Solvent dependent structures of melamine: Porous or nonporous?

Article abstract of DOI:10.1021/acs.cgd.5b00039

Two hydrogen-bonded organic frameworks (HOFs) of the melamine (MA), MA-1-H₂O and MA-2-DMF, have been crystallized in H₂O and DMF (N,N-Dimethylformamide), respectively. Structurally, MA-1-H₂O and MA-2-DMF are condensed and

Full text of DOI:10.1021/acs.cgd.5b00039

Article
pubs.acs.org/crystal
Solvent Dependent Structures of Melamine: Porous or Nonporous?
Peng Li,† Hadi D. Arman,† Hailong Wang,† Linhong Weng,‡ Khalid Alfooty,§ Rehab F. Angawi,§
and Banglin Chen*,†,§
†Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States
‡Department of Chemistry, Fudan University, Shanghai, 200433, People’s Republic of China
§Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia
S
* Supporting Information
ABSTRACT: Two hydrogen-bonded organic frameworks
(HOFs) of the melamine (MA), MA-1-H2O and MA-2-
DMF, have been crystallized in H2O and DMF (N,N-
Dimethylformamide), respectively. Structurally, MA-1-H2O
and MA-2-DMF are condensed and porous, respectively. The
porous MA-2-DMF sustains the porous feature under the
solvent exchange by acetone, dichloromethane (CH2Cl2), and
toluene, while it transforms into the condensed one under
methanol (MeOH) and ethanol (EtOH). Furthermore, MA-2-
DMF and MA-2-Toluene can undergo reversible single
crystal−single crystal transformation with retention of the porous hydrogen-bonded organic frameworks.
about the possibility to make use of melamine as the molecular
building unit to construct porous HOFs. Literature studies
show that melamine sometimes does co-crystallize with other
organic molecules to form “porous” structures, but none of
them has been realized to have flexible/robust structures even
for solvent exchanges.32−34 Herein we report the solvent
dependent crystal structures of melamine in which both
crystallographically condensed and porous structures have
been discovered. More importantly, we realized that these
two phases can be reversibly transformed with each other, while
the porous one can sustain the solvent exchange.
INTRODUCTION
■
Hydrogen-bonded organic frameworks (HOFs) have regained
extensive interest recently because of their potential applica-
tions as organic porous materials for carbon dioxide capture,1
hydrocarbon adsorption and separation,1−10 chiral resolution,11
and luminescent properties.12,13 Compared with other types of
porous materials such as zeolites and metal−organic frame-
works,14−18 HOFs can not only be easily prepared by
crystallization, but more importantly can be straightforwardly
recycled and processed through a solution chemistry approach,
making them unique porous materials for their useful
applications and device fabrications.
Because the hydrogen bonding interactions within HOFs are
much weaker than those ionic bonds with zeolites and
coordination bonds within MOFs, it is challenging to stabilize
the HOFs and thus establish their permanent porosities. A large
number of organic building molecules with typically rigid
backbones19,20 and specific functional groups such as
guanidinium-sulfonate (GS),21−24 pyridyl and carboxyl groups,1
pyrazole,9 pyridone,25 and benzimidazolone26 have been
examined for their construction of HOFs, but with limited
success to establish their gas/vapor sorption capacities. During
our studies based on Wuest’s pioneer work27−30 to construct
robust HOFs for gas separation and enantioselective separation
of chiral alcohols, we have realized that 2,4-diaminotriazinyl
