ONIOM study of ring opening and metal insertion reactions with derivatives of C₆₀: Role of aromaticity in the opening process

We have used the integrated, high accuracy ONIOM(G2MS) molecular orbital method to study the problem of ring-opening and metal-insertions into C₆₀. The concerted ring-opening pathway PrOPosed by Rubin et al. has been examined for hexahydro and hexamethyl derivatives of C₆₀. The related open bistriazoline and bislactam C₆₀ derivatives (Angew. Chem. Int. Ed. Engl. 1999, 38, 2360-2363) have also been studied. In the two possible [2+2+2] ring-fragmentation pathways breaking three sets of either five or six-membered rings, the 5-open hexahydro or hexamethylated species are much more favored than the 6-open analogues, the process being slightly endothermic in respect to the closed species. The orifice generated in these two species are likely to be too small even for the smallest metal cation Li⁺, with insertion barriers ranging from 45 to 60 kcal/mol. On the other hand, the Li⁺ insertion barriers for the bistriazoline and bislactam derivatives are less than 20 kcal/mol, indicating that this small metal cation should be able to insert into the cavity of these systems with stabilization energies near -10 kcal/mol. These results suggest that a larger orifice is required for one of the desirable transition or f-row metals to be taken inside.