Substituent swap affects the crystal structure and properties of: N -benzyl-4-amino-1,2,4-triazole related organic salts

Article abstract of DOI:10.1039/c7nj03445h

An investigation into the effect of switching methoxy and hydroxyl groups on molecular salts is presented in this study. The salts HL₁⁺·NO₃^- (1), L₁·HL₁⁺·ClO₄^- (2), L₁·HL₁⁺·H₂PO₄^-·H₂O (3), HL₂⁺·NO₃^- (4), HL₂⁺·ClO₄^- (5) and HL₂⁺·H₂PO₄^- (6) were synthesized and structurally characterized. The study was carried out by analyzing the crystal structure, properties and intermolecular interactions within each of the salts using IR and fluorescence spectra, TGA, Hirshfeld surface analyses and π?π stacking motifs. Salt 1 has a layered structure, while 4 has a distorted 3-D structure. Salt 2 possesses an edge-to-face type plane interaction, while 5 is formed with a twisted structure. It is important to note that water molecules play a key role in the 1-D chain structure of salt 3. The π?π stacking motif of salts 1-3 have a herringbone structure, while salts 3-6 exhibit a γ-structure. In addition, the competitive crystallization of the synthetic salts was investigated and found to be consistent with the measured solubility.

Full text of DOI:10.1039/c7nj03445h

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Substituent swap affects the crystal structure and
properties of N-benzyl-4-amino-1,2,4-triazole
related organic salts†
Cite this: New J. Chem., 2017,
41, 13846
Jing-Wen Wang, Yao-Jia Li, Chen Chen, Yang-Hui Luo * and Bai-Wang Sun
*
An investigation into the effect of switching methoxy and hydroxyl groups on molecular salts is
+
À
+
À
+
+
À
presented in this study. The salts HL1 ÁNO3 (1), L1ÁHL1 ÁClO4 (2), L1ÁHL1 ÁH2PO4ÀÁH2O (3), HL2 ÁNO3
+
À
+
À
(4), HL2 ÁClO4 (5) and HL2 ÁH2PO4 (6) were synthesized and structurally characterized. The study was
carried out by analyzing the crystal structure, properties and intermolecular interactions within each of
the salts using IR and fluorescence spectra, TGA, Hirshfeld surface analyses and pÁ Á Áp stacking motifs.
Salt 1 has a layered structure, while 4 has a distorted 3-D structure. Salt 2 possesses an edge-to-face
type plane interaction, while 5 is formed with a twisted structure. It is important to note that water
molecules play a key role in the 1-D chain structure of salt 3. The pÁ Á Áp stacking motif of salts 1–3 have
a herringbone structure, while salts 3–6 exhibit a g-structure. In addition, the competitive crystallization
of the synthetic salts was investigated and found to be consistent with the measured solubility.
Received 11th September 2017,
Accepted 6th October 2017
DOI: 10.1039/c7nj03445h
rsc.li/njc
on the triazole group strengthens the oxoanion-binding through
charge-assisted hydrogen bonding. In addition, our previous study
1 Introduction
Anion receptors bearing p electron-deficient species have attracted has demonstrated that the binding ability of N-benzyl-4-amino-1,2,4-
increasing interest during the past decade.1 The anion–p triazole-related ligands with inorganic anions can be affected remark-
interaction, as predicted by researchers, has been shown to ably by the substituent groups at the 4-position of the N-benzyl
exist by experimental investigations involving electron-deficient moiety.10 In other words, the anion–p interactions between triazole-
aromatic rings in both solution and the solid state.2–4 For related ligands and anions can be controlled by altering the sub-
example, in the solid state, when hydrogen-bonding interaction stituents on the triazole-related ligand.
in which anions interact with p electron-deficient arenes,
Hence, two N-benzyl-4-amino-1,2,4-triazole related ligands L1 and
such as a benzene-capped tripodal amide ligand with a pyridyl L2 were synthesized and reacted with various inorganic acids (HNO3,
moiety,3a o-CF3 substituted hexaamide,3b and N4-platform-based HClO4, H3PO4) to form a series of salts, which form the focus of this
polyamine tripodal3c from the periphery of the aromatic rings, is study. A key aim of this article is to study the effect of exchanging the
predominant, typical anion–p interaction, in which an anion groups (methoxy and hydroxy) bound to the para-position of the
(halide) is encapsulated in the middle of host molecule, is N-benzyl ring, with a particular focus on how this affects the binding
observed.5 Despite an increasing number of examples of anion–p of the inorganic anions with L1 and L2, and therefore the spectral
interactions being reported in the literature, to the best of our properties and stability of the resultant salts. The properties of the
knowledge the effect of switching the substituents of the cationic series of salts will be analyzed using TGA and IR, Raman and
species on the directing ability of the anion–p interaction based on Fluorescence Spectroscopy. We qualitatively assess how changes in
charge-neutral, electron-deficient arenes has rarely been studied.
