Tricyanomethane and Its Ketenimine Tautomer: Generation from Different Precursors and Analysis in Solution, Argon Matrix, and as a Single Crystal

Article abstract of DOI:10.1002/anie.201704561

Solutions of azidomethylidenemalononitrile were photolyzed at low temperatures to produce the corresponding 2H-azirine and tricyanomethane, which were analyzed by low-temperature NMR spectroscopy. The latter product was also observed after short thermolysis of the azide precursor in solution whereas irradiation of the azide isolated in an argon matrix did not lead to tricyanomethane, but to unequivocal detection of the tautomeric ketenimine by IR spectroscopy for the first time. When the long-known “aquoethereal” greenish phase generated from potassium tricyanomethanide, dilute sulfuric acid, and diethyl ether was rapidly evaporated and sublimed, a mixture of hydronium tricyanomethanide and tricyanomethane was formed instead of the previously claimed ketenimine tautomer. Under special conditions of sublimation, single crystals of tricyanomethane could be isolated, which enabled the analysis of the molecular structure by X-ray diffraction.

Full text of DOI:10.1002/anie.201704561

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Accepted Article
Title: Tricyanomethane and its Ketenimine Tautomer Generated from
Different Precursors and Analyzed in Solution, Argon Matrix, and
Single Crystal
Authors: Klaus Banert, Madhu Chityala, Manfred Hagedorn, Helmut
Beckers, Tony Stüker, Sebastian Riedel, Tobias Rüffer, and
Heinrich Lang
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Tricyanomethane and its Ketenimine Tautomer Generated from
Different Precursors and Analyzed in Solution, Argon Matrix, and
Single Crystal
Klaus Banert,* Madhu Chityala, Manfred Hagedorn, Helmut Beckers,* Tony Stüker, Sebastian Riedel,
Tobias Rüffer, and Heinrich Lang
Abstract: Solutions of azidomethylidenemalononitrile were
photolyzed at low temperatures to produce the corresponding 2H-
azirine and tricyanomethane, which were analyzed by low-
temperature NMR spectroscopy. The latter product was also
observed after short thermolysis of the azide precursor in solution,
whereas irradiation of the azide isolated in an argon matrix did not
lead to tricyanomethane, but to unequivocal detection of the
tautomeric ketenimine by IR spectroscopy for the first time. When the
long-known “aquoethereal” greenish phase, generated from
potassium tricyanomethanide, dilute sulfuric acid, and diethyl ether,
was rapidly evaporated and sublimed, a mixture of hydronium
tricyanomethanide and tricyanomethane was formed instead of
previously claimed ketenimine tautomer. Under special conditions of
sublimation, isolation of single crystals of tricyanomethane was
possible and enabled analysis of the molecular structure by X-ray
diffraction.
characterized by the latter method. When rapid evaporation of 2
was performed without sublimation, single crystals of hydronium
tricyanomethanide (4) were obtained as revealed by X-ray
diffraction crystallographic studies.[5] Not only removal of the
solvents from 2 followed by transport of the product into the gas
phase, but also treatment of silver salt 1c with pure dry hydrogen
sulfide was utilized to analyze the corresponding products by
microwave spectroscopy[6] and photoelectron spectroscopy.[7]
With the help of both methods, the gas-phase structure of 5 was
confirmed. On the other hand, 5 as well as its tautomer 3 were
discussed to explain the acidic properties of cyanoform.[2,3,8] Thus,
this compound is mentioned in textbooks of organic chemistry as
one of the strongest carbon acids, with pK
relative stabilities, spectroscopic features, and rearrangement
reactions of 3, 5, and other C HN species were studied using
a
of about −5. The
4
3
quantum chemical methods.[9] Recently, the synthesis of 5 by
treating 1d with an excess of anhydrous hydrogen fluoride was
reported by Kornath et al.[10] The product 5 could not be separated
from 6 and turned out to be stable only below −40 °C. After
removal of hydrogen fluoride, however, characterization was
Initial attempts to prepare and isolate tricyanomethane (5), the so-
called cyanoform, were performed as early as 1896 when
Schmidtmann treated sodium tricyanomethanide (1a) with dilute
sulfuric acid (Scheme 1).[1] After addition of diethyl ether, he got a
three-phase system, which included a greenish middle layer 2 that
was claimed to contain cyanoform (5). But first experiments to
remove the solvents did not lead to characterizable products.
