Cross-dimerization of nitrosobenzenes in solution and in solid state

Article abstract of DOI:10.1016/j.molstruc.2008.07.035

Cross-linking of nitrosobenzenes to heterodimers (azodioxides) when they are not sterically crowded with large groups in ortho-position was studied by NMR and FT-IR spectroscopy, X-ray crystallography as well as by cryogenic photolysis. It was found for the first time that cross-linked dimers (heterodimers) exist both in solution and in solid state and that they appear in the crystal together with homodimers. The system is used as a model for studying organic solid solution formation. The role of molecular packing in formation of a weak chemical bond is also discussed.

Full text of DOI:10.1016/j.molstruc.2008.07.035

Journal of Molecular Structure 918 (2009) 19–25
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Cross-dimerization of nitrosobenzenes in solution and in solid state
a
a
a
a
b
b
´
Ivan Halasz , Ivana Biljan , Predrag Novak , Ernest Meštrovic , Janez Plavec , Gregor Mali ,
c
a,
*
ˇ
ˇ
Vilko Smrecki , Hrvoj Vancik
a Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, HR-10000 Zagreb, Croatia
b National Institute of Chemistry, Hajdrihova 19, SI-1115 Ljubljana, Slovenia
c
ˇ
Ruder Boškovi´c Institute, NMR Center, Bijenicka 54, HR-10000 Zagreb, Croatia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 April 2008
Received in revised form 1 July 2008
Accepted 11 July 2008
Available online 19 August 2008
Cross-linking of nitrosobenzenes to heterodimers (azodioxides) when they are not sterically crowded
with large groups in ortho-position was studied by NMR and FT-IR spectroscopy, X-ray crystallography
as well as by cryogenic photolysis. It was found for the first time that cross-linked dimers (heterodimers)
exist both in solution and in solid state and that they appear in the crystal together with homodimers.
The system is used as a model for studying organic solid solution formation. The role of molecular pack-
ing in formation of a weak chemical bond is also discussed.
Keywords:
Nitrosobenzenes
Azodioxides
Ó 2008 Elsevier B.V. All rights reserved.
Solid-state dimerization
Solid solution
Co-crystalization
Cross-dimerization
1. Introduction
same structure (R = R0) or with a different partner (R R0). Some
p
work on such selectivity has been published recently [4b].
The main problem in studies of the possible formation of cross-
linked dimers is how to find the method by which these heterodi-
mers can be unequivocally distinguished from a mixture of homo-
dimers. Our example of the formation of heterodimers includes a
combination of p-bromo- and p-nitronitrosobenzene. The mixture
was obtained by crystallization of equimolar solutions of both
compounds. The obtained crystals were compared with the crys-
tals of pure homodimers (i.e., nitro–nitro or bromo–bromo azodi-
oxides) by IR and CP MAS NMR spectroscopy, as well as by X-ray
diffraction structure determination. Possible appearance of hetero-
dimers in a crystal mixture of two homodimers represents also a
model for studying solid solution formation. To confirm that
cross-dimerization is not only a packing effect, but also a conse-
quence of electronic factors involved in chemical bonding, we also
conducted NMR experiments in solution.
Broad literature is available on the nitroso compound dimeriza-
tion to azodioxides (Scheme 1) [1]. This monomer–dimer equilib-
rium in solution has been studied in detail from kinetic and
thermodynamical aspects. In our recent studies, we found that azo-
dioxides (nitroso dimers) underwent photochromic behaviour not
only in solution but also in solid state [2]. Solid state dimerization
of nitrosobenzenes was also proposed as a model for studying the
solid state reaction mechanisms [3]. Since chemical changes in
crystals include phase transformation and chemical reaction, it is
not quite clear how these processes contribute to the structure of
the product and how they determine the reaction mechanism. A
study of the possibility of formation of cross-linked dimers (R
and R0 are different, Scheme 1) by co-crystallization of monomers
is in line with this investigation.
