Proton-transfer supramolecular salts based on proton sponge 2,2′-dipyridylamine

Article abstract of DOI:10.1016/j.molstruc.2013.07.056

Reactions between proton sponge 2,2′-dipyridylamine and acidic synthons (2,4-dinitrobenzoic acid, 3,4-dinitrobenzoic acid and picronitric acid) afford three proton-transfer supramolecular ammonium salts, (2,4-dinitrobenzoate)?(2,2′-dipyridylammonium) (1), (3,4-dinitrobenzoate)?(2,2′-dipyridylammonium)?(H ₂O) (2) and (picrate)?(2,2′-dipyridylammonium) (3), respectively. During solution crystallization, the proton transfers from the organic acid to the nitrogen atom in the pyridyl ring. It is found that monoprotonated dpaH⁺ has an asymmetrical intramolecular hydrogen bond (IHB) N-H⁺?N, which results in the intramolecular S(6) ring. All supramolecular architectures of 1-3 involve extensive classical hydrogen bonds and display a three-dimensional (3D) framework structure. Robust hydrogen bonding interactions generate various intermolecular ring motifs.

Full text of DOI:10.1016/j.molstruc.2013.07.056

Journal of Molecular Structure 1051 (2013) 124–131
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Proton-transfer supramolecular salts based on proton sponge
2,20-dipyridylamine
a,
a
a,b
Xue-Hua Ding a, Yong-Hua Li a, , Shi Wang , Xing-Ao Li , Wei Huang
⇑
⇑
a Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications,
Nanjing 210046, China
b Jiangsu-Singapore Joint Research Center for Organic/Bio-Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Technology,
Nanjing 211816, China
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
Three supramolecular salts with 3D framework structure have been prepared and characterized.
Intramolecular N–H+ꢁ ꢁ ꢁN hydrogen bonding affords S(6) rings in the proton sponge dpaH+.
Robust intermolecular hydrogen bonding interactions generate various ring motifs, such as R22ð8Þ, R42ð10Þ; R44ð12Þ; R24ð18Þ and R44ð26Þ.
a r t i c l e i n f o
a b s t r a c t
Reactions between proton sponge 2,20-dipyridylamine and acidic synthons (2,4-dinitrobenzoic acid, 3,4-
dinitrobenzoic acid and picronitric acid) afford three proton-transfer supramolecular ammonium salts,
(2,4-dinitrobenzoate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium) (1), (3,4-dinitrobenzoate)ꢁ ꢁ ꢁ(2,20-dipyridylammoni-
um)ꢁ ꢁ ꢁ(H2O) (2) and (picrate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium) (3), respectively. During solution crystalliza-
tion, the proton transfers from the organic acid to the nitrogen atom in the pyridyl ring. It is found
that monoprotonated dpaH+ has an asymmetrical intramolecular hydrogen bond (IHB) N–H+ꢁ ꢁ ꢁN, which
results in the intramolecular S(6) ring. All supramolecular architectures of 1–3 involve extensive classical
hydrogen bonds and display a three-dimensional (3D) framework structure. Robust hydrogen bonding
interactions generate various intermolecular ring motifs.
Article history:
Received 26 June 2013
Received in revised form 31 July 2013
Accepted 31 July 2013
Available online 6 August 2013
Keywords:
Proton-transfer
Supramolecular salts
Hydrogen bonds
Proton sponge
Acidic synthons
3D framework
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
to yield a stabilized N–H+ꢁ ꢁ ꢁN intramolecular hydrogen bond
(IHB) [21].
Since the prototypal compound 1,8-bis(dimethylamino)naph-
thalene (DMAN) was reported by Alder et al. [1], the design of more
basic proton sponges received increasing attention [2–8]. A large
number of proton sponges have been systematically reviewed
[9–13]. Proton sponges have been studied as models of proton sub-
straction [14], as organic base catalysts in green chemistry [15], as
effective H+ scavengers in nanocluster formation [16], and play the
important role in enzymatic catalysis [17], in biological systems
[18], in asymmetric organic synthesis [19] as well as in the field
of gene therapy [20]. A general feature of proton sponge is the
presence of two closely basic nitrogen centers in the molecule,
which have an orientation that allows the uptake of one proton
With regard to proton sponges, particular interest has been
mainly focused on neutral organic bases with chelating proton
acceptor functionalities exhibiting enhanced basicity [5]. Com-
pared to ordinary alkyl/aryl amines, amidines and guanidines, such
proton chelators show a dramatic increase in basicity due to desta-
bilization of the base as a consequence of strong repulsion of un-
shared electron pairs, formation of an IHB in the protonated form
and relief from steric strain upon protonation [21].
Neutral 2,20-dipyridylamine (dpa) has three potential nitrogen
base functionalities. Despite the single bonds to the central nitro-
gen atom, it normally presents as a planar array, which impacts
on its basicity. In comparison with the two pyridyl nitrogen atoms,
the central secondary amino NH group is a little acidic and less
favorable to be protonated. It is likely to be protonated only under
very highly acidic conditions [22]. Therefore, dpa can be employed
as a potential proton sponge on account of the presence of two clo-
sely positioned basic sites.
