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

Abstract

Oxidation of the trans-(Cr(cyca)(OH)2) þ complex, where cyca ¼ meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra- azacyclotetradecane, by (Fe(CN)6) 3� ion in strongly alkaline media, leading to (Cr V O(cycaox)) 3þ 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 OHion 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 OHconcentration. 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 giso ¼ 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 d 1 -electron interactions with four equivalent nitrogen nuclei and (Cr V ¼ O(cycaox)) 3þ formula, where cycaox ¼ oxidized cyca, can be postulated for the intermediate Cr V complex.