A new series of CuII coordination polymers derived from bis-pyridyl-bis-urea ligands and various dicarboxylates and their role in methanolysis of epoxide ring-opening catalysis

Article abstract of DOI:10.1021/cg3011037

A crystal engineering approach has been adopted in synthesizing six new mixed ligand based Cu^II coordination polymers (CPs) derived from two bis-pyridyl-bis-urea ligands, namely, N,N′-bis-(3-pyridyl)ethylene-bis- urea (L1) and N,N′-bis-(3-pyridyl)propylene-bis-urea (L2), and various dicarboxylates. The single crystal structures of the coordination polymers displayed diverse supramolecular architectures such as a one-dimensional (1D) chain, 1D-looped chain, and two-dimensional grid. Although none of them displayed an open-framework structure, which is believed to be conducive for heterogeneous catalysis, almost all of them showed moderate to excellent epoxide ring-opening catalysis. Powder X-ray diffraction indicated structural changes/degradation of the CPs, which might be generating substrate accessible Cu^II species that presumably acted as the catalyst.

Full text of DOI:10.1021/cg3011037

Article
pubs.acs.org/crystal
A New Series of CuII Coordination Polymers Derived from Bis-pyridyl-
bis-urea Ligands and Various Dicarboxylates and Their Role in
Methanolysis of Epoxide Ring-Opening Catalysis
Subhabrata Banerjee, Dhurjati Prasad Kumar, Sabyasachi Bandyopadhay,# N. N. Adarsh,§
and Parthasarathi Dastidar*
Department of Organic Chemistry, Indian Association for the Cultivation of Science (IACS), 2A & 2B Raja S C Mullick Road,
Jadavpur Kolkata −700032, West Bengal, India
S
* Supporting Information
ABSTRACT: A crystal engineering approach has been adopted in
synthesizing six new mixed ligand based CuII coordination polymers
(CPs) derived from two bis-pyridyl-bis-urea ligands, namely, N,N′-bis-(3-
pyridyl)ethylene-bis-urea (L1) and N,N′-bis-(3-pyridyl)propylene-bis-urea
(L2), and various dicarboxylates. The single crystal structures of the
coordination polymers displayed diverse supramolecular architectures such
as a one-dimensional (1D) chain, 1D-looped chain, and two-dimensional
grid. Although none of them displayed an open-framework structure, which
is believed to be conducive for heterogeneous catalysis, almost all of them
showed moderate to excellent epoxide ring-opening catalysis. Powder X-ray
diffraction indicated structural changes/degradation of the CPs, which might
be generating substrate accessible CuII species that presumably acted as the
catalyst.
Scheme 1
INTRODUCTION
■
Coordination polymers (CPs) are hybrid solids with infinite
network structures built from organic bridging ligands (linker)
and metal ions (node).1 In recent years, CPs received a
considerable attention owing to their various potential
applications.2a−d The last decade has witnessed an upsurge of
research activities involving the synthesis of intriguing CPs
derived from ligands equipped with hydrogen bonding
functionality (noninnocent backbone-NIB). The NIB backbone
of the ligands is shown to participate in complementary
hydrogen bonding interactions resulting in internetwork
hydrogen bonding.3 However, such occurrence of internetwork
hydrogen bonding is not so common as the NIB of ligands
might also particiapte in hydrogen bonding interactions with
guests (solvents) molecules and/or counteranions. Such CPs
displayed interesting structures and functions.4 To promote
internetwork hydrogen bonding, two strategies have been
adopted: (i) by increasing the number of hydrogen bonding
funtionality (e.g., from mono to bis-functionalized ligands), the
internetwork interactions could be achieved5 and (ii) by
employing bis-anionic coligands such as dicarboxyates in mixed
ligand systems, it was possible to avoid NIB−counteranion
interactions.6 Urea has a well studied hydrogen bonding
functionality that usually displays a 1D hydrogen bonded
