Supramolecular fixation of NO2 with calix[4]arenes

Article abstract of DOI:10.1039/b207901a

Reaction of NO₂ with simple calix[4]arenes in chloroform in the presence of a Lewis acid rapidly results in intense coloration caused by the encapsulation of nitrosonium cation.

Full text of DOI:10.1039/b207901a

Supramolecular fixation of NO2 with calix[4]arenes
Grigory V. Zyryanov, Yanlong Kang, Stephen P. Stampp and Dmitry M. Rudkevich*
Department of Chemistry & Biochemistry, University of Texas at Arlington, Arlington, TX 76019-0065, USA.
E-mail: rudkevich@uta.edu; Fax: +817 272 3808; Tel: +817 272 5245
Received (in Columbia, MO, USA) 13th August 2002, Accepted 8th October 2002
First published as an Advance Article on the web 25th October 2002
Reaction of NO2 with simple calix[4]arenes in chloroform in
the presence of a Lewis acid rapidly results in intense
coloration caused by the encapsulation of nitrosonium
cation.
1 were recorded as doublets at 4.41 and 3.12 ppm (J = 12.5 Hz).
In complex 3, these were seen as doublets at 4.39 and 3.43 ppm
(J = 12 Hz). The aromatic protons of free 2 were seen as a
doublet and a triplet, 2+1, at 7.00 and 6.68 ppm, respectively (J
= 7.5 Hz). In nitrosonium complex 4, these were transformed
into a triplet and a doublet, 1+2, at 7.17 and 7.08 ppm,
respectively (J = 7.5 Hz). Elemental analysis of extremely
moisture sensitive 3 and 4 proved to be difficult but reproduci-
bly showed CHN ratios corresponding to the presence of only
one NO+ cation in both structures.
Nitrogen dioxide (NO2) is a major component of so-called NOx
gases.1 It is a toxic atmospheric pollutant derived from fossil
fuel combustion, power plants, and large-scale industrial
processes. In combination with nitric oxide (NO), NO2 is
involved in various nitrosation processes in biological tissues.2
Nitrosative mechanisms have been implicated in ion con-
ductance, signal transduction, glycolysis, apoptosis, and DNA
repair. Extensive NO2 circulation in the atmosphere requires not
only its systematic monitoring,3 but also necessitates the
development of improved methods of NO2 fixation. Here, we
describe host–guest complexes, formed upon interaction be-
tween NO2 and simple calix[4]arenes. Our findings offer a
novel process of NO2 utilization and may also lead towards
stable, supramolecular nitrosating reagents, nitrogen oxides
storing materials, and new visual sensory systems for NOx.
Calixarenes are popular building blocks for molecular
containers—cavitands, carcerands, and capsules.4 They provide
quite rigid, p-electron rich inner cavities for complexation of
electron deficient guest-species. We discovered that tetra-
alkylated calix[4]arenes 15 and 2,6 possessing cone and
1,3-alternate conformations, respectively, reversibly interact
with NO2 and trap the highly reactive nitrosonium (NO+) cation.
Bubbling NO2 through the solutions of 1 and 2 in CHCl3
resulted in instant, deep coloration. The UV–vis spectra
changed accordingly: the broad bands appeared at lmax = 560
and 512 nm, respectively. This is in striking contrast to the
colorless solutions of 1 and 2, and the pale yellow solution of
NO2 in CHCl3, and implies a charge-transfer. NO2 is a popular
nitrosating/nitrating agent, and its chemistry is well developed.
Two molecules of NO2 exist in equilibrium with N2O4, which
Independent structural evidence came from the complexation
experiments between calixarenes 1,2 and commercially availa-
ble NO+SbF6 salt (CDCl3, 295 K). The corresponding UV–
2
vis, FTIR and 1H NMR complexation induced changes were in
agreement with the data presented above for complexes 3,4.
In the control experiments with non-cyclic anisole (e.g.,
methoxybenzene), only weak coloration was observed upon
exposure to NO2. Moreover, when mesitylene-derived, Pappa-
lardo’s calixarenes,9 with sterically blocked cavities, were
tested, no coloration was observed either; there was no
indication for strong complexation in the UV–vis and 1H NMR
spectra. These experiments emphasize the importance of the
calixarene cavities in the described transformations and rule out
the possibility of the NO+ coordination outside the cavity.†
According to molecular modeling, only one NO+ can fit inside
the cavities of 1 and 2.
Recently, Rathore, Kochi and co-workers described charge-
transfer complexes between NO+ and structurally similar
calix[4]arenes.10 The cation was indeed found encapsulated
within the cavity (X-ray analysis). Our spectral data are in
agreement with these data. Owing to the fact, that two
molecules of NO2 may disproportionate to NO+NO32, we thus
may disproportionate to ionic NO+NO3 upon reacting with
2
aromatic compounds.7
Interaction of NO2 with 1 and 2 is very dynamic, and the
initial 1H NMR analysis gave rather complex, quickly changing
pictures. The solutions of 1,2 and excess NO2 bleached within
1–2 h, yielding mixtures of p-nitrated calixarenes (TLC, NMR).
To slow down the nitration and to identify the involved
complexes, solutions of 1,2 and NO2 in CHCl3 were treated with
SnCl4. It is known that Lewis acids stabilize arene-nitrosonium
charge-transfer complexes.8 Subsequent precipitation with
hexanes resulted in deeply colored, moisture sensitive solids,
assigned to nitrosonium complexes 3 and 4 ( > 90% yield, Fig.
1). Compounds 3 and 4 were characterized by UV–vis, FTIR,
1H NMR spectroscopy in dry chloroform, and CHN elemental
analysis.
The UV–vis spectra showed broad charge-transfer7 bands at
lmax ~ 563 and 524 nm, and the FTIR spectra exhibited
characteristic7 arene-NO+ stretching at n = 1923 and 1955
cm21 for 3 and 4, respectively. The 1H NMR spectra of 3 and 4
showed new sets of the calixarene signals (Fig. 2). In particular,
aromatic CH protons of guest-free 1 were seen as a singlet at
6.76 ppm. In nitrosonium complex 3, these were transformed
into a singlet at 7.00 ppm. The methylene bridge CH2 protons of
Fig. 1 Chemical fixation of NO2 with calix[4]arenes. Formation of
nitrosonium complexes.
2792
CHEM. COMMUN., 2002, 2792–2793
This journal is © The Royal Society of Chemistry 2002

