Circular Dichroism of trans-Dichlorocobalt(III) Complexes with Chiral 1,2-Diamines

Abstract

The circular dichroism (CD) spectra of trans-dichlorobis(diamine)cobalt(III) complexes were measured in methanol solution in the 700–200 nm region, where the diamines have the general formula of NH₂CH(R)CH₂NH₂(R=isopropyl, isobutyl, sec-butyl, benzyl and phenyl) and belong to the S configuration except the phenyl-substituted one. The spectra are essentially the same as thq spectrum of the (S)-propylenediamine ((S)-pn; R= methyl) complex, except for the (R)-C-phenylethylenediamine ((R)-phenen) complex. The variation of the C-substituted alkyl groups has hardly any influence on the CD of the trans-[CoCl₂(diamine)₂]⁺ type complexes, demonstrating that the vicinal contribution from the C-substituents can be disregarded in these compounds. The trans-[CoCl₂((R)-phenen)₂]⁺ ion shows a CD pattern differing significantly from that of the trans-[CoCl₂((R)-pn)₂]⁺ ion in the first absorption band region, suggesting the significance of the vicinal effect due to the C-substituted phenyl group. The CD curve of trans-[CoCl₂((−)_D-stien)₂]⁺ ion (stien=stilbenediamine) was found to be antimeric to that of the (R)-phenen complex. Taking into account the anomalous vicinal effect of the phenyl groups, the absolute configuration of the (−)_D-stien was assigned to the (S,S) configuration.