Stitching 2D Polymeric Layers into Flexible 3D Metal-Organic Frameworks via a Sequential Self-Assembly Approach

Article abstract of DOI:10.1021/acs.cgd.5b01772

Two-dimensional (2D) coordination polymer [Zn(ATZ)₂]_n (HATZ = 5-amino-1H-tetrazole) featuring a 2D + 2D → 2D pillar-layer array was synthesized, wherein two honeycomb-shaped Zn(ATZ)_1.5 sublayers can be stitched together by dicarboxylate bridging linkers of varied length and type to generate 4 three-dimensional (3D) isoreticular noninterperpentrated frameworks under solvothermal conditions. The interpenetration behavior may be constrained to some extent by the pillar length because a 3D twofold interpenetrated architecture was obtained with a longer ligand using a similar process. The pillar-exchange process enabled the facile synthesis of a family of isoreticular metal-organic framework structures with different flexibilities and interpenetration behaviors through the judicious choice of the type and size of the pillar units. Thermal analysis indicated that [Zn(ATZ)₂]_n also possesses excellent thermostability at a high decomposition temperature up to 356 °C. The kinetic parameters of its exothermic process were studied by Kissingers and Ozawa-Doyles methods. Furthermore, its luminescent properties at room temperature were also studied in detail.

Full text of DOI:10.1021/acs.cgd.5b01772

Article
pubs.acs.org/crystal
Stitching 2D Polymeric Layers into Flexible 3D Metal−Organic
Frameworks via a Sequential Self-Assembly Approach
Hao Zhang,†,‡ Tianlu Sheng,† Shengmin Hu,† Chao Zhuo,†,‡ Haoran Li,†,‡ Ruibiao Fu,† Yuehong Wen,†
and Xintao Wu*,†
†State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, China
‡Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: Two-dimensional (2D) coordination polymer [Zn-
(ATZ)2]n (HATZ = 5-amino-1H-tetrazole) featuring a 2D + 2D → 2D
pillar-layer array was synthesized, wherein two honeycomb-shaped
Zn(ATZ)1.5 sublayers can be stitched together by dicarboxylate bridging
linkers of varied length and type to generate 4 three-dimensional (3D)
isoreticular noninterperpentrated frameworks under solvothermal con-
ditions. The interpenetration behavior may be constrained to some extent
by the pillar length because a 3D twofold interpenetrated architecture was
obtained with a longer ligand using a similar process. The pillar-exchange
process enabled the facile synthesis of a family of isoreticular metal−organic framework structures with different flexibilities and
interpenetration behaviors through the judicious choice of the type and size of the pillar units. Thermal analysis indicated that
[Zn(ATZ)2]n also possesses excellent thermostability at a high decomposition temperature up to 356 °C. The kinetic parameters
of its exothermic process were studied by Kissinger’s and Ozawa−Doyle’s methods. Furthermore, its luminescent properties at
room temperature were also studied in detail.
(3D) MOF.21 These unique advantages of the pillar-layer
INTRODUCTION
■
structure have led us to investigate this intriguing and
Over the last few decades, there has been continued interest in
challenging field.
the rational design and synthesis of coordination polymers
By employing the pillaring strategy, a large number of pillar-
(CPs) or metal−organic frameworks (MOFs) not only because
layer structures have been synthesized that usually adopt
of their applications in fields such as gas storage/separation,
polycarboxylates as grid-forming ligands and diamines or
catalysis, luminescence, and sensors but also because of their
diimines as pillars.31−33 However, interpenetration has been
fascinating topological structures.1−8 Generally, extended
found in these pillar-layered frameworks, especially when a
coordination framework solids allow for the judicious choice
large carboxylate is used.33 This phenomenon achieves close
of metal centers and organic linkers to obtain various
geometries and functionalities.9−11 Nonetheless, a number of
packing and reduces the accessible void, which is undesirable
for their applications.34,35 Although many interesting works
structural and experimental factors, such as the coordination
focusing on the control of the interpenetration of porous
geometry of the metal ions and ligands, pH, metal/ligand ratio,
MOFs have been reported, examples of the control of
and temperature, have been found to play important roles in
the construction of coordination polymers.12−16 Therefore, it is
interpenetration behavior of networks are rare, especially on
pillar-layered MOFs.36−38
still a challenge to rationally design and synthesize target CPs
and understand the rules of self-assembly.