conclude that reaction between NO2 and calix[4]arenes lead to
NO+ encapsulation.
In the experiments between calixarene 2, SnCl4 and a larger
( ~ 50-fold) excess of NO2, one more prereactive complex was
detected, which we assigned to nitronium species 5 (Fig. 2). In
Addition of H2O or alcohols to the freshly prepared CHCl3
solutions of 3–5 resulted in the complete dissociation and
1
recovery of calixarenes 1,2 (TLC, UV–vis, H NMR). Sig-
nificantly, H2O decolorizes 4 within seconds, but takes several
minutes to decompose complex 3. Apparently, t-Bu groups at
the upper rim of the latter pose significant steric hindrances and
protect the encapsulated NO+ species. Such stability of the
arene-NO+ complex is without precedent.7
Primary and secondary amides also bleached the solutions of
3 and 4. On a preparative scale, N-nitrosation of secondary
amides AlkC(O)NHMe (Alk = n-Pr, n-Hex, n-Hept) by
nitrosonium complex 4 in CHCl3 yielded N-nitrosoamides
AlkC(O)N(NO)Me.‡ This not only provides additional struc-
tural evidence for the discovered complexes, but also opens new
perspectives to use them as supramolecular/encapsulated4
nitrosating reagents.
In summary, a novel NO2 fixation process is now available,
which employs simple calixarenes. The resulting complexes can
be used as stable nitrosating and nitrating reagents. Calixarenes
conveniently transmit the information about NO2 binding via
visible light signals. The described charge-transfer interactions
are unique for NO2 and would guarantee its detection in the
presence of such gases as H2O, O2, HCl, SOx, NH3, and even
NO. These findings open wider possibilities towards more
sophisticated NO2/NOx sensing materials, including peptide-
based nanostructures. The latter may be useful to detect NOx
species in biological fluids, provided that the complexes with
sterically hindered calixarenes are quite stable in water.
Financial support is acknowledged from the Donors of The
Petroleum Research Fund, administered by the American
Chemical Society, and the University of Texas at Arlington.
the UV–vis spectrum, a broad charge-transfer band at lmax
=
512 nm was recorded. The FTIR spectrum showed stretches at
n = 2356 cm21, characteristic8 for NO2 species. In the H
NMR spectrum, complex 5 exhibits a broader doublet and a
triplet, 2+1, at 7.10 and 7.00, respectively (Fig. 2). Due to their
extreme reactivity, none of the arene-nitronium p-complexes
have been isolated to date,11 and we attribute the stability of 5
to the encapsulation effects.
+
1
Complex 4 can be converted to 5 when a larger excess of NO2
is employed. As one possible scenario, initially formed
nitrosonium complex 4 yields the electron transfer complex
[2+··NO]NO3 and releases NO. The resulting cation-radical
2
2+·NO3
reacts with an excess of NO2, producing
2
[2+··NO2]NO32 and further charge-transfer nitronium complex
5 ([2·NO2 ]NO32). Bent NO2 cannot fit inside 2, but linear
+
+
NO2 can. Judging from the intense coloration, the calixarene
walls in 5, most probably, encapsulate NO2 , however more
experiments are needed to further support this. At this stage, the
+
structure of 5 was independently confirmed by complexation
