195Pt NMR and Molecular Dynamics Simulation Study of the

DOI: 10.1021/acs.inorgchem.8b01554. Publication Date (Web): September 14, 2018. Copyright © 2018 American Chemical Society. *E-mail for F.M.: ...
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Pt NMR and Molecular Dynamics Simulation Study of the Solvation of [PtCl6]2− in Water−Methanol and Water− Dimethoxyethane Binary Mixtures Leon Engelbrecht,‡,§,† Francesca Mocci,*,‡ Aatto Laaksonen,§,∥,⊥ and Klaus R. Koch*,† †

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa Department of Chemical and Geological Sciences, University of Cagliari, I-09042 Monserrato, Italy § Division of Physical Chemistry, Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Sweden ∥ Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden

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‡

S Supporting Information *

ABSTRACT: The experimental 195Pt NMR chemical shift, δ(195Pt), of the [PtCl6]2− anion dissolved in binary mixtures of water and a fully miscible organic solvent is extremely sensitive to the composition of the mixture at room temperature. Significantly nonlinear δ(195Pt) trends as a function of solvent composition are observed in mixtures of water−methanol, or ethylene glycol, 2methoxyethanol, and 1,2-dimethoxyethane (DME). The extent of the deviation from linearity of the δ(195Pt) trend depends strongly on the nature of the organic component in these solutions, which broadly suggests preferential solvation of the [PtCl6]2− anion by the organic molecule. This simplistic interpretation is based on an accepted view pertaining to monovalent cations in similar binary solvent mixtures. To elucidate these phenomena in detail, classical molecular dynamics computer simulations were performed for [PtCl6]2− in water−methanol and water−DME mixtures using the anionic charge scaling approach to account for the effect of electronic dielectric screening. Our simulations suggest that the simplistic model of preferential solvation of [PtCl6]2− by the organic component as inferred from nonlinear δ(195Pt) trends is not entirely accurate, particularly for water−DME mixtures. The δ(195Pt) trend in these mixtures levels off for high DME mole fractions, which results from apparent preferential location of [PtCl6]2− anions at the borders of water-rich regions or clusters within these inherently micro-heterogeneous mixtures. By contrast in water−methanol mixtures, apparently less pronounced mixed solvent micro-heterogeneity is found, suggesting the experimental δ(195Pt) trend is consistent with a more moderate preferential solvation of [PtCl6]2− anions. This finding underlines the important role of solvent−solvent interactions and micro-heterogeneity in determining the solvation environment of [PtCl6]2− anions in binary solvent mixtures, probed by highly sensitive 195Pt NMR. The notion that preferential solvation of [PtCl6]2− results primarily from competing ion−solvent interactions as generally assumed for monatomic ions, may not be appropriate in general.

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usually the octahedral [PtCl6]2− anion, from an acidic, haliderich aqueous phases into appropriate organic receptor phases.2−7 Some of us have shown that 195Pt NMR spectroscopy is a powerful technique for the chemical speciation of Pt(II/IV) due to ligand-exchange or aquation and/or hydrolysis reactions that may occur in aqueous solutions as relevant to the platinum refining industries.3,4 This is due to the highly sensitive NMR shielding of the 195Pt nucleus, resulting in an extremely large chemical shift range of Pt(II/IV) complexes. This facilitates the unambiguous characterization of platinum complexes in solution by 195Pt NMR, under suitably controlled conditions.3

INTRODUCTION The platinum group metals (PGMs, i.e., Pt, Pd, Rh, Ir, Ru, and Os) are employed in numerous modern technological applications, the industrially most important of which is arguably the extensive use of Pt, Pd, and Rh in automobile emission control catalytic converters.1 The large-scale industrial refining of PGMs involves a complex separation process from aqueous solutions using a variety of chemical processes and techniques, ranging from classical precipitation methods and oxidative distillation to selective solvent extraction, molecular recognition, and ion-exchange techniques.2 The selective solvent extraction of platinum [PtXx]n‑x complexes (X = Cl or Br and n is the Pt oxidation state of II or IV, x = 4 (square-planar) or 6 (octahedral complex geometry)) requires the favorable transfer of the appropriate chemical species, © XXXX American Chemical Society

Received: June 8, 2018

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DOI: 10.1021/acs.inorgchem.8b01554 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry It has long been known that 195Pt chemical shifts are primarily dependent on the oxidation state of the platinum compound, the nature of the donor atoms bound to Pt(II/IV), and the structure of such complexes.8 Of particular interest to us is the remarkable sensitivity of the 195Pt chemical shift of the homoleptic [PtCl6]2− anion to other factors, such as the concentration of the complex in solution,3,14 the nature of the solvent,7,14 the solution temperature,9 ion-pairing,10 including well-resolved isotope effects.11 The remarkably resolved 35/37Cl and 16/18O isotope effects reflected in the 195Pt NMR profiles of inter alia the [PtCl6‑n (H 2 O) n] n‑2 (n = 0−6) and [PtCl6‑m(OH)m]2− (m = 0−6) species in aqueous solution best illustrate the extraordinary sensitivity of the 195Pt nuclear shielding, as has been recently reviewed.12 In the context of understanding the solvation and hydration shells of [PtCl6]2− as relevant to the precious metal refining industry, we found that, in binary mixtures of water and watermiscible organic solvents, very significant changes in the 195Pt NMR chemical shift of the [PtCl6]2− anion at constant concentration and temperature are observed, ranging from ca. 90 to >400 ppm as a function of the mole fraction of organic solvent (XA), varying from 0 to 1.13 Evidently the 195Pt NMR shielding is a remarkably sensitive probe of solute−solvent interactions in single-phase binary solvent mixtures, presumably reflecting subtle changes in the solvation shell of particularly the [PtCl6]2− anion,13,14 and/or other effects such as ion pairing.10,13 The effect of ion-pairing induced on the δ(195Pt) of the [PtCl6]2− anion is, however, relatively small by comparison to those of the binary solvent composition.13 Nevertheless, a detailed understanding of the nature and origin of solvent-induced changes in the 195Pt NMR chemical shift of the [PtCl6]2− anion is still not available.7,13,14 Several computational studies of the fundamental nature of hydration of simple PGM complexes have appeared in the past two decades,15−17 notably of the square-planar Pt and Pd complexes,18 in view of the potential biological activity of such compounds.19 Naidoo and co-workers10,15,17 developed classical force field models for square-planar [PtCl4]2−and octahedral [PtCl6]2− (including [PdCl4]2− and [RhCl6]3−) anions, specifically to investigate the hydration shell structures of these anions and dynamic properties by MD computer simulation. Few detailed computational studies of the solvation of [PtCl6]2− in solvents other than water have been reported, apart from pure methanol.10 In the context of the development of novel and highly efficient, industrially relevant separation schemes for PGMs, such as potential chromatographic or solid-phase extraction methods,5,6 it is expected that a deeper understanding of the detailed nature of solvation of [PtCl6]2− anions not only in water but also in binary solvent mixtures, including waterimmiscible organic phases, will be necessary.3 We here report a study of the significant changes in the 195Pt NMR chemical shift of the octahedral [PtCl6]2− anion in binary mixtures of water and the water-miscible solvents methanol, ethylene glycol (EG), 2-methoxyethanol (MEO), and 1,2-dimethoxyethane (DME). The interesting nonlinear δ(195Pt) trends of [PtCl6]2− as a function of the mole fraction (XA) of the organic component in several solvent mixtures (ranging XA = 0 → 1) were tentatively interpreted as suggesting preferential solvation of the [PtCl6]2− anion by the organic component as XA is increased.13 This idea is based on the well-established interpretation of preferential solvation of cations (vide infra).20

To elucidate the origin of the pronounced nonlinearity of the δ(195Pt) trend observed in water−DME mixtures, which levels off significantly for high XDME values, this system has been examined in detail by means of classical MD computer simulations. The results of these simulations indicate that more subtle effects are at play. Specifically, the microscopic spatial heterogeneity found for DME mixtures, particularly at higher XDME values, in the otherwise macroscopically miscible solvent mixtures, are found to be important in this case. The traditional notion of preferential ion solvation, as implied from studies of monatomic cations in particular,20 may not necessarily be applicable to the polyatomic [PtCl6]2− anions in water−DME and related solvent mixtures.

