Binary and ternary co-crystals and molecular salts of 3,5-dinitrobenzoic acid: A many-faceted supramolecular reagent

Article abstract of DOI:10.1016/j.molstruc.2018.04.066

The co-crystallization of 3,5-dinitrobenzoic acid (DNBA) to form binary and ternary complexes is reported. Binary complexes (DNBA) (flouranthene) (1), (DNBA) (9-bromoanthracene) (2), (DNBA) (9-methylanthracene) (3) and (DNBA) (9-anthroic acid) (4) are all sustained by charge-transfer interactions. By introducing three different pyridine complexes, the ternary complexes (DNBA) (flouranthene) (5-nitro-2-aminopyridine) (5), (DNBA) (9-anthracenecarboxylic acid)?(3-bromo-2-aminopyridinium) (6) and (DNBA) (9-anthroic acid) (3-nitro-2-aminopyridine) (7) were synthesized utilizing an additional classical hydrogen bond from the DNBA to the pyridine. Compound 6 formed a molecular salt via proton exchange between the DNBA and 3-bromo-2-aminopyridine, whereas all other complexes are neutral (co-crystals).

Full text of DOI:10.1016/j.molstruc.2018.04.066

Accepted Manuscript
Binary and ternary co-crystals and molecular salts of 3,5-dinitrobenzoic acid: A many-
faceted supramolecular reagent
Tania N. Hill, Demetrius C. Levendis, Andreas Lemmerer
PII:
S0022-2860(18)30514-3
10.1016/j.molstruc.2018.04.066
MOLSTR 25139
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 19 March 2018
Revised Date: 18 April 2018
Accepted Date: 19 April 2018
Please cite this article as: T.N. Hill, D.C. Levendis, A. Lemmerer, Binary and ternary co-crystals and
molecular salts of 3,5-dinitrobenzoic acid: A many-faceted supramolecular reagent, Journal of Molecular
Structure (2018), doi: 10.1016/j.molstruc.2018.04.066.
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ACCEPTED MANUSCRIPT
Binary and ternary co-crystals and molecular salts of 3,5-
dinitrobenzoic acid: a many-faceted supramolecular
reagent.
Tania N. Hill, Demetrius C. Levendis and Andreas Lemmerer*
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, 2050, Johannesburg,
South Africa
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
To whom correspondence should be addressed:
Dr. Andreas Lemmerer, Molecular Sciences Institute, School of Chemistry, University of the
Witwatersrand, Private Bag 3, 2050, Johannesburg, South Africa.
E-mail: andreas.lemmerer@wits.ac.za. Fax: +27-11-717-6749. Tel: +27-11-717-6711.
1

ABSTRACT:
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The co-crystallization of 3,5-dinitrobenzoic acid (DNBA) to form binary and ternary complexes is
reported. Binary complexes (DNBA)∙(flouranthene) (1), (DNBA)∙(9-bromoanthracene) (2), (DNBA)∙(9-
methylanthracene) (3) and (DNBA)∙(9-anthroic acid) (4) are all sustained by charge-transfer interactions.
By introducing three different pyridine complexes, the ternary complexes (DNBA)∙(flouranthene)∙(5-
nitro-2-aminopyridine) (5), (DNBA)∙( 9-anthracenecarboxylic acid)∙(3-bromo-2-aminopyridinium) (6)
and (DNBA)∙(9-anthroic acid)∙(3-nitro-2-aminopyridine) (7) were synthesized utilizing an additional
classical hydrogen bond from the DNBA to the pyridine. Compound 6 formed a molecular salt via proton
between the DNBA and 3-bromo-2-aminopyridine, whereas all other complexes are neutral (co-crystals).
KEYWORDS:
Charge-transfer, co-crystal, hydrogen bonding, ternary.
Introduction
Crystal engineering, the conception and synthesis of molecular solid state structures, is fundamentally
based upon the discernment and subsequent exploitation of intermolecular interactions. Consequently
non-covalent bonding interactions are primarily used to achieve the organization of molecules and ions in
the solid state in order to produce materials with desired properties. The understanding of how to control
the solid state formation of molecules using a variety of intermolecular interactions is at the very heart of
crystal engineering. The desire to create these assemblies is incorporated in the process of supramolecular
synthesis [1]. Recently, it has been shown that one can synthesize supramolecular assemblies that contain
anywhere from three to six different molecular moieties [2]. Supramolecular synthesis chiefly uses the
hydrogen bond interaction as the most directional of the known intermolecular interactions [3]. An
equally important interaction is that of charge transfer (CT) between an electron rich π-system (donor)
and an electron poor π-system (acceptor) [4]. Classic donor molecules (polycyclic aromatic
2

