Crystal-driven distortion of ligands in copper coordination complexes: conformational pseudo-enantiomers

The ligand in Cu(6)CH3CN]-(ClO4)2 adopts two conformations that may be described as "pseudo-enantiomers" in that they possess approximately mirror-image molecular helicity, although they differ in the orientation of a single methyl substituent. The two conformations differ in energy as judged by analogy to other compounds studied previously, computation of the relative free energies of formation of the isomers, and measurement of solution circular dichroism spectra. The solid-state structures of both single enantiomer and racemic forms of the complex were determined by X-ray crystallography. In the chiral complex, a quasi-racemate was observed with both pseudo-enantiomeric conformers present in the asymmetric unit. Packing forces induce a higher energy conformation in order to achieve higher apparent symmetry in the solid state. In contrast, the racemic complex only displayed a single conformation corresponding to the lower energy one of the two observed in the single enantiomer structure.