Dynamic structural behavior and anion-responsive tunable luminescence of a flexible cationic metal-organic framework

Article abstract of DOI:10.1002/anie.201206724

Guest and anion dependent: Structural dynamism and luminescence of a cationic porous framework (see picture) are investigated by different analytical techniques, such as single-crystal-to-single-crystal structural transformation. The compound shows size-selective sorption of hydrophobic guest molecules, easy exchange of anions of the framework, and interesting anion-responsive luminescence. Copyright

Full text of DOI:10.1002/anie.201206724

.
Angewandte
Communications
DOI: 10.1002/anie.201206724
Dynamic Frameworks
Dynamic Structural Behavior and Anion-Responsive Tunable
Luminescence of a Flexible Cationic Metal–Organic Framework**
Biplab Manna, Abhijeet K. Chaudhari, Biplab Joarder, Avishek Karmakar, and Sujit K. Ghosh*
Dedicated to Professor K. N. Ganesh on the occasion of his 60th birthday
Microporous metal–organic frameworks (MOFs) are of great
current interest, because of their fascinating architectures and
their wide range of potential applications, especially in gas
storage, chemical separation, drug delivery, catalysis, enan-
tioseparation, optical properties, chemosensing etc.[1] Mostly
second-generation rigid porous materials showed exotic
results in the aforementioned areas, with their intact, stable,
and rigid porosity of the overall framework.[2] Recently,
Kitagawa and co-workers, as well as and other research
groups explored the third generation of soft materials and
their advantages over rigid porous frameworks.[3] It is
particularly important to study the guest-responsive structural
dynamism of such materials. Flexible soft porous materials
are sensitive to the chemical environment and undergo
structural variations depending upon the nature of guest
molecules inside the framework.[4] Among various MOFs,
cationic MOFs have advantages over others in the design of
dynamic frameworks. Cationic MOFs are often made of
neutral ligands and metal ions, so extra-framework anions
usually occupy the framework void or are weakly coordinated
to the metal centers.[5] Exchanging anions inside the frame-
work with other anions of different shape and size may lead to
changes in structure and physical properties.[6] Luminescent
MOFs with switchable properties are of great interest because
of their potential applications in chemical sensors.[7] Lumi-
nescent cationic MOFs with extra-framework anions offer
a dynamic framework and tunable luminescent behavior by
exchanging these extra-framework anions with other anions
of different shape, size, and coordinating nature. Until now,
great progress has been made on separation and other
applications of dynamic frameworks, but luminescent
response to guest molecules/anions was rarely reported in
conjunction with structural dynamism.[8]
anion-dependent structural dynamism. Dynamic structural
behavior has been demonstrated by single-crystal-to-single-
crystal (SCSC) structural transformation. The compound
shows slow opening of the framework upon guest inclusion
and size-selective sorption of hydrophobic guest molecules.
Anions of the framework are easily exchangeable, and the
compound shows interesting anion-responsive tunable lumi-
nescent behavior.
Linear bichelating ligand L is synthesized by Schiff-base
condensation of 4,4’-ethylenedianiline and 2-pyridine-carbox-
aldehyde in high yield (see the Supporting Information). The
combination of L and Zn(NO3)2 in a solvent system of
CH2Cl2/MeOH and benzene afforded yellow rod-shaped
single crystals of compound 1a [{Zn(L)(MeOH)2}-
(NO3)2·xG]n (in which G are disordered guest molecules).
Single-crystal analysis of the complex showed the formation
of a 1D chain structure. These 1D chains are H-bonded with
other chains through uncoordinated anions and solvent
molecules, leading to an H-bond-based 3D structure with
1D pore channels. A noteworthy feature of compound 1a is
that, when the crystals of 1a were removed from the mother
liquor and kept at room temperature for one hour, they
showed a drastic structural transformation of the network
without losing their crystalline nature. Single-crystal analysis
of those crystals showed large differences in unit-cell param-
eters, and complete structural analysis showed that two
coordinated MeOH solvent molecules per ZnII escaped and
were replaced by two H2O molecules to form a new phase,
1 [{Zn(L)(H2O)2}(NO3)2·2H2O]n. Although the coordination
networks of complexes 1a and 1 are very similar, the overall
structure, shape, and size of the channels are very different
(Figure 1).
