Carbyne complexes of molybdenum

Dicarbonyl(r -cyclopentadienyI)carbyne Complexes of Molybdenum and Tungsten... 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

Under specific reaction conditions dicarbonyl-(ri -cyclopentadienyl)carbyne complexes of molybdenum and tungsten (R = CH3, C6H5, C6H4CH3) react with trimethylphosphine via substitution... 

If, however, carbonyl(cyclopentadienyl)(trimethylphosphine)carbyne complexes of molybdenum and tungsten are allowed to react with carbon monoxide, carbonylation at the carbyne carbon takes place forming r l-ketenyl complexes in nearly quantitative yields [3,9]. 

Fenske s theoretical studies on cyclopentadienyl-substituted carbyne complexes indicate a remarkable charge on the carbyne carbon atom [25]. This existence of a nucleophilic carbyne carbon becomes evident in the protonation of different carbyne complexes of molybdenum [26], tungsten [27-29]) and osmium [30]. 

DICARBONYL(ti5-CYCLOPENTADIENYL)CARBYNE complexes of MOLYBDENUM AND TUNGSTEN AS BUILDING BLOCKS F. R. KreiBl, J. Ostermeier, W. Schiitt, C. M. Stegmair, N. Ullrich,... 

G. A. McDermott, A. M. Domes, and A. Mayr, Synthesis of Carbyne Complexes of Chromium, Molybdenum, and Tungsten by Formal Oxide Abstraction from Acyl Ligands, Organometallics 6, 925-931 (1987). 

Preparation of the corresponding chlorocarbyne complexes of molybdenum and tungsten was achieved by modification of the classical Fischer synthesis (equation 91), except that the acylmetallate intermediate was isolated as the tetramethylammonium salt, and further reacted with oxalyl chloride in CH2CI2 (equation The carbyne com-... 

CHEMISTRY AND ELECTROCHEMISTRY OF ALKYNE- AND ISOCYANIDE-DERIVED CARBYNE COMPLEXES OF RHENIUM, MOLYBDENUM OR TUNGSTEN... 

When trans-halo-tetracarbonylcarbyne complexes of molybdenum and tungsten, X(CO)4M=CR, are treated with sodium cyclopentadienyl, not only the displacement of the halide, but an additional elimination of two carbonyl ligands is observed, affording dicarbonyl(rj -cyclopenta-dienyl)carbyne complexes [2,3]. A different synthetic approach converts vinylidene ligands into carbyne ligands to yield comparable bisdimethylphosphite substituted complexes of molybdenum [4]. 

Bunz found that molybdenum carbyne complexes of the Schrock-type are highly suitable for this purpose 44 —> 45, 46 (Scheme 11.19) [149]. Alkoxy substituents facilitate the generation of oligomers with a low molecular mass [149]. 

It was noted in Section V,B that the chlorophenyl carbene complex 85 can be prepared by chlorine addition to carbyne complex 80. Treatment of 85 with one equivalent of PhLi does not afford 80, suggesting that the reaction sequence is reduction/substitution rather than substitution/reduc-tion. The recent report (127) of a nucleophilic displacement reaction of the molybdenum chlorocarbyne complex 87 with PhLi to generate phenylcar-byne complex 88 suggests that the intermediacy of the chlorocarbyne complex 86 in the above mechanism is not unreasonable. 

Experimental Procedure 3.1.4. Preparation of a Molybdenum Vinylidene Complex from a Carbyne Complex Tetrabutylanunoniuih Cyano(ethoxycarbonyl) vinylidene (dicarbonyl) hydro-tris(3,5-dimethyl-1 -pyrazblyl)borato molybdenum [526] [37] pp 151 and 188... 

Thermolysis of an allyl-cyclopropyl carbyne complex under the mild conditions used for the acyl carbynes did not result in an oxymetallacycle or a cyclopentenone. Instead, 2-cyclohexenone-5-spirocyclopropane was obtained in low yields (5%) together with moderate yields (43%) of the open-chain (3-allyl V-pentadienal) molybdenum complex (equation 104)163. 

A series of cyclopropyl substituted carbyne complexes was photolyzed in order to follow the regio- and stereochemical outcome of the photooxidation. Irradiation of the deuterated molybdenum complex gave cyclopentenone exclusively labeled at C3 (equation 114). In the analogous tungsten complex only 90% of the deuterium resides on C3 the remaining 10% labeled the 2-position158. 

Deprotonation of some molybdenum-carbyne complexes affords vinylidene derivatives suitable reagents are aryldiazonium salts, trifluoroiodo-methane 34), or n-butyllithium (55) ... 

A recent computational investigation of the [2+2] cycloaddition between a hypothetical molybdenum carbyne complex, CbMo CI I, and ethyne suggests that the formation of the molybdacyclobutadiene, CI3Mo(7/2-C3l l (), is allowed by orbital symmetry. In contrast, the direct formation of the isomeric molybdatetrahedrane (i.e., the if -cyclopropene complex), ChMoft -CsID, is symmetry forbidden <19930M1289> such complexes presumably arise by isomerization subsequent to initial molybdacyclobutadiene formation (Section 2.12.5). 

In the early 1980s, Schrock prepared a series of tungsten- and molybdenum-based carbyne complexes, and demonstrated that they are viable catalysts for performing stoichiometric and catalytic alkyne metathesis [7]. With the defined carbyne complexes, he laid the foundation for the mechanistic understanding of alkyne metathesis, and was the first to demonstrate that vinyl-substituted carbyne complexes are stable [8] and that alkyne metathesis could be performed in the presence of C=C double bonds. 

Scheme 8.13 Preparation of the four-coordinate carbyne tungsten and molybdenum complexes 49 and 50 from tungsten(VI) and molybdenum(VI) compounds as starting materials (M = Li or MgCl)...

E. O. Fischer, G. Kreis, C. G. Kreiter, J. Muller, G. Huttner, and H. Lorenz, trans-Halo[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum and Tungsten. A New Complex Type with Transition Metal-Carbon Triple Bond, Angew. Chem. Int. Ed. Engl. 12, 564-565 (1973). 

Although bis(phosphite) carbyne complex Cp[P(OMe)3]2Mo=C(c-Pr) is incapable of undergoing carbonyl insertion reactions, it adds 1 equivalent of HCl in ether forming the ring-opened f/ -butadiene complex Cp[P(OMe)3](Cl)Mo( / -butadiene) in 15% yield, and P(OMe)3 in equal amounts (equation 108) . Careful analysis of the reaction using two equivalents of HCl reveals the presence of the metal hydride complex Cp[P(OMe)3]2Cl2MoH as the main products (70%), and free butadiene. It was furthermore shown that the two molybdenum complexes are not interconvertible under the reaction conditions and both the yields and products ratio are invariant with temperature in the range of -40 °C to room temperature and the amount of added HCl (1 or 2 equivalents). 

Finally, the octahedral molybdenum-carbyne complex ( j -C5H5)Fe[ 7 -C5H4-C = Mo(CO)4Br] undergoes a one-electron oxidation at E° = -F 0.79 V in DME [34], which is very near to that of the corresponding chromium complex illustrated in Scheme 7-4 and Table 7-5. 

NMR resonances of the alkylidyne carbon atom and selected IR absorptions for new carbyne complexes are given in Table 11. In other studies, the Mo and NMR spectra of molybdenum and tungsten alkylidyne complexes were determined by Enemark and co-workers (23). The metal atoms in the alkylidyne complexes were found to be less shielded than those in nitrido complexes but more shielded than those in compounds containing metal-metal multiple bonds. 

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