Nitrosobenzene cross-dimerization: Structural selectivity in solution and in solid state

Article abstract of DOI:10.1016/j.molstruc.2010.05.034

Possibility of nitrosobenzenes to form dimeric molecular structures (azodioxides) is used as a model for intermolecular selectivity investigations in solution as well as in solid state. Cross-dimerization of different combinations of p- and m-substituted nitrosobenzene pairs was studied by variable temperature ¹H NMR, solid-state NMR (CP MAS), IR, and ab initio calculations. It is evident that p-nitronitrosobenzene behaves nonselectively because it forms dimers with all the studied nitrosobenzene partners. In contrast, p-methoxynitrosobenzene in most cases does not form dimers. The evidence that ability to form dimers is different in solution than in the solid state can be explained by influence of molecular arrangements in the crystal lattice.

Full text of DOI:10.1016/j.molstruc.2010.05.034

Journal of Molecular Structure 979 (2010) 22–26
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Nitrosobenzene cross-dimerization: Structural selectivity in solution
and in solid state
Ivana Biljan a, Gorana Cvjetojevic a, Vilko Smrecki b, Predrag Novak a, Gregor Mali c, Janez Plavec c,
Darko Babic b, Zlatko Mihalic a, Hrvoj Vancik a,
*
a Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, HR-10000 Zagreb, Croatia
b
-
ˇ
Ruder Boškovi´c Institute, Bijenicka 54, HR-10000 Zagreb, Croatia
c National Institute of Chemistry, Hajdrihova 19, SI-1115 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
Possibility of nitrosobenzenes to form dimeric molecular structures (azodioxides) is used as a model for
intermolecular selectivity investigations in solution as well as in solid state. Cross-dimerization of differ-
ent combinations of p- and m-substituted nitrosobenzene pairs was studied by variable temperature 1H
NMR, solid-state NMR (CP MAS), IR, and ab initio calculations. It is evident that p-nitronitrosobenzene
behaves nonselectively because it forms dimers with all the studied nitrosobenzene partners. In contrast,
p-methoxynitrosobenzene in most cases does not form dimers. The evidence that ability to form dimers
is different in solution than in the solid state can be explained by influence of molecular arrangements in
the crystal lattice.
Received 28 April 2010
Received in revised form 24 May 2010
Accepted 24 May 2010
Available online 1 June 2010
Keywords:
Nitrosobenzenes
Azodioxides
Cross-dimerization
Selectivity
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
together with dimers of p-bromo- and p-nitronitrosobenzene
(homodimers) [4b].
Nitrosobenzenes and their dimers (azodioxides) exist in solu-
tion in equilibrium, in which monomers are dominant at the room
temperature [1]. Appearance of dimers, either with (Z) or with (E)
stereochemistry, can be observed by NMR only if the solution
temperature is lowered to 225 K [2]. However, the ability of nitros-
obenzenes to dimerize is structure dependent, and it was proposed
that strong electron-donating para-substituents diminish propor-
tion of dimers. For instance, p-methoxynitrosobenzene does not
dimerize at all. The influence of substituents on the degree of
dimerization was previously explained by contribution of the quin-
oid resonance structure (Scheme 1) [3].
In solid state, dimerization is dependent not only on the elec-
tronic factors but also on the molecular arrangement in the crystal
lattice (topochemical effect). In principle, if nitroso molecules are
well oriented with their nitroso groups in the vicinity, formation
of azodioxides is strongly preferred [4a].
Cross-linking selectivity in solid state between different nitro-
sobenzene pairs has been investigated previously by combinatorial
method [5], but without looking in the details of molecular struc-
tures of heterodimers. In this work we analyze the selectivity in
formation of corresponding heterodimers from dimerizations of
para- and meta-substituted nitrosobenzenes with parent nitroso-
benzene in solution and in solid state. The nitrosobenzenes are
chosen on the basis of the different electron effects of meta- and
para-substituents (Scheme 2). Their selectivity in forming dimers
was studied in solution by low temperature 1H NMR, and in the so-
lid state by 13C CP MAS NMR as well as by IR spectroscopy of the
15N-isotopomers. Experimental results are compared with the re-
sults of ab initio calculations.
2. Experimental
In our recent paper we found that the mixture of p-bromo- and
p-nitronitrosobenzene can combine by forming the cross-linked
dimer (heterodimer), which was observed by NMR in solution. In
solid state, the same heterodimer appears as a solid solution
2.1. Preparation of nitrosobenzenes
Nitrosobenzenes 1, 2, 4, 5, 7, 9, 10 and 11 were prepared by oxi-
dation of corresponding anilines with OxoneÒ (K2SO5 ꢀ K2-
SO4 ꢀ KHSO4) analogously to a method previously described in
the literature [6]. Compounds 3, 6 and 8 were prepared from cor-
responding nitrobenzenes by standard procedures based on Zn
reduction followed by oxidation with FeCl3 [7].
* Corresponding author. Tel.: +385 1 4606400; fax: +385 1 4606401.
E-mail address: vancik@chem.pmf.hr (H. Vancik).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.05.034

