Hydrothermal syntheses, crystal structures and luminescence properties of zinc(II) coordination polymers constructed by bifunctional 4′-(4- carboxyphenyl)-3,2′:6′,3″-terpyridine

Article abstract of DOI:10.1016/j.poly.2012.10.005

Four polymeric frameworks, [Zn(cptpy)₂(Hcptpy) ₂]·2H₂O (1), [Zn(cptpy)Cl]_n (2), [Zn₄(cptpy)₂(3-btc)₂]_n· nH₂O (3) and [Zn₂(cptpy)(2-btc)]_n (4), (Hcptpy = 4′-(4-carboxyphenyl)-3,2′:6′,3″-terpyridine, H ₃(3-btc) = 1,3,5-benzenetricarboxylic acid, H₃(2-btc) = 1,2,4-benzenetricarboxylic acid) have been hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction analyses. Compound 1 has a mononuclear structure, while 2 shows a one-dimensional ladder chain containing 28 member rings with (cptpy)^- anions as bridges. For compounds 1 and 2, weak hydrogen bonding and intra- and/or inter-molecular π?π stacking interactions link the mononuclear or infinite chain structures into high-dimensional supramolecular structures. Compound 3 reveals a (4,4,4)-connected three-dimensional (3D) framework. Compound 4 is a three dimensional network structure comprised of new tetranuclear zinc clusters, exhibiting a {4·6·8}{4 ²·6·8³}{4²·6 ²·8²}{4³·6²· 8⁵} topological structure. In addition, the thermal stabilities and photoluminescence properties of 1-4 were also studied.

Full text of DOI:10.1016/j.poly.2012.10.005

Polyhedron 49 (2013) 207–215
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Hydrothermal syntheses, crystal structures and luminescence properties
of zinc(II) coordination polymers constructed by bifunctional
40-(4-carboxyphenyl)-3,20:60,300-terpyridine
a
a
a
a
Na Li a, Qiao-E Zhu b, Huai-Ming Hu a, , Hui-Lin Guo , Juan Xie , Fei Wang , Fa-Xin Dong ,
⇑
Meng-Lin Yang a, Gang-Lin Xue a
a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University,
Xi’an 710069, China
b Yulin Center for Disease Control and Prevention, Shaanxi, Yulin 719000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four polymeric frameworks, [Zn(cptpy)2(Hcptpy)2]ꢀ2H2O (1), [Zn(cptpy)Cl]n (2), [Zn4(cptpy)2(3-btc)2]nꢀ
nH2O (3) and [Zn2(cptpy)(2-btc)]n (4), (Hcptpy = 40-(4-carboxyphenyl)-3,20:60,300-terpyridine, H3(3-btc) =
1,3,5-benzenetricarboxylic acid, H3(2-btc) = 1,2,4-benzenetricarboxylic acid) have been hydrothermally
synthesized and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction
analyses. Compound 1 has a mononuclear structure, while 2 shows a one-dimensional ladder chain con-
taining 28 member rings with (cptpy)ꢁ anions as bridges. For compounds 1 and 2, weak hydrogen bonding
Received 18 August 2012
Accepted 3 October 2012
Available online 22 October 2012
Keywords:
MOFs
Zinc
Crystal structures
Luminescence
Terpyrdine derivative
and intra- and/or inter-molecular
pꢀ ꢀ ꢀp stacking interactions link the mononuclear or infinite chain
structures into high-dimensional supramolecular structures. Compound 3 reveals a (4,4,4)-connected
three-dimensional (3D) framework. Compound 4 is a three dimensional network structure comprised of
new tetranuclear zinc clusters, exhibiting a {4ꢀ6ꢀ8}{42ꢀ6ꢀ83}{42ꢀ62ꢀ82}{43ꢀ62ꢀ85} topological structure. In
addition, the thermal stabilities and photoluminescence properties of 1–4 were also studied.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
ring nitrogen atoms as multiple interaction sites, can form stable
coordination compounds with various metal ions through coordi-
Exploration of the syntheses and properties of new metal–
organic frameworks (MOFs) has attracted more and more intense
interest from chemists, not only because of their intriguing archi-
tectures, but also their potential applications in heterogeneous
catalysis, molecular magnetism, photoluminescence and molecular
sorption [1–4]. However, it is known that a self-assembly process
is greatly affected by factors such as the nature of the ligand, coor-
dination geometry of the metal center, reaction medium, template,
metal-to-ligand ratio, pH value and counter ion [5]. Therefore, fur-
ther investigation is required to understand the assembly process.
Presently, a rational synthetic strategy widely used in this area is
linking metal ions with polydentate ligands as connectors [6].