It is known from previous work that cross-linked dimers are
formed if a nitroso group is sterically crowded by substituents in
ortho-position in one of the monomer partners [4a]. No-appear-
ance of cross-linked dimers not substituted in the ortho-position
has been observed so far. On the other hand, we still possess no
systematic knowledge about the influence of electronic and/or
topochemical factors on the cross-linking selectivity, i.e., the condi-
tions under which the monomer will combine with a partner of the
2. Experimental
2.1. Preparation of p-nitronitrosobenzene
The compound was prepared by oxidation of p-nitroaniline
with oxoneÒ (K2SO5 ꢀ K2SO4 ꢀ KHSO4) analogously to a method
described in literature [5]. DCM solution of amine (10 mmol in
40 mL) was added in portions to a vigorously stirred mixture of
DCM (10 mL) and water solution of oxone (20 mmol, 100 mL).
* Corresponding author. Tel.: +385 1 4606400; fax: +385 1 4606401.
ˇ
E-mail address: vancik@chem.pmf.hr (H. Vancik).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.07.035

20
I. Halasz et al. / Journal of Molecular Structure 918 (2009) 19–25
10% NaOH (aq) was added until just alkaline. The resulting precip-
O
O
N
N
itate of 15N-p-bromoaniline was filtered with suction and washed
with a small amount of cold water. We obtained 110 mg of 15N-
p-bromoaniline (49%).
N
R
N
+
R
R'
+
+
R'
O
_
O
15N-p-bromoaniline was oxidized to
a nitroso compound
Scheme 1.
according to the method described above and purified by sublima-
tion. We obtained 90 mg of 15N-p-bromonitrosobenzene (74%)
after sublimation.
The reaction mixture was kept open to the atmosphere. At first, the
solution slowly became brownish. Later, it turned green, indicating
the formation of nitroso species. After approximately 5 h, the DCM
layer was separated, washed twice with hydrochloric acid
(c = 1 mol dmꢁ3, 2 ꢀ 20 mL) and water (20 mL). The solution was
dried on anhydrous CaCl2 and the DCM was separated on a rotary
evaporator. We obtained a slightly brownish precipitate, the exact
composition of which we have not determined. p-Nitronitrosoben-
zene was purified by sublimation under reduced pressure
(10ꢁ2 Torr), yielding some green material on the cold finger (tem-
perature of cooling water was around 10 °C). The colour of this
material changed to yellow due to dimerization during approxi-
mately 1 h at room temperature.
2.4. Preparation of crystals of the heterodimer and two homodimers
Twenty milligrams of 15N-p-bromonitrosobenzene and 16.4 mg
of p-nitronitrosobenzene were dissolved in ꢂ1.5 mL of cold (5 °C)
and dry chloroform. This resulted in a green solution, which was
left to evaporate in a refrigerator at 5 °C. After three days, yellow
needle-like crystals were formed.
2.5. IR spectroscopy
IR spectra were recorded on a Bruker FT-IR spectrometer under
2 cmꢁ1 resolution.
2.2. Preparation of p-bromonitrosobenzene
2.6. NMR spectroscopy
The compound was prepared by oxidation of p-bromoaniline
with oxoneÒ (K2SO5 ꢀ K2SO4 ꢀ KHSO4) analogously to a method
described in literature [5]. Ten millimolar of p-bromaniline dis-
solved in 20 mL of DCM was added in one portion to a water solu-
tion of oxone (20 mmol in 50 mL). This resulted in formation of a
white precipitate, which slowly disappeared as the mixture be-
came greener. The mixture was vigorously stirred for 5 h. After-
wards, the layers were separated and the DCM layer was washed
twice with hydrochloric acid (c = 1 mol dmꢁ3, 2 ꢀ 20 mL) and once
with water (20 mL). The solution was dried on anhydrous CaCl2
and DCM was separated on a rotary evaporator. We obtained a
light grey solid, exact composition of which we have not deter-
mined. p-Bromonitrosobenzene was purified by sublimation under
reduced pressure (10ꢁ2 Torr), yielding some green material (tem-
perature of cooling water for the cold finger was around 10 °C).
The green sublimed material turned white due to dimerization
over the following several days while it was stored at ꢁ10 °C.
Solid-state NMR experiments were performed on a 14.1 T Var-
ian Unity Inova spectrometer equipped with a 3.2 mm T3 MAS
probe from Varian/Chemagnetis. 1H 13C CPMAS spectra were re-
corded using a standard ramped cross-polarization pulse sequence
with high-power proton decoupling during acquisition. Contact
time for cross polarization was 5 ms, repetition delay was 8 s and
sample rotation frequency was 10 kHz for all samples. The number
of scans was 2000 for pure p-Br and p-NO2 substances, and 8000
for a physical mixture of the above two substances and for a het-
erodimer. The rotors contained about 100 mg of sample. (Chemical
shifts are reported relative to the position of the signal of
tetramethylsilane.).