⇑
Corresponding authors. Tel./fax: +86 25 85866396.
E-mail addresses: iamyhli@njupt.edu.cn (Y.-H. Li), iamswang@njupt.edu.cn
(S. Wang).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.07.056

X.-H. Ding et al. / Journal of Molecular Structure 1051 (2013) 124–131
125
Moreover, basic pyridyl nitrogen atoms make dpa a promising
synthon in the supramolecular synthesis of multicomponent
molecular complexes. It is central to successful and rational syn-
thesis of multicomponent supramolecular arrays that molecules
with functional groups are often employed, showing a combination
of complementary and specific interactions [23,24]. Supramolecu-
lar synthons with functional groups may be either Brønsted acids
(proton donors, D–H) or Brønsted bases (proton acceptors, :A),
which can interact by sharing the proton and form D–Hꢁ ꢁ ꢁ:A
hydrogen bonding; and either Lewis bases (electron donors, :D)
or Lewis acids (electron acceptors, A) endowed with groups can
interact by sharing a couple of electrons in a similar way, resulting
in D: ? A electron donor–acceptor or charge-transfer interactions
[25]. Carboxylic acids and N-containing molecules have been
proved to be useful and powerful building blocks through hydro-
gen bonding interactions [25–31]. Hydrogen bonding has been
the most common ‘‘tool’’ in crystal engineering due to its strength,
directionality and predictability [32–40].
Particularly attractive are the cases with polytopic potential
hydrogen donors or acceptors in acids or bases. Generally, the
strongest donor will form a bond with the strongest acceptor and
the second strongest donor with the second acceptor [41]. The
assembly of supramolecular compounds will follow this ‘‘rule’’,
though many exceptions would exist because of steric or packing
effects [42]. Proton transfer may occur when the hydrogen accep-
tor is a noticeably stronger base than the deprotonated donor
[41,43,44]. The charge separation leads to the formation of supra-
molecular salts, which have the potential to alter and optimize
physical properties such as crystalline form, solubility and stability
[45–47]. The pKa values of DH and AH+ are commonly used to mea-
sure the relative proton affinities of the donor and the acceptor
atoms [41].
components as supramolecular synthons and report the synthesis
and crystal structure of three supramolecular salts, (2,4-dinitro-
benzoate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium) (1), (3,4-dinitrobenzo-
ate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium)ꢁ ꢁ ꢁ(H2O) (2) and (picrate)ꢁ ꢁ ꢁ(2,20-
dipyridylammonium) (3), respectively (Scheme 1). The proton
transfers from the acid to pyridyl nitrogen atom.
2. Experimental section
2.1. Preparation of the salts
The chemicals and solvents used in this work are of analytical
grade and available commercially and were used without further
purification.
2.1.1. (2,4-Dinitrobenzoate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium) (1)
A solution of 2,20-dipyridylamine (0.05 mmol) in acetonitrile
(3 mL) was added dropwise to a stirred solution of 2,4-dinitroben-
zoic acid (0.05 mmol) in acetonitrile (3 mL). The colorless solution
was stirred for a few minutes at ambient condition, and then re-
fluxed for about half an hour. After the natural cooling, the result-
ing solution was left standing at room temperature for several
days. Colorless rhombus crystals were isolated after slow evapora-
tion in air.
2.1.2. (3,4-Dinitrobenzoate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium)ꢁ ꢁ ꢁ(H2O) (2)
Complex 2 was obtained as colorless rod crystals by the similar
procedure described for 1, except with the addition of 3,4-dinitro-
benzoic acid instead of 2,4-dinitrobenzoic acid.
2.1.3. (Picrate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium) (3)
A solution of 2,20-dipyridylamine (0.05 mmol) in methanol
(3 mL) was added dropwise to a stirred solution of picric acid
(0.05 mmol) in methanol (3 mL). A yellow precipitate appeared
immediately. The mixture was refluxed for about half an hour.
After the powder was dissolved, the solution was filtered. Upon
slow evaporation of the filtrate at room temperature for several
days, well-shaped yellow slice crystals suitable for X-ray diffrac-
tion were obtained.
Recently we have reported the proton-transfer supramolecular
salts assembled from 3,5-dinitrobenzoic acid and (2, 3 and 4)-ami-
nomethyl pyridine [48]. The nitrogen atom of the primary amino
NH2 group in aminomethyl pyridine has been protonated. Contin-
uing our efforts in this line, we choose dpa and acidic organic
2.2. X-ray crystallography
Data collections were made b0y using graphite monochromated
Mo K
a diffraction (k = 0.71073 ÅA) at 293 K. The structures were
solved by direct methods using the SHELXS97 (Sheldrick, 1990)
program and refined by a full-matrix least squares technique based
on F2 using the SHELXL97 (Sheldrick, 1997) program. Primary
atoms were refined by structure-invariant direct methods; second-
ary atoms were located from Difference Fourier maps and hydro-
gen site location was inferred from neighboring sites. Hydrogen
atom positions for the three structures were generated geometri-
cally. Further details of the structural analysis are summarized in
Table 1 for compounds 1–3. The relevant hydrogen bond parame-
ters are listed in Table 2.
