A transition metal carbene complex is a organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (-CR3) and carbynes (≡CR). They feature in many catalytic reactions in the petrochemical industry and are of increasing interest in fine chemicals.
The characterization of (CO)5Cr(COCH3(Ph)) in the 1960s is often cited as the starting point of the area,[1] although carbenoid ligands had been previously implicated.
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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 being 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 valent metals. N-heterocyclic carbenes (NHCs) were popularlized following Arduengo's isolation of a stable free carbene in 1991.[2] 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 with regards to its electrophilicity or nucleophilicity.
Fischer carbenes are found with:
The chemical bonding (scheme 1) is based on electron δ-type donation group of the filled methylene lone pair orbital to an empty metal d-orbital, and pi electron back bonding of a filled metal d-orbital to the empty p-orbital on carbon. An example is the complex (CO)5Cr=C(NR2)Ph.
Fischer carbenes can be likened to ketones, with the carbene carbon being electrophilic, much like the carbonyl carbon of a ketone. Like ketones, Fischer carbene species can undergo Aldol-like reactions. The hydrogen atoms attached to the carbon α to the carbene carbon are acidic, and can be deprotonated by a base such as n-butyllithium, to give a nucleophile which can undergo further reaction.[3]
This carbene is also the starting material for other reactions such as the Wulff-Dötz reaction. These type of carbenes were discovered by E. O. Fischer, and together with other achievements in organometalic chemistry, he was awarded the Nobel prize.[1]
Schrock carbenes do not have π-accepting ligands. These complexes are nucleophilic. Schrock carbenes are typically found with:
Bonding in such complexes can be viewed as the coupling of a triplet state metal and triplet carbene. These bonds are polarized towards carbon and therefore the methylene group is a nucleophile. An example of a Schrock carbene is the compound Ta(=C(H)But)(CH2But)3, with a tantalum(V) center doubly bonded to a neopentylidene ligand as well as three neopentyl ligands. An example of interest in organic synthesis is Tebbe's reagent.
N-heterocyclic carbenes (NHCs) are generally derived from persistent carbenes, which are stable compounds of divalent carbon. Being strongly stabilized by pi-donating substituents, NHCs are good 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 well-established phosphine-based ligands. Like phosphines, NHCs serve spectator ligands that influence catalysis through a combination of electronic and steric effects, but they do not directly bind substrates. Carbenes without a metal ligand have been produced in the lab, promising to reduce costs as required bonds to precious metals are no longer necessary.[4]
The main applications of metal carbenes involves none of the above classes of compounds, but rather heterogeneous catalysts used for alkene metathesis in the Shell higher olefin process. 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. A variety of soluble carbene reagents, especially the Grubbs' and molybdenum-imido catalysts have been applied to laboratory-scale synthesis of natural products and materials science.
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