Thus, a lot of fascinating metal–organic coordination polymers
have been formed by employing either O-donor ligands, such as
1,4-benzenedicarboxylate [7–10] and 1,3,5-benzenetricarboxylate
[11,12], or N-donor ligands, such as 4,4-bipyridine [13–16], 1,10-
phenanthroline and 2,20:60,200-terpyridine derivatives [17], as link-
ers and metal ions or metal clusters as nodes. Oligopyridine, with
nation and hydrogen bonds. Moreover, carboxylic derivatives of
oligopyridine are a fairly well-known type of ligand and have
found widespread application in dye-sensitized photovoltaic cells
[18]. To the best of our knowledge, 40-(4-carboxyphenyl)-
3,20:60,300-terpyridine (Hcptpy, Scheme 1) has not been reported
in the realm of coordination chemistry. However, its following
structural features inspired our research interests: (i) it is a highly
symmetric polydentate bifunctional ligand, the carboxyl group as a
bridging or chelating group may be conductive for the formation of
discrete polynuclear clusters and infinite building blocks by
M–COO–M linkages, while the terpyridyl group can act as a bridg-
ing group which may give the ability to link these clusters to an
intricate extended network. (ii) Hcptpy has a large
p-conjugated
structure that helps to form a good connection between the donor
and acceptor, to fulfill electron transport. Additionally, introduc-
tion of other O-containing auxiliary ligands into a reaction system
may not only form more complicated and interesting compounds,
but might endow the final products with intriguing and useful
properties. The relatively rigid structures of compounds based on
the Hcptpy ligand with active photoluminescence properties are
more available when d10 metals, such as ZnII or CdII, are used.
⇑
Corresponding author. Tel./fax: +86 29 88303331.
E-mail address: chemhu1@nwu.edu.cn (H.-M. Hu).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.10.005

208
N. Li et al. / Polyhedron 49 (2013) 207–215
11.83%. IR (KBr, cmꢁ1): 3445(s), 1715(s), 1631(m), 1494(w),
1401(m), 1388(w), 1118(m), 870(w), 769(w), 702(w), 617(w).
ESI-MS: [M+H]+ calcd. for C22H16N3O2: 354.11; found: 354.12. 1H
NMR (400 MHz, DMSO-d6) d: 9.87 (s, 2H), 9.51 (d, J = 8.0 Hz, 2H),
9.01 (s, 2H), 8.72 (s, 2H), 8.23 (m, 6H). 13C NMR (400 MHz,
DMSO-d6) d: 166.78, 152.67, 149.22, 144.95, 143.43, 140.46,
140.27, 135.60, 131.71, 129.81, 127.75, 125.96, 119.22.
Scheme 1. Molecular structures and abbreviations of the ligands.
2.2.2. Synthesis of [Zn(cptpy)2(Hcptpy)2]ꢀ2H2O (1)
A mixture of ZnCl2 (0.0136 g, 0.1 mmol), Hcptpy (0.0353 g,
0.1 mmol), 1,10-phen (0.0198 g, 0.1 mmol) and deionized water
(10 mL) was stirred for 30 min in air and the pH value was then ad-
justed to 4.0 with 0.5 M NaOH aqueous solution. The reaction mix-
ture was sealed in a Teflon-lined stainless vessel (25 mL), which
was heated to 180 °C for 72 h. After cooling to room temperature
at a rate of 5 °C/h, colorless block crystals were obtained (yield:
21.9 mg, 58% based on Hcptpy). Anal. Calc. for C88H62N12O10Zn
(1512.87): C, 69.86; H, 4.13; N, 11.11. Found: C, 69.83; H, 4.09;
N, 11.03%. IR (KBr, cmꢁ1): 3447(s), 1710(m), 1636(m), 1600(w),
1558(w), 1461(w), 1392(w), 1116(m), 1045(w), 854(w), 776(w),
692(w).
2.2.3. Synthesis of [Zn(cptpy)Cl]n (2)
Scheme 2. Synthetic route to the Hcptpy ligand.
The procedure was the same as that for 1 except that the pH va-
lue was adjusted to 5.0 with 0.5 M NaOH aqueous solution. Color-
less block crystals were obtained (yield: 32.6 mg, 72% based on
Hcptpy). Anal. Calc. (%) for C22H14ClN3O2Zn (453.18): C, 58.30; H,
3.11; N, 9.27. Found: C, 58.37; H, 3.04; N, 9.20%. IR (KBr, cmꢁ1):
3442(s), 1609(m), 1560(w), 1461(w), 1392(m), 1116(w),
1047(w), 778(w), 691(w), 643(w), 488(w).
Therefore, Hcptpy has attracted our interest as a good candidate for
the construction of metal–organic architectures (see Scheme 2).
In this work, four new ZnII structures, [Zn(cptpy)2(Hcptpy)2]-
ꢀ2H2O (1), [Zn(cptpy)Cl]n (2), [Zn4(cptpy)2(3-btc)2]nꢀnH2O (3) and
[Zn2(cptpy)(2-btc]n (4), have been synthesized. Their luminescence
properties and thermal stabilities are studied as well.
