Homogeneous catalysis by heteropoly acids: A redox transformation of H₄[SiMo₁₂O₄₀] in electrophilic reactions

The transformation of the silico-molybdic acid (H₄[SiMo₁₂O₄₀]) used as catalyst in the dehydration of 1,4-butanediol was studied by IR, UV-VIS and ESR spectroscopies. During the reaction, the color of the catalyst changed gradually with the concomitant appearance of a broad new doublet in the 600-900 nm region in the electron spectra. At the beginning, ESR data indicate valence-state change of the molybdenum in the heteropoly acid (Mo(VI)→Mo(V)). After longer reaction times the reaction mixture became diamagnetic. The phenomenon was rooted in the formation of aldehydes and other fragments, which can provide suitable electron transfer linkage for the formation of diamagnetic bridges between molybdenum(V)-containing heteropoly blue species.