Simultaneous presence of both open metal sites and free functional organic sites in a noncentrosymmetric dynamic metal-organic framework with bimodal catalytic and sensing activities

Article abstract of DOI:10.1002/chem.201302084

Assimilation of open metal sites (OMSs) and free functional organic sites (FOSs) with a framework strut has opened up a new route for the fabrication of novel metal-organic materials, thereby providing a unique opportunity to explore their multiple functionalities. A new metal-organic framework (MOF), {[Cu(ina)₂(H₂O)][Cu(ina)₂(bipy)]·2 H₂O}_n (1) (ina=isonicotinate, bipy=4,4′-bipyridine), has been synthesized and characterized. Complex 1 is crystallized in the orthorhombic noncentrosymmetric space group Aba2 and consists of two different 2D coordination polymers, [Cu(ina)₂(H₂O)]_n and [Cu(ina)₂(bipy)]_n, with entrapped solvent water molecules. Hydrogen-bonding interactions assemble these two different 2D coordination layers in a single-crystal structure with interdigitation of pendant 4,4′-bipy from one layer into the groove of another. Upon removal of guest molecules, 1 undergoes a structural transformation in single-crystal-to-single- crystal fashion with expansion of the effective void space. Each metal center is five-coordinated and thus can potentially behave as an OMS, and the free pyridyl groups of pendant 4,4′-bipy moieties and free -C=O groups can act as free FOSs. Thus, owing to presence of both OMSs and free FOSs, the framework exhibits multifunctional properties. Owing to the presence of OMSs, the framework can act as a Lewis acid catalyst as well as a small-molecule sensor material, and in a similar way, owing to the presence of free FOSs, it performs as a Lewis base catalyst and a cation sensor material. Furthermore, owing to noncentrosymmetry with large polarity along a particular direction, it shows strong second-harmonic generation/nonlinear optical (SHG-NLO) activity.

Full text of DOI:10.1002/chem.201302084

FULL PAPER
DOI: 10.1002/chem.201302084
Simultaneous Presence of Both Open Metal Sites and Free Functional
Organic Sites in a Noncentrosymmetric Dynamic Metal–Organic Framework
with Bimodal Catalytic and Sensing Activities
Rajat Saha,[a] Biplab Joarder,[b] Anupam Singha Roy,[c] Sk. Manirul Islam,[c] and
Sanjay Kumar*[a]
In memory of Golam Mostafa (1962–2011)
À
Abstract: Assimilation of open metal
sites (OMSs) and free functional or-
ganic sites (FOSs) with a framework
strut has opened up a new route for
the fabrication of novel metal–organic
materials, thereby providing a unique
opportunity to explore their multiple
functionalities. A new metal–organic
cules. Hydrogen-bonding interactions
assemble these two different 2D coor-
dination layers in a single-crystal struc-
ture with interdigitation of pendant
4,4’-bipy from one layer into the
groove of another. Upon removal of
guest molecules, 1 undergoes a structur-
al transformation in single-crystal-to-
single-crystal fashion with expansion of
the effective void space. Each metal
center is five-coordinated and thus can
potentially behave as an OMS, and the
free pyridyl groups of pendant 4,4’-bipy
moieties and free C=O groups can act
as free FOSs. Thus, owing to presence
of both OMSs and free FOSs, the
framework exhibits multifunctional
properties. Owing to the presence of
OMSs, the framework can act as
a Lewis acid catalyst as well as a small-
molecule sensor material, and in a simi-
lar way, owing to the presence of free
FOSs, it performs as a Lewis base cata-
lyst and a cation sensor material. Fur-
thermore, owing to noncentrosymmetry
with large polarity along a particular
direction, it shows strong second-har-
monic generation/nonlinear optical
(SHG-NLO) activity.
framework (MOF), {[Cu
ACHUTGTNERN(NUG ina)2ACHTUNGTERN(NUGN H2O)]
[Cu(ina)2A(bipy)]·2H2O}n (1) (ina=iso-
A
CHTUNGTRENNUNG
nicotinate, bipy=4,4’-bipyridine), has
been synthesized and characterized.
