Optically active co-ordination compounds. Part 41. Bis(dipeptidato)cobalt(III) complexes

Abstract

The preparation and separation of the meridional isomers of complexes of the type [Co(α₁-α₂)₂]^– is described [Hα₁-Hα₂ is a dipeptide, H₂NCHR′CONHCHR²CO₂H, ranging from glycylglycine (gly-gly) to L-phenylalanyl-L-phenylalanine (L-phe-L-phe)]. With L-α₁-L-α₂, L-α₁-gly, or gly-L-α₂, each of the several preparative methods yields meridional [Co(α₁-α₂)₂]^– as a mixture of its two diastereoisomers which have been separated by ion-exchange chromatography. All the preparative methods give minor products which can be removed by ion-exchange chromatography. The meridional isomer of the complexes has been characterised by electronic, ¹H n.m.r., and circular dichroism (c.d.) spectroscopy. The c.d. spectra of the pairs of diastereoisomers are discussed. Detailed comparison of the ¹H n.m.r. spectrum of the pairs of diastereoisomers enabled determination of their absolute configuration unambiguously for [Co(L-phe-glyO)₂]^– and [Co(gly-L-pheO)₂]^–[L-phe-glyO =L-phenylalanylglycinate(2–)] and for the others by comparison. The protons in the C-terminal residue are activated to exchange in alkaline solution. Knoevenagel reactions have been performed with the bis(glycylglycinato)-complex.