Matthieu Scarpi-Luttenauer

Pierre Mobian
email: mobian@unistra.fr

Laurent Barloy
email: barloy@unistra.fr

Synthesis, structure and functions of discrete titanium-based multinuclear architectures

Keywords:

An overview of organometallic or coordination multinuclear titanium complexes published so far is given herein. The majority of these compounds are supramolecular homonuclear complexes, which are listed according to their nuclearity and are usually synthesized through one-pot procedures; the others are mostly early-late heteronuclear complexes. We have attempted to outline a systematic classification of the architecture of the polyhedra that they form, and to point out their properties in various fields of chemistry.

Ti2M3 pentanuclear complexes

Miscellaneous

Abbreviations: bpym, 2,2'-bipyrimidine; bAnt, 4,4′-bis((E)-2-(anthracen-9-yl)vinyl)-2,2′-bipyrimidine; btmsa, bis(trimethylsilyl)acetylene; Cp', substituted or unsubstituted h 5 -cyclopentadienyl anionic ligand; dpeda, diphenylethene-1,2-diamine; DOSY, diffusion-ordered NMR spectroscopy; EHLB, early-late heterobimetallic; HSAB, hard and soft acids and bases; Kdim, dimerization constant; L, bridging ligand; opda, orthophenylenediamine; py, pyridine.

Introduction

At the end of 19 th century, Alfred Werner established the basement of coordination chemistry by rationalizing the chemistry of cobalt(III)-ammonia compounds. Nowadays, coordination compounds have reached a high degree of complexity and chemists are able to generate sophisticated architectures incorporating many metallic centers. The construction of such multinuclear coordination compounds is a very active field, since it permits to create metallo-architectures that display unique properties that are impossible to access with monomeric transition metal complexes. In catalysis, multinuclear metal complexes are particularly attractive, owing to their synergetic effects arising from the presence of multiple active sites, leading to molecular systems with excellent catalytic activities. [START_REF] Nath | Macrocyclic multinuclear metal complexes acting as catalysts for organic synthesis[END_REF] The presence of several metals within a coordination compound provides original photophysical properties related to a monomeric complex. [START_REF] Rohacova | Photofunctional multinuclear rhenium(i) diimine carbonyl complexes[END_REF] In biology, great efforts are devoted to generate anticancer drugs or probes with polynuclear transition metal-based molecules. [START_REF] Pete | A review on homo multinuclear anticancer Metallotherapeutics[END_REF][START_REF] Li | Dinuclear metal complexes: Multifunctional properties and applications[END_REF][START_REF] Eskandari | A Triangular Platinum(II) Multinuclear Complex with Cytotoxicity Towards Breast Cancer Stem Cells[END_REF] The activation of small molecules, i.e. N2 or CO2, is intensively investigated with binuclear complexes. [START_REF] Takeda | Development of efficient photocatalytic systems for CO2 reduction using mononuclear and multinuclear metal complexes based on mechanistic studies[END_REF][START_REF] Singh | Activation of Dinitrogen by Polynuclear Metal Complexes[END_REF] A plethora of molecular materials based on multinuclear species with various properties including magnetic, [START_REF] Chorazy | Octacyanidometallates for multifunctional molecule-based materials[END_REF] liquid crystal, [START_REF] Serrette | Complementary Shapes in Columnar Liquid Crystals: Structural Control in Homo-and Heteronuclear Bimetallic Assemblies[END_REF] spin crossover, [START_REF] Shen | Redox Modulation of Spin Crossover within a Cobalt Metallogrid[END_REF][START_REF] Wang | Spin transitions in Fe(II) metallogrids modulated by substituents, counteranions, and solvents[END_REF][START_REF] Huang | A mixed-valence metallogrid [CoIII2CoII2] with an unusual electronic structure and single-ion-magnet characterization[END_REF][START_REF] Unruh | Spin state switching of metal complexes by visible light or hard X-rays[END_REF] electrochemical [START_REF] Ube | Synthesis of Hetero-multinuclear Metal Complexes by Site-Selective Redox Switching and Transmetalation on a Homo-multinuclear Complex[END_REF] or nonlinear optical [START_REF] Coe | Nonlinear Optical Properties of Metal Complexes[END_REF] have been reported. Nanosized metal-organic polyhedra are used as molecular building blocks to generate metal organic frameworks (MOFs). Concerning chirality, these multimetallic assemblies are fascinating objects to investigate, in particular, if the architecture is built with chiral metallocorners. [START_REF] Castilla | Stereochemistry in subcomponent self-assembly[END_REF] Several molecular machines and stimuli-responsive devices rely on assemblies containing several metals. [START_REF] Mcconnell | Stimuli-Responsive Metal-Ligand Assemblies[END_REF] Additionally, multinuclear self-assembled architectures are essential scaffolds to synthetize topological non-trivial molecules. [START_REF] Ayme | Template synthesis of molecular knots[END_REF] Clearly, this brief non-exhaustive overview illustrates the crucial role of multinuclear coordination compounds played in many domains of contemporary science.

Three synthetic pathways are followed to access multinuclear coordination compounds.

The step-by-step approach consists in the sequential introduction of metallic ions to expand the framework of an initial building block. This approach that has been extensively used to afford, for instance, metallodendrimers [START_REF] Newkome | Suprasupermolecules with Novel Properties: Metallodendrimers[END_REF] or light harvesting complexes, [START_REF] Frischmann | Powering the future of molecular artificial photosynthesis with light-harvesting metallosupramolecular dye assemblies[END_REF] requires necessarily to perform a "chemistry on the complex". [START_REF] Mede | chemistry-on-the-complex": Functional RuII polypyridyl-type sensitizers as divergent building blocks[END_REF] Therefore, it is not suitable for the preparation of all multinuclear transition metal species, as the chemical transformations employed have to be compatible with the stability of each coordination sub-unit that forms the architecture. A more straightforward pathway to prepare multinuclear coordination compounds is a direct synthesis from a ligand bearing multiple coordination sites and one or several metallic sources. In this case, the size and the number of metal ions that compose their frameworks are limited to the number of ligating groups inscribed in the initial ligands, and the preparation of these ligands usually demands important synthetic efforts. The most powerful method to produce aggregates with a huge number of metal ions is undeniably the self-assembly approach. [START_REF] Lehn | Supramolecular Chemistry -Concepts and Perspectives[END_REF][START_REF] Leininger | Self-Assembly of Discrete Cyclic Nanostructures Mediated by Transition Metals[END_REF] Self-assembled metal-ligand architectures result from the spontaneous mutual programmed recognition between metal ions and rather simple organic ligands. Especially, as demonstrated by Fujita et al., this methodology allows the formation of spectacular giant structures that are impossible to obtain using other approaches. [START_REF] Fujita | Self-Assembly of M30L60 Icosidodecahedron[END_REF] Up to date, the majority of these architectures are based on medium or late transition metals.

Titanium is the second most abundant transition metal, after iron in the Earth's crust.

This first row transition metal has a [Ar] 3d 2 4s 2 electronic configuration and its common oxidation states are +II, +III and +IV. This cheap and relatively non-toxic element is abundantly employed to generate complexes that occupy a major role for various applications.

For this element, the most frequent oxidation state is +4 that corresponds to the stable one in ambient conditions. Consequently, the most abundant titanium coordination complexes are Ti(IV)-species. Organometallic and coordination titanium(IV) complexes are efficient reagents for catalytic enantioselective processes, organic transformations and as initiators for polymerization reactions. [START_REF] Ramón | In the arena of enantioselective synthesis, titanium complexes wear the Laurel wreath[END_REF][START_REF] Manßen | Titanium catalysis for the synthesis of fine chemicals-development and trends[END_REF][START_REF] Azor | Stereoselective synthesis of biphenolate/binaphtolate titanate and zirconate alkoxide species: Structural characterization and use in the controlled ROP of lactide[END_REF] The bioinorganic chemistry of Ti(IV) is rich and diverse, since titanium is found in many biomaterials and numerous Ti(IV)-based complexes are used in a medical or biological context. The readers who are interested in this widespread area are invited to read the excellent review of Valentin et al. [START_REF] Buettner | Bioinorganic chemistry of titanium[END_REF] Importantly, it is noteworthy that over the last decade a resurgence of the research aiming at developing titanium(IV) complexes as anti-cancer drugs occurred. [START_REF] Cini | Using titanium complexes to defeat cancer: The view from the shoulders of titans[END_REF][START_REF] Tshuva | Coordination Complexes of Titanium(IV) for Anticancer Therapy[END_REF] In material science, titanium alkoxides or ligand-modified titanium(IV) alkoxides [START_REF] Schubert | Chemical modification of titanium alkoxides for sol-gel processing[END_REF] are indispensable precursors to form titanium dioxide (TiO2) nanoparticles or useful alloys through sol-gel processes. Indeed, this is an essential domain because TiO2 is one of the most popular semiconductor, mainly due to its well established photocatalytic properties. [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF] It should be mentioned that related to Ti(IV) complexes, Ti(III) species are somewhat less common. These complexes are produced via the direct reaction of a ligand with a Ti(III) source or by electrochemical, [START_REF] Kuhn | Orbital control over the metal: Vs. ligand reduction in a series of neutral and cationic bis(cyclopentadienyl) Ti(iv) complexes[END_REF] chemical [START_REF] Matsuo | Synthesis and structures of Ti(III) and Ti(IV) complexes supported by a tridentate aryloxide ligand[END_REF] and photo reduction starting from Ti(IV) analogues. [START_REF] Krämer | Photo-redox reactivity of titanium-oxo clusters: Mechanistic insight into a two-electron intramolecular process, and structural characterisation of mixed-valent Ti(iii)/Ti(iv) products[END_REF][START_REF] Klahn | Titanocene(iii) complexes with 2phosphinoaryloxide ligands for the catalytic dehydrogenation of dimethylamine borane[END_REF] The +III oxidation state of titanium is often found for organometallic complexes used to develop catalytic transformation. Overall, these Ti(III)-based complexes used on this purpose are dominated by mono-or bimetallic complexes. [START_REF] Ashuiev | Molecular and supported Ti(iii)-alkyls: efficient ethylene polymerization driven by the π-character of metal-carbon bonds and back donation from a singly occupied molecular orbital[END_REF][START_REF] Salvadori | Structure and dynamics of catalytically competent but labile paramagnetic metal-hydrides: The Ti(iii)-H in homogeneous olefin polymerization[END_REF] All these domains drive the design, construction and study of molecular titanium-based multinuclear architectures. Nowadays, the rational synthesis of such architectures remains challenging because of the very specific coordination chemistry of titanium, especially at the most widespread oxidation state +IV. According to the hard and soft acids and bases (HSAB) theory, titanium(IV) is a hard center owing to its high charge and small ionic radius (the Pauling radius of Ti(IV) is 68 pm). Therefore, titanium(IV) accommodates preferably hard ligands, explaining for instance why anionic oxygenated ligands are often associated with titanium(IV) atoms. Also, titanium(IV) has a d 0 electronic configuration, implying that the ligand field does not govern the geometry of a titanium complex. Whereas octahedral or pseudo-octahedral geometries are usually observed for 6-fold coordinated titanium(IV)

atoms, [START_REF] Diebold | Synthesis and characterization of a monomeric octahedral C2-symmetric titanium complex bearing two 3,3′-diphenyl-2,2′-biphenol ligands[END_REF][START_REF] Dubberley | Synthesis and reactivity of calix[4]arene-supported group 4 imido complexes[END_REF] it is not surprising to observe complexes built around 4-fold, [START_REF] Bunge | Synthesis and characterization of a series of rubidium alkoxides and rubidium-titanium double alkoxides[END_REF] 5-fold [START_REF] Diebold | Spontaneous symmetry breaking during self-assembly of a double stranded biphenolate-based Ti(IV)-helicate[END_REF][START_REF] Friedrich | A calix[4]arene monoalkyl ether as a model of a tris(phenolate) ligand with a hemilabile anisole moiety: Syntheses, molecular structures and bonding of calix[4]arene ether supported titanium complexes and their catalytic activity in epoxidation reactions[END_REF] or even 7-fold [START_REF] Pajot | Metal-assisted Activation of the C-0 Bond of 2-Hydroxyethylmethacrylate. Synthesis and Molecular Structure of Ti5(OPri)9(mu-OPri) (mu,eta2-OC2H4O)3(mu3,eta2-OC2H4O)3(mu4,eta2-OC2H4O)[END_REF] coordination Ti-atoms. Fig. 1 illustrates the versatile coordination sphere adopted by titanium atoms within complexes built from an oxygenated coordination sphere. Thus, the precise coordination sphere within a titanium complex is hard to predict when a ligand reacts with a titanium source, but the major difficulty of the Ti(IV) coordination chemistry arises undoubtedly from the pronounced oxophilicity of this element. Titanium(IV) complexes are generally prompt to react with water, forming higher aggregates where the metals are linked through oxo or hydroxo bridges, and under certain conditions the irreversible precipitation of TiO2 occurred. Whereas these hydrolytic processes are the normal way to produce nanosized TiO2 materials, it is a real disadvantage when the conception of molecular titanium coordination compounds is envisioned. Therefore, the synthesis of titanium-based multinuclear architectures requires limited purification steps of the final products. Precipitation and crystallisation methods are largely employed and purifications by chromatography are avoided in general. Post-synthesis transformation of a titanium multinuclear complex is extremely rare because of the low tolerance of titanium complexes towards most of chemical transformations. Hence, these characteristics impose that a majority of multinuclear Ti-architectures are produced through a self-assembly approach.

This review aims to report the principles of construction and the structural description of the architectures composed of several titanium centers. In particular, we inventory different categories of multinuclear Ti(IV) and Ti(III) structures as a function of the nature of the ligands and we detail the properties of these complexes. Importantly, owing to the reasons linked with the synthetic specificities of the Ti complexes mentioned just above, this review primarily focuses on multinuclear self-assembled structures.

Apart from very rare examples, compounds where Ti-O-Ti, Ti-OH-Ti or Ti-OR-Ti linkages are found are not concerned by the scope of this review. Indeed, Ti-aggregates (clusters) containing these bridging ligands represent a central family of compounds because they are intensively investigated as models of bulk TiO2. [START_REF] Coppens | Crystallography and properties of polyoxotitanate nanoclusters[END_REF] Nevertheless, the synthetic aspects allowing the formation of these aggregates are singular since they require oxolation or olation reactions and the addition of a precise amount of water in the course of the synthesis permits to control generally the size of these oxo-clusters. Indeed, for the other types of multinuclear Ti-based architectures, i.e. non-oxo-cluster species, the presence of water is usually avoided. Thus, the synthesis of such aggregates is necessarily carried out under inert atmosphere using classical standard Schlenk or glovebag techniques. [START_REF] Schubert | Chemical modification of titanium alkoxides for sol-gel processing[END_REF][START_REF] Rozes | Titanium oxo-clusters: Precursors for a Lego-like construction of nanostructured hybrid materials[END_REF][START_REF] Matthews | Structure, photochemistry and applications of metal-doped polyoxotitanium alkoxide cages[END_REF][START_REF] Li | Novel properties and potential applications of functional ligand-modified polyoxotitanate cages[END_REF][START_REF] Fang | Synthetic strategies, diverse structures and tuneable properties of polyoxo-titanium clusters[END_REF] More generally, we have excluded the multinuclear titanium complexes which are exclusively based on monoatomic bridges (halido, hydrido, imido or amido,...), or which exhibit a metal-metal bond. Also, the elaboration of polymeric titanium coordination networks or Ti-MOFs is not discussed in this review, since these materials are not discrete assemblies by definition and are already overviewed in several recent publications. [START_REF] Assi | Titanium coordination compounds: From discrete metal complexes to metal-organic frameworks[END_REF][START_REF] Chen | Recent advances in titanium metal-organic frameworks and their derived materials: Features, fabrication, and photocatalytic applications[END_REF][START_REF] Wang | Ti-based MOFs: New insights on the impact of ligand composition and hole scavengers on stability, charge separation and photocatalytic hydrogen evolution[END_REF][START_REF] Nguyen | The chemistry of titanium-based metal-organic frameworks[END_REF] The present review is structured in a rather straightforward manner. The major part of it is devoted to homopolynuclear Ti complexes, whereas the topic of the less abundant heteropolynuclear complexes is addressed at the end of the review. The criteria considered within this major part are the nuclearity of the complexes (up to 8) and the nature of the donor atoms of the bridging ligands.

Homonuclear complexes

Biscatecholato ligands or analogues

The term helicate was first introduced by Lehn et al., [START_REF] Lehn | Spontaneous assembly of double-stranded helicates from oligobipyridine ligands and copper(I) cations: structure of an inorganic double helix[END_REF], and in early comprehensive reviews, Piguet et al. have defined such compounds as discrete helical supramolecular complexes implying covalent organic strands and coordinated ions. [START_REF] Piguet | Helicates as versatile supramolecular complexes[END_REF][START_REF] Piguet | Helicates and Related Metallosupramolecular Assemblies: Toward Structurally Controlled and Functional Devices[END_REF] These architectures are great models to apprehend the rules that govern the self-assembly processes happening in supramolecular chemistry. A plethora of helicates has been reported so far and many reviews have been dedicated to this field. In particular, these architectures have been classified according to several factors. [START_REF] Piguet | Helicates as versatile supramolecular complexes[END_REF] One of them is the type of ligands that are incorporated in these helical structures. Homotopic helicates are formed with identical strands, whereas different strands compose heterotopic systems. The helicates are also discriminated according to the nature of the metal ions employed to gather the organic strands. Homonuclear assemblies are species formed with one type of metal ion and heteronuclear helicates result from the presence of different metal ions within the complex.

