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(C₆F₅)₂ derivatives were successfully isolated as air- and water-stable solids, whereas the BF₂ 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.