Complex 1 is crystallized in the ortho-
rhombic noncentrosymmetric space
group Aba2 and consists of two differ-
ent 2D coordination polymers, [Cu-
Keywords: catalysis · metal–organic
frameworks · Lewis acids · Lewis
bases · sensors
ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(H2O)]n and [CuACHTNUGERTGN(NNU ina)2ACHTNUGTNER(NUGN bipy)]n,
with entrapped solvent water mole-
Introduction
ly depend upon the nature of the framework as well as the
pore surface.[2] Accordingly, multifunctional metal–organic
frameworks (MMOFs) have gained tremendous attention
and are at the heart of current scientific advancements.[3]
Therefore the design and synthesis of MMOFs have become
immense challenges in crystal engineering. Crystal engineers
have focused on surface functionalization to fabricate new
MOFs capable of showing multiple functionalities. Surface
functionalization can be performed in two ways: 1) the im-
mobilization of open metal sites (OMSs) and 2) the intro-
duction of free functional organic sites (FOSs) within the
framework.[4]
In the last two decades, porous metal–organic frameworks
(MOFs) have emerged as a most promising functional mate-
rial on account of their numerous material properties such
as gas and solvent sorption, storage, and separation.[1] In ad-
dition to porosity, researchers have further tried to encom-
pass other properties like magnetism, sensing, drug delivery,
second-harmonic generation/nonlinear optical (SHG-NLO)
activity, ferroelectricity, catalysis, and so forth, which crucial-
[a] R. Saha, Dr. S. Kumar
The presence of OMSs within a framework is quite
common. The generation of OMSs through the removal of
coordinated solvent molecules is also a rather easy process.
It is well known that the presence of OMSs allows the
framework to act as a Lewis acid catalyst.[5] Furthermore, it
helps to improve the adsorption capacity of the framework[6]
and is also responsible for the hysteresis phenomenon.[7] In
addition, Chen and co-workers reported the small-molecule
sensor activity of an MOF with OMSs.[8] By contrast, the
presence of free FOSs within a framework is very rare
owing to their strong affinity towards metal ions and pro-
Department of Physics, Jadavpur University, Jadavpur
Kolkata 700032 (India)
Fax : (+33)2414-6584
E-mail: kumars@phys.jdvu.ac.in
[b] B. Joarder
Department of Chemistry, IISER Pune
Pashan, Pune 411008 (India)
[c] A. S. Roy, S. Manirul Islam
Department of Chemistry, University of Kalyani
Kalyani 741235 (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302084.
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tons. Thus, during the self-assembly process, functional or-
ganic sites bind metal ions and protons; that is, they remain
completely blocked. These free FOSs can serve as active
base sites and thus can be used for selective Lewis basic cat-
alysis.[9] Furthermore, Chen et al. have established the cation
sensing activity of framework materials with free FOSs.[10]
But up to now the simultaneous presence of both OMSs and
free FOSs in a single framework has only been reported
once.[1s]
In this paper we report an unusual simultaneous presence
of both OMSs and free FOSs in a noncentrosymmetric dy-
namic MOF, {[CuACHTUNGTRENNUNG(ina)2ACHTUNRGTE(NGNUN H2O)][CuACTHGUNTERNN(UGN ina)2(4,4’-bipy)]·2H2O}n
(1; ina=isonicotinate, bipy=4,4’-bipyridine) and establish
the multiple functional properties of the material. This is
a hitherto unreported inorganic cocrystal of 2D coordination
polymers; two different coordination layers become stabile
in a framework through interdigitation. The dynamic nature
of 1 has been established by solid-state structural transfor-
mation in single-crystal-to-single-crystal (SC-SC) fashion
and, interestingly, upon removal of disordered guest water
molecules, the framework expands the effective void space.
Owing to the presence of five-coordinated CuII centers
(OMSs) and free pyridyl groups (FOSs), the framework
shows size-selective Lewis acidic catalysis and Lewis basic
catalysis with both metal-ion and solvent (small-molecule)
sensing activities (Scheme 1). Owing to its noncentrosymme-
try along with its high polarity, the framework shows strong
SHG-NLO activity.
Figure 1. a) Asymmetric unit of 1. b) Schemetic representation of a bilay-
er. c) Actual representation of 1. Guest molecules and hydrogen atoms
have been removed for clarity.
