The Influence of the Hofmeister Bias and the Stability and Speciation of Chloridolanthanates on Their Extraction from Chloride Media

<p>The possibility of recovering rare earth elements from solutions containing their chloridometalate anions [LnCl<i>_x</i>]^{(<i>x</i>−3)−} via the process: LnCl<i>_x</i>^{(<i>x</i>−3)−} + (<i>x</i> − 3)<i>L</i>_org + (<i>x</i>–3)H⁺ ⇌ [(LH)<i>_x</i>₋₃LnCl<i>_x</i>]_org has been tested using 2-(1,3-bis(hexylamino)-1,3-dioxopropan-2-yl)-4,6-di-<i>tert</i>-butylpyridine (PMA), tri-<i>n-</i>butylphosphate (TBP), and tri-<i>n</i>-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)₃LaCl₆] 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<i>_x</i>]^{(<i>x</i>−3)−} is not very favorable, particularly so for chloride, compared with nitrato or sulfato systems. Also, they indicate that the formation of <i>outer-sphere</i> assemblies such as {[La(H₂O)₉]·<i>x</i>Cl} in which water ligands are retained in the inner sphere, H-bonded to anions, is more stable than <i>inner-sphere</i> 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<i>_x</i>]^{(<i>x</i>−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.</p

On the Extraction of HCl and H₂PtCl₆ by Tributyl Phosphate: A Mode of Action Study

<p>Combining computational modeling with experimental measurements has revealed the self-assembly of nano-aggregate structures in the transfer of HCl and PtCl₆^2– from an aqueous phase into toluene by the common industrial extractant tributyl phosphate (TBP). Molecular dynamics simulations have been coupled to analytical measurements to provide an atomistic interpretation of the mode of action of TBP under 6 M and 10 M HCl conditions. The structures conform to reverse micelles, where the Cl^– or PtCl₆^2– core is encapsulated by a hydration shell that acts as a mediating bridge to the electronegative oxygen atom in the TBP phosphate groups. For the 6 M HCl extraction model, the data support stable aggregates forming from 2–3 TBP molecules around one chloride anion if the number of water molecules encapsulating the chloride anion is no more than five; increasing the water content to 10 molecules allows a fourth TBP molecule to coordinate. For the 10 M HCl extraction model, stable structures are obtained that conform to the empirical formula (TBP.HCl.H₂O)_3–5. At 6 M HCl, extraction of PtCl₆^2– is achieved by encapsulation by four TBP molecules; the data for extraction at 10 M HCl indicate larger aggregates containing multiple PtCl₆^2– anions are likely to be forming. In all cases, the hydrated core regions of the reverse micelles are considerably exposed. The diameters of the self-assembled structures around chloride ions agree well with available literature data from small-angle neutron-scattering experiments.</p

Outer-Sphere Coordination Chemistry: Amido-Ammonium Ligands as Highly Selective Tetrachloridozinc(II)ate Extractants

Eight new amido functionalized reagents, L¹–L⁸, have been synthesized containing the sequence of atoms R₂N–CH₂–NR′–CO–R″, which upon protonation forms a six-membered chelate with a hydrogen bond between the tertiary ammonium N–H⁺ group and the amido oxygen atom. The monocationic ligands, LH⁺, extract tetrachloridometal­(II)­ates from acidic solutions containing high concentrations of chloride ions <i>via</i> a mechanism in which two ligands address the “outer sphere” of the [MCl₄]^2‑ unit using both N–H and C–H hydrogen bond donors to form the neutral complex as in 2L + 2HCl + MCl₂ ⇌ [(LH)₂MCl₄]. The strengths of L¹–L⁸ as zinc extractants in these pH-dependent equilibria have been shown to be very dependent on the number of amide groups in the R_3‑<i>n</i>N­(CH₂NR′COR″)_<i>n</i> molecules, anti-intuitively <i>decreasing</i> with the number of strong hydrogen bond donors present and following the order monoamides > diamides > triamides. Studies of the effects of chloride concentration on extraction have demonstrated that the monoamides in particular show an unusually high <i>selectivity</i> for [ZnCl₄]^2‑ over [FeCl₄]⁻ and Cl^–. Hybrid-DFT calculations on the tri-, di-, and monoamides, L², L³, and L⁴, help to rationalize these orders of strength and selectivity. The monoamide L⁴ has the most favorable protonation energy because formation of the LH⁺ cation generates a “chelated proton” structure as described above without having to sacrifice an existing intramolecular amide–amide hydrogen bond. The selectivity of extraction of [ZnCl₄]^2‑ over Cl^–, represented by the process 2­[(LH)­Cl] + ZnCl₄^2‑ ⇌ [(LH)₂ZnCl₄] + 2Cl^–, is most favorable for L⁴ because it is less effective at binding chloride as it has fewer highly polar N–H hydrogen bond donor groups to interact with this “hard” anion

A Comparison of the Selectivity of Extraction of [PtCl₆]^2– by Mono‑, Bi‑, and Tripodal Receptors That Address Its Outer Coordination Sphere

