Abstract
The monopalladium(II)‐substituted [Na~2~(H~2~O)~2~PdWO(H~2~O)(α‐AsW~9~O~33~)~2~]^10–^ (1) and the tripalladium(II)‐substituted [Cs~2~Na(H~2~O)~8~Pd~3~(α‐AsW~9~O~33~)~2~]^9–^ (2) were synthesized in aqueous acidic medium (pH 4.8) by reaction of Pd^2+^ ions with [As~2~W~19~O~67~(H~2~O)]^14–^ and [α‐AsW~9~O~33~]^9–^, respectively. The dimeric polyanions 1 and 2 were characterized by complete elemental analysis, FTIR, and electrochemistry. Single‐crystal X‐ray analysis was carried out on Cs~3~Na~7~[Na~2~(H~2~O)~2~PdWO(H~2~O)(α‐AsW~9~O~33~)~2~]·1.5NaCl·22H~2~O (1a), which crystallizes in the triclinic system, space group P$\bar {1}$, with a = 13.2465(16) Å, b = 16.873(2) Å, c = 23.080(3) Å, α = 107.315(2)°, β = 95.025(3)°, γ = 98.040(3)°, and Z = 2; Cs~3~Na~6~[Cs~2~Na(H~2~O)~8~Pd~3~(α‐AsW~9~O~33~)~2~]·NaCl·15H~2~O (2a) crystallizes in the monoclinic system, space group P__2~1~/m__, with a = 13.1842(5) Å, b = 19.6584(7) Å, c = 18.2377(6) Å, β = 100.7660(10)°, and Z = 2. Polyanion 1 is composed of two [α‐AsW~9~O~33~]^9–^ fragments connected by a WO(H~2~O) unit, one square‐planar palladium atom and two sodium ions, leading to a sandwich‐type structure with C~s~ symmetry. Polyanion 2 also consists of two [α‐AsW~9~O~33~]^9–^ moieties, but these are linked by three equivalent, square‐planar Pd^2+^ ions, one sodium ion and two cesium ions, leading to a sandwich‐type structure with C~2__v__~ symmetry. A cyclic voltammetric study of polyanion 2 in a pH 5 medium shows a Pd^0^ deposition process on the glassy carbon electrode surface. The corresponding wave and the waves for the tungsten redox processes could be separated clearly. The thickness of the film increases with the number of potential cycles or the duration of potentiostatic electrolysis. The deposited film proved to be very stable and efficient in the electrocatalytic reduction of dioxygen. Polyanion 1 was not sufficiently stable in solution to warrant electrochemical characterization. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)