hydrocarbons) generally have an electron rich π-system. On the other hand, aromatic hydrocarbons with
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strongly polarizing groups, such as 1,3,5-trinitrobenzene (TNB), have an electron poor π-system and are
classified as the acceptor molecule. Currently, the Cambridge Structural Database (CSD) contains some
95 CT complexes using TNB which have one donor and one acceptor molecule [5]. We have shown
recently that it is possible to introduce a third molecule in a predictable fashion by using hydrogen
bonding to stabilise the ternary arrangement [5]. In the ternary co-crystal (1,3,5-trinitrobenzene)·(1-
naphthanoic acid)·(2-amino-5-nitropyridine) [5], the 1-naphthanoic molecule has an electron rich π-
system as well as a carboxylic acid functional group and hence uses two distinct intermolecular
interactions, one to form a πꢀꢀꢀπ CT interaction to TNB, and one to form a strong O-HꢀꢀꢀN to the pyridine.
We intend to follow up this work by replacing a nitro group on the TNB acceptor molecule with a
carboxylic acid group using 3,5-dinitrobenzoic acid (DNBA) as the acceptor molecule. As a result, the
acceptor molecule itself can be used to hydrogen bond to a third pyridine containing molecule. DNBA has
not been used as extensively as TNB, and so our first step was to make binary CT complexes using the
electron rich acceptors flouranthene, 9-bromoanthracene, 9-methylanthracene and 9-anthracenecarboxylic
acid. Once that was accomplished, we then used three different pyridine containing molecules 5-nitro-2-
aminopyridine, 3-bromo-2-aminopyridine and 3-nitro-2-aminopyridine to make ternary CT complexes
(Scheme 1).
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Fig. 1 The eight molecules used in the supramolecular synthesis of binary and ternary co-crystals of
DNBA.
Experimental
Crystal Synthesis
All reagents used for synthesis and characterization were of analytical grade, purchased from Sigma-
aldrich, unless otherwise stated. Reagents were used as received, without further purification.
For the Binary Systems
3,5-dinitrobenzoic acid (0.1 g, 0.471 mmol) was dissolved in EtOH (absolute, 10 ml) with an equimolar
amount of the donor compound (Table 1), the resulting solution was then heated slowly to 65 °C and held
4

constant for 6 hours. Thereafter, the solution was slow cooled at a rate of 0.033 ° per min to obtain
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crystals suitable of X-ray diffraction analysis.
For the Ternary Systems
3,5-dinitrobenzoic acid (0.1 g, 0.471 mmol) was dissolved in EtOH (absolute, 10 ml) with equimolar
amounts of the donor and ternary compounds (Table 1), the resulting solution was then heated slowly to
65 °C and held constant for 6 hours. Thereafter, the solution was slow cooled at a rate of 0.033 ° per min
to obtain crystals suitable of X-ray diffraction analysis.
Table 1 Donor and Ternary Experimental values
Compound
Donor
Ternary
DNBA·Flouranthene
[C7H4N2O6·C16H10]
(1)
(2)
(3)
(4)
(5)
fluoranthene
0.095 g
-
-
DNBA·9-bromoanthracene [C7H4N2O6·C14H9Br]
DNBA·9-methylanthracene [C7H4N2O6·C15H10]
9-bromoanthracene 0.121 g
9-methylanthracene 0.091 g
DNBA·9-anthroic acid
DNBA·flouranthrene·5-
nitro-2-aminopyridine
DNBA·9-anthroic acid·3-
bromo-2-aminopyridine
DNBA·9-anthroic acid·3-
nitro-2-aminopyridine
[C7H4N2O6·C15H10O2]
[C7H4N2O6·C16H10·C5H5
N3O2]
9-anthroic acid
fluoranthene
0.104 g
0.095 g
-
5-nitro-2-
0.066 g
0.082 g
0.066 g
aminopyridine
3-bromo-2-
aminopyridine
3-nitro-2-
[C7H4N2O6·C15H10O2·C5
H5N2Br]
(6)
(7)
9-anthroic acid
9-anthroic acid
0.104 g
0.104 g
[C7H4N2O6·C15H10O2·C5
H5N3O2]
aminopyridine
Single crystal X-ray crystallography
The Bruker D8 VENTURE PHOTON CMOS area detector diffractometer, equipped with a graphite
monochromated Mo Ka1 radiation (50 kV, 30 mA), was used to collect all the intensity data. Crystal
structures were collected at 173 K to achieve satisfactory thermal ellipsoids. The program SAINT+, vers.
6.02 [15] was used to integrate the data and the program SADABS [16] was used to make empirical
absorption corrections. Space group assignments were made using XPREP [15] on all compounds. In all
cases, the structures were solved in the WinGX [17] suite of programs by direct methods using SHELXS-
97 [18] and refined using full-matrix least-squares/difference Fourier techniques on F2 using SHELXL-97
[18]. All non-hydrogen atoms were refined anisotropically. The positional disorder of the Br atom was
modelled over two positions with occupancies of 0.5922(8) and 0.4078(8). The positional disorder of the
5

