An organic-inorganic hybrid supramolecular framework material based on a [P2W18O62]6- cluster and Yb & Na complexes of pyridine-2,6-dicarboxylic acid: A catalyst for selective oxidation of sulfides in water with H2O2

Article abstract of DOI:10.1039/c5ce02546j

A rare-earth-containing polyoxometalate (RECP) hybrid, {(Yb (PDCH₂)₂(PDCH))·Na(H₂O)₂·(Na(PDCH)(H₂O)₂)}₂[P₂W₁₈O₆₂]·14H₂O (1), based on [P₂W₁₈O₆₂]^6- cluster anions and cationic Yb & Na complex units of pyridine-2,6-dicarboxylic acid (PDCH₂) has been synthesized under normal reaction conditions, which exhibited a supramolecular 3-D framework structure in the crystal lattice. Hybrid 1 acts as a green catalyst for the selective oxidation of sulfides in water with H₂O₂ as the reagent.

Full text of DOI:10.1039/c5ce02546j

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DOI: 10.1039/C5CE02546J
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An organic-inorganic hybrid supramolecular framework material
based on [P2W18O62]6− cluster and Yb & Na complexes of pyridine
2,6-dicarboxylic acid: a catalyst for selective oxidation of sulfides in
water with H2O2†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Pulikanti Guruprasad Reddy, Narsimha Mamidi and Chullikkattil P. Pradeep*
www.rsc.org/
A rare earth contained polyoxometalate (RECP) hybrid, {(Yb larger clusters exhibit better redox properties in comparison to
(PDCH2)2(PDCH))∙Na(H2O)2∙(Na(PDCH)(H2O)2)}2[P2W18O62]∙14H2O
smaller cluster archetypes.6 Nevertheless, RECP hybrids based
(1) based on [P2W18O62]6− cluster anions and cationic Yb & Na on large plenary clusters like [P2W18O62]6− ({P2W18}) are rare in
complex units of pyridine 2,6-dicarboxylic acid (PDCH2) has been the literature.7
synthesized under normal reaction conditions, which exhibited
supramolecular 3-D framework structure in the crystal lattice.
Hybrid 1 acts as a green catalyst for the selective oxidation of
sulfides in water with H2O2 as the reagent.
Meanwhile, the development of green reaction
methodologies is one of the focus areas in synthetic organic
chemistry. Synthetic protocols relying on environment friendly
solvents and reagents are of prime importance in view of
environmental safety. Particularly, reactions that use water as
solvent have great advantages.8 Selective oxidation of sulfides is
an important reaction in organic chemistry because of the
versatile roles of sulfoxides in pharmaceuticals, petroleum
industry, polymer materials, asymmetric catalysis, natural
products and oxygen transfer reagents.9 Sulfoxidation is
generally achieved by using oxidizing agents like m-
chloroperbenzoic acid, NaIO4, UHP, NaClO, dimethyldioxirane
and oxone; many of which are known to exhibit undesirable
traits like toxicity, by-product generation, longer reaction times,
low yields etc.10 Therefore, development of new methodologies
for sulfoxidation using eco-friendly solvents and reagents such
as water and H2O2 would be of considerable interest.11
Polyoxometalates (POMs) are a class of inorganic materials
consisting of discrete, soluble, anionic oxide clusters of early
transition metals like Mo, W, V, Nb and Ta.1 Incorporation of
rare earth (RE) ions into POM framework enhances their
properties including Lewis acidity, polarity, solubility and
electron transfer properties leading to their improved
applications in materials science and catalysis.2 A large number
of RECPs reported over the last few decades have been
reviewed recently and it was noted that a majority of these
RECPs are purely inorganic in nature.3 Therefore, the
development of novel organic–inorganic hybrid RECPs has been
projected as one of the potential new research directions in
RECP chemistry.3 Using appropriate multi-functional organic
ligands, novel organic–inorganic hybrid RECPs or high-
dimensional frameworks with new properties could be
achieved.4 However, the synthesis and characterization of new
hybrid RECPs is a challenging task as the oxophilic lanthanides
often react rapidly with oxygen rich POM units resulting in quick
precipitation. Therefore, the development of new synthetic
strategies that allow the isolation of crystalline hybrid RECPs is
of fundamental importance. Also, synthetic procedures that
allow disassembly and re-assembly of POM building units during
synthesis are important as they lead to totally new structural
assemblies.5 In terms of materials applications, RECP hybrids
containing large plenary POM units are of special interest as
Here in, we report a simple and flexible synthetic procedure
in which coordination complex of a lanthanide metal ion (Yb)
was prepared starting from a multidentate carboxylate ligand
