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(NCMe)₂](OTf)₂ (1), 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 (k_obs) of 7,800 s⁻¹. Replacement of an axial MeCN ligand by CO significantly increases the catalyst stability and turnover number, while the rate of catalysis decreases only slightly (k_obs = 3,100 s⁻¹). Ferrous complexes with one or two axial CO ligands, [LFe(NCMe)(CO)](OTf)₂ (1-CO) and [LFe(CO)₂](OTf)₂ (1-(CO)₂), have been isolated and fully characterized. Based on linear sweep voltammogram (LSV) spectroelectro-IR (SEC-IR) studies for 1 and 1-CO, both under 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 s-donating and redox inactive NHC scaffold leads to metal-centered reduction and population of the exposed d_z² orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.