Group 9 Metallacyclopentadienes as Key Intermediates in [2+2+2] Alkyne Cyclotrimerizations. Insight from Activation Strain Analyses.

The intramolecular oxidative coupling converting a bis-acetylene complex of formula CpM (C2 H2 )2 (Cp=C5 H5- ; M=Co, Rh, Ir) into a 16-electron metallacycle is studied in silico. This reaction is paradigmatic in acetylene [2+2+2] cycloaddition to benzene catalyzed by CpM fragments, being the step with the highest activation energy, and thus affecting the whole catalysis. Our activation strain and quantitative molecular orbital (MO) analyses elucidate the mechanistic details and reveal why cobalt performs better than rhodium and iridium catalysts outlining general principles for rational design of catalysts to be used in these processes.