Crystal packing: An examination of the packing of molecules approximately isosteric with 4,5-dichlorophthalic anhydride

Article abstract of DOI:10.1002/hlca.200390102

The crystal structures of five 5,6-disubstituted benzofurazan 1-oxides are presented and compared with five previously reported structures: three polymorphs of 5,6-dichlorobenzofurazan 1-oxide plus 4,5-dichloro- and 4,5-dibromophthalic anhydride. All but one of these compounds pack with similar two-dimensional layers. The benzofurazan oxides all show disorder about a crystallographic twofold or pseudo-twofold axis. In addition, six complexes of phthalic anhydride and benzofurazan oxides are reported. With the packing in the complexes principally directed by the π complexing, the disorder, invariably found in the uncomplexed benzofurazan oxides, is diminished, and, in two cases, eliminated.

Full text of DOI:10.1002/hlca.200390102

Helvetica Chimica Acta Vol. 86 (2003)
1175
Crystal Packing: An Examination of the Packing of Molecules Approximately
Isosteric with 4,5-Dichlorophthalic Anhydride
by Doyle Britton*, Wayland E. Noland, Matthew J. Pinnow, and Victor G. Young Jr.
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431
(phone: 612-625-9535; fax: 612-626-7541; e-mail: britton@chem.umn.edu)
Dedicated to Professor Jack D. Dunitz on the occasion of his 80th birthday
The crystal structures of five 5,6-disubstituted benzofurazan 1-oxides are presented and compared with five
previously reported structures: three polymorphs of 5,6-dichlorobenzofurazan 1-oxide plus 4,5-dichloro- and
4,5-dibromophthalic anhydride. All but one of these compounds pack with similar two-dimensional layers. The
benzofurazan oxides all show disorder about a crystallographic twofold or pseudo-twofold axis. In addition, six
complexes of phthalic anhydride and benzofurazan oxides are reported. With the packing in the complexes
principally directed by the p complexing, the disorder, invariably found in the uncomplexed benzofurazan
oxides, is diminished, and, in two cases, eliminated.
Introduction. Recently, the crystal structures of 4,5-dichlorophthalic anhydride
(Cl2PA), 4,5-dibromophthalic anhydride (Br2PA), and 4,5-dichlorobenzofurazan 1-
oxide (Cl2BF) were reported by Ojala et al. [1]. The original purpose was to explore the
crystallographic isostructuralism of Cl2PA and Cl2BF [2], an isostructuralism that
requires disordered Cl2BF to mimic the shape of Cl2PA. (see Fig. 1 for the original
compounds plus the related compounds to be reported here). In the course of the
original work, three polymorphs were found for Cl2BF, and a partial isostructural
relationship was found between Cl2PA, which is isostructural with one of the
polymorphs of Cl2BF, and Br2PA.
A summary of the results follows. The molecules in all five structures align in
ribbons along twofold or pseudo-twofold axes. These ribbons appear to be held
together by Lewis acidÀbase interactions between Cl or Br, and O. In four of the five
structures, Cl2PA, Br2PA, Cl2BF-A, and Cl2BF-C, these ribbons align side-to-side to
form roughly planar sheets (in the fifth structure, Cl2BF-B, the sheet is significantly
puckered). The sheets appear to be held together by CÀH ¥¥¥ O and CÀH ¥¥¥ N
interactions. A sheet of Cl2PA is shown in Fig. 2. In Cl2BF-A, the Cl2BF molecules are
disordered about the twofold axis to mimic Cl2PA molecules. The sheets stack together
in similar ways in Cl2PA, Br2PA, and Cl2BF-A, but in a distinctly different fashion in
Cl2BF-C. The similarity in the packing between Cl2PA and Br2PA is quite close even
though the space group is C2/c for Cl2PA while it is C-1 for Br2PA.
This work is extended here (see Fig. 1). First, Br2BF is included, which completes
the previous comparison. Then, BrClBF, BrMeBF, and IMeBF are examined to see
whether removing the equivalence between the X and Y substituents would reduce or
eliminate the disorder in the packing. These were chosen since Cl, Br, I, and Me all have
different Lewis acid strengths, Br> Cl, and Br and I > Me. In every case, one might be

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Helvetica Chimica Acta Vol. 86 (2003)
Fig. 1. The PA and BF molecules referred to in this work. The abbreviations given are those used throughout the
paper. When it is necessary to distinguish between the two isomers, XYBF will refer to the isomer with X in the
6-position, i.e., closest to the exocyclic O-atom, and YXBF to the isomer with X in the 5-position.
Fig. 2. A layer in the structure of Cl2PA. A layer in Br2PA is quite similar, although the two repeat directions in
the latter differ slightly from orthogonal. Layers in Cl2BF-A and Cl2BF-C are similar except for disorder about
the twofold axes.
expected to be favored over the other and lead to less disorder, or, indeed, lead to
ordered or completely different structures. Finally, when it was found that the disorder
was not completely eliminated, pyrene complexes of all of the compounds (except
BrClBF) were examined. The latter extension of the work was based on the idea that it

