Synthesis of 2-Tetrazenes and the Fragmentation Behavior of Their Ions in Tandem Mass Spectrometry

To improve the understanding of the mass spectrometric behavior and hence facilitate the characterization of new 2-tetrazenes, five such compounds, including one novel compound (1,2-di(1H-pyrrol-1-yl)diazene), were synthesized by using the iodine-mediated oxidative N-N coupling of hydrazines via known or slightly modified literature procedures. Electrospray ionization (ESI) positive-ion mode tandem mass spectrometry experiments based on collision-activated dissociation (CAD) were used to delineate the fragmentation behavior of the five ionized 2-tetrazenes as well as several of their fragment ions. Protonated 1,1,4,4-tetramethyl-2-tetrazene and 1,2-di(piperidin-1-yl)diazene exhibit similar fragmentation patterns wherein a direct heterolytic N-N bond cleavage between the terminal and central nitrogen atoms leads to two major fragment ions. However, the fragmentation behavior of protonated 1,2-di(1H-pyrrol-1-yl)diazene and 1,4-dimethyl-1,4,5,6-tetrahydro-1,2,3,4-tetrazine is very different, involving complex rearrangement reactions leading to the elimination of small stable neutral molecules, such as NH₃, HCN, HC≡CH and/or N₂. 1,4-Dimethyl-1,4-diphenyl-2-tetrazene cannot be protonated upon ESI but instead generated a stable molecular radical cation. Its CAD behavior was quite different from the protonated molecules, as expected. The largest fragment ion was formed via the elimination of a N₂ molecule and all subsequent smaller fragment ions formed from this ion. Quantum chemical calculations revealed the likely mechanisms for some of these reactions.