The structures of phenolic oximes and their complexes

This review article describes the rich coordination chemistry of phenolic oxime ligands based on solid state structures in the October 2000 release of the Cambridge Structural Database. The phenolic oxime ligands discussed have the general formula n-(R-1)(x)-2-OH-C6H4-xC(R-2)NOH. Whilst their classical mode of coordination via the deprotonation of the phenol to bind to a the metal centre as a mono-anionic bidentate ligand is frequently observed, the structural survey reveals a plethora of less common binding motifs that can be attained via the deprotonation of both the phenolic and oximic hydrogens resulting in the formation of tridentate ligands and polynuclear bridged complexes. The formation of such polynuclear complexes at surface sites may account for their efficacy as corrosion inhibitors. Also highlighted is the occurrence of hydrogen bonding between oximic hydrogen and phenolic oxygen atoms and its implications on the thermodynamic stability of metal complexes via the formation of pseudo-macrocyclic motifs and on the pre-organisation of dimeric metal-free ligand species. The formation of ligand-ligand hydrogen bonds to define donor cavities of a well defined size accounts for their remarkable selectivity when used in commercial metal recovery processes based on solvent extraction. (C) 2002 Elsevier Science B.V. All rights reserved.