www.rsc.org/dalton | Dalton Transactions
PERSPECTIVE
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Pt NMR and DFT computational methods as tools towards the understanding of speciation and hydration/solvation of [PtX6 ]2− (X = Cl− , Br− ) anions in solution Klaus R. Koch,* M. R. Burger, J. Kramer and A. N. Westra Received 10th April 2006, Accepted 31st May 2006 First published as an Advance Article on the web 15th June 2006 DOI: 10.1039/b605182k 195
Pt NMR together with DFT calculations and MD simulations, offer a powerful toolkit with which to probe the hydration shells of the [PtCl6 ]2− anions, which may lead to a more profound understanding of the solute–solvent interactions of such complexes.
Introduction South Africa is the world’s leading primary producer of platinum, contributing more than three quarters (78%) of the world’s supply in 2005, in addition to producing a large proportion of the associated platinum group metals (PGMs, Pt, Pd, Rh, Ru, Ir and Os).1 Given the complex and diverse chemistry of these noble metals, the process of extraction, concentration, separation and ultimately refining to 99.99% pure metals, is a wonderful example of inorganic chemistry on a large scale. Despite the development of highly efficient processes for the separation and refinement of these metals,2 there is a trend to develop newer, ‘greener’ and more cost-effective methods of separation and refinement. Moreover, increased environmental awareness and stricter legislation for the control of industrial effluents, present some interesting and
Department of Chemistry and Polymer Science, University of Stellenbosch, P. Bag X1, Matieland, 7602, South Africa. E-mail:
[email protected]; Fax: +27 21 8083342; Tel: +27 21 8083020
challenging opportunities to the co-ordination chemist concerned with the speciation and separation chemistry of the PGMs. The large-scale separation and refining of these precious metals is based largely on the favourable properties of their anionic chlorocomplexes such as [PtCl6 ]2− , [PdCl4 ]2− , [RhCl6 ]3− and [IrCl6 ]2/3− , while Ru and Os are generally separated by means of oxidative distillation.2 Currently these complex chloro-anions are separated and refined remarkably efficiently by means of a combination of classical methods (e.g. selective precipitation), more modern solvent extraction, ion-exchange, and even methods of molecular recognition. More recently, there appears to be a trend toward the development of simpler and more integrated separation and recovery schemes for the PGM chloro-anions based on modern solid materials which are PGM selective, as exemplified by the potential chromatographic separation of the PGM anions developed by Schmuckler3,4 and others.5,6 In our view, the development of modern methods involving solid materials for the more efficient separation of the PGMs will require a more detailed understanding of the speciation of the
Professor Klaus R. Koch, born in Namibia, received his PhD in Inorganic chemistry from the University of Cape Town in 1979. He is presently professor of analytical chemistry at the University of Stellenbosch in South Africa. His research interests include the coordination, separation and analytical chemistry of the platinum group metals with an emphasis on inter alia the application of multinuclear NMR methods to furthering the understanding of this chemistry as relevant to the refining industry. Dr Arjan Westra, completed his PhD in 2005 and Maggie Burger is currently completing her PhD, while Dr Jurjen Kramer is a postdoctoral fellow in the Platinum Chemistry Research Group led by Professor Koch at the University of Stellenbosch.
Professor Klaus R. Koch, Dr. Arjan Westra, Maggie Burger and Dr Jurjen Kramer This journal is © The Royal Society of Chemistry 2006
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[MX6 ]2/3− and [M X4 ]2− anions (M = Pt(IV), Rh(III) Ir(IV/III); M = Pd(II), Pt(II), X = Cl− ,OH− ) in aqueous media, as well as the nature and properties of the hydration/solvation of these anions. In this context we have become interested in the use of high-resolution 195 Pt nuclear magnetic resonance (NMR) spectroscopy, combined with modern computational methods, to understand these aspects of hydration, solvation and speciation of [PtX6 ]2− (X = Cl− , Br− ) anions both in solution and the solid state. The favourable NMR properties of the relatively abundant 195 Pt isotope (∼33.8%), viz. a relatively high NMR receptivity (19.17 ), the extraordinarily large chemical shift range (d 195 Pt > 12 000 ppm7 ) showing high sensitivity to the oxidation state of platinum and the nature of the directly bound ligands to the platinum atom/ion, have long been exploited in the coordination chemistry of this metal ion and this work has been widely reviewed.8–10 Earlier work using 195 Pt NMR predominantly focused on the reporting of empirical chemical shift trends while the useful scalar J-couplings between 195 Pt and other NMR active nuclei such as 1 H,13 C, 15 N, 31 P and 119/117 Sn (to mention only a few examples) have proved invaluable