title: Solvation-induced helicity inversion of pseudotetrahedral chiral copper(II) complexes.
abstract: The helicity of four-coordinated nonplanar complexes is strongly correlated to the chirality of the ligand. However, the stereochemical induction of either the Δ- or the Λ-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-κ(2)N,O}copper(II) with X = Cl at the density functional theory level showed a clear dependence of the helicity-determining angle θ 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 Δ, while OCH3-substituted (R)-configured ligands induce Λ 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 Δ, 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.
authors.affiliation: Institut für Pharmazeutische Wissenschaften, Universität Freiburg , Albertstr. 25, D-79104 Freiburg, Germany.
authors.forename: Anne-Christine
authors.initials: AC
authors.lastname: Chamayou
authors.forename: Gamall
authors.initials: G
authors.lastname: Makhloufi
authors.forename: Laurence A
authors.initials: LA
authors.lastname: Nafie
authors.forename: Christoph
authors.initials: C
authors.lastname: Janiak
authors.forename: Steffen
authors.initials: S
authors.lastname: Lüdeke
mesh_terms: D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D012995:Solubility
chemical_list: D009942:Organometallic Compounds; D003300:Copper