Organo-Ruthenium Supported Heteropolytungstates: Synthesis, Structure, Electrochemistry, and Oxidation Catalysis

The reaction of [Ru(arene)Cl2]2 (arene = benzene, p-cymene) with [X2W22O74(OH)2]12− (X = SbIII, BiIII) in buffer medium resulted in four organo-ruthenium supported heteropolytungstates, [Sb2W20O70(RuC6H6)2]10− (1), [Bi2W20O70(RuC6H6)2]10− (2), [Sb2W20O70(RuC10H14)2]10− (3), and [Bi2W20O70(RuC10H14)2]10− (4), which have been characterized in solution by multinuclear (183W, 13C, 1H) NMR, UV−vis spectroscopy, electrochemistry, and in the solid state by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Polyanions 1, 2, and 4 crystallize in the triclinic system, space group P1̅ with the following unit cell parameters: K5Na5[Sb2W20O70(RuC6H6)2]·22H2O (KNa-1), a = 12.1625(2) Å, b = 13.1677(2) Å, c = 16.0141(3) Å, α = 78.9201(7)°, β = 74.4442(8)°, γ = 78.9019(8)°, and Z = 1; Cs2Na8[Bi2W20O70(RuC6H6)2]·30H2O (CsNa-2), a = 11.6353(7) Å, b = 13.3638(7) Å, c = 16.7067(8) Å, α = 79.568(2)°, β = 71.103(2)°, γ = 80.331(2)°, and Z = 1; Na10[Bi2W20O70(RuC10H14)2]·35H2O (Na-4), a = 15.7376(12) Å, b = 15.9806(13) Å, c = 24.2909(19) Å, α = 92.109(4)°, β = 101.354(4)°, γ = 97.365(3)°, and Z = 2. Polyanions 1−4 consist of two (L)Ru2+ (L = benzene or p-cymene) units linked to a [X2W20O70]14− (X = SbIII, BiIII) fragment via Ru−O(W) bonds resulting in an assembly with idealized C 2h symmetry. Polyanions 1−4 are stable in solution as indicated by the expected 183W, 13C, and 1H NMR spectra. The electrochemistry of 1−4 is described by considering the reduction and the oxidation processes. The nature of the arene in Ru(arene) has practically no influence on the formal potentials of the W-centers, which are more sensitive to the Sb or Bi hetero atoms. The results suggest that the respective Sb- and Bi derivatives have very different pK a values, with the reduced form of 1 being the most basic, thus permitting the observation of two well-developed voltammetric waves at pH 6. In contrast, the identity of the arene influences the oxidation processes, thus permitting to distinguish them. A strong electrocatalytic water oxidation peak is observed that is more positive than the one corresponding to the Ru(arene) oxidation process. Also a stepwise oxidation of the Ru(benzene) group could be observed at pH 3. The catalytic efficiency, on the other hand, of 1−4 toward the oxidation of n-hexadecane and p-xylene illustrated the effect of ruthenium substitution on the polyanion catalytic performance.