Metal-organic frameworks (MOFs) constructed from ZnII/CdII-2,2′-bipyridines and polycarboxylic acids: Synthesis, characterization and microstructural studies

Article abstract of DOI:10.1016/j.poly.2008.11.059

Metal-organic frameworks with the compositions [Zn(bpy)(bdc)(H₂O)]_n 1, [Zn(bpy)(btec)_1/2(H₂O)]_n 2, [Cd(bpy)(bdc)(H₂O)]_n 3 and Cd(bpy)(btec)_1/2(H₂O)]_n

Full text of DOI:10.1016/j.poly.2008.11.059

Polyhedron 28 (2009) 600–608
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Metal–organic frameworks (MOFs) constructed from ZnII/CdII-2,20-bipyridines and
polycarboxylic acids: Synthesis, characterization and microstructural studies q
b
c
Rishikesh Prajapati a, Lallan Mishra a, , Keisaku Kimura , Pallepogu Raghavaiah
*
a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
b Graduate School of Material Science, University of Hyogo, Hyogo 678-1297, Japan
c National Single Crystal X-ray Diffractometer Facility, School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal–organic frameworks with the compositions [Zn(bpy)(bdc)(H2O)]n 1, [Zn(bpy)(btec)1/2(H2O)]n 2,
[Cd(bpy)(bdc)(H2O)]n and Cd(bpy)(btec)1/2(H2O)]n (H2bdc = 1,4-benzenedicarboxylic acid = tere-
Received 15 August 2008
Accepted 26 November 2008
Available online 8 January 2009
3
4
phthalic acid, H4btec = 1,2,4,5-benzenetetracarboxylic acid and bpy = 2,20-bipyridine) have been synthe-
sized and characterized using spectroscopic and single-crystal X-ray diffraction techniques. In these
complexes, ZnII/CdII-2,20-bipyridine units and carboxylate anions exists as nodes and spacers respectively.
An infinite 1D zig-zag chain structure is observed for both complexes 1 and 3, whereas complexes 2 and 4
display a 3D supramolecular architecture. The complexes are found to be photoluminescent, porous and
show significant thermal stability.
Keywords:
2,20-Bipyridyl metal complexes
Polycarboxylate ligands
Metal–organic framework
Photoluminescence
Microstructures
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
as molecular tethers for this work. These ligands can link metal
cations into coordination polymer chains, layers or networks, in
Recent years have witnessed rapid progress made in the con-
struction of metal–organic frameworks (MOFs) owing to their
intriguing molecular topologies [1–7] and potential applications
in porosity [8–10], catalysis [11–15], ion exchange [16], chirality
[17–21], luminescence [22–26], non-linear optics (NLO) [27–31],
conductivity [32–35], optoelectronics [36,37], magnetism [38–
42], spin-transition behavior [43–46], gas storage [47–49] and
selective binding of molecules through absorption [13,50]. Most
of the current studies in this area involve d10 transition metal ions
with functional ligands. However, it is still considered a challenge
to explore successful synthetic routes for the preparation of MOFs
that could be exploited for targeted applications.
the process providing the charge balance necessary to avoid the
inclusion of porosity-inhibiting smaller anions. The disposition of
the donor groups within the dicarboxylate moieties, the coordina-
tion geometry preferences at the metal ions and the array of acces-
sible carboxylate binding modes have been shown to act
synergistically in the structure directed self-assembly of coordina-
tion polymers in solution. On the other hand, auxiliary ligands like
2,20-bipyridine also impact on the conformations of the flexible
carboxylic ligand (trans- or cis-), and play an important role in
the formation of the final structure of the complex [54]. The termi-
nal ligand 2,20-bpy tends to construct low-dimension structures
which can be extended into high-dimensional supramolecular net-
In this context, polycarboxylates are widely used in the devel-
opment of supramolecular networks organized through coordina-
works through hydrogen bonds and p–p stacking interactions.
Thus, the present article is thought to be of worth as it deals
with the synthesis and characterization of the structures obtained
on the skeleton of d10 divalent metal (ZnII,CdII)2,20-bipyridines to-
gether with 1,2-benzenedicarboxylic acid and 1,2,4,5-tetra-
carboxylic acid. Since such types of metal–organic frameworks
show the formation of porous structures [5], micro structural stud-
ies have also been carried out on these synthesized complexes.
tion bonds, hydrogen bonds and
p–p stacking interactions.
