Syntheses, structures, and photoluminescent properties of a series of zinc(ii)-3-amino-1,2,4-triazolate coordination polymers constructed by varying carboxylate anions

Article abstract of DOI:10.1039/c3ce00049d

To explore the influence of different carboxylate ligands on the structures of metal-organic frameworks (MOFs), five novel Zn(ii) coordination polymer compounds, namely, {[Zn(atz)(bia)]}_n (1), {[Zn₃(atz) ₃(tpa)_3/2

Full text of DOI:10.1039/c3ce00049d

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Syntheses, structures, and photoluminescent properties
of a series of zinc(II)–3-amino-1,2,4-triazolate
coordination polymers constructed by varying
carboxylate anions3
Cite this: CrystEngComm, 2013, 15, 3261
Received 10th January 2013,
Accepted 14th February 2013
DOI: 10.1039/c3ce00049d
JinYing Gao,a Ning Wang,b XiaHua Xiong,c ChuJun Chen,a WeiPing Xie,a
XingRui Ran,a Yi Long,b ShanTang Yue*a and YingLiang Liud
www.rsc.org/crystengcomm
To explore the influence of different carboxylate ligands on the
structures of metal–organic frameworks (MOFs), five novel Zn(II)
coordination polymer compounds, namely, {[Zn(atz)(bia)]}n (1),
{[Zn3(atz)3(tpa)3/2]}n (2), {[Zn2(atz)2(pda)]}n (3), {[Zn4(atz)2(btc)]?
4H2O}n (4) and {[Zn2(atz)2(btec)]?H2O}n (5) have been successfully
obtained by introducing various secondary auxiliary ligands in
Zn(II)–L systems, characterized by elemental analysis, FT-IR,
thermogravimetric analysis (TGA), powder X-ray diffraction
(PXRD) and single-crystal X-ray diffraction (Hatz = 3-amino-1,2,4-
triazolate, Hbia = benzoic acid, H2tpa = 1,4-terephthalic acid,
H2pda = 1,4-benzenediacetic acid, H3btc = 1,3,5-benzenetricar-
boxylate, H4btec = 1,2,4,5-benzenetetracarboxylic). Compound 1
Introduction
Rational design and successful construction of metal–organic
frameworks (MOFs) has undergone tremendous development in
recent years owing to their intriguing variety of architectures and
captivating topologies as well as their potentially multi-field
applications in magnetism, gas storage, luminescence and
catalysis.1–6 The rapid development of MOFs with versatile
structures and desired properties is inevitable to involve appro-
priate organic ligands which play a key role in directing the
ultimate complex architectures. In addition, the solvent system,
molar ratio of reactants, pH values, temperature and metal ions
also influence the final architectures.7 Up to now, many studies
focused on the use of O or N-donor organic molecules and a large
number of rigid ligands have been employed extensively in
constructing functional coordination polymers because they
exhibit flexible coordination modes and afford more predictable
coordination modes, such as pyrazole, imidazole, tetrazole, and so
on.8 However, reports of MOFs constructed from tridentate
ligands are relatively rare.9 1H-1,2,4-triazole and its derivatives
which unite the coordination geometries of both pyrazole and
imidazole have attracted growing interest in coordination
chemistry because of the fact that the triazole ring with strong
s-donor and weak p-acceptor properties as well as their potential
m1,2-, m2,4- and m1,2,4-bridging fashions.
On the other hand, as an important family of multidentate
O-donor ligands, polycarboxylate organic ligands have been
witnessed to be excellent structural constructors due to their
various coordination modes to metal ions.10 The employment
of mixed ligands of polycarboxylates, which would cause the
structural diversity and construct interesting topologies.11
Moreover, a systematic investigation of the influence of
different aromatic carboxylates on the structures is valuable.
Taking all of the above into account, by introducing five
aromatic carboxylate ligands with different geometric shapes
(Chart 1) as auxiliary ligands into the Zn(II)–atz synthesis system
(Scheme 1), a series of novel MOFs with different structures
and dimensionalities, namely, {[Zn(atz)(bia)]}n (1), {[Zn3(atz)3
(tpa)3/2]}n (2), {[Zn2(atz)2(pda)]}n (3), {[Zn4(atz)2(btc)]?4H2O}n (4)
features
a
2D layer with (4.82) topology, which is further
stabilized by hydrogen bonds between the uncoordinated amino
groups and uncoordinated oxygen atoms from bia ligands.
