Outer-sphere coordination chemistry: Selective extraction and transport of the [PtCl6]2- anion

Article abstract of DOI:10.1002/anie.200703272

(Figure Presented) What's on the outside? Selective extraction and transport of [PtCl₆]^2- from aqueous acidic solutions in the presence of excess chloride has been demonstrated through outer-sphere coordination of the metalloanion by

Full text of DOI:10.1002/anie.200703272

Angewandte
Chemie
DOI: 10.1002/anie.200703272
Solvent Extraction
Outer-Sphere Coordination Chemistry: Selective Extraction and
Transport of the [PtCl6]2ꢀ Anion**
Katherine J. Bell, Arjan N. Westra, Rebecca J. Warr, Jy Chartres, Ross Ellis, Christine C. Tong,
Alexander J. Blake, Peter A. Tasker,*and Martin Schröder*
Selectivity in the transport of platinum group metal ions by
solvent extraction in industrial processes depends critically
upon control of the metal coordination chemistry through the
formation of either inner-sphere complexes with dialkyl
sulfides or hydroxyoximes,[1] or through formation of outer-
sphere organic-soluble salts with hydrophobic trialkylamine
and related reagents of the Alamine type.[1,2] The latter rely
upon control of partition coefficients and solubilities such that
anion exchange can be used to transfer the chlorometallate to
a water-immiscible solvent in a pH-dependent equilibrium
[Eq. (1)].[2]
studies of the solvation and ion pairing of [PtCl6]2ꢀ suggest
that hydrogen-bonding solvate molecules, such as methanol,
address the triangular faces of the hexachloro octahedron,[7]
whereas formation of trifurcated hydrogen-bonds to faces or
bifurcated hydrogen-bonding to edges of the hexachloro
octahedron has been predicted on the basis of the location of
maximum electron density in the anion;[8] such interactions
have indeed been observed in solid-state structures of
chlorometallates.[9]
We report herein a new approach to the selective
complexation and extraction of hexachlorometallates in the
presence of chloride ions using tripodal ionophores incorpo-
rating multiple hydrogen-bond donors linked to a protonat-
able bridgehead N center. Such a design can not only address
the threefold symmetry of the outer coordination sphere of
[PtCl6]2ꢀ by presenting neutral hydrogen-bond donors to the
faces or edges of the hexachloro octahedron (Figure 1), but
n R3NðorgÞ þ n Hþ þ MClx Ð ½ðR3NHÞnMClxŠ
nꢀ
ð1Þ
ðorgÞ
Although the solvent extraction of base metals such as Cu
and Zn usually involves the formation of inner-sphere
coordination complexes,[3] the very slow ligand exchange for
the [PtCl6]2ꢀ ion[4] makes it necessary to address and recognise
the outer coordination sphere of this species to form neutral
anion–ligand packages. Selectivity continues to be a challenge
in the development of supramolecular recognition of anions[5]
and is a pervasive problem in extractive metallurgy because
the generation of electrolytes of high purity is essential for
efficient electrolytic reduction to produce metals.[6] Thus, an
understanding of the nature and disposition of electrostatic
and supramolecular hydrogen-bonding interactions to chlor-
ometallates is essential to the design of selective extractants
for these anions. DFT calculations and NMR spectroscopic
Figure 1. The structure of [PtCl6]2ꢀ (a)and proposed modes of binding
to tripodal multiple hydrogen-bond ionophores through interactions
with the faces (b)or the edges (c)of the hexachloro octahedron. The
protonated amine of the receptor is shown as a black sphere and the
neutral hydrogen-bond donors as gray spheres.
[*] K. J. Bell, Dr. R. J. Warr, Prof. Dr. A. J. Blake, Prof. Dr. M. Schröder
School of Chemistry
University Park
University of Nottingham
Nottingham, NG7 2RD (UK)
Fax: (+44)115-951-3563
E-mail: M.Schroder@nottingham.ac.uk
also provides a positive charge to ensure the solubility of a
neutral 2:1 [LH+]:[PtCl6]2ꢀ complex in water-immiscible
solvents. Our target in this work was to design useful solvent
extractants for the recovery of PtIV from acidic chloride
streams, whereby loading and stripping of the organic phase
are controlled by a “pH-swing” mechanism [Eqs. (2) and (3)].
Dr. A. N. Westra, Dr. J. Chartres, R. Ellis, Dr. C. C. Tong,
Prof. Dr. P. A. Tasker
School of Chemistry
University of Edinburgh
West Main Road, Edinburgh, EH9 3JJ (UK)
Fax: (+44)131-650-6453
2ꢀ
2 LðorgÞ þ 2 Hþ þ ½PtCl6Š Ð ½ðLHÞ2PtCl6Š
ð2Þ
ð3Þ
ðorgÞ
E-mail: P.Tasker@ed.ac.uk
½ðLHÞ2PtCl6ŠðorgÞ þ 2NaOH Ð Na2½PtCl6Š þ 2 LðorgÞ þ 2 H2O
[**] We thank the EPSRC, the Chemistry Innovation Programme, and the
National Research Foundation of South Africa for funding, and the
EPSRC National Mass Spectrometry Service at the University of
Wales, Swansea (UK)for mass spectra. M.S. gratefully acknowl-
edges receipt of a Royal Society Wolfson Merit Award and of a Royal
Society Leverhulme Trust Senior Research Fellowship.
Such a protocol could then be implemented for the recovery
of platinum metal anions from the highly acidic chloride
streams currently used in industry. Furthermore, selective
extraction of [PtCl6]2ꢀ over Clꢀ, which is present in substantial
excess in industrial feed streams, is essential for an efficient
process, and is, therefore, a key design requirement for our
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2008, 47, 1745 –1748
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1745