in structure elucidation of new Pt complexes in solution.9,10 Lesser known or even understood is the origin of the appreciable solvent and concentration dependence of 195 Pt chemical shifts of the same complex in a given solution, as first reported for several [PtX6 ]2− and [PtX4 ]2− complexes by Pesek and Mason,11 as well as the small temperature dependence of d 195 Pt of platinum complexes in solution.12 On the theoretical front, Ramsey13 pioneered early attempts to understand the fundamental nature of magnetic shielding of molecules in the context of NMR spectroscopy. Dean and Green14 in particular, applied these theoretical advances to 31 P and 195 Pt NMR of platinum hydride complexes more than three decades ago. Subsequently several ab initio methods have been developed with which to calculate the 195 Pt chemical shifts of simple octahedral d6 (and square planar d8 ) platinum complexes.15,16 Only relatively recently, however, have theoretical refinements in computational methodology, together with the recognition of the importance of magnetic coupling and relativistic effects in the prediction of 195 Pt chemical shifts, led to the impressively accurate predictions of the 195 Pt magnetic shielding tensor of simple Pt(IV) and Pt(II) complexes.17–20 Generally, modern separation and recovery methods of the PGMs (notably Pt(IV), Rh(III) and Ir(III/IV) on a large scale, essentially depend on the selective distribution of the stable, kinetically inert, chloro-anions of these elements between mostly acidic, chloride-rich, aqueous phases and suitable non-aqueous (organic) receptor phases. The latter receptor phase may be either liquid or solid and it is generally thought that the distribution involves ion-exchange or ion-association mechanisms.2–6 The selectivity of the metal chloro-anion distribution between the two phases, in turn depends on the enthalpy and entropy of ion transfer between the two phases, as well as the nature of the primary (and possibly secondary) hydration shells of the ions.21–23 In this context the utility of high-resolution NMR spectroscopy in probing the primary hydration shells of many simple alkali metal cations has long been demonstrated by the work of, for instance, Bloor et al.24 and Popov and co-workers.25–31 The dependence of the chemical shifts of magnetically active alkali cations on the concentration of the salt in solution, the nature of the solvent, as well as the nature of the anion, was first exemplified by the 3278 | Dalton Trans., 2006, 3277–3284
studies of Bloor and Kid who found a fair correlation of 23 Na chemical shifts with solvent basicity.24 Popov et al. demonstrated a relationship between d 23 Na of sodium salts dissolved in nonaqueous solvents and the Gutmann donor numbers of these solvents.26–29,32 Moreover, the characteristic dependence of d 23 Na on the concentration of salt and the nature of the anion in aqueous and non-aqueous solutions has been ascribed to the formation of ion-pairs in solution.25,29–31 By contrast, relatively few studies of the solvation of simple anions in binary solvent systems by means of NMR spectroscopy have been reported. Of interest here are the investigations of the alkali fluorides33,34 and chlorides33 performed by Covington and co-workers in H2 O/H2 O2 binary mixtures for which the authors developed a model for the calculation of preferential solvation equilibrium constants, based on the observed solute–nucleus NMR chemical shifts. To our knowledge no detailed computational and 195 Pt NMR studies aimed at the understanding of the hydration/solvation of the relatively bulky, complex chloro-anions of the PGMs such as [PtCl6 ]2− , [PdCl4 ]2− , [RhCl6 ]3− and [IrCl6 ]2/3− has been carried out to date. For these bulky anions, there is little detailed information about the nature and structure of their hydration/solvation shells in the literature,35 although in this context, there is considerable interest in anion ‘recognition’ and their associated ‘supramolecular’ chemistry.36 In the past few years we have become interested in the power of experimental 195 Pt NMR, together with the recently developed DFT methods17,19 for predicting 195 Pt chemical shifts, to study the primary hydration/solvation shells of particularly [PtCl6 ]2− anions. In this context we have successfully applied Molecular Dynamics37,38 computational methods together with empirical 195 Pt NMR chemical shift trend measurements to elucidate the nature and structure of the hydration shells of [PtCl6 ]2− , [PtCl4 ]2− , [PdCl4 ]2− and [RhCl6 ]3− in water. Moreover in methanol these methods appear to be a sensitive probe for ion-pairing between simple Na+ and [PtCl6 ]2− .39 A detailed understanding of the nature and structure of the hydration and/or solvation shells of these anions may contribute to the design and preparation of newer, better separation media for these valuable noble-metal anions, leading to hopefully greener and more cost effective refining methods in this industry. In this perspective, we wish to highlight some of our recent work towards this goal.