Among these anionic polycarboxylates, 1,4-benzenedicarboxylic
(H2bdc) acid and 1,2,4,5-benzenetetracarboxylic acid (H4btec) are
extensively exploited as ligands in the construction of supramolec-
ular architectures through their self-assembly [51–53].
In recent years, functional coordination polymers caught the
attention of researchers owing to their potential applications in
technology. Di-anionic dicarboxylate ligands are commonly used
2. Experimental
2.1. Materials and methods
q
A Part of the manuscript has been presented in RSC-CRSI Joint symposium NSC-
10 held during January 31–February 03, 2008 at IISc Bangalore, India.
* Corresponding author. Tel.: +91 542 6702449; fax: +91 542 2368174.
E-mail address: lmishrabhu@yahoo.co.in (L. Mishra).
Reagents of A.R. purity were purchased from standard commer-
cial sources (Sigma–Aldrich, Merck) and were used without further
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.11.059

R. Prajapati et al. / Polyhedron 28 (2009) 600–608
601
purification. Elemental analyses have been carried on a Carbo-Erba
elemental analyzer 1108, and IR spectra were recorded as KBr pel-
lets on a Varian 3100 FT-IR spectrometer. TGA measurements were
performed on a Perkin Elmer Pyris diamond TG-DTA instrument at
a heating rate of 5 °C/min in a nitrogen atmosphere with a flow
rate of 200 ml/min in the temperature range 50–800 °C. Photolu-
minescence (PL) was measured using a Hitachi F-4500 spectroflu-
orometer. M(bpy)(NO3)2 ꢀ H2O (M = ZnII/CdII) were prepared using
the procedure as reported by Sen et al. [55] for analogous
complexes.
6–8 h to give the corresponding complexes in 65–90% yield. The
complexes thus isolated were found to be insoluble in water and
common organic solvents.
2.3.1. Synthesis of [Zn(2,20-bpy)(bdc)(H2O)] 1
Using the above procedure, after heating the components to-
gether at 120 °C for 6 h then cooling to room temperature, tiny
round shaped crystals were obtained after two weeks, which were
washed several times with diethylether. Yield: ꢁ77%. Anal. Calc. for
C18H14N2O5Zn (1): C, 53.55; H, 3.50; N, 6.94. Found: C, 53.99; H,
3.68; N, 7.52%. IR (KBr, cmꢂ1): 3207 (w), 1590 (vs), 1474 (m),
1439 (s), 1376 (vs), 1313 (s) 1152 (w), 1020 (w), 840 (w), 753
(vs), 658 (m), 549 (m).
2.2. X-ray crystallographic studies
Suitable X-ray quality crystals of the complexes were grown di-
rectly from their corresponding reaction mixtures in DMF/EtOH/
H2O (6:2:0.4 by volume) at room temperature and X-ray crystallo-
graphic data were collected by mounting a single-crystal of the
sample on a glass fiber. An Oxford diffraction XCALIBUR-S and Bru-
ker SMART CCD area detector diffractometer equipped with an LN-
2 low-temperature attachment was used for the determination of
cell parameters and intensity data collection. Appropriate empiri-
cal absorption corrections using the programs multi-scan were ap-
2.3.2. Synthesis of [Zn(2,20-bpy)(btec)1/2(H2O)] 2
Complex 2 was also synthesized using a similar method as used
for the synthesis of complex 1 except that H4btec was used instead
of H2bdc. After heating at 120 °C for 8 h and then cooling to room
temperature colorless block shaped crystals were obtained after 10
days, which were washed with diethylether and dried. Yield:
ꢁ93%. Anal. Calc. for C15H11N2O5Zn (2): C, 49.41; H, 3.04; N, 7.68.
Found: C, 49.72; H, 3.38; N, 7.79%. IR (KBr, cmꢂ1): 3409 (w),
3071 (m), 1634 (vs), 1580 (s), 1483 (m), 1381 (vs), 1129 (vs),
1020 (s), 817 (m), 763 (vs), 654 (m), 540 (m), 458 (m).
plied. Monochromated Mo Ka radiation (k = 0.71073 Å) was used
for the measurements. The crystal structures were solved by direct
methods [56] and refined by full matrix least squares SHELXL-97
[57]. Drawings were carried out using MERCURY [58] and DIA-
MOND [59], and special computations were carried out with PLA-
TON [60].