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receptors. Significantly, the simple trialkylamine reagents of
extraction of [PtCl6]2ꢀ from the organic layer into an aqueous
solution was achieved by contacting the loaded CHCl3 layer
with an aqueous solution containing a twofold excess of
NaOH over ligand [Eq. (3)]. Results for the extraction of
[PtCl6]2ꢀ by the receptors are summarized in Figure 3 along
with data for TOA for comparison. Ligands L2 and L6
afforded metal complex salts that precipitated out of solution
and are therefore not discussed.
the Alamine type [Eq. (1)] are known to exhibit poor
selectivity for [PtCl6]2ꢀ over Clꢀ (see below), particularly at
high acid concentrations.[10] In this work we have used
trioctylamine (TOA) as a model for the Alamines[10,11] to
benchmark against our new reagents.
A range of amide and urea tripodal ligands L1–10 derived
from tris(2-aminoethyl)amine (tren) were prepared and
characterized (Scheme 1; see also the Supporting
Information). Ligand L1 was designed with a long spacer
between the protonatable amine bridgehead and the neutral
hydrogen-bond donors to allow these to address three faces of
the [PtCl6]2ꢀ octahedron (Figure 2). The urea ligands L2–5 and
L10 and amide ligands L6–9 were synthesized to access
potential tri- and bifurcating hydrogen-bonding to the
[PtCl6]2ꢀ ion (Figure 2).
Figure 3. Plot of percentage of the total platinum extracted as [PtCl6]2ꢀ
from aqueous 0.6m HCl into CHCl3 as a function of the [L]:[Pt] ratio.
Figure 2. Binding modes of the arms of tren-based urea and amide
receptors that preserve pseudo threefold symmetry (only one arm is
shown for clarity).
TOA is not an effective extractant at high concentrations
of HCl. Although platinum uptake by a threefold excess of
TOA from an aqueous solution of H2[PtCl6] with no added
HCl is around 80% of the theoretical value, it drops to around
only 20% when the aqueous feed solution contains 0.6m HCl,
suggesting strong transfer of Clꢀ instead of [PtCl6]2ꢀ under
these conditions (see the Supporting Information). As a
consequence, the transport efficiency of platinum from acidic
feeds in a flow sheet will be low using TOA, and chloride
concentrations will build up downstream. Significantly, the
Equilibration to form the outer-sphere complexes
[(LH)2PtCl6] [Eq. (2)] was achieved within minutes of
mixing H2[PtCl6] in aqueous 0.6m HCl with a solution of L
in CHCl3 at room temperature. Prolonged mixing (> 16 h) led
to a slight drop in the content of platinum in the CHCl3 layer,
and this observation is attributed to inner-sphere substitution
processes to form insoluble complexes. Quantitative back-
Scheme 1. Synthesis of the receptors L1–L10: a)benzaldehyde, MeOH, RT then NaBH 4;[12] b)2-phthalimidoacetyl chloride, Et 3N, CHCl3, 08C
then RT; c)NH 2NH2·H2O, EtOH, CHCl3, reflux; d) tert-butylbenzoyl chloride, Et3N, CHCl3, 08C then RT; e)phenyl, 3,4-dimethoxyphenyl, 3,5-
dimethoxyphenyl, or 3,4,5-trimethoxyphenyl isocyanate, THF, RT;[13] f)benzoyl, 3,4-dimethoxybenzoyl, 3,5-dimethoxybenzoyl, or 3,4,5-trimeth-
oxybenzoyl chloride, NaOH, H2O, CH2Cl2, or Et2O, RT;[14,15] g) tert-butyl isocyanate, THF, RT. Full details of preparations are given in the
Supporting Information.
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Angewandte
Chemie
tripodal hydrogen-bond donor ligands described herein are
much more effective than the Alamine model TOA[10] in
recovering [PtCl6]2ꢀ in the presence of excess HCl (Figure 3
and Table 1). We note that L3 is especially effective in this
system. Loadings of greater than 90% imply that the
Table 1: Percentage of Pt extracted as [PtCl6]2ꢀ into CHCl3 from aqueous
0.6m HCl in the presence of 3 molar excess of L.
Ligand
