Heterocyclic Carbene Complex with a 528 Ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime[END_REF] Bauer's group included this 1,2,3-triazolebased MIC in a tridentate ligand with a pyridine unit in the middle. The resulting complex

[Fe(btp)2] 2+ (btp=4,4′-(pyridine-2,6-diyl)bis(1-ethyl-3-methyl-1,2,3-triazol-5-ylidene))
exhibited a comparable 3 MLCT lifetime to [Fe(btz)2bpy] 2+ , which could be regarded as a geometrical isomer of the former (Figure 17c). complexes A different way for extending the 3 MLCT lifetimes can be achieved by introducing additional chromophore units on the coordinating ligands, [START_REF] Wang | Photophysical Behavior of Transition Metal Complexes Having Interacting Ligand Localized and Metal-to-Ligand Charge Transfer States[END_REF] as successfully applied for Ru(II) [START_REF] Castellano | Altering Molecular Photophysics by Merging Organic and Inorganic Chromophores[END_REF] and Cu(I) [START_REF] Leydet | Equilibration between Three Different Excited States in a Bichromophoric Copper(I) Polypyridine Complex[END_REF] complexes. The presence of close-lying excited states from the chromophore ( 3 Chrom) allow for a thermal equilibrium with the 3 MLCT state, acting as a reservoir for the excitation energy and contributing to slow down the excited state relaxations. In the case of Fe(II) complexes, a theoretical study of octahedral NHC-based complexes bearing an anthracene at 4 position of the central pyridine, connected by different linkers as electron acceptor unit was carried out by Monerris et al (Figure 18). [START_REF] Francés-Monerris | Photophysical Properties of Bichromophoric Fe(II) Complexes Bearing an Aromatic Electron Acceptor[END_REF] These authors predicted the lowest energy triplet state to be localized on the anthracene. Depending on the spacer the absorption capacities increases or decrease. Among the spacers, the alkynyl one allows a large pidelocalization over the NHC ligand and anthracene. Thus it may act as an energy reservoir for an excited state population delaying the excited-state decay of the complexes, leading to possible extended excited-state lifetimes and enhanced luminescence. 19). [START_REF] Dierks | Ground-and Excited-State Properties of Iron(II) Complexes Linked to Organic Chromophores[END_REF] Unfortunately, no electronic reservoir was observed by equilibrium between 3 MLCT and 3 Chrom, with both complexes presenting 3 MLCT lifetimes ≈ 13 ps. Nevertheless, an antenna effect from 1 Chrom to the 1 MLCT via internal conversion (IC) was reported. Alternatively, Dixon and co-workers reported in 2015 a theoretical investigation of the effects of cyclometallating ligands on the photophysical properties of Fe(II) complexes as an alternative to NHC ligands. [START_REF] Dixon | Reversing the Relative 3 MLCT-3 MC Order in Fe(II) Complexes Using Cyclometallating Ligands: A Computational Study Aiming at Luminescent Fe(II) Complexes[END_REF] Interestingly, a possible inversion of the 3 MC and 3 MLCT states was predicted with tridentate phenyl-bipyridine C^N^N ligands where a double cyclometallation is possible. Additionally, the 5 MC states are not be populated in the relaxation cascade. Experimentally, Bauer and co-workers initially reported a mono-cyclometalated polypyridine Fe(II) version, [Fe(pbpy)(tpy)] 2+ (pbpy=6-phenyl-2,2′-bipyridine) (Figure 20a). [START_REF] Steube | Excited-State Kinetics of an Air-Stable Cyclometalated Iron(II) Complex[END_REF] This mono-cyclometalated complex yielded a 3 MLCT lifetime of 0.8 ps, which represents a significant increase compared to [Fe(tpy)2] 2+ (145 fs), demonstrating its beneficial effect. However, the photophysical characterization revealed that single cyclometallation is no sufficient to reverse the 3 MC-3 MLCT, and only a long-lived 3 MC state (12 ps) was found to be populated in the relaxation cascade. complexes.

Berkefeld and co-workers succeeded in synthesizing the complex [Fe(N phen N'^C')2]

with two cyclometallated units (Figure 20b). [START_REF] Leis | A Photoreactive Iron(II) Complex Luminophore[END_REF] The phenylphenanthroline-derived complex has a strong-field that promotes an MLCT ESL of 1 ns in solution and an outstanding 2.4 ns ESL in the solid state, experimentally proving the computational investigation of Dixon and co.

I.2.4 HOMO inversion

The concept of HOMO inversion, introduced by Jakubikova and co-workers in 2017, was initially conceived to improve the light-absorption properties of Fe(II) species. [START_REF] Mukherjee | HOMO Inversion as a Strategy for Improving the Light-Absorption Properties of Fe(II) Chromophores[END_REF] This theoretical work clearly highlights the importance of the π-donation properties of the ligand.

As previously stated, in Fe(II)-polypyridine complexes, the HOMO possesses a metal-localized t2g character with lower-energy π orbitals (Figure 21, left). Introduction of π-conjugated donor groups entails the destabilization of ligand-centered π orbitals up to about the t2g level, favouring their mixing and resulting in a mixed-character (p+d) HOMO (Figure 21, centre).

Further destabilization of π orbitals can lead to invert the order with t2g metal levels (Figure 21, right) making the HOMO based in the ligand π orbitals. This HOMO inversion could lead to a panchromatic absorption in Fe(II) complexes.

This was recently experimentally demonstrated by Gros and coworkers reporting a series of complexes with variable number of thiophene moieties, which permit the delocalization of the MLCT states on the ligands resulting in an elongation of excited state lifetimes, an increase the molar extinction coefficient and a panchromatic absorption effect (Figure 22). 71 The work of Jakubikova allows us to better understand the work of Herbert and coworkers that designed two complexes by mixing anionic strong σ-donor amido groups with benzannulated N-heterocyclic acceptor units. [START_REF] Braun | Iron(II) Coordination Complexes with Panchromatic Absorption and Nanosecond Charge-Transfer Excited State Lifetimes[END_REF] Interestingly, the complexes [Fe(pqa)2] (pqa= phenanthidin-4-yl)(quinolin-8-yl)amido, R= -t Bu or -CF3) (Figure 23) presented a panchromatic absorption, which correlated well with the idea proposed by Jakubikova. In

I.3 Alternatives to photoactive MLCT Fe(II) complexes

As we have seen, enormous advances have been achieved concerning photoactive Fe(II) complexes, with MLCT ESL attaining even the nanosecond regime. These complexes have proven useful in electron transfer applications (see section §IV.1). However, only the complex reported by Berkefeld exhibits luminescence. [START_REF] Leis | A Photoreactive Iron(II) Complex Luminophore[END_REF] In the search for alternative 3d 6 MLCT complexes, in 2017, Wenger and co-workers synthesized Cr(0) and Mo(0) complexes with chelating isocyanide ligands: the tris(diisocyanide)-chromium complex Cr(CN tBu Ar3NC)3 [START_REF] Büldt | A Tris(Diisocyanide)Chromium(0) Complex Is a Luminescent Analog of Fe(2,2′-Bipyridine)3 2+[END_REF] and the tris(diisocyanide)molybdenum complex Mo(CNAr3NC)3 [START_REF] Büldt | A Molybdenum(0) Isocyanide Analogue of Ru(2,2′-Bipyridine)3 2+ : A Strong Reductant for Photoredox Catalysis[END_REF] (Figures 24a and24b). The Cr(CN tBu Ar3NC)3 complex has a low-spin 3d 6 configuration as a result of a large ligand field splitting provided by the isocyanides. Consequently, the MC states are at sufficiently high energy to avoid nonradiative deactivation process allowing 3 MLCT states with lifetime of 2.2 ns, showing phosphorescence emission at room temperature with an emission quantum yield of φem=10 -5 .

This lifetime is long enough to allow energy transfer reactions from the 3 MLCT state to photon emission by triplet-triplet annihilation up-conversion with anthracene. The Mo 0 complex presents even better properties than its homolog Cr 0 as a consequence of the 4d 6 electronic configuration that confers on it a stronger ligand field than that of the 3d 6 Cr(0) species. In fact, the complex Mo(CNAr3NC)3 presents a 3 MLCT lifetime of 225 ns and a phosphorescence quantum yield of φem=0.045. The oxidation potential of the 3 MLCT is -2.5V (vs Fc/Fc + ), which makes it a strong photoreductant, even stronger than [Ru(bpy)3] 2+ and Ir(ppy)3, which are widely used in photoredox chemistry. Another example developed by Wenger's group in 2021 concerns 3d 6 Mn(I) complexes with MLCT luminescence in solution at room temperature and photoreactivity. [START_REF] Herr | Manganese(i) Complexes with Metal-to-Ligand Charge Transfer Luminescence and Photoreactivity[END_REF] These complexes comprised either bidentate or tridentate isocyanide chelate ligands giving access to air-stable complexes. The complexes [MnL bi ] + (L bi =2,5-bis(3,5-di-tert-butyl-2isocyanophenyl)thiophene) (Figure 25a) and 25b). Both complexes display phosphorescence with quantum yield of φ=0.05% for [MnL bi ] + and φ=0.03% [MnL tri ] + that surpass the isocyanide Cr-complex Cr(CN tBu Ar3NC)3 (Figure 22a). (Figure 26a). [START_REF] Pal | Blue-Emissive Cobalt(III) Complexes and Their Use in the Photocatalytic Trifluoromethylation of Polycyclic Aromatic Hydrocarbons[END_REF] Persson and Wärnmark also reported in 2021 a Co(III) luminescent example,

[MnL tri ] + (L tri =5,5'-(2-isocyano-5-methyl-1,3- phenylene)bis(2-(3,5-di-tert-butyl-2-isocyanophenyl)thiophene)) ( Figure
[Co(PhB(MeIm)3)2] + (PhB(MeIm)3)2=bis(tris(methylimidazolin-2-ylidene)(phenyl)borate)),
bearing an anionic tripodal borate ligand with three carbenes. Most interestingly, the complex presents a rather unusual microsecond emission (≈0.8 μs) from a 3 MC state that can be explained as a consequence of the remarkably strong ligand field (Figure 26b). 77

I.4 Fe(III) complexes

Very recently, remarkable progress has been achieved with Fe(III) d [START_REF] Nazeeruddin | Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO 2 -Based Solar Cells[END_REF] complexes and their photochemical applications, constituting a genuine alternative to previously described Fe(II) d 6 complexes. While essentially having the same frontier molecular orbitals, the partially filled t2g orbitals in low-spin Fe(III) complexes enable low-energy LMCT transitions from a filled ligand π-orbital (Figure 27). A nice example of Fe(III) d [START_REF] Nazeeruddin | Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO 2 -Based Solar Cells[END_REF] complex is [Fe(btz)3] 3+ reported by Wärnmark and coworkers using the ligand btz (btz=3,3'-dimethyl-1,1'-di-p-tolyl) (Figure 28a). 79 Even though [Fe(btz)3] 2+ presented outstanding photophysical properties, [START_REF] Chábera | Fe II Hexa N -Heterocyclic Carbene Complex with a 528 Ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime[END_REF] the properties of the Fe(III) analogue are even more interesting. According to Mössbauer spectroscopy [Fe(btz)3] 3+ has a low-spin 3d 5 electron configuration with a 2 T2 ground state (Figure 28c). Due to its electronic configuration, the complex [Fe(btz)3] 3+ exhibits a LMCT transition with absorption bands peaking at 528 and 558 nm. Excitation of these bands promoted a 2 LMCT emission at room temperature with a lifetime of 100 ps and a quantum yield of 3 x 10 -4 in MeCN becoming the first iron complex that exhibit photoluminescence from a charge transfer state. As for the LMCT state, it presents a weak distortion with respect to the GS, which limits the efficiency of non-radiative relaxation (Figure 28d). The same group described shortly after in 2018 another example of a photoluminescent Fe(III) complex, [Fe(PhB(MeIm)3)2] + , on coordinating the previously mentioned NHC-based borate ligand (Figure 28b). [START_REF] Kjaer | Luminescence and Reactivity of a Charge-Transfer Excited Iron Complex with Nanosecond Lifetime[END_REF] This complex presents a room temperature photoluminescence from a 2 LMCT state with a lifetime of 2.2 ns and a quantum yield of 2%. The anionic nature of the ligand combined with a nearly perfect octahedral NHC coordination and a good metal-

I.5 Use and application of photoactive molecules

Thanks to the reactivity of photoactive molecules with light they can be used in several fields by generating ES that lead to radiative first-order decays such as fluorescence or phosphorescence. When the ES lifetimes are long enough, the molecules can be engaged in second-order processes with other entities such as charge transfer or chemical reactions as in the case of the so-called photosensitizers. Indeed, it is worth saying that electronic ES are much more reactive and with drastically altered properties in comparison to the corresponding GS.

Thus, the coupling of appropriate excited states with appropriate partners can lead to applications in various fields, which range from dye-sensitized solar cells, photochemical catalysis, and phototherapy among others. The next part we detail the most relevant applications.

I.5.1 Dye-sensitized solar cells (DSSCs)

Dye-sensitized solar (DSSCs) are considered good candidates for the next generation of photovoltaic technologies. [START_REF] Muñoz-García | Dye-Sensitized Solar Cells Strike Back[END_REF] These devices are easily fabricated and offer multiple possibilities for modulation. Since the seminal work of Grätzel and O'Regan in 1991, 84 the design of DSSCs has attracted tremendous research interest, improving their efficiency up to a certified 13.5%. [START_REF] Zhang | A Molecular Photosensitizer Achieves a Voc of 1.24 V Enabling Highly Efficient and Stable Dye-Sensitized Solar Cells with Copper(II/I)-Based Electrolyte[END_REF] There are four key components of the DSSCs that can be modified 86 (Figure 29a): i) the photosensitizer (or dye), that is aimed at absorbing and converting the solar energy into electrical one, and is chemically bound to the semiconductor; ii) the semiconducting photoanode, where the oxidation takes place, usually made of a mesoporous oxide layer over a glass substrate with a transparent conducing oxide (TCO) layer (commonly titanium dioxide -TiO2-); iii) the redox mediator, which is needed for the dye regeneration -generally the redox couple iodide/triiodie (I-/I3 -) solution-; and iv) the counter electrode, with those based on platinum being widely used. Modification of each of the components will have an impact on the final performance of the device. However, due to the mutual interdependence of all four components, optimization of the device performance must be tackled as a whole. 86 The operational mechanism of a DSSC 86 begins with the device interacting with light, where the sensitizer (S) is promoted from the ground state to an excited state (S*) (Figure 29b, process 1). After excitation, the excited dye S* injects an electron into the conduction band (CB) of the semiconductor (Figure 29b, process 2) towards the photoanode (Figure 29b, process 3), giving an oxidized dye S + (Figure 29b, process 2'). The redox couple reacts with the oxidized dye, thus recovering S in its original form (Figure 29b, process 4). The electronic charge accumulated in the photoanode is transferred to the cathode at which the redox couple is regenerated, thus completing the electric circuit. Since the total electron transfer process in a DSSC is regenerative, the net charge is zero. [START_REF] Hagfeldt | Dye-Sensitized Solar Cells[END_REF] However, the efficiency of a DSSC depends on the relative energy levels of the components, 86 e.g. the chromophore should be prone and able to inject the electrons into the semiconductor. Thus, it is important to note the side mechanisms that lead to the deactivation of the DSSC. Once the electron density that is

Electrolyte

Redox couple

E + E hv (1) (2) (3) (4) Dye-sensitizer semiconductor (2') (5) (6) (7) a) b) I 3 - 3I - Mesoporous TiO 2 TiO 2 particule
Dye particule

Pt Glass

Redox electrolyte generated after injection of electrons from the S* to the semiconductor, the reduction of S + can happen by reverse electron donation from the CB (Figure 29b, process 5). Similarly, the charge that is in the CB can interact with the oxidized form of the redox couple (Figure 29b, process 6). Furthermore, the excited chromophore may just simply get relaxed in a non-radiative way (Figure 29b, process 7).

Considering the pivotal role of the dye in DSSCs, great efforts have been made to improve DSSC efficiencies by means of dye optimization. Ruthenium-based dyes are far the most widely studied due to their excellent performances. Already in 1993, Nazeeruddin et al. [START_REF] Nazeeruddin | Conversion of Light to Electricity by Cis-X2bis(2,2'-Bipyridyl-4,4'-Dicarboxylate)Ruthenium(II) Charge-Transfer Sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on Nanocrystalline Titanium Dioxide Electrodes[END_REF] reported the N3 dye as standard model of efficient charge transfer sensitizer for TiO2-based DSSCs (Figure 30a). Using N3 as photosensitizer allowed to reach of a photovoltaic efficiency up to 10%, which can be attributed to its absorption from the visible to the near-infrared region.

Moreover, N3 complex shows an efficient photoexcited electron injection into the semiconductor layer via carboxylate anchoring groups. Another remarkable example of the same group is the so-called "black dye", which is a Ru(II) complex bearing thiocyanato and a carboxylated terpyridine leading to an efficiency of 10.4% in a DSSC (Figure 30b). [START_REF] Nazeeruddin | Conversion of Light to Electricity by Cis-X2bis(2,2'-Bipyridyl-4,4'-Dicarboxylate)Ruthenium(II) Charge-Transfer Sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on Nanocrystalline Titanium Dioxide Electrodes[END_REF] In 2005, these authors reported a partially deprotonated N3 dye that led to an improved conversion efficiency up to 11.2%, [START_REF] Grätzel | Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells[END_REF][START_REF] Nazeeruddin | Conversion of Light to Electricity by Cis-X2bis(2,2'-Bipyridyl-4,4'-Dicarboxylate)Ruthenium(II) Charge-Transfer Sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on Nanocrystalline Titanium Dioxide Electrodes[END_REF][START_REF] Nazeeruddin | Combined Experimental and DFT-TDDFT Computational Study of Photoelectrochemical Cell Ruthenium Sensitizers[END_REF][START_REF] Sekar | Metal Complex Dyes for Dye-Sensitized Solar Cells: Recent Developments[END_REF] the so-called dye N719 (Figure 30c), which has become one of the most common benchmarking dyes for DSSCs. [START_REF] Muñoz-García | Dye-Sensitized Solar Cells Strike Back[END_REF][START_REF] Kokkonen | Advanced Research Trends in Dye-Sensitized Solar Cells[END_REF] Many efforts focused on the engineering of the DSSCs, by ligand design of the chromophores seeking to improve their efficiency and stability, [START_REF] Park | Stable Dye-Sensitized Solar Cells by Encapsulation of N719-Sensitized TiO2 Electrodes Using Surface-Induced Cross-Linking Polymerization[END_REF] replacing the anchoring groups by other anions, incorporating like carboxylate, phosphonate or functionalized ancillary groups 92,[START_REF] Huang | Design and Characterization of Heteroleptic Ruthenium Complexes Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells: The Effect of Thiophene and Alkyl Substituents on Photovoltaic Performance[END_REF] have been reported. Moreover, the efficiency of Ru(II)-based DSSCs has remained unchanged over the past ten years. [START_REF] Muñoz-García | Dye-Sensitized Solar Cells Strike Back[END_REF] Even though this kind of DSSCs are stable and reports relatively high efficiency, the complicated synthesis and purification and the high cost of the metals have motivated the development of new types of photosensitizers.

In the case of Fe-based photosensitizers, the group of Gros demonstrated in 2015 the feasibility of the employment of this kind of dyes by reporting the generation of a photocurrent, though very modest (0.13%), with [Fe(cpbmi)2] 2+ under solar irradiation. However, on optimizing the device and the electrolyte, including MgI2 and GuNCS (guanidium thiocyanate)

to improve interfacial charge generation and reducing recombination, a photovoltaic performance of 1% was achieved. [START_REF] Marchini | Recombination and Regeneration Dynamics in FeNHC( II )-Sensitized Solar Cells[END_REF] Following a computational study to understand and improve the dye-to-TiO2 electron injection, [START_REF] Pastore | Interfacial Charge Separation and Photovoltaic Efficiency in Fe(II)-Carbene Sensitized Solar Cells[END_REF] the authors kept working on heteroleptic dyes to favor a vectorial electronic transfer into the semiconductor. As a result, ARM13 reached power conversion efficiency of 1.44% 71 while ARM130 bearing a dimethyloxyphenyl group in the ancillary ligand reached 1.83% [START_REF] Huang | Design and Characterization of Heteroleptic Ruthenium Complexes Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells: The Effect of Thiophene and Alkyl Substituents on Photovoltaic Performance[END_REF] , which is the highest performance reported so far (Figures 31a and 31c respectively). It is worth noting that an interesting performance of 1.3% was also reported by the group of Wärnmark thanks to the

complex [Fe(dtapbmi)(cpbmi)] 2+ (cpbmi=1,10 -(4-carboxypyridine-2,6-diyl)bis(3- methylimidazole-2-ylidene)), dtapbmi=1,10-(4-(di(p-tolyl)amino)pyridine-2,6-diyl) bis(methylimidazole-2-ylidene)) (Figures 31b).

I.5.2 Visible light photoredox catalysis

The term Photoredox catalysis, first used by Michael Grätzel et al. in 1982, refers to a type of catalysis system that can accelerate redox reactions by electron transfer between a photocatalyst and an organic substrate, driven by light. [START_REF] Hockin | Photoredox Catalysts Based on Earth-Abundant Metal Complexes[END_REF] To date ruthenium(II) [START_REF] Kalyanasundaram | Photophysics, Photochemistry and Solar Energy Conversion with Tris(Bipyridyl)Ruthenium(II) and Its Analogues[END_REF] and iridium(III) [START_REF] Dixon | A Family of Luminescent Coordination Compounds: Iridium(III) Polyimine Complexes[END_REF] complexes are the most popular PS due to their intrinsic properties (light absorption in the visible region, long-lived photoexcited states). [START_REF] Prier | Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis[END_REF]100 The principle is the same, after the photosensitizer absorbs a photon in the visible region, an electron of the photocatalyst's on the HOMO t2g metal-centered orbital is promoted to a ligand-centered π* orbital (MLCT). The 1 MLCT goes under ISC to a lower energy 3 MLCT state that should be long-lived enough to engage a SET. This photoexcited triplet states has the property of being more oxidizing and more reducing than the ground-state species. As an example, in oxidative quenching the half reaction is described by the reduction reaction V vs SCE) meaning that this species is a stronger oxidant than the ground state (E1/2 II/I = -1.33

V vs SCE). 101 Thanks to these unique properties [Ru(bpy)3] 2+ * may proceed either by oxidative or reductive quenching (Scheme 1). [START_REF] Prier | Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis[END_REF] This has been exploited in catalysis in redox reactions, C-C bond formations, isomerizations and coupled with other metals to perform several organic transformations. Very recently Wärnmark reported a protocol of addition of alkyl halides to alkenes and alkynes using an Fe-based complex as photoredox catalysis. 103 They took advantage of the long-lived excited state of the 2 LMCT-, 3 MLCT-states and Fe 3+ / 2+ oxidation states via twophoton mechanisms. So far this is the only report where an iron complex is quenched via charge transfer to the ES. The last years, reports of Fe-based photocatalysts have increased finding more application in catalysis. Yet, there are still several challenges to face in order to replace noble metals. First, most of the excited Fe-complexes presents short excited state lifetimes.

Second, most of the reports relay on light-induced electron transfer or atom transfer rather than energy transfer like for the noble metals. Although there is room to improvement to make the 3d earth abundant metals attractive alternatives in this field. 104

I.5.3 Photodynamic therapy

Photosensitizers can be exploited in the field of medicinal chemistry, as for instance in photodynamic therapy (PDT). This therapy relies on the fact that, upon light irradiation, a photosensitizer absorbs one photon and generates an excited state, which triggers the formation of cytotoxic species such as reactive singlet oxygen ( 1 O2) and/or other reactive oxygen species (ROS), causing damage in the vicinity of the cell and eventual cell death (Figure 32). 7,105 Importantly, the photosensitizer should be inactive in the absence of light, allowing for a localized activation and a reduction of the possible side-effects. As general requirements of a PS for PDT are: i) a long-lived electronic excited state, which is responsible of the production of the 1 O2/ROS, ii) minimal cytotoxicity of the non-irradiated PS iii) accumulation of PS in targeted cells (diseased tissue), vi) absorption of the light in the range of relative tissue transparency (700-1100 nm), v) chemical and photochemical stability, vi) prompt PS clearance from the body. In addition, the PS can undergo two-photon excitation (TPE) using NIR light, in this case, the depth of light penetration is greater and the intra cellular components won't be affected by NIR light. 106 The limitations for two-photon PDT include the lack of optical technology for light delivery. 107 To date, thanks to their long excited state lifetimes only heavy TM are used has been explored in this field like Ir(III), Os(II), Pt and Re(I) showing that the metal-based PS are promising candidates for PDT. 7 However, there is no record of any Fe-based complexes in this field. This is another reason to keep in the quest of having Fe-based chromophores with excited long lifetimes.