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EXPERIMENTAL AND COMPUTATIONAL DETAILS

Sample Preparation. Reagent-grade hexachloroplatinic acid, H2PtCl6·H2O (Aldrich), was used without further purification, as were all solvents, i.e., deuterium oxide (D2O, Aldrich), methanol (Riedel de Haën Chromasolv grade), MEO, and DME (both from Aldrich). Ethylene glycol (EG, 1,2-ethanediol, Aldrich) was dried with NaOH, followed by distillation under vacuum, and stored under nitrogen. All organic solvents were stored over freshly activated 4 Å molecular sieves (Aldrich). Samples for NMR spectroscopy were prepared by mass, with total platinum concentration approximately constant at [Pt] ≈ 0.1 M. 195 Pt NMR Spectroscopy. 195Pt NMR spectra (128 MHz, probe temperature 303 K) were obtained using a Varian INOVA 600 NMR spectrometer equipped with a 5 mm broad band probe. All 195Pt NMR chemical shifts are reported relative to that of [PtCl6]2− in a reference solution 500 mg/mL H2PtCl6·H2O in 1 M HCl, 30% v/v D2O, contained in a 1 mm i.d. coaxial insert tube, as recommended by the IUPAC “external” chemical shift referencing procedure.21 195Pt NMR chemical shifts were not corrected for possible differences in magnetic susceptibility between sample and reference, since these effects are likely to be smaller than the estimated error in the measurements of chemical shifts by the chosen referencing procedure.22 Molecular Dynamics (MD) Simulations. The revised metal solution force field (MSFF) developed by Naidoo and co-workers was used to model the [PtCl6]2− complex.17 The classical hydronium (H3O+) model of Sagnella and Voth22 was adopted, with the CHARMM TIP3P water model24 and CHARMM General force field (CGenFF) for methanol.25 The CHARMM ether force field was used to model DME.25 The original full ionic charges of the [PtCl6]2− and H3O+ models were adjusted according to the Molecular Dynamics in Electronic Continuum (MDEC) model of Leontyev and Struchebrukhov.27 This scaling procedure known as Electronic Continuum Correction (ECC), accounts approximately for the effect of electronic dielectric screening of ionic charges in classical MD simulations of condensed phases including liquid solutions. This entails multiplication of full ionic charges by a factor 1/√εel, where εel is the highfrequency (optical) dielectric constant of the medium: 1.78 for pure water and ∼2 for many organic media.27 The model is technically applicable to systems having uniform electronic polarization properties (“electronically uniform”), although this condition has been shown to be appropriate for simulations involving ions in the presence of immiscible liquid phases, such as water/oil interfacial preferences of halide ions, as studied by Jungwirth and co-workers.28 In these studies the ionic charges scaled using the εel value of pure water gave good results. Accordingly in the current study, ionic charges were similarly scaled using εel = 1.78, resulting in a reduction of the charge of the hydronium ion from +1 to +0.75, and that of [PtCl6]2− from −2 to −1.5. Additional details regarding the use of the MDEC model in the simulations reported here can be found in the Supporting Information. For the sake of comparison, we performed a set of simulations of the solutions in water−DME mixtures using the original full ionic charges, shown in Figure S2 in the Supporting Information. B

DOI: 10.1021/acs.inorgchem.8b01554 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry All MD simulations were performed and analyzed with the GROMACS software package (ver. 5.0.5),29 using simulation parameters generally as recommended for use of the CHARMM force field in GROMACS.30 Modifications applied to model the electrostatic interactions better are described below. Simulated systems consisted of approximately 1000−7000 solvent molecules, depending on the particular mixed solvent composition, in cubic simulation cells with dimensions of ca. 60 Å. Detailed system specifications are given in the Supporting Information (Table S3). Initial configurations of the mixed solvent boxes were generated using the “gmx insert-molecules” routine of GROMACS. These were equilibrated using a mixed minimization and MD procedure, as described in more detail in the Supporting Information. Solution systems were prepared with 12 [PtCl6]2− and 24 H3O+ ions added to the equilibrated solvent boxes by randomly substituting solvent molecules with the ions; these ensembles approximate the experimental salt concentration (0.1 M). These solution systems were simulated following a similar protocol as described by Naidoo and co-workers.17 Simulations were performed following a steepestdescent-energy minimization step, using a leapfrog integrator in 1 fs time steps, employing periodic boundary conditions. Bonds involving hydrogen atoms were constrained to specified equilibrium distances using LINCS,31 while SETTLE32 was used for water. Lennard-Jones (LJ) interactions were truncated using the recommended LJ forceswitching method between 14 and 16 Å, with electrostatic interactions computed by the particle-mesh Ewald method (PME, 1 Å grid spacing)33 using short-range Coulomb cutoff at 16 Å. System equilibration was achieved within three consecutive MD simulation steps: (1) a 2 ns NPT ensemble MD simulation (303 K, 1 atm) using the Berendsen temperature/pressure coupling (time constants τT = 0.1 ps, τP = 1 ps).34 (2) A further 2 ns in the NPT ensemble using the Nosé−Hoover35 temperature coupling (τT = 1 ps), and Parrinello− Rahman 36 pressure coupling (τ P = 5 ps, with isothermal compressibility estimated for each specific solvent mixture). (3) A 5 ns NVT ensemble MD simulation with the same Nosé−Hoover thermostat settings. Finally, 40 ns production simulations were similarly performed in the NVT ensemble (303 K), with trajectories stored every 1 ps.

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RESULTS AND DISCUSSION Pt NMR Chemical Shift Trends in Binary Water Solvent Mixtures. The 195Pt NMR chemical shift trends of the [PtCl6]2− anion in binary mixtures of water (D2O) and the series of related water-miscible organic solvents methanol, EG, MEO, and DME are shown in Figure 1a), as a function of bulk mixed solvent composition expressed as the bulk mole f raction of the organic component, XA. The remarkably large variations of δ(195Pt)/ppm of >200 ppm as a function of XA depending on the organic solvent, are due only to changes in the apparent nature of the solvation environment of the [PtCl6]2− complex and are not attributable to substitutional changes in the coordination sphere of the [PtCl6]2− anion. This was checked for and is confirmed by the maintenance of the characteristic 35/37 Cl isotope shift profile of the 195Pt signals11−13 for all solvent mixtures, clearly ruling out ligand exchange and other dynamic processes during these measurements. The four δ(195Pt)/ppm trends of the [PtCl6]2− anion as a function of the bulk mixed solvent composition, shown in Figure 1a, exhibit significant and varying degrees of nonlinear behavior. Similar deviations for specifically NMR active monatomic cations from a linear trend connecting their limiting NMR chemical shift (δ/ppm) values in the respective pure solvents, have previously been interpreted to indicate preferential solvation of the cation concerned in binary solvent mixtiures.37,38 Preferential solvation of a solute in a binary solvent mixture may be defined as a condition in which the 195

