Origin of Enantioselectivity in a Chiral Metal-Organic Framework: A Molecular Simulation Study

We used molecular simulation to examine the nature of enantioselectivity in the homochiral metal organic framework [Ni-2(L-asp)(2)(bipy)] for a number of chiral diols and compared the results to experimental enantiomeric excess (ee) data available in the literature. For all cases, our simulation results not only reproduced the general preference of the R-enantiomers in the framework derived from L-asp but also captured more subtle trends. Studying the adsorption process on the molecular level, we show that the R-enantioselectivity is strongly related to better packing effects at high loadings resulting in higher ee values for 1,3-butanediol in comparison with 1,2-butanediol and 1,2-propanediol. We demonstrate that the level of enantioselectivity is highly dependent on the chain length and the position of hydroxyl groups on the carbon chain. We show that whereas the chirality of the MOP framework assists the separation mechanism, the more dominant factor is the perfect match between guest framework size and shape.