The A-type molecule consists of one CuII ion, two uninega-
tive ina ligands, and one coordinated water molecule, where-
as the B-type molecule consists of one CuII ion, two ina lig-
AHCTUNGTREGaNNNU nds, and one coordinated 4,4’-bipy moiety (Figure S1 in the
Supporting Information). In both A- and B-type molecules,
CuII adopts a five-coordinated square-pyramidal geometry
(for Cu1 t=0.20, for Cu2 t=0.29). In each case (for both
Cu1 and Cu2), the basal plane of the pyramid is built up by
two pyridyl nitrogen atoms of two different ina ligands in
trans mode and two carboxylato-oxygen atoms of two differ-
ent ina ligACTHNUGRTNEUNGands also in trans mode. Each metal center is
bridged by ina ligands to generate 2D coordination layers
with a grid topology in the crystallographic bc plane (Fig-
ure S2 in the Supporting Information). The two layers differ
from each other in the sense that 4,4’-bipy has occupied the
fifth coordination site of one layer (layer B) in monodentate
fashion, whereas in the other layer (layer A) this has been
satisfied by the coordination from the water molecule (Fig-
ure S3 in the Supporting Information). These five-coordinat-
ed metal centers act as OMSs and the free pyridyl groups of
pendant 4,4’-bipy can behave as free FOSs (Figure S4 in the
Supporting Information).
The coordinated water molecules form twisted Y-pat-
terned hydrogen bonds with the uncoordinated oxygen
atoms of carboxylato groups of the adjacent B layer with
the formation of a supramolecular bilayer (Figure S5 in the
Supporting Information). These supramolecular bilayers are
packed in a parallel fashion along the crystallographic a axis
to form a three-dimensional structure (Figures S6–S8 in the
Supporting Information). This 3D decoration of the overall
structure is the reason why the free pyridyl groups of 4,4’-
bipy moieties of layer B are thrown into the groove of adja-
cent layer A; that is, interdigitation occurs (Figure 1). Guest
water molecules are trapped in between such bilayers. The
distance between layers A and B within the supramolecular
bilayers is 5.861 ꢁ and the distance between one A layer
and one B layer of two adjacent bilayers is 5.333 ꢁ. But the
distances between two A layers and two B layers are the
same (11.195 ꢁ). Supramolecular hydrogen-bonding interac-
Scheme 1. The presence of both open metal sites and free functional or-
ganic sites allows the framework to act as a multifunctional material.
Results and Discussion
Crystal and supramolecular structure of complex 1: Single-
crystal X-ray diffraction (SC-XRD) analysis revealed that
1 is crystallized in the orthorhombic noncentrosymmetric
space group Aba2 and is a hydrated cocrystal of two differ-
ent 2D metal–organic coordination polymers (type A: [Cu-
ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(H2O)]n; type B: [CuACTHNGUTREN(NUNG ina)2(4,4’-bipy)]n; Figure 1). The
asymmetric unit contains half an A-type molecule, half a
B-type molecule and one disordered guest water molecule.
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Dynamic Metal–Organic Frameworks
FULL PAPER
tions between A and B layers are responsible for such un-
usual cocrystallization. This is the first example of an inor-
ganic cocrystal of coordination polymers, which is yet to be
explored.
kinetic diameter (3.3 ꢁ) and a larger quadrupole moment
than N2, so it can interact with the host framework, there-
fore there is little uptake and hysteresis behavior (Fig-
ure S12 in the Supporting Information).
Thermal and PXRD analysis: The thermogravimetric analy-
sis of complex 1 indicates the occurrence of four consecutive
steps (Figure S9 in the Supporting Information). In the first
step, the guest water molecules are removed within the tem-
perature range of 80–1508C. The partially dehydrated phase
is also stable up to 1708C and the coordinated water mole-
cules are lost from the framework within the temperature
range of 170–2208C. Consequently, after removal of coordi-
nated water molecules the framework starts to decompose.
The phase purity of the as-synthesized complex was exam-
ined by PXRD analysis. As synthesized compound was
heated at 1508C for 4 h to remove guest water molecules
and was then characterized by PXRD analysis. The PXRD
analysis indicates that after loss of guest water molecules
the framework remains crystalline (2) and is different from
the original complex 1. The PXRD pattern reveals that
upon removal of guest water molecules the peak at 7.908
shifts to a lower angle of 7.728, thus indicating a slight in-
crease in the interlayer separation. All PXRD patterns are
presented in Figure S10 of the Supporting Information.
