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TUTORIAL REVIEW

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A building block strategy to access sulfur-functionalized polyoxometalate based systems using {Mo2S2O2} and {Mo3S4} as constitutional units, linkers or templatesw Emmanuel Cadot,*a Maxim N. Sokolov,b Vladimir P. Fedin,b Corine Simonnet-Je´gat,a Se´bastien Floqueta and Francis Sećheressea Received 17th April 2012 DOI: 10.1039/c2cs35145e The present tutorial review reports on the synthetic approaches for the formation of ‘‘polyoxothiometalate’’ compounds with special emphasis on the unique reactivity of the preformed sulfur-containing cationic building blocks {Mo2O2S2}2+ and {Mo3S4}4+ toward polyoxometalate building blocks. Such simple chemical systems, based on chemical and structural complementarities between ionic reactive moieties have led to the synthesis of a series of relevant clusters with unrivalled large nuclearity structural arrangements, such as loops, triangles, squares and boxes. Specific reaction parameters and considerations will be pointed out showing that a deliberate pure inorganic supramolecular chemistry based on weak interactions, flexibility and dynamic is possible with polyoxometalates.

1. Introduction a

Institut Lavoisier de Versailles, UMR 8180, Universite´ de Versailles Saint Quentin, 45 Avenue des Etats-Unis, 78035 Versailles, France. E-mail: [email protected] b Nikolayev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Prospect Lavrentieva 3, Novosibirsk 630090, Russia w Part of a themed issue covering the latest developments in polyoxometalate science.

Emmanuel Cadot obtained a PhD in the area of catalysis with polyoxometalates under the supervision of Prof. Gilbert Herve´ at the University Pierre and Marie Curie (Paris VI) in 1991. Then, he moved to IRCELYON (CNRS) to take up a postdoctoral fellowship with Dr Michel Lacroix. In 1992, he joined the group of Prof. Francis Sećheresse as assistant professor at the University of Versailles. In 2003, he Emmanuel Cadot was promoted full Professor and directed his own group of research at the University of Versailles. His current research interests include synthetic inorganic chemistry, inorganic supramolecular chemistry, polyoxometalate and thiometalate chemistry.

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The Royal Society of Chemistry 2012

Polyoxometalate (POM) chemistry remains one of the most varied growing areas in inorganic chemistry today, and the enduring interest in the domain is indisputably highlighted by the large number of new POM species (iso- and heteropolyoxometalates) which have been discovered in recent years.1 POM clusters exhibit an unmatched and tunable range of physical and chemical properties, which result in a wide

Prof. Maxim N. Sokolov received his PhD in 1992 from Novosibirsk Institute of Inorganic Chemistry (Prof. V. P. Fedin). He did postdoctoral work with Prof. H. W. Roesky (AvH Fellowship), Prof. A. G. Sykes (ESPRC Fellowship) and Profs. T Saito and Y. Sasaki (JSPS Fellowship). He was invited professor in several universities of Spain, France and Denmark. Currently he holds a position of Chief Researcher Maxim N. Sokolov at Nikolaev Institute of Inorganic Chemistry of the Russian Academy of Sciences and is Professor of Coordination Chemistry at the Novosibirsk State University. His research interests focus on cluster and polynuclear complexes of 4d and 5d transition metals, including polyoxometalates. Chem. Soc. Rev., 2012, 41, 7335–7353

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variety of exciting properties.2 POMs can include in their molecular framework almost all the elements with various electronic configurations and bonding patterns, which can impart diverse electronic,3 magnetic,4 catalytic,5 optical6 or other useful properties to molecular-level structures.7 Owing to such a great structural diversity, new multidisciplinary areas are emerging in POM chemistry, which interface with many fields of science such as biology and medicine, electrochemistry, materials and surface sciences.8 In this context, developing functionalization platforms with POMs constitute a key challenge for the fine tuning and matching of POM properties required for the design of POM-integrated devices. Polyoxometalates are discrete anionic metal–oxygen arrangements, often described as soluble oxide clusters.9 They are built from connections of [MOx} polyhedra, where M is generally an early transition metal in its highest oxidation state. Usually, the metal atoms M are WVI, MoVI, VV, NbV or TaV, which constitute the {MyOx} basic framework of the molecular architecture.10 Functionalization of POMs can be

achieved through different and specific strategies which depend on the nature of the imparted targeted property. For instance, redox behavior can be finely adjusted by the presence of specific metallic addenda within the POM framework. For instance, the mixed Mo/V/W Keggin or Dawson derivatives are a nice example, exhibiting a large redox scale spreading over about 1 V.11 More generally, the inclusion of cationic species as addenda within POM architectures constitutes one of the most explored strategies. In this context, the use of preformed lacunary polyoxotungstates allows developing rational synthetic routes for the tailoring of mixed M–W species, with M belonging to the d- or f-block.12,13 Such an approach gives rise to numerous and diverse POM-based systems ranging from monomeric archetypical POM species (Linqvist, Keggin or Dawson derivatives) to polymodular nanoscopic assemblies. Accumulation and combination of specific metallic centres within polyoxometalate arrangements originate unique properties, which are potentially interesting for relevant applications including catalysis, molecular magnetism,

Prof. Vladimir P. Fedin received his PhD in 1980 from Moscow State University (Prof. A. N. Nesmeyanov). He did postdoctoral work with Prof. A. Mu¨ller (AvH Fellowship), Prof. A. G. Sykes (RCS Rapitza Fellowship) and Prof. T. Saito (JSPS Fellowship). He is currently Director of Nikolaev Institute of Inorganic Chemistry of the Russian Academy of Sciences and Head of Chair of Inorganic Chemistry at Novosibirsk Vladimir P. Fedin State University. His research interests focus on inorganic, coordination, cluster and supramolecular chemistry.

Corine Simonnet-Je´gat received his PhD degree in 1989 at the University of Bordeaux. Her doctoral work dealt with the study by vibrational spectroscopies of CO2 complexes. During her postdoctoral stay at the Max-Planck-Institut fu¨r kohlenforschung under the supervision of Prof. Hoberg in 1990, she studied homogeneous catalytic processes involving carbon dioxide. Since 1991, she has been appointed assistant professor Corine Simonnet-Je´gat in the team of Prof. Sećheresse at the University of Versailles. She works actively with Prof. Cadot and her current research interest is focussed on the reactivity of thiometalates and polyoxometalates.

Se´bastien Floquet was born in Reims, France. He received his PhD degree from University of Paris XI in 2001 in the domain of spin crossover complexes under the supervision of Dr Marie-Laure Boillot before spending two years as a post-doctoral researcher in Geneva in the group of Prof. Claude Piguet, where he studied helicates of lanthanides. He was appointed assistant professor in 2003 and associate professor in 2009 in Se´bastien Floquet the ‘‘Institut Lavoisier de Versailles’’ in the group currently directed by Prof. Emmanuel Cadot. His research interests are mainly focused on the syntheses, the characterizations and the properties of new polyoxothiometalates.

Francis Sećheresse is Emeritus Professor of Chemistry at the University of Versailles St Quentin. His current interest is centered on the synthesis of polyoxo- and polyoxothiometalate based new architectures in view of their applications in the tailoring of nanosystems such as POM-coated nanoparticles and wires, modified electrodes, SMMs and POM hybrids.

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Francis Sećheresse

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electrochemistry and electro/photo/thermochromic systems. Specifically, inclusion of 3d-metallic cations into POM frameworks constitutes additional possibilities for further functionalization through coordination processes. These metallic centres are generally bound to one or several aqua ligands, which are labile enough to be replaced by specific donor atoms, thus giving an access to hybrid organic–inorganic POM arrangements. Another emerging strategy to modify the archetypical POM structures consists in covalent grafting of addenda metallic cations on the surface of metal-saturated POM anions. Such an approach is possible only for anions which exhibit a high surface nucleophilicity and then capable of binding divalent transition metal ions (FeII, MnII, CoII, NiII or ZnII) or trivalent p-block elements such as SbIII or AsIII. This yields a family of complexes based on reduced dodecamolybdate Keggin derivatives {XMo12O40}n, confirming that reversible electron transfers take place within the capped Keggin core. This class of compounds appears especially relevant for their potential implications as key components within electrocatalytic or catalytic devices. Alternately, derivatization of POMs can be achieved through the substitution of some oxo ligands by other functional groups (organic or inorganic). The synthesis of organometallic derivatives of polyoxometalates (including direct M–C bonds) is an area of growing interest, nicely exemplified with cyclopentadienyl derivatives of the Linqvist anion, where one or two terminal O2 oxide ions have been formally replaced by (Z5-C5H5) or (Z5-C5Me5).14 Various nitrogencentered ligands such as amine, nitrido, hydrazido, diazenido or nitrosyl type ligands can be introduced within POM structures leading to a rich and diverse chemistry at the origin of rational synthetic methodologies for the design of covalently bound organic–inorganic POM hybrids. Polyoxoalkoxometalates represent also an important class of derivatized species depending on the composition and the structure of the parent POM species, {R–O} groups are located either at terminal or at bridging positions. Surprisingly, in this field, donor sulfur atoms appear less used within polyoxometalate chemistry, as the inclusion of such a soft donating centre is expected to bring new properties to the POM unit. Although the chemistry of thio- and oxometalates exhibit some similarities, they represent two clearly separated fields in inorganic chemistry with their own concepts, challenges and chemical objectives. Transition metal–sulfide systems are prominent in both biological and industrial catalysts because sulfur-coordinated transition metals engage in facile electron and proton transfer processes, responsible for high active-site turnover in biological systems. For a long time and until the last decade, the chemistry of thiometalates, especially that of molybdate and tungstate has been strongly driven by the petroleum industry because of the central role of MoS2 and WS2 in hydrotreating catalysis, including the removal of sulfur, nitrogen, oxygen and metals from oil fractions. Furthermore, molecular M–S combinations (M = Mo or W) have been vigorously studied, mainly for the apparent importance of Mo–S and W–S coordination in the function of some metallo-enzymes. Nevertheless, metal–sulfide clusters should play an important role as key components in crucial challenging applications using This journal is