I. Biljan et al. / Journal of Molecular Structure 979 (2010) 22–26
23
pling during acquisition. Contact time for cross-polarization was
5 ms, repetition delay was 10 s and sample rotation frequency
was 10 kHz for all samples. Number of scans for different samples
varied between 1000 and 6000. Chemical shifts of 13C signals were
referenced to the signal of tetramethylsilane.
2.5. Methods of calculations
Structures, natural charges and natural bond orders were calcu-
lated by the NBO-NRT analysis using the NBO 5.0 program package
[8a] incorporated into the Gaussian 03 program package [8b].
Scheme 1.
3. Results and discussion
2.2. Preparation of crystals of heterodimers
3.1. Cross-dimerization in solution
The 1D and 2D 1H NMR spectra were recorded in chloroform-d
solution for all the heterodimer combinations in which the referent
partner was always the parent nitrosobenzene, and the other part-
ner was one of the substituted nitrosobenzenes. While at the room
temperature only the mixtures of monomers were observed, low-
ering the temperature to 218 K enabled us to observe signals as-
signed to dimers. Heterodimers were recognized on the basis of
the extra signals, which neither belong to nitroso monomers nor
to any of homodimers.
Heterodimers were prepared by dissolving equimolar quantities
of nitrosobenzenes 2–10 and 15N-nitrosobenzene (11) in cold
(5 °C) and dry chloroform. Green solutions were left to evaporate
in a refrigerator at 5 °C.
2.3. IR spectroscopy
IR spectra were recorded on a Bruker Equinox FT-IR spectrome-
ter under 2 cmꢁ1 resolution.
Because of the spectral complexity, we were not able to deter-
mine the ratio of (Z)- and (E)-dimers, but we successfully estimated
the extent to which heterodimers appear. The results are collected in
Table 1. Looking at the intensity of the heterodimer signals, we could
observe that the most reactive partner for dimerization with the par-
ent nitrosobenzene is p-nitronitrosobenzene (9). On the other side,
p-methoxy- (8), p-methyl (2), p-fluoro (3), p-chloro (4), and p-meth-
ylformylnitrosobenzene (10) do not form heterodimers at all. In
frame of the selectivity concept, 9 is quite unselective, and 8 affords
maximal selectivity: the formation of heterodimer was observed
only in its combination with 9. It means that 8 selects only one of
the studied partners for cross-dimerization.
2.4. NMR spectroscopy
The liquid-state one- and two-dimensional 1H NMR spectra
(600.13 MHz) were measured in CDCl3 at 298 K and 218 K on a Bru-
ker AV600 spectrometer equipped with 5 mm TBI and 5 mm BBO
probes with z-gradients. Chemical shifts, in ppm, were referred to
TMS as the internal standard. The standard 1H and COSY experi-
ments were used. The FID resolution in 1H NMR was 0.29 Hz per
point. The COSY technique with a standard two p/2 pulse sequence
(cosygpqf) in the pulsed field gradient mode (z-gradient) was ap-
plied using 2 k data points in F2 dimension and 512 increments in
F1 dimension. The latter was subsequently zero-filled to 1024
points. Increments were obtained by four scans each, 8 kHz sweep
width and pulse spacing was 1.0 s. The FID resolution was 4.1 Hz/
point and 16.4 Hz/point in F2 and F1 dimensions, respectively.
Solid-state NMR experiments were performed on a 14.1 T
Varian NMR System equipped with a 3.2 mm MAS probe. 1HA13C
3.2. Cross-dimerization in solid state
To study cross-dimerization in solid state we have mixed equi-
molar solutions of different nitrosobenzenes and left the solvent to
evaporate. The composition of the remaining crystals can consists
from the mixture of homodimers and/or the heterodimer. To distin-
guish heterodimer from the mixture of homodimers, we have la-
CPMAS spectra were recorded using
cross-polarization pulse sequence with high-power proton decou-
a
standard ramped
Scheme 2.