These phenomena were confirmed by Hantzsch and Osswald,
who stated that tautomerism of 5 is likely to generate ketenimine
possible by low-temperature IR and Raman spectroscopy.
Convincing NMR spectra (1H, 13C, 14N), which were recorded at
−
6
45 °C by using solutions in [D ]acetone, were also reported. At
[
3]
first glance, the results of Trofimenko seem to be incompatible
with those of the Kornath group. But the presence of the acidic
[10]
salt 6 may have a strong influence on dissociation or exchange
processes of 5 and possibly on the equilibration rate of the
tautomers 3 and 5.
3
.[2] In 1963, Trofimenko repeated the synthesis of 2 by using the
precursor 1b, and called the substance
2 “aquoethereal
cyanoform”.[ He was successful to liberate 2 from solvents by
rapid evaporation and sublimation to get white crystals claimed to
be composed of 3. Structure assignment of 3 was mainly based
on elemental analysis and incomplete IR data, but 1H NMR
3,4]
spectroscopy was missing, although precursor
2
was
[*]
Prof. Dr. K. Banert, Dr. M. Chityala, Dr. M. Hagedorn
Chemnitz University of Technology, Organic Chemistry
Strasse der Nationen 62, 09111 Chemnitz (Germany)
E-Mail: klaus.banert@chemie.tu-chemnitz.de
Homepage: http://www.tu-chemnitz.de/chemie/org/index.html.en
Dr. H. Beckers, T. Stüker, Prof. Dr. S. Riedel
Institut für Chemie und Biochemie, Anorganische Chemie
Freie Universität Berlin
Fabeckstrasse 34−36, 14195 Berlin (Germany)
E-Mail: h.beckers@fu-berlin.de
Scheme 1. Reported syntheses of ketenimine 3 and cyanoform (5).
Dr. T. Rüffer, Prof. Dr. H. Lang
Chemnitz University of Technology, Inorganic Chemistry
Strasse der Nationen 62, 09111 Chemnitz (Germany)
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Herein, we report the first generation and unequivocal
spectroscopic assignment of ketenimine 3, and also the
characterization of pure cyanoform (5) by a single crystal X-ray
diffraction study. Furthermore, we clarify what kind of products are
formed after evaporation and sublimation of 2, and we explain
why special conditions have to be fulfilled to measure NMR
spectra of 5.
At first, we looked for an acid-free method to prepare the
target compounds 3 and 5 at low temperature. We assumed that
photolysis of the known[11] vinyl azide 7 will hopefully lead not only
to the probably highly unstable azirine 8,[12] but also to a product
which results from loss of dinitrogen combined with migration of
the hydrogen atom (Scheme 2). After irradiating a solution of 7 in
CN at −40 °C, H and 13C NMR spectra (−40 and +20 °C)
1
CD
3
indicated the formation of 8 (49% yield) and the aziridine 9 (45%).
Since the latter product was possibly generated by addition of 3
1
Figure 1.
H NMR spectra of cyanoform (5) and 2H-azirine 8 in [D ]THF (400
8
or 5 at 8, we performed a similar photolysis of 7 in [D
−
8
]THF at
MHz) at different temperatures starting with −95 °C (top).
60 °C. In the corresponding 1H NMR spectra (−60 °C), the
signals of 8 were not accompanied by those of 9; however, we
found only an additional broad signal, which could not be
precursor 7 and anhydrous solvent, we were not able to
completely avoid the signals of 4, which might include those of 3
if very rapid exchange processes are supposed. With the help of
our NMR methods, successful detection of 3 was not possible
since a conclusive 15N NMR signal with large doublet coupling
assigned to 3 or 5. Finally, we irradiated a solution of 7 in [D
8
]THF
at −85 °C and measured the NMR spectra of the products at
2
13
15
−
95 °C. The isotopically labeled precursors H-7, C-7, and N-
1
15
1
7
were included into our studies, and this supported assignments
2 8
J( N, H) was not observed, even at −130 °C (Me O, [D ]THF).
