Niobium complexes activation reactions

Finally the ESR spectrum of Nb(7r-allyl)4/alumina was unaffected by the addition of ethylene gas to the ESR sample tube. It is assumed that polyethylene is produced in this process since polymer can be isolated from larger scale reactions under similar conditions. The accepted mechanism for the ethylene growth reaction postulates a steady-state concentration of a a-bonded transition metal-hydrocarbon species which would be expected to modify the ESR spectrum of the supported complex. A possible explanation for the failure to detect a change in the ESR spectrum may be that only a small number of the niobium sites are active for polymerization. Although further experiments are needed to verify this proposition, it is consistent with IR data and radiochemical studies of similar catalyst systems (41, 42, 43). 

Hydrides. According to X-ray and neutron diffraction and metallographic studies of the Nb-H system,524 the H may be considered a lattice gas with phase transitions. In the a-, a -, j6-, and -phases of the system, H occupies tetrahedral interlattice positions. Whereas direct reaction between niobium metal and hydrogen occurs only after repeated activation of the metal by hydrogen absorption at ca. 7 atm and 350 °C, NbH2 is formed at temperatures as low as 22 °C in mixtures of LaNi5H6 7 and Nb.525 The extraordinary catalytic effect of the lanthanum-nickel complex is attributed to the presence of surface-absorbed atomic hydrogen species which are able to diffuse into the niobium lattice. There has been a review of the T a-H system.526... 

Niobium oxide (niobia) is an active catalyst, and can be used as a support for metal nanoparticles or oxides, and it can serve as a promoter in some reactions ([43 5] and references therein). Catalytic applications of niobia include the Fischer-Tropsch synthesis, oxidative dehydrogenation of alkanes, and oxidative coupling of methane. Studies on high-surface-area niobium oxides are complicated by a high degree of complexity because several stable structures (NbO, NbO and Nb O ) exist and the resulting surfaces of high-surface-area niobium oxides are not simple truncations of bulk niobia structures. This is even more so for supported metal oxides when two-dimensional thin films of niobium oxide partially cover a support oxide (Al Oj, SiOj, ZrOj, TiOj, [43]). Nb Oj was also used as a support for V, Cr, Re, Mo, and W oxide overlayers [45, 46]. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Niobium lies directly below vanadium in Group 5 and thus is expected to have high Lewis acidity however, there are few reports on chiral niobium catalysts. This is in contrast to the various asymmetric transformations catalysed by neighbouring Group 4 metal complexes, i.e., titanium and zirconium. In 2005, Kobayashi first reported highly enantioselective niobium Lewis-acid catalysts for the Mannich-type reaction of aldimines 57 with silyl enol ethers 58. To prepare an efficient chiral pentavalent niobium(v) catalyst for the activation of aldimines 57, Kobayashi designed tridentate ligand 56 (Scheme 9.24). 

Decreased activity of alkyne complexes containing 4-6 d electrons in the metallacyclic reaction may be explained by the bonding character of the c orbital (Figure 6.21). Examples are provided by niobium and molybdenum complexes d -d electron... 

A 2000 study from Buffon [69] examined the reaction of Schrock-type, alkoxy-Mo-alkylidenes with the surface OH groups of silica, silica-alumina, and niobium oxide. The specific mode of attachment of the Mo-complex was found to be highly dependent upon the acid-base properties of the support. For silica, it appears to be a Lewis acid-base interaction between the Mo center and surface silanols, whereas in the case of silica-alumina, the attachment appears to occur via the addition of a surface OH group across the Mo-imido bond. In the case of niobium oxide, it is possible that the OH adds across the Mo-alkylidene, deactivating the complex, as the resulting material was completely inactive for metathesis. The activities of both immobilized catalysts were less than the parent homogeneous... 

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