Co-ordination ability of novel tetradentate amide-and-oxime ligands: differential binding to CuII and NiII

Abstract

Four cationic and anionic complexes of Ni^II and Cu^II with new open chain oxime-and-amide ligands, N,N′-bis(2-hydroxyiminopropionyl)-1,2-diaminoethane (H₂pen) and 1,4-diaminobutane (H₂pab) were isolated and characterised by X-ray crystallography and a variety of spectroscopic techniques. The ligands were found to exhibit two types of co-ordination modes depending on pH: N(oxime),O(amide) bis-bidentate and N₂(oxime),N₂(amide) tetradentate. The first bonding mode leads to bridging of the two ligands thus forming binuclear cationic complexes [{Cu(Hpab)(ClO₄)·2H₂O}₂] 1b and Li[Ni(H_–1pen)]·4H₂O 2a which have been determined by single-crystal X-ray analysis. In 1b the bis-bidentate co-ordination mode leads to the formation of a dimeric complex cation containing a loose 22-membered macrocyclic cavity incorporating two Cu^II ions as a part of the ring skeleton. The central atoms in 1b are distorted square-pyramidal with the water molecule in the apical position. N₄-tetradentate co-ordination observed in 2a leads to the formation of a square-planar complex anion with a closed pseudo-macrocyclic environment stabilised by a short intramolecular H-bond between the oxime oxygen atoms.