2.2.4. Synthesis of [Zn4(cptpy)2(3-btc)2]nꢀnH2O (3)
A mixture of Zn(CH3COO)2ꢀ2H2O (0.0219 g, 0.1 mmol), Hcptpy
(0.0353 g, 0.1 mmol), 1,3,5-benzenetricarboxylic acid (0.0220 g,
0.1 mmol) and deionized water (10 mL) was stirred and the pH va-
lue was adjusted to 5.0 with 0.5 M NaOH aqueous solution. The
reaction mixture was then sealed in a 25 mL Telfon-lined stainless
steel container, which was heated to 180 °C for 72 h, then cooled to
room temperature at a rate of 5 °C/h. Colorless block crystals were
obtained (yield: 27.3 mg, 78% based on Zn). Anal. Calc. (%) for C62-
H36N6O17Zn4 (1398.45): C, 53.24; H, 2.59; N, 6.01. Found: C, 53.27;
H, 2.56; N, 6.10%. IR (KBr, cmꢁ1): 3342(s), 1624(m), 1597(w),
1412(w), 1395(m), 1355(m), 1110(w), 1040(w), 773(m), 704(w),
583(w).
2. Experimental
2.1. Materials and methods
The Hcptpy ligand was prepared according to the literature,
with some modifications [19]. All other chemicals were used as ob-
tained from commercial sources without further purification. Ele-
mental analyses (C, H and N) were performed with an Elementar
Vario EL I elemental analyzer. 1H NMR and 13C NMR spectra were
recorded at room temperature using a Varian Inova 400 MHz appa-
ratus and tetramethylsilane (TMS) as a reference. A Micro TOF-QII
mass spectrometer with an electro-spray ionization (ESI) source
(Bruker, Germany) was used. Infrared spectra were obtained from
KBr pellets on a Bruker EQUINOX 55 Fourier transform infrared
spectrometer in the range 400–4000 cmꢁ1. Emission spectra were
recorded on a Hitachi F-4500 spectrophotometer in the solid state
at room temperature. Thermal gravimetric analyses (TGA) were
carried out with a Universal V2.6 DTA instrument under an N2
atmosphere with a heating rate of 10 °C/min.
2.2.5. Synthesis of [Zn2(cptpy)(2-btc)]n (4)
An identical procedure as that for 3 was followed to prepare 4
except that 1,3,5-benzenetricarboxylic acid was replaced with
1,2,4-benzenetricarboxylic acid (0.0192 g, 0.1 mmol). Colorless
block crystals were obtained (yield: 30.7 mg, 89% based on Zn).
Anal. Calc. for C31H17N3O8Zn2 (690.22): C, 53.94; H, 2.48; N, 6.09.
Found: C, 53.87; H, 2.52; N, 6.01%. IR (KBr, cmꢁ1): 3435(s),
1610(w), 1598(m), 1397(m), 1125(w), 922(w), 765(m), 673(w),
534(w).
2.2. Synthesis of the ligand and the compounds
2.2.1. Synthesis of the Hcptpy ligand
2.3. Crystal structure determination
In a typical preparation, 3-acetylpyridine (1.21 g, 10 mmol) was
added to a stirred solution of 4-carboxybenzaldehyde (0.75 g,
5 mmol), KOH (0.8 g, 14 mmol) and NH3 H2O (20 mL) in 150 mL
Suitable single crystals of 1–4 were selected for X-ray diffrac-
tion study. Intensity data were collected on a Bruker Smart Apex
.
ethanol. The reaction mixture was stirred at room temperature
whilst open to air for 48 h, after which the solution was made
slightly acidic (pH 5) with the addition of HCl (aq., conc.). The
resulting light yellow precipitate was filtered off, recrystallized
with a mixture of ethanol and methanol, and dried in vacuo to give
colorless crystals. Yield: 1.31 g (74%). Anal. Calc. for C22H15N3O2
(353.11): C, 74.78; H, 4.28; N, 11.89. Found: C, 74.83; H, 4.31; N,
II CCD diffractometer with graphite-monochromatic Mo Ka radia-
tion (k = 0.71073 Å) at 296 K. The structures were solved by direct
method and refined by full-matrix least-squares on F2 with the
SHELX-97 program package [20,21]. All non-hydrogen atoms were
refined anisotropically, and the hydrogen atoms of organic ligands
were generated geometrically. The crystal data and structural
refinement parameters for 1–4 are summarized in Table 1 and

N. Li et al. / Polyhedron 49 (2013) 207–215
209
Table 1
Crystallographic data and structural refinement parameters for 1–4.