of signals and allowed to also analyze 15N NMR spectra. Besides
signals of the main product 8, we detected at such low
temperatures sharp signals of cyanoform (5) in the NMR spectra
The “intrinsic” stability of cyanoform (5) cannot be
determined when this species is generated by low-temperature
photolysis of 7 because subsequent warming leads to the trapping
reaction 5 + 8 → 9. Whereas vinyl azides generally form 2H-
azirines as primary products after irradiation, it is also well known
that the same substrates often affords mainly nitriles by
(1
H, 13C, 15N).[13] Stepwise going up with the temperature of the
NMR probe led to broader signals of 5; for example, the proton
signal was very broad at −50 °C and undetectable at −30 °C
[14]
(
Figure 1). At the latter temperature, formation of 9 slowly started.
thermolysis. Thus, we tested the stability of 5 by heating diluted
solutions of 7 and 15N-7 in [D ]THF. It turned out that heating at
When the probe was recooled to −95 °C, however, the sharp
signals of 5 returned. Thus, NMR signals of 5 are “hidden” if the
measurements are performed at the “wrong” temperatures.
8
60 °C for 1.5 min followed by rapid cooling to −95 °C and analysis
1
13
15
by NMR spectroscopy ( H, C, N) is a good compromise with
acceptable conversion of the precursors (51−54%) and limited
decay of 5 or 15N-5, which are produced with 49−52% yield based
on reacted 7 or 15N-7, respectively.
8
At low temperatures, photolyses of 7 in [D ]THF yielded also
a
small side product, that was identified as hydronium
tricyanomethanide (4) because addition of highly soluble
trihexyltetradecylphosphonium tricyanomethanide did not lead to
a new set of NMR signals of the anionic part. Even with dried
The unexpected thermal stability of cyanoform (5) and our
difficulties to detect the ketenimine 3 led to the question whether
the former compound is also isolated after workup and
sublimation of 2. When we repeated the experiment described by
Trofimenko,[ low-temperature 1H and C NMR spectra of the
3]
13
sublimed product indicated the formation of 4 and 5 instead of 3.
Iterated sublimation gave
a substance with an increased
proportion of 5. As shown in Figure 2, NMR spectra of such
mixtures of 4 and 5 in [D
8
]THF are highly temperature dependent.
1
Whereas sharp H NMR signals were detected at −95 °C, very
broad signals were observed at −50 °C, and decay started at 0 °C.
By recooling to −95 °C, however, the sharp signals returned.
Broadening of NMR signals at −50 °C was less pronounced when
the mixture of 4 and 5 consisted mainly of 5. Obviously,
Trofimenko[3] isolated at least mixtures which include cyanoform
(
5). But he misinterpreted his results, possibly also because
convincing characterization of 5 by NMR spectroscopy requires
low-temperature measurements.
Scheme 2. Photolysis and thermolysis of azide 7.
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Torr with an open connection to the pump, we obtained brilliant
colorless crystals.[13] It turned out that the product consisted of
single crystals of cyanoform (5), which were appropriate for X-ray
diffraction studies (Figure 3). As a pure solid, 5 obviously is by far
more stable than the same compound in solution. Most likely, this
can be attributed to the presence of layers in the solid state owing
to intermolecular hydrogen bridges. The C−H donor function is
involved in the formation of three different hydrogen bonds with
the correponding N atoms as acceptor function (see Figures
S2−S4 and Table S2 in the Supporting Information). These non-
classical hydrogen bridges are also remarkable in view of the
properties of 5 as one of the strongest carbon acids.
Figure 2. 1H NMR spectra of hydronium tricyanomethanide (4) and cyanoform
8
(5) in [D ]THF (400 MHz) at different temperatures starting with −95 °C (top).
8
In remarkable contrast to our NMR spectra of 5 in [D ]THF,
which indicate very broad signals at −50 °C (see, for example,
Figures 1 and 2), sharp NMR signals were previously reported [10]
and shown for the same compound measured in [D
6
]acetone at
−45 °C. When we prepared 5 from 2 or by photolysis of 7 and tried
to measure low-temperature NMR spectra in [D
6
]acetone, we did
Figure 3. Molecular structure of cyanoform (5) (50% probability level).
not get the corresponding signals because an addition of the
substrate at the solvent molecule completely transformed 5 into
the alcohol 10a (Scheme 3). This surprising result was confirmed
by treating 5 with non-deuterated acetone, which led to the
product 10b. After warming the solutions of 10a or 10b to room
temperature, decay of the adducts was observed, possibly via
liberation and decomposition of 5. The reaction of 5 with
acetaldehyde produced alcohol 10c, which was slightly more
stable and could be characterized by NMR spectroscopy at
ambient temperature.
