Crystal engineering studies on the salts of trans-4,4′- stilbenedicarboxylic acid in the context of solid state [2 + 2] cycloaddition reaction

Article abstract of DOI:10.1039/c0ce00224k

A series of molecular salts of trans-4,4′-stilbenedicarboxylic acid (H₂SDC) with various amines (cyclohexylamine 1, ethylenediamine 2, 1,3-diaminopropane 3, 1,4-diaminobutane 4, cystamine 5, guanidine 6, 4-aminopyridine 7, piperizine 8) have been synthesized and their structures have been determined by X-ray crystallography to understand the influence of various dominant non-covalent interactions in their crystal packing to attain energy minima in the solid state architectures. The well-defined patterns of these interactions have been analysed to assign the corresponding graph set notations and the concept of 'supramolecular synthon' has been justified. The presence of olefinic double bond and its orientations in these molecular salts make it interesting to study their photoreactivity towards UV light for [2 + 2] cycloaddition reaction. In the salt of 1,3-diaminopropane, the dianion was found to align perfectly parallel, congenial for solid state [2 + 2] cycloaddition reaction, undergoing quantitative dimerization under a UV lamp. Although the dianion was found to align perfectly parallel in salt 1, the photostability of this salt justifies Schmidt's distance criterion. The slip-stacked orientation of the dianion makes the molecular salt 4 photostable. A zigzag water cluster chain stabilised by various H-bonding interactions was identified in the molecular salt 6. The photoreactivity of the salts with 3,3′-dipropylamino amine 9 and with ammonia 10 are also accounted to discuss the possible relation between the chain length of the dications.

Full text of DOI:10.1039/c0ce00224k

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PAPER
Crystal engineering studies on the salts of trans-4,40-stilbenedicarboxylic acid
in the context of solid state [2 + 2] cycloaddition reaction†‡
*
Goutam Kumar Kole, Geok Kheng Tan and Jagadese J. Vittal
Received 22nd May 2010, Accepted 9th July 2010
DOI: 10.1039/c0ce00224k
A series of molecular salts of trans-4,40-stilbenedicarboxylic acid (H2SDC) with various amines
(cyclohexylamine 1, ethylenediamine 2, 1,3-diaminopropane 3, 1,4-diaminobutane 4, cystamine 5,
guanidine 6, 4-aminopyridine 7, piperizine 8) have been synthesized and their structures have been
determined by X-ray crystallography to understand the influence of various dominant non-covalent
interactions in their crystal packing to attain energy minima in the solid state architectures. The
well-defined patterns of these interactions have been analysed to assign the corresponding graph set
notations and the concept of ‘supramolecular synthon’ has been justified. The presence of olefinic
double bond and its orientations in these molecular salts make it interesting to study their
photoreactivity towards UV light for [2 + 2] cycloaddition reaction. In the salt of 1,3-diaminopropane,
the dianion was found to align perfectly parallel, congenial for solid state [2 + 2] cycloaddition reaction,
undergoing quantitative dimerization under a UV lamp. Although the dianion was found to align
perfectly parallel in salt 1, the photostability of this salt justifies Schmidt’s distance criterion. The
slip-stacked orientation of the dianion makes the molecular salt 4 photostable. A zigzag water cluster
chain stabilised by various H-bonding interactions was identified in the molecular salt 6. The
photoreactivity of the salts with 3,30-dipropylamino amine 9 and with ammonia 10 are also accounted
to discuss the possible relation between the chain length of the dications.
supramolecular networks or in active pharmaceutical ingredients
Introduction
(APIs) is of primary importance to confirm it to be a neutral
(O–H/N) or ionic (+N–H/Oꢀ) or an in between situation (N/
H/O).4 The formation of a neutral co-crystal or an ionic one
(molecular salt) from an acid–base pair can be predicted by the
difference in pKa values [DpKa ¼ pKa(base) ꢀ pKa(acid)] of the
reactants.5 Although there are many exceptions,6 the formation
of a salt is favoured when DpKa value is sufficiently large viz.
greater than 2 or 3.
