Metal(II) coordination polymers based on a flexible N,N′,N″- tris(3-pyridyl)-1,3,5-benzenetricarboxamide ligand and organic polycarboxylate ligands: Syntheses, structures, and luminescence

Article abstract of DOI:10.1016/j.poly.2012.10.047

Four metal(II) coordination polymers based on a flexible ligand N,N′,N″-tris(3-pyridyl)-1,3,5-benzenetricarboxamide (L), namely, [Co(L¹)(L)]_n (1), {[Cd(L¹)(L)]·0.5H ₂O}_n (2), {[Zn_1.5(L²)(L)(H ₂O)₂]·1.5H₂O}_n (3) and {[Co_1.5(L²)(L)(H₂O)₂]·1. 5H₂O}_n (4), where H₂L¹ = 1,4-benzenedicarboxylic acid and H₃L² = 1,3,5-benzenetricarboxylic acid, have been synthesized under hydrothermal conditions. The structure analyses reveal that compounds 1 and 2 are isomorphic three-dimensional networks and posses (3,5)-connected nets with (4 ²·6)₂(4²·6⁴· 8¹⁰·10) topology. Both 3 and 4 are isomorphic layered structures, which can be simplified as 2D → 2D interpenetrating undulated 6³-hcb nets. The structural differences indicate that the effect of polycarboxylate ligands play an important role in the resulting structures of these networks. Moreover, thermal properties and luminescent properties were also investigated for the compounds.

Full text of DOI:10.1016/j.poly.2012.10.047

Polyhedron 50 (2013) 193–199
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Metal(II) coordination polymers based on a flexible
N,N0,N00-tris(3-pyridyl)-1,3,5-benzenetricarboxamide ligand and organic
polycarboxylate ligands: Syntheses, structures, and luminescence
⇑
Hong-Ye Bai, Shi-Meng Wang, Wei-Qiang Fan, Chun-Bo Liu, Guang-Bo Che
School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Four metal(II) coordination polymers based on a flexible ligand N,N0,N00-tris(3-pyridyl)-1,3,5-benzenetri-
carboxamide (L), namely, [Co(L1)(L)]n (1), {[Cd(L1)(L)]ꢀ0.5H2O}n (2), {[Zn1.5(L2)(L)(H2O)2]ꢀ1.5H2O}n (3) and
{[Co1.5(L2)(L)(H2O)2]ꢀ1.5H2O}n (4), where H2L1 = 1,4-benzenedicarboxylic acid and H3L2 = 1,3,5-benzene-
tricarboxylic acid, have been synthesized under hydrothermal conditions. The structure analyses reveal
that compounds 1 and 2 are isomorphic three-dimensional networks and posses (3,5)-connected nets
with (42ꢀ6)2(42ꢀ64ꢀ810ꢀ10) topology. Both 3 and 4 are isomorphic layered structures, which can be simpli-
fied as 2D ? 2D interpenetrating undulated 63-hcb nets. The structural differences indicate that the
effect of polycarboxylate ligands play an important role in the resulting structures of these networks.
Moreover, thermal properties and luminescent properties were also investigated for the compounds.
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 14 August 2012
Accepted 31 October 2012
Available online 16 November 2012
Keywords:
Coordination polymers
Polycarboxylate ligands
Luminescence
1. Introduction
excellent coordinating ability, and many conformations can be
induced by metal salts due to their flexibility and low symmetry
The design and syntheses of porous coordination polymers
(PCPs), also called metal–organic frameworks (MOFs) are of great
current interest due to their potential applications in storage, rec-
ognition, adsorption, magnetism, and catalysis as well as their
intriguing variety of architectures and fascinating new topologies
[1]. The most common approach to build coordination frame works
(MOFs) is rational combination of suitable organic ligand and me-
tal salt with specific coordination geometry [2]. The organic ligand
is important because changing the structure of the ligand can lead
to various interesting porous MOFs. Accordingly, a careful choice or
design of ligands with suitable groups is the key for the construc-
tion of functional coordination compounds with desired structural
features and physical–chemical properties [3].
