Structural Insight into Guest Binding Sites in a Porous Homochiral Metal-Organic Material

An enantiomeric pair of chiral metal-organic materials (CMOMs) based upon mandelate (man) and 4,4′-bipyridine (bpy) ligands, [Co₂(S-man)₂(bpy)₃](NO₃)₂·guest (1S·guest) and [Co₂(R-man)₂(bpy)₃](NO₃)₂·guest (1R·guest), have been prepared. The cationic frameworks exhibit one-dimensional chiral channels with dimensions of 8.0 Å × 8.0 Å. The pore chemistry is such that chiral surfaces lined with nitrate anions and phenyl groups create multiple binding sites for guest and/or solvent molecules. The performance of 1S and 1R with respect to resolution of racemic mixtures of 1-phenyl-1-propanol (PP) was studied by varying time, temperature, and the use of additives. Selectivity toward PP was determined by chiral HPLC with ee values of up to 60%. The binding sites and host-guest interactions were investigated through single-crystal X-ray structural analyses of guest-exchanged 1S and 1R. Crystallographically observed structural changes (e.g., the absolute configurations of the three PP binding sites switch from R, R, and S to R, R, and R/S) correlate with experimentally observed ee values of 33% and 60% for variants of 1S that contain PP and different solvent molecules, 1S·PPex and 1S·PPex′, respectively. The fact that manipulation of guest solvent molecules, which in effect serve as cofactors, can modify chiral sites and increase enantioselectivity is likely to aid in the design of more effective CMOMs and processes for chiral separations.