‘Crystals engineering’, an interdisciplinary field of research, deals
with the understanding of non-covalent intermolecular interac-
tions that holds molecules together in preferred orientations to
form molecular architectures in the crystalline solid.1 Under-
standing crystal engineering principles and utilizing them to
generate functional molecular solids with desired properties has
become the ultimate goal of researchers in this field. A multi-
component crystal in which the components that are solid at
ambient condition co-exist through hydrogen bonding interac-
tions is known as a co-crystal.2 When different molecules with
complementary functional groups construct hydrogen bonding
that are energetically more favourable than those between like
molecules of each component, then the formation of a co-crystal
is favoured thermodynamically.3 The position of acidic proton
between an acidic molecule and its base partner in such
On the other hand, Schmidt’s topochemical postulates for the
solid state [2 + 2] cycloaddition reaction require parallel orienta-
tion of photoreactive double bonds maintaining centre-to-centre
7
ꢀ
distance in the range 3.5–4.2 A. The parallel orientations of
photoreactive ethylene double bonds and their photodimerization
in organic molecular salts have been reported in literature. The
deprotonation of the acidic protons in the mono- or dicarboxylic
acids by strong base leads to the molecular salts where photo-
dimerization occurs between two anionic moieties.8 Whereas the
pyridyl derivatives containing C]C bonds can be protonated by
adding strong acids to form salts where photodimerization occurs
between two cationic moieties, often stacked by cation–p inter-
actions.9 The counteranions have significant roles in such [2 + 2]
cycloaddition reactions controlling the orientations of the
cationic monomers (e.g. head-to-head or head-to-tail) and hence
dictate the product formation.9c Organic molecular salts are
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore 117543. E-mail: chmjjv@nus.edu.sg; Fax: +65 6779
1691; Tel: +65 6516 2975
† Dedicated to Prof. Fumio Toda.
‡ Electronic supplementary information (ESI) available: Additional
structural diagrams, NMR spectra, TGA, and other characterizations.
CCDC reference numbers 779310–779316. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0ce00224k
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increasingly being popular for [2 + 2] cycloaddition reaction in
the solid state because of several reasons: greater stability due to
robust charge assisted hydrogen-bonded supramolecular ionic
synthons over their neutral counterparts; better control of the
stoichiometry; better solubility and easy of separation of the
dimeric product. Electrostatic interaction, being less directional
compared to other supramolecular interactions, makes it a chal-
lenge to rationalize the orientations of ionic counterparts in solid
molecular salts compared to co-crystals where remarkable
success has been achieved in synthesising functional molecular
solids utilising the directionality of the H-bonding.10
In this contribution, we have chosen trans-4,40-stilbenedi-
carboxylic acid (H2SDC) to synthesize molecular salts with
several amines for various reasons. The identification of the
‘supramolecular synthons’11 and the patterns of the charge assis-
ted hydrogen bonded motifs12 constructing the solid state archi-
tecture is very significant to characterize molecular salts of the
dicarboxylate crystallographically. The ethylenic double bonds in
the linear dicarboxylic acids render itself as functional group for
the synthesis of cyclobutane derivatives via photochemical [2 + 2]
cycloaddition reaction in the solid state. In this respect, we have
recently reported the synthesis of 1,2,3,4-tetrakis-(40-carboxy-
phenyl)-cyclobutane, the dimer of H2SDC, via the salt formation
with 1,3-diaminopropane.13 Herein, we shall present the full
details of the synthetic strategy identifying the ‘supramolecular
synthons’ present in the series of molecular salts.
the amines used were purchased from common commercial
sources and were used without further purification. The amines
used in this study are all moderately strong to snatch acidic
protons from H2SDC to form molecular salts in aqueous
medium15 (pKa values are listed in ESI‡). Single crystals of all the
molecular salts were grown from water. Solid H2SDC was dis-
solved in water by adding equivalent amount of amines with
sonication. The resultant clear solutions were allowed to evap-
orate slowly to obtain suitable single crystals of 1–4 and 7–8. For
the molecular salts 5 and 6, triethylammonium salts of H2SDC
were mixed with corresponding hydrochloride salts of the amines
in aqueous media.
