Model vanadium

Mitchell and Valero (1982, 1983) studied VO-phthalocyanine (VO-PC) and a Schiff base complex, VO-salen, as model vanadium compounds. The VO-PC provides a metal coordination environment comprised of four nitrogens, similar to the porphyrin. The benzo rings at the /3-pyrrolic positions contribute to enhanced aromaticity in the metal ligand. 

The Raman spectra oT model vanadium(V) oxide compounds are very sensitive to vanadium oxygen coordination. A distinct trend in the Raman spectra oT the reference tetrahedral and octahedral vanadium(V) oxide compounds is observed. Vanadium oxide is Tound to interact dit-ferently with the ditterent alumina supports. On the 7-AI2O3 support, the surtace vanadium oxide overlayer consists oT polymeric tetrahedra and distorted... 

Physico-chemical Methods for the Characterisation of Native and Model Vanadium Compounds... 

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). 

Theory has been used predominantly to probe the nature of the sites on vanadium clusters and model vanadium oxide surfaces. Cluster and p>eriodic DFT calculations [68,69] have been carried out in order to imderstand the electronic and structural properties of the exposed (100) surface of (VO)2P207. Both cluster and slab calculations reveal that surface vanadium sites can act as both local acid and base sites, thus enhancing the selective activation of n-butane as well as the adsorption of 1-butene. Vanadium accepts electron density from methylene carbon atoms and, thus aids in the subsequent activation of other C-H bonds. Calculations reveal that that the terminal P=0 bonds lie close to the Fermi level and thus present the most nucleophihc oxygen species present at the surface for both the stoichiometric as well as phosphate-terminated surfaces. These sites may be involved in the nucleophilic activation of subsequent CCH bonds necessary in the selective oxidative conversion of butane into maleic anhydride. Full relaxation of the surface, however, tends to lead to a contraction of the terminal P=0 bonds and a lengthening of the P V bonds. This pushes the P V states, initially centered on the oxygen atoms, higher in energy and thus increases their tendency to be involved in nucleophilic attack . 

A number of nitrogen-fixing bacteria contain vanadium and it has been shown that in one of these, Azotobacter, there are three distinct nitrogenase systems based in turn on Mo, V and Fe, each of which has an underlying functional and structural similarity.This discovery has prompted a search for models and the brown compound [Na(thf)]+[V(N2)2(dppe)2] (dppe = Pli2PCH2CH2PPh2) has recently been prepared by reduction of VCI3 by sodium naphthalenide... 

Sjbberg B-M (1997) Ribonucleotide Reductases - A Group of Enzymes with Different Metallosites and a Similar Reaction Mechanism. 88 139-174 Slebodnick C, Hamstra BJ, Pecoraro VL (1997) Modeling the Biological Chemistry of Vanadium Structural and Reactivity Studies Elucidating Biological Function. 89 51-108 Smit HHA, see Thiel RC (1993) 81 1-40... 

For ail samples, both a.p. and s.o., irrespective of the preparation method, the experimental intensity ratios, V2p/Zr3d, increased proportionally to the V-content up to 3 atoms nm 2 (pjg 2). The ratio approaches those calculated with the spherical model proposed recently by Cimino et al. [27] (full line in Fig. 2). For ZV samples with V-content < 3 atoms nm 2, this finding shows that vanadium species are uniformly spread on the Zr02 surface. On ZV catalysts with a larger V content (not shown in Fig. 2), the intensity ratios were markedly larger than the corresponding values yielded by the spherical model. The results obtained on samples with V-content > 3 atoms nm 2 point therefore to a V surface enrichment. 

The charge transfer model suggested to rationalize the correlati on between i oni zati on potenti al and reacti vi ti es of i ron, vanadium, and niobium with dihydrogen fails for other systems. However a model that takes into account the frontier orbital interactions, although highly simplistic, does account for a variety of observations. This model suggests extensions that include... 

The vanadium complex 159, bearing oligometallasilsesquioxanes, is an interesting model complex for surface species on silica [257]. Polymerization of ethylene with 159/MAO affords polyethylene with an Mn value of 2100 and an Mw value of 47900. An aluminum-vanadium adduct 160 polymerized ethylene without any cocatalysts, affording similar catalytic activity with the 159/MAO system [258]. 

These dimeric complexes involve, in their neutral state, two metal atoms in the (III) oxidation state. In the vanadium complexes such as [CpV(bdt)]2 and [CpV(tft)]2, the V—V bond length, 2.54 A in [CpV(bdt)]2, are shorter than observed in model complexes with a single V—V bond, indicating a partial double-bond character, also confirmed by a measured magnetic moment of 0.6 fiB in [CpV(tfd)]2, lower than expected if the two remaining unpaired electrons contribute to the magnetic susceptibility [20, 49]. This class of complexes most probably deserves deeper attention in order to understand their exact electronic structure. 

All the proposed models for syndiotactic propagation suppose that the active center is a vanadium-carbon bond and that the monomer first coordinates to the metal. Moreover, all of them attribute the stereospecificity to steric factors. However, different driving forces for the syndiospecificity have been proposed. 

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

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