The liquid-state one- and two-dimensional 1H and 13C NMR
spectra (600.13 MHz for 1H, 150.90 MHz for 13C) were measured
in CDCl3 at 298 and 218 K on a Bruker 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 stan-
dard. The standard 1H, APT and COSY experiments were used. The
FID resolution in 1H NMR and 13C NMR spectra was 0.29 Hz and
0.54 Hz per point, respectively. The COSY technique with a stan-
2.3. Preparation of 15N-p-bromonitrosobenzene
15N-p-bromoaniline was prepared according to a method de-
scribed in Vogel’s Preparative Organic Chemistry [6]. 15N-aniline
(0.20 mL) was converted to 15N-acetanilide with acetanhydride
(0.24 mL) in glacial acetic acid (0.24 mL). The reaction mixture
was gently boiled for 40 min and poured into 4 mL of cold water
while still hot. 15N-acetanilide crystallized and was isolated with
suction and washed with ꢂ2 mL of cold water. The product was
slightly pink in colour, probably due to impurities in the starting
15N-aniline. We obtained 210 mg of 15N-acetanilide (70%). 15N-
acetanilide was then dissolved in 0.70 mL of glacial acetic acid;
to this solution, a solution of bromine in glacial acetic acid
(0.50 mL, c = 3.5 mmol mLꢁ1) was added dropwise. The mixture
was stirred at room temperature for ꢂ1 h, resulting in formation
of 15N-p-bromoacetanilide precipitate. The mixture was trans-
ferred to 8 mL of water, filtered with suction and washed with
ꢂ4 mL of water, yielding 280 mg of 15N-p-bromoacetanilide (84%).
15N-p-bromoacetanilide was then dissolved in 0.70 mL of boil-
ing EtOH; to this solution, 0.33 mL of concentrated hydrochloric
acid was added dropwise. The mixture was heated under reflux
for 50 min. Afterwards, 4 mL of water was added and the mixture
was evaporated (removal of EtOH and EtOAc) until ꢂ1 mL re-
mained. The obtained solution was then diluted with water and
dard two p/2 pulse sequence (cosygpqf) in the pulsed field gradi-
ent mode (z-gradient) was applied 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 and 16.4 Hz/point in F2 and F1 dimen-
sions, respectively.
2.7. X-ray diffraction on single crystals
Data were collected by a series of omega scans on an Oxford Dif-
fraction Xcalibur3 CCD X-ray diffractometer with graphite mono-
chromated MoK
a radiation and were processed using the
CrysAlis [7] software package. Crystal structure was solved by di-
rect methods implemented in SHELXS-97 and refined on F2 by full
matrix least squares using SHELXL-97 [8]. Both programs were
used as part of the WinGX [9] software package.
Hydrogen atoms were placed in their geometrically calculated
position with the C–H bond distance of 0.93 Å and with Uiso(-
H) = 1.2Ueq(C). ORTEP3 [10] was used for graphical presentation
of the molecular structure. We performed two data collections of
the co-crystal with crystals from different batches. One set of data

I. Halasz et al. / Journal of Molecular Structure 918 (2009) 19–25
21
was collected at room temperature and the other at 100 K. No dif-
3. Results and discussion
ferences in crystal structures were observed except those which
can be attributed to a change in temperature.
3.1. NMR spectroscopy: cross-dimerization in solution
Crystal structure of the crystal of 1 was solved in the P21/c space
group with two molecules in one unit cell and half a molecule in
the asymmetric unit. Cell dimensions at 100 K are
a = 13.010(3) Å, b = 3.6827(7) Å, c = 12.993(3) Å, b = 103.87(2)°.
Cell dimensions at 293 K are a = 13.018(4) Å, b = 3.760(1) Å,
c = 13.168(4) Å, b = 104.17(3)°.
Crystal structure of dimeric p-nitronitrosobenzene was solved in
the Pbc21 space group with four molecules in one unit cell. Cell
dimensions at 150 K are a = 3.664(1) Å, b = 14.598(4) Å,
c = 22.949(10) Å.
