Interesting organic supramolecular structures constructed by piperazine/N,N′-dimethylpiperazine with aromatic multicomponent acids: Synthon cooperation and structural diversity

Article abstract of DOI:10.1039/c2ce25627d

Crystallographic studies reveal that organic supramolecular structures 1-7 constructed by piperazine/N,N′-dimethylpiperazine with aromatic multicomponent acids are organic salts formed due to the proton transfer from the carboxylic group of the aromatic multicomponent acids to the nitrogen atom of the piperazine/N,N′-dimethylpiperazine. In all salts, the cations and anions are associated via N-H...O(-COO^-) charge-assisted hydrogen bonds, leading to ionic synthons. Competition and cooperation among -COOH, -COO^-, -OH, -NO₂, -NH⁺- and -NH ²⁺- functional groups for the observed hydrogen bond synthons are examined in the seven structures. It is noted that C-H...O hydrogen bonds increase the dimensionality of the supramolecular architectures of the seven salts. The number of acids involved in the complex equals to the number of N-protonated binding sites in the piperazine/N,N′-dimethylpiperazine. Water molecules play a significant role in the formation of the supramolecular architectures. The supramolecular structures of the seven piperazine/N,N′- dimethylpiperazine salts were determined by single-crystal X-ray diffraction, respectively, to understand how variations in their molecular structures and intermolecular interactions influence their supramolecular assemblies. After the formation of salts of 1-7, the melting points are dramatically different, and thermal stability of these compounds were investigated by thermogravimetric analysis (TGA). This journal is

Full text of DOI:10.1039/c2ce25627d

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Table of contents
Interesting organic supramolecular structures constructed by
piperazine/N,N′ꢀdimethylpiperazine with aromatic multiꢀcomponent
acids: Synthons cooperation and structural diversity
by piperazine/N,N′ꢀdimethylpiperazine with aromatic multiꢀcomponent
acids: Synthons cooperation and structural diversity
Lei Wang*, Lei Zhao, Lingyan Xu, Ruixin Chen, and Yu Yang
College of Chemistry and Molecular Engineering, Qingdao University of Science and
Technology, Qingdao 266042, P. R. China
Supramolecular synthons of seven new binary molecular salts and their effect on
crystal packing were discussed in the context.

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PAPER
Interesting organic supramolecular structures constructed by
piperazine/N,N′-dimethylpiperazine with aromatic multi-component
acids: Synthons cooperation and structural diversity
Lei Wang*, Lei Zhao, Lingyan Xu, Ruixin Chen, and Yu Yang
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Crystallographic studies reveal that all complexes are organic salts formed due to the proton transfer from
the carboxylic group of aromatic multi-component acids to the nitrogen atom of the piperazine/N,N′-
dimethylpiperazine. In all salts, the cations and anions are associated via N—H···O(-COO-) charge-
10 assisted hydrogen bonds, leading to ionic synthons in all organic salts (1-7). Competition and cooperation
among -COOH, -COO-, -OH, -NO2, -NH+-, and -NH2+- functional groups for the observed hydrogen bond
synthons are examined in the seven structures. It is noted that C—H···O hydrogen bonds increase the
dimensionality of the supramolecular architectures of the seven salts. The number of acids involved in the
complex equals to the number of N-protonated binding sites in the piperazine/N,N′-dimethylpiperazine.
15 Water molecules play a significant role in the formation of the supramolecular architecture. Seven
supramolecular structures of piperazine/N,N′-dimethylpiperazine were determined by single-crystal X-ray
diffraction, respectively, to understand how variations in their molecular structures and intermolecular
interactions influence their supramolecular assemblies. After the formation of salts of 1-7, the melting
points are dramatically different, and thermal stability of these compounds has been investigated by
20 thermogravimetric analysis (TGA) of mass loss.
Any step taken into predictable molecular assemblies is thereby a
Introduction
practical impetus toward the ultimate goal of solid-based crystal
50 engineering. An advantage in fabricating co-crystals or salts is
that a number of closely related X-ray crystal structures may be
determined to understand the relationship among intermolecular
forces, molecular shapes, and crystal structures, which represents
the fundamental aspect in supramolecular chemistry.11,12
Crystalline materials of supramolecular organic frameworks are
notable representatives in supramolecular chemistry not only
because of their intriguing structural motifs1,2, but also for their
25 useful properties and promising applications as functional
materials3,4. As predominant driving forces, the role of hydrogen
bonds has been well-established.5 We consider two approaches
for a description of crystal structures. Desiraju mentioned the first
possibility of interpretation of crystal packing is a geometrical
30 one,6 which is based on Kitaigorodskii’s close packing principle,7
Etter’s rules for crystals containing hydrogen bonds,8 and the
supramolecular synthon concept.9 The latter approach includes
the recognition of some strongly bonded motifs in the crystal
structure (dimers, trimers, tetramers, chains, etc.) based on strong
35 intermolecular interactions. However, predetermination of even
the simplest molecular structure is a daunting challenge, because
the intermolecular forces for upholding the molecules are poorly
understood and difficult to predict, especially for organic solids.10
55
Hydrogen bonding is one of the most important fundamental
interactions involved in the association of organic molecules,13
giving rise to a variety of different supramolecular synthons.14
Most H-bonds are primarily electrostatic in nature and flexible in
strength according to the different electron donors and acceptors.
60 At the same time, notwithstanding their weak bonding and
angular spreading pattern, H-bonds have been approved as
directional and predictive intermolecular cohesive forces in
molecular packing.15 Among others, H-bonds as main driving
forces in supramolecular assemblies of organic modules featuring
65 functional carboxyl, pyridyl, or a mixture of the two have been
comprehensively explored.16-18 Supramolecular synthons, also
called motifs19 or patterns20, correspond to the regions of a
supramolecular structure where molecular recognition between
the constituent functional groups occurs. Supramolecular
70 synthons exist in two distinct categories: supramolecular
homosynthons that are composed of identical complementary
functional groups, e.g., the carboxylic acid homosynthon21, also
referred to as self-association motifs (dimers, chains, etc.);22
supramolecular heterosynthons composed of different but
40
College of Chemistry and Molecular Engineering, Qingdao University of
Science and Technology, Qingdao 266042, P. R. China. E-mail:
inorchemwl@126.com; Fax: (+86) 532-840-22681
† Electronic Supplementary Information (ESI) available: TGA and DSC
45 data. CCDC reference numbers 876826 and 818047-818052. For ESI and
crystallographic data in CIF or other electronic format see
DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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complementary functional groups,23 such as acid-aromatic
chosen to investigate their supramolecular synthons and provided
nitrogen24.
