By Karl O. Christe
Polynitrogens have the potential for ultrahigh-performing explosives or propellants because singly or doubly bonded polynitrogens can decom- pose to triply bonded dinitrogen (N2) with an extraordinarily large energy
release. The large energy content and relatively low activation energy toward decomposition makes the synthesis of a stable
polynitrogen allotrope an extraordinary
challenge. Many elements exist in different
forms (allotropes)—for example, carbon
can exist as graphite, diamond, buckyballs,
or graphene. However, no stable neutral allotropes are known for nitrogen, and only
two stable homonuclear polynitrogen ions
had been isolated until now—namely, the
N32 anion (1) and the N51 cation (2). On page
374 of this issue, Zhang et al. (3) report the
synthesis and characterization of the first
stable salt of the cyclo-N52 anion, only the
third stable homonuclear polynitrogen ion
In the periodic table, nitrogen is unusual
because its N–N single-bond energy is much
smaller than one half of its double-bond energy or one third of its triple-bond energy. The
same relation holds for the single and double
bonds of oxygen, but the opposite is true for
carbon and all of the other elements. What
might seem like arithmetic facilitates life on
Earth, because we live in a stable atmosphere
of O2 and N2, but our bodies consist of more
stable polymeric carbon compounds.
Although the existence of gaseous cyclo-
N5– had been already demonstrated in 2002
(4) through electrospray ionization–mass
spectrometry of arylpentazoles, its bulk synthesis had not been achieved. In spite of the
relatively large decomposition energy barrier
of 117 kJ/mol predicted for cyclo-N52 (5), the
cleavage of the strong C–N bond in an aryl-pentazole without breaking the considerably
weaker N–N bonds of the pentazole ring
presents a formidable task. A second complicating factor is the presence of five sterically
active free-valence electron pairs in the equatorial plane of cyclo-N52.
Although cyclo-N52 is isoelectronic with
Recently, progress has been made in the
the well-known and highly stable cyclo-
pentadienide anion C5H52, the equatorial
C–H bonds protect the latter against forming
an in-plane s-bond with the cation and favor
the formation of perpendicular h5-p bonds.
The resulting preservation of the high aroma-
ticity of the ring stabilizes it and gives rise
to its well-known metallocene chemistry. In
contrast, in cyclo-N52, h2-s-bond formation is
energetically favored over h5-p-bond forma-
tion, thus destabilizing the ring by diminish-
ing its aromaticity (see the figure).
area of selective C–N bond cleavage in ar-
ylpentazoles. Haas and co-workers (6) have
demonstrated that reductive cleavage of the
C–N bond in phenylpentazole by using alkali
metals in tetrahydrofuran solution gives rise
to the phenylpentazole radical anion. Ther-
mal dissociation yields cyclo-N52, which was
identified with mass spectrometry. In con-
trast, Zhang et al. have used the more tradi-
tional approach of oxidative dearylation (7)
to cleave the C–N bond in a multisubstituted
arylpentazole using m-chloroperbenzoic acid
and ferrous bisglycinate. The key element of
their work was the stabilization of the result-
ing pentazolate anion in the solid state by
hydrogen bonds from neighboring ammo-
nium and hydronium cations, which allowed
its isolation and determination of its crystal
structure and vibrational, nuclear magnetic
resonance and mass spectra.
Zhang et al.’s work demonstrates that the
cyclo-N52 anion is a stable, aromatic, planar
anion and opens the door to interesting
chemistry. However, much work remains to
be done to exploit this breakthrough. The an-
ion needs to be stabilized as salts with sim-
ple counter cations in the form of the more
stable p-complexes. The reductive cleavage
of the weaker C–N bond in an arylpentazole
radical anion might be better suited than the
oxidative C–N cleavage in an arylpentazole.
The ultimate goal of preparing a stable
allotrope of nitrogen remains an extreme
challenge. The synthesis of an ionic nitrogen
allotrope, such as N51N32 or N51N52, is thermodynamically unfeasible (8) because the
first ionization potential of a neutral polynitrogen radical is always much higher than
its electron affinity, and the lattice energy
resulting from the salt formation is insufficient to make up for this difference. Spontaneous exothermic decomposition of an ionic
allotrope to N2 occurs, as was experimentally
demonstrated already for N51N32 (8).
In fact, in the whole periodic table, not
a single element is known to form an ionic
allotrope. Thus, the synthesis of a stable nitrogen allotrope must be aimed at a neutral
covalent species, such as tetrahedral N4 with
a high predicted barrier of 255 to 263 kJ/
mol (9). Although Eremets and co-workers
have observed a single-bonded cubic form
of polynitrogen at 2000 K and 110 GPa in a
diamond anvil cell (10), this allotrope is not
stable under normal conditions, so the quest
for a stable nitrogen allotrope under ambient
conditions continues. j
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R. Boehler, Nat. Mater. 3, 558 (2004).
Polynitrogen chemistry enters the ring
A cyclo-N5– anion has been synthesized as a stable salt and characterized
Loker Hydrocarbon Research Institute and Department of
Chemistry, University of Southern California, LHI 106 Los
Angeles, CA 90089-1661, USA. Email: email@example.com
Keeping orbitals occupied
When cyclo-N5– encounters a metal cation M+, vacant
orbitals can form s-bonds that destabilize and
potentially break its aromatic ring. In cyclo-C5H5–,
these orbitals are tied up in C–H bonds that preserve
its aromatic nature.