A
single-crystal X-ray diffraction (SC-XRD) study
Herein, we report a porous MOF that is made of one-
dimensional (1D) coordination polymers of ZnII and a newly
designed neutral N-donor organic ligand (L) with extra-
framework nitrate anions, and shows interesting guest- and
showed that compound 1a crystallized in the monoclinic
[*] B. Manna, A. K. Chaudhari, B. Joarder, A. Karmakar, Dr. S. K. Ghosh
Indian Institute of Science Education and Research (IISER)
Homi Bhabha Road, Pashan, Pune-411008 (India)
E-mail: sghosh@iiserpune.ac.in
Homepage: http://www.iiserpune.ac.in/~sghosh/
[**] B.M. is thankful to CSIR for a research fellowship. We are grateful to
IISER Pune, DAE (project no. 2011/20/37C/06/BRNS) and DST
(project no. GAP/DST/CHE-12-0083) for financial support.
Figure 1. Crystal structures of compound 1a (top) and 1 (bottom)
showing coordination environment (left) and overall framework (right).
Free solvents and anions are omitted for clarity.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206724.
998
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 998 –1002

Angewandte
Chemie
system, space group C2/c. The asymmetric unit of 1a consists
of one-half of each ZnII and L, one coordinated methanol, one
disordered nitrate anion, and disordered solvents. Each metal
ion exhibits a six-coordinated distorted octahedral geometry
with an N4O2 donor set, bonding from two different L with
four nitrogen-coordinating sites, and the remaining two
coordinating sites are occupied by two methanol molecules.
The ZnII–N(pyridine), ZnII–N(aliphatic), and ZnII–O distan-
ces are 2.156(3), 2.217(3), and 2.081(3) ꢀ, respectively. At
both sides of L, a nitrogen atom from the ortho position of the
aromatic ring and one aliphatic nitrogen atom binds to ZnII in
a bidentate fashion. Coordination from both sides of the
ligand L extended the structure to a 1D chain in zigzag
manner (see Figure S1 in the Supporting Information).
According to the packing diagram, the cationic chains of
compound 1a form 1D channels along the c axis, which
encapsulate disordered nitrate ions and solvent molecules.
SCSC transformation analysis showed that the crystal system
of compound 1 is the same as that of 1a (monoclinic), but the
space group changed to P21/n. The coordination environment
around ZnII remains almost unchanged with a similar N4O2
donor set, but coordinated methanol molecules are now
replaced by water molecules. Bond lengths and angles around
ZnII are also very similar. The 3D packing structure of the
cationic chains of 1 show that this compound also formed
a channel along the a axis, which is occupied by free nitrate
anions and uncoordinated water molecules. Coordinated
water molecules form strong H-bonds with free nitrate
anions, and free water molecules lead to the H-bonds-based
3D structure. Interestingly, close examination of both struc-
tures showed that the M-M-M angles of both compounds are
quite different, as the angle expanded from 101.098 (1a) to
106.618(1) during structural transformation. This transforma-
tion leads to the drastic change in the shape and size of 1D
channel (Figure 2). In single-crystal form, compound 1 is
stable at room temperature for several hours, but in powder
form, the compound is not stable and changes its structure in
open air with time, which is evident from little variations of
powder X-ray diffraction (PXRD) patterns of 1 at different
time intervals. Thermo-gravimetric analysis (TGA) of the
powder sample showed an approximately 5% weight loss at
around 1208C, corresponding to approximately two water
molecules per formula unit. Furthermore, TGA showed that
the sample is stable up to around 3008C. To investigate the
dynamic behavior of the framework, we studied the inclusion
of different guest molecules into compound 1. In a typical
experiment, 1 was exposed to different dry solvents for a few
days. PXRD analysis showed structural changes in the
compound that were exposed to different vapors, suggesting
the flexible behavior of the framework. For small hydro-
phobic guests (acetone, acetonitrile), PXRD patterns are
identical, but different from the PXRD pattern of 1, indicat-
ing new phases after guest inclusion. On the other hand, in the
presence of small hydrophilic guests (ethanol, methanol),
patterns were similar to that of 1, indicating structural
similarity. These results suggest different responses of
1 toward different types of guest molecules (Figure S12). To
confirm the guest-inclusion behavior, solvent sorption experi-
ments were carried out with hydrophobic and hydrophilic
Figure 2. Perspective views of the structural transformation between
compound 1a and 1 (hydrogen atoms and free solvent molecules are
omitted for clarity).