The Raman spectrum of crystalline 5 (see Supp. Info.,
Figure S5) reveals strong and characteristic CH and CN
stretching bands at 2882.8 and 2283.3 cm−1 (Table 1). These
band positions are surprisingly close to those reported
previously[ for a mixture of 5 and 6 (2885 and 2287 cm ,
respectively), indicating that if intermolecular interactions to the
terminal H or N atoms of 5 are considered, there influence on
these two band positions is at least very similar. The influence of
such intermolecular interactions can be evaluated by comparison
with a matrix-isolation spectrum of 5, where molecules of 5 will be
isolated at cryogenic temperatures (< 20 K) in a host of solid argon.
This was achieved by evaporating colorless crystalline 5 at 7 °C
10]
−1
−1
in a flow of argon at a rate of 1 mL min and its deposition on the
matrix support, a rhodium plated mirror cooled to 4 K. The argon-
matrix isolation spectrum of 5 is shown in Figure 4, and the band
positions are listed and compared to quantum chemical results in
Table 1. As expected, the CH stretching vibration of the isolated
Scheme 3. Reaction of cyanoform (5) with acetone or acetaldehyde.
−1
molecule appeared at 2927.6 cm , and thus at significantly higher
A closer look revealed that previous NMR spectra were not
measured in [D
solvent and [D
6
]acetone, but liquid sulfur dioxide was used as
]acetone as external (!) standard (and lock).[
15]
6
When we synthesized 5 from 2 and tried to measure H and 13C
NMR spectra of the corresponding suspension in liquid sulfur
dioxide at −45 °C, we did not get any signal of cyanoform (5), even
after long-term measurement. After addition of trifluoroacetic acid,
however, we detected H and 13C NMR spectra of 5 with sharp
signals, which are very similar to those published[10] previously.
Obviously, the presence of an additional Brønsted acid has a
strong effect on the NMR spectra of 5 in liquid sulfur dioxide, and
measurement of 5 in this solvent at −45 °C is perhaps not possible
without such an acid, for example, acidic salt 6.
1
1
When we completely removed the solvents from 2 at −20 °C
and 10−2 Torr and rapidly used the residue for sublimation into an
uncooled horizontal glass tube by heating from 20 to 50 °C at 10−2
Figure 4. FT-IR spectrum of cyanoform (5) isolated in solid argon at 4 K.
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Table 1. Experimental IR and Raman as well as calculated band positions (in
cm−1) of cyanoform (5) and their tentative assignment.
by secondary photolysis reactions, because i) further photolysis
did not change the relative amount of the primary photolysis
products, and ii) the 8 to 3 conversion is at least a two-step
[
a]
[b]
IR
Raman
2882.8
288,3
B3LYP
IR-Int.)
2909 (12)
Anharm.
[d]
Assignment
process, involving
a
rather high-barrier H-migration and
[c]
(
CCSD(T)
2929
[9a]
subsequent C−N bond cleavage. Thus 3 is a product of the
photolysis of 7. This assumption is verified by our DFT B3LYP/cc-
pVTZ calculations (Figure S8), which revealed a concerted
2
2
927.6
806.1
ν(HC), A
2268 + 556
(CCN), A
(CCN), E
δ(HCC), E
1
2
2302 (0)
2294 (2)
1244 (7)
2260
2261
1266
ν
s
1
2
1
268.1
267.9
2269.6
1254.3
ν
a
2
reaction of 7 by elimination of N with concurrent hydrogen shift
from the carbon to the nitrogen atom (see Figure S9).
1266.7
In conclusion, detection of ketenimine 3 was possible by
irradiation of azide 7 in noble gas matrix only. We assume that
photolysis in solution leads also to the primary product 3 since
similar transformations[16] are known for other vinyl azides. In
solution, however, very rapid tautomerism most probably induces
the reaction 3 → 5. Pure cyanoform (5) is obviously available via
special sublimation of 4. Not only by using neat 5 (see Scheme 3)
or 2, but also by gentle thermolysis of 7, we were able to initiate
nucleophilic attack at different types of carbon electrophiles. We
will report on these reactions, which may be valuable for carbon–
carbon bond formation and heterocyclic ring transformation, in the
near future.
