Crystal Engineering in Supramolecular Polyoxometalate Hybrids through pH Controlled in Situ Ligand Hydrolysis

Article abstract of DOI:10.1021/acs.inorgchem.7b02385

A family of five different three-dimensional polyoxometalate (POM) based supramolecular hybrids were synthesized by a hydrothermal route under different pH using a hydrolyzable naphthalene diimide ligand. The mechanism of crystallographic phase variation of the POM-amino pyridine hybrids under different pH was studied through controlled experiments where the final hydrolyzed products were analyzed through NMR and single crystal X-ray diffraction. Different pH conditions led to variation in the extent of protonation and hydrolyzation of the ligand, yielding different phases. All of these were identified, and the structures of the supramolecular hybrids were characterized extensively. Mechanistic study proved that only the reaction conditions are responsible for the hydrolysis of the ligand and the in situ generated POM species do not have any role in it. Magnetic measurements confirmed the hexavalent oxidation states of the transition metal center (Mo) in the POM. Optical band gap measurements revealed that these hybrids are semiconducting in nature. Two of the compounds were studied for hydrogen peroxide mediated selective oxidation catalysis of small organic molecules and found to exhibit very good activity with high percentage of selectivity for the desired products of industrial importance.

Full text of DOI:10.1021/acs.inorgchem.7b02385

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Crystal Engineering in Supramolecular Polyoxometalate Hybrids
through pH Controlled in Situ Ligand Hydrolysis
Soumyabrata Roy, Dundappa Mumbaraddi, Ankit Jain, Subi J. George, and Sebastian C. Peter*
New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, India
*
S Supporting Information
ABSTRACT: A family of five different three-dimensional
polyoxometalate (POM) based supramolecular hybrids were
synthesized by a hydrothermal route under different pH using a
hydrolyzable naphthalene diimide ligand. The mechanism of
crystallographic phase variation of the POM-amino pyridine
hybrids under different pH was studied through controlled
experiments where the final hydrolyzed products were analyzed
through NMR and single crystal X-ray diffraction. Different pH
conditions led to variation in the extent of protonation and
hydrolyzation of the ligand, yielding different phases. All of these
were identified, and the structures of the supramolecular hybrids
were characterized extensively. Mechanistic study proved that
only the reaction conditions are responsible for the hydrolysis of
the ligand and the in situ generated POM species do not have any role in it. Magnetic measurements confirmed the hexavalent
oxidation states of the transition metal center (Mo) in the POM. Optical band gap measurements revealed that these hybrids are
semiconducting in nature. Two of the compounds were studied for hydrogen peroxide mediated selective oxidation catalysis of
small organic molecules and found to exhibit very good activity with high percentage of selectivity for the desired products of
industrial importance.
1
. INTRODUCTION
POMs to form mixed valent systems with enhanced electronic
properties. In the absence of secondary TMs, the ligands tend to
form supramolecular hybrids having direct, noncovalent
interactions with their inorganic counterparts. The electron
donating capacity of the nitrogen centers can be tuned by
Polyoxometalates (POMs), a class of oxy-anion clusters of early
transition metals (TM = Cr, V, Mo, W, Nb) in the highest
oxidation states, are well-known for their extensive applications
1
−6
in fields ranging from materials to biological sciences.
The
+
variation of H ion concentration in the system, which gives a fine
properties of POMs, originating from the rich redox chemistry of
transition metals, anionic nature of oxygen centers, and
hydrolyzable protons, have found immense applications in
catalysis (chemical, photochemical, electrochemical), single
molecule magnets, molecular electronics, optical properties,
control over the dimensionality, structural interactions, and
crystal phase engineering of the system. However, due to lack of
control over protonation of the ligands, mixed phases and
isomorphs are common problems in such supramolecular
assemblies. An innovative way to solve the issue is to incorporate
the ligand into a hydrolyzable precursor form, which may give
controlled release and protonation of the coordinating ligand. A
simple reaction condition like pH can be used to engineer the
crystallographic phase of such systems, the mechanistic
investigation of which may give valuable insights into the
dynamic role of the reaction conditions.
5
−12
medicinal chemistry, biology, and bioinorganic synthesis.
The sensitivity of POMs-hybrid structural hierarchy toward
reaction conditions like pH, temperature, and reaction time is
often used as an important tool to modulate the structure and
properties of this class of compounds. The structure and
electrochemical properties of the POMs also exhibit a strong
dependence on pH.
and hence basicity of the organic ligands, which eventually
determines the dimensional complexity and nature of
13−15
pH governs the extent of protonation
Here, we report five supramolecular 3D hybrid compounds (1,
4
−
2
, 3, 4, and 5) comprising Keggin POM (SiMo O ) and 4-
12 40
16−19
amino pyridine (4-ap) that have been synthesized under
hydrothermal reaction conditions by in situ POM formation
and hydrolysis of a pyridyl naphthalene diimide (NDI-py) ligand.
pH triggered NDI-py hydrolysis yielded a controlled amount of
the reacting ligand, 4-ap, under the reaction conditions. We have
interactions in these hybrids.
The dimensionality of POM
based hybrids can be increased by coordinating properties of the
organic ligands involved and incorporation of secondary
transition metals, which enhances the catalytic, optical, and
electronic properties of the POMs. The nitrogen containing
ligands with donor properties often act as directing agents in the
20
hybrids and determine the dimensionality of the structures.