Figure 1. (a) 195Pt NMR (128 MHz, 303 K) chemical shift trends (δ195Pt/ppm) of [PtCl6]2− in solutions of H2PtCl6·H2O (∼0.1 M) in binary mixtures of D2O and methanol (MeOH), ethylene glycol (EG), 2-methoxyethanol (MEO), and 1,2-dimethoxyethane (DME). (b) Expansion of δ195Pt of [PtCl6]2− in D2O−methanol mixtures, with best-fit polynomial function and guide lines showing an estimate of the equi-solvation point (red dashed lines, XMeOH ≈ 0.33). It follows that, at a bulk mixture composition XMeOH = 0.50, the local solvation shell composition of [PtCl6]2− is estimated to be XMeOH = 0.66 at (blue dotted lines).

composition of the local solution environment, or specifically the primary/contact solvation shell of the solute, differs from that of the bulk binary solvent mixture, as opposed to the “ideal” behavior, in which the local composition is identical to that of the bulk mixture.37−40 As described in ref 39, the latter situation is expected, with the introduction of certain simplifying approximations, to result in a linear variation of the solute NMR chemical shift δ/ppm as a function of bulk mixed solvent composition, usually expressed as a mole f raction, XA = NA/(NA + NB), where NA and NB are the numbers of solvent molecules A and B, respectively. Based on this concept, the significant curvature of all experimental δ(195Pt) trends as a function of the solvent composition shown in Figure 1a suggests that the [PtCl6]2− anion may be preferentially solvated by the organic solution component in C

DOI: 10.1021/acs.inorgchem.8b01554 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry these binary mixtures as XA increases.37,40 Figure 1b shows an expansion of a detailed trend for the water−methanol binary mixture. Such apparent preferential solvation by the organic solvent molecules in these binary solvent mixtures of the relatively bulky [PtCl6]2− anion, albeit somewhat counterintuitive, may be better assessed qualitatively with reference to the equisolvation point (or “iso-solvation point”). This concept was defined by Frankel et al.37 as that composition of the bulk binary mixed-solvent at which the NMR chemical shift of the solute lies exactly midway between the respective limiting chemical shift values in the pure solvents. At this bulk mixture composition, both solvent components are considered to participate equally in forming the solvation shell of the solute on average, assuming a fast solvent exchange or short solvation shell residence time on the NMR time scale.37 Dechter and Zink40 studied the preferential solvation of Tl+ in formamide− pyrrolidine mixtures by 205Tl NMR, proposing a method to estimate the primary solvation shell composition of the Tl+ cation. Based on their methodology, an estimate of the equisolvation point of the [PtCl6]2− anion in water−methanol mixtures, and what the expected solvent shell composition would be for a bulk composition methanol mole fraction, XMeOH = 0.50, can be derived from data shown in Figure 1b. The estimated local mole fraction of methanol in the primary solvation shell of [PtCl6]2− in an equimolar water−methanol mixture, may be estimated as shown in Figure 1b. Extrapolating horizontally from the δ(195Pt) value at bulk composition XMeOH = 0.5 (at δ(195Pt) ≈ 66 ppm) to the ideal straight line connecting the limiting δ(195Pt) values, indicates a local solvation shell composition XMeOH = 0.66.13 Table 1 lists

binary mixture. These differences, and the surprisingly low equi-solvation point of [PtCl6]2− in water−DME mixtures, suggest that the composition of the solvation shells of the relatively large [PtCl6]2− anion in binary mixtures water−DME and MEO appear to be rather more complicated than anticipated and may not reflect a simplistic variation of the solvation shell composition based on the 195Pt NMR trends. In summary, no further information about the detailed solvation shell structures and the actual solvation numbers may be inferred from these δ(195Pt)/ppm of NMR shift trends alone, indicating a need for other approaches to interpret the experimental chemical shift trends, as well as a need for developing a more realistic understanding of the structure of the solvation of [PtCl6]2− anions in relevant solvents and mixtures. Other quantitative descriptions of the preferential solvation phenomenon based on the determination of solvation equilibrium constants,41 are similarly limited by a lack of information regarding the nature of solvation of [PtCl6]2− in solvents other than water.16,17 Moreover, the binary solvent water−DME mixtures investigated here are known to be nonideal, exhibiting varying degrees of molecular self-association, or micro-heterogeneity under conditions similar to those in this study.42−44 Indeed, the inherent micro-heterogeneity of certain macroscopically miscible binary solvent mixtures are thought to have potentially important implications in the context of preferential solvation phenomena.45,46 Molecular Dynamics Simulations. MD computer simulations are widely used for the modeling of liquids, mixtures, and solutions,47 frequently with the aim of understanding ion solvation,48 and are often compared with NMR studies.46,49 The results of classical MD simulations of [PtCl6]2− in selected water−methanol and water−DME mixtures presented here are intended to model the solutions for which 195Pt NMR data are reported in Figure 1. These systems were chosen for simulation specifically since they apparently represent moderate and strong preferential solvation of the platinum complex, respectively. We first consider the water−DME systems, for which the δ(195Pt) NMR trend in Figure 1a shows significant deviation from linearity, suggestive of a very prominent preferential solvation effect, which is more likely to be reasonably reproduced using a highly transferable general force field.25 MD Simulations of (H3O)2[PtCl6] in Water−DME Binary Mixtures. The final configurations from MD simulations of (H3O)2[PtCl6] (∼0.1 M) in three binary water−DME mixtures, XDME = 0.2, 0.5, and 0.8, are shown in Figure 2a−c. The micro-heterogeneous solvent distributions within these simulated mixed solvent systems are clearly apparent in the employed color scheme. In these configurations the volumes occupied by water and DME molecules are represented by blue and green surfaces, respectively. Comparison of these representations to those of the corresponding water−DME mixed solvent in the absence of (H3O)2[PtCl6] (Figure 2d−f), clearly indicates that the observed solvent micro-heterogeneity is not induced by the solute, but appears to be an inherent feature of these binary mixture simulations. For all simulations, visual inspection of the trajectories shows that the final configurations are representative of the configurations adopted during the production runs. The simulation configurations in Figure 2d−f are broadly consistent with Raman spectroscopy results of Nickolov et al.,44c indicating the presence of water clusters.