guests at 298 K (Figure 3). The dehydrated or guest-free
phase (1’) was generated by heating 1 to 1208C for three hours
to remove guest water molecules. The dynamic behavior of
the framework was confirmed by these solvent sorption
experiments. Small guest molecules of both types showed very
similar sorption patterns. Little sorption was observed below
P/P0 = 0.2, but slowly increased with increasing pressure; all
profiles show hysteric sorption behavior. Among hydrophilic
guest molecules, water showed the highest amount of uptake
( ꢀ 143 mLgꢁ1), whereas methanol ( ꢀ 93 mLgꢁ1) and ethanol
( ꢀ 48 mLgꢁ1) showed less amount of uptake, in accordance
with the size of the guest molecules (Figure S16). Small and
similar-sized hydrophobic guests (acetonitrile and acetone)
showed a similar amount of uptake ( ꢀ 160 mLgꢁ1), but large
hydrophobic guest molecules (benzene and cyclohexane)
could not enter into the channels because of the insufficient
space inside the channels (Figure 3, and Figure S15 in the
Supporting Information).
Angew. Chem. Int. Ed. 2013, 52, 998 –1002
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
999

.
Angewandte
Communications
Figure 3. Sorption curves of hydrophobic (acetonitrile, acetone, ben-
zene, cyclohexane) and hydrophilic (H2O, MeOH, EtOH) guest mole-
cules at 298 K. Filled shapes=adsorption, unfilled shapes=desorp-
tion.
Figure 4. IR spectra of anion-exchanged compounds with highlighted
band positions of the corresponding anions.
SCNꢁ were not exchanged (Figures S20 and 21). In the second
ꢁ
stage of the experiment, with a concentration of 1.0 mm NO3
ꢁ
ꢁ
The flexible nature of these framework was also observed
in anion-exchange studies. As described above, the 1D
channel along the c axis of compound 1 is filled with NO3
in MeOH, only little exchange of N(CN)2 by NO3 was
ꢁ
observed, which was confirmed by the presence of an NO3
ꢁ
ꢁ
band in the FT-IR spectrum of 1ꢂN(CN)2 and a slightly
anions. To study the anion-dependent structural dynamism of
the compound, we performed anion-exchange experiments.
For this purpose, we used two types of anions: 1) anions with
reduced band intensity of N(CN)2ꢁ. On the other hand, no
NO3 uptake into 1ꢂN3 and 1ꢂSCNꢁ was observed,
ꢁ
ꢁ
suggesting strong interactions of N3ꢁ and SCNꢁ anions with
ꢁ
ꢁ
a weak or noncoordinating nature, such as ClO4 and
the framework. However, 1ꢂClO4 was again completely
ꢁ
N(CN)2 (type A), and 2) anions with a strong coordinating
exchanged by NO3ꢁ, as in the previous experiment.
nature, such as N3ꢁ and SCNꢁ (type B). Crystals of compound
1 were immersed in separate methanolic solutions with an
excess of NaN3, KSCN, NaClO4, and NaN(CN)2. We then
monitored anion-exchange experiments by FT-IR spectros-
copy. We observed that complete exchange of anions occurred
within five days. FT-IR spectra of anion-exchanged products
showed strong bands associated with exchanged anions, and
the disappearance of bands of nitrate anions. Other bands in
the spectra remained almost unchanged, suggesting that the
frameworks of the complexes remained intact throughout the
Selective anion exchange with the framework was inves-
tigated by performing anion-exchange experiments with
different binary mixtures of anions. Five different binary
combinations were used to study affinity, N3ꢁ/SCNꢁ, N3ꢁ/
ꢁ
ClO4ꢁ, N3 /N(CN)2ꢁ, ClO4 /N(CN)2ꢁ, and SCNꢁ/N(CN)2
.
ꢁ
ꢁ
In a typical experiment, crystals of 1 were immersed in
a methanolic solution of mixed anions in equimolar concen-
tration (see experimental details in the Supporting Informa-
tion). Anion exchange was further monitored by FT-IR
analysis to examine preferential exchange from the mixture.
Among the five combinations, N3 /SCNꢁ and N3ꢁ/ClO4
showed selective anion exchange with the framework. With
ꢁ
ꢁ
ꢁ
exchange process. Compound 1 with NO3 anions inside the
ꢁ
channels (designated as 1ꢂNO3 ) shows a strong band at
1390 cmꢁ1, corresponding to nitrate anions, and this band
almost completely disappeared in anion-exchanged products.