In this broad research area, helicates, where titanium(IV) ions are associated with catecholato-based strands, occupy a central place and the development of this family of compounds represents a success story. The triscatecholato-titanium(IV) motif has been extensively employed to construct helicates, as it generates robust octahedral nodes when incorporated in metallosupramolecular aggregates. Also, the triscatecholato-titanium(IV) complex is chiral, displaying either a L or D configuration, which makes the stereochemistry of these supramolecular aggregates a central issue. A large variety of triple-stranded binuclear helicates based on biscatecholato ligands containing various spacers, such as alkyl [START_REF] Albrecht | Formation of a "meso-Helicate" by Self-Assembly of Three Bis(catecholate) Ligands and Two Titanium(IV) Ions[END_REF][START_REF] Albrecht | Stabilization of an Unusual Coordination Geometry at Li+ in the Interior of a Cryptand-Type Helicate[END_REF][START_REF] Albrecht | Counter-ion induced self-assembly of a meso-helicate type molecular box[END_REF][START_REF] Albrecht | Alkali-Metal Cation Binding by Self-Assembled Cryptand-Type Supermolecules[END_REF][START_REF] Albrecht | Dynamic behavior of anionic binuclear helicate-and mesohelicate-type coordination compounds[END_REF][START_REF] Albrecht | Template-Directed Self-Recognition of Alkyl-Bridged Bis(catechol) Ligands in the Formation of Helicate-Type Complexes[END_REF],

cycloalkyl [START_REF] Albrecht | An enantiomerically pure dinuclear triple-stranded helicate: X-ray structure, CD spectroscopy and DFT calculations[END_REF], ether [START_REF] Albrecht | Linear Oxygen-Donor Ligands Derived from Gallic Acid Methyl Ester: Ligand Synthesis and Preliminary Coordination Studies[END_REF], aryl [START_REF] Albrecht | Self-assembly of a triple-stranded helicate from a rigid di(catechol) ligand and formation of its dimer in the solid state[END_REF][START_REF] Albrecht | Dinuclear Triple-Stranded Helicates from Rigid Oligo-p-phenylene Ligands: Self-Assembly and Ligand Self-Recognition[END_REF], azine [START_REF] Albrecht | 3,3′-{(1E,2E)-hydrazine-1,2-diylidenedi[(E)methylylidene]} dibenzene-1,2-diol (BCAz-H4): An easy to prepare but very useful building block for the self-assembly of triple-stranded helicates; the X-ray crystal structure of Na4[(BCAz)3Ti2]•7 dmf•H2O[END_REF] or diimine [START_REF] Albrecht | Long-range stereocontrol in the self-assembly of twonanometer-dimensioned triple-stranded dinuclear helicates[END_REF][START_REF] Albrecht | Dicatechol-diimines: Easily accessible ligands for the self-assembly of dinuclear triple-stranded helicates[END_REF][START_REF] Janser | Formation of triple-stranded dinuclear helicates with dicatecholimine ligands: The influence of steric hindrance at the spacer[END_REF] spacers, has been reported.

Hereafter, these complexes are formulated [Ti2L3] 4-, where L is the generic naming of the bridging ligands. This class of compounds has been extensively reviewed, in particular by Albrecht and co-workers. [START_REF] Albrecht | Dicatechol ligands: Novel building-blocks for metallo-supramolecular chemistry[END_REF][START_REF] Albrecht | From molecular diversity to template-directed self-assembly -New trends in metallosupramolecular chemistry[END_REF][START_REF] Albrecht | Let's twist again" -Double-stranded, triple-stranded, and circular helicates[END_REF][START_REF] Albrecht | 5,5′-diamino-2,2′-bipyridine: A versatile building block for the synthesis of bipyridine/catechol ligands that form homo-and heteronuclear helicates[END_REF][START_REF] Albrecht | Supramolecular chemistry -General principles and selected examples from anion recognition and metallosupramolecular chemistry[END_REF][START_REF] Albrecht | Symmetry driven self-assembly of metallo-supramolecular architectures[END_REF][START_REF] Albrecht | Catecholate-Based Helicates[END_REF] Therefore in this section, only selected examples will be discussed, emphasizing mainly on the stereochemistry aspects of these objects. Some representative molecules based on biscatecholato ligands are depicted in Fig. 2. The majority of the complexes constructed with this biscatechol ligand system display a helical shape with the Ti(IV) centers adopting the same configuration, i.e. D,D or L,L.

However, meso compounds are also observed (L,D). In this case, these non-helical compounds are termed meso-helicates or mesocates. Several investigations have proved the key role of the spacer to control the stereochemistry of these binuclear species. For spacers containing alkyl moieties, the formation of one of the two isomers could be predicted. When the alkyl chain contained an even number of carbon atoms, the helicate was obtained, whereas the mesocate was produced for a spacer having an odd number of carbons. Thus, 1a adopted a heterochiral configuration whereas 1b was formed as the chiral homoconfigurated complex.

The same observation was made when the alkyl moiety of the spacer was separated from the catechol units by phenyl groups as for 2a,b (Fig. 2). [START_REF] Albrecht | Long-range stereocontrol in the self-assembly of twonanometer-dimensioned triple-stranded dinuclear helicates[END_REF] In addition to the fascinating questions of stereochemistry tackled with these Ti(IV) helicates, a templating effect of the alkali countercations was demonstrated. A binuclear compound was obtained only when the countercations were able to bind the helicate motif, thus stabilizing the binuclear species, otherwise only undefined products were obtained. These supramolecular architectures revealed a cryptand-like ability to bind alkali cations through the oxygen atoms of the catecholato units. [START_REF] Albrecht | Stabilization of an Unusual Coordination Geometry at Li+ in the Interior of a Cryptand-Type Helicate[END_REF][START_REF] Albrecht | Self-Assembly of Dinuclear CH,-bridged Titanium(Iv)/Catecholate Complexes: Influence of the Counterions and of Methyl Substituents in the Ligand[END_REF] This property was essential to govern self-recognition of the ligand strands [START_REF] Albrecht | Template-Directed Self-Recognition of Alkyl-Bridged Bis(catechol) Ligands in the Formation of Helicate-Type Complexes[END_REF][START_REF] Albrecht | Dinuclear Triple-Stranded Helicates from Rigid Oligo-p-phenylene Ligands: Self-Assembly and Ligand Self-Recognition[END_REF] since the formation of triple-stranded helicates was uncomplete when sterically demanding groups were introduced within the spacer as in 3. Indeed, the choice of alkali cations was a fundamental factor to govern the self-assembly process, as the bulky cation potassium annihilated the formation of the helicate. In this case, the binding of the potassium cations within the cavity of the binuclear complex was prevented. [START_REF] Janser | Formation of triple-stranded dinuclear helicates with dicatecholimine ligands: The influence of steric hindrance at the spacer[END_REF] The influence of the ligand symmetry was also studied for these Ti(IV) catecholato helicates. Albrecht et al. reported the synthesis of the triple-stranded binuclear helicate 4

where the biscatecholato ligands contained a Val-Val dipeptide spacer. Due to the directionality of the ligand strands, a mixture of different species was present as kinetic products at the early stages of the reaction. After two weeks, a thermodynamical rearrangement of the ligand strands yielded only one isomer of 4, with parallel orientation of the ligand strands. [START_REF] Albrecht | The fixation of linear versus loop-type peptidic structures by metal coordination: The coordination chemistry of Val-Val-and Val-Val-Val-bridged dicatechol ligands[END_REF] Besides, Lützen et al. reported the binuclear triple-stranded complex 5 based on a biscatechol ligand incorporating Tröger's base as spacer. As the starting material was a racemic mixture of both homochiral (S,S)-and (R,R)-ligands, no stereoselectivity was observed when these ligands were allowed to react with Ti(IV). [START_REF] Kiehne | Synthesis of bis(catechol) ligands derived from Tröger's base and their dinuclear triple-stranded complexes with titanium(IV) ions[END_REF] The synthesis and the chiroptical analysis by circular dichroism of the triple-stranded helicates 6a,b were described. Noteworthy, the helicate 6b was now constructed around two octahedral TiS6 motifs (Fig. 3). Herein, an enantiopure spacer was introduced, owing to the use of (R,R)-diaminocyclohexane to construct the ligand strand. It was demonstrated for compounds 6a,b that the chiral information contained within the ligand was transferred to the overall helicity of the objects. [START_REF] Albrecht | An enantiomerically pure dinuclear triple-stranded helicate: X-ray structure, CD spectroscopy and DFT calculations[END_REF][START_REF] Kreickmann | Metallosupramolecular chemistry with bis(benzene-o-dithiolato) ligands[END_REF] Further investigations proved that the variation of the spacer nature deeply impacted the structure of the final Ti(IV) products constructed with biscatecholato ligands. For instance, double-stranded instead of triple-stranded binuclear helicates were obtained with catecholato- based ligands where the two oxygenated binding units are bridged by amino acids. [START_REF] Albrecht | Kinetic versus thermodynamic control of the self-assembly of isomeric double-stranded dinuclear titanium(IV) complexes from a phenylalaninebridged dicatechol ligand[END_REF][START_REF] Albrecht | Dinuclear Titanium ( iv ) Complexes from Amino Acid Bridged Dicatechol[END_REF] In addition to these strands, the two metallic centers were coordinated with two alcoholato bridging ligands originated from the solvent used for the reaction (Fig. 4,compound 7). Here, the spacer of the organic strands was short and flexible enough to allow the bridging of the two metallic centers by the two µ-OMe ligands. Another extended class of Ti(IV) helicates concerned architectures built around a TiS6 motif. [START_REF] Seidel | Chances and limits of the coordination chemistry with bis(benzene-1,2dithiolato) ligands, Bioinorg[END_REF][START_REF] Kreickmann | Benzene-o-dithiolate ligands as versatile building blocks in supramolecular chemistry[END_REF][START_REF] Sproules | Tris(dithiolene) Chemistry: A Golden Jubilee[END_REF] In particular, Hahn et al. have deeply investigated the formation of several double-and triple-stranded helicates based on bis(benzenedithiolato) strands. [START_REF] Kreickmann | Metallosupramolecular chemistry with bis(benzene-o-dithiolato) ligands[END_REF][START_REF] Hahn | A dinuclear triple-stranded helicate with a bis(benzene-o-dithiolato) ligand[END_REF][START_REF] Hahn | Self-assembly reactions with a bis(benzene-o-dithiolato) ligand[END_REF][START_REF] Birkmann | Metallosupramolecular complexes derived from bis(benzene-o-dithiol) Ligands[END_REF] Again, the formation of either double-or triple-stranded helicates has been shown to be mainly governed by the bridging unit separating the two benzenedithiolato moieties. [START_REF] Kreickmann | Metallosupramolecular chemistry with bis(benzene-o-dithiolato) ligands[END_REF] Attempts to synthesize a tetrahedral assembly, similar to the one obtained by Raymond et al. [START_REF] Brückner | Symmetry-Driven Rational Design of a Tetrahedral Supramolecular Ti4 L4 Cluster[END_REF] with a bis(benzenedithiol) analogue to the biscatechol ligand used, yielded instead the triple-stranded binuclear helicate such as 8 (see Fig. 5a). The formation of a binuclear species vs. a tetranuclear architecture was explained by a modification of the geometrical parameters linked with the nature of the donor atoms (O vs. S). Further studies on compound 8 highlighted the influence of the countercation on the configuration and geometry of the final product. A mesocate was obtained when the binuclear complex was formed in presence of tetraphenylarsonium or tetraethylammonium. It should be noted that the ethylammonium cation was encapsulated, and this encapsulation led to small structural variations in the meso compound. Contrary to the two first examples, a mixture of mesocate and helicate was observed when tetrabutylammonium was used as a countercation.

Remarkably, only the helical architecture was found in the solid state. [START_REF] Birkmann | Metallosupramolecular complexes derived from bis(benzene-o-dithiol) Ligands[END_REF] Double-and triple-stranded binuclear helicates incorporating catechol/benzenedithiol hybrid ligands were also reported by Hahn et al. [START_REF] Kreickmann | Benzene-o-dithiolate ligands as versatile building blocks in supramolecular chemistry[END_REF] Some examples of architectures based on this particular ligand showed that the formation of either the double-or triple-stranded complex was dependent of the spacer and the reaction conditions. This ligand system was of great interest, because the presence of the two different fragments induced directionality when the coordination compounds were formed. This directionality led to the formation of several isomers.

In the case of compound 9 (Fig. 5a), the two coordinating moieties of the bridging ligand were linked by an unsymmetrical CMe2CH2 spacer. [START_REF] Isfort | Helical complexes containing diamidebridged benzene-o-dithiolato/ catecholato ligands[END_REF] NMR analysis showed only the presence of the isomer with parallel ligand strands in solution. The solid-state structural characterization confirmed this result, which was assigned to the position of the bulky dimethyl substituent of the spacer. The TiS6 octahedron had a larger cavity, thus the insideoriented dimethyl substituents would prefer an orientation towards this octahedron (see Fig. 5b).

The use of a C3-symmetric tris(benzenedithiol) proligand for the synthesis of a "bananashaped" binuclear complex 10 was proposed (Fig. 6). This complex was composed of two ligands, each of them binding a metal with two coordinating units, while the third one was linked to the second metal center. We have displayed on Fig. 7 the alkali-metal-free expanded forms of Ti2 dynamic spring-type molecular switches recently reported by Albrecht et al., and reviewed by them. [START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Albrecht | From Hierarchical Helicates to Functional Supramolecular Devices[END_REF] Those compounds were synthesized from ester-, [START_REF] Chen | A Helicate-Based Three-State Molecular Switch[END_REF][START_REF] Chen | Cation-Controlled Formation and Interconversion of the fac/fac and mer/mer Stereoisomers of a Triple-Stranded Helicate[END_REF][START_REF] Mevissen | Helicates with Ether-Substituted Catechol Esters as Ligands[END_REF] thioester- [START_REF] Kwamen | Catechol Thioesters: Ligands for Hierarchically Formed Lithium-Bridged Titanium(IV) Helicates and Helicate-Based Switches[END_REF] or ketone-functionalized [START_REF] Chen | Cation triggered spring-like helicates based on ketonesubstituted bis-catechol ligands[END_REF] biscatecholato ligands with long flexible linkers. Overall, two switchable different states were described for the binuclear complexes: the compressed one was the anionic [Ti2Li3L3] -complex, where each Li + was coordinated by two carbonyl and two catechol oxygen atoms, as for hierarchical helicates (vide infra, Fig. 70); and the expanded one was typically a lithium-free species such as [Ti2L3] 4-. Another expanded form of complex 11b embedding two potassium cations in its cavity was also mentioned. [START_REF] Albrecht | From Hierarchical Helicates to Functional Supramolecular Devices[END_REF] A cation-translocation based isomerism phenomenon inducing an expansion of compressed helicates was very recently reported. [START_REF] Mevissen | Cationtranslocation based isomerism offers a tool for the expansion of compressed helicates[END_REF] Fig. 7. Helical triple-strand binuclear titanium complexes with biscatechol ligands bearing long spacers (hydrocarbon or oligoether chains). A typical example of M + is Li + encapsulated in a cryptand.

The helical architectures 11c and 14 were built from chiral enantiopure ligands. With regard to 14, the stereocontrol was low because the chiral fragment was too distant from the metals; two steroisomers of the compressed [Ti2Li3L3] -complex were present in solution. [START_REF] Mevissen | Helicates with Ether-Substituted Catechol Esters as Ligands[END_REF] By contrast, three switchable forms of compound 11c were clearly identified, i.e. expanded (L,L)-[Ti2L3] 4-, compressed (L,L)-[Ti2Li3L3] -and expanded (D,D)-[Ti2K2L3] 2-. [START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Albrecht | From Hierarchical Helicates to Functional Supramolecular Devices[END_REF][START_REF] Chen | A Helicate-Based Three-State Molecular Switch[END_REF] Noteworthy, the sodium cation interacted with compounds 13 and 14 to yield an unusual compressed [Ti2Na2L3] 2-topological helicate isomer. [START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Chen | Cation-Controlled Formation and Interconversion of the fac/fac and mer/mer Stereoisomers of a Triple-Stranded Helicate[END_REF][START_REF] Mevissen | Helicates with Ether-Substituted Catechol Esters as Ligands[END_REF]

Oligophenolato ligands

Terphenolato ligands were used to synthesize quantitatively the binuclear Ti(IV) edifices 15 and 15', by allowing them to react with TiCl4. [START_REF] Matsuo | Synthesis and structures of Ti(III) and Ti(IV) complexes supported by a tridentate aryloxide ligand[END_REF][START_REF] Kayal | Ortho-linked polyaryloxide ligands and their titanium complexes[END_REF] In these complexes, each ligand bridged and chelated both metal centers (Fig. 8). Our group has described the synthesis of a neutral titanium(IV)-based double-stranded helicate where the strands contained two 2,2'-biphenolato ligands connected together by a pphenylene spacer (see Fig. 9). [START_REF] Diebold | Synthetic approaches to zigzag-shaped oligophenylene strands laterally decorated with hydroxy functions[END_REF] Fig. 9. Bis-biphenol pro-ligand L 1 H4 used to generate the binuclear Ti complexes 16-20.