In recent years, there has been much attention in the use of three-
connected centers as basic structural units for the construction of
framework materials. Especially flexible tripodal ligands such
as 1,3,5-tris(imidazol-1-ylmethyl)benzene, 1,3,5-tris(imidazol-1-
ylmethyl)-2,4,6-trimethylbenzene, 1,3,5-tris(4-pyridylmethyl)ben-
zene, 1,3,5-tris(pyrazol-1-ylmethyl)-2,4,6-triethylbenzene, etc. [4],
have been widely used in the construction of MOFs. Compared with
the rigid tripodal ligands, the flexible tripodal ligand N,N0,N00-tris(3-
pyridyl)-1,3,5-benzenetricarboxamide ligand (L, Scheme 1), has
[5]. Besides, it may show much more possible coordination modes
because the existence of three carbonyl oxygen atoms and three pyr-
idine nitrogen atoms.
On the other hand, polycarboxylate organic ligands are extre-
mely important in the synthesis of MOFs because of the diversity
of the coordination modes with the metal ions, high structural sta-
bility and sensitivity to pH values of the carboxylate groups [6]. A
large number of metal-carboxylate frameworks have been re-
ported so far, and some groups altered carboxylate ligands in a
two-ligand system in order to obtain different dimensional archi-
tectures ranged from one to three dimensional [7]. However, be-
cause of the complexity and difficult prediction of the resulting
composition or structure, the influential principles in a two-ligand
system are less ascertained and not conclusive [8].
On the basis of the above considerations, in this paper, we fo-
cused our attention on two-ligand assembly reactions of the flexible
tripodal ligands N,N0,N00-tris(3-pyridyl)-1,3,5-benzenetricarboxam-
ide ligand (L), together with different carboxylate ligands and differ-
ent metal salts. Five novel metalꢁorganic frameworks, [Co(L1)(L)]n
(1), {[Cd(L1)(L)]ꢀ0.5H2O}n (2), {[Zn1.5(L2)(L)(H2O)2]ꢀ1.5H2O}n (3)
and {[Co1.5(L2)(L)(H2O)2]ꢀ1.5H2O}n (4), where H2L1 = 1,4-benzenedi-
carboxylic acid and H3L2 = 1,3,5-benzenetricarboxylic acid
(Scheme 2), have been obtained under hydrothermal conditions.
Their single-crystal structures, thermal stability and luminescent
properties are systematically investigated.
⇑
Corresponding author. Tel.: +86 511 887901877.
E-mail address: guangbocheujs@yahoo.com.cn (G.-B. Che).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.10.047

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H.-Y. Bai et al. / Polyhedron 50 (2013) 193–199
11.61. Found: C, 53.25; H, 3.10; N, 11.81%. IR (cmꢁ1): 3455(w),
2. Experimental
3240(w), 1678 (s), 1654(m), 1543(s), 1540(s), 1482(s), 1427(w),
1382(w), 1297(m), 1232(m), 1131(w), 1046(m), 841(s), 746(s),
695(m), 635(s), 520(s).
2.1. Materials and physical measurements
The L ligand was prepared according to the literature method
[9]. All other chemicals purchased were of reagent grade or better
and were used without further purification. Elemental analyses
were performed with a Perkin-Elmer 240C element analyzer. IR
spectra were recorded as KBr pellets in the range 4000–400 cmꢁ1
on Perkin-Elmer spectrometer. Thermogravimetric analysis (TGA)
was performed on a NETZCH STA 449C with a heating rate of
10 minꢁ1 under in an air atmosphere. The Excitation and emission
spectra of the ligands and compounds in the solid state were mea-
sured on a Perkin-Elmer LS 55 luminescence spectrometer. The de-
cay lifetimes and the quantum yields of these compounds were
measured on a Perkin-Elmer FLS-920 spectrometer. The excitation
wavelengths in photoluminescence quantum yield measurement
were set as 330 nm, and the scan range of emission spectra were
350–650 nm with a scan step of 1 nm.
