Olivier Oms

Diversity in structures and properties of 3d-incorporating polyoxotungstates.

This critical review surveys the 3d substituted magnetic polyoxotungstate (POT) molecular compounds reported these last ten years, illustrating the huge variety of topologies that these systems can adopt. Both purely inorganic and organic/inorganic species have been considered, and the cited compounds have been classified according to their nuclearity, i.e., the number of magnetic 3d centres embedded in the POT matrix, which can be as high as 48. This review highlights the synthetic conditions, the solubility and the stability in solution of these species. The magnetic, catalytic and electrocatalytic properties of these complexes are also summarized.

Photo- and electrochromic properties of covalently connected symmetrical and unsymmetrical spiropyran-polyoxometalate dyads.

Polyoxometalates covalently linked to one or two spiropyran entities have been isolated. These organic-inorganic hybrids exhibit multi-electrochromic and photochromic properties.

Route for the elaboration of functionalized hybrid 3d-substituted trivacant Keggin anions.

On the basis of four examples involving various transition-metal cations and various carboxylic acid derivatives, we have shown that hybrid 3d metal-containing polyoxometalate systems can be easily built under mild conditions.

Sulfonium polyoxometalates: a new class of solid-state photochromic hybrid organic-inorganic materials.

For the very first time, sulfonium polyoxometalate (POM) assemblies are shown to develop efficient solid-state photochromism in ambient conditions. The optical properties of the already known Rb(0.75)(NH(4))(5.25)[(Mo(3)O(8))(2)O(O(3)PC(CH(2)S(CH(3))(2))OPO(3))(2)]·8H(2)O (1) and a new material (Me(3)S)(4)[Mo(8)O(26)] (2) under UV excitation are investigated by diffuse reflectance spectroscopy, revealing that the color change effect is highly tunable playing with the nature of the POM. A mechanism involving the photoreduction of Mo(6+) cations associated with electron transfers from the sulfonium cations toward the POMs is proposed.

Properties of a Tunable Multinuclear Nickel Polyoxotungstate Platform

A series of hybrid nickel bisphosphonate (BP) polyoxometalates (POMs) has been isolated. The complexes NaK-Ni7-Ale2 (Ale = [H2O3PC(C3H6NH2)(OH)PO3H2]) and NaNH4-Ni7-Ale2 are both made of two {PW9O34} fragments enclosing a heptanuclear Ni(II) core connected to two alendronate ligands. By pre-functionalising the alendronate moiety through the amino group, the naphthalene (napht) derivative Ni7-(AleNapht)2 and the compounds Ni7-(AlePy2 Ni)2 (py = pyridyl) and Ni7-(AleAc2Ni)2 (Ac = acyl) have been obtained. Compared with the NaK-Ni7-Ale2 species, these last two complexes contain two additional Ni(II) centres connected through two bis(2-pyridylmethyl)amine or two iminodiacetate groups, respectively. This results show that it is possible to functionalise the heptanuclear POM in a controlled manner. Quantification of the magnetic interactions in NaK-Ni7-Ale2 revealed that in the {Ni7} core, ferromagnetic interactions are predominant, with a S = 5 ground state. Magnetisation versus dc field sweeps on a single crystal of NaK-Ni7-Ale2 exhibited hysteresis at low temperature. (1)H and (31)P NMR studies in aqueous solution performed on NaK-Ni7-Ale2 and Ni7-(AleNapht)2 evidenced that the PW9/Ni7/bisphosphonate assembly is stable in solution. This was completed by (31)P magic angle spinning (MAS) investigations and confirmed by (1)H DOSY experiments. The electrochemistry of these compounds proceeds through two well-defined four-electron chemically reversible waves in a medium at pH 6. NaK-Ni7-Ale2 proved to be efficient for the electrocatalytic reduction of nitrate, nitrite and nitrous oxide. Remarkably, its electrocatalytic efficiency for nitrate reduction is approximately three times higher than those previously reported for POMs in a medium at pH >4 under the same potential. The catalytic properties of two representatives of the hybrid family were also examined. It is shown that these nickel bisphosphonate polyoxotungstates are pre-catalysts for the oxidation of alcohols into ketones or carboxylic acids, depending on the classes of alcohols considered, the stoichiometric oxidant used being H2O2. Noticeably, it has been found that an analogous cobalt bisphosphonate polyoxotungstate complex does not present any related activity, highlighting the crucial role of the 3d cations on the catalytic process.

