Influence of N-alkyl substituents in a diamine chelate ring on the optical activity of the [CoIIIN6]chromophore

Abstract

The complexes [Co(diamine)(en)₂]³⁺(en = ethylenediamine; diamine =N-methyl-, NN′-dimethyl, or N-iso-propyl-ethylenediamine) have been obtained and resolved into optical isomers in order to investigate the effect of N-alkyl groups on the d-electron optical activity of the cobalt(III) tris(diamine) complexes. As in other types of complex containing the octahedral [Co^IIIN₆] chromophore, the influence of N-alkyl groups is not large, in contrast to the major chiroptical effects produced by N-methyl substitution in chiral complexes containing the tetragonal trans-[Co^IIIN₄Cl₂] chromophore. A ligand-polarisation model for d-electron optical activity is used to show that, while the changes in the rotational strengths of the individual components of the ¹A₁→¹T₁ octhedral d-electron transition produced by N-alkyl substitution are large, these changes mutually cancel when summed over the three components. The effects of C-alkyl substitution are small, either for the individual components or in sum.