Crystal structure of cyclohexylammonium–stilbenedicarboxylate
(SDC) salt
The structure of the salt of composition C42H57N3O6 (1) was
ꢁ
refined in the triclinic space group P1.x The asymmetric unit
contains one and a half anions C16H10O4 and three cations
C6H14N where one of the cations is disordered. The crystal
structure is stabilized by the charge assisted H-bonding inter-
+
ꢀ
actions between NH3 and CO2 units. Each carboxylate unit
+
interacts with three different NH3 units where one carboxylate
oxygen interacts with two NH3+ proton donors (Fig. 1a). On the
+
other hand, each NH3 unit of the cation donates H-bond to
three carboxylate groups of three different SDC2ꢀ anions
(Fig. 1b). The hydrogen bonding assembly propagates in all the
Result and discussion
directions which result
a complicated three dimensional
network. Among the H-bonding patterns that exist in the
4
packing, the rings are identified with graph set notations R4 (12)
2
and R4 (8) and are described in Fig. 1c. Another interesting
Materials and synthesis
Trans-4,40-stilbenedicarboxylic acid (H2SDC) was synthesized
via dehydrodimerization reaction as reported in literature.14 All
feature of this salt is that the SDC2ꢀ anions are in offset parallel
x Crystal data for 1 at 223 K: C42H57N3O6, M ¼ 699.91, triclinic space
Z ¼ 2, Dcalcd ¼ 1.379 g cmꢀ3, m ¼ 0.292 mmꢀ1, GOF ¼ 1.035, final R1 ¼
0.0320, wR2 ¼ 0.0866 [for 4425 data I > 2s(I)].
ꢁ
ꢀ
ꢀ
ꢀ
group, P1; a ¼ 12.1042(10) A, b ¼ 12.6256(10) A, c ¼ 15.2007(11) A, a ¼
ꢁ
ꢁ
ꢁ
3
ꢀ
110.722(2) , b ¼ 111.880(2) , g ¼ 95.796(2) ; V ¼ 1942.3(3) A , Z ¼ 2,
Dcalcd ¼ 1.197 g cmꢀ3, m ¼ 0.080 mmꢀ1, GOF ¼ 1.034, final R1 ¼ 0.0793,
wR2 ¼ 0.1976 [for 5245 data I > 2s(I)].
Analysis found: C 57.25, H 5.45, N 6.55. C20H24N2O4S2 requires: C
57.12, H 5.75, N 6.66%.
Crystal data for 6 at 223 K: C9H15N3O4, M ¼ 229.24, monoclinic space
ꢀ
ꢀ
ꢀ
Analysis found: C 72.05, H 7.92, N 5.94. C42H57N3O6 requires: C 72.07,
H 8.21, N 6.00%.
group, P21/c; a ¼ 11.6939(8) A, b ¼ 10.6964(7) A, c ¼ 8.8496(6) A,
ꢁ
ꢀ
b ¼ 93.170(2) ; V ¼ 1105.24(13) A , Z ¼ 4, Dcalcd ¼ 1.378 g cmꢀ3, m ¼
0.109 mmꢀ1, GOF ¼ 1.056, final R1 ¼ 0.0482, wR2 0.1260 [for 2279 data
I > 2s(I)].