50 the experimental data to validate the work toward the a priori
prediction of organic salt crystal structures. The solvents play a
key role in the production of these molecular crystals. Seven salts
of piperazine and N,N′-dimethylpiperazine with aromatic multi-
component acids were prepared: [(C8H3O6N2-)·(C4H12N22+)]·H2O
Organic carboxylic acids, especially benzene poly carboxylic
acids represent one of the most prevalent functional groups in
crystal engineering because of their potential for the creation of
extensive arrays through hydrogen bondings. They possess a
5
hydrogen bond donor and acceptor with a geometry that 55 (1), [(C8H4O52-)·(C4H12N22+)] (2), [(C8H5O4 )·(C4H12N2
)0.5] (3),
-
2+
2+
-
2+
facilitates self-association through supramolecular homosynthons.
Indeed, carboxylic acids are well-known to self-associate via
[(C8H4O6N-)·(C6H16N2
)
0.5] (4), [(C8H5O5 )·(C6H16N2
)
0.5]·H2O
-
2+
-
(5),
)·(C6H16N2
[(C8H5O4 )·(C6H16N2
)
]
0.5
(6),
[(C8H7O3
10 centrosymmetric dimer (I) or catemer (II).25-27 Furthermore, in
presence of a number of different complementary functional
groups, such as aromatic nitrogen, and sulfonyls, carboxylic acids
are ideal candidates for multi-component crystals since they form
persistent supramolecular heterosynthons in the resultant multi-
15 component crystals.28
)
0.5·(C8H8O3)] (7). Their structures, melting points,
2+
and thermal stabilities were investigated.
60 Experimental section
General materials and methods
We have explored the possibility of hydrogen bonding
formation of piperazine/N,N′-dimethylpiperazine with molecules
chosen according to the following criteria: the systems employed
All reagents and solvents (analytical grade) for synthesis and
analysis were commercially available and used as received
without further purification. The melting points of all new
in this study are simple aromatic acids similar in shape but 65 compounds were recorded on
a WRS-1B digital thermal
20 differing in their functionality, on passing from 5-
hydroxyisophthalic acid to 5-nitroisophthalic acid and 3-
methylsalicylic acid, we progressively replace the -OH with -NO2
apparatus without correction and refer to the temperature at the
start of the melt. Carbon, hydrogen, and nitrogen microanalyses
were performed with a Perkin-Elmer 2400 elemental analyzer.
Fourier transform infrared (FT-IR) spectra were measured using a
and -CH3 group, suitable for charged assisted hydrogen bonding
with -NH2 -/-NH+- of piperazine/N,N′-dimethylpiperazine. 70 Nicolet Impact 410 FTIR spectrometer, and the samples were
+
25 Piperazine and its derivatives are capable of forming N—H···
(Y = O, N, halogen) hydrogen bonds. Among these, the N—
···O hydrogen bond is one of the most robust and well-studied
synthons in crystal engineering. Piperazine and its derivatives29
Y
prepared as KBr pellets in range 4000-400 cm-1.
Thermogravimetric analysis (TGA) experiments were carried out
on a Perkin-Elmer TGA 7 thermogravimetric analyzer in the
temperature range of 0-900 ºC under N2 atmosphere at a heating
H
consist of a strong hydrogen bond donor, available for hydrogen 75 rate of 10 ºC/min. The crystals were prepared as follows.
30 bonding. Hence, they can be utilized to prepare a salt when the
other component is not acidic enough to form co-crystals.
Piperazine and N,N′-dimethylpiperazine have a tendency to form
Syntheses of the complexes 1–7
[(C8H3O6N2-)·(C4H12N22+)]·H2O (1). To an ethanol/water
solution (v/v = 1.5:1, 25 mL) containing 5-nitroisophthalic acid
(42.2 mg, 0.2 mmol) was added piperazine hexahydrate (38.8 mg,
80 0.2 mmol) with constant stirring for 30 min. The resultant clear
solution was allowed to evaporate under ambient conditions,
affording colorless block crystals after a period of 1 week. Yield:
70%. Anal. Calcd for C12H17N3O7: C, 45.71; H, 5.40; N, 13.33%.
Found: C, 45.82; H, 5.35; N, 13.36%. IR (cm-1): 3424m, 3246m,
85 3102m, 3020m, 2755m, 2645m, 2471m, 2238w, 1690w, 1622m,
1601m, 1560m, 1530m, 1455m, 1421m, 1247m, 1220m, 1162w,
1086m, 1014w, 952w, 923w, 866w, 794w, 781w, 734m, 712m,
690m, 605w, 520w.
salts with acid moiety containing molecules. The acid transfers
the O–H hydrogen to the base, results in the formation of an
35 anion. They are well suited for use in supramolecular syntheses
based on hydrogen bonding interactions with a variety of organic
acids and bases. The presence of the nitrogen atoms in piperazine
and N,N′-dimethylpiperazine afford the generation of
advantageous hydrogen-bonded arrays because they can form
40 predictable patterns useful for constructing a wide range of
supramolecular assemblies. Recently, we and others have made
efforts to use organic salts as molecular building blocks to control
molecular packing in crystalline materials30. In this regard, a
series of salts formed by piperazine/N,N′-dimethylpiperazine
45 with aromatic multi-component acids (Scheme 1), and they were
[(C8H4O52-)·(C4H12N22+)] (2). 5-hydroxyisophthalic acid (91
90 mg, 0.5 mmol) and piperazine hexahydrate (97 mg, 0.5 mmol)
were mixed and dissolved in an ethanol/water solution (v/v = 5:1,
12 mL) with stirring. The resulting solution was evaporated in air
for 5 days to give colorless crystals of 2. Yield: 75%. Anal. Calcd
for C12H16N2O5: C, 53.73; H, 5.97; N, 10.45%. Found: C, 53.65;
95 H, 5.95; N, 10.35%. IR (cm-1): 3432m,3246m, 3020m, 2775m,
2510m, 1640m, 1559s, 1462m, 1437m, 1411s, 1383s, 1299m,
1274m, 1225m, 1162w, 1122w, 1091m, 1061w, 1014w,, 1003w,
987m, 975m, 952w, 921w, 893w, 882w, 816w, 795m, 781m,
721m, 694w, 623w, 603m, 528w, 417w, 446w, 423w.
