Molybdenum species

Important differences distinguish the molybdenum and tungsten systems. In aqueous solution, equilibration of the molybdenum species is complete within a matter of minutes whereas for tungsten this may take several weeks it also transpires that whereas the basic unit of most isopolymolybdates is an MOg octahedron with a pair of cis-terminal oxygens, that of the isopolytungstates is more commonly an MOg octahedron with only one terminal oxygen. The two must therefore be considered separately. 

This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites. 

In order to obtain more structural information about the molybdenum species in Mo/NaY, EXAFS measurements of the cluster 1 and Mo/NaY were carried out. The Fourier transforms of the EXAFS data are shown in Figure 2. Structural parameters (Table 3) showed no change of the Mo-0, Mo-S and Mo-Mo distances, suggesting that there is no significant structural difference between the cluster 1 and the molybdenum compound in the Mo/NaY. From these EXAFS parameters and the UV-visible spectra, it is considered the structure of cluster 1 remained vinually intact after ion exchange. 

Let us now return to MMCT effects in semiconductors. In this class of compounds MMCT may be followed by charge separation, i.e. the excited MMCT state may be stabilized. This is the case if the M species involved act as traps. A beautiful example is the color change of SrTiOj Fe,Mo upon irradiation [111]. In the dark, iron and molybdenum are present as Fe(III) and Mo(VI). The material is eolorless. After irradiation with 400 nm radiation Fe(IV) and Mo(V) are created. These ions have optical absorption in the visible. The Mo(VI) species plays the role of a deep electron trap. The thermal decay time of the color at room temperature is several minutes. Note that the MMCT transition Fe(III) + Mo(VI) -> Fe(IV) -I- Mo(V) belongs to the type which was treated above. In the semiconductor the iron and molybdenum species are far apart and the conduction band takes the role of electron transporter. A similar phenomenon has been reported for ZnS Eu, Cr [112]. There is a photoinduced charge separation Eu(II) -I- Cr(II) -> Eu(III) - - Cr(I) via the conduction band (see Fig. 18). 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

H2 TPR measurements are used to probe the reducibility and may reveal more information on the nature of vanadium and molybdenum species. The assignment of the TPR peaks has been based on the literature study [9, 10] but also by using two reference samples V1-Z15 and Mol-Z15 prepared by solid-state ion exchange. TPR thermograms of V-Mo-Zeolite systems can be divided into two zones of H2 consumption (/) Mo-Zeolites exhibit two reduction peaks at 600 and 850°C corresponding to the reduction of Mo6+ into Mo4+ through the Mo5+ step and to the reduction of Mo4+ into Mo°, respectively while (ii) V-Zeolites led to a broad asymmetric feature around 710°C, which has been previously attributed to the reduction of V5+ into V3+. Finally, the TPR profiles of V-Mo-Zeolite catalysts seem more like a superposition of both Mo and V-catalysts TPR profiles. 

An important study using cyelopentadienyl (Cp) molybdenum species (196) has shown that reductive elimination of saturated product from an alkyl-hydride complex occurs with retention of configuration at the... 

This picture, although conceptually useful, is too simple. Kim et al. [43] showed that it is not the overall pH of the solution that dictates which species deposit on the surface, but the local pH at the support interface. The latter depends on the isoelectric point of the support, the coverage of molybdenum species and the number of NH4+ or H+ counter ions of the negative complexes. The reader is referred to [43] for details. 

Table 7.3 lists only few experimental data that can be used to estimate the accuracy of the theoretical results. The CCSD(T)/II values agree quite well with experiment (note, however, rather large error bars for the measured BDEs of the thiocarbonyl complexes). The MP2/II values are always larger than their CCSD(T)/n counterparts. The latter values show that the tungsten complexes always have the strongest M-Lbond while, in most cases, the molybdenum species have the lowest BDEs. 

DR. HAIGHT We are now about to tackle the problem of distinguishing between the reactivities of mononuclear and polynuclear molybdenum species. We added mononuclear molybdenum- tin(II) complexes to water and to perchlorate and our evidence right now is that it becomes a cluster very fast when you do this. 

The vapor adsorption of Mo(CO),5 in an EMT zeolite followed by nitration with ammonia under a thermal treatment proved to be an appropriate method to introduce molybdenum oxynitrides into the zeolite, preserving the zeolite crystallinity and giving a homogeneous distribution of the molybdenum species in the zeolite [35]. 

Furthermore, chemical oxidation of [Mo(CO)6] with organic molecules containing acidic hydrogen gives rise to molybdenum species in the +2, -i-3 and +A oxidation states. Higher oxidation states (r-5 and -i-6) have been observed when halogens directly oxidize [Mo(CO)6], A Mo tetranuclear complex has been obtained when hydrochloric acid reacts with the tri-p-hydroxy-dimolybdenum complex, as shown in Equation 9.2 ... 

Equation 9.3 summarizes the evolution of the surface molybdenum species ... 

Related, achiral cc,/ -unsaturated molybdenum-( 2-acyl) complexes, such as 8, have been shown to undergo nucleophilic 1,4-conjugatc addition upon treatment with sodium borohy-dride or methyllithium to generate enolate species, such as 9 (produced by addition of hydride). Subsequent alkylation by iodomethane provides the a-alkylated product 1088. Extension of this tandem Michael addition-alkylation sequence to nonracemic molybdenum species has not yet been reported. 

A reaction that has not apparently been attempted with the analogous uranium and molybdenum species is that43 between polyfluoro aromatics and XeF+WF2, e.g. formation of 7, 8 and 9. 

A) Reduction of a high-valent molybdenum species in presence of dinitrogen, e.g. reaction... 

Molybdenum(ll) dimers display a slightly larger span of metal—metal distances than the quadruply bound rhenium(lll) units. Certain ligand types can be associated with fairly specific metal—metal bond lengths in the molybdenum species. Thus molybdenum quadruple bonds differ in this respect from the... 

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