Journal of the American Chemical Society, Vol.142, No.25, 11022-11031, 2020
Stable Olympicenyl Radicals and Their pi-Dimers
An olympicenyl radical, a spin 1/2 hydrocarbon radical with C-2, symmetry and uneven spin distribution, remains elusive despite the considerable theoretical research interest. Herein, we report syntheses of two air-stable olympicenyl radical derivatives, OR1 and OR2, with half-life times (tau(1/2)) in air-saturated solution of 7 days and 34 days. The high stability was ascribed to kinetic blocking of reactive sites with high spin densities. X-ray crystallographic analysis revealed unique 20- center-2-electron head-to-tail pi-dimer structures with intermo- lecular distances shorter than the sum of van der Waals radius of carbon. The ground state of the pi-dimers was found to be singlet, with singlet-triplet energy gaps estimated to be -2.34 kcal/mol and -3.28 kcal/mol for OR1 and OR2, respectively, by variable-temperature electron spin resonance (ESR) spectroscopy. The monomeric radical species were in equilibrium with the pi-dimer in solution, and the optical and electrochemical properties of the monomers and pi-dimers in solution were investigated by UV-vis-NIR spectroscopy and cyclic voltammetry, revealing a concentration-dependent nature. Theoretical calculations illustrated that upon formation of a pi-dimer the local aromaticity of each monomer was enhanced, and spatial ring current between the monomers was present, which resulted in an increment of aromaticity of the interior of the pi-dimer.