Chiral metal complexes. 13. The crystal and molecular structures of four diastereoisomeric forms of β1-{S-alaninato[2,5-diaza-3-methyl-1, 6-di(2-pyridyl)hexanel] cobalt(III)} perchlorate in one single crystal

A crystalline form of β₁-[Co(±picpn)(S-ala)]ClO₄ has been shown by X-ray analysis to contain four diastereoisomeric cations in a single crystal. The crystals are monoclinic, space group P2₁, with a = 17.204(4), b = 9.037(2), c = 17.631(3) Å, β = 116.85(2)° and Z = 4. The structure was refined by block-matrix least-squares methods to R of 0.040 for 2313 non-zero reflexions. The compound crystallizes as a quasi-racemic mixture, with the Λ and Δ cationic species in the asymmetric unit each representing two different diastereoisomers present in an abundance ratio of 0.62:0.38. Differences between the molecular structures of cations of like hand are derived from the alternative positions allowed for the methyl group of picpn when coordinated in β-topology. A pseudo n-glide operation exists between the chiral pairs. The perchlorate groups, which are involved in hydrogen-bonding to amine protons of the cations, are disordered. The four cationic diastereoisomers observed in the crystal, together with their respective abundances, are Δ-β₁-S,S-endo-[Co(S-picpn)(S-ala)]²⁺ (62%), Δ-β₁-S,S-exo-[Co(S-picpn)(S-ala)]²⁺ (38%), Λ-β₁-R,R-endo-[Co(R-picpn)(S-ala)]₂₊ (62%) and Λ-β₁-R,R-exo-[Co(R-picpn)(S-ala)]₂₊ (38%). The S-alanine conformation is different in the two Λ forms, whereas in the Δ forms the S-alanine atomic positions are crystallographically equivalent. The observed preference for endo arrangements in the isomers is consistent with previously reported molecular strain energy calculations. Small angular distortions in the octahedral coordination spheres are consistent with the multidentate behaviour of the ligand. The average CoO, CoN(amine) and CoN(pyridine) bond lengths are 1.89(1), 1.94(3) and 1.93(1) Å respectively. There are no unfavourable intramolecular contacts evident involving the S-alanine groups of the cations.