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.