Helvetica Chimica Acta Vol. 86 (2003)
1177
might be possible to use p complexes to facilitate s interactions between the
benzofurazan (BF) or phthalic anhydride (PA) molecules. This use of p complexes can
be seen, for example, in Desiraju and co-workers [3], or Britton [4].
For an introduction to the history and noncrystallographic properties of benzofur-
azan 1-oxides, see Katritzky and Gordeev [5].
Crystallographic Results. Crystallographic data are given in Table 1. All of the
bond lengths and angles are normal within experimental error. The anisotropic
displacement parameters are reasonable. As an example, a drawing of the anisotropic
displacement ellipsoids in BrMeBF is given in Fig. 3; the rest are similar. As a summary
of the displacement parameters, the ranges of the equivalent isotropic displacement
parameters are given for all of the PA and BF molecules in Table 2. Also shown in
Fig. 3 are the disordered molecules of BrMeBF; again, the other compounds are
similar.
All of the uncomplexed molecules in this study and the previous one, except Cl2BF-
B, form two-dimensional sheets that are isostructural with each other (see Fig. 2),
although they are not all isostructural in three dimensions. There are four different
stacking patterns of the layers. These are shown in Fig. 4. The unit cells given in Table 1
are the conventional cells. In Table 3, the unit cells for the uncomplexed molecules are
converted to new cells in which a' is the repeat distance parallel to the ribbon direction,
b' is the repeat distance closest to perpendicular to a' in the plane of the layer , and c' is
the repeat distance closest to normal to the a'b' plane. The constancy of the a' values
and of the b' values (except for Cl2BF-B, which has puckered sheets), and the close
match of g' to 908 demonstrates the similarity of the layers. The variations in the values
of c', a', and b' testify to the differences in the stacking of the layers.
Fig. 3. Left: Anisotropic displacement ellipsoids (50% probability) for BrMeBF. The atom labelling shown is
used throughout. Right: The four disordered orientations: solid bonds: 0.483 occupancy; solid dashed: 0.217;
open: 0.209; open dashed: 0.091.

Table 1. Crystal Data and Structure-Refinement Details
Br2BF-A
BrClBF-A
BrClBF-C
BrMeBF
IMeBF
Cl2PA/py
Cl2BF/py
Br2BF/py
Br2PA/py
BrMeBF/py IMeBF/py
Empirical formula
C6H2Br2N2O2-A C6H2BrClN2O2-A C6H2BrClN2O2-C C7H5BrN2O2 C7H5IN2O2 C8H2Cl2O3 ¥ C6H2Cl2N2O2 ¥ C6H2Br2N2O2 ¥ C8H2Br2O3 ¥ C7H5BrN2O2 ¥ C7H5IN2O2 ¥
C16H10
monoclinic monoclinic
P21/n P21/n
6.1809(15) 6.3381(16) 6.2985(17) 6.7858(17)
C16H10
C16H10
monoclinic
P21/n
6.8334(17)
15.809(4)
17.147(4)
C16H10
monoclinic triclinic
P21/n
13.516(4)
9.669(2)
14.451(4)
1.5 (C16H10
)
C16H10
monoclinic
P21/c
14.955(4)
17.564(4)
14.329(4)
Crystal system
Space group
a/ä
b/ä
c/ä
a/8
monoclinic
C2/c
11.687(3)
9.087(2)
7.4165(19)
monoclinic
C2/c
11.545(3)
9.010(2)
7.3350(18)
monoclinic
C2/c
7.2868(18)
9.150(2)
11.757(3)
triclinic
P1
triclinic
P1
≈
≈
≈
P1
7.3295(18)
8.550(2)
19.185(5)
88.150(10)
8.041(2)
9.295(2)
8.510(2)
9.605(2)
16.236(4)
17.096(4)
15.487(4)
17.092(4)
67.470(10) 63.770(10)
b/8
g/8
V/ä3
96.050(10)
95.830(10)
96.210(10)
74.430(10) 71.940(10) 96.370(10) 91.260(10)
77.909(010) 74.130(10)
90.580(10)
99.000(10) 79.180(10)
87.080(10)
95.930(10)
3743.7(17)
783.2(4)
4
2.492
7.59.0(3)
4
2.183
779.3(3)
4
2.126
407.3(2)
2
436.1(2)
2
1884.0(8)
4
1795.8(8)
4
1852.3(8)
4
1.779
1865.3(9)
4
1179.0(5)
2
Z
8
Dcalc/g cmÀ3
F(000)
1.868
224
2.102
260
1.478
856
1.506
832
1.810
1000
1.500
542
1.697
1888
552
480
480
976
Crystal size/mm
0.50 Â 0.15
0.50 Â 0.15
0.20 Â 0.20
0.50 Â 0.30 0.50 Â 0.38 0.50 Â 0.32 0.45 Â 0.15
0.40 Â 0.05
0.45 Â 0.40 0.40 Â 0.20
0.40 Â 0.12
 0.05
 0.15
 0.05
 0.08
3.631
 0.22
0.369
 0.26
0.383
 0.10
4.398
 0.05
 0.20
1.777
 0.10
1.733
 0.08
m/mmÀ1
Temp./K
Wavelength/ä
q Range/8
hkl Range
10.30
174(2)
5.725.57
174(2)
5.00
174(2)
4.371
297(2)
0.71073
297(2)
0.71073
174(2)
0.71073
174(2)
0.71073
173(2)
0.71073
1.75 27.55
À 8 < h < 8
173(2)
0.71073
174(2)
0.71073
174(2)
0.71073
1.37 27.49
À 19 < h < 19
0.71073
2.85 27.49
À 15 < h < 15
À 11 < k < 11
À 9 < l < 9
0.71073
2.87 25.03
À 9 < h < 13
À 10 < k < 10
À 8 < l < 8
0.71073
3.49 25.01
À 8 < h < 8
À 9 < k < 10
À 13 < l < 13
2.42 25.00 2.42 25.08 1.73 24.95 1.77 25.11
1.92 25.19 2.16 27.54
À 16 < h < 15 À 9 < h < 9
À 7 < h < 7 À 7 < h < 7 À 7 < h < 8 À 8 < h < 8
À 9 < k < 9 À 10 < k < 9 À 17 < k < 19 À 12 < k < 18 À 20 < k < 20 À 11 < k < 11 À 11 < k < 11 À 22 < k < 22
À 11 < l < 10 À 11 < l < 10 À 17 < l < 20 À 19 < l < 20 À 22 < l < 22 À 17 < l < 17 À 24 < l < 24 À 18 < l < 18
Reflections: collected,
unique, observed
(I > 2s(I))
Number of parameters, 103, 60
constraints
3945, 901,
804
1883, 669,
635
117, 69
1913, 678,
552
122, 69
2789, 1418, 2870, 1522, 9055, 3249, 8742, 3145,
20772, 4249, 21571, 3438, 13745, 5327, 37559, 8548,
2565
253
1061
227, 190
1154
227, 361
2326
320, 35
2400
253
1957
263
2776
443, 213
6001
555, 90
R(F)
0.033
0.086
1.05
0.72, À0.75
0.036
0.095
1.04
0.60, À0.60
0.037
0.102
1.09
0.47, À0.43
0.044
0.128
1.05
0.041
0.127
0.94
0.041
0.111
0.95
0.041
0.111
1.00
0.0520.0502.059
0.104
0.97
0.53, À0.420.96,
0.038
wR(F2) all data
Goodness-of-fit
max/min resid.
dens./e ä3
0.107
0.90
0.144
1.01
0.095
1.02
1.62, À0.45
0.26, À0.35 0.29, À0.28 0.18, À0.22 0.27, À0.27
À0.66 0.36, À0.29
CCDC No.
200604
200695
200696
200920 200921 201419 201417
201418
201420
201534
201421
a) Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre (CCDC). Deposition Nos. are given in the
Table. Copies of the data can be obtained, free of charge, on application to the CCDC, 12Union Road, Cambridge CB21EZ, UK (fax: ‡ 44(1223)336033; e-mail
deposit@ccdc.cam.ac.uk.).