(DAT) group is indeed a very powerful building unit to
assemble and stabilize porous HOFs because of their multiple
sites for the hydrogen bonding. The success of this DAT unit
for the construction of porous HOFs has motivated us to
explore new simple organic building units to develop the HOF
chemistry.31 Given the fact that melamine has very similar
structure features at the DAT motif, we have been wondering
EXPERIMENTAL SECTION
■
All reagents and solvents were used as received from commercial
suppliers without further purification. 1H NMR spectra were recorded
on a Varian Mercury 500 MHz spectrometer. The coupling constants
were reported in hertz. Thermogravimetric analyses (TGA) were
measured using a Shimadzu TGA-50 analyzer under a nitrogen
atmosphere at a heating rate of 5 °C min−1. Powder X-ray diffraction
(PXRD) patterns were recorded by a Rigaku Ultima IV diffractometer
operated at 40 kV and 44 mA with a scan rate of 1.0 deg min−1. The
crystallographic measurement of MA-1-H2O, MA-2-DMF, and MA-2-
Toluene were collected at 100 (2) K on a Rigaku X-ray diffractometer
system equipped with a Mo-target X-ray tube (λ = 0.71073 Å)
operated at 3000 W power (50 kV, 40 mA) with Saturn 724 CCD
detector. The data sets were corrected by empirical absorption
correction using spherical harmonics, implemented in the SCALE3
ABSPACK scaling algorithm. The structure was solved by direct
methods and refined by full matrix least-squares methods with the
SHELX-97 program package. The H atoms on C atoms were
Received: January 8, 2015
Revised: February 20, 2015
© XXXX American Chemical Society
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generated geometrically. X-ray crystallographic data is summarized in
Table 1.
Table 1. Crystallographic Data for the MA-1-H2O, MA-2-
DMF, and MA-2-Toluene
Identification code MA-1-H2O
MA-2-DMF
MA-2-Toluene
Empirical formula C3H6N6
C3H6N6·
C3H7NO
2C3H6N6·C7H8
Formula weight
126.14
100(2)
monoclinic
P21/n
199.21
100(2)
tetragonal
I41/a
344.38
100(2)
tetragonal
I41
Temperature/K
Crystal system
Space group
a/Å
7.2789(3)
7.4799(3)
10.332(4)
90.00
12.150(2)
12.150(2)
24.405(2)
90.00
12.236(6)
12.236(6)
24.574(3)
90.00
b/Å
c/Å
α/°
Figure 1. TGA curves of MA-1-H2O (red), MA-2-DMF (black), MA-
2-toluene (blue), MA-3-Acetone (green), MA-2-CH2Cl2 (purple),
MA-2-EtOH (pink), and MA-2-MeOH (brown).
β/°
γ/°
Volume/Å3
108.495(4)
90.00
90.00
90.00
90.00
90.00
533.46(4)
4
3602.7(9)
16
3679.5(6)
8
Z
Dcal (g cm−3
)
1.570
1.469
1.243
μ (mm−1
)
0.117
0.069
0.086
F(000)
264
1056
1456
Reflections
collected
1947
9049
9511
Data/restraints/
parameters
954/0/106
2053/0/100
1881/1/101
GOF
1.769
1.016
1.981
Final R indexes [I R1 = 0.0856, wR2 R1 = 0.0850, wR2 R1 = 0.1025, wR2
>2σ(I)] = 0.1802 = 0.1756 = 0.1758
Final R indexes [all R1 = 0.0869, wR2 R1 = 0.1000, wR2 R1 = 0.1036, wR2
data]
= 0.1818
= 0.1835
= 0.1954
Largest diff. peak/ 0.53/−0.53
0.28/−0.27
0.20/−0.45
hole/e Å−3
Synthesis of MA-1-H2O. 50 mg Melamine (MA) was dissolved in
6 mL H2O under heating. The resulting solution was cooled to room
temperature and filtered. The filtrate was allowed to evaporate at room
temperature for 2 days. Colorless block-shaped crystals were obtained
in 91% yield.
Synthesis of MA-2-DMF. 50 mg Melamine (MA) was dissolved in
6 mL DMF solvent under heating. The resulting solution was cooled
to room temperature and filtered. The filtrate was allowed to evaporate
at room temperature for 1 day. Large colorless square-shaped crystals
were obtained in 89% yield.
