Transition metal carbyne complex

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Transition metal carbyne complexes are organometallic compounds with a triple bond between carbon and the transition metal. This triple bond consists of a σ-bond and two π-bonds.[1] The HOMO of the carbyne ligand interacts with the LUMO of the metal to create the σ-bond. The two π-bonds are formed when the two HOMO orbitals of the metal back-donate to the LUMO of the carbyne. They are also called metal alkylidynes—the carbon is a carbyne ligand. Such compounds are useful in organic synthesis of alkynes and nitriles. They have been the focus on much fundamental research.[2]

Synthesis[edit]

Transition metal carbyne complexes are most common for the early transition metals, especially niobium, tantalum, molybdenum, tungsten, and rhenium. They can also have low-valence metals as well as high-valence metals.

Protonation of a Re(I) vinylidene complex to give the corresponding cationic Re(V) carbyne derivative.

The first example of a metal carbyne complex was prepared by the Fischer school by treatment of Cr(CO)5(C(OMe)Ph) with boron trichloride:

Cr(CO)5(C(OMe)Ph) + BCl3 → ClCr(CO)4(CPh) + CO + BCl2(OMe)

Many high-valent carbyne complexes have since been prepared, often by dehydrohalogenation of carbene complexes. Alternatively, amino-substituted carbyne ligands sometimes form upon protonation of electron-rich isonitrile complexes. Similarly, O-protonation of μ3-CO ligands in clusters gives hydroxycarbyne complexes. Vinyl ligands have been shown to rearrange into carbyne ligands. Addition of electrophiles to vinylidene ligands also affords carbyne complexes.[2]

Bridging alkylidyne ligands in cluster compounds[edit]

Some metal carbynes dimerize to give dimetallacyclobutadienes. In these complexes, the carbyne ligand serves as a bridging ligand.

Many cluster-bound carbyne complexes are known, typically with CO ligands. These compounds do not feature MC triple bonds; instead the carbyne carbon is tetrahedral. Some of the best known are the tricobalt derivatives, which are prepared by treating cobalt carbonyl with haloforms:[3]

2 HCBr3 + 92 Co2(CO)8 → 2 HCCo3(CO)9 + 18 CO + 3 CoBr2

Structure[edit]

Example of a Fischer and a Schrock carbyne

Monomeric metal carbyne complexes exhibit fairly linear M–C–R linkages according to X-ray crystallography. The M–C distances are typically shorter than the M–C bonds found in metal carbenes. The bond angle is generally between 170° and 180°[4] Analogous to Fischer and Schrock carbenes; Fischer and Schrock carbynes are also known. Fischer carbynes usually have lower oxidation state metals and the ligands are π-accepting/electron-withdrawing ligands. Schrock carbynes on the other hand typically have higher oxidation state metals and electron-donating/anionic ligands. In a Fischer carbyne the C-carbyne exhibits electrophilic behavior while Schrock carbynes display nucleophilic reactivity on the carbyne carbon[5] Carbyne complexes have also been characterized by many methods including infrared Spectroscopy, Raman spectroscopy.[6] Bond lengths, bond angles and structures can be inferred from these and other analytical techniques.

The first Fischer carbyne was isolated in 1973.[7] Two years later in 1975, the first Schrock carbyne was reported.[8]

Metal carbyne complexes also exhibit a large trans effect, where the ligand opposite the carbyne is typically labile.

Reactions and applications[edit]

An example of a transition metal carbyne complex reacting with a nucleophile at the C-carbyne

Metal alkylidyne complexes have mainly been used for specialized reactions in the laboratory, the main used being alkyne metathesis. Triply-bridging carbynes are sometimes prepared by the condensation of terminal carbyne complexes with other metals. Transition metal carbyne complexes usually react with Lewis acids at the C-carbyne. This reaction generally causes them to become transition metal carbene complexes. Depending on the charge of the carbyne complex depends on how well the complex will react with a nucleophile. A cationic carbyne complex will react with a nucleophile right at the C-carbyne, while a nucleophile will not react with the C-carbyne of a transition metal carbyne complex but instead it would react with the metal. This is due to the LUMO of the complexes caused by the electron orbitals of the metal and C-carbyne. Also, the higher the energy of the d-orbitals belonging to an electron-rich metal center the higher the energy of the metal–carbon π-bonds.[9] This improves the conditions for coupling.

