Temperature-controlled assembly of two fluorescent ZnII polymers from 3D pillared-layer framework to 2D (4, 4) layer

Article abstract of DOI:10.1016/j.inoche.2010.11.015

Two ternary coordination polymers with the same ingredients but different structures, [Zn₂(datrz)₂(nip)]_n (1) and {[Zn(Hdatrz)(nip)]?H₂O}_n (2) (Hdatrz = 3,5-diamino-1,2,4-triazole; nip^2- = 5-nitroisophthalate), were respectively obtained by temperature-controlled hydrothermal reactions and fully structural characterized. Interestingly, tetrahedral Zn^II ions in 1 are linked into wavy layers by anionic μ₃-N1,N2,N4-datrz ligands, which are further supported by deprotonated bis-monodentate nip^2- to generate a 3D pillared-layer framework. In contrast, the tetrahedral Zn ^II ions in 2 are aggregated into a (4, 4) covalent layer through alternate μ₂-N1,N4-Hdatrz and bis-monodentate nip^2- bridges. Thus, the competitive binding of the two polydentate ligands to the metal ion plays essential roles on the connectivity and dimensionality of the two complexes. Thermal stability and the fluorescent emissions of the two complexes were also investigated and compared.

Full text of DOI:10.1016/j.inoche.2010.11.015

Inorganic Chemistry Communications 14 (2011) 285–287
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Temperature-controlled assembly of two fluorescent ZnII polymers from 3D
pillared-layer framework to 2D (4, 4) layer
⁎
⁎
En-Cui Yang , Tian-Yu Liu, Qian Wang, Xiao-Jun Zhao
College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Tianjin Normal University, Binshuixi Road 393, Xiqing District, Tianjin 300387, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two ternary coordination polymers with the same ingredients but different structures, [Zn2(datrz)2(nip)]n (1) and
{[Zn(Hdatrz)(nip)]·H2O}n (2) (Hdatrz=3,5-diamino-1,2,4-triazole; nip2−=5-nitroisophthalate), were respec-
tively obtained by temperature-controlled hydrothermal reactions and fully structural characterized. Interestingly,
tetrahedral ZnII ions in 1 are linked into wavy layers by anionic μ3-N1,N2,N4-datrz ligands, which are further
supported by deprotonated bis-monodentate nip2− to generate a 3D pillared-layer framework. In contrast, the
tetrahedral ZnII ions in 2 are aggregated into a (4, 4) covalent layer through alternate μ2-N1,N4-Hdatrz and bis-
monodentate nip2− bridges. Thus, the competitive binding of the two polydentate ligands to the metal ion plays
essential roles on the connectivity and dimensionality of the two complexes. Thermal stability and the fluorescent
emissions of the two complexes were also investigated and compared.
Received 10 September 2010
Accepted 17 November 2010
Available online 1 December 2010
Keywords:
Assembly
Pillared-layer framework
3,5-Diamino-1,2,4-triazole
5-Nitroisophthalate
© 2010 Elsevier B.V. All rights reserved.
Controllable self-assembly of metal-organic frameworks (MOFs)
with the same ingredients but different structures has recently
attracted wide-spread interest due to their potential applications in
magnetism, adsorption, separation, transportation of gases and
liquids as well as catalysis [1–3]. Structurally, these interesting
complexes and their ir-/reversible inter-conversions among different
structures can be carefully manipulated by tuning of some external
factors, such as the ratio of reactants [2,4], solvent [5–8], temperature
and pressure [9,10], pH value [11,12], uncoordinated counter anions
[13–16], and template/guest molecules [17–19]. Undoubtedly, all
these structural control strategies significantly depend on the nature
of the ligands (steric hindrance, cation binding sites, and modes) and
variable coordination polyhedrons of the metal ions. In this regard,
both triazole and polycarboxylate ligands have multideprotonated
forms [20–22], flexible metallic bonding sites [20,21,23] as well as
abundant binding modes [20,21,24] and their combination as ligands
can potentially produce some unpredictable interesting structures. On
the other hand, some inorganic metal ions, such as Cd2+, Zn2+, Cu2+
and so on, can potentially exhibit various coordinated polyhedrons
(tetrahedron, trigonal bipyramid, and octahedron), and easily meet
the binding requirement of the polydentate ligands. Herein, as a
continuation of the structural construction and property investiga-
tions on the functional MOFs [22,25], controllable hydrothermal
reactions of 3,5-diamino-1,2,4-triazole (Hdatrz), 5-nitroisophthalic
acid (H2nip) and ZnII ion were performed. As a result, two novel
polymers with same components and different structural motifs,
three-dimensional (3D) pillared-layer framework for 1 and 2D (4, 4)
covalent layer for 2, were generated by varying the temperature of
hydrothermal reactions. Structural determinations suggest that the
competitive binding of the mixed ligands towards the tetrahedral ZnII
ion changed the spatial arrangement of the mixed ligands and tuned
the overall dimensionality of the resulting complexes.