In order to avoid the harmful interpenetrating behavior and
maximize the accessible space, 5-amino-1H-tetrazole (HATZ)
Regarding the synthesis strategy, the “pillar-layer” method
was chosen as the grid-forming ligand to prevent framework
(pillaring strategy), linking two-dimensional (2D) layers with
interpenetration on the basis of the following considerations:
appropriate pillars, represents one of the most effective
methods to construct MOFs with predictable structures.17−20
(i) According to previous reports, only N1 and N4 are
coordinated to metal centers (the Zn−ATZ−Zn angle is close
Furthermore, this research field is drawing more and more
to 145°, coincident with the Si−O−Si angle) and a tetrahedral
attention owing to the flexibility of pillar-layer structures, whose
framework is adopted that favors the generation of a
pillars can be substituted using a sequential self-assembly (SSA)
noninterpenetrating architecture.43,44 (ii) It can form various
approach to facilely control the dimensions of the channels and
the functionalities of the structures for target applications.21−30
Choe et al. reported the first example of the insertion of a
dipyridyl linker in two sequential steps to transform a 2D
layered MOF to a 2D bilayer and finally to a three-dimensional
Received: December 15, 2015
Revised: April 16, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.5b01772
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Scheme 1. (a) Formation of 3D MOFs from 2D Polymeric Layers by a Sequential Self-Assembly (SSA) Approach and (b)
Molecular Structure of the Co-ligands
Synthetic Procedures. Synthesis of [Zn(ATZ)2]n (A). A solution of
HATZ (730 mg, 8.68 mmol) in isopropyl alcohol (iPrOH, 50 mL) was
poured into a solution of Zn(OH)2 (400 mg, 4 mmol) in aqueous
ammonia (25%, 50 mL) and stirred for several minutes. The resultant
clear solution was left to evaporate slowly at room temperature.
Colorless crystals were collected after 3 days, washed with 3 × 10 mL
of water and then 3 × 10 mL of iPrOH, and air-dried. Colorless block
crystals were obtained and used for X-ray diffraction. Yield: 85%, based
on Zn(ATZ)2. Anal. calcd (%) for C2H4N10Zn (233.54): C, 10.29; H,
1.73; N, 59.98. Found: C, 10.25; H, 1.70; N, 60.03. IR (KBr, cm−1):
3359(s), 3189(m), 1659(s), 1577(s), 1474(m), 1330(w), 1165(m),
1084(s), 1006(m), 750(w), 493(m).
hydrogen bonds with its amino group and nitrogen atoms of
the tetrazolate ring to stabilize the open framework.45−47 (iii)
The combination of the tetrazolate and carboxylate organic
ligands has been rarely reported, and the conventional mixed-
ligand one-pot approach is still a challenge for the tetrazolate
system because the simultaneous incorporation of two
negatively charged bridges is limited by their competition not
only in binding metal ions but also in compensating the metal
charge.48−51 Therefore, the SSA approach (Scheme 1) offers a
new avenue for manipulating complex architectures from a
simple homoligand coordination polymer.
Fortunately, a 2D {66} pillar-layer array of [Zn(ATZ)2]n (A)
was first synthesized directly using the HATZ ligand. After
undergoing exchange with different dicarboxylate pillars, four
pillar-layer noninterpenetrated 3D frameworks of
{[(CH3)2NH2]0.5[Zn(L1)0.5(ATZ)1.5]·1.5H2O·DMF}n (A1),
{[(CH3)2NH2]0.5[Zn(L2)0.5(ATZ)1.5]·1.5H2O·0.5DMF}n (A2),
{[(CH3)2NH2]0.5[Zn(L3)0.5(ATZ)1.5]·H2O·0.5DMF}n (A3),
[(CH3)2NH2]0.5[Zn(L4)0.5(ATZ)1.5]n (A4) were obtained
from compound A as a layer precursor under solvothermal
conditions. Another 3D twofold interpenetrated network of
{[(CH3)2NH2][ZnL5ATZ]·DMF}n (A5) (H2L1 = terephthalic
acid, H2L2 = 2-aminoterephthalic acid, H2L3 = 2,6-naphthalene-
dicarboxylic acid, H2L4 = 2,2′-biphenyldicarboxylic acid, H2L5 =
4,4′-biphenyldicarboxylic acid, and DMF = N,N-dimethylfor-
mamide) using a longer pillar was also prepared. All structures
were evaluated by single-crystal X-ray diffraction and further
characterized by infrared spectroscopy (IR), elemental analysis
(EA), powder X-ray diffraction (PXRD), and thermogravi-
metric analysis (TGA). As a potential explosive material with a
high energy density, the kinetic parameters of the exotherm of
compound A were investigated in detail. In addition, the solid-
state luminescence properties of six CPs were also studied at
room temperature.
Preparation of Compounds A1−A5. A mixture of the H2L1−
H2L5 exchanged ligand (0.3 mmol, 1.0 equiv) and 16.5 mL of DMF−
H2O (v/v = 10:1) was stirred in a screw cap vial. After 30 min,
compound A (0.3 mmol, 1.0 equiv) was added to the above mixture.
Except for the preparation of compound A5, the other solution was
added with 10 drops of 50% HBF4, and the mixture was heated, kept at
85 °C for 3 days, and then slowly cooled to 30 °C at about 5 °C h−1.
{[(CH3)2NH2]0.5[Zn(L1)0.5(ATZ)1.5]·1.5H2O·DMF}n (A1). Colorless
block crystals were obtained and used for X-ray diffraction. Yield:
44%, based on Zn(ATZ)2. Anal. calcd (%) for C9.5H19N9O4.5Zn
(396.72): C, 28.76; H, 4.83, N; 31.78. Found: C, 28.66; H, 4.70; N,
31.80. IR (KBr, cm−1): 3343(s), 3225(m), 1657(s), 1561(s), 1464(w),
1443(w), 1387(m), 1102(m), 1014(w), 889(w), 750(m), 663(m),
483(m).