Size-selective bicatalytic activities: Porous metal–organic
frameworks are highly attractive for heterogeneous size-se-
lective catalytic activities. Generally, MOFs that contain
open metal sites can be used in Lewis acidic catalysis such
as oxidation, cyanosilylation, and so forth.[5,14] And MOFs
that contain Lewis basic free FOSs can be utilized for Lewis
basic catalysis such as Knoevenagel condensation reac-
tions.[4c,9] The framework contains both Lewis acidic five-co-
ordinated CuII centers as well as Lewis basic free pyridyl
À
groups with free C=O groups. Owing to the presence of
both Lewis acidic and Lewis basic functional groups, we
have attempted to study both catalytic oxidation and Knoe-
venagel condensation reactions using the evacuated frame-
work (2) as catalyst.
Catalytic oxidation reactions: Catalytic oxidation reactions
were carried out in the presence of oxidant (tert-butyl hy-
droperoxide (TBHP)) and in the absence of oxidant (aero-
bic oxidation).
Styrene was selected as the model substrate to optimize
the catalytic oxidation reaction conditions. In this study, the
reaction was carried out with 70 wt% TBHP in water as oxi-
dant and acetonitrile as solvent at a constant temperature of
508C. Samples were taken out from the reaction mixture
after certain intervals and were analyzed by gas chromatog-
raphy. Catalytic oxidation of styrene resulted in the forma-
tion of benzaldehyde, styrene epoxide, and phenylacetalde-
hyde (Scheme 2 and Figure S13 in the Supporting Informa-
Dynamic nature—Single-crystal-to-single-crystal transforma-
tion: Guest-templated host frameworks undergo structural
transformation depending upon the nature and amount of
guest molecules within the framework.[11] In most cases, such
a structural transformation upon removal of the guest mole-
cules leads to a shrinking of the effective void space, but in
rare cases expansion has been found to occur.[12]
Complex 1 was heated at 1508C for 1 hour and, interest-
ingly, after heating, the retention of single crystallinity was
observed with a change of color from light blue to very
deep blue (complex 2). SC-XRD analysis indicated that
complex 2 crystallized in the monoclinic space group Pc
(Table S1 in the Supporting Information). Due to heating,
the length of the a axis decreased from 22.389 to 12.999 ꢁ
and also the b angle changed from 90 to 118.858 with
change of the Z value from 4 to 2. Structural analysis also
revealed that upon removal of guest water molecules, both
the A-type and B-type layers remain identical with respect
to their coordination mode like complex 1, but the interlay-
er separation between the 2D layers increases (from 5.861
to 5.962 ꢁ and 5.333 to 5.424 ꢁ; Figure S11 in the Support-
ing Information) through the sliding of 2D coordination
layers along the a axis. As a result, the effective void space
increases by 0.3%. Such dynamicity of this framework arises
from the interdigitation of the 4,4’-bipy unit of the B layer
into the groove of the A layer.[13]
Scheme 2. Catalytic oxidation of styrene at 508C.
tion). The main results are presented in Figure 2 (Table 1).
A blank experiment in the absence of a framework with
TBHP results in low conversion of styrene. Next, another
experiment was carried out with neat styrene without any
solvent. In this case, primarily benzaldehyde was obtained
with low substrate conversion. Conversion of styrene in-
creases with time and reaches a maximum of 80% at 8 h
with an increase in the selectivity towards aldehyde. In this
catalytic reaction, benzaldehyde predominates among the
oxidation products with a selectivity of 63% (24% epoxide
and 13% phenylacetaldehyde). The selectivity of benzalde-
hyde can be increased up to 84% by increasing the TBHP
molar ratio but substrate conversion remains almost con-
stant. Further decreasing the molar ratio of TBHP leads to
Adsorption study: To measure the porosity of the evacuated
framework (2), we carried out a gas adsorption study. It
shows no uptake of N2 (8 mLgÀ1) and little uptake of CO2
(15 mLgÀ1) at 77 and 195 K, respectively. CO2 has smaller
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S. Kumar et al.
When O2 was used as oxidant, the conversion of styrene
was 12% with benzaldehyde as the major product (62% se-
lectivity). The addition of TBHP (0.1 mmol) to the reaction
mixture increased the conversion to 37% with a slight in-
crease in the selectivity of benzaldehyde. Similar aerobic ox-
idation reactions were also carried out for 4-chlorostyrene
and a-methylstyrene (Table S6 in the Supporting Informa-
tion). Furthermore, a comparison of the catalytic efficiency
of this framework in the oxidation of styrene with other re-
ported MOFs clearly indicates that it performs as a better
catalyst than other reported MOFs.[15]
Catalytic Knoevenagel condensation reactions: The frame-
work was also used as catalyst in the liquid-phase Knoeve-
nagel reaction by studying the condensation of benzalde-
hyde with malononitrile to form benzylidenemalononitrile
as the probe reaction (Scheme 3).
