V2O5/SiO2 surface inspired, silsesquioxane-derived oxovanadium complexes and their properties.
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 O=V(O(i)Pr)3 led to a dinuclear compound where the geminal disiloxide functions of two silsesquioxanes are bridging two O=V(O(i)Pr) 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/SiO2 catalysts an anionic complex has been prepared where a divalent dioxovanadium unit replaces one Si corner of a (RSiO1.5)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 v(V=O) bands but also of the v(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.
Ohde
Christian
C
Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489, Berlin, Germany.
Brandt
Marcus
M
Limberg
Christian
C
Döbler
Jens
J
Ziemer
Burckhard
B
Sauer
Joachim
J
D001554:Benzene / Q000737:chemistry; D002384:Catalysis; D018360:Crystallography, X-Ray; D003510:Cyclohexanes / Q000737:chemistry; D003511:Cyclohexanols / Q000138:chemical synthesis / Q000737:chemistry; D003512:Cyclohexanones / Q000138:chemical synthesis / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D017646:Organosilicon Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010636:Phenols / Q000138:chemical synthesis / Q000737:chemistry; D010777:Photochemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D013499:Surface Properties; D017968:Vanadium Compounds / Q000737:chemistry*; D014732:Vibration
D003510:Cyclohexanes; D003511:Cyclohexanols; D003512:Cyclohexanones; D009942:Organometallic Compounds; D017646:Organosilicon Compounds; D010636:Phenols; D017968:Vanadium Compounds; C506365:Cyclohexane; C036468:cyclohexanone; D012822:Silicon Dioxide; C066075:vanadium pentoxide; D001554:Benzene