In order to observe the possible formation of a heterodimer in
solution, we performed NMR experiments in chloroform-d5 at
ambient (298 K) and lowered temperature (218 K). While mono-
mers were found to be dominant species at ambient temperature,
decrease of temperature caused dimerization (as shown in the
COSY spectra of p-nitronitrosobenzene, Fig. 1a and b).
At both temperatures, the rotation of the nitroso group is free
and the two ortho- and meta-protons are indistinguishable.
Restricted rotation leading to distinct chemical shifts of all four
Fig. 2. (a) 1H NMR COSY spectrum of p-bromonitrosobenzene in chloroform at
298 K. Impurity (p-bromonitrobenzene) is labelled by X. (b) 1H NMR COSY spectrum
of p-bromonitrosobenzene in chloroform at 218 K.
Fig. 1. (a) 1H NMR COSY spectrum of p-nitronitrosobenzene in chloroform at 298 K.
(b) 1H NMR COSY spectrum of p-nitronitrosobenzene in chloroform at 218 K.

22
I. Halasz et al. / Journal of Molecular Structure 918 (2009) 19–25
Fig. 3. 1H NMR spectrum of the mixture of p-bromo- and p-nitronitrosobenzene in chloroform at different temperatures.
aromatic protons is expected at a temperature as low as ꢁ100 °C
[11]. Fig. 1b displays two new sets of coupled protons for p-nitro-
nitrosobenzene assigned to homodimers (cis- and trans-) that are
formed by cooling. These signals disappear after warming to room
temperature. Based on previous studies [11,12], we tentatively as-
signed the more abundant homodimer to the cis-form. A similar
observation was also made for p-bromonitrosobenzene (Fig. 2a
and b).
monomers and homodimers, new signals appeared, which can be
attributed to cross-linking products (heterodimers).
The COSY spectrum (Fig. 4) exhibited all the expected connec-
tivities for each form. In case of a heterodimer, two phenyl groups
_
_
O
O
After dissolving equimolar amounts of p-bromo- and p-nitro-
nitrosobenzene in chloroform-d1, the solution was cooled to
218 K and NMR spectra were recorded and analyzed. It is clearly
seen in Fig. 3 that, in addition to proton resonances assigned to
N
N
A
B
+
+
+
+
PHYSICAL MIXTURE
N
N
A
B
_O
_O
SOLVENT
O
O
N
A
N
B
N
A
N
B
O
O
EVAPORATION OF SOLVENT
_
_
_
O
O
O
N
N
N
A
B
A
+
OR
+
+
+
+
+
N
N
N
A
B
B
_O
_O
_O
CHEMICAL MIXTURE
Fig. 4. 1H NMR COSY spectrum of the mixture in chloroform at 218 K.
Scheme 2.

I. Halasz et al. / Journal of Molecular Structure 918 (2009) 19–25
23
were present (A–A or B–B combinations in Scheme 2). In contrast,
if spectra of a chemical mixture differ from those of a physical mix-
ture, we can assume formation of heterodimers (A–B combination
in Scheme 2), or appearance of new polymorphs or crystal phases,
such as mixed crystals of A–A and B–B forms.
It should be pointed out here that, besides co-crystals, the resul-
tant solid chemical mixture can also be a solid solution. The differ-
ence is that a co-crystal has three-dimensional periodicity whereas
a solid solution does not. Solid solution formation arises from sim-
ilarities of supramolecular synthons occurring in the crystal when
synthons are formed with different chemical species. It is an
advantage of solid solutions, as opposed to co-crystals, that their
composition, and thus their properties, can be varied within a cer-
tain range [7a]. Therefore, this gives rise to five potential new prod-
ucts (physical mixture excluded), two co-crystals, two solid
solutions and the disordered structure of pure heterodimer.
The solid-state NMR spectra of pure homodimers as well as of
chemical and physical mixtures are shown in Fig. 5. It is evident
that the physical mixture of homodimer crystals (spectrum b) is
merely a sum of the spectra of pure substances (p-bromonitroso-
benzene/spectrum c/and p-nitronitrosobenzene/spectrum d/).