COOH
H3C
N
N
CH3
HN
NH
COOH
O-phthalic acid
N,N '-dimenthylpiperazine
OH
Piperazine
NO2
COOH
OH
-
2+
100
[(C8H5O4 )·(C4H12N2 )0.5] (3). To a solution of O-phthalic
acid (166 mg, 1.0 mmol) in ethanol (10 mL), a solution of
piperazine hexahydrate (146 mg, 0.75 mmol) in water (2 mL)
was added. The reaction mixture was stirred for 30 min and then
allowed to evaporate at room temperature for 1 week. Colorless
COOH
COOH
CH3
HOOC
HOOC
5-nitroisophthalic acid
3-methylsalicylic acid
5-hydroxyisophthalic acid
Scheme 1 Molecules in this study
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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rod-shaped crystals of 3 were obtained in 85% yield. Anal. Calcd
for C10H11NO4: C, 57.42; H, 5.26; N, 6.70%. Found: C, 57.38; H,
5.28; N, 6.67%. IR (cm-1): 3435m, 3005m, 2898m, 2653m,
2526m, 1694s, 1586m, 1487m, 1456m, 1494s, 1282s, 1141m,
Upon slow evaporation of the solvents, colorless lamellar crystals
suitable for X-ray analysis were produced after 1 week. Yield:
70%. Anal. Calcd for C11H13NO7: C, 59.19; H, 5.83; N, 6.28%.
Found: C, 51.31; H, 5.74; N, 6.26%. IR (cm-1): 3435m, 3017m,
5
1072m, 1006w, 975w, 909m, 851w, 830w, 804m, 759w, 741m, 40 2952w, 2363m, 1963w, 1785w, 1679m, 1601s, 1559s, 1511s,
714w, 697w, 675m, 641w, 581w, 425w.
1471s, 1439s, 1376s, 1365s, 1296m, 1192m, 1108m, 1074s,
1026m, 980m, 861w, 834m, 805m, 729m, 670w, 644m, 608m,
590m, 515w, 455w, 423w..
[(C8H7O3 )·(C6H16N2 )0.5·(C8H8O3)] (7). A solution of 3-
45 methylsalicylic acid (76 mg, 0.5 mmol) in 5 mL of methanol was
mixed with a solution of N,N′-dimethylpiperazine (57 mg, 0.5
mmol) in 5 mL of ethanol. The reaction mixture was stirred for
30 min and allowed to stand for slow evaporation at ambient
conditions, affording colorless block crystals after 7 days in 77%
2+
[(C8H4O6N-)·(C6H16N2 )0.5] (4). 5-nitroisophthalic acid (106
mg, 0.5 mmol) was dissolved in methanol (5 mL) with stirring, to
which a water solution (5 mL) of N,N′-dimethylpiperazine (0.034
-
2+
10 mL, 0.25 mmol) was added. A large amount of white precipitate
was immediately formed, which was dissolved after adding
excessive DMF (10 mL) and 3d HNO3, with pH = 5. The
resultant colorless solution was filtered and left to stand at room
temperature. Colorless block crystals were obtained by slow
15 evaporation of the solvent after 2 weeks. Yield: 85%. Anal. Calcd 50 yield. Anal. Calcd for C19H23NO6: C, 63.15; H, 6.42; N, 3.88%.
for C11H12N2O6: C, 49.25; H, 4.48; N, 10.46%. Found: C, 49.35;
H, 4.53; N, 10.42%. IR (cm-1): 3421m, 3077m, 2622m, 2471m,
2235m,1642m, 1601s, 1535s, 1497s, 1390s, 1265m, 1238m,
1164m, 1141w, 1098w, 999w, 851w, 806w, 785m, 755w, 696w,
20 624m, 540w, 502w, 411w.
Found: C, 62.77; H, 6.68; N, 3.73%. IR (cm-1): 3431m, 3007s,
2855s, 2755s, 2636s, 2475s, 2227w, 1970w, 1888w, 1826w,
1633s, 1603s, 1561s, 1479s, 1394s, 1345s, 1257s, 1225w, 1189m,
1151m, 1080m, 1050m, 1003m, 950w, 914w, 866m, 836m,
55 774m, 761m, 635m, 602m, 573w, 456m.
-
2+
[(C8H5O5 )·(C6H16N2 )0.5]·H2O (5). 5-hydroxyisophthalic acid
(91 mg, 0.5 mmol) and N,N′-dimethylpiperazine (0.034 mL, 0.25
mmol) were dissolved in a methanol/water mixture (v/v = 2:1, 12
mL). The solution was allowed to evaporate at ambient
25 conditions, resulting in clear, colorless block crystals after 5 days
in 80% yield. Anal. Calcd for C11H15NO6: C, 51.36; H, 5.84; N,
5.45%. Found: C, 51.32; H, 5.64; N, 5.49%. IR (cm-1): 3437s,
3060m, 3007m, 2854m, 1719m, 1684s, 1610m, 1585m, 1492m,
1383m, 1364m, 1383m, 1364m, 1276s, 1229s, 1144w, 1126w,
30 1081w, 1031w, 1001w, 971m, 888w, 840w, 798w, 774m, 765m,
690m, 612w, 599w, 491w.
