{AsW9O33}–{Mo3S4} based polyoxometalates including a metal–metal bond with Pd or Ni. Synthesis, structure and studies in solution

Abstract

Heterometallic cuboidal clusters [Mo₃S₄M(H₂O)₉Cl]³⁺ M = Pd or Ni react with the trivacant [AsW₉O₃₃]⁹⁻ anion to give tetramodular complexes [(H₂AsW₉O₃₃)₄{Mo₃S₄M(H₂O)₅}₂]²⁰⁻ (M = Pd for anion 2 and M = Ni for anion 3) in good yield. Both anions crystallized as single crystals of potassium salts to give K-2 and K-3 salts which have been characterized structurally by X-ray diffraction. Both compounds are isomorphous and the anions 2 and 3 are described as two dimeric moeties, associated by internal hydrogen bonds, electrostatic interactions involving four outer potassium ion and coordination bonds within a central {M₂S₂} unit containing a M–M metallic bond. Studies in solution reveal that the dimeric association is maintained in solution in the 2 × 10⁻⁴–2 × 10⁻³ mol L⁻¹ range. Conversely, in the presence of exogeneous ligands, such as iodide or pyridine the UV-vis data are consistent with the dissociation of the anion 2 into monomer through a Pd–L coordination bond (L = I⁻ or Py). Furthermore, ¹⁸³W NMR spectrum of 2 shows that molecular structure of 2 is retained in solution. Elemental analysis and IR are also supplied. Electrochemical behavior of 2 and 3 are given and compared with the Pd or Ni free parent anion. The CVs are dominated mainly by irreversible reduction or oxidation processes, where the peak potentials appear dependent upon the ionic charge of the complex. However, the CV of the Pd-containing anion (2) is consistent with the deposition of Pd metal at the electrode, which gives rise to an oxidation process into palladium oxide.