Helvetica Chimica Acta Vol. 86 (2003)
1179
Table 2. Ranges of Equivalent Isotropic Thermal Parameters for All of the PA and BF Molecules
Molecule
T/K
Range
Molecule
T/K
Range
Ueq.
Ueq.
Cl2PA
Br2PA
297
297
173
173
174
174
174
297
297
0.038 0.065
0.036 0.061
0.020 0.034
0.028 0.051
0.024 0.042
0.020 0.032
0.034 0.056
0.044 0.096
0.075 0.112
Cl2PA/py
Br2PA/py
Cl2BF/py
174
173
174
0.033 0.064
0.029 0.051
0.038 0.055
Cl2BF-A
Cl2BF-C
Br2BF-A
BrClBF-A
BrClBF-C
BrMeBF
IMeBF
Br2BF/py
173
0.029 0.061
BrMeBF/py
IMeBF/py
174
174
0.032 0.055
0.030 0.053
Table 3. Cell Constants Based on Similar Layers in the Uncomplexed Compounds. The first five entries are taken
from [1].
Compound
a'
b'
c'
a'
b'
90
105.08
90
98.56
90
g'
Space group
Matrix
a
Cl2PA
Br2PA
8.994
9.209
8.871
9.011
9.055
9.087
9.010
9.150
9.243
9.605
13.775
13.917
12.937
11.750
13.059
13.165
13.034
13.145
13.066
13.195
7.423
7.628
7.222
7.068
7.223
7.417
7.335
7.287
7.889
8.509
119.65
113.56
118.66
90.
118.10
118.0290
118.22
117.23
114.20
112.47
90
89.89
90
90
90
I2/a
C-1
)
)
)
)
)
)
)
)
b
a
Cl2BF-A
Cl2BF-B
Cl2BF-C
Br2BF-A
BrClBF-A
BrClBF-C
BrMeBF
IMeBF
I2/a
P21/nc)
I2/a
I2/a
I2/a
I2/a
C-1
d
e
a
90
a
90
90
112.08
116.23
90
90
91.04
90.74
e
f
)
f
C-1
)
a) 0,1,0/1,0,1/0,0, À 1. b) 0,1,0/ À 1,0, À 1/0,0, À 1. c) This is (P21/n11). d) 0,0,1/0,1,0/ À 1,0,0. e) 0, À 1,0/1,0,1/ À
1,0,0. f) 0,0,1/ À 2,1,0/0, À 1,0.
The complexes with pyrene show more variety. Cl2BF/py, and Br2BF/py are
isostructural, at least to the extent of similar cell constants and packing; there is a
significant difference in the amount of disorder. Cl2PA/py has cell constants similar to
the previous two, but there is a difference in the packing. Br2PA/py and IMeBF/py are
different from each other, and from the previous three complexes. BrMeBF/py is
distinctly different; the BrMeBF/py ratio is 2:3 rather than the 1:1 found in the other
five. Owing to the limited supply of BrMeBF available, it was not possible to repeat the
preparation with different proportions of the two compounds, although this is worth
doing. It would also be of interest to see if other 2:3 complexes could be formed.
In all of the complexes with pyrene the PA and BF molecules are in stacks in which
they alternate with pyrene molecules in the familiar mode for p complexes. These
stacks are packed in hexagonal arrays except in the BrMeBF/py where the ratio of
molecules is 2:3. In this case, double layers of stacks are separated by layers of pyrene
that lie parallel to the stack direction.
Since our interest in this paper is in the s interactions between BF or PA molecules,
only these molecules are shown in the packing diagrams. In all of the packing diagrams,
only the predominant orientation of the disorder is shown. In all of the packing
diagrams (Figs. 5 9 below), the pyrene (not shown) is stacked directly above and

Fig. 4. The stacking of the layers in the various structural types, drawn normal to the plane of the uppermost molecule. The orientation with the highest
occupancy has been shown in each case (see Table 5). The solid bonds show the top, the open bonds the next, and the dotted bonds the bottom layer. a)
Stacking in BrClBF-A (shown), Cl2PA, Cl2BF-A, Br2BF-A. b) BrClBF-C (shown), Cl2BF-C. c) Br2PA. d) BrMeBF (shown), IMeBF.