These nitrogen donor centers can also reduce Mo or TM of the
Received: September 16, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02385
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Inorganic Chemistry
Article
Table 1. Crystal Data and Structure Refinement Parameters for 1 [H (SiMo O ) ][(C H N ) ]·7H O, 2
8
12 40 2
5
6
2 10
2
[
[
H (SiMo O )][(C H N ) ], 3 [H (Mo O Si) ][(C H N ) ]·7(H O), 4 [H (Mo O Si) ][(C H N ) ]·6(H O), and 5
4
12 40
5
6
2 6
8
12 40
2
5
6
2 9
2
8
12 40
2
5
6
2 7
2
a
H (Mo O Si) ][(C H N ) ]·7(H O)
8
12 40
2
5
6
2 7
2
1
2
3
4
5
empirical formula
formula weight
temperature
C
30
H
36Mo12
N
12
O
40Si
C
50
H
67Mo24
N
20
O
87Si
2
C
45
H
58Mo24
N
18
O
87 Si
2
C
35
H
54Mo24
N
14
O
86Si
2
C
35
H
14 2
50Mo24N O87.61Si
2384.08
4663.69
4601.83
4405.66
100.0 K
0.71073 Å
triclinic
4427.32
293 K
298 K
296.45 K
0.71073 Å
monoclinic
C2/c, 4
100.0 K
wavelength
0.71073 Å
rhombohedral
0.71073 Å
monoclinic
0.71073 Å
triclinic
crystal system
space group, Z
R3
̅
P2
1
/n, 2
P1
̅
̅
P1, 1
unit cell dimensions a = 21.8664(3) Å
b = 21.8664(3) Å
a = 42.1438(19) Å
b = 12.2492(5) Å
c = 25.2517(10) Å
β = 115.802(3)°
a = 21.2231(13) Å
b = 12.3452(7) Å
c = 22.9950(14) Å
β = 113.160(2)°
a = 10.4802(7) Å
b = 11.7748(8) Å
c = 21.0804(15) Å
α = 75.265(4)°
β = 77.340(3)°
γ = 86.720(4)°
2454.6(3) Å3
a = 11.3922(5) Å
b = 15.1799(6) Å
c = 15.9743(7) Å
α = 91.188(2)°
β = 107.447(2)°
γ = 100.744(2)°
2580.52(19) Å3
2.849 g/cm3
c = 10.5150(2) Å
3
11736.0(9) Å3
2.639 g/cm3
2.605 mm−1
5539.2(6) Å3
2.759 g/cm3
2.757 mm−1
volume
4354.06(15) Å
3
2.980 g/cm3
density (calcd)
2.728 g/cm
1
3.101 mm−1
2.952 mm−1
absorption
2.633 mm−
coefficient
F(000)
3414
8816
4372
2086
2095
3
3
3
3
3
crystal size
0.15 × 0.1 × 0.04 mm
2.895−25.597°
0.12 × 0.08 × 0.02 mm
2.215−29.200°
0.14 × 0.06 × 0.04 mm
2.215−26.466°
0.12 × 0.04 × 0.02 mm
2.270−25.000°
0.200 × 0.120 × 0.040 mm
2.551−24.999°
θ range for data
collection
index ranges
−26 ≤ h ≤ 26
−57 ≤ h ≤ 57
−16 ≤ k ≤ 16
−34 ≤ l ≤ 34
261901
−26 ≤ h ≤ 26
−15 ≤ k ≤ 15
−28 ≤ l ≤ 28
122765
−12 ≤ h ≤ 12
−13 ≤ k ≤ 13
−25 ≤ l ≤ 25
46777
−13 ≤ h ≤ 13
−18 ≤ k ≤ 18
−18 ≤ l ≤ 18
89245
−
−
26 ≤ k ≤ 19
12 ≤ l ≤ 12
reflns collected
23764
independent reflns
1812 [Rint = 0.0557]
99.8%
15870 [Rint = 0.0389]
99.8%
11387 [Rint = 0.0736]
99.9%
8573 [Rint = 0.0941]
99.3%
9028 [Rint = 0.0336]
99.3%
completeness to
θ = 26.000°
data/restraints
1812/0/150
15870/0/807
11387/0/811
8573/43/761
9028/72/810
/parameters
refinement method
full-matrix least-squares on
F
2
goodness-of-fit
final R indices
1.137
1.188
1.047
1.011
1.122
Robs = 0.0566
Robs = 0.0408
Robs = 0.0481
Robs = 0.0551
Robs = 0.0505
[
I > 2σ(I)]
wRobs = 0.1429
Rall = 0.0835
wRobs = 0.0982
Rall = 0.0516
wRobs = 0.1042
Rall = 0.0716
wRobs = 0.1026
Rall = 0.1065
wRobs = 0.1237
Rall = 0.0541
R indices [all data]
wRall = 0.1636
largest diff. peak and 0.904 and −0.547 e·Å−
wRall = 0.1081
1.776 and −1.280 e·Å−3
wRall = 0.1155
2.157 and −1.240 e·Å−3
wRall = 0.1202
1.934 and −1.818 e·Å−3
wRall = 0.1269
1.909 and −2.158 e·Å−3
3
hole
a
2
2
2
4
1/2
2
2
o
2
2
o
2
c
R = ∑||F | − |F ||/∑|F |, wR = {∑[w(|F | − |F | ) ]/∑[w(|F | )] and calcd w = 1/[σ (F ) + (0.0318P) + 111.1197P], where P = (F + 2F )/
o
c
o
o
c
o
3
.
used pH as a tool for controlling the protonation and hence the
extent of hydrolyzation of the diimide ligand. Thus, we can vary
the crystallographic phase of the system with pH by in situ release
of variable amounts of amino pyridine in the system. Controlled
blank studies confirmed the mechanism of in situ hydrolysis of
the ligand. Optical absorbance studies show that the hybrids have
considerable band gap decrease from the constituent units
2. EXPERIMENTAL SECTION
2
.1. Reagents. All the chemicals were obtained from commercially
available certified reagent sources and used without further purifications.
Sodium molybdate dihydrate (Na MoO ·2H O, AR, 99%), sodium
silicate pentahydrate (Na SiO ·5H O, LR, 97%), and cupric chloride
2
4
2
2
3
2
dihydrate (CuCl ·2H O, AR, 99.9%) were obtained from Spectrochem.
2
2
Concentrated HCl and acetone (GC, 95%) were purchased from Merck.
Glacial acetic acid (CH COOH, 16 N, 99.99%) was purchased from
3
(POM and 4-ap) resulting in the formation of semiconducting
Sigma-Aldrich. Millipore water of conductivity 18.2 MΩ·cm was used
hybrids. The compounds have been characterized using single
crystal X-ray diffraction (SCXRD), powder X-ray diffraction
for synthesis and all other studies. NDI-pyridine was synthesized
2
1
according to a reported procedure.
.2. Synthesis. The hybrid materials 1: [H (SiMo O )][(C H -
2
4
12 40
5
6
(PXRD), IR spectroscopy, and elemental analysis. Magnetic
N ) ], 2: ([H (SiMo O ) ][(C H N ) ]·7H O), 3: [H (Mo -
2
6
8
12 40
2
5
6
2
10
2
8
12
measurements showed that the compounds are diamagnetic,
thereby confirming the hexavalent oxidation state of the
molybdenum ions in the POM unit. In a proof-of-concept
experiment, the selected compounds were tested for heteroge-
neous catalytic oxidation of small organic molecules, which
showed that these hybrids are good heterogeneous catalysts for
selective oxidation reactions.
O Si) ][(C H N ) ]·7(H O), 4: [H (Mo O Si) ][(C H N ) ]·6-
4
0
2
5
6
2
9
2
8
12 40
2
5
6
2 7
(H O), and 5: [H (Mo O Si) ][(C H N ) ]·7(H O) were synthe-
2
8
12 40
2
5
6
2
7
2
sized under hydrothermal conditions, using 50 mL Teflon lined
autoclave chambers under autogenous pressure. The autoclaves were
filled with 35 mL of distilled water (70% volume capacity), followed by
the addition of 0.5 g (2.07 mmol) of Na MoO ·2H O with stirring.
2
4
2
Glacial acetic acid (16 N) was then added dropwise to adjust the pH of
the solution between 4 and 5. To this solution, 0.5 g (2.36 mmol) of
B
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886 cm− (Mo-O -Mo, corner shared), and 770 cm (Mo-O -Mo, edge
b b
29,30
1
−1
Na SiO ·5H O was added, followed by glacial acetic acid to maintain the
2
3
2
pH between 4 and 5. Consequently, the initial transparent solution
shared).