The two target complexes were obtained in a molar ration of 2:1:2
(Hdatrz:H2nip:ZnII) by decreasing the hydrothermal temperature
[26]. Elemental analysis, FT-IR spectroscopy, thermal gravimetric
analysis (TGA), and X-ray crystallography confirmed the formula of
as-synthesized complexes. In the IR spectra, strong and broad
absorption at 3631 cm−1 in 2 suggests the presence of water
molecules. Two weak bands between 3450 and 3350 cm−1 should
be assigned to the stretch vibrations of the amino group appended on
triazole ring. Compared with the free H2nip, the disappearance of a
band at 1716 cm−1 in the both complexes is indicative of the fully
deprotonation of aromatic coligand. Correspondingly, the asymmetric
(vas) and symmetric stretch (vs) vibrations for the carboxylate are
observed at 1633, 1584 and 1376, 1342 cm−1 for 1 and at 1622 and
1359 cm−1 for 2, respectively. Their differences (Δv=vas −vs =208
and 263 cm−1) suggest the unidentate carboxylate group of nip2−
anion [27].
X-ray single-crystal diffraction analysis [28] shows that complex 1
is a 3D pillared-layer framework with the cationic ZnII-datrz layers
supported by anionic nip2− ligands. As shown in Fig. 1a, the sole
crystallographically independent ZnII ion is four coordinated to one
carboxylate O atom from a nip2− anion and three N donors from three
symmetry-related datrz ligands, exhibiting distorted tetrahedral
coordination geometry. The Zn−O and Zn−N distances are fall in
the normal range of 1.9479(14)−2.0125(16)Å (Table S1), and are
⁎
Corresponding authors. Fax: +86 22 23766556.
E-mail addresses: encui_yang@yahoo.com.cn (E.-C. Yang),
xiaojun_zhao15@yahoo.com.cn (X.-J. Zhao).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.11.015

286
E.-C. Yang et al. / Inorganic Chemistry Communications 14 (2011) 285–287
crystallizes in a chiral space group P41212, exhibits a 2D (4, 4) covalent
sheet alternately extended by neutral Hdatrz and anionic nip2− ligands.
Rather than being in a N3O donor set, the tetrahedral ZnII ion in 2 is
surrounded by two carboxylate O and two triazole N donors from pairs
of symmetry-related Hdatrz and nip2− ligands (Fig. 2a). Moreover, the
Hdatrz ligand in 2 is in a neutral molecule, and only presents its two N
donors to coordinate with the ZnII ion in a bidentate μ2-N1,N4-bridging
mode. Thus, each ZnII ion in 2 is connected together by pairs of bidentate
nip2− and datrz bridges to generate a grid-based (4, 4) layer with a
closet Zn⋅⋅⋅Zn distance of being 7.8645(2) and 6.1235(1) Å, respectively
(Fig. 2b). Furthermore, the separate layers are stacked together into a 3D
supramolecular network through O–H···O hydrogen-bonding interac-
tions between the lattice water and the nitro/carboxylate group of nip2−
ligand (see Fig. S1 and Table S2). Thus, the structural determinations
indicate that the different reaction temperature essentially influenced
the cooperative coordination of the two ligands to the tetrahedron ZnII
ion, which further induced the specific coordination frameworks with
distinct dimensionality and connectivity.
