Structure and Binding in Molecular Complexes of Cyclic Polyethers. 4. Crystallographic Study of a Chiral System: An Inclusion Complex of a Macrocyclic Ligand with Phenylglycine Methyl Ester1

The crystal and molecular structures of the 1:1 complex between an optically pure macrocyclic polyether host (S,S)-1 (C48H40O6) and a hexafluorophosphate salt of (R)-phenylglycine methyl ester has been determined by x-ray diffraction methods at low temperature (-160 °C). The inclusion complex crystallizes with 1 mol of chloroform in the orthorhombic space group P2\2121 with a = 1 1.885 (1), b = 17.039 (2), c = 26.749 (4) ¥, and Z = 4. The structure was solved by Fourier and tangent formula techniques and refined to R = 0.049 for 3204 independent reflections measured by counter diffractome-try. The geometry of the intermolecular “host-guest” type association is described in detail. The hydrophilic alkylammonium substrate is included in and hydrogen bonded to the macrocyclic cavity of the ligand. Steric repulsions involving the rigid aromatic substituents of the interacting host and guest moieties play an important role in determining the overall conformation of the complex. In the crystal, the adduct molecules are arranged in a face-to-face manner; the space between the host-guest entities is filled with chloroform molecules and hexafluorophosphate ions. The structural results are correlated with those of a recent solution study of chiral recognition in α-amino acid and ester complexation.