This architecture formulated as [Ti2L 1 (L 1 H)(HO i Pr)2(O i Pr)] ( 16) was obtained in toluene through a self-assembly process between the pro-ligand L 1 H4 and Ti(O i Pr)4. [START_REF] Diebold | Spontaneous symmetry breaking during self-assembly of a double stranded biphenolate-based Ti(IV)-helicate[END_REF] Unexpectedly, the resulting helicate was formed by two different complex subunits, leading to a noncentrosymmetric assembly as attested by the molecular structure (Fig. 10). The absence of symmetry within the complex was originated from the fact that the two titanium centers adopt a distorted octahedral coordination geometry and a trigonal bipyramid geometry, respectively. For the octahedral complex subunit, the metal was coordinated, in addition to two 2,2'-biphenolato entities, by two HO i Pr ligands occupying two cis positions.

Concerning the trigonal bipyramidal complex, the metal was surrounded by two bidentate units and a third monodentate monodentate ligand (O i Pr) completed the coordination sphere of the titanium center. It should be stressed that the same approach has been tested with Zr(O i Pr)4(HO i Pr). Instead of a double-stranded helicate as observed with Ti(O i Pr)4, the formation of a triple-stranded architecture formulated as [Zr2L 1 2(L 1 H4)] (17) was highlighted. [START_REF] Miao | Identification of Zr(IV)-based architectures generated from ligands incorporating the 2,2′-biphenolato unit[END_REF] Several binuclear double-stranded helicates constructed around TiO4N2 units and incorporating the bis-biphenolato strand L 1 have been described. These complexes were generated by applying a multicomponent self-assembly approach in presence of the pro-ligand L 1 H4, Ti(O i Pr)4 and monodentate or bidentate nitrogen ligands. Enantiopure ((+) or (-) diphenylethene-1,2-diamine (dpeda) [START_REF] Macker | Evaluation of the stereoselectivity for titanium(IV)-based coordination entities induced by the enantiopure diphenylethene-1,2-diamine ligand[END_REF]) or achiral (o-phenylenediamine (opda) [START_REF] Baradel | Titanium(IV)-based helicates incorporating the: Orthophenylenediamine ligand: A structural and a computational investigation[END_REF])

diamino ligands respectively led to helicates [Ti2L 1 2(dpeda)2] (18) and [Ti2L 1 2(opda)2] ( 19
)
where the two nitrogen ligands were disposed from either side of a p-phenylene spacer (see the structure of these two neutral helicates in Fig. 11). For the helicate formed with enantiopure dpeda, it should be noted that the helicity of the titanium-based double-stranded helicate was influenced by the chiral information that is external to the helical framework of the architecture, as demonstrated by the modest diastereomeric excess (1. If the multicomponent self-assembly reactions were performed with monodentate nitrogen heterocycles, double-stranded helicates were also produced but, in this case, each complex subunit was derived from a trans-TiO4N2 motif. [START_REF] Khalil | From monomeric complexes to doublestranded helicates constructed around: Trans -TiO4N2 motifs with intramolecular inter-ligand hydrogenbonding interactions[END_REF] In Fig. 12 Recently, Yashima and co-workers described a water-mediated reversible transformation observed for a double-stranded anionic binuclear titanium(IV) assembly formed from two fully aromatic oligophenol pro-ligands (L 2 H4) and TiO(acac)2. [START_REF] Ousaka | Water-Mediated Reversible Control of Three-State Double-Stranded Titanium(IV) Helicates[END_REF] This sequential controlled process involved three states that correspond to three Ti(IV) helicates:

the chiral racemic helicate rac-[L 2 2(µ-O)Ti2] 2-(rac-21)
where the two metal ions were bridged by a µ-oxo ligand; the meso [L 2 2(µ-OH)2Ti2] 2-(meso-22) assembly containing two µhydroxo ligands ligand; and a second meso binuclear architecture (meso-21) for which a single µ-oxo bridge was reformed (Fig. 13). The hydration/dehydration processes were fully reversible. The authors envisioned relying on this original dynamic three-state device to design supramolecular multinuclear metal catalytic systems governed by water-mediated transformations.

Fig. 13. Chemical equilibria between various forms of a binuclear Ti double-strand complex. The X-ray structures of 23b (Fig. 15a) and 23c revealed planar metallacycles, together with tetrahedral metals centers. Another series of 8-membered cyclic complexes (24a-f) was reported by Roesky et al.,

where Ti(IV) was coordinated by a halide anion and by a Cp' ligand (Fig. 14). [START_REF] Liu | Synthesis and Structure of Eight-Membered Titanium Containing Siloxane Rings[END_REF][START_REF] Liu | Synthesis and structures of cyclopentadienyl fluoro and chloro complexes of a triad (Ti, Zr, Hf) containing acyclic and cyclic siloxane building blocks[END_REF] Note that throughout this review, Cp' refers to the h 5 -cyclopentadienyl anionic ligand or a derivative bearing one or several substituents.

Among the synthetic pathways leading to these compounds, the main one consisted in the reaction of (Cp')TiX3 either with tBu2Si(OH)2 using triethylamine as a base (24b), or with a Li + salt of the silanediol (24a and 24f), or with tBu2Si(OSnMe3)2 (24c-e). A cis/trans isomerism was induced within the Cp'-coordinated binuclear complexes, by contrast with 23a-e. An NMR analysis of complexes 24c-e revealed that the major isomer was trans. [START_REF] Liu | Synthesis and structures of cyclopentadienyl fluoro and chloro complexes of a triad (Ti, Zr, Hf) containing acyclic and cyclic siloxane building blocks[END_REF] This isomer was also the one found in the unit cell after X-ray diffraction analysis of 24d (Fig. 15b); the metallacycle was no longer planar, the bulky cyclopentadienyl ligands occupying equatorial positions. The 6-membered cyclic complex 25 was isolated together with complex 24f. [START_REF] Liu | Synthesis and Structure of Eight-Membered Titanium Containing Siloxane Rings[END_REF] By contrast with the titanasiloxanes mentioned above, 25 contained only one bridging siloxane ligand instead of two.

Si R O O R Ti Ti Si R O O R Y X Y X Si R O O R Ti Ti Si R O O R

Cp' X

The stable Ti(II) titanocene precursors formulated [(Cp')2Ti(btmsa)], where btmsa is the labile bis(trimethylsilyl)acetylene ligand, have been very widely used to develop the chemistry of low-oxidation-state titanium complexes, among which many multinuclear species (see also below, especially § 2.1.3). [START_REF] Rosenthal | The titanocene complex of bis(trimethylsilyl)acetylene: Synthesis, structure, and chemistry[END_REF] One example of these titanocene-generating complexes, viz. the permethylated [(h 5 -C5Me5)2Ti(btmsa)], is displayed on Fig. 16.

[(C5Me5)2Ti(btmsa)] reacted with carbon dioxide, yielding the µ-carbonato binuclear Ti(III) complex 26. The latter complex resulted from a redox reaction, CO2 being reduced into CO present in the by-product [(C5Me5)2Ti(CO)2]. [START_REF] Burlakov | Novel acetylene complexes of titanocene and permethyltitanocene without additional ligands. Synthesis spectral characteristics and X-ray diffraction study[END_REF] Noteworthy, one metal of 26 was coordinated by a single oxygen of the carbonate, whereas the other metal was chelated by the two remaining oxygen atoms. Complex 26 was paramagnetic, unlike other Ti(III) complexes mentioned § 2.1.3. Interestingly, the mixed-valenced Ti(III),Ti(IV) complex 27 was obtained together with 26 as a result of the reaction of [(C5Me5)2Ti IV Cl2] with carbon dioxide, a reducing agent (Mg) and btmsa (Fig. 16). [START_REF] Burlakov | Interaction of carbon dioxide with the bis( trimethylsilyi) acetylene yltitmocene: synthesis and structure of mplex of permethyltitanocene ( Cp; Ti)[END_REF] Fig. 16. An example of titanocene precursor, and the binuclear complexes resulting from its reaction with CO2.

Bridging ligands with N-donor atoms or mixed N,O-donor atoms

In complex 28, both titanium(IV) ions were chelated by a tetraaminodisilazane generated in situ over the reaction of [(C5Me5)TiMe3] with (N-(2,6-diisopropylphenyl)-N-(trimethylsilyl)amino)triaminosilane run at 69°C (Fig. 17). [START_REF] Murugavel | Organosilanetriols: Model compounds and potential precursors for metal-containing silicate assemblies[END_REF][START_REF] Wraag | Preparation and Structural Investigations of[END_REF] Two triaminosilane molecules have condensed into a disilazane ligand, the ammonia by-product entering the coordination sphere as a bridging imido ligand and methane evolving as a by-product. Making the titanocene precursor [(C5H5)2Ti(btmsa)] react with dicyandiamide yielded a C2-symmetric binuclear complex where the Ti(C5H5)2 units were connected with two µ-NH(NH2)C=N-CºN ligands. [START_REF] Ruhmann | Reactions of titanocene with triazines and dicyandiamide: Formation of novel Di-and trinuclear metallacyclic complexes[END_REF] The synthesis of two Ti(IV) double stranded helicates from bis-chelating ligands has to be mentioned here. The reaction of a chiral enantiopure bis-pyrrole-imine nitrogen ligand with Ti(NMe2)4 led to complex 33, [START_REF] Zhang | Synthesis, structure, and catalytic activity of titanium complexes with new chiral 11,12-diamino-9,10-dihydro-9,10-ethanoanthracene-based ligands[END_REF] whereas complex 34 resulted from the reaction of an achiral mixed bis-N,O ligand with an isobutenyl linker with Ti(O i Pr)4 under anhydrous conditions [START_REF] Rao | Self-assembly of double stranded dinuclear titanium( IV )-Schiff base complexes and formation of intramolecular mu-oxo bridges[END_REF] (Fig. 19). The double helical arrangement of the latter ligands was highlighted in the crystal structure of 34 displayed Fig. 20. Noteworthy, the bis-oxo complex 35, which did not exhibit a helical twist of the bis-chelating ligands, was formed together with 34 as a minor product; in the presence of moisture, 35 was the only reaction product. The research domain of binuclear Ti(I), Ti(II) or Ti(III) complexes with a bridging N2 ligand, developed within the general theme of N2 activation, has been the subject of thorough investigations. [125][START_REF]pin, Nitrogen fixation by transition metal compounds: Results and prospects[END_REF][START_REF] Fryzuk | The continuing story of dinitrogen activation[END_REF][START_REF] Mackay | Dinitrogen Coordination Chemistry: On the Biomimetic Borderlands[END_REF][START_REF] Scherer | Low-valent titanium-pentafulvene complexes -Formation of dinuclear titanium-nitrogen complexes[END_REF][START_REF] Studt | Spectroscopic comparison of dinuclear Ti+ and Ti2+ μ-η1:η1 dinitrogen complexes with Cp*/pentafulvene and amine/amide ligation: Moderate versus strong activation of N2[END_REF][START_REF] Oswald | Crystal structure of an isomeric bis[(η5:η1-6,6-di-ptolylpentafulvene)(η5-pentamethylcyclopentadienyl)titanium(III)]-μ2,η1:η1-dinitrogen complex, C60H66N2Ti2[END_REF] The reader find will all details about transition metal-dinitrogen complexes in reference [125]; N2 being often considered as a labile ligand, we will not develop this point further in the present review.

Bridging ligands with C-donor atoms or mixed C,O-, C,N-or C,P-donor atoms

A series of binuclear metallocene complexes containing carbon bridging ligands has been reported, with important contributions of the groups of Rosenthal and of Beckhaus. Typical examples are µ-h(1-3):h(2-4)-trans,trans-tetradehydrobutadiene ("zigzag butadiyne") titanium(IV) complexes (Fig. 21) or bis-s,p-acetylide titanium(III) (Fig. 22). These architectures can somehow be considered as the result of a redox self-assembly, although selfassembly was usually not presented by the authors as their main goal. They are potentially important for the design of carbon-rich networks and of nanoarchitectures for material science.

Several reviews related to the organometallic carbon-rich chemistry of titanium (among other metals) has been published. [START_REF] Rosenthal | The titanocene complex of bis(trimethylsilyl)acetylene: Synthesis, structure, and chemistry[END_REF][START_REF] Ohff | Unusual Reaction of Titanocene-and Zirconocene-Generating Complexes[END_REF][START_REF] Rosenthal | What do titano-and zirconocenes do with diynes and polyynes?[END_REF][START_REF] Rosenthal | Titanocene and zirconocene σalkynyl complexes in C-C single bond coupling and cleavage reactions[END_REF][START_REF] Rosenthal | Five-membered Titana-And zirconacyclocumulenes: Stable 1-metallacyclopenta-2,3,4-trienes[END_REF][START_REF] Rosenthal | Five-membered metallacycles of titanium and zirconium -Attractive compounds for organometallic chemistry and catalysis[END_REF][START_REF] Beckhaus | Pentafulvene complexes of group four metals: Versatile organometallic building blocks[END_REF] In an early publication, Teuben and de Liefde Meijer isolated a complex from the reaction of [(C5H5)2Ti III Cl]2 with sodium phenylacetylide; they suggested that it was a biss,p-acetylide Ti dimer. [START_REF] Teuben | Phenethynyl complexes of titanocene and vanadocene[END_REF] However, a similar reaction was carried out afterwards by Stucky et al. starting from [(C5H4Me)2TiCl]2. [START_REF] Sekutowski | Oxidative Coupling of the Phenylethynyl Group in μ-(1-3η:2-4ηtrans,trans-1,4-Diphenylbutadiene)-bis(bis(η5-methylcyclopentadienyl)titanium) and the Reaction of 1,4-Diphenyl-1,3-butadiene with Bis(cyclopentadienyl)titanium(II)[END_REF] The crystal structure of the product revealed that it was actually the butadiyne dimer 36b, resulting from an oxidative C-C coupling; therefore, it was concluded that the compound synthesized by Teuben and de Liefde Meijer must be 36a (Fig. 23). Indeed, this reaction can be considered as an early serendipitous self-assembly process. It was also checked that another pathway, i.e. the reaction of [(C5H4Me)2Ti II ] (generated in situ from the reduction of [(C5H4Me)TiCl]2 by metallic sodium)

R 2 R 1 Ti(Cp') 2 Ti 36a: Cp' = C 5 H 5 , R 1 = R 2 = Ph 36b: Cp' = C 5 H 4 Me, R 1 = R 2 = Ph 36c: (Cp') 2 = (C 5 H 4 ) 2 SiMe 2 , R 1 = R 2 = Ph 36d: Cp' = C 5 H 5 , R 1 = R 2 = t Bu 36e: Cp' = C 5 H 5 , R 1 = SiMe 3 , R 2 = t Bu 36f: Cp' = C 5 H 5 , R 1 = SiMe 3 , R 2 = Ph 36g: Cp' = C 5 H 5 , R 1 = R 2 = -C≡C-SiMe 3 36h: Cp' = C 5 H 5 , R 1 = R 2 = Me 36i: Cp' = C 5 H 5 , R 1 = R 2 = SBn 36j: Cp' = C 5 H 4 Ad (Ad = 2-adamantyl), R 1 = R 2 = Ph (Cp') 2 Ti R Ti(Cp') 2 R 37a: (Cp') 2 = (C 5 H 4 ) 2 SiMe 2 , R = Ph 37b: Cp' = C 5 H 5 , R = SiMe 3 (Cp') 2
with 1,4-diphenyl-1,3-butadiyne, also yielded 36b. The use of an ansa silylbis(cyclopentadienyl) ligand allowed the isolation and characterization of another analogue, 36c, but also of the isomeric bis-s,p-acetylide titanium(III) complex 37a as an intermediate of synthesis. [START_REF] Cuenca | Reactions of Titanium-and Zirconium(III) Complexes with Unsaturated Organic Systems. X-ray Structure of {[(η5-C5H5)Zr(CH3)[END_REF] The similar zigzag complexes 36d-h were generated either from butadiynes with substituents other than phenyl, [START_REF] Rosenthal | Zu Reaktionen disubstituierter Butadiine mit Titanocen-Komplexen[END_REF][START_REF] Rosenthal | Heterobimetallic σ,π-Acetylide-Bridged Complexes from Disubstituted 1,3-Butadiynes[END_REF][START_REF] Pellny | Dimerization of titanacyclocumulenes to titanium substituted radialenes: Synthesis, stability and reactions of five-membered titanacyclocumulenes with a coupling of two 1,4-diphenyl-1,3-butadiyne between two titanocene molecules to radialene-like fused ti[END_REF][START_REF] Kirchbauer | Synthesis and reactions with carbon dioxide of mono(σ-alkynyl) titanocene(III) complexes Cp*2Ti(C≡CR) (R = Me, t-Bu) and the corresponding "Ate" complexes [Cp*2Ti(C≡CR)2Li(THF)n] (R = SiMe3, t-Bu, Ph)[END_REF] or from an octatetrayne. [START_REF] Pellny | Twofold C -C Single Bond Activation and Cleavage in the Reaction of Octatetraynes with Titanocene and Zirconocene Complexes[END_REF] For that purpose, Rosenthal et al. employed the successful low-valent titanocene precursor [(C5H5)2Ti(btmsa)] mentioned above ( § 2.1.1.3). Incidentally, the synthesis of 36g was regioselective: the two triple bonds of the octatetrayne that were converted into the bridging butadiene were the middle ones. [START_REF] Pellny | Twofold C -C Single Bond Activation and Cleavage in the Reaction of Octatetraynes with Titanocene and Zirconocene Complexes[END_REF] By contrast, they observed that the C-C bond of 1,4-bis(trimethylsilyl)-1,3-butadiyne was cleaved under the same conditions, yielding the bis-s,p-acetylide complex 37b. [START_REF]Die Bildung von [Cp,Ti( C=CSiMe3)]2 aus Titanocen und 1,4-Bis( trimethylsilyl)-1,3-Butadiin[END_REF] The structural differences between the typical zigzag butadiyne complex 36a and the bis-acetylide complex 37b are represented in Fig. [START_REF] Fujita | Self-Assembly of M30L60 Icosidodecahedron[END_REF] Investigations on the equilibria between zigzag butadiyne complexes, bis-s,p-acetylide complexes and transient monomeric Ti(III) monoacetylide complexes were implemented by variable-temperature NMR, [START_REF] Baumann | Dynamic NMR study of a 1,3-diyne bridged dinuclear bis(cyclopentadienyl)titanium complex: Understanding the C -C single bond metathesis reaction[END_REF] DFT theoretical calculations [START_REF] Jemmis | To Couple or Not To Couple : The Dilemma of Acetylide Carbons in Complexes ( M = Ti , Zr ) -A Theoretical Study for R = H, F[END_REF][START_REF] Jemmis | Novel mechanism for interesting C-C coupling and cleavage reactions and control of thermodynamic stability involving [L2M(μ-CCR)2ML2[END_REF] and Tiphotocatalyzed metathesis reactions. [START_REF] Pulst | The First Metathesis of C-C Single Bonds in Homogeneous Solution: Titanocene-Mediated and Photocatalyzed Cleavage and Recombination of Disubstituted Butadiynes[END_REF] It appeared from these studies that energy differences between these structural types were small and an interconversion was achieved easily, under thermodynamic control.

