Mechanistic studies on the cis-[VO2(H2O)4]+ and [Mo(CN)8]3– oxidations of the ReIV2 complex [(C2O4)2Re(µ-O)2Re(C2O4)2]4–

Abstract

Rate constants (25 °C) have been determined for the oxidation of the Re^IV₂ 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 (ε= 6840 M^–1 cm^–1 per Re^IV₂) to give in air the colourless perrhenate(VII)[ReO₄]^– product, peak at 225 nm (ε= 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 Re^IV₂ are obtained indicating conversion through to Re^VII. Two kinetic stages have been monitored with the oxidant in large excess. For the reaction with V^V as oxidant the first stage is dependent on [V^V] and [H⁺] as well as [Re^IV₂], giving the rate constant k₁= 3.31 × 10⁴ M^–2 s^–1. Since in the corresponding reaction with Mo^V as oxidant the reaction is independent of [H⁺](k_Mo= 2.27 × 10⁴ M^–1 s^–1), it is concluded that the V^V reactant introduces the [H⁺]-dependent term. The second stage of the V^V reaction gives the rate law dependence k_2obs=k₂[V^V]+k₀, but with the less strong Mo oxidant k_2obs=k₀ which is ≈0.20 s^–1. The spectrum of the Re^IVRe^V intermediate, peak at 529 nm (ε= 3800 M^–1 cm^–1), was obtained by stopped-flow rapid-scan spectrophotometry. Reactions of less than full stoichiometric amounts of V^V with Re^IV₂, followed by QAE-Sephadex chromatography at 0 °C, gave orange-pink (peak at ≈495 nm) and maroon (≈550 nm) intermediates, which undergo spontaneous decay.