network as shown in Scheme 1. To explore the role of urea in a
mixed ligand systems, we decided to exploit two bis-pyridyl-bis-
urea ligands, namely, N,N′-bis-(3-pyridyl)ethylene-bis-urea
(L1) and N,N′-bis-(3-pyridyl)propylene-bis-urea (L2), in
synthesizing neutral mixed ligand CPs wherein oxalate,
succinate, and 2,6-naphthalene dicarboxylate were used as
coligands (Scheme 2). Thus, a new series of CuII CPs, namely,
[{Cu(μ‐L1)(NO3)(μ‐oxalate)0.5}(H2O)]∞ (CP1)
[Cu(μ‐L1)(DMF)(oxalate)∞]∞ (CP2)
[{Cu(μL2)(H2O)(μ‐oxalate)0.5}NO3]∞ (CP3)
[{Cu(μ‐L1)(H2O)(μ‐succinate)}·H2O]∞ (CP4)
[{Cu(μ‐L2)(H2O)(μ‐succinate)}·3H2O]∞ (CP5)
Received: August 2, 2012
Revised: September 17, 2012
Published: September 26, 2012
© 2012 American Chemical Society
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[{Cu(μ‐L1)(μ‐2, 6‐naphthalene dicarboxylate)H2O}·2MeOH]∞ (CP6)
ravimetric (TG) data showed a weight loss of 12.4%, which did
not match with the calculated value of 6.4% (considering one
solvate water molecules and 0.8 water from SQUEEZE
calculation). This could be due to the fast desolvation of
loosely bound water molecules in the lattice. The ligand
adopted an overall V-shaped geometry presumably because of
the gauche conformation of its ethylene backbone. The relative
orientation of the pyridyl N and urea O atoms may be best
described as syn-syn. The pyridyl and urea plane in each
terminal were found to be reasonably planar with the
corresponding dihedral angles of 17.8 and 22.9°, whereas the
dihedral angles involving terminal pyridyl rings and urea
moieties were 79.2 and 85.9°, respectively. The metal center
CuII showed a distorted square pyramidal geometry wherein the
apical site was occupied by labile nitrate counteranion and the
equatorial sites were occupied by pyridyl N and oxalate O
atoms. The oxalate moiety was found be involved in extended
coordination bond via chelate mode wherein the O atoms came
from both the terminal carboxylates. The overall network may
best described as 1D looped chain topology wherein each loop
was completed by the pyridyl ligands and bridged by oxalate.
The metal−metal distance in the loop was 14.12 Å. The looped
chains were packed in parallel fashion sustained by various
hydrogen bonding interactions involving urea N, oxalate O, and
nitrate O and solvate water O atoms. While one of the urea N−
H moieties was involved in hydrogen bonding with the
carboxylate O [N···O = 2.919(5) Å, ∠N−H···O = 142.8(2)°]
and lattice occluded water [N···O = 3.010(5) Å, ∠N−H···O =
167.5(1)°], the other one made short contact with the O atom
of nitrate counteranion [N···O = 2.970(5) Å, ∠N−H···O =
133.7 (1)°]; one of the urea >CO moieties interacted with
the solvate water molecule via O−H···O interactions [O···O =
2.7996 Å ], while the other one remained free from any
hydrogen bonding interactions (Figure 1).
Scheme 2
have been synthesized and structurally characterized mainly by
single crystal X-ray diffraction (SXRD). It may be noted that
while L1 has been utilized to generate various CPs useful in
anion chemistry,7a−c L2 remains unexplored in the coordina-
tion polymer literature; only the structure of L2 is reported.7a
No mixed ligand based CPs have been reported involving these
two ligands as per CSD search. Interestingly, these CPs
displayed catalytic behavior in ring-opening methanolyses of
epoxides.
Crystal Structure of [Cu(μ-L1)(DMF)2(oxalate)]∞ (CP2).
Interestingly when we changed one of the crystallization
solvents (DMF instead of MeOH) keeping the rest of the
reaction conditions identical as in the case of CP1, crystals of
CP2 were formed. The space group of CP2 was found to be the
centrosymmetric triclinic P1. The asymmetric unit contained
̅
one molecule of L1, one oxalate coordinated to the metal
center in a chelate fashion, and two DMF molecules
coordinated to the metal center. The final electron density
map in SXRD data was clean indicating no solvent occlusion.