X-ray crystallography
Compounds 1-7 were examined under a microscope, and good-
quality crystals were chosen for structure determination by X-ray
diffraction using an optical microscope. The crystals were
60 mounted on a goniometer by gluing to a glass fiber with
cyanoacrylate adhesive, and crystal data were collected on a
Siemens Smart CCD diffractometer equipped with a normal-
focus,
2.4-kW
sealed-tube
X-ray
source
(graphite-
monochromatic MoKa radiation (λ = 0.71073 Å)) operating at 50
65 kV and 40 mA. There was no evidence of crystal decay during
the data collection for all compounds. In each case, all the
measured independent reflections were used in the structural
analysis, and semiempirical absorption corrections were applied
(SADABS) and the program SAINT was used for integration of
-
2+
[(C8H5O4 )·(C6H16N2 )0.5] (6). A methanol solution (10 mL)
of O-phthalic acid (166 mg, 1.0 mmol) was added to a ethanol
solution (8 mL) of N,N′-dimethylpiperazine (0.068 mL, 0.5 mmol)
35 with constant stirring for 10 min and then 2 mL water was added.
70 Table 1 Crystal data and structure refinement summary for compounds 1-7
1
C12H17N3O7
315.29
2
C12H16N2O5
268.27
3
C10H11NO4
209.20
4
C11H12N2O6
268.23
5
C11H15NO6
257.24
6
C11H13NO7
223.22
7
C19H23NO6
361.38
Empirical formula
M
Crystal system
Space group
a/Å
orthorhombic
Pna21 (33)
6.5078(11)
19.897(2)
10.9177(18)
90
monoclinic
P21/c (14)
12.0146(9)
9.6205(7)
11.9970(9)
90
orthorhombic
Pbca (61)
10.3173(4)
7.4516(12)
26.221(4)
90
monoclinic
P21/c (14)
7.5043(18)
20.894(5)
7.5043(18)
90
triclinic
monoclinic
P21/c (14)
12.941(3)
11.979(2)
7.0729(12)
90
monoclinic
P21/c (14)
14.950(2)
7.5242(15)
16.563(3)
90
P-1 (2)
8.4396(14)
8.5941(15)
8.9356(15)
88.535(4)
84.804(4)
66.083(4)
589.98(17)
1
b/Å
c/Å
α/deg
β/deg
90
114.803(10)
90
90
90.580
102.278(4)
90
92.834(2)
90
γ/deg
V/Å3
90
90
90
1413.7(14)
4
1258.78(16)
4
2015.9(5)
8
1176.5(5)
2
1071.3(4)
2
1860.8(6)
4
Z
ρcalcd (g/cm3)
µ(mm-1)
Rint
1.481
1.416
1.379
1.514
1.448
1.384
1.290
0.123
0.109
0.127
0.125
0.119
0.106
0.096
0.0267
0.0194
0.0454
0.0161
0.0136
0..0000378
0.0541, 0.1678
1.033
0.0219
b
R,a Rw
0.0368, 0.0844
1.044
0.0782, 0.2579
1.057
0.0530, 0.1270
1.046
0.0486, 0.1228
1.050
0.0487, 0.1344
1.054
0.0777, 0.2497
1.032
GOF
This journal is © The Royal Society of Chemistry [2011]
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O2N
COOH
NH2
O
O2N
C
O
O
O
H
N
H
H3C
H
O
O
C
O
NH2
N
N
C
OOC
H
N
H
NO2
HOOC
CH3
H
O
C
COOH
H
N
N
H
H
O
Ⅲ
H
O
H2N
H
C
H
COO
O
H2N
O
Ⅱ
C
O
O
H
Ⅰ
COOH
OOC
O
O
O
C
O
H
O
C
NO2
C
O
H
O
OOC
O2N
Ⅵ
O
H
O
H
O
O
O
C
C
O
O
C
O
O
H
H
O
O
H
N
H
H
O
H
H
O
H
C
O
O
H
O
C
H
H
C
H2N
HO
O
Ⅶ
Ⅳ
O
OH
OOC
OH
COO
C
C
O
H
Ⅴ
COOH
O
O
C
O
O
C
HO
O
H
O
OOC
O
O
C
O
H
H
C
O
H
O
OH
Ⅶ
NH
NH
C
O
OH
O
OH
H
O
NH
H
NH
H
C
C
O
H
O
H
H
NH
NH2
H
O
C
O
C
O
O
O
O
O
O
C
Ⅺ
C
O
H
O
O
H C
C
O
OH
HO
O
O
COO
HO
OH
Ⅸ
COO
OH
Ⅹ
NO2
O
O2N
O
O
O
C
H
N
NH
H3C
H
CH3
H
C
C
O
O
N
O
O
N
H3C
CH3
NO2
C
O
C
H
O
HOOC
H
O
Ⅻ
O
C
C
O
H
O
O
NO2
H
NH
NH2
C
C
O2N
O
O
C
O
N
N
H
C
H
O
O
H
H3C
CH3
O
COOH
O
O
C
COOH
O
ⅩⅢ
NO2
ⅩⅣ
Scheme 2 Supramolecular synthons
4
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Table 2 Characteristics of hydrogen-bond geometries observed in the
crystal structures of 1-7
of the hydrogen atoms were first observed in difference electron
density maps and then placed in the calculated sites and included
in the final refinement in the riding model approximation with
15 fixed thermal factors. Further details for crystallographic data and
structural refinement parameters of the compounds are
summarized in Table 1.