{[(CH3)2NH2]0.5[Zn(L2)0.5(ATZ)1.5]·1.5H2O·0.5DMF}n (A2). Pale yel-
low block crystals were formed in 62% yield based on Zn(ATZ)2. Anal.
calcd (%) for C8H16N9O4Zn (367.68): C, 26.13; H, 4.38; N, 34.28.
Found: C, 26.20; H, 4.35; N, 34.18. IR (KBr, cm−1): 3308(m),
3182(m), 2794(w), 2355(w), 1667(s), 1566(s), 1441(m), 1366(m),
1253(w), 1166(w), 1103(m), 1015(w), 777(m), 664(w), 476(m).
{[(CH3)2NH2]0.5[Zn(L3)0.5(ATZ)1.5]·H2O·0.5DMF}n (A3). Colorless
block crystals were formed in 52% yield based on Zn(ATZ)2. Anal.
calcd (%) for C10H15.5N8.5O3.5Zn (376.19): C, 31.93; H, 4.15; N,
31.65. Found: C, 31.77; H, 4.19; N, 31.52. IR (KBr, cm−1): 3300(m),
3184(w), 2357(w), 1656(s), 1556(s), 1393(m), 1350(w), 1253(m),
1192(w), 1095(m), 1015(w), 786(m), 668(w), 578(w), 476(m).
[(CH3)2NH2]0.5[Zn(L4)0.5(ATZ)1.5]n (A4). Colorless block crystals were
formed in 50% yield based on Zn(ATZ)2. Anal. calcd (%) for
C9.5H11N8O2Zn (334.65): C, 34.09; H, 3.31; N, 33.48. Found: C,
34.17; H, 3.29; N, 33.52. IR (KBr, cm−1): 3338(m), 3228(w),
2778(w), 2481(w), 1670(s), 1607(s), 1465(m), 1381(s), 1248(w),
1156(w), 1104(m), 1012(w), 847(w), 750(m), 668(w), 473(m).
{[(CH3)2NH2][ZnL5ATZ]·DMF}n (A5). Colorless block crystals were
formed in 49% yield based on Zn(ATZ)2. Anal. calcd (%) for
C17H18N6O4Zn (435.75): C, 46.86; H, 4.16; N, 19.29. Found: C,
46.77; H, 4.29; N, 19.12. IR (KBr, cm−1): 3312(m), 3184(w),
2779(w), 1672(s), 1597(s), 1534(s), 1383(s), 1253(w), 1176(w),
1104(m), 1014(m), 775(s), 678(m), 483(m).
EXPERIMENTAL SECTION
■
Materials and Methods. All reagents and solvents for the
syntheses were purchased from commercial sources and used as
received. All measurements except for elemental analysis were carried
out in air at room temperature. Elemental analysis (C, H, and N) was
performed on an Elementar Vario MICRO CHNOS elemental
analyzer. IR spectra with KBr pellets were recorded in the range
4000−400 cm−1 on a PerkinElmer Spectrum One FT-IR spectrometer.
PXRD data were collected on a Rigaku Desktop MiniFlexII
diffractometer using Cu Kα radiation (λ = 1.54056 Å) powered at
30 kV and 15 mA. Luminescent spectra were recorded with a Hitachi
F7000 fluorescence spectrometer.
B
DOI: 10.1021/acs.cgd.5b01772
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X-ray Crystallographic Study. Data collection of compounds A
and A1−A5 was performed on a Rigaku Saturn 724HG CCD
diffractometer equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) at room temperature. All structures were
solved by direct methods and refined by full-matrix least-squares on F2
using the SHELX-97 program.54 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were generated geometrically. The
SQUEEZE routine of the PLATON software suite was used to
calculate the contribution to the diffraction from the solvent region
and thereby produced a set of solvent-free diffraction intensities. The
final molecular formulas of compounds A and A1−A5 were estimated
from the results of PLATON−SQUEEZE combined with the data of
TGA and elemental analyses and were included in the molecular
formula directly. The pillared ATZ ligand in compound A, the NH2−
groups of the phenyl rings in compound A2, and the phenyl rings in
compound A5 were disordered and were refined using the atoms split
over two sites with a total occupancy of 1. The bond lengths of certain
Å, which are close to those reported for the Zn−N complexes.