TG data also supported the SXRD data; a weight loss of 12.6%
attributed to the two coordinated DMF molecules exactly
matched with the calculated value of 12.4%. The ligand
displayed a V-shaped conformation presumably due to the
gauche conformation of the ethylene backbone. It also adopted
a syn-anti conformation with respect to the relative orientation
of the pyridyl N and urea O atoms. The terminal pyridyl
moieties were found to be relatively planar with corresponding
urea moieties; the dihedral angles involving these moieties were
14.3 and 22.6°. The dihedral angle involving the terminal
pyridyl rings and urea moieties were 68.78 and 64.45°. The
metal center geometry may be best described as slightly
distorted octahedral wherein the apical positions were
coordinated by the O atoms of the DMF molecules and the
equatorial positions were occupied by the N atoms of pyridyl
rings and O atoms of oxalate. The overall extended
coordination via pyridyl atoms resulted in the formation of a
1D wavy CP. The neighboring 1D chains further packed in a
parallel fashion sustained by hydrogen bonding involving urea
RESULTS AND DISCUSSION
■
We reacted separately L1 and L2 with various dicarboxylates
and Cu(NO3)2 in a 1:0.33:1 (ligand/carboxylate/metal) molar
ratio. It was noted that CP synthesis trials with a 1:1:1 (ligand/
carboxylate/metal) molar ratio lead to the formation of CPs
having only carboxylate ligands in most of the cases. However,
reducing the amount of the carboxylates as stated above, we
could isolate the corresponding mixed ligand CPs CP1−CP6.
Suitably grown single crystals of the CPs were then subjected to
SXRD experiments (see Table 1).
Crystal Structure of [{Cu(μ-L1)(NO3)(μ-oxalate)0.5}-
(H2O)]∞ (CP1). Reaction of L1 and dipotassium oxalate with
Cu(NO3)2 in aqueous MeOH/EtOH mixture resulted in X-ray
quality crystals (green, block shaped) of CP1. SXRD data
revealed that the crystal belonged to the centrosymmetric
triclinic space group P1. The asymmetric unit contained one
̅
ligand, half oxalate molecule residing on a center of symmetry,
one nitrate counteranion coordinated to the metal center, one
solvate water molecule, and some smeared electron densities
that could not be modeled. SQUEEZE8 calculations revealed
the presence of 16 e in the unit cell, which was attributed to 0.8
water molecule in the asymmetric unit. Subsequent thermog-
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Figure 3. Parallel packing of 1-D looped chains in CP3 displaying
various hydrogen bonding (···) interactions.
Figure 1. Parallel packing of 1D looped chains in CP1 displaying
various hydrogen bonding (···) interactions.
unit contained half a molecule of L1, half coligand succinate
(both residing on center of symmetry), one water molecule
coordinated to a half occupied CuII, and a solvate water
molecule. The final electron density map was clean. A weight
loss of 10.5% in TG data clearly supported the loss of three
water molecules (calc. 9.9%) as seen in the crystal structure.
The ligand adopted an overall linear topology due to the
staggered conformation of the ethylene backbone. The relative
orientation of the pyridyl N and urea O atoms in each terminal
was found to be syn−syn. However, anti orientation of the
terminal pyridyl N atoms made the ligand have a linear ligating
topology. Each pyridyl urea segment appears to be quite planar;
the dihedral angles between the pyridyl ring and urea moiety
were 5.9 and 6.6° and the terminal pyridyl rings and urea
moieties made dihedral angles of 5.91°and 0.51°, respectively,
among themselves. The metal center displayed a slightly
octahedral geometry wherein the equatorial positions were
occupied by pyridyl N and succinate O atoms and the apical
positions were coordinated by water molecules. Because of
extended coordination of the pyridyl ligand and succinate
coligand, the crystal structure displayed a 2D grid architecture.
However, there was no space available within the grid for guest
occlusion. The 2D grids were packed in parallel fashion. The
solvate water molecules were embedded within the interstitial
space between the 2D layers sustained by extensive hydrogen
bonding involving urea O [N···O = 2.991(4)Å, ∠N−H···O =
145.1 (1)°], succinate O [O···O = 2.7329 Å ] and metal bound
water molecule [O···O = 2.7329 Å] (Figure 4).
N and oxalate O atoms [ N···O = 2.834−3.095 Å, ∠N−H···O =
136.9 (2)−146.1 (2)°] (Figure 2).
Figure 2. Parallel packing of the 1D chain in CP2 displaying hydrogen
bonding (···) involving urea and oxalate.
Crystal Structure of [{Cu(μ-L2)(H2O)(μ-oxalate)0.5}-
NO3]∞ (CP3). When L2 was used instead of L1 in an identical
condition of synthesizing CP1, crystals of CP3 were obtained.
