Bridging alkylidene complex

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). 

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... 

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 ... 

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 ... 

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). 

Hartner and Schwartz [24] studied long-chain alkylidene-bridged heterobimetallic complexes and found that the addition of Cp2Zr(H)Cl to neohexenylborane 23 gave the new boron—zirconium complex 24 (Scheme 7.7). 

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. 

Thermolysis of the complexes R2M[N(SiMe3)2]2 (M = Zr, Hf) was found to proceed with double activation of the y-CH bonds leading to a dimeric complex containing a bridging alkylidene function (equation 98).252... 

Hahn, James E., Transition Metal Complexes Containing Bridging Alkylidene... 

Gycloreversion to the vinylalkylidene is rarely observed directly, despite being a commonly proposed step in carbene or alkylidene/alkyne reaction cascades. Bridged titanacyclobutene complex 57, however, clearly resists the formation of a strained internal alkylidene from alkyne extrusion, equilibrating instead with vinylalkylidene complex 58, stabilized by the addition of trimethylphosphine (Equation 20) <2003ICA27>. This system provides a very rare simultaneous observation of interconverting metallacyclobutene and vinylalkylidene isomers. 

R CN (Table 2) [86]. The molecular structure of the 2,2-dimethylpropanonitrile derivative contains unsymmetrically bridging alkylidene amide ligands. Reaction of the yttrium and erbium hydride species with isonitrile results in the formation of a formidoyl moiety (Table 2) [87], Surprisingly the Ln-N interaction is in the range of the nitrile product A similar molecular structure was found in the oximato complex [Cp2Gd(/i-t/2-ONCMe2)]2 (Gd-Nav 2.42(1) A) [88]. 

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. 

Griitzmacher, Jansjorg, see De Bruin, Bas Gultneh, Yilma, see Karlin, Kenneth D. Hahn, James E., Transition Metal Complexes Containing Bridging Alkylidene Ligands 31 205... 

In some cases, hydrogenation of the alkylidenes and alkylidynes reduces the metal-carbon multiple bonds to single bonds. The alkyhdene hgand in (29) is converted to an alkyl gronp when exposed to H2, leading to the formation of an interesting tantalum hthium bridging hydride complex. 

The generation and interconversion of hydrocarbon fragments on metal surfaces is an important aspect of transition metal catalysis. In an effort to model and understand these transformations, much attention has been focused on the synthesis and reactivity of organic species coordinated at polynuclear transition metal centers. Organodiruthenium complexes have provided a particularly rich area of study. The availability of a variety of organometallic derivatives of the bis(T) -cyclopentadienyl)diruthenium carbonyl system has allowed extensive examination of the reactivity of bridging alkylidene, alkylidyne, and ethenylidene ligands. 

Formation of the dimer with the bridging carbonyl occurs in 67% yield, and formation of dimer with a bridging alkylidene occurs in 86% yield for R=H, Me . The addition of dry hydrogen halides (HCl or HBr) yields the following complex ... 

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