Polynitrogen chemistry. Synthesis, characterization, and crystal structure of surprisingly stable fluoroantimonate salts of N5+

Article abstract of DOI:10.1021/ja010141g

The new N₅⁺ salt, N₅⁺SbF₆^-, was prepared from N₂F⁺SbF₆^- and HN₃ in anhydrous HF solution. The white solid is surprisingly stable, decomposing only at 70 °C, and is relatively insensitive to impact. Its vibrational spectrum exhibits all nine fundamentals with frequencies that are in excellent agreement with the theoretical calculations for a five-atomic V-shaped ion of C_2v symmetry. The N₅⁺Sb₂F₁₁^- salt was also prepared, and its crystal structure was determined. The geometry previously predicted for free gaseous N₅⁺ from theoretical calculations was confirmed within experimental error. The Sb₂F₁₁^- anions exhibit an unusual geometry with eclipsed SbF₄ groups due to interionic bridging with the N₅⁺ cations. The N₅⁺ cation is a powerful one-electron oxidizer. It readily oxidizes NO, NO₂, and Br₂ but fails to oxidize Cl₂, Xe, or O₂.

Full text of DOI:10.1021/ja010141g

6308
J. Am. Chem. Soc. 2001, 123, 6308-6313
Polynitrogen Chemistry. Synthesis, Characterization, and Crystal
Structure of Surprisingly Stable Fluoroantimonate Salts of N5+
†
†
†
§
†
Ashwani Vij, William W. Wilson, Vandana Vij, Fook S. Tham, Jeffrey A. Sheehy, and
Karl O. Christe*,
†,‡
Contribution from the Propulsion Sciences and AdVanced Concepts DiVision, Air Force Research
Laboratory (AFRL/PRS), Edwards AFB, California 93524, Loker Hydrocarbon Research Institute and
Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089, and
Department of Chemistry, UniVersity of California, RiVerside, California 92521
ReceiVed January 1, 2001. ReVised Manuscript ReceiVed April 13, 2001
Abstract: The new N5+ salt, N5 SbF6 , was prepared from N2F SbF6 and HN3 in anhydrous HF solution.
The white solid is surprisingly stable, decomposing only at 70 °C, and is relatively insensitive to impact. Its
vibrational spectrum exhibits all nine fundamentals with frequencies that are in excellent agreement with the
+
-
+
-
+
-
theoretical calculations for a five-atomic V-shaped ion of C2V symmetry. The N5 Sb2F11 salt was also prepared,
and its crystal structure was determined. The geometry previously predicted for free gaseous N5 from theoretical
calculations was confirmed within experimental error. The Sb2F11 anions exhibit an unusual geometry with
eclipsed SbF4 groups due to interionic bridging with the N5 cations. The N5 cation is a powerful one-
electron oxidizer. It readily oxidizes NO, NO2, and Br2 but fails to oxidize Cl2, Xe, or O2.
+
-
+
+
Introduction
Infrared spectra were recorded on a Mattson Galaxy FT-IR spectrometer
using dry powders pressed between AgCl windows in an Econo press
+
-
The recent discovery of N5 AsF6 as a marginally stable
compound that can be prepared on a macroscopic scale is quite
(Barnes Engineering Co.). Raman spectra were recorded on a Bruker
Equinox 55 FT-RA spectrometer using a Nd-YAG laser at 1064 nm
and Pyrex melting point capillaries as sample containers. The thermal
stabilities were determined using a DuPont model 910 DSC, crimp-
sealed aluminum pans as sample containers, and heating rates of 3 °C/
min. The data were recorded and analyzed with a DuPont model 2000
thermal analyst. Impact sensitivities were measured on an Olin
Mathieson drop weight tester standardized with RDX (hexahydro-1,3,5-
trinitro-1,3,5-triazine, 30 kg‚cm, 50%).
1
+
remarkable. The N5 cation represented only the third readily
-
accessible homonuclear polynitrogen species besides N2 and N3
and as such has received much public acclaim. Since N5AsF6
2
1
is only marginally stable and had given rise to some explosions,
it was of great interest to search for more stable N5+ salts in
order to allow a more thorough characterization of this fascinat-
ing cation and to provide a suitable starting material for the
pursuit of nitrogen allotropes. In this paper, the synthesis and
characterization of surprisingly stable fluoroantimonate salts of
+
-
+
-
The N
2
F SbF
and SbF
described. The HF (Matheson Co.) was dried by storage over BiF
6
and N
2
F Sb
2
F11 starting materials were prepared
from cis-N
2
4-7
F
2
5
in anhydrous HF solution as previously
5
+
+
-
8
N5 and the crystal structure of N5 Sb2F11 are reported.