Complexes 1 and 2 with different dimensionality exhibited
considerably different thermal stability (Fig. S2). For the 3D
framework of 1 without any lattice molecules, only one weight-loss
stage with an obvious exothermic effect was observed between 414 °C
and 561 °C, which is corresponding to the collapse of the pillared-
layer framework (obsd. 70.7%; calcd. 69.6%). In contrast, low-
dimensional 2 displayed two separate thermal weight-loss processes
due to the presence of the lattice water molecules. The first weight-
loss stage with an endothermic effect began at 293 °C and ended at
333 °C, which is due to the removal of the lattice water molecule
Fig. 1. (a) Local coordination environment of the ZnII ion in 1 (Hydrogen atoms were
omitted for clarity, symmetric codes: A=0.5 − x, 0.5 − y, −z; B=x, −y, z − 0.5; C=1 − x,
y, −0.5 − z). (b) 2D ZnII-datrz layer of 1. (c) 3D pillared-layer framework of 1.
comparable to those reported values [29]. The unique Hdatrz ligand in
1 is in a mono-deprotonated anionic form, and presents its three
triazole N donors to coordinate with the ZnII ion in a tridentate μ3-N1,
N2, N4-bridging mode, generating a cationic ZnII-datrz 2D covalent
layer in a crystallographic bc plane (Fig. 1b). Obviously, due to the
specific spatial arrangement of the datrz ligands towards the ZnII atom
(Table S1), the 2D layer is wavy, and consists of 16-membered
macrocycles enclosed by four ZnII ions and four datrz ligands (Fig. 1b).
Furthermore, the aromatic nip2− coligand, lying at a 2-fold rotation
axis, affords its two symmetry-related carboxylate O atoms to support
the wavy ZnII-datrz layer through a bis-monodentate fashion. Thus, a
3D pillared-layer MOF is generated (Fig. 1c).
Fig. 2. (a) Local coordination environment of the ZnII ion in 2 (Hydrogen atoms were
omitted for clarity, symmetric codes: A=1.5 − x, y − 0.5, 0.25 − z; B=2.5 − x, y − 0.5,
0.25 − z). (b) 2D grid-based (4, 4) covalent layer of 2.
Much different from the higher-dimensional framework of 1,
complex 2 generated from the relatively low hydrothermal temperature

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287
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(obsd. 5.7%; calcd. 4.6%). The second stage corresponding to the
decomposition of the mixed ligands was observed between 310 °C
and 564 °C with two consecutive exothermic processes. The final
products of the two complexes are all calculated to be ZnO (obsd.
29.3%; calcd. 30.4% for 1; obsd. 19.5%; calcd. 20.8% for 2). Lumines-
cence complexes are of great current interest because of their various
applications in chemical sensors, photochemistry and structure
electroluminescence (EL) displays and light-emitting diodes (LEDs)
[30]. Thus, the solid UV–vis absorption and luminescent spectra of the
two interesting ZnII complexes were measured (Fig. S3 and Fig. S4).
Upon excitation at 357 nm for 1 and at 355 nm for 2, both complexes
with the same components exhibit intense emissions at 394 nm with
different intensity. Similar emission can also be observed for free
Hdatrz ligand when excited at 355 nm originated from π–π*
transitions. Thus, the emissions of the two complexes should be
assigned to intra-ligand charge transfer and their slight difference in
the intensity is due to the change of the coordination environments
and the fixation of the ligand in a coordination network [30].
In summary, two different fluorescent polymers with the same
components, 3D pillared-layer framework for 1 and 2D (4, 4) grid-layer
for 2, were obtained by changing the temperature of the hydrothermal
reactions. Interestingly, their structural diversity is significantly resulted
from the competitive coordination of the mixed ligands toward the
same metal ion, which encourage us the revenant investigations on the
controllable constructions of the metal complexes with polydentate
mixed ligands.
