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 (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 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.
受异相催化剂表面物种的启发,我们合成了新型
氧化钒(V)
硅烷基sequioxane。含有两个基端羟基的 T8-
硅sequioxane 与 OV(OiPr)3 反应生成了一种双核化合物,其中两个
硅sequioxane 的基端二氧化二官能团桥接了两个 OV(OiPr) 分子(2)。2 的形成表明,与针对
二氧化硅表面提出的建议相反,在分子
化学中,宝石
硅氧烷与一个
钒中心的双齿配位并不有利。有人认为,在 V2O5/SiO2 催化剂上,双锚定在载体上的物种能发挥积极作用,在这种背景下,我们制备了一种阴离子复合物,其中二价二氧
钒单元取代了 (RSiO1.5)8 立方体的一个
硅角(Si-OH 功能仍未实现)(3)。我们通过振动光谱学对 3 进行了深入研究,并不仅对 ν(VO)带,还对ν(V-O-Si)带(其位置在异相催化领域很受关注)进行了赋值,同时还进行了同位素富集研究和 DFT 计算。通过合成和分析 3、4 的
硅烷化衍
生物,对相应的研究提供了帮助。此外,为了研究它们作为结构和光谱模型的潜力,所有复合物都通过单晶 X 射线衍射进行了表征。最后,对 2 和 3 作为
环己烷和苯与
氧气发生光氧化反应的潜在催化剂进行了测试。2 的活性略高于
乙酰丙酮酸钒,而 3 则显著提高了苯转化为
苯酚的周转次数。