Three-dimensional zinc(II) and cadmium(II) coordination frameworks with N, N, N ′, N ′-tetrakis(pyridin-4-yl)methanediamine: Structure, photoluminescence, and catalysis

Article abstract of DOI:10.1021/ic302375z

Coordination polymer networks, i.e., [Zn(tpmd)(H₂O)](NO ₃)₂·7H₂O (1) and [Cd(tpmd)(H ₂O)₂](NO₃)₂·4H ₂O·4CH₃OH (2), were assembled from M ^II(NO₃)₂ hydrates (M = Zn, Cd) and N,N,N′,N′-tetrakis(pyridin-4-yl)methanediamine (tpmd) in CH ₃OH and characterized. 1 and 2 feature three-dimensional networks formed by coordination of the metal ions to the tpmd ligands. 1 exhibits a strong blue emission at ～397 nm, while 2 shows strong emission at ～361 nm. 1 is a more efficient catalyst for the transesterification of various esters than 2.

Full text of DOI:10.1021/ic302375z

Communication
pubs.acs.org/IC
Three-Dimensional Zinc(II) and Cadmium(II) Coordination
Frameworks with N,N,N′,N′‑Tetrakis(pyridin-4-yl)methanediamine:
Structure, Photoluminescence, and Catalysis
Jong Won Shin,† Jeong Mi Bae,‡ Cheal Kim,*,‡ and Kil Sik Min*,†
†Department of Chemistry, Graduate School and Green-Nano Materials Research Center, Kyungpook National University, Daegu
702-701, Republic of Korea
‡Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea
S
* Supporting Information
basic conditions. Four pyridine groups that extend outward
ABSTRACT: Coordination polymer networks, i.e., [Zn-
(tpmd)(H2O)](NO3)2·7H2O (1) and [Cd(tpmd)-
(H2O)2](NO3)2·4H2O·4CH3OH (2), were assembled
from MII(NO3)2 hydrates (M = Zn, Cd) and N,N,N′,N′-
tetrakis(pyridin-4-yl)methanediamine (tpmd) in CH3OH
and characterized. 1 and 2 feature three-dimensional
networks formed by coordination of the metal ions to the
tpmd ligands. 1 exhibits a strong blue emission at ∼397
nm, while 2 shows strong emission at ∼361 nm. 1 is a
more efficient catalyst for the transesterification of various
esters than 2.
indicate that the ligand can bind to four individual metal ions to
form network structures and lead to π−π interactions through
the pyridine groups. Coordination polymer networks 1 and 2
were prepared from MII(NO3)2 hydrates (M = Zn or Cd) and
tpmd ligands in CH3OH at room temperature.6 1 and 2 are
slightly soluble in water but insoluble in acetone, CH3OH, and
Me2SO. Thermogravimetric analysis (TGA) of 1 revealed a
weight loss of 20.9% at 100 °C, which corresponds to a loss of
eight water molecules per unit formula (Figure S1 in the
Supporting Information, SI); no chemical decomposition was
observed up to 250 °C. TGA of 2 revealed a weight loss of 28.6%
at 75 °C, which corresponds to a loss of all solvent molecules per
unit formula (Figure S2 in the SI); no chemical decomposition
was observed up to 320 °C. Accordingly, 2 is more thermally
stable than 1.
1 crystallizes in the tetragonal space group P4/ncc, and the
ORTEP view of 1 is shown in Figure 1a. The asymmetric unit of
1 consists of two crystallographically independent zinc(II)
cations: One zinc(II) ion (Zn1) is tetrahedrally coordinated by
four pyridyl nitrogen atoms from four different tpmd ligands,
while the other zinc(II) ion (Zn2) is octahedrally coordinated by
four pyridyl nitrogen atoms from four different tpmd ligands and
two water molecules. That is, the coordination numbers of Zn1
and Zn2 are 4 and 6, respectively. The average Zn−Npy bond
distances are 1.997(1) and 2.146(1) Å for Zn1 and Zn2,
respectively. The average Zn2−O(water) bond distance is
2.173(3) Å. Each tpmd connects four different zinc(II) ions
because all nitrogen atoms of each tpmd ligand are bonded to
different zinc(II) ions; this gives rise to a 3D network (Figure
1b). The two bpa moieties of the tpmd ligands are linked by a
methylene carbon (C11) at an angle of 110.4(2)°. The shortest
Zn···Zn separation is 7.755(1) Å. Calculations using PLATON
indicate that 30.7% (1850.5 Å3) of the void volume of 1 is
occupied by seven water molecules.7 Guest water molecules and
nitrate anions are contained in the cavities; these were located in
the X-ray structure and were also evident in the IR spectra and
elemental analysis and TGA data.
