Products of the ‘irreversible oxygenation’ of cobalt(II) dipeptide compounds: cobalt(III) compounds of glycylglycine and some other dipeptides

Abstract

The red product of the ‘irreversible oxygenation’ of the bisglycylglycinatocobalt(II) complex is shown to be an anionic cobalt(III) complex with two terdentate peptide ligands, both of which are deprotonated at the peptide nitrogen. A number of salts of the complex anion are described. In acid solution the peptide nitrogen can be re-protonated, and salts of the resulting complex cation also are described. Some of these salts of the complex anion or cation crystallise in a variety of different polymorphic forms or hydrates. For example, three hydrates of the compound calcium bisglycylglycinatocobaltate(III) are described, and four different hydrates of the potassium salt. In addition, a variety of crystalline bisglycylglycinatocobalt(III) compounds were obtained as minor products by following a published method for the oxygenation. These all are rationalised as different crystalline forms containing the same cobalt(III) complex anion or cation. No significant evidence could be obtained from such reaction mixtures for the significant occurrence of other species, in which different bonding modes of the facultative peptide ligands exist. The anionic dipeptide complexes of glycyl-L-leucine, L-alanylglycine, DL-alanylglycine, and L-alanyl-L-alanine also were prepared. What appears to be a marked stereoselectivity has been observed in the formation of these complexes of optically active peptides, and the role of solvation phenomena as the possible source of this stereoselectivity is discussed.