1 Introduction

Polyoxometalate (POM) clusters can be formed in acidic solution by condensation reactions of monomeric oxoanions of transition metals belonging mainly to groups V and VI [1]. In particular, a huge number of species featuring tungsten, molybdenum, vanadium and/or niobium atoms have been isolated. In this family of compounds, it has to be distinguished between the compounds comprising only one type of heavy atom, called isopolyoxometalates, and the compounds obtained in presence of a {XO₄} additional group (X = P^V, As^V, Si^IV, Ge^IV…), called heteropolyoxometalates. To date, four families of compounds and their derivatives dominate the chemistry of POMs, namely Lindquist {M₆O₁₉}, Keggin {XM₁₂O₄₀}, Dawson {X₂M₁₈O₆₂} and Anderson {M’M₆O₂₄} (where M’ is an octahedrally coordinated cation) type systems. For these species, the heavy atoms are generally in their highest oxidation state (W^VI, Mo^VI, V^V, Nb^V,…), but, in some cases, related reduced POMs can be chemically or electrochemically obtained, as shown, for example, by the characterization of the mixed-valence four electron reduced β-[H₄PMo₁₂O₄₀]³⁻ polyanion [2]. It has to be noticed that, beside these archetypal families of POMs, the chemistry of molybdenum mixed-valence POM clusters developed by Achim Müller and his collaborators has afforded spectacular, very high nuclearities compounds, such as a species which contains as many as 368 metal atoms [3]. Focusing on polyoxomolybdate systems where all the molybdenum centers are reduced in their V oxidation state, a large number of species have been isolated both in organic or aqueous media. In all cases, these polyanions contain the dinuclear dicationic [Mo^V₂O₄]²⁺ unit, where a metallic bond connects the two d¹ centers, leading to a diamagnetic behavior of this fragment even at room temperature. The connection of [Mo^V₂O₄]²⁺ units by various organic ligands (oxalate, squarate, polycarboxylate, carbonate, ethanolate, methanolate, etc.) has afforded systems containing from one to six dinuclear fragments [4]. On the other hand, the ability of the related [Mo^V₂O₂S₂]²⁺ oxothiofragment to form polyoxothiometalates has also been studied, affording cyclic systems with nuclearities ranging from 8 to 16 [5].

In our group, we have focused our investigations involving the [Mo^V₂O₄]²⁺ building unit on the polycondensation of this dinuclear fragment via pyrophosphate [O₃POPO₃]⁴⁻ and methylenediphosphonate [O₃P(CH₂)PO₃]⁴⁻ ligands in the eventual presence of other coordinating groups (carboxylate, carbonate, sulfite…) in aqueous media. Our investigations have allowed us to isolate a wide variety of pure Mo^V cyclic POM clusters of various nuclearities. For example, the triangular cluster [(Mo^V₂O₄)₃(O₃P(CH₂)PO₃)₃(CH₃AsO₃)]⁸⁻ has been obtained in presence of methyl arsenate,[6] while ellipsoidal octanuclear clusters have been isolated when the synthetic process is performed in presence of sulfite or carbonate [7]. A larger complex, a dodecanuclear wheel which adopts a cyclohexane-like ring in a chair conformation has been obtained in absence of any additional coordinating groups. Nevertheless, the presence of ammonium cations in the reacting medium is crucial for the obtaining of this species, highlighting the importance of the nature of the counter-ions on the resulting POM [6]. In addition to these cyclic systems, bicyclic compounds have also been obtained. The isolated isostructural complexes [Na₂{(Mo^V₂O₄)₁₀(O₃P(X)PO₃)₁₀(CH₃COO)₈}]²⁴⁻ (X = O, CH₂) are made of two interconnected and nearly perpendicular octanuclear and dodecanuclear wheels, respectively [8]. Recently, we have shown that such diphosphonate Mo^V species are able to form and stabilize metal nanoparticules (NPs), which, from an electrocatalytical point of view, can combine the properties of metal NPs and POM complexes. Moreover, such nanosystems are obtained in a one-pot procedure, in water, at room temperature and without any additional surfactant. For example, by simply mixing the cyclohexane-like dodecanuclear compound mentioned above and Pd^II cations in aqueous solution, NPs with diameter of 9–14 nm containing a Pd⁰ core and capped by POMs are obtained [6]. Additionally, it has also been shown that Ag⁰ [9] and Pt⁰ [10] POM capped NPs can be obtained using this procedure. Beside the observation that this family of POMs allows the formation of NPs, it has also been stated that the sizes and shapes of the obtained nanosystems depend on the nature of the polynuclear Mo^V complex used. In order to synthesize NPs with the desired morphology, it is thus essential to elaborate a library of reduced POMs with different structural and redox characteristics.