substitution influence the intra-molecular interactions of salts 1–6
Inspired by studies carried out by Custelcean et al.6,7 and Das and the position and presence of pÁÁÁp stacking motifs. In addition,
et al.,8 triazole-related ligands were selected for investigating anion–p the competitive crystallization of the two ligands in the presence
interactions within inorganic acids due to their excellent hydrogen- of nitrate, perchlorate and dihydrogen phosphate ions was also
bond acceptor and donor properties.9,10 Moreover, the positive charge explored.
School of Chemistry and Chemical Engineering, Southeast University,
2 Experimental
2.1 General materials and methods
Nanjing 211189, P. R. China. E-mail: peluoyh@sina.com, chmsunbw@seu.edu.cn;
Fax: +86-25-52090614; Tel: +86-25-52090614
† Electronic supplementary information (ESI) available. CCDC 1526868–1526874.
Nitric acid (HNO3) (purity 65–68%), phosphoric acid (H3PO4)
(purity Z 85 wt% in water) and perchloric acid (HClO4)
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7nj03445h
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(purity 70–72%) were purchased from Alfa Aesar and used methanol. The reaction mixture was stirred for 20 minutes,
without further purification. Infrared spectra (IR) was recorded before being left undisturbed at ambient temperature for four
on a SHIMADZU IR Prestige-21 FTIR-8400S spectrometer as days, whereupon an orange stick crystal of 4 was obtained via
KBr pellets in the range 4000–400 cmÀ1. Thermogravimetric the low evaporation–crystallization process. Salts 5, 6 were
analysis (TGA) was performed using a NETZSCH TG 2009 F3 obtained using the same stoichiometric ratio, but using HClO4
system, at a heating rate of 10 K minÀ1 under an atmosphere of (salt 5) and H3PO4 (salt 6) instead of HNO3, respectively. The
dry nitrogen flowing at 20 cm3 minÀ1 over the range from 50 to yields of each salt were similar to those of salts 1–3 (IR is shown
500 1C. Elemental analyses were performed on a Vario-EL III in Fig. S6, ESI†).
elemental analyzer for carbon, hydrogen and nitrogen. NMR
spectra were recorded on a Varian FT-400 MHz instrument. 2.2 Competitive crystallizations of perchlorate/nitrate and
Fluorescence spectra were obtained using a Horiba FluoroMax perchlorate/dihydrogen phosphate
4 spectrofluorometer. Raman spectra were recorded using a
ClO4À/NO3À: 0.1 mmol ligand (L1 or L2) was dissolved in 10 mL
Raman microscope (Kaiser Optical Systems, Inc., Ann Arbor, MI,
of methanol, and then this solution was added to 10 mL of an
USA) with 780 nm laser excitation; each spectrum was obtained
aqueous solution of 0.1 mmol HClO4 and 0.1 mmol HNO3. The
under one 2 min exposure of the CCD detector in the wave-
resulting mixture was then stirred at room temperature for
number range.
24 h, during which a colorless crystalline solid was formed. The
resulting crystalline solid was filtered, washed with water,
and dried under vacuum to provide the product in a yield of
55–60%; PXRD and FT-IR analyses were performed to elucidate
Synthesis of ligand C10H10N4O2 (L1). The ligands L1, L2 were
synthesized according to methods described previously11–13
(Scheme S1, ESI†). 4-Amino-4H-1,2,4-triazole (10 mmol, 0.84 g)
was dissolved in 80 mL ethanol in a round-bottomed flask, and
the composition of the crystalline solid.
