A 4,4',4''-Nitrilotriphenoxyl Radical Derived from Yang's Biradical

Abstract

Phenoxyl radicals have attracted considerable attention due to their unique electronic structures and wide-ranging applications in physics, chemistry, biology, and materials science. Herein, we report the synthesis and in-depth characterization of a previously unreported 4,4',4''-nitrilotriphenoxyl radical 3, structurally derived from Yang’s biradical scaffold. Interestingly, electronic structure analyses reveal that 3 is a monoradical with an open-shell doublet ground state, wherein the unpaired electron is delocalized over the three peripheral 2,6-di-tert-butylphenoxyl units. Notably, radical 3 is metastable in solution, reverting to closed-shell hydroxyl precursor 2 with a half-life of ~116 minutes. The hydroxyl 2 can be deprotonated to form anionic 4 which exhibits a closed-shell singlet ground state. Their unique electronic structures are further elucidated by single-crystal X-ray diffraction and assisted by computational methods, in which 3 is fully conjugated and adopts a C3-symmetric geometry while both 2 and 3 exhibit distinct quinoidal character with apparent C2-symmetry. These findings offer fundamental insights into the electronic structures of nitrogen-bridged polyphenoxyl radicals and establish a new design platform for stable organic open-shell systems.