Compound
2 exhibits a 3D 3,4-connected framework with
{4.82.103}{4.82} topology, constructed from Zn(II) ions and m3-atz
ligand. Compound 3 displays a 3D 3,4-connected network with
{4.6.8}{4.62.83} topology. Compound 4 possesses a 3D 9-nodal
network with
a
point symbol of {4.8.9}2{4.82}{4.83.9.10}
{5.8.9}4{5.82}. Compound 5 shows a 3D 3,4,4-connected frame-
work with {4.82}4{42.83.10}2{84.102} topology. The diverse struc-
tures of these five complexes demonstrate that the skeleton of
carboxylate ligands have a significant impact on the construction
of MOFs. Moreover, the luminescence properties of complexes 1–
5 were investigated in the solid state.
aSchool of Chemistry and Environment, South China Normal University, Guangzhou
510006, P. R. China. E-mail: yuesht@scnu.edu.cn; Fax: +86 20 39310187;
Tel: +86 20 39310187
bSchool of Materials Science and Engineering, Nanyang Technological University, 50
Nanyang Avenue, 639798, Singapore
cCollege of Material Science of Engineering, South China University of Technology,
Guangzhou, 510640, P. R. China
dCollege of Science, South China Agricultural University, Guangzhou 510642, P. R.
China
Electronic supplementary information (ESI) available: Contain the supplementary
3
crystallographic data for 1–5. PXRD patterns, table of selected bond distances and
angles for 1–5, the FTIR spectra figures for 1–5. CCDC 917345–917349. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce00049d
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Chart 1 Structures of the carboxylate ligands used in this work.
Scheme 1 Assembly of five Zn–atz compounds tuned by different carboxylate
ligands.
and {[Zn2(atz)2(btec)]?H2O}n (5), (Hatz = 3-amino-1,2,4-triazolate
Hbia = benzoic acid, H2tpa = 1,4-terephthalic acid, H2pda = 1,4-
benzenediacetic acid, H3btc
= 1,3,5-benzenetricarboxylate,
H4btec = 1,2,4,5-benzenetetracarboxylic), have been successfully
obtained under similar conditions. Their structures were
determined by single-crystal X-ray diffraction analyses and
further characterized by elemental analysis, infrared spectra
(IR), thermogravimetric analysis (TGA) and powder X-ray
diffraction (PXRD). On the basis of synthesis and structural
characterization, the structural and topological analyses of
these complexes as well as the effect of different aromatic
carboxylates on the ultimate frameworks are discussed in detail,
revealing that polycarboxylate ligand skeletons have a great
influence on the architectures of MOFs and can be used as a
tool to tune structures. In addition, the luminescence properties
of complexes 1–5 have also been investigated in the solid state
at room temperature.
diffractometer or Rigaku D/M-2200T automated diffractometer for
Cu Ka radiation (l = 1.54056 Å), with a scan speed of 4u min21 and
a step size of 0.02u in 2h range of 5–50u.
Synthesis of {[Zn(atz)(bia)]}n (1). A mixture of ZnSO4?6H2O
(143.8 mg, 0.5 mmol), 3-amino-1,2,4-triazole-5-carboxylic acid (38.5
mg, 0.3 mmol), Hbia (36.6 mg, 0.3 mmol) and water (10.0 mL) was
placed in a 23 mL Teflon-lined stainless steel vessel. The pH value
of the mixture was adjusted to 6 by triethylamine. The mixture was
sealed and heated at 160 uC for three days, and then cooled to
room temperature at a rate of 5 uC h21. White block-shaped
crystals suitable for X-ray analysis were obtained with 76% yield
based on Zn. Anal. calcd for C9H8N4ZnO2: C, 40.10; H, 2.30; N,
20.78%. Found: C, 40.12; H, 2.09; N, 20.77%. IR (cm21, KBr): 3387
(s), 3175 (m), 1607 (s), 1566 (s), 1395 (s), 1366 (m), 1307 (w), 1224
(m), 1077 (s), 853 (w), 718 (s), 688 (m), 647 (w).