H atoms on the carboxylic acids was modelled over two positions and occupancies fixed at 0.5. After that,
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all C-bound hydrogen atoms were placed at idealized positions and refined as riding atoms with Uiso
parameter 1.2 or 1.5 times those of their parent atoms. Thereafter, the N-bound hydrogen atoms were
located in the difference Fourier map and their coordinates and isotropic displacement parameters refined
freely. The O-bound hydrogen atoms in 1, 3 and 4 were placed at idealized positions and refined as riding
atoms with Uiso parameter 1.5 times those of their parent atoms. Further crystallographic data are
summarised in Table 2. Diagrams and publication material were generated using ORTEP-3 [10],
PLATON [11] and MERCURY [12]. CCDC 1813906-1813912 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures
(or from the Cambridge Crystallographic Data Centre,12, Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033).
6

Table 2 Crystallographic data for co-crystals 1-7.
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1
2
3
Formula
(C7H4N2O6)ꢀ(C16H10)
414.36
(C7H4N2O6)ꢀ(C14H8Br)
(C7H4N2O6)ꢀ(C15H12)
Mr / g mol-1
Temperature / K
Crystal size / mm3
Crystal system
Space group
a / Å
468.24
404.37
173(2)
293(2)
293(2)
0.60 x 0.51 x 0.50
Triclinic
0.34 x 0.17 x 0.16
0.59 x 0.24 x 0.07
Monoclinic
Monoclinic
P-1
C2/c
C2/c
7.0833(2)
8.4467(3)
16.3038(5)
89.991(2)
177.040(1)
79.938(1)
937.73(5)
2
34.806(2)
34.773(3)
b / Å
6.7470(4)
6.7335(5)
c / Å
16.6475(12)
16.4911(10)
90
90
α
β
γ
/ °
/ °
/ °
107.053(3)
106.119(7)
90
90
V / Å3
3737.5(4)
3709.5(5)
Z
8
8
ρcalcd /Mg m-3
1.468
1.664
1.448
/ mm-1
0.108
2.244
0.107
µ
F(000)
428
1880
1680
Theta range for data
collection / °
Completeness to theta
max. / %
2.451 to 27.995
99.9%
2.448 to 28.000
100.0
1.219 to 27.998
99.9
Absorption Correction
semi-empirical from equivalents
0.95 and 0.90
13436
semi-empirical from equivalents
0.72 and 0.53
18616
semi-empirical from equivalents
Max. and min.
transmission
0.95 and 0.90
24827
Reflections collected
No. unique data [R(int)]
4524 [0.0514]
3826
4507 [0.0624]
2642
4466 [0.0505]
2682
No. data with I > 2σ(I)
final R (I > 2σ(I))
final wR2 (all data)
Largest diff. peak and hole
CCDC No.
0.0535
0.0494
0.0562
0.1552
0.1382
0.1669
0.509 and -0.496
1813906
0.519 and -0.259
1813907
0.835 and -0.326
1813908
7

