To Be or Not To Be Protonated: cyclo-N₅^- in Crystal and Solvent

Pentazole (HN₅) and its anion ( cyclo-N₅^-) have been elusive for nearly a century because of the unstable N₅ ring. Recently, Zhang et al. reported the first synthesis and characterization of the pentazolate anion cyclo-N₅^- in (N₅)₆(H₃O)₃(NH₄)₄Cl salt at ambient conditions ( Science 2017, 355, 374 ). However, whether the cyclo-N₅^- in (N₅)₆(H₃O)₃(NH₄)₄Cl salt is protonated or not has been debated ( Huang and Xu, Science, 2018, 359, eaao3672 ; Jiang et al. Science, 2018, 359, aas8953 ). Herein, we employed ab initio molecular dynamics (AIMD) simulations, which can well present the dynamic behavior at realistic experimental conditions, to examine the potential protonated state of cyclo-N₅^- in both crystal and dimethyl sulfoxide (DMSO) solvent. Our simulations revealed that the protonation reaction of (N₅)₆(H₃O)₃(NH₄)₄Cl → (N₅)₅(N₅H)(H₂O)(H₃O)₂(NH₄)₄Cl is thermodynamically spontaneous according to Δ G < 0, and the small energy barrier of 12.6 kJ/mol is not enough to prevent the partial protonation of cyclo-N₅^- due to the temperature effect; consequently, both deprotonated and protonated cyclo-N₅^- exist in the crystal. In comparison, the DMSO solvent effect can remarkably reduce the difference of proton affinities among cyclo-N₅^-, H₂O, and NH₃, and the temperature effect can finally break these hydrogen bonds and lead to the deprotonated cyclo-N₅^- in DMSO solvent. Our AIMD simulations reconcile the recent controversy.