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novel, cheap and ecocompatible processes for energy conversion. Molecular molybdenum sulfide complexes such as dimers, trimers and their cationic derivatives act as efficient and viable materials for electro- and photoelectro catalytic hydrogen evolution.15 Softening of polyoxometalates through inclusion of sulfido groups within the {MxOy} framework is expected to modify significantly the POM properties e.g. structural and electronic, and thereby could be an ‘‘ideal’’ strategy to associate and accumulate complementary properties at the molecular level, such as the electrocatalytic function of the {M–S} core combined with the electron storage capacity of the polyoxometalate. The present review reports on the synthetic approaches for the formation of ‘‘polyoxothiometalate’’ compounds with special emphasis on the unique reactivity of the preformed sulfur-containing cationic building blocks {Mo2O2S2}2+ and {Mo3S4}4+ toward polyoxometalate building blocks. Such simple chemical systems, based on chemical and structural complementarities between ionic reactive moieties have allowed producing a series of relevant clusters with unrivalled large nuclearity structural arrangements, such as loops, triangles, squares and boxes. Specific reaction parameters and considerations will be pointed out in the following section showing that a deliberate pure inorganic supramolecular chemistry based on weak interactions, flexibility and dynamic is possible with polyoxometalates.

2. Polyoxometalates vs. polythiometalates Generally, the approaches used to produce high nuclearity POM based clusters are simple, consisting in acidifying an alkaline aqueous solution of the basic oxoanion [MO4]2 (M = Mo, W) or [VO3]. Nucleation and aggregation processes are considered to be first initiated by the protonation of the tetraoxometalate ions then giving rise to rapid and successive condensation reactions, promoted by expansion of the coordination sphere of the metal ion and auxiliary protonation steps. These polycondensation processes are complex, but readily directed with the use of an assembling group (generally tetrahedral XO4n or pyramidal XO3n) and controlled to some extent by varying specific reaction parameters such as pH, temperature, ionic strength, concentration, etc. Conversely, thiochemistry of molybdenum or tungsten leads to metal– sulfide cluster anions, commonly referred to thiometalates, which contrasts with POM chemistry both in the resulting isolated species and in the synthetic methodologies. Although both chemistries use analogous precursors such as [ME4]2 (with M = Mo or W; E = O or S), it has been established that reactivity of the tetrathiometalate [MoS4]2 or [WS4]2 ions is dominated by prominent and intense induced internal redox processes between sulfur ligands and metal centres. Addition of an electrophile, such as proton, elemental sulfur or organic polysulfide to [MS4]2 leads to the formation of reduced MV or MIV species with low nuclearity, rarely exceeding four metallic centers.16 Condensation and redox internal transfer resulting from the acidification of non-aqueous solutions of [WS4]2 proceed slowly enough to trap transient species such as [W3S9]2,[W3S10]2, [W4S12]2 and [W2S11]2.17 Conversely, electrophilic attack on [MoS4]2 by protons proceeds Chem. Soc. Rev., 2012, 41, 7335–7353

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Fig. 1

Routes of synthesis for the formation of the main thiomolybdates from tetraoxomolybdate and ammonium (poly)sulfide aqueous solutions.

very rapidly and leads almost invariably to amorphous and polymeric substances identified as ‘‘MoS3’’.18 However, formation of reduced binuclear or trinuclear MoV or MoIV species can be achieved with ammonium polysulfide in aqueous solution, using polyoxomolybdate salts as precursors (either Na2MoO42H2O or (NH4)6Mo7O244H2O) and elemental sulfur. Depending on the experimental conditions, several species are formed differing in their nuclearity, oxidation state of the Mo centres, their coordination number and their attached terminal or bridging ligands (oxo, sulfide or disulfido groups). The formation pathway of these species is shown in Fig. 1. Consequently, different attempts to functionalize polyoxometalates through direct sulfurization were frequently frustrated by reduction of the metal centre of the POM, which could lead to its degradation until the formation of the wellknown M–S thiometalate ions. However, the replacement of oxygen by sulfur has proved possible only in less reducible polyoxometalate ions, which contain labile metal–oxygen bonds. Klemperer et al demonstrated that such requirements were fulfilled within the mixed Nb(Ta)–W Linqvist ions [W5MO19]3 (M = NbV or TaV).19 Reacting with hexamethyldisilathiane as sulfurizing agent, the MQO bond in the {MQO}3+ group is selectively replaced by the terminal MQS bond to give the [W5MSO18]3 species (see Fig. 2).20 Such a

Fig. 2 Schematic representation of the O/S substitution for the formation of [W5NbSO18]3 Linqvist anion.

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methodology has been successfully transposed to the Keggin derivative [PW11MO40]4 , thus giving the first sulfur-containing Keggin derivative [PW11MSO39]4.21 Such a synthetic route reveals some limitations such as the weak hydrolytic stability of the {MQS} terminal bonds that imposes the use of nonaqueous solvents. Furthermore, attempts to increase the number of the NbQS groups through O/S exchange from the [PW9Nb3O40]6 parent species22,23 failed because some wellknown acido–basic condensation processes take place leading to connections between Keggin units through the formation of Nb–O–Nb bridges.24

3. The building block approach: available sulfur-bridged thiometallic cations Another, more versatile, route to obtain polyoxothiometalates has been developed by Sećheresse and co-workers who have used a preformed sulfur-bridged {M2S2O2}2+ (M = Mo or W) cationic precursor, chemically and sterically qualified to react with anionic polyoxometallates.25 Such an approach has been then successfully extended by Mu¨ller and co-workers to the trinuclear cation {Mo3S4}4+.26 Actually, thiochemistry of molybdenum and tungsten led to numerous compounds which contain the dinuclear {M2O2S2}2+ and the trinuclear {Mo3S4}4+ cores and such derivatives have been extensively described in several exhaustive reviews.16,27 These compounds differ mainly by the nature of the ligands which complete the coordination sphere of the metal centres, but in our case, we will focus specifically on the aqua derivatives [Mo2O2S2(H2O)6]2+ and [Mo3S4(H2O)9]4+ where the aqua ligands are labile enough to be exchanged for oxo groups belonging to POM compounds. As shown by Coucouvanis, the dicationic core {Mo2O2S2}2+ is quantitatively formed from the dianion [Mo2O2S8]2.28 The reaction proceeds in DMF solution through selective and successive oxidation of the terminal polysulfide ligands S42 and S22 by iodine and led to the dication [Mo2O2S2(DMF)6]2+. This reaction has been adapted successfully to aqueous medium, using the [Mo2S6O2]2 This journal is

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anion as tetramethylammonium salt. Then, the dication [Mo2O2S2(H2O)6]2+ can be hydrolyzed by potassium hydroxide to produce a yellow microcrystalline solid. Characterization of this solid led to an oxothiomolybdic cyclic arrangement associated to two iodide ions, potassium cations and NMe4+, always present as traces. This compound formulated as K2x(NMe4)xI2[Mo10O10S10(OH)10(H2O)5]15H2O has revealed itself as an excellent precursor to develop inorganic macrocycle chemistry with oxothiomolybdenum rings.29 Furthermore, the aqua cation [Mo2O2S2(H2O)6]2+ can be restored reversibly from this solid by controlled acidification. The use of water as solvent is really the key point permitting the control of the condensation processes by simple pH readjustment. The trinuclear aqua cation {Mo3S4(H2O)9]4+ can be also obtained from successive and rational routes of synthesis, adapted from published procedures. The initial Mo–S precursor (NH4)2Mo3S13nH2O is obtained from the direct sulfuration of ammonium heptamolybdate.30 The latter compound reacts with concentrated hydrochloric acid to give (NH4)2Mo3S7Cl6 6H2O,31 which further reacts stoichiometrically with three equivalents of triphenylphosphine in a mixed H2O–CH3CN solvent.32 Then, the aqua ion [Mo3S4(H2O)9]4+ can be readily transferred into appropriate acidic aqueous medium to give characteristic stable green solutions.33 The formation pathways of the two archetypical electrophilic building blocks [Mo2O2S2(H2O)6]2+ and [Mo3S4(H2O)9]4+ are given in Fig. 3. The tungstic analogues can be also prepared and converted into the appropriate aqua cationic clusters [W2O2S2]2+ and [W3S4(H2O)9]4+.34 The use of the [WS4]2 precursor requires high temperature for the activation of the internal redox transfer that promotes condensation reactions. Alternatively, other precursors such as WCl4, W(CO)6 or W metal can be employed either in non-protic solvent or in solid-state synthesis.16 In particular, reaction of W3S7Br4 (made from elements) with H3PO4 in acidic media give reliable yields of blue violet [W3S4(H2O)9]4+ as the only product.35