Compound
1
2
3
4
Empirical formula
Formula weight
T (K)
C
88H62N12O10Zn
C22H14ClN3O2Zn
453.18
296(2)
C62H36N6O17Zn4
1398.45
296(2)
C31H17N3O8Zn2
690.22
296(2)
1512.87
296(2)
k (Å)
0.71073
monoclinic
P2/n
15.596(3)
9.601(2)
23.438(5)
90
95.994(4)
90
3490.4(13)
0.71073 A
monoclinic
P21/m
6.518(3)
12.909(6)
10.687(5)
90
97.423(8)
90
891.7(7)
0.71073
monoclinic
P21/c
14.9228(13)
24.215(2)
14.7464(13)
90
94.124(2)
90
5315.0(8)
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
ꢀ
P1
8.5249(12)
9.6987(14)
16.572(2)
105.971(2)
101.330(2)
90.408(2)
1288.9(3)
a
(°)
b (°)
c
(°)
0
V (ÅA3)
Z
2
2
4
2
Dcalc (g cmꢁ3
)
1.439
0.428
1.688
1.553
1.748
1.870
1.779
1.925
l
(mmꢁ1
)
F (000)
1568
1.000
460
1.016
4498
1646
0.0583
0.0464, 0.1119
0.0637, 0.1221
2824
0.995
29444
10,741
0.0606
0.0483, 0.1060
0.0907, 0.1265
696
1.031
6550
4514
0.0244
0.0449, 0.0964
0.0575, 0.1031
Goodness-of-fit (GOF) on F2
Reflections collected
Reflections unique
17314
6181
0.0637
0.0513, 0.1074
0.1071, 0.1321
Rint
R1a, wR2 [I > 2
r
(I)]
b
R1a, wR2 (all data)
b
a
R =
wR2 = [
R
||F0| ꢁ |FC||/
R|F0|.
b
2
R
[w(F0 ꢁ FC2)2]/
R .
[(F02)2]]1/2
selected bond distances and bond angles are listed in Table S1
(Supporting information).
of 1,10-phen, we obtained metal complexes with the second li-
gand. The aforementioned phenomena did not happen when using
1,10-phen as a second ligand. Therefore, we propose that the 1,10-
phen ligand may play a significant role as a template in the forma-
tion of 1 and 2.
3. Results and discussion
In a comparison of the structures of 3 and 4, it is found that the
coordination sizes of the auxiliary ligands have significant effects
on the formation and structures of the resulting compounds. In or-
der to examine the effect of the counter anion on the syntheses of
compounds 3 and 4, the hydrothermal reactions were carried out
under the same conditions with Zn(OAc)2ꢀ2H2O instead of ZnCl2.
Consequently, compound 3 was obtained, while 4 demonstrated
another structure, which reveals that the counter anion has a cer-
tain influence on the syntheses of the compounds. Based on the
aforementioned facts, reaction conditions such as pH value, the
sizes of the auxiliary ligands and the template exert a synergic ef-
fect on the formation of Hcptpy coordination compounds.
3.1. Syntheses and structural diversity
The syntheses are summarized in Scheme 3. Hydrothermal syn-
thesis is a powerful method for the preparation of coordination
polymers due to their low solubility in common solvents. Com-
pounds 1 and 2 were synthesized by the reactions of ZnCl2 with
Hcptpy and 1,10-phen ligands under the same hydrothermal reac-
tion conditions, but with different pH values. Compound 1 is a
mononuclear structure, while 2 shows a one-dimensional ladder
chain. There are two Hcptpy ligands and two (cptpy)ꢁ anions in
1. The Hcptpy ligand is partly deprotonated when the pH of reac-
tion mixture is 4, as in the preparation of 1, while Hcptpy is com-
pletely deprotonated in the synthesis of 2. Therefore, the different
structures of 1 and 2 could be attributed to the influence of the pH
values in the reaction systems. It is noteworthy that 1,10-phen
does not appear in 1 or 2. A similar situation also occurred in other
systems [22]. In order to study the effect of 1,10-phen, the reac-
tions of the zinc salt with Hcptpy under the same hydrothermal
conditions except in the absence of 1,10-phen were carried out.
The products are faint yellow precipitates which were different
from compounds 1 and 2. Furthermore, when we used other sec-
ondary ligands, such as 2,20-bipyridine or 4,40-bipyridine, instead
3.2. Crystal structures of compounds 1–4
3.2.1. [Zn(cptpy)2(Hcptpy)2]ꢀ2H2O (1)
X-ray crystallography reveals that 1 has a mononuclear struc-
ture. The asymmetric unit of 1 contains half ZnII atom, one (cptpy)ꢁ
anion, one Hcptpy ligand and one lattice water molecule. As shown
in Fig. 1a, each Zn1 atom is located in a distorted octahedral coor-
dination sphere formed by two nitrogen atoms (N4, N4A) from two
Hcptpy ligands and four oxygen atoms (O1, O2, O1A, O2A) from
two (cptpy)ꢁ anions. The N4, N4A, O1 and O1A atoms comprise
the equatorial plane, whilst O2 and O2A atoms occupy the axial
positions [O2–Zn1–O2A = 151.3(1)°]. The Zn1–O bond lengths fall
in the range 2.161(2)–2.219(2) Å, which are similar to those found
in other ZnII carboxylate coordination polymers [23]. The Zn1–N
bond length is 2.073(3) Å, which is a typical value for zinc pyridyl
coordination compounds [24]. In 1, half of the Hcptpy ligands are
deprotonated to form (cptpy)ꢁ anions. Each (cptpy)ꢁ anion acts
as a bidentate-chelating ligand with the pyridyl nitrogen atoms
remaining uncoordinated. The three pyridyl rings from tpy domain
are nearly coplanar; the benzene ring rotates by the small angle of
7.62° relative to the central pyridyl ring of the tpy domain,
Scheme 3. Synthesis of compounds 1–4.

