Triply-Reduced Hexa-peri-hexabenzocoronenes: Solid-State Structures and Magnetic Properties

Discotic polycyclic aromatics have attracted great interest in both fundamental chemistry and material science due to their degenerate frontier orbitals, high molecular symmetry, unique solid-state packing, and ability to bear a certain number of unpaired electrons that can be applied in molecular spintronics and quantum computing. In this work, chemical reduction of hexa-perihexabenzocoronenes, HBC (1) and tBu-HBC (2), as "super benzenes", has been investigated with K metal in THF solution. These reactions readily afforded the triply reduced HBC-based products which have been isolated as single-crystalline materials. The X-ray diffraction study confirmed the formation of pi-complexes [K+(18-crown-6)(THF)2][{K+(18-crown-6)}2(13-)] (3) and [K+(18-crown-6)(THF)2][{K+(18-crown-6)}2(23-)] (4). In 3 and 4, two potassium ions were found to bind the central six membered rings of 13- and 23- with one potassium counterion remaining solvent-separated from the complexes. A notable geometry distortion of the HBC core upon three-electron acquisition and metal binding was revealed in both complexes. The presence of bulky substituents in 2 affected the coordination and crystal packing of otherwise similar building units in 4 vs 3. The magnetic properties of 4 supported the existence of a well-isolated S = 3/2 ground state of the 23- triradical in the solid state.