Helvetica Chimica Acta Vol. 86 (2003)
1181
below the PA or BF molecules. The metric data for the X ¥¥¥ Y (X ˆ Cl, Br, or Me; Yˆ
O or N) interactions are given in Table 4.
Table 4. Metric Parameters for the Intermolecular A ¥¥¥ B Interactions in the Approximate Plane of the Molecule
Compound
A
B
CÀA ¥¥¥ B/8
A ¥¥¥ B/ä
A ¥¥¥ BÀ(or N)/8
Cl2PA
Cl(4)
Cl(4)
Cl(5)
Cl(5)
Br(4)
Br(4)
Br(5)
Br(5)
Cl(4)
Cl(5)
Cl(5)
Cl(4)
Cl(5)
Cl(5)
Br(4)
Br(5)
Br(5)
Cl(4)
Br(5)
Br(5)
Cl(4)
Br(5)
Br(5)
Br(4)
H(8B)
H(8B)
H(6)
O(3)
O(2)
O(2)
O(1)
O(3)
O(2)
O(2)
O(1)
O(2)
O(2)
O(1)
O(2)
O(2)
O(1)
O(2)
O(2)
O(1)
O(2)
O(2)
O(1)
O(2)
O(2)
O(1)
O(2)
O(2)
O(1)
O(1)
O(2)
O(2)
O(1)
O(1)
O(1)
O(1)
O(1)
O(2)
O(1)
O(2)
O(1)
O(2)
O(2)
N(2)a
N(2)b
O(1)b
O(1)a
O(2)b
O(2)a
161
122
122
161
156
119
121
159
131
118
156
134
117
154
131
113
151
131
115
153
135
114
151
138
122
119
123
142
124
123
128
163
127
144
125
146
126
141
109
156
175
173
139
144
147
149
3.33
3.33
3.33
3.33
3.43
3.49
3.44
3.40
3.09
3.59
3.19
3.19
3.83
3.36
3.18
3.73
3.29
3.26
3.63
3.24
3.32
3.83
3.35
3.27
3.69
3.29
2.68
3.38
4.23
3.58
3.28
3.39
3.63
3.26
3.76
3.24
3.73
3.30
3.59
2.74
3.12
3.38
2.84
2.84
3.43
3.31
103
97
97
103
105
96
Br2PA
96
104
107
89
114
108
89
117
106
88
115
106
89
113
110
88
116
112
90
114
123
118
85
123
131
122
136
111
83
Cl2BF-A
Cl2BF-C
Br2BF
BrClBF-A
BrClBF-C
BrMeBF
IMeBF
I(4)
H(8B)
H(8B)
Cl(5)
Br(5)
Br(4)
Cl(5)
Cl(5)
Br(5)
Br(5)
Br(5)
Br(5)
H(7A)
I(5)a
Cl2PA/py
Br2PA/py
Cl2BF/py
Br2BF/py
BrMeBF/py
110
82
101
85
140
136
128
127
134
110
110
IMeBF/pya)
I(5)b
H(6)a
H(6)b
H(7C)a
H(7A)b
a) There are two crystallographically independent molecules, a and b.

1182
Helvetica Chimica Acta Vol. 86 (2003)
A layer of Cl2BF in Cl2BF/py is shown in Fig. 5. The molecules form puckered
ribbons parallel to the b axis. The packing in Br2BF/py is the same except that there is
disorder with Cl2BF and none with Br2BF.
Fig. 5. Packing of Cl2BF in the complex with pyrene. The predominant orientation is shown with solid bonds.
The other orientation is shown with open bonds. The short Cl ¥¥¥ O contact is shown with dashed lines. In the
Br2BF/py complex, the arrangement is the same except there is no disorder.
A layer of Cl2PA in Cl2PA/py is shown in Fig. 6. The molecules form parallel
ribbons similar to those in Cl2BF/py, but a comparison of the two packings shows that
the sideways tilts of the molecules in the ribbons are in the opposite sense in the two
cases. Additionally, the disorder in the Cl2PA is end-for-end rather than side-to-side as
it is in Cl2BF/py. This is the only example of end-for-end disorder that was found.
A layer of Br2PA in Br2PA/py is shown in Fig. 7. This is a double ribbon parallel to
both the (001) plane and the c axis. The contacts perpendicular to the ribbon direction
(dashed lines ) are shorter (stronger) than those parallel to it (dotted lines).
A layer of BrMeBF in BrMeBF/py is shown in Fig. 8. There are double ribbons of
molecules parallel to the b direction. These double ribbons are far from flat, but can be
viewed as the beginning of a sheet similar to those found in the uncomplexed
compounds (see Fig. 2).

Helvetica Chimica Acta Vol. 86 (2003)
1183
Fig. 6. Packing of Cl2PA in the complex with pyrene. Same conventions as in Fig. 5. Note that the arrangement
of the molecules along the chains is not the same as in Fig. 5.
A layer of IMeBF in IMeBF/py is shown in Fig. 9. There are two crystallo-
graphically independent, but similar, ribbons parallel to the b axis. Each ribbon is held
together by I ¥¥¥ N interactions. There are also ring H ¥¥¥ O interactions between
adjacent ribbons. The two ribbons differ in that the ribbon near a ˆ 1/2is ordered, while
the ribbon near a ˆ 0 is disordered, as shown. The two independent molecules are
related to each other through pseudo-symmetry. For example, the two molecules in the
lower left-hand corner of Fig. 9 are related by a pseudo-inversion center near 3/4, 1/8, 3/
4.
Disorder was prevalent in the PA and BF molecules, both uncomplexed and in the
complexes with pyrene (see Fig. 3 for an example). The occupancies of the various
disordered arrangements are given in Table 5. In the XYBF compounds, the disorder
includes both disorder about a twofold or pseudo-twofold axis, and disorder due to the
presence of the two isomers XYBF and YXBF.

