Pentazenium

Within the High Energy Density Matter research program, run by the U.S. Air Force since 1986, systematic attempts to approach polynitrogen compounds began in 1998, when Air Force Research Laboratory at Edwards AFB became interested in researching alternatives to the highly toxic hydrazine-based rocket fuel and simultaneously funded several such proposals. Karl O. Christe, then, a senior investigator at AFRL, chose to attempt building linear N⁺
₅ out of N
₂F⁺
and N⁻
₃, based on the proposed bond structure:[1]

[F−N≡N]⁺ + HN=N=N → [N≡N−N=N=N]⁺ + HF

The reaction succeeded, and N⁺
₅AsF⁻
₆ was created in sufficient quantities to be fully characterized by NMR, IR and Raman spectroscopy in 1999.[2] The salt was highly explosive, but when AsF
₅ was replaced by SbF
₅, a stronger Lewis acid, much more stable N⁺
₅SbF⁻
₆ was produced, shock-resistant and thermally stable up to 60–70 °C. This made bulk quantities, easy handling, and X-ray crystal structure analysis possible.[3]

Reaction of N
₂F⁺
and HN
₃ in dry HF at −78 °C is the only known method so far:

cis-N
₂F
₂ + SbF
₅ → [N
₂F]⁺
[SbF
₆]⁻

[N
₂F]⁺
[SbF
₆]⁻
+ HN
₃ → [N
₅]⁺
[SbF
₆]⁻
+ HF

N⁺
₅ is capable of oxidizing water, NO, NO
₂ and Br
₂, but not Cl
₂ or O
₂; its electron affinity is 10.44 eV (1018.4 kJ/mol). For this reason, N⁺
₅ must be prepared and handled in a dry environment:

4 N⁺
₅AsF⁻
₆ + 2 H
₂O → 4 HF + 4 AsF
₅ + 10 N
₂ + O
₂

2 N⁺
₅SbF⁻
₆ + 2 Br
₂ → 2 Br⁺
₂SbF⁻
₆ + 5 N
₂

Due to stability of the fluoroantimonate, it is used as the precursor for all other known salts, typically accomplished by metathesis reactions in non-aqueous solvents such as HF, SO
₂, CHF
₃, or CH
₃CN, where suitable hexafluoroantimonates are insoluble:

2 N⁺
₅SbF⁻
₆ + A⁺
B⁻
→ N⁺
₅B⁻
+ ASbF
₆

The most stable salts of N⁺
₅ decompose when heated to 50–60 °C: N⁺
₅SbF⁻
₆, N⁺
₅SbF⁻
₅, and N
₅B(CF
₃)
₄, while the most unstable salts that were obtained and studied, N⁺
₅[P(N
₃)⁻
₆] and N⁺
₅[B(N
₃)⁻
₄] were extremely shock and temperature sensitive, exploding in solutions as dilute as 0.5 mmol. A number of salts, such as fluoride, azide, nitrate, or perchlorate, cannot be formed.[1]

In valence bond theory, pentazenium can be described by six resonance structures:

[N≡N⁺−N⁻−N⁺≡N]⁺ ↔ [N⁻=N⁺=N−N⁺≡N]⁺ ↔ [N≡N⁺−N=N⁺=N⁻]⁺ ↔ [N≡N⁺−N⁺≡N⁺−N²⁻]⁺ ↔ [N²⁻−N⁺≡N⁺−N⁺≡N]⁺ ↔ [N⁻=N⁺=N⁺=N⁺=N⁻]⁺,

where the last three pictured have smaller contributions to the overall structure because they have less favorable formal charge states than the first three.[4]

According to both ab initio calculations and the experimental X-ray structure, the cation is planar, symmetric, and approximately V-shaped, with bond angles 111° at the central atom (angle N2–N3–N4) and 168° at the second and fourth atoms (angles N1–N2–N3 and N3–N4–N5). The bond lengths for N1–N2 and N4–N5 are 1.10 Å and the bond lengths N2–N3 and N3–N4 are 1.30 Å.[3]

• Pentazole
• Azide