Electronically Stabilized Nonplanar Phenalenyl Radical and Its Planar Isomer

Stable phenalenyl radicals have great potential as the basis for new materials for applications in the field of molecular electronics. In particular, electronically stabilized phenalenyl species that do not require steric shielding are molecules of fundamental interest, but are notoriously difficult to synthesize. Herein, the synthesis and characterization of two phenalenyl-type cations is reported: planar benzo­[i]­naphtho­[2,1,8-mna]­xanthenium (8⁺) and helical benzo­[a]­naphtho­[8,1,2-jkl]­xanthenium (9⁺), which can be reduced to the corresponding radicals. Radical 9 represents the first stable, helical phenalenyl radical which does not rely on bulky substituents to ensure its stability. Both cations are water-soluble, and the radicals are stable for weeks at room temperature under air. These compounds were characterized crystallographically, and also by NMR, EPR, electrochemistry, and electronic spectra. The synthesis of the previously reported compound benzo­[5,6]­naphthaceno­[1,12,11,10-jklmna]­xanthylium (5⁺), the largest oxygen-containing polycyclic hydrocarbon, was undertaken for comparison with 8⁺ and 9⁺, allowing us to report its crystal structure here for the first time. The different properties of these compounds and their radicals are explained by considering their differing aromaticities using in-depth computational methods.