Figure 2. The time-dependent conversion of styrene and formation of the
three products A–C catalyzed by the framework at 508C.
The Knoevenagel condensa-
tion reaction is an important re-
action for the generation of
Table 1. Oxidation of various olefins with TBHP catalyzed by the framework.[a]
Entry
Substrate
Oxidant
Conv.
[%][b]
Selectivity [%][b]
epoxide
À
C C bonds. As a greener alter-
aldehyde
others
native, the Knoevenagel con-
densation reaction was carried
out at room temperature using
ethanol as solvent. The ratio of
aldehyde and malononitrile is
an important factor in Knoeve-
nagel condensation. The effect
of the molar ratio of benzalde-
hyde/malononitrile on the reac-
1
styrene
TBHP[c]
TBHP[d]
TBHP
3
8
80
82
36
42
79
64
63
84
54
19
23
24
12
30
7
trace
13[g]
13[g]
4[g]
TBHP[e]
TBHP[f]
TBHP
16[g]
–
2
3
a-methylstyrene
75[h]
18[i]
58
4-chlorostyrene
TBHP
60
29
13[j]
[a] Reaction conditions: olefin (5 mmol), complex 1 (20 mg), acetonitrile (5 mL), O2 purged through balloon,
508C, 8 h. [b] Conversion determined by GC and GC-MS. [c] Without catalyst. [d] Without solvent.
[e] 15 mmol TBHP. [f] 5 mmol TBHP. [g] Phenylacetaldehyde. [h] Acetophenone. [i] Propiophenone. tion conversions was therefore
[j] 4-Chlorophenylacetaldehyde.
investigated, and the reactions
a sharp decrease in substrate conversion but the distribution
of products remains almost the same as mentioned earlier.
In the case of 4-cholorstyrene, conversion of styrene de-
creases substantially to 60% with a product distribution that
comprises 4-chlorobenzaldehyde as the major product ac-
Scheme 3. Catalytic Knoevenagel condensation reaction.
companied by 4-chlorophenylacetaldehyde and 4-chlorostyr-
ene oxide. Owing to the larger size of 4-chlorostyrene, we
get a smaller conversion (60%), and also due to the pres-
ence of a chloride group, the selectivity of epoxide forma-
tion is enhanced (29%). a-Methylstyrene showed 42% con-
version with 75% formation of acetophenone and 18% of
propiophenone in 8 h with TBHP (Table 1).
were carried out at molar ratios of 1:0.5, 1:1, and 1:2, respec-
tively. The products were characterized by GC, GC-MS, and
NMR spectroscopic studies (Figures S15 and S16 in the Sup-
porting Information). Experimental results showed that
90% conversion was obtained after 4 h of reaction at
a molar ratio of 1:2. As expected, decreasing the benzalde-
hyde/malononitrile molar ratio to 1:1 resulted in a slight
drop in the reaction rate; however, 90% conversion could
be achieved after 6 h (Table 2). The reaction using half an
equivalent of malononitrile proceeded slowly, with only
61% conversion being observed after 6 h (Figure S17 in the
Supporting Information).
Now, to investigate the effect of the catalyst amount on
the reaction conversion, the reactions were carried out at
room temperature by using a benzaldehyde/malononitrile
molar ratio of 1:1. It was observed that 82% conversion of
benzaldehyde was achieved after 4 h with 25 mg catalyst,
Catalytic aerobic oxidation reaction: Recently, aerobic oxi-
dations of a series of substrates were carried out by using
different MOFs as catalyst under different reaction condi-
tions.[15] Herein, we have also studied the oxidation of ole-
fins under aerobic conditions at 508C in acetonitrile
(Table S6 and Figure S14 in the Supporting Information).
Olefin oxidations using our framework as catalyst were car-
ried out using O2 and small additives of TBHP as initiator
or by using only O2 as oxidant. In both cases, all the olefins
are selectively converted to allylic products (Scheme 2).