D—H···A (Å)
D—H(Å)H···A(Å) D···A(Å) D—H···A (deg)
1
O1W—H1WB···O5a 0.84
1.84
1.93
1.78
1.79
1.85
1.95
2.41
2.41
2.57
1.85
2.40
2.61
1.72
1.72
1.82
1.93
1.72
1.93
2.08
2.45
2.53
1.81
1.82
2.47
2.43
2.54
1.81
1.87
1.91
2.01
1.74
2.43
2.46
1.55
1.77
2.37
2.41
2.42
2.48
1.74
1.78
1.86
1.87
2.35
2.37
2.87
2.96
2.699
1.747
2.677
2.692
2.703
2.814
3.241
3.241
3.393
2.661
3.211
3.189
2.688
2.670
2.712
2.859
2.529
2.769
2.846
2.999
2.973
2.583
2.715
3.118
3.380
3.324
2.566
2.783
2.794
2.694
2.620
3.390
3.385
2.372
2.660
3.141
3.166
3.272
3.318
2.508
2.508
2.584
2.774
2.967
2.057
3.490
3.849
169
159
175
175
158
160
143
143
143
173
150
123
172
165
151
160
171
154
143
120
111
156
169
128
169
158
153
177
169
140
163
173
161
178
165
136
135
147
145
155
147
147
172
125
128
123
160
O1W—H1WA···O4a 0.85
N2—H2B···O1Wb
N1—H1A···O3a
N1—H1B···O6c
N2—H2C···O4a
C10—H10B···O3a
C11—H11B···O5a
C10—H10A···O6c
O5—H5A···O1d
N2—H2D···O1d
N1—H1C···O4d
C4—H4A···O3e
C2—H2B···O4d
C2—H2A···O2f
C4—H4B···O2f
O1—H1···O3g
N1—H1A···O3g
N1—H1B···O2g
N1—H1B···O4g
N1—H1A···O4g
O3—H3···O6f
N1—H1···O5f
N1—H1···O6f
0.90
0.90
0.90
0.90
0.97
0.97
0.97
0.82
0.90
0.90
0.97
0.97
0.97
0.97
0.82
0.90
0.90
0.90
0.90
0.82
0.91
0.91
0.96
0.97
0.82
Results and discussion
Preparation of compounds 1-7
20 In this research, both base-type and acid-type reagents have good
solubility in water and some common organic solvents, such as
CH3OH and C2H5OH. All crystallizations of piperazine/N,N′-
dimethylpiperazine and different aromatic carboxylic acids were
carried out in 1:1, 2:1 and 4:3 ratios, considering the number of
25 hydrogen-bonding donor/acceptor groups in each component. For
the preparation of 2 and 5, 5-hydroxyisophthalic acid was directly
mixed with equivalent base-type reagents in CH3OH/C2H5OH
and H2O solution, which were allowed to evaporate at room
temperature to give the final crystalline products. For compound
30 4, the combination of acid and base lead to a mass of white
precipitation immediately in CH3OH and H2O solution. Therefore,
HNO3 and excessive DMF were added to dissolve the
precipitation. In the case of 1, 3, 6 and 7, acid and base were
dissolved in different solvents individually and then mixed with
35 each other to obtain the single crystals suitable for X-ray
determination. Proton-transfer from acid to base occurs in all
molecular crystals of 1-7. In this work we used water for
crystallizing the molecular complexes and as expected we did
achieved two hydrates 1 and 5. We tried to grow single crystals
40 of these two compounds in absence of water as solvent, but failed.
2
3
4
5
C9—H9C···O2f
C11—H11A···O6f
O2—H2···O3f
O1W—H1WB···O1h 0.92
O1W—H1WA···O4f
O5—H5···O1Wi
N1—H1···O4f
0.90
0.82
0.91
0.97
0.96
0.82
0.91
0.97
0.97
0.97
0.97
0.82
0.82
0.82
0.91
0.91
0.97
0.97
0.93
Molecular and supramolecular structural descriptions of 1-7
The schematic representations of different kinds of hydrogen-
bonding synthons related to this work are summarized in Scheme
2. Hydrogen-bond geometries of 1-7 are listed in Table 2.
C10—H10A···O5j
C9—H9C···O2k
O2—H2···O3l
6
7
45
Structure description of compound 1. Crystallization of 5-
nitroisophthalic acid with piperazine hexahydrate yields a proton-
transfer organic salt 1. The asymmetric unit contains a 5-
nitroisophthalic acid dianion, a piperazine dication, and a water
solvent, as shown in Figure 1a, piperazine dication is chair
N1—H1···O4l
C2—H2B···O2l
C3—H3A···O2l
C2—H2A···O3l
C3—H3B···O3l
O7—H7···O6f
50 conformation and the plane of C9/N1/C12 and the C9-C12 plane
make a dihedral angle of 53.308°. The dihedral angle of the C9-
C12 plane and the C1-C6 aryl ring is 53.609°. As illustrated in
Figure 1b, each water molecule connects two 5-nitroisophthalic
acid dianions via two strong O1W—H1WA···O4(-COO-) and
55 O1W—H1WB···O5(-COO-) hydrogen bonds to form an infinite
zigzag chain along the [001] direction. Then these zigzag chains
are highly interdigitated and further connected through strong
O6—H6···O7f
O2—H2···O3k
N1—H1N···O8f
N1—H1N···O7f
C2—H2A···O3k
C2—H2A···C13f
C9—H9···C8k
N1—H1A···O3(-COO-),
N1—H1B···O6(-COO-),
N2—
a 1/2–x, 1/2+y, -1/2+z. b -1+x, 1+y, -1+z. c 1/2-x, 1/2+y, -1.5+ z. d x, y,
1+z e 1+x, y, 1+z. f x, y, z. g -1+x, y, z. h x, -1+y, -1+z. i1+x, 1+y, -1+ z. j x,
y, -1+z. k x, 1+y, z. l 2–x, 1/2+y, 1/2-z.
