Computational study of acetylene hydration by bio-inspired group six catalyst models

Abstract

A combination of density functional theory and natural bond orbital analysis was employed to study two bio-inspired catalyst models that mimic the acetylene hydratase active site. The Group 6 metals (Cr, Mo, W) and substituents (CN, H, CH3) on the dithiolate supporting ligands were used to evaluate the effect of metal identity and supporting ligand on free energy barriers and the thermodynamics of the reaction. In catalyst model 1, a metal-bound hydroxo ligand (MIV-OH) acts as a general base, while in catalyst model 2, a propionate plays the role of general base. The calculations showed that in both chromium complexes 1 and 2, acetylene does not bind to the metal in the reactant catalysts, suggesting that Cr is not a soft enough acid in the 4+ formal oxidation state to bind acetylene. The comparison of calculated free energies for molybdenum and tungsten complexes reveals that for models 1 and 2, Mo complexes have lower free energy barriers and are more exergonic as compared to the W variant. Comparing the energy profiles of complexes 1 and 2 indicates that on average, complex 2 has lower barrier energies in comparison to 1, while model 1 has a more exergonic ΔGrxn for acetylene hydration. NBO analysis suggests a metallocyclopropene constitutes the more faithful representation of the Mo and W reactants rather than a π-acetylene. Furthermore, calculations indicate that for both complexes 1 and 2, using electron withdrawing substituents such as CN on dithiolate supporting ligands lowers the activation free energies compared to CH3 and H substituents. Analysis of the impact of polarity through use of different continuum solvents on the reaction coordinate suggest that a less polar active site is more favorable for acetylene hydration.