210
N. Li et al. / Polyhedron 49 (2013) 207–215
Fig. 1. (a) The coordination environment of the ZnII ion in 1 with the partial atomic numbering scheme. (b) A view of the 3D supramolecule structure in 1.
Scheme 4. Coordination modes of Hcptpy in 1–4.
indicating the four rings are nearly coplanar. Similar to the
(cptpy)ꢁ anion, each Hcptpy ligand acts as a monodentate ligand,
with the carboxyl group remaining uncoordinated. The three pyri-
dyl rings from the tpy domain are not coplanar, with the angles be-
tween the terminal and central pyridyl rings being 16.24° (N4) and
5.18° (N6), respectively. The benzene ring is twisted with respect
to the central pyridyl (N5), with a dihedral angle of 22.97°, suggest-
ing that the four aromatic rings of the Hcptpy ligand are non-
planar.
Two Hcptpy ligands and two (cptpy)ꢁ anions adopting mono-
dentate (mode I, Scheme 4) and bidentate-chelating modes (mode
II), respectively, coordinate to one ZnII center to form a mononu-
clear structure. The mononuclear structures are further linked to
generate a zigzag chain through hydrogen bonding interactions be-
tween the carboxyl group of a Hcptpy ligand and a lattice water
molecule [O3–H3ꢀ ꢀ ꢀO5i, 2.569(4) Å, 172.8°, symmetry code (i): (x,
y ꢁ 1, z)], and between a lattice water molecule and carboxylate
and pyridyl groups of (cptpy)ꢁ anions [O5–H5Bꢀ ꢀ ꢀO1ii, 2.772(3) Å,

N. Li et al. / Polyhedron 49 (2013) 207–215
211
Fig. 2. (a) The coordination environment of the ZnII ion in 2 with the partial atomic numbering scheme. Hydrogen atoms are omitted for clarity. (b) A view of the
supramolecule layer in 2 along the a axis.
159(5)°, symmetry code (ii): (ꢁx + 1, ꢁy + 1, ꢁz + 1); O5–
H5Aꢀ ꢀ ꢀN3iii, 2.799(4) Å, 174(5)°, symmetry code (iii): (ꢁx + 1/2,
y + 1, ꢁz + 1/2)] (Fig. S2). Additionally, there are two kinds of weak
atoms (N1, N1A), two oxygen atoms (O1A, O1B) from two different
(cptpy)ꢁ anions and one terminal chlorine atom. The coordination
polyhedron around Zn1 atom is a distorted square-pyramid, with
the four atoms N1, N1A, O1A and O1B in the basal plane and the
Cl1 atom in the apical position [25]. The Zn1–O and Zn1–N bond
lengths are 2.235(5) and 2.084(3) Å, respectively. The Zn1–Cl1 dis-
tance is 2.2419(19) Å, which is slightly longer than analogous bond
distances reported in the literature [26]. Each (cptpy)ꢁ ligand,
intermolecular p–p stacking interactions (Fig. S3). One kind of p–p
stacking interaction exist between the central N2 pyridyl and ter-
minal N6 pyridyl rings, with a N2–N6 a ring distance of 3.572 Å,
centroid to centroid, and the centriod to face distance is 3.405 Å.
Another centroid–centroid distance is 3.471 Å, with a 22.6° vertical
displacement angle. Finally, a 3D supramolecular structure is
formed through these weak interactions (Fig. 1b). Thus, the hydro-
adopting the same coordination mode of l3- : :
g1 g1 g2 (mode III),
connects three ZnII ions to form a ribbon chain which contain a
pore with the size of 12.304 ꢂ 11.330 Å, based on the distances
of Zn1ꢀ ꢀ ꢀZn1 and C6ꢀ ꢀ ꢀC6. Finally, the these chains are packed into
gen bonding and p-p stacking interactions play an important role
in the formation of the supramolecular structure.
a supramolecular layer via p–p stacking interactions between the
two terminal pyridyl rings of the (cptpy)ꢁ anions from adjacent
parallel chains (3.638 Å centroid-to-centroid distance and 20.5°
vertical displacement angle) (Figs. S4 and S5) [27]. In the solid
state, the chloride atom is involved in inter-chain C–Hꢀ ꢀ ꢀCl hydro-
gen bonding interactions, with Cꢀ ꢀ ꢀCl separations in the range
3.2.2. [Zn(cptpy)Cl]n (2)
X-ray single-crystal diffraction analysis reveals that compound
2 is a ribbon polymeric coordination chain. The asymmetric unit
of 2 contains half ZnII ion, half (cptpy)ꢁ anion and half Clꢁ anion.
