Three new isostructural coordination polymers with Cd(II) clusters as the SBU: Synthesis, structural characterization, and luminescence properties

Article abstract of DOI:10.1021/cg201028e

Three new isostructural coordination polymers, {Cd_1.33L(Cl) _0.67(H₂O)_0.67}_n (1), {Cd _1.33L(Br)_0.67(H₂O)_0.67}_n (2), and {Cd_1.33L(I)

Full text of DOI:10.1021/cg201028e

ARTICLE
pubs.acs.org/crystal
Three New Isostructural Coordination Polymers with Cd(II) Clusters as
the SBU: Synthesis, Structural Characterization, and Luminescence
Properties
Prem Lama and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
S Supporting Information
b
ABSTRACT: Three new isostructural coordination polymers, {Cd1.33L(Cl)0.67(H2O)0.67 n
}
(1), {Cd1.33L(Br)0.67(H2O)0.67 n (2), and {Cd1.33L(I)0.67(H2O)0.67 n (3), have been
}
}
synthesized using a semiflexible in situ generated tetrazole ligand. All the complexes crystallize
in cubic space group P4132. In complex 1, three Cd2+ ions along with two N atoms each from
three tetrazole units form a trinuclear unit where each metal ion is additionally bridged by
carboxylate O and μ3-Cl units. This Cd(II) cluster along with the ligand form a trigonal
arrangement that extends to form a two-dimensional sheet that further undergoes extensive
interpenetration to generate an overall three-dimensional coordination polymer. All com-
plexes exhibit thermal stability up to ∼330 °C losing coordinated water molecules at
∼140 °C. Upon excitation at 340 nm, the complexes exhibit solid-state luminescence with an
emission peak at ∼415 nm that shows slight red shift with broadening of the band with
changing the halide anion from chloride to bromide to iodide.
’ INTRODUCTION
(2), and {Cd1.33L(I)0.67(H2O)0.67}n (3) using an in situ generated
tetrazole ligand.
Coordination polymers have attracted considerable attention
in recent years because of their potential applications as func-
tional materials as well as the intriguing nature of molecular
connectivities and topologies.1 The design possibilities of organic
ligands and the coordination tendencies of metal ions besides
crystallization conditions have led to a large number of coordina-
tion polymeric structures quite often endowed with novel
structural features as well as unique properties.2 Various
nitrogen3 and carboxylate4 donating ligands have been used for
this purpose. Heterocycles incorporating poly-nitrogen donors
such as tetrazoles5 and triazoles6 have been demonstrated to be
very good ligands for the construction of three-dimensional (3D)
coordination polymers exhibiting a great structural diversity. So,
our main aim was to prepare a ligand that contains both tetrazole
as well as carboxylate groups as donors. The in situ hydrothermal
synthesis to initiate [2 + 3] cycloaddition of azides with nitriles in
presence of metal ions to form tetrazoles are known in the
literature.7 In situ ligand synthesis, first proposed by Champness
and Schroder in 1997,8 is regarded as a new and efficient
approach for the synthesis of both organic compounds and
coordination polymers. Recently, this technique has been devel-
oped rapidly in coordination polymers, as it usually generates
unexpected structures, which cannot be obtained by the direct
reaction of ligands with metal salts.9 Using this approach, we have
synthesized a new tetrazole-based semiflexible ligand that forms
coordination polymers with Cd(II) metal ion (Scheme1).
Herein, we report the synthesis and crystal structural studies of
three coordination polymers of Cd(II) having the formulas
’ EXPERIMENTAL SECTION
Materials and Methods. 4-Fluorobenzonitrile, ethyl isonipeco-
tate, sodium azide, and Cd(II) salts were acquired from Aldrich and used
as received. All solvents and K2CO3 were procured from S. D. Fine
Chemicals, India. All solvents were purified prior to use.
