The [NH3Cl]+ ion

Article abstract of DOI:10.1002/anie.200460544

Stable! The first stable salts containing a simple, inorganic cation with an N-Cl bond are prepared (see picture). They are [NH₃Cl] ⁺[BF₄]^-, [NH₃Cl] ⁺[AsF₆]^-, and [NH₃Cl] ⁺[SbF₆]^- and can be safely prepared from (Me₃Si)₂NCl in mixtures of HF and the corresponding Lewis acids and could be used as storable generators for monochloramine gas.

Full text of DOI:10.1002/anie.200460544

Angewandte
Chemie
published for [ONCl2]+[SbCl6]ꢀ,[10] has been challenged on
theoretical grounds.[12] The paucity of data on simple inor-
ꢀ
ganic N Cl containing cations can be attributed to the general
explosiveness and instability of nitrogen chlorides.[13–15]
Herein, the synthesis and characterization of [NH3Cl]+Mꢀ
salts (M = BF4, AsF6, or SbF6), the first examples of com-
pounds containing a stable, simple inorganic cation with an
ꢀ
N Cl bond, are reported. To our knowledge, the formation of
the [NH3Cl]+ ion has only been postulated based on inves-
tigations of aqueous solutions,[16] by theoretical calcula-
tions,[17] and by mass spectrometric studies.[17,18]
Without doubt, the most important member of the family
of halogenamines is monochloramine, NH2Cl. It is the crucial
intermediate in the industrial synthesis of hydrazine.[13]
Furthermore it is a very powerful disinfectant and germ
killer.[14,19,20] Dilute aqueous solutions of NH2Cl can conven-
iently be prepared by the chlorination of aqueous ammonia
with hypochlorite.[13,14] However, the highest practical NH2Cl
concentration of these solutions is 97%, and purer com-
pounds decompose extremely fast. At ꢀ1108C, NH2Cl begins
to melt with partial decomposition and, at ꢀ408C, it decom-
poses continuously and often explosively, owing to the
formation of ammonium chloride and more highly chlori-
nated products, such as NCl3.[13] Therefore, the use of pure
NH2Cl is not feasible for the preparation of [NH3Cl]+ salts.
The handling problem of pure monochloramine was
overcome by generating it at low temperature from
(Me3Si)2NCl and HF [Eq. (1)].
Inorganic Cations
The [NH3Cl]+ Ion**
Stefan Schneider,* Ralf Haiges, Thorsten Schroer,
Jerry Boatz, and Karl O. Christe*
ðMe3SiÞ2NCl þ 2 HF ! 2 Me3SiF þ NH2Cl
ð1Þ
Dedicated to Professor George Olah
The conversion of a (R3Si)2N group into an H2N group
using a strong acid, such as CF3COOH, has previously been
demonstrated by Wiberg and co-workers for the syntheses of
substituted tetrazenes.[21] When the reaction in Equation (1) is
carried out in the presence of a strong Lewis acid, the
[NH3Cl]+ salts are immediately formed, thus avoiding
significant decomposition of NH2Cl [Eq. (2)].
