Improved Electrocatalytic CO2 Reduction with Palladium bis(NHC) Pincer Complexes Bearing Cationic Side Chains

Stabilizing interactions between charged electrocatalytic intermediates and a series of cationic residues were explored through the synthesis and characterization of six palladium bis(N-heterocyclic carbene) (NHC) complexes bearing unique onium functionalities. The presence of a positively charged, pendant substituent was found to mediate electrode kinetics and facilitate CO2 coordination to the catalytic center in a systematic fashion. The introduction of cationic moieties into this system is shown to enhance catalytic selectivity for the conversion of CO2 to CO by as much as 5 times that of an alkyl-bearing analog. A combination of electrochemical experiments and computational analysis demonstrates that catalyst performance benefits most from a bulky onium unit tethered to the catalyst through a flexible linker. This behavior was interpreted as a preference for a wide, hydrophobic reaction pocket that allows for the unhindered formation of catalytic intermediates and mediated interaction with the solution.