Direct experimental 31P 2D DOSY NMR evidence for oligomerization of transition-metal substituted polyoxotungstates in nonpolar solvents

Abstract

Transition-metal substituted polyoxotungstates (TMSPOTs) dissolved in toluene, where they form surfactant encapsulated clusters (SEC), have been recognized as potential catalysts for carbon dioxide activation. Therefore, their detailed structures in toluene are of great interest. This paper reports the first direct experimental evidence, based on 31-P 2D DOSY NMR, for the oligomerization of TMSPOTs in dry toluene. Two kinds of experiments are described in this paper. First, the 31-P 2D DOSY NMR data in aqueous solutions confirmed the reliability of 31-P 2D DOSY NMR for the determination of diffusion coefficients for the phospho-polyoxotungstates (P-POT) of various shapes. Second, 31-P 2D DOSY NMR for toluene solutions of tetraheptylammonium (THA) salts of cobalt- and nickel- substituted Keggin POTs (PW11Co and PW11Ni) and Weakley Co-POTs (P2W18Co4) showed the existence in the solution of polyoxotungstates with various sizes. For Keggin POTs (with one exchangeable water molecule) the slower and faster diffusing species were identified as the dimers and monomers, respectively. For Weakley POTs, with two exchangeable water molecules, the slower diffusing species is much greater than a dimer. The experimental results are in agreement with the results determined computationally using DFT method.