Effect of Lewis acid on the structure of a diiron dithiolate complex based on the active site of [FeFe]-hydrogenase assessed by density functional theory

The effect of Lewis acid on the structure and H₂ productivity of a diiron dithiolate complex was investigated by using density functional theory (DFT) calculations. When a model molecule of [(CH₃SH)(CO)₂Fe_p(μ-SCH₂NHCH₂S)Fe_d(CO)₃] was geometrically optimized, two isomers were found: one is the unrotated structure (1) with no ligand between two Fe atoms and the other is the rotated structure (1*) with one CO ligand between two Fe atoms. The energy of 1* was higher than 1 by 6.4 kcal/mol in a vacuum. DFT calculations also revealed that all Lewis acids bound to the rotated structure more strongly than to the unrotated structure, leading to the stabilization of the rotated structure. In particular, when AlCl₃ is used, the rotated structure (1*/AlCl₃) is more stable than the unrotated one (1/AlCl₃) by 1.2 kcal/mol in a vacuum. The stabilization of the rotated structure arises from both the stronger basicity of the μ-CO ligand than the axial CO ligand and the increase of the bond strength between the μ-CO ligand and Fe_p atom upon binding of Lewis acid to 1*. Calculation of energy barriers during electrocatalytic H₂ production revealed that 1*/AlCl₃ could efficiently produce H₂via a chemical–electrochemical–chemical–electrochemical mechanism. The analysis of the energy level of the lowest unoccupied molecular orbital showed that 1*/AlCl₃ may produce H₂ at significantly lower reduction potential as compared with 1*. It is also found that the catalytic activity decreases with increasing polarity of the medium.