Synthesis of {[Zn3(atz)3(tpa)3/2]}n (2). The process was similar
to 1 except that Hbia was replaced by H2tpa (33.2 mg, 0.3 mmol).
White block-shaped crystals suitable for X-ray analysis were
obtained with 41% yield based on Zn. Anal. calcd for
C12H10N8Zn2O7: C, 28.31; H, 1.98; N, 22.01%. Found: C, 28.35;
H, 2.02; N, 21.98%. IR (cm21, KBr): 3342 (s), 1643 (w), 1578 (s),
1407 (s), 1224 (m), 1042 (m), 883 (w), 836 (w), 753 (m).
Synthesis of {[Zn2(atz)2(pda)]}n (3). The process was similar to
1 except that Hbia was replaced by H2pda (33.8 mg, 0.2 mmol).
White block-shaped crystals suitable for X-ray analysis were
obtained with 64% yield based on Zn. Anal. calcd for
C14H14N8Zn2O4: C, 34.38; H, 2.88; N, 22.91%. Found: C, 34.40;
H, 2.87; N, 22.91%. IR (cm21, KBr): 3346 (s), 3175 (m), 2794 (w),
1596 (s), 1519 (w), 1366 (s), 1230 (m), 1060 (w), 947 (w), 883 (w), 794
(w), 729 (m), 653 (w), 602 (w).
Experimental section
Materials and general methods
All chemicals employed were commercially available and used as
received without further purification. Elemental (C, H, N) analyses
were performed on a Perkin-Elmer 2400 element analyzer. The
Hatz ligand in compounds 1–3 was derived from the decarboxyla-
tion of 3-amino-1,2,4-triazole-5-carboxylic acid in the solvothermal
process.12 The FT-IR spectra were recorded from KBr pellets in the
4000–400 cm21 ranges on
a Nicolet 5DX spectrometer.
Thermogravimetric analyses were performed on Perkin-Elmer
TGA 7 analyzer with a heating rate of 10 uC min21 in flowing air
atmosphere. Luminescence spectroscopy data were recorded on
an Edinburgh F900 FLS-900 spectrophotometer analyzer with a
xenon arc lamp as the light source. In the measurements of
emission and excitation spectra, the pass width is 5.0 nm. Powder
X-ray diffraction (PXRD) patterns were recorded on a X-pert
Synthesis of {[Zn4(atz)2(btc)]?4H2O}n (4). The process was
similar to 1 except that Hbia was replaced by H3btc (42 mg, 0.2
mmol). White crystals were obtained with 67% yield based on Zn.
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Anal. calcd for C22H18N8Zn4O15: C, 29.49; H, 2.02; N, 12.51%.
Found: C, 29.51; H, 2.03; N, 12.50%. IR (cm21, KBr): 3410 (s), 3151
(m), 2974 (w), 1613 (s), 1578 (s), 1437 (s), 1366 (s), 1213 (w), 1048
(m), 883 (w), 729 (s), 619 (w).
Synthesis of {[Zn2(atz)2(btec)]?H2O}n (5). The process was
similar to 1 except that Hbia was replaced by H4btec (507 mg, 0.2
mmol). White crystals were obtained with 54% yield based on Zn.
Anal. calcd for C14H12N8Zn3O13: C, 24.14; H, 1.74; N, 16.09%.
Found: C, 24.16; H, 1.73; N, 16.11%. IR (cm21, KBr): 3446 (m),
3351 (s), 2980 (w), 1585 (s), 1431 (m), 1389 (m), 1230 (m), 1048 (m),
883 (w), 847 (w), 806 (w), 594 (w).