3. Results and discussion
3.1. Proton sponge 2,20-dipyridylamine
According to the Cambridge Structural Data Base (CSD), differ-
ent kinds of crystal structures related to 2,20-dipyridylamine
(dpa) are shown in Scheme 2. Two pyridyl nitrogen atoms in free
dpa are on both sides of the central NH group and adopt a trans
conformation, which may be caused by severe repulsion between
two nitrogen lone electron pairs [49–51]. In the monoprotonated
Scheme 1. Structures of the proton-transfer ammonium salts.

126
X.-H. Ding et al. / Journal of Molecular Structure 1051 (2013) 124–131
Table 1
Crystallographic data for the complex of 1–3.
Parameter
1
2
3
Formula
C
17H13N5O6
C17H15N5O7
401.34
Triclinic
Pꢃ1
C16H12N6O7
400.32
Triclinic
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
383.32
Triclinic
Pꢃ1
Pꢃ1
7.8583(7)
8.4537(8)
13.0553(12)
96.678 (1)
94.674 (1)
101.906 (1)
837.89(13)
1.519
7.322(8)
11.223(12)
12.265(13)
92.998(14)
107.030(13)
104.631(14)
923.7(17)
1.443
7.892(11)
9.252(13)
12.336(17)
105.045(16)
101.672(16)
96.214(16)
839(2)
1.584
0.13
a
(°)
b (°)
c
(°)
V (Å3)
Dx (Mg mꢃ3
l
)
(mmꢃ1
)
0.12
0.12
Z
2
2
2
T (K)
296
296
296
F (000)
396
416
412
h range for data collection (°)
Index ranges
Measured reflections
Independent reflections
Data/restraints/parameters
Rint
2.5–28.3
1.8–28.8
2.3–28.7
ꢃ10 6 h 6 10, ꢃ8 6 k 6 11, ꢃ13 6 l 6 17
ꢃ9 6 h 6 9, ꢃ14 6 k 6 9, ꢃ13 6 l 6 16
ꢃ10 6 h 6 10, ꢃ11 6 k 6 12, ꢃ16 6 l 6 16
5328
3871
5649
4208
6933
3827
3871/0/261
0.017
2290
0.041
0.100
0.88
4208/2/272
0.115
1018
0.067
0.211
0.75
3827/0/262
0.078
1744
0.147
0.244
1.28
Reflections with I > 2
R1 [I > 2 (I)]
r(I)
r
wR2 (all data)
S
Table 2
Hydrogen Bond Distances and Parameters for the complex of 1–3 (Å, °).
Table 3
Bond lengths (Å) and angles (°) in the proton sponges in compounds 1–3.
D–Hꢁ ꢁ ꢁA
Compound 1
D–H
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA
Proton sponge
Nꢁ ꢁ ꢁN
2.6124(19)
2.591(7)
2.594(9)
N–H
Hꢁ ꢁ ꢁN
1.72(2)
1.92
(N–Hꢁ ꢁ ꢁN)
138.8(19)
134
dpaH+(1)
dpaH+(2)
dpaH+(3)
1.06(2)
0.86
0.86
N1–H1Aꢁ ꢁ ꢁO1
N2–H2ꢁ ꢁ ꢁO5i
N2–H2ꢁ ꢁ ꢁN3
0.86
1.84
2.6961(17)
3.0688(18)
2.6124(19)
3.326(2)
3.073(2)
3.278(2)
3.240(2)
3.386(2)
3.272(2)
171
1.06(2)
1.06(2)
0.93
0.93
0.93
0.93
0.93
0.908(15)
2.55(2)
1.72(2)
2.51
2.50
2.55
2.41
2.54
2.551(15)
109.4(14)
138.8(19
147
120
135
148
151
136.8(12)
1.92
134
C10–H10Aꢁ ꢁ ꢁO6i
C5–H5Aꢁ ꢁ ꢁO5i
C5–H5Aꢁ ꢁ ꢁO3ii
C9–H9Aꢁ ꢁ ꢁO6iii
C2–H2Aꢁ ꢁ ꢁO2
C7–H7Aꢁ ꢁ ꢁO1
Compound 2
N1–H1Aꢁ ꢁ ꢁO7iv
N2–H2ꢁ ꢁ ꢁO2v
N2–H2ꢁ ꢁ ꢁN3
0.86
0.86
0.86
0.86(4)
0.82(3)
0.93
1.85
2.50
1.92
1.92(4)
1.93(4)
2.51
2.712(6)
3.200(6)
2.591(7)
2.738(6)
2.732(6)
3.302(7)
176
139
134
159(6)
167(5)
143
O7–H17ꢁ ꢁ ꢁO1v
O7–H18ꢁ ꢁ ꢁO2vi
C4–H4Aꢁ ꢁ ꢁO5vii
Scheme 3. The graph set for intramolecular ring S(6) in protonated 2,20-dipyridy-
lammonium cation in salts 1–3 through the N–H+ꢁ ꢁ ꢁN hydrogen bond, which are
shown by dashed lines in green. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Compound 3
N1–H1Aꢁ ꢁ ꢁO1iii
N3–H3ꢁ ꢁ ꢁN2
0.86
0.86
0.93
0.93
0.93
2.07
1.92
2.59
2.46
2.43
2.816(7)
2.594(9)
3.212(8)
3.217(10)
3.151(9)
145
134
125
139
135
C2–H2Aꢁ ꢁ ꢁO3vi
C7–H7Aꢁ ꢁ ꢁO4iii
C10–H10Aꢁ ꢁ ꢁO2i
diprotonated salt dpaH22þ, two positioned nitrogen atoms on both
sides of the central NH group are generally in a cis position. How-
ever, trans conformations similar to that in free dpa are found in
Symmetry codes: (i) x, y + 1, z + 1; (ii) ꢃx + 1, ꢃy + 1, ꢃz + 2; (iii) ꢃx, ꢃy + 1, ꢃz + 1;
(iv) x, y ꢃ 1, z; (v) ꢃx + 1, ꢃy + 1, ꢃz + 1; (vi) x ꢃ 1, y, z; (vii) x + 1, y, z + 1.
2ꢃ
form dpaH+, formation of a strong intramolecular hydrogen bond
