Non-Equilibrium Formation of the Elusive Dibridged Diboranyl (B2H5) Radical and Boranes in Low-Temperature Diborane Ices

Abstract

Boranes are prototypical electron-deficient species central to the boron chemistry and chemical vapor deposition. Despite extensive studies of diborane (B₂H₆), key reactive intermediates—particularly the monobridged and dibridged diboranyl (B₂H₅) radicals—have remained incompletely characterized because of their high reactivity. Here, we report the experimental identification of the hitherto elusive dibridged diboranyl radical together with its monobridged isomer in low-temperature diborane ices exposed to energetic electron irradiation. The radicals were identified in irradiated diborane and fully deuterated diborane-d₆ ices at 40 K via Fourier transform infrared spectroscopy, revealing the formation of the monobridged radical through B–H bond cleavage, followed by isomerization to the dibridged isomer. Additionally, utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry combined with isotopic labeling experiments, complex boranes ranging from B₆H₁₀ to B₁₂H₂₆ were detected in the gas phase during temperature-programmed desorption. The formation of these increasingly complex boranes is proposed to proceed through sequential boron-insertion reactions involving BH and BH₃ addition coupled with hydrogenation pathways. These findings highlight the critical role of non-equilibrium chemistry in the synthesis of reactive diboranyl radicals and complex boranes in low-temperature ices, providing fundamental insight into boron chemistry under extreme conditions.