Results and discussion
Structure of 1
The crystallographic analysis reveals that 1 crystallizes in the
orthorhombic Pbca space group. The asymmetric unit consists of
one unique Zn(II) atom, one atz ligand and one bia ligand. The
Zn(II) ion adopts a distorted tetrahedron coordination environ-
ment, which is completed by three nitrogen atoms from three
different atz ligands and one oxygen atom afforded by the
monodentate bia ligand (Fig. 1a). The Zn–N distances range from
1.996(7) to 2.032(7) Å and the Zn–O distance is 2.020(10) Å. The bia
ligand acts as an unexpected terminal coordinated ligand to
construct the distorted tetrahedron geometry. The atz ligand
adopts a m1,2,4-bridging mode, two neighboring Zn(II) centers are
connected by two pyrazole nitrogen atoms, forming a coplanar
6-membered hexagonal ring, and four symmetry-related Zn(II)
centers are linked by four imidazole nitrogen atoms to fabricate a
16-membered non-planar chair-like ring. The interconnection of
the two kinds of rings results in a 2D polymer network along the
ab plane (Fig. 1b). On the basis of the topological approach of the
simplification principle, the 2D polymer can be further simplified
by considering that the atz ligand and Zn(II) center both act as
3-connected nodes with a (4.82) topological network (Fig. 1c). The
uncoordinated oxygen atoms from bia ligands form hydrogen
bonds with uncoordinated amino groups, which has significant
impact on connecting the neighboring layers to generate a 3D
supramolecular architecture in an –ABAB– fashion (Fig. 1d).
Single-crystal structure determination
Data collections were performed at 298 K on a Bruker Smart Apex
II diffractometer with graphite-monochromated Mo KR radiation
(l = 0.71073 Å) for complexes 1–5. Absorption corrections were
applied by using the multiscan program SADABS. Structural
solutions and full-matrix least-squares refinements based on F2
were performed with the SHELXS-97 and SHELXL-97 program
packages, respectively.13 All non-hydrogen atoms were refined
anisotropically. The positions of hydrogen atoms were generated
geometrically. The organic hydrogen atoms were placed in
geometrically idealized positions and refined using a riding
model. Hydrogen atoms attached to water molecules were located
from difference Fourier maps and were also refined using a riding
model. Details of the crystal parameters, data collections, and
refinement for complexes 1–5 are summarized in Table 1.
Hydrogen-bonding data for complexes 1 and 4 are listed in
Table 2. Selected bond lengths and angles are shown in Table S1,
Structure of 2
ESI. Further details are provided in ESI.
3
3
To study the influence of the number of carboxylate groups on the
complex structure, H2tpa was selected in place of Hbia under
similar synthetic conditions and a new complex 2 was obtained.
Single-crystal X-ray diffraction analysis reveals that compound 2
crystallizes in the centrosymmetric tetragonal P4/nnc space group.
Table 1 Crystal data and structure refinement summary for compounds 1–5
Complex
1
2
3
4
5
Empirical formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C9H8N4ZnO2
269.58
296(2)
Orthorhombic
Pbca
9.639(4)
10.219(5)
21.866(10)
90
90
90
2153.8(17)
8
1.663
2.270
1088.0
1.281
0.0775
0.1818
1977/12/145
917345
C12H10N8Zn2O5
477.06
296(2)
Tetragonal
P4/nnc
12.700(3)
12.700(3)
25.364(5)
90
90
90
4091(2)
8
1.549
2.383
1904.0
1.091
0.0585
0.1833
1715/0/118
917346
C14H14N8Zn2O4
489.11
296(2)
Monoclinic
P21/c
10.203(6)
10.005(6)
18.287(10)
90
90.560(8)
90
1866.7(19)
4
1.740
2.609
984.0
C22H18N8Zn4O15
896.00
296(2)
Monoclinic
C2/c
22.579(3)
18.542(3)
13.6018(19)
90
92.614(2)
90
5688.6(14)
8
2.092
3.424
3568.0
1.039
0.0396
0.1000
5004/0/433
917348
C14H12N8Zn3O13
696.49
296(2)
Monoclinic
C2/c
7.9770(9)
16.763(2)
15.8519(19)
90
100.788(2)
90
2082.2(4)
4
2.222
3.521
1384.0
1.068
0.0781
0.2009
1835/611/169
917349
a (u)
b (u)
c (u)
V (Å3)
Z
Dcalcd (g cm23
)
m (mm21
F(000)
GOF
)
0.991
R1 [I . 2s(I)]a
0.0655
0.1860
3013/0/253
917347
wR2 (all data)b
Data/restraints/parameters
CCDC number
a
b
2
R1 = g||Fo| 2 |Fc||/g|Fo|. wR2 = {g[w(Fo 2 Fc2)2]/g(Fo2)2}1.
