Transition metal carbene complex

A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction.

Classification

Metal carbene complexes are often classified into two types. The Fischer carbenes, named after Ernst Otto Fischer, feature strong π-acceptors at the metal and are electrophilic at the carbene carbon atom. Schrock carbenes, named after Richard R. Schrock, are characterized by more nucleophilic carbene carbon centers; these species typically feature higher oxidation state (valency) metals. N-Heterocyclic carbenes (NHCs) were popularized following Arduengo's isolation of a stable free carbene in 1991. Reflecting the growth of the area, carbene complexes are now known with a broad range of different reactivities and diverse substituents. Often it is not possible to classify a carbene complex solely with regards to its electrophilicity or nucleophilicity.

Schrock carbenes

thumb|Structure of , as determined by [[X-ray crystallography. The distances are 2.37 and 2.04 Å, respectively. Color code: blue = Ta, gray = C, white = H.]]

Schrock carbenes do not have π-accepting ligands on the metal centre. They are often called alkylidene complexes. Typically this subset of carbene complexes are found with:

thumb|Orbital interaction in the bonding of a Schrock carbene. Both the metal and carbon provide 2 unpaired electron each, forming the double bond.

Bonding in such complexes can be viewed as the coupling of a triplet state metal and triplet carbene, forming a true double bond. Both the metal and carbon atom donate 2 electrons, one to each bond. Since there is no donation to the carbene atom from adjacent groups, the extent of pi backbonding is much greater, giving a strong double bond. These bonds are weakly polarized towards carbon and therefore the carbene atom is a nucleophile. Furthermore, the major resonance structures of Schrock carbene put the negative charge on the carbon atom, making it nucleophilic. They are popular because they are more readily prepared than Schrock and Fischer carbenes. In fact, many NHCs are isolated as the free ligand, since they are persistent carbenes. Being strongly stabilized by π-donating substituents, NHCs are powerful σ-donors but π-bonding with the metal is weak. For this reason, the bond between the carbon and the metal center is often represented by a single dative bond, whereas Fischer and Schrock carbenes are usually depicted with double bonds to metal. Continuing with this analogy, NHCs are often compared with trialkylphosphine ligands. Like phosphines, NHCs serve as spectator ligands that influence catalysis through a combination of electronic and steric effects, but they do not directly bind substrates.

Bimetallic carbene complexes

An early example of this bonding mode was provided by prepared from diazomethane:

:

Another example of this family of compounds is Tebbe's reagent. It features a methylene bridge joining titanium and aluminum.

Application of Metal Carbenes

Metal carbene complexes have applications in hetereogeneous and homogeneous catalysis, and as reagents for organic reactions.

Catalysis

thumb|262x262px|Catalytic cycle of olefin metathesis. The metal complex alternated between a metallocyclobutane ring and carbene complex, catalyzing the formation of new carbon-carbon double bonds.

The dominant application of metal carbenes involves none of the above classes of compounds, but rather heterogeneous catalysts used for alkene metathesis for the synthesis of higher alkenes. A variety of related reactions are used to interconvert light alkenes, e.g. butenes, propylene, and ethylene. Carbene complexes are invoked as intermediates in the Fischer–Tropsch route to hydrocarbons.

center|thumb|419x419px|Catalytic cycle for the insertion of carbenes into carbon-hydrogen bonds. The metal carbene is generated by nitrogen elimination from the diazo compound, and then inserts into the C-H bond.

Wulff-Dötz Reaction

Fischer carbenes are used with alkynes as the starting reagents for the Wulff–Dötz reaction, forming phenols.

center|thumb|531x531px|General reaction scheme for the Wullf-Dötz reaction, making phenols from Fischer carbene complexes and alkynes.

History

thumb|center|392x392px|The first metal carbene complex, [[Lev Aleksandrovich Chugaev|Chugaev's red salt, was not recognized as such until decades after its preparation.]]

The first metal carbene complex to have been reported was Chugaev's red salt, first synthesized as early as 1925, although it was never identified to be a carbene complex. In 1968, Hans-Werner Wanzlick and Karl Öfele separately reported metal-bonded N-heterocyclic carbenes. The synthesis and characterization of ((CH₃)₃CCH₂)Ta=CHC(CH₃)₃ by Richard R. Schrock in 1974 marked the first metal alkylidene complex. In 1991, Anthony J. Arduengo synthesized and crystallized the first persistent carbene, an NHC with large adamantane alkyl groups, accelerating the field of N-heterocarbene ligands to its current use.