Alina A Grineva

Oleg A Filippov

Yves Canac

Jean-Baptiste Sortais

Sergei E Nefedov

Noël Lugan

Vincent César

Dmitry A Valyaev

Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Since their seminal preparation by Arduengo et al. in 1991 1 stable 2H-imidazol-2-ylidenes (A, Chart 1) became the most prominent class of N-heterocyclic carbenes (NHCs). Ten years later Crabtree and coworkers reported the first examples of transition metal complexes bearing isomeric imidazol-4-ylidene ligands (B) 2 so called "abnormal" NHCs (aNHCs), the first representative free aNHC being isolated in the group of Bertrand in 2009. [START_REF] Aldeco-Perez | Isolation of a C5-deprotonated imidazolium, a crystalline "abnormal" N-heterocyclic carbene[END_REF] Typically aNHC ligands show significantly stronger σ-donor properties than their "normal" NHC analogues and this characteristic feature was successfully exploited in some catalytic processes. 4 In 2006 Arnold et al. reported the preparation of lanthanide complexes supported by a ditopic anionic imidazol-2,4-diylidene (NHDC, C) ligands exhibiting normal and abnormal carbenic functions. [START_REF] Arnold | Deprotonation of N-heterocyclic carbenes to afford heterobimetallic organolanthanide complexes[END_REF] Shortly after Robinson and coworkers showed that lithium type C NHDC complexes could be readily obtained by backbone metallation of free NHCs A with nBuLi. [START_REF] Wang | A Viable Anionic N-Heterocyclic Dicarbene[END_REF] Finally, Whittlesey and coworkers reported in 2007 the first complex bearing the ditopic anionic imidazol-4,5diylidene ligand D [START_REF] Ellul | Abnormally bound N-heterocyclic carbene complexes of ruthenium: C-H activation of both C4 and C5 positions in the same ligand[END_REF] and more recently Ghadwal succeeded generating in situ the corresponding lithium dicarbene species, here by double metallation of 2-arylsubstituted imidazolium salts with two equivalents of nBuLi. 8

Chart 1. Different Types of Mono-and Ditopic NHCs Derived from Imidazolium Salts

Though the chemistry of NHDC species C and their adducts with main group elements have been well developed, [START_REF] Wang | A Viable Anionic N-Heterocyclic Dicarbene[END_REF]9,[START_REF] Conversely ; Mata | bimetallic transition metal complexes bearing the neutral isostructural 1,2,4-triazol-3,5-diylidene ligands have been reported[END_REF] the corresponding binuclear transition metal complexes remain relatively scarce. Bimetallic Zn(II) [START_REF] Wang | NHC-stabilized triorganozincates: syntheses, structures, and transformation to abnormal carbene-zinc complexes[END_REF] and Au(I) [START_REF] Armstrong | Alkali-metal-mediated zincation (AMMZn) meets N-heterocyclic carbene (NHC) chemistry: Zn-H exchange reactions and structural authentication of a dinuclear Au(I) complex with a NHC anion[END_REF] NHDC derivatives based on IPr skeleton (IPr = 1,3-bis (2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) were originally obtained by transmetallation of main group metal cations in appropriate precursors with ZnEt2 and (PPh3)AuCl, respectively. Another approach exploiting C-H 13 or C-I [START_REF] Krüger | Chelating C4-bound imidazolylidene complexes through oxidative addition of imidazolium salts to palladium(0)[END_REF] NHC backbone bond activation in bidentate and pincer complexes resulted in the formation of binuclear Pd(II) or Co(I) NHDC complexes. Finally, a variety of heterobimetallic NHDC species associating Ru(II) to Ag(I), Au(I), Pd(II) or Ir(I) was prepared from cationic complexes [Ru(κ 2 -OAc)(L-L)(aNHC-P)]Br (L-L = NHCphosphine, diphosphine), in which the C2 position of aNHC ligand remained accessible for metal coordination. 15 Clearly, the lack of sufficiently general preparation methods still hamper the development of this chemistry and even the electronic properties of the NHC units in ditopic ligands C have never been fully quantified by experimental or theoretical means.

We have recently shown that the lithiation of the IMes ligand backbone (IMes = 1,3-bis (2,4,6-trimethylphenyl)-2H-imidazol-2-ylidene) in the Mn(I) complex Cp(CO)2Mn(IMes) (1) with nBuLi leads to the quantitative formation of the stable complex Cp(CO)2Mn(μ-dIMes)Li (2) supported by an imidazol-2,4-diylidene ligand denoted here and below as dIMes (Scheme 1). [START_REF] Valyaev | Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2ligand[END_REF] The abnormal carbene function in 2 could readily undergo carboxylation, [START_REF] Valyaev | Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2ligand[END_REF] electrophilic fluorination, [START_REF] Grineva | Direct Access to IMes F and IMes F 2 by Electrophilic Fluorination of Abnormal N-Heterocyclic Carbenes[END_REF] or oxidative coupling [START_REF] Grineva | Oxidative coupling of anionic abnormal N-heterocyclic carbenes: efficient access to Janustype 4,4′-bis(2H-imidazol-2-ylidene)s[END_REF] processes to form the corresponding backbone-decorated IMes derivatives in good yield (Scheme 1, paths i-iii). Scheme 1. Reactivity of the Abnormal Carbene Moiety in Complex Cp(CO)2Mn(μ-dIMes)Li (2) Inspired by the pioneering works of Robinson [START_REF] Wang | NHC-stabilized triorganozincates: syntheses, structures, and transformation to abnormal carbene-zinc complexes[END_REF] and Hevia [START_REF] Armstrong | Alkali-metal-mediated zincation (AMMZn) meets N-heterocyclic carbene (NHC) chemistry: Zn-H exchange reactions and structural authentication of a dinuclear Au(I) complex with a NHC anion[END_REF] we wondered whether transmetallation at the abnormal carbene position in complex 2 (Scheme 1, path iv) could afford a viable route to heterobimetallic transition metal NHDC complexes based on the ubiquitous IMes architecture. We report herein the synthesis of carbonyl-containing Mn(I) and Fe(II) NHDC complexes, the comprehensive study of their electronic structure and the quantification of the donation properties of dIMes ligand. Finally the catalytic activity of a unique series of cationic Fe(II) complexes supported by the normal IMes (A), the abnormal aIMes (B) and the anionic ditopic dIMes (C) ligands (Chart 1, R = Mes) was evaluated in benchmark ketone hydrosilylation.

RESULTS AND DISCUSSION

Synthesis and Characterization of Mn(I) and Fe(II) NHC

Complexes Based on IMes, aIMes and dIMes Scaffolds. We were pleased to find that the reaction of compound 2 with Cp(CO)2FeI led to the formation of the corresponding binuclear complex CpMn(CO)2(μ-dIMes)Fe(CO)2Cp (3) in a good yield (Scheme 2). The manganese fragment attached to the normal NHC position in complex 3 was then selectively decoordinated upon protonation with triflic acid according to our recently developed protocol [START_REF] Grineva | Direct Access to IMes F and IMes F 2 by Electrophilic Fluorination of Abnormal N-Heterocyclic Carbenes[END_REF][START_REF] Grineva | Oxidative coupling of anionic abnormal N-heterocyclic carbenes: efficient access to Janustype 4,4′-bis(2H-imidazol-2-ylidene)s[END_REF] to afford the cationic Fe(II) complex [Cp(CO)2Fe(aIMes)](OTf) ([4](OTf)) representing a rare example of mononuclear transition metal complex featuring aIMes ligand non-substituted at the C2 position. 19,[START_REF] Ghadwal | For related transition metal complexes bearing monodentate imidazol-4-ylidene ligands with other substituents at the nitrogen atoms[END_REF] The later complex was further transformed into the binuclear bis(iron) dIMes derivative [Cp(CO)2Fe(μ-dIMes)Fe(CO)2Cp](OTf) ([5]OTf) upon deprotonation of the C2-H bond of the imidazolium moiety with tAmOK in the presence of Cp(CO)2FeI. Finally, for the sake of comparison of spectral properties, and later on, of catalytic activities, the complex [Cp(CO)2Fe( IMes In contrast to the numerous examples of iron complexes supported by classic 2H-imidazol-2-ylidene ligands, 22 only a few compounds featuring imidazol-4-ylidene ligands are known to date. 20f-g,23 The diamagnetic nature of our NHC Fe(II) complexes made possible their full characterization by 1 H and 13 C NMR spectroscopy (see Table 1 for selected pertinent NMR data). In particular, the signal of the backbone NHC proton of the dIMes ligand in complex 3 is slightly shielded by 0.14 ppm compared to the corresponding value in its IMes antecedent 1 in the same solvent (C6D6). Similar trend is also observed within the Fe(II) IMes → aIMes → dIMes series showing the stepwise decrease of the corresponding δH values by ca. 0.3 ppm in complexes [6](OTf), [4](OTf), and [5](OTf) (Table 1). The abnormal carbenic atoms coordinated to the Fe(II) centers in complexes 3, [4](OTf), and [5](OTf) are observed by 13 C NMR spectroscopy in a range of δC4 124.8-141.8 ppm. Interestingly, the 13 C NMR signals belonging to both normal and abnormal carbene atoms in the cationic dIMes complex [5]OTf are shifted by ca. 5 ppm upfield compared to the corresponding signals in the mononuclear derivatives [6]OTf and [4]OTf. Similar difference for δC4 carbene resonance values in a range 4.4-12.3 ppm was also observed between related Ru(II) abnormal and NHDC complexes. 15 The observed shielding of the backbone imidazolium protons and carbenic resonances in dIMes complexes can be ascribed to the higher electron density within the carbenic heterocycle, which is translated in chemical drawings by a formal negative charge delocalized inside the heterocycle. X-ray diffraction study of the bimetallic NHDC complex 3. The molecular structure of the heterobimetallic complex 3 was established by single-crystal X-ray diffraction analysis (Figure 1). [START_REF]CCDC 2047741[END_REF] Both metal centers display an usual piano-stool coordination environment. While the Mn-NHC bond (2.0133(13) Å) in 3 is slightly longer than the corresponding one in the parent IMes complex 1 (2.003[3] Å), [START_REF] Valyaev | Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2ligand[END_REF] the Fe-aNHC bond distance (1.9936(13) Å) is the shortest among other reported anionic NHDC (2.064(2)-2.110(2) Å) 23b,c or neutral monodentate aNHC iron complexes (2.026(3)-2.117(2) Å), 20g,23a,d and compares well with those found in structurally similar Fe(II) halfsandwich 1,2,3-triazol-4-ylidene derivatives (1.9744( 14)-2.010(6) Å). [START_REF] Johnson | Triazolylidene iron(II) piano-stool complexes: synthesis and catalytic hydrosilylation of carbonyl compounds[END_REF] The normal NHC moiety bonded to Mn1 atom adopts an almost ideal horizontal coordination mode [START_REF] Caulton | Coordination chemistry of the manganese and rhenium fragments (C5H5)M(CO)2[END_REF] (Cpcnt-Mn1-C2-N2 -80.94(12)°), whereas the abnormal one bonded to Fe1 atom shows an eclipsed conformation, in which the NHC ring becomes almost coplanar with one of the CO ligands (C8-Fe1-C4-N2 -23.04(13)°) bringing one of the mesityl group attached to the NHC ring in close proximity to carbonyl carbon atom C8 (C8...C31 3.0284(18) Å). Actually, the combination of this Cipso...C≡O distance being inferior to the sum of the Van der Waals radii (3.4 Å) and the significant bending of the adjacent Fe-C-O fragment (Fe1-C8-O8 172.30 (12)° vs. Fe1-C9-O9 178.46(13)°) suggests the occurrence of an attractive interligand π(C=C)...π*(C≡O) interaction (vide infra) similar to the one previously evidenced in given Fischer-type and NHC complexes. 27 DFT optimized structures of all relevant Mn(I) and Fe(II) complexes obtained in the gas phase using G09/BP86-D3/def2-TZVP approach are pre-sented in Figure 2. The most pertinent structural data are provided in Table S1 (see the Supporting Information). While in the case of the Fe(II) aIMes complex 4 + a single minimum was located on the potential energy surface, two minima were found for both neutral Mn(I) and cationic Fe(II) IMes complexes corresponding to two rotamers exhibiting either an horizontal (1a, 6a + ) or an eclipsed (1b, 6b + ) NHC ligand coordination mode. The same situation was also observed in bimetallic NHDC derivatives 3 and 5 + (Figure 2). The calculated geometries of 1b and 3a show an excellent agreement with the corresponding solid-state structures of complexes 1 [START_REF] Valyaev | Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2ligand[END_REF] and 3 obtained by X-ray diffraction. Since the difference in main thermodynamic characteristics (ΔE, ΔH 298 and ΔG 298 ) between these horizontal and eclipsed isomers (Table S2) does not exceed 1.6 kcal mol -1 , it is not surprising that either one or both conformations have been observed by single crystal X-ray diffraction in parent [Cp(CO)2M(NHC)] n+ (M = Mn, n = 0; [START_REF] Valyaev | Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2ligand[END_REF][START_REF] Grineva | Oxidative coupling of anionic abnormal N-heterocyclic carbenes: efficient access to Janustype 4,4′-bis(2H-imidazol-2-ylidene)s[END_REF][START_REF] Batool | Photochemistry of Cp′Mn(CO)2(NHC) (Cp′ = η 5 -C5H4Me) species: synthesis, time-resolved IR spectroscopy, and DFT calculations[END_REF] M = Fe, n = 1 29 ) complexes bearing N-aryl-substituted NHC ligands and these rotamers are likely to be in fast exchange in solution.

