Oxygen versus nitrogen co-ordination in complexes of MoVI and hydroxamate derivatives of α-amino acids: equilibrium, structural and theoretical studies†

Abstract

Equilibrium and spectroscopic studies showed that Mo^VI (MoO₄^2–) reacts with α-aminohydroxamic acids (glycine-, sarcosine-, α-alanine-hydroxamic acids, Glyha, Sarha and α-Alaha) in the acidic pH range to give species involving O,O co-ordination whereas, as the pH is raised, species involving N,N co-ordination are formed. The crystal structure of [MoO₂(Glyha)₂] confirmed formation of an O,O co-ordination isomer. Theoretical studies of the O,O and N,N isomers of [MoO₂(Glyha)₂] and [MoO₂(Sarha)₂] showed the former to be the more stable but the relative closeness of the calculated energies of the isomers is in accord with the solution studies. Histidine hydroxamic acid (Hisha) forms O,O isomers in the acidic pH range but as the pH is raised forms two new isomers, one containing the amino nitrogen protonated and co-ordinated in a tridentate manner using the two hydroxamate oxygen atoms and the imidazole N while the other probably contains protonated imidazole N and is co-ordinated via the amino and hydroxamate nitrogens.