3
Crystal data for 2 at 100 K: C18H20N2O4, M ¼ 328.36, monoclinic space
ꢀ
ꢀ
ꢀ
group, P21/c; a ¼ 10.156(2) A, b ¼ 9.334(2) A, c ¼ 8.385(2) A, b ¼
ꢁ
3
94.400(5) ; V ¼ 792.5(3) A , Z ¼ 2, Dcalcd ¼ 1.376 g cmꢀ3, m ¼ 0.098
mmꢀ1, GOF ¼ 1.100, final R1 ¼ 0.0795, wR2 ¼ 0.1516 [for 1265 data
I > 2s(I)].
Analysis found: C 47.22, H 6.41, N 18.05. C9H15N3O4 requires: C 47.16,
H 6.60, N 18.33%.
ꢀ
Crystal data for 7 at 223 K: C26H28N4O6, M ¼ 492.52, monoclinic space
ꢀ
ꢀ
ꢀ
Analysis found: C 65.52, H 6.22, N 8.66. C18H20N2O4 requires: C 65.84,
H 6.14, N 8.53%.
group, P21/n; a ¼ 8.5009(6) A, b ¼ 10.6106(7) A, c ¼ 13.7800(9) A, b ¼
ꢁ
ꢀ
101.6510(10) ; V ¼ 1217.34(14) A , Z ¼ 2, Dcalcd ¼ 1.344 g cmꢀ3, m ¼
0.097 mmꢀ1, GOF ¼ 1.042, final R1 ¼ 0.0460, wR2 ¼ 0.1292 [for 2386
data I > 2s(I)].
3
Crystal data for 3 at 223 K: C19H24N2O5, M ¼ 360.40, monoclinic space
ꢀ
ꢀ
ꢀ
group P21/c; a ¼ 16.0565(8) A, b ¼ 11.7861(6) A, c ¼ 9.6935(5) A, b ¼
104.5700(10) ; V ¼ 1775.44(16) A , Z ¼ 4, Dcalcd ¼ 1.348 g cmꢀ3, m ¼
1.362 mmꢀ1, GOF ¼ 1.044, Final R1 ¼ 0.0495, wR2 ¼ 0.1343 [for 4329
data I > 2s(I)].
Analysis found: C 62.96, H 5.60, N 11.25. C26H28N4O6 requires: C 63.40,
H 5.73, N 11.38%.
ꢁ
3
ꢀ
Crystal data for 8 at 100 K: C20H22N2O4, M ¼ 354.40, monoclinic space
ꢀ
ꢀ
ꢀ
Analysis found: C 63.24, H 6.60, N 7.81. C19H24N2O5 requires: C 63.32,
H 6.71, N 7.77%.
group, P21/c; a ¼ 12.5672(16) A, b ¼ 8.3029(10) A, c ¼ 8.2837(11) A,
ꢁ
ꢀ
3
b ¼ 94.708(3) ; V ¼ 861.44(19) A , Z ¼ 2, Dcalcd ¼ 1.366 g cmꢀ3, m ¼ 0.096
mmꢀ1, GOF ¼ 1.130, final R1 ¼ 0.0491, wR2 ¼ 0.1222 [for 1802 data
I > 2s(I)].
Crystal data for 4 at 223 K: C20H28N2O6, M ¼ 392.44, triclinic
ꢁ
ꢁ
ꢀ
ꢀ
ꢀ
space group, P1; a ¼ 7.4542(13) A, b ¼ 9.7070(16) A, c ¼ 13.610(2) A,
ꢁ
ꢁ
3
ꢀ
a ¼ 100.688(2) , b ¼ 96.117(3) , g ¼ 90.070(3) ; V ¼ 962.0(3) A , Z ¼ 2,
Dcalcd ¼ 1.355, m ¼ 0.100 mmꢀ1, GOF ¼ 1.097, final R1 ¼ 0.0629, wR2 ¼
0.1722 [for 3637 data I > 2s(I)].
Analysis found: C 67.88, H 6.12, N 7.94. C20H22N2O4 requires: C 67.78,
H 6.26, N 7.90%.
The composition for the salt 9 was determined from elemental analysis
and was supported by TGA (in ESI‡). Analysis found: C 59.33, H 7.11,
N 6.83. C60H86N6O20 requires: C 59.49, H 7.16, N 6.94.