Computed structures of hydration/solvation shells of [PtCl6 ]2− , [PtCl4 ]2− , [PdCl4 ]2− and [RhCl6 ]3− In collaboration with Naidoo et al.,37,38 we recently reported a detailed DFT and Molecular Dynamics (MD) study of the structure and geometry of the hydration shell of the [PtCl6 ]2− , [PtCl4 ]2− , [PdCl4 ]2− and [RhCl6 ]3− anions. Remarkably these MD calculations revealed relatively well defined hydration shells around all these anions represented here in the form of geometric iso-probability spatial distribution functions (SDFs) at densities ranging from 50–100% greater than that of the bulk density of water, near the chloro-anion. Fig. 1 illustrates the geometric hydration shells obtained from these calculations for the various [PtCl6 ]2− , [PtCl4 ]2− , [PdCl4 ]2− and [RhCl6 ]3− anions in water, at realistic concentrations of Na2 [MCl4/6 ] (M = Pt(II/IV), Pd(II), This journal is © The Royal Society of Chemistry 2006
hydrogen atom of the –OH moiety of each methanol molecule is directed toward the coordinated chloride ions of the complex shown in Fig. 1(f), (blue, H atom iso-probability density, red, O atom density, cyan, representing the C atom of the CH3 moiety of the methanol molecules). 195
Pt NMR spectroscopy as a sensitive probe for the hydration/solvation shells of [PtCl6 ]2−
Fig. 1 The isoprobability density surfaces of O atoms (red), H atoms (blue) and C atoms (cyan) at 100% (a,f) and 50% (b–e) greater than bulk solvent (water or methanol) for complexes (a,b) [PtCl6 ]2− , (c) [PtCl4 ]2− , (d) [PdCl4 ]2− , (e) [RhCl6 ]3− , (f) [PtCl6 ]2− .
Rh(III)) in water. These calculations show that for [PtCl6 ]2− in aqueous solution, for example, eight water molecules are located in close contact proximity to the anion, in such a manner that each water molecule is directed through a hydrogen-bond pointing to a face defined by three coordinated chloride ions of the octahedron of the [PtCl6 ]2− complex at significantly higher iso-probability densities than that for bulk water (Fig. 1a). In these images the blue colour represents the iso-probability SDFs of the H atom of a water molecule, while red represents the O atom iso-probability SDF, with the second H atom of the water molecule pointing away from the complex, being omitted for clarity. Fig. 1(b) shows similar image of a ‘hydrated’ [PtCl6 ]2− anion showing only the O atoms of the water to facilitate easy comparison with similar results obtained for [PtCl4 ]2− , [PdCl4 ]2− and [RhCl6 ]3− anions in Fig. 1(c– e), respectively. It is noteworthy that the hydration shell of the more highly charged [RhCl6 ]3− anion appears to be more extensive at the same iso-probability densities than that of the [PtCl6 ]2− anion, a result which is qualitatively consistent with the relative values of the distribution coefficients of [PtCl6 ]2− > [PtCl4 ]2− ≈ [PdCl4 ]2− [RhCl6 ]3− between acidic chloride rich aqueous (1– 10 M hydrochloric acid) and hydrocarbon phases in the presence of tri-octylamine (as anion exchange agent).40 Comparable calculations of the solvation of [PtCl6 ]2− in methanol results in similar, structured solvation shells of the anion, consisting of eight methanol molecules closely associated with [PtCl6 ]2− in which the This journal is © The Royal Society of Chemistry 2006
The well documented high sensitivity of the chemical shift range of the 195 Pt chemical shifts of Pt(0), Pt(II) and Pt(IV) complexes to the nature of the ligand donor atoms in the primary coordination sphere9 as well as other factors such as concentration, solvent and temperature, suggests 195 Pt NMR spectroscopy as a possible experimental tool with which to probe the hydration/solvation shells of simple [PtCl6 ]2− and [PtBr6 ]2− anions. Pesek and Mason11 reported 195 Pt chemical shifts for [(n-C4 H9 )4 N]2 [PtCl6 ] at 26 ◦ C in D2 O (−11 ppm), CH3 OH (−222 ppm), CH3 CN (−327 ppm) and acetone (−370 ppm), for solutions ranging in concentration from 0.6–0.8 M at 26 ◦ C, relative to the usual reference compound, H2 PtCl6 in dilute hydrochloric acid. The observed temperature dependence of the 195 Pt chemical shift for several platinum complexes in solution, although appreciable, is comparatively much smaller (e.g., for trans-PtCl2 ((n-C4 H9 )3 P)2 , 0.32 ppm K−1 ; trans-PtEtBr((n-C4 H9 )3 P)2 , 0.52 ppm K−1 ).12 In this context, we undertook a systematic study of the effect of solvent on the 195 Pt chemical shift trends of H2 PtCl6 and H2 PtBr6 in a series of seven binary (water-miscible) solvent mixtures at constant temperature and defined concentration of [PtCl6 ]2− and [PtBr6 ]2− range (ca 0.05–0.10 M),41 which is illustrated