The mode of action of iron(III) uptake from sulfate solutions by the commercial extractant CYANEX (R) 272, bis(2,4,4-trimethylpentyl)phosphinic acid, has been studied using conventional solvent extraction methods, together with P-31{(1) H} NMR spectroscopy, electrospray ionisation mass spectrometry, and DFT calculations that have not previously been used to analyse the content of the ISOPAR M water-immiscible phase. The maximum Fe-loading recorded was 177% of the theoretical maximum, based on the formation of an Fe(III) complex [Fe(L)(3)] with a 1:3 molar ratio of Fe to phosphinate (L). Inductively coupled plasma optical emission spectroscopy (ICP-OES) indicates that sulfate or hydrogen sulfate ions are co-extracted in a 1:2 molar ratio with iron across the whole of the Fe-loading range, which helps to account for the loadings being greater than 100% of the theoretical value. P-31{(1) H} NMR spectroscopy indicates that the limiting factor in Fe-loading is the availability of uncomplexed CYANEX (R) 272 (sulfate is present in large excess). In contrast to the behaviour of Co(II) and Zn(II) extraction by CYANEX (R) 272, there is no evidence for the formation of polymeric Fe(III) complexes and highly viscous solutions at high metal loadings. The identity of the extracted species is likely to be a polynuclear Fe(III) sulfate complex. [GRAPHICS] .

The recycling of metals from end-of-life secondary sources such as electronic waste remains a significant environmental and technological challenge currently detrimental to the development of circular economies. The complex nature of electronic waste, containing a myriad of different elemental metals, means that sophisticated yet simple separation methods need to be developed to recycle these valuable and often critical metal resources. In this work, simple primary, secondary, and tertiary amides are appraised as reagents that selectively transport gold from aqueous to organic phases in solvent extraction experiments. While the strength of extraction of gold from single-metal solutions is ordered 3 degrees > 2 degrees > 1 degrees, the 3 degrees and 2 degrees amides are ineffective at gold transport from mixed-metal solutions of concentrations representative of smartphones due to the formation of a third phase. Increasing the polarity of the organic phase can negate third-phase formation but at the expense of selectivity. The identities of the species that reside in the organic and third phases have been studied by a combination of slope analysis, mass spectrometry, NMR spectroscopy, and computational methods. These techniques show that protonation of the amide L occurs at the oxygen atom, resulting in the protonated dimer HL2+, which acts as a receptor for AuCl4- to form dynamic supramolecular aggregates in the organic phase. The characterization of a tin complex in the third phase by X-ray crystallography supports these conclusions and, furthermore, suggests the preference for the chelation of the proton by two amide molecules instead of the transport of hydronium into the organic phase and its subsequent use as a structural template.

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 [( L 1 ·L 1 H)Co( L 1 2 Co) n ( L 1 ·L 1 H)] are formed with µ 2 ‐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 ( L 2 ) in the commercial formulation reduces the viscosity because its incorporation into oligomeric complexes such as [( L 2 )Co{ L 1 3 Co L 1 Co L 1 3 Co} m L 1 ] can terminate chains, resulting in a lower average molecular weight. The uptake of Zn by L 1 H 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.

The possibility of recovering rare earth elements from solutions containing their chloridometalate anions [LnCl(x)]((x-3)-) via the process: LnCl(x)((x-3)-) + (x - 3)L-org + (x-3)H+ [(LH)(x-3)LnCl(x)](org) has been tested using 2-(1,3-bis(hexylamino)-1,3-dioxopropan-2-yl)-4,6-di-tert-butylpyridine (PMA), tri-n-butylphosphate (TBP), and tri-n-octylamine (TOA), which are known to be strong extractants for transition metal chloridometalates. While DFT calculations indicate that the formation of the neutral assembly [(PMAH)(3)LaCl6] in the gas phase is favorable, no uptake of La(III) from 6 M HCl by toluene solutions of PMA (or of TBP or TOA) was observed in solvent extraction experiments. Successful uptake of the [PtCl6](2-) dianion by PMA and the failure to extract the [IrCl6](3-) trianion under the same conditions indicate that the higher hydration energy of the latter makes transfer to the toluene solution less favorable and that this militates against extraction of La(III) chlorido complexes carrying charges of -3 or larger in which all the inner-sphere water molecules have been replaced. Computational results confirm literature observations that, in contrast to transition metal trications, formation of REE metalate anions such as [LnCl(x)]((x-3)-) is not very favorable, particularly so for chloride, compared with nitrato or sulfato systems. Also, they indicate that the formation of outer-sphere assemblies such as {[La(H2O)(9)]xCl} in which water ligands are retained in the inner sphere, H-bonded to anions, is more stable than inner-sphere complexes containing an equivalent number of anions. The high level of hydration of such species disfavors their transfer into nonpolar water-immiscible solvents. It is unlikely that recovery of [LnCl(x)]((x-3)-) from acidic solutions can be achieved efficiently using currently available anion exchange extractants operating in a pH-swing process. Receptors giving very high binding energies to chloridolanthanates will be needed to offset the high dehydration energies required.