Infrared spectra were obtained (KBr disk, 400À4000 cmÀ1) on a
Perkin-Elmer model 1320 spectrometer. 1H NMR and 13C NMR
spectra were recorded on a JEOL-ECX 500 FT (500 and 125 MHz,
respectively) instrument in CDCl3 and DMSO-d6 with Me4Si as the
internal standard. ESI-mass spectra were recorded on a WATERS
Q-TOF premier mass spectrometer. Microanalyses for the compounds
were obtained using a CE-440 elemental analyzer (Exeter Analytical
Inc.) while thermogravimetric analyses (TGA) were obtained using a
Mettler Toledo star system (heating rate of 5 °C/min). Solid-state
photoexcitation and emission spectra were recorded on double UVÀ
vis-NIR spectrophotometer (Varian model Cary 5000) and Jobin Yvon
Horiba Fluorolog-3 spectrofluorimeter at room temperature.
Synthesis. The ligand was synthesized in several steps as
described below.
Synthesis of 1-(4-Cyano-phenyl)-piperidine-4-carboxylic Acid Ethyl
Ester (L1). Ethyl isonipecotate (2 mL, 12.9 mmol) and dry K2CO3 (1.9 g,
13.8 mmol) were taken in a 50 mL round-bottom flask under an inert
atmosphere, and then, 10 mL of dry DMF was added to it. The mixture
Received: August 8, 2011
Revised:
September 30, 2011
{Cd1.33L(Cl)0.67(H2O)0.67}n (1), {Cd1.33L(Br)0.67(H2O)0.67}n
Published: October 07, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/cg201028e Cryst. Growth Des. 2011, 11, 5434–5440
|

Crystal Growth & Design
ARTICLE
Scheme 1. Schematic Representation of an in situ Generated
Tetrazole Ligand
0.3 mmol) in 3 mL of water were sealed in a Teflon-lined autoclave
and heated under autogenous pressure to 180 °C for three days and then
allowed to cool to room temperature at the rate of 1 °C per minute.
Truncated octahedron-shaped brown crystals of 2 were collected in 55%
yield. The crystals were washed with water followed by acetone and air-
dried. Anal. calcd. for C13H13.67N5O2.67Br0.67Cd1.33: C, 32.14%; H,
2.83%; N, 14.41%; found: C, 32.25%; H, 2.76%; N, 14.32%. IR (cmÀ1):
3363(w, br), 2924(s), 2814(s), 1614(m), 1566(m), 1446(m), 1387(m),
1353(m), 1281(s), 1251(s), 1216(m), 1198(m), 1150(s), 1104(s),
1045(s), 956(s), 915(s), 858(s), 836(m), 764(m), 720(s), 654(m).
Synthesis of {Cd1.33L(I)0.67(H2O)0.67}n (3). A mixture containing L1
(0.04 g, 0.15 mmol), NaN3 (0.02 g, 0.30 mmol), and CdI2 (0.12 g, 0.3
mmol) in 3 mL of water were sealed in a Teflon-lined autoclave and
heated under autogenous pressure to 180 °C for three days and then
allowed to cool to room temperature at the rate of 1 °C per minute.
Truncated octahedron-shaped brown crystals of 3 were collected in 52%
yield. The crystals were washed with water followed by acetone and air-
dried. Anal. calcd. for C14H14.67N5O2.67 0.67Cd1.33: C, 31.72%; H,
I
2.78%; N, 13.21%; found: C, 31.65%; H, 2.86%; N, 13.30%. IR
(cmÀ1): 3362(w, br), 2923(s), 2853(s), 2817(s), 1612(m), 1556(m),
1446(m), 1385(m), 1354(m), 1281(s), 1252(s), 1213(m), 1151(s),
1103(s), 1040(s), 952(s), 913(s), 857(m), 834(m), 764(m), 721(s),
652(s).
All attempts to synthesize these complexes in single crystal form using
the ligand H2L with Cd(II) salts remained unsuccessful. Thus, it appears
that the complexes can be synthesized via in situ generated tetrazole as
well as carboxylate.
was stirred for a while at 80 °C followed by addition of 4-fluoro-
benzonitrile (1.6 g, 13.2 mmol), and the resulting mixture was stirred for
24 h in an oil bath at 80 °C. The resulting solution was poured into ice-
cold water (100 mL) whereupon a solid crystalline compound (L1) was
obtained that was collected by filtration, washed several times with
distilled water, and dried in air. Yield: 3.1 g (93%). Melting point: 67 °C.