Fig. 3

4. Combination between {Mo2O2S2}2+ with preformed vacant heteropolyoxotungstate While the self-condensation of the dithiocation {M2O2S2}2+ produces remarkable macrocyclic architectures with striking host–guest properties,36 the same [M2O2S2(H2O)6]2+ unit has been shown to be an efficient ditopic unit for reaction with highly negatively charged heteropolyoxotungstate. The coordination requirements being mainly restricted to the equatorial and axial sites of the two equivalent Mo atoms predispose the coordinating groups to highly predictable selfassembly processes, where the oxothio fragment is found as constitutive unit, linker for modular arrangement or template in original polyoxometalates. The control of the formation of the [Mo2O2S2(H2O)6]2+ cation, by using an appropriate precursor allows to adapt the synthetic conditions to the stability and coordination requirements of the vacant heteropolyoxotungstate. A series of polyoxothiometalate compounds has been obtained through rational and deliberate synthesis via stereospecific addition of the {Mo2O2S2}2+ on preformed vacant heteropolyoxotungstates such as mono-, di-, tri-, tetra- and superlacunary macrocyclic ions. As more than supplementary compounds, such simple chemical systems gave some relevant insights about some important aspects of inorganic chemistry, such as dynamics, self-assembly process or template synthesis. 4.1 Complementary geometries with divacant anions [c-XW10O36]8 Historically, the first isolated heteropolyoxothioanion resulted from the addition of the {M2O2S2}2+ group (M = Mo or W) on the divacant [g-SiW10O36]8 anion that constitutes a nice example of structural and chemical complementarity between reactive moieties.25 The structure of the resulting product [g-SiW10M2S2O38]6 clearly shows the filling of the divacancy of the anion resulting in four Mo–O–W bridges through a desolvation process of the aqua cation (see Fig. 4). This approach has been successfully extended to the phosphato

Reaction pathways for the formation of the two archetypical electrophilic building blocks [Mo2O2S2(H2O)6]2+ (a) and [Mo3S4(H2O)9]4+ (b).

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Fig. 4 Structural representation of the g-[SiW10Mo2S2O38]6 anion highlighting the coordination of the {Mo2O2S2} fragment to the divacant g-[SiW10O36]8 subunit. Color code: Mo = blue spheres; S = yellow spheres; O = red spheres.

analogue [g-PW10O36]7.37 The final oxothioanions retain the original idealized C2v geometry and can be described as a ‘‘metal-saturated’’ Baker–Figgis g-isomer where the {M2O2S2} fragment can be viewed as a constitutional unit of the anion. The geometrical features of the {M2O2S2} fragments are comparable to those observed in other dinuclear compounds and confirm that the localized metal–metal bond is retained within the POM structures. The observed MV–MV distances are of 2.832(1) and 2.815(1) for M = Mo and M = W, respectively. These anions exhibit a reasonable stability in aqueous solution, as was established by UV-vis, electrochemistry and 183W and 31P multinuclear NMR. Such features contrast with those observed with the isocharged divanadium analogue [g-SiV2W10O40]6 which slowly isomerizes into a series of b isomers.39 Such a behavior is probably related to the presence of the MV–MV metal–metal bond, which increases the stability of the {M2O2S2} core, thus acting as a keystone within the g structure. 183W NMR spectra of these compounds contain the expected 183W NMR line-distribution. 183 W NMR spectra of the fully tungsten-containing anion [g-XW12S2O38]6 (X = PV or SiIV) are interesting for showing the resonances of the WVI atoms in an octahedral oxo environment in the usual 100/200 ppm range, and the resonances of the two equivalent WV nuclei belonging to the {W2O2S2} core at +1078 ppm and +1041 ppm and for X = Si and X = P, respectively. Such huge positive chemical shifts are characteristic of reduced tungsten atoms in a sulfur environment,34a supporting the W–W metal–metal bond is still present in the polyoxothiometalates. Furthermore, these compounds gave brown-colored solutions, characteristic of a strong localization of the d-electrons within a metal–metal bond. The formal substitution of two oxo for two sulfido groups leads to the isoelectronic species [g-SiW12O40]6 previously reported by Te´ze´ et al.39 Such a compound gives intense blue solutions characteristic of a heteropolyblue behavior (the two d-electrons are rather delocalized over the POM structure, at the origin of an electronic transition (IVCT) at 1280 nm). 7340

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Fig. 5 Energies of the frontier orbitals calculated for g-[SiW10M2E2O38]6 (M = Mo, W; E = S, O) at the equilibrium geometry (adapted from ref. 40).

DFT calculations, carried out by Rohmer and Benard on the reduced [g-SiW10M2E2O38]6 with M = Mo or W and E = O or S, demonstrate that the presence of the two bridging sulfur atom induces a strong stabilization of the metal–metal bond.40 Such a stabilization is highlighted by the significant increase of the gap separating the HOMO, localized on the {M2O2S2} fragment and the LUMO mainly composed of the d shell orbitals of the WVI atoms (see Fig. 5). These results represent a nice example evidencing how the partial O/S substitution strongly modifies the electronic properties of the POM unit. 4.2 Isomerization processes at the {Mo4S4O4} central core The stereospecific addition of the {Mo2O2S2}2+ oxothiocation to monovacant anions, derived from Keggin or Dawson structures leads to the sandwich-like compounds [(PW11O39)2(Mo4S4O4(H2O)2)]10 and [(P2W17O61)2(Mo4S4O4(H2O)2)]16, respectively.41,42 Both anions are geometrically similar, two {Mo2O2S2} entities are symmetrically bound to the four terminal oxygen atoms, delimiting the vacancy of each monovacant moiety (see Fig. 6). In this arrangement, the POM subunits act as bis-bidentate ligands, constraining the two {Mo2O2S2} to interact through a quasi-linear double Mo–O–Mo bridge. Both POM subunits exhibit a 1801-staggered disposition, consistent with a transoid isomer, according to the terminology proposed by Pope for uranyl-containing complexes.43 31P and 183W NMR solution studies reveal an isomerisation process as depicted in Fig. 7, interpreted as the conversion of the transoid isomers into the corresponding cisoid isomers. A similar behaviour has been recently reported by Kortz and co-workers for two monovacant anions assembled by two close square-planar Pd2+ cations in two distinct cisoid or transoid arrangements.44 A kinetic study showed that at the equilibrium, a 30 : 70 proportion between transoid and cisoid isomers was obtained. The equilibrium state was found to be nearly temperature-independent, indicating that the isomerization process is mainly entropically driven. The activation parameters DHa and DSa have been also determined for both opposite reactions (see Fig. 7). These data reveal that the activation enthalpy is nearly equal for both reactions This journal is

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Furthermore, the {M2O2S2} fragment is shown to be an efficient linker to assemble POM subunits, in order to produce dynamic inorganic supramolecular systems. 4.3 Stereo- and supramolecular chemistry with trivacant anions

Fig. 6 Structural representations of the [(P2W17O61)2(Mo4S4O4(H2O)2)]16 anion: (a) side view showing the central {Mo4S4O4(H2O)2} core sandwiched between two monovacant anions; (b) top view showing the transoid disposition of the two {P2W17O61} moieties. Color code: Mo = blue sphere; S = yellow spheres; O = red spheres; light blue spheres = water molecule.

Fig. 7 Reaction pathway for the transoid/cisoid isomerization of the [(P2W17O61)2(Mo4S4O4(H2O)2)]16 compound.