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Dynamic Metal–Organic Frameworks
FULL PAPER
Table 2. Knoevenagel condensation of benzaldehyde with malononitri-
le.[a]
remains almost unaffected, and small changes were ob-
served for other ions. The prominent broadening of the d–d
transition band is indicative of the distortion of the axial co-
ordination position of CuII of the framework due to stronger
binding of Ag+ with the pyridyl nitrogen of pendant 4,4’-
bipy. The framework shows a broad emission band at ap-
proximately 405 nm upon excitation at approximately
300 nm (Figure 4). A small amount of quenching was ob-
served for Mg2+, Ni2+, Hg2+, and Pb2+, thus indicating
weaker interactions between the framework and these cat-
ions. The Zn2+ ion shows negligible fluorescence enhance-
ment. Importantly, the emission intensity of the Ag+-incor-
porated framework was drastically reduced. This significant
quenching in fluorescence (ꢀ1/4 times) indicates stronger
binding between the framework and the Ag+ ion.
Entry
Aldehyde
Product
t [h]
Conversion [%][b]
1
6
90 (86)
[a] Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), eth-
anol (5 mL), catalyst (20 mg), RT, 6 h. [b] Determined by GC. Isolated
yields are given in parentheses.
whereas 75% conversion was obtained in the case of 20 mg
catalyst over the same reaction time. However, in both cases
the same amount of conversion of benzaldehyde was ach-
ieved after 6 h. It was found that decreasing the catalyst
amount to 15 mg led to a significant drop in the reaction
rate, with 73% conversion being achieved after 6 h. As ex-
pected, a lower reaction rate was observed for the reaction
using 10 mg catalyst, although the reaction afforded 54%
conversion after 6 h (Figure 3).
Small-molecule sensor activity: Owing to the presence of
open metal sites within the framework, the framework can
potentially be used as a small-molecule sensor material.
Upon excitation at approximately 300 nm, the evacuated
Figure 3. Effect of catalyst amount on Knovenagel reaction conversion of
benzaldehyde and malononitrile to benzilidinemalononitrile.
Cation sensor activity: Pyridyl groups and carboxylate
oxygen atoms are efficient in binding cations, and hence the
cation sensing activity of the framework was investigated.
The evacuated framework was immersed in solutions that
contained different metal ions, namely, Mg2+, Ni2+, Zn2+
,
Hg2+, and Pb2+, in DMF with a concentration of 0.1 mmol
each to form metal-ion-incorporated microcrystalline
a
solid. The framework shows selectivity toward the Ag+ ion
over the other tested cations. The selectivity and sensitivity
of the framework towards Ag+ have been proved by solid-
state UV/Vis and fluorescence measurements (Figure S18 in
the Supporting Information). Solid-state UV/Vis measure-
ments have showed that the framework exhibits two reflec-
tance bands at approximately 300 nm and approximately
635 nm, which are assigned to the p–p*/n–p* transition of
the ligand p system and the d–d transition of the framework,
respectively.
For the Ag+-incorporated framework, the lower-energy
band broadens drastically, whereas the higher-energy band
Figure 4. Fluorescence spectra of 2 with different cations.
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S. Kumar et al.
framework (2) shows emission maxima at approximately
420 nm. This evacuated framework was soaked in different
solvents, namely, MeOH, DMF, CH3CN, EtOH, and ace-
tone. The fluorescence studies indicate that in the case of
MeOH, large quenching occurs, whereas for others a small
amount of quenching was found (Figure 5); acetone quench-
ing is the lowest. It may be concluded that MeOH pene-
trates into the void space and strongly binds with the metal
ions due to its small size and large polarity, whereas nonpo-
lar acetone binds with metal centers to a lesser extent and
thus quenching becomes smaller. PXRD analyses also cor-
roborate these findings (Figure S19 in the Supporting Infor-
mation). Upon soaking in MeOH, large changes in the
PXRD pattern were noticed.
mode of bridging ligands. In 1, all pendant water molecules
from A layers and all pendant 4,4’-bipy from B layers are
oriented in the same direction, which develops a large polar-
ity within the framework (Figure S20 in the Supporting In-
formation). Compound 1 is also crystallized in the noncen-
trosymmetric space group Aba2. SHG measurements[16] re-
vealed that 1 displays an SHG efficiency 10.2 times that of
KDP, the second highest among reported coordination poly-
mers until now.[2b]
Conclusion
In conclusion, we have synthesized a new cocrystalline dy-
namic framework that contains both open metal sites
(OMSs) and free functional organic sites (FOSs) and we
have successfully established its multiple functionalities.
