Solvent dependent structures of hydrogen-bonded organic frameworks of 2,6-diaminopurine

Article abstract of DOI:10.1021/cg500602x

Three solvent dependent structures of 2,6-diaminopurine in N,N′-dimethylforamide (DAP-1-DMF), water (DAP-2-H₂O), and methoxybenzene (DAP-3-CH₃OC₆H₅) have been structurally characterized. They exhibit different s

Full text of DOI:10.1021/cg500602x

Article
pubs.acs.org/crystal
Solvent Dependent Structures of Hydrogen-Bonded Organic
Frameworks of 2,6-Diaminopurine
Peng Li,† Osamah Alduhaish,† Hadi D. Arman,† Hailong Wang,† Khalid Alfooty,‡ 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, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia
S
* Supporting Information
ABSTRACT: Three solvent dependent structures of 2,6-diaminopurine in
N,N′-dimethylforamide (DAP-1-DMF), water (DAP-2-H2O), and methox-
ybenzene (DAP-3-CH3OC6H5) have been structurally characterized. They
exhibit different structures because of the different involvement of solvent
molecules in the hydrogen bonded frameworks. The DAP molecules tend to
be self-assembled with other DAP molecules through hydrogen bonding
interactions. DAP has very similar hydrogen bonding interaction patterns to
the established DAT group (2,4-diaminotriazinyl), underlying the potential of
this new unit for the construction of porous hydrogen bonded organic
frameworks.
Scheme 1. Chemical Structures of DAT Unit and DAP
Molecule
INTRODUCTION
■
Hydrogen bonding is essential in biological systems to assemble
complex supramolecules and direct their functionalities.1−3
Although the idea to make use of hydrogen bonding to construct
porous hydrogen bonded organic frameworks (HOFs) has been
proposed more than 20 years, little progress has been made to
design and stabilize porous HOFs to establish their permanent
porosities.4−12 Unlike traditional porous materials such as
zeolites13 and metal−organic frameworks (MOFs),14−17 HOFs
can be easily recovered through simple recrystallization. Further-
more, they have the advantages of solution process ability and
characterization and easy purification, making them very promising
functional materials for gas storage and separation. Recent
discoveries on a few HOFs for hydrogen storage18 and carbon
dioxide capture,19−23 acetylene storage24 and separation of C2H2/
C2H4,25 enantioselective separation of alcohols,26 and luminescent
applications27−29 are really encouraging. These studies have
established the feasibility to construct porous HOFs, although it
is much more challenging than constructing porous zeolites and
MOFs.
As revealed in Wuest’s pioneering work and our recent
discoveries, the 2,4-diaminotriazinyl (DAT) group is a very
powerful building unit to assemble and stabilize porous HOFs
because of their multiple sites for hydrogen bonding.25,26,30 The
success of this DAT unit for the construction of porous HOFs
has motivated us to explore new simple organic building units
to develop HOF chemistry. We speculated that 2,6-diaminopurine
(DAP) might have some similarity to DAT for involvement in
hydrogen bonding. As shown in Scheme 1, DAP also has multiple
sites for their potential hydrogen bonding interactions. Before
we incorporate such a basic unit into organic backbones to
synthesize a variety of organic linkers for the assembly of porous
HOFs, it is necessary to examine the basic hydrogen bonding
motifs of the DAP molecule. Recrystallization of DAP molecules
in N,N′-dimethylforamide (DMF), water (H2O), and methoxyben-
zene (anisole, CH3OC6H5) readily led to crystals of DAP-1-DMF,
DAP-2-H2O, and DAP-3-CH3OC6H5, respectively. Crystal
structures of these different solvent included DAP hydrogen
bonded organic frameworks do reveal some very basic
hydrogen bonding motifs, which are similar to those discovered
in DAT. It is also interesting that different solvents lead to
the different framework structures of DAP molecules
attributed to the different roles of solvent molecules in
the self-assembly of HOFs. Because the methoxybenzene
molecules are not involved in the hydrogen bonding
with DAP molecules, self-assembly of DAP molecules in
methoxybenzene solvent even forms a three-dimensional
HOF DAP-3-CH3OC6H5 with one-dimensional channels.