(DHa (1) = DHa (1) = 68.6 kJ mol1) while the main difference arises in the activation entropy (DSa (1) = 27.7 J K1 mol1; DSa (1) = 35.5 J K1 mol1). In these systems, the kinetic stability between both isomers originates mainly from an entropic effect upon the activated species from either the transoid or the cisoid isomer. It could be of note that the DHa value determined for such an isomerization process is significantly lower than that determined by Anderson and Hill for the b to a isomerization of the Dawson polyanion [P2W18O61]6 (DHa = 83 kJ mol1).45 This should be related to the weakness of the Mo–O bonds within Mo–O–W bridges with respect to the W–O–W junctions in the Dawson polyanion [P2W18O61]6. In conclusion, these sandwich-like systems exhibit a high versatility illustrating the coordinating properties of the MoV and the weakness of the resulting Mo–O–W bridges. This journal is

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A- vs. B-type trivacant isomers. Trivacant POMs can be classified as A- or B-type isomers, both differing by the nature of the triple vacancy.46 Three trivacant anions with A- or B-type isomerism [A-a-PW9O34]9 , [B-P2W15O56]12 and [B-a-AsW9O33]9 have been selected and reacted in the presence of the [M2O2S2(H2O)6]2+. Their structures are represented in Fig. 8. Both types of isomerism exhibit a crown of six terminal oxygen atoms, which are the coordinating centers of the trivacant anions. The perimeter of the {O6} crown in the type-A isomer measures about 21 A˚ and appears significantly reduced to about 17 A˚ in the B-type. In addition, in the [B-P2W15O56]12 compound, a terminal P–O group is pointing towards the center of the six crown terminal oxygen while the center of the {O6} crown is occupied by the lone pair electron of the central AsIII atom in [B-a-AsW9O33]9. These structural and chemical differences are at the origin of unique and specific coordination properties toward metal cations, widely increased by the use of the ditopic {M2O2S2} linker. Thus, depending on the nature of the trivacant anion, modular architectures such as dimer, tetramer and hexamer have been isolated and characterized (see Fig. 8). Fundamental criteria governing the stereochemistry of the self-assembly will be discussed with regard to the structural properties (geometry, symmetry, coordination requirement) of the reactive moieties and with regard to the synthetic conditions such as ionic strength, nature of the counter-cations or temperature. Coordination of {M2O2S2} fragments to the trivacant anion [A-a-PW9O34]9 leads to pillar-like compounds in which two {PW9} subunits are assembled by three parallel {Mo2O2S2(H2O)2} linkers (see Fig. 9a).47 Structural characterization, through X-ray diffraction analysis in the solid state and NMR in solution showed that the overall idealized symmetry of the anion is lowered from D3h to C2v due to the rotation by 1801 of one {Mo2O2S2(H2O)2} fragment with respect to the two other linkers (see Fig. 9b). A similar situation, mainly imposed by steric constraints inside the cluster was also evidenced by 183W NMR for the linking {WO(H2O)} group in [P2W21O71]6.48 Interestingly, changing the [A-a-PW9O34]9 for the [B-P2W15O56]12 affects drastically the stereochemistry of the condensation processes to lead to the spectacular planar tetrameric arrangement depicted in Fig. 10.49 In the tetra-Dawson anion [(H2P2W15O56)4{Mo2O2S2(H2O)2}4{Mo4S4O4(OH)2(H2O)}2]28, the C2h symmetry results from four equivalent {P2W15} subunits without any local symmetry. In short, the anion can be described as two formal dimers, resulting from the connection of two {P2W15} through two {Mo2O2S2(H2O)2} linkers. Then, these dimeric arrangements assemble through two symmetric tetranuclear arms {Mo4S4O4(OH)4(H2O)}. In this specific case, the PQO bond interferes within the condensation process, and prevents the formation of the typical dimeric arrangement. In the tetrameric structure, the four {Mo2S2O2} Chem. Soc. Rev., 2012, 41, 7335–7353

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Fig. 8 Schematic overview of the modular species isolated from the [a-A-PW9O34]9, [B-P2W15O56]12 and [a-B-AsW9O33]9 trivacant heteropolyoxotungstates. Color code: grey sticks = heteropolyoxotungstate subunits; orange sticks = {Mo2O2S2} linkers.

Fig. 9 (a) Structural representation of [(PW9O34)2(Mo2O2S2(H2O)2)3]12 showing the {Mo2O2S2} fragments sandwiched by two trivacant Keggin units; (b) projection in the equatorial plane highlighting the inner and outer distribution of terminal oxygen atoms (red spheres) and water molecules (light blue spheres) attached to the Mo atoms.

groups appear mutually connected to form two symmetric ribbon-like {Mo8} octanuclear cores, decorated by four pendent {P2W15} subunits. Actually, the tetra-Dawson macrocycle is the first multi-unit POM compound to exhibit pairs of hydroxo-bridges, ensuring the inter [Mo2O2S2]-building block connections. Such an arrangement is similar to those observed in oxothiomolybdic rings, resulting from the cyclic 7342

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Fig. 10 Structure of the planar anion [(H2P2W15O56)4{Mo2O2S2(H2O)2}4{Mo4S4O4(OH)2(H2O)}2]28 showing the four {P2W15O56} subunits (grey sticks) anchored to two {Mo8S8(OH)8(OH)2(H2O)5}6+ ribbons (orange polyhedra).

condensation of the {Mo2O2S2} building block.36 The 31P and W NMR spectra of the tetra-Dawson thio derivative exhibit the resonances expected for a C2h idealized symmetry 183

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(two equal 31P resonances and fifteen equal 183W resonances), confirming the very good stability in aqueous solution. Cation-directed synthesis and conformational change with [A-a-AsW9O33]9. We previously reported how a simple inorganic system based on the chemical complementarities between the common trivacant ligand [B-a-AsW9O33]9 (polyoxometalate unit, denoted POM or {AsW9O33}) and the ditopic cation [Mo2O2S2]2+ (linker unit, denoted L or {Mo2}) can produce relevant modular assemblies, stereochemically oriented and conformationally ordered through weak supramolecular interactions.50 Depending on the ionic strength and the nature of the alkali-metal cations used, three architectures formulated as hexamer, tetramer and dimer were isolated and structurally characterized in the solid state. This series of clusters displays the same stoichiometry {POM6L9}36, {POM4L6}24 and {POM2L3}12 and their conditions of formation differ mainly by the nature and the concentration of the alkali-metal cations (from Li+ to Cs+) (see Fig. 11). The {(POM)6L9} hexameric scaffold is obtained exclusively in the presence of Na+, K+ or Li+ cations in a high ionic strength medium. Conversely, the tetrameric {(POM)4L6} and the dimeric {(POM)2L3} arrangements are formed and crystallized from lowest ionic strength solutions, using mixtures of cations such as (Cs+, Li+) or (Rb+, Li+). Synthetic observations (depicted in Fig. 11) supported by crystal structure analysis and solution studies demonstrate that sodium or potassium ions develop intimate interactions with the {POM6L9} cage. Consequently, it can be assumed that these cations direct the self-assembly process toward the formation of the hexameric {POM6L9} arrangement and participate directly to its stabilization. On the other hand, crystallization

carried out in the presence of larger cations such as Rb+ or Cs+ and in 1 mol L1 LiCl solutions allowed to grow crystals of the {POM4L6} tetrameric arrangement. Crystals of {POM2L3} are obtained from 1 mol L1 LiCl solutions after heating to 75 1C, showing that a conversion relationship exists between {POM4L6} and {POM2L3}: in such conditions, the tetrameric arrangement is probably a transient species, and the dimer is in fact the thermodynamic product. In these three arrangements, {Mo2O2S2} exhibits a unique mode of coordination with the trivacant subunits, only the equatorial sites are coordinated to the oxo centers of the POM subunits. The {POM2L3} arrangement corresponds to the simplest topology derived from the {AsW9O33}–{Mo2O2S2} system (Fig. 11). The dimodular anion {POM2L3} results from the connections of three {Mo2O2S2} linkers arranged as parallel pillars sandwiched between the two chelating {AsW9O33} moieties (D3h idealized symmetry). Due to steric hindrance, the six molybdenum atoms exhibit the same square-pyramidal geometry with outwardly directed {MoQO} groups. It should be noted that a similar topology was already found in the [(PW9O34)2(Mo2O2S2(H2O)2)3]12 anion. The tetramodular anion {POM4L6} consists of a tetrahedral ‘‘edge-on’’ arrangement, where the four {AsW9O33} POM units constitute the corners with the six {Mo2O2S2} linkers located on the vertexes (see Fig. 11). Each of twelve molybdenum atoms is in octahedral environment, achieved by a terminal aquo ligand. The structure of the hexamodular anion {POM6L9} consists of two basal triangular subunits {POM3L3}, mutually connected by three additional {Mo2O2S2} linkers acting as pillars within the scaffolding structure. The nine {Mo2O2S2} linkers point their axial MoQO bonds toward the inner cage of the cluster.

Fig. 11 Schematic view of the conditions of formation of the modular anions {POM2L3}, {POM4L6} and {POM6L9} with POM = {AsW9O33} and L = {Mo2O2S2}. The three arrangements are obtained from the same POM/L ratio through crystallization processes differing by ionic strength, nature of the counter ions or temperature.