However, there are significant differences that distinguish the
two mixtures. Namely, the spectrum of the chemical mixture (1)
(spectrum a) exhibits a new signal at 133 ppm, whereas the signals
at 122 and 123 ppm observed for the physical mixture are absent.
Although the solid-state NMR spectra also reflect the molecular
arrangement in the crystal, the difference in spectra a and b could
be an indication of the presence of cross-linked dimers.
Fig. 5. 13C CP MAS NMR spectra of the chemical mixture (a) physical mixture (b), pure
p-bromonitrosobenzene dimer (c), and pure p-nitronitrosobenzene dimer (d).
Crystal structure of 1 was refined in a centrosymmetric space
group (P21/c) with half a ‘‘molecule” in the asymmetric unit and
two ‘‘molecules” in one unit cell. The ‘‘molecular” structure ob-
tained by X-ray analysis is given in Fig. 6. The molecule(s) lies on
an inversion centre bisecting the N–N bond. This site symmetry
on which the molecule lies in the structure is incompatible with
the symmetry of the point group of the heterodimer molecule
but this does not exclude the possibility of its presence in the crys-
tal because the averaged electron density of the heterodimer mol-
ecules having two orientations (related by inversion, for instance)
may fit the symmetry of the P21/c space group.
Fig. 6. ORTEP perspective of the averaged ‘‘molecule” obtained by crystallization
from solution of equimolar amounts of p-bromo- and p-nitronitrosobenzene.
[i = ꢁx, ꢁy, ꢁz]. Occupancies of the bromine atom and the nitro group are 0.5.
are not equivalent, which should give rise to two different sets of
coupled protons for each isomer (cis- and trans-). A close inspec-
tion of the proton spectrum and COSY cross-peaks revealed that
one isomer (probably cis-) of the heterodimer is more populated
than the other. Further experiments are needed to resolve the de-
tails of this issue.
When looking at the crystal structure of 1, it is evident that it is
analogous to the structure of p-bromonitrosobenzene dimer
(Fig. 7). This structure was discussed previously [3b]. Each ‘‘mole-
cule” forms four C–Hmꢃ ꢃ ꢃO contacts (Hm is the hydrogen atom in
meta position to the NO group) with four neighbouring molecules
forming a layer parallel to the (a, b) plane. Structural analogy can
be explained on the basis of similarity of Brꢃ ꢃ ꢃBr and Brꢃ ꢃ ꢃNO2
interactions as supramolecular synthons [13].
The main question, whether we have really obtained the het-
erodimer structure of the molecule with the nitro group on one
side and the bromine substituent on the other side, still remains
unanswered. Namely, because of the orientational disorder, it is
difficult to decide whether the molecule is an expected heterodi-
mer or merely a disordered mixture of homodimers.
3.2. NMR spectroscopy: cross-dimerization in solid state
CP MAS spectra of pure p-bromo- and p-nitronitrosobenzene
and two different kinds of their mixtures were recorded and com-
pared. The first mixture is called a physical mixture, and it consists
of equimolar amounts of crystallized homodimers. The second is a
chemical mixture, obtained by dissolving the physical mixture and
evaporating the solvent. Owing to the fact that only monomers
are present in solution, after evaporation of the solvent they can
recombine with each other either by forming the same mixture
of homodimers or by cross-linking into heterodimers (Scheme 2).
If the spectra of such physical and chemical mixtures are identical
or very similar, this could be an indication that only homodimers
Fig. 7. Packing in 1 (left), in p-bromonitrosobenzene homodimer (middle) and p-nitronitrosobenzene homodimer (right).

24
I. Halasz et al. / Journal of Molecular Structure 918 (2009) 19–25
3.3. FT-IR spectroscopy
of p-bromo- and p-nitronitrosobenzene, all three species (two
homodimers and a heterodimer) are produced, but the cross-linked
molecule is dominant.
The IR signal in the 1260 cmꢁ1 spectral region, assigned to
ON@NO as. stretching, can be used for characterization of the azo-
dioxide dimeric structure. We measured its spectral position in di-
mers obtained from pure p-bromonitrosobenzene, pure p-
nitronitrosobenzene, and their chemical mixture 1 (Fig. 8). It is
interesting that this absorbance obtained from the dimer crystal-
lized from a chemical mixture of different nitrosobenzenes (1) ap-
pears at the lowest frequency (1257 cmꢁ1). If it were just a mixture
of homodimers, we would expect its appearance between the sig-
nals of pure homodimers, i.e., between 1258 and 1261 cmꢁ1. Such
indication of the existence of cross-linked azodioxide is further
supported by the spectra of 15N labelled compounds (Fig. 9).
