Kinetics and mechanisms of the reduction of a cis-dioxoruthenium(VI) complex by [Ni(tacn)2]2+ and [Fe(H2O)6]2+

Abstract

The kinetics of the reduction of cis-[RuVIL(O)2]2+ (L = N,N,N′,N′-tetramethyl-3,6-dimethyl-3,6-diazaoctane-1,8-diamine) by [Ni(tacn)2]2+ (tacn = 1,4,7-triazacyclononane) and [Fe(H2O)6]2+ have been studied in aqueous acidic solutions. Both reactions have the following stoichiometry: 2MII + cis-[RuVIL(O)2]2+ + 2H+ → 2MIII + cis-[RuIVL(O)(OH2)]2+ (M = Ni or Fe). Two distinct steps were observed for both reactions and these are assigned to RuVI → RuV and RuV → RuIV. Both steps are first order in [RuVI] and [MII]. For the reduction by [Ni(tacn)2]2+, the activation parameters (I = [H+] = 0.1 mol dm-3) for the first step are ΔH‡ = 13.4 ± 1.0 kJ mol-1 and ΔS‡ = -111 ± 10 J mol-1 K-1; for the second reaction, ΔH‡ = 28.5 ± 1.5 kJ mol-1 and ΔS‡ = -110 ± 10 J mol-1 K-1. The rate constant for the first step is independent of acid concentration, an outer-sphere mechanism is proposed and a self-exchange rate of 2 × 104 dm3 mol-1 s-1 for the cis-[RuVIL(O)2]2+/cis-[RuVL(O) 2]+ couple is estimated using the Marcus cross-relation. The rate constant of the second step increases with [H+] and it reaches saturation at high [H+]. A mechanism involving a pre-equilibrium protonation of cis-[RuVL(O)2]+ followed by outer-sphere electron transfer is proposed. For the reduction by [Fe(H2O)6]2+, rate constants for both steps are independent of acid concentration in the range of pH = 1-3. The activation parameters (I = 1.0 mol dm-3, pH = 1.0) for the first step are ΔH‡ = 32.5 ± 1.5 kJ mol-1 and ΔS‡ = -52.5 ± 7 J mol-1 K-1; while for the second step, ΔH‡ = 17.3 ± 1.2 kJ mol-1 and ΔS‡ = -140 ± 13 J mol-1 K-1. An outer-sphere mechanism is proposed for the first step and an inner-sphere mechanism is proposed for the second step.