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Nitro(nitroso)cyanomethanides.

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Angewandte
Chemie
Stable Methanides
Nitro(nitroso)cyanomethanides**
Harald Brand, Peter Mayer, Axel Schulz,* and
Jan J. Weigand
Methanide anions of the type [HCR1R2] and [CR1R2R3]
(R1,2,3 = CN, NO, NO2 and all possible permutations of R1,2,3)
can be considered to be resonance-stabilized, nonlinear
pseudohalides.[1] The term pseudohalogen/pseudohalide was
first introduced by Birckenbach in 1925 and applied to linear
species such as CN/CN , CNO/CNO , N3/N3 , OCN/OCN ,
and SCN/SCN .[2] In the same year Grimm developed the
hydride displacement law,[3] according to which the pseudoelements CH3, NH2, and OH have properties similar to those
of fluorine; for example, they form singly charged anions.
Successive substitution of the hydrogen atoms in CH3 (CH3)
and NH2 (NH2) by electron-withdrawing groups in resonance with p bonds, such as CN, NO, and NO2, leads to the
class of resonance-stabilized, nonlinear pseudohalogens
(pseudohalides).[4] Prominent examples of methanides are
di- and tricyanomethanides,[5, 6] di- and trinitromethanides,[7, 8]
dinitrosomethanide,[9] and the mixed substituted species such
as nitrosodicyanomethanide [(ON)C(CN)2] and nitrodicyanomethanide
[(O2N)C(CN)2] .[10]
Nitro(nitroso)cyanomethanide, a methanide with three different characteristic
groups, is not known yet. Here we wish to report the synthesis
and full characterization of the alkali-metal and tetramethylammonium salts of nitro(nitroso)cyanomethanide, a simple
new class of salts with surprising thermal stability.
Nitro(nitroso)cyanomethanides (NNCM) were synthesized readily in a two-step reaction (Scheme 1). First, nitroacetonitrile was nitrosated with sodium nitrite in water to give
the metastable cyanomethylnitrolic acid (1), which was
extracted with diethyl ether. Then an anhydrous ether
solution of 1 was treated with a solution of M’OR (M’ =
alkali metal, NR4 ; R = H, alkyl) in 2-propanol, resulting in a
red precipitate of M’NNCM. The beautiful red alkali-metal
NNCM salts were purified easily by recrystallization from
methanol (yield 50–60 %).
Metastable cyanomethylnitrolic acid and its red color in
basic water solution have already been observed by Steinkopf.[11] Moreover, he postulated the existence of the silver
salt of 1. The intermediate formation of the silver salt was also
postulated by Pillai and Boyer in the reaction of ICH2CN with
AgNO2, which finally gave NCCH2ON=C(NO2)CN as the
only isolable product.[12]
Crystals of LiNNCM and NaNNCM are very hygroscopic
(they deliquesce within seconds), while crystals of KNNCM
and CsNNCM can be handled without inert gas for some
minutes. Interestingly, the red color of the NNCM salts
darkens the heavier the alkali-metal counterion is: LiNNCM
exhibits a bright orange-red color while CsNNCM displays a
purple color.
Similar to the alkali-metal dinitrosomethanide salts,[9]
pure dry alkali-metal NNCM salts are stable at ambient
temperature, are heat and shock sensitive, and decompose
slowly in polar solvents releasing N2O gas (detected by
14
N NMR experiments). To determine the gases formed when
M’NNCM explodes, combined IR and HRMS pyrolysis
experiments were carried out. The only gaseous products
observed were NCCNO, N2O, CO2, NO, and CO. An
intriguing feature of NNCM salts is the liberation of cyanogen
N-oxide (NCCNO)[13] upon gentle heating (e.g. KNNCM:
80 8C < T < Tonset) and the formation of M’NO2 in the solid
state as well as in solution (Scheme 2).[14] Differential scan-
Scheme 2. Decomposition of M’NNCM (M’ = alkali metal) upon gentle
heating.