Each ATZ ligand is bonded to two Zn(II) ions in the μ2-1,4
bridging mode to give a 2D planar sheet along the ab plane
containing 27-membered rings (Zn6C3N18) (Figure 1d). The
cavity of each Zn6C3N18 ring is filled with three amino groups
of the ATZ ligands. The intramolecular Zn(II)··· Zn(II)
distances in the Zn6C3N18 rings are 6.0728(20) and
5.8822(25) Å. Then, the neighboring sheets are pillared by
the ATZ ligand in the c-axis direction with a Zn(II)··· Zn(II)
distance of approximately 6.1798(29) Å to form a wavy 2D
bilayer (Figure 1c). The diffused electrons in the pillaring ATZ
ligand can be satisfactorily refined as twofold disordered ATZ−
(Figure 1b). Taking the Zn1 atoms as four-connected nodes,
the connections representing ATZ− anions, the single wavy 2D
layer of A can be rationalized to form a four-connected “sp”
topology with a short Schlafli symbol of {43.63} and a vertex
+
̈
disordered (CH3)2NH2 countercations in compounds A1−A4 were
symbol or long symbol of [4.6(2).4.6(2).4.6(2)] (Figure 1e).
Compounds A1−A4. X-ray single-crystal diffraction analysis
revealed that compounds A1−A4 are similar in structure;
hence, the results of compound A1 are first given in the ensuing
discussion. Compound A1 crystallizes in the orthorhombic
crystal system with space group Pnma. The asymmetric unit
consists of one crystallographically independent Zn(II) ion, one
and a half ATZ− anions, half of one deprotonated L1 ligand, half
of one (CH3)2NH2+ cation acting as a counteranion, one and a
half free water molecules, and one guest DMF molecule. In
addition, we noticed that the distorted (CH3)2NH2+ cations lie
inside the large solvent-accessible void, which is the byproduct
of in situ decomposition of the DMF solvent, thus leading to
the charge equilibrium. As shown in Figure 2, Zn1 is
restrained to be chemically reasonable using the DFIX command in
SHELXL-97. Notably, the N9 and N9a atoms of the NH2 groups in
compound A2 were refined by using the “‘ISOR’” restraint to make the
ADP values of the disordered atoms more reasonable. Crystallographic
data and structure determination summaries for compounds A and
A1−A5 are listed in Table S1.
Thermal Analyses and Nonisothermal Kinetics Study. TGA
was performed on a TGA/DSC 1 STARe system from room
temperature to 1000 °C at a heating rate of 10 K min−1 under
nitrogen. In the nonisothermal kinetics study, the peak temperature of
the exothermic stage in the DSC curve was obtained at different
heating rates of 5, 8, 10, 13, and 15 K min−1 under nitrogen.
RESULTS AND DISCUSSION
■
Description of the Crystal Structure. Compound A.
Compound A crystallizes in orthorhombic space group Cmcm.
X-ray single-crystal diffraction analysis revealed that there exists
one crystallographically independent Zn(II) ion, one intact
ATZ− anion, and two halves of a deprotonated ATZ ligand in
the asymmetric unit. As shown in Figure 1a, each central Zn(II)
ion is four-coordinate with a distorted [ZnN4] tetrahedral
geometry surrounded by four N atoms from four different ATZ
ligands. The Zn−N distances are from 1.984(5) to 2.0029(10)
Figure 2. View of the Zn(II) coordination environment of A1.
Symmetry codes: (A) −x, −y, 1 − z; (B) x, 0.5 − y, z; and (C) 0.5 − x,
−y, 0.5 + z. Some hydrogen atoms have been omitted for clarity.
coordinated by one oxygen atom from the carboxylate group
of L1 (Zn1−O1 = 1.939(5) Å) and three nitrogen atoms from
distinct ATZ− anions (Zn1−N2 = 1.990(7) Å, Zn1−N5 =
2.017(5) Å, and Zn1−N8C = 2.032(6) Å) to give the distorted
[ZnON3] tetrahedral geometry. Both Zn−N and Zn−O bond
lengths are well-matched with previously reported values.
If the connections through the pillared L1 ligand are ignored
initially, then the deprotonated ATZ− ligands link two Zn1
atoms in a μ2-1,4 bridging mode to give a wavy honeycomb-like
Zn-ATZ 2D extended structure along the a-axis direction with
Zn1−Zn1 distances of 6.1332(23) and 6.1377(35) Å (Figure
3b). These adjacent layers are joined together along the a-axis
direction by deprotonated bridging linkers, producing a 3D
network as shown in Figure 3a. The Zn(II)···Zn(II) distance
between the neighboring layers is 11.0746(59) Å. Additional
careful investigation of this 3D structure indicated that the 3D
Figure 1. (a) Zn (II) coordination environment of A. Symmetry
codes: (A) 1 − x, y, z; (B) x, y, 0.5 − z; and (C) 0.5 − x, 0.5 + y, z.
Some hydrogen atoms have been omitted for clarity. (b) Fragment of
1b showing two Zn atoms linked by a twofold disordered ATZ−
section. (c) View of the 2D bilayer net from the ac plane layer of A.
(d) 2D layer of A. (e) 2D uninodal four-connected network with “sp”
topology and point symbol {43.63} of A.
C
DOI: 10.1021/acs.cgd.5b01772
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Figure 3. (a) View of the 3D framework from the ac plane layer. (b) 2D grid layer of A1.
framework is also stabilized by weak intralayer N−H···O
hydrogen-bonding interactions (N3−H···O2, 2.862(5) Å; N4−
H···O2, 2.850(6) Å) via the amino groups donating H atoms to
the uncoordinated carboxylate oxygen atoms (Figure 2).
