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

Abstract

The tetraheptylammonium (THA) salt of cobalt-substituted polyoxotungstate with Keggin-like structure, (THA)5[PW11O39CoII(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.