Air‐Stable 2,2′‐Azobispyridine Radical–Boron Complexes and Their Near‐Infrared Absorption Properties

Air‐stable nitrogen‐centered radicals are of great interest as building blocks for functional molecular materials. In this study, we developed 2,2′‐azobispyridine radical–boron complexes that exhibit near‐infrared (NIR) absorption. The complexes were synthesized by introducing boron substituents into 2,2′‐azobispyridine frameworks, followed by one‐electron oxidation to generate the corresponding radical species. The B(C 6 F 5 ) 2 derivatives were successfully isolated as air‐ and water‐stable solids, whereas the BF 2 and B( n ‐Bu) 2 analogues could not be obtained. Electron spin resonance (ESR) spectroscopy revealed broad isotropic signals with g ≈ 2.00, indicating that the unpaired electron is delocalized over the 2,2′‐azobispyridine core. Density functional theory (DFT) calculations supported this delocalization and reproduced the observed structural changes, including a N─N bond shortening upon oxidation. Single‐crystal X‐ray diffraction analysis of the methoxy‐substituted complex confirmed these structural features. The radical complexes displayed NIR absorption with λ max values of 800–1140 nm, depending on the substituents. These findings demonstrate that boron complexation effectively stabilizes 2,2′‐azobispyridine radicals and enables precise tuning of their optical properties, providing a promising design principle for NIR functional dyes and radical‐based materials.