The composition of the ammonium salt (10) was determined as
C16H15NO4 from elemental analysis data. Analysis found: C 67.04,
H 5.17, N 4.67. C16H15NO4 requires: C 67.36, H 5.30, N 4.91.
Analysis found: C 61.27, H 7.40, N 7.24. C20H28N2O6 requires: C 61.21,
H 7.19, N 7.14%.
Crystal data for 5 at 100 K: C20H24N2O4S2, M ¼ 420.53, triclinic space
ꢁ
ꢀ
ꢀ
ꢀ
group, P1; a ¼ 8.9392(4) A, b ¼ 10.5683(5) A, c ¼ 12.3610(6) A,
ꢁ
ꢁ
ꢁ
3
ꢀ
a ¼ 71.1670(10) , b ¼ 75.9040(10) , g ¼ 67.6480(10) V ¼ 1012.43(8) A ,
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Fig. 2 Supramolecular synthons that play prominent roles in the
construction of solid state architecture, as observed in 2.
layers. A closer look to the H-bonding pattern reveals that there
4
is H-bonded ring with graph set R4 (12) formed by two NH3+ and
two carboxylate units (See ESI‡).
Fig. 1 Various hydrogen bonding interactions in the crystal structure
of 1.
+
alignment. NH3 unit acts as clipping template towards
SDC2ꢀ anions and organises them with a separation of 4.665 A
ꢀ
between the centres of the olefinic double bonds (Fig. 1d). The
distance is beyond the Schmidt’s criteria for solid state [2 + 2]
cycloaddition reaction, the salt was also found to be
photo-stable.
Crystal structure of ethylenediammonium–SDC salt
Single-crystal X-Ray diffraction analysisx reveals that the
asymmetric unit of the salt of composition C18H20N2O4 (2),
contains one anion C16H10O4 and one cation C2H10N2. The
structure was refined in monoclinic P21/c space group. The
anions and cations are held together by various charge assisted
+
NH–O hydrogen bonds. Each NH3 unit of the diammonium
cation interacts with three carboxylate groups of three different
SDC2ꢀ anions (Fig. 2a). Both the carboxylate units of each
+
SDC2ꢀ anion interact with three NH3 units while two oxygen
atoms in a carboxylate unit interact dissimilarly (Fig. 2b). One
carboxylate oxygen accepts one H-bond from NH3+ unit whereas
+
other one accepts two H-bonds from two NH3 units. Both the
+
NH3 end of one ethylene-diammonium cation act as H-bond
donors toward three carboxylates units that result in an unde-
fined three dimensional hydrogen bonded network. A view on
the ac plane shows the organised arrangement of cations and
anions where the dications are interspersed between the anionic
Fig. 3 Various hydrogen bonding interactions present in the crystal
structure of 3 are highlighted.
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Crystal structure of 1,3-diammonium propane (DAP)–SDC salt
A preliminary version of the structure of the hydrate salt has
been described before.13x The asymmetric unit of the salt
contains one anion C16H10O4 and one cation C3H12N2 and one
ꢀ
water molecule. Each CO2 unit of SDC2ꢀ anion is involved in
+
charge assisted H-bonding interactions with NH3 units of
different DAP cations and H-bonding interaction with water
molecules present in the crystal (Fig. 3a). In terms of supramo-
lecular interactions exhibited, two carboxylate ends of each
+
SDC2ꢀ anion, like two NH3 units of each DAP cations, are
uneven. One carboxylate unit involves in charge assisted
+
H-bonding interaction with three different NH3 units of three
different DAP cations. The other carboxylate unit interacts with
+
two water molecules and two NH3 units of two different DAP
cations. Similarly, one end of DAP interacts with three carbox-
ylate units of three SDC2ꢀ where as the other end interacts with
two carboxylate units of two SDC2ꢀ unit one water molecules
(Fig. 3b). Each water molecule plays the role of H-bond donor to
two carboxylates end as well as acceptor to one NH3+ unit. The
hydrogen bonded assembly of cation, anion and water molecules
propagates in all the directions to result in a three dimensional
network with undefined topology. Thermogravimetric analysis
(TGA) confirms the composition and supports for the strong
H-bonding interactions of lattice water with DAP and SDC
ꢁ
(5.2% wt. loss at ꢂ100 C).