Table 1. Equi-solvation Points Estimated and Their Corresponding 195Pt NMR Chemical Shifts (δ195Pt, 128 MHz, 303 K) for [PtCl6]2− in Solutions of H2PtCl6·H2O (∼0.1 M) in Binary Mixtures of D2O and Co-solvents Methanol, Ethylene Glycol, 2-Methoxyethanol, and 1,2Dimethoxyethane co-solvent (A)

δ195Pt (ppm) at equi-solvation point

equi-solvation point (XA)

methanol EG MEO DME

51 67 102 108

0.33 0.28 0.18 0.11

the equi-solvation points for [PtCl6]2− obtained in this way,37 together with the corresponding δ(195Pt)/ppm values of the [PtCl6]2− anion, for all solvent mixtures examined in this work. All equi-solvation points occur at XA < 0.5, indicating preferential solvation of this complex anion by the organic component in all these binary mixtures. It is interesting that the equi-solvation points decrease significantly from XA = 0.33 in water−methanol (red dashed line in Figure 1b) o only 0.11 for water−DME mixtures obtained in a similar manner. In water−DME mixtures a similar analysis of the δ(195Pt) trends of [PtCl6]2− leads to an estimated solvation shell composition XDME = 0.92 in the corresponding equimolar solvent mixture, suggesting that the [PtCl6]2− anion is predominantly associated/solvated by DME. The significant nonlinearity of the δ(195Pt) vs the XDME mole fractions in Figure 1a levels off rapidly above a XDME ∼ 0.38, in contrast to the more regular trend observed for the water−methanol D

DOI: 10.1021/acs.inorgchem.8b01554 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Representations of final configurations (15 Å thick slices) from classical MD simulations of (H3O)2[PtCl6] in water−DME mixtures: XDME = 0.2 (a), 0.5 (b), and 0.8 (c). Color scheme: water, blue surfaces; DME, green surfaces; [PtCl6]2−, red; H3O+, yellow. (d− f) Corresponding mixed solvent systems without (H3O)2[PtCl6] are similarly represented, illustrating the intrinsic micro-heterogeneity of these solvent mixtures.

In these clusters a minority of single (isolated) water molecules appear to form hydrogen bonds bridging the DME ether oxygen atoms in concentrated DME solutions in water, consistent with the findings of previous computational studies (see Supporting Information).26,44a,50 Of particular importance in the corresponding (H3O)2[PtCl6] (∼0.1 M) containing binary mixtures, is the location of the H3O+ (yellow) and [PtCl6]2− (red) ions within the micro-heterogeneous environments that are formed. Inspection of Figure 2a-c shows that while H3O+ cations are primarily found within water-rich environments hydrogen bonded to surrounding water molecules, the larger [PtCl6]2− complex anions tend to be localized at the borders of water clusters, where they remain on average partially solvated by water molecules. Similar solute distributions are consistently observed throughout the entire trajectory. In a study of the micro-heterogeneity found in water−THF mixtures with low water content, Bragg et al.51 observed that iodide (I−) anions are similarly located (though preferentially hydrated) at the edge of small water clusters, or “pools”, which are characteristic of the water−THF solvent mixture. The authors of the latter study noted that this phenomenon is related to previous simulation studies, in which the I− anion was found to localize at air−water interfaces.52 The local environment of the [PtCl6]2− anion in the simulated water−DME solutions was further characterized by considering appropriate radial distribution functions (RDFs),39 computed from the simulation trajectories. Previous MD simulation studies of the hydration of the chloro complex anions of Pt, Pd, Rh, and Ir in pure water, made particular use of metal-water oxygen (OW) RDFs to estimate average hydration numbers of these complexes.15−17 The Pt−OW and Pt−DME carbon atoms (CDME) RDFs from our simulations of [PtCl6]2− in selected water−DME mixtures are shown in Figure 3, with key RDF parameters and estimated coordination numbers of water and DME molecules closely associated with the [PtCl6]2− anion, are given in Table 2. From the Pt−OW RDFs displayed in Figure 3a, it may be seen that while the position of the first gPt‑Ow maximum

Figure 3. RDFs, g(r), from MD simulations of (H3O)2[PtCl6] in water−DME mixtures for pairs Pt-water oxygen (Ow) (a), DME methylene (CH2) (b), and methyl (CH3) (c) carbon atoms. Legend indicates the mixed solvent composition as mole fraction DME, XDME. Vertical gray dashed line in panel (a) indicates Pt−OW RDF integration range, as discussed in the main text.

remains effectively constant with changing DME content (rmax ≈ 4.5 Å), the second maximum which occurs at 5.4 Å in pure water (solid gray line, Figure 3a),16,17 is slightly shifted and partially merges with the first maximum. Naidoo and coworkers17 found similar merging of metal−OW RDF maxima in simulations of the related, albeit more highly charged octahedral [RhCl6]3− and [IrCl6]3− complexes in pure water. This was interpreted as resulting from a “tighter second hydration shell” for the more highly charged trivalent anionic complexes. In the case of [PtCl6]2− in water−DME mixtures, the similarly merged Pt−OW RDF trends in Figure 3a are more likely to be related to the observed water clustering, as well as the tendency of [PtCl6]2− anions to be located at the interfacial regions of the intrinsic micro-heterogeneity of these solvent mixtures. Such phenomena unfortunately complicate the choice of an appropriate gPt‑Ow integration radius for estimating a primary hydration number in the solvation shell of the [PtCl6]2− complex, a problem also noted for simulations of this anion in pure water.16 In water−DME mixtures the position of the first Pt−OW RDF minimum (which in pure water occurs at ∼5.0 Å, as indicated by the vertical dashed gray line in Figure 3a),17 was chosen as the effective boundary of the primary Pt− OW coordination shell in both pure water and the water−DME mixtures. On the other hand, for the Pt−CDME RDFs shown in Figure 3b,c the broad first maxima occur only between ca. 5.0 and 5.5 Å, with those of the methyl C atoms occurring at slightly shorter distances, showing a distinct indentation at E

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Table 2. Selected RDF Parameters and Coordination Numbers (N) from Simulations of (H3O)2[PtCl6] in Water−1,2Dimethoxyethane (DME) Mixtures (XDME)a XDME parameter rmax (Å) rmin (Å) N(r = 5.0 Å) N(r = 6.0 Å)

0.0 4.55 5.0 8.8 23.5

0.2

0.5

0.8

1.0

4.53 6.1 4.2 8.6

gPt‑Ow(r), Water Molecules 4.45 6.1 2.6 4.8

4.42 6.2 1.8 3.0

− − − −

5.56 6.7 3.5 6.7

5.70 6.7 3.8 7.3

5.20 (5.72) 5.5 4.2 7.5

5.17 (5.73) 5.5 5.1 8.8

gPt‑CH2(r), DME Molecules O−CH2−CH2−O Fragment rmax (Å) rmin (Å) N (r = 5.5 Å) N (r = 6.0 Å)

− − − −

5.60 6.7 2.6 5.0

5.56 6.7 3.4 6.4

gPt‑CH3(r), DME Molecules −OCH3 Fragment rmax (Å) rmin (Å) N(r = 5.5 Å) N(r = 6.0 Å)

− − − −

5.25 (5.78) 5.7 2.8 5.2

5.20 (5.73) 5.5 3.8 6.8

a

Integration ranges r used to calculate the coordination numbers are indicated in parentheses; e.g., N (r = 5.0 Å) refers to the integration number calculated using an RDF integration boundary of 5.0 Å The Pt-CH3 RDFs in Figure 3c show two closely spaced first maxima, and the positions of both these maxima are reported here (the second, occurring at longer distances, is given in parentheses).

