Phenylpyridine-Based Boron Azides: Tuning Reactivity and Accessing Fluorescent Triazoles

We report the synthesis and reactivity of phenylpyridine-based boron azides readily accessible via nucleophilic substitution from in situ generated borenium-type precursors. Three azides were obtained: a hydridic species (L^NC)BHN₃ (L^NC = 2-phenylpyridine), a cyclopentyl-substituted analogue (L^NC)B(cyclopentyl)N₃, and a boron diazide (L^NC)B(N₃)₂ obtained as a byproduct from the synthesis of (L^NC)BHN₃. The prepared borane azides exhibit notable thermal and photochemical robustness, with decomposition temperatures around 140 °C in mesitylene solution and above 170 °C in the solid state, as evidenced by DSC/TGA analysis. Reactivity studies revealed weak electrophilic character compared to organic azides, evidenced by their reluctance to undergo Staudinger-type reactions and relatively high activation barriers in [3 + 2] cycloadditions, even with activated or strained alkynes. In contrast, reactions with nucleophiles afforded unusual head-to-head azide-bridged species. Attempts to reduce the azide functionality to primary amines generally led to elimination and formation of borinic acid. Computational analysis supported the experimental findings and provided insight into frontier orbital interactions and energy profiles during the [3 + 2] cycloadditions. Selected boron-triazole products displayed UV-A/deep-blue fluorescence, with emission maxima at 372-385 nm in solution and 397-438 nm in the solid state, and high quantum yields up to 0.74 (in dichloromethane solution) and 0.48 (in solid state), suggesting their potential as UV-A or deep-blue-emitting fluorophores.