N-bonding of the hydroxamic function in nickel(II) and copper(II) complexes with 2-(hydroxyimino)propanohydroxamic acid

Potentiometric, spectroscopic and X-ray studies of 2-(hydroxyimino)propanoh ydroxamic acid (H2L) and its complexes with Ni2+ and Cu2+ showed that the l igand is a strong chelating agent forming a series of stable complex specie s with remarkably higher formation constants compared to those of either am inohydroxamic acids or oximinocarbonic acids. In the Cu2+-H2L system three dinuclear species [Cu2HL2](+), [Cu2L2] and [Cu2H-1L2](-) with different don or atom sets were found to be dominant at pH 3.5-7.5. In both Cu2+ and Ni2 systems the mononuclear species [MHL2](-), [ML2](2-) and [MH-1L2](3-) are formed in neutral and alkaline solutions which have, according to the UV-VI S spectral data, square-planar structure with a M{N-2(oxime)N-2(hydrox)} co re. Thus, the ligand studied represents a new example in which the adjacent oxime donor group facilitates the N-bonding of the hydroxamic function. Th e crystal structures of two complexes, Na-2[NiL2]. 4H(2)O 1 and [Cu(phen)(h pa)(H2O)]. 4H(2)O 2 [H(2)hpa = 2-(hydroxyimino)propanoic acid] have been de termined by single-crystal X-ray analysis. In 1 the central atom is situate d at the centre of symmetry and in square-planar surroundings of four nitro gen atoms belonging to the deprotonated oxime and hydroxamic groups. The tr ans-disposed ligands are additionally linked by short intramolecular hydrog en bonds featuring the oxime and hydroxamic oxygen atoms. Complex 2 was cry stallised from an alkaline solution containing [Cu(phen)Cl-2] and H2L and f ormed as a result of ligand hydrolysis, The copper(II) ion is in distorted square-pyramidal surrounding, the basal plane bring formed by two nitrogen atoms of 1,10-phenanthroline, an oxime nitrogen and a carboxylic oxygen ato m of the dianion hpa and the water molecule occupies the apical position. B oth organic ligands are co-ordinated in a bidentate chelate mode.