Gas phase oxidation of alkoxo ligands in bis(peroxo) [MO(O2)2(OR)]− and trisoxo [MO3(OR)]− anions (M = Cr, Mo, W)

Abstract

The anions [M^VIO(O₂)₂(OR)]⁻ and [M^VIO₃(OR)]⁻ (M = Cr, Mo, W; R = H, Me, Et, ⁿPr, ⁱPr) were transferred to the gas phase by the electrospray process. Their decomposition was examined by multistage mass spectrometry and collisional activation experiments. The molybdate and tungstate anions [M^VIO(O₂)₂(OR)]⁻ underwent parallel elimination of aldehyde (ketone) and dioxygen while the equivalent chromate underwent loss of dioxygen only. The peroxo ligands were the source of oxidising equivalents in both reactions. For each alkoxo ligand, the total yield of aldehyde for the tungstate system exceeded that for the molybdate system. Collisional activation of [M^VIO₃(OMe)]⁻ led to clean elimination of formaldehyde with the metal centre supplying the oxidising equivalents. For larger alkoxo ligands, only the chromate centre eliminated aldehyde, while the molybdate and tungstate centres underwent clean loss of alkene. Threshold activation voltages indicated that the peroxo ligands of [W^VIO(O₂)₂(OMe)]⁻ are more oxidising than the tungstate centre of [W^VIO₃(OMe)]⁻. ²H and ¹⁸O isotope tracing experiments were consistent with a formal hydride transfer mechanism operating for oxidation of alkoxo ligand in each system. In the solid state, anions [M^VIO(O₂)₂(OR)]⁻ are typically pentagonal pyramidal (oxo in apical site) while [M^VIO₃(OR)]⁻ are tetrahedral. The data indicate that an equatorial ligand position is the site of alkoxo oxidation in [M^VIO(O₂)₂(OR)]⁻ anions. Comparisons of the gas phase data with those for a solution phase system are made.