in this paper for methanol–water (as D2 O) mixtures at 30 ◦ C only.42 Fig. 2a shows selected 195 Pt NMR spectra obtained from freshly prepared methanol–D2 O mixtures as a function of the mole fraction of methanol, relative to an ‘external’ reference solution (500 mg cm−3 H2 PtCl6 ·H2 O in 30% v/v D2 O/1 M HCl) in a 1 mm coaxial capillary inserted into a standard 5 mm NMR tube. The significant, systematic downfield displacement of the d 195 Pt of [PtCl6 ]2− as a function of the mole fraction of methanol in the binary solvent mixture is clear. The invariant 35 Cl/37 Cl isotopomer distribution characteristic of the 195 Pt resonance due to [PtCl6 ]2− species43 shown in Fig. 2b (for an ethylene glycol–water mixture)† confirms that the integrity of the hexachloroplatinum(IV) anion is preserved and that no hydrolysis or Cl− substitution by solvent molecules takes place during this experiment on the NMR timescale. Fig. 3 shows a plot of the relative variation of d 195 Pt at 30 ◦ C (ranging from 8 to 98 ppm downfield relative to the external reference, d 195 Pt = 0 ppm) of freshly prepared solutions of H2 PtCl6 in D2 O–methanol at an approximately constant [PtCl6 ]2− concentration as a function of the mole fraction of methanol. This trend shows a nonlinear, slightly convex relationship (Fig. 3). Although not shown here, qualitatively similar but significantly more convex curves are obtained for a range of other water-miscible organic solvents such as ethylene glycol,
† The isotopomer distribution is shown for ethylene glycol in view of the outstanding resolution obtained; A similar, somewhat more poorly resolved pattern is obtained for all other solvents studied.
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Fig. 2 195 Pt NMR recorded at 303 K using a Varian INOVA 600 MHz spectrometer operating at 129 MHz for 195 Pt using a 5 mm broad-band probe spectrum of (a) the methanol–water mixture containing [PtCl6 ]2− (as H2 PtCl6 ·H2 O); the peak at the left the sample peak, while the peak at the right is the reference peak in a coaxial insert tube containing a [PtCl6 ]2− reference (d 195 Pt = 0 ppm) solution (500 mg cm−3 H2 PtCl6 ·H2 O in 30% v/v D2 O/1 M HCl) (b) an enlargement of the sample peak in ethylene glycol, illustrating the resolution of the 35 Cl, 37 Cl isotopomers.
acetonitrile, acetone, 2-methoxyethanol and hexamethylphosphoramide (HMPA). Moreover, similar results are obtained for the corresponding experiments with H2 PtBr6 , under corresponding conditions.41 The trend of d 195 Pt shift vs. mole fraction of the binary solvent composition in Fig. 3, strikingly resembles similar trends obtained by Dechter and Zink44 who examined the solvation of Tl+ cations in binary mixtures of water and pyrrolidine by means of 205 Tl NMR spectroscopy. These authors convincingly demonstrated that the deviation from linearity of the d 205 Tl vs. solvent composition plot may be interpreted as indicating preferential solvation of thallium(I) cation by pyrrolidine within the primary solvation sphere; by contrast, convex curvature of such trends, are thought to indicate preferential solvation of the Tl+ cation by water. In such studies an ‘iso-solvation point’, defined as that point at which the composition of the primary solvation shell has an identical average solvent composition as the bulk mixture, would be expected to 3280 | Dalton Trans., 2006, 3277–3284
occur at a mole fraction of 0.5 for a given binary mixture of solvents, if no preferential solute–solvent interaction were to occur. The extent of deviation from linearity of such experimental trends from linearity, allows for the estimation of the composition of the solvation shell of a particular cation at any given bulk solvent composition.44 If we assume as a first approximation a similar interpretation for [PtCl6 ]2− anion as shown for cations by Dechter and Zink, then the trends illustrated in Fig. 3 pertaining to the hydration/solvation of the [PtCl6 ]2− , indicate the apparent preferential solvation of the [PtCl6 ]2− anion by methanol in these binary solvent mixtures. The ‘iso-solvation’ point for [PtCl6 ]2− in D2 O–methanol‡ mixture thus occurs approximately at a 0.33 mole fraction of methanol bulk solvent composition. Similar studies with other solvents show isosolvation points for [PtCl6 ]2− of 0.24 in acetonitrile–water and 0.19 in acetone–water mixtures; for the [PtBr6 ]2− anion, iso-solvation points of 0.34, 0.22 and 0.17 in methanol, acetonitrile and acetone– water mixtures are obtained, respectively.41 Based on this analysis, we interpret the considerable 195 Pt chemical shift dependence