Thus, the IR spectra clearly support the presence of 4-ap
became yellowish green. In the next step, 0.238 g (1.4 mmol) of CuCl2·
and the POM in all the compounds and confirm the crystal structures
obtained from SCXRD.
2
H O and 0.59 g (1.73 mmol) of NDI-py were added to obtain a
2
greenish blue solution of pH around 4.6. In the final step, precise pH
control was achieved by successive addition of concentrated and dilute
HCl, very slowly to adjust the pH between 1 and 5, as required for
different reactions. The reaction mixture was stirred for 1 h and was kept
at 180 °C for 5 days.
2.6. UV−vis Spectroscopy. The compounds in powdered form
were used for recording UV−vis spectra in reflectance mode on a
PerkinElmer Lambda750 spectrophotometer. BaSO was used as the
4
reference for solid UV−vis absorbance spectroscopy. The samples were
loaded in a solid sample holder with a transparent quartz window, and
the measurements were performed in the range 200−800 nm. All
The reactions were carried out at pH 1.2, 2.2, 3.2, 4.2, and 4.5 to
understand the formation of different phases without varying the POM/
obtained spectra were subtracted with the BaSO spectrum as the
4
4
-ap ratio. The single crystals were obtained in good yields (55−65%
background.
w.r.t. Na MoO ·2H O, 1: 64%, 2: 62%, 3: 55%, 4: 59%, and 5: 58%)
2.7. Scanning Electron Microscopy (SEM) and Elemental
Analysis. Clean single crystals of the samples were loaded on a carbon
tape, and the FESEM measurement was done on a Quanta3D (FEI)
system, equipped with a monoCL4 cathodoluminescence system
(GATAN). Data were collected with an accelerating voltage of 20
keV and a 50 s accumulation time. The EDAX measurements were
performed on single crystals with a clean surface using a Bruker 129 eV
EDAX instrument (Table S2) to check the presence of the elements for
all 5 compounds. The EDAX analyses were performed using the P/B-
ZAF standardless method (where P/B = peak-to-background model, Z
= atomic no. correction factor, A = absorption correction factor, and F =
fluorescence factor). EDAX of each sample was done over three different
positions, and the overall composition was taken as the average of the
three data sets. CHN analyses were carried out using a Thermo Scientific
Flash 2000 CHN analyzer. The elemental percentages obtained from
SCXRD data broadly matched with the elemental composition obtained
from elemental analyses.
2
4
2
directly from the solution on cooling. They were separated and washed
with distilled water and acetone.
2
.3. Powder X-ray Diffraction (PXRD). Phase identity and purity
of the compounds were checked by PXRD, which was done on a Bruker
X8 Discover diffractometer using Cu Kα radiation (λ = 1.5418 Å). Data
were collected over the angular range 3° ≤ 2θ ≤ 50°, with a step size of
0
.02° at room temperature calibrated against corundum standards. The
comparison of experimental PXRD with the simulated single crystal
XRD patterns of compounds 1−5 is shown in Figure S2.
2.4. Single Crystal X-ray Diffraction (SCXRD). Single crystals of
the compounds 1−5 were mounted on a thin glass fiber with
commercially available super glue. X-ray single crystal data of the
compounds were collected on a Bruker D8 Venture diffractometer
(
Photon CMOS detector) equipped with a microfocus, 2.4 kW sealed
tube X-ray source with graphite monochromatic Mo−Kα radiation (λ =
.71073 Å) operating at 50 kV and 1 mA, with ω scan mode. The
0
22
program SAINT was used for integration of diffraction profiles, and
2.8. Nuclear Magnetic Resonance Spectroscopy (NMR). NMR
was used to analyze the products of the reactions during the synthesis.
NMR spectra were recorded with a Bruker AVANCE 400 (400 MHz)
Fourier transform NMR spectrometer with chemical shifts reported in
parts per million (ppm) with respect to TMS. Splitting patterns are
designated as s, singlet; d, doublet; bs, broad singlet; m, multiplet; t,
triplet. The samples (crystals/powders) were dissolved in chloroform-D
23
absorption correction was done with the SADABS program. The
structures were solved by direct methods, followed by successive Fourier
and difference Fourier syntheses. All the non-hydrogen atoms except a
disordered 4-ap ring were refined anisotropically. Calculations were
24
24
25
carried out using SHELXL 97, SHELXS 97, PLATON, WinGX
26
27
system ver. 1.80.05, and Olex2. The data collection and refinement
parameters for the compounds are summarized in Table 1. Due to high
disorder in the systems, some B level alerts were obtained in the checkcif
of the compounds, particularly in the cases of 3−5. These errors mainly
arose from disordered 4-ap rings and due to hydrogen placements on the
disordered centers.
1
solvent (δ = 7.26 ppm for H). Product of blank NDI-py (Figure S3a−
1
c), H NMR (400 MHz, CD3OD, TMS): δ (ppm) 7.96 (d, 2H), 6.83 (d,
1
2H), crystals of blank NDI-py hydrolysis (Figure S3d), H NMR (400
MHz, CD3OD, TMS): δ (ppm) 7.63 (d, 4H).
2.9. Magnetic Measurement. Magnetic measurements on samples
1−3 were carried out on a Quantum Design MPMS-SQUID
magnetometer. The compounds were powdered and screened by
PXRD to confirm phase purity before doing magnetic measurements.
Temperature dependent magnetic susceptibility data were collected for
field-cooled (FC) and zero-field-cooled (ZFC) modes between 2 and
300 K with an applied field of 1000 Oe. M vs H data were collected at 2
and 300 K between −60 and +60 kOe. Since the compounds showed
very similar magnetic properties, magnetism was studied on the first
three compounds, comprising those being used for catalytic studies to
see the oxidation state of the metal centers.
2
.5. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR
spectra were obtained for 1−5 on a Bruker IFS 66v/S instrument. The
pellets were prepared with dried KBr, and the KBr spectrum was used a
reference. Clean single crystals of the respective compounds were used
for measurements. The KBr−sample mixtures were used to make 13
mm diameter pellets for the measurements. The spectra were recorded
−1
in between 400 and 4000 cm . The IR spectra for the compounds 1−5
−1
are shown in Figure S8 (in the region of 4000−400 cm ). Various
modes of IR stretching frequencies for all the compounds and their
assignment have been tabulated in Table S1. The overall spectra of all
the compounds are similar owing to the commonality of the chemical
species present in the structures. The spectra for compounds 1−5 are a
combination of stretching frequencies for the two units, POM and the 4-
ap group. The stretching frequencies for the 4-ap group mainly fall above
2.10. Catalytic Studies. The catalytic studies were carried out on
small organic molecules (ethyl benzene, cyclohexanol, anthracene,
benzyl alcohol, and cyclooctene) using compounds 1 and 2 (5 mg).
