Geometrical Isomers of Anionic Complexes Favorable to Chiral Discriminatory Ion-Pairing with Λ-[Co(en)3]3+ Ion and the Diastereomeric Ion-Pair Formation Constants in Solution

Abstract The stereoselectivities in the ion pairing of the geometrical isomers of anionic complexes, [Co(ox)(gly)2]− and [Co(ox)(edda)]− (ox=oxalate dianion, gly=glycinate anion, and edda=ethylenediamine-N,N′-diacetate dianion) with [Co(en)3]3+ have been investigated by a chromatographic method. Only the C1-cis(N) isomer of three geometrical isomers of [Co(ox)(gly)2]− and the β-cis isomer of two of [Co(ox)(edda)]−, in which three coordinated oxygen atoms are disposed triangularly along the pseudo C3 axis, have been observed to interact more strongly with [Co(en)3]3+ than the other geometrical isomers. The diastereomeric ion-pair formation constants of C1-cis(N)-[Co(ox)(gly)2]− and β-cis-[Co(ox)(edda)]− with [Co(en)3]3+ in solution have been determined by a conductance method at 25 °C. The results were consistent with those of the chromatographic studies. An enhancement of the circular dichroism (CD) intensity of the A2 component at the first d–d transition region of [Co(en)3]3+ has been observed upon the formation of ion pairing between the complex ions. Therefore, a favorable ion pair is considered to involve the hydrogen bonds between the coordinated oxygen atoms of the anionic complexes and the three N–H protons of [Co(en)3]3+ along the C3 axis.