Structural and spectroscopic studies of the versatile coordination chemistry of the chiral ligand N,N-bis(1-propan-2-onyl oxime)-L-methionine N'-methylamide with Ni(II) and Zn(II).

Abstract

The potentially pentadentate, chiral ligand N,N-bis(1-propan-2-onyl oxime)-L-methionine N'-methylamide (L-MABO) shows remarkable versatility in its coordination chemistry with Ni(II) and Zn(II). In the crystal structure of the ZnCl2 complex of L-MABO, the ligand coordinates to the metal only through its three nitrogen donor groups (one amine and two oximes), with two chloride anions completing the distorted trigonal bipyramidal coordination sphere. In the NiCl2 complex, the three nitrogen donors and the thioether sulfur coordinate, along with two chlorides. The crystal structure of the Ni(NO3)2 complex contains two independent molecules, one of which coordinates the three nitrogens, the thioether sulfur, and the amide oxygen of L-MABO in addition to one nitrate anion. The second molecule coordinates the three nitrogen donors, the amide oxygen, one nitrate anion, and a methanol molecule. Thus, in only three crystal structures, L-MABO demonstrates its ability to provide N3, N3S, N3O, and N3OS donor sets. The thioether-bound complexes are unusual in that they have a predominantly nitrogen environment with a nickel-thioether bond that is not constrained by surrounding donor groups in a macrocyclic or linear polydentate motif. Comparison of the thioether-coordinated and methanol-coordinated molecules in the Ni(NO3) salt of L-MABO demonstrate the effect of the thioether bond on the relative "hardness" of the nickel. The electronic absorption and circular dichroism spectra of the aqueous solutions of the nickel complexes are interpreted in terms of a "descent in symmetry" model based on successive C3v and Cs distortions from octahedral geometry. These ligand field spectra indicate that in aqueous solution all ligand groups except for the three nitrogens of L-MABO are displaced by water. In acetonitrile, the non-nitrogen donors in the nitrate salt may also be displaced, while the chlorides remain coordinated.