This is the first example of an inorganic cocrystal of 2D co-
SHG-NLO activity: Owing to their high thermal stability,
neutral noncentrosymmetric MOFs are considered potential
NLO-active materials. Apparently, the topological structures
of the resultant coordination polymers are mainly depen-
dent on the geometry of metal centers and the binding
ordination polymers. One layer consists of a [Cu
square grid with a pendant water molecule, and the other
layer consists of a similar [Cu(ina)2]n square grid with
ACHTUNGTRENNUNG(ina)2]n
AHCTUNGTRENNUNG
a pendant 4,4’-bipyridine. Hydrogen-bonding interactions
between these 2D layers assemble them in a single-crystal
structure, thereby leading to cocrystallization. Upon remov-
al of guest water molecules, 1 undergoes SC-to-SC transfor-
mation with concomitant interlayer void-space expansion;
the framework is highly flexible in nature.
The five-coordinated metal centers act as open metal sites
and thus the framework shows size-selective Lewis acidic
catalytic activity for the oxidation of styrene derivatives. It
even responds to catalytic oxidation reactions under aerobic
conditions. Owing to the presence of such OMSs, it shows
solvent sensor activity with selectivity and sensitivity toward
small and polar methanol. Consequently, the free pyridyl
groups that are present in the framework act as free func-
tional organic sites, and thus the framework shows Lewis
basic catalytic activity for the condensation reaction of ben-
zaldehyde with malononitrile, yet it also performs as
a cation sensor material with selectivity and sensitivity to-
wards Ag+. Furthermore, owing to its noncentrosymmetric
nature and high polarity on account of the orientation of
pendant 4,4’-bipyridyl and water from the nonpolar 2D
sheets in the same direction, the framework shows strong
SHG-NLO activity. Such structural versatility toward multi-
functionality that has been accomplished here is really un-
precedented and also creates an innovative route toward the
design and fabrication of new multifunctional metal–organic
materials.
Experimental Section
General: Copper (II) nitrate hexahydrate, isonicotinic acid, and 4,4’-bi-
pyridine were purchased from Merck chemical company. All other chem-
icals used were of analytical reagent grade. Elemental analysis (C, H, N)
was carried out using a Perkin–Elmer 240C elemental analyzer. The ther-
mal analysis was carried out using a Mettler Toledo TGA/SDTA 851
thermal analyzer under flow of N2 (30 mLminÀ1). The sample was heated
Figure 5. Fluorescence spectra of 2 with different small-solvent mole-
cules.
16612
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Chem. Eur. J. 2013, 19, 16607 – 16614

Dynamic Metal–Organic Frameworks
FULL PAPER
at a rate of 108CminÀ1 with inert alumina as a reference. IR spectra were
recorded using a Nicolet Impact 410 spectrometer between 400 and
4000 cmÀ1 by using the KBr pellet method. Photoluminescence spectra
were collected using a Shimadzu RF-5301PC spectrophotometer. The ad-
sorption isotherms of N2 at 77 K and CO2 at 195 K were measured using
a Quantachrome Quadrasorb-SI analyzer.
was taken out periodically to monitor the reaction. The products were
analyzed by GC and GC-MS. After the reaction, the catalyst was re-
moved by filtration, washed with acetone, and dried at room temperature
overnight. A similar procedure was followed for the reusability tests.
Catalytic Knoevenagel condensation reaction: The Knoevenagel reaction
between aromatic aldehyde and malononitrile using 2 (catalyst) was car-
ried out in a magnetically stirred round-bottomed flask. A mixture of 2
(20 mg) and benzaldehyde (1 mmol) was placed into a 25 mL flask that
contained ethanol (5 mL). The reactants were stirred for 5 min to dis-
perse 2 in the liquid phase. A solution of malononitrile (1 mmol) in etha-
nol (1 mL) was then added, and the resulting mixture was stirred at room
temperature for 6 h. Reaction conversion was monitored by withdrawing
aliquots from the reaction mixture at different time intervals, analyzing
them by GC, and further confirming the product identity by means of
GC-MS. The catalyst was separated from the reaction mixture by simple
filtration, washed with ethanol and dichloromethane, dried under
vacuum for 6 h, and reused when necessary.