The calculated metal-NHC bond distances in the bimetallic dIMes derivatives are systematically longer by 0.01-0.02 Å than those found in the corresponding conformers of mononuclear IMes and aIMes complexes. Taking into account the presence of several short Cipso...C≡O contacts and associated bent M-C-O fragments (Figure 2, Table S1) for both normal (2.779-3.084 Å, 166.96-172.43°) andabnormal (2.891-2.932 Å, 169.34-171.29°) Cp(CO)2M-carbene moieties in these complexes, we investigated the possible occurrence of attractive interligand π(C=C)...π*(C≡O) interactions 27 using Bader's Atoms In Molecules (AIM) theory 30 (see for details Figures S14-18 and Table S3 in the Supporting information). While such interligand intramolecular interactions were found only for the eclipsed conformations of the Cp(CO)2Mn-NHC moieties in 1b and 3b (Figure S14 and Figure S17, respectively) with an interaction energy Eint of 2.4 kcal mol -1 in both cases as estimated using the Espinosa-Molins-Lecomte correlation, [START_REF] Espinosa | Hydrogen bond strengths revealed by topological analyses of experimentally observed electron densities[END_REF] they are present in all Cp(CO)2Fe-NHC fragments regardless the NHC orientation (Figures S15-18), the interaction energy being in the 1.9-2.6 kcal mol -1 range. According to the bond critical point (BCP) characteristics (Table S3) for the eclipsed conformations of Fe(II) IMes and aIMes fragments π(C=C)...π*(C≡O) interactions are weaker in the latter case (2.6 vs. 2.1-2.2 kcal mol -1 ). Though the estimated interaction energies (1.9-2.0 kcal mol -1 ) found in case of horizontal conformers 5a + and 6a + formally seems to be comparable, these supramolecular bonds show very high values of ellipticity (1.074 and 1.303, respectively vs. 0.122 and 0.088 for the same type of bonding in eclipsed conformers 5b + and 6b + ), which clearly indicate their strongly disturbed character.

In contrast to well-studied normal and abnormal carbene derivatives, to the best of our knowledge the electronic structure and bonding situation for bimetallic transition metal NHDC Table 2. Selected AIM Charges and Bond Orders (BOs) for Optimized Structures of Mn(I) and Fe(II) Complexes Bearing Normal, Abnormal and Ditopic NHCs a total AIM charge of the heterocyclic moiety without aryl groups (C3N2H2 and C3N2H1 for mono-and ditopic carbenes, respectively) complexes has never been investigated by theoretical means. Here, DFT calculations data show virtually the same AIM metal charges and metal-carbene bond orders (BOs) in all Mn(I) and Fe(II) IMes, aIMes and dIMes complexes as well as negatively charged dIMes units in bimetallic species 3a-b and 5a-b + (Table 2). Similar trends in charge distribution can be also deduced from the data obtained by Natural Bond Orbitals (NBO) approach (Table S6).

The metal-carbene bonding in these species was investigated by energy decomposition analysis (EDA) 32 (for details see Experimental section and Table S8 in the Supporting Information). The overall binding energy Eint of the Fe-NHC bonds is ca. 50% higher than for the analogous Mn-NHC interactions mainly due to significantly higher electrostatic contribution Eelst. In the cationic Fe(II) complexes [4-6] + the Fe-NHC bonds are always weaker by 3-4% than Fe-aNHC ones again owing to slightly greater ionic character for the later, in agreement with literature data. 33 Interestingly, binding energies for both Fe-NHC and Fe-aNHC bonds in 5 + are systematically ca. 6% greater than those found in mononuclear congeners 6 + and 4 + , respectively. The bonding parameters of iron-aNHC bonds in dIMes complexes 3 and 5 + were then compared with those of classic Fe-C σ-bonds in model Fe(II) complexes Cp(CO)2Fe-R (R = Me, 2-and 4-imidazolyl). This study revealed the notable increase of both electrostatic Eelst component and σ-bonding contribution in Eorb energy in the series of Fe-aNHC → Fe-heteroaryl → Fe-alkyl bonds (for details see the Supporting Information) thus being consistent with a dative nature of the Fe-aNHC bonds in NHDC species differing significantly from mainly electrostatically-driven Fe-C interactions with pronounced σ-character in model complexes. The overall results obtained in this theoretical investigation confirm that the NHDC (C, Chart 1) in transition metal complexes really behave as anionic L2-type ligands.

Experimental Measurement of Relative Electron Donating Properties of dIMes Ligand and the Rationalization of the Observed Effects by DFT Calculations. Having in hands the first examples of NHDC complexes with carbonyl-containing transition metal fragments in both normal and abnormal positions we then turned our attention to the evaluation of electronic donation properties of this type of NHC ligands by IR spectroscopy (Table 3, Figures 34). Strikingly, we have observed that in complex 3 the CO bands for [Cp(CO)2Mn] and [Cp(CO)2Fe] + moieties are shifted into lower frequency region to ca. 8.5 and 15 cm -1 compared to those of monodentate NHC analogues 1 and [4](OTf), respectively (Figure 3). A similar situation is found for the Fe(II) complex [5](OTf) (Figure 4) and notably the donicity of the aNHC moiety in dIMes ligand was found to be sensitive to the nature of the organometallic fragment on the normal NHC position (<CO> 2005.0 and 2014.3 cm -1 for 3 and [5](OTf), respectively) reflecting the occurrence of an efficient electronic delocalization across NHDC ligand scaffold.

DFT calculations of IR data for all these compounds (Table 3) confirm the initial CO peaks attribution and show the expected low frequency shift for the conformers with intramolecular π(C=C)...π*(C≡O) interactions. 27c The comparison of calculated CO data between the dIMes complexes and their monocarbene congeners is fully consistent with the experimentally observed trends and ΔCO values within the same series of rotamers were well reproduced. These results seem to indicate that both carbene moieties of dIMes are more strongly electron donating than the corresponding normal (IMes) and abnormal (aIMes) NHCs (Scheme 3). This proposition is consistent with the decrease of medium <CO> value for metal-NHC fragments observed by Tamm and coworkers in the rhodium-borane complex (Et4N)[((C6F5)3B)(μ-dIPr)(Rh(CO)2Cl)] (<CO> 2028 cm -1 ) as compared to the classic IPr complex (IPr)Rh(CO)2Cl (<CO> 2038 cm -1 ). [START_REF] Kolychev | Iridium(I) complexes with anionic N-heterocyclic carbene ligands as catalysts for the hydrogenation of alkenes in nonpolar media[END_REF] We also previously observed similar low frequency shift of CO bands in IR spectra of manganese complexes Cp(CO)2Mn(IMes) (<CO> 1885 cm -1 ) as compared to Cp(CO)2Mn(μ-dIMes)Li (<CO> 1871 cm -1 ) in Et2O solution. [START_REF] Valyaev | Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2ligand[END_REF] Yet, taking into account that IR measurements for NHC donicity determination are typically carried out on square-planar Rh(I) or Ir(I) complexes and directly correlated with Tolman's electronic parameter (TEP) 35 and being aware of the possible νCO response perturbations due to interligand interactions in halfsandwich carbene complexes, 27c we decided to further assess our conclusions by another experimental method. Indeed, the measurement of the oxidation potential of metal NHC complexes constitutes an alternative, which could provide sometimes even higher precision than the TEP approach. [START_REF] Wolf | Synthesis of (NHC)Rh(cod)Cl and (NHC)RhCl(CO)2 complexestranslation of the Rh-into the Ir-scale for the electronic properties of NHC ligands[END_REF] While cyclic voltammogram (CV) of Mn(I) IMes complex 1 in acetonitrile reveals a fully reversible one electron oxidation at E1/2 {Mn(I)/Mn(II)} = -0.26 V vs. Fc (Figure S9), its cationic Fe(II) analogue [6](OTf) undergoes under the same conditions an irreversible oxidation at Ep.a. {Fe(II)/Fe(III)} = +1.44 V (Figure S13) probably due to the occurrence of ECE-type process rather typical for this coordinating solvent. [START_REF] Hashidzume | Electrochemistry of octamethyl [3] ruthenocenophane: synthesis and structure of the first dicationic derivative of ruthenocene, [{η 5 :η 5 -C5Me4(CH2)3C5Me4}Ru(NCMe)]-(PF6)2[END_REF] The electrochemical behavior of Fe(II) aIMes derivative [4](OTf) was found to be intermediate compared to its normal IMes analogues and a quasi-reversible (Ia/Ic = 0.6) one electron oxidation wave was observed at E1/2 {Fe(II)/Fe(III)} = +1.12 V (Figure S11) in full agreement with a stronger donation of the aIMes ligand. The analysis of CVs for dIMes complexes (Figures S10 andS12) reveals a similar electrochemical behavior accompanied by a significant decrease of the oxidation potentials for both normal (3: E1/2{Mn(I)/Mn(II)} = -0.39 V; [5](OTf): Ep.a.{Fe(II)/Fe(III)} = +1.34 V) and especially for abnormal metal-NHC fragments (3: E1/2{Fe(II)/Fe(III)} = +0.71 V; [5](OTf): E1/2{Fe(II)/Fe(III)} = +0.86 V). Similarly, a significant decrease of Ru(II)/Ru(III) oxidation potential in binuclear RuIr NHDC complex containing the ruthenium atom coordinated to the abnormal NHC position was very recently attributed to the result of efficient electronic coupling via the NHDC ligand. 15a Though an irreversible electrochemical behavior of normal [Cp(CO)2Fe(NHC)] + moieties precludes a full comparison within the series, lower oxidation potentials observed in the bimetallic dIMes complexes reflect the increased donation properties of ditopic carbene ligand thus being in a perfect agreement with the data obtained by IR spectroscopy.