2
(Ozark Mahoning). The NO and NO (Matheson Co) were purified
by fractional condensation prior to their use. The O
2
and Xe (Matheson
Experimental Section
Co) were used as received. The preparation of HN
3
has previously been
described.1
Caution! HN
3
, azides, and polynitrogen compounds are highly
+SbF -. A Teflon ampule, equipped with a
6
Preparation of N
5
endothermic and can decompose explosiVely. They should be handled
only on a small scale with appropriate safety precautions (face shields,
stainless steel valve and containing a Teflon-coated magnetic stirring
bar, was passivated with ClF . It was attached to the metal vacuum
3
leather gloVes, and protectiVe clothing). Condensation of neat HN
196 °C into Teflon ampules containing oxidizers has repeatedly
resulted in explosions upon condensation or melting of the HN
3
at
line and treated several times with anhydrous HF until no color was
-
observed upon freezing the HF at -196 °C. It was then loaded with
3
.
+
-
6
N
2
F SbF
(4.97 mmol) in the glovebox and attached to the metal
Materials and Apparatus. All reactions were carried out in 0.75-
in.-o.d. Teflon-FEP or -PFA ampules that contained Teflon-coated
magnetic stirring bars and were closed by stainless steel or Teflon
valves. Volatile materials were handled on a stainless steel/Teflon-
vacuum line. The ampule was evacuated and cooled to -196 °C.
Anhydrous HF (∼2 mL) was then condensed into the ampule, and the
contents were allowed to warm to ambient temperature with occasional
+
-
6
stirring. After all the N F SbF had dissolved, the ampule was recooled
2
3
FEP vacuum line. Nonvolatile solids were handled in the dry nitrogen
to -196 °C, and some additional neat HF was condensed onto the upper
atmosphere of a glovebox. HN
3
was generated and handled on a Pyrex
walls of the tube, where the HN was going to be frozen out. The cold
3
glass vacuum line equipped with grease-free Kontes glass/Teflon valves.
ampule was then connected to the glass line, and HN (5.00 mmol)
3
†
Air Force Research Laboratory.
University of Southern California.
University of California, Riverside.
(4) Ruff, J. K. Inorg. Chem. 1966, 5, 1971.
(5) Roesky, H. W.; Glemser, O.; Bormann, D. Chem. Ber. 1966, 99,
1589.
‡
§
(
1) Christe, K. O.; Wilson, W. W.; Sheehy, J. A.; Boatz, J. A. Angew.
Chem., Int. Ed. 1999, 38, 2004.
2) See, for example: Chem. Eng. News 1999, 77, Jan 25, 7; 1999, 77,
Nov 29, 38; 2000, 78, Aug 14, 41.
3) Christe, K. O.; Wilson, W. W.; Schack, C. J.; Wilson, R. D. Inorg.
Synth. 1986, 24, 39.
(6) Pankratov, V. A.; Savenkova, N. I. Zh. Neorg. Khim. 1968, 13, 2610.
(7) Christe, K. O.; Wilson, R. D.; Sawodny, W. J. Mol. Struct. 1971, 8,
245. Christe, K. O.; Wilson, R. D.; Wilson, W. W.; Bau, R.; Sukumar, S.;
Dixon, D. A. J. Am. Chem. Soc. 1991, 113, 1991.
(8) Christe, K. O.; Wilson, W. W.; Schack, C. J. J. Fluorine Chem. 1978,
11, 71.
(
(
1
0.1021/ja010141g CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/09/2001

Surprisingly Stable Fluoroantimonate Salts of N5+
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6309
5 2 11
Table 1. Crystal Data and Structure Refinement for N F
+Sb
-
was added slowly at -196 °C. The reaction mixture was allowed to
warm slowly behind a safety shield to room temperature and kept at
this temperature for about 45 min. The volatile materials were removed
by pumping for several hours at 20 °C, leaving behind a white powder
+Sb
-
identification code
empirical formula
formula weight
temperature
wavelength
crystal system
space group
N
F
5
2 11
F
2
11 5
N Sb
522.55
(
1.502 g, weight calculated for 4.97 mmol of N
was identified by its vibrational spectra as N SbF
This reaction was also carried out by first condensing HN
C into a passivated and preweighed Teflon ampule containing a known
5 6
SbF ) 1.520 g) that
6
213(2) K
0.71073 Å
monoclinic
C2/c
5
.
3
at -196
°
unit cell dimensions
a ) 10.913(8) Å, R ) 90°
amount of HF. The resulting mixture was homogenized at ambient
temperature. The ampule was taken into the glovebox, where a
b ) 12.654(8) Å, â ) 104.715(18)°
+
-
c ) 16.675(11) Å, γ ) 90°
stoichiometric amount of N
2
F SbF
6
was added at -196 °C. The cold
volume
Z
density (calculated)
absorption coefficient
F(000)
crystal size
θ range for data collection
index ranges
2227(3) Å3
8
ampule was attached to the metal vacuum line and evacuated.
