X-ray crystal structures and data bank analysis of Zn(II) and Cd(II) complexes of 2- and 7-nonyl substituted 8-hydroxyquinoline and 8-hydroxyquinaldine extractive agents

Heavy metal ion extractants 2-nonyl-8-hydroxyquinoline (LH1) and 7-nonyl-8-hydroxy-quinaldine (LH2) yield complexes with Zn(II) (LH1 and LH2) and Cd(II) ions (LH2). X-ray crystal structures indicate that the Zn(II) complex of LH1 (1a) as well as the Cd(II) complex of LH2 (2b) are of binuclear five-coordinated type with M2L4 stoichiometry (M=Zn, Cd) whereas the Zn(II) complex of LH2 (2a) is a mononuclear four-coordinated ML2 type. The binuclear complexes 1a and 2b have symmetric build-up. Due to H⋯metal distances observed in the complexes the coordination geometries are also analyzed by the perusal of the Cambridge crystallographic database. There are slightly differing feeble C–H⋯M interactions and a differing pattern of weak C–H⋯O contacts in all three complexes. The different selectivity of ligand LH2 towards Cd2+ and Zn2+, as well as the selectivity of LH1 for both Cd2+ and Zn2+ might possibly be explained by the packing differences in the three crystal structures.