Though this remarkable boosting of the donation properties in NHDC ligands can be intuitively understood in terms of the delocalization of a formal anionic charge within the NHC core, we were pleased to rationalize this phenomenon using theoretical chemistry, namely through the extended transition state natural orbitals for chemical valence (ETS-NOCV) approach. [START_REF] Mitoraj | A Combined charge and energy decomposition scheme for bond analysis[END_REF] Comp.

CO of Cp(CO)2M-NHC, cm 1980 (2005) 2017, 1975 (1996) 2016, 1974 (1995) [5](OTf) 5a + 5b + 2044, 1998 (2021) 2034, 1994 (2014) 2029, 1987 (2008) 2037. 5, 1991 (2014.3) 2026, 1986 (2006) 2023, 1984 (2003.5) The summary of the most relevant data for metal-NHC bonding analysis are presented in Table 4 (for details see Table S8 in the Supporting Information). The ETS-NOCV analysis for Mn(I) complex 1a revealed three primary channels (Figure 5) responsible for IMes σ-donation and two types of π-back bondingorthogonal and coplanar to the NHC ligand plane further denoted as π⊥ and π||, respectively. While the major π⊥ component represents a classic back-donation to the π* orbital of carbene, the minor π|| can be described as an overlap of filled metal orbitals with the anti-bonding σ* orbitals of the NCN moiety. a Additional NOCV channel related to metal-aNHC π-bonding with the associated energy of 2.2 kcal.mol -1 was also found (see Figure S25). b In this case π|| bonding was split in two components and one more NOCV channel related to metal-aNHC π-bonding with the associated energy of 4.2 kcal.mol -1 was also found (see Figure S29). c Additional NOCV channel related to metal-aNHC πbonding with the associated energy of 4.0 kcal.mol -1 was also found (see Figure S30). d Additional NOCV channel related to metal-aNHC π-bonding with the associated energy of 2.4 kcal.mol -1 was also found (see Figure S32).

The associated energies of NOCV channels for 1a (Table 4) are close to calculated alkyl-substituted complex Cp(CO)2Mn(IMe) having the same horizontal orientation of the NHC ligand (σ 35.0, π⊥ 14.1, π|| 5.3 kcal mol -1 ). [START_REF] Fraser | structure and DFT study of asymmetrical NHC complexes of cymantrene derivatives and their application in the dehydrogenative dimerization reaction of thiols[END_REF] For the eclipsed isomer 1b, the energy values for σ and π⊥ components are a little bit higher reflecting slightly stronger Mn-NHC bonding (see Table 2 for the bond orders) and additional NOCV channel responsible for π(C=C)... π*(CO) interligand interaction having the associated energy of 1.5 kcal mol -1 was found (Figure S23). In Fe(II) complexes 6a + and 6b + IMes ligand displays a much stronger σ-bonding energy with a slightly reduced back-donation, which can be explained by a more electron deficient nature of the [Cp(CO)2Fe] + fragment compared to [Cp(CO)2Mn]. As expected, the aIMes ligand in Fe(II) derivative 4 + shows much greater σ-contribution with again slightly reduced π⊥-bonding being both consistent with higher overall donation properties of aNHCs vs. their normal isomers. These results are in agreement with the ETS-NOCV analysis of the bonding of the simplest NHC ligand models IMe and aIMe with different transition metal fragments. 40 The energies of NOCV channels responsible for π(C=C)...π*(CO) interactions in Fe(II) NHC cations (3.3-4.3 kcal mol -1 ) are significantly higher than in the corresponding neutral Mn(I) species being consistent with the results of AIM analysis (vide supra).

Noteworthy, the data obtained for the dIMes complexes within the same series of isomers (Table 4) show a simultaneous increase of the σ-donation and diminution of the π-backdonation for both M-NHC and M-aNHC bonding, which is especially pronounced in the latter case for the abnormal carbene fragment. Though the energy differences in most cases are not very significant, the synergy of both effects is clearly responsible for the experimentally observed amplification of NHC donor properties in NHDC derivatives.

Evaluation of Cationic Fe(II) Complexes Bearing IMes, aIMes and dIMes Ligands in Ketone Hydrosilylation Catalysis. In order to look at the possible incidence of improved donation properties of dIMes ligand we tested a series of cationic Fe(II) complexes [4-6](OTf) in benchmark hydrosilylation of carbonyl substrates. 41 These transformations are well known for half-sandwich Fe(II) complexes bearing normal 22a,b and abnormal 23a,25 carbene ligands and we have already observed a beneficial effect of increased NHC donation on the catalytic performance in such processes. 27b We have chosen as a model the hydrosilylation of 2-acetonaphtone with PhSiH3 (Scheme 4) under the same conditions as previously reported for abnormal 1,2,3triazol-4-ylidene Fe(II) complex. [START_REF] Johnson | Triazolylidene iron(II) piano-stool complexes: synthesis and catalytic hydrosilylation of carbonyl compounds[END_REF]

Scheme 4. Evaluation of Cationic Fe(II) Complexes [4-6](OTf) in Hydrosilylation of 2-Acetonaphtone

To our delight, in the presence of 1 mol % of dIMes complex [5](OTf) a quantitative substrate conversion was achieved after 2 h at 60°C and the corresponding alcohol was isolated in 92% yield after hydrolysis. A decrease of the catalyst charge to 0.5 mol % revealed 94% substrate conversion after 8 h under the same conditions. At the same time aIMes complex [4](OTf) was found to be much less active affording only 60% conversion after 24 h at 1 mol % loading and complex [6]

CONCLUSION

We have shown that bimetallic transition metal complexes based on imidazol-2,4-diylidene scaffold can be prepared from the emblematic IMes by a rational stepwise metallation approach using [Cp(CO)2Mn] moiety as protecting group for C2 carbene position. This gave access to an homogeneous series of cationic Fe(II) complexes bearing IMes, aIMes, and dIMes ligands allowing to show that the donation properties of both normal and abnormal carbenic moieties are actually increased by the metallation at the C4 and C2 positions, respectively. This effect was rationalized by a theoretical study using the ETS-NOCV approach in terms of a simultaneous increase of σ-donation and decrease of π-backdonation being especially pronounced for the abnormal carbenic part. The evaluation of the catalytic properties of the singular series of monometallic Fe(II) IMes and aIMes complexes, and bimetallic Fe(II) dIMes complex in benchmark ketone hydrosilylation revealed the superiority of the derivative bearing the ditopic dIMes ligand. We hope that this contribution will further stimulate the interest to NHDC complexes in the context of homogeneous catalysis.

EXPERIMENTAL SECTION

General considerations. All manipulations were carried out using Schlenk techniques under an atmosphere of dry nitrogen. Dry and oxygen-free organic solvents (THF, Et2O, CH2Cl2, toluene, pentane) were obtained using LabSolv (Innovative Technology) solvent purification system. Acetonitrile for the electrochemical studies was dried by refluxing over P2O5 and distilled under argon prior to use. Solvents used for the purification of organometallic complexes by column chromatography (hexane, toluene, CH2Cl2, THF) were deoxygenated by nitrogen bubbling during 15-20 min. Deuterated chloroform for NMR experiments was passed through a short column of basic alumina, deoxygenated by three freeze-pump-thaw cycles and kept over 4Å molecular sieves. A liquid nitrogen/ethanol slush bath was used to maintain samples at the desired low temperature. Chromatographic purification of the compounds was performed on silica (0.060-0.200 mm, 60 Å) obtained from Acros Organics. Complexes Cp(CO)2Mn(IMes) (1), [START_REF] Valyaev | Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2ligand[END_REF] [Cp(CO)2Fe(IMes)]I [START_REF] Buchgraber | Syntheses, properties, and X-ray crystal structures of piano-stool iron complexes bearing an N-heterocyclic carbene ligand[END_REF] and Cp(CO)2FeI [START_REF] Yasuda | Synthesis of aryliron complexes by palladium-catalyzed transmetalation between [CpFe(CO)2I] and aryl Grignard reagents and their chemistry directed toward organic synthesis[END_REF] were prepared according to literature procedures. All other reagent grade chemicals purchased from commercial sources were used as received.

Solution IR spectra were recorded in 0.1 mm CaF2 cells with 0.5 cm -1 resolution using a Perkin Elmer Frontier FT-IR spectrometer and given in cm -1 with relative intensity in parentheses. [START_REF] Arduengo | A stable crystalline carbene[END_REF] H and 13 C NMR spectra were obtained on Bruker Avance 400 or Avance III HD 400 spectrometers and referenced against the residual signals of deuterated solvents. [START_REF] Fulmer | NMR chemical shifts of trace impurities: common laboratory solvents, organics, and gases in deuterated solvents relevant to the organometallic chemist[END_REF] The signals of quaternary carbon atoms in 13 C NMR spectra were attributed using JMOD and gated 13 C experiments. High-resolution mass spectra (ESI positive mode) were obtained using a Xevo G2 QTof (Waters) spectrometer. Elemental analyses were carried out at the LCC-CNRS (Toulouse) using a Perkin Elmer 2400 series II analyzer.

Cyclic voltammetry and squarewave voltammetry studies were performed using an Autolab PGSTAT100 instrument controlled by GPES 4.09 software in a three-electrode cell consisting of a Pt working electrode (d = 0.5 mm), a platinum wire (S = 1 cm 2 ) as counter electrode, and a SCE electrode as a reference, under Ar atmosphere in acetonitrile solution. 0.1 M solution of Bu4NPF6 was used as the supporting electrolyte and sample concentrations were 1×10 -3 M. All peak potentials are calibrated relative to the ferrocene/ferrocenium couple (E = +0.40 V vs. SCE in MeCN) [START_REF] Connelly | Chemical redox agents for organometallic chemistry[END_REF] by adding ferrocene at the end of the experiments.