(IHB) is beneficial to the arrangement, where two pyridyl nitrogen
atoms are arranged on the same side [22,52]. When it comes to
the isomorphous salts of ½MCl4ꢂ (Co and Cu) [53,54]. Relief from
steric strain upon diprotonation and intermolecular hydrogen
bond may play an key role. In contrast to two twisted rings in free
Scheme 2. Structures in relation to dpa. (a) The free base of dpa; (b) monoprotonated ammonium salt dpaH+; (c) diprotonated ammonium salt dpaH22þ
.

X.-H. Ding et al. / Journal of Molecular Structure 1051 (2013) 124–131
127
Fig. 1. View of the asymmetric unit for 1 with the labelled atoms.
Fig. 3. View of the asymmetric unit for 2, showing the crystallographic numbering
scheme.
dpa and diprotonated dpaH22þ, the two pyridyl rings tend to lie par-
allel and quasi-coplanar in monoprotonated dpaH+.
Reactions between 2,20-dipyridylamine and acidic synthons
(2,4-dinitrobenzoic acid, 3,4-dinitrobenzoic acid and picronitric
acid) have produced salts 1–3. The presence of an electron-donat-
ing secondary amino NH group in the centre raises the charge on
the pyridyl nitrogen atom and hence boosts its ability to remove
a proton from the acid. The pyridyl-N is sufficiently basic to be pro-
tonated, resulting in monoprotonated salts in every case. It is
worthwhile to notice that the 2,20-dipyridylammonium salt has
an asymmetrical and nonlinear intramolecular N–H+ꢁ ꢁ ꢁN hydrogen
bond, known as ‘‘proton sponges’’. Some of the principal geometri-
cal parameters of monoprotonated optimized structures 1–3 are
given in Table 3. The N–H+ distance is much shorter than the
H+ꢁ ꢁ ꢁN distance, which suggests that the proton is located asym-
metrically between pyridyl nitrogen atoms. Compared with that
found in DMANH+, the N–H+ꢁ ꢁ ꢁN hydrogen bonding angles are
smaller in the range of 130–140°. Nonbonded distances r(Nꢁ ꢁ ꢁN)
vary from 2.59 to 2.62 Å, which are slightly higher than the average
value of 2.58 Å in DMANH+ structures [8,10]. Intramolecular N–
H+ꢁ ꢁ ꢁN hydrogen bonding interactions afford similar rings S(6) in
the proton sponge dpaH+ of supramolecular ammonium salts 1–3
(Scheme 3), the nomenclature of which is according to Etter
Fig. 2. (a) View of the 3D layer crystal structure in 1 parallel to the ac-plane, in which hydrogen bonds are shown by dashed lines in green. Various ring motifs are labelled
respectively. (b) The 3D network by effective packing parallel to the ab-plane, where two types of grids are colored as pale pink and light green. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)

128
X.-H. Ding et al. / Journal of Molecular Structure 1051 (2013) 124–131
Fig. 4. (a) View of the 3D crystal structure in 2 parallel to the bc-plane, in which hydrogen bonds are shown by dashed lines in green. The R44ð12Þ rings are labelled between
carboxyl groups and water molecules through N1–H1Aꢁ ꢁ ꢁO7 and N2–H2ꢁ ꢁ ꢁO2 hydrogen bonding interactions. Besides, grids between two components form and are shown in
the pink background. (b) The 3D packing diagram of the salt 2, parallel to the ac-plane. The cations are involved with the R44ð12Þ ring between carboxyl groups and water
molecules via hydrogen bonding, colored as pale pink. (c) The 3D supramolecular framework extending on the ab-plane. The unique structure stacking by cations is in the pale
gray pillar. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
et al. [32,55,56]. The following analysis on the crystal structure
does not include the intramolecular hydrogen bond since it is not
correlated with that of intermolecular interactions.