2.3.3. Synthesis of [Cd(2,20-bpy)(bdc)(H2O)] 3
Complex 3 was also synthesized using a similar procedure as
used for the synthesis of complex 1 but Cd(bpy)(NO3)2 ꢀ H2O was
used instead of Zn(bpy)(NO3)2 ꢀ H2O. Colorless block shaped crys-
tals of 3 were recovered after one week, which were washed sev-
eral times with methanol followed by diethylether. Yield: ꢁ66%.
Anal. Calc. for C18H14N2O5Cd (3): C, 47.97; H, 3.13; N, 6.22. Found:
C, 48.22; H, 3.34; N, 5.92%. IR (KBr, cmꢂ1): 3368 (w), 3199 (w),
1571 (vs), 1434 (m), 1380 (vs), 1310 (m) 1153 (s), 1014 (s), 820
(s), 74 (vs), 651 (m), 534 (m).
2.3. Synthesis of the complexes
The complexes were synthesised by general procedures as re-
ported by Guo et al. [61]. In a typical synthetic procedure,
ZnII(2,20-bipyridyl)(NO3)2 ꢀ H2O (0.363 g, 1 mmol) and CdII(2,20-
bipyridyl)(NO3)2 ꢀ H2O (0.410 g, 1 mmol) were added separately
to the corresponding solutions of H2bdc (0.166 g, 1.0 mmol) and
H4btec (0.127 g, 0.5 mmol) dissolved in DMF (6 ml) and ethanol
(2 ml) at room temperature, containing an equimolar amount of
triethylamine. Few drops of 5N-HNO3 (pH 4–5) was added until
the mixture become clear. The mixture was heated at 120 °C for
2.3.4. Synthesis of [Cd(2,20-bpy)(btec)1/2(H2O)] 4
Using the above procedure, complex 4 resulted in colorless tiny
round shaped crystals after three weeks, which were washed with
methanol followed by diethylether. Yield: ꢁ69%. Anal. Calc. for
Table 1
Summary of crystallographic data for complexes 1–4.
Parameters
1
2
3
4
Formula
M
C18H14N2O5Zn
403.68
C15H11N2O5Zn
364.63
C18H14N2O5Cd
450.73
C15H11N2O5Cd
411.66
Crystal system
Temperature/K
Space group
a (Å)
b (Å)
c (Å)
Triclinic
Triclinic
Monoclinic
133 (2)
P21/c
9.961(2)
16.251(3)
11.901(2)
90
117.41(1)
90
1710.21
4
Monoclinic
298(2)
C2/c
18.991(4)
7.6220(17)
19.921(4)
90
95.695(3)
90
150(2)
150(2)
ꢀ
ꢀ
P1
P1
9.0826(11)
10.1697(12)
10.5345(15)
103.392(11)
113.643(12)
100.673(10)
823.83(18)
2
7.7085(3)
8.8712(5)
10.7317(6)
80.690(5)
69.966(5)
78.027(4)
671.17(6)
2
a
(°)
b (°)
c
(°)
V (Å)3
2869.3(11)
8
1.906
Z
Dc (Mg mꢂ3
)
1.627
1.804
1.750
Reflections collected
Reflections unique
Rint
7670
2895
0.0177
6574
2360
0.0280
16866
3893
0.0251
14148
2821
0.0271
Index ranges
ꢂ10 6 h 6 10
ꢂ12 6 k 6 12
ꢂ12 6 l 6 12
ꢂ9 6 h 6 9
ꢂ10 6 k 6 10
ꢂ12 6 l 6 12
Full-matrix, least squares on F2
0.0549
ꢂ12 6 h 6 12
ꢂ21 6 k 6 21
ꢂ15 6 l 6 15
ꢂ23 6 h 6 23
ꢂ9 6 k 6 9
ꢂ24 6 h 6 24
Refinement method
wR2
R1
0.0568
0.0212
1.059
0.0689
0.0283
0.975
0.0582
0.0251
1.058
0.0231
1.047
Goodness-of-fit (GoF)

602
R. Prajapati et al. / Polyhedron 28 (2009) 600–608
Table 2
1491 (m), 1379 (vs), 1319 (w) 1135 (m), 1014 (s), 869 (s), 769
(vs), 651 (m), 532 (m).
Selected bond lengths (Å) and angles (°) for complexes 1–4.