Subsequent slow warming of the reaction mixture to room temperature
for about 30 min, followed by removal of all volatile material, resulted
3
3.117 Mg/m
4.995 mm
-1
+
-
in the isolation of N
5
SbF
6
in >99% yield.
and reacting it with N
1888
The safest method of generating HN
3
2
F+SbF
6
-
3
0.26 × 0.10 × 0.05 mm
involved the use of two Teflon-FEP U-tubes that were interconnected
through a porous Teflon filter (Pall Corp) and attached to the metal
2.51-25.35°
-12 e h e 13
-15 e k e 15
-20 e l e 17
9125
vacuum line. The first tube contained a weighed amount of NaN
3
and
. Amounts of
anhydrous HF, sufficient to dissolve both solids, were condensed at
+ -
2 6
the second one a stoichiometric amount of N F SbF
reflections collected
independent reflections
absorption correction
max. and min. transmission
refinement method
2022 [R(int) ) 0.0629]
-
196 °C into both U-tubes, and the solids were dissolved in the HF at
+
-
SADABS
room temperature. The second U-tube, containing the N
2
F SbF
6
0.7883 and 0.3567
full-matrix least-squares on F
2022/0/164
1.122
3
solution, was cooled to -196 °C, and the HN , generated in the first
2
U-tube, together with the excess of HF were co-condensed in a dynamic
vacuum into the second U-tube. The resulting mixture was allowed to
warm slowly to room temperature. Removal of the HF in a dynamic
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R1 ) 0.0678, wR2 ) 0.1913
R1 ) 0.0785, wR2 ) 0.2019
0.00026(18)
+
-
vacuum resulted in the isolation of very pure N
5
SbF
6
in >99% yield.
R indices (all data)
This procedure has been carried out repeatedly on a 5-g scale without
incident.
+Sb 11-. Freshly distilled SbF
Preparation of N F (1.449 mmol)
5 2 5
extinction coefficient
largest diff. peak and hole
-
3
4.329 and -2.102 e.Å
was added in the glovebox to a passivated Teflon-FEP ampule, and
HF (1.9 mL liquid) was added on the metal vacuum line at -196 °C.
The mixture was homogenized at room temperature and taken back
into the glovebox. The ampule was cooled inside the glovebox to -196
was controlled by the SMART9 software package. The unit cell
parameters were determined at -60 °C from three runs of data with
3
0 frames per run, using a scan speed of 30 s per frame. A complete
hemisphere of data was collected, using 1271 frames at 30 s/frame,
including 50 frames that were collected at the beginning and end of
the data collection to monitor crystal decay. Data were integrated using
the SAINT10 software package, and the raw data were corrected for
+ -
5 6
C and opened, and N SbF (1.444 mmol) was added. The resulting
°
mixture was allowed to warm to room temperature, and all volatile
material was pumped off. The white solid residue (758 mg, weight
+
-
calculated for 1.444 mmol of N
vibrational spectroscopy to consist of N
5 2
Sb F11 ) 755 mg) was shown by
11
absorption using the SADABS program. The absence of h + k )
odd and h0l reflections (l ) odd) showed the presence of a C-centered
lattice and a c-glide plane parallel and perpendicular to the b-axis,
respectively, indicating Cc or C2/c as the likely space groups. The
+
-
2 11
F .
5
Sb
with NO, NO , O
experiment, a 0.5-in. Teflon-FEP ampule, that was closed by a Teflon
+
-
Reactions of N
5
SbF
6
2
2
, or Xe. In a typical
+
-
valve, was loaded in the drybox with N
5
SbF
6
(0.53 mmol). On the
2
intensity statistics, E - 1 values, indicated a centrosymmetric space
vacuum line, NO (4.2 mmol) was added at -196 °C, and the contents
of the ampule were allowed to warm slowly with intermittent cooling
to room temperature. After the ampule was kept for 2 h at room
temperature, it was cooled back to -196 °C, and the volatile gas (1.34
group, thereby excluding Cc as a possible space group. The space group
was thus unambiguously assigned as C2/c. The structure was solved
12
by the Patterson method using the SHELXS-97 program and refined
2
13
by the least-squares method on F using SHELXL-97. The initial
Patterson map revealed the position of the two Sb atoms linked by a
fluorine atom. The remaining atoms were located from subsequent
difference electron density maps and finally refined anisotropically by
2
mmol of N ) was measured and pumped off. The unreacted NO was
measured (3.6 mmol) and pumped off at room temperature, leaving
+
-
behind 0.53 mmol of NO SbF
6
that was identified by vibrational
spectroscopy.
2
14
the least-squares method on F using the SHELXTL 5.1 software for
Windows NT. The crystal did not show any significant decomposition
during the data collection. The experimental and refinement parameters
are listed in Table 1.
-
In a similar manner, N
with NO and Br
or O
Crystal Structure Determination of N
anhydrous SO was condensed onto 0.200 g of N
5
+SbF
6
was found to react quantitatively
2
2 2
, but no reaction was observed with either Cl , Xe,
2
.
