The structure of mer-bis[di(3-aminopropyl)amine]cobalt(III) perchlorate, and an energy minimization analysis of the [Co(dpt)₂]³⁺ system

Abstract

X-ray crystallographic analysis shows that the isolated [Co(dpt)₂](ClO₄)₃ [dpt = di(3-aminopropyl)-amine] is the mer geometric isomer. All four six-membered chelate rings of the complex adopt flattened chair conformations, and substantial internal strain is evident in the structure. The crystals are monoclinic, space group P2₁/c, with a 15.259(3), b 10.646(2), c 15.367(3) Å, β 9935(1)º. The structure was refined by blocked-matrix least-squares techniques to an R factor (on 1731 F) of 0.042. A molecular mechanics study of the [Co(dpt)₂]³⁺ system shows that the structure (geometric isomer and conformer) having the lowest strain energy is mer-(chair, chair, chair, chair) which is that observed in the solid state, and predicts its coexistence in solution with several other conformers, particularly mer-(chair, chair, chair, λ-skew) and mer-(chair, chair, δskew, chair). The calculated isomer proportions are mer » u-fac > s-fac in accordance with the experimental observation of the mer only. The substantially higher strain energy of the mer isomer compared to mer-[Co(dien)₂]³⁺ accounts for the tendency to hydrolysis of mer-[Co(dpt)₂]³⁺.