Compound 1
Zn(1)–O(4)
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–N(2)
Zn(1)–N(1)
O(1)–H(102)
O(1)–H(101)
O(5)–C(15)
Compound 2
Zn(1)–O(2)
Zn(1)–O(1)
Zn(1)–O(5)#1
Zn(1)–N(2)
Zn(1)–N(1)
O(1)–H(101)
O(1)–H(102)
O(3)–C(11)
Compound 3
Cd(1)–O(1)
Cd(1)–O(3)
Cd(1)–O(4)
Cd(1)–O(5)
Cd(1)–N(1)
Cd(1)–N(2)
O(5)–H(13)
O(5)–H(14)
C(11)–O(2)
Compound 4
Cd(1)–O(4)
Cd(1)–O(2)
Cd(1)–O(1)
Cd(1)–N(1)
Cd(1)–N(2)
Cd(1)–O(7)#3
O(1)–H(1A)
O(1)–H(2B)
C(11)–O(5)
2.0240(12)
2.0346(13)
2.0423(12)
2.0946(15)
2.1650(15)
0.83(3)
O(4)–Zn(1)–O(1)
O(1)–Zn(1)–N(2)
O(4)–Zn(1)–N(2)
O(2)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)
O(4)–Zn(1)–N(1)
O(4)–Zn(1)–O(2)
H(102)–O(1)–H(101)
128.65(5)
128.24(6)
103.05(5)
164.17(6)
76.88(6)
97.33(5)
94.87(5)
104(2)
3. Result and discussion
3.1. IR spectra
Major peaks observed between ꢁ1600 and ꢁ1200 cmꢂ1 are as-
0.91(3)
1.246(2)
signed to
t(bpy) and t(phenyl) of the benzenecarboxylate ligands.
The
t
as(COOꢂ) and
t
s(COOꢂ) bands are observed at ꢁ1570 and
ꢁ1380 cmꢂ1 respectively [62]. The peaks lying in the region
1.9843(14)
2.0356(17)
2.0580(14)
2.0818(19)
2.1649(18)
0.91(3)
O(2)–Zn(1)–N(2)
O(1)–Zn(1)–N(2)
O(2)–Zn(1)–O(1)
O(5)#1–Zn(1)–N(1)
O(2)–Zn(1)–N(1)
O(2)–Zn(1)–O(5)#1
N(2)–Zn(1)–N(1)
H(101)–O(1)–H(102)
125.10(6)
134.33(7)
100.04(7)
162.03(6)
103.70(6)
94.24(6)
76.85(7)
111(3)
ꢁ3400 to ꢁ3200 cmꢂ1 are assigned to
tH2O(coordinated) whereas
wH2O(coordinated) appear at ꢁ655 and
qr
H2O(coordinated) and
q
ꢁ540 cmꢂ1 respectively.
3.2. Structural description of the complexes
0.76(3)
1.215(3)
Crystallographic data and refinement details for the structural
analyses of all the complexes are summarized in Table 1. Selected
bond lengths and bond angles with their estimated standard devi-
ations are presented in Table 2 while selected parameters for weak
interactions are listed in Table 3. Molecular structures (ORTEP dia-
grams) of the complexes 1–4 are shown in Fig. 1.
2.213(10)
2.395(11)
2.371(13)
2.265(12)
2.314(12)
2.336(13)
0.943(4)
O(3)–Cd(1)–N(1)
O(3)–Cd(1)–O(4)
O(4)–Cd(1)–N(2)
N(1)–Cd(1)–N(2)
O(1)–Cd(1)–N(1)
O(5)–Cd(1)–N(1)
O(5)–Cd(1)–N(2)
O(1)–Cd(1)–O(4)
H(13)–O(5)–H(14)
88.10(5)
55.18(7)
95.51(7)
71.35(5)
157.63(6)
126.31(6)
97.41(4)
106.65(7)
112.18(5)
0.902(3)
1.249(4)
3.2.1. Complex 1 and 2
The X-ray single-crystal structure determination reveals that
complex 1 exists as a neutral one-dimensional infinite zig-zag
coordination chain. Each ZnII center is surrounded by two N donors
from the 2,20-bipyridine ring, two oxygen atoms from two carbox-
ylate groups of two different terephthalic acid molecules together
with one water molecule, in a distorted pentacoordinated geome-
try, as depicted in Fig. 1a. The neighboring ZnII centers are con-
nected by a terephthalate dianionic moiety which acts as a
bridge through its benzene ring, as shown in Fig. 2a. The two tere-
phthalate units are almost orthogonal to each other with a dihedral
angle of 88.87° and the two ZnII centres are separated at a distance
of 11.10 Å. In this structure, the 2,20-bipyridine molecule acts as a
2.2286(18)
2.2753(18)
2.309(2)
2.312(2)
2.332(2)
2.589(2)
0.79(4)
O(1)–Cd(1)–N(2)
O(2)–Cd(1)–O(1)
O(2)–Cd(1)–O(7)#3
O(1)–Cd(1)–N(1)
N(1)–Cd(1)–N(2)
O(4)–Cd(1)–N(2)
N(2)–Cd(1)–O(7)#3
O(4)–Cd(1)–O(7)#3
H(1A)–O(1)–H(2B)
88.07(9)
77.03(9)
53.18(6)
142.71(9)
70.83(7)
157.23(8)
106.24(7)
86.34(7)
108(4)
0.82(5)
1.241(3)
C15H11N2O5Cd (4): C, 43.76; H, 2.69; N, 6.80. Found: C, 44.23; H,
2.61; N, 7.02%. IR (KBr, cmꢂ1): 3405 (w), 3065 (m), 1560 (vs),
Table 3
Selected parameters for weak interactions in complexes 1–4.