Synthesis of complex 3. To a solution of complex 1 (640 mg, 1.33 mmol) in THF (5 mL) a 1.6 M solution of nBuLi (1.0 mL, 1.6 mmol, 1.2 equiv.) was added dropwise at room temperature. After stirring for 15 min the resulting deep orange solution of complex 2 was evaporated under vacuum and the yellow-brown residue was dissolved in toluene (5 mL). The resulting solution of 2 was then added dropwise for ca. 15 min under vigorous stirring to a solution of Cp(CO)2FeI (404 mg, 1.33 mmol, 1.0 equiv.) in toluene (5 mL) at -10 °C. The cooling bath was removed and the reaction mixture was allowed to reach room temperature. The volatiles were removed under vacuum and the orange residue was purified by column chromatography on silica (2×15 cm). The first elution with 1:1 hexane/toluene mixture afforded yellow band containing traces of Cp(CO)2FeI and starting complex 1 (discarded), followed by a broad orange band of the product 3 contaminated with traces of [Cp(CO)2Fe]2 eluted with 95:5 toluene/ether mixture. The eluate was evaporated under vacuum and the residue was dissolved in 1:1 THF/heptane mixture (ca. 50 mL). The solution was filtered through Celite and concentrated to ca. 20 mL under vacuum to induce the crystallization of the target product finished at -20 °C overnight. The supernatant was removed by decantation and the precipitate was washed with hexane (2×10 mL) and dried under vacuum to afford complex 3 (725 mg, 83%) as orange microcrystalline solid. Single crystals suitable for X-Ray diffraction experiment were obtained by slow vapor diffusion of hexane into the solution of complex 3 in ether at room temperature. [START_REF] Arduengo | A stable crystalline carbene[END_REF] H NMR (400.1 MHz, C6D6, 25°C): δ 6.99 (s, 2H, CHMes), 6.97 (s, 2H, CHMes), 6.04 (s, 1H, CHIm-5), 4.09 (s, 5H, Cp-Fe), 3.89 (s, 5H, Cp-Mn), 2.28 (s, 3H, CH3 o-Mes), 2.23 (s, 9H, CH3 o-Mes), 2.08 (s, 6H, CH3 p-Mes); 13 C{ 1 H} NMR (100.6 MHz, C6D6, 25°C): δ 235.1 (Mn-CO), 213.8 (Fe-CO), 204.4 (Mn-CN2), 142. 5, 139.7 (Ci-Mes), 138.1, 137.9 (C-CH3 p-Mes), 137.6, 136.3 (C-CH3 o-Mes), 132.9 (CHIm-5), 129.6, 129.3 (CHMes), 124.8 (CIm-4-Fe), 85.3 (Cp-Fe), 81.7 (Cp-Mn), 21.4, 21.2 (CH3 p-Mes), 19.2, 18.8 (CH3 o-Mes). IR (CH2Cl2): CO 2030.0 (s), 1980.0 (s), 1900.5 (s), 1827.0 (s) cm -1 . Elemental analysis calculated (%) for C35H33FeMnN2O4: C, 64.05; H, 5.05; N, 4.25; found: C, 64.05; H, 5.30; N, 4.14.

Synthesis of complex [4](OTf).

To a solution of complex 3 (394 mg, 0.6 mmol) in CH2Cl2 (10 mL) triflic acid (65 μL, 0.72 mmol) was added via microsyringe at -40 °C. The orange color of the solution became red-brown and IR spectrum show the transformation of νCO bands of 3 (2030.0 (s), 1980.0 (s), 1900.5 (s), 1827.0 (s) cm -1 ) into those of the product [4](OTf) (2044.0 (s), 1996.0 cm -1 (s)) and CpMn(CO) 3 (2022.5 (s), 1935.0 cm -1 (vs)). The reaction mixture was stirred for 10 min at -40 °C and then allowed to reach room temperature over ca. 15-20 min. The resulting brown solution was loaded on a column of silica (2×10 cm) and several yellow-brown bands containing CpMn(CO)3 and other unidentified impurities were eluted with pure CH2Cl2. The desired product [4](OTf) was then eluted with a CH2Cl2/THF 20:1 mixture as a yellow band. The eluate was evaporated under vacuum and the residue was dissolved in THF (5 mL). The solution was filtered through Celite, evaporated under vacuum and the product was thoroughly dried under vacuum to afford [4](OTf) (337 mg, 89%) as yellow powder. [START_REF] Arduengo | A stable crystalline carbene[END_REF] 17.6, 17.3 (CH3 o-Mes). IR (CH2Cl2): νCO 2044.0 (s), 1996.0 (s) cm -1 . Elemental analysis calculated (%) for C29H29F3FeN2O5S: C, 55.25; H, 4.65; N, 4.45; found: C, 55.11; H, 4.65; N, 4.41.

Synthesis of complex [5](OTf).

To a solution of [4](OTf) (126 mg, 0.2 mmol) and Cp(CO)2FeI (61 mg, 0.2 mmol) in THF (4 mL) a ~1.7 M solution of tAmOK in toluene (175 μL, 0.3 mmol) was added dropwise at -80 °C. The reaction mixture was stirred at -80 °C for 30 min and then allowed to warm up to room temperature. IR spectrum showed the formation of [5](OTf) (νCO 2035.5 (s), 2027.0 (sh), 1988.0 (s), 1878.5 (sh) cm -1 ) as the main organometallic product. The volatiles were removed under vacuum and the residue was purified by chromatography on silica column (1×8 cm). Several yellow-brown bands containing traces of Cp(CO)2FeI and other unidentified impurities were eluted with pure CH2Cl2. The product was then eluted with a CH2Cl2/THF 20:1 mixture as a yellow band. Evaporation of the solvent and drying the residue under vacuum afforded complex [5](OTf) (119 mg, 74%) as yellow powder. [5] 139.9, 137.9 (Ci-Mes), 136.7, 136.65 (C-CH3 o-Mes), 136.6 (CIm-4-Fe), 135.7 (CHIm-5), 130.1, 129.8 (CHMes), 86.2, 86.1 (Cp-Fe), 21.35, 21.3 (CH3 p-Mes), 18.9, 18.4 (CH3 o-Mes). IR (CH2Cl2): νCO 2044.0 (s), 2037.5 (s), 1998.0 (s), 1991.0 (sh) cm -1 . HRMS (ESI) 657.1141 (M + ), calculated for C35H33Fe2N2O4 657.1139 (εr = 0.3 ppm).

Synthesis of complex [6](OTf). To a solution of [Cp(CO)2Fe(IMes)]I (177 mg, 0.25 mmol) in CH2Cl2 (5 mL) a solution of NaOTf (60 mg, 0.35 mmol) in a nitrogen saturated water (5 mL) was added and the resulting biphasic mixture was vigorously shaken for 2 min. Then the aqueous mixture was removed and the organic mixture was washed with water (5 mL), dried over MgSO4 and evaporated to dryness under vacuum. The residue was dissolved in THF (5 mL), filtered through Celite and evaporated to afford after drying under vacuum [6](OTf) (146 mg, 93%) as yellow powder. 18.1 (CH3 o-Mes). IR (CH2Cl2): νCO 2050.5 (s), 2006.5 cm -1 (s) cm -1 . Elemental analysis calculated (%) for C29H29F3FeN2O5S: C, 55.25; H, 4.65; N, 4.45; found: C, 55.03; H, 4.41; N, 4.34.

X-ray diffraction experiment for complex 3. X-ray diffraction data were collected on a Bruker D8/APEX II/Incoatec Mo IµS Microsource diffractometer using MoKα radiation (λ = 0.71073 Å, graphite monochromator). All calculations were performed on a PC compatible computer using the WinGX system. [START_REF] Farrugia | WinGX and ORTEP for Windows: an update[END_REF] The structures were solved using the SIR92 program, [START_REF] Altomare | SIR92 -a program for automatic solution of crystal structures by direct methods[END_REF] which revealed in each instance the position of most of the non-hydrogen atoms. All the remaining non-hydrogen atoms were located by the usual combination of full matrix least-squares refinement and difference electron density syntheses using the SHELX program. [START_REF] Sheldrick | A short history of SHELX[END_REF] Atomic scattering factors were taken from the usual tabulations. Anomalous dispersion terms for Mn and Fe atoms were included in Fc. All non-hydrogen atoms were allowed to vibrate anisotropically. The hydrogen atoms were set in idealized positions (R3CH, C-H = 0.96 Å; R2CH2, C-H = 0.97 Å; RCH3, C-H = 0.98 Å; C(sp 2 )-H = 0.93 Å; Uiso 1.2 or 1.5 times greater than the Ueq of the carbon atom to which the hydrogen atom is attached) and their positions refined as "riding" atoms.

Computational details. Calculations were performed with the Gaussian 09 50 package with the BP86 51 functional with D3 version of Grimme's dispersion [START_REF] Grimme | A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[END_REF] (DFT/BP86-D3) without any ligand simplification. For all atoms the Def2-TZVP 53 basis set was applied. The structures of the all complexes were fully optimized on this level without any symmetry restrictions applying ultrafine integration grid and a tight SCF option. Vibrational frequencies were calculates at the same theory level and reported without applying any scaling factors.

Topological analysis of the electron-density distribution function ρ(r) was performed using the AIMALL 54 and Multiwfn v3.6 [START_REF] Lu | Multiwfn: a multifunctional wavefunction analyzer[END_REF] program packages based on the wave function obtained by the BP86 calculations. The energies of non-covalent interactions were calculated using the correlation between the binding energy (EBCP) and the value of the density-functional potential energy VC(r) in the corresponding critical point (3, -1): E = 0.5•VC(r). [START_REF] Espinosa | Hydrogen bond strengths revealed by topological analyses of experimentally observed electron densities[END_REF] Bond ellipticity, ε was defined as ε = (λ1/λ2-1), where λ1 and λ2 are the negative eigenvalues of the Hessian of the electron density at the bond critical point ordered such that λ1 < λ2 < 0. 30 NBO analysis performed with the NBOPro 6.0 program. 56 For bonding analysis all complexes were fully reoptimized with the ADF2014 [START_REF] Baerends | Theoretical Chemistry[END_REF] program using BP86-D3 functional and TZP 58 basis set. The overall geometry of complexes, as well as relative vibrational frequencies (Table S7) are similar to those of G09/BP86-D3/def2-TZVP calculations. With the energy decomposition analysis (EDA) 32 method, the fragments interaction energy, ΔEint, was decomposed into terms ΔEint = ΔEelst + ΔEPauli + ΔEorb+ ΔEdisp. ΔEorb was further decomposed with ETS-NOCV approach to the electron density transfer channels, Δρi(r). [START_REF] Mitoraj | A Combined charge and energy decomposition scheme for bond analysis[END_REF] Hydrosilylation of 2-acetonaphtone catalyzed by Fe(II) NHC complexes [4-6](OTf). A 10 mL oven-dried Schlenk tube containing a stirring bar, was charged with catalyst, 2-acetonaphtone (85.1 mg, 0.5 mmol), PhSiH3 and toluene (1 ml, C = 0.5 M). The reaction mixture was placed in a pre-heated oil bath at 60°C and stirred for the indicated time. After cooling to room temperature MeOH (1 mL) and 1M aqueous NaOH solution (2 mL) were added. The reaction mixture was vigorously stirred for 2 hours at room temperature and then extracted with Et2O (3×10 ml). The combined organic layers were dried over MgSO4, filtered and evaporated under vacuum. The conversion was determined by [START_REF] Arduengo | A stable crystalline carbene[END_REF] H NMR spectroscopy in the crude mixture by the ratio of proton signals at δH of 8.47 ppm of 2-acetonaphthone (s, 1H, CHAr) and δH of 5.08 ppm of α-methyl-2-naphthylmethanol (q, 1H, CHCH3). The isolation of the product was carried out by column chromatography on silica (5×1 cm) using a 4:1 petroleum ether / EtOAc mixture to afford after drying under vacuum α-methyl-2-naphthylmethanol (78 mg, 92%).

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. 1 H, 13 C NMR spectra and cyclic voltammograms for all NHC complexes, additional DFT calculations data (PDF). Cartesian coordinates for all G09 and ADF optimized structures (xyz).

Table of Contents artwork and text

Superior electronic donation of imidazol-2,4-diylidene ligand vs. the monocarbenic ligands was quantified by IR spectroscopy and cyclic voltammetry on the corresponding mono-and bimetallic Mn(I) and Fe(II) complexes. The observed effect was rationalized by DFT calculations in terms of a synergy between increased σ-donation and decreased π-backdonation of both normal and abnormal carbene fragments being especially pronounced in the latter case.