rhombus salt 1 and crystallizing in the triclinic space group Pꢃ1.
The asymmetric unit consists of one cation of 2,20-dipyridylammo-
nium and one anion of 2,4-dinitrobenzoate with the labelled
atoms, as shown in Fig. 1.
3.2. Structural descriptions
Extensive hydrogen bonding interactions have drawn our keen
interest between anionic and cationic moieties. The secondary
amino group (NH) forms the N–Hꢁ ꢁ ꢁOꢃ hydrogen bond with depro-
tonated carboxylate group (COOꢃ) and the Hꢁ ꢁ ꢁOꢃ distance is
1.84 Å. Another N–H+ꢁ ꢁ ꢁO hydrogen bond is produced between
the positively charged pyridyl-NH+ and nitro group (NO2) as a
acceptor in the para position with H+ꢁ ꢁ ꢁO distance of 2.55 Å. Com-
pared with the above ionic N–Hꢁ ꢁ ꢁOꢃ hydrogen bond, this hydro-
gen bond is more weaker. Besides the ionic N–Hꢁ ꢁ ꢁO hydrogen
3.2.1. X-ray structure of (2,4-dinitrobenzoate)ꢁ ꢁ ꢁ(2,20-
dipyridylammonium) (1)
The carboxylic acid features the important hydrogen bonding
functional group COOH. It is interesting to exploit the robust and
directional recognition of carboxylic acids with N-containing moi-
eties. 2,4-Dinitrobenzoic acid as a acidic synthon reacts with 2,20-
dipyridylamine in 1:1 ratio in acetonitrile, affording the colorless

X.-H. Ding et al. / Journal of Molecular Structure 1051 (2013) 124–131
129
3.2.2. X-ray structure of (3,4-dinitrobenzoate)ꢁ ꢁ ꢁ(2,20-
dipyridylammonium)ꢁ ꢁ ꢁ(H2O) (2)
The similar synthesis procedure described for 1 is suitable for
salt 2 as well, except with the use of 3,4-dinitrobenzoic acid in-
stead of 2,4-dinitrobenzoic acid. The colorless rod crystal 2 has
captured a water molecule from the solvent or ambient atmo-
sphere, which crystallizes in the triclinic space group Pꢃ1. The
asymmetric unit is occupied by one anionic 3,4-dinitrobenzoate,
one cationic 2,20-dipyridylammonium and one water molecule,
where the proton of the carboxylic acid has transferred to the N
atom of pyridyl group (Fig. 3).
Compared to the ring motifs in salt 1, the ring motifs in 2 are not
so abundant and multiple. Even though only one type of the R44ð12Þ
ring exists as shown in Fig. 4a, the molecular architecture is very
fascinating. Generally, carboxylic acids aggregate in the solid state
as dimer, catemer and bridged motifs [57–59]. Herein, two car-
boxyl groups and two inserted water molecules are both bifurcate
and construct the R44ð12Þ rings via O–Hꢁ ꢁ ꢁO hydrogen bonds, which
extremely differ from those rings offered by both acidic and basic
components together in 1. Moreover, N–Hꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁO hydro-
gen bonds are associated to form the crystal packing, which pos-
sesses two types of similar grids, colored as pale pink and light
green, respectively. Running along the a axis, the 2,20-dipyridylam-
monium cation layer is slightly disordered and alternates with the
3,4-dinitrobenzoate anion layer.
Fig. 5. View of the asymmetric unit for 3, showing the crystallographic numbering
scheme.
bonds, weak C–Hꢁ ꢁ ꢁO hydrogen bonds are common in the struc-
ture, values of which are all above 2.40 Å (Table 3).
The directional hydrogen bonding determines the position and
orientation of synthons and plays a crucial role on the formation
of 3D structure. Viewed along the b-axis (Fig. 2a), every one
2,4-dinitrobenzoate anion layer is followed by one 2,20-dipyridy-
lammonium cation layer. The adjacent ions in the same layer are
antiparallel to each other, no matter in the cation layer or in the
anion one. The robust hydrogen bonding interactions generate
various ring motifs in complex 1, for instance, R12ð6Þ, R22ð8Þ, R21ð5Þ
and R32ð7Þ between the neighboring two molecules while R24ð10Þ,
R24ð18Þ and R44ð26Þ among the adjoining four ones. Stacked along
the c axis as shown in Fig. 2b, two kinds of grids form in the close
packing via hydrogen bonding, colored as pale pink and light green,
respectively. The grids in the light green shadow are much ‘‘slim’’
than those in the pale pink ones. Such interesting grids alternate
to be like ladders, producing a 3D supramolecular network.