3-hydroxy-4-methoxybenzaldehyde (10 mmol, 1.52 g) was added
to the solution whilst under continuous stirring. The resulting
solution was refluxed at 80 1C for 3 h. The solvent was removed
under reduced pressure and the residue recrystallized from an
ethanol solution (15 ml) by heating to 60 1C to dissolve solids and
then leaving to cool (under ambient conditions). The resulting
white, microcrystalline solid (L1) was dried under vacuum condi-
tions (2.00 g, yield 85%). Elemental analysis for L1: Anal. calcd
(%): C, 55.04; N, 25.68; H, 4.62. Found: C, 55.06; N, 25.64; H, 4.63.
1H-NMR (300 MHz, d6-DMSO) d (ppm): 9.97(s, 1H), 9.04(s, 1H),
8.97(s, 2H), 7.52(d, 1H), 7.34(d, 1H), 6.35(s, 1H), 3.83(s, 3H)
(see S1, ESI†).
Synthesis of ligand C10H10N4O2 (L2). Ligand L2 was prepared
following a similar process as described above for L1, except
that the 3-hydroxy-4-methoxybenzaldehyde is replaced with
3-methoxy-4-hydroxybenzaldehyde (10 mmol, 1.52 g) to get a
white solid product L2 (2.00 g yield 85%).
Ligand L1 was dissolved in a small amount of methanol
solvent. The resulting clear solution was evaporated at room
temperature over four days and colorless, blocky crystals of L1,
suitable for X-ray diffraction, were obtained. However, only the
L2 powder was collected for further measurements.
Synthesis of salts 1–3. The preparation of 1–3 was carried out
in solution by slow evaporation-crystallization using pure methanol
as solvent. Salt 1 was obtained by dissolving a 3 : 5 stoichiometric
ratio of L1 (45.60 mg, 0.3 mmol) and HNO3 (48.46 mg, 0.5 mmol) in
À
ClO4À/H2PO4À: competitive crystallizations of ClO4À/H2PO4
were performed by similar procedures as described above, except
that H3PO4 was used instead of HNO3. Again, PXRD measurements
were performed to investigate the composition of the crystalline
solid. Yield 58–62%.
2.3 X-ray crystallography
Crystallographic diffraction of L1 and salts 1–6 was performed
at 293 K with graphite-monochromated Mo Ka radiation
(l = 0.071073 nm) equipped with Rigaku SCXmini diffractometer,
using the o-scan technique.14 The absorption correction was
carried out using the Bruker SADABS program with a multi-scan
method. The structures were solved by full-matrix least-squares
methods on all F2 data, and the SHELXS-2014 and SHELXL-2014
programs were used for the structure solution and structure
refinement, respectively.15 All non-hydrogen atoms were refined
anisotropically and hydrogen atoms (except the hydrogen atoms
from the water molecule) were inserted at their calculated posi-
tions and fixed at their positions.16 Mercury program17 was used
to produce the molecular graphics. Further details of the crystallo-
graphic data and structural refinement results of the L1 and salts
1–6 are given in Table S1 (ESI†), and selected H-bond lengths and
angles of ligand L1 and salts 1–6 are listed in Table S2 (ESI†).
2.4 The Hirshfeld surface
20 mL methanol solution by continuously stirring for 20 minutes at The Hirshfeld surface serves as a powerful tool for elucidating
room temperature. The resulting homogeneous solution was kept molecular crystal structures,18,19 and is determined using the
undisturbed at ambient temperature over four days to obtain a CrystalExplorer26 program in this study. When the CIF files
colorless transparent block crystal. The crystals of salts 2 and 3 were were read into the CrystalExplorer program for analysis, all
obtained in a similar way to that described for crystal 1, with an bond lengths to hydrogen were automatically modified to
identical stoichiometric ratio, but using HClO4 (salt 2) and H3PO4 typical standard neutron values (C–H = 1.083 Å and N–H =
(salt 3) instead of HNO3. Salts 1, 2 and 3 were produced in 73–76%, 1.009 Å). The Hirshfeld surfaces were generated using standard
80–85%, 60–63% yields, respectively.
(high) surface resolution, and the 3-D de surfaces were mapped
Synthesis of salts 4–6. Salt 4 was prepared by dissolving L2 using a fixed color scale of 0.76 Å (red) to 2.4 Å (blue). The 2-D
(45.60 mg, 0.3 mmol) and HNO3 (48.46 mg, 0.5 mmol) in 20 ml fingerprint plots were displayed using the standard 0.6–2.6 Å
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view with the de and di distance scales displayed on the in the formation of the structure by hydrogen bonding. The 3-D
graph axes.
packing of salt 4 shows that the layers are not parallel to each
other (Fig. 2d).