Table 2 contd. Crystallographic data for co-crystals 1-7.
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4
5
6
Formula
(C7H4N2O6)ꢀ(C15H10O2)
(C7H4N2O6)ꢀ(C16H10)ꢀ(C5H5N3O2)
(C7H3N2O6)ꢀ(C15H10O2)ꢀ(C5H6BrN2)
Mr / g mol-1
Temperature / K
Crystal size / mm3
Crystal system
Space group
a / Å
434.35
553.48
607.37
293(2)
293(2)
293(2)
0.65 x 0.12 x 0.05
0.34 x 0.17 x 0.16
0.37 x 0.10 x 0.09
Monoclinic
Monoclinic
Monoclinic
P21/c
P21/n
P21/n
7.2041(3)
7.5090(8)
7.0877(2)
b / Å
8.9925(5)
14.3212(14)
23.8518(8)
c / Å
29.6402(13)
22.677(2)
14.6382(4)
90
90
90
α
β
γ
/ °
/ °
/ °
92.009(3)
90.470(7)
90.220(2)
90
90
90
V / Å3
1919.00(16)
2438.6(4)
2474.63(13)
Z
4
4
4
ρcalcd /Mg m-3
1.503
1.508
1.630
/ mm-1
0.117
2.113
1.723
µ
F(000)
896
1144
1232
Theta range for data
collection / °
Completeness to theta
max. / %
1.375 to 27.994
99.8%
1.682 to 25.497
100.0
1.623 to 27.997
100.0
Absorption Correction
semi-empirical from equivalents
0.99 and 0.93
16092
semi-empirical from equivalents
0.94 and 0.99
22205
semi-empirical from equivalents
0.57 and 0.86
39082
Max. and min.
transmission
Reflections collected
No. unique data [R(int)]
4623 [0.0625]
2734
4540 [0.1896]
2110
5988 [0.0494]
4168
No. data with I > 2σ(I)
final R (I > 2σ(I))
final wR2 (all data)
Largest diff. peak and
CCDC No.
0.0566
0.0723
0.0378
0.1490
0.2072
0.1060
0.272 and -0.206
1813909
0.306 and -0.278
1813910
1.152 and -0.551
1813911
7
Formula
(C7H4N2O6)ꢀ(C15H10O2)ꢀ(C5H5N3O2)
Mr / g mol-1
573.47
Temperature / K
Crystal size / mm3
Crystal system
173(2)
0.56 x 0.13 x 0.07
Monoclinic
8

Space group
a / Å
P21/n
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7.0404(7)
12.814(2)
14.8004(13)
90
b / Å
c / Å
α
β
γ
/ °
/ °
/ °
93.515(6)
90
V / Å3
2476.8(4)
4
Z
ρcalcd /Mg m-3
1.538
/ mm-1
0.120
µ
F(000)
1184
Theta range for data
collection / °
Completeness to theta
max. / %
1.622 to 27.995
100.0%
Absorption Correction
semi-empirical from equivalents
Max. and min.
transmission
0.99 and 0.93
23953
Reflections collected
No. unique data [R(int)]
5958 [0.0833]
3377
No. data with I > 2σ(I)
final R (I > 2σ(I))
0.0526
final wR2 (all data)
Largest diff. peak and
CCDC No.
0.1488
0.314 and -0.299
1813912
9

