Optically active co-ordination compounds. Part XXX. Mono-dipeptide complexes of cobalt(III)

Abstract

Chloride and perchlorate salts of the cations [Co(NH₃)₃(α₁α₂)]⁺ and [Co(dien)(α₁α₂)]⁺, where α₁α₂H₂ is a dipeptide, and dien = diethylenetriamine, have been prepared and characterised. A general structure for these complexes is proposed, in which the dipeptide ligand is terdentate and planar, having lost the amide proton. The protonation of [Co(dien)(glygly)]⁺ and [Co(NH₃)₃(glygly)]⁺ has been studied in detail by electronic absorption and ¹H n.m.r. spectroscopy; complexes in which the oxygen atom of the amide group is protonated have been isolated from solutions in concentrated perchloric acid. In hot concentrated hydrochloric or hydrobromic acid [Co(NH₃)₃(glygly)] ClO₄ gives crystalline [Co(NH₃)₃(glyglyH₂)X]X₂(X = halide) containing a bidentate peptide ligand. Concentrated aqueous solutions of this product deposit crystalline [Co(NH₃)₃(glyglyH)X]X, in which the peptide remains bidentate. Dilute aqueous solutions of [Co(NH₃)₃(glyglyH₂)Cl] Cl₂ undergo aquation as well as deprotonation; on the addition of base ring closure occurs to give [Co(NH₃)₃(glygly)]⁺. [Co(NH₃)₃(glygly)]-ClO₄ can be recovered from dilute aqueous solutions of [Co(NH₃)₃(glyglyH₂)Cl]Cl₂ after ageing and addition of sodium perchlorate. The disproportionation of [Co(dien)(α₁α₂)]⁺ and [Co(NH₃)₃(α₁α₂)]⁺ in alkaline solution and the base-catalysed exchange at the methylene group of the C-terminal amino-acid are reported, together with some preliminary observations on the activation of the peptide towards condensation with aldehydes at the C-terminal residue.