TOPOS analysis revealed that the 3D framework of compound
macrometallocycle that is linked through sharing ATZ ligands,
resulting in the wavy 2D sheet, [Zn(ATZ)1.5]n (Figures S1c and
S2c). Then, the neighboring sheets are pillared by the different
dicarboxylate ligand in longitudinal axes. Consequently,
compounds A1−A4 built from the same layered motif,
[Zn(ATZ)1.5]n, are linked by linear bridges into porous 3D
networks with a typical diamondoid network (Figures S1d and
S2d). Topologically, with a similar simplified rule, the overall
structures of compounds A1−A3 can be considered to have a
3D uninodal four-connected network with the same lon
topology and Pnma space group (Figure S1d). However, an
interesting phenomenon was found whereby compound A4
crystallizes in space group Pnnm and forms a “crb” topology
A1 can be rationalized to form a “lon” topology with a Schlafli
̈
symbol of {66} and a vertex symbol or long symbol of
[6(2).6(2).6(2).6(2).6(2).6(2)], which is typical of a diamond-
oid network (Figure 4). If the ATZ and L1 ligands are both
with a Schlafli symbol of {4.65} (Figure S2d), which may be
̈
caused by the special zigzag conformation of the L4 ligand. A
comparison of the cell parameters of compounds A1−A3
reveals that the a axis (along the bridging ligand) of compound
A3 is longer, commensurate with the relative longer bridge.
More importantly, the frameworks of compounds A1 and A2
exhibit two kinds of interconnected 1D rectangular-shaped
channels, with dimensions of 10.4 × 7.4 and 10.2 × 6.2 Å2
(Figure 5). The larger dimensions of the two channels for
compound A3 are calculated to be 12.8 × 6.2 and 12.2 × 6.6 Å2
(Figure S1e,f). However, compound A4 has two kinds of 1D
rectangular-shaped channels along only the [001] direction
with dimensions of 10.6 × 7.0 and 5.6 × 3.6 Å2 (the dimensions
were all measured without taking van der Waals radii into
consideration) (Figure S2e). The solvent-accessible volumes in
these dehydrated structures are 67.4 and 44.6% for compounds
A3 and A4, respectively, as calculated by PLATON (Table 1).
Compound A5. X-ray single-crystal diffraction analysis
shows that compound A5 crystallizes in the orthorhombic
crystal system with space group Pbca, whose asymmetric unit
includes one Zn(II) cation, two halves of one deprotonated L5
Figure 4. 3D uninodal four-connected network with “lon” topology
and point symbol {66} of A1.
considered two-connected spacers, then the Zn atom can be
simplified to a four-connected node. A computation of the
voids using PLATON suggests a value of 4488.0 Å3,
corresponding to 57.5% of the unit cell volume, which is
occupied by guest DMF molecules (Figure 5).
Compounds A2−A4 show a similar Zn (II) coordination
environment and pillar-layer structure as those of compound
A1 (Figures S1a and S2a). Their structures could be considered
isoreticular, differing only with respect to the pillared ligand. Six
ATZ ligands and six Zn(II) ions generate a 27-membered
+
ligand, one deprotonated ATZ ligand, and one (CH3)2NH2
anion, which are omitted for clarity. It is worth noting that the
charge of the whole framework in compound A5 is negative.
Therefore, we assume that there must be a disordered
+
(CH3)2NH2 countercation randomly lying inside the large
solvent-accessible void, which is the byproduct of in situ
decomposition of the DMF solvent, thus leading to the charge
equilibrium. The diffused electrons of the L5 ligand in the
structure can be satisfactorily refined as a twofold disordered
mode.
In the asymmetric unit, the Zn(II) cation is coordinated by
two carboxylic O atoms from two L5 ligands and two nitrogen
atoms from two ATZ ligands to form a distorted tetrahedron
geometry. The two carboxylate groups of the L5 ligand take the
same monodentate coordination mode, whereas the ATZ
ligand takes a μ2-1,4 bridging mode. The bond lengths of Zn−
Figure 5. Space-filling model of A1 showing two kinds of
interconnected 1D rectangular-shaped channels with dimensions of
(a) 10.4 × 7.4 Å2 (c axis) and (b) 10.2 × 6.2 Å2 (b axis).
D
DOI: 10.1021/acs.cgd.5b01772
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Table 1. Structrual Comparison of Compounds A1−A5
compound
interpenetration
space group
topology
a (Å)
accessible vol. (calcd; %)
A1
A2
A3
A4
A5
no
Pnma
Pnma
Pnma
Pnnm
Pbca
lon
lon
lon
crb
dia
28.13(3)
57.5
49.9
67.4
44.6
55.6
no
27.250(15)
31.928(17)
no
no
twofold
Figure 6. (a) Zn(II) coordination environment of A5. Symmetry codes: (A) −x, −y, −z; (B) −x, −y, 1 − z; and (C) 0.5 − x, −0.5 + y, z. Some
hydrogen atoms have been omitted for clarity. (b) View of a 1D zigzag chain in A5. (c) 3D single net of A5. (d) View of the 3D structure of A5
along the crystallographic b axis. (e) Space-filling model of the 3D single net from the bc plane layer. (f) View of the 3D twofold interpenetrating
framework from the bc plane layer. (g) 3D uninodal four-connected dia-type topological network with a Schlafli symbol of {66}.