1
023.4
1025.9
987 (37)
1012
a 3
ν (CC ), E
[
e]
1013.9
8
8
89.5
22.5
556 + 334
(CC ), A
(CC ), A
(CC ), E
(CCN), A
(CCN), E
(C(CN) ), A
(C(CN) ), E
822.4
811 (5)
558 (0)
552 (1)
345 (0)
344 (0)
162 (25)
128 (15)
822
556
553
336
334
161
128
ν
s
3
1
570.9
δ
s
3
1
5
56.4
561.2
δ
a
3
[
f]
3
3
48.8
48.8
δ
s
2
[f]
δ
a
164.4
δ
δ
s
3
1
149.0
a
3
[
a] Isolated in solid argon at 4 K (strongest matrix sites are listed); [b] solid
sample, see Figure S5; [c] This work: DFT B3LYP/cc-pVTZ level, IR intensities
in parentheses (in km mol−1); [d] Ref. 9a, computed by adding VPT2 anharmonic
corrections calculated at MP2/cc-pVTZ level to harmonic frequencies calculated
at the CCSD(T)/cc-pVTZ level; [e] strong matrix site; [f] overlapped.
Conflict of Interest
wavenumbers than in the solid state. The matrix-isolation
spectrum is in excellent agreement with previously calculated
anharmonic vibrational frequencies obtained by adding VPT2
anharmonic frequency corrections (predicted at the MP2/cc-pVTZ
level) to the CCSD(T)/cc-pVTZ harmonic frequencies.[9a] In
agreement with preliminary TD-DFT B3LYP/cc-pVTZ calculations
The authors declare no conflict of interest.
Keywords: azides • cyanides • cyanoform • ketenimines •
reactive intermediates
(
see Supp. Info., Table S3), our attempts to initiate a photo-
rearrangement of 5 failed (Figure S6).
After we have successfully proved that 5 can not only be
obtained from 2, but also by heating or irradiating azide 7 in
Acknowledgements
We thank Jana Buschmann for her skill to isolate single crystals
of 5, and we gratefully acknowledge the Zentraleinrichtung für
Datenverarbeitung (ZEDAT) of the Freie Universität Berlin for the
allocation of computer time. M. C. is grateful to DAAD (Deutscher
Akademischer Austauschdienst) for a Ph.D. fellowship. This work
is Part 38 in the series “Reactions of Unsaturated Azides”, for Part
8
[D ]THF solution, the question arises whether the conversion of 7
to 5 is a pure monomolecular process that can be studied under
matrix-isolation conditions. Photolysis of 7 probably involves the
azirine intermediate 8, where the rearrangement from 8 to 5 has
recently been shown requires an appreciable energy to surmount
a barrier of about 60 kcal mol−1 at the CCSD(T)/cc-pVTZ level.[9a]
We have studied the photolysis of 7 isolated in a solid argon
matrix, prepared by gently heating (30−50 °C) 7 and its deposition
with an excess of argon gas (ca. 1:1000) onto the matrix support
held at 151 K. To facilitate the band assignment, the experiments
37, see K. Weigand, N. Singh, M. Hagedorn, K. Banert, Org.
Chem. Front. 2017, 4, 191−195.
______________________________________________________________
[1]
[2]
[3]
[4]
H. Schmidtmann, Ber. Dtsch. Chem. Ges. 1896, 29, 1168−1175.
A. Hantzsch, G. Osswald, Ber. Dtsch. Chem. Ges. 1899, 32, 641–650.
S. Trofimenko, J. Org. Chem. 1963, 28, 217–218.
were repeated using 15N labelled 7, in which either the terminal or
the -nitrogen atoms of the azide group were substituted by 15N.
For a more detailed description of the cyanoform story, see the excellent
introduction in ref. [5].
