Polynitrogen compounds have received extensive attentions as potential candidates for high energy density materials (HEDMs). Pentazole (HN5), with a five-membered ring composed solely of nitrogen atoms, is expected as promising building blocks for high-performance explosives or propellants. For the whole 20th century the only stable form of N5 was in the form of arylpentazole molecule, and the isolation for cyclo-N5- from the aryl group is difficult for C-N bond is stronger than N-N bond. The first synthesis of pentazolate salt was reported in 2017 through selective cleavage of C-N bond in arylpentazoles. Yet the mechanistic interpretation for this bond-breaking process is not given and the yield for cyclo-N5- is still low. We carried out quantum chemical study for the synthetic mechanism of arylpentazoles and cyclo-N5- based on the DFT and DLPNO-CCSD(T) methods. The complete pathways for the arylpentazole synthesis was theoretically explored. We found three possible intermediates which can interconversion to each other, and only one is crucial for the cyclo-N5 production. The substituent effects for the ring-formation reaction was discussed to minimize the by-product while increase the cyclo-N5 production. Our theoretical calculations for the selective C-N bond cleavage in arylpentazole through treatment of [Fe(Gly)2] and m-CPBA reveal that a direct C-N bond activation by m-CPBA while keeping the pentazole ring intact is kinetically inaccessible. The [Fe(Gly)2] complex is not a stabilizer, but a key ingredient that participates in the bond cleavage mechanism. The high-spin [Fe(Gly)2] can be efficiently oxidized by m-CPBA and leads to the formation of a Fe(IV)-oxo complex [Fe(O)(Gly)2], which was further verified by our UV-vis absorption spectrum and TDDFT calculations. We anticipate that these insights into the C-N bond activation mechanism will be beneficial to the experimental synthesis of cyclo-N5--based energetic materials.

Mechanistic study on the routes to arylpentazoles and pentazolate anion

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