Results and Discussion
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The atomic numbering scheme for the four binary (1 - 4), two ternary (5 & 7) co-crystals and one ternary
molecular salt 6 are given in Figures 1 and 2 respectively, which additionally shows the contents of the
asymmetric unit in each case. The distances and angles within the compounds reported are generally as
expected [13]. All compounds have a Z' = 1 with all molecules placed in general positions. The packing
of the unit cells of the complexes follows a donor (D) / acceptor (A) π∙∙∙π interaction, which was found
to be the major driving force in the formation of these complexes, as seen in Figure 3 for the binary co-
crystals 1 - 4 and in Figure 4 for the ternary co-crystals 5 - 7 (donor molecules shown in blue and acceptor
in green). All four compounds have a general face-to-face π-stacking, with Table 2 summarizing the
closest centroid∙∙∙centroid distances between the DNBA acceptor and aromatic donor systems. The
intermolecular interactions of the D∙∙∙A stacks can be quantified using Hirshfeld surface analysis as well
as resulting fingerprint plots using the programme Crystal Explorer 17.5 [14]. In the paper by Chen et al.
[15] the authors describe those regions of blue and red triangles on the Hirshfeld surface using the shape
index as evidence of π∙∙∙π interactions. Figure 5 shows such surfaces plotted for the DNBA molecules in
1 - 7. The red triangles show concave regions indicative of ring carbons of the π stacked molecule above
it, and 1, 4 - 7 show the most pronounced red triangles. This can be quantified by looking at the
contribution that C∙∙∙C contacts make up in the fingerprint plots. Compounds 1 – 3 have the greatest
values of the binary co-crystals, and significantly greater than observed for the ternary co-crystals 5 – 7.
Table 3 summarizes the percentages for C∙∙∙C, H∙∙∙H, C∙∙∙H and O∙∙∙H contacts and the relevant
fingerprint plots are given in the supplementary information. In terms of molecular geometry, the DNBA
molecules show some changes from the geometries encountered in the pure compound. The pure
compound has torsion angles for the nitro group of -23.6° and +3.44°, whereas in the CT complexes
described here they range from -11.3° to +3.0°. The packing and hydrogen bonding interactions are
further described below individually for each compound.
10

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Figure 1 Perspective views of the binary co-crystals 1-4, showing the atom-numbering schemes.
Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres
of arbitrary radii. Only one position of the disordered Br atom is shown in 2.
Figure 2 Perspective views of the ternary co-crystals 5-7, showing the atom-numbering schemes.
Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres
of arbitrary radii. The dashed line indicates the symmetry independent N-H∙∙∙O and O-H∙∙∙N hydrogen
bonds.
11

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Table 3. Closest contacts between DNBA acceptor and aromatic polycyclic donors and proportion (%) of
intermolecular contacts between donor and acceptor molecules in the Hirshfeld fingerprint plots.
1
2
3
4
5
6
7
3.5405(1)a
3.5308(3)b
3.5225(3)b
3.5332(2)b
3.3148(4)c
3.4266(1)d
3.3736(3)d
CgꢀꢀꢀCg
C···C
/ Å
14.0
22.0
8.4
15.4
14.4
5.9
14.0
23.8
7.2
11.6
13.8
6.5
12.5
15.3
10.4
40.0
13.5
10.9
6.2
13.2
14.1
6.4
H···H
C···H*
O···H*
40.1
43.2
42.3
45.3
54.9
47.8
* Reciprocal contacts included. (a) central five-membered ring (C8-C17); (b) central six-membered ring of anthracene (C8-
C21); (c) eleven-membered ring (C9-C23); (d) ten-membered ring (C8-21)
12