Photochromic Properties of Polyoxotungstates with Grafted Spiropyran Molecules

The first systems associating in a single molecule polyoxotungstates (POTs) and photochromic organic groups have been elaborated. Using the (TBA)4[PW11O39{Sn(C6H4I)}] precursor, two hybrid organic-inorganic species where a spiropyran derivative (SP) has been covalently grafted onto a {PW11Sn} fragment via a Sonogashira coupling have been successfully obtained. Alternatively, a complex containing a silicotungstate {PW11Si2} unit connected to two spiropyran entities has been characterized. The purity of these species has been assessed using several techniques, including (1)H and (31)P NMR spectroscopy, mass spectrometry, and electrochemical measurements. The optical properties of the hybrid materials have been investigated both in solution and in the solid state. These studies reveal that the grafting of SPs onto POTs does not significantly alter the photochromic behavior of the organic chromophore in solution. In contrast, these novel hybrid SP-POT materials display highly effective solid-state photochromism from neutral SP molecules initially nonphotochromic in the crystalline state. The photoresponses of the SP-POT systems in the solid state strongly depend on the nature and the number of grafted SP groups.

Synthesis of Planar Five‐Connected Nodal Ligands

Synthetic routes to a penta(4-pyridyl)cyclopentadienyl ligand are explored. The most successful route uses a palladium-catalysed pentapyridation of di(tert-butyl)phosphinoferrocene by using a procedure developed by Hartwig. The same method allows the synthesis of cyclopentadiene ligands substituted with 4-benzaldehydes or 4-phenylthiols. The pyridine ligands are formally five-connected nodes that may be linked by linear coordination metals to give closed spherical complexes of composition [(metal)(30)(ligand)(12)] as shown by molecular modelling. Experiment shows that the ligand complexes copper(I) and silver(I) with the expected 1:2.5 stoichiometry, and the (1)H NMR spectrum of the resulting product shows the ligands to be equivalent. NMR diffusion and light-scattering measurements support the formation of a species with a hydrodynamic radius of the order of 15 A, in agreement with the modelling studies. The resulting complex would be topologically identical to the C(60) fullerene structure.

New photoresponsive charge-transfer spiropyran/polyoxometalate assemblies with highly tunable optical properties

Seven new photochromic hybrid organic–inorganic supramolecular assemblies have been synthesized by combining a photoswitchable cationic spiropyran (SP) with different polyoxometalate (POM) complexes, and the structures of five of them have been solved by single-crystal X-ray diffraction analysis. These materials differ by the nature of the POM (e.g. [Mo8O26]4−, [M(OH)6Mo6O18]3− (M = Al, Fe), and [Mo6O19]2−), the SP/POM ratio and the crystallized solvent molecules. Their optical properties before and under low-power UV irradiation have been thoroughly investigated by diffuse reflectance spectroscopy, and compared with those of the previously reported compound (SP)3[PMo12O40]. All seven SP/POM assemblies exhibit improved solid-state photochromic performances. The latter vary with the composition and the design of the hybrid frameworks, and structure–property relationships have been evidenced. Especially, the coloration of the materials before UV exposure is governed by a low-energy intermolecular charge-transfer (CT) transition between SP donor and POM acceptor. The CT transition energy can be tailored by tuning the intrinsic ligand-to-metal charge-transfer (LMCT) of the POM unit, which allows drastic improvement of the photocoloration contrasts. Besides, the coloration kinetics has been systematically quantified revealing that the SP photoisomerization strongly varies in the series. These results have been interpreted taking into account several physical parameters (SP structural characteristics, SP/POM and SP/solvent solid-state interactions, molar volume). Finally, based on studies involving two of the reported compounds, it has been evidenced that, under ambient conditions, such materials are remarkable quasi-bistable systems which exhibit fast and comparable coloration and fading rates, and show very good cyclabilities.