SXRD revealed that the crystals belonged to the centrosym-
metric triclinic space group P1. In the asymmetric unit, one
̅
ligand, half a molecule of oxalate (residing on a center of
symmetry), one water molecule coordinated to the metal center
and one free nitrate counteranion were located. The final
electron density map was found be clean indicating absence of
lattice included solvents. A weight loss of 3.5% attributed to
one coordinated water molecule (calc. 3.6%) in TG data
supported the crystal structure. The ligand adopts a V-shaped
geometry presumably due to the gauche and staggered
conformation of the propylene backbone. It also displayed
anti-syn conformation with respect to the relative orientation of
the pyridyl N and urea O atoms. In this case also, the terminal
pyridyl rings and urea moieties were found to be planar
displaying dihedral angles of 15.3 and 9.1°. The terminal
pyridyl rings and urea moieties display dihedral angles of 62.8
and 61.3°, respectively. The metal center adopted a distorted
square pyramidal geometry wherein the apical position was
occupied by a water molecule and the equatorial sites were
coordinated by pyridyl N and oxalate O atoms. Extended
coordination by the pyridyl N and oxalate O atoms resulted in a
1D looped chain topology. The metal−metal distance was
16.87 Å, slightly longer than that of in CP1. The chains were
packed in parallel fashion sustained by hydrogen bonding
involving urea O atom and metal bound water [O···O
2.761(4)−3.064(4) Å, ∠O−H···O = 175.0(4)−177.0 (4)°],
and the urea N atom and nitrate O atoms [N···O = 2.977(3)−
3.170(3) Å, ∠N−H···O = 144.2 (2)−170.3 (2)°] (Figure 3).
Crystal Structure of [{Cu(μ-L1)(H2O)2(μ-succinate)}·-
H2O]∞ (CP4). When the molecular length of the coligand
increased as compared to oxalate such as in succinate, reaction
of L1 under identical conditions for the synthesis of CP1
resulted in block-shaped blue crystals of CP4. SXRD indicated
Crystal Structure of [{Cu(μ-L2)(H2O)(μ-succinate)}·3-
H2O]∞ (CP5). When L2 was reacted with Cu(NO3)2 under
identical conditions of CP4 synthesis, block shaped blue
crystals of CP5 were obtained. SXRD indicated that they
Figure 4. (a) 2D grid architecture displaying one grid in space-filling
model, (b) parallel packing of the grids (shown in alternating orange
and purple color) displaying the solvate water molecules (red ball)
occupying the interstitial space sustained by various hydrogen bonding
(···), (c) TOPOS9 diagram of two offsetly packed 2D sheets in CP4.
the centrosymmetric triclinic space group P1. The asymmetric
̅
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belonged to the noncentric orthorhombic space group Pca21.
The asymmetric unit contains one CuII metal center, one L2,
one succinate, one metal bound water, and three solvate water
molecules. The final refinement cycle did not show any
significant electron density. TG data also supported the crystal
structure [weight loss for four water molecules = 12.7%
(expected), 12.8% (found)]. The propylene backbone dis-
played gauche and staggered conformation resulting in an overall
V-shape in the molecule. The relative orientation of the pyridyl
N and urea O atoms in each terminal was found to be syn-syn.
However, anti orientation of the terminal pyridyl N atoms gave
the molecule a linear ligating topology. Each pyridyl urea
moiety displayed reasonably planar conformation; the dihedral
angles between the pyridyl ring and urea moiety in each
terminal were 17.5 and 14.3°. The dihedral angles between the
urea moieties and pyridyl rings were 48.9 and 70.2°. The metal
center displayed a slightly distorted square pyramidal geometry
wherein the equatorial positions were occupied by succinate O
and pyridyl N atoms and the apical position was coordinated by
a water molecule. Because of the extended coordination
involving both the pyridyl ligand and succinate coligand, the
crystal structure displayed a grid architecture which had a highly
undulating topology. In this case also, there was no space
available within the grid. The solvate water molecules were
located within the interstitial space available between the 2D
grids which were parallelly packed. Extensive hydrogen bonding
interactions involving urea N [N···O = 2.897(3)−3.081(3) Å,
∠N−H···O = 149.0 (4)−176.8(2)°] and O [O···O =
2.847(3)−2.790(3) Å, ∠O−H···O = 167(3)°] and succinate
O [O···O = 2.695(2)−2.825(3) Å, ∠O−H···O = 164(3)−
173(3)°] with the solvate water molecule were observed
(Figure 5).
