Coordination modes of hydroxamic acids in copper(II), nickel(II) and zinc(II) mixed-ligand complexes in aqueous solution

Abstract

The stability constants and coordination modes of the mixed-ligand complexes formed by Cu(II), Ni(II), Zn(II), ethylenediamine (en), 2,2′-bipyridine (bpy), glycinate (Gly), disodium salt of 4,5-dihydroxybenzene 1,3-disulfonate (Tiron), diethylenetriamine (dien) or 2,2′:6,2″-terpyridine (terpy) (=ligand B) and acetohydroxamate (Aha), N-methylacetohydroxamate (MeAha) or N-phenylacetohydroxamate (PhAha) (=ligand A) were determined in water (25°C, I=0.2 M KCl) by pH-metric, spectrophotometric, EPR and calorimetric methods. Mixed-ligand complexes with typical hydroxamate type chelation mode involving the NHO − moiety are formed in all systems. However, further copper(II) induced deprotonation of the NHO − moiety of Aha in the presence of en or bpy results in the formation of mixed-ligand complexes with hydroximato chelates at high pH. The results show the favoured coordination of a hydroxamate to metal(II)–en and especially to a metal(II)–bpy moiety. If ligand B is Gly, the increase of stability of the mixed-ligand complexes is as expected on statistical basis, whereas the formation of complexes involving O,O-coordinated hydroxamate and O,O-coordinated Tiron is unfavoured. The tridentate coordination of dien or terpy results in five-coordinated mixed-ligand copper(II) complexes in which, most probably, the hydroxamate moiety adopts an equatorial–axial coordination mode. This quite unstable hydroxamate chelate can not hinder the hydrolysis of the complex above pH 8. Under very basic conditions acetohydroximato moieties (CONO 2−) displace the rigid terpy ligand from the coordination sphere and complexes, [Cu(AhaH −1) 2] 2− involving hydroximato chelates are formed.