Selective Electrocatalytic CO2 Reduction to CO by an NHC-Based Organometallic Heme Analogue

Molecular first-row transition metal complexes for electrocatalytic CO2 reduction mostly feature N-donor supporting ligands, iron porphyrins being among the most prominent catalysts. Introducing N-heterocyclic carbene (NHC) ligation has previously shown promising effects for some systems, yet the application of NHC iron complexes for electrochemical CO2 reduction has so far remained unreported. Herein, we show that the macrocyclic tetracarbene iron complex [LFe­(MeCN)2]­(OTf)2 (1(OTf)2), which can be described as an organometallic heme analogue, mediates selective electrocatalytic CO2-to-CO conversion with a faradaic efficiency of over 90% and a very high initial observed catalytic rate constant (kobs) of 3300 s–1. Replacement of an axial MeCN ligand by CO increases the catalyst stability and turnover number, while the rate of catalysis decreases only slightly (kobs = 3100 s–1). Ferrous complexes with one or two axial CO ligands, [LFe­(MeCN)­(CO)]­(OTf)2 (2(OTf)2) and [LFe­(CO)2]­(OTf)2 (3(OTf)2), have been isolated and fully characterized. Based on linear sweep voltammogram spectroelectro-IR studies for 12+ and 22+, both under the N2 and CO2 atmosphere, a mechanistic scenario in anhydrous acetonitrile is proposed. It involves two molecules of CO2 and results in CO and CO32– formation, whereby the first CO2 binds to the doubly reduced, pentacoordinated [LFe0(CO)] species. This work commences the exploration of the reductive chemistry by the widely tunable macrocyclic tetracarbene iron motif, which is topologically similar to hemes but electronically distinct as the strongly σ-donating and redox inactive NHC scaffold leads to metal-centered reduction and population of the exposed dz2 orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.