of [PtCl6 ]2− in binary solvent mixtures as reflecting a gradual change of the composition of the primary hydration/solvation shell of the [PtCl6 ]2− anion from mainly eight water molecules as obtained from our MD simulations shown in Fig. 1(a) in pure water, to water : methanol ratios of 6 : 2, 4 : 4 to 6 : 2 at bulk solvent mole fractions of methanol of ca 0.15, 0.33 and 0.61, respectively, thus providing qualitatively at least a measure of the extent of hydration of the [PtCl6 ]2− anions in the water–methanol mixtures. In pure methanol, at the limiting chemical shift of d 195 Pt ≈ 98 ppm, the [PtCl6 ]2− anion is completely solvated by methanol, as indicated in Fig. 1(f). A detailed account of these results for a series of other solvents will be published elsewhere. Interestingly, while carrying out these MD simulations of the solvation of the Na2 [PtCl6 ] in methanol, we assumed Na2 [PtCl6 ] to be initially completely dissociated into 2Na+ and [PtCl6 ]2− ions. However, for simulations longer than 500 ns, final convergence of the model system shows that it eventually tends toward an energy minimum with the formation of a {Na+ [PtCl6 ]2− }− contact ion-pair; significantly when using water as the solvent a similar ionpaired state is not obtained under corresponding MD conditions.39 Whilst it is not unreasonable to expect the formation of ion-pairs in non-aqueous solvents, this is the first computational study reported confirming the formation of, and revealing the likely structure of such contact ion-pairs between Na+ and [PtCl6 ]2− in methanol solution. Further MD simulations show that it is indeed possible to follow computationally at least, the formation of the various stages anticipated in the formation of ion-pairs, ranging from solvent separated (SSIP), solvent shared (SSHIP) to finally contact ion-pairs (CIP) between Na+ and [PtCl6 ]2− in methanol.39 Representative spatial distribution function from these simulations in methanol for a CIP and a SSHIP are shown in Fig. 4(a) and 4(b), respectively. In order to verify experimentally the existence of such ion-pairs in methanol solutions, we found 195 Pt NMR to be a sensitive probe for such a system, confirming the formation of ion-pairs between Na+ and [PtCl6 ]2− as illustrated by the effect of increasing
‡ Neglecting a possible small isotope effect as a result of D2 O being used here.
This journal is © The Royal Society of Chemistry 2006
Fig. 3 The variation of d 195 Pt (for [PtCl6 ]2− ) as a function of solvent composition in the D2 O–methanol binary solvent system is shown (a best-fit polynomial trend-line has been inserted). By relating the observed experimental data with the straight line drawn through the shift extremes, the solvation sphere composition at a given bulk composition may be estimated on the x-axis.
Fig. 4 Representative SDFs from MD simulations of (a) a contact ion pair (CIP) and (b) a solvent separated ion pair (SSHIP) between [PtCl6 ]2− and Na+ in methanol.
Na+ concentration on the d 195 Pt of the [PtCl6 ]2− at constant concentration and temperature, shown in Fig. 5. Similar results have been obtained for [PtBr6 ]2− and from a detailed analysis of the changes induced in d 195 Pt by increasing Na+ concentrations in methanol and acetonitrile, it has been possible to estimate relative formation constants for the {Na[PtX6 }2− }− ion-pair (X=Cl− , Br− ), which will be published elsewhere. Although the NMR data confirms the formation of {Na[PtX6 }2− }− ion-pairs, it has not been possible to distinguish between solvent separated (SSIP), solvent shared (SSHIP) and contact ion-pairs (CIP) by means of NMR spectroscopy currently, since on the NMR timescale, these types of ion-pairs are likely to be in fast exchange. Nevertheless the various types of ion-pairs are accessible computationally, and studies to elucidate their possible structure and to estimate the relative stabilities of SSIP, SSHIP and CIPs are in progress. The effect of addition of the crown-ether, 18-crown-6, on the trends of d 195 Pt of the [PtCl6 ]2− shown in Fig. 5 is interesting, and confirms the apparent formation of ion-pairs between Na+ and [PtCl6 ]2− ; addition of crown ether at 2 : 1 and 1 : 1 This journal is © The Royal Society of Chemistry 2006
Fig. 5 The variation of the 195 Pt chemical shift as a function of Na+ concentration: (䉬) H2 PtCl6 ·H2 O + NaClO4 in methanol, (䊊) H2 PtCl6 ·H2 O + NaClO4 + 18-crown-6 in methanol (ratio Na+ : crown ether at 1 : 1), () H2 PtCl6 ·H2 O + NaClO4 + 18-crown-6 in methanol (ratio Na+ : crown ether at 2 : 1). Best fit Excel trend-lines are shown for each series.