̈
O vary in the range of 1.945(5)−1953(4) Å, and the Zn−N
bond distance is 1.995(5)−2.039(9) Å. All of these distances
and bond angles are within the normal ranges. The terminal
amino groups of each ATZ ligand donate H atoms to the O and
N atoms of the adjacent coordinated ligand to generate
intramolecular N−H···O (2.843(9) Å) and N−H···N (2.949(9)
Å) hydrogen bonds (Figure 6a). In the structure, neighboring
Zn(II) atoms are connected by ATZ ligands to generate 1D
zigzag chains that are stabilized by the above-mentioned N−
H···N hydrogen bonds (Figure 6b); the diagonal adjacent 1D
zigzag chains are bridged by L5 ligands, forming a 3D network
(Figure 6c). In the 3D network, 58-membered rings occur with
six Zn(II) ions, two ATZ ligands, and four L5 ligands. If ATZ
and L5 are considered to be connectors, then the Zn(II) centers
can be considered four-connected nodes. Thus, the topology of
the structure can be simplified as a 3D uninodal four-connected
ATZ ligand not only acts as grid-forming segments but also
links two layers to form the 2D + 2D → 2D network. Inspired
by this pillar-layer structure, we select various dicarboxylate
ligands as the exchanging pillars to obtain a series of isoreticular
3D pillar-layer coordination polymers (A1−A4) having the
same diamondoid network. For comparison, we used the longer
terephthalic acid to fabricate a similar pillar-layer network;
however, we obtained an interesting 3D twofold inter-
penetrated network. According to a report of the SSA approach,
this substitution was probably triggered by the threading of
dicarboxylate ligands through the layers and driven by the
thermodynamics of coordination.33 The result proves that the
SSA method is also constrained to linear pillars of an
appropriate length for isoreticular structures.
Surprisingly, we could not obtain compounds A1−A5 using
the same ratio of the ATZ ligand and zinc salt. Perhaps
compound A also plays a templating role in constructing the
pillar-layer structure. We also found that fluoroboric acid plays
an important role in the syntheses of compounds A1−A4 and
its absence leads to a large amount of precipitate, which is not
preferred for X-ray crystallographic study. As described above,
the SSA strategy is an efficient and controllable route to
construct and adjust a series of isoreticular CPs.
dia-type topological network with a Schlafli symbol of {66} and
̈
a
v e r t e x s y m b o l o r l o n g s y m b o l o f
[6(2).6(2).6(2).6(2).6(2).6(2)] (Figure 5g). Because of the
spaced nature of the single net, two identical nets mutually
interpenetrate each other to form a twofold interpenetrating
framework (Figure 6e,f). The calculated free volume of
compound A5 upon removal of guest solvent molecules is
55.6%, determined using PLATON.
Synthetic Discussion. Compound A was synthesized using
a solvent evaporation method, whereas compounds A1−A5
were synthesized using compound A as a precursor at 85 °C
under similar solvothermal conditions. In compound A, the
IR, PXRD, and Thermal Analyses. The broad absorption
bands at about 3300 cm−1 can be assigned to the characteristic
vibrations of O−H and N−H, suggesting the presence of water
molecules and the amino group of ATZ ligands. Compared
with free polycarboxylate acids, the absence of a strong
E
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Article
absorption at around 1700 cm−1 suggests the complete
deprotonation of the carboxylate ligands. The strong bands in
the range of 1650−1380 cm−1 are assigned as the asymmetric
and symmetric stretching vibrations of the carboxylate groups.
The bands at about 1580 cm−1 can be associated with υ(C
significantly higher than that of most of commonly employed
energetic materials such as HMX (287 °C), RDX (210 °C),
and TNT (244 °C);55,56 1D MOFs (CHP, 194 °C);57 2D
MOFs (CHHP, 231 °C);58 and 3D MOFs (ARTZ-1, 244 °C;
ARTZ-2, 257 °C).59 The relatively high thermal stability is
presumably caused by the strong structural reinforcement, and
the extensive network may act as an energy sink to suppress
decomposition.
N)/ring stretching vibrations plus δ(N−H)NH,NH of the ATZ
2
ligand.52,53 The IR spectra are in good agreement with the
single-crystal X-ray diffraction results.
