Nickel Complexes of C-Substituted Cyclams and Their Activity for CO₂ 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 CO 2 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 CO 2 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.

NiII Complexes of C‐Substituted Cyclam as Efficient Catalysts for Reduction of CO2 to CO

“…The selected electrolysis potential resulted in lower FE but higher TOF for Ni II (MPC) and Ni II (cyclam) compared to our previous report . While each experiment showed significant and highly variable H 2 production, a catalyst‐free CPE experiment revealed that the H 2 production is mostly due to the background current instead of the catalysts.…”

“…They all share the common features of the loss of reversibility for the Ni(+1/+2) couple, significant increase in current and the growth of an additional reduction peak at more negative potentials. These features are indicative of catalytic reduction of CO 2 . Furthermore, while the catalytic currents at –1.76 V are about the same for 1 (56 µA) and Ni II (MPC) (49 µA), it is nearly doubled for 2 (96 µA).…”

“…The improved catalytic currents of 1 and 2 compared to Ni II (MPC) prompted further examination of their catalytic proficiency through controlled potential electrolysis (CPE) experiments, which allow for the establishment of the composition of reduction products through headspace analysis and ensuing estimation of Faradaic efficiency and catalytic turnover. The conditions of CPE experiments were identical to those described in our previous report, except for a AgCl coated silver wire being used as the reference electrode in place of a Ag wire pseudoreference electrode. The potential for each experiment was held at a constant –1.76 V vs. Fc/Fc + , which is slightly more positive than Ni(+1/+2) couples for each complex under CO 2 .…”

“…Through controlled‐potential electrolysis, Kubiak and co‐workers recently verified that CO is the major electrolysis product in Ni II (cyclam) catalyzed CO 2 reaction . Seeking more robust and efficient catalysts, our laboratory recently reported the preparation of three new nickel(II) complexes of cyclams bearing C ‐aryl substituents, and demonstrated that of the new complexes, Ni II (MPC) is significantly more efficient than Ni II (cyclam) in reducing CO 2 to CO under the same conditions . We postulate that the improved catalytic ability results from the increased electron‐donor ability of the aryl functionalized cyclam ring, and that it can be further improved through reduction of the steric interference of the substituents with the active catalytic site.…”

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

“…While these configurations may change upon reduction of Ni70 and Ni71 to their Ni + active species, this example shows how minor changes in ligand structure can have a dramatic effect on catalytic activity. 473…”