Three-Level Aromaticity Switching in a Molecular Möbius Band

Stępień, M.; Szyszko, B.; Latos-Grażyński, L.
J. Am. Chem. Soc. 2010, 132, 3140–3152 | view at publisher's site

Möbius π-conjugation in cyclic molecules leads to the reversal of Hückel aromaticity rules and affects the electronic and magnetic properties of these systems. We found the first example of a medium-sized macrocyclic structure that is sufficiently flexible to switch between three distinct π-conjugation topologies, planar (T0), Möbius (T1), and twisted Hückel (T2), without changing its oxidation level. The switching is under thermodynamic and kinetic control and can be realized in a three- or four-step cycle. On titration with trifluoroacetic acid (TFAH) or dichloroacetic acid (DCAH), the Möbius free base (T1-H2), which is the preferred structure in dichlorofluoromethane at 150 K, undergoes a series of acid−base reactions involving changes of the π-conjugation topology. The forms observed in the course of titration involve a Möbius aromatic monocation ([T1-H3]+), an antiaromatic twisted Hückel species ([T2-H4(A)]+) containing a coordinated carboxylate anion (A = TFA, DCA), and two additional Möbius forms ([T1-H4(A)(HA)n]+ (n = 1, 2)), containing complex carboxylate anions. The protonated forms undergo a thermally activated ring planarization to yield an antiaromatic quasi-planar dication [T0-H4]2+, characterized in the solid state as a TFA salt. The corresponding free base (T0-H2) is metastable but can be trapped by addition of triethylamine at low temperatures.