2.2.3. Synthesis of {[Zn1.5(L2)(L)(H2O)2]ꢀ1.5(H2O)}n (3)
A mixture of ZnCO3 (0.075 mmol, 0.0125 g), H3L2 (0.05 mmol,
0.0105 g), L (0.05 mmol, 0.0219 g), and water (10 mL) was adjusted
and then heated at 120 °C in a Teflon-lined autoclave for 3 days,
followed by slow cooling to room temperature. The resulting color-
less prismatic crystals were collected. Elemental Anal. Calc. for C66-
H56Zn3N12O25: C, 49.13; H, 3.50; N, 10.42. Found: C, 49.26; H, 3.68;
N, 10.65%. IR (cmꢁ1): 3426(m), 3086(w), 1683(s), 1627(s), 1542(s),
1486(s), 1411(s), 1311(m), 1298(m), 1232(s), 1191(m), 1068(w),
917(m), 884(w), 805(s), 720(m), 649(m), 569(m).
2.2.4. Synthesis of{[Co1.5(L2)(L)(H2O)2]ꢀ1.5(H2O)}n (4)
The synthesis of compound 4 was carried out as described
above for compound 3 except that CoCO3 and heated at 120 °C
were used instead of ZnCO3 and heated at 150 °C, respectively.
The resulting purple prismatic crystals were collected (yield, ca.
30% based on Co). The yield of the colorless prismatic crystals is
2.2. Syntheses of the metal compounds
2.2.1. Synthesis of [Co(L1)(L)]n (1)
A mixture of CoCO3 (0.1 mmol, 0.0119 g), H2L1 (0.1 mmol,
0.0166 g), L (0.05 mmol, 0.0219 g), and water (10 mL) was adjusted
and then heated at 120 °C in a Teflon-lined autoclave for 3 days,
followed by slow cooling to room temperature. The resulting pur-
ple prismatic crystals were collected. Elemental Anal. Calc. for C32-
H22CoN6O7: C, 58.10; H, 3.35; N, 12.70. Found: C, 58.25; H, 3.24; N,
12.81%. IR (cmꢁ1): 3280(w), 1684(s), 1644(m), 1583(m), 1540(s),
1481(s), 1424(w), 1391(w), 1294(s), 1239(m), 1127(w), 1047(m),
801(s), 754(s), 700(s), 642(m), 588(m).
ca. 30% based on Zn. Elemental Anal. Calc. for C66H56Co3N12O25
:
C, 49.73; H, 3.54; N, 10.54. Found: C, 49.57; H, 3.72; N, 10.61%. IR
(cmꢁ1): 3452(m), 2979(w), 2926(w), 1681 (s), 1619(s), 1558(s),
1479(s), 1417(s), 1356(m), 1330(m), 1295(s), 1233(m), 1128(w),
1058(w), 917(w), 803(m), 724(m), 575 (m).
2.3. Crystal structure determination
Single-crystal X-ray diffraction data of the four compounds
were recorded on an Oxford Diffraction Gemini R Ultra diffrac-
tometer with graphite-monochromated Mo
2.2.2. Synthesis of {[Cd(L1)(L)]ꢀ0.5H2O}n (2)
K
a
radiation
The synthesis of compound 2 was carried out as described
above for compound 1 except that CdCO3 were used instead of
CoCO3. The resulting colorless prismatic crystals were collected.
Elemental Anal. Calc. for C32H23CdN6O7.5: C, 53.09; H, 3.20; N,
(k = 0.71073 Å) at 293 K. Absorption corrections were applied using
multi-scan technique. All the structures were solved by Direct
Method of SHELXS-97 and refined by full-matrix least-squares tech-
niques using the SHELXL-97 program [10]. Non-hydrogen atoms of
the compounds were refined with anisotropic temperature param-
eters except the disordered atoms. N6 and C29 to C33 in com-
pounds 3 and 4 are disordered and were split over two sites with
a total occupancy of 1. The hydrogen atoms of the disordered
atoms were not included in the model. The hydrogen atoms at-
tached to carbon atoms were generated geometrically and refined
using a riding model with d(C–H) = 0.93 Å, Uiso = 1.2 Ueq(C) for aro-
matic atoms. The hydrogen atoms attached to oxygen and nitrogen
atoms were located from difference Fourier map. The hydrogen
atoms were assigned with common isotropic displacement factors
and included in the final refinement by using geometrical restrains.
However, some water H atoms of compounds 2, 3, and 4 were not
included in the model. The detailed crystallographic data and
structure refinement parameters for these compounds are summa-
rized in Table 1. Selected bond distances and angles and hydrogen
bonds for them are listed in S1a–S4b (Supporting Information).