Synthesis of complex 1 {[CuCAHTNUGNRETU(NNG ina)2ACHTUNGTRENU(NG H2O)][CuACHTUNGERTNN(GNU ina)2ACHTNUGTERN(UNGN bipy)]·2H2O}n: An
aqueous solution of sodium isonicotinate (0.190 g, 2 mmol) and 4,4’-bipyr-
idine (0.078 g, 0.5 mmol) in methanol (H2O/MeOH 2:3 v/v) was carefully
layered on an aqueous solution of copper(II) nitrate hexahydrate
(0.241 g, 1 mmol). Deep blue block-shaped single crystals suitable for
X-ray diffraction were grown at the junction of the two layers after one
week. Yield: 83%. IR: n˜ =3444 (br), 2926 (w), 1643 (s), 1556 (m), 1376
(s), 1218 (w), 774 (m), 698 cmÀ1 (s); elemental analysis calcd (%) for
C34H30N6O11Cu2: C 49.45, H 3.64, N 10.19; found: C 49.52, H 3.68, N
10.12.
Synthesis of complex 2 {[CuACHTNUGTENRNU(GN ina)2ACHTUGTNRENNUG(H2O)][CuACHTUNGERTG(NNNU ina)2ACHTNUGTREN(NNGU bipy)]}n: Complex 2
was synthesized by heating complex 1 at 1508C for 1 h. IR: n˜ =3444 (br),
2926 (w), 1643 (s), 1556 (m), 1376 (s), 1218 (w), 774 (m), 698 cmÀ1 (s); el-
emental analysis calcd (%) for C34H26N6O9Cu2: C 51.64, H 3.29, N 10.63;
found: C 51.68, H 3.32, N 10.60.
Acknowledgements
Crystallographic data collection and refinement: Single crystals of com-
plexes 1 and 2 were mounted on a Bruker SMART CCD diffractometer
equipped with a graphite monochromator and MoKa (l=0.71073 ꢁ) radi-
ation. The structures were solved by the Patterson method and followed
by successive Fourier and difference Fourier synthesis. Full-matrix least-
squares refinements were performed on F2 using SHELXL-97 with aniso-
tropic displacement parameters for all non-hydrogen atoms. The hydro-
gen atoms were refined isotropically, and their locations were determined
from a difference Fourier map. All calculations were carried out using
SHELXL 97,[17] SHELXS 97,[18] PLATON 99,[19] ORTEP-32,[20] and
WinGX system v1.64.[21]
R.S. is thankful to the CSIR for a research fellowship (09/096ACTHNUGRTNEUNG(0565)2008-
EMR-I). Thanks to Prof. P. K. Das and his student Mr. R. Pandey, IISc.,
Bangalore, India for SHG measurements. We are grateful to Dr. D. Das
of the UGC-DAE CSR Kolkata Center for Magnetic Studies. Thanks to
S. K. Ghosh, IISER Pune, Prof. S. Paul, Department of Life Science and
Bio-Tech., J. U., and Dr. K. Pramanik, Department of Chemistry, J. U.
India, for scientific suggestions. B.J. is also thankful to CSIR for a re-
search fellowship (09/936ACTHNUGTRNEUNG(0085)/2013 EMR-I).
Crystal data for 1: C34H30Cu2N6O11; orthorhombic, space group Aba2-cba;
a=22.3890, b=13.2170, c=11.8610 (5) ꢁ; V=3509.853 ꢁ3; Z=4; T=
293 K; R=0.0560, wR2 =0.1610, GoF=1.13.
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CCDC-917902 (1) and -917903 (2) contain the supplementary crystallo-
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Second-harmonic measurements: For SHG experiments, powder samples
were illuminated by a Q-switched Nd:YAG laser beam of fundamental
wavelength 1064 nm (Spectra Physics, PROLAB 170, pulse width 10 ns,
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Experimental details for catalysis—Materials and methods: Olefins, alde-
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Merck Chemical Company and used without further purification. Product
analysis was performed using a Varian 3400 gas chromatograph (GC)
equipped with a 30 m CP-SIL8CB capillary column and a flame ioniza-
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DSQ II GC-MS.
Catalytic oxidation reaction: Catalytic experiments were carried out in
thermostatted (508C) glass vessels under vigorous stirring. Typically, the
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(catalyst 20 mg). In the experiments with molecular oxygen, TBHP
(0.1 mmol) was added to a mixture that contained 2 (20 mg) and styrene
(5 mmol) and had been preliminarily blown with O2 (1 atm). A sample
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Received: May 31, 2013
Published online: October 23, 2013
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Chem. Eur. J. 2013, 19, 16607 – 16614