Synthesis and characterization of cobalt(III) complexes with a stereospecific ligand derived from (S)-valine

Abstract

Complexes of the type [CoLX 2] + and [CoLXY], where L is the quadridentate ligand ethylenediamine-N,N′-di-(S)-α-isovalerate (H 2ven) and X = H 2O or ethylenediamine (en) and Y = NO 3 have been prepared. Three geometrical isomers, cis-α, cis-β, and trans forms, may in principle be obtained. The complex [Co(ven)(NO 3)H 2O] exists only as the cis-α geometrical isomer, while in the case of [Co(ven)-(H 2O) 2] + and [Co(ven)en] + complexes both cis-α and cis-β are found to exist. The yields of the cis-α isomers exceeded considerably those of the cis-β isomers. 13C-NMR and absorption spectral measurements confirm these arrangements. In addition, the IR spectra of the complexes are discussed. The compounds exhibit a string negative CD bond under the 1A 1g → 1T 1g cubic absorption band, which is due to the 1A 1g → 1E g tetragonal component, consistent with the Δ absolute configuration imposed by the (S) configuration of the asymmetric carbon atom in the (S)-valine moiety.