Insights into the Mechanism of CO2 Electroreduction by Molecular Palladium-Pyridinophane Complexes

Herein we report the synthesis, characterization, and electrocatalytic CO2 reduction activity of a series of PdII complexes supported by tetradentate pyridinophane ligands. In particular, we focus on the electrocatalytic CO2 reduction activity of a PdII complex supported by the mixed hard/soft 2,11-dithia[3.3](2,6)pyridinophane (N2S2) ligand . This is one of the few examples of a Pd complexes supported by a mixed hard-soft ligand which selectively produces CO from the electrocatalytic reduction of CO2. Notably, unlike previously reported molecular Pd complexes, selective CO2RR occurs in presence of weak proton sources such as 2, 2, 2 trifluoroethanol (TFE) and phenol, at mild overpotentials (~160 mV) and with high rates (kobs = 4.5 x 103 s-1, with phenol as proton source) and at Faradaic efficiencies of up to 70% for CO, without any H2 being detected. As the catalyst was not stable to long term electrolysis, we analyzed possible decomposition routes for this catalyst and, based on the characterization of its reaction with CO by UV-vis, NMR, and IR spectroscopy, we propose the intermediacy of a binuclear [(N2S2)PdI(η2-CO)]2 species toward the ultimate decomposition of the catalyst into free ligand and Pd0. Overall, these studies offer important insights into Pd catalyst decomposition and may explain the historically poor performance of related Pd molecular catalysts for CO2 reduction. In addition, the structurally-related hard N-donor diazapyridinophane (RN4)Pd complexes are shown to be unstable towards bulk electrolysis at cathodic potentials, suggesting that such compounds are ill-suited for CO2 electroreduction.