1
5
Spectra of 7 and of N-7 are shown in Figure S7. Band positions
and their 14/15N isotope shifts are compared to calculated values
in Table S4. As expected, photolysis of 7 or 15N-7 using a solid-
state laser of  = 266 nm efficiently produces the corresponding
[
5]
D. Šišak, L. B. McCusker, A. Buckl, G. Wuitschik, Y.-L. Wu, W. B.
Schweizer, J. D. Dunitz, Chem. Eur. J. 2010, 16, 7224–7230.
B. Bak, H. Svanholt, J. Mol. Struct. 1977, 37, 153–156.
[
[
6]
7]
H. Bock, R. Dammel, Z. Naturforsch. 1987, 42b, 315−322.
a) R. H. Boyd, J. Phys. Chem. 1963, 67, 737–744; b) E. Raamat, K.
Kaupmees, G. Ovsjannikov, A. Trummal, A. Kütt, J. Saame, I. Koppel, I.
Kaljurand, L. Lipping, T. Rodima, V. Pihl, I. A. Koppel, I. Leito, J. Phys.
Org. Chem. 2013, 26, 162–170; c) A. Kütt, T. Rodima, J. Saame, E.
Raamat, V. Mäemets, I. Kaljurand, I. A. Koppel, R. Y. Garlyauskayte, Y.
L. Yagupolskii, L. M. Yagupolskii, E. Bernhardt, H. Willner, I. Leito, J. Org.
Chem. 2011, 76, 391−395.
1
5
isotopologues 8 and N-8, respectively (Figure S8 and Table S5).
However, a second photolysis product was detected, showing a
rather strong N−H and a strong ketenimine absorption at 3360
[8]
−1
and 2096 cm , respectively (Figure S8). Based on these
vibrations and their characteristic 14/15N isotope shifts, we
assigned this species to the hitherto unknown ketenimine 3 (Table
S6). We note that absorptions of cyanoform (5) were not detected
in these experiments, and attempts to produce 5 by irradiating the
deposit failed.[13] We can also exclude the formation of 3 from 8
[
9]
a) M. Szczepaniak, J. Moc, J. Phys. Chem. A 2017, 121, 1319–1327; b)
H. Brand, J. F. Liebman, A. Schulz, Eur. J. Org. Chem. 2008, 4665−4675;
c) B. Bak, C. Bjorkman, J. Mol. Struct. 1975, 25, 131−140; d) S. S.
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Stoyanov, J. A. Tsenov, I. G. Binev, Asian Chem. Lett. 2009, 13,
1
55−162; e) S. A. Elroby, Chem. Cent. J. 2016, 10, 20.
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3980; Angew. Chem. Int. Ed. 2015, 54, 13775–13776.
[
[
1
11] a) H. Schubert, M. Regitz, Synthesis 1982, 149–151; b) K. Friedrich,
Angew. Chem. 1967, 79, 980; Angew. Chem. Int. Ed. Engl. 1967, 6, 959–
960; c) P. P. Koriuta, A. I. Kalenskaya, V. I. Shevchenko, Zh. Obshch.
Khim. 1971, 41, 2390−2395.
[
[
[
12] For investigation of 8 by quantum chemical methods, see ref. [9a].
13] For details, see the Supporting Information.
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Azides, Syntheses and Applications (Eds.: S. Bräse, K. Banert), Wiley,
Chichester, 2010, pp. 115–166; b) S. J. Collier in Science of Synthesis,
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Schaumann), Thieme, Stuttgart, 1993, pp. 818–875; d) A. Hassner in
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83, 103−109; Angew. Chem. Int. Ed. Engl. 1971, 10, 98–104; h) B. Hu,
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Mitschke, C. Wentrup, J. Org. Chem. 2002, 67, 1084–1092.
[
[
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Held incommunicado in argon
matrix, a long-sought ketenimine can
be detected after irradiation of an
appropriate vinyl azide, whereas
photolysis or thermolysis of the same
precursor in solution leads to
Klaus Banert,* Madhu Chityala, Manfred
Hagedorn, Helmut Beckers,* Tony
Stüker, Sebastian Riedel, Tobias Rüffer,
and Heinrich Lang
Page No. – Page No.
cyanoform, which is also available as
single crystals from potassium
tricyanomethanide.
Tricyanomethane and its Ketenimine
Tautomer Generated from Different
Precursors and Analyzed in Solution,
Argon Matrix, and Single Crystal
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