As shown in Figure S9, the TGA curve of A1 displays a
weight loss of 6.6% (calcd, 6.8%) at 30−110 °C, corresponding
to the loss of one and a half free water molecules. Its second
weight loss of 19.2% occurs from 130 to 200 °C, which implies
the release of one guest DMF molecule (calcd, 18.4%). Above
this temperature, the framework begins to decompose, and the
remaining weight is ascribed to the formation of Zn(CN)2
(found, 28.7%; calcd, 29.6%). For compound A2, an initial
weight loss of 7.1% occurs between 30 and 100 °C due to the
loss of guest water (calcd, 7.3%); the second weight loss step
(found, 10%) begins at 140 °C and is ascribed to guest DMF
molecules. The framework then collapses and decomposes as
the temperature increases, and the remaining weight corre-
sponds to the formation of Zn(CN)2 (found, 32.5%; calcd,
31.9%). For compound A3, the weight loss of 15.4% from room
temperature to 200 °C is ascribed to the loss of free water and
DMF molecules (calcd, 14.5%). The framework then collapses
and decomposes as the temperature increases. Compound A4
bears a weight loss of 63.4% from 30 to 400 °C (calcd, 65.5%)
befor the network of A4 decomposes. For compound A5, the
loss of (CH3)2NH2+ countercation and guest solvent molecules
(found, 21.8%; calcd, 21.4%) is observed before 240 °C. Upon
further heating, rapid weight loss occurs due to the
decomposition of the compound. It should be noted that all
of the 3D MOFs (compounds A1−A5) show larger pores and
higher accessible surface areas; thus, they have lower thermo-
stability than that of compound A. Furthermore, the framework
of compound A exhibits a close-packed structure in which the
hydrogen and π···π* stacking interactions between ATZ ligands
have unique advantages in enhancing its stability. Selecting the
proper co-ligand to increase the stacking interactions between
ligands may provide a promising alternative to enhance their
stability.
The phase purity of the bulk samples was confirmed by
PXRD. As shown in Figures S3−S8, all of the peaks displayed
in the measured patterns closely match those in the simulated
patterns generated from single-crystal diffraction data, which
indicates that it is single phase. Additionally, compounds A1−
A4 can be stabilized for more than 3 days in HBF4 solution
(Figures S3−S8). To characterize the thermal stability of all of
these compounds, their thermal behavior was investigated by
TGA (Figure S9). The experiments were performed on samples
consisting of numerous single crystals of A and A1−A5 under a
nitrogen atmosphere at a heating rate of 10 °C min−1.
For compound A, the melting point is not observed in the
DSC plots and the DSC curve shows that one intense
exothermic process starts at 350 °C and ends at 385 °C, with
the peak at a temperature of 371 °C. The TG curve reveals that
compound A undergoes one mass-loss stage. An abrupt process
of weight loss is observed from 320 to 380 °C, which is
considered to be the collapse of the main framework and to
correspond to the intense exothermic process in the DSC
curve; the final product containing Zn(CN)2 with a residual
mass percentage of 51.1% is in good agreement with the
calculated value, 50.28% (Figure 7). Its thermal stability is
Nonisothermal Kinetics Analysis. Nitrogen-rich hetero-
cycles have usually been applied to synthesize high-perform-
ance explosives; thus, the nitrogen content of 82.3 wt % in the
HATZ ligand allows compound A to act as a novel high-energy-
density material.39−42 In order to investigate the thermokinetics
for the decomposition of A, we employed Kissinger’s method60
Figure 7. TG (red) and DSC (blue) curves of compound A.
Figure 8. Thermokinetics of the decomposition of compound A with (a) Kissinger’s and (b) Ozawa−Doyle’s methods.
F
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Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Article
and Ozawa−Doyle’s method61,62 to obtain the apparent
activation energy (E) and the pre-exponential factor (A) in
the current work, which are universally applied in this field,
using the peak temperatures measured at five different heating
rates of 5, 8, 10, 13, and 15 K min−1 (Figure 8). The Kissinger
and Ozawa−Doyle equations are as follows, respectively:
⎛
⎞
β
P
AR
E
E 1
ln⎜ ⎟ = ln
−
T2
R T
⎝
⎠
(1)
P
0.4567E
log β +
= C
RT
(2)
P
where Tp is the peak temperature (K), A is the pre-exponential
factor (s−1), E is the apparent activation energy (kJ mol−1), R is
the gas constant (J mol−1 K−1), β is the linear heating rate (K
min−1), and C is a constant. The apparent activation energies Ek
and E0, the pre-exponential factor Ak, and the linear coefficients
Rk and Ro were calculated using each method (where subscript
k is Kissinger’s method and subscript o is Ozawa−Doyle’s
method). The results of the two methods listed in Table 2
Figure 9. Emission spectra of all compounds in the solid state at room
temperature.
red-shifted (44 nm for A1 and 72 nm for A4). The difference in
the emission behavior for compounds A1 and A4 is probably
derived from the various dicarboxylate co-ligands, which may
affect the rigidity of solid-state crystal packing and further
influence their luminescence emission bands. Compound A3
shows an obvious blue shift of 25 nm compared with that of the
L3 ligand, but it exhibits an intense emission with two band
peaks that can be assigned to the naphthalene unit of the L3
ligand. Furthermore, compared with isoreticular compound A1,
the emission intensity of compound A2 becomes stronger,
which is probably due to the fact that the incorporation of N
atoms with lone-pair electrons promotes the production of
triplet excitons through an n−π* transition.