∼5.5 Å (Figure 3c). Inspection of the solvation shell configurations around [PtCl6]2− for selected trajectory frames reveals that the DME molecules which solvate a [PtCl6]2− anion, adopt several variable conformations within this region. This is exemplified in Figure 4. Clearly, the Pt−OW RDF

Given the uncertainty in the definition of the integration ranges of the RDFs in Figure 3, a second integration boundary of 6.0 Å was used for both the Pt−OW and Pt−CDME RDFs for the sake of comparison,16 in spite of the risk that atoms of solvent molecules outside the primary solvation shell might be included in the coordination number count on increasing the integration radius. Such comparisons have previously been shown to be useful in simulation studies of preferential solvation.46 The resulting Pt−OW and Pt−CDME coordination numbers are plotted as a function of solvent composition XDME in Figure 5a (and in Figure S4, using a uniform RDF integration boundary r = 6.0 Å in the Supporting Information). It may be seen that the coordination number trends shown in Figure 5a for the integration radii ranging from 5.0 to 5.5 Å do not vary significantly on increasing the integration radii to 6.0 Å (numerical results in Table 2). Significantly, comparison of the Pt−OW (red) and Pt−CDME (methylene blue; methyl red) coordination number variations in Figure 5a to the corresponding experimental δ(195Pt) NMR data in Figure 1a (green set), shows that the sharp initial increase in δ(195Pt) in the range XDME = 0−0.2, parallels the substantial changes in the simulated [PtCl6]2− solvation shell composition rather well. The total Pt−CDME coordination numbers (i.e., the sum of methyl and methylene group contributions) are shown in Figure 5 by a gray dashed line. The red dashed line similarly corresponds to the total Pt−OW coordination number contributions from both water molecules and H3O+ cations, the latter only contributing significantly at higher DME content from XDME > 0.8 onward.11 Between the DME mole fraction range, XDME = 0.2−0.8 the significant leveling of the experimental δ(195Pt) chemical shift data in Figure 1a, is reflected by the more gradual changes in the computed Pt−OW/CDME coordination numbers in as shown in Figure 5a (and Table 2). The slightly greater-than-expected changes in the coordination number trends found between XDME = 0.8−1.0 as reflected by the Pt−OW (solid red line) in

Figure 4. Representative conformations of DME molecules in the solvation shell of [PtCl6]2− in water−DME mixture (XDME = 0.5), using a standard atom coloring scheme (C atoms in cyan, O red, Pt violet, Cl green). The “liquorice” representation, with hydrogen atoms omitted in the interest of clarity, shows that solvating DME molecules may adopt various conformations and orientations within this region. Pt−C distances (in Å) to carbon atoms within 5.5 Å of Pt are indicated using labeled dotted black lines.

integration boundary described above (5.0 Å) cannot be used for the more bulky solvating methyl and methylene groups of first-shell DME molecules, so that a range of Pt−OW up to 5.5 Å was chosen for the estimation of Pt−CDME coordination numbers, identified with the number of DME carbon atoms in the primary solvation shell of the [PtCl6]2− complex. F

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For comparison, the Pt−OW and Pt−CDME coordination number trends from a series of simulations performed using full ionic charges of [PtCl6]2− and H3O+ are shown in Figure 5b. These are based on original models using full ionic charges as previously successfully implemented in the simulations of these respective ions in pure water.17,23 It may be seen that in water−DME mixtures the use of full ionic charges for [PtCl6]2− leads to results substantially at variance with the experimental NMR data for this system. The computed Pt− OW/CDME coordination numbers as a function of XDME (Figure 5b) show the largest changes in the range XDME = 0.8−1.0; this finding does not reasonably correlate with the δ(195Pt) trends, in which the most significant chemical shift changes occur in the range XDME ≈ 0.2−0.6 (Figure 1a). Moreover, the estimated equi-solvation point of XDME ≈ 0.88 obtained using the full-ionic charge model implies that the [PtCl6]2− anion is more strongly hydrated in these simulations (Figures S2 and S3), which is clearly incompatible with the value inferred from the experimental 195Pt NMR trends (XDME = 0.11, Figure 1a).13 Although we did not perform MD calculations for the water−MEO binary mixtures in this study, the experimental δ(195Pt) NMR chemical shift trend in Figure 1a (blue data points) similarly levels off as a function of higher mole fractions of 2-methoxyethanol, XMEO, something not surprising in view of the similarity of the MEO and DME structures. This suggests similar behavior for MEO−water mixtures which must, however, await confirmation. The limitations of MD simulations using full ionic charges of ions in mixed solvents as illustrated above, have also previously been pointed out: Bagno and co-workers, 46 in their investigation of the apparent preferential solvation of tetramethylammonium (TMA+) and Cl− ions in water− acetonitrile mixtures, specifically report an overestimation in the strength of chloride-water interactions in their classical simulations, which was ascribed to the neglect of electronic polarizability effects and the use of point charges on atoms.46b More recent computational studies emphasize the importance of the use of appropriate polarizable models for accurate classical modeling of ion solvation.28,53 On the other hand, the ionic charge-scaling method (MDEC) used in this study has been shown to account approximately for the important effect of electronic polarizability, specifically the electronic dielectric screening of ionic charges in condensed phases.27 To date very few studies of the solvation of inorganic polyatomic ions using the MDEC charge scaling method have been reported to our knowledge. Pegado et al.54 in their comparative MD simulation study of the hydration of the SO42− anion, demonstrated that the MDEC model provides results comparable to those obtained with a fully polarizable shell model (charge-on-spring model), as well as to ab initio MD (AIMD) simulations. These authors conclude that since the MDEC method is computationally simpler than a shell model, and it was found in some respects to provide superior results, it should be recommended as the method of choice.54 MD Simulations of (H3O)2[PtCl6] Water−Methanol Binary Mixtures. Given the more regular curvature of the δ(195Pt) NMR chemical shift trends for water−methanol mixtures shown in Figure 1, we undertook similar MDEC ionic charge-scaled simulations for ∼0.1 M (H3O)2[PtCl6] in representative water−methanol mixtures at XMeOH = 0.2, 0.5, 0.8. The final configurations of these simulations, representa-

Figure 5. Pt−OW (red), CDME, methylene (CH2, blue), and methyl (CH3, green) coordination numbers, N(r), for [PtCl6]2− in water− DME mixtures (composition XDME), obtained using RDF integration ranges r = 5.0 Å (Pt−OW) and 5.5 Å (Pt−CDME). Dashed red lines indicate total Pt−Ow (water and H3O+) and dashed gray lines the total Pt−CDME coordination numbers. The computed N(r) values using MDEC-scaled ionic charges are shown in (a). The corresponding values from similar simulations with the original full ionic-charges are shown in panel (b). The vertical black dashed lines indicate the position of the total Pt−OW and total Pt−CDME coordination number intercepts, used for the estimation of the equi-solvation points, giving 0.18 for MDEC-scaled ionic charges simulations, compared to 0.88 for simulations with full ionic charges (see text).

Figure 5a are noteworthy. In the XDME = 0.8−1.0 mole fraction range, the average Pt−OW coordination number decreases almost 2 molecules (1.8→0) as compared to only 0.8 (2.6→ 1.8) between XDME = 0.5 and 0.8, with a concomitant slight increase in the coordination number of both fragments of the DME molecules (Table 2). Indeed, a similar, albeit less pronounced increase in δ(195Pt) is just perceptible between XDME ≈ 0.9−1.0 (Figure 1a, green data set). The Pt−OW/CDME coordination numbers shown in Figure 5 may be used to estimate the [PtCl6]2− equi-solvation point in the simulated water−DME mixtures. A comparison of the estimated total Pt−OW (dashed red) and Pt−CDME (dashed gray) coordination numbers indicates an equi-solvation point of XDME ≈ 0.18, found from the intersection of these trends (vertical black dashed lines in both figures) in Figures 5a and S4, in reasonable agreement with the value of XDME ≈ 0.11, obtained from the experimental δ(195Pt) trends (Table 1).37 G

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Figure 7. Representative solvation shell arrangement of [PtCl6]2− in a simulated water−methanol mixture (XMeOH = 0.5), using a standard atom coloring scheme (C in cyan, O red, Pt violet, Cl green). Water molecules with oxygen atoms within 5.0 Å, and methanol molecules with O atoms within 5.3 Å and/or C within 5.5 Å of the Pt center (violet) are shown. Selected typical Pt−O and Pt−C distances (in Å) are given as indicated by the black-labeled, dotted lines. Methanol molecules labeled (1)−(3) are discussed in the main text..

