V2O5/SiO2 surface inspired, silsesquioxane-derived oxovanadium complexes and their properties

Abstract

Inspired by surface species proposed to occur on heterogeneous catalysts novel oxovanadium(V) silsesquioxanes were synthesised. Reaction of a T₈-silsequioxane containing two geminal OH groups with OV(OⁱPr)₃ led to a dinuclear compound where the geminal disiloxide functions of two silsesquioxanes are bridging two OV(OⁱPr) moieties (2). Formation of 2 shows that—in contrast to proposals made for silica surfaces—in molecular chemistry a bidentate coordination of geminal siloxides to one vanadium centre is not favourable. With the background that species being doubly anchored to a support have been suggested to play active roles on V₂O₅/SiO₂catalysts an anionic complex has been prepared where a divalent dioxovanadium unit replaces one Si corner of a (RSiO_1.5)₈ cube (a Si–OH function remains pending) (3). 3 has been intensely investigated by vibrational spectroscopy, and to support assignments not only of the ν(VO) bands but also of the ν(V–O–Si) bands, whose positions are of interest in the area of heterogeneous catalysis, isotopic enrichment studies and DFT calculations have been performed. The corresponding investigations were aided by the synthesis and analysis of a silylated derivative of 3, 4. Moreover, with regard to their potential as structural and spectroscopic models all complexes were characterised by single crystal X-ray diffraction. Finally, 2 and 3 were tested as potential catalysts for the photooxidation of cyclohexane and benzene with O₂. While 2 shows a slightly higher activity than vanadylacetylacetonate, 3 leads to significantly increased turnover numbers for the conversion of benzene to phenol.