Toward a Rational Design of Half-Sandwich Group 9 Catalysts for [2+2+2] Alkynes Cycloadditions

Filling the gap between molecular structure and reactivity is a well-known challenging task in chemistry. The rational design of catalysts may greatly benefit of computational aid, provided state-of-the-art methodologies are employed. The case of metal catalyzed [2+2+2] cycloaddition of alkynes/alkynes-nitriles to benzene/pyridine is investigated in detail, due to the paramount importance of these reactions for the synthesis of cyclic and polycyclic organic compounds. Catalysts of general formula Cp’M are considered, where Cp’ is the cyclopentadienyl anion or the cyclopentadienyl moiety of larger polycyclic aromatic/heteroaromatic ligands, and M=Co, Rh, Ir. Energy profiles of the whole cycles with a number of intermediates ranging from 5 to 9 connected by the corresponding transition states are computed and the catalyst performance is evaluated based on its turnover frequency (TOF), by implementing the equations of the energy span model. TOF values are related to peculiar structural features of the Cp’M fragment, i.e. to the M-Cp’ bonding mode which results in slippage phenomena during the catalytic cycle. In fact, the metal is never coordinated to the five carbon atoms ring in highly symmetric fashion (eta5), but is slipped and the amount of this distortion changes during the various steps of the catalysis. This fluxionality is found to affect importantly the efficiency of the catalyst.