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Surprisingly, the hexameric scaffold exhibits a rare flexibility evidenced in the solid state by two distinct conformations, either eclipsed (D3h symmetry or regular) or off-staggered (D3 symmetry or distorted) (see Fig. 12). From the structural point of view, the modularity of the chemical system is directly related to the ‘‘angle of approach’’ between two vectors representing the orientation of the {Mo2O2S2} linkers and that of the POM subunit, respectively. The {Mo2O2S2} linker is defined by a vector crossing both the Mo atoms and along the Mo–Mo bond, while the POM ligand is characterized by a vector normal of the plane delimited by the six crown oxygen atoms as shown in Fig. 13. For the dimer, the three a angles are equivalent and close to zero while they are significantly opened for the tetramodular arrangement, a = 35.2 21, close to the idealized value of 35.31, as calculated for a perfect tetrahedron. For the regular D3h hexameric scaffold, the situation is quite different. The angles within the two apical {POM3L3} triangular subunits are significantly larger (a = 40 11) while the connecting angle involving the three {Mo2O2S2} pillars ensuring the linking between the two apical {POM3L3} subunits increases to 62 11. Thus, the values of the a angle appears directly related to the presence of alkali-metal cations as guest components associated to the structures. The dimodular anion {POM2L3} corresponds to the most compact arrangement without inner cations. The a angle increases in the tetramodular arrangement, mainly related to steric constraints due to the presence of 2.4 rubidium cations disordered over four inner positions. Finally, the regular hexameric scaffold, characterized by the largest angle of approach traps a large inner cationic cluster composed of six sodium cation and fourteen coordinated water molecules. These three structural examples, deriving from an unique simple chemical system

Fig. 13 Schematic view showing the angle of approach a between the linker {Mo2O2S2} and the vector normal of the plane delimited by the six crown oxygen atoms of the POM ligand.

constitute a nice example illustrating the ‘‘template effect’’ often invoked as responsible for the outcome of the selfassembly process. In this case, the structuring role of cations arises from the formation of ionic aggregates with the highly negatively charged [AsW9O33]9 anion. For polarizing cations with small ionic radius, such as Li+, Na+ and K+, electrostatic interactions within the ionic aggregate should induce some steric congestion in the vicinity of the coordinating centres leading to large angles of approach and to the highmodularity {POM6L9} arrangement. The use of largest cations (Rb+ and Cs+) associated to weak ionic strength induces the closure of the a angle observed for the tetra- and dimodular species. Such a series of compounds illustrates the high structural versatility of such assemblies, which arises (i) from the possibility to control the angle of approach for the design of the resulting geometry, and (ii) from the weakness of the W–O–Mo bridges which confers to the system a great

Fig. 12 Side and top views of the eclipsed (a) and staggered (b) conformations of the {POM6L9} arrangement. The eclipsed structure (a) encloses six sodium cations (pink spheres) and fourteen water molecules (light blue spheres) while the 401 staggered conformer (b) contains eight potassium cations and eight water molecules statistically disordered within the cavity (green spheres).

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sensitivity upon the nature of the alkali-metal cations. The {Mo2O2S2} linkers act as flexible hinges while the weakness of the bridges allows the system to self-repair through templation around ionic aggregates. Thus, the enriched lithium salt KLi35{POM6L9} spontaneously converts in aqueous solution into the dimeric {POM2L3} species with the concomitant release of the encrypted potassium cation. The conversion process is kinetically slow and has been quantitatively studied by 183W and 39K NMR. Another interesting feature corresponds to the existence of two distinct conformers for the {POM6L9} hexameric scaffold. The regular arrangement (D3h symmetry) exhibits two eclipsed triangular subunits enclosing the large cationic cluster {Na6(H2O)14}6+. In the presence of potassium, the hexameric arrangement adopts a strongly distorted conformation (D3 symmetry) where both triangular basal units adopt a pronounced 401 staggered conformation (see Fig. 13). The staggered conformation contains a smaller cationic aquo cluster guest composed of eight K+ ions and eight water molecules. In this case, the driving force governing the conformational change from eclipsed to staggered could be attributed to the reaction of the system to minimize the empty volume of the cavity. Such a constraint is reinforced by the presence of inner potassium ions resulting in attractive electrostatic interactions between the flexible {POM6L9} host and the inner cationic guest cluster. The experimental structural data have been supported by DFT calculations. Geometries and energies of the three assemblies {POM6L9}36, {POM4L6}24 and {POM2L3}12 have been computed by Poblet and Lopez to highlight the crucial influence of the cations for the stabilization of the host–guest supramolecular assemblies and for the POM-linker coordination and conformation.50 Calculations carried out on the tetrameric structure {POM4L6}24 reveal that inclusion of two Rb+ ions is required to produce bond lengths close to those obtained from X-ray diffraction study. Therefore, solvent effects have also been introduced to maintain the four anionic {AsW9O33}9 in a realistic tetrahedral arrangement. Similar results have been obtained with the regular hexameric scaffold. Geometry optimisation started from the X-ray structure reveals that the cluster tends to open (as in a symmetric breathing mode) leading to an unrealistic model with very long {AsW9O33}– {AsW9O33} distances. The vertical {Mo2O2S2} pillars also tend also to orientate outwards, both gas-phase and COSMO

calculations leading to a similar result. The presence of the solvent does not suffice to keep the cluster in a realistic compact form, illustrating the fundamental role of the internal Na+ cations and water molecules for the stability of the {POM6L9} scaffold as in the tetrameric compound. Nevertheless, a constrained optimization leads to a realistic structural model characterized by six potential energy minima coinciding with the positions of the Na atoms located by X-ray diffraction. 4.4 Control of the modularity by deprotecting process with tetravacant anion Reaction of the tetravacant anion51 [b-HAs2W8O31]7 with the [Mo2O2S2(H2O)6]2+ oxocation quantitatively produces the trimodular species {POM3L3}15 shown Fig. 14.52 Structural characterizations carried out by X-ray diffraction method reveal that the POM subunit corresponds to a trivacant anion {AsOH(b-AsW9O33)}7 indicating that the tetravacant anion used as precursor converts into a trivacant anion with the retention of the b isomerism. The three {Mo2O2S2(H2O)} linkers display the usual equatorial coordination with the oxo groups of the POM units. In addition, the two remaining oxygen atoms bordering the vacancies of each POM subunit are coordinated to a pendent {AsIII–OH}2+ group, the latter acting as protecting group to hinder any further aggregation process with {Mo2O2S2}. These outer As(III) centres reacted with iodine to give formally As(V) based oxoanions labile enough to permit further condensation processes. Such a reaction formally corresponds to a deprotecting process allowing the formation of a tetrahedral {POM4L6} arrangement based on four {b-B-AsW9O33}7 POM subunits (see Fig. 14). 4.5 {Mo2O2S2}-directed synthesis of heteropolyoxotungstoarsenate The acidification of As(III) oxo precursors (such as As2O3 or sodium metaarsenite NaAsO2) with tungstate WO42 provides numerous heteropolyoxometalates ions. Their formation depends mainly upon the pH and the nature of the alkalimetal counter-cations. In the presence of potassium ions, a series of sandwich-type anions [As2W21O69(H2O)]6, [As2W20(H2O)2O68]10 and [As2W19(H2O)O67]14 is selectively obtained under conditions ranging from a strongly acidic medium to pH = 6.53a–c When condensation processes are conducted in the presence of sodium as structuring cation, the crown-shape anion [As4W40O140]28 previously reported by

Fig. 14 Scheme of the structural rearrangement of a trimodular into a tetramodular species through selective oxidation of the three ‘‘protecting’’ {AsIIIOH}2+ groups by iodine.

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Herve´ and co-workers is selectively isolated.54 Besides, the use of the preformed K14[As2W19(H2O)O67] compound at pH B 2 has led to the largest tungstoarsenate [As6W65(H2O)7O217]26 anion.55 We reported that the condensation process of the preformed trivacant anion [a-B-AsW9O33]9 as potassium salt with tungstate can be selectively directed by the presence of the {Mo2O2S2(H2O)6}2+ dication.56 At pH = 1.4, a {Mo2O2S2}supported tungstoarsenate arrangement was isolated and fully characterized. The discrete anion [(a-AsW9O33)3(WO(H2O))3(Mo2O2S2(H2O)4)]16 exhibits a new polyoxotungstate framework consisting of three {AsW9O33} subunits linked together by three {OQW–OH2} groups (see Fig. 15). Such a macrocyclic assembly delimits a large inner cavity lined by the remaining six terminal oxygen atoms belonging to the three [AsW9O33]9 subunits. Interestingly, the structure reveals the presence of the {Mo2O2S2(H2O)4}2+ core, grafted on the polyoxoanion surface and spanning two adjacent [AsW9O33]9 subunits. The remaining void of the cavity is occupied by a potassium cation closely embedded between terminal oxo groups and aquo ligands of the {Mo2O2S2(H2O)4}2+ core. Structural analysis of the cell packing reveals that close interactions exist between the two anions, symmetrically arranged to form a large dimeric association (see Fig. 16) in which ionic interactions through O–K–O bridges and short hydrogen bonds ensure the cohesion of the supramolecular assembly. Interestingly, two tetramethylammonium cations interact symmetrically with the opposite faces of the dimer where each triangular {As3W30} oxo-backbone designs the contour of a small anionic pocket (see Fig. 16). Actually, although the {Mo2O2S2}2+ core (and also potassium ions) appears to be weakly bound to the [(a-AsW9O33)3(WO(H2O))3]15 backbone, this dication plays a crucial role in the self-assembly processes, thus justifying the used statement ‘‘{Mo2O2S2}2+ directed synthesis’’. Reproducing similar syntheses without {Mo2O2S2}2+ under otherwise similar conditions of pH, reagent concentrations, ionic strength and temperature give colorless well-shaped crystals whose 183W NMR characterization as the Li salt gives

Fig. 15 Structural representation of the anion [(AsW9O33)3(WO(H2O))3(Mo2O2S2(H2O)4)]13 showing the {Mo2O2S2(H2O)4]2+ cation (orange sticks) grafted on the {As3W30} backbone (grey sticks). The cavity of the triangular cluster contains a encrypted potassium ion (green sphere).