Scheme 1. Synthesis of nitro(nitroso)cyanomethanide salts (M’ = alkali
metal, NR4 ; R = H, alkyl).
[*] H. Brand, Dr. P. Mayer, Dr. A. Schulz, J. J. Weigand
Department Chemie und Pharmazie
Ludwig-Maximilians-Universitt Mnchen
Butenandtstrasse 5–13 (Haus D), 81377 Mnchen (Germany)
Fax: (+ 49) 89-2180-77492
E-mail: axel.schulz@cup.uni-muenchen.de
[**] A.S. thanks Professor Dr. T. M. Klaptke (LMU Mnchen) for his
generous support and Professor Dr. W. Beck for his interest and
advice.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 3929 –3932
ning calorimetry (DSC) experiments revealed that M’NNCM
salts (M’ = K, Cs, NMe4) undergo exothermic decomposition.
For example, for the explosion of KNNCM: DH =
37.25 kcal mol1; onset of decomposition: 101.7 8C (b =
5 8C min1) in a temperature range of 90–117 8C; estimated
activation energy of ca. 31.9(0.5) kcal mol1.[15] We assume
that the presence of a p system delocalized over the entire
anion accounts for the remarkable kinetic stability of NNCM
salts (see below). Nitro-, nitrosodicyanomethanide, and
dinitrosomethanide salts possess a higher thermal stability
than M’NNCM salts.[9, 10] It is interesting to note that we were
able to detect the NNCM anion in a FAB mass spectrometry
experiment that was carried out with crystals of KNNCM.
14
N NMR spectra of the NNCM anion in DMSO solution
display three resonances in the ranges typical for CNO
(265 ppm, cf. 332 ppm [HC(NO)2]),[9] CNO2 (15 ppm, cf.
25 ppm H2C(NO2)2),[16] and CCN species (107 ppm, cf.
105 ppm [C(NO)(CN)2]).[16] The 13C NMR spectrum shows
DOI: 10.1002/anie.200500495
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3929
Communications
singlet resonances at 149.6 ppm (C-CN, cf. 150.3 ppm
[C(NO2)3])[17] and 119.5 ppm (C-CN, cf. 117.2 ppm
[C(NO2)(CN)2]).[10] The characteristic frequencies of the
NO (1380–1400 cm1), NO2 (1490–1510 cm1), and CN (2210–
2230 cm1) groups can be detected in the Raman and IR
spectra of all M’NNCM salts (M’ = alkali metal, Me4N).[18a]
KNNCM crystallizes in beautiful red rods in the monoclinic space group P21/c with four units per cell.[19] The
structure consists of an infinite three-dimensional network of
repeating K[C(NO2)(NO)(CN)] units. Each anion is bonded
to six potassium cations and vice versa (Figure 1). Specifically,
Figure 2. Packing diagram of KNNCM (view along 100).
Figure 1. Coordination environment of NNCM in KNNCM; bond
lengths []: C1–N1 1.418(3), C1–N2 1.323(3), C1–C2 1.422(4), C2–N3
1.138(3), N1–O1 1.233(3), N1–O2 1.236(3), N2–O3 1.273(3); angles
[8]: C1-C2-N3 173.2(3), C1-N1-O1 117.5(2), C1-N1-O2 119.7(2), C1-N2O3 115.6(2), N1-C1-N2 117.4(2), N1-C1-C2 117.7(2), N2-C1-C2
124.9(2).
the oxygen atoms of the nitro group are bonded to three
potassium centers with KO distances of 2.790(2) to
3.586(2) , while the oxygen atom of the NO group is
attached to two potassium centers (d(KO) = 2.814(2) and
2.744(2) ). Additionally, two KN interactions are found for
the N atom of the NO group (3.003(2) and 3.276(2) ). The
nitrogen atom of the CN group coordinates to two neighboring potassium centers with KN distances of 2.832(2) and
3.275(3) . Hence, as shown in Figure 1, each potassium
center is surrounded by four nitrogen and five oxygen atoms.