II. Thesis objectives II. Thesis objectives

As detailed in the introduction, especially during the last decade, there has been a continuously growing interest in developing Earth-abundant metal-based photosensitizers as an alternative to noble metals. So far, the most important advances concern the photophysics of 3d TMC of Cr( 0 Therefore, this PhD thesis aims at preparing photoactive iron complexes with a controlled coordination sphere and including original ligand motifs in order to rationalize their photophysical properties, providing thus important guidelines for their successful use in energy conversion applications.

This manuscript is divided in three main parts:

-Bidentate azine-NHC Fe(II) complexes.

Bidentate ligands instead of tridentate represents an improvement in the interaction between the ligand and the metal, hence a higher destabilization of the MC states could be achieved in order to reduce their deleterious non-radiative EE deactivation. A big challenge that addresses the use NHC-py bidentate ligands in Fe complexes is the non-separable mixture of facial and meridional isomers -Statistically 1:3 fac:mer ratio-(Figure 34). Henceforth, development of a selective protocol for a facial and/or meridional isomer is pivotal to study the impact of the geometrical influence in the photophysical and photochemical properties. -Fe(III) complexes bearing tridentate anionic ligands.

In order to increase the lifetime of the excited states the use of strong donor ligands would be applied, whether amides or carbanions will be subject of study in tridentate ligands (Figure 35). Moreover, the possibility of the improvement in the symmetry and interaction between ligand and metal by ligand design. The homoleptic and heteroleptic versions will be tackled. In parallel to the fundamental studies developed previously, this last chapter deals with a more target-oriented work. In fact, capitalizing on previous results on photoactive tridentate iron(II)-NHC complexes from our laboratory, and in the frame of a French-German consortium, we targeted the preparation of Fe-Co dyads as photocatalysts for solar H2 production. It is desirable that all these compounds, strategies and techniques would be applied to solve a problematic. The problematic addressed will be the search of production of molecular H2 by using solar light as an energy source. We will design supramolecular, heteroleptic and ditopic catalysts (Figure 36). That would be constituted by a photosensitizer and linked to a catalyst. That further solar irradiation would perform energy transfer from the Fe-chromophore through the liker into the catalyst that will perform the H2 formation.

III. 1 Introduction

The stereoselective synthesis of octahedral complexes with unsymmetrical bidentate ligands M(A^B)3 is attractive due to the intrinsic relationship between molecular structure and resulting properties. Indeed, it is well-known that the coordination of this type of ligands often results in a mixture of facial and meridional stereoisomers in a statistical 1:3 ratio (Figure 37). 113 However, these isomers can have different photophysical, electrochemical and magnetic properties, [114][115][116][117][118] with appealing applications in OLEDs, 114,119,120 catalysis 120,121 or supramolecular chemistry [122][123][124] . Considering that isolation of these isomers can be extremely challenging, 113 especially in the case of labile complexes, attaining control over the fac/mer stereoisomerism is thus of great importance. Especially in the case where the nature of donor atoms are remarkably different, selectivity towards a specific isomer can be achieved more easily. Among the parameters controlling the stereoisomerism, temperature is frequently used since it does not require any chemical modification of the complexes. In 2003, Thompson reported a synthesis of triscyclometalated Ir(III) complexes (Ir(C^N)3) in which they manage to control the isomerism thorough temperature. 118 The meridional isomers were obtained at low temperature, suggesting that their formation was kinetically favored, the facial isomers were obtained at higher temperatures, indicating a thermodynamic control in this latter case (Scheme 3a). Once the complexes are formed, isomerization conversions can be performed as well upon heating (with or without acid-base catalysis) or light irradiation. 125 In this regard, the same group reported isomerization reactions from a fac:mer mixture towards single isomer (Scheme 3b). In particular, the authors succeeded in the mer→fac isomerization by refluxing the mer isomer in glycerol, supporting the idea that the facial isomer is the compound thermodynamically favored. An explanation of this is that the meridional configuration places phenyl groups trans to each other, which has a strong trans-effect and destabilizes the compounds, 126 while in the facial complexes all the three phenyl groups are opposite pyridyl or pyrazolyl groups (Scheme 3b). Interestingly, they also reported the mer → fac isomerization by photochemical conversion using an UV lamp for 2h at room temperature.

Other factors such as steric or additional intra-/intermolecular interactions can further modulate the fac/mer ratio, with effects that can largely counterbalance that of the trans-effect, especially in the case of bidentate ligands composed of similar coordinating units, 127 Scott in 2011 reported a nice example where intermolecular interactions rules the stereoisomerism. 128 Facial Fe(II)-complexes are obtained thanks to inter-ligand π-π stacking interactions between phenyl and pyridine moieties. As well, they reported the same results for other metals like Mn(II), Co(II/III), Ni(II) and Zn (II) (Figure 38). Alternatively, the template strategy has been used also for aligning non-symmetric ligands around a metal center. 129,130 In 1998 Weizman three tethered bipyridine units to a benzene scaffold using ester linkages, giving at the end a tripodal ligand. 131 After coordination with Ru III , the ester was hydrolyzed affording a tris-bidentate 2-Ru-D complex with a facial ligand orientation (Scheme 4). This kind of strategy is very useful for so-called "inert" late transition metal with diimine ligands. This strategy takes relevance when the metal ion belongs to the first-row transition due to their higher lability. Thus, the formation of a (hemi)cage complex impedes the rapid equilibration fac-mer in solution.

A different strategy to isolate either mer of fac complex can relies upon either selective crystallization or differential solubility. 127 Although either form can be favored in some cases, a dual action of the anion and solvent permitted selective isolation of fac isomers. 132

Scheme 4. Synthetic procedure to [2-Ru-D] 2+ complex using a template ligand.

Recently Nabeshima and co-workers reported a fac/mer isomerization with tripodal ligands, where they revealed that the fac:mer ratio depends on the solvents. 123 Interestingly, the solvent-dependent selectivity does not relies on the counteranion, metal center and the substituents on the bipyridine (Table 1). In this contribution solvation plays an important role in supramolecular systems that adapt their structures in response to the environmental change.

They conclude that the dispersion term d of the Hansen solubility parameter (HSPs) showed a good correlation with G of the fac:mer isomerization. mer ratio was obtained. 134 In sharp contrast to typical Fe II complexes, no lability was observed for this kind of complexes. Unfortunately, it was not possible to isolate these geometric isomers. Therefore, molecular modeling was chosen to offer some insights into the photophysical properties of these distinct species, particularly regarding their excited state properties. These results showed that, upon electronic excitation, the meridional configuration leads to faster relaxation kinetics, resulting in shorter excited state lifetimes than the facial isomer. 134 Scheme 5. Overview of previous ligand designs explored in the group and their impact on the fac/mer ratio of the corresponding iron(II) complexes.

Prompted by these results, an alternative strategy was selected to favor facial coordination. This geometry was nicely achieved by means of the tripodal ligand afforded complex C1 thanks to the preorganized bidentate units (Scheme 5). 135 A longer excited-state MLCT lifetime (up to 20 ps) was obtained in comparison to that of the mostly mer iron(II) complex bearing C0 (10 ps), which confirms the crucial role of the fac:mer isomerism in the overall decay process. However, the influence of the connecting scaffold and the increased rigidity on the final properties of the facial isomer cannot be ruled out.

In a subsequent study, Kévin Magra selected a series of isomeric bidentate ligands L2a-2c with increased π-conjugated systems. 136 The conjugation of the benzannulated ring have a distinct effect on the energies of the frontier orbitals, as reflected in cyclic voltammetry and UV-Vis spectroscopy. Noteworthy, the increased steric demand of the ligands leads to a modification of the stereoselectivity towards higher fac:mer ratios respect L0 (Scheme 5), highlighting the importance of rational ligand design. Taking into account the interesting photophysical properties of pyridine-NHC iron(II) complexes, the necessity of developing a stereoselective methodology for the preparation of these geometrical isomers became clear to us, that would subsequently preclude the isomer isolation issues. To tackle this challenging objective, the synthesis of these complexes was carried out: i) on modifying the reaction conditions;

ii) upon modulation the stereo-electronic properties of the ligands.

III. 2 Synthesis of Fe(II) complexes with pyridine-NHC ligands.

The prototypical ligand precursor HL0, was selected for our first coordination study.

Its synthesis was carried out following two steps. With HL0 in hand, the complexation conditions previously established in our group (See scheme 7) were applied i.e. the use of FeCl2 as iron source, DMF as solvent and t-BuOK as the base and 1 h of stirring at room temperature under Argon atmosphere (Schlenk). Under these conditions, C0 was obtained in 40% yield with a 6:94 fac:mer ratio.

Scheme 7. Reaction conditions for synthesis of complex C0.

The ratio between fac:mer isomers can be easily determined by comparing the 1 H-NMR spectra of HL0 and C0. In figure 28 on top (green) there is the 1 HNMR of the ligand precursor HL0, and at the bottom that of the complex C0 (red). After the complexation reaction the proton "a" (≈ 9.26 ppm, singlet) corresponding to the proton of the pre-carbenic carbon disappears, signing the formation of the NHC. Then proton "b" (≈ 3.96 ppm, singlet) assigned to the methyl group undergoes a high field shift and gets desymmetrized displaying a unique singlet for the facial isomer due to its symmetry and three singlets for the meridional isomer (Figure 39). The peak integration gives a quick access to the fac:mer ratio showing here a large preference for the mer isomer (6:94).

III. 2.1 Modification of the reaction's conditions

From this first experiment favouring almost exclusively the meridional isomer, we moved to the investigation of the impact of the reaction conditions on the selectivity. Several relevant parameters were varied such as the solvent, the base and the reaction temperature. The main results are collected in Table 2. As already mentioned complex C0 was obtained in 40% yield with a 6:94 fac:mer ratio (Table 2, entry 1) using FeCl2 as metallic source, t-BuOK as base and DMF as solvent, with the reaction being performed at room temperature overnight. 137 We have screened several solvents (entries 2-5). Complex C0 was obtained in slightly lower yield (20-27%) except when using toluene in which the reaction was inhibited (entry 2). Interestingly, the solvent was found to modify the isomers ratios. In fact, the formation of the more apolar meridional isomer appeared to be less favored in solvents such as CH3CN or DMSO, yielding a 25:75 fac:mer mixture. Changing the base also plays an important role, (entries 1, 6 and 7). While no complex was observed with Cs2CO3, the use of KHMDS resulted in an increased facial isomer amount (25:75) respect to that of the reported condition with t-BuOK (6:94) (Entries 1 and 6). From these results, combining DMSO and KHMDS was expected to further favour the formation of the facial isomer (entry 8) but a lower fac:mer ratio (17:83) was obtained instead. Performing this same reaction at high temperature had the most noticeable impact in the stereoselectivity (entry 9). In fact, mixing the ligand and FeCl2 at 180° for 20 min before the addition of the base afforded C0 as a facial-enriched mixture (fac:mer 44:56) reinforcing the hypothesis that the facial isomer is indeed the thermodynamic product.

We envisioned that the presence of the carbene unit prior to the addition of the metallic source might have had an impact in the coordination mechanism (entry 10). Indeed, in the previous experiments, the carbene was only formed after a likely initial coordination of the pyridine unit to the metal center. The carbene preformation reaction was carried out in THF at -20°C using LiHMDS as the base, further addition of FeCl2 and stirring for 1h. The coordination resulted in a statistical fac:mer ratio with C0 in 40% yield. An effect favouring facial complex is observed. This can be attributed to the change of the base, as entry 3 where the fac percentage increases from 8% to 25%, or to the preformation of the carbene prior to iron addition (entry 10). However, the statistical ratio does not mean that there is no preference for any of the isomers.

Despite of having changed several parameters, we did not succeed in achieving selectivity neither in favour of the fac or the mer isomers. The reported protocol (Scheme 7)

remained the most selective, in this case, towards a meridional configuration. The best selectivity towards the facial configuration were achieved with DMSO as a solvent, KHMDS as a base and at high temperature (180°C), attaining an almost equimolar fac/mer mixture.

Temperature is an important factor to favour one isomer over another however, total selectivity was not achieved, and it was not a determinant factor as for other metals. 118

III. 2.2 Isomerization reactions

As stated in the introduction of this chapter, it is possible to convert a kinetic isomer into the thermodynamic one, which frequently corresponds to respectively meridional and facial configurations. Thus, we decided to investigate such mer → fac isomerization in our case.

The isomerization process was first attempted applying some protocols based on either protic 138 or aprotic acid activation with UV-light. 139 Table 3 shows the results with malonic and trifluoro acetic acids (TFA) chosen as protic acids (HA). C0 was first solubilized in EtOAc), then the acid was added and the mixture heated at 70°C (Table 3, entries 1-2). Even after 6 days of stirring, we did not observe any change in the fac:mer ratio. In the case of aprotic acid, C0 was dissolved in CH3CN-d3 and SiO2 was added. Then the mixture was irradiated with UV-light (at 254 and 366 nm)) during 6h at r.t.. No signs of interconversion between isomers could be observed (Table 3, entries 3-4). Of great interest here is the non-lability of the PyNHC ligands in this complex, which is unexpected by comparison with the bipyridine analogue. 113

SiO2 (hv = 366 nm) ACN-d3 6 h 25:75 a Fac:mer ratio determined by 1 H-NMR.

The thermal conversion of C0 (7:93 fac:mer ratio) was also investigated. The 1 H-NMR spectra recorded in DMSO-d6 at various temperatures are depicted in Figure 40. The mixture was heated at 150 °C and analyzed over time. Interestingly, the fac:mer ratio evolved into 13:83 ratio after 12h (green spectrum), 30:70 ratio after 24h (cyan blue spectrum) and 58:42 fac:mer ratio after six days (purple spectrum). These results confirm, in agreement with our previous investigations, the lower thermodynamic stability of the meridional ligand arrangement.

Unfortunately, total conversion could not be reached after extended periods of time where decomposition of the complex occurred.

Even though a total conversion was not achieved, it is remarkable that through thermal conversion, the facial isomer increases its proportion more than 800% from going from 7% to 58% (6 days). On the other hand, this study proved the remarkable thermal stability of C0.

Since we failed in isomerizing or achieving a selective synthesis by modifying the reaction conditions, we decided to develop a ligand-controlled selective synthesis.

III. 3 Influence of the stereoelectronic properties of the ligands on the complexation reaction

Modification of py-NHC ligands was previously investigated in the group, but only at the NHC unit, where the steric hindrance seemed to play a key role. 140 Assuming that the azine unit may coordinate the Fe(II) ion on the first place if the carbene is not preformed (Table 2, entry 10), we reasoned that modifications of this coordination site could have a prominent impact on the stereoselectivity. Thus, we decided to explore the effects of modifying both the substitution nature and pattern in the azine unit on the final complex configuration as shown in Scheme 8. Also, we studied the modification of the carbene unit to ascertain if the selectivity was further affected by its nature. Scheme 8. General overview of the modification strategy of ligand L0.

III. 4 Substituent-induced control of fac/mer isomerism in azine-

NHC Fe(II) complexes.

To begin with, a systematic substitution pattern over the pyridine was planned. For this purpose, two different substituents intended to impulse two levels of steric hindrance were targeted. A low steric hindrance, methyl (HL3-HL8) was selected in order to minimize the inductive or resonance effects (Scheme 9a). As a high level, mesityl (Mes) group (HL6-HL8) was chosen instead, not only due to its high steric demand, but also because of the additional possibility of promoting π-stacking interactions (Scheme 9b). 141 The introduction of the substituents was done at position 4,5 and 6 of the pyridine moiety, since an anticipated weaker metal coordination could be anticipated for functionalization at position 3 due to a reduction of ligand coplanarity. 117 Scheme 9. Synthesis of (a) methyl-substituted ligands HL3-HL5, and (b) mesityl-substituted ligands HL6-HL8.

The synthesis of Me-based ligands HL3-HL5 started with the precursors 3a-5a, which were obtained in good yields by an Ullmann coupling within the imidazole on the corresponding methylated bromopyridine, then a quaternization with iodomethane in DMF at 110°C and finally metathesis with potassium hexafluorophosphate (Scheme 9a). Mes-based ligands HL6-HL8 were prepared from the appropriate halogenated 2-chloropyridines. After the initial selective introduction of the mesityl group by means of a Suzuki coupling affording the intermediates 6a-8a. The imidazole unit was introduced via a SNAr mechanism at the Clsubstituted pyridine position (6b-8b). Once the imidazole was introduced, the quaternization with MeI was carried out followed by I -to PF6 -counterion metathesis. The target precursors were obtained in good yields (71-84%, Scheme 9b).

Based on our previous study on the reaction conditions with unsubstituted ligands (see §III.2.1), we selected FeCl2 as iron source, DMF as solvent and KHMDS as the base since the target complex was obtained in good yield with no preference for any of the isomers, which could thus allow to better evaluate the neat ligand effect. The complexation results of HL3-HL8 precursors are collected in Table 4. All the ligands coordinated successfully, except L8.

The overall yield for complexes C3-C8 is around 45% regardless the ligand nature. However, a clear impact in the fac:mer selectivity was observed depending on the substituent. For the methyl series, the methyl group at position 4 and 5 (L3, L4) did not influence the selectivity, affording C3 and C4 with near statistic 25:75 fac:mer ratio (entries 1 and 2). In sharp contrast, the presence of the methyl group in position 6, adjacent to the pyridinic N atom (L5) led to the exclusive formation of the facial isomer fac-C7 despite a non-negligible steric hindrance towards the iron center (entry 3). We can consider this result as atypical and unexpected since the coordination of 6-methyl-2,2'-bipyridine with iron resulted in complexes with meridional isomerism. 142 As well, the complex presents a low-spin configuration, which differs as well from other similarly substituted facial complexes. 143,144 On the other hand, the Mes group did not show any influence in the final configuration when substituted at position -4 (L6), we can explain this due to its outward directionality. The corresponding C6 complex showed a statistical 25:75 ratio (entry 4). Howbeit, the presence of the Mes group at position -5 results in a sterically hindered environment, particularly in facial configuration. As a result, a direct impact on the final configuration leads to almost exclusively mer-C7 complex was after coordination of L7 (entry 5).

The coordination of L8 with the Mes group in position 6 next to the pyridinic N atom did not proceed, even heating the reaction or performing the carbene before addition of the iron source, the coordination was unsuccessful (entry 6). It is worth to note that similar substitutions at the 5-membered NHC moiety in related Fe II Py-NHC complexes have been described. 141 However, the smaller angle formed between the Mes group and the Fe-N bond due to the 6membered pyridine ring is likely to increase the steric hindrance to such an extent that coordination is completely inhibited.

III. 4.1 Influence of the NHC unit

Having identified the excellent selectivity promoted by the 6-Me (L5) and 5-Mes (L7) substitutions towards respectively fac and mer geometries, , the impact of the NHC electronic nature was then evaluated. Thus, two carbene precursors were envisaged for each series derived from benzimidazole (bIm) and 4-methylimidazole (4-MeIm) to afford more π-acceptor and more σ-donor NHC units respectively. In Scheme 10 is depicted the synthesis of these new ligand precursors HL9-HL12.

The coordination reactions were performed under the same conditions as in section §II.2.1, and the results are collected in Table 4. The coordination of L9-L12 ligands led to the expected complexes C9-C12 in moderate yields (40-49%) (Table 5). The electronic nature of the NHC unit did not affect the facial configuration selectivity with a 6-Me substitution (entries 1-3) or the meridional selectivity in the case of the 5-Mes substitution (entries 4-6), attaining even a complete selectivity for both configurations while keeping the facial or meridional congeners remaining at best below the detection limit in 1 H-NMR. Therefore, these results nicely indicated that the final configuration of the system is mostly determined by the azine substitution pattern in py-NHC complexes.

III. 4.2 Substituent nature variation at position 6

The counterintuitive and unanticipated facial selectivity exerted by the 6-Me substituted ligands L5, L9 and L10 encouraged us to investigate the exact role played by this group, allowing us to understand if a possible agostic interaction between 6-Me with a pyridine group or another non-covalent interaction was the reason of facial selectivity. As a result, a new series of ligand precursors was prepared with varying functionalities at this position. The introduction of a more sterically hindered isopropyl ( i Pr) chain was performed by reacting 2,6-dibromo pyridine with iPrMgCl. The introduction of imidazole via an Ullman coupling followed by quaternization and I -/PF6 -exchange gave ligand HL13 (Scheme 11). Ligands HL14-HL16 bearing halogenated substituents (CF3, F, Cl) and HL17 with a dimethylamino (NMe2) group were also prepared to examine the impact of the electronic effect. The synthesis of these ligands is described in Scheme 11. The synthetic protocols were similar to those previously described starting from the appropriate 2-halogenopyridine except for HL17 where the previous introduction of the NMe2 groups was necessary.

Precursors HL13-HL17 were subsequently involved in the iron coordination step (Table 6). The introduction of the i Pr group induced the same selectivity as the methyl group, i.e. the exclusive facial isomerism, affording fac-C13 in 30% yield (entry 1). With electronwithdrawing halogenated substituents, the results were different (entries 2-4). No complex was obtained with L14 (6-CF3) and L15 (6-F) attributable to an excessively reduced coordinating ability of the pyridine due to high-electron withdrawing character of the substituents, while L16 (6-Cl) led exclusively to fac-C16 albeit in poor yield (18%). This latter example is, however, of particular interest since it would allow for complex functionalization via crosscouplings or SNAr reactions for instance. In presence of the electron-donating NMe2 group, no coordination complex could be obtained (entry 5). A possible origin of such a fate could be coordination by the amino group competing with those of the NHC moiety. From these results we can infer that facial selectivity is not necessarily led by an agostic interaction between the hydrogens of the methyl group and the pyridine ring since the selectivity was not compromised with -Cl and -i Pr substituents.

III. 4.3 Use of π-extended azines

In order to evaluate the scope of this unprecedented ligand-induced stereoselectivity, we anticipated that the steric hindrance afforded by a -CH-unit of a benzannulated ring could be comparable to that of the 6-CH3 (Figure 41). thus, we have prepared quinoline and quinoxaline-based ligand precursors (Scheme 12), together with one based on isoquinoline for comparison reasons (Scheme 13). However, it is worth nothing that other bidentate quinolinebased ligands such as 2-pyridin-2yl)quinoline lead to the exclusive formation of the meridional iron(II) complex. 145 HL18 and HL19 were prepared as we previous ligand precursors by means of a SNAr reaction. However, while HL18 was prepared stepwise in good yield by reacting 2-chloroquinoline with in-situ generated potassium imidazolate followed by quaternization with MeI, HL19 was directly obtained from 2-chloroquinoxaline with N-methylimidazole in 77% yield due to the higher reactivity of this azine. An isoquinoline-based ligand (HL20) was also synthesized with the study the effect of having a conjugated aromatic ring in positions 3 and 4

of the pyridine (Scheme 12) and elucidate if the facial or meridional preference could be guided by having an aromatic ring in these positions. Possibly, the substitution pattern of the isoquinoline can lead to a series of intra or inter ligand interactions conferring certain selectivity allowing us to reach a plausible explanation for the facial selectivity. The synthesis of HL20 was performed via the previously described Ullmann coupling to afford 20a, then a later quaternization to give the target ligand precursor in 77% yield.

Scheme 12. Synthesis of ligand precursors HL18 and HL19 and synthesis of complexes fac-C18 and fac-C19.

The complexation of both π-extended HL18H-HL20 precursors was carried out under the aforementioned conditions using FeCl2 and KHMDS in DMF at r.t. (Table 4).

Complexation of L18 and L19, which implies an important steric hindrance close to the metal, resulted in an almost complete selectivity towards the facial isomer, affording exclusively fac-C18 and fac-C19 complexes in 37% and 34% yield, respectively. Gratifyingly, complex C20

was obtained instead as a mostly meridional mixture (8:92 fac:mer), which allowed us to validate our hypothesis (Figure 41).

As in the pyridine-substituted series, we further investigated the effect of the NHC unit for these type of ligand precursors. The synthesis of two additional quinoxaline-based precursors HL21 and HL22 is presented in Scheme 14, together with their complexation reactions. Selection of the quinoxaline derivatives was based on their improved optical properties respect to pyridine-based complexes (see §III.4.6). Interestingly, complexation was achieved with full stereoselectivity in both cases, affording fac-C21 and fac-C22 in 37% and 34% yield, respectively, and being completely consistent with previous results.

