Vanadium oxo

Scheme 6.1-2 Vanadium oxo-exchange chemistry in a basic [EMIM]CI/AICl3 ionic liquid...

Catalytic turn-over [59,60] in McMurry couplings [61], Nozaki-Hiyama reactions [62,63], and pinacol couplings [64,65] has been reported by Fiirst-ner and by Hirao by in situ silylation of titanium, chromium and vanadium oxo species with McaSiCl. In the epoxide-opening reactions, protonation can be employed for mediating catalytic turn-over instead of silylation because the intermediate radicals are stable toward protic conditions. The amount of Cp2TiCl needed for achieving isolated yields similar to the stoichiometric process can be reduced to 1-10 mol% by using 2,4,6-collidine hydrochloride or 2,6-lutidine hydrochloride as the acid and Zn or Mn dust as the reduc-tant (Scheme 9) [66,67]. 

Solutions to similar problems of achieving catalytic turnover [22] in McMurry couplings [23], Nozaki—Hiyama reactions [24], and pinacol couplings [25] have been reported by Fiirstner and by Hirao. The key step in these reactions is the in situ silylation of titanium and vanadium oxo species with Me3SiCl and reduction of the metal halides by suitable metal powders, e. g. zinc and manganese dust, as shown in Scheme 12.13. 

The central shaded part of Figure 1.12 describes the catalytic operation of active sites. These are considered as isolated sites of a cluster of vanadium oxo species (Figures 1.3 and 1.4). They are adaptive in the sense that they need first to be activated... 

Isocyanate formation through multiple bond metathesis of C02 with carbodiimide has been also demonstrated [112]. This transformation can be promoted by titanium isopropoxide, at 383 K, in THF as solvent. It is worth noting that the reverse process, which opens an entry into carbodiimide synthesis, is a well-known process that is catalyzed by several other systems, including trialkylphosphine oxides [113] or vanadium-oxo or -imido complexes [114]. 

Bourikas, K. et al.. Mechanism of deposition of vanadium-oxo species on the anatase/electrolytic solution interface, J. Phys. Chem. B, 101, 8499, 1997. 

There are many different pieces of code available for molecular mechanics, ranging from the simple, such as MM2, to the elaborate, such as Cerius S YBYL, Spartan, and HyperChem. The code chosen for a particular model of catalytic processes depends on two factors (1) the complexity of the system that is to be studied, and (2) the amount of computer expertise available. Complicated structures, such as surfaces and zeolites, generally require specialized software packages for their visualization typically workers use commercial code with perhaps minor modifications. Simpler systems, such as modeling vanadium oxo species, are amenable to study using simpler codes, such as MM2, that are customized to suit the specific needs of the research group. It should be noted that the various available packages employ different force assumptions and some force fields are more suitable to one kind of application than to another (see Chapter 2). 

Shape selectivity Vanadium oxo complexes Custom code MM2... 

The formation of 3-methyl-2-buten-l-ol, 4, and of 3-methyl-2-butenal, 5, in some experiments can be related to the isomerization of la and subsequent oxidation of product 4, thus formed, to 5. We observed that the vanadium oxo-alkoxide, [OV(OC3H7)3], is a good precursor for the catalytic epoxidation of substrates la, lb and Ic with systems involving tert-butyl hydroperoxide and tertiary allylic alcohols. These systems proved to be more active than those involving [VO(acac)2] usually associated with anhydrous r-BuOOH in benzene or toluene (Figure 1 and Table 2). 

Chen, Q. Salta, L Zubieta, J., Synthesis and Structural Characterization of Binuclear Vanadium Oxo... 

Chen, Q. and Zubieta, J., Investigations of the Vanadium-Oxo-Organophosphonato System - Preparation... 

The H transfer in Equation 1.13, to a vanadium oxo ligand, is surprisingly slow. The transfer requires a great deal of structural reorganization, as the V-O distance increases by 0.264A when V=0 is converted to V-OH. The barrier to transfer of an H from V-OH to V=0 (in Marcus theory, the intrinsic barrier to selfexchange) is thus large, and its size is reflected in the size of the barrier to the C—>0 H" transfer in Equation 1.13 [46],... 

Kodama and co-workers have used vanadium-oxo-isopropoxide in the presence of a Lewis base for cyanocylation (reaction 7.23) of aromatic aldehydes with trimethylsilyl cyanide, 113 [73]. 

A different type of polymer is obtained when the 2-bromopropionate-substituted titanium zirconium and vanadium oxo clusters mentioned above are employed as macroinitiators for copper-catalyzed atom transfer polymerizations of MMA or styrene. Each carboxylate ligand at the cluster surface acts as an initiator for the growth of a polymer chain from the cluster surface, resulting in coreshell nanoparticles with an inorganic (cluster) core and a polymer shell. These reactions proceed in a controlled manner and rather low polydispersities are achieved. ... 

Although synthesis of VO(OR)3 by the reaction of V2O5 with alcohols was described as early as 1913, " only a few more vanadium oxo-alkoxides " appear to have been synthesized as shown by Eqs (2.77) and (2.78) ... 

The chemistry of vanadium is replete with 0x0 vanadium complexes including a large number of vanadium oxo-alkoxides. 

Boghosian, S., Chrissanthopoulos, A. and Fehrmann, R. (2002). Structure of Vanadium Oxo-sulfato Complexes in V2O5-M2S2O7-M2SO4 (M = K, Cs) Melts. A High Temperature Spectroscopic Study, J. Phys. Chem. B, 106, pp. 49-56. 

Crepaldi E.L., de A.A. Soler-IUia G.J., Grosso D., Sanchez C. Nanocrystallised titania and zirconia mesoporous thin films exhibiting enhanced thermal stabtiity. New J. Chem. 2003b 27 9-13 Crepaldi E.L., Grosso D., de A.A. Soler-IlUa G.J., Albouy P.-A., Amenitsch H., Sanchez C. Formation and stabilization of mesostructured vanadium-oxo-based hybrid thin films. Chem. Mater. 2002 14 3316-3325... 

V2O5 Oxides. - Supported V2O5 oxides are extremely important industrial catalysts for environmental pollution control, and are used in catalytic scrubbers for SO2 oxidation and NO reduction. During the operation of the catalyst (usually at 400-600 °C) in the SO2 oxidation reactions, pyrosulfate melts are formed in the pores of the catalysts, and V2O5 can dissolve in these melts forming vanadium oxo-sulfate complexes ... 

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