An example of a transition metal carbyne complex reacting with an electrophile

Transition metal carbyne complexes can also react with electrophiles. The electrophile reacts with the C-carbyne to form a transition metal carbene complex.

Photooxidation of a transition metal carbyne complex

These complexes can also undergo photochemical reactions. This means that the transition metal carbyne complexes react with light. One example of a photochemical reaction of a carbyne complex is the formation of a cyclopropenyl complex by an addition of an alkyne. This is also known as photooxidation. Generally when a transition metal carbyne complex undergoes photooxidation a new organic ring structure is formed.

There are other photochemical reactions with carbyne complexes as well. Some of these include coupling of the carbyne ligand to a carbonyl, protonation of the carbyne carbon and conversion of the carbyne ligand into a π-allyl.[10]

References[edit]

  1. ^ Kim, Heesook P.; Angelici, Robert J. (1987). "Transition Metal Complexes with Terminal Carbyne Ligands". Adv. Organomet. Chem. 27: 51–111. doi:10.1016/S0065-3055(08)60026-X. 
  2. ^ a b Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2. 
  3. ^ Seyferth, Dietmar; Nestle, Mara O.; Hallgren, John S. (1980). "μ3-Alkylidyne-Tris(Tricarbonylcobalt) Compounds: Organocobalt Cluster Complexes". Inorg. Synth. 20: 224–226. doi:10.1002/9780470132517.ch52. 
  4. ^ Spessard, Gary O.; Miessler, Gary L. Organometallic Chemistry (2nd ed.). pp. 439–449. ISBN 9780199342679. 
  5. ^ Crabtree, R. H. (2014). The Organometallic Chemistry of the Transition Metals (6th ed.). New York, NY: Wiley. pp. 290–315. ISBN 9781118138076. 
  6. ^ Kreißl, F. R. Transition Metal Carbyne Complexes. ISBN 9789401047289. 
  7. ^ Fischer, E. O.; Kreis, G.; Kreiter, C. G.; Muller, J.; Huttner, G.; Lorenz, H. (1973). "trans-Halogeno-alkyl(aryl)carbin-tetracarbonyl-Komplexe von Chrom, Molybdän und Wolfram–Ein neuer Verbindungstyp mit Übergangsmetall-Kohlenstoff-Dreifachbindung" [trans-Halogenoalkyl(aryl)carbynetetracarbonyl complexes of chromium, molybdenum and tungsten–A new type of compound with a transition metal–carbon triple bond]. Angew. Chem. 85 (14): 618–620. doi:10.1002/ange.19730851407. 
  8. ^ Guggenberger, L. J.; Schrock, R. R. (1975). "Tantalum carbyne complex". J. Am. Chem. Soc. 97 (10): 2935–2935. doi:10.1021/ja00843a072. 
  9. ^ Mayr, A.; Bastos, C. M. (1992). "Coupling Reactions of Terminal Two-Faced π Ligands and Related Cleavage Reactions". Prog. Inorg. Chem. 40: 1–98. doi:10.1002/9780470166413.ch1. 
  10. ^ Kingsbury, K. B.; Carter, J. D.; McElwee-White, L. (1990). "Formation of cyclopentenone upon photo-oxidation of the cyclopropyl (c-C3H5) carbyne complex [(η5-C5H5){P(OMe)3}(CO)W≡C(c-C3H5)]". J. Chem. Soc., Chem. Commun. 1990: 624–625. doi:10.1039/C39900000624.