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Helvetica Chimica Acta Vol. 86 (2003)
Fig. 7. Packing of Br2PA in the complex with pyrene. The shorter (stronger) Br¥¥¥ O distances are shown as
dashed lines; the longer as dotted lines.
Discussion. Phthalic Anhydride Structures. The PA compounds and complexes
were intended to serve as reference structures. No disorder was anticipated, but in
Cl2PA/py (Fig. 6) there is end-for-end disorder that is not present in any of the other
structures. In the uncomplexed structures, which are isostructural, both halogen atoms
are in contact with two O-atoms each, for a total of four X ¥¥¥ O contacts (Fig. 2 and
Table 4). In the complexed structures, the two PA complexes are no longer
isostructural, In the Cl2PA/py, there is only one Cl ¥¥¥ O contact, at about the same
distance as in Cl2PA. In the Br2BF/py, there is one Br¥¥¥ O contact at about the same
distance as in Br2PA, and a second, put in for comparison only, at a longer distance
than a normal Van der Waals contact.
The Uncomplexed Structures. In Cl2BF-A and -C, and Br2BF-A, the disorder is
across a crystallographic two fold axis, so the disorder must be 50 :50. In BrClBF-A,
there is also close to complete disorder between the two isomers, BrClBF and ClBrBF.
In BrClBF-C, the disorder is even closer to complete; there is no significant difference
among any of the four possible orientations or isomers (Table 5). The replacement of
two Cl or two Br by one Cl and one Br has no significant effect on the ordering.
That BrClBF-C is slightly more disordered is consistent with the larger a' and b'
dimensions (see Table 3). The looser packing is also consistent with the larger

Helvetica Chimica Acta Vol. 86 (2003)
1185
Fig. 8. Packing of BrMeBF in the complex with pyrene. Contacts between O-atoms, and Me H- and Br-atoms
are shown as dashed lines.
displacement parameters (see Table 2). Dunitz [6] has pointed out that the most closely
packed polymorph should be the low-temperature form. The melting-point behavior
(see Crystallography :Experimental) is consistent with this; A is the low and C the high-
temperature form.
In BrMeBF and IMeBF, the occupancies are far from equal. In both cases, the
MeXBF isomer is more-abundant than the XMeBF isomer, and one of the two
orientations of the MeXBF is more abundant than the other. It is surprising that the Me
groups, which replace halogen atoms while leaving the structure otherwise isostructural
with all the other BF compounds, do not have any short contacts to the O-atoms. The H
¥¥¥ O distances (Table 4) are much larger than the usual CÀH ¥¥¥ O Van der Waals
distances.
The change from BrClBF to BrMeBF might be expected to have a greater effect on
the packing than the change from Cl2BF to BrClBF. It did, but only to the extent of
favoring one isomer over the other in the same disordered arrangement. Likewise, the
change from BrMeBF to IMeBF might also be expected to have an effect on the

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Helvetica Chimica Acta Vol. 86 (2003)
Fig. 9. Packing of IMeBF in the complex with pyrene. The rows along a ꢀ 0 and 1 are crystallographically
independent from the row at a ꢀ 1/2. I ¥¥¥ N and ring H ¥¥¥ O interactions are shown with dashed lines. The
weaker CH3 ¥¥¥ O interactions are shown with dotted lines.
packing, but it did not. The general arrangement is the same, and the distribution of
occupancies is not greatly different between the two.
The Pyrene Complexes. In four of the six complexes, the molecular volume is larger
than the sum of the molecular volumes of the pyrene plus the PA or BF; only in the
Br2PA and IMeBF complexes is the packing more efficient for the complex than for
the separated structures. There is very little disorder in the two efficiently packed
complexes, but even in the more loosely packed complexes there is considerably less
disorder than in the uncomplexed compounds (see Table 5). The result found,
decreased disorder in the complexes, is what was looked for, but it is still surprising.
In the Cl2BF/py crystal, one orientation of the Cl2BF is strongly favored over the
other. In the isostructural Br2BF/py, the same preferred orientation is found in an
ordered crystal. In each case there is only one significant X ¥¥¥ O contact, although a
second distance is given for comparison (Table 4). The short distances are slightly
longer than in the uncomplexed structures.

Helvetica Chimica Acta Vol. 86 (2003)
1187
Table 5. Degree of Disorder in the Phthalic Anhydride and Benzofurazan 1-Oxide Molecules
Compound
Orientation in the plane of the moleculea)
ONON
XY
NONO
YX
ONON
YX
NONO
XY
Cl2PA
Br2PA
Cl2BF-A
Cl2BF-Ba)
Cl2BF-C
ordered
ordered
0.5
0.622(4)
0.5
0.5
0.378(4)
0.5
Br2BF
0.5
0.5
BrClBF-A
BrClBF-C
BrMeBFa)
IMeBFa)
Cl2PA/py
Br2PA/py
Cl2BF/pya)
Br2BF/py
BrMeBF/pya)
IMeBF/py
IMeBF/pya)
0.282(9)
0.254(11)
0.091(4)
0.119(4)
0.282(9)
0.254(11)
0.209(4)
0.105(3)
0.218(9)
0.246(11)
0.483(4)
0.435(3)
0.218(9)
0.246(11)
0.217(4)
0.341(4)
end-for-end disorder 0.716(2)/0.284(2)
ordered
0.822(3)
ordered
0.510(5)
ordered
0.654(3)
0.178(3) around a pseudo-twofold axis
0.056(4)
< 0.001
0.311(4)
0.346(3)
0.123(5)
< 0.001
a) The molecule lies on a pseudo-twofold axis.
The change from Br2BF/py (ordered) to BrMeBF/py might have been expected to
lead to an ordered arrangement. Given that one of the Br-atoms in the Br2BF complex
does not interact with N or O, the Me could replace it with very little change in the
packing. But there is disorder and it is such that both of the two major occupancies have
the N2O2 end of the molecule in the same orientation. The disorder is primarily
between Br and Me; that is, Me apparently has replaced Br on a nearly equal basis in
interacting with the O-atoms. In contrast to the uncomplexed BrMeBF, there is a short
CH3 ¥¥¥ O distance. There is also a short ring H ¥¥¥ O distance (Table 4). (See Desiraju
and Steiner [7], for a discussion of CÀH H-bonds).
When the change in the complex is from BrMeBF to IMeBF, the interaction
changes from a Br¥¥¥ O to an I ¥¥¥ N interaction, the only example of an X ¥¥¥ N
interaction in all the compounds described here. By contrast, in the series 5-
halobenzofurazan 1-oxide (halo ˆ Cl, Br, I) [8], there are Cl ¥¥¥ N, Br¥¥¥ N, and I ¥¥¥ O
interactions. In this structure, there are two crystallographically independent IMeBF
molecules. Both are involved in similar interactions, but one of the two shows
significant disorder such that the I ¥¥¥ N interaction is replaced by a CH3 ¥¥¥ I interaction.
The I ¥¥¥ N contacts are at different distances but are geometrically similar. Again, as in
the uncomplexed IMeBF, the CH3 ¥¥¥ O distances are too long to be regarded as
important contacts. There are also two short ring H ¥¥¥ O distances that could be
regarded as CÀH H-bonds.
In these compounds, taken as a whole, the methyl group appears to be comparable
in strength to Br as a Lewis acid or, at least, in its effect on the structures. On the other
hand, in 5-methylbenzofurazan 1-oxide [9] there are no significant CH3 ¥¥¥ O or
CH3 ¥¥¥ N interactions.

