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195Pt NMR and Molecular Dynamics Simulation Study of the Solvation of [PtCl6]2– in Water–Methanol and Water–Dimethoxyethane Binary Mixtures
journal contribution
posted on 2018-09-14, 14:50 authored by Leon Engelbrecht, Francesca Mocci, Aatto Laaksonen, Klaus R. KochThe
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, 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 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.