H2B···O1W, N2—H2C···O4(-COO-) hydrogen bonds between
60 acid and base subunits, resulting in a 2D layer network (Figure
1c), in which two additional synthons Ⅱ [R5 (14)] (N2—
5
3
the diffraction profiles31. All structures were solved by direct
methods using the SHELXS program of the SHELXTL package
and refined with SHELXL32. The final refinement was performed
by full-matrix least-squares methods with anisotropic thermal
H2B···O1W, N2—H2C···O4(-COO-), N1—H1A···O3(-COO-),
N1—H1B···O6(-COO-), and O1W—H1WB···O5(-COO-)) and Ⅳ
[R4 (16)] (N2—H2B···O1W, N2—H2C···O4(-COO-), O1W—
3
65 H1WA···O4(-COO-), and O1W—H1WB···O5(-COO-)) are
included. These 2D layers are stabilized by weak C10—
H10A···O6(-COO-), C11—H11B···O5(-COO-) and C10—
10 parameters for all the non-hydrogen atoms on F2. All non-
hydrogen atoms were refined anisotropically. In both cases, most
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H10B···O3(-COO-) hydrogen bonds, and the resultant crystal
structure can also be envisaged as a 3D network (Figure 1d).
crystal packing of 2 suggests that 5-hydroxyisophthalic acid
20 dianions are connected by O5—H5A···O1(-COO-) to generate a
1D zigzag chain along the [001] direction (Figure 2b). Adjacent
1D zigzag chain and piperazine dications are connected through
strong N1—H1C···O4(-COO-) and N2—H2D···O1(-COO-)
interactions (synthon Ⅸ [R5 (34)]) between acid and base
6
25 subunits, resulting in a 2D layered network viewing from the ac
plane (Figure 2c). Further analysis of the crystal packing indicate
that these 2D layers are connected by weak C2—H2B···O4(-
COO-) hydrogen-bonding to form 3D network. Through weak
C2—H2A···O2(-COO-), C4—H4A···O3(-COO-), and C4—
30 H4B···O2(-COO-) interactions to stabilize the 3D network (Figure
2d).
Figure 1 (a) Molecular structure of 1 with atom labeling of the
asymmetric unit. (b) An infinite zigzag chain along the [001] direction. (c)
2D layered network. (d) 3D hydrogen-bonded supramolecular structure of
1. (O, red; N, blue; C, gray; H, light gray in this and the subsequent
figures) Hydrogen bonds are shown as dashed lines.
5
Structure description of compound 2. The structural
10 determination of 2 reveals an asymmetric two-component
molecular crystal, in which each 5-hydroxyisophthalic acid
dianion crystallizes with one piperazine dication (Figure 2a).
Within 2, 5-hydroxyisophthalic acid was deprotonated to form
dianion compound. Meantime, piperazine was protonate to obtain
15 dication component, and it keeps chair conformation. The
dihedral angle between the plane of C1/C2/C3/C4 and the
C1/N1/C4 plane is 51.989°. The C1-C4 plane and the C6-C11
aryl ring make a dihedral angle of 66.432°. Analysis of the
Figure 2 (a) Molecular structure of 2 with atom labeling of the
asymmetric unit. (b) A 1D zigzag chain along the [001] direction. (c) 2D
35 layered network viewing from the ac plane. (d) 3D hydrogen-bonded
supramolecular structure of 2.
Structure description of compound 3. As depicted in Figure
3a, the molecular structure of 3, crystallize with an O-phthalic
acid monoanion and a half piperazine dication. In 3, piperazine
6
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dications are centrosymmetric and keep chair conformation. The
dihedral angle between the plane of C9/C10/C9/C10 and the
C9/N1/C10 plane is 52.572°. Piperazine dications and the
benzene ring of O-phthalic acid make a dihedral angle of 88.408°.
As shown in Figure 3b, O-phthalic acid monoanions are
combined into an infinite zigzag chain via O1—H1···O3(-COO-)
along [010] direction. Meanwhile, an 1D infinite chain is formed
5
by synthon ⅩⅢ [R2 (4)] (strong N1—H1A···O3(-COO-) and
1
N1—H1A···O4(-COO-) hydrogen bonds) and synthon Ⅺ [R2 (7)]
1
10 (strong N1—H1B···O4(-COO-) and N1—H1A···O2(-COO-)
hydrogen bonds) and synthon
Ⅰ
[R2 (8)] (strong N1—
2
H1B···O4(-COO-) and N1—H1A···O4(-COO-) interactions),
viewing from [010] direction (Figure 3c). The two kinds of
infinite chains are further linked into a 2D layered network along
15 bc plane (Figure 3d).
Figure 4 (a) Molecular structure of 4 with atom labeling. (b) 1D infinite
chain of along the [100] direction. (c) 2D hydrogen-bonding
4
25 architecture of 4, viewing along the ab plane. (d) 3D supramolecular
architecture of 4.
Structure description of compound 4. In the local structure
(Figure 4a) of 4, the N,N′-dimethylpiperazine dication exhibits
chair conformation and is centrosymmetric. The asymmetric unit
30 contains a half N,N′-dimethylpiperazine dication and a 5-
nitroisophthalic acid monoanion. The plane consisting of 4 atoms
C10/C11/C10/C11 makes a dihedral angle of 50.673° with the
C10/N1/C11 plane. The dihedral angle between acid and base
components is 76.940°. In 4, analyses of the intermolecular
35 interactions show that 5-nitroisophthalic acid monoanions are
assembled together via strong hydrogen-bonding. Thus, an
infinite 1D chain is afforded, running along the [100] direction
(Figure 4b). Each two acid monoanions connected a N,N′-
dimethylpiperazine dication by N1—H1···O5(-COO-) and N1—
Figure 3 (a) Molecular structure of 3 with atom labeling of the
asymmetric unit. (b) 1D infinite zigzag chain via O1—H1···O3(-COO-)
along the [010] direction. (c) 1D infinite chain is formed by strong N—
20 H···O hydrogen-bonding view along the [010] direction. (d) Perspective
view of the 2D hydrogen-bonded layer in 3.
40 H1···O6(-COO-) strong interactions [synthons Ⅲ R2 (4) and ⅩⅣ
1
R6 (46)] to form a 2D layer network viewed from the ab plane, as
8
illustrated in Figure 4c. Adjacent layers are linked into 3D
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supramolecular arrays via weak hydrogen-bond C11—
H11A···O6(-COO-) and strong interaction N1—H1···O5(-COO-)
(Figure 4d). Further analysis of this structure reveals that two
weak hydrogen bonds C9—H9C···O2(-NO2) and C10—
H10B···O1(-NO2) between N,N′-dimethylpiperazine dication and
5-nitroisophthalic acid monoanion also consolidate the 3D
architecture.