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dicarboxylate ligands in the formation of Zn–atz polymers, and
complex 3 was synthesized which exhibits a 3D entangled
architecture and crystallizes in the monoclinic P21/c space group
with an asymmetric unit that contains two crystallographically
independent Zn(II) ions, two atz ligands and one pda ligand
(Fig. 3a). Both Zn1 and Zn2 ions are four-coordinated with
distorted tetrahedron coordination geometries by three nitrogen
atoms deriving from three distinct atz ligands, and one oxygen
atom originating from a pda ligand. The Zn–N distances vary in
the range of 1.977(5) to 2.048(5) Å, and the Zn–O bonds are
1.960(5) and 2.004(5) Å. Like compound 2, the Zn(II) centers are
linked by atz ligands to form a 1D chain along the ab plane. These
1D zigzag chains are connected by tpa ligands to form 2D layers
(Fig. 3b). Furthermore, 2D layers are also linked by atz ligands to
form a 3D framework (Fig. 3c). To further understand the
structure, we define Zn(II) centers as 4-connected nodes, atz
ligands as 3-connected nodes, and pda ligands as bridge linkers.
Such 3D structures can be described in a 3,4-connected topology
Table 2 Hydrogen bond lengths/Å and angles/u for 1–2
…
…
…
Donor–H Acceptor
d(D–H)
d(H A)
/DHA
d(D A)
Compound 1
N4–H3 O2
…
0.86
2.01
150.1
2.789(16)
Compound 2
…
O2W–H2B O12
0.85
0.85
0.85
1.78
2.00
2.02
173.6
132.2
165.4
2.620
2.651(4)
2.847(4)
…
O1W–H1A O10
…
O3W–H3A O7
The asymmetric unit of 2 includes one crystallographically
independent Zn(II) atom, one tridentate atz ligand and one half
tpa ligand (Fig. 2a). The Zn(II) atom has a slightly distorted
tetrahedron geometry coordinated by three nitrogen atoms from
three atz ligands and one oxygen atom from the carboxylate group
of the tpa ligand. The Zn–N distances are ranging from 1.984(5) to
2.031(5) Å and the Zn–O distance is 1.947(5) Å. The structure of 2 is
best described in terms of layers formed by the bridging atz ligand
which presents the m3-1kN: 2kN: 4kN coordination mode (Fig. 2b).
Such layers are further pillared by the aromatic backbones of tpa
ligands to generate an overall 3D network (Fig. 2c). Furthermore,
topological analysis is carried out for the complicated network of
2. The Zn(II) atoms are simplified as 4-connected nodes, atz
ligands as 3-connected nodes, and tpa ligands as lines,
respectively. As a result, the 3D framework of 2 can be simplified
2
3
¨
with Schlafli symbol {4.6.8}{4.6 .8 } (Fig. 3d).
Structure of 4
Compared with the former complexes, H3btc, which has three
carboxylate groups, was introduced in place of H2tpa to investigate
the role of the number of carboxylate groups in the formation of
Zn–trz polymers, then complex 4 was synthesized. Single-crystal
X-ray crystallographic analysis reveals that 4 crystallizes in the
monoclinic system with space group P21/c and exhibits a 3D
framework. The asymmetric unit of 4 is composed of four
crystallographically independent Zn(II) centers, two dissimilar atz
ligands, two btc ligands and three coordinated water molecules
(Fig. 4). The four Zn(II) atoms are all in the tetrahedral
2
3
2
¨
as a 3,4-connected topology with Schlafli symbol {4.8 .10 }{4.8 }
(Fig. 2d).
Structure of 3
Compared with complex 2, the flexible H2pda ligand was
introduced in place of H2tpa to investigate the role of flexible
Fig. 1 (a) Coordination environment of complex 1, all hydrogen atoms were omitted for clarity [symmetry code: a = 0.5 + x, 0.5 2 y, 1 2 z; b = 0.5 2 x, 20.5 + y, z]; (b)
a polyhedral view of the 2D layer structure constructed of 1D chains; (c) the topological (4.82) net of the 2D layer; (d) the supermolecular 3D framework constructed
by intermolecular hydrogen bonds.