In this article, we present our results concerning the synthesis and the characterization of polynuclear Mo^V POMs obtained by condensation of the dinuclear [Mo^V₂O₄]²⁺ fragment with the 1-hydroxyethane 1,1-diphosphonate (or etidronate) ligand [O₃P(C(CH₃)(OH))PO₃]⁴⁻ (Fig. 1a). Our goal was to answer two questions: (a) do the potentially available Mo^V/etidronic POMs display the same topologies as those obtained by condensation of dinuclear [Mo₂O₄]²⁺ fragments with methylenediphosphonic ligands; and (b) can the hydroxo group grafted on the carbon center linking the two phosphorous atoms in [O₃P(C(CH₃)(OH))PO₃]⁴⁻ act as a ligand for the Mo^V cations? We thus report here the structural characterization of the tetranuclear compound (CN₃H₆)₆[(Mo^V₂O₄)₂(O₃P(C(CH₃)O)PO₃)₂]·12H₂O (1), the dodecanuclear complex (NH₄)₁₈[(Mo^V₂O₄)₆(OH)₆(O₃P(C(CH₃)(OH))PO₃)₆]·35H₂O (2) and the tetradecanuclear species (CN₃H₆)₁₈Na₃[(Mo^V₂O₄)₇(O₃P(C(CH₃)O)PO₃)₇(CH₃COO)₇]·5CH₃COONa 52H₂O (3). Their stability in aqueous media has also been studied by ³¹P NMR.

2 Experimental section

3 Results and discussion

Compound 1 has been obtained by adding solid etidronic acid to a solution of [Mo^V₂O₄(H₂O)₆]²⁺. The pH of the mixture was raised to 6.4 with sodium hydroxyde, followed by an addition of guanidinium chloride. After slow evaporation of the filtrate, red crystals of 1 were obtained. A single crystal X-ray diffraction study revealed that 1 is made of a [(Mo^V₂O₄)₂(O₃P(C(CH₃)O)PO₃)₂]⁶⁻ tetranuclear polyanion (Fig. 1b) with six guanidinium counter-ions. Two crystallographically non-equivalent [(Mo^V₂O₄)₂(O₃P(C(CH₃)O)PO₃)₂]⁶⁻ subunits have been found in the unit cell. Nevertheless, these two subunits are highly similar and will be considered as identical for the forthcoming discussion. Each [(Mo^V₂O₄)₂(O₃P(C(CH₃)O)PO₃)₂]⁶⁻ unit contains a centrosymetric, rhomboid {Mo₄O₁₆} core with localized metal–metal bonds. Such topology is recurrent in the Mo^V chemistry [15]. The two crystallographically equivalent etidronate groups constituting each [(Mo^V₂O₄)₂(O₃P(C(CH₃)O)PO₃)₂]⁶⁻ fragment in 1 act as tetradentate ligands, respectively (see Scheme 1a). For each [O₃P(C(CH₃)O)PO₃]⁵⁻ ligand, one {PO₃} group is monocoordinated to a Mo^V center, while the other phosphonato group is coordinated via two oxygen atoms to an adjacent metallic center. Additionally, the hydroxo group bound to the carbon atom connecting the P^V centers is found to be deprotonated, and acts as a μ₂O ligand. The characterization of 1 then allows one to answer the questions asked in the introduction section: the hydroxo group of an etidronate ligand can be deprotonated and connect Mo^V centers even in slightly acidic medium, and then the reactivity of this ligand can be different from the reactivity of the methylenediphosphonate ligand. Moreover, it should be mentioned that, despite the large number of Mo^V / methylenediphosphonate ligands characterized to date, no complex possessing a {Mo^V₄O₁₆} core had been observed using this ligand, confirming that the Mo^V chemistry involving the methylenediphosphonate ligand cannot be transposed to that involving the etidronate ligand. The stability of 1 in aqueous medium has been studied. The ³¹P NMR spectrum in water is represented in Fig. 2. It exhibits a doublet of doublets at 32.28, 32.10, 22.12 and 21.93 ppm (J = 22.1 Hz). Such a spectrum is in agreement with the presence of two independent and coupled phosphorous atoms, indicating that the structure observed in the solid state for the anion constituting 1 is maintained in solution.

Compound 2 has been synthesized using a similar procedure to that described for compound 1, but in this case the pH of the solution was adjusted to 7.3 with ammonia and no additional cationic species was added. Its structure (Fig. 3) is similar to that previously described for the [(Mo^V₂O₄)₆(OH)₆(O₃P(CH₂)PO₃)₆]¹⁸⁻ methylenediphosphonato complex that we recently reported [6]. It consists in six connected {(Mo^V₂O₄)(O₃P(C(CH₃)(OH))PO₃} units which form a cyclohexane-like ring in a chair conformation and where the tetracoordinated etidronate ligands (see Scheme 1b) are at the vertices and the edge-sharing metallic octahedra are at the edges, leading to a wheel with a pseudo S₆ symmetry. Bond valence summations indicate that six Mo^V bridging oxygen atoms are protonate [16]. Additionally, contrary to compound 1, in 2 the hydroxo groups of the etidronate ligands are not deprotonated, explaining why the analogous methylenediphosphonate complex can be obtained. As previously found for this last compound, ³¹P NMR studies indicate that 2 degrades rapidly in pure water into unknown products. Moreover, we have not been able to find an aqueous medium (1 M LiCl, 1 M NaCl…) where compound 2 can be stabilized.