Solvent Soaking Experiment. A small amount of as-synthesized
MA-2-DMF crystals were soaked in 10 mL different solvent (CH2Cl2,
acetone, toluene, EtOH, and MeOH), replacing the soaking solution
every 24 h for 3 days. After soaking, the samples were filtered and
placed in inert atmosphere until further analysis. The isolated samples
are named as MA-2-Solvent (solvent = CH2Cl2, acetone, toluene,
EtOH, and MeOH).
Figure 2. X-ray crystal structure of MA-1-H2O showing (a) local
hydrogen bonding environments of one MA molecule (yellow); (b)
packing of the 3D framework.
spaces of the crystal structures (Figure 1). The decomposition
temperature for MA-1-H2O (190 °C) is much lower than that
of MA-2-DMF and MA-2-Toluene (250 °C) (Figure 1). NMR
studies of the dissolved MA-2-DMF and MA-2-Toluene show
that there exist an equivalent DMF solvent molecule and half
an equivalent toluene per melamine molecule within their
frameworks, respectively (Figure S3). The chemical formulas of
MA-1-H2O, MA-2-DMF, and MA-2-Toluene are determined
to be C3H6N6, C3H6N6·C3H7NO, and C3H6N6·0.5C7H8,
respectively, based on their single crystal structures, TGA and
NMR data.
RESULTS AND DISCUSSION
■
Single crystals of MA-1-H2O and MA-2-DMF were easily
recrystallized from H2O and DMF, respectively (Figure S1).
The single crystal of MA-2-Toluene was obtained by solvent
exchange of MA-2-DMF with toluene. Their phase purities
were confirmed by the powder X-ray diffraction patterns which
match well with their simulated ones from X-ray single crystal
structures (Figure S2). Thermogravimetric analysis (TGA)
indicates that there is no solvent molecule within MA-1-H2O.
The weight losses are 36.7% for MA-2-DMF and 26.7% for
MA-2-Toluene between 70 and 120 °C, corresponding to the
lost DMF molecules (calc. 36.6%) and toluene molecules (calc.
26.8%), respectively, which are encapsulated in their void
X-ray Crystal Structure of MA-1-H2O. The similar
structures of MA-1-H2O have been reported in previous
literature.35−37 The crystallization methods in the literature and
here are essentially identical (using water as solvent to
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Figure 4. X-ray crystal structure of MA-2-Toluene showing (a) the
local hydrogen bonding environments of MA molecules; (b) packing
of the framework in which the solvent toluene molecules (CPK
models) reside in the channels of 3D hydrogen-bonded organic
framework along the a axis.
Figure 3. X-ray crystal structure of MA-2-DMF showing (a) the local
hydrogen bonding environments of MA molecules; (b) three-
dimensional hydrogen-bonded organic framework; (c) packing of
the framework in which the solvent DMF molecules (CPK models)
reside in the channels of 3D hydrogen-bonded organic framework
along a axis.
Scheme 1. Basic Hydrogen Bonding Motifs within HOFs
MA-1-H2O and MA-2-DMF
recrystallize melamine). The reported structure of melamine
crystallizes in P21/a space group with slightly different unit cell
parameters (a = 10.54, b = 7.45, c = 7.25, β = 112.03)
compared with that of MA-1-H2O (Table 1), attributed to the
different crystallography setting. As shown in Figure 2a of MA-
1-H2O, each MA molecule connects with four neighboring MA
molecules through 8-fold hydrogen bonds to form a three-
dimensional HOF (Figure 2b). The dihedral angles between
planes of neighboring MA aromatic rings in MA-1-H2O are 34°
or 180°. MA-1-H2O is a condensed structure in which there is
no solvent molecule inside the pores. The hydrogen bonds and
dihedral angles between planes of neighboring MA in MA-1-
H2O and reported melamine are comparable.