Table 4. Geometrical parameters for hydrogen bonds in 1-7 [Å and °]
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Symmetry
transformations
D-H···A
d(D-H)
d(H···A)
d(D···A)
<(D-H···A)
1
O1-H1···O2
O2-H2···O1
2
0.84
0.84
1.82
1.81
2.6283(15)
2.6283(15)
162
163
-x + 2, -y, -z
-x + 2, -y, -z
O1-H1···O2
3
0.82
1.81
2.632(3)
177
-x, -y, -z + 1/2
O1-H1···O2
O2-H2···O1
4
0.82
0.82
1.83
1.83
2.633(2)
2.633(2)
167
168
-x, y, -z + 1/2
-x + 2, -y, -z
O1-H1···O2
O7-H7···O8
O8-H8···O7
5
0.82
0.82
0.82
1.83
1.81
1.82
2.644(2)
2.614(2)
2.614(2)
177
167
163
-x, -y + 1, -z + 1
x + 1, -y + 1, -z + 1
x + 1, -y + 1, -z + 1
O1-H1···N3
N4-H4A···O2
C25-H25···O5
C25-H25···O6
N4-H4A···O3
N4-H4B···O4
6
0.82
0.86
0.93
0.93
0.86
0.86
1.82
2.16
2.63
2.46
2.69
2.67
2.631(4)
2.984(5)
3.471(5)
3.332(5)
3.122(5)
3.388(5)
170
160
152
157
112
142
-
-
x - 1/2, -y + 1/2, z - 1/2
x - 1/2, -y + 1/2, z - 1/2
-x + 1/2, y - 1/2, -z + 1/2
-x + 1/2, y - 1/2, -z + 1/2
O7-H7···O2
N4-H4B···O6
N4-H4A···O1
N3-H1···O1
7
0.79(4)
0.79(3)
0.87(3)
0.99(4)
1.90(4)
3.52(3)
2.96(3)
1.58(4)
2.659(2)
4.205(3)
3.524(3)
2.576(3)
161(4)
147(3)
124(2)
176(3)
-
x - 1/2, -y + 1/2, z + 1/2
-
-
O1-H1···N3
O7-H7···O2
N4-H4A···O2
N4-H4B···O9
N4-H4B···O6
0.84
1.72
2.546(2)
2.677(2)
2.917(3)
2.656(3)
2.949(3)
165
-
-
-
-
0.88(3)
0.91(3)
0.91(3)
0.91(3)
1.82(3)
2.02(3)
1.98(3)
2.48(3)
163(3)
169(3)
130(2)
112(2)
x - 1/2, -y + 1/2, z + 1/2
Description of binary co-crystals 1-4
Structure 1 crystallizes in the P space group with both DNBA and flouranthene molecules in the
asymmetric unit. The donor and acceptor molecules pack in a checker-board fashion parallel to the ab-
plane (Fig. 3a). The acceptor molecules form dimers using the well-known carboxylic acid functional
group. In this case, the H atom was found to have a positional disorder, and is bonded to both O atoms of
the carboxylic acid group in a 50/50 ratio (Fig. 6a). This disorder is seen most clearly in the C-O bond
lengths of the two bonds, those being 1.2546(18) Å for C7-O1 and 1.2760(18) Å for C7-O2. This
phenomenon was observed in other co-crystals of DNBA. The first reported is 4-(N,N-
diethylamino)benzoic acid in 1993 (CSD ref. code GAUTAM), in which the authors refer to this type of
hydrogen bonded interaction as “O-H∙∙∙O hydrogen-bonded carboxy homodimers” [16]. In our database
13

study, this type of homodimer is seen in five other co-crystals (KIZQIT, KIZQOZ, KIZQUF & KIZREQ)
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using carbozole derivatives [17] and with 1,4-diiodobenzene (PUQQIB) [18].
Structure 2 crystallizes in the C2/c space group with both DNBA and 9-bromoanthracene in the
asymmetric unit. The packing of the structure shows a well-defined needle axis in the b-axis direction
(Fig. 3b), in addition to a clear overlapping of the donor (blue) and acceptor (green) molecules. The Br
atom displays a positional disorder, with a refined occupancy of 0.59:0.41. No short contacts between the
bromine atoms were observed. The acceptor molecules form dimers by means of the carboxylic acid
functional group, and as a result do not show the same positional disorder as observed in 1, but still have
C-O bond lengths that are not indicative of a completely neutral functional group. The relevant bond
distances are 1.282(3) Å for C7-O1, and 1.239(3) Å for C7-O2. In the plane parallel to the stacking, there
are C---H∙∙∙O2N interactions between the DNBA and 9-bromoanthracene molecules (Fig. 6b).
Structure 3 crystallizes in the C2/c space group with both DNBA and 9-methylanthracene in the
asymmetric unit. The packing of 3 is isostructural to compound 2, with similar unit cell lengths and a
simple exchange of the steric space as occupied by the Br atom in 2 with a methyl group in 3 (the r.m.s.
parameter of an overlay done in MERCURY gives a value of 0.1. The acceptor molecules again form
dimers utilizing the carboxylic acid functional group, and display a similar positional disorder as
observed for 1. The relevant bond distances were found to be 1.251(3) Å for C7-O1, and 1.279(3) Å for
C7-O2. In the plane parallel to the stacking, there are C-H∙∙∙ O2N interactions between the DNBA and 9-
bromoanthracene molecules (Fig. 6c) similar to those found in 2.
Structure 4 crystallizes in the P21/c space group with both DNBA and 9-anthracenecarboxylic acid in the
asymmetric unit. The packing of the structure displays a clear needle axis in the a-axis direction (Fig. 3b),
along with the overlapping of the donor (blue) and acceptor (green) molecules. In this structure, a
carboxylic acid group replaces of the Br atom in 2 and methyl group in 3. The acceptor molecules again
form dimers using the carboxylic acid functional group, which show bond lengths indicative of a neutral
14

