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

Abstract

Potentiometric, spectroscopic and X-ray studies of 2-(hydroxyimino)propanohydroxamic acid (H2L) and its complexes with Ni2+ and Cu2+ showed that the ligand is a strong chelating agent forming a series of stable complex species with remarkably higher formation constants compared to those of either aminohydroxamic acids or oximinocarbonic acids. In the Cu2+–H2L system three dinuclear species [Cu2HL2]+, [Cu2L2] and [Cu2H–1L2]– with different donor 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-VIS spectral data, square-planar structure with a M{N2(oxime)N2(hydrox)} core. Thus, the ligand studied represents a new example in which the adjacent oxime donor group facilitates the N-bonding of the hydroxamic function. The crystal structures of two complexes, Na2[NiL2]·4H2O 1 and [Cu(phen)(hpa)(H2O)]·4H2O 2 [H2hpa = 2-(hydroxyimino)propanoic acid] have been determined by single-crystal X-ray analysis. In 1 the central atom is situated at the centre of symmetry and in square-planar surroundings of four nitrogen atoms belonging to the deprotonated oxime and hydroxamic groups. The trans-disposed ligands are additionally linked by short intramolecular hydrogen bonds featuring the oxime and hydroxamic oxygen atoms. Complex 2 was crystallised from an alkaline solution containing [Cu(phen)Cl2] and H2L and formed as a result of ligand hydrolysis. The copper(II) ion is in distorted square-pyramidal surrounding, the basal plane being formed by two nitrogen atoms of 1,10-phenanthroline, an oxime nitrogen and a carboxylic oxygen atom of the dianion hpa and the water molecule occupies the apical position. Both organic ligands are co-ordinated in a bidentate chelate mode.