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Fig. 16 Supramolecular arrangement resulting from the interactions of two [(AsW9O33)3(WO(H2O))3(Mo2O2S2(H2O)4)]13 anions through hydrogen and K–O bonds. Two tetramethylammonium cations interact on both sides of the cluster through weak electrostatic contacts. Color code: grey sticks = {As3W30} backbone; orange stick = Mo2O2S2(H2O)4 core; Blue dotted lines = hydrogen bonds; K = green sphere; van der Waals spheres (white and purple) = NMe4+ cations.

a complex spectrum containing more than forty resonances between 80 and 210 ppm, inconsistent with the [(a-AsW9O33)3(WO(H2O))3]15 anion.56 4.6 Vectorial growth on the superlacunary [H7P8W48O184]33 anion The combination of the dithiocation {Mo2O2S2(H2O)6]2+ and the macrocyclic [H7P8W48O184]33 anion (abbreviated {P8W48}) leads to the capping of the {P8W48} structure by two [Mo4O4S4(OH)2(H2O)3]2+ oxothiomolybdenum clusters.57 In the crystal structure, the [Mo4O4S4(OH)2(H2O)3]2+ hemicycle appears distributed with a statistical occupancy factor of 50% over two equivalent positions related through a C4 symmetry axis. 31P NMR studies in solution and in the solid state are consistent with the presence of two anions located on a single crystallographic site, both identified as perpendicular (denoted perp) and parallel (denoted para) (see Fig. 17). Additionally, four potassium cations, symmetrically distributed over eight equivalent positions were found in the cavity. The molecular structure of the {P8W48} oxothio-derivative closely resembles the full oxo analogue previously described by Mu¨ller and co-workers58 which represents a rare example where the oxo cation {Mo2O4}2+ exhibits similar modes of junction as those usually observed for oxothio {Mo2O2S2}-based molecules. Solution studies carried out by 31P (and 183W) NMR and UV-visible spectroscopies demonstrate that coordination of the {Mo2S2O2} on the P8W48 leads to an intermediate, consistent with the mono-handle derivative prior to the formation of the saturated adducts. Moreover, the influence of the counter cations K+ vs. Na+ upon the perp/para isomeric ratio and upon the dynamic properties of the saturated bishandle thio derivatives was also qualitatively demonstrated. This journal is

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Fig. 17 (a) Representation of the structural disorder involving two {Mo4S4(OH)2(H2O)3}2+ arms grafted on both sides of the {P8W48} surface as the superimposition of two geometrical isomers with perpendicular (b) and parallel disposition (c). Color code: grey sticks = P8W48 framework; orange sticks = {Mo4S4(OH)2(H2O)3} ‘‘basket handle’’; green spheres = disordered potassium cations.

These results differ from those obtained by 183W and 31P NMR on the fully oxo analogue, which agrees with two [Mo4O10(H2O)3] clusters strongly grafted on the {P8W18} surface in a perpendicular fashion.

5. Trapping oxo-building blocks by {Mo2O2S2} coordination Condensation of molybdates or tungstates in aqueous solution leads to the formation and isolation of numerous isopolyoxometalates, differing by their size, shape and structure,59 but in the presence of exogeneous species such as weakly chelating ligands, reducing agent, metallic cations, etc., the formation of unusual building blocks, intrinsically unstable components of a virtual dynamic library can be engaged within nanosized

discrete species. One of the nicest examples which illustrates such a concept is probably the striking pentagonal motif {M6O21}6 (M = Mo or W) at the origin of numerous spectacular compounds such as molybdenum oxide based nanowheels or ball-shaped Keplerate capsules.60 Using a trapping component such as the [Mo2O2S2(H2O)6]2+ cation should be an attractive approach to further explore the dynamic library of aqueous solutions of tungstate or molybdate ions. As illustrated by many examples previously presented in this review, the MoV centers within the {Mo2O2S2} unit behave as ‘‘soft’’ metals that confer a labile character to the WVI–O–MoV bridges with respect to the WVI–O–WVI junctions. Although the {Mo2O2S2} component cannot be considered as ‘‘non-innocent’’, we could assume that it should smoothly steer polycondensation of the oxo building blocks. In addition, the specific coordination requirements of the dithiocation should be helpful to properly integrate the oxo component during the course of self-assembly processes. The first example combining the {Mo2O2S2} core and metalates within polycondensed arrangements corresponds to the cyclic arrangement {M8S8O8(OH)8} (M = Mo or W) based on double-hydroxo junctions of four {Mo2O2S2} clusters encapsulating a single {MVIO6} octahedron (see Fig. 18).61 Such types of [M9S8O10(OH)10(H2O)]2 anions were synthesized in aqueous solution at pH = 4.5 for [MVIO4]2/ [M2O2S2(H2O)6}2+ ratios close to the stoichiometry. The central {MVIO6} octahedron exhibits a linear OQM–OH2 axis, and the presence of two protonated oxygen atoms in the {MVIO4} equatorial plane. This example deriving rather from the templated-oxothiomolybdic ring, widely investigated by our group,29,36 corresponds to the simplest possible combination, but it could be anticipated that oxothiomolybdic cyclic systems should be able to stabilize and trap transient polyoxometalates. The change of the synthetic conditions such as the sequence of addition of the reagents, the use of acetic buffer and the increase of the [MVIO4]2/[M2O2S2(H2O)6}2+ ratio allowed isolation of the first high nuclearity polyoxothiomolybdate ion.62

Fig. 18 Species obtained from the virtual dynamic library of aqueous molybdates or tungstates: (a) cyclic oxothiomolybdate {Mo8S8O8(OH)8} trapping an isolated {MO6} octahedron (M = Mo or W); (b) cyclic arrangement built on the alternating connections of four {Mo8O28} building blocks and four {Mo2O2S2} linkers; (c) {W72Mo60} type-Keplerate resulting from connections of 20 pentagonal {W6O21(H2O)6} motifs and 30 {Mo2O2S2} linkers. Color code: orange sticks: {Mo2O2S2} linkers; grey sticks = MVI-oxo building blocks (M = Mo or W).

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The [(Mo8O28)4(Mo2O2S2)4]24 macrocyclic anion can be described as a Mo40-membered ring built from the connections between four octamolybdate building blocks and four {Mo2O2S2} linkers (see Fig. 18). Interestingly, the Mo40S8-ring contains a rare isomer of the octamolybate {Mo8O28}8 fragment. Solution studies, carried out with various electrolytes showed that whatever the conditions, the {Mo40S8} anion converts rapidly into complex mixtures, as recently observed by Cronin and co-workers.63 Lately, Mu¨ller and co-workers demonstrated that the thirty oxo {Mo2O4} linkers can be replaced by the oxothio {Mo2O2S2} fragment within the W72Mo60-type Keplerate.64 The presence of softer sulfide ligands provides a new option to change the properties and reactivity of the Keplerate capsules. The synthesis was carried out at pH 4 in acetate buffer, using an acidic solution of [Mo2O2S2(H2O)6]2+ and sodium tungstate as precursors (see Fig. 18). As anticipated by the authors, reactions under the confined environment should be influenced by the presence of the 60 sulfide ligands, which increases the internal shell polarizability of the capsule. Another important properties of the Keplerate ions concerns their predisposition to form structured aggregates, so-called ‘‘blackberries’’.65 Softening with sulfur the frontiers of the 20 available crown ether-like {W3Mo6S3O6} pores should also influence their affinity for cations and thus their self-recognition during aggregation processes.

