Synthesis of the sym-fac-[Co(medien)(S)-Asp]+ isomer and its stereochemical comparison with other ternary cobalt(III) complexes containing (S)-aspartic acid and linear triamine

Abstract

The sym-fac-[Co(medien)(S)-Asp]ClO4 (medien = 4-methyl-1,4,7-triazaheptane) isomer was prepared from [Co(medien)Cl3] and Ag2(S)-Asp in the presence of charcoal. Since sym-fac isomer does not isomerize when equilibrated with charcoal, its formation is of thermodynamic origin. The sym-fac configuration was assigned from electronic, NMR, and circular dichroism spectra. The d-d circular dichroism spectrum which is very similar both in shape and relative intensity to that of the [Co(NH3)3(S)-Asp]+ ion exhibits vicinal contribution from S centre, conformational contributions either from aspartic acid (λ, δ-twist boat) either from medien rings (λ, δ or δ, λ asymmetric envelope). the last contributions are canceled. NMR parameters are consistent with symmetrical conformation which is favoured due to the staggered placement of N-CH3 group with respect to oxygen atoms of aspartic acid. The preferred formation of sym-fac isomer over unsym-fac ones is discussed in relation to the nonbonded interactions of N-CH3 group with Hax of adjacent NH2 group of aspartic acid in each of the two unsym-fac isomers.