(Invited) Conducting the Scholl Reaction: Spironanographenes Vs Helically Arranged Nanographenes

Abstract

The stepwise synthesis of graphene molecular fragments, molecular nanographenes (NGs), allows the design and monodisperse preparation of sophisticated molecules with perfectly defined shapes and properties.1 The introduction of defects to the honeycomb pattern allows the preparation of nanographenes with finely tunable characteristics, as chirality. Helicenes and non-hexagonal rings are the most usual motifs to this end, as they generate axial chirality by diverting the structures from the 2D plane.In 2021 we reported a new family of molecular nanographenes in which the axial chirality results from the helical arrangement of the graphitized moieties covalently connected through a tetrafluorobenzene core 1.2 Although two enantiomers were confirmed by x-ray diffraction and chiral HPLC, racemization was observed.3 The isomerization process was evaluated, and the isomerization barrier resulted of ∆G≠= 24.6 kcal/mol at 40 ºC, which corresponds to a half-life time of t½ = 107 min.In order to modulate both optoelectronic and chiroptical properties we decided to increase the rigidity of the structure by introducing a 9,10-anthracene as central core. However, unexpected spironanographene 2 was obtained during the key step in the synthesis of nanographenes, the Scholl reaction. Under different reaction conditions we were able to obtain spirocompounds with different graphitization degree. Herein we reported the first spirocycles formation under Scholl reaction conditions without interrupting the graphitization process.4 To further understand the formation of the unexpected nanographenes, theoretical calculations were carried out and revealed the formation of a trityl cation that is involved in the formation of the spirocycles.Furthermore, in order to conduct the reaction towards the formation of the expected rigid helically arranged nanographene, an electron deficient core was designed. The substitution of the central anthracene with eight electron withdrawing fluorine atoms destabilizes the trityl cation and the expected helically arranged nanographene 3 was obtained.4 [1] Y. Gu, K. Müllen. J. Am. Chem. Soc. 2022 , 144, 11499–11524.[2] P. Izquierdo-García, J. M. Fernández-García, I. Fernández, J. Perles, N. Martín. J. Am. Chem. Soc. 2021, 143, 11864–11870.[3] J. M. Fernández-García, P. Izquierdo-García, M. Buendía, S. Filippone and N. Martín. Chem. Commun. 2022, 58, 2634-2645.[4] P. Izquierdo-García, J. M. Fernández-García, J. Perles, I. Fernández, N. Martín. Manuscript in preparation. Figure 1