+Sb
F
11-. About 1 mL of
SbF at -196 °C in
5
2
2
5
6
Results and Discussion
a 0.5-in.-o.d. sapphire tube (Tyco Corp.) closed by a stainless steel
+
-
valve. The contents of the tube were warmed to -78 °C, causing all
Synthesis and Properties of N5 SbF6 . The synthesis of
+
-
of the N
SO
5
SbF
6
to dissolve and form a pale yellowish solution. Anhydrous
N5 SbF6 was carried out in the same manner as previously
reported for N AsF by reacting N F SbF with HN in
5 6 2 6 3
1
+
-
+
-
2
ClF (∼1.5 mL) was then slowly condensed onto this solution under
vacuum. The solvents were then slowly removed under a static vacuum
at -64 °C over a period of ∼16 h, leaving behind platelike colorless
crystals. These crystals were extremely reactive to perfluoropolyether
oil and showed an instantaneous evolution of nitrogen gas. The majority
of the crystals were very soft and difficult to handle, but a few crystals
appeared to exhibit a different habit and better mechanical strength.
One of these crystals was immersed in halocarbon grease and mounted
on the goniometer head using a precentered Nylon Cryoloop equipped
with a magnetic base. The structure of the salt was determined using
a Bruker diffractometer equipped with a CCD detector and a low-
temperature, LT3, device. The three-circle platform with a fixed c-axis
(
9) SMART V 4.045, Software for the CCD Detector System, Bruker
AXS, Madison, WI, 1999.
10) SAINT V 4.035, Software for the CCD Detector System, Bruker
(
AXS, Madison, WI, 1999.
(11) SADABS, Program for absorption correction for area detectors,
Version 2.01, Bruker AXS, Madison, WI, 2000.
(
12) Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal
Structure, University of G o¨ ttingen, Germany, 1997.
13) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
(
Crystal Structure, University of G o¨ ttingen, Germany, 1997.
(14) SHELXTL 5.1 for Windows NT, Program library for Structure
Solution and Molecular Graphics, Bruker AXS, Madison, WI, 1997.

6310 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Vij et al.
anhydrous HF solution at -78 °C, followed by removal of the
and experimentally unknown lattice energy values for N5SbF6
+
-
16
volatile products at room temperature. The yield of N5 SbF6
and NO2SbF6 were estimated using the method of Jenkins and
Passmore based on Bartlett’s volume-based relationship.17 It
should be kept in mind, however, that the structure of N5(g) is
unknown and that the final reaction product is 2.5 mol of N2.
The conversion of N5 to N2 is also strongly exothermic, as is
HF
+
-
+
5
-
6
N F SbF + HN3 -78 to 25 °C8 N SbF + HF (1)
2
6
is essentially quantitative, and the product purity is high. It is
essential that the reaction system is completely anhydrous, as
water hydrolyzes the N5+ salt, generating free SbF5, which in
combination with HF protonates HN3 under formation of
1
+
apparent from the calculated heat of formation of gaseous N5
351.1 kcal/mol) and the above-derived minimum electron
(
+
affinity value of N5 (211 kcal/mol).
+
- 15
H2N3 SbF6 .
+
-
The N5 SbF6 salt is a colorless hygroscopic solid that is
stable at ambient temperature and, based on the DSC data, starts
to decompose at 70 °C. It is surprisingly insensitive to impact.
Even at the maximum setting of our apparatus (200 kg‚cm),
only partial thermal decomposition due to adiabatic heating of
the sample was observed, but no explosions. The salt is soluble
in and compatible with HF, SO2, and CHF3.
The above results show that N5+ is a weaker oxidizer than
+ 18
+
either PtF6 that can oxidize O2 to O2
or O2 that can oxidize
+
19,20
Xe to Xe2 under similar conditions.
Despite the first
ionization potential (IP) of N2 (15.51 eV) being 3.44 eV higher
+
-
The oxidizing properties of N5 SbF6 were examined in the
solid state and in HF solution by treating it with 2 atm of either
oxygen or xenon gas between -78 °C and ambient temperature.
+
than that of O2 (12.07 eV), the electron affinity of N5 is
+
+
substantially lower than that of O2 because in N5 the positive
charge is spread over a larger number of atoms, thereby
+
+
No oxidations to O2 and Xe2 , respectively, were observed
+
decreasing its oxidizing power. Nevertheless, the fact that N5
+
-
under these conditions. Furthermore, solid N5 SbF6 did not
oxidize liquid or gaseous chlorine, but is capable of oxidizing
NO (2), NO2 (3), and Br2.
can quantitatively oxidize either NO, NO2, or Br2 renders it a
very strong one-electron oxidizer. Although it is not quite as
+
powerful as PtF6 or O2 salts, it offers the great advantage of
not acting as a fluorinating or oxygenating agent, which can be
a very important consideration when dealing with substrates that
are easily fluorinated or oxygenated.