experiments (SXRD and TG). The ligand adopted a staggered
conformation in its ethylene backbone. The relative orientation
of the pyridyl N and urea O atoms in each terminal was found
to be syn-syn. However, the ligand molecule displayed a linear
ligating topology because of the relative anti orientation of the
pyridyl N atoms. Each pyridyl urea moiety displayed reasonably
planar conformation; the dihedral angles between the pyridyl
ring and urea moiety in each terminal were 0° and 0°. The
dihedral angles between the urea moieties and pyridyl rings
were 4.1 and 4.1°. The metal center displayed a distorted
octahedral geometry wherein the equatorial positions were
occupied by O atoms of 2,6-naphthalene dicarboxylate and
water molecules and the apical positions were coordinated by
pyridyl N atoms. Extended coordination involving both the
pyridyl ligand and 2,6-naphthalene dicarboxylate resulted in 2D
grid architecture. Interestingly, in this case there was a space of
∼3.9 × 7.9 Å available for guest occlusion. The disordered
electron densities were located within this space. The grids
were packed in offset fashion sustained by hydrogen bonding
involving urea N and 2,6-naphthalene dicarboxylate O [N···O =
2.947(4)−3.149(4) Å, ∠N−H···O = 151.1(1)−159.8 (1)°]
atoms and C−H···π interactions involving naphthyl ring and
pyridyl C−H (Figure 6).
Figure 6. (a) 2D grid architecture displaying one grid in space-filling
model, (b) offset packing of the grids displaying hydrogen bonding
and C−H···π interactions in CP6, (c) TOPOS9 diagram of the offset
packed grids.
It is clear from the above crystallographic description that
most of these crystals (CP1 to CP4 and CP6) crystallized in
triclinic P1 space group, whereas CP5 crystallized in
̅
Figure 5. (a) 2D grid architecture displaying one grid in space-filling
model, (b) parallel packing of the grids displaying lattice occluded
water molecules (red balls) within the interstitial space, (c) TOPOS9
view of the highly undulating grids.
orthorhombic Pca21 space group. Basically two kinds of
architecture were evident after analyzing all the structures viz.
CP1 and CP3 showed one-dimensional looped chain topology
whereas CP4, CP5, and CP6 displayed 2D grid architecture;
CP2 showed a one-dimensional wavy chain structure. These
structural diversities among the CPs arose because of the
flexible nature of the ligands L1 and L2 that displayed two
different coordination modes (angular and linear). In addition
to the ligand flexibility, varied coordination modes of oxalate
(bridging and chelating) were also responsible for these
structural diversities. It is interesting to note that internetwork
hydrogen bonding involving urea synthon is not observed in
any of these structures.
Crystal Structure of [{Cu(μ-L1)(μ-naphthalene
dicarboxylate)H2O}·2MeOH]∞ (CP6). Reaction of L1 with
2,6-naphthalene dicarboxylate under identical conditions for
CP1 synthesis resulted in block shaped bluish-green crystals of
CP6. The space group was found to be the centrosymmetric
triclinic P1. The asymmetric unit contained half occupied CuII,
̅
half L1, half 2,6-naphthalene dicarboxylate, one metal bound
water, and smeared electron densities that could not be
modeled. The electrons (26 e/asymmetric unit) found in the
SQUEEZE8 calculations were assigned to two solvate MeOH.
However, TG data were found to be inconsistent with the
calculated results (calc. 12.6%; experimental 21.8%). This could
be because of the difference in solvent loss in two different
Catalysis. CPs acting as catalysts in ring-opening alcoholysis
of epoxides have been reported by various groups.2a,10 To the
best of our knowledge, none of the reports dealing with
alcoholysis of epoxides involves CP as catalyst wherein the
ligand backbone is equipped with hydrogen bonding
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functionality (NIB−noninnocent backbone, see Introduction).
Since epoxide can participate in hydrogen bonding interactions
with urea functionality as depicted in Scheme 3, we thought it
catalytic efficiency particularly after a third run for CP4 and
CP5.
An attempt to correlate the catalytic performances of the CPs
based on their crystal structures revealed interesting observa-
tions. Since all the CPs displayed nonporous architecture, the
catalytic metal center is not expected to be exposed to the
upcoming substrate epoxides. Therefore, in order for the CPs
to act as catalyst, some kind of changes in the crystalline phases
wherein the metal center is somewhat exposed must be
happening during the reactions. In fact, a close look at the
PXRD patterns of the CPs (simulated, bulk, activated, and
reacted) in all the cases displayed considerable differences
meaning that the crystal structures of the CPs underwent some
change/collapse/degradation; in the cases of CP1, CP3, and
CP4, although quite a few peaks do match with that of in the
simulated pattern, there still exist considerable differences
indicating some kind of change in crystalline phase/degradation
in these cases (Figure 7). Thus, these data indicate that a
substrate accessible CuII species might be generating during
reaction, which acted as catalyst.