An analysis of 552 structures of metal complexes of alkyl and arylphosphinates in the Cambridge Crystallographic Database shows that the phosphinate ligating group is remarkably versatile and is able to adopt ten different binding motifs in both mono- and polynuclear complexes in which an individual phosphinate group can bind to up to five metal atoms. The majority of both homo- and heteroleptic complexes contain M-O-PR2=O-M units in oligomeric and polymeric structures. In many heteroleptic complexes ligands containing hydrogen bond donors form strong bonding interactions with the phosphinate, generating pseudochelated structures. Similar pseudochelates, O--PR2=O center dot center dot center dot H-O-PR2=O, are formed when both a phosphinate and its parent phosphinic acid are coordinated to a single metal atom. Such structures feature also in the solution chemistry involved in metal extraction processes using phosphinate ligands. As might be expected, many of the binding motifs found in phosphinate complexes are similar to those in carboxylate complexes but there are fewer examples of phosphinates being used to form metal organic frameworks. (C) 2016 Elsevier B.V. All rights reserved.

Waste electrical and electronic equipment (WEEE) such as mobile phones contains a plethora of metals of which gold is by far the most valuable. Herein a simple primary amide is described that achieves the selective separation of gold from a mixture of metals typically found in mobile phones by extraction into toluene from an aqueous HCl solution; unlike current processes, reverse phase transfer is achieved simply using water. Phase transfer occurs by dynamic assembly of protonated and neutral amides with [AuCl4 ](-) ions through hydrogen bonding in the organic phase, as shown by EXAFS, mass spectrometry measurements, and computational calculations, and supported by distribution coefficient analysis. The fundamental chemical understanding gained herein should be integral to the development of metal-recovery processes, in particular through the use of dynamic assembly processes to build complexity from simplicity.

High anion selectivity for PtCl6(2-) over Cl(-) is shown by a series of amidoamines, R(1)R(2)NCOCH2CH2NR(3)R(4) (L1 with R(1) = R(4) = benzyl and R(2) = R(3) = phenyl and L3 with R(1) = H, R(2) = 2-ethylhexyl, R(3) = phenyl and R(4) = methyl), and amidoethers, R(1)R(2)NCOCH2CH2OR(3) (L5 with R(1) = H, R(2) = 2-ethylhexyl and R(3) = phenyl), which provide receptor sites which extract PtCl6(2-) preferentially over Cl(-) in extractions from 6 M HCl solutions. The amidoether receptor L5 was found to be a much weaker extractant for PtCl6(2-) than its amidoamine analogues. Density functional theory calculations indicate that this is due to the difficulty in protonating the amidoether to generate a cationic receptor, LH(+), rather than the latter showing weaker binding to PtCl6(2-). The most stable forms of the receptors, LH(+), contain a tautomer in which the added proton forms an intramolecular hydrogen bond to the amide oxygen atom to give a six-membered proton chelate. Dispersion-corrected DFT calculations appear to suggest a switch in ligand conformation for the amidoamine ligands to an open tautomer state in the complex, such that the cationic N-H or O-H groups are also readily available to form hydrogen bonds to the PtCl6(2-) ion, in addition to the array of polarized C-H bonds. The predicted difference in energies between the proton chelate and nonchelated tautomer states for L1 is small, however, and the former is found in the X-ray crystal structure of the assembly [(L1H)2PtCl6]. The DFT calculations and the X-ray structure indicate that all LH(+) receptors present an array of polarized C-H groups to the large, charge diffuse PtCl6(2-) anion resulting in high selectivity of extraction of PtCl6(2-) over the large excess of chloride.