Dimerization of mono-ruthenium substituted α-Keggin-type tungstosilicate [α-SiW11O39RuIII(H2O)]5− to µ-oxo-bridged dimer in aqueous solution: synthesis, structure, and redox studies

Abstract

We report the dimerization of a mono-ruthenium(III) substituted α-Keggin-type tungstosilicate [α-SiW₁₁O₃₉Ru^III(H₂O)]⁵⁻ to a µ-oxo-bridged dimer [{α-SiW₁₁O₃₉Ru^m}₂O]ⁿ⁻ (m = III, n = 12; m = IV/III, n = 11; m = IV, n = 10). Single crystal X-ray structure analysis of Rb₁₀[{α-SiW₁₁O₃₉Ru^IV}₂O]·9.5H₂O (triclinic, P, with a = 12.7650(6) Å, b = 18.9399(10) Å, c = 20.2290(10) Å, α = 72.876(3)°, β = 88.447(3)°, γ = 80.926(3)°, V = 4614.5(4) ų, Z = 2) reveals that two mono-ruthenium substituted tungstosilicate α-Keggin units are connected through µ-oxo-bridging Ru–O–Ru bonds. Solution ¹⁸³W-NMR of [{SiW₁₁O₃₉Ru^IV}₂O]¹⁰⁻ resulted in six peaks (−63, −92, −110, −128, −132, and −143 ppm, intensities 2 : 2 : 1 : 2 : 2 : 2) confirming that the µ-oxo bridged dimer structure is maintained in aqueous solution. The dimerization mechanism is presumably initiated by deprotonation of the aqua-ruthenium complex [α-SiW₁₁O₃₉Ru^III(H₂O)]⁵⁻ leading to a hydroxy-ruthenium complex [α-SiW₁₁O₃₉Ru^III(OH)]⁶⁻. Dimerization of two hydroxy-ruthenium complexes produces the µ-oxo bridged dimer [{α-SiW₁₁O₃₉Ru^III}₂O]¹²⁻ and a water molecule. The Ru(III) containing dimer is oxidized by molecular oxygen to produce a mixed valence species [{α-SiW₁₁O₃₉Ru^IV-III}₂O]¹¹⁻, and further oxidation results in the Ru(IV) containing [{α-SiW₁₁O₃₉Ru^IV}₂O]¹⁰⁻.