+
+
À
L1ÁHL1 ÁClO4À (2) and HL2 ÁClO4 (5). As shown in Fig. 3a,
%
salt 2 crystallizes in the triclinic space group P1, the ASU
contains one L1 molecule (neutral), one protonated HL1 and
3 Results and discussion
+
3.1 Description of crystal structures
one perchlorate ion. Compared with salt 2, the structure of 5
C10H10N4O2 (L1). Ligand L1 crystallizes in the monoclinic space has a 1 : 1 (L2: perchloric acid) molecular salt in the monoclinic
group P21/c, revealing a 1-D O2–H2AÁ Á ÁN2 hydrogen bond- Pn space group with Z = 2 and the ASU consists of a protonated
+
interaction structure. As shown in Fig. 1a, the asymmetric unit molecule of HL2 and a perchlorate ion (Fig. 3b).
consists of only one L1 molecular. The O2–H2AÁ Á ÁN2 bond
For salt 2, each protonated molecule of HL1+ is connected by
length is 2.72 Å, which is comparable to what has been O1–H1AÁ Á ÁN1, N6–H6Á Á ÁN2 and O3–H3BÁ Á ÁO5 interactions to
previously reported for O–HÁÁÁN hydrogen bonding in compounds form an edge-to-face type plane structure (Fig. S4a, ESI†). The
with similar structures (Fig. S2, ESI†). From the view of Fig. 1b, the 2-D plane structures are then stacked in a bridged fashion of
L1 molecules interact with each other via hydrogen bonds, form- the perchlorate ion by C–HÁ Á Áp weak intermolecular hydrogen
ing a cross-connected structure; the cross-connected structures bonds into the layered structures (Fig. 3c). In this regard, salt 5
experience additional stacking due to pÁÁÁp intermolecular shows a visible difference from 2. Single-crystal X-ray diffrac-
+
interactions, ultimately creating an interlaced 3-D layer structure. tion analysis showed that the protonated HL2 molecules in
The plane separation between the stacked L1 molecules in the salt 5 have twisted structures (they are not ‘flat’), with the 3-D
adjacent units is 4.165 Å and 3.498 Å in the horizontal direction structure held together via O1Á Á ÁO6 lone electron pair, and
and tilt direction, respectively.
N1–H1Á Á ÁO2 and O1–H1AÁ Á ÁN1 hydrogen bonding interactions
+
À
+
À
Salts HL1 ÁNO3 (1) and HL2 ÁNO3 (4). Single-crystal X-ray (Fig. 3d). Compared with 2, both methoxy and hydroxyl groups
diffraction analysis reveals that salt 1 crystallizes in the mono- in salt 5 participate in the formation of the packing structure
clinic space group P21/c, showing a 2-D network structure. via strong O–HÁ Á ÁN interactions.
+
+
As shown in Fig. 2a, the asymmetric unit of salt 1 contains a
L1ÁHL1 ÁH2PO4ÀÁH2O (3) and HL2 ÁH2PO4À (6). Salt 3 features a
single molecule of protonated HL1+ per nitrate ion. The ASU of 3-D anionic network based on dihydrogen phosphate ions. Single-
salt 4 is similar to that of salt 1, consisting of a molecule of crystal X-ray diffraction analysis reveals that salt 3 crystallizes in
+
protonated HL2 per nitrate ion. Salt 4 crystallizes in the the orthorhombic space group Pca21. As shown in Fig. 4a, each
monoclinic space group Pn. The structure of 4 is different from asymmetric unit of salt 3 contains one dihydrogen phosphate
+
that of 1 owing to the position switch of the methoxy and ions, one neutral L1 molecule, one protonated HL1 and one
hydroxyl groups. Salt 1 is mainly stabilized by intramolecular lattice water. Compared to that of salt 3, there is no water in the
O5–H5AÁ Á ÁO2 and N5–H5Á Á ÁO3 hydrogen bonds and is stacked ASU of salt 6, and it only consists of one protonated HL2+ and one
in a parallel fashion, forming a layered 3-D structure held dihydrogen phosphate ion in the monoclinic space group Pn.
