Molybdenum complexes carbyne

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). 

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. 

The molybdenum carbyne complex 24 is the product of the unusual rearrangement, shown in Eq. (23) (66), involving formal a,) -migration of hydrogen in a vinyl ligand. Heating of the cyclopentadienyl complex 23 to 80°C in hexane results in loss of one trimethylphosphite ligand and... 

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... 

However, the new phenoxide-ligated molybdenum carbyne complexes have been used to form unsaturated macrocycles. In particular, the molybdenum catalyst containing phe-noxide ligands catalyzes the reaction in Equation 21.32. ° This reaction occurs in high yield because the macrocycUc product precipitates from solution. 

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... 

Isocyanides are rather more reactive than CO at low-valent molybdenum centres and may react with nucleophiles, e.g. R NH2, to give carbene complexes,la e.g. equation (2), or with electrophiles to give carbyne complexes,1,4 e.g. (3). 

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. 

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. 

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). 

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). 

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. 

Metal alkylidyne complexes undergo a variety of oxidation and reduction reactions as well as redox-induced transformations of the alkylidyne ligands. A method for the direct transformation of Fischer-type carbyne complexes into Schrock-type alkylidyne complexes was developed in our laboratory. Bromine oxidation of the /ra/7, -carbyne bromo tetracarbonyl complexes 49 of molybdenum and tungsten in the presence of dimethox-yethane affords the dme-stabilized alkylidyne tribromo metal complexes 50 [Eq. (42)] (81). For alkyl-substituted complexes (R = Me, CH2CMe3)... 

Irradiation of 69 in chlorinated solvents in the presence of tri-methylphosphine gives rise to the oxidized products 70. The reaction was postulated to proceed via short-lived 17-electron carbyne complex intermediates. Electrochemical evidence for the existence of 17-electron species was reported. In the absence of trimethylphosphine no products could be characterized for the molybdenum phenylcarbyne complex. In the absence of trimethylphosphine the tungsten cyclopropylcarbyne complex affords W( / -C5H5)Cl3(CO)[P(OMe)3] and cyclopentenone [Eq. (59)] (99). 

Recent work by Stone provides additional examples of the photoreactivity of carbyne carbonylmetal complexes. Irradiation of the 2,6-xylylcar-byne molybdenum complex 209 in the presence of triphenylphosphine affords the ketenyl complexes 210, which slowly transform into the tri-phenylphosphine-substituted carbyne complexes 211 [Eq. (165)] (J5). In... 

An actual metathesis reaction of a low-valent carbyne complex was observed by Stone and co-workers 124). The molybdenum neopentyli-dyne complex 247 could be isolated from the reaction of complex 246 with lert-butylphosphanitrile [Eq. (185)]. 

Treatment of the carbyne complexes (// -cyclopentadienyl)(CO)[P(OMe)3]M = CR 1, where M = molybdenum or tungsten and R = cyclopropyl, with hydrogen chloride in diethyl ether results in a rearrangement reaction to give 7 -acyl complexes 2 in quantitative yield. 

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