Transition metal alkyne complex

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

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.

PK reaction

Structure and bonding

Structures of various metal-alkyne complexes.

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.^[4] Because the bending induced by complexation, strained alkynes such as cycloheptyne and cyclooctyne are stabilized by complexation.^[5]

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.^[6][7]

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.^[8]

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).^[7]

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.^[9]

Applications

Metal alkyne complexes are intermediates in the semihydrogenation of alkynes to alkenes:^[10]

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.^[11]

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:^[12]

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.^[13]