together by pÁ Á Áp intermolecular interactions with a plane
The structure of salt 3 can be described as follows: the
+
separation of 3.497 Å (Fig. 2c). The absence of interaction molecules of protonated HL1 are connected through
between the methoxy group and the N atom of another ligand N8–H8AÁ Á ÁO2 (distance of 2.61 Å) and O5–H5Á Á ÁO3 (distance
molecule leads to a completely different crystal structure of salt of 2.64 Å) hydrogen bonds, with the water molecules playing an
1 compared to that of salt 4. Salt 4 is mainly held together by important bridging role by connecting the two dihydrogen
N1–H1BÁÁÁO1 and N1–H1BÁÁÁO2 hydrogen bonding interactions: phosphate ions in a 1-D chain structure through numerous
the oxygen atoms of the methoxy and hydroxyl groups are involved O1–H1AÁÁÁO2W and O2W–HÁÁÁO2 hydrogen bonds (Fig. S5a, ESI†).
Fig. 1 (a) The asymmetric unit of ligand L1 with the atomic labeling scheme. (b) Hydrogen bonding patterns of L1; the hydrogen atoms are omitted for
clarity.
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Fig. 2 X-ray crystal structure of the salts 1 and 4. ORTEP representation showing (a) the structure of the cation and (b) the structure of the anion. (c) The
crystal packing of salt 1 viewed along the b-axis. (d) The crystal packing of salt 4, as viewed along the a-axis, and with the intermolecular hydrogen bond
interactions highlighted (pale blue dotted marking). The hydrogen atoms have been omitted for clarity.
Fig. 3 (a) The asymmetric unit of salts 2 and 5 with the atomic labeling scheme (a and b). 3-D pack structure of salts 2 (c). Crystal packing of salt 5,
viewed down the crystallographic a-axis (d). The hydrogen atoms are omitted for clarity.
Salt 3 forms an edge-to-edge type interaction, with the dihydrogen difference, as can be observed in Fig. 4c and d. This difference
phosphate ions located on the inner position of the opposite chain can be ascribed to the position swap of methoxy and hydroxyl
+
unit, while the protonated HL1 molecules are on the two sides groups between ligands L1 and L2. The hydrogen bond interactions
(Fig. 4c). The structure of salt 6, is largely constructed of direct of ligand L1 and salts 1–6 are shown in Fig. S2–S5 (ESI†).
N3–H3AÁÁÁO1 and N3–H3AÁÁÁO6 hydrogen bonds, which originate
3.2 The influence of the positional switching of the methoxy
and hydroxyl groups on the structure and spectral properties of
ligands L1, L2 and salts 1–6
in L2. In addition, 6 forms edge-to-face type interactions, which are
similar to those found in salts 4 and 5. The dihydrogen phosphate
ions are located on the inner position of the reversed chain unit,
while the protonated HL2+ molecules are on the two sides. Further- As is shown in Fig. 5, a comparison of the structures of salts
more, the 3-D structures of salts 3 and 6 show a significant 1–6 shows that the hydrogen bond connection mode of the
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Fig. 4 The asymmetric unit of (a) salt 3 and (b) salt 6, showing the atomic labeling scheme. The crystal structures of (c) salt 3 and (d) salt 6, viewed down
the crystallographic c-axis. The hydrogen atoms are omitted for clarity (a–d).
Fig. 5 Hydrogen bonding patterns of the ligand L1/L2 stacks with inorganic anions to form salts 1–6.
+
inorganic anions with ligands L1 and L2 has a profound effect The nitrate ion in salt 4 interacts with five protonated HL2
on the overall structure of each salt. The same ligand can be molecules, two above and two below the main plane, forming
connected with different anions to form different structures, an overall twisted 2-D structure. In salt 2, there are two
and the same anion can also be connected with different independent L1 molecules connected through O1–H1AÁ Á ÁN1
ligands to form different salt structures. In salt 1, each nitrate and N6–H6Á Á ÁN2 hydrogen bonds; beyond that, the perchlorate
+
ion interacts with three neighboring protonated HL1 species, anion interacts with L1 by weak hydrogen bonds. However,
forming a 2-D network plane (Fig. 2c), with each hydroxyl group perchlorate anions interact strongly with the hydroxyl group in
of L1 forming two hydrogen bonds (N5–H5Á Á ÁO3 and salt 5 via hydrogen bonding, and O3 and O5 (between the
O5–H5AÁ Á ÁO2), while the hydroxyl group in L2 forms a perchlorate groups) can also be connected by lone electron
O1–H1AÁ Á ÁO4 hydrogen bond within salt 4 (Fig. S3, ESI†). pairs. For salts 3 and 6, the main difference is that 3 contains
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lattice water, and dihydrogen phosphate ions are linked by This caused us to investigate the potential for selective oxoanion
water molecules whilst for salt 6, the dihydrogen phosphate separation using the two N-benzyl-4-amino-1,2,4-triazole related
ions are connected directly without water. Thus, the position ligands (L1 and L2). Table S3 (ESI†) lists the measured aqueous
swap of methoxy and hydroxyl groups on L1 and L2 results in solubilities of the dihydrogen phosphate, nitrate and perchlorate
distinct crystal packing structures (Fig. 5).