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Fig. 2 (a) Coordination environment of complex 2, all hydrogen atoms were omitted for clarity [symmetry code: a = 20.5 + x, 1 2 y, 1 2 z; b = 2 2 x, 2y, 1 2 z; c = 1.5
2 x, y, 1.5 2 z]; (b) a polyhedral view of the 2D layer structure constructed of 1D chains; (c) the supermolecular 3D framework constructed by H2tpa; (d) hexagon-like
net with the topological notation {4.82.103}{4.82}. All amino groups were omitted for clarity in (b) and (c).
coordination geometry. Both Zn1 and Zn4 centers are coordinated
to one nitrogen atom deriving from the atz ligand, two carboxylate
oxygen atoms from two separate btc ligands, and one oxygen atom
from the terminal coordinated water. While Zn2 is completed by
two nitrogen atoms from two atz ligands, one oxygen atom from
the monodentate carboxylate of btc ligand and one oxygen atom
from the terminal coordinated water. Zn3 is surrounded by two
nitrogen atoms from two atz ligands and two oxygen atoms
afforded by two btc ligands. The Zn–N bond lengths vary from
1.977(4) to 1.999(4) Å, and Zn–O bond lengths vary from 1.910(3)
Fig. 3 (a) Coordination environment of complex 3, all hydrogen atoms were omitted for clarity [symmetry code: a = 2 2 x, 0.5 + y, 0.5 2 z; b = 1 + x, 0.5 2 y, 0.5 + z; c
= 1 2 x,20.5 + y, 0.5 2 z]; (b) a polyhedral view of the 2D layer structure constructed of 1D chains; (c) the supermolecular 3D framework constructed by H2pda; (d)
diamond-like net with Schlafli symbol {4.6.8}{4.62.83}. All amino groups were omitted for clarity in (b) and (c).
¨
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is performed on complex 4, the Zn(II) centers are treated as 3 or
4-connected nodes, atz ligands as 3-connected nodes, and btc
ligands as 3-connected nodes, the 3D structure can be described
2
3
¨
as 9-nodal net with a Schlafli symbol of {4.8.9}2{4.8 }{4.8 .9.10}
{5.8.9}4{5.82} (Fig. 5c). The infinite 3D architectures exhibit a
layered arrangement, in which O–H–O type hydrogen bond
interactions have been observed due to the interaction between
the coordinated water molecules and uncoordinated carboxylato-
oxygen atoms playing a key role in the supramolecular architecture
(Fig. 5d).
Structure of 5
Single-crystal X-ray crystallographic analysis reveals that 5 crystal-
lizes in the monoclinic system with the space group P21/c and
exhibits a 3D framework. The asymmetric unit of 5 contains two
crystallographically independent Zn(II) atoms, two atz ligands, one
completely deprotonated btec and one coordinated water (Fig. 6).
It can be clearly seen that Zn1 is coordinated to two nitrogen
atoms from symmetry-related atz ligands, and two oxygen atoms
from symmetry-related carboxylate groups of two btec ligands,
displaying a distorted tetrahedron geometry. While Zn2 displays a
distorted octahedral coordination geometry, which is provided by
two nitrogen atoms from two different atz ligands, and three
oxygen atoms from two carboxylate groups of two btec ligands as
well as one oxygen atom from a terminal coordinated water
molecule. The atz ligand holds two Zn(II) ions together and is
responsible for the formation of the 1D ribbon structure. These 1D
ribbons are further extended by btec ligands to generate a 3D
framework. The Zn–N bond lengths in the range of 1.973(7) to
2.031(6) Å, and the Zn–O bond lengths range from 1.966(7) to
Fig. 4 Coordination environment of complex 4, all hydrogen atoms were
omitted for clarity [symmetry code: a = 20.5 + x, 0.5 + y, z; b = 0.5 2 x, 0.5 2 y, 1
2 z; c = 2x,2 y, 1 2 z; d = 20.5 + x, 20.5 + y, z; e = 0.5 2 x, 20.5 2 y, 1 2 z].
