Base hydrolysis of mer-trispicolinatoruthenium(III): kinetics and mechanism

Abstract

The mer-[Ru(pic)3] isomer, where pic is 2-pyridinecarboxylic acid, undergoes base hydrolysis at pH > 12. The reaction was monitored spectrophotometrically within the UV–Vis spectral range. The product of the reaction, the [Ru(pic)2(OH)2]− ion, is formed via a consecutive two-stage process. The chelate ring opening is proceeded by the nucleophilic attack of OH− ion at the carbon atom of the carboxylic group and the deprotonation of the attached hydroxo group. In the second stage, the fast deprotonation of the coordinated OH− ligand leads to liberation of the monodentato bonded picolinate. The dependence of the observed pseudo-first-order rate constant on [OH−] is given by $$ k_{{{\text{obs}}1}} = \frac{{k_{ + } k_{1} [{\text{OH}}^{ - } ] + k_{ + } k_{2} K_{1} [{\text{OH}}^{ - } ]^{2} }}{{k_{ - } + k_{1} + \left( {k_{ + } + k_{2} K_{1} } \right)[{\text{OH}}^{ - } ] + k{}_{ + }K_{1} [{\text{OH}}^{ - } ]^{2} }} $$ and $$ \left( {k_{{{\text{obs}}2}} = \frac{{k_{ca} + k_{cb} K_{2} [{\text{OH}}^{ - } ]}}{{1 + K_{2} [{\text{OH}}^{ - } ]}}} \right) $$ for the first and the second stage, respectively, where k 1, k 2, k -, k ca , k cb are the first-order rate constants and k + is the second-order one, K 1 and K 2 are the protolytic equilibria constants.