D–HꢀꢀꢀA
D–H (Å)
HꢀꢀꢀA (Å)
DꢀꢀꢀA (Å)
DHA (°)
Symmetry code
1
O(1)–H(101) ꢀꢀꢀO(3)
O(1)–H(102)ꢀꢀꢀO(5)
C(1)–H(1)ꢀꢀꢀO(5)
C(7)–H(7)ꢀꢀꢀO(4)
C(10)–H(10)ꢀꢀꢀO(2)
C(18)–H(18)ꢀꢀꢀO(1)
2
0.91
0.83
0.93
0.93
0.93
0.93
1.74
1.86
2.54
2.49
2.45
2.54
2.6056
2.6895
3.3322
3.3072
3.0188
3.4108
158
177
143
146
119
156
–
1 ꢂ x, 1 ꢂ y, 1 ꢂ z
1 ꢂ x, 1 ꢂ y, 1 ꢂ z
ꢂx, ꢂy, ꢂz
–
1 ꢂ x, 1 ꢂ y, 1 ꢂ z
O(1)–H(101)ꢀꢀꢀO(2)
O(1)–H(102)ꢀꢀꢀO(4)
O(1)–H(102)ꢀꢀꢀO(4)
C(3)–H(3)ꢀꢀꢀO(3)
C(7)–H(7)ꢀꢀꢀO(4)
C(10)–H(10)ꢀꢀꢀO(5)
3
0.91
0.76
0.76
0.93
0.93
0.93
1.75
2.11
2.59
2.55
2.42
2.52
2.656
176
141
122
125
164
118
1 ꢂ x, 1 ꢂ y, ꢂz
–
ꢂx, 1 ꢂ y, ꢂz
1 ꢂ x, 1 ꢂ y,1 ꢂ z
ꢂx, 1 ꢂ y, 1 ꢂ z
–
2.7422
3.0607
3.1815
3.325
3.716
O(5)–H(13)ꢀꢀꢀO(2)
O(5)–H(14)ꢀꢀꢀO(4)
C(4)–H(4)ꢀꢀꢀO(3)
C(7)–H(5)ꢀꢀꢀO(3)
C(16)–H(11)ꢀꢀꢀO(3)
C(14)–H(10)ꢀꢀꢀO(4)
4
0.94
0.90
0.95
0.95
0.95
0.95
1.75
1.79
2.35
2.59
2.49
2.56
2.662
2.688
3.275
3.498
2.805
2.853
162
178
165
160
131
112
–
1 ꢂ x, ꢂy, ꢂz
x, 1 ꢂ y, 1 ꢂ z
ꢂx, ꢂy, ꢂz
–
x, ½ ꢂ y, ½ + z
O(1)–H(1A)ꢀ ꢀ ꢀO(5)
O(1)–H(2B)ꢀꢀꢀO(2)
C(2)–H(2)ꢀꢀꢀO(7)
C(3)–H(3)ꢀꢀꢀO(2)
C(4)–H(4)ꢀꢀꢀO(5)
C(8)–H(8)ꢀꢀꢀO(7)
0.79
0.82
0.93
0.93
0.93
0.93
2.07
1.89
2.44
2.59
2.49
2.58
2.7904
2.6867
3.3543
3.5000
3.3802
3.1141
152
162
166
165
162
117
½ ꢂ x, ½ + y, ½ ꢂ z
½ ꢂ x, ꢂ½ + y, ½ ꢂ z
ꢂx, 2 ꢂ y, ꢂz
x, 2 ꢂ y, ꢂ½ + z
½ ꢂ x, 3/2 ꢂ y, -z
5
½ ꢂ x,
/
2 ꢂ y, ꢂz

R. Prajapati et al. / Polyhedron 28 (2009) 600–608
603
Fig. 1. Molecular structures of complexes 1(a), 2(b), 3(c) and 4(d).