IR: sharp peaks were observed at 2211 and 1721 cmÀ1 corresponding to
υCN and υCO (str), respectively. 1H NMR (CDCl3), δ (ppm): 1.26 (t,
J = 7.1 Hz, 3H); 1.86À1.93 (m, 2H); 2.11À2.13 (m, 2H); 2.54À2.59 (m,
1H); 3.04 (t, J = 10.3 Hz, 2H); 3.75À3.79 (m, 2H); 4.16 (q, J = 7.3 Hz,
2H); 7.00À7.04 (m, 2H,); 7.52 (d, J = 8.8 Hz, 2H). 13C NMR (CDCl3),
δ (ppm): 14.1, 27.2, 40.4, 47.1, 60.5, 100.0, 114.5, 119.9, 133.4, 152.8,
174.1. Elemental analysis: calcd. for C15H18N2O2 (258.13): C, 69.74%;
H, 7.02%; N, 10.84%; found: C, 69.61%; H, 7.15%; N, 10.76%. ESI-MS
m/z: calcd. for C15H18N2O2, 258.1368; found, 259.1368 [M + 1]+.
Synthesis of 1-(4-(2H-tetrazol-5-yl)phenyl)piperidine-4-carboxylic
acid (H2L). Complex 1 (0.2 g), which was prepared in situ, was dissolved
by adding 1 M HCl solution such that the final pH is maintained ∼3. To
this solution, 1 M NaOH solution was added dropwise under ice-cold
condition to give light brown compound at pH 7. The resulting solid
compound (H2L) obtained was collected by filtration, washed several
times with distilled water, and dried in air. Yield: 0.09 g (75%). Melting
point, 208 °C. IR: peaks around 1700 cmÀ1 corresponding to υCO (str)
Single-Crystal X-ray Studies. Single-crystal X-ray data on L1 and
complexes 1À3 were collected at 298 K on a Bruker SMART APEX
CCD diffractometer using graphite monochromated MoKα radiation
(λ = 0.71073 Å). The linear absorption coefficients, scattering factors for
the atoms, and the anomalous dispersion corrections were taken from
the International Tables for X-ray Crystallography.10 The data integra-
tion and reduction were carried out with SAINT11 software. For each
data set, empirical absorption correction was applied to the collected
reflections with SADABS,12 and the space group was determined using
XPREP.13 The structure was solved by the direct methods using
SHELXTL-9714 and refined on F2 by full-matrix least-squares using
the SHELXL-9715 program package. In complexes 1À3, all non-hydro-
gen atoms were refined anisotropically. The H atoms have been refined
as follows: the hydrogen atoms attached to carbon atoms were posi-
tioned geometrically and treated as riding atoms using SHELXL default
parameters. The one H atom of water molecule was located from
difference Fourier maps but the other one could not be located even
from difference Fourier maps. Several DFIX commands were used to fix
the bond distances of ligand and water molecule. The higher R1 values
and low data of completeness in all Cd2+ complexes are due to the poor
diffraction quality of crystals. Data collection, lattice parameters, and
structure solution parameters are collected in Table 1 while selective
bond distances and angles are given in Table S1 (Supporting
Information).
1
of carboxylic acid group. H NMR (DMSO-d6), δ (ppm): 1.54À1.62
(m, 2H); 1.85À1.88 (m, 2H); 2.45 (m, 1H); 2.88 (t, J = 10.9 Hz, 2H);
3.79 (d, J = 13.1 Hz, 2H); 7.07 (d, J = 8.9 Hz, 2H); 7.82 (d, J = 8.9 Hz,
2H); 12.22 (s, 1H). 13C NMR (DMSO-d6), δ (ppm): 27.7, 47.3, 47.4,
114.2, 115.4, 128.6, 129.4, 152.9, 176.4. Elemental analysis: calcd. for
C15H18N2O2 (273.13): C, 57.13%; H, 5.5%3; N, 25.63%; found: C,
57.21%; H, 5.40%; N, 25.77%. ESI-MS m/z: calcd. for C13H15N5O2,
273.1226; found, 272.1226 [M À 1]+.
