Towards understanding the stability of the N5+-containing salts: the role of counterions

Abstract

The thermal stabilities of the N₅⁺M⁻ species (M = Sb(OH)₆, Sb(OH)₄F₂, AlF₆, AlF₄, BF₄, B(CF₃)₄, PF₆, AsF₆ and SbF₆) have been studied by means of density functional theory. The present calculations indicate that their thermal stabilities (represented by activation enthalpy, ΔH^≠ in kcal mol⁻¹) decrease in the order N₅⁺ (47.2) > N₅B(CF₃)₄ (34.1) ≈ N₅SbF₆ (31.6) ≈ N₅AsF₆ (31.5) > N₅PF₆ (30.5) > N₅AlF₄ (27.1) ≈ N₅BF₄ (27.1) > N₅AlF₆ (8.5). Only N₅SbF₆ has a small positive reaction enthalpy, Δ_rH. The thermal stability of the N₅⁺ salt depends on the amount of electron transfer from the counterion to N₅⁺ in the reaction. The high electronegativite atoms or groups in the counterion are essential. Another crucial factor is bond strength between the central ion and the ligand. Studies of the N₅Sb(OH)₄F₂ isomers indicate that the OH rather than the fluorine ions in the axial coordination positions could stabilize the decomposition transition structure through partial dissociation of the Sb–OH bond, which is used to explain the reason why there is an obvious difference in the decomposition activation enthalpies of the three isomers.