Fig. 2. ID zig-zag chain (a) and supramolecular 2D sheet network of complex 1 containing hydrogen bonds and p–p stacking (b).

604
R. Prajapati et al. / Polyhedron 28 (2009) 600–608
peripheral ligand. It prevents the chain growth in a straight for-
ward direction; hence a zigzag chain is formed.
It is very interesting to note that two independent chains are
linked through hydrogen bonds formed between coordinated
Fig. 3. 1D linear chain with the continuous cavity of diameter ꢁ5.6 Å (a), crystal packing in complex 2 through inter- and intra-molecular H-bonding and
p–p stacking (b).

R. Prajapati et al. / Polyhedron 28 (2009) 600–608
605
water molecules and the carbonyl oxygen atoms of theterephtha-
in a monodentate fashion, together with one oxygen atom (O1W)
of the coordinating water molecule and in addition to two nitrogen
atoms (N1 and N2) from the 2,20-bipyridine ring. As shown in Table
2, one Zn–O (carboxylate) distance, Zn(1)–O(2) 1.9843(14) Å, is
found to be shorter than the Zn–O (carboxylate) distance Zn(1)–
O(2) = 2.0423(12) Å as found in complex 1. Other distances in this
complex are comparable with the bond distances observed in com-
plex 1.
It is quite interesting to observe from Fig. 3a, that each car-
boxylate group is acting as a linker between four Zn(bpy) units,
out of these four units, two units are common in the formation
of a 16-membered macrocyclic ring of diameter ꢁ5.6 Å. This 16-
membered macrocyclic ring extends along the molecular axis in
a linear fashion and provides a chain structure. The distance be-
tween two ZnII centres across the cavity is found to be 8.144 Å,
whereas adjacent ZnII centres are separated by a distance of
7.709 Å (Fig. 3b).
late dianion, in addition to
p–p stacking interactions between
the 2,20-bpy rings at a distance of 3.80 Å. It results in the formation
of a 2D sheet-like supramolecular structure (Fig. 2c).
Crystal packing analysis of complex 1 shows the formation of
independent intra-chain (O–HꢀꢀꢀO) as well as inter-chain (O–HꢀꢀꢀO)
hydrogen bonds (Table 3). The intra-chain hydrogen bond is
formed between the O–H group of the coordinated water molecule
and an uncoordinated carboxylate oxygen at a DꢀꢀꢀA distance of
2.6056 Å and an O–H–O angle of 158°. However, in the formation
of the inter-chain O–HꢀꢀꢀO hydrogen bonds, the O–H group of the
coordinated water molecule of one chain is linked through the
uncoordinated oxygen atoms of the terephthalate dianion of the
adjacent chain at a DꢀꢀꢀA distance of 2.6895 Å and an O–H–O angle
of 177°. The KPI for this complex, found to be 71.8%, indicates com-
pact packing in its crystal lattice.
The molecular structure of the ZnII complex with benzenetetra-
carboxylic acid (H4btec), 2, is depicted in Fig. 1b. This complex is
also pentacoordinated, but the two carboxylate oxygen atoms
(O2 and O5) come from two different carboxylate groups, acting
Weak force studies reveal that six hydrogen bonds are formed
in the molecular packing of complex 2. Three hydrogen bonds
are conventional hydrogen bonds whereas the remaining three
Fig. 4. 1D zig-zag chain (a), view of the crystal packing along the a-axis, the packing is stabilized through p–p stacking between bipyridyl rings and H-bonding in complex 3.

606
R. Prajapati et al. / Polyhedron 28 (2009) 600–608
bonds are non-conventional hydrogen bonds. The non-conven-
tional hydrogen bonds involve C–H units as donor groups, whereas
the acceptors are oxygen atoms in each case (Table 3). This com-
plex has a Kitagorodskii Packing Index (KPI) of 76.6%, indicating a
compact packing with no solvent accessible voids. In complex 1
the KPI value is found to be 71.8%. Thus the Zn(bpy)-benzenetetra-
carboxylate complex 2 possess more compact packing as compared
to the Zn(bpy)-dicarboxylate complex 1.