As shown in Fig. 2a, the Zn1 atom is surrounded by two nitrogen

212
N. Li et al. / Polyhedron 49 (2013) 207–215
3.673–3.695 Å [28], which contribute to the stability of the supra-
molecular structure of 2 (Fig. 2b).
and 1.903(3)–1.960(3) Å, respectively, which are comparable to
other reported ZnII compounds [30]. Compared to the Zn1 atom,
the Zn3 atom is surrounded by three oxygen atoms (O7, O8, O9A)
from two bridging-chelating 3-btc3ꢁ anions, O3 and N3A atoms
from two (cptpy)ꢁ anions. The coordination geometry of Zn3 centre
can be described as a slightly distorted trigonal bipyramid, with the
three atoms O3, O8 and N3A in the equatorial plane and two oxygen
atoms, O9A and O7, in the axial positions [O9A–Zn3–
O7 = 160.6(1)°]. The four aromatic rings of the (cptpy)ꢁ anion are
non-planar. The benzene ring is twisted with respect to the central
pyridyl ring (N2 pyridyl) with a dihedral angle of 21.55°; each outer
pyridyl group (N1 pyridyl) is twisted with respect to the central one
with a dihedral angle of 21.77°, and the dihedral angle between the
central and the other outer pyridyl ring (N3 pyridyl) is 13.20°.
In 3, the two 3-btc3ꢁ anions take different coordination modes.
3.2.3. [Zn4(cptpy)2(3-btc)2]nꢀnH2O (3)
X-ray crystallographic analysis reveals that compound 3 is a
three-dimensional framework with interconnecting cavities con-
taining water molecules. As shown in Fig. 3a, the asymmetric unit
of 3 contains four crystallographically independent ZnII ions, two
(cptpy)ꢁ anions, two 3-btc3ꢁ anions and one lattice water molecule.
The Zn1 atom is surrounded by two oxygen atoms (O14, O16A)
from two bridging 3-btc3ꢁ anions, N1 and O4A atoms (Zn1–N1,
2.059(3) Å) from two (cptpy)ꢁ anions. The Zn1–O distances fall in
the range 1.942(3)–2.000(3) Å, which are similar to those found
in other Zn carboxylate coordination polymers [29]. The coordina-
tion polyhedron around the Zn1 ion is a distorted tetrahedron.
The coordination geometry of Zn2 and Zn4 are similar to that of
Zn1, except for slight differences in bond lengths and angles. The
Zn–N and Zn–O bond distances are in the range 2.003(3)–2.053(3)
One 3-btc3ꢁ anion adopts a l4
and Zn3 atoms, and forming a kind of chain. Similarly, the other
3-btc3ꢁ anion adopts a l4 g1 mode, linking the Zn4
-
g1
:
g1
:
g1
:g
2 mode, linking the Zn2
-
g1 g1
:
:
g1
:
Fig. 3. (a) The coordination environment of the ZnII ions in 3 with the partial atomic numbering scheme. Lattice water and hydrogen atoms are omitted for clarity. (b) View of
the 3D network. (c) View of four 42-membered rings. (d) View of the 3D topological network in 3. The (cptpy)ꢁ anions, 3-btc3ꢁ anions and ZnII ions are marked as blue, pink
and bright green, respectively. (Color online.)

N. Li et al. / Polyhedron 49 (2013) 207–215
213
and Zn1 centers to form another kind of chain (Fig. S6). It is note-
worthy that there are no further connections between the different
3.2.4. [Zn2(cptpy)(2-btc)]n (4)
Single-crystal X-ray analysis revealed that 4 is a 3D metal–organ-
ic framework based on tetranuclear ZnII cluster units. The asymmet-
ric unit of 4 contains two ZnII ions, one (cptpy)ꢁ anion and one 2-
btc3ꢁ anion (Fig. 4a). Zn1 is surrounded by one nitrogen atom (N1)
from a (cptpy)ꢁ anion [Zn1–N1 = 2.040(3) Å] and three oxygen
atoms (O3, O6A, O8A) from two 2-btc3ꢁ anions. The Zn1–O bond
lengths are in the range 1.900(3)–2.013(3) Å. The coordination
geometry of the Zn1 center can be described as a distorted tetrahe-
dron. Similarly, the Zn2 atom is also four-coordinated, to N3A and
O1A from the (cptpy)ꢁ anion [Zn2–N3A = 2.058(3) Å, Zn2–O1A =
1.974(3) Å] and two oxygen atoms (O4 and O5A) from two 2-btc3ꢁ
anions [Zn2–O4 = 1.982(3) Å, Zn2–O5A = 1.981(3) Å] to form a dis-
torted tetrahedral geometry. Obviously, the three pyridyl rings from
the tpy domain are not coplanar, the torsion angles between the ter-
minal and central pyridyl rings (N2) are 14.61° (N1 pyridyl ring) and
chains. Three (cptpy)ꢁ anions adopting the l4 g1 g1 g1 1-bridg-
- : : :g
ing coordination mode IV (Scheme 4) connect to four different ZnII
ions to form a layer which contains a 36-membered [Zn4O4N4C25
]
ring. A peculiar structural feature is that two layers with different
extending directions are further linked by two types of chains to
form a 3D network (Fig. 3b). Interestingly, two (cptpy)ꢁ anions,
two 3-btc3ꢁ anions and four different ZnII ions connected end to
end to form a 42-membered (Zn4O6N2C30) ring which produces a
grid pore with a size of 16.783 ꢂ 16.092 Å, based on the distances
of Zn2ꢀ ꢀ ꢀZn4 and Zn3ꢀ ꢀ ꢀZn1, in the 3D open framework (Fig. 3c). To
reduce the multidimensional structure to simple node and connec-
tion nets, the all Zn atoms, (cptpy)ꢁ ligands and 3-btc3ꢁ anions can
be regarded as 4-connected nodes. By analysis, compound 3 has a
{4ꢀ85}2{42ꢀ84}{86} topology (Fig. 3d).