Extending on the ac plane, 2,20-dipyridylammonium cations
seem to be comparatively ordered arrangement (Fig. 4b). As illus-
trated in the pale pink background, the cations insert themselves
into the R44ð12Þ ring between carboxyl groups and water molecules
through N1–H1Aꢁ ꢁ ꢁO7 and N2–H2ꢁ ꢁ ꢁO2 hydrogen bonding interac-
tions. When viewed along the c axis, the orderly assembly of
ammonium cations results in a striking architecture just like a pil-
lar, colored as pale gray (Fig. 4c). These unique pillars supported
the layers packing by 3,4-dinitrobenzoate anions.
Fig. 6. (a) View of the 3D supramolecular networks in 3, in which hydrogen bonds are shown by dashed lines in green. (b) The 3D wavelike structure in 3. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web version of this article.)

130
X.-H. Ding et al. / Journal of Molecular Structure 1051 (2013) 124–131
3.2.3. X-ray structure of (picrate)ꢁ ꢁ ꢁ(2,20-dipyridylammonium) (3)
As an extension of our study of hydrogen bonding interactions
concerning organic acids, we chose the picric acid (PAc) with the
strong withdrawing groups. PAc is a good candidate of polytopic
hydrogen acceptor. The pKa(O–H) of 0.36 indicates its strong acid-
ity and ensures proton transfer to amines to form a rather strong
hydrogen bond between the protonated cation and the picrate an-
ion. Meanwhile, the nitro groups are left free to interact with other
potential hydrogen donors in the structure, even though they are
not ideal H-bond acceptors due to their pKa of nearly ꢃ12
[25,41,60].
In conclusion, from this we can gather clues about the influence
of different functional groups as well as the steric effect of elec-
tron-withdrawing groups on the overall packing, which offer the
favorable and reliable information for the design of
supramolecules.
Acknowledgments
The authors gratefully acknowledge financial support from the
Natural Science Foundations of China (51173082), the National Ba-
sic Research Program of China (2009CB930601), Program for Post-
graduates Research Innovation in University of Jiangsu Province
(CXZZ12-0456) and the Starting Fund from Nanjing University of
Posts and Telecommunications (NY211027 and NY211135). We
also thank our co-workers for their distinct contributions and help-
ful discussions.
Well-shaped salt 3 is obtained from 2,20-dipyridylamine and
picric acid in methanol, which crystallizes as triclinic yellow crys-
tals in the centrosymmetric space group Pꢃ1. As shown in Fig. 5,
one crystallographically independent 2,20-dipyridylammonium
cation and one picrate anion compose the asymmetric unit, with
no solvent molecules included. In comparison with those in the
compounds 1–2, hydrogen bonds in 3 are not so rich in variety.
Parallel to the ac plane, the picrate anion layer by random stacking
treads on the heels of the 2,20-dipyridylammonium cation layer,
leading to the formation of the 3D supramolecular framework
through N–Hꢁ ꢁ ꢁOꢃ and C–Hꢁ ꢁ ꢁO hydrogen bonds (Fig. 6a). The
strength of these hydrogen bonding interactions is relatively weak-
er, with Hꢁ ꢁ ꢁO distance of over 2.00 Å. The adjacent cations in the
same layer are antiparallel to each other.
Especially attractive is the wavelike architecture viewed along
the a-axis, where everyone transverse anion layer is surrounded
by two endwise cation layers (Fig. 6b). The neighboring anions in
the same layer are antiparallel to each other while all the cations
are homo-orientated in the same column. In addition, the direc-
tions of the molecular arrangement are opposite between the
two close columns.
Appendix A. Supplementary material
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic data center, CCDC
Nos. 92890 for 1, 92892 for 2, and 92891 for 3. Copies of this infor-
mation may be obtained free of charge from the +44 1223 336 033
or Email: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk.
Supplementary data associated with this article (details of crystal-
lographic data and hydrogen bond information for the complex of
1-3) can be found, in the online version, at http://dx.doi.org/
10.1016/j.molstruc.2013.07.056.
References
[1] R.W. Alder, P.S. Bowman, R.W.S. Steele, D.R. Winterman, J. Chem. Soc., Chem.
Commun. (1968) 723.
[2] H.A. Staab, A. Kirsch, T. Barth, C. Krieger, F.A. Neugebauer, Eur. J. Org. Chem.
(2000) 1617.
[3] A.F. Pozharskii, O.V. Ryabtsova, V.A. Ozeryanskii, A.V. Degtyarev, O.N. Kazheva,
G.G. Alexandrov, O.A. Dyachenko, J. Org. Chem. 68 (2003) 10109.