  )] (OTf)([6]OTf)) (Scheme 2, inset) was prepared by an anion exchange in the known [Cp(CO)2Fe(IMes)]I[START_REF] Buchgraber | Syntheses, properties, and X-ray crystal structures of piano-stool iron complexes bearing an N-heterocyclic carbene ligand[END_REF] salt with NaOTf in CH2Cl2/water. Scheme 2. Synthesis of Normal, Abnormal and Ditopic NHC Complexes of Mn(I) and Fe(II). Mes = 2,4,6-Trimethylphenyl Reaction conditions: (i) 1.2 equiv. nBuLi, THF, r.t., 15 min; (ii) Cp(CO)2FeI, toluene, -10°C to r.t., 0.5h; (iii) 1.2 equiv. TfOH, CH2Cl2, -40°C to r.t., 0.5h; (iv) 1.5 equiv. tAmOK, Cp(CO)2FeI, THF, -80°C to r.t., 1h.

Figure 1 .

 1 Figure 1. A perspective view of a complex 3 (30% probability ellipsoids). Theoretical Investigation of the Electronic Structure of Transition Metal dIMes Complexes and the Comparison with Their Mononuclear Congeners. DFT optimized structures of all relevant Mn(I) and Fe(II) complexes obtained in the gas phase using G09/BP86-D3/def2-TZVP approach are pre-

Figure 2 .

 2 Figure 2. DFT optimized geometries for the cationic parts of Mn(I) and Fe(II) IMes, aIMes and dIMes complexes calculated at G09/BP86-D3/def2-TZVP level. Hydrogen atoms are omitted for clarity, color element codes: C -grey, Nblue, Ored, Mnrose, Feorange. Non-covalent interligand π(C=C)...π*(C≡O) interactions detected by AIM approach are indicated by red dashed lines.

Figure 3 .

 3 Figure 3. Superposition of normalized experimental IR spectra of complexes 1, 3 and [4](OTf) in 2100-1750 cm -1 range (CH2Cl2, 0.5 cm -1 resolution).

Figure 4 .

 4 Figure 4. Superposition of normalized experimental IR spectra of complexes [4-6](OTf) in 2100-1950 cm -1 range (CH2Cl2, 0.5 cm -1 resolution)

Scheme 3 .

 3 The Fragmentation Method Used for the Comparison of Electronic Donation of Anionic dIMes Ligand with Those of Related Neutral IMes and aIMes Carbenes (R = Mes, [M] = Cp(CO)2Mn, Cp(CO)2Fe)

Figure 5 .

 5 Figure 5. Electron deformation density corresponding to primary NOCV channels for Mn(I) complex 1a responsible for σ-donation (left) and two types of π-back donation -π⊥ (center) and π|| (right). Isosurfaces set at 0.003 and 0.001 a.u. for σand π-bonding, respectively. Red and blue areas correspond to electron density depletion and concentration, respectively. Table 4. Associated Energies of Primary NOCV Channels (kcal mol -1 ) Responsible for the σ-Donation and π-Back Bonding of NHC Ligands in Mn(I) and Fe(II) Complexes Bearing IMes, aIMes and dIMes Ligands (ADF/BP86-D3/TZP)

Table 1 . Selected 1 H and 13 C NMR Data for Mn(I) and Fe(II) Complexes Bearing Normal, Abnormal and Ditopic NHC Ligands

 1

	Complex δC5-H	δC2	δC4	δC5	δCO
	1 a	6.18	205.4	123.8	123.8	234.4
	[6](OTf) b 7.37	171.4	128.1	128.1	209.4
	[4](OTf) b 7.07	138.2	141.8	132.2	211.8
	3 a	6.04	204.4	124.8	132.9	235.1, 213.8
	[5](OTf) b 6.79	166.7	136.6	135.7	212.7, 210.3
	a C6D6 solution; b CDCl3 solution

Table 3 . Experimental (CH2Cl2 Solution) and Calculated (G09/BP86-D3/Def2-TZVP, italic) IR Spectroscopy Data for Mn(I) and Fe(II) IMes, aIMes and dIMes Complexes (Average <νCO> Values Are Given in Parenthesis in Red and Blue for Experimental and Theoretical Data, Respectively)

 3

  (OTf) bearing IMes ligand was totally inactive. To the best of our knowledge, this preliminary study represents the first example

	of direct comparison of the catalytic performance for transition
	metal complexes bearing structurally related normal, abnormal
	and ditopic NHCs. 42,43
	Comp. Cp(CO)2M-NHC bond	Cp(CO)2M-aNHC bond
	1a	σ 38.0, π⊥ 12.5, π|| 5.7 -
	1b	σ 38.8, π⊥ 13.9, π|| 5.4 -
	6a +	σ 54.3, π⊥ 9.6, π|| 3.8	-
	6b +	σ 54.9, π⊥ 9.8, π|| 4.1 a -
	4 +	-	σ 64.8, π⊥ 9.1, π|| 4.4
	3a	σ 38.6, π⊥ 11.2, π|| 5.7	σ 78.2, π⊥ 8.7, π1|| 2.5, π2|| 2.8 b
	3b	σ 39.4, π⊥ 12.9, π|| 5.5 σ 77.9, π⊥ 8.9, π|| 3.5 c
	5a +	σ 57.3, π⊥ 9.2, π|| 4.1	σ 71.5, π⊥ 8.7, π|| 3.2
	5b +	σ 57.5, π⊥ 9.9, π|| 4.1 d σ 71.3, π⊥ 9.0, π|| 3.4

  (OTf): 1 H NMR (400.1 MHz, CDCl3, 25°C): 1 H NMR (400.1 MHz, CDCl3, 25°C): δ 7.07 (s, 2H, CHMes), 7.05 (s, 2H, CHMes), 6.79 (s, 1H, CHIm-5), 4.98 (s, 5H, Cp-Fe), 4.79 (s, 5H, Cp-Fe), 2.39 (s, 3H, CH3 p-Mes), 2.38 (s, 3H, CH3 p-Mes), 2.06 (s, 6H, CH3 o-Mes), 1.96 (s, 6H, CH3 o-Mes) 13 C{ 1 H} NMR (100.6 MHz, CDCl3, 25°C): δ 212.7 (Fe-CO), 210.3 (Fe-CO), 166.7 (Fe-CN2), 140.6, 140.3 (C-CH3 p-Mes),

ACKNOWLEDGMENT

We thank Centre National de la Recherche Scientifique (CNRS) and Institut Universitaire de France (IUF) for a general support of this project. A.A.G. is grateful to French Embassy in Moscow for a joint PhD fellowship (Vernadski program). Computational studies were performed using HPC resources from CALMIP (Grant no. P18038). O.A.F. thanks the Ministry of Science and Higher Education of the Russian Federation for partial support of this research.

AUTHOR INFORMATION

Corresponding Authors * E-mail for N.L.: noel.lugan@lcc-toulouse.fr * E-mail for V.C.: vincent.cesar@lcc-toulouse.fr * E-mail for D.A.V.: dmitry.valyaev@lcc-toulouse.fr ORCID Oleg A. Filippov: 0000-0002-7963-2806 Yves Canac: 0000-0002-3747-554X Jean-Baptiste Sortais: 0000-0003-1178-8588 Noël Lugan: 0000-0002-3744-5252 Vincent César: 0000-0002-6203-6434 Dmitry A. Valyaev: 0000-0002-1772-844X

Notes

The authors declare no competing financial interest.


Claudia Gonçalves

Raphaël Mereau

Virginie Nazabal

Catherine Boussard-Plédel

Claire Roiland

Eric Furet

Michaël Deschamps

Bruno Bureau

Marc Dussauze

Catherine Boussard-Pledel

Study of the Ge20Te80-xSex glassy structures by combining

Keywords: Chalcogenide glasses, Structure, ss-NMR spectroscopy, Vibrational spectroscopies, DFT modelling

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

The first studies on the Ge-Te-Se ternary glassy system presenting an extensive vitreous domain were focused on the thermal and electrical properties. [START_REF] Bhanu Prashanth | Composition dependent electrical switching in glassesthe influence of network rigidity and thermal properties[END_REF][START_REF] Ganesan | Composition Dependence of the Glass Transition in Ge-Se-Te Glasses[END_REF] It is only with the growing interest in information storage that these glasses have been considered for their phase change properties. [START_REF] Rubin | Progress and issues of phase-change erasable optical recording media[END_REF][START_REF] Sharma | Far-infrared transmission and bonding arrangement in Ge10Se90-xTex semiconducting glassy alloys[END_REF][START_REF] Vinod | Effect of selenium addition on the GeTe phase change memory alloys[END_REF] Recently, Ge-Te-Se glasses have been investigated for space and biological research. [START_REF] Maurugeon | Te-rich Ge-Te-Se glass for the CO2 infrared detection at 15 μm[END_REF][START_REF] Danto | A Family of Far-Infrared-Transmitting Glasses in the Ga-Ge-Te System for Space Applications[END_REF] Indeed, chalcogenide glasses are known for their extended range of transmission in the infrared (up to a wavelength of 14 μm for bulk selenide glasses with thicknesses of few mm, and even beyond wavelength of 18 μm for telluride glasses). Several studies are still going on to further extend the transmission range of these glasses. It is the reason why tellurium-rich glasses have been the subject of much work in recent years.

Despite a smaller range of transparency, selenide glasses are known to synthesize easily, with a broad vitreous domain. They have been the subject of numerous experimental and theoretical studies, particularly for GeSe 4 and GeSe 2 by NMR and Raman spectroscopies, neutrons and density-functional based molecular dynamics simulations. [START_REF] Gjersing | Structure, Connectivity, and Configurational Entropy of GexSe100-x Glasses: Results from 77Se MAS NMR Spectroscopy[END_REF][START_REF] Edwards | A combined 77Se NMR and Raman spectroscopic study of the structure of GexSe1-x glasses: Towards a self consistent structural model[END_REF][START_REF] Bureau | Germanium selenide glass structures studied by 77Se solid state NMR and mass spectroscopy[END_REF][START_REF] Kibalchenko | Structural Composition of First-Neighbor Shells in GeSe2 and GeSe4 Glasses from a First-Principles Analysis of NMR Chemical Shifts[END_REF][START_REF] Micoulaut | Structure, topology, rings, and vibrational and electronic properties of GexSe1-x glasses across the rigidity transition: A numerical study[END_REF][START_REF] Sykina | Network connectivity and extended Se chains in the atomic structure of glassy GeSe4[END_REF]. In Ge x Se 100-x glasses, the dominant coordination number for Ge is four and Se is two, and coordination defects would also be present for these two atoms in a small proportion. The Ge five-fold coordinated would be less J o u r n a l P r e -p r o o f than 5%, while the Ge and Se three-fold coordinated would be even smaller but would increase for GeSe 2 glass to ∼11 and 8%, respectively [START_REF] Micoulaut | Structure, topology, rings, and vibrational and electronic properties of GexSe1-x glasses across the rigidity transition: A numerical study[END_REF]. In the case of stoichiometric GeSe 2 glass, the [GeSe 4/2 ] tetrahedral (Td) can be linked by corners (CS) or an edge (ES). In addition, the presence of Ge-Ge (forming (GeGeSe 3 ) entities only) and Se-Se homopolar bonds allowed by the relatively close electro-negativities of Ge and Se atoms are expected and probably over-or under-coordinated atoms as well, as mentioned. For GeSe 4 in particular, the occurrence of a fully bonded network is suggested, in which Se-Se-Ge linkages ensure the connection between Se-Se-Se chains and Ge-Se-Ge tetrahedral arrangements [START_REF] Kibalchenko | Structural Composition of First-Neighbor Shells in GeSe2 and GeSe4 Glasses from a First-Principles Analysis of NMR Chemical Shifts[END_REF].The proportion of these [GeSe 4/2 ] Td CS and ES motifs, homopolar bonds and coordination defects evolve according to the composition of the Ge x Se 100-x glasses forming odd-and even-sized rings.