Fig. 4. (a) The coordination environment of the ZnII ions in 4 with the partial atomic numbering scheme. Hydrogen atoms are omitted for clarity. (b) View of the 3D network
in 4. (c) Perspective of the 3D framework along the b axis, with three kinds of channels. (d) View of the 3D topological network in 4. The l
3-(cptpy)ꢁ anions, 2-btc3ꢁ anions
and ZnII ions are marked as light orange, lavender and turquiose, respectively.

214
N. Li et al. / Polyhedron 49 (2013) 207–215
COOꢁ groups are at 1550–1599 and 1470–1392 cmꢁ1, respectively.
The difference between as(COO) and s(COO) (
< 200 cmꢁ1) of
m
m
D
compounds 1 and 2 indicates the carboxylate groups coordinate
to the ZnII ions in a bidentate-chelating coordination mode. In con-
trast, the separation between the mas(COO) and ms(COO) bands in 4 is
more than 200 cmꢁ1, confirming a mondentate bridging mode of
the carboxylate groups in 4 [33]. These results are in good agree-
ment with those of the X-ray structural analyses.
3.4. Thermogravimetric analyses
To study the thermal stabilities of these compounds, thermal
gravimetric analyses (TGA) of compounds 1–4 were performed
(Fig. S8, Supporting Information). Compound 1 first lost its uncoor-
dinated water molecules below 130 °C, the weight loss of 2.1% was
consistent with that calculated (2.3%). The framework is stable up
to 420 °C, after which temperature the framework begins to col-
lapse. The remaining residue is 5.48%, corresponding to the forma-
tion of ZnO (calcd 5.35%). For compound 2, weight loss in stages
was not observed since the compound does not contain any sol-
vent molecules. The framework started to collapse, accompanied
by the decomposition of the organic components, at 410 °C and
ended at 800 °C, leading to the formation of ZnO as a residue (obsd
17.10%, calcd 17.87%). The TGA curve of 3 shows a continuous
weight loss of 1.30% in the temperature range 30–120 °C, which
is attributed to the loss of a lattice water molecule (calcd 1.29%).
The network of this compound started to decompose when the
temperature was higher than 315 °C. The remaining weight corre-
sponds to the formation of ZnO (obsd 18.52%, calcd. 20.27%). For
compound 4, the framework is stable up to 300 °C, when the
framework begins to collapse. The remaining weight of 22.67% cor-
responds to ZnO (calcd. 23.40%).
Fig. 5. The solid-state emission spectra of Hcptpy, 1, 3 and 4 at room temperature.
5.48° (N3 pyridyl ring). The pendant benzene ring is rotated by
36.05° relative to the central pyridyl ring.
Notably, each (cptpy)ꢁ anion, which adopts a l3 g1 g1 1 coor-
- : :g
dinationmode(modeV), connectstwoZn2andoneZn1atoms, while
the 2-btc3ꢁ anion, acting as a pentadentate ligand, links two Zn2 and
three Zn1 atoms. Based on these connection modes, two Zn2 centers
are bridged by four carboxylate groups of two 2-btc3ꢁ anions to form
a {Zn2O4C8} unit, while two Zn1 centers are connected to the {Zn2O4
C8} unit by another two oxygen atoms of the same two carboxylate
groups to form a [Zn4(2-btc)4] unit. Thus, such subunits join each
other to construct a chain of [Zn4(2-btc)4]n. In 4, the tetranuclear ZnII
subunits are further interconnected via two nitrogen atoms of
tridentate-bridging (cptpy)ꢁ anions to generate a layer (Fig. S7). Fi-
nally, these layers are linked by carboxylate groups of (cptpy)ꢁ and
2-btc3ꢁ anions to form ultimately a 3D network (Fig. 4b). It is worth
noting that the 3D framework contains three kinds of channels,
namely A, B and C, along the b axis, which include a 18-membered
ring (Zn2O4C12), a 28-membered ring (Zn4O4N6C14) and a 36-
membered ring (Zn4O4N6C22), respectively (Fig. 4c). In these rings,
the largest is formed by two (cptpy)ꢁ and two 2-btc3ꢁ anions con-
nected end to end (Zn4O4N6C22), which produced a grid pore with
a size of 13.657 ꢂ 13.811 Å. From a topological view, the Zn1 and
Zn2 atoms can be regarded as 4-connected nodes, with the (cptpy)ꢁ
and 2-btc3ꢁ anions acting as 3-connecting and 5-connecting nodes.