Reaction conditions for each reaction were obtained from various
−
1
33−35
1
000 cm (except for the bending modes), while those for the POM
literature procedures
and are shown in Scheme 2. The products
−1
28−32
unit are observed below 1000 cm .
The broad O−H band at 3400
were analyzed by drawing 1 mL aliquots after reaction completion,
which was injected into a Shimadzu GCMS-QP 2010 Plus gas
chromatography instrument (coupled with a mass analyzer) to know
quantitative amounts of products formed. The percentage conversion
and yields were determined on the basis of the concentration of starting
materials remaining after the reactions. Compounds 1 and 2 were taken
as the representative of this class of hybrids, and catalytic studies were
done using these compounds as the systems are completely
undisordered. Experimental details on catalytic studies are given in the
Supporting Information.
−1
cm , observed for the compounds 2−5, is due to the presence of the
lattice water molecules which somewhat masks the N-H stretch
expected in the same region. The N-H stretch in the case of compounds
−1
1
and 2 is quite sharp at 3446 cm . The other important stretching
frequencies originating from the 4-ap groups in all the compounds are
−1
observed at ∼2900−3000 cm , which are characteristic of C-H
stretching (sym and asym) of the aromatic group. Other important
−1
peaks observed are aromatic CC stretch at ∼1520 cm , CN
−
1
−1
stretch at ∼1650 cm , N-H bending at ∼1587 cm , and C-N at 1380
−1
cm . The slight shift in the positions of N-H, C-H, and CC stretching
frequencies is due to the extensive hydrogen bonding interactions
present in all the structures. The peaks corresponding to the POM unit
2.11. Adsorption Measurements. Adsorption isotherms of N at
2
77 K were recorded with the dehydrated sample using a
QUANTACHROME QUADRASORB-SI analyzer. About 50 mg of
each of the samples 1, 3, and 5 were taken in a sample holder and
−1
−1
(
SiMo O ) are observed at ∼1045 cm (Mo-O ), 940 cm (Si-Oc),
12
40
t
C
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Inorganic Chemistry
Article
degassed at 100 °C under 10− Pa vacuum for about 12 h before the
measurements to dehydrate the samples. Dead volume of the sample cell
was measured using helium gas of 99.999% purity. Adsorbed gas amount
was calculated from the pressure difference (Pcal − P ), where P is the
1
environment in the in situ hydrolysis of NDI-py, the following
three controlled reactions were performed: (a) blank NDI-py
reaction (without POM precursors), (b) the reaction of 4-ap
with POM precursors, and (c) the reaction of NDI-py with POM
precursors maintaining the same reaction conditions (temp: 180
e
cal
calculated pressure with no gas adsorption and P is the observed
e
equilibrium pressure. All the operations were computer-controlled and
automatic. The BET surface areas of the compounds were found to be
very low, as expected from the nonporous nature of the hybrids. The
respective BET surface areas of the compounds 1, 3, and 5 were found to
°
C, pH: 1−2, time: 5 days). The product solutions were analyzed
through NMR (Figure S3) which shows that all the three
reactions generated the same organic species in solution, i.e., 4-
ap. NMR spectra of the product obtained from the three control
reactions confirm the in situ generation of 4-ap or equivalent
species through hydrolysis of NDI-py and show how the
hydrolysis is affected by the chemical environment. The two solid
compounds shown, orange color crystalline mass (Figure S3a,
inset) and the red crystals (at the bottom, Figure S2d, left inset),
were obtained in the blank NDI-py reaction. In the inset of
Figure S3d, the optical image and solved structure of the red
crystals are shown. The blank NDI-py reaction yielded two solid
products (Figure S3, insets of (a) and (d)), both of which were
also analyzed through NMR and SCXRD. The NMR of the
orange color crystalline product (Figure S3a, inset) was found to
be the same as the solution NMR spectra obtained for the three
control reactions, signifying the in situ formation of 4-ap. The
NMR data of the red crystals (Figure S3d, inset) showed that it is
the other half of hydrolyzed NDI-py, i.e., naphthalene acid
dianhydride, which was later reconfirmed from the SCXRD
analysis (Figure S3d, right inset). This proves that NDI-py is
indeed generating the 4-ap in situ and this hydrolysis is affected
only by conditions like temperature, pressure, and pH of the
reaction, and not by any chemical precursor.
2
be 4.394, 2.842, and 2.298 m /g. The adsorption isotherms of the
compounds are shown in Figure S10.
2
.12. Thermogravimetric Analysis (TGA). The TGA analysis of
compounds 1−5 was performed using a Mettler Toledo TGA 850 under
a nitrogen atmosphere at a heating rate of 5 °C/min between 25 and 800
°
C. The samples showed similar thermogravimetric behavior with
sequential decays of lattice water molecules, followed by degradations of
the organic ligands. The loss of water molecules (2−4%) mainly
happened between 25 and 120 °C. The loss of organic ligands broadly
occurred in two steps between 200 and 400 °C (∼10%) and between
5
S11.
20 and 770 °C (20%). The TGA curves for 1−5 are provided in Figure
3
. RESULTS AND DISCUSSIONS
.1. Synthesis and Crystal Growth. Phase modulation by
3
the slow release of ligand and controlled protonation may be an
innovative technique to engineer the crystal structure of POM-
organic hybrids. However, such fine modulations are difficult in
rigid indirect covalent POM hybrid systems as the secondary
TMs bind the ligands into high dimensional rigid structures.
Supramolecular structures on the other hand, are far less rigid
and give wider windows for structural fine-tuning and
consequent engineering of crystal phases. To have a better
control on the overall reactivity of the ligand, here we have used
the simple ligand, 4-ap pyridine, in the form of a hydrolyzable
precursor, NDI-Py. NDI-py is well-known for its photophysical
The results of the controlled synthetic studies indicated the
occurrence of an acid catalyzed hydrolysis of NDI-py, which
prompted us to use pH as a tool to tune the extent of hydrolysis
and modulate the phase of the resulting hybrids. Hydrothermal
reactions were set at different pH conditions (1.2, 2.3, 3.2, 4.2,
and 4.5), maintaining otherwise identical reaction conditions.