6. Combination of the {Mo3S4}4+ building blocks with preformed heteropolyoxotungstates The Mo3S44+ cluster core belongs to the {M3(m3-Q)(m2-Q)3} type which contains a triangular metal cluster M3 and four chalcogenide ligands (one m3-Q and three m2-Q). Numerous examples of sulfide and selenide clusters of molybdenum are known, but only one Mo3Te44+ cluster has been reported so far.27d,66 The number of valence electrons available for metal–metal bonding is six (i.e. formally three two-electron bonds), which are supplied by three MoIV centers (d2 configuration). If the M–M bonds are ignored, the metal coordination polyhedron is a distorted octahedron. Alternately, this cluster core can be referred to as a one-metal depleted or incomplete cube. In a more general context, clusters with the {Mo3Q4} cluster core belong to the family of [M3X13] complexes as represented in Fig. 3, and the structure can be described as consisting of three MX6 coordinated octahedra, fused together so that each octahedron shares one vertex (m3-X) and two edges (the m2-X). In this way each metal atom has three terminal ligands which are easily exchangeable. This type of clusterization is common for early transition metals and does not necessarily involve M–M bonding, as it occurs in polyoxometalates (the trimetallic core {M3O13} building blocks commonly found in the Keggin and Dawson type structures). This analogy is very important since it allows us to predict the existence of hybrid systems based upon combinations of the {Mo3S4} and {M3O4}10+ cores. Incorporation of various heterometals into {Mo3Q4}4+ derivatives gives rise to heterometallic clusters of cuboidal structure of the {Mo3(M 0 Ln)Q4(H2O)9} type. This is the most explored side of the [Mo3Q4(H2O)9]4+ reactivity which implies 7348

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a core transformation, and altogether some 25 chalcogenophilic middle-, late- and post-transition metals have been incorporated. The chemistry of the heterometallic clusters with {M3M 0 Q4} cores was initially predominantly developed in aqueous media, and major advances have been made in the field of molybdenum clusters, where the heterometals incorporated into the incomplete cuboidal [Mo3S4(H2O)9]4+ aqua ion according to [3+1] cluster building strategy range from group 6 to 15 elements. 6.1 Mo3S44+ as a constitutional unit within a Dawson-like structure The arrangement of metal, chalcogen and oxygen atoms in the cluster aqua complex [Mo3S4(H2O)9]4+ is the same as in the {W3O13}8 units which forms standard building blocks for Keggin- and Dawson-type structures. No stable lacunary POMs based on complementary structures to the {M3S4} moiety are known. The main reason is that the m3-S atom must take the place, upon incorporation, of one of the oxygen atoms of the tetrahedral units EO4 (E = P, Si etc.) which is not chemically viable. The only suitable candidates appear to be the POMs with a Dawson-like {EW18O60} core (E = Se(IV), Te(IV), As(III), Sb(III), Bi(III), Sn(II)),67 because in their structures the stereochemically active lone pair of the {EW9O33} subunits prevents the incorporation of the second heteroelement into the other half of the structure, which thus simply consists of three {W3O13} units linked together through common vertices. In that case, the replacement of at least one of such units becomes viable. In fact, a unique family of hybrid chalcogenide cluster-incorporated polyoxometalates (POM) has been synthesized, in which one {W3O4}10+ unit is replaced by topologically similar chalcogenide cluster fragments {Mo3S4}4+ and {Mo3S2O2}4+.68 This family includes complexes [EW15Mo3S4(H2O)3O53]9 (E = As, Sb); [TeW15Mo3S4(H2O)3O53]8 and [AsW15Mo3O2S2(H2O)3O53]9. As seen in Fig. 19, [AsW15Mo3S4(H2O)3O53]9 can be described as derived from the known prototype69 [H2AsW18O60]7 by replacing one of the six {W3O13} units

Fig. 19 Structure of the pseudo-Dawson [AsW15Mo3S4(H2O)3O53]9 anion. (a) Isolated species showing the presence of the {Mo3S4(H2O)3} as a constitutional trimetallic unit; (b) view of the dimeric arrangement observed in the solid state built on four specific van der Waals S S contacts. Color code: grey sticks = {AsW15} framework; green sticks = {Mo3S4} core; yellow spheres = S atoms.

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with common vertices in the As-free hemisphere by a {Mo3S4O6(H2O)3} unit, so that one of the polar belts includes one Mo atom and the equatorial belt the other two Mo atoms. The three m2-S bridges of the cluster replace the three m2-O bridges of the parent arsenotungstate, and the position of the capping m3-S matches the position occupied by one of the m3-O bridges. As in the archetypical [H2AsW18O60]7, both hemispheres in [AsW15Mo3S4(H2O)3O53]9 belong to the B-type, from the way in which the octahedral units (15{WO6} and 3{MoO3S3}) are linked together. However, an important difference is that the Mo atoms in the POM are in +IV oxidation state, instead of +VI, and three Mo–Mo bonds (2.764(3)–2.763(3) A˚) are present in the Mo3 subunit. The bonding is not localized over the remainder of the POM structure: closest Mo W distances are as long as 3.8 A˚. The As–(m3-S) distance of 3.648(7) A˚ is shorter than the sum of their van der Waals radii and two of the Mo–(m3-S) bonds (which are usually quite independent of the environment around Mo in other {Mo3S4}4+ clusters) are shortened to 2.27 A˚ from their usual value of around 2.33–2.34 A˚. Shortened As–O distances (1.79(1)–1.82(2) A˚ vs. 1.82(2)–1.90(2) A˚ in [H2AsW18O60]7) show that the As atom is pushed away from the S atoms. In the crystal, two [AsW15Mo3S4(H2O)3O53]9 ions are weakly associated into a dimer through four specific m2-S m2-S contacts (3.42–3.49 A˚). The hybrid POM [AsW15Mo3S4(H2O)3O53]9 is directly obtained by hydrothermal reaction of [Mo3S4(H2O)9]4+ with [AsW9O33]9 in 1 : 1.5 molar ratio. This reaction clearly involves partial decomposition of some of the [AsW9O33]9 to furnish the tungstate necessary to achieve the As : W = 1 : 15 stoichiometry required in the final product. Attempts to streamline the reaction pathway by reacting [Mo3S4(H2O)9]4+ and [AsW9O33]9 in 1 : 1 molar ratio in the presence of another six equivalents of Na2WO4 only favored the formation of side products, mainly [H2AsW18O60]7. The Sb analogue [SbW15Mo3S4(H2O)3O53]9 was similarly obtained from [Mo3S4(H2O)9]4+ and [SbW9O33]9. Telluritotungstate [TeW9O33]8 gives, depending on experimental conditions, [TeW15Mo3S4(H2O)3O53]8 and [TeW15Mo3S4(H2O)2ClO53]9. The mixed oxo–sulfido cluster core was incorporated by the reaction of [Mo3O2S2(H2O)9]4+ with [AsW9O33]9 in 1 : 2 molar ratio. All these new sulfur-containing POMs were isolated in the solid state and structurally characterized as mixed Na–Cs salts Cs6.5K(NH4)1.5[{(Mo3S4(H2O)3)AsW15O53}] 14H2O, Cs5.85Na3.15[{Mo3S4(H2O)3}SbW15O53]14.85H2O, Cs6Na2[{Mo3S4(H2O)3}TeW15O53]11.7H2O, Cs7.15Na1.85[{Mo3S4(H2O)2Cl}TeW15O53]11.2H2O, Cs6(H3O)3[{Mo3O2S2(H2O)3}AsW15O53]9.85H2O. The packing in the crystals is dominated by S S contacts (3.3–3.5 A˚) between the outwardlooking m2-S atoms between two anions, resulting in centrosymmetric dimers, except in the case of Cs6(H3O)3[{Mo3O2S2(H2O)3}AsW15O53]9.85H2O where such contacts are absent. According to 183W NMR spectrum, the structure of [AsW15Mo3S4(H2O)3O53]9 anion is preserved in solution. The built-in sulfide ligands represent a new center of coordination for ‘‘soft’’ metal ions, and represent a potential avenue to combine high-oxidation state POMs with low-oxidation state metal centres: it was shown that [AsW15Mo3S4(H2O)3O53]9 reacts with Cu+ with the formation of a new heterometal This journal is

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Fig. 20 Structural representation of [AsW15Mo3(CuCl)S4(H2O)3O53]9 resulting from the coordination of the Cu+ ion on the three bridging m2 sulfur atoms. Color code; Cu = violet sphere; Cl = light green sphere.

cluster [AsW15Mo3(CuCl)S4(H2O)3O53]9 containing metals in localized high (W+6), middle (Mo4+) and low (Cu+) oxidation states (see Fig. 20).49 The Cu–S and Cu–Cl bond lengths show no unusual features when compared with those observed in [Mo3CuS4(Hnta)3]2.70 6.2 Non-complementary binding of {Mo3S4}4+ to B-[AsW9O34]9: interplay of coordination and supramolecular chemistry As previously shown (see Fig. 8 and 12), the trivacant anion [B-AsW9O33]9 contains a crown of six terminal oxygen atoms, constituting the coordinating centers. The perimeter of the {O6} crown is about 17 A˚. This arrangement in principle could favour complementary binding between the [Mo3S4(H2O)9]4+ and [AsW9O33]9 since the former has six water molecules pointing in the same direction, potentially exchangeable for the six terminal oxygen atoms of [AsW9O33]9. However, the center of the {O6} crown is occupied by the active lone pair of the AsIII atom preventing any close approach of m3-S from the cluster and making the abovementioned complementary coordination impossible. Actually, the reaction of [Mo3S4(H2O)9]4+ with arsenitotungstate [AsW9O33]9 gives an impressive hybrid POM-cluster supramolecular complex {[(H4AsW9O33)2(Mo3S4(H2O)5)]2}12, depicted in Fig. 21.71 Two {H4AsW9O33}5 subunits sandwich a {Mo3S4(H2O)5}4+ moiety forming the complex [(H4AsW9O33)2(Mo3S4(H2O)5)]6. The supramolecular dimeric complex consists of two such units held together by hydrogen bonding between terminal hydroxo groups of the [H4AsW9O33]5 subunits and coordinated water molecules and by S S contacts between two Mo3S4 cluster cores (see Fig. 21). The complex [(H4AsW9O33)2(Mo3S4(H2O)5)]2}12 reacts with extra arsenite to give the closely related anion {[(H2As2W9O34)(H4AsW9O33)(Mo3S4(H2O)5)]2}14 through a grafting of one {AsOH}2+ group to one of the {AsW9} subunits (see Fig. 22). Electronic spectra show that the dimeric {[(H2AsW9O34)(H4AsW9O33)(Mo3S4(H2O)5)]2}14 and the monomeric [(H4AsW9O33)(H2AsW9O33)(Mo3S4(H2O)5)]7 species are in equilibrium in solution, characterized by a dissociation constant Kd B930 (see Fig. 23). Ag+ and Cu+ react Chem. Soc. Rev., 2012, 41, 7335–7353