5
Structure description of compound 5. The molecular
structure of 5 is depicted in Figure 5a, the asymmetric unit of
10 each polymorph is made up of one 5-hydroxyisophthalic acid
monoanion, a half N,N′-dimethylpiperazine dication, and one
water molecule. Similar to 4, N,N′-dimethylpiperazine dication
makes a dihedral angle of 52.121° between the plane of
C10/C11/C10/C11 and C10/N1/C11 plane, and it is also
15 centrosymmetric. The dihedral angle between the plane of C1-C6
aryl ring of acid molecule and the plane consisting of atoms
C10/C11/C10/C11 of base component is 88.818°. First, 5-
hydroxyisophthalic acid monoanions are connected together via
strong hydrogen-bond O2(-COOH)—H2···O3(-COO-) to form a
20 1D chain along the crystallographic [010] direction (Figure 5b).
Through further O1W—H1WA···O4(-COO-) and O1W—
H1WB···O1(-COOH) interactions [synthon Ⅹ R6 (28)], two
4
adjacent such 1D chains are combined into a 1D double chain
(Figure 5c), and we can clearly see that the lattice water located
25 in the space of the 1D double chain. As shown in Figure 5d, each
water moiety behave as
(O1W—H1WA···O4(-COO-), O1W—H1WA···O1(-COOH), and
O5(-OH)—H5···O1W) with two carboxylic groups and
a hydrogen-bonding 3-connector
a
hydroxyl group from distinct 5-hydroxyisophthalic acid hydroxyl
30 group from distinct 5-hydroxyisophthalic acid monoanions, and
two water molecules link two 5-hydroxyisophthalic acid
monoanions by O1W—H1WA···O1(-COOH) and O5(-OH)—
H5···O1W strong hydrogen-bindings to result in a closed pattern
with an orbicular synthon Ⅴ [R6 (30)]. Through the orbicular
6
35 synthon, the double chains are connected to form a 2D layer
network. Each adjacent 2D layer array connect together via N1—
H1···O4(-COO-) to result in a 3D supramolecular array (Figure
5e). Two weak C9—H9C···O2(-COOH) and C10—H10A···O5(-
OH) interactions are further connected to consolidate the 3D
40 network.
Structure description of compound 6. With respect to 6, a
half symmetric N,N′-dimethylpiperazine dication crystallizes
with an O-phthalic acid monoanion in the local structure (Figure
6a). The dihedral angle which is made by the plane of
45 C2/C3/C2/C3 and the C2/N1/C3 plane is 52.203°. In crystal of 6,
N,N′-dimethylpiperazine dication is centrosymmetric, chair
conformation and has two different directions. The dihedral angle
made by them is 62.528°, with the dihedral angle of the two
kinds of N,N′-dimethylpiperazine dication from the benzene
50 plane is 87.650° and 88.293°, respectively. In 6, the carboxylic
acid
H atoms of O-phthalic acid transfer to the N,N′-
dimethylpiperazine molecular to form O-phthalic acid monoanion,
Due to the deprotonation effect, the carboxyl O atom forms a
intramolecular S(7) ring [synthon Ⅵ] with the carboxyl group via
55 O2(-COOH)—H2···O3(-COO-) strong hydrogen-bonding. Two
O-phthalic acid monoanions and a N,N′-dimethylpiperazine
dication were connected together by N1—H1···O4(-COO-) to
form a subunit structure (Figure 6b). Weak C2—H2B···O2(-
60 Figure 5 (a) Molecular structure of 5 with atom labeling of the
asymmetric unit. (b) 1D chain along the crystallographic [010] direction.
(c) 1D double chain along the [010] direction. (d) Perspective view of the
2D layer in 5. (e) 3D hydrogen-bonded supramolecular structure of 5.
COOH) and C3—H3A···O2(-COOH) interactions applied to
65 connect the subunit structures to form a 2D layer network (Figure
8
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6c). The 2D layer array, interacted with O-phthalic acid
monoanion via weak C3—H3B···O3(-COO-) and C2—
H2A···O3(-COO-) hydrogen bonds, which stabilize the total
supramolecular structure (Figure 6d).
5
Figure 6 (a) Molecular structure of 6 with atom labeling of the
asymmetric unit. (b) The subunit structure formed by two O-phthalic acid
monoanions and
a N,N′-dimethylpiperazine dication. (c) and (d)
25 Figure 7 (a) Molecular structure of 7 with atom labeling of the
Perspective view of the 2D hydrogen-bonded layer in 6.
asymmetric unit. (b) 0D hydrogen-bonded architecture along the
crystallographic [001] direction. (c) 2D supramolecular network viewing
along the b axis in the unit cell. (d) A side view of the 3D architecture.
10
Structure description of compound 7. Crystallization of 7,
gave single crystals of good quality, by a slow evaporation
process at ambient conditions. The crystal structure of 7 is
interesting crystallographically. As depicted in Figure 7a, crystal
7 crystallizes in monoclinic space group P21/c(14) with half
15 dication of N,N′-dimethylpiperazine, one 3-methylsalicylic acid
monoanion, and one 3-methylsalicylic acid molecule. In 7, the
COO- group plane of 3-methylsalicylic acid form dihedral angles
of 2.034° and 2.809° from co-planarity, respectively, with the
two benzene rings of 3-methylsalicylic acid anion and 3-
20 methylsalicylic acid molecule. On the other hand, the benzene
ring plane is nearly coplanar, and the angle between the two
planes is 2.468°. Whereas for N,N′-dimethylpiperazine dication,
the plane defined by the piperazine ring is almost perpendicular
30 (82.679°) with the benzene ring plane of 3-methylsalicylic acid
anion. The crystal packing of 7 can be envisaged as an interesting
3D network resulted from the stacking of infinite 0D dimers. The
crystal structure is based on a five-component centrosymmetric
assembly involving two molecules of 3-methylsalicylic acid, two
35 anions
of
3-methylsalicylic
acid
and
one
N,N′-
dimethylpiperazine dication linked via N1—H1N···O8, N1—
H1N···O7 and C2—H2A···O3 hydrogen bonds (Figure 7b). A S(6)
ring [synthon Ⅵ] followed by the ring motif S(6) [synthon Ⅶ].
