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.