The potassium coordination sphere consists of six NNCM
ligands which leads to the interesting three-dimensional
network arrangement of the ions (Figure 2). A view along
the a axis reveals stacked chains of K+ and NNCM ions.[18b,c]
As depicted in Figure 1, the NNCM anion is nearly
planar[20] (sum of angles at C1 = 359.98) like most of the
known NO-, NO2-, and CN-substituted methanide anions
(except trinitromethanide, in which the balance between
resonance stabilization and steric repulsion results in the
torsion of one NO2 group out of the plane).[8a, 21] Inspection of
the conformational space at the B3LYP/aug-cc-pvTZ level
yielded two different planar structures of the NNCM ion: a cis
isomer (as shown in Figure 1, O3 in cis position to C2) and a
trans arrangement. In agreement with the experiment our
calculations indicated that the cis form is the most stable
isomer (DEtot(cistrans) = + 4.6 kcal mol1). As expected, all
3930
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
NO bonds (KNNCM: 1.233(3), 1.236(3), and 1.273(3) ) are
significantly shorter than the C1N1, C1N2, and C1C2
bonds (KNNCM: 1.418(3), 1.323(3), and 1.422(4) ).[18c] This
is comparable to the situation found in the structure of
dinitrosomethanide[9] and the nitroso- and nitrodicyanomethanides.[10] The CNnitroso bond is significantly shorter than
the CNnitro bond, indicating a stronger p interaction along
the CNnitrosoO moiety. These short CN bonds together
with the planarity indicate that the p bonds are delocalized
over the whole anionic species.
MO[18d] and natural bond orbital (NBO)[22] calculations of
the NNCM ion support the existence of a 10p-electron, eightcenter bond (Scheme 3 and Figure 3).[23] According to NBO
analysis A and B in Scheme 3, are the energetically preferred
Lewis representations. Investigation of the intramolecular
donor–acceptor interactions utilizing the NBO partitioning
scheme clearly indicates a highly delocalized 10p-system
Scheme 3. Lewis structures of NNCM corresponding to the results of
NBO analysis.
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 3929 –3932
Angewandte
Chemie
at 0 8C for 30 min, and then 1 was extracted with diethyl ether (4 25 mL). The combined extracts were dried over anhydrous calcium
chloride, filtered, and cooled to 0 8C with stirring. A solution of
KOtBu (1.40 g, 12.5 mmol) in 2-propanol (10 mL) was added dropwise, resulting in a red precipitate, which was recrystallized from
methanol to give red crystals (rods) of KNNCM (1.82 g, 11.9 mmol,
57 %). M.p. 105 8C (decomp.), Raman (200 mW, 25 8C, 100 scans):[18a]
ñ(Irel) = 2225 (4), 1501 (3), 1400 (7), 1344 (10), 1258 (2), 1214 (9), 848
(2), 834 (2), 806 (1), 777 (1), 570 (0.5), 537 (1), 489 (0.3), 258 (1), 180
(2), 154 cm1 (1); UV/Vis (methanol):[18d] lmax(e) = 489 (60), 328
(11 055), 245 nm (5493 cm2 mmol1); 13C NMR ([D6]DMSO,
101 MHz, 25 8C): d = 119.5 (s, C-CN), 149.6 ppm (s, C-CN); 14N
NMR ([D6]DMSO, 28.9 MHz, 25 8C): d = 107 (s, Dn1/2 = 240 Hz, CCN), 15 (s, Dn1/2 = 80 Hz, C-NO2), 265 ppm (s, Dn1/2 = 320 Hz, CNO); MS (FAB , xenon, 6 keV, mNBA matrix): 267 [NNCM +
NBA] , 114 [NNCM] ; MS (EI, 70 eV, > 5 %, explosion gases); m/
z (%): 68 (6) [NCCNO]+, 52 (28) [(CN)2]+, 44 (100) [CO2]+ and
[N2O]+, 32 (8) [O2]+, 30 (41) [NO]+, 28 (30) [CO]+, 27 (6) [HCN]+;
Elemental analysis for KC2N3O3 (153.139) calcd C 15.69, N 27.44;
found C 16.16, N 26.24.
