Cyclotrimerization and linear oligomerization of phenylacetylene on the nickel(I) monocyclopentadienyl complex CpNi(PPh3)2

The reactions of the CpNi(PPh₃)₂ monocyclopentadienyl complex with phenylacetylene and diphenylacetylene in toluene have been studied by ESR. When an alkyne is in twofold molar excess over nickel, it substitutes rapidly for PPh₃ ligands to form the bisalkyne π complex CpNi(η²-C₂PhR)₂, where R = H or Ph. In the case of phenylacetylene, two structural isomers of the Ni(I) π complex have been identified. Irreversible clustering occurs in the system as time passes. When phenylacetylene is in excess, it oligomerizes actively at ambient temperature. The composition of the oligomerization products depends substantially on the reaction temperature: at T = 20°C, the main product is 1,2,4-triphenylbenzene (97% of the conversion products); at T = 40°C, the main products are linear oligomers with an average molecular weight of 1050. The formation and stabilization of active complexes in the system take place when the substrate is in excess. Phenylacetylene trimerization and linear oligomerization schemes in which the Ni(I) monocyclopentadienyl complex stabilized by substrate molecules is the active species are suggested.