mole ratios of Na+ : 18-crown-6, respectively, has a significant effect on the d 195 Pt of the [PtCl6 ]2− in methanol, with increasing Na+ concentration. Remarkably, at very high Na+ : 18-crown-6 concentrations upfield shifts of d 195 Pt are induced, suggesting the disruption of the postulated {Na[PtX6 }2− }− ion-pairs, as a result of the well-known competitive binding of the Na+ ions by the 18-crown-6 in solution.45 This may be due to the formation of an ion-pair of presumably a different structure in solution between [Na(18-crown-6)]+ and [PtCl6 ]2− , prior to precipitation of a salt from these solutions. In the absence of further data, however, any comment about the nature of the interaction of [Na(18-crown6)]+ with [PtCl6 ]2− must remain speculative, although a salt-like structure as has been characterised by X-ray diffraction for [K(18crown-6)][PtCl5 (H2 O)]46 or even (H3 O)2 [PtCl6 ]·2(18-crown-6),47 is not inconceivable. Dalton Trans., 2006, 3277–3284 | 3281
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Pt NMR and computational studies of the speciation of [PtCl6−m Brm ]2− and [PtCl6−n (OH)n ]2− (n and m = 0–6) Knowledge of the detailed ‘speciation’ of the complexes dissolved in acidic, chloride-rich solutions is critical to the success in separation and refining of the PGM chloro-anions. 195 Pt NMR has been shown to be a powerful tool for determining the speciation of Pt(IV) chloro-bromo complexes8 as well as for the product complexes resulting from aquation and hydrolysis of [PtCl6 ]2− .48–50 We have recently extended some of this work to focus on the unambiguous assignment of d 195 Pt to the series of aquation complexes resulting from the substitution of Cl− and/or Br− ions by water from the anionic [PtCl6−m Brm ]2− complexes, to yield [PtCl5−m Brm (H2 O)]− (m = 0–5) species, as relevant to the extraction of such Pt(IV) complexes by means of silica-based poly(amine) ion-exchange materials.51 As previously observed by Carr et al.48 we find that it is a relatively simple matter to assign all d 195 Pt for a series of [PtCl6−n (OH)n ]2− complexes since replacement of the Cl− ions by hydroxide ions within the coordination sphere of Pt(IV) leads to a monotonic downfield shift with a well defined 2nd order correlation of d 195 Pt as a function of n (d 195 Pt = 13.5 + 671.5n–21.4n2 , with R2 = 1.0000, relative to the chemical shift of [PtCl6 ]2− ), as shown in Fig. 6. These strikingly monotonic, quasilinear correlations are very useful for the rapid and unambiguous
Fig. 6 195 Pt chemical shifts of [PtCl6−n (OH)n ]2− (䊊, n = 0–6) complexes in an aqueous solution (0.5 M) of H2 PtCl6 ·H2 O in 2.0 M NaOH.
assignment of the 195 Pt NMR spectra resulting from hydrolysis of the entire series of mixed chloro/bromo/hydroxo complexes of Pt(IV); details of which will be published elsewhere.52 Recent work on the prediction of 195 Pt chemical shifts by density functional theory computational methods notably as pioneered by Gilbert and Ziegler,17 together with the commercial availability of efficient software such as the ADF package53,54 has greatly improved the accuracy of computed d 195 Pt compared to the experimentally observed chemical shifts for simple platinum(II) and platinum(IV) complexes. These advances in theory have considerably increased the potential understanding and interpretability of measured chemical shifts for a given series of related Pt(IV) complexes. In an attempt to obtain theoretical support of our interpretation of the nature of the solvation/hydration shells of [PtCl6 ]2− anions as discussed above, we embarked on a systematic computational study aimed at understanding the various factors which determine the measured values of the d 195 Pt of the series of complexes such as [PtCl6−m Brm ]2− and [PtCl6−n (OH)n ]2− which are of relevance to the large scale PGM separation and production industry. Confidence in our own calculations of the 195 Pt chemical shifts of the series of mixed chloro/bromo [PtCl6−m Brm ]2− complexes was recently confirmed by an independent study of Penka Fowe et al.20 who published an assessment of the theoretical prediction of the NMR shielding tensor of the 195 Pt chemical shifts of this series of complexes. The calculated shifts in the gas phase compare remarkably well with the experimentally determined d 195 Pt values in dilute aqueous acid medium as shown in Table 1. Inspection of this data shows that the agreement between 195 Pt experimental and calculated shifts by Penka Fowe et al.20 and our work§ is excellent, and is generally within a 1.5% relative difference w.r.t. experimental chemical shifts. Recalculation of the 195 Pt chemical shifts of this series of complexes including the conductor-like screening model § Basis sets used in this study: All electron ZORA TZP; Scalar ZORA relativistic correction55 for geometry optimizations; Spin–orbit ZORA relativistic correction for shielding tensor; functionals employed local density approximation of Volko, Wilk and Nusair56 and with gradient correction from Perdew and Wang.57