Scheme 13. Synthesis of ligand HL20 and complex C20. Scheme 14. Synthesis of ligand precursors HL21 and HL22 and synthesis of complexes fac-C21 and fac-C22.

III. 4.4 X-Ray analysis

Complexes fac-C5, mer-C7 and fac-C18 were characterized by X-Ray analysis confirming the structural information given by 1 H-NMR. Suitable crystals were obtained grown by slow evaporation of the corresponding solvent; methanol for fac-C5 and acetonitrile for mer-C7 and fac-C18 complexes. Figure 42 shows the structures for the three complexes displaying a distorted octahedral geometry and confirming the anticipated configurations.

Some selected structural parameters are collected in Table 7. Calculation of the stretching (𝜁), angular (𝛴) and trigonal (𝛩) distortions from the ideal octahedral structure (Table 7) 147 allows us to have a more global comparison. Taking as reference the calculated structures of fac-C0, it is evident that the deformation induced by the sterically-demanding azine unit, with an increased asymmetry in bond lengths (𝜁values) and worse M-L interaction (𝛴 and 𝛩 values, with 𝛴=0° and 𝛩=0° for a perfect octahedron).

Interestingly, complex mer-C7 displays the most regular coordination sphere even when compared with parent mer-C0. , with 𝜃 𝑖 being a torsional angle between the ligand atoms on opposite triangular faces of the octahedron viewed along the pseudo-threefold axis.

III. 4.5 Theoretical calculations

In order to gain more insights into the selectivity of the facial coordination, the structural and energetic differences between the fac and mer isomers for the most relevant complexes of this work have been studied using quantum chemistry calculations (collaboration with Dr. Antonio Francés-Monerris, Valencia). The fac vs mer thermodynamic stabilities are

shown in Figure 43. The most stable configuration for most of the Fe II -NHC family apparently is the fac arrangement. A trend that globally agrees with the fac preference observed experimentally. However, more specific analyses reveal that the thermodynamic stabilities of the isomers are not the only factor ruling the fac/mer preferences, suggesting that other factors also play an important role. These factors would include complexation kinetics and/or intramolecular interactions as previously suggested.

The Gibbs energy differences between the two isomers are not equal for all complexes (Figure 43). Whereas C5, C7, C9, and C13 clearly favor the fac disposition (ΔG ~3 kcal/mol), complexes C0, C10, and C18 show almost degenerated energies for both fac and mer arrangements (ΔG ~±1 kcal/mol). This is coherent with the fac/mer isomer distribution (6:94) observed for C0 135 and the fac preference (>98:2) exhibited by C5, C9 and C13. An eyechasing case is C7, which exists mostly as a mer isomer, although the fac disposition is thermodynamically more stable. This evidences that the complexation mechanism penalizes the fac isomer. Meanwhile, the ΔG values for C10 and C18 also do not explain the fac/mer ratio higher than 94:6 as observed experimentally in terms of thermodynamic stabilities, again pointing to extrinsic factors in the complexation process. Comparing complexes C0 and C5 is particularly interesting. The only difference in both complexes is the methyl group at the 6 position of the pyridine ring in C5, which is key to induce opposite isomer preferences: C0 is mostly mer whereas C5 is fac.

A detailed analysis of the Fe-N and Fe-C connections as well as the non-covalent interactions between the methyl groups and the C and N dative atoms coordinated to iron is presented in Figure 44 and Table 8. 8). In general, the net structural deformations upon methylation from C0 to C5 are almost equal for both isomers. These subtle -yet important-differences impact the relative stability of both isomers. As a matter of fact, the thermodynamic stability for fac-C5 is about ~2 kcal/mol larger than that of fac-C0 with respect to their respective mer counterparts (Figure 43). This difference can be attributed to the combination of diverse attractive and repulsive interactions. In this regard, the more pronounced stretching of the more labile Fe-N bonds (as compared to Fe-C) in fac-C5, as well as the more favorable Pyr-CH3•••N non-covalent interactions (as compared to the Pyr-CH3•••C), in which the methyl group can act as a Lewis base 148 and the N as a Lewis acid, may explain the slight energetic differences. In the facial conformation exists a series of non-covalent interactions exists, which are not present in the mer arrangement that can also explain the stability of the fac-C7 (Figure 45) over the meridional isomer. The fac arrangement clearly shows triple T-stacking interactions between the methyl groups of the mesityl substituent and the aromatic rings (Figure 45), whereas in the mer disposition only two 5-Mes groups interact due intrinsic reasons. Thus, can be hypothesized that principal reason to obtain mainly mer-C7 instead fac-C7 isomer (Table 5) is likely due to the concomitant steric congestion induced by the Mes groups in the complexation mechanism. This analysis evidences again the need to consider mechanistic and other extrinsic factors besides the intrinsic stability of the isomers to fully understand the fac/mer isomerism in this kind of Fe II -NHC complexes.

III. 4.6 Ground-state characterization

Optical and electrochemical properties of all facial and meridional complexes were investigated by UV-Vis spectroscopy (Figure 46) and cyclic voltammetry, and the main results are collected in Table 9. As previously reported compounds, 149 the UV-Vis spectra of all complexes present three main absorption bands. The intense bands below 300 nm correspond to 1 (π→π*) transitions centered on the ligands. At longer wavelengths, two distinct broader and less intense MLCT bands are found at longer wavelengths corresponding to Fe-carbene 1 (d→ π*NHC) transitions (320-420 nm) and Fe-azine 1 (d→ π*azine) transitions, which extend well into the visible region (380-650 nm). 149 However, several differences can be distinguished arising from the ligand configuration:

-Less intense absorptions are observed for facial complexes due to their higher symmetry. 135 -Modification of the NHC unit results in a different 1 (d→π*azine) band energy variation in mer and fac series. In fact, an inverse correlation between NHC donation and energy is nicely observed in facial complexes, with MLCT energies varying as fac-C10 (BIm) > fac-C5 (Im) > fac-C9 (4MeIm) (Figure 43, left) and fac-C22 (BIm) > fac-C18 (Im) > fac-C21 (4MeIm) (Figure 46, left). In contrast, no correlation between the MLCT band energy and the NHC donation is identified in the meridional series, with MLCT band energy varying as mer-C7 (Im) > mer-C11 (4MeIm) > mer-C12 (BIm)) (Figure 46, right) (vide infra). As a result, while a MLCT blue-shift is obtained with the BIm carbene in a facial coordination (fac-C10 vs fac-C5 or fac-C22 vs fac-C19), the same carbene induces a MLCT red-shift in meridional coordination (mer-C12 vs mer-C7).

The effect of the 6-substitution on the pyridine unit appears to be related to the relatively elongated 6-Me-pyridine-Fe (Figure 43, left). As a matter of fact, fac-C5 (6R=Me) and fac-C16 (6R=Cl) present an almost identical spectra no matter the distinct electronic nature of their substituents. The increased steric hindrance of the i Pr in complex fac-C13, can be the reason of the less intense UV-vis spectrum. As for the benzannulation of the azine unit, a noticeable shift to lower energies is clearly noticed in the absorbance spectrum as a result of the more stabilized π* orbitals in comparison to the pyridine-based complexes. For instance, bands appeared red-shifted ca. 3200 cm -1 for fac-C20 (quinolyl) and ca. 5000 cm -1 for fac-C21 (quinoxalyl) respect to fac-C7 (pyridyl), also reflecting the higher accepting character of the quinoxaline moiety (Figure 46). When comparing fac-C20 (quinolyl) and C22 (isoquinolyl), a slight blue-shift is observed for the latter (Figure 47). Concerning the redox properties, cyclic voltammetry of the complexes was carried out with SCE as standard electrode and ferrocene as internal reference (Fc + /Fc = 0.39V/SCE in acetonitrile) (see experimental part for further details). At positive potentials, these bidentate fac/mer complexes display reversible Fe II to Fe III oxidations in most cases with potentials ranging between 0.62-1.30 V (Table 8). These values can be nicely correlated to the π-back donation of Fe II to both NHC and azine moieties, which results in higher reduction potentials due to the stabilization of the t2g-like orbital (HOMO). Thus, the increasing electron withdrawing character on going from 4MeIm to Im to bIm nicely correlates with an anodic shift of the oxidation potential in both facial (fac-C9 < fac-C5< fac-C10 < fac-C21< fac-C19< fac-C21) and meridional series (mer-C11 < mer-C7< mer-C12). The same effect is observed when only the azine unit is modified; e.g. 0.69 V for fac-C5 (pyridyl-Im), 0.79 V for fac-C18 (quinolyl-Im) and 1.08 V for fac-C19 (quinoxalyl-Im). On the cathodic range, remarkable differences are obtained as well (-1.91 to -0.82 V).

As for parent C0 (-1.94V, -1.97V), reduction is believed to take place at the azine unit for mer-C7 (-1.86 V) and mer-C11 (-1.90 V), together with those bearing π-extended azines (fac-C18, fac-C19, C20, fac-C21, fac-C22), which show drastic cathodic shifts that correspond well with their increased acceptor character and thus, lower π* orbitals. Moreover, the introduction of more π-conjugated NHC units (BIm) seems to contribute to more delocalized LUMO, resulting in lower potentials as well (fac-C10 vs fac-C5 and fac-C9; fac-C22 vs fac-C19 and fac-C21; and mer-C12 vs mer-C7 and mer-C11). 116 However, the interpretation is less straightforward for fac-C14 and fac-C16, exhibiting different 6R substituents. It is reasonable to think that the steric hindrance exerted by the azine unit in these facial complexes will partially hinder their coordination to the metallic center and therefore, the exact nature of the Fe-L interactions. In consequence, these results highlight the subtle relationship between molecular structure and the corresponding electronic effects. For facial pyridyl-based complexes, and evident dependence of the MLCT energies on the carbene donor strength was identified (Figure 46, left and right). However, meridional complexes are more sensitive to the π-conjugation of the carbene unit, whit opposing effects for BIm-based complexes fac-C10 (Figure 46, rigth)

and fac-C22 (Figure 47) with respect to mer-C12 (Figure 46, right). This result could be attributed to a different localization of the LUMO, i.e. on the NHC moiety rather than on the azine, as it has previously reported for related complexes, 150 precluding any direct comparison.

The unexpected redshift of the MLCT band in mer-C12 could thus be consistent with the higher stabilization of the t2g-like orbital as a result of the π-accepting character of the BIm moiety that is largely offset by a noticeable stabilization of the π* orbital.

Moreover, selection of π-extended azines (fac-C18, fac-C19, C20, fac-C21, fac-C22) leads to a sizeable stabilization of the ligand π* orbital. In consequence, their UV-vis spectra are systematically shifted towards lower energies, with their MLCT band extending over a good part of the visible range. More interesting, considering that these complexes may have high-lying MC states due to their greatly stabilized t2g-like orbitals, an inversion of their relative order of their MLCT and MC excited states could be readily conceivable. The dynamics of the excited state relaxation is currently been investigated by transient absorption spectroscopy in a partnership with Prof. Dr. Stefan Haacke in the University of Strasbourg.

III. 5 Conclusions and perspectives

In this chapter we have described the ligand design guidelines to selectively access both facial and meridional isomers in azine-NHC Fe(II) complexes by means of a room-temperature protocol. As we have demonstrated, the origin of this outstanding selectivity stems from the bulkiness of the azine unit in nearby positions to the N coordinating atom. Interestingly, increase of the steric hindrance at the alpha position (6-substituted derivatives) with relatively small substituents such as a methyl group or a chlorine atom resulted in a surprising fac configuration. In fact, considering the common orientation of these substituents, similar substitution patterns of bidentate ligands have been typically reported to lead to the mer isomer.

In the case of the highly sterically demanding mesitylene group, target complexes were only achieved upon moving away the steric hindrance to position 5 of the azine unit, where mer isomers were mostly obtained instead. In spite of some preliminary computational calculations, explanation of the fac/mer selectivity requires a deeper investigation on the complexation mechanism, which is currently ongoing in collaboration with Dr. Antonio Francés Monerris.

Structural characterization of these complexes has revealed that fac isomers possess more distorted coordination spheres than the mer isomers, which could be deleterious for their EE dynamics. Nevertheless, it is also reasonable to think that the presence of more interligand interactions in fac derivates could prevent Fe-N bond elongation, which has been shown to be the main coordinate driving the MLCT excited states decay. 133 On the other hand, the nature of both the NHC unit and the extension of π-conjugation appeared to be the key parameters controlling their optical and redox properties, with both isomers exhibiting distinct behaviors.

The use of π-extended azines is of particular relevance due to their low-lying MLCT manifold, which results in an improved absorbance in the visible range. In addition, the possible reduction of the electronic coupling with the MC states could contribute to longer-lived MLCT. A proper analysis of fac/mer influence on the EE kinetics will be the object of a following contribution to be taken in Strasbourg by Prof. Dr. Stefan Haacke.

Thus, this chapter highlights the great potential of this type of complexes that capitalize on the synergetic effect of combining a certain substitution pattern at the azine unit together with NHC. As a result, not only non-labile iron(II) complexes with a ligand-encoded stereochemistry can be prepared, but also with opto-electronic properties that can be easily fine-tuned as a function of both the azine and the NHC moieties. Moreover, π-extended quinoline and quinoxaline moieties exerted a similar preference for facial isomerism.

IV. Fe(III) complexes bearing tridentate anionic ligands IV. Fe(III) complexes bearing tridentate anionic ligands IV. 1 Introduction

As already mentioned in the introduction, the destabilization of the MC states is crucial for the development of photoactive iron complexes. Thus, the use of ligands inducing a sufficiently strong ligand field is a common feature of the different reported synthetic strategies (see §I.2.3). In this regard, the nature of the donor units plays a pivotal role on the energy splitting of the metal d orbitals. As theoretically shown by the group of Jakubikova, C-donor ligands induce the strongest ligand fields, followed by N-donors and O-donors, being the latter the weakest ligands (Figure 48). However, imposing a strong electron-donating coordinating sphere results in a concomitant destabilization of Fe(II) respect to higher oxidation states, making the complexes easier to oxidize into Fe(III), as observed for neutral NHC 79 or (formally) anionic ligands such as cyclometallating rings, 153 amides, [START_REF] Braun | Iron(II) Coordination Complexes with Panchromatic Absorption and Nanosecond Charge-Transfer Excited State Lifetimes[END_REF] or phenoxides. 154 For instance, Figure 51 depicts some Fe(IV) exemplars. Interestingly, while [Fe IV ( benzNHC OCO)2] has been described to be stable only in solid state (Figure 51a), 154 anionic [Fe IV (L-6H)] 2-is exceptionally stable under aqueous, acidic or basic media due to the shielding effect provided by the macropolycyclic cage on around the iron center. 155 Nevertheless, it is possible to counteract the overall metal orbital destabilization by means of the introduction of electron withdrawing groups. In fact, Jakubikova presented a theoretical analysis of Fe-pyridylcyclometallated complexes, in which the introduction of nitro (-NO2) and carboxylic acids (-COOH) onto the ligand scaffold stabilized the orbital energies via electrostatic effects, conferring them oxidative stability in the Fe(II) oxidation state. But more importantly, the ligand field was maintained or even improved, constituting a very useful design strategy when using very strong ligands. 153 [Fe(L CNC )2] + complex (Figure 52). 156 The IR absorption was attributed to a 2 LMCT transition instead. Unfortunately, the authors characterized the excited state, revealing a 2 LMCT ESL of only 1.3 ps. However, they did not study in detail such a fast decay.

Thus, the objective of this chapter was to develop iron complexes bearing tridentate anionic ligands that could stabilize Fe(II). To modulate the electron-donation character of the coordination sphere, several strategies will be investigated including the substitution of NHC by imine donors or the introduction of electron-withdrawing moieties within the ligand scaffold.

IV. 2 Amido tridentate anionic ligands

Motivated by the outstanding panchromatic absorption and nanosecond ESL of the pincer-like amido complexes reported by Herbert, [START_REF] Braun | Iron(II) Coordination Complexes with Panchromatic Absorption and Nanosecond Charge-Transfer Excited State Lifetimes[END_REF] we decided to explore the effects of other anionic N-donor ligands on the properties of Fe complexes. To this end, we selected the carbazole-based ligands H3L23, H3L23 and HL25 shown in scheme 16. These ligands were expected to induce strong ligand-fields as a result of strong donor units (carbazolate together with NHC in H3L23 and H3L24) as well as an improved M-L interaction due to the 6membered chelate rings (Figure 53). Modulation of the donor properties of the ligands was envisaged at the lateral coordinating units. It is important to note that two articles from Wenger's group were published while developing the work described in this section: the first one concerning HL25 (pyridine, See scheme 15), 157 then a second one in early 2022 concerning H3L24 (imidazole). 156

IV. 2.1 Ligand synthesis

The synthesis of the ligands is depicted in Scheme

IV. 2.2 Complex synthesis: homoleptic series

With the target ligands in hand, we proceeded with the complexation reactions starting with H3L23 (imidazolium) (Scheme 16). Capitalizing on our experience, we opted for a stepwise protocol with the formation of the ligand L23 in THF at low temperature (-40ºC) prior to metal coordination following FeCl2 addition (See §III.2.1). [START_REF] Housecroft | Inorganic Chemistry: Solutions Manual[END_REF] Selection of LiHMDS was motivated to favor LiCl formation as a driving force for the reaction. After purification of the reaction crude, a green solid was obtained. The 1 H-NMR analysis was not consistent with the initial target ferrous complex C23 in Fe(II) oxidation state, with no signals from the ligand being neither observed (Figure 54). Instead, the corresponding paramagnetic Fe(III) species C23 + was obtained (40% yield) as established by HRMS and X-ray analysis and further confirmed by UV-vis spectroscopy and electrochemistry (see below). The formation of the Fe(III) form resulted from the strong donor character of the ligand. A difference in 1 H-NMR between ligand H3L23 and complex C23 + can be observed in Figure 54, attributed to the paramagnetic nature of the complex. 158 Crystals suitable for X-ray diffraction were obtained by slow evaporation of CH3CN.

Nicely, the formation of the homoleptic complex C23 + was confirmed. The associated X-ray structure is shown in Figure 55 and some selected structural parameters are gathered in Table 10. Interestingly, the octahedral geometry around the central iron atom is close to an ideal octahedron, with an average bite angle of ≈176.4° approaching the ideal 180°. The Fe-N bond distances are slightly shorter (1.898(2) Wärnmark. [START_REF] Kjaer | Luminescence and Reactivity of a Charge-Transfer Excited Iron Complex with Nanosecond Lifetime[END_REF] On the other hand, a pronounced torsion of 34.18º is observed for the NHC units respect to the carbazole moiety, resulting in an helical conformation for the tridentate ligand.

Thanks to the X-ray structure we could envisage that both isomers exists, for more clarity in After having validated the complexation conditions, we switched to the other ligands of the series, i.e. H3L24 (triazolium) and HL25 (pyridine). To our surprise, these

N1 N2 N3 C2 Fe C19 N5 N4 C2 N2 N1 N3 Fe C19 N5 N4
complexations were unsuccessful. Therefore, a new set of optimization reactions was performed, with the results being collected in Table 11. Since HL25 (pyridine) was supposed to be a ligand with better coordination ability, we began the new optimization process with this precursor upon adjusting the number of base equivalents to 1.2 per number of acidic-H (three for H3L24 and one for HL25). In order to favor the coordination to the metal center, a series of reactions were performed on changing the iron source for more labile species: FeBr2 (entry 2) and FeOTf2 (entry 3) respect to FeCl2 (entry 1). However, no complex was obtained, with the initial ligand being recovered after the reaction. Considering the slightly lower bite angle imposed by L25 (6-membered pyridine) in comparison to L23 (5-membered imidazole), the metal coordination might be more difficult. As a result, an attempt was done with FeCl3 instead since Wenger contemporarily reported a Cr(III) complex with the same ligand (entry 4). 157 Nevertheless, no complex was neither obtained, so ion size seemed not to play an important role. Following the same train of thought, another test was carried out increasing the temperature of the coordination step up to 50°C, but without success (entry 5).

IV. 3 Complex synthesis: heteroleptic series

In order to stabilize Fe(II), we opted for the reduction of the number of strong donor ligands by conceiving tpy-based heteroleptic Fe-complexes (Figure 56). We envisaged that the improved M-L interaction afforded by L23 (imidazole) could result in a sufficiently high field splitting in spite of the tpy ligand, which would provide with low-energy * orbitals.

Furthermore, tpy ligands allow for an easy and controlled access to heteroleptic complexes as it has previously reported in our lab. 159 The presence of the pendant pyridine in 4'-(pyridin-4yl)-2,2':6',2''-terpyridine (4'-pytpy) would offer additional opportunities for property finetuning due to its electron-withdrawing character, along with the possibility of protonation or even additional metal coordination.

IV. 3.1 Ground state characterization

The optical and electrochemical properties of complexes C23 + , [Fe(L23)(tpy)] 2+ and [Fe(L23)(4'-pytpy)] 2+ were investigated by UV-Vis spectroscopy and cyclic voltammetry and the main results are collected in Table 12 and Figure 57. The UV-vis spectra of the 3 complexes present a set of intense absorption bands in the UV-region (270-400 nm) that are assigned to 1 (π→π*) transitions. Concerning the visible region, different absorptions are observed. In the case of C23 + , a series of absorption bands extend up to the green region with relatively high  values (> 5•10 4 M -1 cm -1 ) that seem to be the result of an admixture of metal 3d and orbital ligands recalling the "HOMO inversion" concept as already described for other similar complexes. [START_REF] Mengel | A Heteroleptic Push-Pull Substituted Iron(II) Bis(Tridentate) Complex with Low-Energy Charge-Transfer States[END_REF][START_REF] Braun | Iron(II) Coordination Complexes with Panchromatic Absorption and Nanosecond Charge-Transfer Excited State Lifetimes[END_REF] The lowest-energy band at 700-900 nm is attributed to a LMCT transition. 156 As for the heteroleptic complexes [Fe(L23)(tpy)] 2+ and [Fe(L23)(4'-pytpy)] 2+ , an absorption band is observed at ca. 560 nm, that can be attributed to the MLCT transition 1 (dπ → π*tpy). 159 The very broad band at the lowest-energy part of the spectrum is tentatively attributed to a LMCT transition, though contributions from LL'CT (ligand-to-ligand charge transfer) transitions are likely as well. Calculations are currently ongoing in collaboration with Dr.

Antonio Francés Monerris to gain insight into the exact nature of these electronic transitions.

Cyclic voltammetry of homoleptic and heteroleptic complexes were recorded with SCE as standard electrode and ferrocene as internal reference (Fc + /Fc = 0.39V/SCE in acetonitrile) (see experimental part for further details). For complex C23 + , at positive potentials a first reversible anodic peak appears at 0.51 V while for [Fe(L23)(tpy)] 2+ and [Fe(L23)(4'-pytpy)] 2+ complexes at 0.94 V and 0.96 V respectively, which can be attributed to Fe IV/III oxidation. A second anodic event is observed for the three complexes around 1.02-1.17 V that can presumably be attributed to ligand oxidation. Especially in the case of the heteroleptic complexes, these results could support the idea of metal-ligand orbital mixtures as described in the "HOMO inversion" concept. [START_REF] Mukherjee | HOMO Inversion as a Strategy for Improving the Light-Absorption Properties of Fe(II) Chromophores[END_REF][161][162][163] Due to the strong donor character of the amido ligand we obtained three Fe(III) complexes. In the following section we will abord the cyclometalated ligands that are anionic strong donor ligands as well.

IV.4 Cyclometalated tridentate anionic ligands

Bauer's group has recently reported a nice Fe(III) example in which the complex 58), 163 shows a Janus-type dual emission by combining MLCT and LMCT luminescence. Interesting, the corresponding excited states exhibited a record 4.2 ns ESL for the MLCT state and a 0.2 ns ESL for LMCT state. Furthermore, not only luminescence is very rare for Fe-based compounds, [START_REF] Leis | A Photoreactive Iron(II) Complex Luminophore[END_REF]79,[START_REF] Kjaer | Luminescence and Reactivity of a Charge-Transfer Excited Iron Complex with Nanosecond Lifetime[END_REF] but a dual emission is a rather uncommon feature even for noble metals. 165 The unusual and unique properties of [Fe(ImP)2] + complex can be attributed to the ligand design upon combining four NHC units with two cyclometalating phenyl rings with both strong σ-donor and acceptor capabilities (Figure 58). In the framework of a 2-month internship at Bauer's lab within the DrEAM program, we envisaged new cyclometalated complexes by incorporating electron-accepting cyclometalating azine units while maintaining the strong σ-donor character of the ligands (Figure 58). This modification was driven by our interest in stabilizing the MLCT excited states for subsequent applications such as n-type DSSC [START_REF] Marri | A Series of Iron(II)-NHC Sensitizers with Remarkable Power Conversion Efficiency in Photoelectrochemical Cells*[END_REF]166 or photocatalysis (see § I.6).

