Chirality at metal and helical ligand folding in optical isomers of chiral bis(naphthaldiminato)nickel(ii) complexes

Abstract

Enantiopure bis[{(R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ²N,O}]nickel(II) complexes {Ar = C₆H₅ (1R or 1S), p-OMeC₆H₄ (2R or 2S), and p-BrC₆H₄ (3R or 3S)} are synthesized from the reactions between (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldimine and nickel(II) acetate. Circular-dichroism spectra and their density-functional theoretical simulation reveal the expected mirror image relationship between the enantiomeric pairs 1R/1S and 3R/3S in solution. CD spectra are dominated by the metal-centered Λ- or Δ-chirality of non-planar four-coordinated nickel, this latter being in turn dictated by the ligand chirality. Single crystal structure determination for 1R and 1S shows that there are two symmetry-independent molecules (A and B) in each asymmetric unit that give a Z′ = 2 structure. Two asymmetric and chiral bidentate N^O-chelate Schiff base ligands coordinate to the nickel atom in a distorted square planar N₂O₂-coordination sphere. The conformational difference between the symmetry-independent molecules arises from the “up-or-down” folding of the naphthaldiminato ligand with respect to the coordination plane, which creates right- (P) or left-handed (M) helical conformations. Overall, the combination of ligand chirality, chirality at the metal and ligand folding gives rise to discrete metal helicates of preferred helicity in a selective way. Cyclic voltammograms (CV) show an oxidation wave at ca. 1.30 V for the [Ni(L)₂]/[Ni(L)₂]⁺ couple, and a reduction wave at ca. −0.35 V for the [Ni(L)₂]/[Ni(L)₂]⁻ couple in acetonitrile.