N3 /SCNꢁ, NO3 is preferentially exchanged by SCNꢁ (Fig-
ꢁ
ꢁ
ꢁ
ꢁ
ure S22), and with N3ꢁ/ClO4
, NO3 is preferentially
ꢁ
ꢁ
New bands appeared at ꢀ 2050 cmꢁ1 (1ꢂN3 ), ꢀ 2080 cmꢁ1
exchanged by N3 (Figure 5b). With the other combinations,
the presence of both anions inside the framework was
confirmed by their respective IR bands (Figures S24–26). It
is worth noting that anion exchange could not be reverted in
(1ꢂSCNꢁ), ꢀ 1100 cmꢁ1 (1ꢂClO4ꢁ), and ꢀ 2160 cmꢁ1
ꢁ
(1ꢂN(CN)2 ) as expected for different anions (Figure 4).
PXRD patterns of anion-exchanged products are different for
different anions because of their different shape, size, and
coordinating tendency, thus showing the highly flexible nature
of the framework. The framework can easily adjust its channel
dimension to encapsulate different guest anions because of its
dynamic nature.
1ꢂSCNꢁ and 1ꢂN3 because of strong coordination of the
ꢁ
corresponding anions. Thus, the above-mentioned experi-
ments show the order of affinity of guest anions to the
ꢁ
framework: SCNꢁ > N3ꢁ > N(CN)2ꢁ > ClO4
(Figure 5a).
These differences in affinity to the framework arise from
several properties of the anions, such as their size, shape,
geometry, their coordinating tendency to ZnII, and also their
different p–p interaction and hydrogen-bonding abilities with
the framework.
Anion-exchanged materials showed interesting anion-
dependent luminescent behavior (Figure 6a). Solid-state
UV absorptions were measured for all anion-exchanged
Reversibility of the anion exchange was studied in a two-
stage experiment with two solutions of tetrabutylammonium
nitrate in methanol (concentrations: 0.5 mm and 1.0 mm; see
experimental details in the Supporting Information). In the
first stage of the experiment, with a concentration of 0.5 mm
ꢁ
ꢁ
NO3 in MeOH, FT-IR spectra showed that ClO4 was
completely exchanged by NO3ꢁ, while N(CN)2ꢁ, N3ꢁ, and
1000 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 998 –1002

Angewandte
Chemie
Figure 5. a) Schematic representation of the anion selectivity with
combinations of two anions. b) Selective uptake of N3ꢁ from the
combination of anions evidenced by FT-IR spectrum.
compounds, and these absorption curves were very similar to
that of compound 1 (Figure S27). Furthermore, solid-state
emission spectra of powder samples of L, 1, and all anion-
exchanged compounds were measured at room temperature
(Figure 6b). Upon excitation at 394 nm, L shows two emission
bands, at 512 nm and 533 nm; L is weakly fluorescent
compared to the other samples. Compound 1 showed an
intense band at 541 nm with a red-shift with respect to L,
which may be attributed to the coordinating effect of L to
ꢁ
ZnII. Anion-exchanged compounds 1ꢂN3
,
1ꢂSCNꢁ,
1ꢂN(CN)2ꢁ, and 1ꢂClO4ꢁ display broad bands with intensity
maxima at 514, 542, 543, and 536 nm, respectively. Compound
1ꢂN3ꢁ showed a blue-shift with respect to L and 1. In case of
compound 1, p*-n or p*–p intraligand transitions are possible.
The emission intensity of anion-exchanged compounds was
very different to the emission intensity of 1. For anion-
Figure 6. a) Effects of anion exchange on the solid-state luminescence
properties of 1. b) Luminescence intensity of different anion-exchanged
samples. c) Bar diagram showing luminescence enhancement and
quenching of anion-exchanged samples with respect to 1.
ꢁ
exchanged compounds of type A, that is, 1ꢂN(CN)2 and
using the technique for the powder samples described by Bril
et al.,[9] with the following expression:
1ꢂClO4ꢁ, high enhancement of fluorescence was observed
(70.72 and 84.14% respectively), on the other hand, for anion-
exchanged compounds of type B, that is, 1ꢂN3ꢁand 1ꢂSCNꢁ,
fluorescence quenching was observed (29.53% and 80.36%,
respectively; Figure 6c).
1 ꢁ Rst Ix
ð1Þ
Fx ¼
Fst
1 ꢁ Rx Ist
Quantum yields were measured for
1
and anion-
Rx and Rst represent diffuse reflectance of the complex
exchanged compounds in the solid state at room temperature
and the standard, respectively (with regard to a particular
Angew. Chem. Int. Ed. 2013, 52, 998 –1002
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1001

.
