Three-dimensional aromaticity in an antiaromatic cyclophane

Abstract

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems.

Fig. 1

Cyclophanes. a Representative examples of aromatic cyclophanes (AR-cyclophanes) and antiaromatic cyclophanes (AN-cyclophanes). b Theoretical studies on AN-cyclophanes. c Attempted unsuccessful syntheses of AN-cyclophanes

Fig. 2

Norcorroles and a norcorrole cyclophane. a Chemical structures of norcorrole Ni(II) complex 1. b Tethered norcorrole Ni(II) dimer 2 and its dynamic equilibrium. c AN-cyclophane 5 which contains two stacked norcorrole Ni(II) units

Fig. 3

Synthesis and structure of 5. a One-pot synthesis of 5 from 3. b, c Top and side views of the X-ray diffraction structure of 5 (hydrogen atoms are omitted for clarity; thermal ellipsoids are shown at 50% probability)

Fig. 4

Computational results supporting the aromatic nature of AN-cyclophane 5. a, b Two-dimensional NICS plots of 6 and 5 in the xy plane. c, d Top view and side view of the calculated magnetically induced current in the model system 5′ obtained using the GIMIC method. e Schematic visualization of the through-space current channels in 5′

Fig. 5

Interactions between the two antiaromatic norcorrole subunits. a UV/Vis/NIR absorption spectra of 6 (black) and 5 (red) in dichloromethane. b The HOMO–1 of 5 calculated at the CAM-B3LYP/6-31G(d) level of theory. c Decomposition of the total intermolecular interaction energy in kJ mol^–1 of 5′ for the crystal-structure geometry (left) and the crystal-structure geometry without the Ni atoms (right)