Ni^II Complexes of <i>C</i>‐Substituted Cyclam as Efficient Catalysts for Reduction of CO₂ to CO

Abstract

Two new nickel(II) complexes of cyclams bearing C ‐alkyl groups, Ni(MEC)OTf 2 ( 1 , MEC = 5,12‐diethyl‐7,14‐dimethyl‐1,4,8,11‐tetraazacyclotetradecane) and Ni(CTMC)OTf 2 ( 2 , CTMC = 5,7,12,14‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane), were prepared, and their similarity to Ni II (MPC) (MPC = 5,12‐dimethyl‐7,14‐diphenyl‐1,4,8,11‐tetraazacyclotetradecane) in ring conformation was revealed through single‐crystal X‐ray diffraction studies. Solution electronic absorption spectroscopy indicates the retention of octahedral coordination mode for both 1 and 2 in 20 % aqueous acetonitrile. Cyclic voltammetry studies of 1 and 2 under CO 2 in 20 % aqueous acetonitrile revealed significantly increased catalytic currents compared to previously studied Ni II (cyclam) and Ni II (MPC). Controlled potential electrolysis studies of 2 revealed a 250 % increase in CO turn over frequency from that of Ni(cyclam)OTf 2 and a 40 % increase from that of Ni(MPC)OTf 2 . Such improvements establish the benefit of electronically donating substituents that minimize steric interference around the axial catalytic sites.