Tailored ligands for zinc coordination

Abstract

Selective coordination to zinc over other members of the first row transition metal series is important for the commercial application of lipophilic ligands for the hydrometallurgic recovery of the metal. In order to achieve selectivity for zinc(n) over other ions, particularly copper(II) and iron(III), ligands have to be designed which take advantage of zinc's preferred tetrahedral coordination geometry and the borderline donor atom preference of the metal. With such ligands in mind, three new classes of ligand system have been designed and synthesised based on the benzimidazole, quinoline and pyridine ring systems with appended phosphinic and thiophosphinic acid anionic donor groups. Benzimidazole ligands which bind zinc in a 2:1 manner in solution have been made and their complexation ability assessed by ESMS, (^31)p NMR, liquid-liquid extraction, fluorescence and UV absorption spectroscopic methods. A bisbenzimidazole ligand with a C(_3) spacer in the 2,2' position was synthesised and shown to bind zinc initially in a 2:1 manner, at low metal concentrations, and predominantiy as a 1:1 species at higher metal concentrations. The formation constant for the ML complex was shown to be logK(_ML)=5 by analysis of the NMR titration curve, which was in close agreement with the value obtained from liquid-liquid extraction studies. A directly linked 2,2'- bisbenziraidazole system was also synthesised as an extension to previous work, however isolation of the target ligand proved to be difficult due to the insolubility of the desired bis-acid.8-(Quinolinyl)phenylphosphinic acid and 8-(2-metiiylquinolinyl)phenylphosphinic acid were also synthesised and their solution complexation behaviour studied in detail. The unsubstituted ligand appeared to form 1:1 complexes with zinc at all the metal concentrations studied, and the absence of the methyl substituent does not inhibit coordination to the ferric ion. In contrast the metiiyl substituted ligand initially forms a 2:1 L:M complex, and 1:1 complexes at higher metal concentrations. The initial ML(_2) complex probably involves coordination of two of the phosphinic acid moieties. Three pyridyl derived ligand systems were also synthesised varying the bulk of the C-6 substituent and the effect of the phosphinic versus the thiophosphinic acid moiety towards zinc coordination was examined. Both of the methyl appended ligands were shown to bind zinc in a 2:1 manner, with the thiophosphinic acid exhibiting a greater avidity for zinc. In contrast, the unsubstituted ligand predominantly forms 1:1 complexes at all metal concentrations.