Scheme 1. Structure of the N,N0,N00-tris(3-pyridyl)-1,3,5-benzenetricarboxamide (L)
ligand.
3. Results and discussion
3.1. Syntheses of the compounds
The hydrothermal method has been proved to be an effective
way to synthesize compound crystals. Under hydrothermal condi-
tions, the properties of the reactants are quite different from those
under conventional conditions. So some fantastic structures and
novel topologies may be produced under hydrothermal reactions.
Compounds 1–4 were prepared by reactions of polycarboxylic
Scheme 2. Structures of the carboxylic acids used in this work.

H.-Y. Bai et al. / Polyhedron 50 (2013) 193–199
195
Table 1
Crystal data and structure refinement for compounds 1ꢁ4.
Compound
1
2
3
4
Formula
C
32H22CoN6O7
C32H23CdN6O7.5
723.96
C66H56Zn3N12O25
1613.34
C66H56Co3N12O25
1594.02
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
661.49
triclinic
P1
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
8.7692(18)
11.736(2)
15.474(3)
105.41(3)
105.41(3)
106.62(3)
1443.6(5)
2
1.522
678
1.050
0.0406
8.6957(17)
11.849(2)
16.089(3)
97.21(3)
104.71(3)
108.56(3)
1481.1(5)
2
1.623
730
1.058
0.0261
10.282(2)
11.063(2)
14.673(3)
99.64(3)
91.00(3)
100.59(3)
1615.4(6)
1
1.658
826
1.032
0.0438
0.1077
10.228(2)
11.060(2)
14.757(3)
100.10(3)
90.91(3)
100.58(3)
1613.5(6)
1
1.640
817
1.038
0.0424
0.1090
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalcd (g cmꢁ3
F(000)
GOF on F2
)
R1a[I > 2
r
(I)]
r
wR2b[I > 2
(I)]
0.0877
0.0559
a
R1
wR2 = |
=
R
||Fo| ꢁ |Fc||/
R|Fo|.
b
R
w(|Fo|2 ꢁ |Fc|2)|/
R
|w(Fo2)2|1/2
.
Fig. 1. (a) Coordination environment around the Co(II) centers in 1. Hydrogen atoms are omitted for clarity. Symmetry code: #1 x, y ꢁ 1, z; #2 ꢁx + 2, ꢁy + 1, ꢁz + 1; #3
ꢁx + 1, ꢁy, ꢁz + 2; #4 ꢁx + 1, ꢁy + 1, ꢁz + 1. (b) The chain constructed by Co(II) atoms and L1 ligands. (c) The ladder-like chain constructed by Co(II) atoms and L ligands. (d)
The 3D structure of compound 1. (e) The (3,5)-connected net with (42ꢀ6)2(42ꢀ64ꢀ810ꢀ10) topology of compound 1.

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H.-Y. Bai et al. / Polyhedron 50 (2013) 193–199
acids (H2L1 and H3L2), metal carbonates (CoCO3, CdCO3 and ZnCO3),
and L ligand under hydrothermal condition.
In addition, hydrogen bonding interactions (N1ꢀ ꢀ ꢀO2#6 =
3.042 Å, N2ꢀ ꢀ ꢀN4#1 = 2.947 Å and N3ꢀ ꢀ ꢀO2#6 = 2.895 Å) (symme-
try code: #1 x, y ꢁ 1, z; #6 x + 1, y + 1, z) contained in this structure
presumably help to further reinforce the 3D framework
(Table S1b).
3.2. Description of crystal structures
3.2.1. Crystal structure of [Co(L1)(L)]n (1)
3.2.2. Crystal Structure of {[Cd(L1)(L)]ꢀ0.5H2O}n (2)
ꢀ
Compound 1 crystallizes in the monoclinic space group P1. The
CdCO3 was selected to react with L1 and L under similar syn-
thetic conditions, and a new compound 2 was obtained. Compound
2 is isomorphous with 1. Nevertheless, some differences exist be-
tween the two compounds.
results of crystallographic analysis reveal that the asymmetric unit
of compound 1 consists of one Co(II) atom, two half of L1, and one L
ligand. As shown in Fig. 1a, each Co(II) atom is six-coordinated by
three O atoms of two carboxylate groups from two L1 and two N
atoms and one O atom from three L ligands in octahedral coordina-
tion geometry. The Co–N bond distances are 2.159(2) and
2.180(2) Å and the Co–O bond lengths are in the range of
2.0537(17)–2.1902(19) Å (Table S1a).
