Dimerization of cobalt-substituted Keggin phosphotungstate, [PW₁₁O₃₉Co(X)]^5-, in nonpolar solvents

The tetraheptylammonium (THA) salt of cobalt-substituted polyoxotungstate with Keggin-like structure, (THA)₅[PW₁₁O₃₉Co^II(X)], was investigated by ³¹P NMR and UV/VIS spectroscopies in toluene. A solution obtained after transferring PW₁₁Co from aqueous solution into toluene, using THABr as a transfer agent, and rigorous drying, gives multiple peaks in the ³¹P NMR spectrum. The spectrum in dry toluene significantly simplifies, after extracting a hexane solution several times with water, which removes simple inorganic ions. However, even the dry toluene solution after purification has ³¹P NMR and UV/VIS spectra which cannot be explained using the known equilibrium between (THA)₅[PW₁₁O₃₉Co(H₂O)] and (THA)₅[PW₁₁O₃₉Co(-)]. Both ³¹P NMR and UV/VIS spectroscopies indicate the existence of dimers in dry toluene solutions, (THA)₁₀[(PW₁₁O₃₉Co)₂]. Pure (THA)₅[PW₁₁O₃₉Co(-)] with five-coordinate cobalt can, however, be obtained in dry toluene in the presence of excess inert salt (high ionic strength conditions). Quantum mechanical/molecular mechanical calculations also support dimer formation in toluene.