Herein we report the X-ray crystal structures of these three
solvent dependent hydrogen-bonded organic frameworks of
2,6-diaminopurine (DAP).
Received: April 28, 2014
Revised: May 27, 2014
Published: May 28, 2014
© 2014 American Chemical Society
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solution was cooled to room temperature and filtered. The filtrate was
allowed to evaporate at room temperature for 1 day. Light blue block
shaped crystals were obtained in 82% yield.
EXPERIMENTAL SECTION
■
All reagents and solvents were used as received from commercial
suppliers without further purification. FTIR spectra were performed
on a Bruker Vector 22 spectrometer at room temperature. Thermo-
gravimetric analyses (TGA) were measured using a Shimadzu TGA-50
analyzer under a nitrogen atmosphere with a heating rate of 5 °C min−1.
Powder X-ray diffraction (PXRD) patterns were recorded on 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 DAP-1-DMF,
DAP-2-H2O, and DAP-3-CH3OC6H5 were collected at 298(2),
298(2), and 293(2) K respectively, 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 solvent molecules in DAP-3-CH3OC6H5 are highly disordered.
The SQUEEZE subroutine of the PLATON software suit was used to
remove the scattering from the highly disordered guest molecules in
DAP-3-CH3OC6H5. The resulting new files were used to further refine
the structures. The H atoms on C atoms were generated geometrically.
X-ray crystallographic data is summarized in Table 1.
RESULTS AND DISCUSSION
■
Single crystals of DAP-1-DMF, DAP-2-H2O, and DAP-3-
CH3OC6H5 were easily recrystallized from DMF, H2O, and
CH3OC6H5, respectively. 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 S1,
Supporting Information). Thermogravimetric analysis (TGA)
indicates that different amounts of solvent molecules exist within
these frameworks, about 6.1% wt (from 60 to 150 °C), 10.5% wt
(from 130 to 150 °C), and 6.7% wt (from 70 to 140 °C), respec-
tively, in DAP-1-DMF, DAP-2-H2O, and DAP-3-CH3OC6H5
(Figure 1). NMR studies of the dissolved DAP-3-CH3OC6H5
Table 1. Crystallographic Data for the DAP-1-DMF, DAP-2-
H2O, and DAP-3-CH3OC6H5
identification
code
DAP-1-DMF
DAP-2-H2O
DAP-3-CH3OC6H5
empirical formula C11H20N8O2
C5H8N6O
168.17
C5H6N6
150.16
formula wt
temp, K
cryst syst
space group
a, Å
296.35
298(2)
monoclinic
P21/n
298(2)
293(2)
trigonal
triclinic
P1
̅
R3
̅
13.978(4)
8.0838(19)
13.895(4)
90.00
8.861(7)
9.424(6)
9.490(6)
69.25(6)
73.03(6)
73.75(6)
694.9(8)
4
18.711(3)
18.711(3)
10.843(2)
90.00
b, Å
c, Å
α, deg
Figure 1. TGA curves of DAP-1-DMF (blue), DAP-2-H2O (black),
and DAP-3-CH3OC6H5 (red).