Figure 6. Representations of final configurations (15-Å-thick slices) from classical MD simulations of (H3O)2[PtCl6] in water−methanol (MeOH) mixtures: XMeOH = 0.2 (a), 0.5 (b), and 0.8 (c). Color scheme: water, blue surfaces; MeOH, orange surfaces; [PtCl6]2−, red; H3O+, yellow. (d−f) The corresponding mixed-solvent systems without (H3O)2[PtCl6] show a degree of intrinsic micro-heterogeneity of the systems.

coloring scheme as in Figure 2, except with methanol molecules colored orange. The water and methanol molecules, clearly, are also not uniformly distributed in these systems (Figure 6d−f), exhibiting a certain degree of microheterogeneity, in agreement with the general conclusions of previous studies.42,43d The extent of micro-heterogeneity in the water−methanol mixtures, however, appears to be smaller compared to that found in the corresponding water−DME mixtures (Figure 2). While a detailed quantitative comparison of the structural micro-heterogeneities exhibited by these two systems was not undertaken, additional preliminary information supporting this view is provided in the Supporting Information, Figures S6 and S7. A representative MD “snapshot” of a typical solvation shell arrangement of [PtCl6]2− by water and methanol at XMeOH = 0.5 is shown in Figure 7, illustrating that solvating methanol molecules adopt various orientations within this region. While some methanol molecules show their O−H dipoles directed toward the [PtCl6]2− anion with their methyl groups pointing outward (see for example molecule labeled (1) in Figure 7), others show both hydroxyl and methyl groups within the solvation shell region (2); yet others are oriented with their methyl groups directed toward the platinum complex (3), presumably allowing these molecules to engage in hydrogen-bonding interactions with similarly oriented methanol, or water molecules in the surrounding bulk solvent. The computed Pt−OW, Pt−methanol oxygen (OM), and carbon (CM) RDFs in the water−methanol mixtures, as well as in pure water and methanol, are shown in Figure 8. The Pt− OW trend shows merging of first and second maxima for XMeOH in the range 0.5→0.8, similar to that found for the water− DME systems (Figure 3a). For the Pt−OM and Pt−CM RDFs two important features may be noted: (1) the first Pt−OM RDF minimum that occurs at 5.6 Å in pure methanol, moves systematically to shorter distances with increasing water content (5.1 Å at XMeOH = 0.2), and (2) the Pt−CM RDFs

Figure 8. (a) Pt−OW, (b) Pt−methanol oxygen (OM), and (c) carbon (CM) RDFs, g(r), from MD simulations of (H3O)[PtCl6] in water− methanol (MeOH) mixtures. Legend indicates the mixed solvent composition as mole fraction of methanol, XMeOH.

display a significant first maximum at ∼5 Å, indicating that some methyl groups of methanol are found within the first solvation shell region of [PtCl6]2−, as more clearly illustrated in Figure 7. H

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Inorganic Chemistry As discussed for the water−DME systems above, the RDF features described for the water−methanol mixtures also complicate the choice of integration ranges to be used for estimating [PtCl6]2− solvation numbers. As before, two sets of RDF integration ranges were employed in computing the coordination numbers: (1) gPt‑Ow(r) integrated up to 5.0 Å, gPt‑Om(r) up to their first minima, and gPt‑Cm(r) to 5.5 Å (same boundary used for DME methyl and methylene groups in the previous section); (2) all RDFs integrated up to 6.0 Å. The Pt−X (X = OW, OM, and CM) coordination numbers estimated by this procedure are listed in Table 3 and shown graphically Table 3. Selected RDF Parameters and Coordination Numbers (N) from Simulations of (H3O)[PtCl6] in Water− Methanol Mixtures at Various Values of XMeOHa XMeOH parameter

0.8

1.0

rmax (Å) rmin (Å) N (r = 5.0 Å) N (r = 6.0 Å)

gPt‑Ow(r), Water Molecules O 4.55 4.55 4.55 5.0 5.1 6.0 8.8 5.8 2.9 23.5 13.8 6.2

4.52 6.0 1.1 2.0

− − − −

rmax (Å) rmin (Å) N (r = rmin) N (r = 6.0 Å)

gPt‑Om(r), Methanol Molecules O − 4.66 4.69 − 5.1 5.3 − 1.2 2.7 − 2.7 5.5

4.70 5.5 4.4 7.9

4.70 5.6 5.3 7.2

gPt‑Cm(r), Methanol Molecules CH3 − 5.16 5.12 5.10 − 7.2 7.1 7.1 − 2.9 5.4 7.0 − 4.9 8.9 11.2

5.06 7.0 7.9 12.5

rmax (Å) rmin (Å) N (r = 5.5 Å) N (r = 6.0 Å)

0.0

0.2

0.5

Figure 9. Pt−OW (red), OM (blue), and CM (green) coordination numbers, N(r), for [PtCl6]2− in water−methanol mixtures (composition XMeOH), obtained by integration of appropriate RDFs with integration radii r = 5.0 Å (Pt−OW) and rmin (Pt−OM) 5.5 Å (Pt− CM) (Figure 8, Table 3). Dashed red and gray lines indicate respectively the total Pt−OW (water and H3O+) and Pt−methanol (OM + CM) coordination numbers. The vertical black dashed line indicates the position of the total Pt−OW and Pt−(OM + CM) coordination number intercept, used for the estimation of the equisolvation point.

chemical shift trends lends confidence in the methodology adopted in this study. Overall, the MD calculations performed largely support the notion of preferential solvation of [PtCl6]2− by methanol over water in binary mixtures, as inferred from the 195Pt NMR chemical shift trends as a function of XMeOH reported previously.13 The degree of micro-heterogeneity found in methanol−water mixtures, as well as the tendency of solvating methanol molecules to orient themselves in such a way that their more highly charged hydroxyl groups are directed away from the anionic complex, facilitating hydrogen-bonding interactions with surrounding solvent molecules, emphasizes the important role of the detailed solvent mixture microstructure as well as solvent−solvent interactions in determining solvation phenomena in these solutions.46 Moreover, the notion that experimental 195Pt NMR chemical shift trends for the [PtCl6]2− anion in binary solvent mixtures are a sensitive probe for the microscopic solvation shell composition of the [PtCl6]2− anion is supported by our MD simulations. The results of these simulations are generally consistent with the idea of preferential solvation of the complex by the organic component in water−methanol and water−DME mixtures. Thus, under appropriate conditions, the approximate solvent shell composition of the [PtCl6]2− complex anion may be inferred from the δ(195Pt) NMR trends with reasonable confidence. However, for solvent mixtures such as water−DME and water−MEO, a detailed description of the solvation shell becomes somewhat more complex, given the greater degree of micro-heterogeneity of these solvent mixtures. While our simulations of the water−DME mixtures suggest that the [PtCl6]2− anions tend to associate significantly at the interface between the solvent’s domains, the notion of a degree of preferential solvation by DME is nevertheless also supported by MD simulations. The role of ion-pairing between the solvated [PtCl6]2− anion and cations that are present in

a Integration ranges r used to calculate the coordination numbers N are indicated in parentheses.

