THE EFFECT OF N-METHYLATION ON THE SPECTROSCOPICAL AND ELECTROCHEMICAL PROPERTIES OF 1,4,8,11-TETRAAZACYCLOTETRADECANE CHROMIUM(III) COMPLEXES

trans-[Cr(tmc)X2] complexes (tmc = 1,4,8,11-tetramethyl-1.4,8,11-tetraazacyclotetradecane, X = F, Cl, Br, H2O, OH, NO3) have been prepared and spectroscopically characterized. The redox potentials of the tmc complexes and of the analogous cyclam complexes (cyclam = 1,4,8,11-tetraazzacyclotetradecane) were determined by cyclic voltammetry. The potentials of the time complexes are 1.1+/-0.1 V more positive. Their long wavelength absorption is red-shifted by 50 to 100 nm with respect to that of the analogous cyclam complexes. The shift was consistent with a linear dependence of the polarographic half-wave potentials on the transition energies of the first spin allowed ligand field bands, observed on a series of chromium(III) amine complexes. The reaction entropies for the reduction of trans-[Cr(cyclam)(H2O)2]3+, trans-[Cr(cyclam)(OH)2]+, and trans-[Cr(tmc)(Oh)2]+, DELTA-S = 88+/-2, 200+/-40 and 53+/-8 J mol-1 K-1, respectively, were obtained from the temperature dependence of the redox potential. It was shown that trans-[Cr(cyclam)(H2O)2]3+ catalyzes the electroreduction of nitrate.