In the present complex 3, the CdN2O4 coordinated core consists
of two nitrogen atoms (N1 and N2) from 2,20-bpy, one oxygen atom
[O(5)] from a water molecule together with one oxygen atom (O1)
from one terephthalate dianion and other two oxygen donors
atoms [O(3) and O(4)] from another terephthalate unit which lies
nearly orthogonal to the first terephthalate unit (dihedral angle:
80.16 °). It is interesting to note that each terephthalate unit serves
as a bridging ligand between two cadmium atoms, forming a one-
dimensional zig-zag polymeric chain (Fig. 4a). However, the bridg-
ing terephthalate dianion linked to two CdII ion, acts as monoden-
tate at one coordination end and bidentate to another coordination
end. Thus, one bridging terephthalate unit overall acts as a triden-
tate ligand and links two CdII ions at a distance of 11.341 Å. This
distance is a little larger than the distance of 11.10 Å observed be-
tween the two ZnII ions in complex 1. This difference may be ex-
plained by the larger size and higher coordination mode of the
CdII ion as compared to the ZnII ion.
Both inter and intra molecular hydrogen bonds, in addition to
p–p stacking interactions, provide a 3D supramolecular architec-
ture (Fig. 3b) in complex 2.
3.2.2. Complex 3 and 4
Complex 3 is a CdII analogue of complex 1. It also possesses an
infinite zig-zag one-dimensional chain structure, but each CdII ion
is hexa-coordinated in a highly distorted octahedral geometry
(Fig. 4a). The formation of a seven coordinated CdN2O5 core has
been reported [63] for a complex of CdII with the terephthalate
dianion, but it contains 1,10-phenanthroline as one of the compo-
nents instead of 2,20-bipyridine as used in the present complex.
The cell packing analysis, as depicted in Fig. 4b, shows that both
coordinated planes, as well as the bridging ligand plane, are more
tilted from the crystallographic plane as compared to that
Fig. 5. 1D linear chain containing the continuous cavity of diameter of ꢁ5.8 Å (a), packing of molecules in the crystal lattice; H-bonding and
p–p stacking in complex 4.

R. Prajapati et al. / Polyhedron 28 (2009) 600–608
607
observed in the analogous ZnII complex 1. This may be understood
due to the different modes of coordination (monodentate and
bidentate) of the same ligand.
The crystal packing of complex 3 consists of two conventional
and four non-conventional hydrogen bonds. In all cases, oxygen
atoms acts as the acceptor atoms. The intra-molecular hydrogen
bonding is formed between O(5)–H(13) and O(2) at a HꢀꢀꢀA distance
of 1.75 Å with an O–H–O angle of 162°, whereas inter-molecular
hydrogen bonding between O(5)–H(14) is formed at a HꢀꢀꢀA dis-
tance of 1.79 Å with an O–H–O angle of 178°. It indicates that
two structural units are attached closely through hydrogen bond-
ing. Out of the four non-conventional hydrogen bonds, two intra-
molecular hydrogen bonds are formed at HꢀꢀꢀA distances of 2.35
(C4–H4ꢀꢀꢀO3) and 2.59 Å (C7–H5ꢀꢀꢀO3), whereas inter-molecular
non-conventional hydrogen bonds are formed at the larger dis-
tances of 2.49 and 2.56 Å.
Complex 4 also possesses a distorted octahedral coordination
geometry with a CdN2O4 coordination core around the CdII ion. In
the construction of this structure, btec acts as hexadentate ligand,
bis (bidentate + monodentate). However, again in a recently re-
ported CdII complex containing 1,10-phenanthroline as a terminal
ligand, btec acted in a tetrakis(bidentate) fashion [63]. Thus, the
presence of the 2,20-bipyridine ring as a terminal ligand alters
the mode of coordination of the btec ligand. Additionally, one btec
ligand links four CdII ions in the present complex 4 whereas the
same btec ligand linked six CdII ions in the earlier reported com-
plex [63]. Thus, the effect of the terminal ligand on the coordinat-
ing mode of the tetradentate ligand is quite evident.
The unit cell packing mode of complex 4, depicted in Fig. 5a,
again shows the formation of a 16-membered ring, but a little
change in the ring size is observed (5.8 Å diameter in complex 4
as compared to 5.6 Å in complex 2). Six hydrogen bonds (two con-
ventional and four non-conventional) are found in the weak force
studies of complex 4.