[Fe(ImP)2] + (HImP=1,1'-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene)) (Figure
Furthermore, the eventual stabilization of cyclometalated Fe(II) species could be also of utmost importance due to their photocatalytic or luminescent properties. 69

IV.4.1 Synthetic strategies to access cyclometalated iron complexes

In comparison to other metals, in particular those of the platinum group such as Ru, Os, Rh, Ir, Pd or Pt, cyclometallation reactions involving Fe are synthetically very challenging mainly to the low stability of the Fe-C bond, even when stabilized through multidentate chelates. 167 Since 1959 with the work of Hübel and Braye 168 to 2005 with the work of Klein,169 iron cyclometalated complexes have been relatively rare. One methodology widely used is the one based of iron( 0) precursors bearing phosphine ligands as Fe(PMe3)4 [169][170][171] or carbonyls like Fe(CO)5 170 that, in combination with ortho-directing groups, afford the corresponding cyclometallated intermediate has been used by Che 152 and Bauer 172 to obtain heteroleptic complexes upon complexation of a second ligand as reported (Scheme 18). 169,170,173 Scheme 18. Assisted synthesis of orthometallation using with Fe(PMe3)4 and Fe(CO)5 with different ligands.

Methodologies to obtain cyclometalated-iron complexes using transmetalation reagents and bases are scarcer. In regard to transmetalation reactions, metals like zirconium 163 and mercury 174 has been used, but the price of the former and high toxicity of the latter prompt the research for greener alternatives. For instance, Fout reported the use of base as LiHMDS has been employed to form an NCH-Fe(III) bond followed by a further reduction to Fe( 0

IV.4.3 Synthesis of homoleptic complexes

At first, we followed the Bauer's metalation/transmetalation strategy 163 that starts by reacting the ligand HL-ImP with Zr(NMe)4 (Scheme 22). Two possible intermediate organozirconium species could be formed (A or B) 176 from which a transmetalation would happen after the addition of FeCl2. Once the complex is obtained, a presumably air oxidation occurred followed by an anion exchange, affording the final [Fe(ImP)2] + complex with a 31% yield.

Based on the previous results from Bauer, the preparation of complex C26 + was initially attempted by means of the metalation/transmetalation strategy (Table 13).

Ligand H3L26 was first mixed with Zr(NMe2)4 in THF for 2h, then FeCl2 was added and the reaction mixture was stirred during 16h (Table 13, entry 1). Only byproduct 28 was formed. The high electrophilicity of the position 4 and 6 of the pyrimidine ring in combination with the imidazolium as leaving group clearly favor the double SNAr reactions of the amides instead of deprotonating the procarbenic proton. Then we changed the reaction times in order to decrease this undesired side reaction. As in entries 2 and 3, the ligand H3L26 was mixed with Zr(NMe2)4 in THF for 1 h instead, then FeCl2 was added and stirred for 16h and 2h, resulting in the same 28 product. The formation of byproduct 28 as only product regardless the time of metalation/transmetalation prompted us to change of strategy.

Scheme 21. Schematic representation via a metalation/transmetalation strategy in [Fe(ImP)2] + complex formation.

Aiming to avoid byproduct 28, we tried to inhibit its formation decreasing the number of NMe2 surrounding the Zr center by addition of 2 equiv of MeI to form the intermediate Zr(NMe2)2I2. So, we mixed Zr(NMe2)4 with MeI (entry 3) for 1h, then ligand H3L26 was added and stirred over 3h, after this time FeCl2 was added and led stirring overnight. Unfortunately, the only product was still byproduct 28.

Additionally, instead of -NMe2 we opted for a more sterically hindered base. ZrCl4 was mixed with 4 equiv of TMPMgCl (2,2,6,6-tetramethylpiperidinylmagnesium chloride lithium) in THF with the aim to form Zr(TMP)4 and thus prevent the substitution of the imidazolium (Entry 5). However, the reaction did not proceed either, giving a decomposition of the ligand and no substitution of the TMP over the pyrimidine. In view of these results, the transmetalation/metalation strategy was abandoned and we decided to change the approach by using [Fe(HMDS)2] (HMDS=hexamethyldisilazane), so as to deprotonate-coordinate as fast as possible to promote the complex formation. 177 Contrary to other reported protocols, we opted by an in situ formation of this starting material due to its high reactivity, which makes difficult its proper storage. The [Fe(HMDS)2] was formed under Schlenk conditions by mixing FeCl2 and LiHMDS at low temperature (Scheme 22). In parallel, in another Schlenk the ligand H3L26 was deprotonated with LiHMDS to generate the carbenes (Scheme 22, L26), then it was transferred into the Schlenk containing the preformed [Fe(HMDS)2] at -40°C. The reaction mixture was stirred overnight allowing it to reach room temperature, and complex C26 + was obtained in 36% yield after a filtration over Celite.

IV.4.4 Ground state characterization

Optical and electrochemical properties of complexes C26 + and C27 + were investigated by UV-Vis spectroscopy and cyclic voltammetry, and the main results are collected in Table 14 andFigure In the range of 300-450 nm, there are two additional bands that are can be ascribed to MLCT transitions. 163 In fact, the electron accepting character of the pyrimidine backbone results in a bathochromic shift of these MLCT bands respect to that of [Fe(ImP)2] + . In addition, a further bathochromic shift is observed for C27 + in comparison to C26 + , which agrees well with the lower ligand-based * orbital of the CF3 derivative. )

Wavelength (nm)

C26 + C27 +
At longer wavelengths (590-670 nm), the broad bands with very low extinction coefficients are tentatively assigned to LMCT transitions, which are again red-shifted in comparison to [Fe(ImP)2] + . 163 In the case of C26 + , the presence of the phenyl substituent leads to an increased absorption, especially in the UV region.

As for the electrochemistry, at positive values, both complexes present one reversible oxidation potential at 0.71 V and 1.03 V for C26 + and C27 + , respectively, attributed to the couple redox Fe III /Fe IV , while for [Fe(ImP)2] + the much lower value of 0.47 V indicates a clear effect of the electron withdrawing pyrimidine backbone. However, no the ligand-centered oxidation was observed as in parent [Fe(ImP)2] + (1.62 V).Concerning the cathodic potentials, one reversible wave reduction is observed at -0.35 V and -0.063 V for C26 + and C27 + , respectively, that is attributed to the redox couple Fe II /Fe III . From these results it becomes clear the stabilizing orbital effect of having an electron withdrawing group as -CF3 in comparison to the one with -Ph group, without a remarkable change in the electrochemical band gap.

Interestingly, the reduction to the divalent species becomes noticeable easier for the pyrimidine series respect to the phenyl-based [Fe(ImP)2] + .

IV.5 Conclusions

In this chapter, we synthesized and characterized a set of strong anionic ligands bearing NHC units, three N-amido (carbazolate series) and two C-anionic (cyclometalated series). In regard to the N-amido series, we successfully afforded three complexes, one homoleptic and two heteroleptic. The metal-ligand interaction was maximized due to the formation of 6membered chelates. The strength of the coordination sphere was modulated as a function of the number of coordinated N-amido ligands, although in all cases Fe(III) species were obtained.

Interestingly, both homoleptic and heteroleptic compounds seemed to have metal-based and ligand-based orbital mixing as evidenced by their oxidation behavior, suggesting that this type of ligands could be useful to achieve a full "HOMO inversion" as defined by Jakubikova. More distinct properties arose from their reduction behavior, with the heteroleptic complexes featuring a remarkably anodically shifted first reduction potential that could reflect the less enrich metal ion respect to the homoleptic complex. In addition, reduction of the tpy moieties was observed as well due to their strong acceptor character. Interestingly, both series of complexes displayed an enlarged absorbance until the IR region, with the heteroleptic complexes seemingly showing a higher contribution of outwards electronic transfers (from the carbazolate and the metal towards the tpy, MLCT+LL'CT) respect to the inwards LMCT transitions.

As for the C-anionic ligands, a new methodology to get two new bis-cyclometalated Fe-CCC complexes was developed without the use of glove box and avoiding the use of any transmetallating or reducing reagent. These complexes feature a cyclometalating pyrimidine unit that allows for stabilization orbital effects induced by the pyrimidine backbone and the substituents in position 2. This is reflected on the redox potentials between C26 + and C27 + and [Fe(ImP)2] + : as the electron withdrawing character increases, an anodically shift is observed their reduction potential wave. The UV-vis absorption display three distinct absorption bands corresponding to π-π* (200-250 nm), MLCT (300-450 nm) and LMCT (590-670 nm)

transitions which are lower energy in comparison to [Fe(ImP)2] + . Finally, the presence of the phenyl substituent leads to an increased absorption specially in the UV region.

As a perspective, further optimization reactions are required to perform the coordination of the different N-amido ligands. Moreover, the dynamics of the excited state relaxation of all complexes will be investigated (TAS experiments) in collaboration with Prof.

Dr. Haacke in Strasbourg and with Dr. Antonio Francés Monerris in Valencia will determine the nature of the electronic transitions corresponding to the UV-vis transitions. These calculations and experiments will allow us to understand better the impact of the ligand nature on the ESL. In order to complete this part of the work.

V. Iron complexes as photosensitizers towards H 2 -evolving bimetallic photocatalysts V.1 Introduction

Hydrogen is the most abundant element in the universe. Consisting only in one proton and one electron, it is the building block for the formation of all other elements in the stars by means of nuclear reactions. It is also one of the most abundant elements in the Earth's crust as well as in living organisms, although it is mostly found combined to other elements as in the case of water (H2O). Furthermore, hydrogen plays a pivotal role in acid-base reactions, where protons are exchanged between molecules.

Molecular dihydrogen (H2) is used for numerous industrial purposes such as the production of fertilizers, food processing, pharmaceutics, metallurgy and refining just to name a few. However, H2 is not abundant in nature and must be produced from hydrogenated compounds (NH4, MeOH, i.e.). Interesting, H2 is a particularly light energy carrier that can deliver or store enormous amount of energy. For instance, 1 kg of H2 (energy density = 120 MJ kg -1 ) generates more than three times energy than 1 kg of gasoline (energy density = 44 MJ kg -1 ). 178 In addition, H2 can be used as power generator in fuel cells to produce electricity, power or heat, justifying its growing interest as a key actor for the energy transition. Indeed, the demand of H2 has grown more than threefold since 1970 and is continuously rising (Figure 61). 179 Currently, most of the production of H2 comes from fossil fuels (black and grey hydrogen, Figure 61) and, in a lower scale, water electrolysis and biomass (yellow and brown hydrogen). Unfortunately, the production via fossil fuels generates a massive production of greenhouse gases as by products. Thus, there is an urgent need to find environmentally friendly processes for its sustainable production. In this regard, water splitting (WS) is a very promising alternative (Eq 1). Splitting water evolves in H2 and oxygen (O2) as products and can be separated in two half reaction: proton reduction (Eq 2) and water oxidation (Eq 3) with relatively low carbon footprint is associated to this production methodology, ultimately depending on the electricity source (purple hydrogen from nuclear power; green hydrogen from renewable sources, with sunlight receiving increasingly attention.

𝐻 2 𝑂 ⇌ 𝐻 2 + 1 2 𝑂 2 ∆𝐸°= 1.23 𝑉 𝑬𝒒 𝟏 2𝐻 + + 2𝑒 -⇌ 𝐻 2 𝐸 0 = 0 𝑉 𝑣𝑠 𝑅𝐻𝐸 𝑬𝒒 𝟐 𝐻 2 𝑂 ⇌ 1 2 𝑂 2 + 2𝐻 + + 2 𝑒 - 𝐸 0 = 1.23 𝑉 𝑣𝑠 𝑅𝐻𝐸 𝑬𝒒 𝟑
Nevertheless, WS into H2 and O2 is an extremely difficult process both from a thermodynamic and kinetic point of view. In this regard, alternatives are in constant development such as molecular catalysis, 181 photoelectrochemical at solid or semiconductor surfaces, 2 biomass conversion or enzymatic [3][4][START_REF] Nazeeruddin | Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO 2 -Based Solar Cells[END_REF] as alternatives in the quest for greener hydrogen production. Among these alternatives, the molecular systems provide the tools to obtain atomefficient catalysts with outstanding intrinsic activities. 181 Indeed, molecular systems can be subjected to high level of design and modification through chemical synthesis. In addition, the mechanisms and kinetics can be probed through spectroscopic measurements, allowing us to have a better understanding of hydrogen evolution systems. Thus the approach here addressed, will be the photocatalytic molecular homogeneous systems. The photocatalytic H2 production can be classified depending on the origin and nature of the proton and electron. According to literature, four main types can be identified (Scheme 23). 182 i) Direct R-H Substrates. Hydrogen production from R-H substrates produces electron and proton equivalents from homo-or heterolysis of R-H bonds. This process often uses inner sphere mechanisms, with the catalyst directly bounded to the substrate. The common substates are alkanes, primary and secondary alcohols, which after H2 generation produce alkenes, aldehydes or ketones as by-products.

ii) Acids. Hydrogen can come directly from acids, the main drawback here is the degradation of the catalyst by oxidation of the conjugate base.

iii) Carbon Monoxide. Reaction between water-gas-shift (WGS) couples the oxidation of CO to CO2 with the reduction of water to H2. iv) Indirect RH substrates and acids. This is the most common approach. 6 The first approach was a three-system component that consisted on 1) A redox photosensitizer (PS), which absorbs light and make the electron transfer to 2) An electron relay (ER) that transfer the electrons to the 3) Proton reduction catalyst (Cat).

Scheme 23. Schematic representation of H2 production in a molecular homogeneous catalytic system. Inspired by reference 182 .

The first reported systems were the three-component ones, containing a PS, Cat and an electron relay (ER). Afterwards, further research that showed that the electrons could be transferred directly from PS to Cat, without the necessity of the ER, 182 and the three-component system was simplified into a two-component one. In the two-component system the electron transfer is limited by the diffusion (Figure 62a), whereas in the single-component system (dyad), the photosensitizer and the proton reduction catalyst (PRC) are chemically linked and thus, a vectorial electron transfer through a conjugated linker is required (Figure 62b). In both systems, a sacrificial reagent/electron donor (SR) is required to regenerate the photo-oxidized chromophore. For the two-component system, a wide range of PS have been studied and developed in order to allow thermodynamically-favored electron transfer (Figure 63), such as organic molecules (e.g. Eosin Y) [183][184][185] or metal complexes. The former possess rather intense, though narrow, absorptions and suffer from photostability issues. The latter comprise Co or Znporphyrin pigments 180,186,187 or noble metal-based complexes. PS based on Ru 188,189 and Ir 190,191 have been widely used because they combine long-lived charge transfer excited states, broad light absorption (UV-visible-NIR) and favorable ground and excited state redox properties (Figure 63). especially Pt, Ru and Rh based due to the low overpotential towards proton reduction requiring less energy to perform the reduction (Figure 64). 181,182,192,193 However, their high cost has stimulated the development of more sustainable and affordable alternatives. 187 Particularly molecular catalysts bioinspired by hydrogenases, have been developed based on their active site which are binuclear complexes with two iron or a combination of nickel and iron centers (Figure 64). 194 Inspired by these enzymes, chemists use first row transition metals like Fe Ni, Co, Mn and Cu for the preparation of noble metal-free hydrogen-evolving catalysts. 196 In particular cobaloximes played a pivotal role in H2 evolution due to their performance and easy synthesis, but typically with low stability. 180,197,198 In the last decade, Co-polyamines catalysts have received a great deal of attention due to their robustness and better catalytic activities (Figure 64). A two-component system were both PS and Cat comprising complexes based on nonnoble metals was reported by Wärnmark in 2022. 199 The system is composed of as source of protons (Figure 65). The reported TON is remarkable for an Earth abundant photosensitizer, enabling TON over >1000 without degradation. The H2 evolution proceeds through reductive quenching of the 2 LMCT state of the iron complex instead of via the MLCT states. The fact that Fe(III)-NHC complex has an efficient energy transfer from the reactive 2 LMCT state opens even more the door to possible applications in solar energy conversion processes for catalyst based on Earth abundant metals. In regard to dyads for hydrogen evolution, different models have been synthesized so far. 181,188,194 In late 1980s, one of the first dyads was reported by Rillema and coworkers in a system composed by a polypyridyl Ru(II)-Pt(II) complex (Figure 66), 200 although they did not report any H2 production, these dinuclear complex drew attention due to their potential application as photo-hydrogen-evolving molecular device. photocatalyst for photochemical H2 production (Figure 66). 203 The production of H2 in this system showed a TON of 85 with TEA as SR. The connection between the PS and the Cat is assured by a fully conjugated bridge, which facilitates electron transfer both through conjugated bonds or by outer-sphere electron transfer pathways. In particular, these authors reported a Fe(II)-Co(III) dyad where the reduction proceeds with TEOA as sacrificial reductant and the proton source comes from a solution ACN/Water. The photocatalytic performance of the dyad is increased by a factor of two in respect to the equivalent two-component system. This increase was directly assigned to the directional charge effect of the linker between the Fe(II) and Co(III) metal centers (Figure 67). K. Heyne (Free Univ. Berlin, Germany) and our group was created aiming at developing noble metal-free Fe-Co multi-metallic assemblies for H + reduction. The general scheme for the assemblies as well as the initial molecular targets is depicted in Figure 68.

At a first stage, and capitalizing on our group's expertise, we decided to start our investigation with tridentate Fe(II) complexes bearing CNHC^Npy^CNHC ligands with different linker units to tune the -conjugation (Figure 68, green). Concerning the Co PRC, the most straightforward approach would consist on a cobaloxime at which the Fe(II)-PS would be axially coordinated via a pyridine moiety (Figure 68, orange). As an alternative, a more stable polyimine Co PRC could be obtained upon coordination of the Fe(II)-PS through a bipyridine unit (Figure 68). Interestingly, this approach may simplify the 2-electron transfer required for the H + reduction by having two PS next to the Co center. Thus, with the long-term target of developing noble metal-free Fe-Co dyads in mind, this chapter will deal with the synthesis of some ditopic ligands and the corresponding ferrous complexes as a starting point for this international project.

V.2 Ditopic ligand synthesis

It is relevant to mention that this part of the thesis started almost at the same time than the bidentate azine-NHC Fe(II) complexes (Section §III), and the works of Bauer and Wärnmark group's had not been reported at that time (Sections §IV. 1 and IV.4). Within the framework of the project, we focused our attention to the ligand synthesis and preparation of heteroleptic complexes. We conceived three different carbene-based ligands, of which the imidazolium precursors (pre-ligands) H2L28, H2L29 and H2L30 are shown in Figure 69.

Electron donor Tridentate ligand

PRC-Cobaloxime

Co-Polypyridine PRC

Fe-PS Co-Cat Linker

V.2.1 Iron complexes for monodentate coordination of Cobalt

The synthesis started with the reaction of Since the introduction of the alkyne linker was unsuccessful, we then examined the direct introduction of pyridine at the 4-position of A and we moved to the synthesis of H2L29.

A Suzuki coupling between A and 4-pyridinylbroronic acid afforded C in 79% yield (Scheme 26). The quaternization of the imidazole units was performed reacting C with 2.1 equivalents of MeI. After anion exchange with aqueous solution of KPF6, the tris-methylated product H2L31 was isolated as a single product instead of H2L29 (Figure 70, top). Efforts to avoid the quaternization of the pyridine were made. First addition of only 1 equiv of MeI followed by NMR showed a competition between the pyridine and the imidazole moieties, then a decrease on the temperature reaction from 120°C to 50°C yet the result was the ligand H2L31 (Scheme 26). In order to study the relative reactivity of both imidazole and 4-pyridine units, we carried An alternative synthetic strategy to prepare the ligand precursors H3L28 and H3L29,

where the quaternization step is avoided, is depicted in Scheme 27. In order to overcome this selectivity issue, we started from 2,6-dichloropyridine instead of 4-amino-2,6-dichloropyridine that can be selectively iodinated in position -4 affording a reactive intermediate susceptible to react in coupling reactions and nucleophilic substitutions (Scheme 27). In addition, 2,6dichloropyridine is much cheaper (14 times cheaper than 4-amino-2,6-dichloropyridine ). 205,206 We started introducing an iodine atom in position 4 on the 2,6-dichloropyridine by using n- After bibliographic research we found a couple of methodologies for the same reactant (2,4,6-trifluoropyridine) in which they use a base in an protic media (MeOH and EtOH) 209,210 or aprotic media (THF, DMSO) 211,212 mixed with an amine as nucleophile, in anhydrous conditions deprotonating the nucleophile, 213,214 or even without a base just taking advance of the nucleophilicity of the reactants. 210,215,216 . With this in mind, reactions without a base in EtOH and DMF at r.t. and heating at 90°C were tried, but the reaction did not proceed as expected obtaining only the starting nucleophile 2,2'-dipyridylamine (entries 3-4). Another attempt was done by deprotonating the amine prior to the addition of the 2,4,6-trifluoropyridine seeking to increase the amine nucleophilicity (Entry 3) but without success. As a last attempt in order to promote a SNAr we followed a microwave-assisted protocol. The two reactants were mixed in a mw tube in ACN and heated to 90°C for 1h (60w). Unfortunately, the result was the same without any trace of trifluoropyridne or product. A possible explanation for this lack of reactivity could be due to the nature of the 2,4,6-trifluoropyridine, which has a flash point of 17°C at 1 atm. 217 The flash point is related to vapor pressure, which is dependent on the temperature, meaning that at T°>17°C there is the lowest necessary concentration of vapor to ignite. Thus, even when the system was closed it is possible that 2,4,6-trifluoropyridine was out of the reaction mixture and did not react, which agrees well with the fact that we did not observe any trace of 2,4,6-trifluoropyridine after reaction time.

V.3 Synthesis of homoleptic and heteroleptic complexes bearing ditopic ligands toward H2 evolution

The work described in this part was carried out during an internship in the laboratory of Prof. Dr. Bauer at the University of Paderborn in the framework of the international DrEAM program of Lorraine Université d'Excellence (LUE).

Although the final goal is to prepare a heteroleptic complex with the ligand H2L30 and the so called Bulky Ligand (BuL) reported by Bauer,204 we first synthesized the homoleptic version in order to optimize the complexation conditions as well as for comparison reasons.

The different reaction conditions used for the synthesis of [FeL302] 2+ are collected in Table 17. We first chose some already reported conditions for the complexation of this tridentate ligands [START_REF] Marri | Towards Panchromatic Fe(II) NHC Sensitizers via HOMO Inversion[END_REF]218,219 preconizing FeBr2 as iron source (0.5 equiv) and t BuOK as the base in DMF, but after 5-10 min of stirring no complex formation was detected (entry 1). A plausible explanation could be the coordination of the iron center by dipyridine side thus competing with the C^N^C ligand (Figure 71a). To avoid the competitive coordination, we decided to first form the carbene by deprotonation with LiHMDS in THF before iron addition. One could expect that once the carbenes would be formed and available to complex the metal, the probability to obtain the complex [Fe(L30)2] 2+ would be higher. The deprotonation was carried out at -20°C and let it arrive to r.t. (later we realized that temperature is key for this ligand, beyond 0°C it starts to decompose, see Heteroleptic complex). Unfortunately, after iron addition no complex was observed (entry 2). A plausible explanation is the formation of an unstable intermediate that compete with Fe-center (Figure 71b). In order to reduce the competitive coordination between the iron center and the bipyridine, we changed the iron source to FeCl2. The lower lability of the chlorine ligands respect to the bromo ones might have an impact. The ligand H2L30 and FeCl2 were mixed in DMF for 5 min at r.t., then t BuOK was added at r.t. and stirred for 3h (entry 3). Gratifyingly, the complex [Fe(30)2] 2+ was obtained with 40% yield. With the conditions for the coordination of L30 in hands, we proceeded to synthesize the heteroleptic version. The synthesis of heteroleptic Fe(II)-NHC complexes described by Bauer and coworkers 204 was based on the use of a tridentate bulky ligand (BuL) to kinetically inhibit the formation of the homoleptic complex. Thus, we followed the same protocol starting with the formation of an iron-base Fe(HMDS)2. This latter is obtained by mixing FeCl2 and LiHMDS in dry THF at -70°C and, after a couple of hours, the reactant Fe(HMDS)2 is obtained. It can be stored under inert conditions or used immediately after its preparation (Scheme 30, A). 204 The synthesis continues with addition of a stoichiometric quantity of Fe(HMDS)2 to BuL in THF. Monosubstituted M is formed after 7h, which is stabilized due to the steric hindrance exerted by the diisopropylphenyl groups. In parallel, in another schlenk the ligand H2L30 was suspended in THF and deprotonated with 2 equiv of LiHMDS at -40° allowing it to reach 0°C during 7h. A particular mention for H2L30 is that temperature should not go beyond 0°C or it decomposes. After this time the deprotonated L30 was transfer into the schlenk with M and it was stirred overnight reaching room temperature. Nicely, the heteroleptic complex [Fe(BuL)(L30)] 2+ was obtained with a 33% yield.