excess NO
+
-
6
+
-
NO + N SbF
8 NO SbF6 + 2.5N2 (2)
196 to 25 °C
5
-
Ongoing studies in our laboratory show that the potential
hazards of handling neat HN3 in the synthesis of N5 can be
and
+
excess NO2
avoided by either replacing HN3 with the insensitive (CH3)3-
SiN3 or generating the desired HN3 from a weighed amount of
NaN3 and excess HF in a separate ampule and transferring all
volatiles into the reaction vessel containing an HF solution of
+
-
6
+
-
NO + N SbF
8 NO SbF + 2.5N2 (3)
2 6
2
5
-196 to 25 °C
Since NO, NO2, Br2, Cl2, O2, and Xe have first ionization
potentials of 9.26, 9.75, 10.52, 11.48, 12.07, and 12.13 eV,
respectively, these experiments indicate that the N5+ cation can
oxidize substrates having a first ionization potential of 10.52
eV or less, but cannot oxidize substrates with a first ionization
potential of 11.48 eV or more. When solid starting materials or
reaction products are involved in these redox reactions, the first
ionization potential of the substrate does not equal the electron
affinity of the oxidizing agent, as any lattice energy changes
must also be taken into account. For the calculation of the
electron affinity, the appropriate Born-Haber cycle must be
used, as shown in kilocalories per mole for reaction (3).
+
-
N2F SbF6 . The reactions with (CH3)3SiN3 are carried out in
+
either HF or SO2 solution and produce N5 in high yield. When
HF is used as the solvent, the first reaction step most certainly
involves the formation of (CH3)3SiF and HN3; i.e., HN3 is
generated in situ in the reactor.
+
-
Synthesis and Properties of N5 Sb2F11 . To preclude a
-
potential side reaction of Sb2F11 with HF and HN3 to give
-
+
-
+
-
SbF6 and H2N3 SbF6 , a sample of N5 SbF6 was reacted
with an equimolar amount of SbF5 in HF solution at room
+
-
temperature. The resulting N5 Sb2F11 salt is a colorless solid
HF
-
+
-
N SbF + SbF5 25 °C8 N Sb F
(4)
5
6
5
2 11
that is stable at room temperature and undergoes, according to
its DSC data, thermal decomposition at 70 °C; i.e., its thermal
+
-
stability is comparable to that of N5 SbF6 , but, in contrast to
+
-
N5 SbF6 , it undergoes a reversible endotherm (melting) at
about 30 °C. Consequently, the replacement of SbF6 by Sb2F11
-
-
did not result in increased thermal stability and does not appear
to offer any significant advantages for studying the reaction
+
chemistry of N5 salts.
Crystal Structure of N5 Sb2F11 . The structure of N5 Sb2F11
+
-
+
-
is shown in Figures 1 and 2, and the important bond lengths
For the redox reaction to proceed, the enthalpy change, or more
precisely the free energy change if entropy changes are included,
of the reaction must be negative. Assuming ∆Hr to be zero, the
electron affinity of N5 (g) is calculated to have a minimum value
of 211 kcal/mol or 9.16 eV, a value significantly lower than
the first ionization potential of NO2 (9.75 eV). The required
(16) Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg.
Chem. 1999, 38, 3609.
(17) Mallouk, T. E.; Rosenthal, G. L.; Muller, G.; Busasco, R.; Bartlett,
+
N. Inorg. Chem. 1984, 23, 3167.
(18) Bartlett, N.; Lohmann, D. H. Proc. R. Chem. Soc. (London) 1962,
277; J. Chem. Soc. 1962, 5253.
(19) Stein, L.; Norris, J. R.; Downs, A. J.; Minihan, A. R. J. Chem. Soc.,
(
15) Christe, K. O.; Wilson, W. W.; Dixon, D. A.; Khan, S. I.; Bau, R.;
Chem. Commun. 1978, 502.
(20) Stein, L.; Henderson, W. W. J. Am. Chem. Soc. 1980, 102, 2856.
Metzenthin, T.; Lu, R. J. Am. Chem. Soc. 1993, 115, 1836.