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Fig. 21 Structure of the dimeric association {[(H4AsW9O33)2(Mo3S4(H2O)5)]2}12. The covalent unit consists of two {AsW9O33} subunits bridged by one {Mo3S4(H2O)5} linker. The dimerization of the covalent monomer results from the presence of twelve hydrogen bonds (blue dotted lines) and four S S contacts (yellow dotted lines).

with {[(H2AsW9O34)(H4AsW9O33)(Mo3S4(H2O)5)]2}14, leading to {[Ag2(H2As2W9O34)(H2AsW9O33)(Mo3S4(H2O)5)]2}16 and {[Cu(H2As2W9O34)(H4AsW9O33)(Mo3S4(H2O)5)]2}13, respectively (Fig. 22). In the silver complex Ag+ is in a unique environment of two sulfur atoms from the two {Mo3S4} units, plus two oxygen atoms and one central arsenic atom from one of the {AsW9O33} subunits. This is the first example of direct coordination between silver and {Mo3S4} moieties. UV-vis and potentiometric titrations confirm that the incorporation of Ag+ ions into {[(H2AsW9O34)(H4AsW9O33)(Mo3S4(H2O)5)]2}14

proceeds in two successive steps with K1 = 4.1 106 and K2 = 2.3 105. For both the Ag- and Cu-containing complexes 183W NMR pattern agrees with a dynamic hopping of the heterometal cations between equivalent coordination sites. Cuboidal clusters [Mo3(MCl)(H2O)9]3+ (M = Pd or Ni) which are topologically similar to [Mo3S4(H2O)9]4+ react with the trivacant [AsW9O33]9 Keggin-type anion to give nanosized complexes [(H2AsW9O33)4{Mo3S4M(H2O)5}2]20. As shown in Fig. 22, this assembly displays a similar dimeric arrangement as observed in the above-discussed {[(H2As2W9O34)(H4AsW9O33)(Mo3S4(H2O)5)]2}14 which can be considered as the parent compound. The structural relationship between these species corresponds to the formal inclusion of a {M–M} core within the dimeric arrangement (see Fig. 22). The central double-cuboidal cluster core {Mo3S4M–MS4Mo3}8+ is embedded between four {AsW9O33}9 grafted onto the central cluster by W–O–Mo bridges. The four {AsW9O33} subunits act as bidentate ligands distributed in two equivalent groups. Two of them coordinate a single molybdenum atom with two terminal oxygen atoms whereas the other two bridge the two remaining molybdenum atoms. Five water molecules are located at the remaining coordination sites of the {Mo3S4} clusters to complete the octahedral coordination sphere of the molybdenum atoms. These coordinated water molecules are involved in internal hydrogen bonds with the terminal oxygen atoms, and the resulting twelve hydrogen bonds are characterized by short O O separations of 2.613(2)–2.684(2) A˚. Studies in solution reveal that the double cuboidal core structure is maintained in solution.

Fig. 22 Reaction pathways illustrating the coordination properties of the supramolecular {[(H4AsW9O33)2(Mo3S4(H2O)5)]2}12 dimer. (a) {M–M} insertion (M = Pd or Ni) at the centre of the cluster; (b) grafting of two outer {AsOH}2+ groups on the periphery of the cluster; (c) coordination of two Ag+ ions spanning the {AsW9O33} and {Mo3S4} subunits; (d) coordination of a single Cu+ ion bridging two {Mo3S4} fragments.

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Fig. 25 View of the sandwich compound [(SiW11O39)(Mo3S4(H2O)3(m-OH))2]10. Fig. 23 UV-vis change resulting from the dissociation of the dimeric arrangement {[(H4AsW9O33)2(Mo3S4(H2O)5)]2}12. Experimental (J) and calculated values (—) for the dissociation constant Kd = 930 (—) are shown.

In the presence of exogeneous ligands, such as iodide or pyridine, dissociation of the double cubane into single cubanes (type A) is observed through a M–L coordination bond (L = I or Py).72 6.3

Macrocycle assembly from {Mo3S4}4+ and c-[SiW10O36]8

The brown ‘‘macrocycle’’-like complex [{Mo3S4(H2O)5}4(g-SiW10O36)4]16 shown in Fig. 24 was obtained from equimolar amounts of [Mo3S4(H2O)9]4+ and [g-SiW10O36]8 at pH 1–5 and isolated as (Me2NH2)16[{Mo3S4(H2O)5}4(SiW10O36)4] 20H2O. According to X-ray data, the size of the cyclic anion is 16 30 A˚, and a inner cavity of 4.4 A˚ in diameter surrounded by 12 m2-S atoms is present.73 6.4 Coordination-induced condensation of two clusters: combination between [Mo3S4(H2O)9]4+ and monovacant Keggin or Dawson heteropolytungstates An interesting aggregation resulting from the combination of trinuclear clusters [Mo3S4(H2O)9]4+ with monovacant Keggin

Fig. 24 Representation of the cyclic arrangement resulting from the connections of four {SiW10O36} subunits and four {Mo3S4(H2O)5} linkers.

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and Dawson-type lacunary anions [SiW11O39]8 and [P2W17O61]10 yields nanosized hybrid cluster species [(SiW11O39)Mo3S4(H2O)3(m-OH))2]10 and [(P2W17O61)Mo3S4(H2O)3(m-OH))2]14 isolated and structurally characterized as Me2NH2+ salts.26 The central core results from two {Mo3S4(H2O)3}4+ fragments connected through two bridging OH groups (cis to m3-S) (see Fig. 25). Such an arrangement is reminiscent of that obtained from monovacant Keggin or Dawson ions with [Mo2O2S2(H2O)6]2+ where two sandwiched thiofragments interact closely through linear {MQO Mo} junctions. These large clusters are stable in solution at pH between 1 and 7 and are quantitatively converted into [Mo3S4(H2O)9]4+ in more acidic media. The structure of the reaction products was explained by taking into account the preferred octahedral coordination of molybdenum and the number and arrangement of oxygen atoms in the lacunary-type anions acting as strong donors. Linking two {Mo3S4}4+ clusters with hydroxo bridges is probably the only way to maintain two clusters and two POMs in the same structure while reducing the number of extra ligands to ease steric repulsion.

7. Summary and outlook The field of polyoxothiometalates is still recent, because only a very limited number of archetypical {M–S} cationic building blocks have been engaged into combination with polyoxometalates as regards to the richness of inorganic thiochemistry. Besides, other synthetic approaches should be deployed depending on the nature of the metal and the targeted chemical system. The use of non-aqueous medium or specific organic sulfurizing agent or preassembled M–S clusters should be useful to develop rational synthetic strategies. Nonetheless, this pioneering work demonstrates that the simplest {Mo2O2S2} and {Mo3S4} core can be appropriately combined with polyoxometalate either as constitutional units into Keggin or Dawson-like structures, or as linking groups in polymodular assemblies and also as template components for the synthesis of new POM arrangements. With this rational building block strategy, a large number of oxo-thio derivatives have been obtained. Structural investigations, supported by solution studies, such as UV-vis, multinuclear NMR (31P, 183W, 39K) showed that these systems are able to push forward some current challenges, important for the expansion of POM chemistry. Self-assembly processes, supramolecular Chem. Soc. Rev., 2012, 41, 7335–7353

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chemistry, host–guest properties and molecular recognition represent many fields of applications where this class of sulfurcontaining POM is relevant. Efforts on exploring the chemical–physical properties of this class of molecular materials are necessary. For instance, the {Mo3S4}-containing POMs represent a unique platform for further functionalization. The labile terminal aqua ligand, suitable to be substituted for functional groups, and the possibility to coordinate soft metals with zero-valent character, supported by the fact that {Mo3S4} is a redox active component, should open the way for the design of POM-hybrids and for the realization of functioning POM-based integrated nanosystems (PINs).

Acknowledgements We gratefully acknowledge the Centre National de la Recherche Scientifique (CNRS), the Ministe`re de l’Education Nationale de l’Enseignement Supe´rieur et de la Recherche (MENESR) and the University of Versailles Saint Quentin for their financial support. Dr Je´roˆme Marrot is also gratefully acknowledged for X-ray diffraction studies.

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