Angewandte
Communications
~
(KBr pellet): n ¼3406.42(m), 2921.91(w), 1630.18(w), 1599.65(m),
wavelength). Fx and Fst are the quantum yields of solid
samples and standard phosphor sample, respectively. Ix and Ist
represent the integrated emission spectrum of the sample x
and the standard, respectively. KBr was used as a reflecting
standard, and perylene was used as a standard phosphor
sample with an emission maximum at approximately 569 nm
(Fst = 0.98,[10] excitation range 390 to 440 nm). The measured
values of quantum yields of 1 and anion-exchanged com-
pounds are in good agreement with their respective lumines-
1502.91(w), 1384.21(s), 1156.59(w), 1101.12(w), 1019.91(m),
907.19(m), 847.26(m), 775.34(m), 639.08(m), 564.10 cmꢁ1 (m).
Crystal data for 1a: Formula C28H30N6O8Zn, monoclinic, space
group C2/c, a = 17.746(5), b = 12.105(4), c = 15.306(5) ꢀ, b =
90.244(7)8, V= 3287.7(19) ꢀ3, Z = 4, T= 150(2) K, R = 0.0608,
wR2 = 0.1933, GOF = 1.087. Crystal data for 1: Formula
C26H30N6O10Zn, monoclinic, space group P21/n, a = 15.305(4), b =
10.6119(3), c = 18.043(5) ꢀ, b = 93.120(7)8, V= 2926.0(14) ꢀ3, Z = 4,
T= 150(2) K, R = 0.0480, wR2 = 0.1182, GOF = 1.041. CCDC 883148
(1a) and 883147 (1) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
ꢁ
cence intensity profiles. 1ꢂClO4 has the most intense
emission profile (F = 0.921), and 1ꢂSCNꢁ has the least
intense emission profile (F = 0.0015). Quantum yields of
1ꢂN(CN)2ꢁ and 1ꢂN3ꢁ are 0.357 and 0.0250, respectively, and
in good agreement with their corresponding luminescence
intensities. The reasons for the order of observed lumines-
cence of the different anion-exchanged compounds are not
very obvious; the differences are probably a result of various
electronic interactions of the anions with the framework walls
and the ZnII center, depending upon their size, shape,
Received: August 20, 2012
Published online: December 5, 2012
Keywords: anion exchange · cationic framework ·
dynamic framework · luminescence · sorption
.
ꢁ
geometry, and coordinating tendencies. As ClO4 and
ꢁ
[1] a) H.-C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112,
673 – 674; b) Q.-K. Liu, J.-P. Ma, Y.-B. Dong, J. Am. Chem. Soc.
2010, 132, 7005 – 7017; c) A. C. McKinlay, R. E. Morris, P.
Horcajada, G. Ferey, R. Gref, P. Couvreur, C. Serre, Angew.
Chem. 2010, 122, 6400 – 6406; Angew. Chem. Int. Ed. 2010, 49,
6260 – 6266; d) J. An, S. J. Geib, N. L. Rosi, J. Am. Chem. Soc.
2009, 131, 8376 – 8377; e) J. Y. Lee, O. K. Farha, J. Roberts, K. A.
Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38,
1450 – 1459; f) L. Ma, J. M. Falkowski, C. Abney, W. Lin, Nat.
Chem. 2010, 2, 838 – 846; g) Z. Guo, R. Cao, X. Wang, H. Li, W.
Yuan, G. Wang, H. Wu, J. Li, J. Am. Chem. Soc. 2009, 131, 6894 –
6895; h) G. Li, W. Yu, Y. Cui, J. Am. Chem. Soc. 2008, 130, 4582 –
4583.
[2] a) A. R. Millward, O. M. Yaghi, J. Am. Chem. Soc. 2005, 127,
17998 – 17999; b) S. M. Humphrey, J.-S. Chang, S. H. Jhung, J. W.
Yoon, P. T. Wood, Angew. Chem. 2007, 119, 276 – 279; Angew.
Chem. Int. Ed. 2007, 46, 272 – 275.
[3] S. Horike, S. Shimomura, S. Kitagawa, Nat. Chem. 2009, 1, 695 –
704.
[4] a) N. Yanai, K. Kitayama, Y. Hijikata, H. Sato, R. Matsuda, Y.
Kubota, M. Takata, M. Mizuno, T. Uemura, S. Kitagawa, Nat.
Mater. 2011, 10, 787 – 793; b) C.-L. Chen, A. M. Beatty, J. Am.