Therefore, the whole structure can thus be represented as a
{4ꢀ6ꢀ8}{42ꢀ6ꢀ83}{42ꢀ62ꢀ82}{43ꢀ62ꢀ85} network topology (Fig. 4d).
3.5. Luminescent properties
Inorganic–organic hybrid coordination compounds, especially
with d10 metal centers, have been investigated for their fluorescent
properties owing to their potential application as luminescent
materials [34]. The photoluminescent properties of Hcptpy and
compounds 1–4 were investigated at room temperature. Unfortu-
nately, compound 2 exhibits only a very weak emission. The emis-
sion spectra of compounds 1, 3, 4 and Hcptpy are depicted in
Fig. 5. The ligand Hcptpy exhibits one emission band at 403 nm upon
excitation at 290 nm. The emission peak at 407 nm (kex = 300 nm) in
1 shows a slightly red shift relative to that of free Hcptpy, which may
be assigned to the
p ?
p⁄ intraligand fluorescence emission of the
Hcptpy ligand [35]. The maximum emission spectra of 3 and 4 can
be observed at 403 and 384 nm, respectively, under the same exci-
tation conditions (kex = 290 nm). On the contrary, a relatively weak
band in compound 3 is due to Zn atom fluorescence quenching,
resulting from the abundant vibrating lattice water molecules,
which give rise to the loss of energy by a non-radiative transition.
Compared with the Hcptpy ligand, the enhancement and blue-shift
in 4 is probably due to the unique coordination of the Hcptpy ligand
to the Zn center, increasing the ligand’s conformational rigidity and
thereby reducing the non-radiative decay of the intraligand
3.3. IR spectroscopy
In the IR spectrum of Hcptpy, the characteristic bands which ap-
pear at 3445 and 1715 cmꢁ1 can be attributed to O–H and C@O
stretching vibrations. The bands at 1631 and 1494 cmꢁ1 can be
attributed to the typical C@C and C@N stretching vibrations of pyr-
idyl and benzene groups. The band at 1401 cmꢁ1 is due to the ring
deformation mode of C–C vibrations, and the peak at 769 cmꢁ1 is
assigned to the out-of-ring bend of the C–H stretching vibrations
of pyridyl groups [31]. The IR spectra of 1–4 also exhibit character-
(p ?
p⁄) excited state [36]. To this point, the fluorescence quench-
ing can be minimized with a rigid fluorophore-ligand architecture,
which electronically decouples the metal cation from the fluores-
cence emitter.
istic bands for (cptpy)ꢁ at about 1600 (
m(C@C)), 1460 (m(C@N)) and
1400 cmꢁ1 (C–C ring deformation mode). The peak at about
770 cmꢁ1 is assigned to the out-of-ring bend of C–H stretching
vibrations of the aromatic rings, which means the para-position
of the benzene ring is substituted. The absence of any strong bands
at around 1700 cmꢁ1 for 2, 3 and 4 indicate that the carboxyl groups
of the organic-acid and Hcptpy ligand are completely deprotonated
[32]. The asymmetric and symmetric stretching vibrations of the
4. Conclusions
In summary, a series of coordination polymers 1–4 with various
architectures were constructed from a new type of bifunctional

N. Li et al. / Polyhedron 49 (2013) 207–215
215
ligand, Hcptpy, and the transition metal ZnII ion. It is clearly veri-
fied by compounds 1 and 2 that Hcptpy has the ability of adjusting
its coordination configuration and mode under different pH condi-
tions. It was found that compound 3 is distinguished from 4 due to
the effect of different coordination sizes of the auxiliary ligand.
Moreover, the difference in the fluorescence intensities of com-
pounds 1 and 2 is caused by the diversity of the coordination
structures. Compound 1 demonstrates notable fluorescent proper-
ties and possesses high thermo-stability, which indicates that it
might be used in the area of photoluminescence. These results also
provide a valuable insight into the rational design of variable
dimensional materials and the synthesis of polynuclear clusters
of other transition or lanthanide metals. Further studies on synthe-
sizing other metal–organic frameworks with this ligand are under
way in our laboratory.
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Appendix A. Supplementary material
CCDC 883237-883240 contain the supplementary crystallo-
graphic data for compounds 1–4. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
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