Scheme 30. Schematic synthetic representation of [Fe(BuL)(L30)] 2+ complex.

Following this procedure we were able to synthesize an heteroleptic and ditopic complex that can be used to further coordinate a cobalt ion at the dipyridyl site. 204 The main goal will be to have a dyad and/or a triad that can be tested in photocatalytic H2 evolution reaction, which won't be addressed in this thesis due to lack of time.

V.4 Ground state characterization

Optical and electrochemical properties of both complexes were investigated by UV-Vis spectroscopy (Figure 72) and cyclic voltammetry, and the main results are collected in Table 18. complexes was recorded with SCE as standard electrode and ferrocene as internal reference (Fc + /Fc = 0.39V/SCE in acetonitrile) (cyclic voltammograms are depicted in Experimental part, Section §5). In both cases, the oxidation can be attributed to the couple redox Fe III/ Fe II . For complex [Fe(L30)2] 2+ , its oxidation potential is found at 0.59 V while that of the heteroleptic complex [Fe(BuL)(L30)] 2+ is shifted to more anodic values (0.76V). The more stabilized pseudo-t2g level of the latter is attributed to the coordination of the BuL ligand which compares well that of [Fe(pbmi)2] 2+ , thus suggesting the less acceptor character of L30.

In the negative potentials' region, one irreversible reduction at -1.

V.5 Conclusions and perspectives.

In this chapter we described different synthetic routes to obtain ligands H2L28-H2L30.

We have explored diverse synthetic approaches that have led us to finally succeeded in obtaining two ligands, namely H2L29 and H2L30. The complex with the ligand H2L29 was reported during this doctoral work, so we focused on ligand H2L30 from which we obtained two complexes, the homoleptic [Fe(C30)2] 2+ and the heteroleptic version [Fe(BuL)(L30)] 2+ . It is remarkably that in both cases, from the two possible coordinative sites (C^N^C and N^N) it was possible to selectively coordinate the iron center to the tridentate one. On the other hand, the heteroleptic complex was obtained through a stepwise protocol, in which a sterically demanding tridentate ligand (BuL) kinetically inhibited the formation of the homoleptic complex allowing us to introduce a subsequent step.

For both complexes the ground state characterization was made. We could observe from the necessity to increase the electronic differences between the terminal (push) and ditopic (pull) ligands. For instance, the use of an anionic terminal ligand and/or the introduction of electron reservoir units within the ditopic ligand could be envisaged.

VI. General conclusion and perspectives

VI. General conclusion and perspectives

In this doctoral work we had a fundamental approach developing a stereoselective synthesis of Fe(II)-complexes bearing bidentate azine-NHC asymmetric ligands and the synthesis of Fe(III)-complexes bearing two different types of anionic ligands (N-amido, Canionic). Moreover, we dabbled into more applied chemistry as for the synthesis heteroleptic Fe(II) complexes towards H2 production in water splitting reactions.

In regard to the stereoselective synthesis of bidentate azine-NHC Fe(II) complexes, we developed a room-temperature methodology to selectively access complexes with facial or meridional geometry. To achieve this selective fac/mer control we designed several ligands with modified substitution of the azine with groups of various degrees of bulkiness such as Mes and CH3 (See §III.4, Scheme 9). We discovered that the selectivity is closely related to the bulkiness in the proximity to the azine's N coordinating atom. The fac selectivity was achieved when the CH3 (smaller steric hindrance) was at the alpha position of the nitrogen coordinated to the iron center (See §III.4, Table 4). Interestingly, increasing the steric hindrance in this position (6-substituted derivative series) with substituents like isopropyl or chlorine groups afforded the same selectivity (See §III.4.3, Table 6). In fact, comparable substitution sequences of bidentate ligands have often been observed to lead to the mer isomer, considering the common orientation of these substituents. In case of a more sterically demanding group (Mes), a mer selectivity was afforded only with Mes in position 5 of the azine (See §III.4, Table 4). To evaluate the scope of our fac/mer selective methodology, we used carbenes of varied electronic nature from more accepting (bIm) to more donating (4MeIm) moieties. We observed that regardless the NHC electronic nature, the selectivity was maintained, affording a set of facial and meridional complexes with different carbenes (See §III.4.1, Table 5). To improve the optical properties of the pyridine-based complexes, quinoline and quinoxaline-based ligands were prepared. Interestingly, complexation with both ligands afforded the facial stereoselectivity exclusively (See §III. 4.3, Schemes 12 and 13). As in the pyridine-substituted series, we further investigated the effect of the NHC. The synthesis of two additional quinoxaline-based ligands with bIm and 4MeIm as NHC was performed. Interestingly, the complexation proceeded with fac stereoselectivity, being consistent with previous results (See §III.4.3, Scheme 14).

In spite of preliminary computational calculations performed by Dr. Antonio Francés Monerris (Valencia), understanding the fac/mer selectivity requires a deeper investigation on the complexation mechanism, which is currently on going. So far structural characterization has shown that the fac isomers display more distorted coordination spheres (See §III.4.4, Figure 42, Table 7), than the mer ones, which could be deleterious to their EE dynamics. However, it is also conceivable that the facial configurations results in interligand interactions that are no present in the meridional one (See §III.4.5, Figure 44, Table 8). These interactions in the facial conformation might prevent the Fe-N bond elongation, which has been demonstrated to be the primary factor causing the MLCT excited states decay. On the other hand, the nature of both NHC and π-conjugation appeared to be the key parameters controlling their optical and redox properties, with both isomers exhibiting distinct behaviors (See §III. 4.6). The use of π-extended azines is of particular relevance due to their low-lying MLCT manifold, resulting in an improved absorbance in the UV-vis spectrum (See §III. 4.6, Figures 46 and 47, Table 9).

Furthermore, the possible reduction of the electronic coupling with the MC states could contribute to longer-lived MLCT. In this regard, a collaboration with Prof. Dr. Stefan Haacke in Strasbourg is being carried out to study the influence on the EE kinetics. This part of the thesis highlights the great potential of these complexes, which can take advantage of the synergistic effect of coupling a specific substitution pattern at the azine unit with the NHC, is thus highlighted. As a result a synthesis of non-labile iron(II) complexes with stereoselectivity presumably given by inter-ligand interactions. Additionally, the opto-electronic characteristics that can be easily modified as a function of both the azine and the NHC moieties.

In regard to the Fe(III) complexes bearing tridentate anionic ligands, we aimed at obtaining iron complexes with increased long-lived excited state lifetimes. The strategy was the use of strong donor ligands such as amides and cyclometallating units in order to induce strong ligand field and destabilize the MC states (See §IV). Hence, we synthesized and characterized different anionic ligands that bear NHC units together with N-amido donors Interestingly, both homoleptic and heteroleptic compounds seemed to have metal-based and ligand-based orbital mixing as evidenced by their oxidation behavior (See §IV.4.4, Figure 57 and Table 12), suggesting that this type of ligands could be useful to achieve a full "HOMO inversion" as defined by Jakubikova. More distinct properties arose from their reduction behavior, with the heteroleptic complexes featuring a remarkably anodically shifted first reduction potential that could reflect the less enrich metal ion respect to the homoleptic complex. In addition, reduction of the tpy moieties was observed as well due to their strong The presence of the phenyl substituent on the pyrimidine leads to an increase of absorption intensity (See §IV.4.4, Figure 60, Table 14). Calculations are ongoing in collaboration with Dr.

Antonio Francés Monerris to get insights into the nature of the observed UV-vis absorptions for both the N-amido and cyclometallated iron complexes. As complementary measurements in collaboration with Prof. Dr. Haacke in Strasbourg are conducted to measure the dynamics of the excited state relaxation (TAS experiments). Next in order, a series of reductions test will be conducted to the cyclometallated complexes in order to obtain Fe(II)-CCC complexes, then the ground state and excited state properties of the complexes will be investigated. Finally, the synthesis of heteroleptic complexes with push-pull ligands will be attempted aiming to obtain Fe(II)-CCC complexes with vectorial electron flux.

Finally, yet important in this doctoral work, we get into the skyrocketing trend of green H2 production, due to its huge energy density (120 MJ kg -1 ) and its large industrial application (pharmaceutical, refining, agriculture, etc), pursuing the production of green H2 with zero carbon print. In order to fulfil this goal, we planned the synthesis of a set ligands in order to design a bimetallic heteroleptic ditopic Fe-Co dyads that will be tested as photosensitizers in water splitting reaction in the reduction of protons (2𝐻 + + 2𝑒 -⇌ 𝐻 2 𝐸 0 = 0 𝑉 𝑣𝑠 𝑅𝐻𝐸).

This part of the synthesis of the ligands was carried out in our laboratory (L2CM) while the heteroleptic synthesis of the complex was made in the framework of the DrEAM programme in Germany with Prof. Dr. Bauer.

After having explored different approaches to obtain the targeted ligands (See §V. It is worth noting that in both cases, we selectively coordinate the iron center to the tridentate one from the two possible coordinative sites (C^N^C and N^N). Moreover, we were able to synthesize a heteroleptic ditopic complex that may coordinate a cobalt ion using Bauer's protocol. The ground state characterization revealed that the electron-rich nature of the dipyridyl units appears to favor the MLCT implying the BuL ligand rather than L30, preventing the target electronic transfer towards the Co coordinating site. As a result, while actual Co coordination may exert a sufficient pull effect, these findings clearly demonstrate the need to increase the electronic differences between terminal (push) and ditopic (pull) ligands.

For example, using an anionic terminal ligand and/or the introduction of electron reservoir units within the ditopic ligand could be envisaged.

The project is currently ongoing, targeting the differences in electron character between the terminal and ditopic ligand so as to favor a vectorial electron transfer towards the Co coordination site. As for our complex, it will be coordinated to cobalt in order to obtain a triad (Figure 78) and thus evaluate:

-Stability of the triad, ground state characterization and ESL.

-Their performance in photocatalytic H2 evolution and compare activities between dyad and triads. The reaction was then quenched by addition of H2O (15 mL) and extracted with CH2Cl2 (3 x 10 mL). The organic phases were dried over Mg2SO4 and purified over silica column (3:1 Hex:

EtOAc + 1% Et3N).

Method C: General Suzuki cross-coupling reaction for mesitylene introduction

A round flask charged with 20 mL of MeOH:Toluene (

VII.3.1 Compounds 3a-17b

2-(imidazole-1-yl)-4-methyl pyridine (3a). 220 According to aforementioned method B: 0.3 g (75%) of a white solid. Elution mixture for flash chromatography 3:1 Hex: EtOAc + 1% Et3N. In a round flask charged with 2,6-dibromopyridine (0.5 g, 3.38 mmol, 1equiv), imidazole (0.276 g, 4.05 mmol, 1.2 equiv) and Cs2CO3 (2.2 g, 6. 76 mmol, 2equiv) and Cu2O (0.032 g, 0.338 mmol, 0.1 eq) were mixed and stirred at 150°C overnight. After this time the reaction was quenched by addition of H2O (10 mL) and extracted with CH2Cl2 (5mL x 3), the organic phase was washed with NaHCO3aq (5 mL x 2), dried over MgSO4 and the solvent was removed under vacuum, affording 16a (0.48 g, 80%) as a white powder. Elution mixture for flash chromatography 1:1 Hex: EtOAc + 1% Et3N. 1 H NMR (400 MHz, CDCl3) δ 8.32 (s, 1H), 7.72 (t, J = 7.9 Hz, 1H), 7.57 (s, 1H), 7.22 (dd, J = 15.8, 7.9 Hz, 2H), 7.13 (s, 1H).

6-bromo-N,N-dimethylpyridin-2-amine (17a) was synthesized using following literature procedure 229 . 1 H NMR (400 MHz, CDCl3) δ 7.18 (dd, J = 7.6, 0.6 Hz, 1H), 6.60 (d, J = 7.4 Hz, 1H), 6.31 (d, J = 8.4 Hz, 1H), 2.99 (s, 6H).

MHz, CD3CN) δ 159. 2, 145.4, 140.1, 133.4, 133.2, 124.3, 115.5, 110.0, 33.5, 22.9, 8.0 6, 146.1, 140.5, 140.3, 132.4, 129.7, 127.7, 127.3, 124.5, 115.5, 113.3, 113.3, 33.4, 23.0 6, 146.1, 142.3, 139.1, 137.0, 134.7, 134.5, 134.3, 129.3, 116.8, 114.4, 34.8, 21.0, 20.8, 9.2 1, 146.8, 142.4, 141.9, 139.3, 139.2, 137.0, 134.4, 133.7, 131.0, 129.4, 129.1, 128.6, 117.6, 116.6, 114.6, 34.8, 21.1, 20.9. HRMS (ESI-TOF) m/z: [M -PF6 -] + Calcd for C22H22N3 m/z: 328.1805; Found: 328.1773.

Synthesis of 3-methyl-1-(quinoxalinyl)imidazolium hexafluoro phosphate (HL19 PF6 -). 230

In a sealed tube 2 with 2-chloroquinoxaline (1.82 mmol, 0.3 g, 1 equiv) and N-methylimidazole (2.18 mmol, 0.19 g, 1.2 equiv) were stirred in a sand bath at 150 °C overnight.

Then a small portion of MeOH was added (minimum for solubilizing) followed by H2O (5 mL) and KPF6 (2 mmol, 0.37 g, 1.1 equiv). The solution was stirred for 30 min and the precipitated was filtered through a glass funnel filter #3, washed with diethyl ether (3x10mL) and the solid was recovered affording HL19 PF6 -in 0.5 g, (77%) of an orange powder. 25, 141.90, 139.54, 137.92, 133.02, 130.79, 128.55, 125.11, 125.05, 124.11, 124.01, 123.09, 37.45.HRMS (ESI-TOF) m/z: [M -PF6 -] + Calcd for C13H12N3 210.1026; Found: 210.1043.

3,4-dimethyl-1-(quinoxalin-2-yl)-1H-imidazol-3-ium hexafluoro phosphate (HL21 PF6 -). -). According to aforementioned method to ligand HL18. 0.5 g (94%). 1 H NMR (400 MHz, CD3CN) δ 9.45 (s, 1H), 9.28 (s, 1H), 8.30 -8.25 (m, 1H), 8.17 -8.14 (m, 1H), 8.11 (t, J = 3.0, 1.3 Hz, 1H), 8.05 -7.97 (m, 2H), 3.93 (s, 3H), 2.46 (s, 3H). 13C{ 1 H} NMR (100 MHz, CD3CN) 13 C NMR (101 MHz, Acetonitrile-d3) δ 143. 55, 141.88, 140.60, 138.16, 135.56, 135.10, 133.46, 132.52, 130.35, 129.72, 117.05, 35.12, 9.31 In a round flask charged with 4,6-dichloro-2-phenylpyrimidine (2.28 mmol, 0.51 g, 1 equiv) was dissolved in 25 mL of ACN and N-methylimidazole (5.7 mmol, 0.45 mL, 2.5 equiv) were stirred in a sand bath at 90 °C in a sand bath for 5h, after this time a precipitate was formed. The solvent was removed under vacuum and the solid was recovered and washed with acetone (3 x 10 mL) and diethylether (3 x 10 mL). The was recovered affording H3L26 2Cl -in 0.79 g, (90%) of an withe powder. %). 1 H NMR (400 MHz, DMSO-d6) δ 9.72 (s, 1H), 8.67

(td, J = 2.2, 0.9 Hz, 1H), 8.27 (t, J = 1.9 Hz, 1H), 7.92 (s, 0H), 7. 75 -7.62 (m, 3H), 4.06 (s, 2H). 13C{ 1 H} NMR (100 MHz, DMSO-d6) δ 166. 29, 156.77, 136.72, 134.71, 134.03, 129.78, 129.59, 126.55, 119.77, 98.01, 37.63 60, 155.28 (q, J = 681.1, 78.3, 39.4 Hz), 137.93, 126.36, 120.21, 116.33 (q, J = 688.7, 474.1, 229.7 Hz), 104.12, 37.31.

VII.5 General schemes for H2L28-H2L31 synthesis (IV. Iron complexes as photosensitizers towards H2-evolition)

Compounds 1A-C and H2L31 ( §V.2)

VII.5.2 Synthesis of complexes

General complexation method.

Ligand HLx (1 equiv, 0.27 mmol) and FeCl2 (0.3 equiv, 0.09 mmol) were submitted to three vacuum/Ar cycles, after that 1.5 mL of dry and degassed DMF were added, the mixture was stirred under Ar at r.t for 5 min then KHMDS (1.2 equiv, 0.325 mmol) were added. The reaction mixture was stirred at r.t. for 1 h. Afterwards, H2O (10 mL) was added and 1 mL of KPF6 (saturated solution) the reaction was vigorously stirred and the precipitated formed was filtered in a glass funnel filter #4 or #3 (depending on particle aggregation: #4 for small, #3 for bigger). The solid was washed with H2O (10 mL) and diethyl ether (3 x 15 mL) until the solid was a fine powder. The solid was recovered by adding acetonitrile and purified by column chromatography [for 100 mg of ligand 20 cm of high and around 4cm of diameter] using a mixture 10:2:1 (Acetone:water:KNO3(Saturated solution)). Recovering of the phases: All the desired phases were mixed in a balloon and an extra amount of H2O was added. Acetone was subsequently removed under vacuum and a small quantity of NH4PF6 was added until a precipitated appeared. The solid was filtered in a glass funnel filter #4, washed with diethyl ether (3 x 10 mL) and recovered with acetonitrile in a round flask.

Fac-and mer-C20 in 37:63 ratio. General complexation method afforded 76 mg (34%) of an intense reddish solid. 15, 211.85, 209.27, 206.82, 155.00, 154.60, 153.92, 153.20, 146.42, 144.87, 142.15, 140.39, 139.15, 138.75, 138.19, 137.74, 133.00, 132.80, 132.56, 132.30, 130.32, 130.24, 130.04, 129.97, 128.69, 128.61, 128.54, 128.47, 128.41, 128.36, 123.64, 123.53, 122.77, 122.74, 122.51, 122.31, 122.04, 121.67, 121.39, 120.95, 120.91, 120.26, 119.86, 119.51, 119.42, 37.01, 36.83, 36.07, 35.85. HRMS (ESI-TOF) m/z: [M -2PF6 -] 2+

Calcd for C39H33N9Fe 341.6099 ; Found: 341.6099.

Fac-C21. General complexation method afforded 45 mg (34%) of a purplish solid. 1 H NMR (400 MHz, CD3CN) δ 9. 35 (s, 1H), 8.26 (dd, J = 8.4, 1.5 Hz, 1H), 8.10 (t, J = 1.2 Hz, 1H), 7.70 (ddd, J = 8.4, 7.0, 1.2 Hz, 1H), 7.11 (ddd, J = 8.6, 7.0, 1.5 Hz, 1H), 6.79 (dd, J = 8.7, 1.2 Hz, 1H), 2.66 (s, 3H), 2.14 (s, 3H). 13 C{ 1 H} NMR (100 MHz, CD3CN) δ 203. 60, 149.14, 143.17, 142.65, 138.87, 136.59, 133.40, 132.43, 131.10, 123.91, 117.07, 33.22, 10.46 19, -20 ≤ l ≤ 20 -25 ≤ h ≤ 24, -11 ≤ k ≤ 11, -33 ≤ Cyclic voltammetry of complex mer-C7. Cyclic voltammetry of complex mer-C11.

Cyclic voltammetry of complex mer-C12.

Résumé

Cette thèse de doctorat s'inscrit dans le domaine de la chimie organométallique. D'un point de vue fondamental, nous avons étudié le contrôle de la géométrie moléculaire dans les complexes azine-NHC bidentés de Fe(II) lors de la modification du ligand, accédant sélectivement aux complexes soit faciaux, soit méridiens. En outre, les caractéristiques optoélectroniques peuvent être facilement modifiées en fonction des parties azine et NHC, ce qui entraîne une meilleure absorbance dans le spectre UV-vis. De plus, nous avons exploré l'effet de deux unités de coordination (formellement) anioniques, la N-amido et la C-anionique, en obtenant un ensemble de complexes Fe(III). En ce qui concerne la série N-amido, nous avons rapporté la synthèse de trois complexes, un homoleptique et deux hétéroleptiques. Il est intéressant de noter que les composés homoleptiques et hétéroleptiques semblent présenter un mélange d'orbitales basé sur le métal et sur le ligand, ce qui suggère que ce type de ligands pourrait être utile pour obtenir une "inversion HOMO" complète telle que définie par Jakubikova. En conséquence, les deux séries de complexes ont présenté une absorbance élargie jusqu'à la région IR. Quant à la série C-anionique, nous avons développé une méthodologie qui nous a permis d'obtenir de nouveaux complexes Fe-CCC bis-cyclométallisés, en soulignant que l'utilisation de la boîte à gants, d'un réactif transmétallant ou réducteur a été évitée.

De plus, nous nous sommes lancés dans une chimie plus appliquée comme pour la synthèse de complexes Fe(II) hétéroleptiques pour la production de H2 dans des réactions de fractionnement de l'eau. Deux complexes ont été obtenus, un homoleptique, et un hétéroleptique qui a été réalisé en suivant une procédure par étapes rapportée par Bauer.

Mots Clés : Complexes de fer, conversion d'énergie solaire, carbènes Nhétérocycliques

Abstract

This doctoral thesis falls within the field of organometallic chemistry. From a fundamental point of view, we have investigated the control of the molecular geometry in bidentate azine-NHC Fe(II) complexes upon ligand modification, accessing selectively either facial or meridional complexes. Additionally, the opto-electronic characteristics can be easily modified as a function of both the azine and the NHC moieties, resulting in an improved absorbance in the UV-vis spectrum. In addition, we explored the effect of two (formally) anionic coordinating units, the N-amido and C-anionic, obtaining a set of Fe(III) complexes. Concerning the N-amido series, we reported the synthesis of three complexes, one homoleptic and two heteroleptic. Interestingly, both homoleptic and heteroleptic compounds seemed to have metal-based and ligand-based orbital mixing, suggesting that this type of ligands could be useful to achieve a full "HOMO inversion" as defined by Jakubikova. As a result, both series of complexes displayed an enlarged absorbance until the IR region. As for the C-anionic series, we developed a methodology that allowed us to get new bis-cyclometalated Fe-CCC complexes, highlighting that the use of glove box, a transmetallating or reducing reagent was avoided.

Furthermore, we dabbled into more applied chemistry dealing with the synthesis heteroleptic Fe(II) complexes towards H2 production in water splitting reactions. Two complexes were obtained, one homoleptic, and one heteroleptic that was achieved by following a stepwise procedure reported by Bauer.

