Theoretical Study of the Acetylene Trimerization with CpCo

A theoretical investigation of the mechanism of the CpCoL 2 (L = CO, PR 3, olefin) catalyzed acetylene cyclotrimerization reaction has been carried out at the ab initio and density functional theory (DFT) levels. The mechanism begins with a stepwise pair of ligand substitution reactions in which precatalyst CpCo(PH 3 ) 2 ( 1 ) is converted, via CpCo(PH 3 )(η 2 -C 2 H 2 ) ( 2 ), to CpCo(η 2 -C 2 H 2 ) 2 ( 3 ) with the liberation of 11.3 kcal/mol at the B3LYP level. Oxidative coupling of the alkyne ligands in 3 to give a cobaltacyclopentadiene complex ( 4 ) is exothermic by 13.1 kcal/mol and is predicted to occur in a facile manner (Δ H ⧧ = 12.8 kcal/mol). Reductive cyclization of the bidentate C 4 H 4 ligand in 4 to generate CpCo(η 4 -cyclobutadiene) ( 8 ) is considerably exothermic (Δ H = −34.0 kcal/mol). However, the least motion pathway that transforms 4 directly into 8 which conserves a mirror plane is found to be symmetry forbidden, implying the presence of a large barrier. Coordination of a third acetylene to 4 results in the formation of CpCo(C 4 H 4 )(η 2 -C 2 H 2 ) ( 5 ). Energetically, this third acetylene is weakly bound (12.4 kcal/mol). This is attributed to the parallel orientation the acetylenic C−C bond vector occupies with respect to the Co−Cp bond axis. Collapse of 5 to CpCo(η 4 -C 6 H 6 ) ( 7 ) occurs in a kinetically very facile process (Δ H ⧧ = 0.5 kcal/mol) reflecting the extremely exothermic nature of this transformation (Δ H = −81.4 kcal/mol). An alternate path converting 5 to 7 via the intermediacy of a cobaltacycloheptatriene complex ( 6 ) was found to be energetically prohibitive due to the symmetry-forbidden nature of the reductive elimination converting 6 to 7 . In addition, a stationary point corresponding to 6 on the B3LYP potential energy surface could not be located. Completion of the catalytic cycle is achieved via a stepwise ligand substitution process in which two acetylene molecules displace the arene in 7 to regenerate 3 with the release of 7.4 kcal/mol at the B3LYP level. Two alternative pathways leading to arene formation in which a phosphine intercepts 4 and remains attached to the Co atom throughout the arene construction process were found to be unlikely mechanistic candidates.