Namely, we prepared the 15N p-bromonitrosobenzene and mixed
it with the 14N p-nitronitrosobenzene. If the cross-linked azodiox-
ide appears, it must have the 14N = 15N signal (14N = 14N peak be-
longs to p-nitronitrosobenzene homodimer, and 15N = 15N signal
is assigned to p-bromonitrosobenzene). As it is evident from
Fig. 9, all three peaks appear in the spectrum. These findings
strongly support the conclusion that by mixing equimolar amounts
3.4. Co-crystal versus solid solution
In principle, if there are co-crystals of dimers that are periodi-
cally ordered, then any process of dissociation followed by recom-
bination must disorder such a structure, and the IR spectrum
before and after recombination must be different. Conversely, if
there is a solid solution, it is already disordered and statistically
mixed, and such recombination processes will not change the spec-
tra. Here, we use two such recombination tests based on recombi-
nation after photolysis and after sublimation.
We know that photolysis of p-bromonitrosobenzene dimer
yields monomers below 170 K [2a] that recombine upon warming.
While photolysis is
a single-crystal-to-single-crystal reaction,
recombination is not, and it yields polycrystalline homodimers.
The structure of photolyzed p-bromonitrosobenzene is such that
recombination occurs with either of the two neighbouring mono-
mer molecules. If there is a solid solution of two homodimers, then
we may expect that recombination would not proceed selectively
and would yield at least some amount of heterodimer. However,
it is evident from Fig. 10 that the IR spectra of the chemical mixture
before the cryogenic photolysis with the spectra obtained after
photolysis followed by warming above 170 K are identical. Accord-
ingly, dissociation and recombination in the solid state yield the
same composition of homo- and/or heterodimers. If the reactant
were a mixture of homo-co-crystals, and the product after recom-
bination were a solid solution of homo and hetero dimers, then the
position of the ONNO as. stretching peak at 1257 cmꢁ1 would
change its position. On the other side, if the starting mixture is al-
ready a solid solution, then the product mixture must be the same,
and the spectrum will remain the same, as this is the case in our
experiment.
A similar recombination test can be done also by sublimation. In
general, sublimation of nitrosobenzene dimers yields monomer
molecules that crystallize as metastable crystals on the cold finger
of the sublimator. Warming to room temperature induces redimer-
ization in the solid state [11]. When the equimolar mixture of two
homodimers is sublimed and redimerized, the IR spectrum is iden-
tical to the spectrum obtained by crystallization of the same mix-
ture from solution. If it is assumed that dimerization in these two
experiments proceeded nonselectively, they strongly support the
Fig. 8. FT-IR spectra of the chemical mixture (mixed dimer), pure p-bromonitros-
obenzene dimer, and pure p-nitronitrosobenzene dimer.
Fig. 9. FT-IR spectra of the chemical mixture obtained after mixing equimolar
amounts of pure 15N labelled p-bromonitrosobenzene and pure 14N p-
nitronitrosobenzene.
Fig. 10. IR spectra of the photolysis experiment of 1. The dashed spectrum is
displaced for clarity.

I. Halasz et al. / Journal of Molecular Structure 918 (2009) 19–25
25
conclusion that there is a solid solution of a heterodimer and two
References
homodimers.
