Unraveling the Mechanism of <i>cyclo</i>-N₅^– Production through Selective C–N Bond Cleavage of Arylpentazole with Ferrous Bisglycinate and <i>m</i>-Chloroperbenzonic Acid: A Theoretical Perspective

Abstract

Very recently, the bulk synthesis of <i>cyclo</i>-N₅^- from arylpentazole through the treatment with <i>m</i>-chloroperbenzonic acid (<i>m</i>-CPBA) and ferrous bisglycinate ([Fe(Gly)₂]) (Zhang, C., et al. <i>Science</i> <b>2017</b>, <i>355</i>, 374) has greatly promoted the application of pentazolate anion as a novel high-performance energetic material. Yet the mechanism for this reaction is still unexplored. Herein we perform mechanistic studies on the selective C-N bond cleavage in arylpentazole by using density functional theory methods. The direct C-N bond activation by <i>m</i>-CPBA was computed to be kinetically inaccessible. Instead, the oxidation of [Fe(Gly)₂] by <i>m</i>-CPBA is much favorable, which leads to the generation of a high-valent iron(IV)-oxo product. The Fe(IV)-oxo intermediate has been examined by UV-vis absorption spectra experiments and further verified by excited-state calculations. It is found that the Fe(IV)-oxo serves as the key intermediate for the C-N bond activation of arylpentazole and the <i>cyclo</i>-N₅^- generation. Our calculations clarified the key mechanistic details of the <i>cyclo</i>-N₅^- generation, and the factors that affect the production yield are further discussed.