X-ray Crystal Structure of MA-2-DMF. The X-ray crystal
structure of MA-2-DMF is completely different from that of
MA-1-H2O. Each MA molecule forms 9-fold hydrogen bonds
with six nearby MA molecules and one DMF molecule (Figure
3a). Close examination on the structure of MA-2-DMF reveals
that there exist two sets of double-layered MA molecules
orthogonal to each other (Figure 3b). The dihedral angles
between planes of neighboring MA aromatic rings in MA-2-
DMF are 90° or 180°. Packing diagram shows that the DMF
molecules reside in the two-dimensional channels of about 5.0
Å in diameter along a and b axes (Figure 3c).
X-ray Crystal Structure of MA-2-Toluene. The X-ray
crystal structure of MA-2-Toluene crystallizes in a space group
(I41), which is close to that of MA-2-DMF (I41/a). The local
environment of the MA molecule in MA-2-Toluene is similar
to that of the MA molecule in MA-2-DMF. The hydrophobic
toluene molecule interacts with MA molecule by a π−π
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Table 2. Summary of Hydrogen Bonding Motifs Revealed in MA HOFs and Hydrogen Bonding Interactions Details and
Geometries
motif
donor−H···acceptor
H···A (Å)
MA-1-H2O
D−H···A (Å)
angle (deg)
II
I
N3−H3A···N5i
N2−H2A···N4ii
N1−H1B···N6iii
2.22
2.08
2.20
3.06
3.01
3.08
162
175
163
I
symmetry codes
(i) 1/2 − x, −1/2 + y, 1/2 − z; (ii) −1 − x, 1 − y, −z; (iii) −x, −y, −z
MA-2-DMF
I
N5−H5B···N1i
N4−H4B···N3ii
N5−H5A···N2iii
N6−H6A···N2iv
2.19
2.12
2.32
2.40
3.05
2.98
3.09
3.22
178
178
148
161
I
II
II
symmetry codes
(i) 1/2 + x, y, 1/2 − z; (ii) −1/2 + x, y, 1/2 − z; (iii) 5/4 − y, 1/4 + x, 1/4 − z; (iv) 3/4 − y, 1/4 + x, 1/4 + z
MA-2-Toluene
I
N4−H4B···N9i
N2−H2B···N11ii
N1−H1A···N12iii
N3−H3A···N10iv
N1 −H1B···N8
N7iii−H7A···N8
2.08
1.99
2.87
2.07
2.58
2.25
3.00
2.90
2.97
2.97
2.90
3.12
168
177
88
I
I
I
177
103
165
II
II
symmetry codes
(i) y, 1/2 − x, 1/4 + z; (ii) 1 + y, 1/2 − x, 1/4 + z; (iii) 1/2 − y, x, −1/4 + z; (iv) 1/2 − y, −1 + x, −1/4 + z
Crystal Transformation by Solvent Soaking. Solvent-
induced crystal transformation experiments were conducted at
room temperature by soaking the crystals of MA-2-DMF in
different solvents for 3 days: acetone, CH2Cl2, toluene, EtOH,
and MeOH. The solid samples isolated after solvent soaking are
named as MA-2-Solvent (solvent = acetone, CH2Cl2, toluene,
EtOH, and MeOH). During the soaking experiments, no
obvious dissolution−precipitation process was observed in any
solvent.
Scheme 2. Schematic Illustration of Crystal−Crystal
Transformations of the Melamine Based HOFs
In toluene, the crystal sample of MA-2-DMF can keep its
crystallinity very well. Optical microscopy revealed that single
crystals did not exhibit any change in transparency or crystal
integrity after the soaking experiment. Single crystal X-ray
diffraction analysis revealed that the lattice parameters of MA-
2-toluene had changed to be a = 12.236 Å, b = 12.236 Å, c =
24.574 Å in I41, which is also slightly larger than the one of
MA-2-DMF (Table 1). The NMR data indicated that the
solvent exchange process can be finished in 48 h (Figure S3a).
