Transition metal alkyne complex
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In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.[1]
Synthesis
Transition metal alkyne complexes are often formed by the displacement of labile ligands by the alkyne. For example, a variety of cobalt-alkyne complexes arise by the reaction of alkynes with dicobalt octacarbonyl.[2]
- Co2(CO)8 + R2C2 → (R2C2)Co2(CO)6 + 2 CO
Many alkyne complexes are produced by reduction of metal halides:[3]
- Cp2TiCl2 + Mg + Me3SiC≡CSiMe3 → Cp2Ti[(CSiMe3)2] + MgCl2
Complexes of dichloroacetylene are produced by in situ reduction of the metal by the alkyne:[4]
- 4 C2Cl2 + 2 WCl6 → [WCl4(C2Cl2)]2 + 2 C2Cl4.
Reactions
Transition metal alkyne complexes participate in many reactions.
Protonation of alkyne ligands gives alkenyl complexes:
- LnM(RC≡CR) + H+ → [LnM−CR=CRH]+
Complexes of terminal alkenes are prone to rearrange to vinylidene derivatives:
- LnM(HC≡CR) → LnM=C=CRH
The Pauson–Khand reaction provides a route to cyclopentenones via the intermediacy of cobalt-alkyne complexes.
Structure and bonding
The coordination of alkynes to transition metals is similar to that of alkenes. The bonding is described by the Dewar–Chatt–Duncanson model. Upon complexation the C-C bond elongates and the alkynyl carbon bends away from 180º. For example, in the phenylpropyne complex Pt(PPh3)2(MeC2Ph), the C-C distance is 1.277(25) vs 1.20 Å for a typical alkyne. The C-C-C angle distorts 40° from linearity upon complexation.[5] Because the bending induced by complexation, strained alkynes such as cycloheptyne and cyclooctyne are stabilized by complexation.[6]
The C≡C vibration of alkynes occurs near 2300 cm−1 in the IR spectrum. This mode shifts upon complexation to around 1800 cm−1, indicating a weakening of the C-C bond.
η2-coordination to a single metal center
When bonded side-on to a single metal atom, an alkyne serves as a dihapto usually two-electron donor. For early metal complexes, e.g., Cp2Ti(C2R2), strong π-backbonding into one of the π* antibonding orbitals of the alkyne is indicated. This complex is described as a metallacyclopropene derivative of Ti(IV). For late transition metal complexes, e.g., Pt(PPh3)2(MeC2Ph), the π-backbonding is less prominent, and the complex is assigned oxidation state 0.[7][8]
In some complexes, the alkyne is classified as a four-electron donor. In these cases, both pairs of pi-electrons donate to the metal. This kind of bonding was first implicated in complexes of the type W(CO)(R2C2)3.[9]
η2, η2-coordination bridging two metal centers
Because alkynes have two π bonds, alkynes can form stable complexes in which they bridge two metal centers. The alkyne donates a total of four electrons, with two electrons donated to each of the metals. And example of a complex with this bonding scheme is η2-diphenylacetylene-(hexacarbonyl)dicobalt(0).[8]
Benzyne complexes
Transition metal benzyne complexes represent a special case of alkyne complexes since the free benzynes are not stable in the absence of the metal.[10]
Applications
Metal alkyne complexes are intermediates in the semihydrogenation of alkynes to alkenes:[11]
- C2R2 + H2 → cis-C2R2H2
This transformation is conducted on a large scale in refineries, which unintentionally produce acetylene during the production of ethylene. It is also useful in the preparation of fine chemicals. Semihydrogenation affords cis alkenes.[12]
Metal-alkyne complexes are also intermediates in the metal-catalyzed trimerization and tetramerizations. Cyclooctatetraene is produced from acetylene via the intermediacy of metal alkyne complexes.
Acrylic acid was once prepared by the hydrocarboxylation of acetylene:[13]
- C2H2 + H2O + CO → H2C=CHCO2H
With the shift away from coal-based (acetylene) to petroleum-based feedstocks (olefins), catalytic reactions with alkynes are not widely practiced industrially.
Polyacetylene has been produced using metal catalysis involving alkyne complexes.
Ti-catalyzed polymerization of acetylene, inspired by Ziegler–Natta catalysis.
Cuprous chloride also catalyzes the dimerization of acetylene to vinylacetylene, once used as a precursor to various polymers such a neoprene. Mechanistic studies suggest that this reaction proceeds by insertion of acetylene into a copper(I) acetylide complex.[14]