Kinetic and mechanistic studies on the substitution of aqua ligands from cis-diaqua-bis-(bypyridyl)-ruthenium(II) ion by vic-dioximes

Abstract

Kinetics of aqua ligand substitution from cis-[Ru(bpy)2(H2O)2]2+ by three vicinal dioximes, namely dimethylglyoxime (L1H), 1,2-cyclohexane dionedioxime (L2H) and α-furil dioxime (L3H) have been studied spectrophotometrically in the 45–60 °C temperature range. The rate constants increase with increasing dioxime concentration and approach a limiting condition. We propose the following rate law for the reaction in the 3.5–5.5 pH range: \( {\text{Rate }} = k_{ 2} K_{\text{E}} \left[ {{\text{Ru}}\left( {\text{bpy}} \right)_{ 2} \left( {{\text{H}}_{ 2} {\text{O}}} \right)_{2}{}^{2+} } \right]{{\left[ {\text{LH}} \right]} \mathord{\left/ {\vphantom {{\left[ {\text{LH}} \right]} {\left( { 1+ K_{\text{E}} \left[ {\text{LH}} \right]} \right)}}} \right. \kern-\nulldelimiterspace} {\left( { 1+ K_{\text{E}} \left[ {\text{LH}} \right]} \right)}}, \) where k 2 is the interchange rate constant from outer sphere to inner sphere complex and K E is the outer sphere association equilibrium constant. Activation parameters were calculated from the Eyring plots for all three systems: ΔH ≠ = 59.2 ± 8.8, 63.1 ± 6.8 and 69.7 ± 8.5 kJ mol−1, ΔS ≠ = −122 ± 27, −117 ± 21 and −99 ± 26 J K−1 mol−1 for L1H, L2H and L3H, respectively. An associative interchange mechanism is proposed for the substitution process. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants give negative ΔG 0 values for all the systems studied at all the temperatures (ΔH 0 = 30.05 ± 2.5, 18.9 ± 1.1 and 11.8 ± 0.2 kJ mol−1; ΔS 0 = 123 ± 8, 94 ± 3 and 74 ± 1 J K−1 mol−1 for L1H, L2H and L3H, respectively), which also support our proposition.