Elsevier

Chemical Physics Letters

Volume 445, Issues 4–6, 13 September 2007, Pages 193-197
Chemical Physics Letters

Structure and dynamics of the hydrated palladium(II) ion in aqueous solution A QMCF MD simulation and EXAFS spectroscopic study

https://doi.org/10.1016/j.cplett.2007.08.009Get rights and content

Abstract

The pharmacologically and industrially important palladium(II) ion is usually characterised as square-planar structure in aqueous solution, similar to the platinum(II) ion. Our investigations by means of the most modern experimental and theoretical methods give clear indications, however, that the hydrated palladium(II) ion is hexa-coordinated, with four ligands arranged in a plane at 2.0 Å plus two additional ligands in axial positions showing an elongated bond distance of 2.7–2.8 Å. The second shell consists in average of 8.0 ligands at a mean distance of 4.4 Å. This structure provides a new basis for the interpretation of the kinetic properties of palladium(II) complexes.

Graphical abstract

The pharmacologically and industrially important palladium(II) ion has been investigated by means of the most modern experimental and theoretical techniques proving the existence of a hexacoordinated palladium(II) hydrate.

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Introduction

Palladium(II) and platinum(II) complexes are utilised in numerous industrial processes because of their catalytic properties [1], [2]. They have also cytotoxic properties, and several types of palladium(II) and platinum(II) complexes are employed as anticancer drugs [3]. The hydrated palladium(II) and platinum(II) ions are so far only known in acidic aqueous solution and they hydrolyse readily with pKa values of 2.3 and ∼2.5 [4], [5], respectively. No solid-state structures containing fully hydrated palladium(II) and platinum(II) ions are reported so far. This is most probably due to the fact that the water molecules are too loosely bound, in a similar way as found for the equally soft silver(I) ion [6]. Electronic spectra [7] and the physico-chemical and kinetic substitution behaviour [8], [9], [10] appeared compatible with square-planar configuration. The water exchange rate of the hydrated palladium(II) and platinum(II) ions differs by six orders of magnitude, kex = 5.6 × 102[11] and 4.4 × 10−4 s−1[12], [13], respectively. The volume of activation of the water exchange in the hydrated palladium(II) and platinum(II) ions are small and negative, −2.2 and −4.6 cm3 mol−1, respectively, strongly indicating a weak associative interchange, Ia, mechanism for the water exchange of both ions [11], [12], [13]. The structure of the hydrated palladium(II) ion in aqueous solution was reported recently with four tightly bound water molecules in the equatorial plane with the oxygens forming a square-plane with a Pd–O bond distance of 2.00(1) Å. Additionally two (or one) water molecule(s) are weakly bound in the axial positions at 2.5 Å[14].

A vast majority of the crystal structures of the palladium(II) complexes and compounds display a square-planar configuration around palladium [15], [16]. The dimethylsulfoxide solvated palladium(II) ion is the only solvate structure studied by X-ray methods in solution showing a square-planar configuration with two oxygen- and two sulphur-bound dimethylsulfoxide molecules [17]. This clearly shows the soft binding properties of the palladium(II) ion in its square-planar binding positions. A search in the CSD data base [15] shows several palladium(II) complexes with the coordination numbers five and six in strongly square-pyramidal and tetragonally elongated octahedral configurations, and with axial Pd–O bond distances of ca. 2.7 Å[18], [19], [20]. The presence of additional one or two fast-exchanging water molecules in the axial position(s) had been proposed based on theoretical investigations without any experimental proofs [21]. The very long axial Pd–O bond distances in the five- and six-coordinated palladium(II) complexes observed in the solid state strongly indicate that these bonds have mainly electrostatic character, and they are therefore difficult to observe with kinetic and thermodynamic experimental methods.

Density functional theory at the local density approximation level (LDA-DFT) calculations have been applied to model the water exchange of the [Pd(H2O)4]2+ complex in gaseous phase in order to correlate with experimental observations [10]. However, the experimentally determined exchange rates refer to the in-plane water molecules which exchange too slow to be investigated by molecular dynamics, in particular with hybride quantum mechanical/molecular mechanical (QM/MM) methods. More recently, a classical molecular dynamics (MD) study of the palladium(II) ion in water by Sanchez-Marcos [22] gave strong indications that two more ligands are coordinated at longer distance to the ion, forming a tetragonally elongated octahedron. As different potentials were employed for equatorial and axial positions in this study, one cannot determine, however, to what extent this results could be ‘biased’ by the potentials. A subsequent ab initio QM/MM MD simulation of the same system predicted only one additional (axial) ligand and thus a square-pyramidal structure, but it was concluded that inclusion of only one hydration shell in the quantum mechanical treatment could not have been sufficient for this system [23]. In the present study we have performed a simulation with the ab initio quantum mechanical charge field (QMCF) MD formalism [24], which works without potential functions except the one for solvent–solvent interactions and includes two full hydration layers into the quantum mechanical treatment. Experimentally, a 40 mmol dm−3 Pd(ClO4)2 solution in 1.0 mol dm−3 HClO4, in order to supress hydrolysis, was investigated by extended X-ray absorbtion fine structure (EXAFS) methods, and the X-ray spectroscopic data were analysed on the basis of different models for the hydrate complex.

Section snippets

Theoretical

The ab initio QMCF MD simulation was performed for one palladium(II) ion in a cubic box containing 499 water molecules, applying periodic boundary conditions and maintaining the NVT ensemble at room temperature by the Berendsen algorithm [25]. The starting structure was obtained from a previous ab initio QM/MM MD simulation of the same system [22], and after re-equilibration of 4 ps, a total simulation time of 12 ps was used for sampling, which is sufficient to obtain reliable data for structural

Results and discussion

The Pd–O and Pd–H radial distribution functions (RDFs) resulting from the QM/MM MD simulation, shown in Fig. 1 and Table 1, clearly show that the first hydration shell consists of four ligands located at an average distance of 2.05 Å, and two further water molecules ∼2.71 Å away from the metal ion. A second shell peaking at 4.4 Å contains only eight ligands in average. A more detailed analysis showed that only the four ligands in the plane are capable of forming this second hydration shell, not

Acknowledgement

Financial support by the Austrian Science Foundation (Project 18429) and the Swedish Research Council is gratefully acknowledged.

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