Whereas at least seven simple inorganic cations, [NH3F]+,[1,2]
[NH2F2]+,[3] [NF4]+,[4] [N2F]+,[5] [N2F3]+,[6] [ONF2]+,[7] and
[N3NOF]+,[8] which contain N F bonds, have been prepared
ꢀ
ꢀ
and well characterized, the existence of corresponding N Cl
bond containing cations is not well established. Thus, only two
+ [9]
N Cl containing cations, [NCl4]
and [ONCl2]+,[10,11] have
ꢀ
þ
ꢀ
NH2Cl þ HF þ M ! ½NH3ClŠ ½MFŠ ðM ¼ BF3, AsF5, or SbF5Þ
been reported, however, our repeated attempts to duplicate
their syntheses were unsuccessful, and the crystal structure,
ð2Þ
[*] Dr. S. Schneider, Dr. R. Haiges, Dr. T. Schroer, Prof. Dr. K. O. Christe
Loker Research Institute
The [NH3Cl]+ salts are formed in high yields, with small
amounts of the corresponding [NH4]+ salts being the only
impurities, which can be detected by vibrational or NMR
spectroscopy. In one of our [NH3Cl]+[BF4]ꢀ preparations, the
formation of [NH4]+[BF4]ꢀ as a by-product was also con-
firmed by its X-ray crystal structure. All attempts to obtain
single crystals of the [NH3Cl]+ salts, suitable for a crystal-
structure determination, failed. The formation of some
[NH4]+ ions as a by-product is difficult to avoid because the
acid-catalyzed decomposition of NH2Cl starts already at
ꢀ1108C. This observation is in accord with the report by
Allenstein and Goubeau that neat solid NH2Cl explodes on
contact with BF3 even at ꢀ1208C.[15]
University of Southern California
Los Angeles, CA 90089-1661 (USA)
Fax: (+1)213-740-6679
E-mail: stefan.schneider@edwards.af.mil
kchriste@usc.edu
Dr. J. Boatz
Space and Missile Propulsion Division
Air Force Research Laboratory (AFRL/PRSP)
10 East Saturn Boulevard
Bldg 8451, Edwards Air Force Base, CA 93524 (USA)
[**] This work was funded by the Defense Advanced Research Projects
Agency, with additional support from the Air Force Office of
Scientific Research and the National Science Foundation. We thank
Drs. A. Morrish, D. Woodbury, and M. Berman, for their steady
support, and Dr. R. Wagner for his help and stimulating discussions.
Dedicated to Professor George Olah on the occasion of winning the
Priestley Award.
All the [NH3Cl]+ salts, prepared in this study, are stable
above room temperature. Unfortunately, reliable melting
points could not be determined because of the [NH4]+
impurities. The salts readily dissolve in water with the
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DOI: 10.1002/anie.200460544
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5213

Communications
formation of the corresponding oxonium salts and mono-
chloramine. The monochloramine was identified by gas-phase
IR spectroscopy and its characteristic intense smell [Eq. (3)].
CH3Cl which is experimentally well characterized.[26] As can
be seen from Table 1, the MP2 and CCSD(T) geometries
deviate by less than 0.01 Š and 0.38 from the experimental
values, while the B3LYP distances are, as expected, slightly
longer. Therefore, we expect the geometry, predicted for
[NH3Cl]+ (Table 1), to be also a good approximation of the
þ
þ
½NH3ClŠ ½MFŠꢀ þ H2O ! ½H3OŠ ½MFŠꢀ þ NH2Cl
ð3Þ
The reaction in Equation (3) is
in accord with the observation by
Table 1: Calculated geometries of [NH3Cl]+, compared to observed[a] and calculated geometries of
Muench that even (CH3)2NCl,
which is considerably more basic
than NH2Cl,[22] can be displaced
from [(CH3)2NClH]+[CF3SO3]ꢀ by
water.[23] These displacement reac-
tions are somewhat surprising
because NH2Cl possesses a higher
gas-phase basicity (GB = 761 ꢁ
5 kJmolꢀ1)[17b] than H2O (GB =
691 kJmolꢀ1)[22] and, therefore,
H2O should not displace NH2Cl
from its [NH3Cl]+ salts. However,
in aqueous solution or in solid–gas
isoelectronic CH3Cl.
[NH3Cl]+
r(N-Cl) r(N-H) a H-N- a H-N- r(C-Cl) r(C-H) aH-C- aH-C-
CH3Cl
[Š]
[Š]
Cl [8]
H [8]
[Š]
[Š]
Cl [8]
H [8]
MP2/aug-cc-pvtz
CCSD(T)/aug-cc-pvtz
CCSD(T)/6-
1.735
1.743
1.747
1.025
1.023
1.026
109.2
109.1
109.3
109.8
109.8
109.7
1.780 1.084 108.4
1.784 1.084 108.3
110.6
110.6
–
–
–
–
311++G(3df,3pd)[b]
B3LYP/aug-cc-pvtz
observed
1.755
–
1.025
–
109.1
–
109.8
–
1.802 1.085 108.2
1.776 1.085 108.6
110.7
110.4
[a] Data from ref. [26]. [b] Data from ref. [17b].