Synthesis of {Cd1.33L(Cl)0.67(H2O)0.67}n (1). A mixture containing L1
(0.04 g, 0.15 mmol), NaN3 (0.02 g, 0.30 mmol), and CdCl2 xH2O (0.06
3
g, 0.30 mmol) in 3 mL of water was sealed in a Teflon-lined autoclave
and heated under autogenous pressure to 180 °C for three days and then
allowed to cool to room temperature at the rate of 1 °C per minute.
Truncated octahedron-shaped brown crystals of 1 were collected in 51%
yield. The crystals were washed with water followed by acetone and air-
dried. Anal. calcd. for C13H13.67N5O2.67Cl0.67Cd1.33: C, 34.23%; H,
3.02%; N, 15.35%; found: C, 34.35%; H, 3.11%; N, 15.26%. IR (cmÀ1):
3366(w, br), 2924(s), 2854(s), 2812(s), 1615(m), 1565(m), 1446(m),
1386(m), 1351(m), 1281(s), 1251(s), 1216(m), 1197(m), 1149(s),
1103(s), 1045(s), 990(s), 956(s), 915(s), 858(s), 836(m), 764(m),
720(s), 653(m).
’ RESULTS AND DISCUSSION
The semiflexible ligand L1 has been designed to provide some
rotational freedom for the carboxylate and tetrazole moieties that
can facilitate bridging of Cd(II) centers. Under hydrothermal
reaction conditions applied, the ester group is hydrolyzed to
carboxylic acid and the nitrile group reacts with added azide to
form a tetrazole moiety (L2À) both of which coordinate to metal
ions. Once isolated, the coordination polymers are stable in air
and insoluble in common organic solvents and water. The IR
spectra of all the compounds show strong absorption bands
between 1422 and 1616 cmÀ1 that are diagnostic16 of coordinated
Synthesis of {Cd1.33L(Br)0.67(H2O)0.67}n (2). A mixture containing L1
(0.04 g, 0.15 mmol), NaN3 (0.02 g, 0.30 mmol), and CdBr2 4H2O(0.11 g,
3
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Crystal Growth & Design
ARTICLE
Table 1. Crystal and Structure Refinement Data for L1 and 1À3
L1
1
2
3
empirical formula
formula wt
crystal system
space group
a, Å
C
15H18N2O2
C13H13.67Cl0.67N5O2.67Cd1.33
456.13
C13H13.67Br0.67N5O2.67Cd1.33
485.76
C13H13.67I0.67N5O2.67Cd1.33
258.31
517.09
monoclinic
P21/c
cubic
cubic
cubic
P4132
P4132
P4132
12.637(2)
13.230(5)
8.256(6)
90.000
16.499(5)
16.499(5)
16.499(5)
90.000
16.635(3)
16.635(3)
16.635(3)
90.000
16.614(5)
16.614(5)
16.614(5)
90.000
b, Å
c, Å
α (deg)
β (deg)
γ deg)
102.010(5)
90.000
90.000
90.000
90.000
90.000
90.000
90.000
U, Å3
1350.1(11)
4
4491(2)
12
4603(2)
12
4586(2)
12
Z
F
calc, Mg/m3
μ, mmÀ1
1.271
2.024
2.103
2.247
0.085
2.058
3.628
3.245
F(000)
552
2680
2824
2968
refl. collected
independent refl.
GOOF
8270
21403
20422
20782
3061
1358
1352
1199
1.042
1.003
1.010
1.086
final R indices [I > 2σ(I)]
R1 = 0.0596
wR2 = 0.1422
R1 = 0.0892
wR2 = 0.1708
À
R1 = 0.0750
wR2 = 0.1723
R1 = 0.1561
wR2 = 0.2222
0.001(10)
R1 = 0.0849
wR2 = 0.2088
R1 = 0.1488
wR2 = 0.2538
0.06(8)
R1 = 0.0969
wR2 = 0.2144
R1 = 0.1236
wR2 = 0.2341
0.2(2)
R indices (all data)
Flack parameter
Figure 2. Section showing connection between two trinuclear units via
bridging O atom in 1.