Table 2. Peak Temperatures of the Exothermic Stage at
Different Heating Rates and Kinetic Parameters
experimental data
β (K min−1
Tp (K)
calculated result
)
kinetic parameters
5
634.95
641.35
644.35
647.75
649.65
Ek (kJ mol−1
)
78.516
8
log Ak (s−1
)
7.889
0.9992
76.801
0.9993
10
13
15
Rk
E0 (kJ mol−1
)
Ro
CONCLUSIONS
■
We synthesized a simple 2D bilayer coordination polymer (A)
based on HATZ and then used it as a starting template to
obtain four 3D isoreticular noninterpenetrating MOFs (A1−
A4) and one 3D twofold interpenetrating stucture (A5) with
various aromatic dicarboxylate ligands. The investigation not
only illustrates that structural diversities of desired coordination
polymers can be achieved by employing a SSA approach but
also provides new examples for the control of the inter-
penetration behavior of pillar-layered networks. Thermal
analysis results also showed that compound A is a good
candidate for high-energy-density material applications because
it features superior thermostability with high decomposition.
The kinetic parameters of its exothermic process were studied
by Kissinger’s and Ozawa−Doyle’s methods. An investigation
of the fluorescent properties of these compounds revealed that
they may be potential blue fluorescent materials.
correspond well with each other, and they are also all in the
normal range for kinetic parameters of the thermal decom-
position of solid materials.63 Obviously, the exothermic peak,
Tp, shifts to a higher temperature as the heating rate increases,
and the linear correlation coefficients are very close to 1,
predicting that the results are credible. The value of Ea (the
average of Ek and E0) is 165.1725 kJ mol−1. The Arrhenius
equation for compound A is expressed as follows: ln k = 7.889
− 77.658 × 103/RT, which can be applied to estimate the rate
constants of the initial thermodecomposition processes.
Photoluminescent Properties. The luminescent proper-
ties of CPs with d10 metal centers and π-conjugated organic
linkers have attracted intense interests due to their potential
applications in chemical optical sensors and light-emitting
devices.64−66 In our work, photoluminescent properties were
investigated in the solid state at ambient temperature. On
complexation of organic ligands with Zn(II) ions, intense blue
emission was observed at 446 nm for A, 430 nm for A1, 440
nm for A2, 380 and 400 nm for A3, 430 nm for A4, and 427
nm for A5 under excitation at 330 nm (Figure 9). The free
HATZ ligand displays light photoluminescence with an
emission maximum at 325 nm, in accordance with previous
reports,67,68 whereas Zn(II) ions are difficult to oxidize or
reduce due to their d10 configuration. Taking the emission
bands of these free carboxylate co-ligands into consideration,
they may be assigned to a characteristic of intraligand emission,
as reported for other Zn(II) CPs constructed from mixed N
and O donor ligands.69−71 The maximum emission peaks of
compounds A and A5 are similar to those of the free co-ligands,
and the emission bands of compounds A1 and A4 are relatively
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.5b01772.
■
S
Additional structure figures, PXRD patterns, and TGA
curves (PDF)
Accession Codes
CCDC 1413530−1413535 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif or by e-
mailing data_request@ccdc.cam.ac.uk or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
G
DOI: 10.1021/acs.cgd.5b01772
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
(30) Song, J. L.; Zhao, H. H.; Mao, J. G.; Dunbar, K. R. Chem. Mater.
2004, 16, 1884−1889.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wxt@fjirsm.ac.cn.
Notes
■
(31) Lee, J. Y.; Olson, D. H.; Pan, L.; Emge, T. J.; Li, J. Adv. Funct.
Mater. 2007, 17, 1255−1262.
(32) Lee, J. Y.; Pan, L.; Huang, X. Y.; Emge, T. J.; Li, J. Adv. Funct.
Mater. 2011, 21, 993−998.
The authors declare no competing financial interest.
(33) Zhang, Z.-X.; Ding, N.-N.; Zhang, W.-H.; Chen, J.-X.; Young, D.
J.; Hor, T. S. A. Angew. Chem., Int. Ed. 2014, 53, 4628−4632.
(34) Hafizovic, J.; Bjørgen, M.; Olsbye, U.; Dietzel, P. D. C.; Bordiga,
S.; Prestipino, C.; Lamberti, C.; Lillerud, K. P. J. Am. Chem. Soc. 2007,
129, 3612−3620.
ACKNOWLEDGMENTS
■
We are thankful for financial support from the 973 Program
(2012CB821702 and 2014CB845603), the National Natural
Science Foundation of China (21233009 and 21203194), and
Fujian Province (2013J05039).
(35) Zhang, J.; Wojtas, L.; Larsen, R. W.; Eddaoudi, M.; Zaworotko,
M. J. J. Am. Chem. Soc. 2009, 131, 17040−17041.
(36) He, H.; Yuan, D.; Ma, H.; Sun, D.; Zhang, G.; Zhou, H.-C.
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