reactions, the relative basicities might be different. Unfortu-
nately, the basicity of NH2Cl in water is difficult to measure
and, as yet, has not been reliably determined because of its
instability in acidic solutions. Arguments have been presented
that NH2Cl should be either slightly more basic[17b,22,23] or
more acidic than water.[23,24] That even in the case of the
stronger base (CH3)2NCl, the dis-
true geometry of the free gaseous ion. Similarly, a comparison
of the observed and calculated vibrational frequencies of
CH3Cl shows very good agreement (Table 2). Note, however,
that the calculated frequencies are harmonic values for the
free gas at 0 K, and that the experimentally observed
frequencies require large anharmonicity corrections, partic-
placement reactions with water
Table 2: Calculated harmonic and experimental anharmonic and harmonic vibrational frequencies and
calculated IR and Raman intensities of CH3Cl.[a]
proceed could be explained by the
reaction in Equation (3) being an
equilibrium which is shifted to the
right by an excess of water and
continuous removal of NH2Cl
owing to either its volatility or
rapid decomposition. Equation (3)
might also explain why, in the
presence of water, protonation of
NH2Cl and formation of [NH3Cl]+
salts have not been observed.
Although knowing the pKa value
of [NH3Cl]+ would be desirable, its
Band
Calculated harmonic frequency
B3LYP
Experimental frequency
MP2
CCSD(T)
anharmonic
harmonic
A1
E
n1
3111.2 (22) [150]
1401.1 (11) [0.04]
764.2 (24) [17]
3222.5 (4.6) [95]
1511.0 (11) [7.5]
1050.0 (4.0) [0.98]
3071.0 (23) [155]
1375.6 (12) [0.004]
707.3 (27) [17]
3165.7 (7.9) [107]
1482.8 (12) [7.7]
1027.3 (4.1) [1.1]
3098.5 (23)
1394.7 (12)
749.1 (22)
3176.4 (7.8)
1510.2 (11)
1039.4 (3.5)
2953.9
1354.9
732.8
3039.3
1452.2
1018.1
3088.4
1396.3
751.2
3183.3
1496.2
1036.8
n2
n3
n4
n5
n6
[a] For all calculations, the aug-cc-pvtz basis set was used; frequencies in cmꢀ1, IR and Raman
intensities in kmmolꢀ1 and Š4 amulꢀ1, respectively.
experimental measurement would be very difficult because of
the above problems and the unavoidable presence of [NH4]+
impurities.
ularly for the vibrations involving hydrogen atoms. Therefore,
most of the differences between the observed and calculated
frequencies can be attributed to anharmonicity effects, and
the agreement between the harmonic values is much better.
The stability of the [NH3Cl]+ salts and their ability to
generate NH2Cl, when exposed to atmospheric moisture,
make them ideally suited for NH2Cl gas generation. This
property could be exploited for a convenient gas-phase
method of deactivating spores, such as anthrax.[25] Further-
more, previous work by Snyder and Margerum has indicated
that the [NH3Cl]+ ion is a very reactive chlorinating agent for
the transfer of chlorine to other amines, such as methylamine,
amino acids, and peptides, while being a less reactive oxidant
than Cl2 or HOCl.[16c]
A
comparison between the observed (Table 3 and
Figure 1) and calculated vibrational frequencies of [NH3Cl]+
is given in Table 4. The differences between the observed
anharmonic and the calculated harmonic frequencies are
comparable to those in CH3Cl and establish the new species as
the [NH3Cl]+ ion. The slight variation in the observed
vibrational frequencies of the [NH3Cl]+ ion in the different
salts is attributed to solid-state effects, such as various degrees
of anion–cation interactions and hydrogen bonding. Further
support for the presence of the [NH3Cl]+ ion comes from the
Conclusive evidence for the [NH3Cl]+ ion comes from the
observed IR, Raman and NMR spectra and their comparison
with theoretical calculations. To assess the accuracy of these
calculations, we have tested these methods for isoelectronic
35
37
ꢀ
Cl– Cl isotopic shift of the N Cl stretching vibration. The
ꢀ
N Cl stretching vibration (Figure 1) shows a splitting of
approximately 6 cmꢀ1, in accord with the calculated harmonic
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Table 3: Observed vibrational spectra[a] of solid [NH3Cl]+ Mꢀ (M=BF4, AsF6, SbF6) and their assignments.