to 2.075(4) Å. All the atz ligands show uniform m1,2,4-bridging
modes, connecting Zn2 and Zn3 ions in the m1,2-mode, Zn1 and
Zn2 as well as Zn3 and Zn4 in m1,4-modes, respectively. Each bta
ligand links three Zn(II) centers through three carboxylate groups,
then the Zn(II) centers are linked via btc and atz ligands into an
infinite ziplike chain. Thus, the interconnection of the two kinds
of chains results in a 2D polymeric network (Fig. 5a). It should be
noted that the btc ligand can only link the adjacent two layers into
an infinite 3D architecture by the monodentate carboxylate
(Fig. 5b). To further understand the structure, topological analysis
Fig. 5 (a) A polyhedral view of the 2D layer structure constructed of 1D chains for 4; (b) two layers constructed by H3btc; (c) 4-nodal (3,3,3,4)-connected framework
with Schlafli symbol {4.8.9} {4.82}{4.83.9.10}{5.8.9} {5.82}, red: Zn1; pink: Zn2; green: Zn3; yellow: Zn4. (d) The supermolecular 3D framework constructed by hydrogen
¨
2
4
bonds. All amino groups were omitted for clarity in (a), (b) and (d).
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Influence of carboxylates on the structures of the complexes
In light of the structure description, a novel family of Zn(II)
compounds have been successfully synthesized under similar
conditions except the used polycarboxylate ligands. On the basis of
the X-ray analysis results, the crystal structures of these five
compounds ranging from 2D layers to 3D coordination architec-
tures indicate that the series of different polycarboxylate ligands
are critical factors in the formation of such coordination
architectures, and previous reports have mentioned the role of
organic carboxylate anions.14 Generally, the role of polycarboxylate
ligands can be explained in terms of their differences in the
number of carboxylate groups, the lengths of the spacer and the
flexible abilities. Complexes 1 and 2 are mainly attributed to the
effect of the spacer group of the carboxylate ligands, complex 1 is a
2D layer by using a bia ligand which has only one carboxylate
group. When H2tpa with a rigid spacer and an angle of 180u
between the two carboxylic groups acting as a linker is introduced
into the synthetic procedure, the 3D 4-connected structure of 3 is
obtained. Complexes 2 and 3 imply the influence of the spacer
length and flexibility of the carboxylate ligands on the resulting
frameworks. Although both H2tpa and H2pda are based on
benzene spacers, the length and flexibility of the spacers are
different. H2pda possesses flexibility owing to the presence of the –
CH2– spacer, which allows the two carboxylate groups to bridge
the metals from different directions. Obviously, the structural
difference of compounds 4 and 5 is mainly attributed to the
different angles between the carboxylate groups in each ligand.
Fig. 6 Coordination environment of complex 5, all hydrogen atoms were
omitted for clarity [symmetry code: a =2 2 x, y, 0.5 2 z; b = x, 2y, 20.5 + z; c = 2
2 x, 1 2 y, 1 2 z; d = 1 + x, y, z; e = 1.5 2 x, 0.5 2 y, 1 2 z; f = 1 2 x, 2y, 1 2 z; g
= 2x, 2y, 1 2 z; h = 21 + x, y, z].
2.276(6) Å. The Zn(II) centers are linked by btec and atz ligands to
form a infinite chain (Fig. 7a). These infinite chains are connected
by atz ligands to form 2D layers (Fig. 7b). Furthermore, another
two carboxylate groups of btec ligands show a chelate–bidentate
fashion to connect the 2D layers to generate a 3D pillar-layered
framework (Fig. 7c). In order to better understand the final
architecture of the complex, the dinuclear Zn(II) unit, which is
linked by carboxyl groups, can be defined Zn atoms as
4-connected nodes, atz ligands as 3-connected nodes, btec ligands
as 4-connected nodes. In this way, the 3D structure can be
¨
a Schlafli symbol of
described as 3,4,4-connected with
{4.82}4{42.83.10}2{84.102} (Fig. 7d).
Fig. 7 (a) The Zn(II) centers are linked by btec and atz ligands to form an infinite chain for 5; (b) the atz ligands as pillars to extend the 1D chains to be 2D net; (c) the
supermolecular 3D framework constructed by H btec; (d) 4,4-connected framework with Schlafli symbol {4.82} {42.83.10} {84.102}. All amino groups were omitted for
¨
4
4
2
clarity in (a), (b) and (c).
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Fig. 10 TGA curve of compound 3.
Fig. 8 TGA curve of compound 1.