Figure 1. Trinuclear Cd2+ unit formed by the μ3-chloride ion and
tetrazole moiety of the ligand in complex 1.
the three Cd2+ ions (Figure 1). All CdÀN, CdÀO, and CdÀCl
bond distances are normal18 within statistical errors. The dis-
tance between the ClÀ ions is 3.372 Å; the covalent radius of
chlorine is 1.75 Å while van der Waals radius19 of ClÀ ion is 1.81
Å. The short distance suggests considerable interactions between
the triply coordinated chloride ions. A similar situation is found
in the case of coordination polymers with bromide and iodide
(i.e., 2 and 3, respectively). Such short distances between triply
bridged halide ions have been reported20 in case of discrete
complexes of Cu(II) with the bis(diphenylphosphino)methane
ligand. Each metal ion of this cluster is bridged via a carboxylate
to another cluster containing three Cd2+ ions (Figure 2).
In the later cluster, each metal ion shows distorted hexagonal
bipyramidal CdO8 coordination from three chelating carboxy-
lates (CdÀO = 2.444(16) Å) and two water molecules (CdÀO =
2.272(12) Å). A clear picture of the coordination geometry
around each type of Cd2+ ion is shown in Figure 3.
carboxylate groups.17 The IR spectra of 1À3 give broad peaks in
the range of 3360À3370 cmÀ1 due to presence of coordinated
water molecules.16
Since all the complexes are isostructural, the detail structural
description of only complex 1 is discussed here. Complex 1
crystallizes in cubic space group P4132 with two crystallographi-
cally independent Cd(II) ions (Cd1 with half occupancy and
1
1
Cd2 with /6 occupancy), half L2À, one chloride ion with /3
1
occupancy, and one coordinated water molecule having /3
occupancy in the asymmetric unit. Three Cd(1) ions form a
trinuclear unit where each metal ion is coordinated to two O
atoms from a bridging carboxylate (CdÀO = 2.271(14) Å) and
two ClÀ ions (CdÀCl = 2.696(5) Å) while the axial sites are
occupied by two N atoms each from three tetrazole moiety
(CdÀN = 2.177(17) Å). Each of the ClÀ ions is triply bridged
occupying the vertex of the pyramid where the base is formed by
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The Cd2+ clusters form a unit, and three such units are
connected to three L2À ligands to form a triangular unit that
connects other triangular units to form a two-dimensional (2D)
sheet structure (Figure 4a and b). The triangular units are
interpenetrated (Figure 5a) to afford an overall 3D structure
(Figure 5b). There are few examples of 3D self-penetrated
structures reported in the literature.21
The precursor (L1) has an ester group and a benzene ring bearing
a nitrile group attached to a piperidine ring in trans arrangement to
each other. The piperidine ring is in chair form in the structure
(Figure 6a). Under hydrothermal conditions in the presence of
sodium azide, the cyano group is converted to a tetrazole moiety
while the ester group is hydrolyzed to a carboxylic acid. The chair
conformation of the piperidine ring gets somewhat flattened making
a dihedral angle of 27.7(5)o between the flattened ring and the
phenyl moiety (Figure 6b) in L2À bound to metal ions.17
It has been observed that the tetrazole moiety can form
different types of trinuclear and tetranuclear metal clusters in
coordination polymers similar to that of carboxylate-based
ligands.22 In the present case, the tetrazole moiety and the
carboxylate group along with Cd2+ ions form a triangular
unit that propagate to form the coordination polymers. Each
coordination polymer here provides an uncommon example
of 6-connected self-penetrating 3D polymer with an in situ
generated tetrazole moiety having 612 83 topology. The 3D
3
topological network of the complexes are analyzed by the OLEX
Figure 3. Coordination mode for (a) Cd1 and (b) Cd2 in 1.
program,23 and the results show that the long topological vertex
Figure 4. (a) Triangular unit present in 1 built using three L2À, three trinuclear Cd(II) cluster units. (b) 2D layered network along the plane in 1.
Figure 5. (a) View showing an interpenetration of ligand within a triangular unit. (b) 3D view of complex 1.
