Polyoxomolybdodiphosphonates: Examples Incorporating Ethylidenepyridines

We have synthesized and structurally characterized three pyridylethylidene-functionalized diphosphonate-containing polyoxomolybdates, [{MoVIO3}2{MoV2O4}{HO3PC(O)(CH2-3-C5NH4)PO3}2]6– (1), [{MoVI2O6}2{MoV2O4}{O3PC(O)(CH2-3-C5NH4)PO3}2]8– (2), and [{MoV2O4(H2O)}4{O3PC(O)(CH2-3-C5NH4)PO3}4]12– (3). Polyanions 1–3 were prepared in a one-pot reaction of the dinuclear, dicationic {MoV2O4(H2O)6}2+ with 1-hydroxo-2-(3-pyridyl)ethylidenediphosphonate (Risedronic acid) in aqueous solution. Polyanions 1 and 2 are mixed-valent MoVI/V species with open tetranuclear and hexanuclear structures, respectively, containing two diphosphonate groups. Polyanion 3 is a cyclic octanuclear structure based on four {MoV2O4(H2O)} units and four diphosphonates. Polyanions 1 and 2 crystallized as guanidinium salts [C(NH2)3]5H[{MoVIO3}2{MoV2O4}{HO3PC(O)(CH2-3-C5NH4)PO3}2]·13H2O (1a) and [C(NH2)3]6H2[{MoVI2O6}2{MoV2O4}{O3PC(O)(CH2-3-C5NH4)PO3}2]·10H2O (2a), whereas polyanion 3 crystallized as a mixed sodium–guanidinium salt, Na8[C(NH2)3]4[{MoV2O4(H2O)}4{O3PC(O)(CH2-3-C5NH4)PO3}4]·8H2O (3a). The compounds were characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analyses. The formation of polyanions 1 and 3 is very sensitive to the pH value of the reaction solution, with exclusive formation of 1 above pH 7.4 and 3 below pH 6.6. Detailed solution studies by multinuclear NMR spectrometry were performed to study the equilibrium between these two compounds. Polyanion 2 was insoluble in all common solvents. Detailed computational studies on the solution phases of 1 and 3 indicated the stability of these polyanions in solution, in complete agreement with the experimental findings.