Surprisingly Stable Fluoroantimonate Salts of N5+
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6311
+Sb
F
5 2 11
-
Table 2. Bond Lengths and Angles for N
Bond Lengths (Å)
Sb(1)-F(5)
Sb(1)-F(4)
Sb(1)-F(2)
Sb(1)-F(3)
Sb(1)-F(1)
Sb(1)-F(6)
Sb(2)-F(10)
Sb(2)-F(7)
1.839(8)
1.845(8)
1.851(9)
1.854(9)
1.856(10)
2.031(7)
1.844(7)
1.849(9)
Sb(2)-F(9)
Sb(2)-F(8)
1.849(10)
1.866(8)
1.866(8)
2.007(7)
1.102(19)
1.303(19)
1.295(19)
1.107(19)
Sb(2)-F(11)
Sb(2)-F(6)
N(1)-N(2)
N(2)-N(3)
N(3)-N(4)
N(4)-N(5)
Bond Angles (deg)
F(5)-Sb(1)-F(2)
F(4)-Sb(1)-F(2)
F(5)-Sb(1)-F(3)
F(4)-Sb(1)-F(3)
F(2)-Sb(1)-F(3)
F(5)-Sb(1)-F(1)
F(4)-Sb(1)-F(1)
F(2)-Sb(1)-F(1)
F(3)-Sb(1)-F(1)
F(5)-Sb(1)-F(6)
F(4)-Sb(1)-F(6)
F(2)-Sb(1)-F(6)
F(3)-Sb(1)-F(6)
F(1)-Sb(1)-F(6)
F(10)-Sb(2)-F(7)
F(10)-Sb(2)-F(9)
94.8(4) F(7)-Sb(2)-F(9)
169.9(4) F(10)-Sb(2)-F(8)
95.4(5) F(7)-Sb(2)-F(8)
89.7(4) F(9)-Sb(2)-F(8)
89.3(5) F(10)-Sb(2)-F(11)
93.6(5) F(7)-Sb(2)-F(11)
91.0(5) F(9)-Sb(2)-F(11)
88.4(5) F(8)-Sb(2)-F(11)
170.8(4) F(10)-Sb(2)-F(6)
179.9(5) F(7)-Sb(2)-F(6)
84.7(4) F(9)-Sb(2)-F(6)
85.1(4) F(8)-Sb(2)-F(6)
84.5(4) F(11)-Sb(2)-F(6)
86.4(4) Sb(2)-F(6)-Sb(1)
91.2(5) N(1)-N(2)-N(3)
90.8(5) N(4)-N(3)-N(2)
170.7(5)
171.3(4)
87.9(4)
88.8(5)
94.8(4)
96.7(5)
92.2(5)
93.9(4)
85.0(3)
85.9(4)
85.3(5)
86.3(4)
177.5(4)
155.0(4)
168.1(15)
111.2(11)
5 2
Figure 1. ORTEP diagram of N F
+Sb 11-, showing the thermal
ellipsoids at the 30% probability level and the close-range N‚‚‚F contacts
within the crystal lattice.
The observed geometry supports the previously given ratio-
1
+
nale that the exceptional stability of N5 is largely due to
resonance stabilization, resulting in relatively high bond orders
+
for all the bonds. The bonding in N5 can be described by the
following six resonance structures:
+
2
F
11-, showing the close
Figure 2. Space-filling representation of N
5
Sb
+
-
packing of the N
5
cation within the Sb
2
F
11 cavity.
and angles are summarized in Table 2. The observed V-shaped
geometry of the N5+ cation is in excellent agreement with the
1
+
theoretical predictions for the free gaseous N5 cation at the
B3LYP level of theory, with the calculated terminal and central
N-N bond distances of 1.11 and 1.30 Å being close to the
observed values of 1.105(19) and 1.299(19) Å, respectively.
Although structures IV-VI possess one or two terminal nitrogen
atoms with only six valence electrons, it must be kept in mind
that conventional Lewis structures are oversimplifications and
often do not adequately describe the actual bonding. Inclusion
of structures IV-VI is required to account for the charge distri-
+
Furthermore, the terminal N-N distance of 1.105(19) Å in N5+
is only slightly longer than that of 1.089(9) Å found for N2F
+
- 21
in N2F Sb2F11
and compares well with the N-N bond
22
distances of 1.0976(2) Å in N2 and 1.0927 Å found in
26
butions calculated at the NBO(B3LYP/aug-cc-pVDZ) level,
+
23,24
HN2 ,
indicating that the terminal bonds approximate triple
+
bonds. The central N-N bond length of 1.299(19) Å in N5 is
somewhat longer than typical N-N double bonds (1.17-1.25
Å) but is significantly shorter than typical N-N single bonds
1.43-1.75 Å). Additionally, the agreement between calculated
112.3 and 166.7°] and observed [111.2(11) and 167.2(15)°,
25
(
[
as well as the relative shielding of the N NMR signals (the
respectively] bond angles is very good.
shielding increases from the terminal nitrogen to the â-nitrogen
1
(
21) Vij, A.; Vij, V.; Tham, F.; Christe, K. O. Unpublished results.
to the central nitrogen) and the terminal N-N-N bond angles
(22) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van
of about 167°.
Nostrand Reinhold: New York, 1979.
23) Owrutsky, J. C.; Gudeman, C. S.; Martner, C. C.; Tack, L. M.;
Rosenbaum, N. H.; Saykally, R. J. J. Phys. Chem. 1986, 84, 605.
A least-squares plane analysis for N5+ shows that the cation
is essentially planar. The N2 atom exhibits a maximum deviation
of 0.11 Å from the average mean plane that shows a root-mean-
(
(
24) Botschwina, P. Chem. Phys. Lett. 1984, 107, 535.
(25) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd
ed.; Butterworth, Heinemann: Oxford, 1998.
(26) Fau, S.; Bartlett, R. J. J. Phys. Chem. A 2001, 105, 4096.

6312 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Vij et al.