he self-assembly of functional coordination polymer
Tnetworks has attracted much attention because of
fundamental scientific interest and their potential applications
in catalysis, absorption, magnetism, and luminescence.1 Of these,
the self-assembly of compounds that include zinc(II) and
cadmium(II) ions and organic ligands has been extensively
studied because of their interesting framework structures and
luminescent properties.2 A variety of zinc(II) and cadmium(II)
coordination polymers containing aromatic moieties exhibit
unique ligand-to-metal charge transfer luminescent properties;
these are very dependent upon the geometry of the ligands and
counteranions.3 Furthermore, zinc(II) and cadmium(II) com-
pounds with functional organic ligands can potentially be used in
applications such as light-emitting diodes and heterogeneous
catalysts.4 Thus, the rational design of new building blocks for
self-assembly is very important. To explore the preparation of
bis(4-pyridyl)amine (bpa) derivatives that could be luminescent
chromophores, we obtained a new ligand containing methylene
and bpa moieties, i.e., N,N,N′,N′-tetrakis(pyridin-4-yl)-
methanediamine (tpmd),5 in which two bpa units are linked by
a methylene moiety. Herein, we report three-dimensional (3D)
coordination polymer networks, i.e., [Zn(tpmd)(H2O)]-
( N O 3 ) 2 · 7 H 2 O ( 1 ) a n d [ C d ( t p m d ) ( H 2 O ) 2 ] -
(NO3)2·4H2O·4CH3OH (2), which were self-assembled in
CH3OH from zinc(II) or cadmium(II) ions and tpmd. These
coordination networks feature interesting solid structures,
luminescence, and catalytic properties.
2 crystallizes in the monoclinic space group C2/c, and the
ORTEP view of 2 is shown in Figure 2a. The tpmd ligand utilizes
all of its pyridyl nitrogen atoms to coordinate four cadmium(II)
The ligand tpmd was obtained as a pale-yellow solid by the
treatment of sodium hydride with bpa in CH2Cl2.5 The
methylene group is abstracted from CH2Cl2 by NaH under
Received: October 31, 2012
© XXXX American Chemical Society
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Inorganic Chemistry
Communication
ions. Each cadmium(II) has a slightly distorted octahedral
coordination geometry with four pyridyl nitrogen atoms
coordinated in the equatorial plane and two water molecules
coordinated at the axial positions. The Cd−N1 and Cd−N3
bond distances are 2.351(5) and 2.348(6) Å, respectively, and
the Cd−O1 bond length is 2.323(5) Å. Through the linkage of
four tetraconnected tpmd ligands and four tetraconnected
cadmium(II) ions, a 3D network structure is formed (Figure
2b). Topologically, one cadmium(II) ion could be viewed as a
square-planar tetraconnected node and the two central nitrogen
atoms in the tpmd ligand as two trigonal triconnected nodes; this
generates an unprecedented 3D bimodal 3,4-connected network.
The two bpa groups in the ligands are connected by a methylene
carbon at an angle of 109.7(7)°. The shortest Cd···Cd separation
is 8.623(1) Å. Calculations using PLATON indicate that 33.8%
(1148.7 Å3) of the void volume of 2 is occupied by four water and
four methanol molecules.7 The guest solvent molecules and
nitrate anions are included in the cavities; these were located in
the X-ray structure and also evident in the IR spectra and
elemental analysis and TGA data.
The solid-state luminescent properties of 1 and 2 were
investigated at room temperature. The fluorescence spectra of 1
and 2 are shown in Figure 3. Upon excitation at λex = 300 nm, 1
Figure 1. (a) ORTEP view of 1. The atoms are represented by 50%
−
probability thermal ellipsoids. Hydrogen atoms, NO3 , and solvent are
omitted for clarity. (b) Perspective view of 1 showing an extended 3D
structure via the coordination of zinc(II) ions and tpmd ligands.
Figure 3. Solid-state photoinduced emission spectra of 1 (solid line,
blue), dried 1 (1′, dashed line, blue), 2 (solid line, red), and dried 2 (2′,
dashed line, red) at room temperature with λex = 300 nm.
displays a strong fluorescent emission band with a maximum at
∼397 nm, while 2 shows an intense fluorescent signal at 361 nm.