Heteroanionic Materials Based on Copper Clusters, Bisphosphonates, and Polyoxometalates: Magnetic Properties and Comparative Electrocatalytic NOx Reduction Studies

Three compounds associating for the first time polyoxotungstates, bisphosphonates, and copper ions were structurally characterized. They consist in heteropolyanionic monodimensional materials where [Cu6(Ale)4(H2O)4](4-) (Ale = alendronate = [O3PC(O)(C3H6NH3)PO3](4-)) complexes alternate with polyoxometalate (POM) units. In Na12[{SiW9O34Cu3(Ale)(H2O)}{Cu6(Ale)4(H2O)4}]·50H2O (SiW9CuAle), the polyoxometalate core consists in a {SiW9Cu3} monomer capped by a pentacoordinated Ale ligand, while sandwich-type Keggin {(SbW9O33)2Cu3(H2O)(2.5)Cl(0.5)} and Dawson {(P2W15O56)2Cu4(H2O)2} complexes are found in Na8Li29[{(SbW9O33)2Cu3(H2O)(2.5)Cl(0.5)}2{Cu6(Ale)4(H2O)4}3]·163H2O (SbW9CuAle) and Na20[{(P2W15O56)2Cu4(H2O)2}{Cu6(Ale)4(H2O)4}]·50H2O (P2W15CuAle), respectively. A comparative magnetic study of the SiW9CuAle and SbW9CuAle compounds enabled full quantification of the Cu(II) superexchange interactions both for the POM and non-POM subunits, evidencing that, while the paramagnetic centers are anti-ferromagnetically coupled in the polyoxometalate units, both anti-ferromagnetic and ferromagnetic interactions coexist in the {Cu6(Ale)4(H2O)4} cluster. All the studied compounds present a good efficiency upon the reduction of HNO2 or NO2(-), the POM acting as a catalyst. However, it has been found that SbW9CuAle is inactive toward the reduction of nitrates, highlighting that both the {(SbW9O33)2Cu3} unit and the {Cu6(Ale)4(H2O)4} cluster do not act as electrocatalysts for this reaction. In contrast, SiW9CuAle and P2W15CuAle have shown a significant activity upon the reduction of NO3(-) and thus both at pH 1 and pH 5, evidencing that the chemical nature of the polyoxometalate is a crucial parameter even if it acts as precatalyst. Moreover, comparison of the activities of P2W15CuAle and [(P2W15O56)2Cu4(H2O)2](16-) evidenced that if the [Cu6(Ale)4(H2O)4](4-) cluster does not act as electrocatalyst, it acts as a cofactor, significantly enhancing the catalytic efficiency of the active POM.

Sequential Synthesis of 3 d–3 d, 3 d–4 d, and 3 d–5 d Hybrid Polyoxometalates and Application to the Electrocatalytic Oxygen Reduction Reactions

The Co7 (AlePy)2 polyoxometalate, which encloses a {(PW9 )2 Co(II) 7 } core covalently bound to two free aminopyridine groups through bisphosphonate ligands (AlePy), has been isolated. It can be used as a precursor, allowing the synthesis of heterometallic hybrid compounds, as illustrated by the characterization of cobalt/zinc (Co7 (AlePyZn)2 ), cobalt/palladium (Co7 (AlePyPd)2 ), and cobalt/platinum (Co7 (AlePyPt)2 ) species. A composite based on the water-insoluble precious metal-free Co7 (AlePyZn)2 compound and the low-cost carbon material Vulcan XC-72 has been selected as a cathode material (Co7 Zn/C) for oxygen reduction reaction studies. The electrocatalytic performances of the Co7 Zn/C hybrids were assessed at neutral and basic pH, showing that Co7 Zn/C exhibits high selectivity for the four-electron reduction of O2 . Moreover, its durability is superior to that of a commercial Pt/C catalyst with 20 % loading. Also, comparative studies performed in the presence of methanol have indicated that Co7 Zn/C has a much better tolerance to the crossover effect than Pt/C. Altogether, these results indicate for the first time that, even in neutral media, polyoxometalate/carbon composites can represent low-cost oxygen reduction catalysts that can function stably, for a long time, and with high performance.