Keywords: Iron complexes, solar energy conversion, N-heterocyclic carbenes

  A l'heure actuelle, Bauer et ses collègues ont rapporté le complexe hétéroleptique avec l'un des ligands pour une coordination monodentée du Co (Voir §V.2.1, Schéma 27, ligand H2L29). Par conséquent, nous nous sommes concentrés sur la synthèse des complexes homoleptiques et hétéroleptiques (voir §V.3, Schéma 30) avec le ligand ditopique à coordination bidentate pour le centre Co. Le complexe homoleptique [Fe(L30)2] 2+ a été obtenu (voir §V.3 Tableau 17). Ensuite, le complexe hétéroleptique a été obtenu en suivant une procédure rapportée par Bauer dans laquelle ils utilisent un ligand volumineux tridenté (BuL) pour inhiber cinétiquement la formation du complexe hétéroleptique. Le [Fe(HMDS)2(THF)] en solution a été utilisé comme source combinée de fer et de base pour coordonner le BuL, puis le ligand H2L30 a été parallèlement déprotoné et ajouté à la solution de [FeBuL] 2+ , ce qui a permis d'obtenir le complexe hétéropolitique [Fe(BuL)(L30)] 2+ (voir §V.3, schéma 30). Il est intéressant de noter que dans les deux cas, nous coordonnons sélectivement le centre ferreux au tridenté parmi les deux sites de coordination possibles (C^N^C et N^N). Nous avons réussi à synthétiser un complexe ditopique hétéroleptique qui peut coordonner un ion cobalt en utilisant le protocole de Bauer. La caractérisation de l'état fondamental a révélé que la nature riche en électrons des unités dipyridyle semble favoriser le MLCT, impliquant un ligand BuL plutôt que L30, empêchant le transfert électronique cible vers le site de coordination du Co. En conséquence, alors que la coordination réelle du Co peut exercer un effet d'attraction suffisant, ces résultats démontrent clairement la nécessité d'augmenter les différences électroniques entre les ligands terminaux (push) et ditopiques (pull). Par exemple, l'utilisation d'un ligand terminal anionique et/ou l'introduction d'unités réservoirs d'électrons au sein du ligand ditopique pourraient être envisagées. Le projet est actuellement en cours, ciblant les différences de caractère électronique entre le ligand terminal et ditopique afin de favoriser un transfert vectoriel d'électrons vers le site de coordination du Co. Quant à notre complexe, il sera coordonné au cobalt afin d'obtenir une triade (Voir §VI.5, Figure 78) et ainsi évaluer :

  Abbreviation list ...................................................................................................................... I. General Introduction .............................................................................................................

  thiophene) LUMO = Lowest unoccupied molecular orbital MC = Metal centered MLCT = Metal-to-ligand charge-transfer NHC = N-heterocyclic carbenes NIR = Near-infrared o-DCB-d4 = Tetra-deuterated 1,2-dichlorobenzene OLEDS = Organic light-emitting diodes PALCT = πanti-bonding-to-ligand charge transfer' pbbi = (pyridine-2,6-diyl)bis(1-tert-butyl)-imidazol-2-ylidene pbbi = (pyridine-2,6-diyl)bis(1-tert-butyl)-imidazol-2-ylidene) pbmbi = (pyridine-2,6-diyl)bis(1-methyl-benzimidazol-2-ylidene pbmi = (pyridine-2,6-diyl)bis(1-methyl-imidazol-2-ylidene) pbpy = 6-phenyl-2,2′-bipyridine pdmmi = 3,3'-pyridine-2,6-diyl(methylene)bis(1-methylimidazolylidene) PDT = Photodynamic therapy PhB(MeIm)3)2 = Bis(tris(methylimidazolin-2-ylidene)(phenyl)borate) pqa = Phenanthidin-4-yl)(quinolin-8-yl)amido PRC = Proton reduction catalyst PS = Photosensitizer pzbhbi = (pyrazine-2,6-diyl)bis(1-hexyl-benzimidazol-2-ylidene) pzbmi = (pyrazine-2,6-diyl)bis(1-hexyl-benzimidazol-2

3

 3

Figure 1 .

 1 Figure 1. Estimate of finite and renewable energy reserves (Terawatt-years) taken from reference 3.

13

 13

Figure 2 .

 2 Figure 2. Global production of rare earth oxides from 1950 to 2000. Taken from reference 12.

  As a result, the development of robust and efficient photoactive molecular materials based on Earth-abundant metals constitute a genuine milestone to replace environmentally harmful process towards sustainability.[START_REF] Wenger | Photoactive Complexes with Earth-Abundant Metals[END_REF]

Figure 3 .

 3 Figure 3. Abundance (atom fraction) of the chemical elements in Earth's crust as a function of atomic number. Graphic taken from reference 15.

Figure 4 .

 4 Figure 4. a) Overlap between metal d-orbitals and ligands. b) Simplified potential energy surface for 3d-metal complexes c) Simplified potential energy surface for 4d-or 5d-metal complexes.

23 Figure 5 .

 235 Figure 5. Molecular orbital diagram for an octahedral transition metal complex. MC, LMCT, MLCT and LC transitions based on localized MO configurations.

Figure 6 .

 6 Figure 6. a) Tanabe-Sugano diagram for the d 6 electron configuration. Simplified molecular orbital diagrams of b) [Ru(bpy)3] 2+ and c) [Fe(bpy)3] 2+ complexes.

Figure 7 .

 7 Figure 7. Simplified potential energy surface diagrams with key electronic states in [Ru(bpy)3] 2+ (left) and [Fe(bpy)3] 2+ (right).

Figure 8 .

 8 Figure 8. Metal atomic orbitals dx 2 -y 2 , dz 2 matched by symmetry with ligand group orbitals for an octahedral complex with only σ-bonding picture taken from reference 35.

Figure 9 .

 9 Figure 9. Structure, angles and MLCT lifetimes of a) [Ru(bpy)3] 2+ and b) [Ru(bpy)3] 2+

Figure 10 .

 10 Figure 10. Structure, angles, MLCT and MC lifetimes of [Fe(dcpp)2] 2+ , [Fe(dcpp)(ddpd)] 2+ , and [Fe(dqp)3] 2+ complexes.

  interesting example of the improvement of the ES dynamics on increasing the rigidity was reported by McCusker and coworkers, comprising the macrocyclic cage ligand shown in Figure 11. 41 Following the initial complexation of Fe(II) by the bpy units, the additional coordination of two Cu(I) ions at the amine-imine sites contributed to the rigidification of the structure. More importantly, the vibrational modes associated to the MLCT relaxation were reduced, leading to an impressive ESL for the MLCT state of 2.6 ps, which is more than 20-fold longer than the analogous unconnected [Fe(bpy)3] 2+ (100 fs).

Figure 11 .

 11 Figure 11. Structure of ligand L. Single-crystal X-ray structure of the cation of [FeCu2(L)](PF6)4 with MLCT lifetime. For the metal ions, color coding is as follows: Fe, red; Cu, green; N, purple; C, grey. All hydrogen atoms and counteranions have been removed for clarity. Taken from reference 41.

Figure 12 .

 12 Figure 12. Structure of [Fe(bpy)(CN)4] 2-complex and 3 MLCT lifetime.

Figure 13 .

 13 Figure 13. a) singlet carbene structure, b) triplet carbene structure, c) linear carbene, d) NHCcarbene structure (singlet).

Figure 14 .

 14 Figure 14. Structure and MLCT lifetimes of a) [Fe(pbmi)2] 2+ and b) [Fe(pbbi)2] 2+ complexes.

Figure 15 .

 15 Figure 15. Structures and MLCT lifetimes of a) [Fe(cpbmi)2] 2+ , b) [Fe(pbmbi)2] 2+ , b) [Fe(cpbmbi)2] 2+ and c) [Fe(pzbhbi)2] 2+ complexes.

Figure 16 .

 16 Figure 16. Structure and MLCT life time of [Fe(dpmi)2] 2+ complex.

62

 62

Figure 17 .

 17 Figure 17. Structures and MLCT lifetimes of [Fe(bpy)(btz)2] 2+ , [Fe(btz)3] 2+ , and [Fe(btp)2] 2+

Figure 18 .

 18 Figure 18. Structures of different Fe(II) heteroleptic complexes with different spacers linked to anthracene.

Figure 19 .

 19 Figure 19. Structure and MLCT lifetime of [Fe(bim-ant)2] 2+ and [Fe(bim-pyr)2] 2+ complexes.

Figure 20 .

 20 Figure 20. Structure and MLCT life time of a) [Fe(pbpy)(tpy)] 2+ and b) [Fe(N phen N'^C)2]

Figure 21 .

 21 Figure 21. Interactions between Fe centered t2g orbitals with ligand centered π orbitals (HOMO metal-centered, left). Mixed character between Fe centered t2g orbitals with ligand centered π orbitals (HOMO with mixed character, middle). Interactions between Fe centered t2g orbitals with ligand centered π orbitals (HOMO ligand-centered, right). Inspired from reference 70.

Figure 22 .

 22 Figure 22. Structures of complexes C, CTh, CTh2, Ch3 and UV-Vis spectra of complexes in acetonitrile. Inset: normalized absorbance for the lowest energy band. Picture taken from reference 71.

Figure 23 .

 23 Figure 23. Structure of [Fe(pqa)2] complex with excited lifetime and simplified molecular orbital diagram showing a partial HOMO-inversion character by mixing the orbitals metalbased t2g and π ligand orbitals.

Figure 24 .

 24 Figure 24. Structures and MLCT lifetime of a) Cr(CN tBu Ar3NC)3 and b) Mo(CNAr3NC)3 complexes.

  Complex [MnL bi ] + has a3 MLCT lifetime of 740 ps and the tridentate version [MnL tri ] + presents a 3 MLCT of 1730 ps both complexes were able to perform photoinduced electron transfer derived for the 3 MLCT. Further characterization of the excited states suggests the existence of a lower energy 3 (π -π*) state (Figure25c) which is the responsible for triplet-triplet energy transfer (TTET) observed in presence of triplet quenchers meanwhile the 3 MLCT luminescence remained unchanged.

Figure 25 .

 25 Figure 25. Structures and MLCT-lifetime of a) [MnL bi ] + and b) [MnL tri ] + complexes. c) Simplified energy level scheme for [MnL bi ] + and [MnL tri ] + complexes with relevant electronic states.

Figure 26 .

 26 Figure 26. Structures and excited lifetimes of a) [Co(L1)2] 3+ , [Co(L2)2] 3+ and, b) [Co(PhB(MeIm)3)2] + complexes.

78

 78

Figure 27 .

 27 Figure 27. Schematic molecular orbital diagram for a prototype low-spin d 5 complex exhibiting and -back donation interactions. The green arrow indicates a LMCT transition from a ligand π-orbital to metal-based t2g orbital.

Figure 28 .

 28 Figure 28. a) Structure and MLCT-lifetime of [Fe(btz)3] 3+ complex. b) Structure and MLCTlifetime of [Fe(PhB(MeIm)3)2] + complex. c) Tanabe-Sugano diagram for a d 5 configuration octahedral ML6 complex. d) Simplified potential energy surface diagram with relevant electronic states for a d 5 configuration octahedral FeL6 complex. Taken from reference 20.

  account for these impressive results. With respect to [Fe(btz)3] 3+ , the MC states ( 4 T1g and 6 A1g) in [Fe(PhB(MeIm)3)2] + are pushed to higher energies, reducing the ISC from the 2 LMCT to the 4 MC and 6 MC states (Figure28d). In addition, [Fe(PhB(MeIm)3)2] + in the 2 LMCT excited state is a strong oxidant and reductant with excited state redox potentials of E1/2*([Fe III ]*/[Fe II ]) = 1.0 V and E1/2*([Fe IV ]*/[Fe III ]*) = -1.29V. As a result, this complex has been employed in several photochemical investigations, raising the prospect of applying Fe-complexes as an truly sustainable alternative to Ru(II) polypyridines complexes.[START_REF] Rosemann | Tracing the Full Bimolecular Photocycle of Iron(III)-Carbene Light Harvesters in Electron-Donating Solvents[END_REF][START_REF] Aydogan | Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light[END_REF]

Figure 29 .

 29 Figure 29. a) Schematic overview of a dye-sensitized solar cell. b) Simplified operating diagram for a DSSC. The basic electron transfer processes are indicated by numbers (1-7).

  Photoanode

Figure 30 .

 30 Figure 30. Structure and energy conversion efficiency of a) N3 dye, b) black dye and c) N719 dye. TBA=tetrabutylammonium.

Figure 31 .

 31 Figure 31. Structure and conversion efficiency of a) ARM13, b) [Fe(cpbmi)(dtapbmi)] 2+ and c) ARM130 complexes.

96

 96

[

  Ru(bpy)3] 3+ + e --> [Ru(bpy)3] 2+ * with reduction potential of -0.81 V (E1/2 III/*II vs the saturated calomel electrode (SCE). This means that the excited-state [Ru(bpy)3] 2+ * is stronger reductant than the ground-state [Ru(bpy)3] 2+ (E1/2 III/II = +1.29 V vs SCE). 101 On the other hand, the reduction potential of the excited state [Ru(bpy)3] 2+ * + e --> [Ru(bpy)3] + (E1/2 *II/I = +0.77

Scheme 1 .Scheme 2 .

 12 Scheme 1. Oxidative and reductive quenching cycles of [Ru(bpy)3] 2+ .

Figure 32 .Figure 33 .

 3233 Figure 32. Schematic representation of process and mechanism of PDT. Light irradiation to the PS, formation of active drug (PS*) that produces ROS leading to targeted cell death.

  ), Mn(I), Fe(II/III) or Co(III) have been achieved, exhibiting long-lived EE of varying nature ranging from MLCT, LMCT, PALCT, or even 3 (π -π*) in the case of manganese. The case of iron-based compounds is particularly appealing for low-cost devices and industrial applications since it is the most abundant metal on the Earth crust. As a result, research on photoactive iron complexes continues to stimulate the scientific community due to their particularly challenging ultrafast non-radiative deactivation. While remarkable achievements have been made as it has been previously evocated, systematic studies relating chemical structure and the resulting photophysical properties are still rare. Moreover, most of the reported compounds are cationic complexes bearing tridentate NHC-based ligands.

Figure 34 .

 34 Figure 34. Representation of fac-FeL3 and mer-FeL3 complexes.

Figure 35 .

 35 Figure 35. Schematic representation of tridentate anionic ligands

Figure 36 .

 36 Figure 36. Schematic representation of Fe-L-Co dyads for H2 production

Figure 37 .

 37 Figure 37. Representation of fac-FeL3 and mer-FeL3 complexes.

Scheme 3 .

 3 Scheme 3. a) Synthesis of mer-Ir(C^N)3 and fac-Ir(C^N)3. b) Thermal and light triggered isomerization reactions from mer-Ir(C^N)3 to fac-Ir(C^N)3.

Figure 38 .

 38 Figure 38. Structure of fac-[ML 3 3] 2+ .

2 Following

 2 the research line of our laboratory dealing with photoactive Earth-abundant metals, iron(II) complexes bearing bidentate pyridyl-NHC were initially studied during the PhD work of Kévin Magra (Scheme 5).133 Interestingly, these complexes exhibited MLCT excited state lifetimes of up to 20 ps that compare well with those of their tridentate analogs(Figures 14, see §I.2.3) in spite of having one NHC unit less. These results can be interpreted as a result of the better M-L interaction afforded by the bidentate ligands, highlighting their interest to afford photoactive iron complexes. However, the asymmetry of these bidentate ligands resulted in the formation of fac/mer complexes. For instance, in the case of the ligand reference (Scheme 5) comprising pyridine-imidazol-2-ylidene ligands (L0/C0), a 1 fac to 14

Scheme 6 .

 6 Scheme 6. Synthesis of the ligand precursor HL0.

Figure 39 .

 39 Figure 39. 1 NMR spectra of HL0 (top) and mixture of fac:mer-C0 complex (bottom) in CH3CN-d3 (or CD3CN).

Figure 40 .

 40 Figure 40. 1 H-NMR monitoring of the mer → fac conversion in C0 as a function of the temperature in DMSO-d6. Only the 2.20-3.60 ppm window is shown for clarity.

Scheme 10 .

 10 Scheme 10. Synthesis of ligands HL9-HL12.

Figure 41 .

 41 Figure 41. Steric hindrance of CH3 compared to aromatic CH.

Facial

  complexes fac-C5 and fac-C18 show three bidentate ligands with the same orientation, confirming the facial arrangement with each Fe-C bond trans to a Fe-N bond. In comparison to the computationally-optimized structure of fac-C0, 134 average Fe-C bond distances (1.904 Å for fac-C5 and 1.899 Å for fac-C19 vs 1.948 Å for fac-C0) are significantly shorter, while average Fe-N bond distances are only slightly shorter (2.141 Å for fac-C5 and 2.138 Å for fac-C18 vs 2.072 Å for fac-C0). The steric hindrance exerted by the imine unit is more evident when analyzing the conformation of the bidentate units, which exhibit marked tilting angles of 4.4-13.1° and 5.1-10.4° for fac-C5 for fac-C18, respectively. Thus, it would be tempting to correlate the enhanced structural trans-effect in fac-C5 and fac-C18 respect to fac-C0 with a higher kinetic trans-effect, which would explain the obtained selectivity, but analysis into the complexation mechanism is required.126 We can clearly observe the presence of several short contacts in these facial complexes. These interactions involve mostly the coordinated atoms (Ccarbene or Nazine) to the iron center with a neighboring CH3 group (d + 0.2 Å < sum of the Van der Waals radii) for both fac-C5 for fac-C18, or Nquinoline•••H-C 8 quinoline (d + 0.4 Å < sum of the Van der Waals radii) in the case of fac-C18. Since Ccarbene•••Cl intramolecular interactions have been reported, 146 this type of short contacts might be also envisaged for fac-C18. Therefore, it is therefore possible that these interactions contribute significantly to the here-obtained facial selectivity.

Figure 42 .

 42 Figure 42. (left) View from the (pseudo)-C3 axis and (right) side view of the X-ray crystal structures of complexes (a) fac-C5, (b) mer-C7 and (c) fac-C18 with a partial labelling. Periphery atoms on the ligands are omitted for more clarity. CCDC for fac-C5: 2126499; mer-C7: 2126506; fac-C18: 2126500.

  The fact of having a methyl in position 6 of the pyridine in fac-C5 clearly indicates an elongation of the three Fe-N distances by 0.134 Å (on average), due to steric congestion of the three methyl groups. This effect is illustrated in Figure 44b. As a result, on the opposite side of the molecule, the Fe-C and the Im-CH3•••C distances decrease by -0.005 and -0.051 Å, respectively (Table

Figure 43 .

 43 Figure 43. Gibbs energy difference (ΔG) between the fac and mer isomers for some selected complexes synthesized in this work. Optimized geometries and vibrational frequencies computed at the DFT/B3LYP-D3 level of theory. Energies refined with the DLPNO-CCSD(T) method.

Figure 44 .

 44 Figure 44. a) DFT/B3LYP-D3 optimized structures of the fac (left) and mer (right) isomers for C7 and nomenclature of the intramolecular non-covalent interactions. b) Side (left) and top

Figure 45 .

 45 Figure 45. Side (left) and top (right) views of the fac-C7 structure. 5-Mes groups are highlighted as cyan (carbon) and white (hydrogen) spheres.

Figure 46 .

 46 Figure 46. UV-Vis spectra in air-equilibrated acetonitrile solution at room temperature to show: (left) NHC influence and azine variation through substitution in facial series; (right) NHC influence in meridional series.

Figure 47 .

 47 Figure 47. UV-Vis spectra in air-equilibrated acetonitrile solution at room temperature to show azine variation through substitution or π-conjugation extension and NHC influence in facial series.

42

 42

Figure 48 .Figure 49 .

 4849 Figure 48. Fe(II)-polypyridines with different ligand donor strength investigated by Jakubikova and co-workers inspired from reference 42.

²Figure 50 .

 50 Figure 50. Structure and ESL of [Fe(bpy^C)(bpy^NHC)] + and [Fe( phen N,N^C)2] complexes.

Figure 51 .

 51 Figure 51. Structures of a) [Fe IV ( benzNHC OCO)2] and b) [Fe IV (L-6H)] 2-complexes. Taken from references 154 and 155.

Figure 52 .

 52 Figure 52. (Top) Complexes aminamide-Fe(II) [Fe(PyrNH)2] and [Fe(BzPyrNH)2]. (bottom) Structures and ESL of [Fe(L CNC )2] + complexes.

Figure 53 .

 53 Figure 53. Electronical schematic representation of bisNHC-carbazolate ligand.

Scheme 15 .

 15 Scheme 15. Synthetic route of ligands H3L23, H3L24 and HL25.

Scheme 16 .

 16 Scheme 16. Synthesis of complex C23 + . Counteranion is PF6 -for all charged compounds.

Figure 54 .

 54 Figure 54. 1 H-NMR of complex C23 + (top, cyan) and ligand H3L23 (bottom, red) recorded in in ACN-d3.

Figure 55

 55 Figure 55 only one is shown.

Figure 55 .

 55 Figure 55. X-Ray crystal structure view of C23 + complex. Hydrogen atoms and PF6 -molecules are omitted for clarity.

Figure 56 .Scheme 17 .

 5617 Figure 56. Schematic representation of electronic effects in heteroleptic [Fe(L23)L] 2+ complexes. L= tpy and 4-pytpy.

Figure 57 .

 57 Figure 57. UV-Vis spectra of C23 + , [Fe(L23)(tpy)] 2+ and [Fe(L23)(4'-pytpy)] 2+ complexes in acetonitrile at room temperature.

Figure 58 .

 58 Figure 58. Structure of [Fe(ImP)2] + complex and ESL. Target iron bis-cyclometallated complexes.

  Scheme 20. a) Synthesis of H3L26 and H3L27 ligands, b) Steric demand in position -2 at the pyrimidine moiety prevents bidentate coordination.

Figure 59 .

 59 Figure 59. 1 H-NMR in ACN-d3 of ligand H3L26 (top) and complex C26 + (bottom).

60 .

 60 The data of complex [Fe(ImP)2] + reported by Bauer is added for comparison reasons. Both complexes display three distinct absorption bands. At the highest energy side of the spectra (200-250 nm), intense absorption bands assigned to π-π* transitions are observed.

Figure 60 .

 60 Figure 60. UV-vis spectra in air-equilibrated acetonitrile solution of C26 + and C27 + complexes. Corresponding data for [Fe(ImP)2] + (dotted lines at 351 nm for MLCT and 585 nm for LMCT transitions) are shown for comparison reasons.

Figure 61 .

 61 Figure 61. Colors of the hydrogen production processes. Taken from reference 180.

Figure 62 .

 62 Figure 62. General schemes for light-driven proton reduction catalysts: a) two-component system; b) dyad. PS = Photosensitizer, Cat = Catalyst (PRC), SR = Sacrificial reagent.

Figure 63 .

 63 Figure 63. Selected representation of organic, metal-porphyrin and metal-based photosensitizers.

Figure 64 .

 64 Figure 64. Selected Ru-and Pt-based catalysts, hydrogenase model catalysts and Co-based catalysts for hydrogen production.

[

  Fe(phtmeimb)2] + acting as PS, mixed with the cobaloxime [CoCl(dmgH)2(py)] as PRC. As sacrificial electron donor they reported triethanolamine (TEOA) and a mixture of [HNEt3][BF4]

Figure 65 .

 65 Figure 65. Structures of [Fe(phtmeimb)2] + complex and cobaloxime [Co(dmgH)2pyCl] in two-system component for hydrogen production, inspired from reference 199.

Figure 66 .

 66 Figure 66. Selected examples of Ru(II)-Pt(II), Ru(II)-Fe(I), Ru(II)-Pt(II) and Ru(II)-Co(II) dyads for H2 evolution.

  204 According to their measurements of dyad [Fe-BL-Co], the electron transfer occurred from3 MLCT state of [Fe-BL-Co] to the cobalt moiety.

Figure 67 .

 67 Figure 67. Structures of single-component system ([Fe-BL-Co]) in hydrogen evolution and two-component system ([Fe-BL]+[Co]). DIPP=2,6-diisopropylphenyl.

Figure 68 .

 68 Figure 68. Schematic representation of Fe(II)-Co(II) dyad. Iron-based photosensitizer in blue, linkers in green, (orange) different coordinative ways to Co, dyad Fe-Co or triad Fe2-Co.

Figure 69 .Scheme 24 .Scheme 25 .

 692425 Figure 69. Structures of ligands precursors H2L28, H2L29 and H2L30.

  out a new reaction by adding only one equivalent of MeI at room temperature and following the reaction by 1 H-NMR. After the addition we could observe the apparition of two signals corresponding to the quaternized imidazole and pyridine (Figure 70, bottom), and the signals of the starting material intermediate C. The integrals between the imidazolate and methylpyridium were 3:2, indicating that the pendant pyridine is competing with the imidazole in the nucleophilic substitution over MeI.

Figure 70 .

 70 Figure 70. 1 H-NMR of ligand H2L31 (green) and mixture of intermediate C and ligand H2L31 with 1 equiv of MeI (red).

  BuLi and I2 at -78°C in THF, affording the intermediate D with 60% yield. D was then reacted with 4-pyridinylboronic acid under Suzuki coupling conditions affording the intermediate E with 78% yield. Afterwards, intermediate E was mixed with 2.1 equiv of N-methylimidazole at 150°C giving the desired ligand H2L29 after anion exchange with saturated aqueous solution of KPF6. The fact that we used N-methylimidazole to introduce the imidazolium unit avoided the aforementioned problem of the quaternization selectivity as in ligand H2L31.

