Molybdenum system

Catalysts used in hydrotreatment (hydrodesulfurization, HDS) processes are the same as those developed in Germany for coal hydrogenation during World War II. The catalysts should be sulfur-resistant. The cobalt-molybdenum system supported on alumina was found to be an effective catalyst. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

Recent developments have been reported for imido-molybdenum systems. When X = 2,5-dimethylpyrrolyl, more stable surface species can be obtained (although a second minor species, formed by the addition of a surface hydroxyl group to the alkylidene ligand, is also present). Moreover, by tuning the imido ligand, the catalytic activity has been improved dramatically (Table 11.3, bottom rows) [20]. 

Nawa, M., Yamazaki, K., Sekino, T. et al, A new type of nanocomposite in tetragonal zirconia polycrystal-molybdenum system, Mater. Lett., 1994, 20(5-6) 299. 

Polymers of 1,4-dienes are obtained in the presence of titanium, and also with Co, Ni, and Rh, where allyl complexes can be isolated. 1,2-Polybutadiene can be produced in the presence of Pd, which is not generally regarded as a Ziegler catalyst. Chromium and molybdenum systems have also been used. Whereas structural isomerism is controlled by the metal in the catalyst center, the geometric isomerism is determined by the ligands and counterions. 

Alkyne linkage is also displayed by the molybdenum systems. Complex 45 (R1 = R2 = Ph) (138) is obtained in a thermal reaction of 41a (R1 = R2 = Ph), while 45 (R1 = R2 = Et) can be made from 41a (R1 = R2 = Et) both thermally and photochemically (144). Chain extension continues to complexes 46 (Fig. 11) containing four linked alkynes in reactions involving the highly active Me02CC=CC02Me (138,142,146,147). 

The activity of the catalyst systems increases as the dissociation energy of the labile ligand is reduced. The catalytic performance of these systems is comparable with Schrock s highly active yet very sensitive molybdenum system [45]. So even tetrasubstituted olefins are accessible by ring-closing metathesis using these optimized air- and moisture-stable ruthenium systems [46, 47]. 

A major concern is the association of Pu with the Zr-molyb-denum precipitates, which results in relatively high losses of Pu. In an effort to determine the mechanism for the Pu association, several possibilities were explored. Presented here are chemical/ structural considerations of the molybdenum systems, the experimental evidence collected to date on these materials and the initial conclusions that have been reached about them. 

Degras and Lecante (81) in an electron-probe study of the CO-molybdenum system has suggested that the ionic desorption product... 

It is clear that these model systems begin to approach real catalytic systems. They have demonstrated that solid-state Mo NMR spectroscopy can be used effectively to extract structural information from molybdenum systems that have been difficult to investigate by other spectroscopic means. 

A very related molybdenum system [39] Mo(NO)(CO)(PMe3)3H qualitatively revealed the same reaction patterns, but showed an enhanced reactivity. In particular, the reactions with metal carbonyls do not only require shorter reaction times, but also have significantly higher K values of the corresponding insertion equilibria. According to equation 14, the reaction with Fe(CO)3 is completely irreversible in this case, while the reaction with Re2(CO),p is still at equilibrium, however lying much more on the product side as compared to the insertion reaction with the tungsten compound. The value for the equilibrium with Re2(CO)jg could be determined as AH = -47 kJ/mol. 

The formation of bridging nitrogen-hydride ligands as intermediates on route to the reduction of dinitrogen to ammonia and hydrazine has been suggested by many authors. The unique feature of this molybdenum system is the pathway by which the bridge is constructed. Loss of dinitrogen from an intermediate such as D followed by N-N bond homolysis leads eventually to a molybdenum(III) amide F that should be readily converted to ammonia in the presence of acid. In no step is intermolecular electron transfer a requirement. 

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