+
2
between 2 and NO2 SbF6 salt in CDCl3. The obtained UV–
vis, FTIR and 1H NMR spectra were similar to those of complex
5.
We then reexamined the reaction between 1,2 and NO2, in the
absence of SnCl4. As an excess NO2 was passed through the
solution of 1, spectral features of nitrosonium complex
[1·NO+]NO3 were recorded (UV–vis, H NMR), along with
the nitration products. For 2, no signals for nitrosonium
2
1
complex [2·NO+]NO3 were seen, but nitronium complex
2
+
2
[2·NO2 ]NO3 was detected. Apparently, while formed the
nitrosonium species quickly react with excess NO2. Both
reactions subsequently yield p-nitrated calixarenes.
Notes and references
† Even slight excess of NO+SbF62 results in complete complex formation
in CDCl3, and no free calixarenes 1,2 were observed after equilibration; Kass
> 106 M21 for both complexes was estimated. The experimental details
will be given in a full paper.
‡ Spectral data for the obtained N-nitrosoamides are in agreement with
those published, see ref. 12. Mixing nitronium complex 5 and AlkC(O)NH2
(Alk = Me, t-Bu) in MeCN at 295 K resulted in the nitro derivatives
AlkC(O)NHNO2 (1H NMR analysis).
1 M. T. Lerdau, J. W. Munger and D. J. Jacob, Science, 2000, 289,
2291.
2 M. Kirsch, H.-G. Korth, R. Sustmann and H. de Groot, Biol. Chem.,
2002, 383, 389.
3 D. Filippini, M. Rosch, R. Aragon and U. Weimar, Sens. Actuators, B,
2001, 81, 83; A. Bradford, P. L. Drake, O. Worsfold, I. P. Peterson, D.
J. Walton and G. J. Price, Phys. Chem. Chem. Phys., 2001, 3, 1750; C.
M. Dooling, O. Worsfold, T. H. Richardson, R. Tregonning, M. O.
Vysotsky, C. A. Hunter, K. Kato, K. Shinbo and F. Kaneko, Mater.
Chem., 2001, 11, 392.
4 D. M. Rudkevich, Bull. Chem. Soc. Jpn., 2002, 75, 393 (review).
5 P. J. A. Kenis, O. F. J. Noordman, H. Schonherr, E. G. Kerver, B. H. M.
Snellink-Ruel, G. J. van Hummel, S. Harkema, C. P. J. M. van der Vorst,
J. Hare, S. J. Picken, J. F. J. Engbersen, N. F. Van Hulst, G. J. Vancso
and D. N. Reinhoudt, Chem. Eur. J., 1998, 4, 1225.
6 A. Casnati, A. Pochini, R. Ungaro, F. Ugozzoli, F. Arnaud, S. Fanni, M.-
J. Schwing, R. J. M. Egberink, F. de Jong and D. N. Reinhoudt, J. Am.
Chem. Soc., 1995, 117, 2767.
7 E. Bosch and J. K. Kochi, J. Org. Chem., 1994, 59, 3314; G. I. Borodkin
and V. G. Shubin, Russ. Chem. Rev., 2001, 70, 211 (review).
8 J. C. Evans, H. W. Rinn, S. J. Kuhn and G. A. Olah, Inorg. Chem., 1964,
3, 857.
9 S. Pappalardo, G. Ferguson and F. Gallagher, J. Org. Chem., 1992, 57,
7102.
10 R. Rathore, S. V. Lindeman, K. S. S. P. Rao, D. Sun and J. K. Kochi,
Angew. Chem., Int. Ed., 2000, 39, 2123.
11 S. V. Rosokha and J. K. Kochi, J. Org. Chem., 2002, 67, 1727–1737.
12 J. Garcia, J. Gonzalez, R. Segura, F. Urpi and J. Vilarrasa, J. Org.
Chem., 1984, 49, 3322.
Fig. 2 Proposed structures and portions of the 1H NMR spectra (500 MHz,
CDCl3, 295 K) of: (a) calix[4]arene 1. (b) nitrosonium complex 3. (c)
calix[4]arene 2. (d) nitrosonium complex 4. (e) nitronium complex 5. The
residual CHCl3 signals are marked ‘4’. In the MacroModel 7.1 representa-
tions, long alkyl chains and hydrogen atoms are omitted for viewing
clarity.
CHEM. COMMUN., 2002, 2792–2793
2793