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Helvetica Chimica Acta Vol. 86 (2003)
When this study was initiated, it was not expected to be this difficult to find ordered
5,6-disubstituted benzofurazan 1-oxides. In the structures of 4,7-dihalobenzofurazan 1-
oxides (two polymorphs of the dichloro and one of the dibromoderivatives) there is no
disorder in the N2O2 arrangement [9]; in these molecules, involving the same sorts of
intermolecular interactions, the interactions are sufficient to order the molecules.
The formation of complexes did allow the unambiguous structures of the Br2BFand
IMeBF molecules to be determined, something that could not be done in the
disordered pure compounds. This is a possible route for the determination of other
molecular structures that cannot be determined in the pure state owing to disorder or
difficulty in preparing acceptable single crystals.
Crystallography: Experimental. For all of the compounds given in Table 2, data
were collected on a Siemens SMART area detector diffractometer [10], data reduction
was done with SAINT [10], and solution and refinement were done with SHELXTL
[11].
The sample of Br2BF was provided by Prof. F. B. Mallory. Crystals were grown from
MeOH, EtOH, i-PrOH, benzene, acetone, CHCl3, and CCl4. Fifteen crystals of varying
morphology were examined, and all were isomorphous with the A polymorph of Cl2BF.
The crystal used for the structure determination was grown from EtOH.
Crystals of BrClBF were grown from the same solvents as mentioned above.
Thirteen crystals of varying morphology were examined, and two polymorphs were
found, isomorphous with the A and C polymorphs of Cl2BF. The crystal used for
BrClBF-A was a needle, and that used for BrClBF-C was a prism, both grown from
CHCl3. A prism (C form) melted at 1288, while a needle (A form) turned opaque at
75 808 and then melted at 1288. This suggests that the A form transformed to the C
form at 75 808, and that the C form is the stable one at high temperature.
The BrMeBF crystal used was a plate grown from CCl4. No search for polymorphs
was made.
The IMeBFcrystal used was a prism grown from acetone. No search for polymorphs
was made.
The crystals used for all six complexes with pyrene were grown from acetone. No
searches for polymorphs were made. The crystal used for Br2PA/py was twinned. The
data were integrated with SAINTV6.35A [12] as a two-component nonmerohedral
twin. TWINABS [13] found 15030 data from component 1, 15182data from component
2, and 2721 including both components. Rint was 0.11 based on component 1. All
systematic absences were removed with SYSABSFILTER [14]. All unpaired redun-
dancies were removed with STRIP_REDUNDANT [14].
In almost every structure, disorder of the BFand PA molecules was present to some
extent. Since the molecular dimensions could be estimated based on the earlier work,
constraints could be used to help the refinement. In general, bond-length and angle
restraints were introduced early in the refinement and then relaxed as far as possible
without leading to an unstable refinement. The amount of relaxation possible varied
from structure to structure and led to different numbers of restraints. In every case,
overlapping atoms from the different orientations were constrained to have the same
anisotropic displacement parameters. H-Atoms were included at idealized positions
with isotropic displacement parameters 20% larger than the attached C-atoms for the

