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

Abstract

Molecular first-row transition metal complexes for electrocatalytic CO<sub>2</sub> 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 CO<sub>2</sub> reduction has so far remained unreported. Herein we show that the macrocyclic tetracarbene iron complex [LFe(NCMe)<sub>2</sub>](OTf)<sub>2</sub> (<b>1</b>), which can be described as an organometallic heme analogue, mediates selective electrocatalytic CO<sub>2</sub>-to-CO conversion with a faradaic efficiency of over 90% and a very high initial observed catalytic rate constant (<i>k</i><sub>obs</sub>) of 7,800 s<sup>−1</sup>. Replacement of an axial MeCN ligand by CO significantly increases the catalyst stability and turnover number, while the rate of catalysis decreases only slightly (<i>k</i><sub>obs</sub> = 3,100 s<sup>−1</sup>). Ferrous complexes with one or two axial CO ligands, [LFe(NCMe)(CO)](OTf)<sub>2</sub> (<b>1-CO</b>) and [LFe(CO)<sub>2</sub>](OTf)<sub>2</sub> (<b>1-(CO)<sub>2</sub></b>), have been isolated and fully characterized. Based on linear sweep voltammogram (LSV) spectroelectro-IR (SEC-IR) studies for <b>1</b> and <b>1-CO</b>, both under N<sub>2</sub> and CO<sub>2</sub> atmosphere, a mechanistic scenario in anhydrous acetonitrile is proposed. It involves two molecules of CO<sub>2</sub> and results in CO and CO<sub>3</sub><sup>2−</sup> formation, whereby the first CO<sub>2</sub> binds to the doubly reduced, pentacoordinated [LFe<sup>0</sup>(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<sub>z²</sub> orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.