A thorough examination of the H-bonding patterns in the
packing structure (See ESI‡) reveals that there is a H-bonded
2
ring with graph set R4 (8) as shown in (See ESI‡). The most
interesting property of this salt is the parallel orientation of
the SDC2ꢀ anions where the centre-to-centre distance between
two parallel aligned SDC2ꢀ anions is 3.715 A that satisfies the
ꢀ
Fig. 4 Various aspects in the crystal structure of 4 are analyzed.
Schmidt’s criteria for solid state photochemical reaction
+
(Fig. 3c). Here, NH3 units of the diamonium act as clipping
stacking interactions. The distance between two planes of parallel
aligned offset SDC2ꢀ anions is 3.592 A and between the centroids
ꢀ
of two nearest aromatic rings is 3.867 A (Fig. 4c). However, the
templates for aligning dicarboxylate units with the help of
water molecules present in the crystal. Upon irradiation under
UV light for 30 h the salt was observed to undergo quantitative
photodimerization to yield 1,2,3,4-tetrakis-(40-carboxyphenyl)-
cyclobutane salt. The mechanochemical synthesis and the char-
acterizations of this compound from this salt has been described
where we have shown it to be a potential candidate for co-crystal
and metal organic framework synthesis.13
ꢀ
crystal is found to be photostable as the olefinic double bonds of
two parallel aligned SDC2ꢀ anions are slipped away.
Crystal structure of 2,20-dithiobis(ethanammonium)–SDC salt
Single crystal X-ray diffraction analysis showsx that the asym-
metric unit of the salt of composition C20H24N2O4S2 (5),
contains two halves of the anion C16H10O4 and one cation
C4H14S2N2. Both the carboxylate units of SDC2ꢀ anions are
Crystal structure of 1,4-diammonium butane (DAB)–SDC salt
The structure of this salt of composition C20H28N2O6 (4) was
+
ꢁ
refined in the triclinic space group P1.x The asymmetric unit
interacting with three NH3 units of three 2,20-dithiobis
contains two halves of the anion C16H10O4, one cation C4H14N2
and two water molecules. Each carboxylate oxygen atom of each
SDC2ꢀ is interacting with one water molecule and one NH3+ unit
of a DAB (Fig. 4a). Each water molecule acts as H-bond donor
to two carboxylate oxygen atoms and H-bond acceptor to one
(ethanammonium)cations. One carboxylate oxygen accepts
+
H-bonding from two NH3 units and while the other one from
+
a single NH3+ unit only (Fig. 5a). On the other hand, each NH3
ꢀ
unit of the cation is hydrogen bonded to three CO2 units
(Fig. 5b). In the crystal structure of the salt, we find two
dimensional sheets where SDC2ꢀ ions orient in such a way that
they undergo point-to-face p–p interactions with a distance of
+
+
NH3 unit. Each NH3 unit of DAB is connected to two
carboxylate units of two different SDC2ꢀ anions through charge
assisted H-bonding and one water molecule (Fig. 4b).
ꢀ
3.908 A (Fig. 6) between carbon to centroid of the phenyl ring
A detailed scrutiny to the packing (see ESI‡) of the crystal
(see ESI‡). Two dimensional sheets of such kind are stacked
structure reveals the presence of two types of H-bonded rings with
3
parallel also by point-to-face p–p interaction between phenyl
ꢀ
5
graph set R3 (8) and R7 (16) (see ESI‡). Another interesting
feature of the salt is that the SDC2ꢀ anions are involved in p–p
rings of the SDC2ꢀ anions having a distance of 3.673 A (Fig. 5c).