Interestingly, we found pH to play a crucial role in the phase
formation of the hybrid structures where the amount of 4-ap
incorporation is changing with changing pH conditions, leading
to the different crystal structures of the POM-amino pyridine
hybrid. Compounds 1: [H (SiMo O )][(C H N ) ], 2:
2
1
properties, having rich electronic properties and is known to be
21
quite stable under acidic conditions. However, it failed to
survive the harsh hydrothermal acidic reaction conditions and
degraded (pH 1−4.5, 180 °C for 5 days) to yield 4-ap and
naphthalene tetracarboxylic acid (Scheme 1). The dissociation
leads to the in situ formation of crystals of hybrid compounds of
6
−
4
12 40
5
6
2 6
POM (Keggin: SMo O and 4-ap). Neither solution phase
1
2
40
[
[
(
H (SiMo O ) ][(C H N ) ]·7H O, 3: [H (Mo O Si) ]-
8 12 40 2 5 6 2 10 2 8 12 40 2
(C H N ) ]·7(H O), 4: [H (Mo O Si) ] [(C H N ) ]·6-
synthesis at milder reaction conditions nor reactions at relatively
lower temperature of 160 °C yielded any NDI-py incorporated
systems. To check the effect of reaction conditions and chemical
5
6
2 9
2
8
12 40
2
5
6
2 7
H O), and 5: [H (Mo O Si) ][(C H N ) ]·7(H O) were
2
8
12 40
2
5
6
2 7
2
formed at pH 1.2 and 2.2, 3.2, 4.2, and 4.5, respectively. More
POM hybrid crystal structures with varying 4-ap arrangements
were obtained until pH 4.5, after which the formation of POM
gets affected due to higher pH conditions. All the compounds
Scheme 1. Schematic Representation of the Mechanism of
a
NDI-py Hydrolysis
(
1−5) crystallized as dark blue colored crystals. SEM images of
single crystals of the compounds 1−5 are shown in Figure
S1.The crystals were found to be stable under normal conditions
and in common laboratory solvents. These crystals were
insoluble in water and common organic solvents. The controlled
phase modulation study showed that pH plays an important role
in the overall crystal phase formation of the hybrid by controlling
the extent of hydrolyzation of the ligand NDI-pyridine.
Alternatively, when we carried out the reactions with 4-ap
directly, in the majority of the cases (75%), we have ended up
with a mixture of different phases of POM-4-ap supramolecular
hybrids.
3
.2. Crystal Structure and pH Controlled Phase
Modulation. Compound 1 synthesized at pH 1 crystallizes in
a rhombohedral crystal system having R3 space group with lattice
a
The naphthalene tetracarboxylic acid might exist as the dianhydride
as well which was confirmed from the NMR spectra and SCXRD
analysis.
̅
parameters, a = b = 21.8664(3) Å and c = 10.5150(2) Å. The
D
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Figure 1. Crystal structure of the compound 1: [H (SiMo O )][(C H N ) ]. (a) Molecular structure, (b) arrangement of 4-ap molecules along the b
4
12 40
5
6
2 6
direction, (c) unit cell, and (d) central projection of the ligand arrangement along the c direction.
Figure 2. Crystal structure of the compound 2: [H (SiMo O ) ][(C H N ) ]·7H O. (a) Asymmetric unit, (b) arrangement of 4-ap molecules along
8
12 40
2
5
6
2
10
2
the c direction, (c) unit cell, and (d) central projection of the ligand arrangement and lattice water molecules along the b direction.
structural arrangement in compound 1 is simpler as compared to
the other hybrids of this family. The molecular structure of 1
connected by supramolecular interactions. The asymmetric unit
of 1 comprises one-third of a POM cluster and one 4-ap
molecule. The molecular structure is generated through
(Figure 1a) consists of one POM unit and a 4-ap molecule
E
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symmetry operations, the formula unit of which has one POM
unit and six 4-ap molecules. The unit cell structure and the 3D
arrangement of the bipyridine rings are shown in Figures 1b−d
and S4. The molecular structure of 1 is devoid of any lattice water
molecules. The POM unit in 1 exists as disordered Keggin where
the heteroatom Si is coordinated by eight half-occupied oxygen
atoms. The 3D structure of 1 is formed mainly through
supramolecular interactions like hydrogen bonding and C−H−
O interactions. The amine groups and C-H of the 4-ap rings form
hydrogen bonds with the terminal oxygen atoms of the POM
unit. There is also parallel-displaced π−π stacking (4.2−4.4 Å)
present among the 4-ap rings which adds further stability to the
supramolecular hybrid structure.
Compound 2 was synthesized at pH 2.2. It crystallized in a
monoclinic system with space group C2/c having lattice
parameters a = 42.1438(2) Å, b = 12.2492(5) Å, c =
25.2517(10) Å, and β = 115.802(3)°. The formula unit (Figure
2a) contains two such asymmetric units. Each one consists of one
unit of the POM (SiMo O ) and five 4-ap molecules with four
1
2
40
lattice water molecules. The unit cell structure of 2 and the
arrangement of 4-ap molecules in the hybrid compound are
shown in Figure 2b−d and Figures S5 and S6. Some of the
terminal and corner shared oxygen atoms of the POM unit form
strong hydrogen bonds with the water molecules and the amine
groups of 4-ap molecules. One of the 4-ap molecules (N10) is
disordered at the amine position (N10 and N10A, Figure S5).
The 2D and 3D hybrid structures are made up of supramolecular
interactions like intermolecular hydrogen bonding, C−H−O
interaction (C−H of 4-ap molecules and O of water or POM),
π−π stacking, and electrostatic interactions. Moreover, there are
moderately strong π−π interactions present between the
aromatic 4-ap rings (π−π distance ∼ 3.65−3.75 Å) which
provide further stabilization to the 3D structure.
Figure 3. Molecular crystal structure of (a) 3: [H (Mo O Si) ]-
8
12 40
2
[
(C H N ) ]·7(H O), (b) 4: [H (Mo O Si) ][(C H N ) ]·6(H O),
5 6 2 9 2 8 12 40 2 5 6 2 7 2
and (c) 5: [H (Mo O Si) ][(C H N ) ]·7(H O). Hydrogen atoms
8
12 40
2
5
6
2
7
2
and lattice water molecules are omitted.
Compound 3, synthesized at a pH of 3.2, crystallized in P2 /n
1
space group with lattice parameters a = 21.2231(13) Å, b =
1
2.3452(7) Å, c = 22.995(14) Å, and β = 113.16(2)°. The
formula unit shows the presence of two POM units, nine 4-ap
moieties, and seven lattice water molecules. The molecular unit
(Figure 3a) consists of half of the formula unit. The 3D
supramolecular hybrid structures are formed through non-
covalent interactions like H-bonding (b/w the 4-ap amine
protons, lattice water hydrogen atoms, and terminal O atoms of
the POM), C−Hπ and π−π interactions between the 4-ap
molecules, and CH−O interactions between the 4-ap and the
lattice water molecules. The arrangement of the 4-ap units and
lattice water molecules in the molecular structure is shown in
Figure 4c. There are two strongly disordered 4-ap units in the
structure of 3 (Figure 4c). The disorder in the two units was
successfully resolved where, in one unit, there was a super-
imposition of two 180° rotated 4-ap rings, while, in the other, two
laterally half-translated 4-ap moieties were superimposed (Figure
S7a).
Figure 4. Arrangement of the 4-ap units and lattice water molecules in
the molecular crystal structure of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5.