Chem. Soc. 2008, 130, 17222 – 17223; c) J.-P. Zhang, X.-M. Chen,
J. Am. Chem. Soc. 2008, 130, 6010 – 6017; d) N. Yanai, T.
Uemura, M. Inoue, R. Matsuda, T. Fukushima, M. Tsujimoto,
S. Isoda, S. Kitagawa, J. Am. Chem. Soc. 2012, 134, 4501 – 4504.
[5] a) K. S. Min, M. P. Suh, J. Am. Chem. Soc. 2000, 122, 6834 – 6840;
b) B.-C. Tzeng, T.-H. Chiu, B.-S. Chen, G.-H. Lee, Chem. Eur. J.
2008, 14, 5237 – 5245; c) Q.-Y. Yang, K. Li, J. Luo, M. Pana, C.-Y.
Su, Chem. Commun. 2011, 47, 4234 – 4236.
N(CN)2 are weakly coordinating, they can occupy different
positions within the dynamic framework after exchange with
NO3ꢁ, thus helping to change the framework in such a way
that intraligand interactions may increase, and luminescence
may subsequently be enhanced. As N3 and SCNꢁ are
ꢁ
strongly coordinating with ZnII, their coordination can lead
to drastic structural change (also observed in PXRD, Fig-
ure S13), thus diminishing intraligand interactions within the
framework, which may reduce luminescence intensity com-
pared to compound 1.
In conclusion, we have synthesized a dynamic luminescent
cationic porous framework using a newly designed chelating
N-donor ligand. The compound shows guest- as well as anion-
dependent structural dynamic behavior. Guest-dependent
dynamic behavior has been demonstrated by SCSC experi-
ments. Anions of the compound encapsulated in the channels
are completely exchangeable by other anions. Completely
reversible, partially reversible, and nonreversible anion
exchange was observed depending upon the nature of the
anions. Furthermore, different kinds of affinities of the anions
toward the framework were observed. Anion-exchanged
compounds showed interesting anion-responsive tunable
luminescent properties, which might have important biolog-
ical and environmental applications.
Experimental Section
[6] a) T. K. Maji, R. Matsuda, S. Kitagawa, Nat. Mater. 2007, 6, 142 –
148; b) J.-P. Ma, Y. Yu, Y.-B. Dong, Chem. Commun. 2012, 48,
2946 – 2948.
[7] a) Y. Cui, H. Xu, Y. Yue, Z. Guo, J. Yu, Z. Chen, J. Gao, Y. Yang,
G. Qian, B. Chen, J. Am. Chem. Soc. 2012, 134, 3979 – 3982; b) C.
Wang, W. Lin, J. Am. Chem. Soc. 2011, 133, 4232 – 4235; c) J. An,
C. M. Shade, D. A. Chengelis-Czegan, S. Petoud, N. L. Rosi, J.
Am. Chem. Soc. 2011, 133, 1220 – 1223.
Synthesis of ligand L: The condensation of 4,4’-ethylenedianiline (5 g,
0.0235 mol) and 2-pyridine carboxaldehyde (5.034 g, 0.047 mol) in
EtOH/MeOH (1:1, 100 mL) for 3 h at 708C led to a light-yellow solid,
which was filtered, washed with EtOH and MeOH, and dried under
vacuum to afford the product as a light-yellow solid (7 g). Calcd for
C26H22N4: C, 79.89; H, 5.67; N, 14.33; found: C, 80.40; H, 5.07; N,
14.52.
[8] Y. Takashima, V. M. Martꢁnez, S. Furukawa, M. Kondo, S.
Shimomura, H. Uehara, M. Nakahama, K. Sugimoto, S. Kita-
gawa, Nat. Commun. 2011, 2, 168.
[9] A. Bril, A. W. De Jager-Veenis, J. Res. Natl. Bur. Stand. Sect. A
1976, 80A, 401 – 407.
Synthesis of compound 1a: In a glass tube, benzene (1 mL) and
a solution of Zn(NO3)2 (29.74 mg) in MeOH (1 mL) were carefully
layered over a solution of L (39 mg) in CH2Cl2 (1 mL). Rod-like
yellow crystals suitable for X-ray studies were obtained in approx-
imately 60% yield after 15 days.
[10] W. H. Melhuish, J. Opt. Soc. Am. 1964, 54, 183 – 186.
When parent crystals were taken out of the mother liquor and
kept in open air for about 1.5 h, they changed to compound 1. FT-IR
1002 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 998 –1002