The supramolecular and coordination chemistry of cobalt(II) extraction by phosphinic acids

A combination of mass spectrometry, DFT calculations and 31P{1H} NMR spectroscopy has been used to define the mode of action of the commercial cobalt extractant, bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX®272, L1H) in Co recovery. The nature of the Co(II)-complexes formed in the water-immiscible phase is determined largely by the propensity of phosphinates to form strong interligand H-bonds in the outer coordination sphere and also to form stable µ2-Co-O-P-O-Co bridges. At low Co loading levels, the predominant species is the 4:1 complex, [Co(L1.L1H)2], in which coordinated neutral phosphinic acid ligands form strong H-bonds to adjacent anionic phosphinates. At higher Co loading, oligomers such as [(L1.L1H)Co(L12Co)n(L1.L1H)] are formed with µ2-phosphinate bridging, resulting in a substantial increase in the viscosity of the water-immiscible phase. The presence of tris(2,4,4-trimethylpentyl)phosphine oxide (L2) in the commercial formulation reduces the viscosity because its incorporation into oligomeric complexes such as [(L2)Co{L13CoL1CoL13Co}mL1] can terminate chains, resulting in a lower average molecular weight. The uptake of Zn by L1H shows a very similar dependence of viscosity on loading, and DOSY spectra and mass spectrometry demonstrate that higher molecular weight species are present at high Zn loading.