The zigzag butadiyne complexes 36i and 36j reported by Beckhaus et al. were generated through pathways that were different from those mentioned above: the synthesis of the sulfur-containing complex 36i from bis(benzylsulfanyl)acetylene involved C-S cleavage and C-C coupling, [START_REF] Altenburger | Examples of different reactions of benzylsulfanyl-substituted alkynes with selected complexes of TiII and CoI[END_REF] and 36j was prepared from the reaction of a titanaaziridine with phenylacetylene followed by thermal decomposition. [START_REF] Manßen | Reactions of Secondary Amines with Bis(η5:η1-pentafulvene)titanium Complexes: Formation of Titanium Amides and Titanaaziridines[END_REF] In Fig. 26 are displayed various binuclear complexes akin to the zigzag butadiynes of Fig. 21. The spiro compounds 40a-c, made from tetraalkynylsilanes, comprised two zigzag butadiyne motifs linked by a silicon atom; the analogy of Si with Ti was underlined by the authors. [START_REF] Pellny | Reactions of tetraalkynylsilanes (RC≡C)4Si (R = Ph, tBu, SiMe3) with titanocene and zirconocene complexes[END_REF] 41 and 42a,b were also complexes with a zigzag carbon-rich bridging ligand, but with a central C=C bond (respectively zigzag butatriene [START_REF] Burlakov | Reduction of 1,4-dichlorobut-2-yne by titanocene to a 1,2,3-butatriene[END_REF] and hexapentaene [START_REF] Beckhaus | Pentafulvene complexes of group four metals: Versatile organometallic building blocks[END_REF][START_REF] Oswald | Formation of Binuclear Zigzag Hexapentaene Titanium Complexes via a Titanacumulene [Ti=C=C=CH2] Intermediate[END_REF] complexes). Coming back to the reaction conditions that had led to complex 36a, a change in the Ti:diphenylbutadiyne stoichiometry to 1:1 led to annellated binuclear complexes, viz. the symmetric radialene 44a and the unsymmetric 45a (Fig. 28). [START_REF] Pellny | Dimerization of titanacyclocumulenes to titanium substituted radialenes: Synthesis, stability and reactions of five-membered titanacyclocumulenes with a coupling of two 1,4-diphenyl-1,3-butadiyne between two titanocene molecules to radialene-like fused ti[END_REF][START_REF] Burlakov | Reactions of the phenyl-substituted five-membered titanacyclocumulene -Unusual coupling of a 1,4-disubstituted 1,3butadiyne with two titanium atoms[END_REF] These fused complexes resulted from the spontaneous evolution (dimerization) of unstable monomeric species in equilibrium, i.e. a five-membered titanacyclocumulene and an h 2 -alkyne complex. Complex 44b was synthesized in order to get single crystals suitable for X-ray diffraction analysis.

Changing diphenylbutadiyne to 2,4-hexadiyne led to a mixture of the novel complex 45b and of 36h (vide supra), among others; but no radialene compound was isolated. pyridines or difluorotriazine (Fig. 30). [START_REF] Piglosiewicz | Selective C-H and C-F bond activation reactions of pyridine and fluoropyridines -Formation of binuclear μ-X titanocene complexes (X = H, F) with α-functionalized N-heterocycles[END_REF] The Beckhaus et al. made one of their µ-N2 Ti(I) dimers react with white phosphorus. Indeed, the displacement of the labile dinitrogen ligand led to the Ti(III) binuclear complex 55 , where two P-P bonds of P4 were split and two P-C bonds were formed, generating an uncommon mixed Cp,P-bridging chelating ligand (Fig. 31). [START_REF] Beckhaus | Pentafulvene complexes of group four metals: Versatile organometallic building blocks[END_REF][START_REF] Oswald | Crystal structure of a P4-bridged (η5-pentamethylcyclopentadienyl)(η5-adamantylcyclopentadienyl) titanium(III)complex, C50H66P4Ti2[END_REF] Fig. 31. Synthesis of a Ti(III) complex with a mixed Cp,P-bridging ligand.

Trinuclear complexes 2.2.1. Bridging ligands with O-donor atoms

To our knowledge, only two trinuclear complexes generated from catechol-containing ligands have been reported by Albrecht et al. (Fig. 32). In complex 56, three triscatecholato ligands chelated three aligned Ti atoms within a classical anionic triple-stranded helicate, such as the ones we had mentioned earlier ( § 2.1.1.1); it was determined that it racemized in solution in a stepwise manner. [START_REF] Albrecht | Let's twist again" -Double-stranded, triple-stranded, and circular helicates[END_REF][START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Albrecht | Racemisation behaviour of trinuclear helicates formed from ethylenebridged tris(catechol) ligands and titanium(IV) ions[END_REF] Complex 57, which was isolated from a mixture of oligomers in DMF after a crystallization step, exhibited a more unusual structure; [START_REF] Albrecht | Selecting different complexes from a dynamic combinatorial library of coordination compounds[END_REF] a simplified drawing of this structure is represented Fig. 32c. The three Ti centers occupied the Throughout their research work on asymmetric catalysis, Corey et al. reported an early genuine self-assembly of three titanium(IV) ions by a chiral enantiopure tetraol synthesized from D-mannitol (Fig. 33). [START_REF] Piguet | Helicates as versatile supramolecular complexes[END_REF][START_REF] Corey | Synthesis and X-ray structure of a novel chiral trinuclear titaniumtetraol complex[END_REF] The crystal structure of the air-stable compound 58 revealed that it was a left-handed helicate where the metal atoms were aligned and hexacoordinated. Both "central" oxygen atoms of the ligands coordinated two Ti atoms in a bridging mode. A rather similar reaction starting from another tetraol was depicted later, and the authors deduced from NMR and MS data that a cyclic trinuclear Ti(IV) complex was obtained; [START_REF] Hegarty | A commentary on the self assembly and properties of a chiral titanium alkoxide cyclic trimer[END_REF] however, in the absence of X-ray structure, a linear complex similar to 58 is a hypothesis that cannot be excluded. It was reported that a quaterphenolato ligand imparted a helical twist to a trinuclear Ti and forced a bent structure (Ti...Ti...Ti angle of ca. 130°). [START_REF] Kayal | Ortho-linked polyaryloxide ligands and their titanium complexes[END_REF] A circular trinuclear helicate was obtained from the pro-ligand L 1 H4 (vide supra Fig. 9)

and Ti(O i Pr)4 by modifying slightly the experimental protocol allowing the formation of the already mentioned double-stranded helicate 16. Instead of using toluene, the self-assembly reaction performed in dichloromethane afforded the circular helicate [Ti3L 1 3(HO i Pr)6] (59) as a crystalline solid when n-pentane was used as precipitant. [START_REF] Weekes | A remarkable solvent effect on the nuclearity of neutral titanium(IV)-based helicate assemblies[END_REF] The molecular structure of the complex displayed in Fig. 34 underlined that three homochiral octahedral sub-units composed this circular structure. The crystal system of this species was found to be achiral with an unit cell formed with an equal proportion of the (M-D,D,D) and (P-L,L,L) complex. Crystals suitable for X-ray analysis of complex 61 appeared in the medium at elevated temperature if toluene was used as solvent. The molecular structure of 61 displayed in Fig. 36 highlights the formation of homochiral assemblies where each vertex within a complex displays the same D or L configuration. In solution, complex 61 was highly stable and no bowl-to-bowl inversion was observed. It also showed a good robustness towards hydrolysis or alcoholysis. This contrasted with the usual low stability of neutral titanium(IV) coordination compound formed with oxygenated ligands. Remarkably, if the stoichiometry of the initial reaction permitting the formation of 61 was modified, binuclear aggregates were characterized where the bpym ligand bridged two metallic centers. These complexes were formulated [Ti2L

Bridging ligands with N-donor, Sb-donor or mixed N,O-donor atoms

On Fig. 37 The A-type coordination mode has been observed in complexes 62a,b with imidazolato bridging ligands (Fig. 38a). [START_REF] Sander | Self-Assembly Reactions To Form Multinuclear Zirconium(III) and Titanium(III) Complexes with Imidazole Derivatives as Bridging Ligands[END_REF] Titanium was at the +III oxidation state in these complexes; they resulted from the spontaneous dehydrogenation of imidazole or 4,5-diphenylimidazole following their reaction with [(C5Me5)2Ti(btmsa))] or [(C5H5)2Ti(btmsa)], respectively.

Closely related architectures were tetrameric (vide infra); it is clear that the selectivity of the reaction towards a trimer or a tetramer was under steric control, an increased steric demand (methyl groups borne by the h 5 -cyclopentadienyl ligand, or phenyl groups by the µimidazolato ligand) favoring the formation of triangles. The crystal structure of 62a showed that the metal atoms were located at the vertices of an almost equilateral triangle (Fig. 38b). Concerning B-type chemical architectures, several 6-membered nitrogen heterocycles have led to trinuclear complexes (Figs. 39 and40). [START_REF] Kraft | Directed reduction of six-membered nitrogen heterocycles -Selective formation of polynuclear titanium complexes[END_REF][START_REF] Sander | Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNPh6)-Bridged Titanium Complexes[END_REF][START_REF] Piglosiewicz | Dehydroaromatization of quinoxalines: One-step syntheses of trinuclear 1,6,7,12,13,18-hexaazatrinaphthylene titanium complexes[END_REF] These could be considered as selective redox self-assemblies where the ligand precursors underwent C-C couplings over the assembly process; the driving force favoring the formation of the trinuclear complex was attributed to the formation of the six Ti-N bonds. [START_REF] Jung | Unexpected trimerization of pyrazine in the coordination sphere of low-valent titanocene fragments[END_REF] The mechanism of synthesis of 63 is similar to the one proposed for the formation of the binuclear complexes 31a,b (vide supra), starting with the initial reduction of pyrazine into a radical anion and resulting with a loss of aromaticity in the final bridging hexahydrodipyrazinoquinoxaline ligand. Crystallographic data and theoretical considerations indicated that Ti was at the + IV oxidation state in the dominant form of 63. [START_REF] Kraft | Directed reduction of six-membered nitrogen heterocycles -Selective formation of polynuclear titanium complexes[END_REF][START_REF] Jung | Unexpected trimerization of pyrazine in the coordination sphere of low-valent titanocene fragments[END_REF] The central cyclohexane moiety of the bridging ligand was in a chair conformation: the H atoms along the newly formed C-C bonds adopted a trans position. Noteworthy, the outcome of the reaction of pyrazine with [(C5Me5)2Ti(btmsa)] was considered as surprising by the authors, this heterocycle being a typical bridging ligand. Indeed, pyrazine was incorporated as a bridging ligand within a tetranuclear Ti complex when [(C5H5)2Ti(btmsa)] was used instead of [(C5Me5)2Ti(btmsa)] as the reagent (vide infra).

Complexes 64a-c were synthesized from quinoxalines (Fig. 40). By contrast with 63, in 64a-c the 7-rings aromatic hexaazatrinaphthylene bridging ligands were generated by a spontaneous dehydroaromatization of the quinoxalines. In theory, complexes 64a-c could be represented under two mesomeric forms where Ti is either at the +II or the +IV oxidation state. Some of the X-ray data supported one form while others support the other one; [START_REF] Piglosiewicz | Dehydroaromatization of quinoxalines: One-step syntheses of trinuclear 1,6,7,12,13,18-hexaazatrinaphthylene titanium complexes[END_REF] however, those complexes were sensitive to air and moisture, and for that reason were later rather referred to as Ti(II) compounds. [START_REF] Piglosiewicz | Selective oxidation and reduction of trinuclear titanium(II) hexaazatrinaphthylene complexes -Synthesis, structure, and electrochemical investigations[END_REF] Thorough electrochemical and spectroscopic investigations were carried out on these complexes; intervalence charge transfer bands were observed by near-infrared spectroscopy. [START_REF] Sander | Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNPh6)-Bridged Titanium Complexes[END_REF] Fig. 40. Synthesis of B-type titanium complexes 64a-c.

A spectacular chemical transformation leading selectively to the Ti(IV) trimers 65a and 65b was reported by Rosenthal et al. [START_REF] Becker | Formation of Tri-and tetranuclear titanacycles through decamethyltitanocene-mediated intermolecular c-c coupling of dinitriles[END_REF] It involved the C-C coupling of dinitriles, viz.

either 1,3-dicyanobenzene or adiponitrile, triggered by 3 equiv. of [(C5H5)2Ti(btmsa)] (Fig. 41). These compounds comprising three 1-titana-2,5-diazacyclopenta-2,4-dienes also belonged to the B-type category. 65a was poorly stable, it underwent decomposition at room temperature whereas 65b did not. It is remarkable that the latter was generated selectively in spite of the flexibility of the adiponitrile hydrocarbon chain (see the X-ray structure Fig. 42).

It was established by DFT computation that a concerted mechanism was likely for the synthesis of 65a. Noteworthy, some other dinitriles rather led to tetrameric complexes (vide

Breuning et al. worked on the reaction of [(C5H5)2Ti(btmsa)] with

tetraorganodistibenes R4Sb2 and found out that the outcome of the reaction depended on the nature of the R groups. In particular, they isolated and characterized the B-type trimeric complex 66 when R was a benzylamino group (Fig. 41), which probably resulted from recombinations of radical intermediates. [START_REF] Breunig | Polynuclear titanocene complexes with antimony ligands: [(Cp2Ti)2(SbR2)2(R = Et)[END_REF] Its crystal structure revealed a pyramidal central antimony and a chair conformation of the peripheral six-membered Ti3Sb3 ring.

Chelidamic acid was an interesting and simple pro-ligand, prone to bridging metals with a hydroxy function on one side and a chelating dicarboxypyridine on the other side. We have represented in Fig. 43 the spontaneous transformation of a mononuclear chelidamato titanium complex into the trinuclear species 67 in DMSO at room temperature. [START_REF] Fandos | Experimental and Theoretical Studies on the Reactivity of Titanium Chelidamate Complexes: The Significant Role of the Hydroxide Pyridine Moiety[END_REF] In its crystal

Bridging ligands with C-donor atoms

To our knowledge, the only example which has to be mentioned herein is that of the C3symmetric complex [(C5H5)2Ti III (CN)]3 (68, Fig. 44), resulting from the reaction of [(C5H5)2Ti(btmsa)] with cyanuric chloride. [START_REF] Ruhmann | Reactions of titanocene with triazines and dicyandiamide: Formation of novel Di-and trinuclear metallacyclic complexes[END_REF] In this species, the three Ti(C5H5)2 moieties were connected with three µ-cyano ligands. Its crystal structure confirmed a triangular pattern, but the atoms of the CN ligands were disordered.