Table 1 Experimental and calculated d 195 Pt of [PtCl6−m Brm ]2− and [PtCl6−n OHn ]2− complexes Computed
a
195
Species
Experimental d Pt
d 195 Pt COSMO (% diff)a
d 195 Pt GAS (% diff)a
d 195 Pt GAS Penka Fowe20
[PtCl6 ]2− [PtCl5 Br]2− cis-[PtCl4 Br2 ]2− trans-[PtCl4 Br2 ]2− fac-[PtCl3 Br3 ]2− mer-[PtCl3 Br3 ]2− cis-[PtCl2 Br4 ]2− trans-[PtCl2 Br4 ]2− [PtClBr5 ]2− [PtBr6 ]2− [PtCl5 (OH)]2− fac-[PtCl3 (OH)3 ]2− [PtCl(OH)5 ]2− [Pt(OH6 )]2−
0 −286 −583 −585 −891 −893 −1212 −1215 −1546 −1890 660 1843 2824 3291
0 (0.0) −266 (−1.0) −552 (−1.6) −570 (−0.8) −850 (−2.2) −867 (−1.4) −1176 (−1.9) −1198 (−0.9) −1521 (−1.3) −1897 (0.4) −668 (−0.2) 1658 (5.6) 2376 (13.7) 2617 (20.5)
0 (0.0) −274 (−0.7) −560 (−1.2) −578 (−0.4) −864 (−1.4) −886 (−0.3) −1208 (−0.2) −1231 (0.9) −1568 (1.2) −1962 (3.8) 698 (−1.2) 1638 (6.2) 2175 (19.8) 2273 (30.9)
0 −277 −566 −583 −866 −888 −1201 −1223 −1552 −1915 — — — —
Differences calculated as a percentage of the relevant total chemical shift range.
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(COSMO),58–60 in an attempt to compensate for the effect of an electrostatic field due to the presence of water molecules in an aqueous solution on the calculated 195 Pt shielding (Table 1), shows, on average, no significant improvement between the calculated and the experimentally observed chemical shifts for the [PtCl6−m Brm ]2− series of complexes. Sensitivity to the use of differing basis sets (TZP; TZ2P and QZ4P) in the computations was tested and found to be negligible. In agreement with Penka Fowe et al.,20 we also find that differences in both the paramagnetic and the spin–orbit shielding terms contribute significantly to the overall 195 Pt chemical shift, with the differences in the diamagnetic shielding contribution being almost negligible. To assess the accuracy of the calculated 195 Pt chemical shifts of the hydrolysis products [PtCl6−n (OH)n ]2− obtained experimentally by treatment of [PtCl6 ]2− solutions with variable amounts of NaOH(aq), we calculated 195 Pt chemical shifts and compared these with experimental d 195 Pt values for this series of complexes. In view of the expected capability of the coordinated OH− ligand to participate in hydrogen bonding with bulk water, our aim was to assess the accuracy of the calculated195 Pt chemical shifts for the [PtCl6−n (OH)n ]2− series of complexes. Preliminary results show (Table 1) interestingly that the agreement between calculated and experimental chemical shifts for the series of [PtCl6−n (OH)n ]2− complexes is somewhat poorer, notably as n (the number of OH− ligands bound to the Pt(IV) ion) increases. In the case of the [Pt(OH)6 ]2− complex, the relative percent difference between calculated (gas phase) and measured chemical shifts is ca 31%; using the COSMO model improves the agreement between the computed and experimental data marginally, to 20% difference. Although the COSMO model incorporates the impact of the electrostatic effect of the bulk solvent on the d 195 Pt, as shown by this improvement, it does not account for the anticipated effect of hydrogen bonding of the coordinated OH− moiety with bulk water, as well as other factors such as exchange reactions in real water solutions. Work aimed at understanding the reasons for this poor agreement is underway. Penka Fowe et al.20 found that the calculated 195 Pt shift of [PtBr6 ]2− is extraordinarily sensitive to the effective Pt–Br bond length, at ca 150 ppm per picometer, which is also confirmed by our calculations for the [PtCl6 ]2− species at ca 180 ppm per picometer. It is thus tempting to speculate that the extremely high sensitivity of the experimental 195 Pt chemical shifts of [PtCl6 ]2− complexes to solvent composition, concentration and temperature may, in part at least, be due to the intimate solvent–solute interactions, which can conceivably lead to subtle changes in the nature of, and thus the length of, the Pt–X (X = simple anionic ligand) bond, which may in turn be reflected in the d 195 Pt for given conditions in solution. To test this hypothesis, we are in the process of assessing the effect of water molecule interactions on the calculated 195 Pt chemical shift of the [PtCl6 ]2− anion through specifically oriented hydrogen bonding to the coordinated chloride anions within the inner hydration shell of [PtCl6 ]2− . Although the required calculations are computationally expensive and provide only a ‘static’ view of such postulated interactions, our preliminary calculations in vacuum show that a single water molecule approaching the [PtCl6 ]2− complex from infinity through an axial Pt–Cl · · · H– O type interaction, induces a significant downfield shift (ca 90– 100 ppm) in the computed d 195 Pt values relative to the isolated This journal is © The Royal Society of Chemistry 2006