Figure 71 .

 71 Figure 71. Possible intermediates in the [Fe(L30)2] 2+ complex formation.

  Results for [Fe(BuL)(L29)] 2+204 and [Fe(pbmi)2] 2+ (See §I.2.3) 31 are included as well for comparison reasons.

Figure 72 .

 72 Figure 72. UV-Vis spectra of [Fe(L30)2] 2+ and [Fe(BuL)(L30)] 2+ complexes in airequilibrated acetonitrile solution at room temperature. Corresponding data for [Fe(BuL)(L29)] 2+ (blue-dotted lines) and [Fe(pbmi)2] 2+ (green-dotted lines) are shown for comparison reasons.

  The UV-Vis spectra of [Fe(L30)2] 2+ and [Fe(BuL)(L30)] 2+ complexes present three main absorption bands. The intense bands below 300 nm correspond to 1 (π-π*) transitions centered on the ligands. At longer wavelengths, two different broader and less intense MLCT bands are found corresponding to Fe-carbene 1 (d-π*NHC) (320-420 nm) and Fe-azine 1 (dπ*azine) (380-550 nm) transitions. A less intense absorption is observed for the heteroleptic [Fe(BuL)(L30)] 2+ complex, which could be explained by the slightly elongated Fe-CNHC bond distances due to the bulky substituents (DIPP) on the imidazole part.204 It is noteworthy that a similar effect was observed for complex fac-C13 (See section III.4.2). On the other hand, a redshift is observed for the Fe-azine MLCT transition in the homoleptic complex [Fe(L30)2] 2+ respect to the heteroleptic complexes [Fe(BuL)(L30)] 2+ . Cyclic voltammetry of homoleptic [Fe(L30)2] 2+ and heteroleptic [Fe(BuL)(L30)] 2+

  UV-Vis spectra of [Fe(L30)2] 2+ and [Fe(BuL)(L30)] 2+ that the complexes present three main absorption bands corresponding to 1 (π-π*) transitions centered on the ligands, two different broader and less intense bands corresponding to Fe-carbene 1 (d-π*NHC) MLCT transitions, and transitions corresponding to Fe-azine 1 (d-π*azine). It is worth to note the slight reduction of the absorbance intensity of [Fe(BuL)(L30)] 2+ attributed to the slightly elongated Fe-CNHC bond distances due to the bulky substituents (DIPP) on the imidazole part. Further, a redshift is observed for the Fe-azine MLCT transition in the homoleptic complex [Fe(L30)2] 2+ respect to the heteroleptic complexes [Fe(BuL)(L30)] 2+ . A more distinct behavior was revealed by the electrochemical investigation. Indeed, both homoleptic [Fe(L30)2] 2+ and heteroleptic [Fe(BuL)(L30)] 2+ present a metal-based oxidation. However, reduction was only observed for the heteroleptic species under the conditions of our experiments. The electron-rich character of the dipyridyl units in L30 must cathodically shift the reduction potential out of the electrochemical window. A more critical result comes from the comparison of [Fe(BuL)(L30)] 2+ with [Fe(BuL)(L29)] 2+ . In fact, the electron-rich character of L30 seems to favour the MLCT implicating BuL ligand instead of L29, thus precluding the target electronic transfer towards the Co coordinating site. Therefore, although actual Co coordination may exert a sufficient pull effect, these results clearly show

(

  Scheme 17) with maximized metal-ligand interactions due to the formation of 6-membered chelates. The strength of the coordination sphere was modulated as a function of the number of coordinated N-amido ligands, although in all cases Fe(III) species were obtained.

  acceptor character. Interestingly, both series of complexes displayed charged transfer bands of different nature almost over the whole visible range and extending until the IR region. As for the C-anionic series, during a 2-month internship with Prof. Dr. Bauer in Paderborn in the framework of the DrEAM program of Lorraine University of Excellence (LUE), we developed a methodology that allowed us to get new bis-cyclometalated Fe-CCC complexes (See §IV.4.3, Scheme 22), highlighting that the use of glove box, a transmetallating or reducing reagent was avoided. The ground state properties of the obtained homoleptic complexes were investigated. These complexes feature a cyclometalating pyrimidine unit that allow for an orbital stabilization induced by the pyrimidine backbone and the substituents in position 2 (-Ph, -CF3), as the electron withdrawing character increases, an anodically shift is observed in their reduction potential waves. Both complexes display three distinct absorption bands corresponding to π-π*, MLCT and LMCT transitions that are found at lower energy by comparison with the one reported by Bauer with a phenyl backbone instead of a pyrimidine.

Figure 73 .

 73 Figure 73. Structure of triad [(Fe(BuL)(L30))2CoCl2].

  . HRMS (ESI-TOF) m/z: [M -PF6 -] + Calcd for C19H22N3 m/z: 292.1814; Found: 292.1828. 1-(5-mesitylpyridin-2-yl)-3-methyl-1H-benzoimidazol-3-ium hexafluorophosphate (HL12 PF6 -). General quaternization method D afforded 0.44 g, (75 %) of a yellowish powder.

  . HRMS (ESI-TOF) m/z: [M -PF6 -] + Calcd for C13H13N4 225.1135; Found: 225.1094. Synthesis of 1,1'-(2-phenylpyrimidine-4,6-diyl)bis(3-methyl-1H-imidazol-3-ium) bis chloride (H3L26)

19 F

 19 NMR (376 MHz, DMSO-d6) δ -69.38. HRMS (ESI-TOF) m/z: [M -2 Cl -] + Calcd for C13H13 F3N6 155.0572; Found: 155.0787.

  . HRMS (ESI-TOF) m/z: [M -2PF6 -] 2+ Calcd for C39H36N12Fe 364.1262; Found: 364.1291. and solvent were removed under vacuum. The green product of the column, was purified by vapor diffusion crystallization with acetone and diethylether as pair of solvents (acetone inside to solubilize the product/diethylether outside the mixture). The crystallization afforded complex C23 + in 35 mg (40%). HRMS (ESI-TOF) m/z: [M -2PF6 -] 2+ Calcd for C56H64N10Fe 932.4659; Found: 932.4670. X-Ray in §VI.6.Synthesis of Complex C26 +In a schlenk with FeCl2 (8.1 mg, 0.064 mmol) three vacuum-Argon cycles were made, then dry THF (3 mL) was added and the suspension was cooled to -78°C, then LiHMDS [1M in THF] (0.13 mL, 0.128 mmol) was added dropwise, the reaction was allowed to reach r.t. and stirred for 4h forming [Fe(HMDS)2 . THF] . In parallel, in a Schlenk charged with H3L26 (50 mg, 0.128 mmol) 3 mL of dry THF were added and cooled to -40°C, then a solution of LiHMDS [1M in THF] (0.28 mL, 0.28 mmol) was added dropwise and stirred for 3h reaching r.t. Then the both schlenk's were cooled to -60°C and the mixture of [HL26] was transferred via canula into the schlenk with [Fe(HMDS)2 . THF]. The mixture was stirred overnight, after this time, the strong violette reactions was quenched with addition of MeOH (5 mL) and 1 mL of KPF6 (saturated solution). The solvents were removed under vacuum and supported on neutral alumina. The complex was isolated thorough a neutral alumina column, starting with mixture diethylether: ACN (4:1), gradually increasing to only ACN. The green product of the column, was purified by vapor diffusion crystallization with acetone and diethylether as pair of solvents (acetone inside to solubilize the product/diethylether outside the mixture). The crystallization afforded a green solid C26 + in 39 mg (36%). 1 H NMR (400 MHz, CD3CN) δ 12.11 (t, J = 7.5 Hz, 2H), 10.85 (s, 6H), 2.92 (s, 2H), 1.17(d, J = 7.5 Hz, 2H), -1.11 (t, J = 7.4 Hz, 1H), -4.02 (s, 2H). HRMS (ESI-TOF) m/z: [M -PF6 -] 2+ Calcd for C36H30N12Fe 686.2060; Found: 686.2054.

  crystal structure of C23 + . Displacement ellipsoids are at the 50% probability level. Counter ions and solvent molecules are omitted for clarity. C: black; N: blue; Fe: red. Suitable crystals were grown by slow evaporation in acetonitrile. H were omitted for clarity Cyclic voltammetry of complex fac-C5.

  complex fac-C18.Cyclic voltammetry of complex fac-C19.

  Cyclic voltammetry of complex fac-C21.

[Fe(pbmi)2] 2+ and [Fe(pbbi)2] 2+ ,

  These authors reported a series of homoleptic complexes,

	comprising tridentate pyridine-bis(NHC) ligands (pbmi = (pyridine-2,6-
	diyl)bis(1-methyl-imidazol-2-ylidene)	and	(pbbi=(pyridine-2,6-diyl)bis(1-tert-butyl)-
	imidazol-2-ylidene)) (Figure 14a and 14b). The presence of the strong NHC units in the
	coordination sphere resulted in an outstanding increase of the 3 MLCT lifetime up to 9 ps, which
	is almost two

orders of magnitude compared to that of [Fe(tpy)2] 2+ . Moreover

	, the quaternizing
	substituent of the NHC moiety was shown to play a significant role in the MLCT lifetime. In
	fact, the tert-butyl groups ( t Bu) present in [

Fe(pbbi)2] 2+ reduced the 3 MLCT lifetime to 300 fs in comparison to [Fe(pbmi)2]

Table 1 .

 1 Schematic

	Solvent	TCE-d2	CDCl3	o-DCB-d4	Dioxane-d8	DMF-d7	THF-d8
	fac:mer	10:90	10:90	13:87	20:80	32:68	34:66
	Solvent	CD2Cl2	CD3CO2D	CD3OD	Acetone-d6	CD3CN
	fac:mer	40:60	53:47	64:36	69:31	71:29

representation of the interconversion between. Ratios of the fac:mer isomers of [2bFe](TFPB)2. (TFBP=tetrakis[3,5-bis(trifluoromethyl) phenyl]borate.

Table 2 .

 2 Modification of parameters in complexation reaction conditions.

	Entry	Solvent	Base	T (°C)	Yield(%) a	fac:mer b (%)
	1	DMF	t-BuOK	r.t.	40	6:94
	2	Tol	t-BuOK	r.t.	No reaction	No reaction
	3	THF	t-BuOK	r.t.	20	8:92
	4	MeCN	t-BuOK	r.t.	25	14:86
	5	DMSO	t-BuOK	r.t.	27	25:75
	6	DMF	KHMDS	r.t.	44	25:75
	7	DMF	Cs2CO3	r.t.	-	-
	8	DMSO	KHMDS	r.t.	45	17:83
	9	DMSO	KHMDS	180	64	44:56
	10 c	THF	LiHMDS	-20	40	25:75

a Isolated Yield; b fac/mer ratio determined by

1

H-NMR; c Reaction carried at -20°C.prior deprotonation of the ligand with LiHMDS and FeCl2 addition and temperature allowed to reach r.t.

Table 3 .

 3 Isomerization reactions with protic/aprotic acids.

	Entry	Conditions	Solvent	Time	After fac:mer a
	1	Malonic acid (10 eq)	EtOAc	6 days	25:75
	2	TFA (10 eq)	EtOAc	6 days	25:75
	3	SiO2 (hv = 254 nm)	ACN-d3	6 h	25:75

Table 4 .

 4 Effect of the ligand steric demand on Fe II complex formation. Isolated yield. b fac/mer ratio determined by

	Entry	Ligand precursor	Yield	fac:mer	Complex
			(%) a	ratio b
	1	HL3 (4-Me)	44	25:75	C3
	2	HL4 (5-Me)	44	20:80	C4
	3	HL5 (6-Me)	45	98:2	fac-C5
	4	HL6 (4-Mes)	48	25:75	C6
	5	HL7 (5-Mes)	45	4:96	mer-C7
	6	HL8 (6-Mes)	-	-	-
	a

Table 5 .

 5 Effect of the nature of the NHC unit on the Fe II complex formation. Data from HL7 and HL19 are presented for comparison reasons. Isolated Yield. b fac/mer ratio determined by

	Entry	Ligand precursor	Yield (%) a	fac:mer ratio b	Complex
	1	HL5 (6-Me, Im)	45	98:2	fac-C5
	2	HL9 (6-Me, 4-MeIm)	48	100:0	fac-C9
	3	HL10 (6-Me, bIm)	43	100:0	fac-C10
	4	HL7 (5-Mes, Im)	45	4:96	mer-C7
	5	HL11 (5-Mes, 4-MeIm)	49	1: 99	mer-C11
	6	HL12 (5-Mes, bIm)	40	1: 99	mer-C12

a

Table 6 .

 6 Coordination with different substituents at position 6 on the pyridine moiety.

	Entry	Ligand precursor	Yield (%) a	fac:mer ratio b	Complex
	1	HL13 (R=iPr)	40	100:0	fac-C13
	2	HL14 (R=CF3)	-	-	-
	3	HL15 (R=F)	-	-	-
	4	HL16 (R=Cl)	18	100:0	fac-C16
	5	HL17 (R=NMe2)	-	-	-

a Isolated Yield. b fac/mer ratio determined by

1

H-NMR, with ≥1% as detection limit.

Table 7 .

 7 Selected structural and deformation parameters for complexes fac-C5, fac-C18 and mer-C7. The deformation parameters were calculated with OctaDist.147 For the sake of comparison, data for DFT optimized fac-C0 and mer-C0 are provided as well, with partial labelling analogous fac-C5 and mer-C7, respectively.135 Trigonal distortion calculated as𝛩 = ∑ |60 -𝜃 𝑖 |

		fac-C0	fac-C5	fac-C18	mer-C0	mer-C7
	Fe-N1 (Å)	2.073	2.105(3)	2.137(2)	2.038	1.975(3)
	Fe-N2 (Å)	2.072	2.131(2)	2.135(2)	2.082	2.015(2)
	Fe-N3 (Å)	2.070	2.188(2)	2.142(2)	2.042	1.988(3)
	Fe-C1 (Å)	1.949	1.893(3)	1.907(2)	1.942	1.903(3)
	Fe-C2 (Å)	1.950	1.910(3)	1.899(2)	1.992	1.934(3)
	Fe-C3 (Å)	1.947	1.910(3)	1.892(2)	1.983	1.956(3)
	C1-Fe-N2 (°)	172.6	174.1(1)	168.6(9)	173.2	173.4(1)
	C2-Fe-N3 (°)	172.8	169.9(1)	173.6(9)	-	-
	C3-Fe-N1 (°)	172.7	176.6(1)	176.1(9)	-	-
	C2-Fe-C3 (°)	-	-	-	170.0	169.9(1)
	N1-Fe-N3 (°)	-	-	-	177.0	177.8(1)
	(Å) a	2.010	2.023	2.019	2.013	1.962
	𝜁 (Å) b	0.368	0.710	0.717	0.245	0.185
	𝛴 (°) c	64.9	81.4	85.9	67.7	61.2
	𝛩 (°) d	205.1	241.2	257.0	213.3	196.4

a Average metal-ligand bond. b Length distortion calculated as 𝜁 = ∑ (𝑑 𝑖 -〈𝐹𝑒 -𝑋〉

6 𝑖=1

, with 𝑑 𝑖 being a metalligand bond length. c Angle distortion calculated as ∑ |90 -𝛷 𝑖 |

Table 8 .

 8 Relevant

	Bond/interaction	fac-C0	fac-C5	mer-C0	mer-C7
	Fe-N	2.057	2.195	2.065	2.126
		2.057	2.190	2.030	2.200
		2.057	2.189	2.025	2.135
	Fe-C	1.931	1.926	1.927	1.977
		1.930	1.926	1.968	1.917
		1.931	1.926	1.974	1.963
	Im-CH3•••C	3.175	3.117	3.205	3.166
		3.174	3.135
		3.172	3.117
	Im-CH3•••N			3.224	3.136
				3.209	3.121
	Pyr-CH3•••C				3.047
					3.005

interatomic distances (in Å) for the fac and mer isomers of C0 and C5 as defined in Figure

44

.

Table 9 .

 9 Photophysical and electrochemical data for the facial or meridional Fe II complexes.

	Complex	λabs-max (nm) [ε(M -1 .cm -1 )] a	Eox b [V vs SCE]	Ered [V vs SCE]	∆E [V] c
	fac-C5	285 [26795]	0.69 (rev)	-1.59 (qr)	2.28
		355 [5891]		-1.87 (rev)
		435 [7957]
	fac-C9	286 [24398]	0.62 (rev)	-1.59 (rev)	2.21
		357 [6237]		-1.90 (qr)
		436 [7928]
	fac-C10	293 [29471]	0.95 (rev)	-1.19 (rev)	2.14
		340 [11185]		-1.60 (rev)
		419 [7839]
	fac-C13	283 [16359]	0.81 (rev)	-1.42 (rev)	2.23
		351 [3274]		-1.91 (irr)
		416 [3810]
	fac-C16	278 [25731]	0.86 (rev)	-1.59 (irr)	2.45
		293 [27383]
		362 [7470]
		428 [9332]
	fac-C18	317 [22077]	0.79 (rev)	-1.41 (irr)	2.20
		367 [5157]		-1.63 (irr)
		508 [6354]		-1.75 (irr)
	fac-C19	338 [25764]	1.08 (qr)	-0.82 (irr)	1.90
		428 [1916]		-1.16 (irr)
		555 [10856]		-1.51 (irr)
	C20	323 [18675]	0.86 (rev)	-1.59 (rev)	2.45
		371 [4202]		-1.79 (irr)
		503 [6443]
	fac-C21	328 [26324]	1.01 (rev)	-1.00 (irr)	2.01
		363 [16001]		-1.17 (irr)
		564 [7139]		-1.69 (irr)
	fac-C22	308 [28777]	1.30 (qr)	-1.62 (irr)	2.92
		334 [27996]
		354 [25738]
		372 [20212]
		511 [6281]
	mer-C7	289 [35856]	0.73 (rev)	-1.86 (rev)	2.59
		362 [6987]
		444 [16187]
	mer-C11	296 [19460]	0.66 (rev)	-1.90 (rev)	2.56
		368 [4589]
		451 [12640]
	mer-C12	293 [47267]	1.06 (rev)	-1.37 (rev)	2.43
		360 [16852]		-1.84 (irr)
		464 [8562]
	a Measured in CH3CN at 25°C; b First oxidation potential. Potentials are quoted vs SCE. Recorded in CH3CN using
	NBu4PF6 (0.1M) as supporting electrolyte at 100 mVs -1 ; under these conditions, E1/2 (Fc+/Fc) = 0.39V / SCE; rev =
	reversible ; qr = quasi-reversible; irr = irreversible. c Electrochemical band gap (∆E=Eox-Ered1).

  and 1.900(2) Å) than the Fe-C bonds lengths (1.991(2)-2.014(2) Å). The bond lengths are in the expectable range of values according to previously reports of iron(III) complexes containing Fe-Namido and Fe-CNHC reported by Herbert 72 and

Table 10 .

 10 Selected structural and deformation parameters for C23 + (esd in parenthesis).

	Atoms	Bond distance (Å)	Atoms	Angle (°)	Torsion angle (°)
	Fe-C2	1.991(2)	C2-Fe-C19	175.1(8)	-
	Fe-N3	1.900(2)	N3-Fe-N3'	178.3(7)	-
	Fe-C19	2.010(2)	C2'-Fe-C19'	175.8(8)	-
	Fe-C2'	1.999(2)	N2-N5	-	45.5(2)
	Fe-N3'	1.898(2)	N2'-N5'	-	38.7(2)
	Fe-C19'	2.014(2)	-	-	-

Table 11 .

 11 Optimization of the complexation reaction for H3L24 (triazolium) and HL25 (pyridine).Similar results were obtained for the complexation reactions of H3L24 (triazolium), with 3.6 equivalents of LiHMDS in the case. In comparison to HL25 (pyridine), no starting ligand was recovered, which indicated that deprotonation of the NHC units was accomplished in spite of the failed coordination. Considering that the complexation reaction was successfully achieved with L23, it seems unlikely that a too strong coordination of Li + to L24 and L25 could explain these results. Due to a lack of time, additional tests with other bases haven't been possible.

	Entry	Ligand	Base	T (°C)	Time Reaction (h)	FeXn	Product a
	1 2	py tz	LiHMDS	rt	24	FeCl2	s.m. -
	3 4	py tz	LiHMDS	rt	48	FeBr2	s.m. -
	5 6	py tz	LiHMDS	rt	72	FeOTf2	s.m. -
	7 8	py tz	LiHMDS	rt	72	FeCl3	s.m. -
	9 10	py tz	LiHMDS	50	48	FeCl2	s.m. -

a s.m.=starting material, -= any product recovered.

Table 12 .

 12 Photophysical and electrochemical data for carbazole-based complexes C23 + , complex [

Fe(L23)(tpy)] 2+ and [Fe(L23)(4'-pytpy)] 2+ .

	Complex	λabs-max (nm) [εM-1.cm-1)] a	Eox b [V/SCE]	Ered [V/SCE]	∆E [V] c
	C23 +	307 [24771]	0.51 (rev)	-0.93 (rev)	1.44
		342 [8559]	1.02 (rev)
		364 [9991]
		396 [7373]
		426 [5450]
		496 [1771]
		752 [6808]
		822 [13050]
	[Fe(L23)(tpy)] 2+	319 [28873]	0.94 (rev)	-0.06 (rev)	1.00
		555 |4884]	1.11 (rev)	-1.26 (qr)
		934 [5178]		-1.42 (qr)
				-1.59 (qr)
	[Fe(L23)(4'-pytpy)] 2+	284 [32784]	0.96 (rev)	-0.13 (rev)	1.09
		323 [15670]	1.17 (qr)	-1.43 (qr)
		568 [6944]		-1.65 (qr)
		946 [1240]		-1.86 (qr)

a Measured in CH3CN at 25°C; b First oxidation potential. Potentials are quoted vs SCE. Recorded in ACN using NBu4PF6 (0.1M) as supporting electrolyte at 100 mVs -1 ; under these conditions, E1/2 (Fc+/Fc) = 0.39V / SCE; rev = reversible; qr = quasi-reversible; irr = irreversible. c Electrochemical band gap (∆E=Eox-Ered1).

Table 13 .

 13 Synthesis of complex C26 + metalation/transmetalation method.

	Entry	t1, t2 (h)	Additive (equiv)	Product
	1	2, 16	-	28
	2	1, 16	-	28
	3	1, 3	-	28
	4 a	(Zr + MeI 1 h)	MeI (2)	28
	5 b	2, 16	TMPMgCl (4)	ligand degradation

a Zr(NMe2)4 with MeI (2 equiv) with for 1h, then ligand H3L26 for 2h, then FeCl2 overnight. b ZrCl4 + TMPMgCl (TMPMgCl= 2,2,6,6-Tetramethylpiperidinylmagnesium chloride lithium chloride).

Table 14 .

 14 Photophysical and electrochemical data for C26

+ , C27 + and [Fe(ImP)2] + complexes.

	Complex	λabs-max (

nm) [ε(M -1 .cm -1 )] a Eox b [V/SCE] Ered [V/SCE] ∆E [V] c

	C26 +	275 [23735]	0.71 (rev)	-0.35 (rev)	1.06
		329 [10922]
		386 [12767]
		557 [521]
		606 [741]
		665 [720]
	C27 +	268 [20973]	1.03 (rev)	-0.063 (rev)	1.09
		346 [8375]
		421 [5569]
		595 [393]
	[Fe(ImP)2] +	351 [6000]	0.47 (qr)	-0.77 (rev)	1.24
		585 [540]	1.62 (irr)

a Measured in CH3CN at 25°C; b First oxidation potential. Potentials are quoted vs SCE. Recorded in CH3CN using NBu4PF6 (0.1M) as supporting electrolyte at 100 mVs -1 ; under these conditions, E1/2 (Fc+/Fc) = 0.39V / SCE; rev = reversible; irr = irreversible; qr = quasi-reversible. c Electrochemical band gap (∆E=Eox-Ered). d Data taken from reference 204 considering E1/2(Fc+/Fc) = 0.39 V/SCE.

Table 15 .

 15 Sonogashira reactions between intermediate A with B.

	2,6-dichloro-4-aminopyridine with in-situ

Table 16 .

 16 Optimization of the reactions conditions of the SNAr.

	Entry	Solvent	Temperature (°C)	Base (n equiv)	Yield (%)
	1	DMF	0 -90	NaH (1.2)	n.r.
	2	DMF	0 -r.t.	K2CO3	n.r.
	3	EtOH	0 -r.t.	-	n.r.
	4	DMF	r.t.-90	-	n.r.
	5	THF	0 -reflux	LiHMDS (1.2)	n.r.
	6 a	ACN	90°C	-	n.r.
	a=Microwave, power 60W, T°=90°C, 1h, ACN; n.r. = no reaction.