Firstly, as shown in Fig. 2a, compound 2 contains one Cd(II)
atom, two halves of L1, one L ligand, and two quarters of uncoordi-
nated solvent molecule. While compound 1 does not contain any
free water molecules. Secondly, Co(II) atom in compound 1 is
six-coordinated displaying distorted octahedral coordination
geometry. Whereas Cd(II) atom in compound 2 is seven-coordi-
nated by four O atoms from two carboxylate groups of two inde-
pendent L1 ligands and two N atoms and one O from three
independent L ligands in a pentagonal bipyramid environment
(Fig. 2a). The Cd–N bond distances are 2.3044(16) and 2.343(2) Å,
respectively and the Cd–O bond lengths are 2.3046(19)–
2.5174(18) Å (Table S2a). Moreover, both of the two kinds of L1
in compound 1 bridge two Cd(II) atoms in a chelating coordination
mode to generate a chain (CdꢁL1 chain) (Fig. 2b), whereas the two
kinds of L1 in compound 1 adopt different coordination modes.
In addition, the structural stability of compound 2 is also stabi-
lized by hydrogen bonding interactions between the N atom of the
The carboxylate groups of one kind of L1 connect the Co(II) in
monodentate mode, while those of the other kind of L1 adopt a
chelating coordination mode. The two kinds of L1 ligands link the
Co(II) atoms alternately, forming a chain (Fig. 1b). In addition, each
ligand L coordinates to three Co(II) atoms through one Co–O bond
and two Co–N bonds in mode I as a tridentate ligand (Scheme 3),
forming a ladder-like chain along b axis (Fig. 1c). The two kinds
of chains are further connected by the sharing Co(II) centers,
resulting in a 3D network of compound 1 (Fig. 1d).
From the topological view, the L ligands can be viewed as 3-
connected nodes, while the Co(II) centers can be considered as 5-
connected nodes. Thus, the whole 3D framework of compound 1
can
be
simplified
as
a
(3,5)-connected
net
with
L
ligand and
O
atoms of the L1 (N4ꢀ ꢀ ꢀO4#6 = 2.946 Å and
(42ꢀ6)2(42ꢀ64ꢀ810ꢀ10) topology (Fig. 1e).
N6#6ꢀ ꢀ ꢀO4 = 3.245 Å) and between the N atom of the two L
(N5ꢀ ꢀ ꢀN1#5 = 2.925 Å) (symmetry codes: #5 x, y ꢁ 1, z; #6 x + 1,
y, z) (Table S2b).
3.2.3. Crystal structures of {[Zn1.5(L2)(L)(H2O)2]ꢀ1.5H2O}n (3) and
{[Co1.5(L2)(L)(H2O)2]ꢀ1.5H2O}n (4)
In order to explore the influence of organic polycarboxylate li-
gands on the structures of the resultant compounds, carboxylate li-
gands H3L2 were introduced to react with MCO3 and the L ligand
under similar reaction conditions, and two new isomorphic com-
pounds {[Zn1.5(L2)(L)(H2O)2]ꢀ1.5H2O}n (3) and {[Co1.5(L2)(L)(H2-
O)2]ꢀ1.5H2O}n (4) were obtained. Here we just take compound 3
for example and discuss its structure in detail.
The X-ray crystallographic study shows that there are two crys-
tallographically independent Zn(II) centers with different coordi-
nation environments, one crystallographically independent
ligands L, one L2 ligand, two coordinated water molecules, and
Scheme 3. The coordination modes of the L ligand.
Fig. 2. (a) ORTEP view of 2 showing the local coordination environment of Cd(II) atom with hydrogen atoms omitted for clarity. Symmetry code: #1 x, y + 1, z; #2 ꢁx + 1, ꢁy, ꢁz;
#3 ꢁx, ꢁy + 1, ꢁz; #4 ꢁx, ꢁy, ꢁz + 1. (b) The CdꢁL1 chain in compound 2.