In order to take advantage of the structural similarities between tellurium and selenium, research was then conducted along the pseudo-binary line GeSe 4 -GeTe 4 (i.e. Ge 20 Se 80-x Te x ) of the Ge-Te-Se ternary diagram, in order to find the best compromise between the wide GeTe 4 transparency window and the ease of synthesis and shaping of GeSe 4 . Maurugeon et al. [START_REF] Maurugeon | Te-rich Ge-Te-Se glass for the CO2 infrared detection at 15 μm[END_REF] showed that the addition of a small percentage of selenium in the vitreous matrix of Ge-Te system stabilized the material and kept the extended range of transparency characteristic of telluride glasses. It is important to mention that beyond x = 30 and before x = 50, there is a nonmixing zone, where it seems extremely delicate to form glass under conventional synthesis conditions.

A number of structural analyzes have already been done on this Ge-Te-Se ternary system. Either the work focused on a single composition of the ternary system, or on its selenium or telluriumrich part. For instance, the study conducted by Jovari et al. [START_REF] Jóvári | Structure of Te-rich Te-Ge-X (X = I, Se, Ga) glasses[END_REF] on the tellurium-rich part began by investigating the structure of Ge 20 Se 10 Te 70 glass by X-ray and neutrons diffraction and J o u r n a l P r e -p r o o f Extended X-Ray Absorption Fine Structure (EXAFS) measurements, producing data simultaneously fitted by the Reverse Monte-Carlo (RMC) simulation technique. These results show that the glass is built up of Ge(Te, Se) 4 tetrahedra linked by Te-Te and Te-Se bonds. The coordination number of Ge is close to 4, the Te is predominantly two-fold coordinated and a coordination number of Se higher than 2 is also compatible with their model. Moreover, some Se atoms in the model are shared by two Ge(Te, Se) 4 tetrahedra without forming Te-Se bonds. To go further using the same approach, Ratkai et al. [START_REF] Rátkai | Microscopic origin of demixing in Ge20SexTe80-x alloys[END_REF] have tried to understand the presence of the non-mixing zone for Ge 20 Se 80-x Te x (x = 75, 70, 65, 60) glasses. They suggested that for lower Se concentration, the structure can be described as essentially built up of GeTe 4 tetrahedra connected to each other by Te-Te dimers. Se preferentially binds to Ge for all compositions investigated. From 20% of Se, Se-Se bonds start to be present without Se-Te bonding and the Te-Te coordination starts to increase. For this concentration, nanoscale phase separation is envisaged due the fact that Se and Te avoid each other in the presence of Ge. Lately, studies have been carried out to explore the crystal growth process that limits the fiber-drawing procedure for the tellurium-rich glasses (up to 10 at.% of Se ) [START_REF] Svoboda | Thermal behavior of Ge20SeyTe80-y infrared glasses (for y up to 8 at.%)[END_REF][START_REF] Svoboda | Se-doped GeTe4 glasses for far-infrared optical fibers[END_REF][START_REF] Sun | Structural investigation of Te-based chalcogenide glasses using Raman spectroscopy[END_REF]. They confirmed that selenium atoms will bind preferentially to Ge atoms and proposed, based on Raman spectroscopy investigation, an increase with Se addition in the amount of edge-shared tetrahedra and the short Te chains interconnecting the tetrahedral networks, excluding the presence of GeSe 4 tetrahedra and Se n chains.

On the selenium-rich part of the system, Moharram et al. [START_REF] Moharram | Short and intermediate range order of Ge20Se80-xTex glasses[END_REF] studied four compositions Ge 20 Se 80-x Te x (x = 0, 10, 20 and 30) by XRD technique and Raman spectroscopy considering the chemical order network model approach. [START_REF] Moharram | Short and intermediate range order of Ge20Se80-xTex glasses[END_REF] They concluded with the presence of mixed chalcogenide chains interconnecting the germanium tetrahedral. GeTe 4/2 entities are proposed to J o u r n a l P r e -p r o o f be excluded from their Raman spectroscopy analysis. Recently, NMR studies on Ge-Te-Se glasses with have been carried out to understand the structure of the vitreous network. Bouëssel du Bourg et al. [START_REF] Du Bourg | Impact of Te on the structure and 77Se NMR spectra of Se-rich Ge-Te-Se glasses: a combined experimental and computational investigation[END_REF] studied the impact of tellurium on the structure of selenium-rich part of the GeSe 4 -GeTe 4 pseudo-binary line (i.e. Ge 20 Se 80-x Te x , x = 0, 10, 30 and 60). This study, comparing experimental and theoretical results, concluded that Ge atoms are mainly 4-fold coordinated with a single maximum centered at 109° for the angular distribution functions and that a single tellurium at the maximum can be found in its coordination sphere with a very low occurrence. This analysis of the molecular dynamics structures has shown that the Te atoms avoid binding to Ge atoms. The tellurium would rather disperse in the selenium chains, which causes a subsidence of the number of Se-Se-Se environment.

In the present study, we have focused our attention on the structure of the entire pseudo-binary GeSe 4 -GeTe 4 glassy system. We will present the results of 73 Ge solid-state NMR characterizations and IR / RAMAN vibrational spectroscopies measurements combined to DFT calculations.

EXPERIMENTAL SECTION

Glass Preparation

Ge 20 Te 80-x Se x glasses (with x varying from 0 to 80) were prepared in silica tubes, sealed under vacuum from high purity Te (6N, JX Nippon Mining), Se (5N, Umicore) and Ge (5N, Umicore) in the exact appropriate stoichiometry. The initial chemicals were not further purified before being introduced into the silica set-up. The tubes were placed into a rocking furnace at 800°C, to homogenize the melt. Then the batch is annealed for 2h (at Tg-10°C) in order to remove residual J o u r n a l P r e -p r o o f stress and finally cooling down slowly to the room temperature. The amorphous character of samples was verified by X-ray diffraction (XRD) and Differential Scanning Calorimetry (DSC).

Solid-State NMR measurements

A series of 73 Ge NMR measurements were acquired on a 20T NMR spectrometer (850 MHz for 1 H and 29.66 MHz for 73 Ge -spin I = 9/2 , natural abundance = 7.76%-), and in static conditions inside a 5 mm diameter solenoid probe. The chemical shift was referenced to Ge(OCH 3 ) 4 . The pulse sequence was a QCPMG sequence (Quadrupolar Carr-Pucell-Meiboom-Gill echo sequence) with a 90° RF pulse of 2 µs (with a 600 W output power from the RF amplifier) that is selective on the central transition (no contribution is observed from the satellite transition). The echo delay between consecutive 180° pulses was set to 700 µs to allow for the recording of the entire full echo signal. Each Free Induction Decay (FID) signal was stored in a 2D buffer and FT was performed in a 2D mode as described elsewhere [START_REF] Petracovschi | Structural study by Raman spectroscopy and 77Se NMR of GeSe4 and 80GeSe2-20Ga2Se3 glasses synthesized by mechanical milling[END_REF]. The recovery delay was set to 0.5 s for full relaxation, and Double Frequency Sweep (DFS) was used to enhance the intensity of the central transition signal [START_REF] Kentgens | Advantages of double frequency sweeps in static, MAS and MQMAS NMR of spin I=3/2 nuclei[END_REF]. The line widths remain considerable (although 1000 ppm correspond to 30 kHz) and when needed, the Variable Offset Cumulative Spectrum (VOCS) technique is used to acquire the spectra due to their broadening. [START_REF] Massiot | 71Ga and 69Ga nuclear magnetic resonance study of β-Ga2O3: resolution of four-and six-fold coordinated Ga sites in static conditions[END_REF]

Raman measurement

Raman spectroscopy measurements were carried out using the 785 nm laser on a Labram HR spectrometer, equipped with one filter which allow a global reflection of the laser for measurement in microscopy (NoiseBlock ASE Suppressor filter, Ondax®) and two SureBlock Notch filters (Ondax®) to have a sufficient optical density over a short spectral range). The acquisition of Raman spectra was, therefore, made with caution due to the photosensitive nature of the glasses. To avoid any modification of the network, it was necessary to use very low laser J o u r n a l P r e -p r o o f powers (from 24 µW to 170 µW) as well as to apply low counting times per point. The spectra presented in this article are the average of 10 measurements, for 3 acquisitions of several minutes by points. The overall acquisition was approximately 60 minutes per spectrum.

Infrared measurement

Infrared spectra were conducted by specular reflection method on a Bruker Vertex 70V vacuum spectrometer, equipped with a mercury source, producing radiation in the far infrared.

The spectral range of study is from 30 to 1000 cm -1 . The signal was recorded with the help of an external reflection tool with an almost normal angle of incidence (11°). The results were subjected to the Kramers-Kroning analysis, to obtain the complex refractive index (n, k). The spectra will be presented in the form of absorption coefficient α=4πk/λ. [START_REF] Kamitsos | Infrared reflectance spectra of lithium borate glasses[END_REF]

DFT calculation

Studies of optimizations and harmonic vibrational frequency calculations were carried out on small sized clusters using Density Functional Theory (DFT) calculations, using the functional B3LYP. Prior to any geometry optimizations, the dangling bonds originating from singly bonded chalcogen atoms were neutralized by adding hydrogen-like atoms with masses equal to germanium to provide better agreement between calculated and experimental vibrational frequencies. After geometry optimizations of the clusters, harmonic vibrational frequencies were calculated from the equilibrated geometries of the different clusters, in order to obtain the Raman and Infrared spectra. Qualitative vibrational assignments were made by visualizing the atom displacement vectors for each mode likely responsible for experimental IR and Raman bands. All calculated vibrational modes including the dangling bonds of the clusters were not taken in account. In addition, a Potential Energy Distribution (PED) analysis was performed to also J o u r n a l P r e -p r o o f quantify the contribution of internal coordinates to a specific vibrational mode. The germanium, gallium and selenium atoms have been treated by the "all-electron" 6-311 ++ G (3df, 2p) basis set except for tellurium which was treated with the help of pseudo-potential, LANL2DZ. All the calculations presented in this article were perform using the Gaussian 09 software.

RESULTS: EXPERIMENT

NMR results

The 73 Ge nucleus is not that present in literature because it combines several difficulties: low natural abundance (7.76%), low gamma (-0.936x107 rad.T -1 .s -1 ) and a high quadrupolar constant (-19.6x10-30 m 2 ). This nucleus is a real challenge to probe. Three compositions were probed.