The H-bonded motifs present in the structure can be represented
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Fig. 6 Two dimensional sheet-like structure and point-to-face p–p
interaction in 5.
half an anion C16H10O4 and one cation CN3H6 and two water
molecules. Each carboxylate unit interacts with two guanidinium
cations and two water molecules where one carboxylate oxygen is
bonded to two N–H (guanidinium) and one O–H (water) and the
other one with one N–H (guanidinium) and one O–H (water)
(Fig. 7a). Generally, the guanidinium cation due to its inherent
3-fold axial symmetry, is able to form three pairs of strong
hydrogen bonds to various oxyanions to construct (6,3) con-
nected Rosette like networks with C3-symmetric oxoanion
building blocks.17 Again, guanidine hydrochloride was utilised
by Ito to make two different co-crystals of fumaric acid that
undergo photodimerization partially under UV light.8d
Fig. 5 The analysis of different features in the crystal structure of 5.
2
4
by graph set notation as R4 (8) and R4 (12) as described in (also
see ESI‡).
Another interesting feature of the packing is the conformation
of the dication. Dithiobis-(ethanammonium) dications present in
both the M- and P-type helical conformation and this particular
conformation is stabilised by the weak interactions between
sulfur and NH3+ units of the two types of conformers (Fig. 5d).
+
ꢀ
The distances between sulfur and nitrogen of NH3 are 3.283 A
ꢀ
and 3.487 A that are in order of the sum of the van der Waal
radii of nitrogen and sulfur. Hence, the existing weak interaction
+
between sulfur and NH3 is responsible for the helical confor-
mation as shown in Fig. 6. The existence of helical conformation
of this cation has been noted before in the literature.16
Crystal structure of guanidinium–SDC salt
Fig. 7 Various hydrogen bonding interactions around various func-
tional groups (a and b) and existence of 1D water chain in the crystal
structure of 6.
The asymmetric unit of the title salt of composition C9H15N3O4
(6) crystallized in the monoclinic space group P21/cx contains
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However, in the present study, guanidinium cation has been
used for synthesising molecular salts with SDC2ꢀ where each
guanidinium cation is bonded to two SDC2ꢀ anions and two
water molecules. The structure resembles neither with the (6, 3)
connected Rosette networks nor the SDC2ꢀ anions are found
parallel. The supramolecular interactions between neutral
fumaric acids with guanidinium cations in the co-crystals (where
Clꢀ ions balances the charge)8d are different from that of between
SDC2ꢀ anions and the same cations in 6. H-bonding centred
around each guanidinium cation can be represented by graph set
2
2
notation as R2 (8) and R3 (8) (Fig. 7b). A through perusal of the
H-bonding patterns in the packing of this salt reveals that there is
1D zigzag water chain passing along c-direction. Various
H-bonds stabilise the water chain in the crystal. Each water
molecule acts as H-bond donor to one carboxylate oxygen and
one water molecule; whereas it accepts two H-bond from another
water molecule and one N–H from guanidinium ion (Fig. 7c).
A discrete (H2O)32 cluster, encapsulated in an organic salts of
guanidinium cations, has been reported in literature.18
Fig. 9 A perspective view of packing diagram along c-direction shows
the supramolecular synthons present in 8.
bonded to four carboxylate units of four SDC2ꢀ anions in C(2)
fashion (Fig. 9).
