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 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 as compared to 1, while model 1 has a more exergonic thermodynamics for acetylene hydration. Moreover, calculations indicate the effect of metal and catalyst model are more significant than the impact of supporting ligand substituents. NBO analysis suggests a highly reduced metallocyclopropene as opposed to a p-acetylene constitutes the more faithful representation of the Mo and W reactants. Furthermore, attempts to converge fully optimized geometries of CH3 and H substituted Mo and W complexes 2, suggests that the role of second-shell residues, environmental factors such as the nature of the protein active site are essential to the reactivity of acetylene hydratase. The results of effect of polarity of continuum solvent on the reaction coordinate suggest that a less polar active site is more favorable for acetylene hydration.

Impact of metal identity and supporting ligand on acetylene hydration by group six catalyst models

256th National Meeting (2018)

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