The sequence then repeats again along the b-axis. Hence, the
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five-component assembly is formed, which then are held together
binding sites in the piperazine/N,N′-dimethylpiperazine molecule.
+
by weak C2—H2A···C13 hydrogen bonds to form a 2D layer
Moreover, -NH2 -/-NH+- on piperazine/N,N′-dimethylpiperazine
(Figure 7c) through a R2 (4) [synthon Ⅻ] ring. The resultant
cation boost the ability of donors to form H-bonds to direct the
1
crystal structure can also be envisaged as a 3D network, wherein 60 molecular assemblies, which has been demonstrated as a
5
the 2D network of fused grids are further arranged with the
formation of C9—H9···C8 bonds between N,N′-
successful synthetic strategy to build new supramolecular organic
frameworks.
dimethylpiperazine dication, 3-methylsalicylic acid anion and 3-
methylsalicylic acid molecule.
In all salts 1-7, the O—H hydrogen of aromatic multi-
component acid was transferred to the piperazine/N,N′-
65 dimethylpiperazine and stabilized through neutral and charge-
assisted hydrogen bonds. The present study shows the remarkable
coordinating properties of the piperazine and N,N′-
dimethylpiperazine molecules, and illustrates their ability to
Thermal behavior of the seven crystalline forms
10 The thermal stability of the compounds previously presented was
assessed by combining data from different techniques, such as
TGA and DSC.
achieve
a suitable balance between the driving force to
70 coordinate to -COOH/-COO- binding sites of the acids and their
tendency to self-assemble via supramolecular heterosynthons in
the structure of the complexes described herein. Therefore, the
main interactions present in the crystalline systems reported
herein are of the O—H···O, N—H···O and N—H···O types. The
75 only exception to this is salt 7, in which C—H···C bond is
observed in addition to the ones previously mentioned.
Additionally, the relative orientation and the nature of the
different carboxylic acids also have a significant effect on
forming the final diverse networks. Survey of the hydrogen-
80 bonding arrays easily reveals the significant role of water
molecules in the intermolecular hydrogen bonding interactions.
However, salts 1-7 have melting points which are between those
of the starting materials, and the melting points are dramatically
different after formation of salts.
All compounds 1-7 are air stable and can retain their structural
integrity at ambient conditions for a considerable length of time.
15 TGA experiments were implemented to investigate their thermal
stability. The TGA measurement of 1 shows a weight loss of 5.63
% in the temperature range 80 ℃-120 ℃, which corresponds to
the loss of lattice water molecules, and the host framework
remains stable to 200 ℃, then two consecutive weight losses
20 occur from 200 ℃ to 900 ℃ (peak positions: 300 ℃ and 370 ℃).
As for 2 and 6, the TGA results indicate that they remain intact
until 120 ℃ and 140 ℃, respectively, and then there are a sharp
weight loss ending at 250 ℃ for 2 and 240 ℃ for 6 (peaks: 228
℃ for 2 and 210 ℃ for 6), corresponding to the explosion of all
25 base and acid components. For compound 3, four consecutive
weight losses of all substance in the 160 ℃-480 ℃ (peaking at
197 ℃, 219 ℃, 303 ℃ and 454 ℃), correspond to the stepwise
removal of the host components. The TGA curve of 4 and 7 show
two consecutive weight loss of all crystalline sample from 210 ℃
30 to 900 ℃ (peaking at 249 ℃ and 308 ℃, respectively) and from
5 ℃ to 242 ℃ (peaking at 80 ℃ and 220 ℃, respectively),
respectively. For compound 5, the TGA curve shows the first
weight loss of 5.20 % from 70 ℃ to 100 ℃ (peaking at 86 ℃),
corresponding to the release of lattice water. Four consecutive
35 weight losses occur in the range of 100-600 ℃ (peak position:
107 ℃, 234 ℃, 299 ℃, 512 ℃), corresponding to the loss of host
components.
85
In conclusion, this study shows that piperazine and N,N′-
dimethylpiperazine are both excellent supramolecular reagents
that can produce salts with aromatic acids in high yield. Further
studies on such flexible building blocks are desirable to
understand the crystal engineering of hydrogen-bonded solids,
90 and related exploration on this perspective is underway. The
intermolecular hydrogen-bonded motifs appear reliable and will
be useful in the design of analogues for specific applications. We
are currently extending this strategy by using polycarboxylic
acids as the hydrogen-bonding participators to prepare new
95 crystalline materials. Here, we have shown that the
supramolecular synthon play an important role in complexation
and packing of molecules in the solid state. Density functional
theory calculations to interpret their relative stability of
supramolecular synthons using the Amsterdam Density
100 Functional program package are currently in progress33.
DSC measurements on crystalline 1-7 show that species are
stable and do not undergo any changes until melting are observed,
40 immediately followed by decomposition. In salts 1-7, the melting
points are 230 ℃, 110 ℃, 180 ℃, 225 ℃, 175 ℃, 115 ℃ and 90
℃, respectively.
Conclusions and perspectives
Acknowledgments
The supramolecular synthesis of the piperazine/N,N′-
45 dimethylpiperazine complexes in the solution unambiguously
lead to the organic salts. In this study, we have investigated the
possibility of forming new crystal forms between
We are grateful to the financial support by the National Natural
Science Foundation of China (No. 20701023 and 20971076), and
the Natural Science Foundation of Shandong Province, China
105 ((No. 2009ZRB019KH).
piperazine/N,N′-dimethylpiperazine
and
aromatic
multi-
component acids, such as 5-nitroisophthalic acid, 5-
50 hydroxyisophthalic acid, O-phthalic acid and 3-methylsalicylic
acid. These molecules carry oxygen and/or nitrogen atoms that
can act as hydrogen bonding acceptors and can therefore take part
in extended hydrogen bonded networks, suitable for the study of
synthons competition and cooperation established in this kind of
55 structures. The number of aromatic multi-component acid
involved in the complex equals to the number of N-protonated
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