in Figures 9 and S5 respectively for integration ranges (1) and (2) as defined above. The Pt−X coordination number changes in the water−methanol mixtures as a function of shown in Figure 9 (and Figure S5), to be identified with changes in composition of the solvation shell of the [PtCl6]2− complex, are noticeably more gradual compared to those for [PtCl6]2− in water−DME mixtures in Figure 5. These trends reflect the more gradual changes in the corresponding experimental δ(195Pt) NMR chemical shifts very well (black sets, Figure 1a,b), as compared to what is found for the water−DME mixtures. The total Pt−OM/CM coordination numbers (dashed gray lines), as well as the total Pt−OW coordination number contributions from both water molecules and H3O+ ions (dashed red lines), is also shown in Figure 9. The equi-solvation points estimated from this MD data in methanol−water, occurring at XMeOH = 0.28 using RDF integration radii r = 5.0 Å for water oxygen, the Pt−OM RDF first minimum for methanol oxygen, and 5.5 Å for methanol carbon (or at 0.33 using larger integration radii of r = 6.0 Å, Figure S5), are in excellent agreement with the experimental value XMeOH = 0.33 derived from δ(195Pt) NMR data (Figure 1b). This satisfactory agreement between the equi-solvation point obtained from the appropriate MDEC ionic-charge scaled MD simulation procedure accounting for electronic polarizability effects,27 and the experimental δ(195Pt) NMR I

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counterintuitive, considering the lack of appropriate positively charged interaction sites in the DME molecule, compared to those of water.55 Kudin and Car,56 in a computational study of the interfacial preferences of hydronium and hydroxide ions, noted that while simple electrostatic considerations may dictate that ions should prefer to be located in bulk water as opposed to interfaces with low dielectric media, this is not necessarily the case. Indeed, experiments reveal that water interfaces with oil droplets and hydrophobically assembled structures are usually electrically charged. Similar interfacial ion solvation phenomena have been reported by other workers,52 including the preference of halide anions for the water−oil interface, as studied by Jungwirth and co-workers.28 Caleman et al.53b recently investigated the preference of halide anions (excluding F−) for surface vs bulk water solvation in water droplets, in a vapor phase. The authors demonstrated that while the phenomenon is determined by a complex interplay between energetic components, it is driven by favorable water−water interactions.53b Our MD simulations results suggest that similar phenomena may be operative in the “preferential solvation” of [PtCl6]2− by DME in binary mixtures with water. As the bulk water content in these mixtures is reduced to water-rich regions, eventually resulting in dynamic small water clusters with increasing XDME, the [PtCl6]2− anions remain associated with these water clusters at the borders of bulk DME, affording minimal additional perturbation of the hydrogen-bonded water network. This finding is consistent with a leveling of the experimental δ(195Pt) NMR chemical shift, indicating no further changes in the local solvation environment of the [PtCl6]2− anion. In water−methanol mixtures, on the other hand, MD simulations of [PtCl6]2− anions indicate more gradual changes in the solvation shell composition with increasing methanol content, consistent with the more regular curved experimental δ(195Pt) trend as a function of XMeOH.13 Interestingly, the solvated methanol molecules tend to adopt orientations allowing them to participate in hydrogen bonding interactions with surrounding solvent molecules instead, with the less polarized weakly interacting methyl groups directed toward the [PtCl6]2−anion. This observation, together with the notion that in water−methanol mixtures a certain degree of water selfassociation takes place,42 suggests that the preferential solvation of [PtCl6]2− by methanol, may essentially be driven by the need for maximal preservation of energetically favorable hydrogen-bonded structures within these particular water− methanol mixtures.45 Similar phenomena are expected to be operative in the solvation of [PtCl6]2− in the other binary water−EG and water−MEO (Figure 1a) mixtures, for which heterogeneous microstructures have also been suggested.43 Finally, the MDEC ionic charge scaling method used in our MD simulations to account for the electronic dielectric screening of ionic charges (an important consequence of electronic polarizability in condensed phases,27) results in simulations which are both qualitatively and quantitatively consistent with the interpretation of the corresponding experimental δ(195Pt) NMR data. By contrast, similar MD simulations performed for H3O+ and [PtCl6]2− in water−DME mixtures using full ionic charges, originally intended for modeling of these ions in pure water,17,23 results primarily in apparent hydration of the [PtCl6]2− anion. This overestimation of water−ion electrostatic interactions in water−DME mixtures reported before for other systems,46 supports the current conclusion that accurate modeling of ion solvation

such solutions, should of course also not be neglected, particularly as Xorg increases to 1, since ion-pairing has been shown to take place in such solutions.3,10,13 Nevertheless, the effect of ion-pairing on changes in the δ(195Pt) trends of the [PtCl6]2− anion is comparatively small to those induced by the solvent composition, unless the ion-pairing interaction is very strong. Moreover, ion-pairing depends significantly on the relative concentration of the cation involved, the relative polarity of the solvent and most likely also on the nature of the cations present in solutions containing [PtCl6]2−. We have previously shown that ion-pairing between [PtCl6]2− and Na+ cations takes place, although in practice only at very high Na+ concentrations.3,10,13 Interestingly, two decades ago Hawlicka and Swiatla-Wojcik found in their MD simulations of NaCl in water−methanol mixtures that Cl− anions also appear to be preferentially solvated by methanol, particularly in mixtures with a low water content.45 These authors concluded that since Cl− ion and methanol molecules cannot easily be accommodated within the energetically favored hydrogen-bonded water network, Cl− ions tend to be “excluded” and forced to associate. Consequently the apparent preferential solvation of the ions is not due to direct differences in ion−solvent interactions, but rather due to their incompatibility with water and methanol solvent structures.45 A similar situation may pertain to the significantly larger [PtCl6]2− anion, although the size and the overall charge density of this complex anion may play an important role;13 assessment of such effects will require more detailed study.

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CONCLUSIONS The 195Pt NMR chemical shift δ(195Pt) of the industrially relevant [PtCl6]2− complex anion in binary mixtures of water (D2O) and selected water-miscible solvents such as methanol, ethylene glycol, 2-methoxyethanol, and 1,2-dimethoxyethane (DME) has been found to exhibit significant, nonlinear variations with solvent composition XA. According to the established view37−39 concerning the effect on the NMR chemical shift of ions in mixed solvents, the observed δ(195Pt) trends are consistent with varying degrees of preferential solvation of [PtCl6]2− complex by the organic molecules in these binary solvent mixtures. Particularly in water−DME mixtures the large δ(195Pt) variation shows pronounced deviation from linearity, indicating apparently significant preferential solvation of [PtCl6]2− anions by DME. Classical MD computer simulations performed to understand the nature of solvation of [PtCl6]2− in water−DME mixtures, lead to findings summarized as follows: (i) The water−DME mixtures are microscopically heterogeneous. (ii) The [PtCl6]2− anions appear, on average, to localize near or at the interface between the water and DME domains, where they are partially solvated by both water and DME. (iii) The [PtCl6]2− solvation shell composition changes sharply between XDME = 0 and 0.2, then more gradually with increasing XDME, with [PtCl6]2− predominantly solvated by DME for XDME > 0.2, reflecting the sharp initial increase and subsequent plateau of the corresponding experimental δ(195Pt) data. The preferential location of [PtCl6]2− within DME-rich environments at the edge of water clusters may seem initially J

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phenomena mandates taking into account the electric polarization and dielectric screening.52−54 Jungwirth et al.57 demonstrated that MDEC ionic charge-scaled models derived from nonpolarizable classical force fields, may benefit from additional parameter adjustment. It follows that the MDEC ion models used in the present study must be regarded as a necessary first step toward incorporating the effects of electronic polarization of relatively more complex and bulky anions such as the [PtCl6]2− reported here. Moreover, such models may similarly benefit from further optimization for use in nonaqueous solution environments in future computational studies.

