Solvation-Induced Helicity Inversion of Pseudotetrahedral Chiral Copper(II) Complexes

The helicity of four-coordinated nonplanar complexes is strongly correlated to the chirality of the ligand. However, the stereochemical induction of either the Delta- or the Lambda-configuration at the metal ion is also modulated by environmental factors that change the conformational distribution of ligand rotamers. Calculation of the potential energy surface of bis{(R)-N-(1-(4-X,phenyl)ethyl)salicylaldiminato-kappa N-2,O}copper(II) with X = Cl at the density functional theory level showed a clear dependence of the helicity-determining angle theta between the two coordination planes on the relative population of different ligand conformers. The influence of different substituents (X = H, Cl, Br, and OCH3) on complex helicity was studied by determination of the absolute configuration at the metal ion in complexes with either (R)- or (S)-configured ligands. X-ray single-crystal analysis showed that,(R)-configured ligands with H, Cl, Br induce Delta, while OCH3- substituted (R)-configured ligands induce Lambda in the solid state. According to:vibrational circular dichroism and electronic circular dichroism studies in solution, however; all tested complexes with (R)-ligands exhibited a propensity for Delta, With high diastereomeric ratio for X = Cl and X = Br and moderate diastereomeric ratio for X = H and X = OCH3 substituted ligands. Therefore, solvation of copper complexes with X = OCH3 goes along with helicity inversion. This solid-state versus solution study demonstrates that it is not sufficient to determine the chiral-at-metal configuration of a compound by X-ray crystallography alone, because the solution structure can be different. This is particularly important for the use of chiral-at-metal complexes as catalysts in stereoselective synthesis.