Strong Solvent-Dependent Preference of Δ and Λ Stereoisomers of a Tris(diamine)nickel(II) Complex Revealed by Vibrational Circular Dichroism Spectroscopy

In the present study, we use vibrational
circular dichroism (VCD) spectroscopy to investigate the metal-centered
Δ and Λ chirality of a tris­(diamine)­nickel­(II) complex.
Chiral diphenylethylenediamine is chosen as the ligand, which puts
the Δ and Λ isomers of the complex in a diastereomeric
relationship. X-ray crystallography indicates an equal preference
of both stereoisomers in the solid state. This equal preference is
also supported by the related density functional theory calculations.
A comparison between the experimental and calculated VCD spectra also
proves the existence of both isomers in an acetonitrile solution.
However, a significant shift of the equilibrium toward the Λ
diastereomer is found for the complex in dimethyl sulfoxide. This
solvent-induced preference for a particular absolute configuration
is hypothesized to arise from a stronger and more effective solvation
of the Λ isomer. The observation that the solvent can significantly
influence and shift an equilibrium between two diastereomeric forms
is expected to have important implications on structural analysis
and on how reaction mechanisms are rationalized.