Structures of polynuclear complexes of palladium(ii) and platinum(ii) formed by slow hydrolysis in acidic aqueous solution

Abstract

The aqua ions of palladium(II) and platinum(II) undergo extremely slow hydrolysis in strongly acidic aqueous solution, resulting in polynuclear complexes. The size and structures of these species have been determined by EXAFS and small angle X-ray scattering, SAXS. For palladium(II), the EXAFS data show that the Pd-O and Pd⋯Pd distances are identical to those of crystalline palladium(II) oxide, but the intensities of the Pd⋯Pd distances in the Fourier transform at 3.04 and 3.42 Å are significantly lower compared to those of crystalline PdO. Furthermore, no Pd⋯Pd distances beyond 4 Å are observed. These observations strongly indicate that the polynuclear palladium(II) complexes are oxido-and hydroxido-bridged species with the same core structure as solid palladium(II) oxide. Based on the number of Pd⋯Pd distances, as derived from the EXAFS data, their size can be estimated to be approximately two unit cells, or ca. show Pt-O and Pt⋯Pt distances identical to those of amorphous platinum(II) oxide, precipitating from the solution studied. The Pt⋯Pt distances are somewhat different from those reported for crystalline platinum(II) oxide. The polynuclear platinum(II) complexes have a similar structure to the palladium ones, but they are somewhat larger, with an estimated diameter of 1.5-3.0 nm. It has not been possible to precipitate any of these species by ultracentrifugation. They are detectable by SAXS, indicating diameters between 0.7 and 2 nm, in excellent agreement with the EXAFS observations. The number of oxido-relative to hydroxido bridges will increase with increasing size of the complex. The charge of the complexes will remain about the same, +4, at growth, with approximate formulas [Pd 10 O 4 (OH) 8 for complexes with a size of 2 and 3 unit cells of the corresponding solid metal oxide, respectively. Their high ionic charge in acidic aqueous solution will result in a stabilizing hydration shell.

In-situ Quantification of Nanoparticles Oxidation: A Fixed Energy X-ray Absorption Approach

“…Although a relevant aliquot of the Pd(II) species investigated by FEXRAV is probably Pd(OH) 4 2− (due to the dissolution process already mentioned) the preparation of a reliable standard solution is impossible in the sense that it could affect the reproducibility of the measurements. Still, PdO and Pd(OH) 4 2− share the same coordination geometry, number and distances and the same ligand species, showing the same features in the respective XAS spectra [47][48][49][50][51][52]. All this considered PdO should be considered the best standard for the XANES of Pd(OH) 4 2− .…”

The Mixed Gold–Palladium Polyoxo‐Noble‐Metalate [NaAuIII4PdII8O8(AsO4)8]11−

“…proposed the formation of a tetranuclear species formulated as {Pd(OH)} 4 4+ upon alkaline hydrolysis of H 2 [PdCl 4 ] in NaCl medium with a Pd II concentration higher than 2.5 m M , as based on potentiometric titration studies 3. A number of physicochemical measurements indicated the possible formation of polynuclear Pd II assemblies also in Pd(OH) 2 solutions in HClO 4 4 and recently Persson and co‐workers suggested the formula [Pd 10 O 4 (OH) 8 (H 2 O) 12 ] 4+ for the cationic species formed in a 15 years aged Pd(ClO 4 ) 2 solution in 1 M HClO 4 as based on EXAFS studies 5. Using a combination of multinuclear ( 17 O, 23 Na, 35 Cl, and 133 Cs) NMR and UV/Vis spectroscopy together with EXAFS, SAXS and TEM studies, Troitskii et al.…”

“…In the isomer (Figure 2 a). Hence, the structure of Au 4 Pd 8 _1 resembles that of Pd 13 As 8 , [8a] and also that of the solid-state assembly Na 5 (Figure 2 a).…”

“…[3] A number of physicochemical measurements indicated the possible formation of polynuclear Pd II assemblies also in Pd(OH) 2 solutions in HClO 4 [4] and recently Persson and co-workers suggested the formula [Pd 10 O 4 (OH) 8 (H 2 O) 12 ] 4 + for the cationic species formed in a 15 years aged Pd(ClO 4 ) 2 solution in 1 m HClO 4 as based on EXAFS studies. [5] Using a combination of multinuclear ( 17 O, 23 Na, 35 4 }] 3 + (OAc = CH 3 COO À ), reported by Vargaftic and co-workers. [7] In 2008, the Kortz group discovered a simple synthetic strategy for stabilizing products of Pd II condensation in aqueous media by using arsenate to terminate the agglomeration process, leading to the discrete [Pd 13 As 8 O 34 (OH) 6 ] 8À , which crystallized as a sodium salt.…”

Breaking the Gordian Knot in the Structural Chemistry of Polyoxometalates: Copper(II)–Oxo/Hydroxo Clusters

“…It is remarkable that to date there are no reported data of noble‐metal‐based isoPOMs, which can be associated with an extremely slow formation of these metal–oxo clusters in solution in the absence of heterogroups. In this respect, more easily isolable POCus featuring structural motifs comparable with those of the above‐mentioned POPds and POAus can be used to translate their construction principles to noble‐metal‐based isoPOMs in order to explore the structure‐property relationships in the latter.…”