salts at 25 1C. The salts decrease in their solubility in the order
The solution-state emission spectra of compounds L1, L2 and dihydrogen phosphate salts 4 nitrate salts 4 perchlorate salts.
salts 1–6 at ambient temperature are shown in Fig. 6. The main Thus, perchlorate salts have the lowest aqueous solubility, indi-
emission peaks of the ligands L1 and L2 are at 415 nm (lex
321 nm) and 400 nm (lex = 320 nm), respectively, which may be tive crystallization of the salts L1ÁHL1 ÁClO4 and HL2 ÁClO4
due to p*–p or p*–n intra-molecular transitions. The same emis- To verify the abovementioned proposition, a series of com-
=
cating the possibility of aqueous perchlorate separation via selec-
+
À
+
À
.
sions occur at 420 nm for 1, 420 nm for 2, 414 nm for 3, 427 nm petitive crystallization experiments were performed involving
for 4, 433 nm for 5 and 403 nm for 6 (lex = 322 nm). The emissions the crystallization of ligands L1 and L2 in aqueous solution
of these salts may be assigned to the conjugate effects of in the presence of various anion mixtures (Table S4, ESI†).
intra-molecular transitions and differences in the conjugate The composition of the resulting crystalline products was
environments. As is shown in Fig. 6, the fluorescence wavelength confirmed by PXRD and FT-IR measurements. For ligand L1, salt
+
of L1 is longer than that of L2, which may be ascribed to the L1ÁHL1 ÁClO4À (2) was exclusively crystallized from the perchlorate/
molecular structure of L1 on the same plane. Compared to the free nitrate and perchlorate/dihydrogen phosphate mixtures in 76%
L1 and L2 ligands, the peaks are red-shifted by 5 nm, 5 nm, 27 nm and 78% yield, respectively (entries 1–2), while foÀr ligand L2,
+
À
+
and 33 nm for salts 1, 2, 4, 5, respectively. It can be seen that when crystalline mixtures of HL2 ÁNO3 (4) and HL2 ÁClO4 (5) as well
NO3À and ClO4À are associated with L2, the emission has a larger as HL2 ÁH2PO4 (6) and HL2 ÁClO4 were obtained from the
fluorescence shift, because the methoxy and hydroxyl groups in ‘competition experiment’ using perchlorate/nitrate and perchlorate/
the L2 are all linked by hydrogen bonding with the oxo-anion, dihydrogen phosphate, respectively (entries 3–4). These results
thereby increasing the conjugation capacity (Fig. 5). The emission may be attributed to the similar crystal structures and aqueous
peaks for salts 3 and 6 are essentially the same as the corres- solubilities of the salts 1, 2 and 3. Thus, the anion selectivity
ponding ligand, which implies that H2PO4À has little effect on the from these paired competitive crystallizations are generally
+
À
+
À
fluorescence emission of the ligands.
consistent with the measured solubilities.
3.3 Competitive crystallization
3.4 The Hirshfeld surface
According to the above mentioned crystallization experiments, it The effects of switching methoxy and hydroxyl groups on L1,
is apparent that both the ligands L1 and L2 can generate relatively L2 can also be visualised by the Hirshfeld surface, which is
insoluble salts with various oxoanions, providing a range of yields. a useful tool for describing the surface characteristics of
Fig. 6 Fluorescence emission spectra of ligands L1, L2 and salts 1–6 (lex = 322 nm, c = 2.5 Â 10À5).
