Mechanistic studies on the cis-[VO2(H2O)4]+and [Mo(CN)8]3–oxidations of the ReIV2complex [(C2O4)2Re(µ-O)2Re(C2O4)2]4–

Abstract

Rate constants (25 °C) have been determined for the oxidation of the ReIV2 complex di-µ-oxo-bis[dioxalatorhenate(IV)], [(C2O4)2Re(µ-O)2Re(C2O4)2]4–, I= 1.00 M (ClO4– or Cl–). Whereas the complex is itself stable in air at pH 7 over many days, at lower pH there is a slow decay of the 446 nm peak (Iµ= 6840 M–1 cm–1 per ReIV2) to give in air the colourless perrhenate(VII)[ReO4]– product, peak at 225 nm (Iµ= 3400 M–1 cm–1). The kinetics indicate a protonation step, equilibrium constant K= 2.4 M–1, followed by the decay process k= 4.3 × 10–5 s–1. With the one-equivalent oxidants cis-[VO2(H2O)4]+(1.0 V) and [Mo(CN)8]3–(0.80 V), stoichiometries of 6 mol of oxidant per ReIV2 are obtained indicating conversion through to ReVII. Two kinetic stages have been monitored with the oxidant in large excess. For the reaction with VV as oxidant the first stage is dependent on [VV] and [H+] as well as [ReIV2], giving the rate constant k1= 3.31 × 104 M–2 s–1. Since in the corresponding reaction with MoV as oxidant the reaction is independent of [H+](kMo= 2.27 × 104 M–1 s–1), it is concluded that the VV reactant introduces the [H+]-dependent term. The second stage of the VV reaction gives the rate law dependence k2obs=k2[VV]+k0, but with the less strong Mo oxidant k2obs=k0 which is ≈0.20 s–1. The spectrum of the ReIVReV intermediate, peak at 529 nm (Iµ= 3800 M–1 cm–1), was obtained by stopped-flow rapid-scan spectrophotometry. Reactions of less than full stoichiometric amounts of VV with ReIV2, followed by QAE-Sephadex chromatography at 0 °C, gave orange-pink (peak at ≈495 nm) and maroon (≈550 nm) intermediates, which undergo spontaneous decay.