Hybrid quantum mechanics/molecular mechanics investigation of (salen)Mn for use in metal-organic frameworks

Hybrid quantum mechanics/molecular mechanics (QM/MM) methods were employed to study electronic effects of 5,5′-substituents on the enantioselectivity of (salen)Mn catalysts. These methods were shown to effectively capture electronic effects of substituents if the substituents were treated with a simple electrostatic coupling using point charges. Correlations between calculated properties of (salen)Mn=O indicative of its reactivity and the modified Hammett constant σpara⁺ were applied to study electronic framework effects on enantioselectivity for (salen)Mn heterogenized in a metal-organic framework (MOF). The results suggest that these effects are too small to account for all of the decrease in enantioselectivity observed experimentally and that steric constraints of the framework likely play a role. The reactivity correlations also enabled fast ligand screening for the prediction of highly enantioselective (salen)Mn catalysts that could be used in MOFs. A modified version of the DREIDING force field was also developed to allow QM/MM calculations of (salen)Mn species.