Circular Dichroism Studies of a Series of (Ammine)[(<i>RR</i>)-2,4-pentanediamine]cobalt(III) Complexes

Abstract

Abstract The circular dichroism spectra of a series of complexes of the type [Co(NH3)2n(RR-ptn)3−n]3+(n=0, 1,2), where the RR-ptn is (RR)-2,4-pentanediamine, were recorded in aqueous solutions with and without the addition of electrolytes such as chloride or sulfate, and in DMSO solutions. The circular dichroism spectra in the region of the first absorption band of Δ-[Co(RR-ptn)3]3+ and cis-Δ-[Co(NH3)2(RR-ptn)2]3+ were extremely sensitive to the environment of the complexes and showed a remarkable variation, while those of the corresponding diastereomers, Λ-[Co(RR-ptn)3]3+ and cis-Λ-[Co(NH3)2(RR-ptn)2]3+ were little affected by the given environment. The other complexes, trans-[Co(NH3)2(RR-ptn)2]3+ and [Co(NH3)4(RR-ptn)]3+ exhibited small variations in the circular dichroism spectra. These variations seem to be accounted for by the conformational change of the flexible sixmembered chelate ring brought about by ion-pair formation between the complex cations and anions.