Thermal analysis and PXRD patterns
loss of free water molecules (observed, 7.72%; calculated, 7.75%),
and then a plateau region follows until 390 uC. The further weight
losses represent the decomposition of the ligands.
In order to characterize the complexes more fully in terms of
thermal stability, TGA experiments were investigated on poly-
crystalline samples in the temperature range of 30–800 uC with a
heating rate of 10 uC min21 in dry air atmosphere (Fig. 8–12). For
compounds 1 and 3, no weight loss is observed until it reaches 320
and 310 uC, respectively, corresponding to the decomposition of
the organic ligands. For compound 2, the weight loss from 200 to
245 uC is consistent with the loss of three crystal-lattice water
molecules (observed, 11.34%; calculated, 11.31%). Then no
significant weight loss is observed until the decomposition of
the framework occurs at 355 uC. Compound 4 undergoes two
weight loss steps. The first weight loss is observed in the range of
120–225 uC, which is attributed to the release of three coordinated
water molecules (observed, 5.98%; calculated, 6.03%), and then
the decomposition of the ligands at 390 uC. The TGA curve of 5
shows a weight loss process under 225 uC, corresponding to the
X-Ray powder diffraction (PXRD)
PXRD experiments were carried out for 1–5 to confirm whether the
crystal structures are truly representative of the bulk materials (Fig.
S1–S5, ESI ). By comparison, the experimental patterns and the
3
simulated patterns are in good agreement with each other, which
confirms the phase purity of the as-synthesized products.
Luminescent properties
Luminescent properties of compounds containing d10 metal
centers are of intense interest due to their potential applications,
such as in chemical sensors, photochemistry and electrolumines-
cent display, and so on.15 Hence, the solid-state luminescent
properties of complexes 1–5, together with the free Hatz ligand,
Fig. 9 TGA curve of compound 2.
Fig. 11 TGA curve of compound 4.
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Fig. 12 TGA curve of compound 5.
Fig. 14 Solid-state emission spectra at room temperature for 1, 2, 3.
have been determined at room temperature (Fig. 13 and 14). The
free ligand Hatz exhibits an emission band at 411 nm (lex = 325
nm). Complexes 1–5 show broad emission bands with the
attributed to the coordination environment around them and even
the structural topologies. These compounds may be good blue-
light emitting candidate materials, since they display lumines-
cence in the blue region.18
maximum peak at 343 nm (lex = 297 nm) for 1, 385 nm (lex
=
314 nm) for 2, 354 nm (lex = 274 nm) for 3, 421 nm (lex = 327 nm)
for 4 and 447 nm (lex = 366 nm) for 5. Compared with free Hatz
ligand, the emissions for 1, 2 and 3 are blue-shifted by 68 nm, 26
nm and 57 nm, respectively. The results may be attributed to
intraligand luminescent emission, and the lightly blue-shifted may
be attributed to the chelating of the Hatz ligand-to-metal charge
transfer (LMCT), which effectively increases the rigidity of the
ligand and reduces the loss of energy by radiationless decay of the
intraligand emission excited state.16 While the emissions for 4 and
5 are slightly red-shifted by 10 nm and 36 nm, respectively, which
may be assigned to metal-to-ligand charge transfer (MLCT), and
similar red shifts have been observed before.17 From above
discussion, all the complexes with the same Zn(II) center and
N-donor ligand show different behaviors, which are probably
Conclusion
In summary, five novel coordination polymers of Zn(II)–atz have
been successfully synthesized under hydrothermal reactions
assisted with a series of auxiliary carboxylate ligands. They show
diverse structures and dimensionalities from 2D network to 3D
coordination architecture. This investigation demonstrates that
the number of the carboxylate groups, the length and flexibility of
carboxylate ligands play significant roles in constructing the
structures of coordination polymers. Therefore, a new paradigm
for further rational design and synthesis of new entangled MOFs
can be accomplished by careful control of auxiliary ligands. The
thermogravimetric analyses show that complexes 1–3 exhibit high
thermal stability. Furthermore, their interesting luminescent
behavior indicates that these complexes may be suitable as
excellent candidates for optical materials.
Acknowledgements
We are thankful for financial support from NSFC (Grants
20971047, U0734005 and 21271076), and Key Research
Program of Guangdong Provincial Universities Science and
Technology innovation (Grant cxzd1020).
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