More surprisingly, after immersion of the single crystal of MA-
2-toluene in DMF for 3 days, the NMR data showed that the
toluene molecules in MA-2-toluene can be reversely exchanged
by DMF (Figure S3b). Single crystal X-ray diffraction of the
crystal sample revealed that the framework reverted back to the
original I41/a unit cell. This reversible single crystal to single
crystal transformation (SCSCT) is very rare in HOF materials.
In acetone and CH2Cl2, although the crystal sample of MA-
2-DMF becomes opaque in 5 h, the acetone and CH2Cl2
exchanged MA-2 still keep the high crystallinity and have the
same basic similar structures as MA-2-DMF, as confirmed from
their PXRDs which match well with that of MA-2-DMF
(Figure S2). The NMR data also indicate that the DMF guests
in MA-2-DMF can be fully exchanged with acetone or CH2Cl2
(Figure S4). However, soaking MA-2-DMF in MeOH and
EtOH leads to completely different structures. Given the fact
that the PXRD patterns of MA-2-MeOH and MA-2-EtOH are
quite similar to that of MA-1-H2O, structures of MA-2-MeOH
and MA-2-EtOH might be close to that of the condensed MA-
1-H2O as well. Apparently, the much stronger hydrogen-
bonded interactions with MeOH/EtOH with melamine lead to
stacking interaction (3.2 Å in distance). The packing diagram
shows that the toluene molecules also reside in channels along
the a and b axes in the 3D hydrogen-bonded organic
frameworks (Figure 4b).
Basic Hydrogen Bonding Motifs. The detailed X-ray
crystal structural studies of these HOFs of MA enable us to
rationalize some basic hydrogen bonding motifs to connect MA
molecules. As summarized in Scheme 1, there are two basic
hydrogen bonding motifs (I) and (II). In motif I, the
neighboring MA molecules are parallel with each other; while
in motif II, the two nearby MA molecules are in orthogonal
positions. Some more structural features of the hydrogen
bonding interactions are listed in Table 2 as well.
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Article
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such framework transformations. The NMR data suggested that
no solvent molecules are included in the MA-2-MeOH and
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(Figure S4).
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In summary, two hydrogen-bonded organic frameworks
(HOFs) of the melamine (MA), MA-1-H2O and MA-2-DMF,
have been crystallized in H2O and DMF, respectively. The X-
ray crystal structural studies indicate that the formation of
different HOF isomers is attributed to the different hydrogen-
bonded patterns of melamine molecules: structurally, MA-1-
H2O and MA-2-DMF are condensed and porous, respectively.
The porous MA-2-DMF sustains the porous feature under the
solvent exchange by acetone, dichloromethane (CH2Cl2), and
toluene, while it transforms into the condensed state under
methanol (MeOH) and ethanol (EtOH). Furthermore, MA-2-
DMF and MA-2-Toluene can undergo reversible single
crystal−single crystal transformation with retention of the
porous hydrogen-bonded organic frameworks (Scheme 2). We
are working on the construction of even more robust melamine
based HOFs for their recognition of small molecules and their
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ASSOCIATED CONTENT
■
S
* Supporting Information
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X-ray crystallographic files (CIF), PDF of check-cif, power X-
ray diffraction analysis (PXRD), and NMR data of MA-1-H2O,
MA-2-DMF MA-2-Toluene, MA-2-Acetone, MA-2-CH2Cl2,
MA-2-MeOH, and MA-2-EtOH. This material is available free
of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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(25) Saied, O.; Maris, T.; Wuest, J. D. J. Am. Chem. Soc. 2003, 125,
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Corresponding Author
*E-mail: Banglin.Chen@utsa.edu. Fax: (1)-210-458-7428.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This project was funded by the Deanship of Scientific Research
(DSR), King Abdulaziz University, under grant No. (70-130-
35-HiCi). The authors, therefore, acknowledge technical and
financial support of KAU. This work was also partially
supported by the grant AX-1730 from the Welch Foundation
(B.C.).
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