TOA
5
L1
L3
L4
L7
L9
% Pt extracted
89
98
90
85
80
Figure 4. Plot of logDPt vs. log{[(LH)Cl]/[Clꢀ]}.
triamide/urea ligands show high selectivity for [PtCl6]2ꢀ over
Clꢀ because the latter is present in a 60-fold excess in the
aqueous feed solution. Generally, extractant strengths are
observed to be greater for the urea-containing ligands than
their amido analogues (Table 1).
We used a procedure similar to that described by
Yoshizawa and co-workers[11] to determine the stoichiometry
of the platinum-containing complex formed in the water-
immiscible phase and to probe further the selectivity of
[PtCl6]2ꢀ over Clꢀ. At low pH values, where it can be assumed
that the ligand is fully protonated, the extraction of [PtCl6]2ꢀ
is a competitive process, as shown by Equation (4).
CHCl3. Whilst the data for these ligands are consistent with
the formation of [(LH)2PtCl6] complexes as the extracted
species, for other ligands, L3, L5, L7, and L9 in this work, and
for TOA at high ligand concentrations,[11] the Yoshizawa plots
show some deviation from linearity (see the Supporting
Information). Such deviations may be due to the formation of
outer-sphere complexes with alternative stoichiometries such
as 3:1:1 [LH]+:[PtCl6]2ꢀ:Clꢀ at high ligand concentrations or
may involve the incorporation of a hydroxonium ion into the
outer-sphere complex giving a 1:1:1 assembly [LH]+:[H3O]+:
[PtCl6]2ꢀ, which would be favored at low ligand concentra-
tions. Alternatively, the receptors, their hydrochloride salts,
and/or their platinum complexes with some solubility in the
aqueous phase cannot be discounted at this stage. These
caveats notwithstanding, the triamide/urea ligands L1 and L4
show very high selectivity for [PtCl6]2ꢀ over Clꢀ, the latter
being present in a 60-fold excess.
K
2ꢀ
Š
½PtCl6
½PtCl6Š2ꢀ þ 2 ½ðLHÞClŠ
½ðLHÞ PtCl ŠðorgÞ þ 2Cl ꢀ
ƒƒƒƒ!
ƒƒƒƒ
ðorgÞ
2
6
½ðLHÞ2PtCl6Š ½ClꢀŠ
2
ð4Þ
K
2ꢀ
¼
½PtCl6
Š
2ꢀ
2
½PtCl6 Š ½ðLHÞClŠ
Assuming no inner-sphere substitution on the timescale of
the extraction, the distribution coefficient for platinum is
DPt = [(LH)2PtCl6]/[PtCl62ꢀ], and from Equation (4), logDPt
has values defined by Equation (5).
The formation of a 2:1 [LH]+:[PtCl6]2ꢀ package in organic
media was supported further by a single-crystal X-ray
structure determination of [(L10H)2PtCl6].[16] The structure
determination confirms the presence of significant hydrogen-
bonding between the urea moieties of [L10H]+ and the
[PtCl6]2ꢀ ion, and, as anticipated, the bridgehead N atom in
L10 is protonated to afford a complex with a net neutral charge
(Figure 5). A bifurcated hydrogen bond is observed between
ꢀ
ꢁ
½ðLHÞClŠ
½ClꢀŠ
log DPt ¼ log K
2ꢀ þ 2log
ð5Þ
½PtCl6
Š
Plots of logDPt versus log{[(LH)Cl]/[Clꢀ]} (Figure 4) for
TOA, L1, and L4 show slopes close to 2, which is in line with
formation of the anticipated 2:1 [LH]+:[PtCl6]2ꢀ assemblies in
Figure 5. View of the structure of [(L10H)2PtCl6]. Hydrogen-bond lengths: i)2.791, ii)2.799, iii)2.805, iv)2.664, v)2.180, vi)2.096 ꢀ.
Angew. Chem. Int. Ed. 2008, 47, 1745 –1748 ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Communications
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Received: July 21, 2007
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[16] Crystal data for [(L10H)2PtCl6]: Mr = 1379.2, triclinic, a =
8.8396(5), b = 13.2745(7), c = 14.4379(8) Š, a = 93.865(2), b =
Keywords: anions · ionophores · metal extraction ·
97.368(2), g = 95.263(2)8, V= 1667.7(3) Š3, T= 150(2) K, space
.
metal transport · platinum
group P1, Z = 1, 1calcd = 1.373 gcmꢀ3, m(MoKa) = 2.397 mmꢀ1
,
¯
7540 unique reflections (Rint = 0.013) used in all calculations.
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Angew. Chem. Int. Ed. 2008, 47, 1745 –1748