Enantioselectivity in Ni(II) Schiff-base complexes derived from amino-acids and (S)-o-N-(N-benzylprolyl)aminobenzophenone: molecular structure of several chiral Ni(II) Schiff-base complexes, circular dichroism and molecular mechanics studies.
Several Ni(II) complexes derived from (S)-o-N-(N-benzylprolyl)aminobenzophenone ((S)-BBP) and amino acids of general formula [Ni((S)-BBP-L-(or D-)-aa)] were prepared. The crystal and molecular structures of [Ni((S)-BBP-Gly)], [Ni((S)-BBP-L-Ser)] and [Ni((S)-BBP-L-aaIm)](aaIm =L-2-amino-3-(imidazol-1-yl)propanoate were determined by X-ray diffraction analysis. In the three complexes the nickel atoms display a square-planar coordination and the overall structure around the metal indicates that the entire Schiff-base ligands form quite rigid frameworks. Molecular mechanics calculations were carried out for complexes [Ni((S)-BBP-Gly)], [Ni((S)-BBP-Ser)] and [Ni((S)-BBP-aaIm)] containing either the L- or D-amino acid forms, and the factors controlling the stereoselectivity are discussed. Several other [Ni((S)-BBP-L-aa)] complexes are also prepared and their circular dichroism spectra in solution and of the solids dispersed in KBr disks are measured and discussed. In agreement with other studies in solution with similar [Ni((S)-BBP-aa)] complexes, the Cotton effects for the bands with lambda(max) at 520--530 nm are positive when the amino acids have the L-configuration at the alpha-carbon. The same is observed in this work for the solid-state CD spectra of all compounds.