Fig. 6. Photoluminescence spectra of complexes 1–4 in the solid-state at room
temperature.
which only 45% residue remains, which may be due to carbon res-
idues as well as metal oxides. Complex 2 shows a weight loss
(47.9%) only after 230 °C, corresponding to the removal of the coor-
dinated water molecule together with the 2,20-bpy ligand (calcu-
lated 47.71%) from the structural framework. At 800 °C complex
2 also shows 41.25% residue remains. Multiple steps of weight loss
occur upon heating complexes 3 and 4.
3.5. Microstructural studies
Since such structural frameworks of complexes are con-
structed from hetero components, the formation of porosity in
their bulk structure may be looked upon. Therefore, their SEM
images were recorded. The SEM images of the complexes show
3.3. Photoluminescent properties
the formation of micro-sized grains varying from 10–30
lm in
The photoluminescent properties of complexes 1–4 are exam-
ined at room temperature and the corresponding spectral features
are shown in Fig. 6. Upon excitation at kex = 340 nm, complex 1
emits at 365 nm with a shoulder at 430 nm, while in complex 2
the corresponding peaks are red shifted and appear at 370 and
435(sh) nm. Since solid-state emissions from the free 2,20-bpy
group are also observed in a similar region at kem = 360 and
complex 1 to 15–70 m in complex 2. In between these grains,
l
pores can be seen which are dispersed throughout the structure
(marked as ‘P’) (S2). The SEM micrograph of complex 3 shows
the presence of parallelepipeds crystals with an average size of
25 ꢄ 5 ꢄ 4
l
m3 whereas in complex 4, the grain size varies in
the 15–30 lm range.
385 nm due to the
p–
pꢃ transition, the strong emissions arising
4. Conclusion
from complexes 1 and 2 are assigned to intra-ligand fluorescence
of the coordinated 2,20-bpy ligand. However, weak emissions from
complexes 1 and 2 observed at 430 (sh) and 435 (sh) nm respec-
tively are considered to arise from a charge-transfer transition,
consistent with earlier reports [64–66]. Additionally, multiple
emissions arising at 400, 462(sh) and 520(sh) nm for complex 3,
and 401, 465(sh) and 523(sh) nm for complex 4 are assigned to a
In summary, the reactions of ZnII/CdII-2,20-bipyridine with di
and tetra carboxylates under mild conditions (pH 4–5) provide a
series of photoluminescent polymeric complexes with diverse
architectures from zig-zag chains (complexes 1 and 3) to straight
chains (complexes 2 and 4). The weak forces of inter-molecular
and intra-molecular hydrogen bonding interactions together with
combination of strong
p–
pꢃ transitions of the coordinated 2,20-
pꢃ transition from both bdc and btec
p–p aromatic stacking interactions provide a two-dimensional
bpy ligand along with weak
ligands.
p–
layer for complex 1, whereas for complexes 2, 3 and 4, three-
dimensional supramolecular networks result as a secondary struc-
ture. The complexes with the tetracarboxylate ligand provide a 16-
membered metallomacrocyclic cavity of diameter 5.6 Å in 2 and
5.8 Å in 4. Additionally, the presence of 2,20-bipyridine rings is
found to alter the coordination behavior of the carboxylates.
3.4. Thermogravimetric (TG) analysis
Thermogravimetric analyses (TGA) performed on crystalline
samples under a nitrogen atmosphere are depicted in S1. In com-
plex 1, a weight loss of 4.40% at 100–220 °C is assigned to the loss
of coordinated water molecules (calculated per formula unit
weight loss = 4.45%). This loss is further followed by a second
weight loss of 38.60% in the temperature range 241–270 °C, corre-
sponding to the loss of the terminal 2,20-bpy group. The final
decomposition starts at 280 °C and continues up to 800 °C, at
Acknowledgements
Authors thank the authorities of SAIF, CDRI, Lucknow and IIC, IIT
Roorkee, India for providing analytical data. Help rendered by Prof.
T. Shinmyozu, Kyushu University, Japan for X-ray analysis is grate-

608
R. Prajapati et al. / Polyhedron 28 (2009) 600–608
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Appendix A. Supplementary data
CCDC 646994, 650284, 685998 and 696004 contain the supple-
mentary crystallographic data for 1–4. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at doi:10.1016/
j.poly.2008.11.059.
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