Kinetic study of reaction of [ReN(H₂O)(CN)₄]²⁻ with quinoline-2-carboxylate and pyridine-2,3-dicarboxylate anions

Abstract

The kinetics of the reaction between [ReN(H₂O)-(CN)₄]²⁻ with different κ² N,O-donor ligands (quin⁻ and 2,3-dipic⁻, respectively) have been studied in the pH 4–12 range in aqueous solution. Two consecutive reaction steps with the formation of the [ReN(η¹-quin)(CN)₄]³⁻ and [ReN(μ²-quin) (CN)₃]²⁻ complexes, respectively, were spectrophotometrically observed and kinetically investigated. The same reaction mechanism is proposed for these two ligands. The first fast reaction (for quin⁻) is attributed to the aqua substitution of [ReN(H₂O)(CN)₄]²⁻ with forward and reverse rate constants of 1.96(5) × 10⁻¹ M⁻¹ s⁻¹ and 5.6(3) × 10⁻² s⁻¹, while a rate of 2.64(3) M⁻¹ s⁻¹ was observed for the reaction between the conjugate base [ReN(OH)(CN)₄]³⁻ and quin⁻ at 40.2 °C. Due to small absorbance changes, it was difficult to obtain any good quality data for the fast reactions for 2,3-dipic⁻. The second, slower reaction is attributed to cyano substitution with rate constants (k ₃ K ₁) of 4.17(4) × 10⁻³ for quin⁻ and 4.68(7) × 10⁻³ M⁻¹ s⁻¹ for 2,3-dipic⁻, at 80.02 °C, respectively. The acid dissociation constant for the aqua complex was spectrophotometrically determined as 11.58(3) and 11.54(2) and kinetically as 11.51(8) and 11.41(1), at 80.4 °C, respectively. Negative $\Updelta{S}^{\neq}$ values of −83.5(2) and −144.1(2) J K⁻¹ mol⁻¹ as well as the $\Updelta {H}^{\neq}$ of 71.4(3) and 47.3(3) kJ mol⁻¹, for the slow quin⁻ and 2,3-dipic⁻ reactions, respectively, point to an ordered transition state where bond formation is responsible for the major driving force of the reaction. The $\Updelta{H}^{\neq}=39.6(2)\,\hbox{kJ}\,\hbox{mol}^{-1}$ and $\Updelta{S}^{\neq}=-142.8(1)\,\hbox{J\,K}^{-1}\,\hbox{mol}^{-1}$ for the fast forward reaction of quin⁻ is indicative of expected associative activation in the transition state.