Acid-base catalyses by dimeric disilicoicosatungstates and divacant γ-Keggin-type silicodecatungstate parent:: Reactivity of the polyoxometalate compounds controlled by step-by-step protonation of lacunary W=O sites

The catalytic properties of disilicoicosatungstates, [{gamma-SiW10O32(H2O}(2))(2)(mu-O)(2)](4-) (2) and [H(gamma-SiW10O32)(2)(mu-O)(4)](7-) (3), and their parent divacant gamma-Keggin type silicodecatungstate, [7-SiW10O34(H2O)(2)](4-) (1), toward C-C bond formation reactions have been investigated. The disilicoicosatungstate 2 with aquo ligands exhibits the acidic nature and catalyzes the Mukaiyama-aldol condensation and carbonyl-ene reaction, while 1 and 3 are rather basic and catalyze the Knoevenagel condensation. Therefore, the acid-base properties of a series of lacunary gamma-Keggin silicotungstate derivatives 1-3 are clearly different, and the catalyses of 1-3 depend on the molecular structures while 1-3 are composed of a common [SiW10O32] fragment. (c) 2006 Elsevier B.V. All rights reserved.