Structural insights into the coordination and selective extraction of copper(II) by tertiary amine ligands derived from 2-aminomethylpyridine

Abstract

To develop selective ligands for the separation of copper(II), nickel(II) and cobalt(II), the solvent extraction and coordination chemistry of a bidentate chelating ligand, N,N-dioctyl-2-aminomethylpyridine (AMPO), and a tridentate chelating ligand, tert-butyl 2-(N-octyl-2-picolyamino)acetate (AMPA), were investigated. The ligands exhibit excellent extraction ability and high selectivity towards copper(II) over nickel(II) and cobalt(II) in the simulated electrolysis anolytes. Two novel copper(II) complexes, [Cu2(AMPO)2Cl4] (1) and [Cu(AMPA)Cl2] (2), were synthesized and fully characterized. The solid-state structures of the complexes 1 and 2 were established by single crystal X-ray diffraction. The results show that the complex 1 consists of isolated dinuclear molecules unit with two bridging chloro ligands and the geometry around Cu(II) displays a slightly distorted square pyramid, and the complex 2 is also best described as a slightly distorted square pyramid in reasonable approximation distance. The stoichiometric ratios and association constants of the copper(II) complexes with AMPO and AMPA were also determined in CH3OH/H2O mixtures by UV−Vis spectrometry, respectively. The interactions between the ligands and the metal ions (Cu2+, Ni2+ and Co2+) were correspondingly evaluated by X-ray photoelectron spectroscopy (XPS) and electrospray ionization-mass spectrometry (ESI-MS), and the results reveal that AMPO can be correspondingly coordinated through tertiary amine N atom and pyridine N atom with the three metal ions of copper(II), nickel(II) and cobalt(II), and AMPA can be coordinated through tertiary amine N atom, pyridine N atom and the carbonyl O atom only with copper(II) not with nickel(II) or cobalt(II). All the results indicate that AMPA could be a better candidate for the deep removal of copper(II) from the nickel or cobalt electrolysis anolyte.