Chiral metal complexes. 10. The inversion of Λ-α-{dichloro[3R-methyl-1,6-di(2-pyridyl)-2,5-diazahexane]cobalt(III)} to a Δ-β isomer via an isolated Δ-β-dinitro intermediate

Λ-α-{Dichloro[3R-methyl-1,6-di(2-pyridyl)-2,5-diazahexane]cobalt(III)}, Λ-α[Co(R-picpn)Cl₂]⁺, reacts with nitrite ion in aqueous solution, inversion occurs and the main product is Δ-β-[Co(R-picpn)(NO₂)₂]⁺. This reaction is stereospecific in that only one of four possible Δ-β isomers forms. A small amount of Λ-α[Co(R-picpn)(NO₂)₂]ClO₄ was isolated from the reaction solution. The Λ-α-diastereomer readily isomerizes to the favoured Δ-β form, the reaction being light-catalysed. Reaction of the isolated Δ-β-dinitro species with concentrated hydrochloric acid results in the stereospecific formation of one isomer of Δ-β[Co(R-picpn)Cl₂]⁺. All of the complexes were investigated using circular dichroism and high resolution proton NMR methods.