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Fig. 7 Fingerprint plots for salts 1–6 resolved into HÁ Á ÁH interaction, the full fingerprint appears beneath each decomposed plot as a grey shadow.
molecules. All the intermolecular interactions within salts 1–6
are summarized in Fig. 8, which enables the influence of
substituent position and different inorganic anions on the
intermolecular interactions to be more fully appreciated. The
HÁ Á ÁO interaction, which forms the greatest contribution to
the total Hirshfeld surface, comprises 45.6%, 33.1%, 39.0%, 37.6%,
35.3% and 35.6% of the total Hirshfeld surfaces for salts 1–6,
respectively. Compared to other salts, salt 2 has a significantly
lower value; this can be explained by the fact that NÁ Á ÁH
interactions in salt 2 play an obvious role in the construction
of the structure (Fig. 5). Furthermore, the OÁ Á ÁO interaction,
which comprises 7.8% of the total Hirshfeld surface, also has a
unique role in the connection of perchlorate ions in salt 2.
Fig. 8 The percentage contributions from the individual intermolecular
Interestingly, the CÁ Á ÁO interaction contributes 1.9%, 2.9%,
interactions to the Hirshfeld surfaces of salts 1–6.
4.1%, 5.4% and 5.6% of the total Hirshfeld surfaces for salts 1,
2, 4, 5, 6, respectively, with the exception of salt 3, due to the
bridging effects of water molecules. The HÁ Á ÁH contacts com-
ratios in the range 1.2–2.7, and b-structures have a ratio between
prise 17.9%, 23.2%, 26.6%, 23.2%, 23.3%, and 29.3% of the total
Hirshfeld surfaces for salts 1–6, respectively. It can be seen that
the values depend strongly on the substituent position of the
methoxy and hydroxyl groups (Fig. 7).
0.46 and 1.0. The ratios of salts 1–6 are found to be 5.47, 4.59,
4.26, 1.56, 1.89 and 1.94, respectively; i.e., the pÁÁ Áp stacking motif
of salts 1–3 has a herringbone structure whilst salts 4–6 have
a g-structure. According to Desiraju and Gavezzotti,21 each motif
represents a particular p-interaction geometry: the herringbone-
motifs are rich in C–HÁÁÁp interactions and often form edge-to-
3.5 The pÁ Á Áp stacking motifs
Aromatic compounds usually crystallize in one of four possible face interactions, while the g-motifs are rich in pÁÁÁp (CÁÁÁC)
structural motifs: herringbone, sandwich-herringbone, g-, or interactions and the molecules in them often form infinite face-
b-structures (Fig. 9).20–22 The determination of structural motifs to-face or offset stacks along one axis and edge-to-face stacks
is based on the calculated ratios between C–HÁ Á Áp and pÁ Á Áp along another axis. This implies that different types of inorganic
(CÁ Á ÁC) interactions. According to the literature,22 herringbone anions have little effect on pÁÁÁp stacking motifs, whereas the
structures have a ratio greater than 4.5, sandwich-herringbone substituted positions make a big difference to the pÁÁÁp stacking
structures have a ratio between 3.2 and 4.0, g-structures have motifs of organic salts.
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pairwise competitive crystallizations are generally consistent with
the measured solubilities.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the Natural Science Foundation
of China (Grant no. 21371031 and 21628101), the International
S&T Cooperation Program of China (No. 2015DFG42240) and
the Priority Academic Program Development (PAPD) of Jiangsu
Higher Education Institutions.
Fig. 9 Diagrammatic representation of the (a) herringbone, (b) sandwich-
herringbone, (c) g (g), and (d) b (b) packing motifs.
3.6 Thermogravimetric analysis (TGA) of ligands L1, L2
and salts 1–6
The thermogravimetric behaviors of the ligands and salts are
shown in Fig. S8 (ESI†). Compounds L1 and L2 start to decom-
pose at 232 1C and 235 1C, respectively. Salts 1 and 4 undergo
two sets of mass loss (in steps) at temperatures of 135 1C and
157 1C, respectively. The first weight loss step is mainly due to
the decomposition of nitrate and further weight loss can
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˜
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4 Conclusions
In summary, a series of organic salts HL1 ÁNO3À (1), L1ÁHL1 ÁClO4À
+
+
+
+
À
+
À
(2), L1ÁHL1 ÁH2PO4ÀÁH2O (3), HL2 ÁNO3 (4), HL2 ÁClO4 (5) and
+
À
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13854 | New J. Chem., 2017, 41, 13846--13854 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017