Organo-Ru supported sandwich-type tungstoarsenates: synthesis, structure and catalytic properties

Abstract

Five organo-Ru supported sandwich-type 17-tungsto-2-arsenates [{B-α-AsW₉O₃₃(OH)}{B-β-AsW₈O₃₀(OH)}{M₄(OH)₂(H₂O)₂}{(RuC₆H₆)₃}]⁶⁻ (M = Ni^II, 1; Zn^II, 2; Cu^II, 3; Mn^II, 4; Co^II, 5) have been synthesized by reacting the salts of the corresponding transitional metals with [(RuC₆H₆)AsW₉O₃₄]⁷⁻ in sodium acetate solutions at pH 6.0. The synthetic method is different from the previously reported transition metal-derivatives. We instead used the organo-Ru functional lacunary species [(RuC₆H₆)AsW₉O₃₄]⁷⁻ as the starting material. The salts of 1–5 were characterized by powder XRD, IR spectroscopy and elemental analysis. The structures of 1 and 2 have been solved by single-crystal XRD, comprising a rhomblike (M₄O₁₆) group sandwiched between two different lacunary Keggin moieties, [B-α-AsW₉O₃₄]⁹⁻ and [B-β-AsW₈O₃₁]⁹⁻, leading to the asymmetric sandwich-type frameworks, to which three (RuC₆H₆) units are grafted via Ru–O–W, Ru–O–M and Ru–O–M(W) bonds. Most importantly, the salts of 1–5 show high catalytic activities towards the oxidation of n-hexadecane with air without any additives and solvents. Comparing with the catalytic activities of the corresponding transition metals salts of [As₂W₁₈M₄(H₂O)₂O₆₈]¹⁰⁻ (M = Ni^II, 6; Zn^II, 7; Cu^II, 8; Mn^II, 9; Co^II, 10) and their precursors [AsW₉O₃₄]⁹⁻ (11) and [(RuC₆H₆)AsW₉O₃₄]⁷⁻ (12), the synergistic effect of Ru and transition metal ions in the compounds leads to the enhanced catalytic properties.