Scheme 3
To probe it further, we have carried out EPR experiments on
a selected CP, namely, CP5. We recorded EPR spectra under
three different conditions: (a) CP5 as synthesized (before using
it as catalyst), (b) CP5 isolated after carrying out methanolysis
of styrene oxide, and (c) the mother liquor of the reaction
mixture devoid of any solid catalyst. The EPR spectra obtained
in conditions (a) and (b) are identical displaying rhombic
symmetry having g1 = 2.03819, g2 = 2.06912, and g3 = 2.2848
indicating the existence of CuII species with its unpaired
electron lying in dx −y orbital with s = 1/2 meaning that the
CuII center is square pyramidal as also seen in the single crystal
structure of CP5. Thus, the coordination of the metal center
does not seem to change after reaction. EPR spectrum in
condition (c) does not give any characteristic EPR signal
meaning that CP5 is not leaching into the reaction mixture.
Therefore, these results indicate that the metal center is getting
exposed to the substrate due to some dynamic structural
change during the reaction  the exact nature of which could
not be established (see Supporting Information).
might facilitate the reaction by bringing the substrates
(epoxides) in close proximity of the catalytic CuII metal center
in the CPs studied herein. Therefore, we performed
methanolysis of various epoxides using the CPs reported
herein. Except CP2, all the CPs displayed catalytic activity in
such reactions (Table 2).
2
2
Thus, it is evident from Table 2 that all the CPs acted as
excellent catalyst for styrene oxide methanolysis. They also
catalyzed the methanolysis of cyclohexene oxide quite
efficiently. In the case of trans-stilbene oxide, the major product
was the methanolysis product, whereas the alpha diketone
product (benzil) was the minor one as evident from GC, mass
1
and H NMR and 13C NMR (DEPT) analyses. On the other
CONCLUSIONS
hand, when cis-stilbene was used as substrate, the GC-MS
showed only one peak for product (benzil). Further analyses
(mass and 1H NMR and 13C NMR (DEPT)) revealed that the
methanolysis product was also present which could not be
detected in the GC trace. It may be mentioned that the alpha
diketone product is not commonly observed in methanolysis of
stilbene oxides.11 Both cyclopentene and cyclooctene oxides
were found to be unreactive under these catalytic conditions.
As-synthesized CPs did not show any catalytic activity.
However, when activated, all of them except CP2 did display
reasonable catalytic properties. Typically, the as-synthesized CP
was subjected to heating under a high vacuum at ∼140 °C for
about 2 h. In most of the cases, TG data revealed that the lattice
included and in some cases, the metal bound solvents except in
CP2, CP3 escaped the crystal lattice. The resultant activated
catalyst was then placed in a reaction vessel containing the
epoxide substrate and MeOH and the mixture was stirred at
room temperature. The reaction progress was monitored by
GC analyses. Attempt to recycle the catalyst after isolation from
the reaction mixture followed by activation was unsuccessful.
But if the reaction was performed without isolating the catalyst
from the reaction mixture, the reaction (recyclability) was
running smoothly, however, with a gradual decrease in the
■
On the basis of crystal engineering rationale, we have
synthesized mixed ligand based CuII coordination polymers
CP1−CP6 derived from bis-pyridyl-bis-urea ligands and
various carboxylates. All of them were structurally characterized
by single crystal X-ray diffraction. Urea synthon could not be
observed in any of these structures. Except CP2, all the CPs
displayed moderate to excellent epoxide ring-opening meth-
anolysis with various substrates. Styrene oxide turned out to be
the best substrate resulting in 100% conversion within 12 h.
Since single crystal structures of the these CPs did not display
any open channel conducive for ring-opening catalysis and
PXRD data under various conditions indicated structural
changes/degradation, it is logical to presume that a substrate
accessible CuII species might be generated during reactions,
which acted as catalyst.
EXPERIMENTAL SECTION
■
Materials and Methods. All chemicals were commercially
available and used without further purification. Ligands L1 and L2
were synthesized were previously reported.7 The elemental analysis
was carried out using a Perkin-Elmer 2400 Series-II CHN analyzer.