Crystal structure of 4-aminopyridinium–SDC salt
Discussion
Single-crystal X-ray diffraction analysis revealsx that the asym-
metric unit of the title salt of composition C26H28N4O6 (7),
contains half of the SDC2ꢀ anion, one cation C5N2H7 and
a water molecule. Two-point ionic synthon PyNH+/ꢀO2C plays
the key role in the building of hydrogen-bonded network in the
solid state structure. The protonation to Py-N rather than NH2
group can be rationalized from all possible pKa values of
4-aminopyridine (see ESI‡). Both the carboxylate groups are
bonded to pyridinium cation via charge assisted two-point
(cyclic) PyNH+/ꢀO2C synthon and also bonded to two water
molecules via H-bonding. The NH2 groups of the cations are
pointed towards each other and are bridged by two water
molecules via H-bonding (Fig. 8). From the packing of the
crystal structure (see ESI‡), it can be observed that the cations
and anions involve p-stacking interaction and cations are
interspersed between two SDC2ꢀ anions. A closer look to the
H-bonding pattern reveals that there are three types of H-bonded
Having aimed in mind to synthesize 1,2,3,4-tetrakis(40-carboxy-
phenyl) cyclobutane via [2 + 2] cycloaddition reaction of H2SDC,
we have studied this series of molecular salts. The photostable
nature of H2SDC in the solid state demands crystal engineering
and the limited solubility of H2SDC in common organic solvents
prompted us to employ the salts formation route over co-crystals
strategy. In the crystal structures of all these molecular salts, we
have identified the supramolecular synthons that are exploited in
the robust three dimensional ionic architectures in the solid state.
The complex nature of these synthons are due to the less direc-
tional charge-assisted hydrogen bonding generated by the
+
ꢀ
interactions between NH3 and CO2 units of two ionic coun-
terparts. The formation of molecular salts in aqueous media is in
good agreement with the differences in the pKa values (see ESI‡)
of H2SDC and the amines used for this study. The orientation of
ionic counterparts in crystalline solids built on electrostatic
interaction is less directional and hence imposes designing chal-
lenge as opposed to hydrogen bonds in co-crystals. In the salt
with cyclohexylamine, SDC2ꢀ anions are aligned parallel with
2
motifs and can be represented by graph set notation as R2 (7),
2
R4 (8), R4 (12) (also see ESI‡).
4
ꢀ
a distance of 4.665 A, which is over the Schmidt’s distance limit
Crystal structure of piperizine–SDC salt
and thus found to be photostable under UV light. Whereas in the
case of DAP, the centre-to-centre distance of two SDC2ꢀ anions
ꢀ
aligned parallel is 3.715 A and undergoes quantitative dimer-
The asymmetric unit of this salt of composition C20H22N2O4 (8)x
contains half an anion C16H10O4 and half a cation C4H12N2. The
ionic synthon, NH2+/ꢀO2C plays principal role in the formation
of the solid state architecture. Each carboxylate unit is bonded to
ization under UV light obeying Schmidt’s topochemical postu-
lates. Although, in the case DAB the SDC2ꢀ anions are parallel,
the crystal was found to be photostable as the olefinic double
bonds of two parallel aligned SDC2ꢀ anions are slip-stacked. In
the series of linear primary diamines, only DAP was found to
result the parallel alignment of SDC2ꢀ anion that suggest the
possibility of some correlation between the chain length of the
diamine and the parallel orientation of the ionic counterparts in
the solid state. One can rationalize our findings to the n ¼ 3 rule
for the intramolecular excimer formation in bichromophoric
molecules linked by a short flexible chain.19 Also, our result can
be correlated with the enhancement of photocyclization of tri-
methylene dicinnamates.20 In 5, 2,20-dithiobis (ethanammonium)
+
four NH2 units of four cations and each piperizine cation is
Fig. 8 A view of the hydrogen bonding interactions forming a chain in
the solid state architecture in 7.
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cations are identified to adopt a rare finite helical shape (called
‘helicate’) which is stabilised by the weak interactions between
+
sulfur and NH3 units. The so called ‘ladder-like’ hydrogen-
Notes and references
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Acknowledgements
We gratefully acknowledge the Ministry of Education, Singapore
for the financial support through NUS FRC grant R-143-000-
371-112. We sincerely thank Hong Yimian for her kind help with
X-ray crystallography.
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