Optical activity and stereochemistry of (S)-aspartato-dipropylenetriaminecobalt(III) isomers

Abstract

The sym-fac-, unsym1-fac-(Λ)-and unsym2-fac-(Δ)-[Co(dpt)(S)-Asp]+ isomers, where dpt is dipropylenetriamine and (S)-Asp is (S)-aspartate, which are formed in the ratio 60:18:22, show very similar circular dichroism spectra due to similar distortions of the triamine nitrogen atoms from the Cartesian coordinates and domination by the vicinal effect from (S)-aspartic acid. Absolute configurations of the unsymmetrical isomers have been tentatively assigned using the calculated configurational contributions of the very low rotatory strenght, which depends on the magnitude of NH2-Co-N(H) triamine bond angles. Distorted chair conformations of one of the two dipropylenetriamine chelate rings provide chirally displaced nitrogen donor atoms which are thought to be responsible for the CD similarity in the isolated isomers. The isomer population is discussed in terms of trigonal bipyramid intermediates with the coordinated oxygen atoms of (S)-Asp preferentially in the trigonal plane and the differences in acidity of α- and β-chain carboxylates.