Monohydroxamic acids and bridging dihydroxamic acids as chelators to ruthenium(iii) and as nitric oxide donors: syntheses, speciation studies and nitric oxide releasing investigation

Abstract

The synthesis and spectroscopic characterisation of novel mononuclear Ru^III(edta)(hydroxamato) complexes of general formula [Ru(H₂edta)(monoha)] (where monoha = 3- or 4-NH₂, 2-, 3- or 4-Cl and 3-Me-phenylhydroxamato), as well as the first example of a Ru^III-N-aryl aromatic hydroxamate, [Ru(H₂edta)(N-Me-bha)]·H₂O (N-Me-bha = N-methylbenzohydroxamato) are reported. Three dinuclear Ru^III complexes with bridging dihydroxamato ligands of general formula [{Ru(H₂edta)}₂(μ-diha)] where diha = 2,6-pyridinedihydroxamato and 1,3- or 1,4-benzodihydroxamato, the first of their kind with Ru^III, are also described. The speciation of all of these systems (with the exception of the Ru–1,4-benzodihydroxamic acid and Ru–N-methylbenzohydroxamic systems) in aqueous solution was investigated. We previously proposed that nitrosyl abstraction from hydroxamic acids by Ru^III involves initial formation of Ru^III-hydroxamates. Yet, until now, no data on the rate of nitric oxide (NO) release from hydroxamic acids has been published. We now describe a UV-VIS spectroscopic study, where we monitored the decrease in the ligand-to-metal charge-transfer band of a series of Ru^III-monohydroxamates with time, with a view to gaining an insight into the NO-releasing properties of hydroxamic acids.