V₂O₅/SiO₂surface 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 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.

From Surface‐Inspired Oxovanadium Silsesquioxane Models to Active Catalysts for the Oxidation of Alcohols with O2—The Cinnamic Acid/Metavanadate System

“…[7e, 8,9] If on the other hand the protons of H 3 R T 7 are replaced by transition-metal cations, the resulting complexes simulate situations in which such cations are bound to silica supports, but have the advantage of being treatable with the methods of molecular chemistry. [5][6][7] A second ligand system that is often employed for the more general modeling of an oxidic coordination platform are calixarenes, [10] and in the context of the above-mentioned supported oxovanadium catalysts employed for ODH processes, we have recently reported the results of structure-function analyses performed for calixarene-based model compounds with respect to the ODH of alcohols.…”

Progress in the Compilation of an Oxovanada‐Silsesquioxane Portfolio and Catalytic Activity of Organometallic Representatives in Ethylene Polymerisation

“…Crystals obtained were suitable for X-ray diffraction studies, and the result of a corresponding analysis is shown in Figure 5. It revealed that beside VI a tetranuclear complex [(OV) 4 (OH) 2 ( c-C5H9 T 7 ) 2 ], 5, is formed as a second product. Every vanadium atom in 5 is coordinated by five oxygen atoms, one of which is doubly bound; they occupy the corners of a square pyramid (see Figure 6).…”

“…With the background of the successful synthesis of 1 despite sterical constraints, one may ask whether a VO 2 + unit can act as a bridge between two silsesquioxanes: In all reactions of PPh 4 VO 2 Cl 2 with silsequioxane-based silanols investigated so far only 1:1 conversions occurred [4,6], and there were never indications for the formation of products with two silsequioxane ligands bound to one V centre. Hence, HOSi c-C5H9 T 7 containing only one silanol function was treated with 0.…”

“…Inter alia, these could be utilised as spectroscopic models to discuss certain assignments made in vibrational spectra of corresponding heterogeneous catalysts [4]. Moreover we managed to prepare polynuclear oxovanadium(IV) compounds (see for instance VI), which can be regarded as models for reduced aggregates formed on silica surfaces in course of the reduction of corresponding catalysts in contact with hydrocarbons and whose behaviour in contact with O 2 was thus investigated to simulate the reoxidation step for the real catalysts [5].…”

“…On the one hand supported vanadiumoxide catalysts are employed for the oxidative dehydrogenation (ODH) of alcohols and alkanes [2], and for a couple of years we have been interested in molecular compounds, which can simulate the structure, reactivity and/or the spectroscopic properties of corresponding surfaces. For the construction of suitable models we are pursuing two approaches: oxovanadium units are attached on the one hand to deprotonated forms of calixarenes [3] or on the other hand to deprotonated forms of silsesquioxanes [4,5,6], both of which are capable of mimicking the oxidic environments, that vanadiumoxo moieties experience on the surface of oxidic support or bulk materials. This contribution focuses on oxovanada silsequioxanes.…”