FT-IR spectra were recorded using Perkin-Elmer Spectrum GX, and
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Table 2. Methanolysis of Various Epoxides Catalyzed by CP1, CP3−CP6
TGA analyses were performed on a SDT Q Series 600 Universal
VA.2E TA Instruments. Powder X-ray patterns were recorded on a
Bruker AXS D8 Advance Powder (Cu Kα1 radiation, λ = 1.5406 Å)
diffractometer. GC was recorded on a PERICHROM PR:2100. GCMS
was recorded on GCMS-shimazdu-QP 5050A.The mass spectrum was
recorded on QTOF Micro YA263. NMR spectra were recorded using
a 300 MHz Bruker Avance DPX300 spectrometer EPR spectra was
recorded on JES-FA SERIES.
water and later layering a methanolic solution of Cu(NO3)2 (24 mg,
0.1 mmol). The resultant trilayer solution, thus obtained, was kept
undisturbed. After 1 week, plate shaped green crystals appeared.
Elemental analysis calcd for C15 H20 Cu N7 O9(%): C,35.47; H,
4.37; N, 19.30; found: C 36.25, H 3.70, N 19.20. FT-IR (KBr pellet)
3394m, 3306(m, water ν O−H), 2960w, 2798w, 1699s (urea CO
stretch), 1645s (s, urea CO stretch), 1589, 1556 (s,urea N−H
bending), 1506, 1483, 1429, 1384, 1330, 1924, 1240 (NO3-
asymmetric stretch), 1134, 1105, 1068, 937, 906, 804, 694, 653, 615,
495.
CP1[{Cu(μ-L1)(NO3)(μ-oxalate)0.5}(H2O)]∞ was synthesized by
layering an ethanolic solution of L1 (30 mg, 0.1 mmol) over an
aqueous solution of diammonium oxalate (4.7 mg, 0.033 mmol) in
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experimental data was inconsistent, FT-IR (KBr pellet): 3360 w,
3315w, 3080 (w, aliphatic C−H stretch), 1695m, 1683m (s, urea C
O stretch), 1670 (s, urea CO stretch), 1585w, 1541s, 1483s, 1425m,
1384s, 1327s, 1300m, 1273s, 1226m, 804 m, 694m cm−1.
CP5[{Cu(μ-L2)(H2O)(μ-succinate)}·3H2O]∞ (CP5) was synthe-
sized by layering an ethanolic solution of L2 (31 mg, 0.1 mmol) over
an aqueous solution of dipotassium succinate (6.5 mg, 0.033 mmol) in
water and later layering a methanolic solution of Cu(NO3)2 (24 mg,
0.1 mmol). The resultant trilayer solution, thus obtained, was kept
undisturbed. After 10 days, neddle shaped blue crystals appeared,
Elemental analysis calculated for C19 H30 Cu N6 O10 (%): C 40.32,
H 5.32, N 14.85; found: C, 41.17; H, 4.73; N, 15.59; FT-IR (KBr
pellet): 3365m 3298m, 3263 (s, aromatic C−H stretch), 3134w,
3080w, 2966w, 2947, 1693s (s, urea CO stretch), 1683 (s, urea C
O stretch), 1583 (s, urea N−H bend), 1566s, 1487s, 1427s, 1404s,
1327s, 1298s, 1271w, 1234s, 1193 m, 1234s, 1109m, 1060 w, 879w,
806, 696s, 650s, 680w, 609w cm−1.
CP6 [{Cu(μ-L1)(μ-2,6-naphthalene-dicarboxylate)-
H2O}·2MeOH]∞ was synthesized by layering an ethanolic solution
of L1 (30 mg, 0.1 mmol) over an aqueous solution of 2,6-dipotassium
naphthalene dicarboxylate (7 mg, 0.033 mmol) in water and later
layering a methanolic solution of Cu(NO3)2 (24 mg, 0.1 mmol). The
resultant trilayer solution, thus obtained, was kept undisturbed. After
1−2 weeks, neddle shaped bluish green crystals appeared. C,H,N
experimental data was inconsistent FT-IR (KBr pellet): 3504m, 3389,
3356m, 3082m, 1672s (urea CO stretch), 1591m (s, urea N−H
bend), 1552s, 1514w, 1481s, 1431s, 1384s, 1352s, 1300m, 1265m,
1244m, 1139w, 1116, 1062, 1028, 999w, 918m, 879w, 833w, 821m,
806m, 696s, 655w cm−1.
Catalytic Reaction Conditions. Reaction conditions: All the
reactions were carried out at room temperature under vigorous
magnetic stirring. Catalyst/substrate (epoxide) ratio was maintained at
1:10. In a typical reaction 6.5 × 10−3 mmol of the catalyst and 6.5 ×
10−2 mmol of the epoxide was taken in 5 mL of methanol and stirred
at room temperature in a stoppered container.
