Determinant Factors of Three-Dimensional Aromaticity in Antiaromatic Cyclophanes

Three-dimensional aromaticity arising from the close stacking of two antiaromatic pi-conjugated macrocycles has recently received considerable attention. Here, a cyclophane consisting of two antiaromatic Ni(II) norcorrole units tethered with two flexible alkyl chains was synthesized. The norcorrole cyclophane showed crystal polymorphism providing three different solid-state structures. Surprisingly, one of them adopted an aligned face-to-face stacking arrangement with negligible displacement along the slipping axis. Although the exchange repulsion between two pi-clouds should be maximized in this orientation, the pi-pi distance is remarkably close (3.258 angstrom). Three-dimensional aromaticity in this conformation has been supported experimentally and theoretically as evidenced by small bond length alternations as well as the presence of a diatropic ring current. An analogous cyclophane with two aromatic Ni(II) porphyrin units was prepared for comparison. The porphyrin cyclophane exhibited a slipped-stacking conformation with a larger displacement (2.9 angstrom) and a larger interplanar distance (3.402 angstrom) without noticeable change of the aromaticity of each porphyrin unit. In solution, the norcorrole cyclophane forms a twist stacking arrangement with effective interplanar orbital overlap and exists in an equilibrium between stacked and nonstacked structures. Thermodynamic parameters of the stacking process were estimated, revealing an inherently large attractive interaction operating between two norcorrole units, which has been further supported by energy decomposition analysis.

Automated summary

The recent focus has been on the three-dimensional aromaticity arising from the close stacking of two antiaromatic pi-conjugated macrocycles. An antiaromatic Ni(II) norcorrole cyclophane was synthesized and exhibited crystal polymorphism with one structure showing an aligned face-to-face stacking arrangement. Experimental and theoretical evidence supported the presence of three-dimensional aromaticity in this conformation. A comparison with an aromatic Ni(II) porphyrin cyclophane revealed larger intermolecular interactions in the norcorrole cyclophane in solution.