[NH3Cl]+[BF4]ꢀ [NH3Cl]+[AsF6]ꢀ [NH3Cl]+[SbF6]ꢀ
[NH3Cl]+ (C3v)
Mꢀ
[SbF6]ꢀ (Oh)
Raman
IR
Raman
IR
Raman
IR
[BF4]ꢀ (Td) [AsF6]ꢀ
3247.6(18)
3188.6(9)
1552.2(1)
1454.8(0+)
n.o.
759.0(82)
753.8(50)
1079.0(0+)
772.0(100)
3221vw
3241.2(16)
3167.7(3)
1566.7(0+)
1447.0(0+)
1071.0(0+)
766.4(15)
3209w
3172w
1564w
1435s
3229.6(8)
3168.0(4)
1557.0(0+)
1433.5(0+)
1068.8(0+)
766.2(49)
3217vw
3112vw
1569w
1435s
1072m
767w
n4 (E)
n1 (A1)
n5 (E)
n2 (A1)
n6 (E)
1570w
1458m
n.o.
1071w
[b]
763w
n335Cl (A1)
n337Cl (A1)
[b]
761.2(9)
761.2(30)
762w
1035vs,vb
769w
n˜3 (F2)
n˜1 (A1)
703vs,b
659vs
n˜3 (F1u)
n˜1 (A1g)
n˜2 (Eg)
688.6(100)
573.8(22)
654.4(100)
570.1(28)
528.8(14)
354.5(18)
530/524m
n˜4 (F2)
n˜2 (E)
373.0(43)
281.6(38)
n˜5 (F2g)
[a] Frequencies in cmꢀ1 and uncorrected relative intensities. [b] Observed as shoulders on the very intense 703 cmꢀ1 band; n.o.=not observed.
splittings, ranging from 6.6 (B3LYP) to 7.1 (MP2) cmꢀ1. If the
observed isotopic shifts were corrected for anharmonicity, the
agreement would be even better. In CH3Cl, anharmonicity
corrections increase the observed 35Cl–37Cl isotopic shift by
0.29 cmꢀ1 from the anharmonic value, Dn = 5.83, to the
harmonic value, Dw = 6.12 cmꢀ1.[26] The complexity of the
Raman bands of [NH3Cl]+[BF4]ꢀ in the region of the N H
ꢀ
stretching modes (Figure 1) can be explained by Fermi
resonance between n1(A1) and 2n5(A1) and the possible
presence of some [NH4]+ impurity.
Additional support for the [NH3Cl]+ ion comes from the
results of a normal coordinate analysis (Table 5). The general
harmonic force field, calculated for the [NH3Cl]+ ion at the
CCSD(T) level, corresponds very closely to that of isoelec-
tronic CH3Cl.[25] All vibrations are highly characteristic, and
ꢀ
only the N Cl stretching vibration mixes, as expected, to a
small extent with the NH3 umbrella deformation mode.
Figure 1. Raman spectra of [NH3Cl]+[SbF6]ꢀ (upper) and
1
The 14N and H NMR spectra of [NH3Cl]+[SbF6]ꢀ in HF
[NH3Cl]+[BF4]ꢀ (lower). The enlarged sections of the spectra show a
and DF solutions (Table 6) exhibit single resonances at d =
ꢀ
35/37 chlorine isotopic splitting in the N Cl vibration.