[4] S.A. Reiter, S.D. Nogai, K. Karaghiosoff, H. Schmidbaur, J. Am. Chem. Soc. 126
(2004) 15833.
4. Conclusions
The 2,20-dipyridylamine (dpa) has been employed as a proton
sponge due to the presence of two closely positioned basic sites
that can accept a proton between the two nitrogen atoms. Basic
pyridyl nitrogen atoms make it a promising synthon in the supra-
molecular synthesis of multicomponent molecular complexes.
Reactions of dpa and acidic synthons with suitable different func-
tional groups afford three 3D supramolecular frameworks by ro-
bust and predictable hydrogen-bonding interactions. The
strength and directionality of the strong ionic N–Hꢁ ꢁ ꢁOꢃ hydrogen
bonds between 2,20-dipyridylamine and acidic components, de-
scribed herein, are sufficient to give rise to proton transfer and for-
mation of organic acid–base salts. It is worthy of notice that the N–
H+ꢁ ꢁ ꢁN hydrogen bonding offers the intramolecular ring S(6) in the
proton sponge dpaH+ of supramolecular ammonium salts 1–3.
Under similar reaction conditions, all crystallize in the triclinic
space group Pꢃ1. However, large discrepancy has been observed
among them. The robust hydrogen bonding interactions generate
various ring motifs in the salt 1, e.g. R12ð5Þ, R21ð6Þ, R23ð7Þ, R22ð8Þ,
R22ð11Þ, R42ð10Þ, R42ð18Þ and R44ð26Þ. Only one type of the R44ð12Þ ring
exists in ammonium carboxylate 2. The ammonium picrate 3 does
not form any of recognized types of ring motifs between cations
and anions. From the three cases, we can draw the inference that
variations in molecular shape on the acidic components lead to dif-
ferent architectures. Functional carboxyl group with two oxygen
atoms can act as a bidentate potential hydrogen-bonding donor
so that it has an advantage over the deprotonated picrate in con-
structing hydrogen bonding rings, which are well agreement with
these results. As for the difference of ring motifs between 1 and 2,
the position of nitro group in the benzene ring may play an impor-
tant role on it.
[5] V. Raab, E. Gauchenova, A. Merkoulov, K. Harms, J. Sundermeyer, B. Kovacevic,
Z.B. Maksic, J. Am. Chem. Soc. 127 (2005) 15738.
[6] A. Singh, S. Chakraborty, B. Ganguly, Eur. J. Org. Chem. (2006) 4938.
[7] A. Singh, B. Ganguly, J. Phys. Chem. A 111 (2007) 6468.
[8] J. Galeta, M. Potacek, J. Org. Chem. 77 (2012) 1010.
[9] A.L. Llamas-Saiz, C. Foces-Foces, J. Elguero, J. Mol. Struct. 328 (1994) 297.
[10] A.F. Pozharskii, Russ. Chem. Rev. 67 (1998) 1.
[11] A.F. Pozharskii, V.A. Ozeryanskii, PATAI’s Chemistry of Functional Groups,
2009, p. 1.
[12] J.C. Chambron, M. Meyer, Chem. Soc. Rev. 38 (2009) 1663.
[13] Z.B. Maksic, B. Kovacevic, R. Vianello, Chem. Rev. 112 (2012) 5240.
[14] A.E. Wille, K. Su, P.J. Carroll, L.G. Sneddon, J. Am. Chem. Soc. 118 (1996) 6407.
[15] A. Corma, S. Iborra, I. Rodriguez, F.J. Sanchez, Catalysis 211 (2002) 208.
[16] S. Ozkar, R.G. Finke, Langmuir 19 (2003) 6247.
[17] S.O. Shan, S. Loh, D. Hershchlag, Science 272 (1996) 97.
[18] M. Haro, P. Cea, I. Gascon, F.M. Royo, M.C. Lopez, J. Phys. Chem. B 111 (2007)
2845.
[19] Y. Horbatenko, S.F. Vyboishchikov, ChemPhysChem 12 (2011) 1118.
[20] T.H. Kim, J.E. Ihm, J.W. Nah, C.S. Cho, J. Controlled Release 93 (2003) 398.
[21] V. Raab, J. Kipke, R.M. Gschwind, J. Sundermeyer, Cryst. Growth Des. 8 (2002)
1682.
[22] P.C. Junk, W.M. Lu, L.I. Semenova, B.W. Skelton, A.H. White, Z. Anorg, Allg.
Chem. 632 (2006) 1303.
[23] G.R. Desiraju, Nature 412 (2001) 397.
[24] J.A. Bis, O.L. Mclaughlin, P. Vishweshwar, M.J. Zaworotko, Cryst. Growth Des. 6
(2006) 2648.
[25] V. Bertolasi, P. Gilli, G. Gilli, Cryst. Growth Des. 11 (2011) 2724.