[PtCl6 ]2− in the gas phase (Fig. 7a). Moreover the extent of this downfield shift depends on the effective Cl · · · H–O distance, as shown graphically. Differences in the angle and geometry of approach of the postulated Pt–Cl · · · H–O interactions, as shown for some other hypothetical situations in Fig. 7, also result in surprisingly large differences in the computed 195 Pt shielding. Thus a water molecule approaching the [PtCl6 ]2− complex to form two H-bond type interactions to two adjacent coordinated chloride ions (Fig. 7b), or in a manner in which the water molecule straddles three chloride ions defining a face of the octahedron defined by the [PtCl6 ]2− complex (Fig. 7c), results in progressively upfield 195 Pt chemical shifts as illustrated graphically in Fig. 7. Calculations show that these effects induced in the computed 195 Pt shielding as a result of different types of Pt–Cl · · · H–O interactions result primarily from changes in the paramagnetic deshielding term contributing to the total shielding of the 195 Pt nucleus.
Fig. 7 The impact and extent of changes induced by an approaching water molecule on the computed 195 Pt shielding in three hypothetical situations: (a) axial approach; (b) double H-bonded and (c) straddled across an octahedral face of the [PtCl6 ]2− , complex relative to the calculated shielding of an isolated [PtCl6 ]2− complex in the gas phase.
These preliminary results are very interesting and suggest that the nature and extent of water molecule interactions with the [PtCl6 ]2− anion within the 1st hydrations shell probably do have a significant effect on the overall chemical shifts of [PtCl6 ]2− complexes in solution, lending theoretical support to the experimental findings and interpretations of these reported in this paper. Moreover in the case of [Pt(OH)6 ]2− , in which the OH− ligands may participate in relatively strong H-bonds with bulk water, and be subject to exchange dynamics, it appears that such effects are even more significant. We wish to emphasize that the above calculations concerning these aspects are preliminary at the time of writing, and require refinement as well as more experimental validation before too much significance is attached to them at this time. Nevertheless, results obtained to date are in our view encouraging, and we are actively continuing these ADF computations combined with experimental studies of deceptively simple Pt(IV) and Pt(II) anionic complexes as relevant to the PGM separation and recovery. In conclusion, we believe the results obtained to date present a convincing case for the power of this combined computational and experimental approach using high field 195 Pt NMR spectroscopy as a probe for speciation of simple anionic Pt(IV)chloro/bromo/hydroxo complexes in solution. Moreover DFT calculations together with MD simulations, offer a unique Dalton Trans., 2006, 3277–3284 | 3283
tool with which to probe the 1st hydration/solvation shells of the chloro-anions of the PGMs, which may lead to a more profound understanding of the solute–solvent interactions of such complexes, with the ultimate goal of leading to better, more efficient and ‘greener’ separation methods for these precious metals. The methods for the calculation of 195 Pt NMR shieldings have improved greatly in accuracy, which will in our view, lead to greater interpretability of experimental 195 Pt NMR chemical shifts of octahedral [PtCl6 ]2− (and their related aqua and hydroxo species), particularly in respect to their hydration/solvation and speciation in solution. The results of such studies may thus improve prospects of achieving more accurate and quantitative prediction of phenomena such as hydration/solvation, ion-pairing and hopefully, the nature and thermodynamics of phase distribution equilibria of the PGM chloro-anions.
Acknowledgements We are greatly indebted to the collaboration of and fruitful discussions with Professors Kevin J Naidoo (Univ. of Cape Town), Jan Dillen (Univ. of Stellenbosch) as well as Dr David Robinson (Angloplatinum Ltd.). Financial support from the National Research Foundation (GUN 2053351), THRIP (Project 2921) and Angloplatinum Ltd., for this project is acknowledged.
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