H.-Y. Bai et al. / Polyhedron 50 (2013) 193–199
197
Fig. 3. (a) ORTEP view of 3 showing the local coordination environment of Zn(II) atom with hydrogen atoms omitted for clarity. N6 and C29–C33 are statistically disordered
over two sites, and here only one disorder position is shown for clarity. Symmetry code: #1 ꢁx, ꢁy ꢁ 2, ꢁz + 2; #2 x + 1, y, z; #3 ꢁx + 2, ꢁy, ꢁz + 2. (b) The one-dimensional
structure of Zn–L2. (c) The four-membered ring of (Zn2)2L2. (d) The layer of compound 3. (e) The 63-hcb net of compound 3. (f) A view of the 2D ? 2D interpenetrating
undulated 63-hcb net.
one and a half of lattice water molecules in the asymmetric unit of
compound 3 (Fig. 3a). N6 and C29–C33 of one pyridyl ring in L li-
gand are disordered over two sites. The Zn1 is six-coordinated by
four water molecules in the equatorial positions and two O atoms
from carboxylate groups of two L2 in the apical positions showing
octahedral coordination geometry with the Zn–O bond distances in
the range of 2.051(2)–2.175(3) Å. The Zn2 is four-coordinated by
two N atoms from two independent L ligands and two O atoms
from carboxylate groups of two L2 showing an slightly distorted
tetrahedral geometry with the Zn–N bond distances of 2.034(3)
and 2.038(3) Å and the Zn–O bond lengths of 1.983(2) and
1.989(2) Å respectively.
10.282 Å) (Fig. 3b). Each L ligand coordinates to two Zn2(II) atoms
as a bidentate ligand in mode II (Scheme 3) through two Zn–N
bonds, forming a four-membered ring (Zn2)2L2 (Fig. 3c). Further,
the (Zn2)2L2 rings are bridged by the neighboring ladder-like
chains to construct a layer of compound 3 (Fig. 3d).
Topologically, the layer of compound 3 can be simplified as an
undulated honeycomb net (63-hcb) (Fig. 3d), if the Zn2 and L2
are considered as 3-connected nodes and the Zn1 and L are viewed
as linkers. Furthermore, the a more impressive structural charac-
teristic of compound 3 is that there are two identical 2D undulated
single nets interpenetrating in a 2D ? 2D parallel fashion, as
shown in (Fig. 3f).
Each L2 ligand connects two Zn2(II) atoms and one Zn1(II)
atoms to form a ladder-like chain with the Zn1 on the rungs
(Zn1ꢀ ꢀ ꢀZn1 10.282 Å) and the Zn2 on the siderails (Zn2ꢀ ꢀ ꢀZn2
Besides, the crystal structure of compound 3 is further strength-
ened through hydrogen bonding interactions between the uncoor-
dinated water molecules and the ligands (Table S3b).

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H.-Y. Bai et al. / Polyhedron 50 (2013) 193–199
structures of the compounds in this experimental system. Never-
theless, the structural differences between compounds 1 and 4
clearly illustrate that the carboxylate anions play an important
roles in structures. In this work, compounds 1 and 4 were synthe-
sized under similar reaction conditions, except using different aux-
iliary ligands (H2L1 for 1; H3L2 for 4), which result in two
distinctive structures, Compound 1 shows a 3D structure and 4
possesses a layer structure. Obviously, the distinctive structures
are attributed to the difference of carboxylate groups’ numbers of
the carboxylate ligands.
3.4. Thermal analyses
To examine the thermal stability of these compounds, thermo-
gravimetric analysis (TGA) were carried out. As shown in Fig. S1,
the TGA curves of compounds 1 and 2 are similar, as well as 3
and 4 due to their similar structures. For compound 1, the decom-
position of the compound occurs at 346 °C, indicating high thermal
stability of the frameworks. For compound 2, the decomposition of
the framework occurs from 372 °C. The loss of lattice water mole-
cules of 2 was not observed. It is probably that the lattice water
molecules lost in the air. For 3 and 4, the gradual weight loss be-
tween about 87 and 154 °C can be attributed to the release of
water molecules (obsd 5.8%; calcd 7.81% for 3, and obsd 5.4%; calcd
7.90% for 4). Then no obvious weight loss is observed until the
decomposition of the frameworks occurs at 340 °C.
