Structure of the Molecular Receptor 1,4,7,10-Tetrakis[(S)-2-hydroxy-2-phenylethyl]-1,4,7,10-tetraazacyclododecane: A Combined X-Ray Crystallographic and Theoretical Study Producing an Assessment of the Crystal Packing Energy

Abstract

X-Ray crystallography demonstrates that the guest molecule binding cavity within the molecular receptor ligand 1,4,7,10-tetrakis[(S)-2-hydroxy-2-phenylethyl]-1,4,7,10-tetraazacyclododecane, (S)-thpec12, is a poorly defined conical region stabilized by three O–H···O hydrogen bonds and a single O–H···N hydrogen bond. Two similar, but crystallographically independent, molecules exist within the unit cell. Ab initio calculations, using Gaussian 03 (LanL2DZ basis set at the Hartree–Fock level of theory), predict that these have steric energies of 97.73 and 97.06 kJ mol⁻¹, respectively, above that of the minimum energy (gas phase) conformer of the same hydrogen-bonding configuration, which is believed to be the structure of global minimum energy. The mean of these energies (97.4 kJ mol⁻¹) represents a best estimate of the crystal packing energy for (S)-thpec12, some of which is seen to be expended in rotating the phenyl rings away from the positions favoured in the gas phase. The ability of the Cd^II complex of (S)-thpec12 to act as a molecular receptor for aromatic anions is demonstrated by the isolation of two inclusion compounds in which p-nitrophenolate and anthraquinone-2-sulfonate are retained.