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Figure 6. (a) Schematic view of ligand L1 in chair form. (b) View showing flattening of piperidine ring when the carboxylate and tetrazole moiety is
bound to Cd2+ ions (H atoms are omitted for clarity).
Figure 7. Topological view showing the interpenetration in complex 1.
symbol is 66 66 64 64 62 66 62 64 64 64 62 64 816 816 816
Decomposition of this compound is accomplished only above
330 °C. Similarly, both 2 and 3 exhibit weight loss corresponding
to removal of coordinated water in the temperature range
80À130 °C and thereafter stable up to at least 300 °C.
3
3
3
3
3
3
3
3
3
3
3
3
3
3
considering all Cd1 + halide as one node and 66 66 62 64
3
3
3
3
64 66 64 62 64 64 64 62 816 816 816 for Cd2 + oxygen atoms,
3
3
3
3
3
3
3
3
3
3
giving a short vertex symbol 612 83 for both cases. Thus, the
3
overall 3D framework can be abstracted into a 6-connected
network (Figure 7).
Luminescence Properties. Luminescent compounds are of
immense current interest because of their diverse applications in
chemical sensors, photochemistry, as well as electroluminescent
display.24 The luminescence properties of carboxylate containing
systems with a d10 metal have been reported in the literature.25
The solid-state luminescence of compounds 1À3 are
Thermal stabilities of the complexes are examined.17 Complex
1 is thermally stable up to ∼330 °C. It shows a weight loss of
2.5% (expected = 2.6%) in the temperature range 90À150 °C
that corresponds to loss of coordinated water molecules.
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’ ACKNOWLEDGMENT
We gratefully acknowledge the financial support received from
the Department of Science and Technology, New Delhi, India
(to P.K.B.) and SRFs from the CSIR to P.L. We also would like to
thank Prof. Oleg Dolomanov for his help in finding the topology
and Prof. R. Butcher for giving his valuable suggestion while
solving crystal structures.
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2010, 49, 9191. (b) Su, C.-Y.; Goforth, A. M.; Smith, M. D.; Pellechia,
P. J.; zur Loye, H.-C. J. Am. Chem. Soc. 2004, 126, 3576.
Figure 8. Solid-state emission at room temperature.
investigated at room temperature and are shown in Figure 8.
Upon excitation at 340 nm, solid 1 exhibits strong luminescence
in the visible region centered at λmax = 415 nm. Compared with
the ligands, which have no luminescence, the greatly enhanced
photoluminescence intensities are attributed to the charge-
transfer transition between ligands and metal centers26 or
intraligand fluorescence emission because ligand coordination
to the metal center effectively increases the rigidity of the ligand
and reducing the loss of energy by radiation-less decay.27
Interestingly, the complexes show anion-dependent variation
in luminescence property and are thus attributable to metalÀ
ligand charge transfer transition. The Cd Cd distances are
3 3 3
long enough (more than 3.6 Å) to explain cluster-centered
transition caused by Cd Cd interaction in all the complexes.
3 3 3
Therefore, in addition to charge-transfer transition between
ligands and metal centers, there exists some mixing of halide-
to-metal charged transfer characters.
’ CONCLUSION
In summary, we have synthesized three isostructural 3D
coordination polymers with Cd2+ ions under hydrothermal
conditions using a new semiflexible in situ generated tetrazole
ligand. All the complexes display a rare μ3-halide bridging that is
extended in all dimensions to generate highly interpenetrated 3D
coordination polymers. They show interesting anion-dependent
luminescence property. Such type of cluster containing coordi-
nation polymers with other transition metal ions are under
investigation with this and other ligands containing both carboxy-
late and nitrogen donor groups in our laboratory.
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’ ASSOCIATED CONTENT
(10) International Tables for X-Ray Crystallography; Kynoch Press:
Birmingham, England, 1952; Vol. III.
S
Supporting Information. Crystallographic data for L1
b
and 1À3 in CIF format, table for selected bonds and distances
for complexes 1À3, IR, TG analysis, ESI-MS, and NMR and
additional figures. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Refinement; University of G€ottingen: G€ottingen, Germany, 1997.
’ AUTHOR INFORMATION
Corresponding Author
*Email: pkb@iitk.ac.in.
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