Dimerization of cobalt-substituted Keggin phosphotungstate, [PW_(11)O_(39)Co(X)]~(5-), in nonpolar solvents

摘要

The tetraheptylammonium (THA) salt of cobalt-substituted polyoxotungstate with Keggin-like structure, (THA)_5[PW_(11)O_(39)Co~Ⅱ(X)], was investigated by ~(31)P NMR and UV/VIS spectroscopies in toluene. A solution obtained after transferring PW_(11)Co from aqueous solution into toluene, using THABr as a transfer agent, and rigorous drying, gives multiple peaks in the ~(31)P 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 ~(31)P NMR and UV/VIS spectra which cannot be explained using the known equilibrium between (THA)_5[PW_(11)O_(39)Co(H_2O)] and (THA)_5[PW_(11)O_(39)Co( )]- Both ~(31)P NMR and UV/VIS spectroscopies indicate the existence of dimers in dry toluene solutions, (THA)_(10)[(PW_(11)O_(39)Co)_2]. Pure (THA)_5[PW_(11)O_(39)Co( )] 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.