Tetranuclear complexes 2.3.1. Bridging ligands with chalcogen donor atoms 2.3.1.1. Bis-or triscatecholato ligands or analogues

A series of anionic metallosupramolecular tetrahedral Ti4 complexes built from ligands L with two (Fig. 45 for complexes 69a [START_REF] Davis | The big squeeze: Guest exchange in an M4L6 supramolecular host[END_REF] and 69b [START_REF] Scherer | Triple Helicate-Tetrahedral Cluster Interconversion Controlled by Host-Guest Interactions[END_REF]) or three (Table 1 for complexes 70a-i, and Fig. 46 2012. The metal atoms were located at the vertices of the tetrahedra. The complexes were formulated Ti4L6 in the first case, where µ 2 -L was located on an edge of the tetrahedron, and Ti4L4 in the second case where µ 3 -L was located on a face. Table 1.

Tetrahedral Ti4L4 complexes derived from C3-symmetric triscatechol ligands, where the catechol units were bound to the triangular backbone through their 3-position. The countercations were alcali metal or polyalkylammonium ions. 70f O [START_REF] Albrecht | Supramolecular [M 4L 4] tetrahedra based on triangular acylhydrazone catechol ligands[END_REF] 70g O [START_REF] Albrecht | Supramolecular [M 4L 4] tetrahedra based on triangular acylhydrazone catechol ligands[END_REF] 70h O [START_REF] Albrecht | Tuning the size of supramolecular M 4L 4 tetrahedra by ligand connectivity[END_REF] 70i none S [START_REF] Birkmann | Assembly of a tetranuclear host with a tris(benzene-o-dithiolato) ligand[END_REF] The countercations were alcali metal or polyalkylammonium ions.

Compound X References

We refer again the reader to the numerous reviews written on self-assembled architectures generated from titanium and catechol-based ligands or sulfur analogues, some of them being previously mentioned in § 2.1.1.1 and § 2.2.1. [START_REF] Albrecht | Let's twist again" -Double-stranded, triple-stranded, and circular helicates[END_REF][START_REF] Albrecht | Supramolecular chemistry -General principles and selected examples from anion recognition and metallosupramolecular chemistry[END_REF][START_REF] Caulder | The rational design of high symmetry coordination clusters[END_REF][START_REF] Albrecht | Symmetry driven self-assembly of metallo-supramolecular architectures[END_REF][START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Kreickmann | Benzene-o-dithiolate ligands as versatile building blocks in supramolecular chemistry[END_REF][START_REF] Sproules | Tris(dithiolene) Chemistry: A Golden Jubilee[END_REF][START_REF] Albrecht | Catechol imine ligands: From helicates to supramolecular tetrahedra[END_REF][START_REF] Albrecht | Into the next dimension: Nanometer-sized, oligonuclear coordination compounds with C3symmetric ligands[END_REF][START_REF] Albrecht | Supramolecular Templating in the Formation of Helicates[END_REF][START_REF] Caulder | Supermolecules by design[END_REF] In particular, Caulder and Raymond have thoroughly set out the principles that govern self-assembly processes leading to tetrametallic tetrahedral architectures based on catecholcontaining ligands. [START_REF] Caulder | Supermolecules by design[END_REF][START_REF] Caulder | The rational design of high symmetry coordination clusters[END_REF] Thus, such architectures could be obtained via a rational synthetic approach. We will confine ourselves to providing hereafter a short overview of the syntheses, analyses and properties of these Ti4 container-type architectures. Indeed, the tetranuclear Ti complexes were synthesized by self-assembly using stoichiometric amounts of ligand and titanium(IV) source. A template effect has often been observed; the cation involved was eventually encapsulated within the complex as a guest (vide infra). For instance, the "Ti4L6type" compound 69b was obtained from a biscatechol ligand only in the presence of an alkylammonium guest; otherwise, the self-assembly product was a dimeric triple-strand complex.

For the synthesis of Ti4L4 species from rigid triscatechol ligands, the procedure was usually straightforward with or without template effect. Yet, as mentioned § 2.2.1, a mixture of oligomers resulted from the reaction of the flexible triscatechol ligand L 3 with Ti(IV) and potassium carbonate in DMF; it was necessary to switch to DMSO as the solvent to generate the tetranuclear 70d, owing to its K + solvating ability.

Overall, the complexes were clearly identified from MS analyses as anionic metal/ligand 4:4 adducts. Solution NMR analyses usually revealed highly symmetrical species. The tetrahedral geometry and other structural characteristics (e.g. Ti … Ti distances, indicative of the cavity size) were evidenced by crystallography or computational chemistry methods. They were generally highly stable in solution, e.g. in water or methanol, even for a long time; however, when their size was high, they might collapse upon removal of the solvent (e.g. 71b). The size and nature of the ligands played an important role in the volume of the Ti4 container, but other factors may interfere, such as ligand conformation (e.g. comparison of highly condensed 70a with its much larger sulfur analogue 70i) or ligand connectivity. [START_REF] Albrecht | Tuning the size of supramolecular M 4L 4 tetrahedra by ligand connectivity[END_REF] A comparison of Ti4L6 vs. Ti4L4 architectures has been extensively discussed. [START_REF] Yeh | Large M4L4 (M = Al(III), Ga(III), In(III), Ti(IV)) tetrahedral coordination cages: An extension of symmetry-based design[END_REF] With regard

71a: M = K, 71b: M = Na, = = N CH 2 3 CH 2 R R R Ti Ti Ti Ti M 8 O O Ti O O Ti
to encapsulation, there is obviously a link between the size of the containers and the presence of guests. Thus, no evidence of a guest was found for a small molecule like 70a, whereas supramolecular 69a and 69b hosted an ammonium cation, and huge tetrahedra like 70c or 70i respectively hosted four [K(DMF)3] + , or one K + and four NMe4 + cations. Guest exchange processes were also evaluated. [START_REF] Davis | The big squeeze: Guest exchange in an M4L6 supramolecular host[END_REF][START_REF] Yeh | Large M4L4 (M = Al(III), Ga(III), In(III), Ti(IV)) tetrahedral coordination cages: An extension of symmetry-based design[END_REF][START_REF] Albrecht | A metallosupramolecular tetrahedron with a huge internal cavity[END_REF][START_REF] Albrecht | Self-assembly and host-guest chemistry of big metallosupramolecular M4L4 tetrahedra[END_REF] The sole mention of a heterochiral (D,D,L,L) tetranuclear complex of this category was found for compound 70i; otherwise, many of those that had crystallized and were analyzed by X-ray diffraction were racemic homochiral complexes. [START_REF] Scherer | Triple Helicate-Tetrahedral Cluster Interconversion Controlled by Host-Guest Interactions[END_REF][START_REF] Caulder | Design, Formation and Properties of Tetrahedral M4L4 and M4L6 Supramolecular Clusters[END_REF][START_REF] Yeh | Large M4L4 (M = Al(III), Ga(III), In(III), Ti(IV)) tetrahedral coordination cages: An extension of symmetry-based design[END_REF]

Oligophenolato ligands

Stable anionic tetranuclear Ti4L6 cages 72a,b have been self-assembled from a bridging aromatic bis-hydroxycarboxylic (embonic) acid and either from Ti(O i Pr)4 (72a) or from TiCl4 (72b), the electric charge depending on the oxidation state of each Ti ion of the cage (IV or III) (Fig. 47). [START_REF] He | Water-Soluble and Ultrastable Ti4L6 Tetrahedron with Coordination Assembly Function[END_REF] 72a was used as a selective and efficient homogeneous photocatalyst, e.g. The molecular structure of a neutral tetranuclear titanium-based assembly built from pro-ligand L 1 H4 (vide supra, Fig. 9) was recently reported. This giant circular helicate constructed around TiO4N2 units incorporated four 2,2'-bipyrimidine chelates substituted by two (anthryl)vinyl moieties (bAnt, see Fig. 48). [START_REF] Khalil | Symmetry Decrease between Self-Assembled Circular TiO4N2-Based Helicates[END_REF] O The multicomponent self-assembly process involving L 1 H4, Ti(O i Pr)4 and bAnt ended in the quantitative formation of a thermodynamically stable dissymmetric circular helicate [Ti3L 1 3(bAnt)3] (74), highlighting a symmetry decrease going from tetrameric 73 to trimeric 74. DFT calculations showed that it is the atomic spatial disposition in the trimer displaying a C1-symmetry that strongly minimized electronic repulsions, which explained why a loss of symmetry occurred for this self-assembly process. 2); [START_REF] Murugavel | Hetero-and metallasiloxanes derived from silanediols, disilanols, silanetriols, and trisilanols[END_REF][START_REF] Murugavel | Organosilanetriols: Model compounds and potential precursors for metal-containing silicate assemblies[END_REF] they are represented in Fig. 50. For 75, X-ray diffraction analysis revealed that the four titanium(IV) atoms were coplanar, yet in two different environments: both central metal atoms were hexacoordinated whereas the peripheral metal atoms were pentacoordinated. [START_REF] Voigt | Soluble Molecular Titanosilicates[END_REF] The coordination sphere included six silanetriolato ligands. By contrast, the titanium(IV) atoms formed almost regular tetrahedra within Ti4Si4 cubes in 76a-g, [START_REF] Winkhofer | Stable Silanetriols[END_REF][START_REF] Voigt | Facile and rational route for high-yield synthesis of titanasiloxanes from aminosilanetriols[END_REF] as revealed by the X-ray crystallographic analysis of 76a and 76c (e.g. for 76a, dTi...Ti = 4.80 ± 0.03 Å). The pentamethylcyclopentadienyl-coordinated complex 76a was synthesized from tBu2Si(OSnMe3)3 and [(C5H4Me)TiCl3]. The other complexes (76b-g) were issued from the reaction of an (N-aryl-N-(trimethylsilyl)amino)silanetriol with titanium isopropoxide; the metal centers were tetracoordinated, in a tetrahedral geometry. As mentioned in § 2.1.1.2, the IR analysis of these complexes helped assign some corresponding vibrations in titanium-containing zeolites. Actually, the architectures 76a-g can also be represented as the Ti4L4 tetrahedra which had been represented Table 1 and Fig. 47; here, the The structure of 77, obtained after reaction of (N-(2,6-diisopropylphenyl)-N-(trimethylsilyl)amino)silanetriol with titanium tetrachloride, is that of a Ti4Si3 open cube. [START_REF] Voigt | Soluble Molecular Titanosilicates[END_REF] As for 75, the coordination number of the metal centers depended on their position: 5 for the central titanium and 6 for the three peripheral ones. As expected considering the bridging ligands, the Ti … Ti distances were much shorter between two peripheral metals (ca. 3.35 Å) than between central and peripheral metals (4.3-4.5 Å). [START_REF] Burlakov | Interaction of carbon dioxide with the bis( trimethylsilyi) acetylene yltitmocene: synthesis and structure of mplex of permethyltitanocene ( Cp; Ti)[END_REF] four (C5H5)2Ti units assembled around two in situgenerated µ-carbonato ligands to yield the tetranuclear complex 78 (Fig. 51). As in the analogous dimeric complex 26 (vide supra), Ti(II) had been oxidized into Ti(III). In the crystal structure of 78, the four Ti atoms were coplanar and formed a diamond; each carbonato ligand bridged three titanium atoms. No antiferromagnetic interactions between the Ti atoms were detected and the complex was paramagnetic. Noteworthy, the dimer 26 bearing bulky C5Me5 ligands was formed under similar conditions; the origin of the selectivity of the self-assembly process towards tetramer 78 was probably steric.

Other bridging ligands with O-donor atoms

Si O Ti O R' O Si Si O R Ti O R' O R O Si Ti O Ti O O O O R' R
Starting from CO2 and from either [(C5H5)2Ti II (CO)2][202] or [(C5H5)2Ti II ( t BuCºCSiMe3)],

Bridging ligands with N-donor atoms

A dichotomy between two main types of self-assembled architectures, related to the types A and B mentioned Fig. 37 with regard to trinuclear complexes, was also observed in the field of tetranuclear complexes with N-based ligands (Fig. 52). The same naming is used hereafter, although with respective generic formulas Ti4L4 and Ti4L. Note however that these A-type complexes are distinct from the Ti4L4 tetrahedra mentioned § 2.3.1.1, because the L ligand is now µ 2 -bridging instead of µ 3 -bridging; as a consequence, these complexes appear more flexible. 53). [START_REF] Sander | Self-Assembly Reactions To Form Multinuclear Zirconium(III) and Titanium(III) Complexes with Imidazole Derivatives as Bridging Ligands[END_REF] It is interesting to point out the contrast of the selectivity of this reaction with that of the synthesis of the analogous trinuclear species 62a,b (vide supra); those had formed specifically because of steric strain in the coordination sphere. As in 62a,b, titanium was at the +III oxidation state, and the imidazoles had been converted into imidazolato ligands with spontaneous dehydrogenation over the self-assembly process. Although the chemical environment of each metal center was identical, they were arranged as rectangles rather than as squares in their crystal structures. This was attributed to the varying angle that the ligand planes made with the mean Ti4 plane. The formally titanium(II) complexes 81-82 were obtained starting from 6-membered instead of 5-membered nitrogen heterocycles (pyrazine, tetrazine, 2,2'-bipyridine), and from [(Cp')2Ti(btmsa)] (Fig. 54). [START_REF] Kraft | Directed reduction of six-membered nitrogen heterocycles -Selective formation of polynuclear titanium complexes[END_REF][START_REF] Kraft | Titanium-based molecular squares and rectangles: Syntheses by self-assembly reactions of titanocene fragments and aromatic N-heterocycles[END_REF] It is striking that by contrast, the B-type trinuclear species 63 or 64a-c had formed under similar conditions. Indeed, with ligands which were not sterically demanding, no C-C coupling took place over the self-assembly process, and the

(C 5 H 5 ) 2 Ti N N Ti(C 5 H 5 ) 2 R R R R R R R R
heterocyclic ligand remained intact. The mixed-bridged rectangular complexes 83a,b could be synthesized using a step-by-step procedure, making [(Cp')2Ti III Cl]2 react with pyrazine and adding then a reducing agent followed by bipyridine, in this order. Incidentally, using t Bu substituents on the h 5 -cyclopentadienyl ligand increased the solubility of the complexes in organic solvents. A close examination of the crystal data of 81-83 led to the conclusion that the bridging ligands were actually reduced to radical anions in the complexes, Ti being actually at the +III oxidation state. 4,4′-Azobispyridine is a peculiar ligand bearing central donor atoms (azo group), in addition to the peripheral pyridines. Thus, it could be converted into the formally-Ti(II) complexes 84a,b by reaction with 4 equiv. [(Cp')2Ti(btmsa)] (Fig. 55); a cis-trans isomerisation of the ligand was concomitant with the self-assembly process. [START_REF] Theilmann | Reactions of low-valent titanocene(II) fragments with trans-4,4'-azobispyridine (RN=NR, R = C5H4N): Formation of tetranuclear molecular squares by transcis isomerization[END_REF] Contrary to the other tetranuclear complexes presented in this Section, they incorporated 2 bridging ligands instead of 4; as a consequence, the titanocene moieties were in two different N-donor environments within a Ti4L2 architecture. Similar to 81-83, the azo group was in a reduced state (radical anion). In parallel with the synthesis of the trinuclear compounds 65a and 65b ( § 2.2.2), the Btype Ti(IV) complexes 85a and 85b were selectively obtained using 1,4-dicyanobenzene and 1,3-dicyanopyridine as reagents, under similar conditions (Fig. 56). [START_REF] Becker | Formation of Tri-and tetranuclear titanacycles through decamethyltitanocene-mediated intermolecular c-c coupling of dinitriles[END_REF][START_REF] Tomaschun | Group 4 metallocene complexes and cyanopyridines: Coordination or coupling to metallacycles[END_REF] The macrocyclic central ligand was generated in situ through intermolecular C-C couplings of the dinitriles. It appeared that 85a was poorly stable, whereas 85b did not decompose at ambient temperature. The stability of the latter species was attributed to a large free energy of formation, on the basis of DFT calculations performed on a model. X-ray diffraction of single crystals was the tool that allowed to determine whether the four Ti atoms of all the supramolecular structures described in this Section were coplanar or not. Roughly speaking, coplanarity with more or less deviations was the rule for most of these molecular squares and rectangles containing pseudotetrahedrally coordinated corner units.