+SbF
-, N
+Sb
11 and N
-
+AsF
-
and Their Assignments
Table 3. Observed Infrared and Raman Spectra of N
5
6
5
2
F
5
6
observed frequency (cm-1) and relative intensity
+SbF
-
N
5
+Sb
F
11
-
N
5
+AsF
-
assignments (point group)
N
5
6
2
6
+
-
-
IR
Raman
IR
Raman
IR
Raman
N
5
(C2V
)
MF
6
(O
h
)
2 11
Sb F
3
3
3
2
2
2
1
1
357 vw
334 vw
079 w
681 vw
270 m
205 s
(ν
(ν
(ν
(ν
(ν
(ν
(ν
(ν
1
+ ν
+ ν
+ ν
+ ν
3
8
7
9
9 2
+ ν )(B ) ) 3358
1
2
1
1
7
3
8
)(B
)(B
)(B
2
2
2
) ) 3323
) ) 3077
) ) 2682
3069 w
2671 vw
2268 (9.4) 2260 m
2205 (2.0) 2203 s
1919 vw
2261 (9.0)
2202 (1.9)
2270 m 2271 (4.4)
)(A
)(B
1
)
)
2210 s
2211 (0.8)
2
921 vw
891 vw
+ 3ν
+ 2ν
9
)(B
)(B
2
2
) ) 1914
) ) 1883
1883 vw
9
1
1
366 w
288 vw
1
240 vw
}
comb. bands comb. bands
1
1
9
8
8
092 ms
1089 s
1064 s
892 vvw
867 w
1088 s
872 w
(ν
3
+ ν
9
6
)(B
2
2
) ) 1086a
) ) 903
064 s
02 vvw
71 w
ν
8
(B
+ ν
(A
(2ν )(A
2
)
(ν
5
)(B
872 (0.6)
837 (0+)
866 (0.6)
871 (0.7)
ν
2
1
)
35 vw
824 vw
824 (0+)
9
1
) ) 828b
692 (5.5)
7
25-650 vs,br 654 (10)
νSbF
}
598 (1.4)
6
72 (1)
664 (∼1)
680 sh
704 vs
669/672 (1.8)
686 (10)
ν
3
(A
1
)
6
55 vs
82 w
ν
ν
3
(F1u
)
)
6
52 (10)
1
(A1g
5
5
96 mw
37 mw
}
νSbF
5
571 (0.8)
78 (0+)
575 w
420 sh
579 (1.6)
ν
2 g
(E )
4
97 s
νSb-F-Sb
4
470 (0+)
417 (0+)
ν
?
ν
ν
5
(A
2
)
4
4
4
47 w
25 ms
12 mw
449 w
417 ms
409 sh
6
9
(B
(B
1
2
)
)
416 (0+)
2
2
2
2
95 (2.1)
83 sh
72 sh
31 (2.0)
δSb-F
}
2
84 vs
394 vs
ν
ν
4
(F1u
(F2g
)
)
2
2
82 (2.8)
04 (5.0)
372 (3.4)
209 (4.4)
5
200 (3.6)
ν
4 1
(A )
1
9
35 sh
7 (5.0)
1
07 (5)
125 (5.5)
lattice vibrations
}
a
). b In Fermi resonance with ν
(A ).
2 1
In Fermi resonance with ν
8
(B
2
square deviation of 0.0058 Å. The N5+ mean plane is almost
perpendicular (78.1°) to the plane containing the F5-Sb1-F6-
Sb2-F11 atoms. The latter is also almost perfectly planar and
shows a root-mean-square deviation of 0.014 Å.
The geometry of the Sb2F11- anion also deserves special
comment. This anion is known to possess little rigidity and can
exist in either an eclipsed or a staggered conformation and
exhibit a wide range of Sb-F-Sb bridge angles, depending
27
upon the counterion present in the crystal lattice. The eclipsed
conformation is rare but has previously also been observed for
+
- 21
BrF4 Sb2F11 . In the latter compound, the eclipsed structure
results from a packing effect in which one equatorial fluorine
-
ligand of each antimony atom of Sb2F11 bridges to a different
+
+
-
BrF4 cation. Since the two BrF4 cations and the Sb2F11 anion
are coplanar, the bridging equatorial fluorine ligands around
the antimonies become also coplanar, resulting in the eclipsed
-
configuration. The eclipsed conformation of the Sb2F11 anion
found for N5 Sb2F11 is also due to fluorine bridging but
+SbF
-
, recorded as an AgBr
+
-
28
Figure 3. Infrared spectrum of solid N
5
6
-
1
pellet at room temperature. The band at 493 cm , marked by an
-
asterisk, is due to a small amount of Sb
F
2 11
.