Hydrogen atoms are omitted for clarity. In the structure for compounds
3, 4, and 5, the disordered 4-ap units are circled in red.
lattice water molecules (6 in compound 4 and 7 in compound 5)
and the molecular arrangements involving different supra-
molecular interactions as mentioned in the above structural
descriptions. Both compounds had severe disorders in two 4-ap
units. The kinds of disorder are different and have been
pictorially resolved as shown in Figure S7b,c. In the case of 4,
both were two different kinds of 180° rotational disorders, while,
in 5, one disorder was pivotal ∼30° rotation of the ring about one
atom as the center.
pH dependent phase modulation study yielded many
interesting observations which shine light on the mechanism of
the crystal engineering in this class of supramolecular hybrids. As
Figure 5 and molecular formula of the compounds 1−5 suggest,
there is a clear role of pH in the extent of hydrolysis of the NDI-
py molecule, thereby varying the amount of 4-ap available for the
molecule formation. This occurs as the pH controls the
protonation of the nitrogen center of the NDI-py unit, which
Compounds 4 and 5 were synthesized at pH of 4.2 and 4.5,
respectively. Both compounds crystallized in the triclinic system
of space group P1
11.7748(8) Å, c = 21.0804(15) Å, α = 75.265(4)°, β =
7.340(3)°, γ = 86.720(4)° for compound 4 and a = 11.3922(5)
Å, b = 15.1799(6) Å, c = 15.9743(7) Å, α = 91.188(2)°, β =
07.447(2)°, γ = 100.744(2)° for compound 5. The molecular
̅
with the lattice parameters a = 10.4802(7) Å, b
=
7
1
structures of compounds 4 and 5 are shown in Figure 3b,c.
Owing to their similar synthesis conditions, both the compounds
have a similar 4-ap-to-POM ratio of 3.5 in the formula unit. The
difference between the two compounds lies in the number of
F
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Figure 5. pH dependent controlled phase variation in the POM-AP hybrids. As we move from lower to higher pH, due to lower protonation and
hydrolyzation of the NDI ligand, the AP-to-POM ratio is decreasing. However, with increase in pH, the hybrids are crystallizing with lesser symmetry.
Figure 6. Kubelka−Munk plots of compounds 1−5. The band gaps were determined from the K-M plots of the DRS spectra of (a) 1: 2.7 eV, (b) 2: 2.68
eV, (c) 3: 2.53 eV, (e) 4: 2.48 eV, and (e) 5: 2.52 eV.
catalyzes its hydrolysis by water attack at the amide center
Scheme 1). This has also been proved by the blank controlled
study of the NDI-py where we could identify the naphthalene
anhydride through SCXRD along with the presence of 4-ap in
the solution. It can be clearly observed that, as we move lower
system as we move from lower to higher pH, i.e., high to low 4-
ap:POM ratio. Also, it is observed that, upon moving from
compound 1 to 5, the extent of molecular disorder in the 4-ap
units increases. This may be attributed to a lower concentration
of 4-ap and a slower crystallization phenomenon.
(
(
1.2) to higher pH (4.5), the 4-ap-to-POM ratio also decreases
3.3. UV−vis Spectroscopy and Band Gap. Electronic
spectra of the solid powdered samples of 1−5 were recorded in
reflectance mode in the range of 200−1000 nm (Figure S9). The
two absorption bands observed at around 213 and 250 nm can be
attributed to the LMCT and n−π*/π−π* transitions of the
almost linearly as 6 in compound 1, 5 in compound 2, 4.5 in
compound 3, and 3.5 in compounds 4 and 5. From this
observation, it is evident that lowering the H concentrations
decreases the extent of 4-ap formation, thereby lowering the 4-
ap:POM ratio in the molecular crystal structure. Another
interesting fact is the successive lowering in symmetry of the
+
36
ligand 4-ap, respectively. The broad absorption band at ∼335−
360 nm can be assigned to the charge transfer transition in the
G
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Figure 7. Temperature dependent magnetic susceptibility in FC mode (top) and field dependent magnetization (bottom) of the compounds 1 (a), 2
(
b), and 3 (c).
3
6
compounds. The DRS spectra were converted to a Kubelka−
other hand, the field dependent magnetization data at 2 K
represents a typical sigmoid curve which saturates at higher fields.
The sigmoid curve confirms the presence of a small amount of
paramagnetism. The paramagnetic contribution at 2 K can be
attributed to the crystallized paramagnetic oxygen present in the
system. The saturation might be coming from some oxygen
vacancies present at the sites of the 8 half-occupied oxygen atoms
around Si present at the core of the POM. The vacant positions
may contain some residual electrons which might account for
such sigmoid curves at low temperatures. Overall, from the χ vs T
data and the M vs H data at 300 K, it is clear that the compounds
are diamagnetic and all the Mo in the compounds are in their +6
oxidation state.
32,37
Munk (K-M) function plot
(Figure 6), and the optical band
gaps were determined as the energy value (in eV) for the first
prominent peak maxima in the K-M plot. The optical band gap
was determined to be 2.7, 2.68, 2.53, 2.48, and 2.52 eV for 1−5,
respectively, which falls in the semiconducting region. It is to be
noted that the two comprising units of the hybrid structures, i.e.,
32
4
-ap and SiMo O , are known to exist as insulators. There is a
12 40
clear transition from insulator to semiconductor state as we move
from the individual units to the hybrid structure. This is probably
occurring because of the enhanced band structure in the case of
the hybrids than the individual molecules in the solid state. Due
to interactions between the POM and ligand units, new bands
arise above and below the valence and conduction bands, which
eventually leads to the decrease in band gap of the hybrids.
38
3.6. Catalytic Studies. POMs have emerged as excellent
catalysts for various types of catalytic reactions like acid catalyzed
reactions, selective oxidations, hydrogenation reactions, and also
for industrial catalysis because of their acidic and redox facile
3
.5. Magnetic Properties. Magnetic measurements were
performed on powdered crystalline samples of compound 1−3 in
the range 2−300 K at an applied magnetic field of 1000 Oe (field-
cooled) to check the oxidation states of molybdenum atoms and
to see the overall magnetic properties of the hybrid. It is expected
that all the Mo ions are in their highest oxidation state (+6) and
hence nonmagnetic, devoid of any d-electrons. The variation of
molar magnetic susceptibility (χ ) with temperature (T) for 1, 2,
and 3 is depicted in Figure 7a−c, respectively. The plots clearly
show that the compounds are diamagnetic in nature as the χ
40−42
nature.
Out of these, selective oxidation of small molecules
is of immense importance in the field of catalysis as the end
products are highly useful both in pharmaceuticals and as starting
materials for various kinds of molecules, which forms the basis of
chemical industries. Selective oxidation of molecules is
challenging for both physical and synthetic chemists since
most catalytic transformations have many oxidized products out
of which the desired ones, in most cases, are not the
thermodynamic or kinetic end product. Thus, the need for
controlling the reaction pathways to get the desired products,
arresting the reactions from further oxidations, demands the use
of suitable catalysts. POMs have catered to this catalytic need for
a long time. However, the major drawback while using POMs in
catalysis is their high solubility in aqueous and polar organic
M
M
values are negative in the entire temperature range of 100−300
K. Below 70 K, there is an increase in magnetic susceptibility
small hump, more pronounced in Figure 7b,c), which may be
(
originating from a small amount of paramagnetic γ oxygen
impurity (arising from insufficient purging) present in the system
which crystallizes at low temperatures. Paramagnetic solid γ
oxygen is a paramagnetic phase with an A15 cubic structure.