Helvetica Chimica Acta Vol. 86 (2003)
1189
ring H and 50% larger for the Me H. No attempt was made to look for disorder in the
Me groups.
The values of the occupancies of the various disordered species were determined by
standard least-squares procedures [11]. The uncertainties quoted in Table 5 are the
standard uncertainties of least-squares refinement. Many of the parameters in the
disordered structures are highly correlated, and, judging from the sensitivity of the
occupancy parameters to small changes in other variables, the uncertainties should
probably be twice as large. In spite of the uncertainties in the accuracies. the differences
are clear as to the relative importance of the various occupancies in the disordered
structures.
Synthesis: Discussion. All three benzofurazan 1-oxides (BrMeBF, IMeBF, and
BrClBF) were prepared conveniently by dropwise addition of aqueous 5% NaOCl to a
red solution of the K salt of the corresponding 2-nitroaniline in ethanolic KOH.
Disappearance of the red color of the anion indicates the end point, where the yellow
color of the product remains in solution and in the precipitate. The NaOCl oxidation
procedure (which probably goes through a nitrene intermediate) is essentially that of
Green and Rowe [15] as used subsequently by us [9][16]. Two of the 2-nitroanilines
were prepared (Scheme 1) by direct para-halogenation of 5-methyl-2-nitroaniline with
N-bromosuccinimide (NBS) (giving 4-bromo-5-methyl-2-nitroaniline and then
BrMeBF) or N-iodosuccinimide (NIS) (giving 4-iodo-5-methyl-2-nitroaniline and,
then IMeBF) in refluxing glacial AcOH according to the procedure of Gershon et al.
[17]. The third preparation proceeded (Scheme 2) through acetylation of 4-bromo-3-
chloroaniline in aqueous solution [18] to give 4-bromo-3-chloroacetanilide, which was
nitrated with KNO3 in cold H2SO4 to the known 4-bromo-5-chloro-2-nitroacetanilide
(previously prepared by Wheeler and Valentine [19] by nitration in cold fuming HNO3
(d 1.52)). Our product appeared by its wide melting point and 1H-NMR spectrum to be
a mixture of isomers, possibly including the other product of ortho-nitration, 4-bromo-
3-chloro-2-nitroacetanilide, which was removed by two crystallizations. Hydrolysis of
the purified product in refluxing aqueous H2SO4 gave the known 4-bromo-5-chloro-2-
nitroaniline (previously prepared by hydrolysis in boiling EtOH containing some conc.
HCl [19]). This product was then subsequently oxidized with aqueous 5% NaOCl to
BrClBF.
The complexes with pyrene were prepared by dissolving approximately equimolar
amounts of both components in the solvent, with heating if necessary, and allowing
evaporation of the solvent at room temperature. Initially, complexes were prepared of
Cl2BF with hexamethylbenzene, pentamethylbenzene, 2,6-dimethylnaphthalene, pyr-
ene, anthracene, phenanthrene, and triphenylene. Of these, the pyrene complex gave
the best-quality crystals for diffraction experiments. Based on this, pyrene was used as
the complexing agent for the remaining complexes.

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Helvetica Chimica Acta Vol. 86 (2003)
Scheme 1
Me
NH2
NO2
Me
Br
NH2
NO2
NBS, AcOH
reflux
81%
(commercial)
(yellow, m.p. 175–176°)
NIS, AcOH
reflux
1. KOH, EtOH
2. 5% NaOCl
91%
37%
O
Me
I
NH2
NO2
Me
N
O
N
Br
BrMeBF
(yellow, m.p. 112.5–113°)
(yellow-orange, m.p. 166–169°)
1. KOH, EtOH
2. 5% NaOCl
47%
O
Me
N
O
N
I
IMeBF
(yellow, m.p. 79–80°)
Experimental Part
1. General. M.p.: uncorrected. NMR Spectra: Varian Unity 200- and 300-MHz spectrometers, chemical
shifts in d [ppm] downfield from TMS.
2. 4-Bromo-5-methyl-2-nitroaniline (Scheme 1): A soln. of 5-methyl-2-nitroaniline (1.00 g, 6.57 mmol) and
N-bromosuccinimide (NBS; 1.15 g, 6.46 mmol) in glacial AcOH (100 ml) was refluxed, with stirring, for 80 min
and then allowed to cool. The resulting mixture was poured into H2O (900 ml), and the mixture was stirred for
10 min. The precipitate was suction-filtered and dried overnight to give a yellow solid (1.21 g, 5.24 mmol; 81%).
M.p. 175 1768. 1H-NMR ((D6)DMSO): 8.05 (s, 1 H); 7.46 (s, 2H); 6.94 ( s, 1 H); 2 .2 4s(, 3 H).
3. 5-Bromo-6-methylbenzofurazan 1-Oxide (BrMeBF; Scheme 1). Ice-water-cooled aq. 5% NaOCl was
added dropwise to a red, ice-cold soln. of 4-bromo-5-methyl-2-nitroaniline (76 mg, 0.329 mmol) in ethanolic
KOH (35 ml). A precipitate formed but redissolved and, as addition continued, the soln. turned yellow, and a
final precipitate formed. The mixture was extracted with CH2Cl2 (2 Â 50 ml), and the extracts were dried
(MgSO4), filtered, and evaporated in a rotating evaporator, leaving a crude solid (76 mg). Recrystallization
1
from EtOH gave BrMeBF as a yellow solid (28 mg, 0.122 mmol; 37%), M.p. 112.5 1138; H-NMR (CDCl3):
7.79 (s, 1 H); 7.30 (s, 1 H); 2.46 (s, 3 H).
4. 4-Iodo-5-methyl-2-nitroaniline (Scheme 1). A soln. of 5-methyl-2-nitroaniline (500 mg, 3.29 mmol) and
N-iodosuccinimide (NIS; 665 mg, 2.96 mmol) in glacial AcOH (30 ml) was refluxed for 70 min and then allowed
to cool for 5 min. The mixture was poured into ice-water (300 ml) and stirred for 10 min. The resulting
precipitate was filtered, washed with cold H2O (3 Â 70 ml), and dried to give a yellow-orange solid (752mg,
2.70 mmol; 91%). M.p. 166 1698. 1H-NMR ((D6)DMSO): 8.23 (s, 1 H); 7.42 (s, 2H); 6.92( s, 1 H); 2 .2 4 s(, 3 H).
13C-NMR ((D6)DMSO): 148.78; 146.28; 134.70; 130.11; 119.60; 82.83; 28.01.
5. 5-Iodo-6-methylbenzofurazan 1-Oxide (IMeBF; Scheme 1): Ice-water-cooled aq. 5% NaOCl was added
dropwise to a red, ice-cold soln. of 4-iodo-5-methyl-2-nitroaniline (100 mg, 0.360 mmol) in ethanolic KOH

