Base hydrolysis of macrocyclic chromium(iii) complexes. Importance of ion-pair formation for kinetics and mechanism

Abstract

A series of cis and trans complexes of the general formula [Cr(cyc)(OH₂)X]²⁺, where cyc is the macrocyclic tetraamines meso- or rac- 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane and X⁻ = NCS⁻, N₃⁻ and Cl⁻, have been prepared in solution via anation of the diaqua complexes. The kinetics of the monodendate X ligand release in alkaline media have been studied at a range of temperatures and hydroxide concentrations at ionic strengths of 1.0 or 2.0 M using NaClO₄, NaBr or NaCl as “inert” electrolytes. A higher order than linear dependence of the pseudo-first-order rate constant on the hydroxide concentration has been observed for all the systems investigated. These results have been rationalized in terms of specific ion-pair interactions between the macrocyclic chromium(III) reactants and counterions of the supporting electrolytes, followed by proton transfer in the hydroxide ion pair to give a reactive conjugate base.