The Supramolecular and Coordination Chemistry of Cobalt(II) Extraction by Phosphinic Acids

Abstract

A combination of mass spectrometry, DFT calculations and 31 P{ 1 H} 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, L 1 H) 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 μ 2 -Co-O-P-O-Co bridges. At low Co loading levels, the predominant species is the 4:1 complex, [Co(L 1 ·L 1 H) 2 ], in which coordinated neutral phosphinic acid ligands form strong H-bonds to adjacent anionic phosphinates. At higher Co loading, oligomers such as [a]

Co-extraction of Iron and Sulfate by Bis(2,4,4-trimethylpentyl)phosphinic Acid, CYANEX®272

“…Unlike other recently reported work, [21,26] electrospray ionisation mass spectra of Fe(III)loaded organic phases provide ambiguous results in identifying the major species present in the condensed phase. The species identified in positive-and negative-ion spectra are very different.…”

“…This behaviour contrasts markedly with the uptake of Co(II) by CYANEX®272, where the evidence points to the formation of polymeric species of significantly different molecular weights at different Co-loading levels, and as a consequence the viscosity varies considerably. [21][22][23] It also marks a further deviation from the Fe:2L 1 or Fe:4L 1 ratios reported for the extracted complex by Biswas and Ahmed. [2,13]…”

“…In the current work, the Feconcentrations used mirrored those of studies of Co(II) and Zn(II) extraction, which demonstrated that polymeric complexes were formed and which accounts for the high viscosity observed at high Co(II) and Zn(II) loading. [21] Formation of similar complexes by Fe(III) could present problems in systems where recovery of base metals is accompanied by Fe(III)-uptake that is not readily acid-stripped. However, in accordance with other studies, [2,13,15,16] there is no evidence for increased viscosity with Fe-uptake, even at the 177% loading level achieved in this study by undertaking multiple contacts with an Fe2(SO4)3 solution.…”

“…In a more general context, as the extractions in this work were conducted under very similar conditions to those used earlier [21] on Co(II) and Zn(II) uptake by CYANEX® 272, they draw attention to remarkable differences in loading mechanisms. Unlike for Co and Zn, there is no evidence in Fe-extraction for the formation of complexes that contain a pseudochelate ring (Figure 8a).…”

“…[6,7] Such pseudochelate units are also found in solid-state structures of phosphinate complexes of M 2+ metals, but not in those of M 3+ metals. [28] In contrast to Co(II) and Zn(II) extraction, [21] uptake of Fe(III) from sulfate solutions is always accompanied by the incorporation of sulfate into the assembly formed in the organic phase. Unlike the complexes formed by Co(II) and Zn(II) in extractions by CYANEX® 272, there is no evidence that Fe(III) forms polymeric complexes with high molecular weight and viscous solutions at high metal loading.…”