[26] T. Yuge, T. Sakai, N. Kai, I. Hisaki, M. Miyata, N. Tohnai, Chem. Eur. J. 14 (2008)
2984.
[27] S. Kohmoto, Y. Kuroda, Y. Someya, K. Kishikawa, H. Masu, K. Yamaguchi, I.
Azumaya, Cryst. Growth Des. 9 (2009) 3457.
[28] C. Mchugh, A. Erxleben, Cryst. Growth Des. 11 (2011) 5096.
[29] A. Lemmerer, CrystEngComm 13 (2011) 2849.
[30] Y. Kobayashi, T. Kinuta, T. Sato, T. Harada, R. Kuroda, Y. Matsubara, Y. Imai,
CrystEngComm 14 (2012) 1468.
[31] A. Lemmerer, J. Bernsteinz, M.A. Spackman, Chem. Commun. 48 (2012) 1883.
[32] M.C. Etter, Acc. Chem. Res. 23 (1990) 120.

X.-H. Ding et al. / Journal of Molecular Structure 1051 (2013) 124–131
131
[33] C.B. Aakeroy, K.R. Seddon, Chem. Soc. Rev. 22 (1993) 397.
[34] L.J. Prins, D.N. Reinhoudt, P. Timmerman, Angew. Chem. Int. Ed. 40 (2001)
2382.
[35] D.C. Sherrington, K.A. Taskinen, Chem. Soc. Rev. 30 (2001) 83.
[36] G.R. Desiraju, Acc. Chem. Res. 35 (2002) 565.
[48] X.H. Ding, L.F. Cui, Y.H. Li, S. Wang, W. Huang, New J. Chem. 36 (2012) 1884.
[49] J.E. Johnson, R.A. Jacobson, Acta Crystallogr. Sect. B 29 (1973) 1669.
[50] G.J. Pyrka, Acta Crystallogr. Sect. C 48 (1992) 91.
[51] H. Schodel, C. Nather, H. Bock, F. Butenschon, Acta Crystallogr. Sect. B 52
(1996) 842.
[37] T. Steiner, Angew. Chem. Int. Ed. 41 (2002) 48.
[38] D. Braga, L. Maini, M. Polito, F. Grepioni, Supramolecular Assembly Via
Hydrogen Bonds II, vol. 111, 2004, p. 1.
[39] A.D. Burrows, Supramolecular Assembly Via Hydrogen Bonds I, vol. 108, 2004,
p. 55.
[52] I. Ucar, A. Bulut, O. Buyukgungor, Acta Crystallogr. Sect. E 63 (2007) o3377.
[53] R.D. Willett, Acta Crystallogr. Sect. C 51 (1995) 1517.
[54] S.F. Haddad, B.F. Ali, R.H. Al-Far, Polyhedron 30 (2011) 1061.
[55] J. Bernstein, R.E. Davis, L. Shimoni, N.L. Chang, Angew. Chem. Int. Ed. 34 (1995)
1555.
[40] M. Meot-Ner, Chem. Rev. 105 (2005) 213.
[56] K. Kinbara, Y. Hashimoto, M. Sukegawa, H. Nohira, K. Saigo, J. Am. Chem. Soc.
118 (1996) 3441.
[57] S.V. Kolotuchin, E.E. Fenlon, S.R. Wilson, C.J. Loweth, S.C. Zimmerman, Angew.
Chem. Int. Ed. 34 (1995) 2654.
[58] S.S. Kuduva, D.C. Craig, A. Nangia, G.R. Desiraju, J. Am. Chem. Soc. 121 (1999)
1936.
[59] S.W. Jin, W.B. Zhang, L. Liu, H.F. Gao, D.Q. Wang, R.P. Chen, X.L. Xu, J. Mol.
Struct. 975 (2010) 128.
[41] V. Stilinovic, B. Kaitner, Cryst. Growth Des. 11 (2011) 4110.
[42] O. Ugono, N.P. Rath, A.M. Beatty, CrystEngComm 13 (2011) 753.
[43] S. Mohamed, D.A. Tocher, M. Vickers, P.G. Karamertzanism, S.L. Price, Cryst.
Growth Des. 9 (2009) 2881.
[44] C.B. Aakeroy, A. Rajbanshi, Z.J. Li, J. Desper, CrystEngComm 12 (2010) 4231.
[45] C.B. Aakeroy, I. Hussain, J. Desper, Cryst. Growth Des. 6 (2006) 478.
[46] D.P. Mcnamara, S.L. Childs, J. Giordano, A. Iarriccio, J. Cassidy, M.S. Shet, R.
Mannion, E. O’donnell, A. Park, Pharm. Res. 23 (2006) 1888.
[47] S.W. Jin, L. Liu, D.Q. Wang, J.Z. Guo, J. Mol. Struct. 1005 (2011) 59.
[60] K.I. Nättinen, K. Rissanen, Cryst. Growth Des. 3 (2003) 339.