Explaining Asymmetric Solvation of Pt(II) versus Pd(II) in Aqueous Solution Revealed by Ab Initio Molecular Dynamics Simulations

Abstract

The solvation behavior of Pt(II) versus Pd(II) has been studied in ambient water using ab initio molecular dynamics. Beyond the well-defined square-planar first solvation shell encompassing four tightly bonded water molecules as predicted by ligand field theory, a second coordination shell containing about 10 H2O is found in the equatorial region. Additional solvation in the axial regions is observed for both metals which is demonstrated to be induced by the condensed phase. For the Pt(II) aqua complex, however, this water molecule is bonded with one of its hydrogen atoms toward the cation, thus establishing a typical anionic solvation pattern, which is traced back to the electronic structure of Pt(2+) versus Pd(2+) cations, in particular to the anisotropic polarizability of their tetrahydrates. Systematic model calculations based on suitable aqua complex fragments embedded in a polarizable continuum solvent support the idea that anionic hydration is facilitated by the liquid. Furthermore, transient protolysis of water molecules in the first shell is observed for both divalent transition metal cations, being more pronounced for Pt(II) versus Pd(II). The relevance of these solvation features is discussed with respect to the different acidity of Pt(2+) versus Pd(2+) aqua ions in water, their different water ligand exchange rates, and force field modeling approaches.

Dispersion‐Driven Hydrogen Bonding: Predicted Hydrogen Bond between Water and Platinum(II) Identified by Neutron Diffraction

“…[22] Aquation rates for analogous palladium(II) diamine complexes do not show any appreciable charge effect either. [23] Based on our present results and previous calculations, [12,[24][25][26] we dare to speculate that this surprising absence of a charge effect could be at least partly ascribed to "inverse hydration", whose inhibitory effect on the aquation should decrease with increasing charge, and could therefore counterbalance the effect of charge separation. The relatively slow aquation of cisplatin, which is the ratedetermining step for its pharmacologically relevant reaction with DNA, [27] could be therefore related to the "inverse hydration".…”

Ab initio molecular dynamics simulation and free energy exploration of copper(I) complexation by chloride and bisulfide in hydrothermal fluids

“…On the other hand, since periodic boxes were employed in the ab initio MD simulations, the Cu-Cu interaction is negligible, and Cu can be considered to be at infinite dilution with respect to Cu-Cu interactions. This approach has been widely used in many ab initio MD studies of low concentration metal ions in aqueous solutions (e.g., Ayala et al 2010;Beret et al, 2008;Terrier et al 2010). …”

Modeling the interactions of phthalocyanines in water: From the Cu(II)-tetrasulphonate to the metal-free phthalocyanine

“…For highly oriented water molecules following an ion-dipole interaction, 49 that value is near 0.7 Å. This tendency has already been observed in solvent molecules axially hydrating Pd(II) and Pt(II) square planar hydrated ions 29,38,52 and understood on the basis of the hydrogen-bonded network defined by the rest of solvent molecules. The previous planar complexes were hydrophilic (aquaions) while now the complex is neutral and of hydrophobic character, and interestingly in both the cases a labile situation is found for the water molecules located in that region.…”

“…In this context, polarization effects present in a metal center simultaneously interacting with two water molecules have been previously detected in square planar complexes of Pt(II) (Ref. 38) leading to asymmetric solvation environments around the metal ion. With that purpose a set of geometries in which two water molecules located in the axial region above and below the molecular CuPC plane have been tested.…”

“…In these angular DFs (distribution functions), the space around the central position, geometrical center of the macrocycle in our case, is split into different regions, which are defined by an azimuthal angle θ relative to the axis perpendicular to the molecular plane. 38 This way, distribution functions, equivalent to the global RDF, for the solvent present in each of these regions can be obtained separately. A global, but not spherically averaged, view of the solvent distribution can be obtained by means of spatial distribution functions (SDFs).…”