24
I. Biljan et al. / Journal of Molecular Structure 979 (2010) 22–26
Table 1
1H NMR shifts of aromatic protons of compounds (1–10) and their combinations recorded at 218 K in CDCl3.
c
c
Compound
Dominant speciesa,b
dH
Combination
New signalse
dH
2
3
4
5
6
2,6 3,5 4 20,60 30,50
1
2
3
4
5
6
7
(Z)-D
M
(Z)-D
M
(Z)-D
M
(Z)-D
M
(Z)-D
M
(Z)-D
M
(Z)-D
M
M
7.35–7.45 complex multiplet
1 + 2
1 + 3
1 + 4
1 + 5
1 + 6
1 + 7f
1 + 8
7.96
7.26
ꢂ7.90
7.43
ꢂ8.00
7.40
7.89
7.31
ꢂ7.80
7.14
7.68
7.57
7.59
6.58
7.65
8.27
8.61
7.50
8.03
7.69
7.16
7.45
7.13
7.36
7.36
7.65
7.56
ꢂ7.80
7.76
8.08
7.80
7.69
7.16
7.45
7.13
7.36
7.36
7.65
7.56
ꢂ7.80
7.76
8.08
7.36
8.26
7.30
8.29
8.51
8.16
8.07
8.36
7.96
7.26
ꢂ7.90
7.43
ꢂ8.00
7.40
7.89
+
+
+
7.42–7.50
7.40–7.50
?
7.29
7.15
?
7.52
7.72
?
7.31
ꢂ7.80
7.14
7.68
7.19
7.47
7.72–7.77
8d
9
6.88
8.29
8.51
8.16
8.07
8.36
9.39
7.65
8.27
8.61
7.50
8.03
1 + 9
1 + 10
+
+
7.42–7.56
7.72
7.64
8.37
8.30
(Z)-D
(E)-D
M
(Z)-D
M
10
8 + 9
7.35
6.89
-
a
M = monomer and D = dimer.
b
Assignation was based in accordance with the previous study which suggested that (Z)-dimers were more abundant over (E)-dimers in solution of nitrosobenzenes at low
temperatures [2].
c
CDCl3 solutions measured at 218 K.
Ref. [3b].
New signals that appeared in five combinations are attributed to heterodimers, presumably the (Z)-dimers.
Six additional signals were detected at d = 7.56, 7.45, 7.43, 7.31, 7.29, 7.20 but could not be assigned with certainty.
d
e
f
Table 3
beled the NO group of the parent nitrosobenzene with 15N isotope.
Appearance of heterodimers in solution and in solid state.
Since all the other nitroso partners have only 14N nitroso group,
the appearance of the 15NA14N signal of the asymmetric ON@NO
vibration in the IR spectrum of the solid mixture is an indication that
a heterodimer is formed in solid state. Note, that while the parent
nitrosobenzene homodimer must have 15NA15N signal, other homo-
dimers must have only 14NA14N signals. The 15NA15N signals which
belong to homodimers of parent nitrosobenzene are at higher wave-
numbers than 14NA14N signals because it crystallizes as Z-dimer.
The only exception is combination 11 + 9 where position of the
15NA15N signal of nitrosobenzene homodimer is really at lower fre-
quency (1233 cmꢁ1). This could be an indication that nitrosoben-
zene homodimer cocrystallizes with 9 in form of E-dimer. Such
cocrystallization was described previously [4b]. The combination
11 + 6 is somewhat different because p-iodonitrosobenzene can
crystallize in both forms, as monomer as well as dimer [1j]. From
the results represented in Table 2, most of the combinations are able
to form heterodimers with the exception of p-methoxynitrosoben-
zene (8) and p-methylformylnitrosobenzene (10). As it could be seen
in Table 3, the ability to form heterodimers in solid state is higher
than in solution and the derivatives which do not form heterodimers
in solid state also are not able to form heterodimers in solution too.
Combination
Solution
Solid state
1 + 2
1 + 3
1 + 4
1 + 5
1 + 6
1 + 7
1 + 8
1 + 9
1 + 10
d
d
d
d
d
d
Ç
Ç
Ç
Ç
d
Ç
d
Labels heterodimers formed in solution.
Labels heterodimers formed in solid state.
Evidently, the topochemical factors are crucial for the ON@NO bond
formation in these combinations.
For three combinations (p-methylformyl- (10), p-nitro- (9), and
m-chloronitrosobenzenes (7)) with the parent nitrosobenzene we
have recorded and analyzedthe 13C NMR CP MAS spectra. Solid-state
NMR spectroscopy can provide a wealth of information on different
solid forms and their mixtures and is now considered as one of the
Table 2
Wavenumbers and assignments of bands in the IR spectra of 15N-nitrosobenzene (11) combined with compounds (2–10).
14N@OÞ=cmꢁ1
m
ð
14N14NÞ=cmꢁ1
m
ð
15N15NÞ=cmꢁ1
14N15NÞ=cmꢁ1
~
mð
Combinations
~
~
~
m
ð
11 + 2
11 + 3
11 + 4
11 + 5
11 + 6
11 + 7
11 + 8
11 + 9
11 + 10
1255
1266
1259
1260
1260
1413
1365, 1382a
1248
1248
1247
1248
1248
1394
1357, 1365, 1382a
1357, 1365, 1382a
1357, 1382a
1482
1506
1357, 1365, 1382a
1363a
1357, 1365, 1382a
1233b
1266
1266
1248
1357, 1382a
a
Z-dimer.
E-dimer.
b