Compound 3 was prepared by mixing sodium molybdate, etidronic acid, guanidinium chloride and hydrazine in 4 M CH₃COONa/CH₃COOH buffer (pH = 4.9). After slow evaporation of the filtrate for one day, crystals of 3 were obtained. In 3, seven {(Mo^V₂O₄)(O₃P(C(CH₃)(OH))PO₃)(CH₃COO)} units (Fig. 4a) are interconnected, forming a tetradecanuclear wheel (Fig. 4b) with an internal diameter of ca. 12 Å. In each dinuclear building unit, the oxygen atoms of the μ_1,1 acetato ligand are located in axial position of the distorted {Mo^VO₆} octahedra, in trans position of the terminal Mo = O oxo groups. As in complex 2, the hydroxo groups of the etidronate ligands are not deprotonated. It follows that in each {(Mo^V₂O₄)(O₃P(C(CH₃)(OH))PO₃)(CH₃COO)} building unit, the [O₃P(C(CH₃)(OH))PO₃]⁴⁻ groups are doubly coordinated to the [Mo^V₂O₄]²⁺ fragment (see Scheme 1b), the oxygen atoms of the etidronate ligand connected to the Mo^V center occupying two equatorial positions. The connection of seven building units affords an almost planar cyclic system. From a crystallographic point of view, a mirror plane passing through the phosphorous atoms of an etidronic ligand and the carbon atoms of an acetate ligand (see Fig. 4b) is observed. The presence of this symmetry element induces a crystallographic disorder of the methyl and the hydroxo group of the etidronate ligand located on the mirror plane, these groups occupying two crystallographic sites with an occupancy factor of 0.5, respectively. It then follows that 3 exhibits only a C_s symmetry. The solution behaviour of this compound has also been studied. As for compound 2, ³¹P NMR studies revealed that 3 degrades into unknown products in aqueous media.

4 Conclusion

We have shown here that it is possible to isolate Mo^V/etidronate cyclic systems of various shapes and nuclearities. Moreover, depending on the synthetic conditions, it appears that the hydroxo group of the [O₃P(C(CH₃)(OH))PO₃]⁴⁻ can either be deprotonated and then connect Mo^V centers (compound 1) or remain protonated (compound 2 and 3). The experimental factors governing the possible deprotonation of this ligand in presence of Mo^V cations have not been clearly elucidated, but it can be remarked that the deprotonation processes cannot be attributed exclusively to the pH of the reacting medium. Indeed, 1 has been isolated at pH = 6.4 while 2 was obtained at pH = 7.4. The role of the counter-ions on the formation of the final product then seems crucial, as previously observed for the family of methylenediphosphonate POMs [8]. Obviously, when deprotonation occurs, systems that cannot be obtained using the methylenediphosphonate ligands are isolated, but it can be noticed that to date, surprisingly, no methylenediphosphonato Mo^V complex analogous to complex 3 has been isolated. Finally, to the best of our knowledge, complex 3 represents the homometallic Mo^V polyoxometalate cyclic system with the highest nuclearity reported to date. The next step is now to study the ability of these systems to form and stabilize metal nanoparticules. Additionally, the synthesis of analogous POM compounds containing functionnalized diphosphonate ligand [O₃P(C(OH)R)PO₃]⁴⁻ where R is an alkyl group bearing amino, phenolato, thiophene, etc. functional groups will be published soon. Finally, the reactivity of these diphosphonate ligands with MoVI cations must be studied [17].

5 Supplementary materials

The supplementary material has been sent to the Cambridge Crystallographic Structural Database, 12 Union Road, Cambridge, CB2 1EZ, UK, as supplementary material CCDC Nos. 734898, 734899 and 734900, for 1, 2 and 3, respectively, and can be obtained by contacting the CCDC, (fax: +44-1223-336033; e-mail: deposit @ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

6 Table of content

The reactivity of the [Mo^V₂O₄]²⁺ dinuclear unit with the [O₃P(C(CH₃)(OH))PO₃]⁴⁻ etidronate ligand has been investigated for the first time. Three complexes of nuclearities ranging from four to 14 have been isolated and characterized by IR spectroscopy, elemental analysis and single crystal X-Ray diffraction studies. Their stability in aqueous solution has been studied by ³¹P NMR.