Fig. 4. Solid-state photoluminescent spectra of L and compounds 2 and 3 at room
temperature.
3.3. Discussion
3.3.1. Coordination modes of L ligand
From the structure description above, we can see that the
N,N0,N00-tris(3-pyridyl)-1,3,5-benzenetricarboxamide (L) ligand
can bend and rotate freely when coordinating to the central metals
due to the flexible nature of the ligand. Moreover, L have six poten-
tial coordination nodes (three carbonyl oxygen atoms and three
pyridine nitrogen atoms), which can participate in coordination
and greatly beautify and enrich the MOFs’ structures. Thus, the L
ligand can show many possible coordination modes. In compounds
3.5. Luminescent properties
Coordination compounds with d10 metal centers and conju-
gated organic linkers are promising candidates for photoactive
materials with potential applications in chemical sensors, photo-
chemistry and electroluminescent display [11]. Hence, the solid-
state photoluminescence properties of ligand L, together with
and compounds 2 and 3 were investigated at room temperature
(Fig. 4). The main emission peak of L is at 441 nm (kex = 335 nm),
1–4 the
L ligand show two different coordination modes
(Scheme 3). In 1 and 2, L links the metal centers through one Co–
O bond and two Co–N bonds in mode I as a tridentate ligand. Nev-
ertheless, in compounds 3 and 4, the L ligands act as bidentate li-
gands bridging two metal ions through pyridyl nitrogen atoms in
mode II.
which is ascribed to the p⁄ ? n or p⁄
? p electronic transition
[12]. Compared with the luminescence of free ligand, the emissions
of 2 (kem = 401 nm, kex = 326 nm), and 3 (kem = 410 nm, kex = 332 -
nm) undergo some blue-shifts. Because the Zn(II) and Cd(II) ions
are difficult to oxidize or to reduce, the emissions of compounds
2 and 3 is neither metal-to-ligand charge transfer (MLCT) nor li-
gand-to-metal charge transfer (LMCT) in nature, which can be as-
signed to the intraligand transitions of the L ligands. The blue
3.3.2. Influence of the auxiliary ligand on the structures of compounds
We have selected two polycarboxylate ligands (H2L1 to H3L2) as
auxiliary ligands. Though different metal carbonates are used, but
the structures of compounds 1 and 2 are the same, as well as 3 and
4. Thus, the metal centers do not show influence in the resultant
Fig. 5. The decay lifetime curves of 2 and 3 in the solid state. The black circles represent experimental data, and the solid red lines represent fitting results). (Color online.)

H.-Y. Bai et al. / Polyhedron 50 (2013) 193–199
199
shifts may be attributed to the coordination effects of the L ligands
Appendix A. Supplementary data
to metal cations, which increases the ligand conformational rigid-
ity and reduces the non-radiative decay of the intraligand [13].
The fluorescence lifetimes of 2 and 3, s, are investigated in the
solid state at room temperature, and the curves of the fluorescence
CCDC 886475–886478 contains the supplementary crystallo-
graphic data for 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, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2012.10.047.
decay of them are illustrated in Fig. 5. The decay curves of them are
well fitted into a single-exponential function as I = Aexp(ꢁt/
s) + y0
with luminescence lifetimes 2 = 1.53 ns and 3 = 1.80 ns, respec-
s
s
tively. The luminescent lifetimes of these compounds are much
shorter than the lifetime of the emission resulting from a triplet
state (>10ꢁ3 s), indicating the emissions should arise from the sin-
glet state [14]. The nanosecond range of the lifetimes in the solid
state at room temperature reveals that their emissions are fluores-
cent in nature.
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Acknowledgments
The authors are grateful for the National Natural Science Foun-
dation of China (Grant No. 21201085), Natural Science Foundation
of Jiangsu Province (BK2012294), Start-Up Foundation of Jiangsu
University (11JDG105, 11JDG153, 11JDG104 and 09JDG001), Post-
doctoral Science Foundation of China (2012M511202), and Post-
Doctoral Research Funding Schemes of Jiangsu Province
(1102124C) for Support.
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