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

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.