Chiral metal complexes. 25. Cobalt(III) complexes of some linear mesomeric tetramine ligands

The complexes of Λ-α-[Co(R,S-picbn)Cl₂] ClO₄ (where R,S-picbn is 3R,4S-dimethyl-1,6-di(2-pyridyl)-2,5-diazahexane) together with its Δ-Λ-α and Δ,Λ-β exo congeners,Δ-Λ-β-exo-[Co(picchmn)Cl₂] ClO₄ (where picchmn is N,N'-di(2-picolyl)-1R, 2S-diaminocyclohexane) as well as Δ,Λ-β-endo-[Co(R,S-picstien)Cl₂] ClO₄·2H₂O, [Co(R,S-picstien)(ox)] ClO₄·0.5H₂O and [Co(R,S-picstien)(mal)] ClO₄·3H₂O (where picstien is 3R,4S-diphenyl-1,6-di(2-pyridyl)-2,5-diazahexane, ox is the oxalate dianion and mal is the malonate dianion) have been synthesised. The nature of the compounds was determined using a combination of ¹H NMR and, for certain chiral species, chiroptical techniques. In the various β complexes, the tetradentate is observed to adopt either the exo or endo geometry, specifically. Factors which influence coordination geometry include steric interactions and hydrophobic bonding effects. A number of chemical transformations between dinitro and dichloro complexes of Co(III) with R,S- picbn have been examined, as has the reaction of Δ,Λ-β-exo-[Co(R,S-picbn)Cl₂]⁺ with S-alanine in aqueous solution. The resulting product mixture contains eight of the sixteen possible β diastereoisomers, of which three have been isolated and characterised. The eight are composed of four β₁ and four β₂ isomers, however, and it is noted that isomerisation at the inplane amine nitrogen atom is restricted by the overall geometry of the complexes formed. Discriminatory forces in these complexes are small in magnitude, and exo/endo isomerisation is somewhat dependent upon the choice of ligand(s) used to complete the coordination sphere.