81a: Cp' = C 5 H 5 , X = CH 81b: Cp' = C 5 H 4 t Bu, X = CH 81c: Cp' = C 5 H 5 , X = N N X N X Ti (Cp') 2 N X N X Ti (Cp') 2 N X N X Ti(Cp') 2 N X N X Ti(Cp') 2 N Ti (Cp') 2 N Ti(Cp') 2 N N N Ti(Cp') 2 N N Ti (Cp') 2 N N Ti (Cp') 2 N Ti(Cp') 2 N N Ti(Cp') 2 N Ti (Cp'
Exceptions were complexes 81c, 85a and 85b which look folded in the unit cell ("butterflylike" arrangement; see for instance Fig. 57). In 85a, the titanium atoms even nearly formed a regular tetrahedron. [START_REF] Becker | Formation of Tri-and tetranuclear titanacycles through decamethyltitanocene-mediated intermolecular c-c coupling of dinitriles[END_REF] Noteworthy, complex 82a was flat in single crystals grown from a tetralin solution, but strongly distorted when the crystals were obtained from a toluene

Bridging ligands with C-donor atoms or mixed C,N-donor atoms

Rosenthal et al. took advantage of the organometallic titanium chemistry mentioned above ( § 2.1.3) leading especially to complexes 36d and 37b, to synthesize the analogous self-assembled tetranuclear species 86 and 87 from octatetraynes following a similar experimental procedure (Fig. 58). [START_REF] Pellny | Twofold C -C Single Bond Activation and Cleavage in the Reaction of Octatetraynes with Titanocene and Zirconocene Complexes[END_REF] Unsurprisingly, the outcome of the reaction depended on the nature of the R substituents borne by the octatetrayne: the Ti(IV) complex 86 was built around a "zigzag octatetraene" ligand with alternating C-C single and double bonds, whereas the Ti(III) complex 87 resulted from a twofold C-C cleavage of 1,8-bis(trimethylsilyl)-1,3,5,7-octatetrayne, bearing one central butadiyne ligand and two lateral (trimethylsilyl)acetylide ligands. As in complexes 84a,b, the metal centers of 86 and 87 occupied two different environments. The resolution of crystal structures showed that in 87 the four metal atoms were coplanar (Fig. 59), but in 86 an angle of 68.6° was measured between both almost planar Ti2tetrahydrobutadiene units linked through a single C-C bond. With regard to square tetranuclear complexes, a few examples were provided in the literature with various bridging ligands. [START_REF] Schinnerling | Die Struktur von [Cp2TiIII(CN)]14[END_REF][START_REF] Stang | Supramolecular chemistry: self-assembly of titanium molecular squares[END_REF] Among them, it was interesting to compare the cyano-bridged tetramer [(C5H5)2Ti III (µ-CN)]4 that Schinnerling and Thewalt synthesized from [(Cp)2TiCl]2 and NaCN, [START_REF] Schinnerling | Die Struktur von [Cp2TiIII(CN)]14[END_REF] with the corresponding trimer mentioned above and which had been prepared through a redox reaction ( § 2.2.3). The crystal structure of the tetrametallic complex showed that the four metal atoms were not coplanar, and it adopted a butterfly-shaped conformation.

Pentanuclear complexes and beyond

Such examples of very large molecules with more than 4 titanium atoms were relatively scarce, and it became somehow a challenge to beat records in order to self-assemble the highest number of titanium atoms. [START_REF] Kraft | Directed reduction of six-membered nitrogen heterocycles -Selective formation of polynuclear titanium complexes[END_REF] This objective has been met either purposefully with specific ligands through well-known procedures [START_REF] Birkmann | Metallosupramolecular complexes derived from bis(benzene-o-dithiol) Ligands[END_REF], or by fortuitous discovery (88, 90) (Fig. 60). groups. [START_REF] Breunig | Polynuclear titanocene complexes with antimony ligands: [(Cp2Ti)2(SbR2)2(R = Et)[END_REF] The Ti atoms are located at the vertices of an irregular pentagon. It is striking that it was synthesized under conditions similar to those that had led to the trimer 66 (vide supra), the difference lying only in the nature of the R group.

By analogy with the outcome of the reaction of 1,4-di(tert-butyl)-1,3-butadiyne with [(C5H5)2Ti(btmsa)] (vide supra § 2.1.3), the three diynyl substituents of 1,3,5-tris(tertbutylbutadiynyl)benzene reacted with 6 equiv. [(C5H5)2Ti(btmsa)], leading to the elegant hexanuclear Ti(IV) architecture 89. [START_REF] Pellny | Novel Ti6, Zr3, and Zr6 complexes from branched polyynes and titanocene as well as zirconocene[END_REF] Its X-ray structure revealed that the zigzag Ti2 butadiynes were nearly planar as in 36d; their mean planes made various angles with the central benzene ring, ranging from 0° to 96°.

In the square compound 90, the eight [(C5H5)2Ti III ] fragments occupied two different environments; four of them were located at the vertices of the square, and the other four were on its edges, chelated by the in situ-generated nitrogen bridging ligand. [START_REF] Kraft | Directed reduction of six-membered nitrogen heterocycles -Selective formation of polynuclear titanium complexes[END_REF] It resulted from the reaction of [(C5H5)2Ti(btmsa)] with pyrimidine. As in the synthesis of 31a ( § 2.1.2), C-C couplings of the nitrogen-containing precursor happened during the supramolecular process leading to non-aromatic bis-heterocycles. The X-ray structure of 90, exhibiting a puckered spatial arrangement, was represented Fig. 61; two of the "edge" Ti atoms pointed towards the Another octanuclear self-assembled compound was recently reported as an anionic cubic Ti8L 4 12 architecture (91, Fig. 62) where H3L 4 was the commercial bis-chelating ligand precursor 2,6-dihydroxybenzoic acid. [START_REF] Zhu | Host-Guest and Photophysical Behavior of Ti8L12 Cube with Encapsulated [Ti(H2O)6] Species[END_REF] This species was obtained after heating a DMF/acetonitrile solution of H3L 4 and Ti(O i Pr)4. X-ray photoelectron spectroscopic analysis of 91 revealed that it comprised six Ti(IV) and two Ti(III) ions. Furthermore, modifying the synthesis solvent led to the stable neutral host-guest system [Ti(H2O)6)]@Ti8L 4 12, where all the titanium atoms were now at the +IV oxidation state; its X-ray diffraction analysis showed that the [Ti(H2O)6)] 4+ complex was encapsulated in the center of the cube. Encapsulation influenced its photophysical behavior, e.g. larger photocurrent and longer excited-state lifetime. In this section, we will detail only mixed-metal supramolecular architectures including at least 2 titanium atoms. However, we can cite beforehand several examples of heteronuclear complexes (mostly binuclear) with only one Ti atom, the other metal or fragment being : an alkali metal; [START_REF] Albrecht | Selecting different complexes from a dynamic combinatorial library of coordination compounds[END_REF][START_REF] Albrecht | Protonation of tris(iminocatecholato) complexes of gallium(III) and titanium(IV)[END_REF][START_REF] Burk | Selective inclusion of cesium ion in a cryptand-type Ti(IV) complex derived from a tripodal tris-2,3-dihydroxynaphthalene ligand[END_REF] Ni, Pd or Pt; [START_REF] Rosenthal | Heterobimetallic σ,π-Acetylide-Bridged Complexes from Disubstituted 1,3-Butadiynes[END_REF][START_REF] Pulst | Formation of the first heterobimetallic phosphido-bridged σ-π-acetylide complex [(C5H5)2Ti(μ-σ,η2-C≡CPh)(μ-PPh2)Ni(PPh3)] by cleavage of the central C-C single bond in PhC≡C-C≡CSiMe 3 and of the P-C bond in Ph3P, and coupling of the fragments to[END_REF][START_REF] Pulst | Nickel(0) Complexes of Five-Membered Titana-and Zirconacyclocumulenes as Intermediates in the Cleavage of C-C Bonds of Disubstituted Butadiynes[END_REF][START_REF] Mokuolu | Amidophosphine ligands as constructs for earlylate hetero-bimetallic complexes; the formation of an anisotropic metallocage[END_REF][START_REF] Mokuolu | Earlylate, mixed-metal compounds supported by amidophosphine ligands[END_REF][START_REF] Delgado | Thiolate derivatives of bis(diphenylphosphinocyclopentadienyl)titanium TiPt and TiPd heterobimetallic compounds[END_REF][START_REF] Sakata | Site-selective ligand exchange on a heteroleptic TiIV complex towards stepwise multicomponent self-assembly[END_REF] Al; [START_REF] Krüger | Tetraalkoxytitanium-Carbene Complexes with Double Intramolecular Et2AI Bridging[END_REF] Ga; [START_REF] Albrecht | Controlling the orientation of sequential ligands in the self-assembly of binuclear coordination compounds[END_REF] La; [START_REF] Albrecht | Self-assembly of heterodinuclear triplestranded helicates: Control by coordination number and charge[END_REF] Zr; [START_REF] Burlakov | The First Titanacyclic Five-Membered Cumulene. Synthesis, Structure, and Reactivity[END_REF] Mo; [START_REF] Leblanc | Lausanne -Printed in[END_REF][START_REF] Hahn | Heterobimetallic triple-stranded helicates with directional benzeneo-dithiol/catechol ligands[END_REF][START_REF] Graham | Diaryl-and Dialkylphosphino)alkyl)cyclopentadienyl Ligands and Their Use in the Preparation of Heterobinuclear Ti/Mo and Zr/Mo Complexes[END_REF][START_REF] Szymoniak | Synthesis of bent titanocene metalloligands with the (diphenylphosphino)tetramethylcyclopentadienyl moiety. X-ray structure of [(η5-C5Me4PPh2)2TiCl2]Mo(CO)4[END_REF] Fe; [START_REF] Oswald | Reaction of Pentafulvene Titanium and Zirconium Complexes with Phosphorus Ylides: Stoichiometric Reactions and Catalytic Intramolecular Proton Shuttles[END_REF] Rh; [START_REF] Fandos | Early-late heterobimetallic alkoxides as model systems for late-transition-metal catalysts supported on titania[END_REF] or various metal carbonyls. [START_REF] Beckhaus | Surprising Differences in the Cycloaddition Reactions of Cumulenes and Heterocumulenes to Vinylidenetitanocene: Formation of Four-or Five-Membered Titanacycles[END_REF][START_REF] Beckhaus | Fischer-Carbenkomplexe -gebildet aus einem Vinylidentitanocen-Fragment und Metallcarbonylen[END_REF][START_REF] Beckhaus | Zur Reaktivität isomerer Heterodinuclearer Fischer-Carben-Komplexe mit Titanaoxetan-und Titanaoxolenteilstrukturen -Cycloreversions-und Insertionsreaktionen[END_REF][START_REF] Beckhaus | Intermolecular Carbene -Carbene Coupling of a Nucleophilic Vinylidene and an Electrophilic 1-Methoxyethylidene Ligand[END_REF][START_REF] Spannenberg | Crystal structure of bis(η-3-trime-thyl-propinylcarboxylato)-bis(tricarbonylcobalt)-titan], C33H39CO2O8Ti[END_REF] Most of the compounds resulting from multicomponent self-assembly processes detailed hereafter were highly-symmetric early-late heterobimetallic (ELHB) complexes; in general terms, such species display many interesting geometries and properties. [START_REF] Leininger | Self-Assembly of Discrete Cyclic Nanostructures Mediated by Transition Metals[END_REF][START_REF] Stephan | Early-late heterobimetallics[END_REF][START_REF] Lang | Bis(alkynyl) Titanocenes as Organometallic Chelating Lignads for the Stabilization of Monomeric Organo Copper(I) Compounds[END_REF][START_REF] Gade | Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic Complexes[END_REF][START_REF] Gade | Transition metal complexes with polydentate amido ligands: Novel structural building blocks and chemical reagents[END_REF][START_REF] Wheatley | Structure and Reactivity of Early-Late Heterobimetallic Complexes[END_REF][START_REF] Cooper | Catalytic applications of early/late heterobimetallic complexes[END_REF][START_REF] Ferrer | Pyridinethiolate Titanocene Metalloligands and Their Self-Assembly Reactions to Yield Early-Late Metallamacrocycles[END_REF] In these ELHB complexes, the late metal usually had a d 8 electronic configuration in a squareplanar geometry, and was coordinated with one end of a bifunctional bridging ligand bearing a dative coordinating group. Conversely, the titanium was always linked to the bridging ligand by at least one strong covalent bond. In agreement with the HSAB theory, the hard Ti(IV) ion was preferentially bound to a hard donor atom (e.g. O, N...), whereas the soft late metal ion (e.g. Pd(II)) was rather coordinated by a soft donor group like a phosphine fragment or, to a lesser extent, like a pyridine fragment. Therefore, several bridging ligands presented an ambivalent hard/soft character, allowing a site-selective complexation. [START_REF] Albrecht | From Hierarchical Helicates to Functional Supramolecular Devices[END_REF][START_REF] Caulder | Supermolecules by design[END_REF] Some of the EHLB complexes could not be characterized by X-ray diffraction, but computational chemistry methods were helpful to get some insight of their stereochemical structure.

Heteronuclear complexes

These heteropolynuclear supramolecular architectures were most often obtained following a stepwise procedure, where Ti was first coordinated by the bridging ligand, yielding a metalloligand building block that was then allowed to react with the second metal. However, a few attempts (successful or not) of one-pot self-assembly syntheses are detailed below.

Ti2M2 tetranuclear complexes

One example of the hierarchical self-assembly leading to ELHB complexes is shown on In complexes 94 and 95a,b, the phosphine groups coordinated Rh(I) in a trans geometry.

The isolation of compound 94 by Love et al. was remarkable in comparison with the reaction of the titanadiphosphine ligand with a group 10 metal instead of rhodium; the latter led to a heterobinuclear complex instead of a heterotetranuclear complex. The authors attributed this difference to the trans configuration of the late metal and to p-stacking. The four coplanar metal atoms were arranged in a rhomboidal motif (close to a rhombus), as revealed by the crystal structures. A modification of the synthetic procedure, i.e. coordination of the late metal (here rhodium) in the first place, and then coordination of titanium, was unsuccessful.

High dilution and low temperature were necessary conditions to isolated successfully compounds 95a and 95b; for the latter, two isomers were distinguished in solution by NMR.

A methyltitanium monomer chelated by the pincer dipicolinato ligand was protonolyzed by a µ-hydroxo Rh I or Ir I starting material to afford cleanly 98a,b (Fig. 65). Noteworthy, the pincer ligand has become dissymmetric in the heterobimetallic complexes. The X-ray structure of 98a was resolved, showing that the coplanar metal atoms form a rhomboid rather close to a rectangle. Besides, two mesomeric forms accounted for its crystal data. underlining that the stepwise route is the only efficient way to this compound.

Ti2M3 pentanuclear complexes

On Fig. 67 palladium(II) or platinum(II). [START_REF] Albrecht | Catecholate-Based Helicates[END_REF] The bridging ligands were either phosphinylcatecholates (101a-d [START_REF] Sun | Exploiting incommensurate symmetry numbers: Rational design and assembly of M2M′3L6 supramolecular clusters with C3h symmetry[END_REF][START_REF] Sun | Rational design and assembly of M2M′3L6 supramolecular clusters with C3h symmetry by exploiting incommensurate symmetry numbers[END_REF]) or pyridylcatecholates (102[243] and 103a-d [START_REF] Nakagawa | Construction and structural analysis of monoand heterobimetallic bis(titanate) molecular cages[END_REF]). From the considerations developed by Raymond, Wong et al. concerning 101a-d, we have represented on Fig. 68 a general stepwise synthesis of Ti2M3 cages. In particular, it was demonstrated that the fac and mer isomers of their triphosphine titanoligand are very probably in fast equilibrium at room temperature. Noteworthy, an analogous scheme specifically related to Ti2Li3 cages (vide infra) has been represented in ref [START_REF] Albrecht | Hierarchically Assembled Titanium(IV) Helicates[END_REF]. and 101d were also generated through analogous experimental procedures, whereas the hexaiodo complex 101c was obtained by halide metathesis from 101b.

