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

Abstract

Silsesquioxane dioxovanadate(V) complexes were investigated with respect to their potential as a catalyst for the oxidative dehydrogenation of alcohols with O(2) as an oxidant. The turnover frequencies determined were comparatively low, but during the oxidation of cinnamic alcohol an increase in activity was observed in the course of the process, which was inspected more closely. It turned out that during the oxidation of cinnamic alcohol, not only was the aldehyde formed but also cinnamic acid, which in turn reacts with the silsesquioxane complex employed to give NBu(4)[O(2)V(O(2)CC(2)H(2)Ph)(2)], which can also be obtained from NBu(4)VO(3) and cinnamic acid and represents a far more active catalyst, not only for cinnamic alcohol but also for other activated alcohols and hydrocarbons. The rate-determining step of the conversion corresponds to an hydrogen-atom abstraction from the C-H units, as shown by the determination of the kinetic isotope effect in case of 9-hydroxyfluorene, and the reoxidation of the reduced catalyst proceeds via a peroxo intermediate, which is also capable of oxidizing one alcohol equivalent. Furthermore the influence of the organic residues at the carboxylate ligands on the catalyst performance was investigated, which showed that the activity increases with decreasing pK(s) value. Moreover, it was found that during the oxidation the catalyst slowly decomposes, but can be regenerated by addition of excessive carboxylic acid.

Surface‐Inspired Molecular Vanadium Oxide Catalysts for the Oxidative Dehydrogenation of Alcohols: Evidence for Metal Cooperation and Peroxide Intermediates

“…This finding is consistent with recent investigations concerning the H-atom abstraction reactivity of ligated [VO 2 ] + units. [8,10] Isolation of the reduced catalyst: As in the case of Ia, we also tried to identify reaction intermediates of the catalytic system involving II. Interestingly, omitting O 2 during conversion of 9-fluorenol led to isolation of (PPh 4 ) 2 …”

“…[2][3][4][5][6][7][8] For simulation of the oxidic environments that oxidovanadium moi-A C H T U N G T R E N N U N G eties experience on the surfaces of oxidic support materials we have successfully employed two different types of ligands: The deprotonated forms of calixarenes [2][3][4][5] and the deprotonated forms of incompletely condensed silsesquioxanes. [6][7][8] Recently we reported the results of structure-function analyses of calixarene-based oxidovanadium model compounds with respect to the ODH of alcohols. [2,4] It turned out that dinuclear compounds are more active than mononuclear ones, and that thiacalixarene complexes are more efficient than complexes containing the classical calixarene ligands.…”

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

“…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.…”

“…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]. Very recently we have described the results of studies testing available oxovanadium(V) silsesquioxane complexes as functional models [6]. While the employed silsesquioxane complexes were found to be only slightly active as catalysts for the ODH of alcohols, indepth studies showed that leaching caused by carboxylic acids (formed as products) leads to a very active catalyst, which was identified.…”

“…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.…”