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REFERENCES

(1) Kendal, T. Platinum 2005: Interim Review; Johnson Matthey PLC, 2005. (2) Berfeld, G. J.; Bird, A. J.; Edwards, R. I. Gmelin Handbook of Inorganic Chemisty, 8th ed.; Springer-Verlag: Berlin, 1986. (3) Koch, K. R.; Burger, M. R.; Kramer, J.; Westra, A. N. 195Pt NMR and DFT computational methods as tools towards the understanding of speciation and hydration/solvation of [PtX6]2− (X = Cl−, Br−) anions in solution. Dalton Trans. 2006, 3277−3284. (4) (a) Kramer, J.; Koch, K. R. 195Pt NMR Study of the Speciation and Preferential Extraction of Pt(IV)-Mixed Halide Complexes by Diethylenetriamine-Modified Silica-Based Anion Exchangers. Inorg. Chem. 2006, 45, 7843−7855. (b) Kramer, J.; Koch, K. R. 195Pt NMR Chemical Shift Trend Analysis as a Method to Assign New Pt(IV)Halohydroxo Complexes. Inorg. Chem. 2007, 46, 7466−7476. (c) Preetz, W.; Peters, G.; Bublitz, D. Preparation and Spectroscopic Investigations of Mixed Octahedral Complexes and Clusters. Chem. Rev. 1996, 96, 977−1025. (d) Xian, L.; Engelbrecht, L.; Barkhuysen, S.; Koch, K. R. Room temperature photo-induced deposition of platinum mirrors and nano-layers from simple Pt(II) precursor complexes in water−methanol solutions. RSC Adv. 2016, 6, 34014− 34018. (5) (a) Schmuckler, G. U.S. Patent 4 885 143, 1989. (b) Schmuckler, G.; Limoni-Relis, B. Interseparation of Platinum Metals in Concentrated Solution by Gel Permeation Chromatography. Sep. Sci. Technol. 1995, 30 (3), 337−346. (6) (a) Grant, R. A.; Taylor, Y. Eur. Patent 756013 A1, 1997. (b) Bernardis, F. L.; Grant, R. A.; Sherrington, D. C. A review of methods of separation of the platinum-group metals through their chloro-complexes. React. Funct. Polym. 2005, 65, 205−217. (7) Pesek, J. J.; Mason, W. R. Platinum-195 Magnetic Resonance Spectra of Some Platinum(II) and Platinum(IV) Complexes. J. Magn. Reson. 1977, 25, 519−529. (8) (a) Pregosin, P. S. Platinum-195 Nuclear Magnetic Resonance. Coord. Chem. Rev. 1982, 44, 247−291. (b) Still, B. M.; Kumar, P. G. A.; Aldrich-Wright, J. R.; Price, W. S. 195Pt NMRtheory and application. Chem. Soc. Rev. 2007, 36, 665−686. (c) Priqueler, J. R. L.; Butler, I. S.; Rochon, F. D. An Overview of 195Pt Nuclear Magnetic Resonance. Appl. Spectrosc. Rev. 2006, 41, 185−226. (9) Cohen, S. M.; Brown, T. H. Temperature dependence of 195Pt nuclear resonance chemical shifts. J. Chem. Phys. 1974, 61, 2985− 2986. (10) Naidoo, K. J.; Lopis, A. S.; Westra, A. N.; Robinson, D. J.; Koch, K. R. Contact Ion Pair between Na+ and PtCl62‑ Favored in Methanol. J. Am. Chem. Soc. 2003, 125, 13330−13331. (11) (a) Gerber, W. J.; Murray, P.; Koch, K. R. 195Pt NMR isotopologue and isotopomer distributions of [PtCln(H2O)6−n]4−n (n = 6,5,4) species as a fingerprint for unambiguous assignment of isotopic stereoisomers. Dalton Trans. 2008, 4113−4117. (b) Murray, P.; Gerber, W. J.; Koch, K. R. 35/37Cl and 16/18O isotope resolved 195Pt NMR: unique spectroscopic ‘fingerprints’ for unambiguous speciation of [PtCln(H2O)6−n]4−n (n = 2−5) complexes in an acidic aqueous solution. Dalton Trans. 2012, 41, 10533−10542. (c) Engelbrecht, L.; Murray, P.; Koch, K. R. Isotope Effects in 195Pt NMR Spectroscopy: Unique 35/37 Cl- and 16/18 O-Resolved “Fingerprints” for All [PtCl6−n(OH)n]2− (n = 1−5) Anions in an Alkaline Solution and the Implications of the Trans Influence. Inorg. Chem. 2015, 54, 2752− 2764. (12) Koch, K. R.; Engelbrecht, L. Intrinsic 35/37Cl and 16/18O isotope effects in high-field 195Pt and 103Rh NMR of purely inorganic Pt and Rh complexes: unique spectroscopic fingerprints for unambiguous assignment of structure. Dalton Trans. 2017, 46, 9303−9315. (13) (a) Koch, K. R.; Westra, A. N.; Lopis, A. S.; Naidoo, K. J.; Robinson, D. J. Proceedings ISEC, Beijing, China Sept, 19−23, 2005. (b) Westra, A. N. High Resolution NMR Studies Concerning the Solvation/Hydration and Coordination Chemistry of Pt(II/IV) Compounds. Ph.D. Thesis, University of Stellenbosch, South Africa, 2005.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01554.

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Article

Additional, detailed MD simulation parameters (Table S1); radial distribution functions (RDFs) pertaining to water−DME mixtures (Figure S1) and selected RDF parameters (Table S2); detailed simulation system specifications (Table S3); representations of system configurations (Figure S2) and computed RDFs (Figure S3) for simulations using full ionic charges; plots of simulated Pt−X coordination numbers using alternative RDF integration ranges (Figures S4 and S5); and preliminary evaluation of differences in mixed-solvent microheterogeneity for water−methanol and water− DME mixtures (Figures S6 and S7) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail for F.M.: [email protected], Tel. +390706754390. *E-mail for K.R.K.: [email protected]. Tel.++27 21 808 3020. ORCID

Klaus R. Koch: 0000-0003-3845-4406 Present Address ⊥

Centre of Advanced Research in Bionanoconjugates and Biopolymers Dept. Petru Poni Institute of Macromolecular Chemistry Aleea Grigore Ghica-Voda, 41A, 700487 Iasi Romania

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.R.K. gratefully acknowledges financial support from the South African National Research Foundation (GUN no. IRF2011032800040), Anglo Platinum Ltd., and Stellenbosch University, as well as the Visiting Professor Program financed by the Regione Autonoma Sardegna. A.L. gratefully acknowledges support from the Swedish Science Council, VR. Computational work was performed using resources provided by the Swedish National Infrastructure for Computing (SNIC). F.M. acknowledges financial support from Progetto Fondazione di Sardegna (CUP F71I17000170002). F.M. and A.L. thank for a partial support by a grant of Ministry of Research and Innovation, CNCS - UEFISCDI, project number PN-III-P4-ID-PCCF-2016-0050, within PNCDI III. K

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DOI: 10.1021/acs.inorgchem.8b01554 Inorg. Chem. XXXX, XXX, XXX−XXX