Selective electrochemical CO₂ reduction to CO by a Co(II) dimer catalyst by metal-ligand cooperativity

An approach to reducing greenhouse gas emissions that shows promise is the electrochemical conversion of CO₂ to products with added value. Here, we present [Co(8HQ-Tpy)(H₂O)]₂(PF₆)₂ ([Co1]), a cobalt-based molecular electrocatalyst that can convert CO₂ to CO in a DMF/H₂O mixture (4.8 : 0.2 v/v) in a selective manner (8HQ-Tpy = 2-([2,2':6',2''-terpyridin]-4'-yl)quinolin-8-ol). At an overpotential of 760 mV, the catalyst shows a TOF_max of 2575 s^-1 and a high Faradaic efficiency of 94 ± 2%. The CO₂ reduction follows both ECEC and EECC-type routes, involving stepwise proton and electron transfer, according to a mechanistic investigation that combines DFT calculations, infrared spectroelectrochemistry (IR-SEC), and kinetic isotope effect (KIE) observations. Sequential protonation and CO₂ activation are made possible by the reduction of a hexa- to penta-coordinate Co centre. According to DFT studies, protonation at the ligand O^- site, which takes place before CO₂ coordination and favours an EECC pathway, becomes thermodynamically favourable following reduction. Both deprotonated and protonated CO₂-derived intermediates are captured by IR-SEC measurements, and proton transfer is not rate-limiting as the KIE is low (k_H/k_D = 1.17). When taken as a whole, these results offer a comprehensive mechanistic understanding of CO₂-to-CO conversion as well as design guidelines for creating advanced molecular electrocatalysts for carbon capture and utilization.