Molybdenum carbonyl, decomposition

The presence of a 2-substitutent in 3-phenylazirines (17, R —H in Scheme 21) modifies the mode of reaction with molybdenum carbonyl.47 In contrast to pyrazine formation for (17, R =H see Section V,C,2), the alkenyl azirine (18, Scheme 22) is transformed in excellent yield into 2-phenyl-5-carboxy-methylpyrrole. This product probably arises by intramolecular cyclization within an intermediate dienylnitrene intermediate, and related reactions have been devised to synthesize isoxazoles (see Section IV,E,2) and pyrazoles (see Section IV,D,1).47 The molybdenum carbonyl-promoted formation of 2,5-disubstituted pyrroles47 has analogy in uncatalyzed thermal, but not photochemical decomposition of 3-phenyl-2//-azirine 2-acrylate.49... 

Pyrroles among other products are also formed from diiron enneacarbonyl or molybdenum carbonyl-induced decomposition of the Z-ketovinylazirine... 

In 1966 Kemball returned to Scotland to take up the chair of chemistry at the University of Edinburgh. His choice of catalysts became more versatile. With D. A. Whan he studied molybdenum obtained by carbonyl decomposition on alumina. Oxides and zeolites featured strongly among the solids whose catalytic properties he investigated, notably with assistance from... 

Molybdenum Carbonyl Catalysts. The elucidation of the structure and activity of molybdenum carbonyl catalyst systems, the first catalysts reported active for metathesis, continues to be the subject of investigation. Brenner and BurwelE " have identified the dominant surface species formed during the interaction of Mo(CO)6 with 7-alumina at various temperatures. Sub-carbonyl species, Mo(CO)y ads Cy = 3,4, 5, or 6), are formed in the reversible decomposition of Mo(CO)e at temperatures below 100° Activation at 100° C of Mo(CO)6 on partially hydroxylated alumina forms... 

Pdioo-xMox/C nanoparticle catalysts were synthesized by a simultaneous thermal decomposition with palladium acetyla-cetonate, platinum acetylacetonate, and molybdenum carbonyl in o-xylene in the presence of Vulcan XC-72R carbon. At first, the metal precursor and carbon support were mixed together and refluxed and Anally dried, followed by a heat treatment up to 900 °C in H2 atmosphere. ... 

Molybdenum carbide (M02C) by the decomposition of molybdenum carbonyl (Mo(CO)6)l l... 

Molybdenum. Molybdenum is another refractory metal with low resistivity (5-7 iohm-cm) now under investigation for metallization of IC s. It is usually deposited by the decomposition of the carbonyl, Mo(CO)6, or by the hydrogen reduction of the halide (M0CI5 or MoFg). These reactions are described in Ch. 6. 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

Tricarbonyltris(pyridine)molybdenum(0) is a yellow to orange crystalline compound that can be handled in the ambient atmosphere without noticeable decomposition. It can be stored indefinitely at 0-5°C under an inert atmosphere. It decomposes at 205-210°C (a gradual change in color from yellow to brown is already observed starting at 100-105°C). Its IR spectrum shows two carbonyl bands at 1901 and 1764 cm. ... 

The use of highly dispersed catalysts from soluble salts of molybdenum is another approach to the reduction of catalyst amount because of their excellent activity despite their higher price. Recently, metal carbonyl compounds, such as Fe(CO)5, Ru3(CO)i2, and Mo(CO)6 have been investigated as metal cluster catalysts. Preparation involved their deposition and decomposition on catalyst support surfaces (71-73). 

The principal use of the carbonyls is that of obtaining pure metals. The Mond process for refining nickel and the preparation of pure iron for special pui oses, such as magnet cores, involve the formation of a volatile carbonyl, transport of the vapors away from impurities in the original metal, and subsequent decomposition to obtain the pure metal. The carbonyls of chromium, molybdenum, and timgsten have been used in mass spectroscopy to determine the stable isotopes of the respective metals. Nickel carbonyl has been used to obtain metallic mirrors and to coat objects with a thin film of metal. Iron carbonyl has been used as an antiknock agent in gasoline. 

In 2010, Buchmeiser [56] developed a similar system that capitalized on the thermally reversible carboxylation [11] of NHCs (Scheme 31.13, inset). By employing the NHC-CO2 adduct (which essentially is a protected NHC), the reaction conditions did not have to be stringently air- and moisture-free to prevent NHC decomposition. Synthesis of the norbornene-functionalized monomer 37 allowed the molybdenum-catalyzed ROMP with l,4,4a,5,8,8a-hexahydro-l,4,5,8-exo-ewdo-dimethanonaphthalene (a ditopic norbornene) to produce crossHnked polymer 38 with pendant CO2-masked NHCs (Scheme 31.13). Upon heating in the presence of Rh, Ir, or Pd species, the NHC-metal-functionalized polymers 39 were formed and found to contain >20mol% metal, as determined with inductively coupled plasma optical emission spectrometry (ICP-OES). The C02-masked NHC material was found to catalyze the carboxylation of carbonyl compounds and the trimerization of isocyanates upon thermal deprotection (i.e., decarboxylation). Moreover, the NHC-metal-crosslinked materials were found to catalyze Heck reactions, transfer hydrogenations, and also the polymerization of phenylacetylene (M = 8.4 kDa, PDI = 2.45, as determined with GPC in DMF against PS standards). This modular system provides an array of options for catalysis from simple modifications of polymer-supported, C02-masked NHCs. 

The drying in air should not be overdone, as the material is volatile at room temperature and is probably toxic. Loss of carbonyl may be incurred if the ethereal solutions arc allowed to stand a brownish deposit precipitates on standing overnight in the dark decomposition in sunlight is rapid. There is also the possible hazard of violent decomposition if ethereal solutions are allowed to stand for extended periods. The authors have been advised by English workers that solutions of molybdenum hexacarbonyl have detonated on standing. 

Group 6 metal (chromium, molybdenum, tungsten) carbonyls trapped in zeolites have been found to undergo partial thermal decomposition to produce M(CO)3 species which react with phosphines, polyolefins, and arenes to form the expected complexes, e.g. ( / -C6H6)Cr(CO)3 [210]. 

Metals can also be introduced by adsorption of the elemental vapor or melt, for instance in the case of mercury or alkali metals. Adsorption of molecular "precursors such as carbonyls of iron, cobalt, nickel and molybdenum, and subsequent thermal or photochemical decomposition has become an important approach for metals that are difficult to reduce. Other ligands such as alkyls or acetylacetonates have also been used for this purpose. In all these cases, thermal decomposition carries the risk of excessive mobility of the precursors or intermediates such that agglomeration and particle formation at the external surface of the zeolite crystals can occur. Barrer has described the synthesis of salt-bearing zeolites including the famous dry synthesis of ultramarin in 1828, which is sodalite containing intercalated Na-polysulphides. Adsorption of numerous non-ionic and salt species into zeolites was also described, either as such or as precursors for oxides, hydroxides, or metals. 

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