Field dependent magnetic moment studies were done at 2 and
00 K (Figure 7a−c, bottom). The negative slope of field
dependent magnetization data (M vs H) at 300 K clearly
indicates that the systems are of diamagnetic nature at room
temperature. This is expected as the systems are devoid of any
unpaired electrons particularly on the Mo metal centers. On the
39
43
solvents. Due to their solubility and homogeneous mode of
activity, recovering the catalyzed products and their application
in practical devices is hampered. Though the homogeneous
catalytic activities of POMs are very well studied and usually
higher than that of heterogeneous cases, for practical
applications, heterogeneous catalysis is more desirable. To
overcome this issue, various methods have been adopted for the
3
H
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a
Scheme 2. Scheme of the Various Organic Catalysis Reactions
a
Here, we studied the selective oxidations of small organic molecules by the catalysts. The amount of catalyst used in each reaction was 5 mg.
a
Table 2. Catalytic Conversion and Selectivity of the Expected Products for the Oxidation of Various Small Organic Molecules
constituents
reagents amount
ethyl benzene 0.3 mL
conditions
conversion (%)
selectivity (%)
code
reaction name
blank
30
1
2
blank
45
1
2
C1 oxidation of ethyl
12 h,
70 °C
65
60
acetophenone
(100)
acetophenone
(85)
benzene
(2.4 mmol)
acetonitrile
0% TBHP
8 mL
7
0.745 mL
(4.8 mmol)
C2 oxidation of cyclo-
cyclohexanol
0% H O
0.1 g
8 h, 85 °C negligible
inactive
40.5
inactive
49.3
40
hexanol
3
0.17 g
5 mL
0.05 g
2
2
dioxane
anthracene
C3 oxidation of
12 h,
75 °C
20
50
52
anthraquinone
(100)
anthraquinone
(70)
anthracene
7
0% TBHP
0.125 g
5 mL
benzene
C4 oxidation of benzyl
benzyl alcohol 0.108 g
(1 mmol)
8 h, 85 °C 34
67.79
65.3
benzaldehyde
(53)
benzaldehyde
(100)
alcohol
3
1
0% H O
0.17 g (5 mmol)
5 mL
2
2
,4-dioxane
C5 epoxidication of
cyclooctene
1 g (10 mmol) 10 h,
85 °C
negligible
20
cyclooctene
epoxide (41)
cyclooctene
epoxide (32)
cyclooctene
TBHP
0.136 g
(
4 mmol)
1
,4-dioxane
10 mL
a
5
mg catalyst.
immobilization of POMs involving processes like forming POM
hybrids with organic ligands, use of ionic liquids, loading POMs
onto metal organic frameworks/mesoporous materials, and use
reactions are given in Scheme 2, Table 2, and Figure 8.
Comparing the blank and catalyzed reactions, we found that both
+
40,43,44
of water insoluble salts of POMs (e.g., Cs ).
Here, we chose selective oxidation of small organic molecules
to check catalytic property of the hybrids. A set of five peroxide
based catalysis reactions, (1) oxidation of ethyl benzene, (2)
oxidation of cyclohexanol, (3) oxidation of anthracene, (4)
oxidation of benzyl alcohol, and (5) epoxidation of cyclooctene,
were screened to study the oxidative properties of these hybrids.
All catalytic studies were performed using pure powdered
samples of compounds 1 and 2 as the catalyst. These compounds
were chosen as the representative of this class of hybrids for
catalytic studies as these were systems without disorders. The
products of the catalysis reactions were analyzed through gas
chromatography. The products, percentage conversions, and
selectivity of the catalysis reactions by 1, 2 and the blank
Figure 8. Comparison of the percentage conversion and selectivity of
blank and catalyzed (compounds 1 and 2) oxidation of small organic
molecules.
I
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Table 3. Comparison of Catalytic Activity of Similar POM Based Systems for Small Molecule Oxidation
catalysis
catalyst
conversion (%)
selectivity/yield (%)
ref
alkylation of 2-methylnaphthalene
isobutyraldehyde oxidation
isopolytungstate supported on zirconia
TBA HPW CoO
>92
94
>95%
54
46
42
4
11
39
39
TBA PW CoO
39
71
40
5
11
Na PW CoO
39
5
11
formaldehyde oxidation
TBA HPW CoO
20
12
19
60
5
4
11
TBA PW CoO
39
3
5
11
Na PW CoO
39
9
5
11
liquid-phase oxidation of anthracene
benzyl alcohol oxidation
cyclohexanol oxidation
cyclooctene oxidation
H [PMo V2O ]·32.5H O, supported on SBA-15
100
98
99
100
47
5
34
47
5
10
40
2
H [PMo V2O ]·32.5H O, + SBA-15 + [C mim]+
5
10
40
2
4
ZrO and Keggin-type 12-phosphomolybdic acid
82
35
2
cyclohexene oxidation
30
styrene oxidation
30
ethyl benzene oxidation
oxidation of ethyl benzene
oxidation of cyclohexanol
oxidation of anthracene
oxidation of benzyl alcohol
epoxidication of cyclooctene
{[Cu (4,4-bipy) (H O) ](SiW O )(H O) }n
56.8
65
88.1
100
33
2
4
2
4
12 40
2
18
catalyst 1
this report
inactive
40.5
67.79
65.3
100
53
41
the compounds are active for the selective oxidation of small
organic molecules. Catalysts 1 and 2 could successfully catalyze
all the reactions except oxidation of cyclohexanol. Compound 1
showed excellent catalytic activity for the oxidation of ethyl
benzene and anthracene with 100% selectivity toward the
formation of the respective desired products of acetophenone
and anthraquinone (Table 2, Figure 8). The percentage
conversions were also much higher (∼65% and 40%,
respectively) in these two reactions compared to the blank
reactions (∼30% and 20%, respectively). For the other two
reactions, i.e., oxidation of benzyl alcohol and epoxidation of
cylooctene, the catalyzed reactions showed higher conversions (2
times) than the blank reactions, but selectivity of the specific
products was similar to that of the uncatalyzed reactions.
Compound 2, on the other hand, showed excellent selectivity for
oxidation of benzyl alcohol (100%). Broadly, the catalytic activity
of compound 1 was found to be better than that of compound 2.
Thus, as we move from uncatalyzed to catalyzed reactions, the
selectivity for the desired products was greatly enhanced,
inferring that both the compounds are good catalysts for the
selective oxidation of small organic molecules. A comparison of
the catalytic activity of catalyst 1 with similar reported catalysts
for heterogeneous catalysis of small molecule oxidation has been
presented in Table 3. It is well-known that homogeneous
catalysis of selective oxidation by POMs in the presence of
peroxides proceeds through the formation of the active catalytic
hybrid, the structure or nature of the active oxygen species is not
completely understood. The marginally higher activity of 1 over
2 may be due to the different extent of selective substrate
activations by the respective catalysts due to their different
crystallographic structures. However, a complete understanding
of the entire reaction mechanism needs further in-depth studies.
