Catalyst metal-free organic ligands

Vanadyl salen is readily converted at 100°C with H2S in the absence of a catalyst to a vanadium sulfide and a free organic ligand (or decomposition products). Vanadyl phthalocyanine is more stable with respect to ring attack and demetallation. Rates relative to catalytic reactions have not been measured. If VO-salen is an appropriate model of vanadium binding in asphaltenes, asphaltenic metals are more readily converted to sulfides under hydrotreating conditions than the porphyrinic metals. This suggests... 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

As mentioned previously, the role of the metal is not specific. Even transition metal free layered double hydroxides are suitable catalysts for the olefin epoxidation with O2 and a sacrificial aldehyde (205). Leaching of the metal from the solid catalyst is a serious problem since organic acids that are potential metal ligands accumulate during the reaction. Leaching occurs, for example, with the polybenzimidazole-supported Ni2+ catalyst (199). 

Many transition metals and their compounds with organic ligands initiate the polymerization of alkenes and/or dienes. Some of them do not need any special treatment to this end while others require the presence of some organic or mineral compound or a special physical modification. In contrast to ZN catalysts, they are active without an organometal of Groups I—III. They are commonly known as metal alkyl free (MAF) catalysts. Many of their features are, of course, in common with ZN catalysts. MAF catalysts initiate stereoselectively controlled polymerization. Even less is known of their operating mechanism than that of ZN catalysts. It is assumed that propagation also occurs on the transition metal-carbon bond. 

Catalysis by metal-organic complexes often involves as a first step the loss of a ligand from the catalyst, freeing a coordinative site that can bind a reactant ... 

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