The 73 Ge QCPMG NMR spectra are gathered in the Figure 1. First, the presence of Ge-Ge homopolar bonds within the tetrahedron was identified by Sen et al.. [START_REF] Sen | Chemical order around Ge atoms in binary germanium selenide glasses: Results from 73Ge solid-state NMR spectroscopy at 19.6 Tesla[END_REF] If this structural arrangement is to be found in the glass, it means germanium may be bound to either another germanium or Te/Se. Such a variety of coordination environments is certainly likely to widen the 73 Ge spectrum.

A second hypothesis is based on the greater variability in the symmetry of the GeTe 4 tetrahedra compared to the GeSe 4 tetrahedra due to the higher polarizability of tellurium: such a theory will be preferred at this stage as it does not require extensive homopolar Ge-Ge bonding in the sample.

Results on Vibrational Spectroscopy/DFT calculations

The Raman and Infrared (IR) spectra of Ge 20 Te 80-x Se x (with x from 0 to 80) glasses are presented in Figure 2. Regarding the compositional evolution of the Raman spectra (Figure 2a.), starting from Ge 20 Te 80 , two contributions are observed at 126 and 155 cm -1 . The peak at 126 cm -1 decreases by the selenium addition, until the Ge 20 Te 50 Se 30 composition where it ends up disappearing. The peak at 155 cm -1 seems to widen by the addition of selenium towards higher frequencies making it asymmetrical. When the evolution of the spectra is examined starting from Ge 20 Se 80 , three contributions are observed at 190, 210 and 260 cm -1 . The addition of tellurium in the vitreous matrix reduces the peak at 260 cm -1 . The shoulder observed at 210 cm -1 intensifies by the addition of tellurium. A new vibration mode is recorded at 170 cm -1 , starting from Ge 20 Te 5 Se 75 , J o u r n a l P r e -p r o o f and intensifies by the tellurium addition in the matrix. One can also note the shoulder at 155 cm -1 beginning to appear for the composition Ge 20 Te 30 Se 50.

J o u r n a l P r e -p r o o f

For the selenium-rich part of the infrared spectra (Figure 2.b), there is no notable change. Two principal peaks are identifiable at 260 and 310 cm -1 and do not change to a significant degree with the tellurium addition in the matrix. It is necessary to wait for the Ge 20 Te 60 Se 20 composition to start observing significant differences in the spectrum. However, from 5% of tellurium in the selenium matrix, a new low intensity band can be observed for low frequency range at 210 cm -1 .

On the other hand, in the tellurium-rich part, significant modifications are observable. A wide band at 220 cm -1 is recorded, extended on a broad frequency range where several contributions can be suspected. At 250 cm -1 , a new peak is recorded, starting from the Ge 20 Te 70 Se 10 composition, and increase until the non-mixing zone at Ge 20 Te 50 Se 30 .

Few results are present in the literature to help us explaining the two tendencies observed.

Either the work focused on a single composition of the Ge-Te-Se ternary system, or on a selenium or tellurium rich part of the system along the pseudo-binary line GeSe 4 -GeTe 4 . [START_REF] Rátkai | Microscopic origin of demixing in Ge20SexTe80-x alloys[END_REF][START_REF] Svoboda | Thermal behavior of Ge20SeyTe80-y infrared glasses (for y up to 8 at.%)[END_REF][START_REF] Svoboda | Se-doped GeTe4 glasses for far-infrared optical fibers[END_REF][START_REF] Sun | Structural investigation of Te-based chalcogenide glasses using Raman spectroscopy[END_REF][START_REF] Moharram | Short and intermediate range order of Ge20Se80-xTex glasses[END_REF][START_REF] Petracovschi | Structural study by Raman spectroscopy and 77Se NMR of GeSe4 and 80GeSe2-20Ga2Se3 glasses synthesized by mechanical milling[END_REF][START_REF] Andrikopoulos | Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials[END_REF] However, different assignments of vibration bands have been reported showing the difficulties of interpretations. To interpret these results, the methodology used combine experimental acquisitions (Raman and IR) and DFT calculations on selected clusters to help in the assignment of the vibrational modes observed. Initially, we will make sure that DFT calculations can describe correctly binary glasses Ge 20 Se 80 and Ge 20 Te 80 . The spectral refinement on this study will focus both (i) on the local stretching modes of tetrahedral units and (ii) on the vibration due to their interconnection.

2.2.1.

Binary glasses Ge [START_REF] Du Bourg | Impact of Te on the structure and 77Se NMR spectra of Se-rich Ge-Te-Se glasses: a combined experimental and computational investigation[END_REF] According to the literature, these modes are assigned respectively to the symmetrical stretching of the GeSe 4 tetrahedra, the symmetrical stretching of the GeSe 4 edge-sharing tetrahedra, and finally, the stretching of the Se-Se homopolar bond into chains of variable length. [START_REF] Gjersing | Structure, Connectivity, and Configurational Entropy of GexSe100-x Glasses: Results from 77Se MAS NMR Spectroscopy[END_REF][START_REF] Petracovschi | Structural study by Raman spectroscopy and 77Se NMR of GeSe4 and 80GeSe2-20Ga2Se3 glasses synthesized by mechanical milling[END_REF] In the infrared spectrum, two broad peaks are apparent at 255 and 320 cm -1 ,mainly due to the observation of asymmetrical stretching modes of the GeSe 4 tetrahedra. presented in the Table 1. The "Potential Energy Distribution" (PED) give the relative weight of each vibration describing the stretching mode of the clusters (Table 1).

J o u r n a l P r e -p r o o f According to DFT calculation, the active mode of stretching tetrahedra calculated at 187 cm -1 and 215 cm -1 from cluster (a) and (e), respectively, can be attributed to those observed in Raman spectra of the Ge 20 Se 80 glass. Thus, the main band at 195 cm -1 is associated to symmetric stretching modes of [GeSe 4 ] tetrahedra while the shoulder observed on the Raman spectrum at 210 cm -1 can be associated with the symmetrical vibration modes of tetrahedra linked by an edge (Figure 4).

Concerning infrared results, the apparent maxima at 260 and 310 cm -1 in the infrared spectra can be related to asymmetrical stretching vibration modes of the [GeSe 4 ] tetrahedra of cluster (a)

and cluster (e) (Figure 4). More to the point, the calculated asymmetrical stretching modes at 258 cm -1 and 299 cm -1 of classical [GeSe 4 ] tetrahedral (cluster a) mainly contribute to the main band at 260 cm -1 and broad band at 310 cm -1 (Figure 4). The calculated modes at 285 and 311 cm -1 represent the two asymmetrical stretching modes, both present on infrared spectra (Figure 4), of the cluster (e), i.e. corresponding to the [GeSe 4 ] tetrahedra connected by an edge.

J o u r n a l P r e -p r o o f

Considering the clusters (b, c and d) with chains of two, three and four selenium interconnecting the [GeSe 4 ] tetrahedra, the selected vibrational mode of Se-Se bonds will range from 245 to 270 cm -1 , depending on the length of chain. PED analysis shows that for short chains containing only two selenium the calculated frequency is 270 cm -1 while for a longer chain, represented by cluster (d), the spectroscopic signature of ν s (Se)-Se-Se-(Se) was extracted at 245 cm -1 . This is even more noticeable with the mode ν s (Se)-Se-Se-(Ge), where their frequencies are higher (263, 265 cm -1 ) than the ν s (Se)-Se-Se-(Se) mode (245 cm -1 ). To sum-up, the longer the -(Se-Se) n -chain, the lower the frequency is, whereas a Se-Se bond linked to one or two germaniums will see its calculated frequency of vibration gradually increase.

The Ge 20 Se 80 Raman spectrum has a broad band centered at 260 cm -1 usually associated with the vibrational modes of Se-Se small chains (Figure 4). The width of this band ranging from 240 to 280 cm -1 can be perfectly explained by bearing in mind the spread of DFT calculated frequency of the Se-Se vibration modes strongly impacted by chain length and atomic mass effect.

J o u r n a l P r e -p r o o f The Raman spectrum of Ge 20 Te 80 glass is shown in Figure 6 where two main modes have been observed at 126 and 155 cm -1 in accordance with the literature [START_REF] Andrikopoulos | Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials[END_REF][START_REF] Sen | Physical properties of GexAs2xTe100 -3x glasses and Raman spectroscopic analysis of their short-range structure[END_REF]. Due to its high reflectivity behavior, the IR spectrum of the Ge 20 Te 80 glass could not be recorded. The composition of Ge 20 Te 78 Se 2 was then chosen to assign the IR modes as the composition is very close to Ge 20 Te 80 . From this IR spectrum, a main wide band centered at 220 cm -1 was identified.

Using the same methodology than for DFT calculations of the Ge 20 Se 80 glass, a stoichiometric cluster [GeTe 4 ] was calculated (Figure 5a.) as well as GeTe 4 tetrahedra interconnected by chains of two, three, and four tellurium (Figure 5b. c. d. respectively). The increase of the tellurium chain will show the effect on the frequency calculation for the homopolar Te-Te bond vibration.

The results of the DFT calculation presented in Table 2 reveal a main active Raman mode at 131 cm -1 and two main active IR modes at 216 and 232 cm -1 related to the symmetrical and J o u r n a l P r e -p r o o f antisymmetrical stretching of the GeTe 4 tetrahedra (cluster (a)), respectively. Thus, the ν s GeTe 4 tetrahedra mode can be associated to the Raman band at 125 cm -1 (Figure 6). while the other two ν as GeTe 4 tetrahedra modes can be observed in the infrared wide band peaking at 220 cm -1 and presenting a shoulder around 240 cm -1 . From DFT calculation, the selected modes of the Te-Te bond vibration range from 145 to 164 cm -1 (Table 2). The lowest frequency at 145 cm -1 corresponds to the Te-Te vibration surrounded by tellurium on both sides, whereas that at 164 cm -1 is attributed to the vibration of the Te-Te bond but this time surrounded by germanium on both side. The gradual shift to lower frequencies can be mainly explained by a predominant atom weight effect between Te and Ge. The 155 cm -1

J o u r n a l P r e -p r o o f band observed in Raman spectra can be logically attributed to the stretching vibrational mode of Te-Te bonds.

From the results on binary glasses, the used of small clusters calculated by DFT calculation allow a good description of the structure. The same methodology is used for ternary glasses, with first, the focus on the rearrangement of the chain and second, on the reorganization of the tetrahedra, with the addition of Te or Se in the glassy matrix. According to DFT calculations, the Raman and Infrared spectra of the clusters were calculated.

Ternary system Ge

To be clearer, only the vibrational modes of the mixed chains were extracted (Table 3).

J o u r n a l P r e -p r o o f The clusters [Ge-Te-Te-Se-Ge] and [Ge-Se-Se-Te-Ge] were studied to identify modes when Se or Te is inserted in a homopolar chain. For cluster (a), the presence of the modes νTe-Te at 157 cm -1 and νSe-Te at 219 cm -1 were indexed. When selenium is bonded to germanium (cluster b), the modes νSe-Te at 210 cm -1 and the homopolar vibration νSe-Se at 266 cm -1 were calculated. If the element added in the matrix binds to germanium, the homopolar mode is recorded.

The Se-Te-Se and Te-Se-Te modes are characteristic of the substitution of an atom by another, in the center of Se-Se or Te-Te homopolar chain. According to PED analysis, only the presence of the mixed mode Se-Te is observed between 208 and 222 cm -1 .

