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Compelling evidence has been found for the formation and direct detection of the cyclopentazole anion (cyclo-N5- ) in solution. The anion was prepared from phenylpentazole in two steps: reduction by an alkali metal to form the phenylpentazole radical anion, followed by thermal dissociation to yield cyclo-N5- . The reaction solution was analyzed by HPLC coupled with negative mode mass spectrometry. A signal with m/z 70 was eluted about 2.1 min after injection of the sample. Its identification as N5 was supported by single and double labeling with 15 N, which yielded signals at m/z=71 and 72, respectively, with identical retention times in the HPLC column. MS/MS analysis of the m/z=70 signal revealed a dissociation product with m/z=42, which can be assigned to N3- . To our knowledge this is the first preparation of cyclo-N5- in the bulk. The compound is indefinitely stable at temperatures below -40 °C, and has a half-life of a few minutes at room temperature.
Bazanov et al. (2016) studied this question.