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Möbius pi-conjugation in cyclic molecules leads to the reversal of Huckel 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 pi-conjugation topologies, planar (T0), Möbius (T1), and twisted Huckel (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-H(2)), which is the preferred structure in dichlorofluoromethane at 150 K, undergoes a series of acid-base reactions involving changes of the pi-conjugation topology. The forms observed in the course of titration involve a Möbius aromatic monocation (T1-H(3)(+)), an antiaromatic twisted Huckel species (T2-H(4)(A)(+)) containing a coordinated carboxylate anion (A = TFA, DCA), and two additional Möbius forms (T1-H(4)(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-H(4)(2+), characterized in the solid state as a TFA salt. The corresponding free base (T0-H(2)) is metastable but can be trapped by addition of triethylamine at low temperatures.
Stępień et al. (Mon,) studied this question.