Derivatization of polyoxotungstates in aqueous solution. Exploration of the kinetic stability of cobalt(II)- and cobalt(III) derivatives of lacunary anions with pyridine and pyridine-type ligands

Four complexes of cobalt(II) with lacunary polyoxotungstate ligands, alpha-[PW11O39Co(H2O)](5-), alpha-[SiW11O39Co(H2O)](6-), alpha (1)-, and alpha (2)-[P2W17O61Co(H2O)](8-) have been examined as potential precursors to substitutionally inert derivatized polytungstates that are stable in aqueous solution at near neutral pH. Replacement of the terminal water ligand by pyridine is readily achieved and the rates of exchange between bound and free pyridine were determined from NMR line widths at temperatures between 24 and 48 degreesC. Oxidation to the aquacobalt(III) and pyridinecobalt(III) polytungstates occurs at potentials between +0.64 V (for alpha (1)-[P2W17O61Co(py)](8-)) and +1.00 V (for alpha (2)-[P2W17O61Co(H2O)](8-)). The cobalt(III) complexes (with the exception of alpha-[PW11O39Co(H2O)](4-) which undergoes slow reduction by the solvent) were prepared by controlled potential electrolysis and by oxidation with potassium perdisulfate. The rates of pyridine substitution on the aquacobalt(III) complexes were determined and follow the sequence alpha(1)-[P2W17O61Co(H2O)](7-) (similar to0.5 M-1 s(-1)) >> alpha (2)-[P2W17O61Co(H2O)](7-) similar to alpha-[SiW11O39Co(H2O)](5-). Cesium salts of alpha (1)-[P2W17O61CoIII(L)](7-) (L = pyrazine, 4,4'-bipyridine, isonicotinic acid, and the ethyl ester of isonicotinoylglycylglycine) and the "dumbbell" 2:1 complex with bridging 4,4' -bipyridine were isolated and characterized by cyclic voltammetry and H-1 NMR spectroscopy.