Synthetic Modeling of Carbon Monoxide Dehydrogenase (CODH) Active Site and CO2 Reactivity of Tetranuclear Fe(II) Cluster

Abstract

Part I: It is important to make useful substances such as CO, formal acid, and oxalic acid through CO2 reduction reactions from both energy and environmental perspectives. In this regard, studies have been conducted on molecular catalysts for CO2 reduction, and many researchers are interested in the structure of active site of enzyme in nature for more efficient catalyst development. In nature, [NiFe] carbon monoxide dehydrogenase (CODH) is well known as an enzyme that effectively catalyzes reversible reduction from CO2 to CO. The active site of [NiFe] CODH consists of [Fe4S4Ni], N donor, and S donor ligands, and several metal complex inspired by this structure has been reported as a CO2 reduction catalyst. In particular, four-coordinated Fe(II) center has tetrahedral geometry, and model complexes that imitate this structure are not well known. In this regard, we synthesized the model complex by imitating the Fe center of active site of [NiFe] CODH. The synthesized model complex (NNS-FeI) is based on NNS type ligand (NNS = 2,6-diisopropyl-N-((2-isopropylphenyl)(6-(2-(methylthio)phenyl)pyridin-2-yl)methyl)aniline) and is expected to have tetrahedral geometry with one iodide. For the synthesized NNS-FeI complex, to electrocatalytic CO2 reduction, cyclic voltammetry was carried out in the presence of 2,2,2-trifluoroethanol as a proton source. Furthermore, a CO2 reduction reaction was carried out under CO2 of ambient pressure conditions on the flask. Part II: We report the preparation of alkoxo-bridged tetranuclear Fe(II) cluster, [Fe4I2-alkoxo] ([Fe4I2(C14H11O2)6]) and its magnetic property and reactivity. The Fe(II) cluster is derived from a benzoin-based scaffold that provides two different Fe(II) coordination environments of a trigonal bipyramid and a trigonal prism. The X-ray photoelectron spectroscopy revealed that the binding energy of Fe(II) 2p electron in the different coordination sites appeared by ~1.43 eV away. The effective magnetic moment (μeff) of the Fe(II) cluster was measured as 9.06 μB in solution, and the similar μeff value of 9.49 μB was obtained in a solid state. Total spin (S) values was obtained through spin-only formula are 4.06 and 4.28, respectively, in solution and a solid state measurements. DFT calculation provided insights for the distribution of electron spins. The simulation of the exchange coupling through the broken symmetry approach was provided a spin alignment of S = 4.43 state along with the calculation of Mulliken spin densities. The trigonal bipyramidal Fe(II) center was activated by a halide (iodide) abstraction, which exhibited a stoichiometric reactivity for the CO2 hydrogenation to formate under ambient atmosphere of CO2 and H2.