Ru(iii) -based polyoxometalate tetramers as highly efficient heterogeneous catalysts for alcohol oxidation reactions at room temperature

Abstract

A novel ruthenium-containing polyoxometalate-based organic–inorganic hybrid, K₄Na₉H_7.4[(AsW₉O₃₃)₄(WO₂)₄{Ru_3.2(C₃H₃N₂)₂}]·42H₂O (1), was successfully synthesized by a one-step hydrothermal method under acidic conditions, which applied a self-assembly strategy between inorganic polyoxometalate based on trivacant [B-α-AsW₉O₃₃]⁹⁻ {AsW₉} fragments and an organic ligand, imidazole (C₃H₄N₂). Compound 1 was further characterized by single-crystal X-ray diffraction, PXRD, IR spectroscopy, UV-Vis spectroscopy, ESI-MS, elemental analysis and TGA. Single-crystal X-ray diffraction data reveal that the polyanion consists of four trivacant Keggin-type polyanion {AsW₉} building blocks bridged by four {WO₆} units, leading to a crown-shaped tetrameric structure [(AsW₉O₃₃)₄(WO₂)₄{Ru_3.2(C₃H₃N₂)₂}]^20.4−. The ESI-MS result reveals that the polyanion unit has excellent structural integrity in water. Moreover, the catalysis study of 1 was also further investigated, and the experimental results indicate heterogeneous catalyst 1 presents high efficiency (yield = 98%), excellent selectivity (>99%), and good recyclability for the oxidation of 1-(4-chlorophenyl)ethanol to 4′-chloroacetophenone with commercially available 70% aqueous tert-butyl hydroperoxide {TBHP (aq.)} as the oxidant at room temperature.