Figure 3. Selected molecular orbitals (B3LYP/aug-cc-pvTZ) of cis
NNCM (the five occupied p MOs are those with A’’ symmetry).
corresponding to the resonance structures A–G. The calculated natural atomic orbital population (NAO) net charges
are q(O) = 0.459 (O1), 0.430 (O2), 0.464 (O3); q(N) =
0.475 (N1), 0.022 (N2), 0.382 (N3); q(C) = 0.025 (C1),
0.263 e (C2). This indicates that the negative charge is found
mainly on the three O atoms and the N atom of the CN group.
The UV/Vis spectra of the red solutions of NNCM alkalimetal salts in methanol exhibit one very strong characteristic
p!p* and one weak n!p* electronic transition at roughly
328 and 489 nm, respectively, which could be assigned on the
basis of TD-B3LYP calculations (Figure 3).[18d] The red color
arises from the weak n!p* HOMO–LUMO electronic
transition in the anion. The HOMO describes a lone pair
that lies in the anion plane. A closer inspection of the orbital
coefficients of the HOMO shows rather large coefficients for
the nitroso group; hence, it can be concluded that the nitroso
group is mainly responsible for the red color.
In conclusion, we present here an easy, high-yielding
synthetic procedure and first full characterization of alkalimetal and tetramethylammonium salts of nitro(nitroso)cyanomethanides. The presence of a p system
delocalized over the entire anion accounts for the remarkable
thermal stability of alkali-metal NNCM salts, and the NNCM
anion can be regarded as a nonlinear, resonance-stabilized
pseudohalide. Because of the three different functional
groups, NNCM salts may have great synthetic potential, for
example, as a source for NCCNO release in situ.[14b]
Experimental Section
Caution: Although ammonium and alkali-metal NNCM salts are
kinetically stable compounds, they are nonetheless high-energy
materials and appropriate safety precautions should be taken,
especially when these compounds are prepared on a larger scale
(see the Supporting Information).
Synthesis of KNNCM:[24] A solution of H2SO4 (100 %; 1.10 g,
11.2 mmol) in water (3 mL) was added dropwise to a stirred solution
of nitroacetonitrile (1.80 g, 20.9 mmol)[25] and sodium nitrite (1.50 g,
21.7 mmol) in water (30 mL) at 0 8C. The reaction mixture was stirred
Angew. Chem. Int. Ed. 2005, 44, 3929 –3932
Received: February 9, 2005
Published online: May 17, 2005
.
Keywords: bonding analysis · methanides · pseudohalogens ·
structure elucidation
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www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3931
Communications
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
3932
e) Z. Berkovitch-Yellin, L. Leiserowitz, Acta Crystallogr. A
1984, 40, 159 – 165.
H. Brand, P. Mayer, K. Polborn, A. Schulz, J. J. Weigand, J. Am.
Chem. Soc. 2005, 127, 1360 – 1361.
N. Arulsamy, D. S. Bohle, B. G. Doletski, Inorg. Chem. 1999, 38,
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W. Steinkopf, Ber. Dtsch. Chem. Ges. 1909, 42, 617 – 621.
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a) For detailed temperature-dependent data for gas-phase IR
spectra and HRMS, see the Supporting Information; b) C.
Grundmann, H. D. Frommeld, J. Org. Chem. 1966, 31, 4235 –
4237; c) B. Guo, T. Pasinszki, N. P. C. Westwood, K. Zhang, P. F.
