Synthesis, structures and speciation studies of ruthenium(III) hydroxamate/hydroximato complexes. Crystal and molecular structure of hydrated [Ru(H2edta)(2-methoxyphenylhydroxamate)], the first structurally characterised ruthenium(III)-hydroxamate complex

Reaction of K[Ru(Hedta)Cl]·1.5H2O with various phenylhydroxamic acids, R-PhaH, in aqueous solution affords the hydroxamate complexes [Ru(H2edta)(R-Pha)]·xH2O, the crystal and molecular structure of one of which i.e. hydrated [Ru(H2edta)(2-OMe-Pha)], where 2-OMe-Pha = 2-methoxyphenylhydroxamate, has been determined. In this complex, the first reported structure of a Ru(III)hydroxamate, the carboxylato groups of the tetradentate H2edta are trans to each other and the amino nitrogen and hydroxamate oxygen donor atoms are roughly coplanar. Relevant bond lengths ((angstrom)) are: Ru-O(carboxylato) 2.016(4) and 2.044(3), Ru-O (hydroxamato O-) 1.964(4), Ru-O (hydroxamato CO) 2.019(4), Ru-N 2.060(4) and 2.156(4). Addition of R-PhaH to an aqueous solution of K[Ru(Hedta)Cl]·1.5H2O resulted in [Ru(edta)(R-Pha)]2- as the major species at pH 4-7. At higher pH the hydroxamate NH groups in these complexes undergo deprotonation to give the hydroximato complexes [Ru(edta)(R-PhaH-1)]3- as the major species at pH 7-11. This deprotonation, which has previously been reported in only a very small number of cases for mononuclear complexes, is accompanied by marked shifts to longer wavelengths in the ligand to metal charge transfer bands. At pH > 10 hydrolysis to give [Ru (edta)(R-PhaH-1)(OH)]4- in which an edta carboxylato group has been replaced by hydroxide ion is observed. Formation constants for the various species in solution are reported. The affinity of Pha for [Ru(edta)(H2O)]-(hexacoordinated) is much greater than for [Fe (edta)(H2O)]-(heptacoordinated) but this is largely due to differences in charge and coordination numbers of the immediate metal ion environments rather than intrinsic affinity differences between Ru(III) and Fe(III) for hydroxamate ligands.