X-ray Crystallography: X-ray single crystal data were collected using
MoKα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer
equipped with CCD area detector. Data collection, data reduction,
structure solution/refinement were carried out using the software
package of APEX II. The structures of CP1−CP6 were solved by
direct method, respectively, and refined in a routine manner. In all
cases, non-hydrogen atoms were treated anisotropically. Whenever
possible, the hydrogen atoms were located on a difference Fourier map
and refined. In other cases, the hydrogen atoms were geometrically
fixed. CCDC Nos. 885293−885298 contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB21EZ, UK; fax: (+44) 1223−336−033; or deposit@ccdc.cam.ac.
uk).
Figure 7. PXRD patterns of CP1, CP3−CP6 under various
conditions, e.g., simulated (violet), bulk (black), activated (olive),
reacted (red).
CP2 [Cu(μ-L1)(DMF)2(oxalate)]∞ was synthesized by layering an
DMF solution of L1 (30 mg, 0.1 mmol) over an aqueous solution of
diammonium oxalate (4.7 mg, 0.033 mmol) in water and later layering
a ethanolic solution of Cu(NO3)2 (24 mg, 0.1 mmol). The resultant
trilayer solution, thus obtained, was kept undisturbed. After 10 days,
block shaped green crystals appeared. Elemental analysis calcd C22
H30 Cu N8 O8%: C,44.18; H, 5.06; N, 18.74; found: inconsistent
experimental data FT-I.R (KBr pellet): 3389s, 3279s,1707m (urea C
O stretch), 1649s (urea CO stretch), 1552s (s,urea N−H bending),
1483s, 1427s, 1384w, 1330w, 1300s, 1232s, 1222, 1130, 1111 cm−1.
CP3[{Cu(μ-L2)(H2O)(μ-oxalate)0.5}NO3]∞ was synthesized by
layering an ethanolic solution of L2 (31 mg, 0.1 mmol) over an
aqueous solution of diammonium oxalate (4.7 mg, 0.033 mmol) in
water and later layering a methanolic solution of Cu(NO3)2 (24 mg,
0.1 mmol). The resultant trilayer solution, thus obtained, was kept
undisturbed. After 10 days, block-shaped green crystals appeared.
Elemental analysis calcd C16 H20 Cu N7 O8 (%): C, 38.21; H, 4.21;
N,19.50; found: C, 38.68; H, 3.58;N,19.51; FT-IR (KBr pellet):
3396m, 3333 (m, water ν O−H), 3113w, 1710s (CO stretch), 1683
(s, N−H bending), 1651s, 1612w, 1587w, 1556s, 1539s, 1494m,
1479m 1423(NO3-asymmetric sretch), 1373w, 1350w, 1319w, 1274w,
1228m, 1215m, 1109, 1064, 1028w, 995w, 809s, 698s, 655m, 640m,
486 m, 457w, 420w cm−1.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detail experimental conditions, SXRD details and TG curves,
PXRD patterns. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: parthod123@rediffmail.com, ocpd@iacs.res.in.
Present Addresses
#Department of Inorganic Chemistry, Indian Association for
the Cultivation of Science (IACS), 2A and 2B Raja S C Mullick
Road, Jadavpur Kolkata −700032, West Bengal, India.
§Institute of Condensed Matter and Nanosciences, Universite
Catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-la-
Neuve, Belgium.
CP4[{Cu(μ-L1)(H2O)2(μ-succinate)}·H2O]∞ was synthesized by
layering an ethanolic solution of L1 (30 mg, 0.1 mmol) over an
aqueous solution of dipotassium succinate (6.5 mg, 0.033 mmol) in
water and later layering a methanolic solution of Cu(NO3)2 (24 mg,
0.1 mmol). The resultant trilayer solution, thus obtained, was kept
undisturbed. After 6 days, block shaped blue crystals appeared, calcd.
C18 H20 Cu N6 O10 (%): C, 39.17; H, 5.11; N,15.11; found:
́
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Department of Science & Technology (DST), New
Delhi, India for financial support. S.B and N.N.A. thank IACS
for research fellowships. D.P.K. thanks DST for his fellowship.
Single crystal X-ray diffraction was performed at the DST-
funded National Single Crystal Diffractometer Facility at the
Department of Inorganic Chemistry, IACS.
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