Stereochemistry of optically active nickel(ii) and cobalt(ii) coordination compounds derived from N-acetyl aminoalcohols

Abstract

Herein, we report the stereoselective synthesis and stereochemical analysis of nickel(II) and cobalt(II) coordination compounds derived from N-acetyl-(−)-ephedrine 1(−) and N-acetyl-(+)-ephedrine 1(+), N-acetyl-(+)-pseudoephedrine 2(+) and N-acetyl-(−)-phenylephrine 3(−). Two ligands are coordinated to the metal ion in a tridentate mode giving place to optically active coordination compounds ML₂, in a fac octahedral geometry. In all cases, the coordination to the metal ion occurs through the nitrogen atom, the OH group and the deprotonated carboxylic group, forming two five-membered rings. The coordination compounds are characterized in the solid state by UV-Vis-NIR reflectance spectra, IR, X-ray crystallography and magnetic susceptibility measurements. The metal becomes a stereogenic center and, for each ligand, coordination to the metal ion produces two new stereogenic nitrogen and oxygen. Different modes of ligand bonding are present, the fac all-trans, which originates from the fact that the two new stereogenic atoms in each ligand present opposite configurations (mirror effect), as a consequence the bonded ligands are diasteromeric and the complexes have C1 symmetry. The second bonding mode is the fac–cis, which results in the new stereogenic centers acquiring the same configuration; the two bonded ligands are stereochemically identical, producing a complex with C2 symmetry. The use of enantiomeric ligands 1(−) and 1(+) produces separately and stereoselectively the corresponding enantiomeric metallic complexes which are crystallized pure, whereas the use of a racemate of compound 1 produces a different isomer where the metal has a different configuration. Complexes prepared with N-acetyl-ephedrines are diasteromers of those obtained from N-acetyl-pseudoephedrine. The phenylephrine complexes show that the acquired N-configuration is independent of the substitution of the nitrogen vicinal carbon atom and depends only of the cis or trans ligand coordination mode. Analysis of the conformation of the aminoalcohol rings shows that, in all compounds, the phenyl groups are in the equatorial position determining the conformation of the chelates.