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-[Ru VI L(O) 2 ] 2ϩ (L = N,N,NЈ, diazaoctane-1,8-diamine) by [Ni(tacn) 2 ] 2ϩ (tacn = 1,4,7-triazacyclononane) and [Fe(H 2 O) 6 ] 2ϩ have been studied in aqueous acidic solutions. Both reactions have the following stoichiometry: 2M II ϩ cis-[Ru VI L(O) 2 ] 2ϩ ϩ 2H ϩ 2M III ϩ cis-[Ru IV L(O)(OH 2 )] 2ϩ (M = Ni or Fe). Two distinct steps were observed for both reactions and these are assigned to Ru VI Ru V and Ru V Ru IV . Both steps are first order in [Ru VI ] and [M II ]. For the reduction by [Ni(tacn) 2 ] 2ϩ , the activation parameters (I = [H ϩ ] = 0.1 mol dm Ϫ3 ) for the first step are ∆H ‡ = 13.4 ± 1.0 kJ mol Ϫ1 and ∆S ‡ = Ϫ111 ± 10 J mol Ϫ1 K Ϫ1 ; for the second reaction, ∆H ‡ = 28.5 ± 1.5 kJ mol Ϫ1 and ∆S ‡ = Ϫ110 ± 10 J mol Ϫ1 K Ϫ1 . The rate constant for the first step is independent of acid concentration, an outer-sphere mechanism is proposed and a selfexchange rate of 2 × 10 4 dm 3 mol Ϫ1 s Ϫ1 for the cis-[Ru VI L(O) 2 ] 2ϩ /cis-[Ru V L(O) 2 ] ϩ couple is estimated using the Marcus cross-relation. The rate constant of the second step increases with [H ϩ ] and it reaches saturation at high [H ϩ ].A mechanism involving a pre-equilibrium protonation of cis-[Ru V L(O) 2 ] ϩ followed by outer-sphere electron transfer is proposed. For the reduction by [Fe(H 2 O) 6 ] 2ϩ , 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 Ϫ3 , pH = 1.0) for the first step are ∆H ‡ = 32.5 ± 1.5 kJ mol Ϫ1 and ∆S ‡ = Ϫ52.5 ± 7 J mol Ϫ1 K Ϫ1 ; while for the second step, ∆H ‡ = 17.3 ± 1.2 kJ mol Ϫ1 and ∆S ‡ = Ϫ140 ± 13 J mol Ϫ1 K Ϫ1 . An outer-sphere mechanism is proposed for the first step and an inner-sphere mechanism is proposed for the second step.

26 Mechanisms of reactions in solution

“…. Each stage is outer-sphere, except Ru() to Ru() reduction by [Fe(H 2 O) 6 ] 2ϩ 38. Excellent agreement has been achieved between the theoretically derived and experimentally determined rate constants for the reactions of[(bpy) …”