For the Se-rich part, the presence and rise of this band confirm the mode Se-Te. Moreover, by the decrease of the Se-Se band at 260 cm -1 , only the insertion of Te inside the Se-rich chain J o u r n a l P r e -p r o o f 21 (cluster c.) seems to be the arrangement present in the glassy matrix. Another statement can be made for tellurium-rich part, if selenium gets into the homopolar chain, the modes between 208 and 211 cm -1 would be shown on the Raman spectra. However, there is no mode at this frequency, the mixed νSe-Te mode should not be present in the Te-rich part of the system in agreement with Ratkai et al. [START_REF] Rátkai | Microscopic origin of demixing in Ge20SexTe80-x alloys[END_REF].

• Mixed tetrahedra study

In this part, we have defined mixed Se/Te clusters based on the GeSe 4 and GeTe 4 Td clusters depicted in Figure 3 and 5. As detailed in Figure 8, these mixed clusters are named: GeTe 8-x Se x with x varying from 0 to 8. For each x value, several arrangements of the chalcogenide atoms can be predicted. All possibilities are listed in Figure 8, the index number of the cluster together with its x value allow to find the details of the calculation results in a sum-up table provided in the supporting information section. In regard to these observations, one can conclude that the vibrational frequency of the tetrahedral internal modes is mostly sensitive to the chalcogen atoms directly bonded to the germanium. This observation was expected; however, it denotes that if the chalcogen introduction in the glassy matrix is not preferentially links to Ge, these modes are almost not sensitive to this change of composition.

DISCUSSION

A careful analysis of the data presented in this study allows to give some conclusions concerning the respective affinity of Ge, Se and Te along the pseudo-binary line GeSe 4 -GeTe 4 .

For selenium-rich glasses, the introduction of tellurium induces the appearance of a Raman mode at 210 cm -1 , attributed to Se-Te stretching, and a decrease of the large band at 260 cm -1 assigned to the Se-Se bonds. This is in accordance with the 77 Se NMR study reported by J o u r n a l P r e -p r o o f

Bouëssel du Bourg et al. [START_REF] Du Bourg | Impact of Te on the structure and 77Se NMR spectra of Se-rich Ge-Te-Se glasses: a combined experimental and computational investigation[END_REF] showing that the chemical shift at 860 ppm corresponding to the Se-Se-Se chains collapses upon tellurium addition in a GeSe J o u r n a l P r e -p r o o f

For the tellurium-rich part, the Se-Te mode around 210 cm -1 is not observed in Raman. We have noted a gradual decrease of the ν s GeTe 4 around 130 cm -1 and an enlargement of the band pointing at 155 cm -1 which spread from 135 up to 175 cm -1 . In this spectral range, the DFT calculations have shown that we can expect Te-Te stretching but also the symmetric stretching of the Se/Te mixed Ge tetrahedra with 1, 2 or 3 selenium directly attached to germanium which were calculated around 145, 160 and 170 cm -1 respectively. This presence of mixed tetrahedral units is also confirmed by the continuous change of the IR spectra which shift gradually towards higher wavenumbers upon insertion of Se in the Te-rich glasses. This trend of the IR response is also expected from the DFT calculations of the asymmetric stretching modes of mixed tetrahedral units. These findings agreed perfectly with the X-ray, neutron diffraction and extended X-ray fine structure (EXAFS) measurements reported by Ratkai et al. [START_REF] Rátkai | Microscopic origin of demixing in Ge20SexTe80-x alloys[END_REF].

Finally, NMR and vibrational data reported in this study can confirm and clarify a whole structural description of the GeSe 4 / GeTe 4 glassy system. As observed form both sides of the demixing zone, all studies point out that the highest affinity of Se with Ge governs the glassy structure. It denotes that a homogeneous glassy network, i.e. with an amount of Se/Te mixed germanate tetrahedral units following the glass composition, might be obtained only for the Terich part of the glassy system. For the selenium-rich part, mixed Se-(Te) x -Se chains will be preferred.

CONCLUSION

Vibrational spectra and NMR study allow to make a structural assessment of the Ge 20 Te 80-x Se x glass series. For selenium-rich glasses, when tellurium is added, it would tend to be inserted into the Se-Se chains, to form mixed chains, of the Se-Te type. However, above a certain percentage J o u r n a l P r e -p r o o f

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Figure 1 .

 1 Figure 1. 73 Ge QCPMG NMR spectra of Ge 20 Te 80-x Se x glasses (with x = 2, 70, 80).

Figure 2 .

 2 Figure 2. (a) Unpolarized Raman spectra and(b) Infrared spectra of Ge 20 Te 80-x Se x glasses.

  Se 80 and Ge 20 Te 80 J o u r n a l P r e -p r o o f • Ge 20 Se 80 Ge 20 Se 80 glass presents three principal peaks in Raman spectra: 195, 210 and 260 cm -1 .

Figure 3

 3 Figure3shows the representative clusters of the GeSe 4 glass structure used for DFT

Figure 3 .

 3 Figure 3. DFT-optimized geometries of (a) [GeSe 4 ], (b) [Ge-Se-Se-Ge], (c) [Ge-Se-Se-Se-Ge],

Figure 4 .

 4 Figure 4. Assignments of vibration modes for IR and Raman spectra of Ge 20 Se 80 glass.

Figure 5 .Table 2 .Fig 2 )Figure 6 .

 5226 Figure 5. DF-optimized geometries of (a) [GeTe 4 ], (b) [Ge-Te-Te-Ge], (c) [Ge-Te-Te-Te-Ge]

  20 Te 80-x Se x (with x from 0 to 80) • Mixed chains study To provide a consistent study, different clusters were studied in this part with mixed chalcogen sequences linking two germanium of the type, Se-Te-Se, Te-Se-Te, Te-Te-Se, Se-Se-Te, for the ternary glasses. The selected clusters are represented in Figure 7 and are named respectively [Ge-Se-Te-Se-Ge], [Ge-Te-Se-Te-Ge], [Ge-Te-Te-Se-Ge] and [Ge-Se-Se-Te-Ge].

Figure 7 .Table 3 .

 73 Figure 7. DF-optimized geometries of (a) [Ge-Se-Se-Te-Ge], (b) [Ge-Se-Se-Te-Ge], (c) [Ge-

Figure 8 .

 8 Figure 8. Schematic presentation of the DFT clusters calculated for the mixed Se,Te,

Figure 9 .

 9 Figure 9. Calculated vibration wavenumbers of the internal modes of all the mixed Se/Te

Finally, vibrational

  spectroscopy,77 Se and73 Ge NMR analysis are consistent and conclude that for the selenium-rich part of the pseudo-binary GeSe 4 -GeTe 4 , the Te is not connected directly to germanium and inserted in the vicinity of selenium chains.Above 10% of Te in the glassy matrix, Se/Te mixed Ge tetrahedracan be expected has shown by the Raman mode at 170 cm -1 corresponding well to a mixed-unit with only one Te atoms directly connected to Ge. Closer to the de-mixing zone, for the glass Ge 20 Te 30 Se 50 the IR response remains almost unchanged which could indicate that mixed germanate units with more than two Te in the vicinity of the germanium are not expected in agreement with Bouëssel du Bourg et al. In this case, the two strong Raman modes at 155 and 170 cm -1 , which dominates the Raman response for this particular composition, could be attributed to (Ge)-Se-Te-Te-(Ge) or (Ge)-Se-Te-Te-Se-(Ge) chains which modes are expected at these two wavenumbers.

Table 1 .

 1 Selected

	Calculated Frequency (cm -1 )	Vibrational Mode PED (%)	Cluster
			ν(1,2): 16,1%, ν(1,5): 16,1%,
	187	ν s GeSe 4		Cluster a
			ν(1,8): 16,1%, ν(1,11): 16,1%
			ν(2,1): 4,9%, ν(2,3): 6,9%, ν(2,7):
	215	ν s GeSe 4 (Edge-Sharing)	6,7%,ν(2,5): 6,6%, ν(4,1): 8,6%, ν(4,3): 5,0%, ν(4,9):	Cluster e
			5,8%, ν(4,11): 5,0%
	245	ν(Se)-Se-Se-(Se)	ν(11,23): 21,0%, ν(12,11): 11,1%, ν(23, 24) : 11,8%	Cluster d
	252	ν

Raman and infrared-active vibrational modes calculated by DFT for [GeSe 4 ] cluster, long-chain clusters and edge-sharing tetrahedral clusters shown in Fig. 3 and the relative weight of each individual vibration extracted from a Potential Energy Distribution analysis. as Se-Se-Se ν(12,11): 10,9%, ν(11,23): 12,4% Cluster c J o u r n a l P r e -p r o o f

  [START_REF] Sharma | Far-infrared transmission and bonding arrangement in Ge10Se90-xTex semiconducting glassy alloys[END_REF] glassy matrix. Both studies denote that Se-Te-Se arrangements are formed preferentially to Se-Se-Te linkages. This is confirmed by NMR and vibrational characterizations focused on the germanate response. The 73 Ge NMR results has shown that adding 10% of tellurium in GeSe 4 does not modify the shape of the 73 Ge spectrum. Moreover, the Se/Te mixed Ge tetrahedral study by DFT calculations points out that if the chalcogen substitution is done on the second shell around the germanium, it does not affect significantly the internal mode of the tetrahedral units. It explains why the infrared spectra do not show significant spectral differences up to 20% of Se to Te substitution. Similarly, in Raman the symmetric stretching mode of GeS 4/2 tetrahedral units peaking at 190 cm -1 does not shift in frequency.

Tél. : 33(0)5 4000 6361 -e-mail : marc.dussauze@u-bordeaux.frwww.ism.u-bordeaux.fr 3 rd December 2020 University of Bordeaux FRANCE

ACKNOWLEDGMENT

The authors gratefully acknowledge the financial support of: l'Agence Nationale de la Recherche for the project IRTeGlass (ANR-14-CE07-0013-03), IDEX Bordeaux (Cluster of Excellence LAPHIA and the allocated grant referred to as ANR-10-IDEX-03-03), and the CNRS project EMERGENCE @INC2019. All IR and Raman experiments have been performed at the platform SIV at University of Bordeaux, funded by the FEDER and the Region Aquitaine. ABBREVIATIONS NMR, Nuclear magnetic resonance; DFT, Density Functional Theory; EXAFS, Extended X-Ray Absorption Fine Structure; IR, Infrared; XRD, X-ray Diffraction; DSC, Differential scanning calorimetry; QCPMG, Quadrupolar Carr-Pucell-Meiboom-Gill echo sequence; VOCS, Variable Offset Cumulative Spectrum; PED, Potential Energy Distribution.

Data Availability Statement

J o u r n a l P r e -p r o o f

of tellurium, it would eventually bind to germanium to form mixed tetrahedra, like those of [GeTeSe] clusters. In the case of tellurium-rich glasses, it would appear that when selenium is added, given the strong affinity of selenium with germanium, there would be the formation of Se/Te mixed Ge tetrahedra connected to each other by Te-Te homopolar chains. Moreover, no vibrational signature of the mixed-chain modes (210 cm -1 ) is observed, confirming the only presence of the vibration mode νTe-Te.

ASSOCIATED CONTENT

Supporting Information.

-A table containing all calculated values for each mixed GeTe 8-x Se x clusters depicted in Figure 8 and plotted in Figure 9 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: marc.dussauze@u-bordeaux.fr, virginie.nazabal@univ-rennes1.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of interests

To, The Editor-in-chief, Journal of Solid State Chemistry

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.