Chiral resolution of 1-phenylethylamine in Schiff base form within a mixed ligand complex of Ni(II)

Abstract

Racemic 1-phenylethylamine was converted to bidentate Schiff base ligand by condensing with imidazole-2-carboxaldehyde (rac-L2). The reaction between Ni(II)(ClO4)2, rac-L2 and another Schiff Base S-L1 (ratio:1:2:2) yielded crystals of fac-Δ-[Ni(S-L1)2(S-L2)](ClO4)2 (1). The ligand S-L1 is a condensation product of S-1-phenylethylamine and pyridine-2-carboxaldehyde. Enantiomeric excess (ee) of bulk crystals of 1 was found to be ~91%. The values were calculated from from Circular dichroism (CD) spectra and optical rotation experiments. The ee measured on a non-crystalline form of 1, isolated by rapid precipitation, was found to be ~59%. The value supports the presence of chiral enhancement without the crystallization. Compared to this, only 40–60% ee in crystals and 0% in precipitated form was achieved in our earlier report. Substituting rac-L2 with R-L2, in the reaction did not yield the [Ni(S-L1)2(R-L2)](ClO4)2 diastereoisomer. The solid from the reaction contains [Ni(S-L1)3](ClO4)2 complex among others. The enantiomer, fac-Δ-[Ni(R-L1)2(R-L2)](ClO4)2 (2), has been synthesized and structurally characterized. Cyclic voltammetry (CV) of the complexes were analyzed for a probable method for electrochemical detection of enantiomers.