Extraction and binding studies of [PtCl₆]^2– are reported for 24 mono-, bi-, and tripodal extractants containing tris­(2-amino­ethyl)­amine (TREN) or tris­(3-amino­propyl)­amine (TRPN) scaffolds. These reagents are designed to recognize the outer coordination sphere of [PtCl₆]^2– and to show selectivity over chloride anion under acidic conditions. Extraction from 0.6 M HCl involves protonation of the <i>N</i>-center in tertiary amines containing one, two, or three urea, amide, or sulfonamide hydrogen-bond donors to set up the following equilibrium: 2L_(org) + 2H⁺ + [PtCl₆]^2– ⇌ [(LH)₂­PtCl₆]_(org). All reagents show higher Pt loading than trioctylamine, which was used as a positive control to represent commercial trialkylamine reagents. The loading of [PtCl₆]^2– depends on the number of pendant amides in the extractant and follows the order tripodal > bipodal > monopodal, with urea-containing extractants outperforming amide and sulfonamide analogues. A different series of reagents in which one, two, or three of the alkyl groups in tris-2-ethylhexylamine are replaced by 3-<i>N</i>′-hexylpropanamide groups all show a comparably high affinity for [PtCl₆]^2– and high selectivity over chloride anion in extractions from aqueous acidic solutions. ¹H NMR titration of three extractants [LH·Cl] with [(Oct₄N)₂­PtCl₆] in CDCl₃ provides evidence for high selectivity for [PtCl₆]^2– over chloride for tri- and bipodal extractants, which show higher binding constants than a monopodal analogue

A Comparison of the Selectivity of Extraction of [PtCl₆]^2– by Mono‑, Bi‑, and Tripodal Receptors That Address Its Outer Coordination Sphere

Extraction and binding studies of [PtCl₆]^2– are reported for 24 mono-, bi-, and tripodal extractants containing tris­(2-amino­ethyl)­amine (TREN) or tris­(3-amino­propyl)­amine (TRPN) scaffolds. These reagents are designed to recognize the outer coordination sphere of [PtCl₆]^2– and to show selectivity over chloride anion under acidic conditions. Extraction from 0.6 M HCl involves protonation of the <i>N</i>-center in tertiary amines containing one, two, or three urea, amide, or sulfonamide hydrogen-bond donors to set up the following equilibrium: 2L_(org) + 2H⁺ + [PtCl₆]^2– ⇌ [(LH)₂­PtCl₆]_(org). All reagents show higher Pt loading than trioctylamine, which was used as a positive control to represent commercial trialkylamine reagents. The loading of [PtCl₆]^2– depends on the number of pendant amides in the extractant and follows the order tripodal > bipodal > monopodal, with urea-containing extractants outperforming amide and sulfonamide analogues. A different series of reagents in which one, two, or three of the alkyl groups in tris-2-ethylhexylamine are replaced by 3-<i>N</i>′-hexylpropanamide groups all show a comparably high affinity for [PtCl₆]^2– and high selectivity over chloride anion in extractions from aqueous acidic solutions. ¹H NMR titration of three extractants [LH·Cl] with [(Oct₄N)₂­PtCl₆] in CDCl₃ provides evidence for high selectivity for [PtCl₆]^2– over chloride for tri- and bipodal extractants, which show higher binding constants than a monopodal analogue

A Comparison of the Selectivity of Extraction of [PtCl₆]^2– by Mono‑, Bi‑, and Tripodal Receptors That Address Its Outer Coordination Sphere

Extraction and binding studies of [PtCl₆]^2– are reported for 24 mono-, bi-, and tripodal extractants containing tris­(2-amino­ethyl)­amine (TREN) or tris­(3-amino­propyl)­amine (TRPN) scaffolds. These reagents are designed to recognize the outer coordination sphere of [PtCl₆]^2– and to show selectivity over chloride anion under acidic conditions. Extraction from 0.6 M HCl involves protonation of the <i>N</i>-center in tertiary amines containing one, two, or three urea, amide, or sulfonamide hydrogen-bond donors to set up the following equilibrium: 2L_(org) + 2H⁺ + [PtCl₆]^2– ⇌ [(LH)₂­PtCl₆]_(org). All reagents show higher Pt loading than trioctylamine, which was used as a positive control to represent commercial trialkylamine reagents. The loading of [PtCl₆]^2– depends on the number of pendant amides in the extractant and follows the order tripodal > bipodal > monopodal, with urea-containing extractants outperforming amide and sulfonamide analogues. A different series of reagents in which one, two, or three of the alkyl groups in tris-2-ethylhexylamine are replaced by 3-<i>N</i>′-hexylpropanamide groups all show a comparably high affinity for [PtCl₆]^2– and high selectivity over chloride anion in extractions from aqueous acidic solutions. ¹H NMR titration of three extractants [LH·Cl] with [(Oct₄N)₂­PtCl₆] in CDCl₃ provides evidence for high selectivity for [PtCl₆]^2– over chloride for tri- and bipodal extractants, which show higher binding constants than a monopodal analogue