β, deg
105.158(4)
90.00
90.00
γ, deg
vol, Å3
120.00
3287.3(9)
18
1515.4(7)
4
Table 2. Summary of Hydrogen Bonding Motifs Revealed in
These Three HOFs and Hydrogen Bonding Interaction
Details and Geometries
Z
Dcal (g cm−3
)
1.299
1.608
1.365
μ (mm−1
)
0.095
0.122
0.098
F(000)
632.0
352.0
1404.0
4517
motif
type
D−H
donor−H···acceptor (Å)
H···A
D−H···A
angle
(deg)
(Å)
(Å)
reflns collected
8040
3769
a
DAP-1-DMF
0.89
data/restraints/
params
2807/0/250
2701/0/265
1672/0/103
II
N1−H1A···N4i
N5−H5A···N2ii
N5−H5B···N3iii
2.11
2.08
2.08
3.14
3.12
3.12
143
170
150
GOF
0.992
0.999
1.015
III
III
0.96
final R indexes
[I > 2σ(I)]
R1 = 0.0577,
wR2 = 0.1309
R1= 0.0503,
wR2 = 0.1130
R1 = 0.0927,
wR2 = 0.2186
0.86
b
DAP-2-H2O
0.90
Final R indexes
[all data]
R1 = 0.0630,
wR2 = 0.1344
R1 = 0.0653,
wR2 = 0.1241
R1 = 0.0997,
wR2 = 0.2337
I
N5−H5B···N7i
N11−H11A···N1ii
N3−H3B···N8iii
N9−H9A···N2iv
2.13
2.15
1.98
3.03
3.04
2.86
2.92
175
175
170
169
I
0.89
largest diff. peak/ 0.54/−0.28
0.33/−0.28
0.53/−0.37
hole, e Å−3
II
II
0.89
0.85
2.08
c
DAP-3-CH3OC6H5
Synthesis of DAP-1-DMF. 2,6-Diaminopurine (DAP) (50 mg)
was dissolved in DMF (6 mL) 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 block
shaped crystals were obtained in 69% yield.
Synthesis of DAP-2-H2O. 2,6-Diaminopurine (DAP) (50 mg) was
dissolved in H2O (6 mL) 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 yellow block shaped
crystals were obtained in 91% yield.
III
III
IV
V
N6−H6A···N4iv
N6−H6B···N1v
N5−H5A···N1ii
N3−H3B···N2i
N5−H5B···N4iii
0.86
0.86
0.86
0.86
0.86
2.19
3.02
3.02
3.08
2.90
3.12
161
158
140
145
162
2.20
2.37
2.15
2.29
VI
a
Symmetry codes: (i) 1 − x, −y,1 − z; (ii) 3/2 − x, 1/2 + y,1/2 − z;
b
(iii) 3/2 − x, −1/2 + y,1/2 − z. symmetry codes (i) x, y, 1 + z; (ii) x,
c
y, −1 + z; (iii) 1 + x, y, z; (iv) −1 + x, y, z. symmetry codes (i) y,
Synthesis of DAP-3-CH3OC6H5. 2,6-Diaminopurine (DAP)
(50 mg) was dissolved in anisole (6 mL) under heating. The resulting
−x + y, −z; (ii) 1/3 − x + y, 2/3 − x, −1/3 + z; (iii) 1/3 + y, 2/3 − x + y,
2/3 − z; (iv) x − y, x, 1 − z; (v) y, −x + y, 1 − z.
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Figure 3. X-ray crystal structure of DAP-2-H2O showing (a) a two-
dimensional water incorporated framework sheet and the local
hydrogen bonding environments of DAP molecules and H2O and
(b) packing of the 3D framework.
considered as part of the framework, this HOF can be
considered as a three-dimensional water incorporated frame-
work in which the hydrogen bonding interactions between
water molecules link the two-dimensional sheets (shown in
Figure 3a) together (Figure 3b).
Figure 2. X-ray crystal structure of DAP-1-DMF showing (a) the local
hydrogen bonding environments of DAP molecules and DMF, (b)
two-dimensional hydrogen bonded organic framework, and (c)
packing of the framework in which the solvent DMF molecules reside
between the 2D hydrogen bonded organic framework sheets.
