Alkylidene complexes Bridged

Terminal alkylidene complexes generated in this manner can react further with the organometallic substrate when steric interactions are not unfavorable, forming dinuclear species. Herrmann has used this methodology extensively in the preparation of bridging alkylidene complexes (84). 

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). 

Unusual bridging (//-cyclopropyIidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis248. The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(/(-Co)(//-C,H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and mww-vinylidene complexes [CpCoFe](//-CO)(//-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent /z-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

Diazoalkanes are important sources of reactive CR2 fragments in alkylidene chemistry. Bridging alkylidenes are extensively reviewed (116-118). A number of diazoalkanes add across the metal-metal triple bond associated with 2. The reactions are, however, highly complex (as shown by the formation of 30 above), with reaction products heavily dependent on the R substituents of R2CN2 as well as the substituents on the cyclopentadienyl ring. While many interesting products arise in these reactions, the consequence is a minimum ability to predict reaction products. 

Most of these reactions are reversible under either thermal or photochemical conditions. Interestingly, the bridging alkylidene complex (32) is redox active. 

R H, R" = CH CH2 R = H, R = C02Et) to give bridging alkylidene complexes (51). The latter undergo dynamic behaviour in solution. Herrmann et al(31) have prepared a number of related dirhodium complexes with bridging alkylidene groups by reacting either... 

Addition of 2 eq HC1 to (22) in ether produced a thermally unstable bis(alkylidene) complex, which upon addition of excess PMe3 generated binuclear (41). The bridging alkylidyne ligands of (41) exhibited inequivalent Ta—C bond lengths of 2.073(6)-2.099(6) A and 1.922(6)-1.942(7) A, consistent with the different nature of the Ta centers.73,74... 

Titanium derivatives containing carbazole (cb) ligands have been isolated and studied. Reaction of Ti(CH2SiMe3)4 with carbazole yields the alkylidene-bridged dimer [Ti(/i-CHSiMe3)(cb)2]2 (Scheme 106). A singlet at 8 14.75 is observed in the 1H NMR spectrum of the alkylidene complex. The reaction of this compound with 2,6-dimethyl-phenyl isocyanide leads to titanium derivatives containing new carbon-carbon bonds (Section 4.05.2.3).109,110... 

Bridging alkylidene complexes similarly react with alkynes to afford isolatable derivatives arising from the formal insertion into one metal-carbon bond. So, UV irradiation of M2(CO)2(/t-CO)( U-CHCH3)(>j -Cp), in the presence of acetylene results in the fomation of a new carbene and a three-carbon bridging system, XXII ... 

Bu and Ph) are reduced under N2 to generate [ (Pr 2PCH2Sihfc2)2N Ta=CHR]2(fi-N2) in which the dinitrogen is bound in an end-on, bridging mode. 243 Treatrrrent of the alkylidene complex 54 with ZnCl(Np) and ZnNp2 results in the bimetallic species 55.244... 

Fiirstner and coworkers [181] report heterobimetaUic gold(l)-chromium(0) and gold(l)-tungsten(O) complexes bridged by alkylidene ligands. Scheme 11.9. Complexes of both group 6... 

With R = CH2SiMe3, the complexes slowly decompose at ambient temperature, with abstraction of an H atom in ato one of the ligands, giving a bridging alkylidene complex ... 

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