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[26] Synthesis of 1 and 2: A mixture of Hdatrz (9.9 mg, 0.1 mmol), H2nip (10.5 mg,
0.05 mmol), and Zn(NO3)2·6H2O (29.7 mg, 0.1 mmol) was dissolved in doubly
deionized water (10 mL) and the initial pH of the mixture was adjusted to 7 by
slow addition of triethylamine with constant stirring. The mixture was then
transferred into a parr Teflon-lined stainless steel vessel (23 mL) and heated to
160 °C for 72 h under autogenous pressure. After the mixture was cooled to room
temperature, yellow block-shaped crystals suitable for X-ray analysis were
generated directly, washed with ethanol, and dried in air. Yield: 49% based on
H2nip. Anal. calcd for C12H11N11O6Zn2: C 26.89, H 2.07, N 28.74%. Found: C 26.88,
H 2.10, N 28.75%. IR (KBr, cm−1) 3430(w), 3342(w), 1633(vs), 1584(s), 1517(s),
1376(s), 1342(s), 1073(m), 845(w), 728(s), 585(w), 473(w). The reaction of 2
was carried out in the procedures similar to those of 1, only heating to 140 °C.
Yellow block-shaped crystals suitable for X-ray analysis were generated directly,
washed with ethanol, and dried in air. Yield: 36% based on H2nip. Anal. calcd. for
C10H10N6O7Zn: C 30.67, H 2.57 N 21.46%. Found: C 30.70, H 2.55, N 21.50%. IR (KBr,
cm−1) 3631(br), 3419(w), 3334(w), 3087(w), 1622(vs), 1359(vs), 1076(w), 823
(w), 726(m), 557(w), 444(w).
Acknowledgments
This present work was financially supported by the National
Natural Science Foundation of China (20703030, 20871092 and
20973125), the Key Project of Chinese Ministry of Education (Grant
No. 209003), the Program for New Century Excellent Talents in
University (NCET-08-0914), and the Natural Science Foundation of
Tianjin (10JCZDJC21600, and 10JCYBJC04800), which are gratefully
acknowledged.
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Appendix A. Supplementary material
[28] Crystal data for 1: C12H11N11O6Zn2, M=536.06, monoclinic, space group C2/c,
a=19.579(6), b=9.512(2), c=9.641(2) Å, β=99.142(6), V=1772.6(8) Å3,
Z=4, Dc=2.009 g cm−3, F000=1072, Mo Kα radiation, λ=0.71073 Å, T=296
X-ray crystallographic file in CIF format (CCDC 789249 and 789250
for 1 and 2, respectively), additional Tables, Figures, TG-DTA curves and
solid state UV–vis and fluorescent spectra for 1−2. Supplementary data
to this article can be found online at doi:10.1016/j.inoche.2010.11.015.
(2) K, μ=2.770 mm−1
, GOF=1.041, R1=0.0183, wR2=0.0467 [IN2σ(I)].
Absorption correction: SADABS (Tmin / Tmax =0.6503 / 0.5809). Crystal data for
2: C10H10N6O7Zn, M=391.61, tetragonal, space group P41212, a=9.7885(3),
b = 9.7885(3), c = 28.9618(10) Å, β = 90, V = 2774.97(15) Å3, Z = 8,
Dc=1.870 g cm−3, F000=1576, Mo Kα radiation, λ=0.71073 Å, T=296(2) K,
μ=1.824 mm−1, GOF=1.047, R1=0.0246, wR2=0.0663 [IN2σ(I)]. Absorption
correction: SADABS (Tmin / Tmax=0.7468 / 0.6791). X-ray data collection was
carried out using a Bruker APEX II diffractometer with a CCD area detector. The
structure was solved by direct methods and refined with the full-matrix least-
squares technique using the SHELXS-97 and SHELXL-97 programs. All non-
hydrogen atoms were treated anisotropically. C-bound H atoms of the ligands
were placed geometrically refined as riding atoms.
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