ꢀ364 and 7.91 ppm, respectively. The observed chemical
shifts are in good agreement with our expectations for the
[NH3Cl]+ ion: the nitrogen atom in
the [NH3Cl]+ ion is slightly
Table 4: Calculated harmonic and experimental anharmonic vibrational frequencies and calculated IR
deshielded compared with that in
and Raman intensities of [NH3Cl]+.[a]
[NH4]+ (d = ꢀ367 ppm), but signif-
Band
Calculated harmonic frequency
Range of
icantly more shielded than that in
[NH3F]+ (d = ꢀ252.1 ppm).[27] The
proton shift (d = 7.91 ppm) falls in
between those of the [NH4]+ (d =
5.71 ppm) and [NH3F]+ (d =
10.4 ppm) ions.[2] The similarity of
the 14N shifts of the [NH3Cl]+ and
[NH4]+ ions cannot be attributed to
signal averaging between the
[NH3Cl]+ ion and either the
[NH4]+ ion or the solvents, because
in all spectra separate signals were
observed for the [NH3Cl]+ and
experimental
anharmonic
frequency
MP2
B3LYP
CCSD(T)
aug-cc-pvtz
3404.1 (78)
6-31++G(3df,3pd)[b]
A1 n1 3374.7 (85) [87]
3357.1 (79) [91]
3355.1
1467.9
741.5
3112–3188
1435–1458
759–767
3209–3247
1552–1570
1069–1072
n2 1475.9 (59) [0.48] 1466.5 (57) [0.45] 1474.0 (56)
n3 785.0 (2.6) [12] 737.5 (2.4) [13] 762.9 (2.1)
n4 3480.6 (386) [42] 3445.0 (356) [47] 3484.1 (349) 3441.9
n5 1642.1 (101) [6.1] 1628.9 (105) [6.5] 1646.7 (100) 1628.8
E
n6 1054.8 (37) [1.35] 1037.0 (36) [1.69] 1045.8 (35)
1039.2
[a] For the MP2 and B3LYP calculations, the aug-cc-pvtz basis set was used; frequencies in cmꢀ1
intensities (infrared) and [Raman] in kmmolꢀ1 and Š4 amuꢀ1, respectively. [b] Data from ref. [17b].
,
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Table 5: General harmonic force field[a] of C3v [NH3Cl]+ and potential energy distribution[b] calculated at
the CCSD(T)/aug-cc-pvtz level of theory.
All reactions were carried out in Teflon–FEP
(FEP = perfluoro ethylene propylene polymer)
ampules that contained Teflon-coated magnetic
stirring bars and were closed by stainless steel
valves. Volatile materials were handled on a
stainless steel vacuum line. Nonvolatile solids
were handled in the dry nitrogen atmosphere of a
glove box. IR spectra were recorded on a Midac,
M Series, FT-IR spectrometer using AgCl pellets.
The pellets were prepared inside the glove box
using an Econo press (Barnes Engineering Co.).
Ramaꢀn1 spectra were recorded in the range 4000–
8 0 cm on a Bruker Equinox 55 FT-RA spec-
trometer using a Nd-YAG laser at 1064 nm with
power levels of 800 mW or less. Pyrex melting
point capillaries, glass NMR or 9 mm Teflon–FEP
tubes were used as sample containers. NMR
spectra were recorded unlocked on a Bruker
AMX 500 NMR spectrometer at room temper-
Band
Approximate mode Frequency [cmꢀ1
description
]
Symmetry force constants
PED
F11 F22
F11 6.746
F22
F33
0.138
0.619 ꢀ0.454 99.7 (2)
A1 n1 n sym NH3
n2 d sym NH3
n3 n N–Cl
3404.1
1474.0
762.9
0.100 99.6 (1)
F33
3.997 86.3 (3) + 13.7 (2)
F44 F55
F66
E
n4 n asym NH3
n5 d asym NH3
n6 d wag NH3
3484.1
1646.7
1045.8
F44 6.591 ꢀ0.136
0.000 98.3 (4)
F55
F66
0.610 ꢀ0.011 95.3 (5)
0.668 95.2 (6)
[a] Stretching constants in mdynŠꢀ1, deformation constants in mdynŠ/rad2, and stretch-bend
interaction constants in mdyn/rad. [b] PED in percent. Symmetry coordinates contributing less than
5% are omitted. Symmetry coordinates, taken from ref.[26], are defined as follows: S1 =n sym (N-H), S2 =
d sym (H-N-H–H-N-Cl), S3 =n (N-Cl), S4 =n asym (N-H), S5 =d asym (H-N-H), S6 =d asym (Cl-N-H).
1
ature. The 14N and H NMR spectra were refer-
enced to external samples of neat nitromethane and tetramethylsilane
in CDCl3, respectively.
Table 6: Observed NMR spectra of HF/DF solutions of
[NH3Cl]+[SbF6]ꢀ.[a]
The (Me3Si)2NCl starting material was prepared from
(Me3Si)2NH and tBuOCl using a literature method.[29] The HF/DF
solvents (Matheson Co./Ozark Mahoning) were dried[30] by storage
over BiF5 (Ozark Mahoning). SbF5 (Ozark Mahoning) was purified
by distillation prior to use. BF3 (Matheson) and AsF5 (Ozark
Mahoning) were used as received.
