Ratio-Controlled Precursors of Anderson-Evans Polyoxometalates: Synthesis, Structural Transformation, and Magnetic and Catalytic Properties of a Series of Triol Ligand-Decorated {M2Mo6} Clusters (M = Cu2+, Co2+, Ni2+, Zn2+).

Abstract

A series of triol ligand [CH3C(CH2OH)3] covalently decorated polyoxometalates (POMs), which could be ascribed to the primary complexes with structural formulas {M2[Mo2O4(CH3C(CH2O)3)2]3}2- (M = Cu2+, Co2+, Ni2+, Zn2+), have been synthesized in organic solvents. Single-crystal X-ray structural analysis reveals that the synthesized polyanionic clusters are comprised of three {Mo2} units and two divalent transition-metal ions connecting to each other in an alternating style, where all {Mo2} blocks were covalently decorated by two triol ligands in the trans conformation. The 1/3 molar ratio of M/Mo in the prepared complexes was higher than those ratios in typical Anderson-Evans, Wells-Dawson, and Keggin POMs. With a decrease in the M/Mo molar ratio of a Mo-contained reactant to 1/6 and/or the addition of acetic acid to the reaction solution, the primary complexes acting as precursors transformed continuously into the corresponding triol-ligand-decorated Anderson-Evans POMs. Detailed investigations were conducted by using different isopolymolybdates in various solvent environments, and several Anderson-Evans POMs in different triol-ligand-decorated fashions were obtained from the primary complexes. In addition, we also realized the transformation between the Anderson-Evans clusters in different decoration fashions by simply controlling the acidity in solution. Magnetic measurement showed a general property, but the catalytic experiments demonstrated that CoII- and Zn II-containing POMs displayed a higher efficiency for the selective oxidation of thioanisole to sulfoxide.