195Pt NMR and Molecular Dynamics Simulation Study of the Solvation of [PtCl6]2-in Water-Methanol and Water-Dimethoxyethane Binary Mixtures

The experimental195Pt 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, 2-methoxyethanol, 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 sensitive195Pt 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.