Half-Sandwich Metal-Catalyzed Alkyne [2+2+2] Cycloadditions and the Slippage Span Model.

Half-sandwich Rh compounds display good catalytic activity toward alkyne [2+2+2] cycloadditions. A peculiar structural feature of these catalysts is the coordination of the metal to an aromatic moiety, typically a cyclopentadienyl anion, and, in particular, the possibility to change the bonding mode easily by the metal slipping over this aromatic moiety. Upon modifying the ancillary ligands, or proceeding along the catalytic cycle, hapticity changes can be observed; it varies from η, if the five metal-carbon distances are identical, through η+η, in the presence of allylic distortion, and η, in the case of allylic coordination, to η, if a σ metal-carbon bond forms. In this study, we present the slippage span model, derived with the aim of establishing a relationship between slippage variation during the catalytic cycle, quantified in a novel and rigorous way, and the performance of catalysts in terms of turnover frequency, computed with the energy span model. By collecting and comparing new data and data from the literature, we find that the highest performance is associated with the smallest slippage variation along the cycle.