Computational prediction of pentadentate iron and cobalt complexes as a mimic of mono-iron hydrogenase for the hydrogenation of carbon dioxide to methanol

Homogeneous Catalytic CO2 Hydrogenation by [Fe]-Hydrogenase Bioinspired Complexes: A Computational Study

Computational modeling at the DLPNO-CCSD(T)/CBS//M06-L/def2-TZVP level of theory was used to propose four different iron catalysts whose structures were inspired on the [Fe]-hydrogenase active site: [Fe(^MeP^NNH^NP)(acmp)] (C(1), ^MeP^NNH^NP = 2,6-bis(dimethylphosphine), acmp = acylmethylpyridine), [Fe(C^NNH^NC)(acmp)] (C(2), C^NNH^NC = 2,6-bis(methylimidazol-2-ylidene)), [Fe(^MeP^NN^NP)(acmp)] (D(1), ^MeP^NN^NP = 2,6-bis((dimethylphosphine)pyridine)), and [Fe(C^NN^NC)(acmp)] (D(2), C^NN^NC = 2,6-bis((methylimidazol-2-ylidene) pyridine)). Through these electronic structure calculations, the catalytic mechanism of the reaction was explored. The intermediates and transition states present along the reaction coordinate were identified and described as to their equilibrium geometries, vibrational frequencies, and energies. Quasi-harmonic corrections were performed considering conditions analogous to those used experimentally. To compare the catalytic activities of the studied catalysts, turnover frequencies (TOFs) were calculated. Based on the explored catalytic cycles and TOF values (D(1) > C(1) > D(2) > C(2)), the most suitable iron catalysts are those with tridentate phosphine pincer-type ligands coordinated to the metal center. These systems are new promising iron catalysts to promote the CO₂ hydrogenation to formic acid without any use of bases or additives.

Bioinspired Design and Computational Prediction of SCS Nickel Pincer Complexes for Hydrogenation of Carbon Dioxide

Inspired by the structures of the active site of lactate racemase and H2 activation mechanism of mono-iron hydrogenase, we proposed a series of sulphur–carbon–sulphur (SCS) nickel complexes and computationally predicted their potentials for catalytic hydrogenation of CO2. Density functional theory calculations reveal a metal–ligand cooperated mechanism with the participation of a sulfur atom in the SCS pincer ligand as a proton receiver for the heterolytic cleavage of H2. For all newly proposed complexes containing functional groups with different electron-donating and withdrawing abilities in the SCS ligand, the predicted free energy barriers for the hydrogenation of CO2 to formic acid are in a range of 22.2–25.5 kcal/mol in water. Such a small difference in energy barriers indicates limited contributions of those functional groups to the charge density of the metal center. We further explored the catalytic mechanism of the simplest model complex for hydrogenation of formic acid to formaldehyde and obtained a total free energy barrier of 34.6 kcal/mol for the hydrogenation of CO2 to methanol.