The authors underlined the prime importance of the choice of the countercation and of the solvent for clean reactions. Like Love et al. (vide supra, §2a), they tried a late-metal-first self-assembly procedure by coordinating palladium (II) to the phosphinylcatechol ligand, but this attempt failed also. Yet, they met success by running a spectacular one-pot synthesis of an analogue of 101b with NH4 + instead of Cs + cations, starting from eleven components. Single crystals of complex 101b were analyzed by X-ray diffraction, which revealed that the five metal atoms were spatially arranged in a trigonal bipyramidal geometry (Fig. 69). The cage compound 102 could be easily interconverted with the tetranuclear ring 99a under appropriate experimental conditions (either addition of Ti(IV) or Pd(II) starting material). The configuration of all the palladium fragments in the molecules displayed Fig. 67 was trans; this could be ascertained by NMR spectroscopy and/or crystallography (101a-d), or by far-IR spectroscopy [START_REF] Miao | Identification of Zr(IV)-based architectures generated from ligands incorporating the 2,2′-biphenolato unit[END_REF]. With regard to the configuration of titanium, the homochiral isomer 102 (D,D or L,L) was the most stable one according to molecular modeling, as for 99a;[243] by contrast, 101b was a mesocate (D,L) according to its crystal structure, and molecular modeling confirmed its stability on the basis of steric considerations. [START_REF] Sun | Rational design and assembly of M2M′3L6 supramolecular clusters with C3h symmetry by exploiting incommensurate symmetry numbers[END_REF] As mentioned § 2.1.1.1, Albrecht et al. had noticed the importance of interactions of binuclear titanium helicates built from biscatechol bridging ligands with alcali metal cations, especially Li + . Later on, they described Ti2Li3 hierarchically assembled helicates, where the two Ti and three Li atoms were arranged in a trigonal bipyramidal geometry, as for the Ti2Pd3 species 101b mentioned previously. Following a sequential pathway similar to the one depicted in Fig. 68, three catecholato ligands L substituted with carbonyl functions in position 3 of the aromatic ring were first converted into mononuclear titanium complexes Li2[TiL3] which were in equilibrium with the expected binuclear complexes Li[Ti2Li3L6]. We have represented in Fig. 70 the generic formula of the very high number of heterodimetallic Ti2Li3 complexes formulated 104. They were synthesized from an aldehyde (X = H) [START_REF] Albrecht | Hierarchical Assembly of Helicate-Type Dinuclear Titanium ( IV ) Complexes[END_REF], from about twenty ketones (X = R) [START_REF] Albrecht | Hierarchical Assembly of Helicate-Type Dinuclear Titanium ( IV ) Complexes[END_REF][START_REF] Kwamen | Solvent Dependence of the Monomer-Dimer Equilibrium of Ketone-Substituted Triscatecholate Titanium(IV) Complexes[END_REF] or from as many thioesters (X = SR) [START_REF] Kwamen | Catechol Thioesters: Ligands for Hierarchically Formed Lithium-Bridged Titanium(IV) Helicates and Helicate-Based Switches[END_REF] with R = alkyl, cycloalkyl, phenyl or benzyl. But the overwhelming majority of them (>100 complexes) was prepared from more than a hundred esters (X = OR with R = alkyl, cycloalkyl, alkenyl, alkynyl, benzyl-, ether-or amine-type, or analogues). [START_REF] Mevissen | Helicates with Ether-Substituted Catechol Esters as Ligands[END_REF][START_REF] Albrecht | Hierarchical Assembly of Helicate-Type Dinuclear Titanium ( IV ) Complexes[END_REF][START_REF] Kwamen | Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution[END_REF][START_REF] Schlottmann | Stability of Hierarchically Formed Titanium(IV) Tris(catecholate ester) Helicates with Cyclohexyl Substituents in DMSO[END_REF][START_REF] Van Craen | Hierarchically assembled helicates as reaction platform -from stoichiometric Diels-Alder reactions to enamine catalysis[END_REF][START_REF] Albrecht | Hierarchical self-assembly of metallo-dendrimers[END_REF][START_REF] Albrecht | Induced fit" in chiral recognition: Epimerization upon dimerization in the hierarchical self-assembly of helicate-type titanium(IV) complexes[END_REF][START_REF] Albrecht | Stereocontrol in dinuclear triple lithium-bridged titanium(IV) complexes: Solving some stereochemical mysteries[END_REF][START_REF] Albrecht | Terpenols as substituents for the diastereoselective formation of enantiomerically pure triple lithium-bridged helicate type-coordination compounds[END_REF][START_REF] Van Craen | Expanding the Size of Catecholesters -Modified Ligands for the Hierarchical Assembly of Dinuclear Titanium(IV) Helicates[END_REF][START_REF] Van Craen | A Supramolecular Chiral Auxiliary Approach: "remote Control" of Stereochemistry at a Hierarchically Assembled Dimeric Helicate[END_REF][START_REF] Van Craen | Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions[END_REF][START_REF] Van Craen | Kinetic investigation of the dissociation of dinuclear hierarchically assembled titanium(iv) helicates[END_REF] The reader will find again in the recent reviews of Albrecht et al. [START_REF] Albrecht | Supramolecular chemistry -General principles and selected examples from anion recognition and metallosupramolecular chemistry[END_REF][START_REF] Albrecht | Symmetry driven self-assembly of metallo-supramolecular architectures[END_REF][START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Albrecht | From Hierarchical Helicates to Functional Supramolecular Devices[END_REF][START_REF] Albrecht | Hierarchically Assembled Titanium(IV) Helicates[END_REF][START_REF] Giese | Alkyl-Alkyl Interactions in the Periphery of Supramolecular Entities: From the Evaluation of Weak Forces to Applications[END_REF] many details about such hierarchical helicates obtained by two successive recognition steps; we will summarize these considerations hereafter. These mainly comprised their stepwise syntheses and analyses in solution (NMR), in the gas phase (ESI-MS, revealing inter alia a high stability of the dimers in the gas phase), or in the solid state (crystallography); the factors that governed the monomer/dimer equilibrium; and stereochemical considerations.

Noteworthy, the X-ray structures of the helicates revealed twisted cylinders with grooves formed by the aromatic platforms of the ligands. Hierarchical helicates that possess dendrimer structures have also been described. [START_REF] Albrecht | Hierarchically Assembled Titanium(IV) Helicates[END_REF] The substituent X (Fig. 70) was of paramount importance with regard to the dimerization constant (Kdim). [START_REF] Albrecht | Supramolecular chemistry -General principles and selected examples from anion recognition and metallosupramolecular chemistry[END_REF][START_REF] Albrecht | Symmetry driven self-assembly of metallo-supramolecular architectures[END_REF][START_REF] Mevissen | Helicates with Ether-Substituted Catechol Esters as Ligands[END_REF][START_REF] Kwamen | Catechol Thioesters: Ligands for Hierarchically Formed Lithium-Bridged Titanium(IV) Helicates and Helicate-Based Switches[END_REF][START_REF] Kwamen | Solvent Dependence of the Monomer-Dimer Equilibrium of Ketone-Substituted Triscatecholate Titanium(IV) Complexes[END_REF] The thermodynamics of the dimerization equilibrium leading to 104 complexes has been evidenced by variable-temperature NMR. The forces. [START_REF] Kwamen | Solvent Dependence of the Monomer-Dimer Equilibrium of Ketone-Substituted Triscatecholate Titanium(IV) Complexes[END_REF][START_REF] Van Craen | Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions[END_REF][START_REF] Kwamen | Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution[END_REF][START_REF] Schlottmann | Stability of Hierarchically Formed Titanium(IV) Tris(catecholate ester) Helicates with Cyclohexyl Substituents in DMSO[END_REF][START_REF] Giese | Alkyl-Alkyl Interactions in the Periphery of Supramolecular Entities: From the Evaluation of Weak Forces to Applications[END_REF] Additionnally, it was established that electron-donating substituents of the aromatic core of the catechol ligand shifted the equilibrium towards the dimeric 104; conversely, electron-withdrawing substituents favored the monomer. [START_REF] Van Craen | Expanding the Size of Catecholesters -Modified Ligands for the Hierarchical Assembly of Dinuclear Titanium(IV) Helicates[END_REF] The solvent also strongly influenced the value of Kdim; typically, the [Ti2Li3L6] -anion was favored in methanol, whereas DMSO which captures easily Li + led preferentially to the [TiL3] 2- dianion.

With regard to stereochemistry, [START_REF] Albrecht | From Hierarchical Helicates to Functional Supramolecular Devices[END_REF] it was shown that the dimers were homochiral, illustrating thus a chiral recognition process; by contrast with the monomers, the stereochemistry of the Ti2Li3 helicates was locked. Using chiral enantiopure auxiliaries (chiral ammonium countercations or catechol ligands with chiral substituents) allowed some stereocontrol that was revealed through circular dichroism analysis. The case of the ester where R = (S)-CHMePh was a clear-cut one. Diastereomeric excesses were very high, but the configuration of the metal center was opposite for the monomer (L, in DMSO) or for the dimer (D,D, in methanol), due to a change of conformation of the CHMePh fragment. [START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Albrecht | Hierarchically Assembled Titanium(IV) Helicates[END_REF][START_REF] Albrecht | Induced fit" in chiral recognition: Epimerization upon dimerization in the hierarchical self-assembly of helicate-type titanium(IV) complexes[END_REF] Interestingly, an opposite stereocontrol mechanism favoring the dimer in both solvents was observed using an ester with R = (S)-CHMe(2-bromophenyl), due to the bulkiness of the bromo group. [START_REF] Albrecht | Stereocontrol in dinuclear triple lithium-bridged titanium(IV) complexes: Solving some stereochemical mysteries[END_REF] Some other kinds of enantiopure esters, including those with terpenol substituents, rather led to mixtures of monomers and dimers, and/or lower diastereomeric excesses. [START_REF] Albrecht | Induced fit" in chiral recognition: Epimerization upon dimerization in the hierarchical self-assembly of helicate-type titanium(IV) complexes[END_REF][START_REF] Albrecht | Stereocontrol in dinuclear triple lithium-bridged titanium(IV) complexes: Solving some stereochemical mysteries[END_REF][START_REF] Albrecht | Terpenols as substituents for the diastereoselective formation of enantiomerically pure triple lithium-bridged helicate type-coordination compounds[END_REF] Such enantiopure esters, together with achiral esters, were applied to the preparation of heteroleptic titanium complexes, that were applied afterwards to chiral induction in stoichiometric Diels-Alder and catalytic nitro-Michael reactions. [START_REF] Albrecht | Catecholate-Based Helicates[END_REF][START_REF] Van Craen | A Supramolecular Chiral Auxiliary Approach: "remote Control" of Stereochemistry at a Hierarchically Assembled Dimeric Helicate[END_REF][START_REF] Van Craen | Hierarchically assembled helicates as reaction platform -from stoichiometric Diels-Alder reactions to enamine catalysis[END_REF] In addition, kinetic investigations on the dissociation of compounds 104 (X= OR) allowed to determine activation energies, that were correlated with the size of R; bulky ester substituents led to high energy barriers.

[257]

Miscellaneous

The substitution of the acetylacetonato ligand of 99a by a bistropolonato bridging ligand, with a tether shorter than the one used for the synthesis of 100 (vide supra), led to the heterooctanuclear Ti4Pd4 species 105 (Fig. 71). [START_REF] Sakata | Site-selective ligand exchange on a heteroleptic TiIV complex towards stepwise multicomponent self-assembly[END_REF] The size difference between 100 and 105 was evidenced by DOSY measurements of a mixture of both of them. Molecular modeling of 100 revealed that the most plausible isomer has a (D,D,L,L) configuration. Like 100, 105

could not be obtained cleanly through a one-pot synthetic procedure.

The linear trinuclear Ti2Zn helicate 106 was synthesized from a bis-catecholato ligand with a central bipyridine moiety (Fig. 71). [START_REF] Albrecht | 5,5′-diamino-2,2′-bipyridine: A versatile building block for the synthesis of bipyridine/catechol ligands that form homo-and heteronuclear helicates[END_REF] Among the molecules reported in § 2, it was a rare example of heterobimetallic complex that was exclusively obtained by one-pot synthesis.

In addition, the complex 72a mentioned above ( § 2.3.1.2) was converted into Ti4Mn2, [START_REF] He | Coordination Assembly of the Water-Soluble Ti 4 (embonate) 6 Cages with Mn 2+ Ions[END_REF] Ti4Agn (n = 7, 9 or 12), [START_REF] Chen | Self-Assembly of a Ti4(embonate)6Cage toward Silver[END_REF] Ti4Co3 and Ti4Ln2 [START_REF] He | Water-Soluble and Ultrastable Ti4L6 Tetrahedron with Coordination Assembly Function[END_REF] heterobimetallic cages upon reaction with transition metal or lanthanide salts. The third-order non-linear optical properties of the titanium-silver compounds were investigated.

Conclusion and perspectives

In this review article, we have described the structural aspects of the large family of non-oxo-cluster discrete titanium-based multinuclear architectures. This systematic highlight has permitted to underline the different synthetic strategies allowing the formation of organometallic or coordination multinuclear Ti-compounds. Mainly for metallosupramolecular complexes, the aggregates are obtained by reacting a pro-ligand with a titanium source as titanium alkoxides, TiCl4, Ti(NMe2)4 or TiO(acac)2. For organometallic assemblies, Ti-cyclopentadienyl derivatives are the most common precursors to afford multinuclear architectures. Contrary to the preparation of heteromultinuclear species that generally required step-by-step approaches, the synthesis of complexes containing exclusively titanium was carried out through one-pot procedures. These procedures often required an inert atmosphere, the reagents or products being usually air-and/or moisture-sensitive. Overall, the synthetic pathway adopted to construct Ti species is linked with the moderate stability usually displayed by most of them, which made the isolation of synthetic intermediates challenging.

Furthermore, this instability is incompatible with further classical synthetic transformations or purification steps. Thus, the post-synthetic modification of titanium multinuclear species represents obviously a challenging task in the field of titanium multinuclear architectures to increase the complexity and diversity of molecular titanium-based multinuclear systems.

Nevertheless, this required the development beforehand of robust titanium precursors. [START_REF] Scarpi-Luttenauer | Bent 1,10-phenanthroline ligands within octahedral complexes constructed around a TiO4N2core[END_REF] Also, in the course of this review, we have pointed out that organometallic architectures incorporated titanium centers having oxidation states ranging from +IV to +II, whereas for non-organometallic titanium complexes, the oxidation state of the metal is almost always +IV.

It should be mentioned that species containing low-oxidation-state titanium centers are usually highly air and moisture sensitive, implying that their synthesis and storage involve specific conditions. However, the presence of Ti(III) or Ti(II) centers in these architectures confers unexpected structural features.

From the analysis of the Ti architectures, the number of Ti atoms found in these complexes varied from two to eight. Thus, two categories could be distinguished for these species: the binuclear and trinuclear complexes, and the aggregates with higher nuclearities.

For low nuclearity Ti-complexes, their structures are rather predictable. It is particularly true for catecholato-based helicates for which their development represents a success story. Apart from rare cases, the precise structure of a final compound that incorporates more than three metallic centers is difficult to anticipate when the ligand(s) reacted with a titanium source, the construction of such multinuclear architectures being partially governed by serendipity.

Sometimes, the initial purpose was clearly not related to supramolecular chemistry (e.g. organometallic low-valent Ti chemistry, [START_REF] Rosenthal | Titanocene and zirconocene σalkynyl complexes in C-C single bond coupling and cleavage reactions[END_REF][START_REF] Beckhaus | Pentafulvene complexes of group four metals: Versatile organometallic building blocks[END_REF] or precursors of iminosilicate materials [START_REF] Murugavel | Organosilanetriols: Model compounds and potential precursors for metal-containing silicate assemblies[END_REF]); but even in these cases, self-assembly could be considered as a goal. [START_REF] Kraft | Directed reduction of six-membered nitrogen heterocycles -Selective formation of polynuclear titanium complexes[END_REF] We counted only eight metallic atoms for the complexes reported in this review incorporating the highest number of Ti-atoms. This marks an actual limitation concerning the possibility to access high nuclearity titanium assemblies. By comparison, the Ti oxo-cluster with the highest nuclearity reported until now contains 52 atoms. [START_REF] Fang | A 3.6 nm Ti52-Oxo Nanocluster with Precise Atomic Structure[END_REF] Also, self-assembled structures built from late transition metals are frequently constructed with an impressive number of metallic connexions. The Pd(II)-nano-spheres reported by Fujita et al. are representative examples of these high nuclearity self-assembled structures. [START_REF] Fujita | Self-Assembly of M30L60 Icosidodecahedron[END_REF] Undeniably, the introduction of higher degrees of rationality to construct these Ti-compounds may offer the possibility to form architectures with much more titanium atoms.

Multinuclear Ti architectures are characterized by their high symmetry. From the most frequent structures compiled throughout this review (Ti3 and Ti4), schematic representations of the connections of the ligands between the metallic corners found within complexes are retrieved in Figure 72. This overall picture emphasizes the polyhedra or polygons adopted by these complex. Again, by comparison with the numerous structural types of oxo-clusters as spherical, cyclic, prismatic or rectangular, the structural diversity for non-oxo-clusters Tiassemblies is less pronounced. Indeed, this constitutes a motivation and a real synthetic challenge to access Ti-architectures possessing unprecedented frameworks. As presented in this article, the early structures were synthetized for applications mainly in the fields of dendrimers, material sciences, catalysis, photocatalysis, photophysics or for the comprehension of self-assembly processes. Additionally, many stereochemical investigations (asymmetric induction, recognition of chiral drugs, molecular reaction platforms) relied on di, tri, or tetranuclear Ti species were performed. Nowadays, dynamic molecular Ti systems have emerged and represent an active domain for potential major breakthroughs.

So, herein, we took stock of the different types of multinuclear Ti architectures and we highlighted the state of the art and perspectives associated with this family of molecules.

Despite, the large variety of Ti assemblies reported, there are still some synthetic challenges and opportunities to reach unique properties or functions lying ahead.

Nota Bene: During the reviewing process of this manuscript, we became aware of very recent articles depicting interesting applications of novel Ti multinuclear species (non-linear optical properties, precursors of dye-containing titania composites, permanent porosity), [START_REF] Chen | Synthesis and Third-Order Nonlinear Optical Properties of Metal-Organic Zeolites Built from Ti 4 (embonate) 6 Tetrahedra[END_REF][START_REF] Fang | Titanium compounds containing naturally occurring dye molecules[END_REF][START_REF] Lerma-Berlanga | Permanent Porosity in Hydroxamate Titanium-Organic Polyhedra[END_REF] which highlight an increasing interest of the chemistry research community towards this field.

Fig. 1 .

 1 Fig. 1. Ball-and-stick representation of the solid-state structures obtained from single crystal X-ray crystallography of several complexes illustrating the diverse oxygenated coordination spheres surrounding a titanium center. The carbon atoms are represented in dark grey, oxygen atoms are in red, all titanium atoms are in green (except in d, where they are represented in green and light grey) and hydrogen atoms are omitted for clarity. a) 4-fold coordinated Ti-atom in Ti(OC6H3Me2-2,6)4[41] b) 5-fold coordinated Ti-atom in [Ti(OC6H3-2,6-i Pr2)(p-tert-butyl calix[4]arene monomethyl ether)][43] c) 6-fold coordinated Ti-atom in [Ti(O-4-C6H4Me)2(p-tert-butylcalix[4]arene dimethyl ether)][40] d) 7-fold coordinated Ti-atom in Ti5(OPr i )10(OC2H4O)5[44] (for this structure the 7-fold coordinated titanium atom is in green and the four other Ti atoms are in light grey).