functional group (1.294(3) Å for C7-O1, and 1.234(3) Å for C7-O2). In contrast, the donor molecule
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forms the hydrogen-bonded carboxy homodimers featuring the same positional disorder as seen in the
DNBA acceptor molecules in 1 and 3 (1.268(3) Å for C22-O7, and 1.264(2) Å for C22-O8). In the plane
parallel to the stacking, C-H∙∙∙O2N interactions were observed between the DNBA and 9-anthracene
carboxylic acid molecules (Fig. 6d).
Fig. 3 The packing diagrams for all four binary compounds 1-4. The acceptor molecules are shown in
green and the donor molecules in blue.
15

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Fig. 4 The packing diagrams for all three ternary compounds 5-7. The acceptor molecules are shown in
green, the donor molecules in blue and the pyridine derivatives in red.
16

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Fig. 5 The molecular Hirshfeld surfaces mapped with shape index for the DNBA acceptor molecule in 1-
7.
17

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Fig. 6 The hydrogen bonding diagrams for all four binary compounds 1-4.
Description of ternary co-crystals 5 and 7, and molecular salt 6
In the CSD [19], there are 12 ternary co-crystals and 67 binary co-crystals of DNBA (excluding. solvents,
host-guest complexes). The ternary complexes are listed in Table 5, and show that hydrogen bonding was
used as the primary means of assembly between the three components. A noteworthy exception is the
ternary complex of anthracene, DNBA and the solvent molecule benzene. Here, all three interact with via
D···A charge transfer interactions. In the ternary complexes 5 - 7, a combination of D···A CT interactions
along with hydrogen bonding was attempted.
18

Table 5 Summary of ternary complexes of DNBA and the intermolecular interactions binding them.
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CSD Ref.
Code
Molecule Molecule 2
1
Molecule 3
1-2
1-3
2-3
AJAKIF
DNBA
DNBA*
DNBA
isonicotinamide
4-methylbenzoic acid
COOH···Npy
Namide-H···O=C(2)
COOH···Npy
Npy-H···OOCdeprot
COOH···Npy
C-H···O2N
acid···amide
heterodimer
BEYZAI
BIZTIP
4,4-‘bipryidinium 4-aminosalicyclic acid
COOH···OOCd none
eprot
isonicotinamide
3-aminobenzoic acid
acid···amide
π···π
Namide-H···O=C(2)
heterodimer
N-H(3)···O=C(2)
acid···amide
heterodimer
acid···amide
heterodimer
BUDZUV
BUFBIP
DNBA
DNBA
isonicotinamide
isonicotinamide
3-methylbenzoic acid
4-(N,N-
COOH···Npy
Namide-H···O=C(2)
COOH···Npy
π···π
π···π
dimethylamino)benzoic Namide-H···O=C(2)
acid
BUFQAU
QUQCUB
SASBAQ
SASBEU
SASBIY
DNBA
DNBA
DNBA
DNBA
DNBA
isonicotinamide
4-hydroxy-3-
methoxycinnamic acid
COOH···Npy
Namide-H···O2N(1)
Ophen
H···O=C(1)
-
acid···amide
heterodimer
hemikis(4,4’-
bipyridine)
4-
COOH···Npy
acid···acid
homodimer
π···π
none
none
none
dimethylaminobenzoic
acid
1-((3-
4-nitrobenzoic acid
COOH···Nimidazole
COOH···Npy-2
COOH···Npy-2
COOH···Npy-2
Pyridyl)methyl)
benzimidazole
1-((3-Pyridyl)
methyl)
benzimidazole
1-((3-
π···π
3-
COOH···Nimidazole
(dimethylamino)benzoi
c acid
π···π
4-nitrobenzoic acid
COOH···Nimidazole
Pyridyl)methyl)-
2-
π···π
methylbenzimidaz
ole
WUZKIM
ZUPJOJ
DNBA
DNBA
ethenzamide
mesitylene solvate
benzene
acid···amide
heterodimer
D···A
C-H···O2N
none
π···π
anthracene
D···A
Note: Both protonated and deprotonated DNBA molecules in the asymmetric unit.
Structure 5 crystallizes in the P21/n space group with both DNBA and flouranthene in the asymmetric
unit as seen in 1, but now a third molecule, with significant hydrogen bonding ability, 5-nitro-2-
aminopyridine is included in the asymmetric unit. 5-nitro-2-aminopyridine has a pyridine N atom that is
able act as a hydrogen bonding acceptor, and an amine group as a hydrogen bond donor. The DNBA and
flouranthene interact in a similar manner as described for 1. The pyridine, a hydrogen from the amine
functional group and a DNBA molecule are involved in a hydrogen bonded ring, with graph set notation
20