[1] (a) Th.A.J. Wajer, Th.J. De Boer, Recueil 91 (1972) 565;
(b) F.D. Greene, K.E. Gilbert, J. Org. Chem. 40 (1975) 1409;
4. Conclusions
(c) M.L. Greer, H. Sarker, M.E. Medicino, S.C. Blackstock, J. Am. Chem. Soc. 117
(1995) 10460;
(d) B.G. Gowenlock, W. Lütke, Quart. Rev. 12 (1958) 321;
(e) J.P. Snyder, M.H. Heyman, E. Suciu, J. Org. Chem. 40 (1975) 1395;
[f] P. Singh, ibid. 1405;
(g) D.A. Fletcher, B.G. Gowenlock, K.G. Orrell, J. Chem. Soc. Perkin Trans. 2
(1998) 797;
We have shown here that by mixing p-bromo- and p-nitronitr-
osobenzenes, both homo- and heterodimers may be formed in the
solid state and in solution. The appearance of cross-linked hetero-
dimers of nitrosobenzenes sterically not crowded with ortho-sub-
stituents is in solid state governed by crystal packing. Since the
crystal consists of all three combinations of nitroso molecules,
two homodimers and one cross-linked dimer, the formation of a
dimer is not a selective process, especially if one of the homodi-
mers and the heterodimer are isomorphic by molecular arrange-
ment. Recombination tests also suggest that there is a solid
solution of a heterodimer and two homodimers. The predominant
form in solution, at ambient temperature, is a monomer while a
mixture of monomers, homodimers and heterodimers exists at
lowered temperature. The appearance of cross-linked dimers in
solution demonstrates that cross-linking is governed not only by
crystal packing but also by chemical bonding.
(h) D.A. Fletscher, B.G. Gowenlock, K.G. Orrell, V. Sik, D.E. Hibbs, M.B.
Hursthouse, A.K.M. Malik, J. Chem. Soc. Perkin Trans 2 (1996) 191;
(i) B.G. Gowenlock, G.B. Richter-Addo, Chem. Soc. Rev. 34 (2005) 797–809.
[2] (a) H. Vancik, V. Simunic-Meznaric, I. Caleta, E. Mestrovic, S. Milovac, K.
Mlinaric Majerski, J. Veljkovic, J. Phys. Chem. B 106 (2002) 1576–1580;
(b) G. Orrell, D. Stephenson, J.H. Velarque, J. Chem. Soc. Perkin Trans. 2 (1990)
1297.
ˇ
´
ˇ
´
´
[3] (a) H. Vancik, V. Šimunic-Meznaric, E. Meštrovic, I. Halasz, J. Org. Chem. 69
(2004) 4829–4834;
ˇ
ˇ
´
ˇ
(b) I. Halasz, E. Meštrovic´, H. Cicak, Z. Mihalic, H. Vancik, J. Org. Chem. 70
(2005) 8461–8467.
[4] (a) V.A. Batyuk, T.I. Shabatina, Yu.N. Morozov, S.V. Ryapisov, G.B. Sergeev,
Vestn. Moscow. Univ. Seriya 2. Chimia. 29 (5) (1988) 270–274;
ˇ
ˇ
´
ˇ
´
´
´
´
(b) V. Šimunic´-Meznaric, E. Meštrovic, V. Tomišic, M. Zgela, D. Vikic-Topic, H.
ˇ
ˇ
Cicak, P. Novak, H. Vancik, Croat. Chem. Acta 78 (2005) 511–518.
[5] (a) B. Priewisch, K. Ruck-Braun, J. Org. Chem. 70 (2005) 2350–2352;
(b) G. Orrell, D. Stephenson, J.H. Velarque, J. Chem. Soc. Perkin Trans. 2 (1990)
1297.
Acknowledgements
[6] Vogel’s, Textbook of Practical Organic Chemistry, Fourth ed., Longman, 1978.
[7] (a) M. Dabros, P.R. Emery, V.R. Thalladi, Angew. Chem. Ind. Ed. 46 (2007) 4132;
(b) 5 Oxford Diffraction (2003). Oxford Diffraction Ltd., Xcalibur3 CCD system,
CrysAlis Software system, Version 1.171.26 beta.
[8] G.M. Sheldrick, SHELXL97 and SHELXS97. Programs for Crystal Structure
Analysis (Release 97-2), University of Goettingen, Germany. 1997.
[9] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837–838.
We gratefully acknowledge the financial support to this work
from the Ministry of Science, Education, and Sports of the Republic
of Croatia, No. 119-1191342-1334 and 098-0982929-2917.
[10] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[11] D.A. Fletcher, B.G. Gowenlock, K.G. Orrell, J. Chem. Soc. Perkin Trans. 2 (1997)
2201–2205.
Appendix A. Supplementary data
[12] M. Azoulay, E. Fischer, J. Chem. Soc. Perkin Trans. 2 (1982) 637–642.
[13] G.R. Desiraju, Angew. Chem. Ind. Ed. 34 (1995) 2311.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2008.07.035.