Kinetics and mechanism of a trans-tetraazamacrocyclic chromium(III) complex oxidation by hexacyanoferrate(III) in strongly alkaline media

Oxidation of the trans-Cr(cyca)(OH)₂]⁺ complex, where cyca = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by Fe(CN)₆ ]^3- ion in strongly alkaline media, leading to Cr^V O(cyca_ox )]³⁺ ion, has been studied using electronic and e.p.r. spectroscopy. The kinetics of the Cr^III → Cr^IV transformation have been studied using a large excess of the reductant and OH^- ion over the oxidant. The reaction is a second order process: first order in Cr^III] and Fe^III] at constant OH^-]. The second order rate constant is higher than linearly dependent on the OH^- concentration. The mechanism of the reaction has been discussed. A relatively inert intermediate chromium(V) species was detected based on characteristic bands in the visible region and the e.p.r. signal at g_iso = 1.987 for the systems where an excess of oxidant was used. The hyperfine structure of the main e.p.r. signal is consistent with the d¹ -electron interactions with four equivalent nitrogen nuclei and Cr^V = O(cyca_ox)]³⁺ formula, where cyca_ox = oxidized cyca, can be postulated for the intermediate Cr^V complex.