Axial Structure of the Pd(II) Aqua Ion in Solution

Abstract

Solution chemistry of Pd(II) and Pt(II) complexes is relevant to many fields of chemistry given the widespread applications of their compounds in homogeneous and heterogeneous catalysis, intermediate reaction synthesis, and antitumoral drugs. The well-defined square-planar arrangement of their complexes contrasts with the rather diffuse axial environment in solution. A theoretical proposal for a characteristic hydration shell in this axial region, called the meso-shell, stimulated further experimental and theoretical studies which have led to different pictures. The present work characterizes the structure of the axial region of the Pd(II) aqua ion in solution using a combination of neutron and X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy, with empirical potential structure refinement (EPSR). The results confirm the existence of the axial region and structurally characterize the water molecules within it. An important finding not previously reported is that the counterion, in this case the perchlorate anion, competes with water molecules for the meso-shell occupancy. The important role played by the axial region in many ligand substitution reactions is therefore intimately connected with the presence of the counterion and not just hydration water. This must call the attention of the experimental community to the important role that the counterion of the precursor salt must play in the synthesis.

Highly compressed water structure observed in a perchlorate aqueous solution

“…For the EPSR computational modeling, a cubic simulation box was constructed at the same concentration, temperature and atomic number density as the experimentally measured samples, using relevant starting reference (seed) potentials and molecular structures 29 – 31 . Note that the measured neutron data for the three different isotopic mixtures are compared to the fits produced by the EPSR simulation in Fig.…”

“…For the perchlorate ion the values derive from those used in ref. 29 modified to use full electronic charges on the ions, while keeping the O–Ow nearest neighbor distance similar to the Ow–Ow nearest neighbor distance. For the perchlorate ion, a Cl–O bond distance of 1.42 Å with an O–Cl–O angle of 109.47° was observed, as in ref.…”

A cartography of the van der Waals territories

“…For instance, the Pd II ion has been found by a combination of diffraction and EXAFS techniques to be coordinated in aqueous solution by four water molecules at Pd-O distances of 2.04 Å (cf. the sum of covalent radii of 2.05 Å 10 ), while two additional water molecules are axially semi-coordinated at distances in the range of 2.7-3.5 Å, 41 consistent with crystallographic characterization of axially coordinated molecules in some square planar Pd complexes at less than 2.80 Å. 42 It can also be seen that the short Pd⋯O intermolecular distances come mostly from O atoms nearly aligned with the symmetry axis of the Pd coordination plane (Fig.…”

On the structure of an aqueous propylene glycol solution

“…34,35 The use of EPSR to analyse neutron diffraction data has been frequently exploited to study the solution properties of a range of organic liquids, 11,12,14,[36][37][38][39][40][41][42][43][44][45][46] biological and drug molecules [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] and ionic species. [63][64][65][66] A set of reference potentials are used to start the EPSR process and these potentials are subsequently modified to provide a fit to the neutron diffraction data through a reverse Monte Carlo process. The Lennard-Jones potentials and Coulombic charges for each atomic component are listed in Table II for both R-and S-propylene glycol molecules, which were obtained from the CHARMM forcefield 67 and their intramolecular geometry optimised using the Avogadro software 68 and the SPC/E model was used for the water molecules.…”