Metal alkylidene/alkylidyne

B. The Synthesis and the Chemistry of Metal-Alkylidene and -Alkylidyne Functionalities... 

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. 

Because the allenyl fragment is constructed from C (carbide), CH (alky-lidyne) and CH2 (alkylidene) units, several possible routes to allenyl complexes via C-C coupling reactions can be envisaged for instance, reaction of a metal C fragment (alkylidene, alkylidyne) with a C2 substrate... 

Alkynes insert into both metal-alkylidene and metal-alkylidyne bonds, as shown in reactions (s) and (t), respectively ... 

For a recent survey on the synthesis of high-valent metal-alkylidene and -alkylidyne complexes, see R. R. Schrock, Chem. Rev., 2002,102, 145. 

Besides the usual jt bonding, there are special types of bonding metal-to-carbon double and triple bonds known in metal alkylidene and alkylidyne complexes and these are included in Scheme 1.1. 

Reactions of metal alkylidene and alkylidyne complexes with olefins and alkynes. 

Migratory insertion does not lead to a change in formal oxidation state, unless the inserting ligand Y is an alkylidene, alkylidyne, or isoelectronic ligand bound by a metal-ligand multiple bond. 

Despite their obvious similarity to alkenes and alkynes, transition-metal alkylidene and alkylidyne complexes have not been used as building blocks for the synthesis of lowdimensional materials analogous to polyenes and polyynes. We have begun to explore the syntheses, structures, and properties of conjugated complexes and polymers derived from metal-alkylidyne complexes as part of our effort to develop the chemistry of transition-metal analogues of conjugated organic compounds. 

Extensive studies on the coordination sphere of the metal using Mo-bisdiphenylamido and dipyrrolyl complexes for olefin metathesis revealed how to improve the activity and selectivity of these catalysts [62-68]. This seminal work led to the synthesis of a new generation of highly active olefin metathesis catalysts [69, 70]. Using a similar strategy, various surface-metal alkylidene and alkylidyne complexes have also been tested in alkane metathesis to determine if they can be employed as catalyst precursors [11, 71]. This section describes the structure-activity relationship of different metal-alkyl complexes with oxide surfaces in alkane metathesis. 

The bond between tetrahedral carbon and transition metals mainly exists in the form of terminal metal-alkyl complexes such as [WCCHs) ]. - Metal-alkyl complexes are now numerous. It has been understood that they are stable if the p-elimination path is blocked either because the complexes have 18 valence electrons, - or have less valence electrons, but no p hydrogen. - Complexes of other tetrahedral carbon are also known in which two, three or four metal substituents are metal fragments. They are respectively bridging alkylidene, alkylidyne and carbide complexes, below ... 

Early transition metal, lanthanide and actinide alkoxy and amido complexes are common, and they often are stable because of the interaction between the filled p orbital of the O or N atom of the ligand and an empty d metal orbital. The alkoxy and aryloxy ligands play a crucial role in the catalytic properties of group 5-7 metal-alkylidene and metal-alkylidyne complexes for the metathesis of simple, double and triple bonds. - On the other hand, the behavior of late transition-metal alkoxy and amido complexes is less known. Many of them are stable, however, in spite of the possible repulsion between the filled d orbital and the p orbital of the heteroatom. The metal-heteroatom bonds are robust, and the main characteristic of these is that they are strongly polar and possess a significant ionic character. They exhibit nucleophilic reactivity and sometimes form strong bonds to proton donors (they even deprotonate relatively weak acids). 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

Caulton and Eisenstein have examined the interconversion of metal-alkyl, metal-alkylidene and metal-alkylidyne species using combined experimental and DFT theoretical approaches. This useful strategy has yielded examples of such interconversions. For instance, in the first reaction below, McsSiCFs is a source of nucleophilic CF3 ligand. It is proposed that the transient Ru-CFs bond does form and further rearranges by a-F elimination producing the difluorocarbene-metal-fluoride product. The latter reacts with CO which provokes the reverse reaction, insertion of the carbene into the Ru-F bond leading to the Ru-CFs product. ... 

Alkyne polymerization can be initiated by metal-alkylidenes or metal-alkylidyne complexes. The mechanism involves metallacyclobutene or metallacyclobutadiene key intermediates, in the same way as alkene metathesis and ROM involve metal-lacyclobutane intermediates Katz mechanism, bottom of this page, an extension of the hauvin mechanism for alkyne metathesis. For instance, Schrock s catalyst W - u O - u 3, shown in section. as catalyst of disynunetrical alkyne metathesis, also initiates the polymerization of acetylene and terminal alkynes rather than metathesizing them. Indeed, the molecules of terminal alkynes successively insert into the W- bond of the metallacyclobutadiene intermediate... 

Metal-alkylidene and alkylidyne complexes bonded to silica via silyloxy ligands are also well-defined active alkene and alkyne metathesis catalysts. Recall that Schrock had turned metathesis-inactive alkylidene complexes into active ones by the introduction of alkoxy ligands. In Basset s catalysts, this beneficial role is played by a silyloxy ligand from silica. 15.2,15.31a the catalysts [(SiO)M(=CH-r-Bu)... 

For the purpose of assigning oxidation states, organometallic ligands such as cyclopentadienyl, alkyl, and allyl are often considered to be anionic. Thus, the formal oxidation states of the metals in Ti(Cp), RjTa (R = Bu CH ), and [Ni(allyl)J are Ti, Ta, and Ni + provided the ligands are treated as Cp, R, and. With alkylidene, alkylidyne, and NHC carbenes, the assignment of the oxidation state is a little more complicated. As discussed later, it is best understood in conjunction with the electron counting scheme. 

The synthetic method leading to Nb-alkylidenes and Nb-alkylidynes was particularly successful, due to a quite remarkable difference in the reaction rate of 29 with ketones or aldehydes, vs the subsequent reaction of the alkylidene with ketones and aldehydes (see Scheme 37). The former reaction takes a few minutes at -40°C, while the latter one occurs in hours at room temperature.88 The reaction between 178 and benzaldehyde led to triphenylethylene and the niobyl derivative 184. Due to the difference in reaction rates between a and b in Scheme 37, it was found that the sequential addition of two different ketones or aldehydes to a THF solution of 29 produced a nonsymmetric olefin in a stepwise McMurry-type reaction.84 This is exemplified in the coupling shown in reaction c (Scheme 37). The proposed reaction pathway does not involve the intermediacy of a pinacolato ligand and therefore differs from the mechanism of the McMurry reaction and related reductive couplings at activated metal sites.89... 

Main-group elements X such as monovalent F, divalent O, and trivalent N are expected to form families of transition-metal compounds MX (M—F fluorides, M=0 oxides, M=N nitrides) that are analogous to the corresponding p-block compounds. In this section we wish to compare the geometries and NBO descriptors of transition-metal halides, oxides, and nitrides briefly with the isovalent hydrocarbon species (that is, we compare fluorides with hydrides or alkyls, oxides with alkylidenes, and nitrides with alkylidynes). However, these substitutions also bring in other important electronic variations whose effects will now be considered. 

For a general review of metal-ligand multiple bonds, see W. A. Nugent and J. M. Mayer, Metal-Ligand Multiple Bonds The Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido, Alkylidene, or Alkylidyne Ligands (New York, Wiley, 1988). 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

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