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{N_2(oxime)N_2(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.