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 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-κ2N,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.