The recyclability and stability studies were carried out over three
cycles after washing the filtered catalyst each time before reuse.
The recyclability test showed that the catalyst is quite stable and
there was negligible drop in catalytic activity (3−5%) after three
cycles. This slight decrease in catalytic activity can be attributed
to the poisoning of the catalyst by the organic molecules present/
formed in the solution.
4
. CONCLUSION
Five different POM based supramolecular 3D hybrid compounds
with varying crystallographic phases were synthesized using a
hydrolyzable ligand under hydrothermal conditions of different
pH. Mechanistic study on the degradation of the starting ligand
proved that only the reaction conditions and not the in situ
generated POM species are responsible for the hydrolysis of the
ligand. pH dependent synthetic study yielded crystallographic
phase modulation in the system. The mechanism of phase
variation and role of pH were studied to find that varying pH
conditions led to variation in the extent of protonation and
hydrolyzation of the ligand, yielding different phases. Such
controlled phase modulation studies will find many applications
in crystal engineering. Crystal structure analysis revealed that the
hybrid structures have been stabilized in three dimensions
through supramolecular interactions like H-bonding, C−H−O
interactions, and π−π stacking. The optical absorption studies
showed that the hybrids formed are semiconducting in nature.
Catalytic studies were done for small organic molecule oxidation
with the catalysts. Selected compounds showed good catalytic
activity and excellent selectivity for some of the desired products
with 1 being slightly better than 2. In this work, we have
demonstrated a simple way of immobilizing POM by forming
supramolecular hybrids with organic ligands, which can be used
as good heterogeneous catalysts for oxidation reactions.
41
species like the Ishii−Venturello complex, where the peroxide
reacts with the POM cluster before oxygen transfer to the
substrates. However, reaction mechanism in the case of
heterogeneous catalysis by POM is less understood and may
45
vary from system to system. There are two processes which
need to be understood in order to analyze the overall mechanism
of the selective oxidation, i.e., the mechanism of substrate
activation and the generation of active oxygen species. POMs,
being redox active species, accept/donate electrons through the
various steps of catalysis to activate the peroxide, which is the
oxygen donating species. The integrated structure of the POM-
hybrid complex will determine the pathways, kinetics, and extent
of substrate oxidation in this case. Here, though we expect some
interactions between the peroxide species and the POM of the
J
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supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.uk,
or by contacting The Cambridge Crystallographic Data Centre,
Closer to Photosystem II: A Co O Cubane Catalyst with Flexible
4
4
Ligand Architecture. J. Am. Chem. Soc. 2013, 135, 18734−18737.
12) Sadakane, M.; Steckhan, E. Electrochemical properties of
polyoxometalates as electrocatalysts. Chem. Rev. 1998, 98, 219−237.
13) Xu, B. B.; Xu, L.; Gao, G. G.; Yang, Y. B.; Guo, W. H.; Liu, S. P.;
(
(
1
3
2 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
36033.
Sun, Z. Z. Multicolor electrochromic and pH-sensitive nanocomposite
thin film based on polyoxometalates and polyviologen. Electrochim. Acta
2
009, 54, 2246−2252.
AUTHOR INFORMATION
Corresponding Author
Phone: 080-22082998. Fax: 080-22082627. E-mail:
(14) Liu, S. Q.; Kurth, D. G.; Volkmer, D. Polyoxometalates as pH-
sensitive probes in self-assembled multilayers. Chem. Commun. 2002, 9,
■
*
9
(
76−977.
15) Chi, Y. N.; Cui, F. Y.; Jia, A. R.; Ma, X. Y.; Hu, C. W. pH-
sebastiancp@jncasr.ac.in.
Dependent syntheses of copper-quinoxaline-polyoxotungatate hybrids:
variable role of Keggin-type polyanion in different pH conditions.
CrystEngComm 2012, 14, 3183−3188.
ORCID
Dundappa Mumbaraddi: 0000-0003-3099-9348
Subi J. George: 0000-0003-4429-5237
Sebastian C. Peter: 0000-0002-5211-446X
(16) Yang, L.; Hou, L. L.; Ma, P. T.; Niu, J. Y. pH-dependent assembly
of saturated to dilacunary Keggin-based silicotungstate: [Cu-
en) (H O)][H en]{-SiW O [Cu(en) (H O)] }center dot 7.5H O.
(
2
2
2
10 36
2
2
2
2
Notes
J. Coord. Chem. 2013, 66, 1330−1339.
The authors declare no competing financial interest.
(17) Filippov, A. P.; Kovalenko, A. S.; Il'in, V. G. Formation of
nanoperiodic structures based on keggin and lacunary anions of
phosphotungstic acid and cationic surfactants. Theor. Exp. Chem. 1998,
ACKNOWLEDGMENTS
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3
(
4, 229−234.
We thank the Jawaharlal Nehru Centre for Advanced Scientific
Research, Sheikh Saqr Laboratory, and Department of Science
and Technology (DST), India, for financial support. S.R. thanks
the University Grants Commission, India, for the research
fellowship, and D.M. thanks UGC-DAE CSR, Mumbai Centre,
for the project fellowship. The authors thank Mr. Abhijit Sen for
magnetic measurements and Prof. Tapas K. Maji and Mr.
Subhajit Laha for the BET surface area measurements. S.R thanks
Dr. Arpan Hazra for fruitful discussions regarding SC data
analysis. S.C.P. acknowledges support from DST (Grant SB/FT/
CS-07/2011). We are grateful to Prof. C. N. R. Rao for his
constant support and encouragement.
18) Yu, F.; Kong, X. J.; Zheng, Y. Y.; Ren, Y. P.; Long, L. S.; Huang, R.
B.; Zheng, L. S. pH-dependent assembly of 0D to 3D Keggin-based
coordination polymers: Structures and catalytic properties. Dalton T
2009, 9503−9509.
(19) Zheng, P. Q.; Ren, Y. P.; Long, L. S.; Huang, R. B.; Zheng, L. S.
pH-dependent assembly of Keggin-based supramolecular architecture.
Inorg. Chem. 2005, 44, 1190−1192.
(20) Chen, Z. Y.; Lu, J. H.; Yu, K.; Zhang, H.; Wang, L.; Wang, C. M.;
Zhou, B. B. Nonclassical Phosphomolybdates with Different Degrees of
Reduction: Syntheses and Structural and Photo/Electrocatalytic
Properties. Inorg. Chem. 2016, 55, 8309−8320.
(21) Guha, S.; Saha, S. Fluoride Ion Sensing by an Anion-pi
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(
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22) SAINT 6.02; Bruker AXS Inc.: Madison, WI, 2000.
23) Sheldrick, G. M. SADABS: Empirical Absorption Correction
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