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, XF, 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-tetraazacyclotetradecane) were determined by cyclic voltammetry. The potentials of the tmc 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]⁺, ΔS=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.