Transition metal carbene complex

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A transition metal carbene complex is an 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.[dubious ] Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (−CR3) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.

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 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 popularized following Arduengo's isolation of a stable free carbene in 1991.[1] 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

Fischer carbenes are found with:

The chemical bonding (Scheme 1) is based on σ-type electron donation of the filled lone pair orbital of the carbene C atom to an empty metal d-orbital, and π back bonding of a filled metal d-orbital to the empty p-orbital on the carbon atom. An example is the complex (OC)5Cr=C(NR2)Ph.

Fischer carbenes can be likened to ketones, with the carbene carbon atom being electrophilic, much like the carbonyl carbon atom of a ketone. Like ketones, Fischer carbene species can undergo aldol-like reactions. The hydrogen atoms attached to the carbon atom α to the carbene carbon atom 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 the starting material for other reactions such as the Wulff-Dötz reaction.

Schrock carbenes

Structure of (C5H5)2TaCH3(CH2), as determined by X-ray crystallography. The Ta−CH3 and Ta=CH2 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. These complexes are nucleophilic at the carbene carbon atom. Schrock carbenes are typically found with:

  • high oxidation state metal center
  • early transition metals Ti(IV), Ta(V)
  • π-donor ligands
  • hydrogen and alkyl substituents on carbenoid carbon.

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 carbene atom is a nucleophile. Complexes consisting of methylene (CH2) as a ligand are, in a sense, the parents of all Schrock carbene complexes.

Examples include Os(PPh3)2(NO)Cl(=CH2)[4] and (C5H5)2TaCH3(=CH2).[5]

N-Heterocyclic carbenes

IMes is a common NHC ligand.

N-Heterocyclic carbenes (NHCs) are particularly common carbene ligands.[6] 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.[7][8] Being strongly stabilized by π-donating substituents, NHCs are powerful σ-donors but π-bonding with the metal is weak.[9] 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.[10][11]

Bimetallic carbene complexes

An early example of this bonding mode was provided by [C5Me5Mn(CO)2]2(μ−CO) prepared from diazomethane:

2 C5Me5Mn(CO)2(thf) + CH2N2 → [C5Me5Mn(CO)2]2(μ−CH2] + N2 + 2 thf

Another example of this family of compounds is Tebbe's reagent. It features a methylene bridge joining titanium and aluminum.[12]

Applications of carbene complexes

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. In the nucleophilic abstraction reaction, a methyl group can be abstracted from a Fischer carbene for further reaction.

Diazo compounds like methyl phenyldiazoacetate can be used for cyclopropanation or to insert into C-H bonds of organic substrates. These reactions are catalyzed by dirhodium tetraacetate or related chiral derivatives. Such catalysis is assumed to proceed via the intermediacy of carbene complexes.[13][14]

History

The first metal carbene complex, Chugaev's red salt, was not recognized as such until decades after its preparation.[6]

The characterization of (CO)5W(COCH3(Ph)) in the 1960s is often cited as the starting point of the area,[15] although carbenoid ligands had been previously implicated. Ernst Otto Fischer, for this and other achievements in organometalic chemistry, was awarded the 1973 Nobel Prize in Chemistry.[15]

See also

References

  1. ^ Arduengo III AJ, Harlow RL, Kline M (1991). "A stable crystalline carbene". J. Am. Chem. Soc. 113 (1): 361–363. doi:10.1021/ja00001a054.
  2. ^ Hegedus LS, McGuire MA, Schultze LM. "1,3-Dimethyl-3-methoxy-4-phenylazetidinone". Organic Syntheses. 65: 140. doi:10.15227/orgsyn.065.0140.
  3. ^ Crabtree RH (2005). The Organometallic Chemistry of the Transition Metals (4th ed.). New Jersey: Wiley-Interscience. ISBN 978-0-471-66256-3.
  4. ^ Hill, Anthony F.; Roper, Warren R.; Waters, Joyce M.; Wright, Anthony H. (1983). "A mononuclear, low-valent, electron-rich osmium methylene complex". Journal of the American Chemical Society. 105 (18): 5939–5940. doi:10.1021/ja00356a050.
  5. ^ "A neutron diffraction study of bis(cyclopentadienyl)(methyl)(methylene)tantalum(V) at 15 K". Acta Crystallographica Section C Crystal Structure Communications. 44 (3): 439–443. 1988. Bibcode:1988AcCrC..44..439.. doi:10.1107/S0108270187010527. hdl:1808/17672.
  6. ^ a b Hahn FE, Jahnke MC (2008). "Heterocyclic carbenes: synthesis and coordination chemistry". Angewandte Chemie. 47 (17): 3122–72. doi:10.1002/anie.200703883. PMID 18398856.
  7. ^ Aldeco-Perez E, Rosenthal AJ, Donnadieu B, Parameswaran P, Frenking G, Bertrand G (October 2009). "Isolation of a C5-deprotonated imidazolium, a crystalline "abnormal" N-heterocyclic carbene". Science. 326 (5952): 556–9. Bibcode:2009Sci...326..556A. doi:10.1126/science.1178206. PMC 2871154. PMID 19900893.
  8. ^ Arduengo AJ, Goerlich JR, Marshall WJ (2002-05-01). "A stable diaminocarbene". J. Am. Chem. Soc. 117 (44): 11027–11028. doi:10.1021/ja00149a034.
  9. ^ Fillman KL, Przyojski JA, Al-Afyouni MH, Tonzetich ZJ, Neidig ML (February 2015). "N-heterocyclic carbene complexes". Chemical Science. 6 (2): 1178–1188. doi:10.1039/c4sc02791d. PMC 4302958. PMID 25621143.
  10. ^ Przyojski JA, Veggeberg KP, Arman HD, Tonzetich ZJ (2015-09-08). "Mechanistic Studies of Catalytic Carbon–Carbon Cross-Coupling by Well-Defined Iron NHC Complexes". ACS Catalysis. 5 (10): 5938–5946. doi:10.1021/acscatal.5b01445.
  11. ^ Przyojski JA, Arman HD, Tonzetich ZJ (2012-12-18). "NHC Complexes of Cobalt(II) Relevant to Catalytic C–C Coupling Reactions". Organometallics. 32 (3): 723–732. doi:10.1021/om3010756.
  12. ^ Herrmann, W. A. (1982). "The methylene bridge: A Challenge to Synthetic, Mechanistic and Structural Organometallic Chemistry". Pure and Applied Chemistry. 54: 65–82. doi:10.1351/pac198254010065.
  13. ^ Davies, H. M. L.; Morton, D. (2011). "Guiding Principles for Site Selective and Stereoselective Intermolecular C–H Functionalization by Donor/Acceptor Rhodium Carbenes". Chemical Society Reviews. 40 (4): 1857–1869. doi:10.1039/C0CS00217H. PMID 21359404.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ^ Selvaraj, Ramajeyam; Chintala, Srinivasa R.; Taylor, Michael T.; Fox, Joseph M. (2014). "3-Hydroxymethyl-3-phenylcyclopropene". Org. Synth. 91: 322. doi:10.15227/orgsyn.091.0322.
  15. ^ a b Fischer EO, Maasböl A (1964). "On the Existence of a Tungsten Carbonyl Carbene Complex". Angew. Chem. Int. Ed. Engl. 3 (8): 580–581. doi:10.1002/anie.196405801.