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

preprint
submitted on 18.10.2020, 11:23 and posted on 19.10.2020, 12:24 by Allyssa A. Massie, Claudia Schremmer, Isabelle Rüter, Sebastian Dechert, Inke Siewert, Franc Meyer
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 (kobs) 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 (kobs = 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 orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.

Funding

University of Göttingen

Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via 217133147/SFB 1073, project C01

History

Email Address of Submitting Author

franc.meyer@chemie.uni-goettingen.de

Institution

University of Göttingen

Country

Germany

ORCID For Submitting Author

0000-0002-8613-7862

Declaration of Conflict of Interest

The authors declare no conflicts of interest.

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