Helvetica Chimica Acta Vol. 86 (2003)
1191
Scheme 2
H
N
Cl
Br
NH2
Cl
Br
O
Ac2O, AcONa
H2O, r.t.
28%
Me
(commercial)
(light-gray, m.p. 120–123°;
[19]: m.p. 125° (sharp))
KNO3, H2SO
0°
37%
H
Cl
Br
NH2
NO2
Cl
Br
N
O
H2SO4
H2O, reflux
93%
Me
NO2
(dark-yellow, m.p. 205–206°;
[19]: yellow, m.p. 202–203°)
(yellow, m.p. 129–130°;
[19]: colorless, m.p. 129–130°
1. KOH, EtOH
2. 5% NaOCl
50%
O
Cl
N
O
N
Br
BrClBF
(yellow, m.p. 125–126°)
(20 ml). A precipitate formed but redissolved and, as addition continued, the soln. turned yellow, and a final
precipitate formed. This precipitate was extracted with CH2Cl2 (2 Â 50 ml), and the combined org. extracts were
dried (MgSO4), filtered, and evaporated in a rotating evaporator, leaving a tan solid (84 mg). Crystallization
from EtOH gave IMeBF (47 mg, 0.170 mmol; 47%). Yellow crystals. M.p. 79 808. 1H-NMR (CDCl3): 8.11 (s,
1 H); 7.31 (s, 1 H); 2.49 (s, 3 H).
6. 4-Bromo-3-chloroacetanilide (Scheme 2): The method of acetylation in aq. soln. was used [18]. 4-Bromo-
3-chloroaniline (5.00 g, 24.2 mmol) and conc. HCl (1.99 ml, 24.2 mmol) in H2O (100 ml) were stirred at r.t. for
20 min. Ac2O (3.09 g, 30.3 mmol) and then anh. AcONa (3.29 g, 24.2 mmol) were added, producing a fluffy
white precipitate. The mixture was stirred for 30 min, and the precipitate was filtered, washed with H2O
(100 ml), and dried to give a light tan solid (5.31 g, 21.4 mmol, 88%). Recrystallization from EtOH gave a light
gray solid (1.70 g, 6.84 mmol; 28%). M.p. 120 1238 ([19]: m.p. 1258 (sharp)). 1H-NMR ((D6)acetone): 9.42( s,
1 H); 8.06 (d, J ˆ 2.6, 1 H); 7.61 (d, J ˆ 8.8, 1 H); 7.42( dd, J ˆ 8.8, 2.4, 1 H); 2.02 (s, 3 H).
7. 4-Bromo-5-chloro-2-nitroacetanilide (Scheme 2). This compound has been reported previously in 71%
yield by nitration in cold fuming HNO3 (d ˆ 1.52) [19]. An ice-cold soln. of KNO3 (230 mg, 2.02 mmol) in concd.
H2SO4 (5 ml) was added, with stirring, to a soln. of 4-bromo-3-chloroacetanilide (500 mg, 2.02 mmol) in conc.
H2SO4 (5 ml) at r.t. over a period of 2min. The soln. turned brown and was stirred for 20 min and then poured
onto ice (30 g). The resulting yellow precipitate was suction-filtered, washed with cold H2O (80 ml), and dried
on the filter for 30 min to give a yellow solid (530 mg, 1.84 mmol; 89%), which appeared from the wide m.p. and
the 1H-NMR spectrum to be a mixture of isomers (possibly including 4-bromo-3-chloro-2-nitroacetanilide).
Two crystallizations from EtOH separated a single isomer as yellow crystals (270 mg, 0.92 mmol; 46%). M.p.
1
129 1308 ([19] colorless, m.p. 129 1308). H-NMR ((D6)DMSO): 10.35 (s, 1 H); 8.32( s, 1 H); 7.94 (s, 1 H);
2.07 (s, 3 H). 13C-NMR ((D6)DMSO): 169.28; 141.15; 138.85; 132.18; 130.11; 126.13; 116.56; 24.03.

1192
Helvetica Chimica Acta Vol. 86 (2003)
8. 4-Bromo-5-chloro-2-nitroaniline (Scheme 2). Preparation of this compound has been reported previously
in unstated yield by hydrolysis of 4-bromo-5-chloro-2-nitroacetanilide with some conc. HCl in boiling EtOH
[19]. A soln. of 4-bromo-5-chloro-2-nitroacetanilide (100 mg, 0.341 mmol) in H2O (10 ml) and conc. H2SO4
(1.5 ml) was refluxed for 2.5 h, at which time TLC (silica gel; CH2Cl2) indicated that no starting material was
present. The soln. was cooled, made alkaline to litmus with KOH pellets, and extracted with CH2Cl2. The extract
was dried (MgSO4) and evaporated in a rotating evaporator to leave a dark yellow solid (80 mg, 0.318 mmol;
93%). M.p. 205 2068 ([19]: yellow, m.p. 202 2038). 1H-NMR (CDCl3): 8.38 (s, 1 H); 6.98 (s, 1 H); 6.08 (s, 2 H).
9. 5-Bromo-6-chlorobenzofurazan 1-Oxide (BrClBF; Scheme 2). Aq. 5% NaOCl was added dropwise to a
red soln. of 4-bromo-5-chloro-2-nitroaniline (200 mg, 0.795 mmol) in ethanolic KOH (60 ml), and a yellow
precipitate started to form, but addition was continued until precipitation stopped, and the solution turned
yellow. The mixture was extracted with CH2Cl2 (3 Â 100 ml), and the extracts were dried (MgSO4), filtered, and
evaporated in a rotating evaporator to leave a residue (180 mg) shown by TLC to contain a little starting
material. Chromatography (silica gel; hexanes/CH2Cl2 2:1) gave a better, but still slightly impure sample
(160 mg). Crystallization from EtOH gave a yellow solid (100 mg, 0.401 mmol; 50%) . M.p. 125 1268. 1H-NMR
(CDCl3): 7.91 (s, 1 H); 7.68 (s, 1 H).
We thank Prof. Frank B. Mallory for the sample of Br2BF. We thank the Wayland E. Noland Research
Fellowship Fund of the University of Minnesota Foundation for research support to M. J. P.
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[14] SYSABSFILTER and STRIP_REDUNDANT, W. Brennessel, V. G. Young Jr., unpublished work,
University of Minnesota, Minneapolis, MN. 2003.
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Received January27, 2003