Indenyl effect due to metal slippage? Computational exploration of rhodium-catalyzed acetylene [2+2+2] cyclotrimerization

Abstract

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 precatalyst 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 cocyclotrimerization of different alkynes, is considered. Strong hapticity variations occur in both 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. Caution, wet ligand! An alternative mechanism for acetylene [2+2+2] cyclotrimerization to benzene catalyzed by RhI half-metallocenes is proposed (see figure), based on an extensive computational exploration and electronic structure analyses. The mechanism explains the experimentally observed indenyl effect in terms of a more facile slippage of [RhI] on the indenyl versus cyclopentadienyl ligand