OXIDATIVELY INDUCED ISOMERIZATION OF SQUARE-PLANAR [NI(1,4,8,11-TETRAAZACYCLOTETRADECANE)](CLO4)(2)

Previous proton NMR and electronic spectroscopy studies have demonstra ted that the square-planar complex of Ni(II) and 1,4,8,11-tetraazacycl otetradecane (cyclam) exists in two stable isomeric conformations, nam ely the R,S,R,S form or trans-I isomer and the R,R,S,S form or trans-m isomer. Electrochemical analysis of the trans-I isomer of Ni(II)-cycl am has demonstrated rapid conversion to the trans-III cyclam conformat ion following oxidation to Ni(III). The mechanism and kinetics for thi s oxidatively-induced isomerization were studied by the technique of c yclic voltammetry (CV) with simulation of CV traces by finite-differen ce computations. The most crucial mechanistic indicator was found to b e the transient trans-I-Ni(III) species. This intermediate was detecte d by CV over a pH range 2-4 and was found to have an apparent half-lif e of ca. 400 ms at room temperature. Remarkably, this life-time was ap proximately a billionfold shorter than the corresponding trans-I-Ni(II ) species. Measurements made at varied solution temperature and pH dem onstrated that the oxidatively induced isomerization followed an appar ent square-scheme, where the trans-I/III isomerization process of Ni(I II) was independent of pH. This finding precluded a base-catalyzed iso merization process that has been previously identified for the Ni(II) system. Arrhenius plots of the forward isomerization rate constant all owed the extraction of activation parameters for the Ni(III) process. These parameters are discussed with respect to possible rate-determini ng steps.