Table 17 .

 17 Complexation reactions of ligand precursor H2L30 to obtain [

Fe(L30)2] 2+ .

	Entry	FeX2	Solvent	Base (equiv)	T (°C)	[Fe(L30)2] 2+ (%)
	1	FeBr2	DMF	t BuOK (2.1)	r.t.	n.p.
	2	FeBr2	THF	LiHMDS (2.1)	-20 -rt	n.p.
	3	FeCl2	DMF	t BuOK (2.1)	r.t	40

n.p. = no product.

Table 18 .

 18 Photophysical and electrochemical data for [

Fe(L30)2] 2+ and [Fe(BL)(L30)] 2+ complexes.

  Measured in CH3CN at 25°C. b First oxidation potential. Potentials are quoted vs SCE. Recorded in CH3CN using NBu4PF6 (0.1M) as supporting electrolyte at 100 mV s-1 ; under these conditions, E1/2 (Fc+/Fc) = 0.39V/SCE; rev = reversible; qr = quasi-reversible; irr = irreversible; n.d.= no detected. c Electrochemical band gap (∆E=Eox-Ered 1 ). d Data taken from the paper from reference 204 considering E1/2(Fc+/Fc) = 0.39 V/SCE. e Data taken from the paper from reference 159.

	Complex	λabs-max (nm) [ε(M -1 .cm -1 )] a	Eox b [V/SCE]	Ered [V/SCE]	∆E [V] c
	[Fe(L30)2] 2+	295 [35614]	0.59 (rev)	n.d.
		401 [10901]
		472 [25724]
	Fe(BuL)(L30)] 2+	297 [35550]	0.76 (rev)	-1.97 (irr)	2.73
		399 [9349]
		464 [18723]
	[Fe(BuL)(L29)] 2+ d	294 [16800]	0.86 (rev)	-1.58 (rev)	2.44
		398 [8900]
		486 [11200]
	[Fe(pbmi)2] 2+ e	287 [31400]	0.80 (rev)	-1.95 (irr)	2.75
		393 [9000]
		460 [15900]

a

  2+ reported by Bauer may be misleading. In fact, the same trends, i.e. more positive potentials for both anodic and cathodic events, are found for both heteroleptic complexes respect to [Fe(L30)2] 2+ . Nevertheless, the reduction must take place on L29 instead of BuL due to the presence of the 4,4'-bipyridyl moiety as suggested by the noticeable more positive reduction potential.159

	97 V wave was
	observed for [Fe(BuL)(L30)] 2+ that is almost identical to that of [Fe(pbmi)2] 2+ . However, no
	reduction was recorded for [Fe(L30)2] 2+ within the electrochemical window, which is in line
	with a more destabilized * level.
	Therefore, these results seem to indicate that the photoinduced charge transfer in
	heteroleptic [Fe(BuL)(L30)] 2+ takes place in the opposite direction to what was initially
	intended (d-π*azine-BuL instead of d-π*azine-L30), though the coordination of the cobalt ion could
	well invert this behavior. Moreover, it is important to note that comparison with the heteroleptic
	complex [Fe(BuL)(L29)]

  2.1, Schemes 26, 27, and 29), we succeeded in having two out of three ligands: one for a monodentate Co coordination to give a dyad (See §.V.2, Figure68), and one for a bidentate cobalt coordination, possibly allowing the access to both a dyad and a triad (See §V.2, Figure68). At this time, Bauer and co-workers reported the heteroleptic complex with one of the ligands for monodentate Co coordination (See §V.2.1, Scheme 27, ligand H2L29). So we focused in the synthesis of the homoleptic and heteroleptic (See §V.3, Scheme 30) complexes

with our ligand. The homoleptic complex [Fe(L30)2] 2+ was afforded in 40% yield (See §V.3 Table

17

). Then, the heteroleptic complex was achieved by following a procedure reported by Bauer in which they use a tridentate bulky ligand (BuL) to kinetically inhibit the formation of the heteroleptic complex. [Fe(HMDS)2(THF)] in solution was used as a combined source of iron and base to coordinate BuL, then ligand H2L30 was in parallel deprotonated and added into the solution of [FeBuL] 2+ affording the heteroleptic complex [Fe(BuL)(L30)] 2+ (See §V.3, Scheme 30).

  compound(0.280 g, 74 % yield).1 H NMR (400 MHz, CDCl3) δ8.36 (s, 1H), 7.97- 7.93 (m, 1H), 7.64-7.63 (m, 1H), 7.54 (d, J= 7.59 Hz, 1H), 7.5 (d, J= 8.28 Hz, 1H), 7.16 (s, 1H). 13 C{ 1 H} NMR (100 MHz, CDCl3) δ 149.2, 148.0(q, J = 35.8 Hz), 140.8, 135.2, 131.3, 121.0 (d, J = 274.3 Hz), 118.4 (q, J = 2.8 Hz), 116.3, 115.1. 19 F NMR (400 MHz, CDCl3) δ -68.39. HRMS (ESI-TOF) m/z: [M+H] + Calcd for C9H7F3N3 214.0592; Found: 214.0597. 2-fluoro-6-(1H-imidazol-1-yl)pyridine (15a) was synthesized according to a literature procedure 227 . 1 H NMR (400 MHz, CDCl3) δ 8.31 (s, 1H), 7.90(q, J = 7.9 Hz, 1H),7.59 (s, 1H), 7.22 (dd, J = 7.8, 1.8 Hz, 1H), 7.17 (s, 1H),6.85 (dd, J = 8.1, 2.7 Hz, 1H).Synthesis of 2-chloro-6-(1-imidazol-1-yl)pyridine (16a).228

	1 H NMR (400 MHz, CDCl3) δ
	8.35 (s, 1H), 8.31 (d, J = 5.1 Hz, 1H), 7.62 (s, 1H), 7.17 (d, J = 3.9 Hz, 2H),
	7.04 (d, J = 5.0 Hz, 1H), 2.42 (s, 3H) ppm.
	2-(imidazole-1-yl)-5-methyl pyridine (4a). 220 According to aforemen-
	tioned method B.: 0.4 g (64%) of a yellowish solid. Elution mixture for
	flash chromatography 3:1 Hex: EtOAc + 1% Et3N.

1 H NMR (400 MHz, CDCl3) δ 8.28 (s, 2H), 7.60 (d, J = 7.7 Hz, 2H), 7.23 (d, J = 8.3 Hz, 1H), 7.16 (s, 1H), 2.35 (s, 3H) ppm. desired

  . HRMS (ESI-TOF) m/z: [M -PF6 -] + Calcd for C11H14N3 m/z: 188.1182; Found: 188.1178. 3-methyl-1-(6-methylpyridin-2-yl)-benzimidazolium hexafluorophosphate (HL10 PF6 -). General quaternization method D afforded 1.1 g, (75 %) of a yellowish powder. 1 H NMR (400 MHz, CD3CN) δ 9.52 (s, 1H), 8.44 -8.36 (m, 1H), 8.05 (t, J = 7.9 Hz, 1H), 7.99 -7.91 (m, 1H), 7.78 (td, J = 6.4, 3.6 Hz, 2H), 7.63 (d, J = 8.0 Hz, 1H), 7.51 (d, J = 7.7 Hz, 1H), 4.17 (s, 3H), 2.66 (s, 3H). 13 C{ 1 H} NMR (100 MHz, CD3CN) δ 159.

  . HRMS (ESI-TOF) m/z: [M -PF6 -] + Calcd for C14H14N3 m/z: 224.1188; Found: 224.1175. 1-(5-mesitylpyridin-2-yl)-3,4-dimethyl-1H-imidazol-3-ium hexafluorophosphate (HL11 PF6 -). General quaternization method D afforded 0.38 g, (80 %) of a yellowish powder. 1 H NMR (400 MHz, CD3CN) δ 9.23 (d, J = 1.8 Hz, 1H), 8.36 (d, J = 2.2 Hz, 1H), 7.89 (dd, J = 8.3, 2.2 Hz, 2H), 7.74 (d, J = 8.3 Hz, 1H), 7.02 (s, 2H), 3.86 (s, 3H), 2.39 (s, 3H), 2.32 (s, 3H), 2.01 (s, 6H).

13 C{ 1 H} NMR (100 MHz, CD3CN) δ 150.

  1 H NMR (400 MHz, CD3CN) δ 9.47 (td, J = 1.6, 0.8 Hz, 1H), 9.29 (s, 1H), 8.29 (t, J = 2.0 Hz, 1H), 8.29 -8.21 (m, 1H), 8.13 (ddt, J = 6.9, 3.6, 1.8 Hz, 1H), 8.05 -7.94 (m, 2H), 7.65 (t, J = 1.9 Hz, 1H), 4.03(d,

	J = 0.6 Hz, 3H).
	1-(isoquinoline-1-yl)-3-methyl-1H-imidazolium hexafluoro phosphate
	(HL20 PF6 -). According to aforementioned method to ligand HL18. 0.3
	g (77%) of a yellowish solid. 1 H NMR (400 MHz, CD3CN) δ 9.00 (s, 1H),
	8.51 (d, J = 5.7 Hz, 1H), 8.20 -8.15 (m, 1H), 8.09 (d, J = 5.6 Hz, 1H),
	7.98 -7.93 (m, 2H), 7.91 (t, J = 1.8 Hz, 1H), 7.86 -7.80 (m, 1H), 7.64 (t,

J = 1.

8 Hz, 1H), 4.02 (s, 3H)

. 13C{ 1 H} NMR (100 MHz, CD3CN) δ 146.

  . HRMS (ESI-TOF) m/z: [M -2 Cl -] + Calcd for C18H18N6 H3L26. 0.48 g (95%). 1 H NMR 1H NMR (400 MHz, MeOD4) δ 10.40 (s, 1H), 9.32 (d, J = 3.3 Hz, 2H), 8.85 (t, J = 4.5, 2.4 Hz, 2H), 8.06 (t, J = 2.7, 2.5, 2.0 Hz, 0H), 4.24 (s, 10H). (400MHz, DMSO-d6) δ 10.68 (s, 2H), 10.16 (s, 1H), 9.36 (t, J = 2.1 Hz, 2H), 8.16 (t, J = 1.9 Hz, 2H), 4.06 (s, 6H). 13C{ 1 H} NMR (100 MHz, DMSO-d6) δ 157.

	159.0791; Found: 159.0797.
	1,1'-(2-trifluoropyrimidine-4,6-
	diyl)bis(3-methyl-1H-imidazol-
	3-ium) bis chloride (H3L27) was
	obtained	according	to
	aforementioned method to ligand

1 H

 1 NMR (400 MHz, CD3CN) δ 8.81(d, J = 2.4 Hz, 1H), 8.78 (dd, J = 6.8, 3.3 Hz, 3H), 8.74 (d, J = 2.3 Hz, 1H), 8.71 (d, J = 2.3 Hz, 2H), 8.69 -8.64 (m, 1H), 7.63 (d, J = 0.8 Hz, 1H), 7.55 (dd, J = 6.4, 0.9 Hz, 2H), 7.50 (d, J = 2.3 Hz, 1H), 7.46 (d, J = 2.2 Hz, 1H),7.38 (d, J = 2.4 Hz, 1H), 7.33-7.23 (m, 5H), 6.90 (d, J = 6.4 Hz, 2H), 6.79 (d, J = 6.5 Hz, 1H), 3.24 (s, 3H), 3.03 (s, 5H), 2.76 (s, 3H), 2.72 (s, 3H). 13 C{ 1 H} NMR (100 MHz, CD3CN) δ 212.

Table of

 of Crystal data and structure refinement.

	Complex			fac-C5			mer-C7	fac-C18	C23 +
	Identification code		UC249			UC254	UC258	UC-400
	CCDC number			2126499			2126506	2126500
	Empirical formula		C30H33F12FeN9P2 C54H63F12FeN9O2.5P2 C39H33.67F12FeN9O0.33P2 C56H64FeN10,F4.53P,1.52
	Formula weight			865.44			1223.92	979.52	1077.99
	Temperature/K			100.0(1)			100.0(1)	100.0(1)	100(1)
	Crystal system			monoclinic		triclinic	monoclinic	monoclinic
	Space group			P21/n			P-1	P21/c	P 21/c
	a/Å			13.5072(4)			13.7392(5)	19.1360(14)	12.4479(13)
	b/Å			12.9500(4)			14.5195(5)	8.7370(7)	16.4163(3)
	c/Å			20.3565(6)			15.7361(5)	24.799(2)	25.333(15)
	α/°			90			88.127(2)	90	90
	β/°			103.098(3)			68.396(3)	95.418(3)	91.344
	γ/°			90			86.209(3)	90	90
	Volume/Å 3			3468.09(18)		2912.11(18)	4127.7(6)	5175.34
	Z			4			2	4	4
	ρcalcg/cm 3			1.658			1.396	1.576	1.385
	μ/mm -1			0.631			0.401	0.541
	F(000)			1760.0			1268.0	1989.0
	Crystal size/mm 3		0.204 × 0.123 × 0.024	0.368 × 0.105 × 0.061 0.55 × 0.15 × 0.08
	Radiation			MoKα 0.71073)	(λ	=	MoKα (λ = 0.71073) MoKα (λ = 0.71073)
	2Θ collection/° range	for	data	3.756 to 61.132		3.938 to 56.652	4.276 to 56.56
	Index ranges			k ≤ 18, -29 ≤ l ≤ 29 -19 ≤ h ≤ 19, -18 ≤	-17 ≤ h ≤ 18, -19 ≤ k ≤

𝑖=1with 𝛷 𝑖 being a cis ligand-metal-ligand angle.

Compounds HL13-HL17 ( §III.4.2)

Compounds HL18-HL22 ( §III.4.3)

Acknowledgements

Scheme 19.

Stepwise synthesis of heteroleptic Fe(II) complexes with transmetalation and basereducing approaches.

IV.4.2 Ligand synthesis

The synthesis of the ligand precursors H3L26 and H3L27 is shown in Scheme 20. For both syntheses, 3,6-dichloropyrimidines differently functionalized in position 2, i.e. a phenyl ring and a trifluoromethyl group were chosen as starting materials. The interest for this heterocyclic unit was multifold: i) it would better stabilize Fe(II) over Fe(III); ii) the cyclometallation could take place in an easier manner; and iii) the pronounced electron acceptor character would stabilize the MLCT excited state. Furthermore, apart from their electronic modulation, the selected substitutions were intended to prevent a possible coordination of the ligand to the iron center in a bidentate fashion upon increasing the steric hindrance (R = Ph) or Scheme 22. Synthetic route to C26 + and C27 + complexes, base approach.

The complex formation was confirmed by mass spectrometry and bottom). The 1 H-NMR of C26 + (Figure 59, bottom) shows a shift in all its signals in comparison to the ligand H3L27 (Figure 59, top), appearing over a very large chemical shift range (-4 to 13 ppm). Furthermore, the coupling loss and the peak broadening can be clearly attributed to the paramagnetic Fe(III) center. With this stablished protocol, we proceeded to do the same with ligand H3L27 affording the complex C27 + with 30% yield. This result is remarkable since at first glance the protocol is similar to the one reported by Fout (Scheme 19), 175 but with this approach we avoid the use of an external reducing reagent to get the cyclometallated complex. When the same Sonogashira conditions described for B were used, no product was obtained (entry 1). We decided to increase the reaction temperature to force the reaction, selecting then a mixture of solvents DMF+Et3N (1:2) (both degassed) but no product was observed (entry 2). Moreover, a change on the catalyst for the more reactive Pd(PPh3)4 and the solvents-base mixture to THF:DIPA:MeOH (5:3:3) (DIPA=diisopropylamine) under reflux conditions did not lead to product formation either (entry 3). At this point, we changed the strategy and instead of reacting A with B, we decided to react A with TMSA (entries 4 and 5).

V. Iron complexes as photosensitizers towards

Whether by using the first conditions of entries 2 or 3 the reaction did not proceed. Since we did not observe any indication of Sonogashira coupling product, we decided to start working on the ligand H2L29. Scheme 27. Schematic representation of synthesis to get H2L28 and H2L29 with intermediates

D-F.

In order to access H2L28, D was subjected to a Sonogashira coupling with TMSA, giving the intermediate F in 86% yield. After a fast deprotection of the -SiMe3 group with K2CO3 in MeOH at room temperature, the subsequent terminal alkyne was reacted without further purification with 4-iodopyridine under Sonogashira conditions giving the intermediate G with 36% yield. Unfortunately, attempts to react intermediate G with N-methylimidazole under the same conditions as previously described for H2L29 did not lead to H2L28.

Compound G decomposed into intractable mixture of side products; the triple bond probably suffered from nucleophilic attacks.

The numerous issues faced with the chemistry of the alkyne-containing intermediates, drew us to interrupt our investigations on these types of ligands.

V.2.2 Iron complexes for bidentate coordination of Cobalt

The proposed synthetic route for H2L30 is shown in Scheme 28. First a nucleophilic substitution of the reactant 2,2'-dipyridylamine over position -4 of the 2,4,6-trifluoropyridine would afford intermediate I, then a double nucleophilic substitution of previously formed imidazolate on positions -2 and -6 giving intermediate J to finally perform a quaternization over the imidazoles to get the final ligand H2L30.

Scheme

28.

Schematic representation of synthesis

for ligand H2L30.

Attempts to deal with SNAr of 2,2'-dipyridylamine over 2,4,6-trifluoropyridine are collected in Table B. The first attempt was following a reported protocol, 207 in which they form the nucleophile under Schlenk conditions, in our case mixing 2,2'-dipyridylamine with NaH in DMF at 0°C (Entry 1), followed by the addition of 2,4,6-trifluoropyridine and heating up at 90°C overnight. However, the reaction did not lead to the target compound. It is important to note that neither the desired product or the starting 2,4,6-trifluoropyridine were observed.

Another try was carried out upon changing the base to K2CO3 in DMF at 0°C as reported for another similar protocol (Entry 2). 208 The reaction mixture was slowly warmed to room temperature and followed by TLC, but there were no signs of product and starting material, suggesting that the starting material was lost from the reacting mixture.

Based on these results, a new synthetic approach was investigated (Scheme 29).

Compound D obtained in scheme 27, was reacted under Ullmann conditions with 2,2'dipyridylamine in DMF at 150°C affording K in 76% yield. Introduction of the imidazole units in K was performed using the potassium imidazolate salt in diglyme yielding L in 91% yield.

The quaternization of the imidazole moieties was selectively achieved, leading to the expected ligand precursor H2L30 in 70% yield Scheme 29. Alternative synthetic approach to obtain H2L30.

Approximately, at this moment of the thesis, Bauer reported the synthesis and complexation of the heteroleptic version and dyad of our ligand L29. 204 As a result, we continued our investigation focusing only on the ligand H2L30.

VII. Experimental part VIII. Experimental part VII.1 Materials and methods

Solvents and commercially available reagents were used as received without further purification. All complexation reactions were performed under argon atmosphere using standard Schlenk techniques. Solvents were dried using standard protocols when necessary.

All reactions were monitored by thin layer chromatography using TLC 60 F-254 (Merk) plates.

Purifications by chromatography were performed with silica gel 60 (0.063-0. After measurement, ferrocene was added as the internal reference for calibration. All potentials were quoted versus SCE. In these conditions, the redox potential of the couple Fc + /Fc was found at 0.39 V. In all the experiments, the scan rate was 100 mV/s.

Synthesis of 6-(1H-imidazol-1-yl)-N,N-dimethylpyridin-2-amine (17b).

In a 50 mL round bottomed flask was charged with 17a (0.3 g, 1. 49 mmol, 1eq), imidazole (0.254 g, 3.73 mmol, 2.5 eq), CuI (0.029 g, 0.149 mmol, 0.1 eq), L-proline (0.086 g, 0.746 mmol, 0.5 eq), K2CO3 (0.618 g, 4.48 mmol, 3 eq) and dry DMSO (10 mL). Then the reaction mixture was heated at 90 °C for 24 h. After completion of reaction, the mixture was poured into ice cold water. The cooled mixture was partitioned between ethyl acetate and saturated NH4Cl, the organic layer was washed with brine, dried over MgSO4 and concentrated in vacuo to get desired compound 17b (0.2 g, 50% yield) and used for next step without purification.

VII.3.2 Synthesis if compounds HL3-HL22

Method D: General quaternization method at the imidazole moiety 2-imidazole-6-methylpyridine (3.69 mmol, 0.54 g, 1 equiv) and MeI (4.43 mmol, 0.63 g, 1.2 equiv) in DMF (10 mL) were mixed in a 25 mL round flask and heated at 80 °C in a sand bath overnight with constant stirring. After this time, the solvent was almost removed in vacuum, then H2O (7 mL) was added, followed for KPF6 (4.06 mmol, 0.75 g, 1.1 equiv). The solution was stirred for 30 min and the precipitate was filtered through a glass funnel filter #3, washed with diethyl ether (3 x 10mL) and the solid was recovered.

Synthesis of 3-methyl-1-(quinoxalin-2-yl)-1H-benzo [d]imidazol-3-ium hexafluoro phosphate (HL22 PF6 -). According to aforementioned method to ligand HL18. 0.38 g (50%). 1 H NMR (400 MHz, CD3CN) δ 9.78 (s, 1H), 9.37 (s, 1H), 8.71 -8.64 (m, 1H), 8.34 -8.22 (m, 3H), 8.08 -7.99 (m, 3H), 7.87 (t, J = 4.3 Hz, 1H), 7.85 (t, J = 3.6 Hz, 1H), 4.25 (s, 3H). 13C{ 1 H} NMR (100 MHz, CD3CN) δ 143. 25, 143.19, 142.12, 140.92, 139.89, 133.59, 133.42, 132.77, 130.96, 130.37, 129.92, 129.58, 129.06, 117.21, 114.77, 35. In a round flask with a degassed mixture of 25 mL toluene:MeOH (1:1), 4-bromo-2,6di(1H-imidazol-1-yl)pyridine (A) (0.5 g, 1.72 mmol), 4-pyridinylbroronic acid (0.21 g, 1.72 mmol), K2CO3 (5.17 g, 3 mmol), Pd(dppf)Cl2 (0.12 mg, 0.17 mmol) were added. The mixture was heated at 80°C in a sand bath for 24h. The solid was used without purification in the subsequent reaction.

2,6-dichloro-4-iodopyridine (D) was synthesized according to a literature procedure 0.51 g (60%). 235 1 H-NMR (400 MHz,, CDCl3) δ 7.68 (s, 1H).

Synthesis of 2,6-dichloro-4,4'-bipyridine (E).

In a round flask with a degassed mixture of 8 mL toluene:MeOH (1:1), 2,6-dichloro-4iodopyridine (D) (0.26 g, 0.96 mmol), 4-pyridinylbroronic acid (0.12 g, 0.96 mmol), K2CO3 (0.39 g, 2.8 mmol), Pd(dppf)Cl2 (0.07 mg, 0.0.09 mmol) were added. The mixture was heated at 80°C in a sand bath for 24h. After this time the mixture was filtered through celite and washed with CH2Cl2 (3 x 10 mL) and the solvents were removed under vacuum. The crude was purified by silica chromatographic column using a mixture Hexane:EtOAc + 1%Et3N

Complex C27 + Complex C27 + . Following the complexation method for C26 + afforded 32 mg, (30 %) of an green solid. In schlenk with FeCl2 (9.1 mg, 0.07 mmol) and ligand H2L30 (100 mg, 0.14 mmol), 2 mL of dry DMF were added. The mixture was stirred for 5 in, then t-BuOK (48 mg, 0.43 mmol). The reaction color turned into dark-reddish. The reaction was stirred for 3h, then solvent was removed under vacuum. The mixture was dissolved in a minimum amount of acetone (0.5 mL) and poured diethylether. The precipitated formed was filtered in a glass funnel filter #4 or #3 (depending on particle aggregation: #4 for small, #3 for bigger). The solid was washed with diethyl ether (3 x 15 mL) until the solid was a fine powder. The crude was purified over Hz, 12H). 13 C{ 1 H} NMR (100 MHz, CD3CN) δ 201. 57, 201.39, 157.15, 155.06, 154.94, 153.41, 150.32, 145.90 (d, J = 2.1 Hz), 139.78, 138.68, 134.99, 131.38, 131.28, 126.55, 124.68, 122.62, 120.84, 119.53, 116.61, 106.63, 97.67, 35.64, 28.56, 26.70 (d Cyclic voltammetry of complex fac-C16.