Molybdenum complexes acetylene

Molecular recognition, calmodulin, 46 447 Mollusks, arsenic in, 44 150, 167, 168, 170 Molten salts electrolysis, 31 11 oxygen activation, 44 328-329 Molybdate ions, tetrahedral, 39 194-195 Molybdenite, 17 108 Molybdenum, 45 1 acetylene complexes of, 4 104 alkoxides... 

Molybdenum(iv) and Tungsten(iv).—A wide variety of molybdenum(iv) complexes dissolved in non-aqueous media have been shown to catalyse the reduction of acetylene by sodium borohydride. The optimum yield was 2.8 moles of C2H2 reduced per g. atom of MoIV, with both C2H4 and C2H6 being formed.265... 

Molybdenum bisalkyne products form directly when Mo(CO)2-(S2CNEt2)2 and excess alkyne are heated in methylene chloride for 18 hours (52), or, more quickly, in refluxing benzene or toluene [Eq. (31)] (87). The parent acetylene complex, Mo(HC=CH)2(S2CNMe2)2, has... 

Alkyne-substituted quinoxalines (10) and pterins have been converted to molybdenum-dithiolenes via Eq. (2) (54) and Eq. (3) (55) by exploiting the reaction of activated acetylenes with molybdenum polysulfide complexes 9, 27, 28). The Mo-S vibration of 11, the final... 

The metallacyclobutadiene formation from acetylene and molybdenum carbyne complex CI3M0CH was studied [45] along a postulated least-motion pathway as shown in Fig. 12. The CI3M0CH was brought together with the acetylene molecule such that the three carbon atoms and the metal center remained coplanar throughout the course of the reaction. It has been found... 

Several reactions of metal-carbene complexes with alkynes leading to five-membered ring compounds have been described. The action of acetylenes on the chromium phenyl(pyrrolidino)carbene complex 472 results in mixtures of indanones 473 and indenes 474297 Terminal alkynes (pent-l-yne or hex-l-yne) react with the molybdenum carbene complex 475 to afford, after oxidative work-up, indanones 476 in contrast, trimethylsilylacetylene gave only the naphthoquinone 477. ... 

Scheme 2.23 Cycloaddition reaction of molybdenum thiolate and acetylene derivative forming molybdenum dithiolene complex.

Very related to the titanium chemistry described above are experiments with resembling molybdenocene complexes. If the molybdenum-diphenyl-acetylene complex Cp2Mo(PhC=CPh) is used as starting material, no C-C linkage reaction takes place with carbon dioxide. Instead, a crystalline solid was isolated whose x-ray proved the formation of a molybdenum-carbon dioxide complex [85] (Equation 15). 

The active site of nitrogenase may involve a cycle between Mo and Mo during catalysis. Reversible binding of acetylene in the molybdenum(iv) complex [OMo(S2CNEt2)2] is a prerequisite to borohydride reduction to ethylene. It is... 

The molybdenum and tungsten acetylene complexes [M(RCCR)3 (CO)] and [W(RCCR)3 (MeCN)] are very interesting. All diphenylacetylene molecules in... 

In attempting to extend this reaction to derivatives of chromium and molybdenum, completely different results were obtained under comparable reaction conditions. Reaction of 3-hexyne with (CH CN) Mo(CO) did not appear to give a compound corresponding to the tungsten acetylenic complex, Hexaethylbenzene isolated from the reaction mixture indicates that the molybdenum complex causes cyclotrimerization of the acetylenic molecule. 

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). 

The formation of 2H-pyrroles (21) and a pyrrole derivative (22) from the reaction of 3-phenyl-2//-azirines and acetylenic esters in the presence of molybdenum hexacarbonyl is intriguing mechanistically (Schemes 24, 25).53 Carbon-nitrogen bond cleavage must occur perhaps via a molybdenum complex (cf. 23 in Scheme 26) but intermediate organometallic species have not yet been isolated.53 Despite the relatively poor yields of 2H-pyrrole products, the process is synthetically valuable since the equivalent uncatalyzed photochemical process produces isomeric 2H-pyrroles from a primary reaction of azirine C—C cleavage54 (Scheme 24). 

Devoto 115)has described an indirect procedure for the determination of 0.1 ppm arsenic in urine. The arsenomolybdic acid complex is formed and extracted from 1 ml of urine at pH 2 into 10 ml of cyclohexanone. The molybdenum in the complex is then measured. Before extracting the arsenic, phosphate in the urine is separated by extracting the phosphomolybdic acid complex at pH 1 into isobutyl acetate. The direct determination of arsenic in biological material and blood and urine is best done using a nitrous oxide-acetylene flame 116>. The background absorption by this flame is low at 1937 A, and interferences are minimized due to the high temperature of the flame. 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

As part of a study of the reactions of metallacyclic y-ketovinyl complexes of molybdenum and tungsten with acetylenes, directed toward the synthesis of complexed -/-lactones, Stone has reported92 the isolation of several vinyl-ketene complexes. When complex 72 was heated with 2-butyne, one molecule of the alkyne was incorporated into the complex with concomitant carbonylation. X-ray analysis of the product (73) has shown unequivocally that the C-l to C-4 vinylketene fragment is bonded in a planar, rj4-configu-ration. In contrast to the thermal reaction, ultraviolet irradiation of 72 or 74 in the presence of 2-butyne affords the complexes 75 and 76, respectively, where the lone carbonyl remaining after alkyne insertion had been replaced by a third molecule of the alkyne. 

Alkynes have two v and two it orbitals that can potentially interact with metal orbitals, and in some instances, it is thought that all of these are involved at the same time in a mononuclear complex. An extended Hiickel calculation on Mofmexo-tetra-p-tolylporphyrinXHC=CH) supports this view (Fig. I5.25).8" Thus both bonding orbitals of the alkyne (6, and at) can donate electron density to molybdenum to form (he 16, and lo, MOs. and both anti bonding orbitals (b2 and a2) can accept electron density to form the lf>2 and Ui2 MOs. Notice that both it bonding orbitals (a, and bt) of acetylene interact significantly with metal d orbitals of the same symmetry. 

A closely related formation of >/3-cydopropenyl molybdenum complex Cp P(OMe)3)2Mo( 3-C3Ph2Me), possibly involving an interamolecular ligand cycloaddition within a carbyne-acetylene intermediate complex, has been accomplished by treating the acetylenic cation complex [Cp f P(OMe)3 2Mo( 2-PhC=CPh)]BF4 with vinylmagne-sium bromide in THF (equation 253)321. 

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