Solvent, T
Chemical shift [ppm] (line width [Hz])
d14N
d1H
[b]
HF, 208C
DF, 208C
ꢀ363 (188)
ꢀ364 (125)
7.91 (3.8)
[a] In addition to the resonances arising from the [NH3Cl]+ ion, d14N
resonance signals arising from the [NH4]+ ion were observed at ꢀ368 (q,
54.7 Hz) in HF and at ꢀ367 (q, 54.8 Hz) ppm in DF; the d1H resonance
signals from the [NH4]+ ion were observed at 5.65 (tr, 54.6 Hz) in HF and
at 5.71 (tr, 54.4 Hz) ppm in DF. [b] Resonance obscured by the HF
solvent signal.
[NH3Cl]+Mꢀ [M = BF4, AsF6, SbF6]: In a typical experiment,
anhydrous HF (2 mL of liquid) and BF3, AsF5, or SbF5 (1.44 to
3.176 mmol) were combined at ꢀ1968C in a 9 mm Teflon–FEP
ampule closed by a stainless steel valve. The mixture was warmed to
258C and then recooled to ꢀ1968C. A stoichiometric amount of
(Me3Si)2NCl was added to the ampule at ꢀ1968C, and additional HF
was condensed on top of it at a very slow rate to avoid contact of the
frozen silyl compound with liquid HF during the condensation
process. The frozen mixture was warmed first to ꢀ788C and then
slowly to 258C. During warm-up, a colorless precipitate was formed,
which was only partially soluble in the HF. The ampule was
immediately recooled to ꢀ648C and all volatiles were pumped off
at this temperature. Colorless stable solids of [NH3Cl]+[BF4]ꢀ,
[NH3Cl]+[AsF6]ꢀ, or [NH3Cl]+[SbF6]ꢀ were left behind which con-
tained small amounts of the corresponding [NH4]+ salts as the only
impurities, detectable by vibrational spectroscopy.
[NH4]+ ions which were always separated by the same
amount, and the [NH4]+ ion proton resonance consisted of
very narrow triplets of equal intensity arising from 14N–1H
spin–spin coupling. The similarity of the 14N shifts in the
[NH4]+ and [NH3Cl]+ ions is attributed to nitrogen and
chlorine having very similar electronegativities, resulting in a
ꢀ
low polarity of the N Cl bond and a weak electron-with-
Theoretical calculations were performed using the GAMESS,[31]
Gaussian98,[32] and ACES II[33] program systems, and the augmented
correlation-consistent polarized vvalence triple-zeta basis set (aug-cc-
pvtz) of Dunning et al.[34] Computational methods included density
functional theory with the hybrid B3LYP functional,[35] second order
perturbation theory (MP2, also known as MBPD(2))[36] and coupled-
cluster singles and doubles[37] with perturbative estimates of triple
excitations (CCSD(T)).[38]
drawing effect of chlorine. In contrast, substitution of one
hydrogen atom by a highly electronegative fluorine atom
results in strong deshielding of the nitrogen atom. A similar
trend is also reflected, although to a lesser degree, in the 13C
shifts of CH4 (d = ꢀ2.1 ppm), CH3Cl (d = 25.6 ppm), and
CH3F (d = 71.6 ppm).[28]
In summary, this study provides [NH3Cl]+, the first stable,
ꢀ
simple, inorganic cation containing an N Cl bond. For the
syntheses of the [NH3Cl]+ salts, the explosiveness and thermal
instability of the parent molecule NH2Cl was circumvented by
using a safe organosilicon derivative, (R3Si)2NCl, as a
precursor. Conclusive evidence for the existence of the
[NH3Cl]+ ion is given by its vibrational and NMR spectra
and theoretical calculations.
Received: May 4, 2004
Revised: June 26, 2004
Keywords: cations · nitrogen chlorides · NMR spectroscopy ·
.
theoretical chemistry · vibrational spectroscopy
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Experimental Section
Caution! Neat chloramines are highly unstable and often can
decompose explosively. They should be handled on a small scale
with appropriate safety precautions.
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