Possible products of the end-on addition of N3- to N5+ and their stability

The recent synthesis of N-5(+)[AsF6](-) suggests the addition of N-5(+) and N-3(-) as a possible route to N-8. Because homoleptic polynitrogen compounds with more than three nitrogen atoms are nearly unknown and generally not very stable, we investigated possible products of this addition reaction. We also computed transition states for various interconversions and dissociative reactions to assess the stability of the addition products. Seven structures are minima at the B3LYP/aug-cc-PVDZ and MBPT(2)/aug-cc-pVDZ levels of theory: Five diazidyldiazenes and two diazidylaminonitrenes or N-3-N-5 complexes. Gibbs free energies based on CCSD(T)/aug-cc-pVDZ single-point calculations strongly suggest that only four of the diazidyldiazene structures are minima at higher levels of theory. We show that CCSD(T) produces good energies for molecules with some multireference character using DIP-STEOM-CCSD. In diazidyldiazenes, the loss of N-2 from one of the azidyl end groups is as likely as the loss of both azidyl groups. Isolation of covalently bonded Ns from N-5(+) and N-3- will be difficult because the most likely product has a decomposition barrier of only 18 kcal/mol. It may not be formed at all, because one of the approach pathways has great potential for mutual neutralization and subsequent fragmentation.