Abstract
The planar bis-[ N-alkyl-α-amineoxime] complexes of nickel(II) are shown to be relatively stable in either the meso( RS/ SR) or dl( RR/ SS) form by 1H NMR and X-ray analysis. They can be isolated, quantitatively separated and analyzed for by HPLC. As bromide salts they crystallize separately having different crystal morphology and color. The X-ray crystal structures have different unit cell symmetry: meso, orthorhombic, Aba2 while the dl isomer is monoclinic, P2 1/ c. The unit cells contain similar monomeric nickel(II) cations, bromide ion and a non-coordinated water molecule. They differ only in the geometric arrangement around the asymmetric nitrogens. In solution, the isomers slowly convert to equilibrium mixtures ( t 1/2≈1–5000 min at 0°), with the rate and K eq being highly dependent on the solvent and the solution acidity. The equilibrium constant, K eq=[ meso]/[ dl], measured in various solvents, combined with the observed first-order rate constants for rearrangement give k f and k r values. These are compared under different conditions and possible mechanisms for racemization are discussed. Both the rates of interconversion and the equilibrium ratios are very sensitive to solvent composition.
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