Wydział Chemii

Marcin Stępień

Bartosz Szyszko

Lechosław Latos-Grażyński

2010

Journal of the American Chemical Society

132

3140-3152

10.1021/ja909913y

Naukowa

Angielski

Artykuł

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.

https://doi.org/10.1021/ja909913y

https://www.acs.org/content/acs/en.html