Indenyl Effect Due to Metal Slippage? In Silico Exploration of Rhodium-Catalyzed Acetylene [2+2+2] Cyclotrimerization

The mechanism of CpRh (Cp = cyclopentadienyl) and IndRh (Ind = indenyl) catalyzed acetylene [2+2+2] cyclotrimerization has been revisited aiming at finding an explanation for the better performance of the latter catalyst found experimentally. The hypothesis that an ancillary ligand of the pre-catalyst remains bonded to the metal center throughout the whole catalytic cycle, based on the experimental evidence that the nature of this ligand can exert some control in cocylotrimerization of different alkynes, has been considered here. Strong hapticity variations occur both in the CpRh- and IndRh-catalyzed processes. As the Ind ligand undergoes a more facile slippage than Cp, the energy profile is far smoother in the IndRh-catalyzed cyclotrimerization. This difference in the energetics of the process translates into an enhanced activity of the IndRh catalyst, in nice agreement with experiment.