2(8)
[20]
(Fig. 7a). The amine group, using both hydrogen atoms, acts as a donor to a nitro group of a
R2
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2 (6)
second DNBA molecule, forming a
ring. The packing of this ternary co-crystal illustrates the two
R2
distinct intermolecular interactions. The DNBA (green) acceptor and flouranthene (blue) donor molecules
are overlapping using π∙∙∙π interactions, and are interspersed by the aminopyridine molecule shown in
red and hydrogen bonding to the DNBA acceptors (Fig. 4a).
Structure 6 is a ternary molecular salt, which has a dintrobenzoate anion, a neutral 9-anthracenecarboxylic
acid and a 3-bromo-2-aminopyridinium cation, exposing the disrupting influence of the aminopyridine. In
the original binary co-crystal 4, the DNBA and the 9-anthracenecarboxylic acid form carboxylic acid
homodimers, with the anthracene having the carboxy homodimer. In salt 6, due to the calculated pKa
difference [21] (∆pKa = pKa(protonated base) - pKa(acid) = 1.37) between the 3-bromo-2-aminopyridine
(4.14±0.36) and DNBA (2.77±0.1), a proton from the carboxylic acid (1.258(3) Å for C7-O1, and
1.254(3) Å for C7-O2) has been transferred to the pyridine [21]. This forms a charge-assisted hydrogen
2(8)
bonded
ring (Fig. 7b). Additionally, the 9-anthracenecarboxylic acid was found to have a neutral
R2
carboxylic acid group (1.326(3) Å for C22-O7, and 1.213(3) Å for C22-O8) which hydrogen bonds to the
carboxylate group of the dinitrobenzoate cation (Fig. 7b). Overall, the packing of the ternary molecular
salt 6 is similar to 5, having overlapping pairs of donor/acceptors interspersed by the aminopyridinium
cation (Fig. 4b).
The last ternary complex 7 to be described is isostructural to 6, with similar unit cell parameters. The
major difference is that 7 does not form a molecular salt, but a ternary co-crystal. Structure 7 has a
DNBA, 9-anthracenecarboxylic acid and 3-nitro-2-aminopyridine in the asymmetric unit. The resulting
pKa difference is less than zero (∆pKa = 2.40 - 2.77 = -0.37) and predicted not to result in proton transfer,
2(8)
which is in fact the case. Subsequently, a neutral hydrogen bonded ring, with graph set notation
,
R2
is formed similar to the one seen in 5. The 9-anthracenecarboxylic acid hydrogen bonds to the carbonyl of
the carboxylic acid on DNBA (Fig. 7c) and the packing of 7 is in the same vein as 5 and 6.
21

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Fig. 7 The hydrogen bonding diagrams for all three ternary compounds 5-7.
22

Conclusion
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Preparing neutral multi-component molecular complexes is an active field in the crystal engineering
community. It is well known that charge transfer complexes, which are essentially binary co-crystals, can
be formed readily provided the donor and acceptor molecules are correctly chosen. Hydrogen bonding is
also an interaction that is primarily used to make a binary co-crystal, and very often it is the carboxylic
acid∙∙∙pyridine hydrogen bonding synthon that is applied. In this work, we have shown that since the two
intermolecular interactions are different in nature, the former involving conjugated aromatic systems and
the latter an electrostatic interaction, they are complimentary and can be used in a concerted effort to
make ternary co-crystals.
Acknowledgements
The University of the Witwatersrand and the Molecular Sciences Institute are thanked for providing the
infrastructure and financial support required to do this work.
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25

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1) DNBA is co-crystallized with aromatic hydrocarbons to form binary CT complexes.
2) DNBA can also form ternary complexes using strong hydrogen bonds to pyridines
3) Hirscheld analysis was used to quantify the charge-transfer interactions.