X-ray crystal structure of DAP-3-CH3OC6H5. The X-ray
crystal structure of DAP-3-CH3OC6H5 is completely different
from DAP-1-DMF and DAP-2-H2O because of the hydro-
phobic nature of CH3OC6H5 molecules, which excludes their
hydrogen bonding interactions with DAP molecules. That
enables DAP molecules to interact with nearby DAP molecules
in as many hydrogen bonding interactions as possible. As
shown in Figure 4a, each DAP molecule can link with eight
DAP molecules through weak hydrogen bonding interactions
to form a three-dimensional HOF without the involvement of
solvent molecules (Figure 4b). More interestingly, one-
dimensional channels even exist along the c axis (Figure 4c)
of about 3.8 Å.
Basic Hydrogen Bonding Motifs. The detailed X-ray
crystal structural studies of these three solvent dependent
HOFs of DAP enable us to rationalize some basic hydrogen
bonding motifs to connect DAP molecules. As summarized in
Scheme 2, the hydrogen bonding motifs I, II, and III are very
similar to those revealed in DAT based HOFs, underlying the
show that the solvent molecules within this framework are only
water molecules (Figure S3, Supporting Information).
X-ray Crystal Structure of DAP-1-DMF. As shown in
Figure 2, DAP molecules are self-assembled together to form a
two-dimensional HOF (Figure 2b). There exists one free −NH2
site that is not involved in the hydrogen bonding within DAP
molecules but interacts with DMF molecules through hydrogen
bonding (Figure 2a). The packing diagram shows that the DMF
molecules reside between the two-dimensional sheets of DAP
molecules (Figure 2c).
X-ray Crystal Structure of DAP-2-H2O. The stronger
preference of H2O molecules for their hydrogen bonding
interactions with DAP leads to its different framework structure
from DAP-1-DMF. As shown in Figure 3a, water molecules
even bridge several DAP molecules together. In fact, without the
considerations of water molecules, the framework of self-assembled
DAP molecule is a zigzag chain. Once water molecules are
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Scheme 2. Basic Hydrogen Bonding Motifs in DAP-1-DMF,
DAP-2-H2O, and DAP-3-CH3OC6H5 Molecules
Scheme 3. Potential New Building Unit for the Construction
of Porous Hydrogen Bonded Organic Frameworks
systematically tuned in order to realize their specific recognition of
small molecules and their applications.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic files (CIF), power X-ray diffraction
analysis (PXRD) and IR spectra of DAP-1-DMF, DAP-2-H2O,
■
S
Figure 4. X-ray crystal structure of DAP-3-CH3OC6H5 showing (a)
the local hydrogen bonding environments of DAP molecules, (b)
three-dimensional hydrogen bonded organic framework, and (c)
packing of the framework showing one-dimensional channels of about
of about 3.8 Å along the c axis.
1
and DAP-3-CH3OC6H5, and H NMR spectra of DAP-3-
CH3OC6H5 and DAP without solvent digested in DMSO-d6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: (1)-210-458-7428. E-mail: Banglin.Chen@utsa.edu.
■
feasibility of DAP unit for the synthesis of DAP containing
organic linkers for the construction of porous HOFs. Some
more structural features of the hydrogen bonding interactions
are listed in Table 2 as well, once again confirming the richness
of DAP molecules for their hydrogen bonding interactions.
In summary, X-ray crystal structural studies of 2,6-diaminopurine
(DAP) in different solvents indicate that DAP molecules are indeed
easily self-assembled with each other through hydrogen bonding
interactions. As revealed in the detailed hydrogen bonding motifs in
DAP HOFs, their intermolecular hydrogen bonding interaction
patterns are quite similar to those discovered within DAT
containing HOFs. That is really encouraging, because it shows
the promise to make use of this DAP unit (Scheme 3) to synthesize
a variety of DAP containing organic compounds to construct
porous HOFs. DAP can be considered as an expanded DAT unit,
so the porosities and pore surfaces of the resulting HOFs can be
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the grant AX-1730 from the Welch
Foundation (B.C.).
■
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