Fig. 2 .

 2 Fig. 2. Selected binuclear biscatecholato triple-stranded helicates.

1 , 5 :

 15 M = Li, Na 1b: n = 2, M = Li, Na, K 2a: n = 1, M = Li, Na, K 2b: n = 2, M = Li, Na, K M = Li, Na, K

Fig. 3 .

 3 Fig. 3. Binuclear triple-stranded helicates based on biscatechol and benzenedithiol proligands incorporating an enantiopure cyclohexyl bridge in the spacer.

  = O, M = Li, Na; spacer = 6b: X = S, M = Ph 4 As; spacer

Fig. 4 .

 4 Fig. 4. Binuclear double-stranded helicate with a spacer incorporating a phenylalanine fragment (M = Li, Na, K).

Fig. 5 .

 5 Fig. 5. a) Triple-stranded binuclear helicates composed of bis(benzenedithiolato) ligands or of mixed ligand strands incorporating a catechol and a benzenedithiol fragment (PNP = bis(triphenylphosphoranylidene)ammonium). b) Schematic representation of two of the possible isomers for Na(PNP)3[Ti2(9)3]. The bulky methyl substituents (green spheres) of the -CH2CMe2-spacers are oriented towards the larger [TiS6] octahedron (yellow) rather than the [TiO6] octahedron (red).

M 3 M' 8 : 9 :

 89 X = S, M = M' = Ph 4 As, Et 4 N, Bu 4 N; spacer = X = O, M = PNP, M' = Na; spacer =

  [START_REF] Hupka | A banana-shaped dinuclear complex with a tris(benzene-o-dithiolato) ligand[END_REF]

Fig. 6 .

 6 Fig. 6. Binuclear "banana-shaped" complex 10 (Me3PhN)4[Ti2(ligand)2]. Each Ti(IV) is chelated by two different strands (black lines).

4 Fig. 8 .

 48 Fig. 8. Binuclear titanium complexes with terphenolato ligands.

Fig. 10 .

 10 Fig. 10. Molecular structure of 16. H atoms are omitted for clarity. Ti atoms are in green, O atoms in red. L strands are in two colors (turquoise and purple) for clarity. The carbon atoms of the monodentate ligands are in grey.

  2:1) measured by 1 H NMR when the crystals of [Ti2L 1 2((1S,2S)-dpeda)2] or [Ti2L 1 2((1R,2R)-dpeda)2] were dissolved in acetone.

Fig. 11 .

 11 Fig. 11. Molecular structures of two double-stranded helicates incorporating diamino bidentate chelates. a) Helicate 18 bearing the (1S,2S)-dpeda ligand; b) Helicate 19 bearing the opda ligand. H atoms are omitted for clarity. Ti atoms are in green, O atoms in red, N atoms in blue. L 1 strands are in two colors (turquoise and purple) for clarity. The carbon atoms of the bidentate nitrogen ligands are in grey.

Fig. 12 .

 12 Fig. 12. Molecular structure of helicate 20. Ti atoms are in green, O atoms in red, N atoms in blue. L strands are in two colors (turquoise and purple) for clarity. The carbon atoms of the py ligands are in grey.

Fig. 14 .

 14 Fig. 14. Titanasiloxanic dimers.

Fig. 15 .

 15 Fig. 15. Molecular structures of complexes a) 23b; b) 23d. H atoms are omitted for clarity. Ti atoms are in green, O atoms in red, Si atoms in off-white, C atoms in grey, Cl or F atoms in light green.

X

  24a: R = t Bu, Cp' = C 5 H 5 , X = Cl 24b: R = t Bu, Cp' = C 5 H 4 Me, X = Cl 24c: R = t Bu, Cp' = C 5 Me 5 , X = F 24d: R = t Bu, Cp' = C 5 Me 4 Et, X = F 24e: R = t Bu, Cp' = C 5 H 4 Me, X = F 24f: R = Ph, Cp' = C 5 Me 5 , X = Cl Cp' = OSi(O t Bu) 3, X = Y = O i Pr 23b: R = t Bu, X = Y = Cl 23c: R = t Bu, X = Y = Br 23d: R = t Bu, X = Y = I 23e: R = t Bu, X = Y = NEt 2 23f: R = t Bu, X = Cl, Y = N(SiMe 3 ) 2 b) a)

5 Fig. 17 . 28 Fig. 18 .

 5172818 Fig. 17. Binuclear tetraaminodisilazane Ti(IV) complex.

(C 5 Fig. 19 .

 519 Fig. 19. Titanium helical (33, 34) or non-helical (35) complexes built from chelating imine ligands.

Fig. 20 .

 20 Fig. 20. Molecular structure of complex 34. H atoms are omitted for clarity. Ti atoms are in green, O atoms in red, N atoms in blue, C atoms in grey, Cl or F atoms in light green. The chelating ligand strands are in two colors (turquoise and purple) for clarity. The carbon atoms of the isopropoxide ligands are in grey.

Fig. 21 .

 21 Fig. 21. Zigzag butadiyne Ti(IV) complexes.

Fig. 22 .

 22 Fig. 22. Bis-acetylide Ti(III) complexes.

Fig. 23 .

 23 Fig. 23. Seminal syntheses of Ti zigzag complexes.

Fig. 24 .

 24 Fig. 24. Molecular structures of complexes a) 36a; b) 36b. H atoms are omitted for clarity. Ti atoms are in green, Si atoms in off-white, C atoms in grey.

Fig. 25 .

 25 Fig. 25. Synthesis of binuclear Ti complexes with bridging cyclopentadienyl ligands.

Fig. 26 .

 26 Fig. 26. Zigzag butadiyne Ti(IV) complexes.

Fig. 27 .

 27 Fig. 27. Molecular structure of complex 43. H atoms are omitted for clarity. Ti atoms are in green, N atoms in blue, C atoms in grey, Cl atoms in light green.

43 Fig. 28 .Fig. 29 .

 432829 Fig. 28. Annellated binuclear Ti complexes.

Fig. 30 .

 30 Fig. 30. Ti(III) and Ti(IV) complexes with short bridges.

  triangle and were bridged by only two L 3 ligands; the coordination sphere was completed by two µ-oxo ligands. Noteworthy, an oxygen atom of one of the catechol moieties bridged two metal atoms. As underlined by Albrecht et al. in reviews,[START_REF] Albrecht | Symmetry driven self-assembly of metallo-supramolecular architectures[END_REF][START_REF] Albrecht | Catechol imine ligands: From helicates to supramolecular tetrahedra[END_REF] two of the large potassium cations were bound to the nitrogen atoms of L 3 , allowing the ligand to accomodate the Ti3O2 fragment.

Fig. 32 .

 32 Fig.32. a) Formula of complex 56. b) Formula of pro-ligand L 3 H6 used to synthesize complex K2[K2Ì{Ti3O2L 3 2}] (57). c) Simplified partial formula of complex 57 drawn from its crystal structure; the catecholato fragments of one ligand L 3 are represented in blue, and those of the other ligand L 3 , in red.

Fig. 33 .

 33 Fig. 33. a) Formula and b) molecular structure of complex 58. H atoms are omitted for clarity. Ti atoms are in green, and the three strands respectively in purple, light blue and grey.

Fig. 34 .

 34 Fig. 34. Molecular structure of complex 59. Ti atoms are in green, O atoms in red. L strands are in three colors (turquoise, grey and purple) for clarity. The carbon atoms of HO i Pr ligands are in grey.

Fig. 35 .

 35 Fig. 35. Schematic representation of the conversion of complex 59 into complex 16 in solution and of the release of isopropanol molecules (the chirality is arbitrary).

Fig. 36 .

 36 Fig. 36. Molecular structure of compound 61. Ti atoms are in green, O atoms in red, N atoms in blue. L strands are in three colors (turquoise, grey and purple) for clarity. The carbon atoms of the bpym ligands are in salmon.

  are represented the main coordination possibilities met among trinuclear triangular Ti complexes addressed in this Section.[175] In form A, three bridging ligands remained unmodified during the chemical self-assembly process and constituted the edges of the triangle; it has been already encountered in § 2.2.1. Conversely, form B comprised one central bridging ligand usually generated in situ as a result of C-C couplings, that coordinated the three metal atoms. In both cases, the metal centers occupied the three vertices of the triangles. Various examples of both types of complexes have been reported in the literature, particularly by the group of Beckhaus.

Fig. 37 .

 37 Fig. 37. Coordination modes in triangular trinuclear Ti complexes, after ref [175]: a) A type (Ti3L3); b) B type (Ti3L).

Fig. 38 .

 38 Fig. 38. a) Ti(III) trinuclear complexes with imidazolato bridging ligands. b) Molecular structure of complex 62a. H atoms are omitted for clarity. Ti atoms are in green, N atoms in mauve, C atoms in grey.

Fig. 39 .

 39 Fig. 39. Synthesis of Ti3L-type complex 63.

  C 5 H 5 ) 2 Ti(btmsa)] infra); the reason for the selectivity of the reaction towards one way or another remained unclear.

Fig. 41 .

 41 Fig. 41. Self-assembled titanium trimers.

Fig. 42 .

 42 Fig. 42. Molecular structure of complex 65b. H atoms are omitted for clarity. Ti atoms are in green, N atoms in mauve, C atoms in grey.

  appeared as a triangle with Ti vertices and chelidamato edges; the crystal data suggested a substantial contribution of a keto mesomeric form of the chelidamato ligand in the complex.

Fig. 43 .

 43 Fig. 43. Trimerization of a Ti(IV) chelidamato complex.

Fig. 44 .

 44 Fig. 44. Cyano-bridged trinuclear titanocene complex.

  for complexes 71a,b[185]) catechol fragments was published from 1998 to

Fig. 45 .

 45 Fig. 45. Tetrahedral Ti4L6 complexes derived from biscatechol ligands.

Fig. 46 .

 46 Fig. 46. Tetrahedral Ti4L4 complexes derived from C3-symmetric triscatechol ligands, where the catechol units were bound to the triangular backbone through their 4-position.

  triggering the complete decomposition by visible light of acid blue 93 within 10 min. Its optical resolution as enantiopure (D,D,D,D) and (L,L,L,L) isomers was carried out; the separated homochiral cages presented enantioselective recognition toward chiral organic molecules.[START_REF] He | Optical Resolution of the Water-Soluble Ti4(embonate)6 Cages for Enantioselective Recognition of Chiral Drugs[END_REF]

Fig. 47 .

 47 Fig. 47. Tetrahedral Ti4L6 complexes derived from embonic acid.

Fig. 48 .

 48 Fig. 48. Substituted 2,2'-bipyrimidine ligand used to generate the tetranuclear Ti complex 73.

Fig. 49 .

 49 Fig. 49. Molecular structure of complex 73. Ti atoms are in green, O atoms in red, N atoms in blue. L 1 strands are in different colors (turquoise, grey and purple) for clarity. The carbon atoms of the bAnt ligands are in salmon.

bAnt

  Various stable tetranuclear titanasiloxanes have been reported by Roesky et al., within the scope of the metallasiloxane research field (cf § 2.1.1.

Fig. 50 .

 50 Fig. 50. Tetranuclear titanasiloxanes.

R 75 µ 3 -

 753 76a: R = t Bu, R' = C 5 Me 5 76b: R = (2,4,6-Me 3 C 6 H 2 )N(SiMe 3 ), R' = OEt 76c: R = (2,6-Me 2 C 6 H 3 )N(SiMe 3 ), R' = OEt 76d: R = (2,6-i Pr 2 C 6 H 3 )N(SiMe 3 ), R' = OEt 76e: R = (2,4,6-Me 3 C 6 H 2 )N(SiMe 3 ), R' = O i Pr 76f: R = (2,6-Me 2 C 6 H 3 )N(SiMe 3 ), R' = O i Pr 76g: R = (2,6-i Pr 2 C 6 H 3 )N(SiMe 3 ), R' = O i L ligands on the faces of the tetrahedra are silanetriolato ligands instead of the chelating triscatecholato ligands.

Fig. 51 .

 51 Fig. 51. Carbonato-bridged Ti tetranuclear complex.

78 ( 2 Fig. 52 .

 78252 Fig. 52. Coordination modes in tetranuclear Ti complexes bearing nitrogen ligands: a) A type (Ti4L4); b) B type (Ti4L).

Complexes 79 - 83 (

 7983 Figs. 52-53) belong to the first category (A), with the generic formula Ti4L4. Beckhaus et al. have reported the reaction of [(C5H5)2Ti(btmsa)] with relatively small 5-membered nitrogen heterocycles, viz. imidazoles or benzimidazoles, yielding complexes 79a,b and 80a,b (Fig.

Fig. 53 .

 53 Fig. 53. Imidazolato-and benzimidazolato-bridged Ti tetranuclear complexes.

Fig. 54 .

 54 Fig. 54. Ti tetranuclear complexes bridged by 6-membered heterocycles.

Fig. 55 .

 55 Fig. 55. Azobispyridine-bridged Ti tetranuclear complexes.

Fig. 56 .

 56 Fig. 56. Tetrameric 1-titana-2,5-diazacyclopenta-2,4-dienes.

Fig. 57 .

 57 Fig. 57. Molecular structure of complex 85b. H atoms are omitted for clarity. Ti atoms are in green, N atoms in blue, C atoms in grey.

Fig. 58 .

 58 Fig. 58. Tetranuclear Ti complexes obtained from substituted octatetrayne.

Fig. 59 .

 59 Fig. 59. Molecular structure of complex 87. H atoms are omitted for clarity. Ti atoms are in green, Si atoms in off-white, C atoms in grey, Cl or F atoms in light green.

Fig. 60 .

 60 Fig. 60. Penta-, hexa-and octanuclear Ti architectures.

  ring, whereas the two others pointed outwards. The H atoms along the newly formed C-C bonds were in a trans position.

Fig. 61 .

 61 Fig. 61. Molecular structure of complex 90. H atoms are omitted for clarity. Ti atoms are in green, N atoms in mauve, C atoms in grey.

Fig. 62 .

 62 Fig. 62. Octanuclear Ti cubic architecture.

Fig. 63 ,

 63 Fig. 63, where the metalladiphosphine 92 obtained from titanocene dichloride [(C5H5)2TiCl2] displaced the labile acetonitrile ligands of a cyclooctadiene Ir I complex, affording the d 0 -d 8 metallamacrocycle 93.[239] A preference for the bridging vs. chelating coordination mode of the hydroxyphosphine was observed by the authors.

Fig. 63 .

 63 Fig. 63. Stepwise synthesis of complex 93.

Fig. 64 .

 64 Fig. 64. Tetranuclear Ti2M2 architectures.

95a:

  [M] = Rh(CO)Cl, n = 1 95b: [M] = Rh(CO)Cl, n = 2 95c: [M] = PdCl 2,

Fig. 65 .

 65 Fig. 65. Synthesis of Ti2Rh2 and Ti2Ir2 complexes 98a,b.

Complexes 95c,[ 240 ]

 240 96a,b,[242] 97a-d,[238] 99a,b and 100[217,243] were Ti2M2 ring-shaped heterobimetallic complexes where M belonged to group 10 of the periodic table,

  viz. Pd and Pt (Figs.64 and 66).

Fig. 66 .

 66 Fig. 66. Tetranuclear Ti2Pd2 architectures.

  are depicted various heterobimetallic cages where two monomeric titanium(IV) fragments were tethered by three metal fragments, the metal being usually

Fig. 67 .

 67 Fig. 67. Pentanuclear Ti2M3 cages.

Fig. 68 .

 68 Fig. 68.Stepwise synthesis of heterobimetallic pentanuclear complexes.

Fig. 69 .

 69 Fig. 69. Molecular structure of complex 101b. H atoms are omitted for clarity. Ti atoms are in green, Pd atoms in pink, O atoms in red, C atoms in grey, P atoms in orange and Br atoms in brown.

Fig. 70 .

 70 Fig. 70. Ti2Li3 hierarchically assembled helicates.

  influence of the nature and size of the R group on Kdim came under close scrutiny; in chains aggregated and stabilized the dimer through London dispersion

Fig. 71 .

 71 Fig. 71. Ti4Pd4 and Ti2Zn heterobimetallic complexes.

Fig. 72 .

 72 Fig.[START_REF] Albrecht | From molecular diversity to template-directed self-assembly -New trends in metallosupramolecular chemistry[END_REF]. Schematic representations of the most frequent Ti3 and Ti4 architectures mentioned herein, related to the category of bridging ligand (without any symmetry consideration). The term "squares" should be taken in a broad sense, i.e. including structures where the Ti atoms may form parallelograms or may not be coplanar.

Finally, this review

  has detailed the main motivations driving the construction of Tiassemblies. The properties of the multinuclear architectures were related to their structures (e.g. encapsulation of guests and molecular recognition, molecular springs...) or to the valence of Ti atoms (redox chemistry, intervalence charge transfer...). = µ 2 -ligand L = µ 3 -ligand L = µ 4 -ligand L

a) b)

Acknowlegments

We thank Dr. Stéphane Baudron and Dr. Midhun Mohan for many sensible advice. We are also indebted to the reviewers for valuable suggestions. Financial support of the University of Strasbourg and of the CNRS is gratefully acknowledged. M. Scarpi-Luttenauer thanks the French Ministry of Research for his PhD fellowship.