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

Abstract

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 (Å) 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 10 hydrolysis to give [Ru(edta)(R-PhaH₋₁)(OH)]⁴⁻ in which an edta carboxylato group has been replaced by hydroxide ion is observed.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.