(27) Willner, H.; Bodenbinder, M.; Broechler, R.; Hwang, G.; Rettig,
S. J.; Trotter, J.; von Ahsen, B.; Westphal, U.; Jonas, V.; Thiel, W.; Aubke,
F. J. Am. Chem. Soc. 2001, 123, 588.
+
-
results from N5+ acting as a spacer between the two equatorial
(
28) N5 Sb2F11 shows numerous N‚‚‚F contacts shorter than the sum
of the van der Waals radii of 3.0 Å. The shortest contacts, 2.723(15) and
-
SbF4 units of Sb2F11 (Figure 2). In accord with the resonance
a
1
1
1
2
+
2
.768(14) Å, arise from N2‚‚‚F8 and N2‚‚‚F10 (a ) /2 + x, /2 - y, /2
+
b
3
structures and the calculated charge distributions of N5 (see
z), while the N4‚‚‚F2 and N4‚‚‚F4 (b ) x, 1 - y, /2 - z) distances,
.887(15) and 2.813 Å, respectively, are slightly longer.
above), the positively charged nitrogen atoms interact with the

Surprisingly Stable Fluoroantimonate Salts of N5+
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6313
Table 4. Comparison of Observed and Unscaled Calculated CCSD(T)/6-311+G(2d) Vibrational Frequencies (cm 1) and Intensities (km mol-1
-
4
-1
+
5
and Å amu ) for N
abs int
Raman
rel int
approx mode description in point group C2V
calcd freq
IR
13
0.5
2
0.3
0
6
105
138
1
obsd freq
IR
Raman
A
1
ν
ν
ν
ν
ν
ν
ν
ν
ν
1
2
3
4
5
6
7
8
9
in-phase terminal stretches
sym central stretch
central bending
in-phase terminal bends
out-of-phase, out-of-plane bend
in-phase, out-of-plane bend
out-of-phase term stretches
asym central stretch
2229
818
644
181
475
215
5
1
6
1
0
42
2
2260-2271
866-872
664-672
200-209
470-478
417-425
2203-2211
1055-1064
412-417
m
w
10.0
0.6
a
obscd
1
4
b
-
A
2
-
ms
s
0+
B
B
1
405
0
2
2175
1032
399
1.9
s
not obsd
out-of-phase term bends
0.5
mw
0+
a
Obscured in infrared and interference in Raman by anion bands. b Outside of the frequency range of our spectrometer.
predictions for point group C2V. The splittings observed for ν8-
B2) and ν2(A1) can be attributed to Fermi resonance. The
(
-
-
presence of Sb2F11 impurities in the SbF6 salt can be readily
detected by Raman bands at 692, 598, and 231 cm and
infrared bands at 708 and 497 cm that are characteristic for
Sb2F11 and do not overlap with the SbF6 bands.
-
1
-
1
-
-
Conclusion
+
-
The synthesis and thorough characterization of N5 SbF6 and
+
-
+
N5 Sb2F11 demonstrate that the N5 cation can form excep-
tionally stable salts with fluoroantimonate anions and that these
salts are surprisingly insensitive to impact. The N5+ cation is a
powerful one-electron oxidizer that can oxidize NO, NO2, and
Br2 and does not give rise to undesirable fluorination or
oxygenation side reactions. The ready availability of a stable
polynitrogen cation in addition to the long-known azide anion
opens a venue to neutral polynitrogen compounds and may
provide the basis for the first synthesis of stable nitrogen
allotropes.
-
Figure 4. Raman spectrum of solid N
temperature.
5
+SbF
6
recorded at room
negatively charged fluorine ligands (Figure 1). Thus, the
â-nitrogens, N2 and N4, bridge to the two eclipsed fluorine
atoms, F2 and F8, but since the N5+ plane is not perfectly
perpendicular to the Fax-Sb-F-Sb-Fax plane, the N2-F8 and
N4-F2 distances are somewhat shorter than the N2-F2 and
Acknowledgment. The authors thank the Defense Advanced
Research Project Agency, the U.S. Air Force Office of Scientific
Research, and the National Science Foundation for financial
support and Drs. T. Schroer, S. Schneider, and M. Gerken from
the University of Southern California for experimental support
and stimulating discussions.
N4-F8 distances.28 In contrast to BrF4 Sb2F11 , which has an
+
-
2
1
almost linear Sb-F-Sb bridge angle of 175°, that of 155.0(4)°
+
-
in N5 Sb2F11 is much closer to those usually found for
-
27
Sb2F11 .
+
Vibrational Spectra of N5 . The infrared and Raman spectra
of solid N5 SbF6 are shown in Figures 3 and 4, respectively.
The experimentally observed frequencies of N5 SbF6 , N5 -
Sb2F11 , and N5 AsF6 , together with their assignments, are
listed in Table 3. A comparison of the observed and calculated
frequencies and intensities of N5 is given in Table 4. As can
be seen, the previously missing remaining four fundamental
vibrations and numerous combination bands of N5 have been
+
-
+
-
+
Supporting Information Available: Tables of structure
determination summary, atomic coordinates, bond lengths and
angles, and anisotropic displacement parameters of N5Sb2F11
-
+
-
+
(PDF). This material is available free of charge via the internet
1
at http://pubs.acs.org.
+
observed and are in excellent agreement with the theoretical
JA010141G