Bernath, J. Chem. Phys. 1996, 105, 4457 – 4460; d) T. Pasinszki,
N. P. C. Westwood, J. Phys. Chem. 1996, 100, 16 856 – 16 863; e) T.
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a) The release of nitrile oxides has also been described in the
decomposition of nitrolic acids (RC(=NOH)NO2) yielding
HNO2 and RCNO; b) C. Grundmann, P. Gnanger, The Nitrile
Oxides, Springer, New York, 1971.
a) T. Ozawa, Bull. Chem. Soc. Jpn. 1965, 38, 1881 – 1886; b) H. E.
Kissinger, Anal. Chem. 1957, 29, 1702 – 1706; c) Standard Test
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Spectrosc. 1993, 31, 1 – 480.
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Struct. 2000, 520, 221 – 228.
a) For the approximate assignment of all normal modes on the
basis of DFT calculations and trends see Table S8 in the
Supporting Information. b) When the coordination environment
on the anion was changed by introducing the bulky cation
Me4N+, noncoordinated cations and anions were observed in
solid state instead of the three-dimensional network, and the
anions do not adopt the parallel eclipsed stacking found in
KNNCM (Figure S4). c) For comparison, see X-ray data of
(NMe4)NNCM (Tables S4 and S5). d) See Figure S7 and
Table S10.
Crystal data for KNNCM: Mr(C2N3O3K) = 153.14, crystal
dimensions ca. 0.32 0.10 0.03 mm, monoclinic, space group
P21/c, a = 4.8112(3), b = 7.5510(5), c = 14.9095(10) , a = g =
90.0,
b = 93.086(4)8,
V = 540.87(6) 3,
Z = 4,
1calcd =
1.881 g cm3, F(000) = 304, Nonius Kappa CCD, l(MoKa) =
0.71073 , T = 200 K, m = 0.909 mm1, 3.84 q 25.98, of 4842
reflections collected, 1041 were independent, 804 were observed,
(Rint = 0.053), The R values are R1 = 0.0359 (I > 2s(I)) and wR2 =
0.0787 (all data); max./min. residual electron density: 0.258/
0.378 e 3. All structures were solved by direct methods
(structure solution program: SIR97)[26] and refined by fullmatrix least squares methods with SHELXL-97[27] and anisotropical thermal parameters for all atoms. Crystal data for
(NMe4)NNCM are given in the Supporting Information.
CCDC 261891 and CCDC 261892 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+ 44) 1223-336-033; or
deposit@ccdc.cam.ac.uk).
Largest deviations from the ideal least-square plane (all atoms
included) are 0.040(2) (O1, NMe4NNCM) and 0.090(2) (O2, KNNCM).
a) Calculations predict a propeller-type D3-symmetric anion: J.
Cioslowski, S. T. Mixon, E. D. Fleischmann, J. Am. Chem. Soc.
1991, 113, 4751 – 4755; b) Additionally, our B3LYP/aug-cc-pvTZ
calculations revealed a nonplanar C2v structure as the global
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[22]
[23]
[24]
[25]
[26]
[27]
minimum (0.5 kcal mol1 lower in energy than the D3-symmetric
isomer).
a) E. D. Glendening, A. E. Reed, J. E. Carpenter, F. Weinhold,
NBO Version 3.1; b) A. E. Reed, L. A. Curtiss, F. Weinhold,
Chem. Rev. 1988, 88, 899 – 926.
All MOs are shown in Figure S7 of the Supporting Information.
The other ammonium and alkali-metal NNCM salts can be
obtained either by ion-exchange reactions or by the described
synthetic procedure (see the Supporting Information).
G. H. Reidlinger, H. Junek, Synthesis 1991, 10, 835 – 838.
A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C.
Giacorazzo, J. Appl. Crystallogr. 1999, 32, 115 – 119.
G. M. Sheldrick, SHELXL-97, Program for Solution of Crystal
Structures; University of Gttingen: Gttingen, Germany, 1997.
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