I. Biljan et al. / Journal of Molecular Structure 979 (2010) 22–26
25
Fig. 1. 13C NMR CP MAS spectra of solids obtained after mixing the equimolar solutions of two different nitrosobenzenes and evaporation of the solvent (labeled as
‘‘synthesis”) and the sum of the spectra of pure homodimers.
most powerful tool in characterization of solid samples. This tech-
nique has already proven useful to differentiate homo- and hetero-
dimers of nitrosobenzenes [4b]. In Fig. 1 are shown the spectra of
solids obtained after mixing the equimolar solutions of different
nitrosobenzenes and evaporation of the solvent (labeled as ‘‘synthe-
sis”) together with the sum of the spectra of pure homodimers. In the
case of p-methylformylnitrosobenzene (10) no new signals have ap-
peared in the spectra indicating no heterodimer formation. On the
other hand, the other two derivatives provided the spectra which
were different from the sum of the spectra of pure homodimers.
New signals which appeared have aroused as a consequence of het-
erodimer formation either as co-crystals or as solid solutions. Our
previous investigations suggested that a solid solution of heterodi-
mer together with two homodimers was preferred over the co-crys-
tals [4b].
Fig. 2. Relative stabilities of hetero- versus homodimers calculated by B3LYP/6-
311++Gꢃꢃ method.
3.3. Quantum-chemical calculations
Structures and energies of all the possible dimers of nitroso-
nitrosobenzene, and almost negligible affinity to dimerization af-
fords p-methoxy- (8), and p-halonitrosobenzenes.
benzenes and their hetero-partners were calculated by B3LYP/6-
311++Gꢃꢃ method [8]. The stability of hetero- versus homodimers
was estimated by calculating DDGr for the corresponding isodes-
mic reaction (Scheme 3, Fig. 2).
In principle, for all dimers, hetero combination is more stable
than homo combination. As in experiments in solution, p-nitronitr-
osobenzene (9) forms the most stable heterodimer with the parent
The difference in ability of 8 and 9 to form dimers can be ex-
plained by comparison of the calculated group natural charges
and the bond orders in going from nitrosobenzene to 8 or to 9,
respectively (Table 4). It is evident that while p-NO2 group in the
benzene ring increases the positive charge on the nitroso group,
Scheme 3.

26
I. Biljan et al. / Journal of Molecular Structure 979 (2010) 22–26
Table 4
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Acknowledgements
We gratefully acknowledge the financial support to this work
from the Ministry of Science, Education, and Sports of the Republic
of Croatia, Nos. 119-1191342-1334, 119-1191342-1083, 119-
1191342-1339 and 098-0982929-2917.