Nickel Complexes of C-substituted Cyclams and Their Activity for CO$_2$ and H$^+$ Reduction

Abstract

Several nickel(II) complexes of cyclams bearing aryl groups on the carbon backbone were prepared and evaluated for their propensity to catalyze the electrochemical reduction of CO2 to CO and/or H$^+$ to H$_2$, representing the first catalytic analysis to be performed on an aryl–cyclam metal complex. Cyclic voltammetry (CV) revealed the attenuation of catalytic activity when the aryl group bears the strong electron-withdrawing trifluoromethyl substituent, whereas the phenyl, p-tolyl, and aryl-free derivatives displayed a range of catalytic activities. The gaseous-product distribution for the active complexes was determined by means of controlled-potential electrolysis (CPE) and revealed that the phenyl derivative is the most active as well as the most selective for CO2 reduction over proton reduction. Stark differences in the activity of the complexes studied are rationalized through comparison of their X-ray structures, absorption spectra, and CPE profiles. Further CV studies on the phenyl derivative were undertaken to provide a kinetic insight.