The λmax value of 1 is longer than that of 2, which can be
attributed to the heavy-atom and coordination effects due to the
different types and coordination environments of the metal ions.8
To assess the photoluminescence of desolvated 1 and 2, they
were dried at 110 °C for 3 h. The maximum peak of dried 1 is
shifted very slightly and broadened (403 and 420 nm, split). For
dried 2, the maximum peak is red-shifted by ∼25 nm to 386 nm.
The emissions of both compounds undergo a red shift due to
desolvation of 1 and 2. This indicates that the local environments
are changed through drying and energy is lost through a
nonradiative pathway by contraction due to the coordination
effect. The emissive behaviors in 1 and 2 can be mostly attributed
to ligand-centered π → π* or π → n transitions within the
aromatic groups of the tpmd ligands.9
Figure 2. (a) ORTEP view of 2. The atoms are represented by 50%
probability thermal ellipsoids. Hydrogen atoms, NO3 , and solvent are
omitted for clarity. (b) Perspective view of 2 showing an extended 3D
structure via the coordination of cadmium(II) ions and tpmd ligands.
−
B
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These compounds are potential candidates for incorporation
in luminescent sensors and photoactive materials, e.g., for the
detection of aromatic organic molecules.10 When the dried 1 and
2 solids were exposed to water vapor, the original luminescence
spectra were regenerated; this means that the original frame-
works of 1 and 2 are fully restored (Figures S3−S5 in the SI).
To assess the catalytic effects of 1 and 2, we applied them to
the transesterification of various esters in CH3OH at 50 °C. The
reaction of 4-nitrophenyl acetate and methanol in the presence of
1 and 2 quantitatively produced methyl acetate within 2 and 5 h
(eq 1), respectively, under neutral conditions (entry 1 of Table
S1 in the SI).
structural data, and X-ray crystallographic data for 1 and 2 in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: minks@knu.ac.kr (K.S.M.), chealkim@seoultech.ac.kr
(C.K.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea funded by
the Ministry of Education, Science and Technology (Grant 2010-
0003672). X-ray crystallography at the PLS-II 2D-SMC beamline
was supported, in part, by MEST and POSTECH.
The catalytic results indicate that 1 has a higher catalytic
activity for ester synthesis by transesterification than 2. This is
attributed to the fact that the second-period transition metals are
much less labile, thereby hindering the substitution reaction.11
Importantly, the transesterification reactivity of zinc-containing 1
is the best among the previously reported catalytic systems with
zinc-containing polymeric compounds, to our knowledge.12a−c
Moreover, 2 is recyclable, although its activity is somewhat
reduced after recycling (Table S2 in the SI). The amount of
cadmium that leached into the filtrate after two cycles was
measured via inductively coupled plasma spectroscopic analysis
and determined to be 15 ppm.12c The loss of catalyst due to
leaching was calculated to be ∼2 wt % and thus may be
considered negligible. Transesterification reactions of other
esters using catalysts 1 and 2 were also carried out, and the results
are given in Table S1 in the SI. Various esters were converted to
their corresponding products within 2−14 and 5−240 h using 1
and 2, respectively. On the basis of our present and previous
results,12a−c we assume that the transesterification reactions
occur outside the framework because the results of the
transesterification reactions catalyzed by 2 showed a trend
similar to those observed using our previously reported
homogeneous catalysts.12a,b Specifically, 2 did not display any
size effects that could possibly occur inside the framework.
Therefore, the substrate phenyl acetate substitutes the labile
water molecules of the exterior cadmium ions to produce the
cadmium−substrate adduct. Then, the methanol nucleophile
attacks the carbon atom of the carbonyl moiety of the adduct to
produce the methyl phenyl acetate product. These results
suggest that 1 and 2 could be used as catalysts for the
transesterification of various esters.
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In conclusion, two coordination polymer networks, 1 and 2,
with different metal ions were constructed using tpmd. The
zinc(II) ions in 1 show octahedral and tetrahedral coordination
geometries, while 2 displays only octahedral coordination
geometries. The topologies of 1 and 2 are 3D network structures.
1 and 2 have strong emission at ∼397 and 361 nm, respectively. 1
shows more efficient catalytic activity for the transesterification
of various esters than 2 does. Further studies on the mechanism
of the catalytic transesterification and luminescence and the
fabrication of new coordination networks are ongoing.
ASSOCIATED CONTENT
* Supporting Information
Details of the synthesis, analytical results, TGA data,
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S
luminescence data, transesterification results, PXRD patterns,
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dx.doi.org/10.1021/ic302375z | Inorg. Chem. XXXX, XXX, XXX−XXX