Planarization of a bowl-shaped molecule by triple-decker stacking

A bowl-shaped π-conjugated molecule typically undergoes bowl-inversion motion via a planarized transition state (TS), the highest-energy state along the reaction coordinate between two minimum bowl structures. Isolation of such planarized state has been challenging. Here, we report that a platinum(II) complex of antiaromatic porphyrin, norcorrole, adopts bowl-shaped conformation in solution but can become planar in solid. In the crystalline state, the norcorrole Pt(II) complex forms triple-decker stacking structures, in which the planarized norcorrole is stabilized and sandwiched between two bowl-shaped norcorroles. The planar conformation corresponds to the TS structure in the dynamic bowl-to-bowl inversion motion of the bowl-shaped molecule. Detailed theoretical studies have revealed that the intrinsically unstable planar conformation is stabilized by the dispersion force among the substituents and enhanced Pt–Pt interactions created by the antiaromatic macrocyclic ligand.