(pyridine-2,6-dicarboxylic acid, PDCH2) in water, which on in situ
reaction with a lacunary POM building block [P2W15O56]12−
generated
(PDCH2)2(PDCH))∙Na(H2O)2∙(Na(PDCH)(H2O)2)}2[P2W18O62]∙14H2
O ( ) containing a plenary POM cluster {P2W18} (PDCH denotes
a new organic-inorganic hybrid RECP {(Yb
1
singly deprotonated PDCH2). Under the present reaction
conditions, the lacunary POM building block [P2W15O56]12− gets
transformed into a plenary cluster {P2W18}. Hybrid
1 exhibits
an interesting supramolecular framework structure in solid
state assembled through H-bonding interactions between the
cluster oxygens and PDCH2/PDCH units of Yb & Na complexes.
Catalytic property of hybrid 1 towards the oxidation of a variety
of sulfides under green reaction conditions has also been
tested.
a.School of Basic Sciences, Indian Institute of Technology Mandi, Mandi - 175001,
Himachal Pradesh, India
.
E-mail: pradeep@iitmandi.ac.in; Fax: (+91)1905 267 009
† Electronic Supplementary Information (ESI) available: [Synthetic details and
characterization data are available in Electronic Supplementary Information]. See
DOI: 10.1039/x0xx00000x
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The synthetic scheme of hybrid
1 is shown in Figure 1.
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DOI: 10.1039/C5CE02546J
Hybrid
1 was characterized by various analytical and
spectroscopic techniques including IR, UV-visible, EDX, thermo
gravimetric analyses (TGA) and single crystal X-ray diffraction
studies, see ESI† for details.
Figure 2. Asymmetric unit of hybrid
1
,
(
A
); cationic complex
cat3+
). All
{(Yb(PDCH2)2(PDCH))∙Na(H2O)2∙(Na(PDCH)(H2O)2)}3+
(
)
(B
hydrogens have been omitted for clarity. Colour code: WO6 - dark blue
polyhedra, PO4 – pink polyhedra, Na - cyan, Yb - pale green, C - grey, O -
red and N - blue.
connected to the terminal oxygens (O15 and O41) of two
{P2W18} units and hence act as a bridging unit between two
adjacent {P2W18} structural sub-units. The remaining two
coordination sites of Na1 are satisfied by water molecules. In
the Yb complex ion [Yb(PDCH2)2(PDCH)]2+, each of the three
ligands (of which one is singly deprotonated) coordinate to the
Yb center through two carbonyl oxygens and pyridine nitrogen
in the expected tridentate coordination mode resulting in a
tricapped trigonal prismatic Yb complex having D3h symmetry as
shown in Figure 3(B). This type of coordination geometry is
common for nine-coordinated Ln(III) complexes with tris
chelated tridentate ligands.12 The coordinated carbonyl oxygen
atoms from the three PDCH2/PDCH ligands form the two
trigonal faces of the trigonal prism defined by the atoms {O30,
O34, O38} and {O32, O36, O40} while the coordinated pyridine
nitrogens (N1, N2, and N3) form a plane perpendicular to the
molecular C3 axis. The trigonal faces of the trigonal prism are
almost parallel, being inclined by ca. ∠1.2°. The distortion from
the ideal D3h symmetry arises due to a rotation of the trigonal
faces with respect to each other about the C3 axis, by ca. 15°.
The Yb complex ion present in hybrid
1 is structurally very
Figure 1. Synthetic route of hybrid 1. Colour code: WO6 - dark blue polyhedra,
PO4 – pink polyhedra, Na – cyan, Yb - pale green, C - grey, O - red and N - blue.
similar to some of the previously reported Ln(III) complexes
based on PDCH2 based ligands.12 The sodium center Na2 is penta
coordinated with one PDCH ligand coordinated to Na2 in
tridentate manner. The remaining coordination sites of Na2 are
satisfied by two water molecules, see Figure 2(B).
The single crystal X-ray analysis revealed that hybrid
1
crystallizes in monoclinic C2/c space group, see ESI† Table S1 for
crystallographic data and Table S2 for selected bond lengths and
angles. The asymmetric unit consists of a) half of the {P2W18}
In the crystal structure of hybrid 1, each {P2W18} cluster
unit is connected to four cat3+ moieties as shown in Figure 3(C).
However, each of these cat3+ units is shared between two
adjacent {P2W18} cluster units. Therefore, in effect, there are
cluster
anion,
b)
a
cationic
complex
(
{(Yb
(PDCH2)2(PDCH))∙Na(H2O)2∙(Na(PDCH)(H2O)2)}3+
cat3+) and c)
seven water molecules. Major contents of the asymmetric unit
are shown in Figure 2(A). Out of the four PDCH2 units present in two cat3+ units per {P2W18} cluster anions in hybrid
1 balancing
cat3+, two are singly de-protonated.
the charge. In the crystal lattice, each {P2W18} building unit is
connected to two adjacent {P2W18} units on either side
through bridging cat3+ units in a zig-zag 1-D chain fashion as
shown in Figure 3(D). Each of these independent chains are
further connected to its neighbouring chains in a 3-D manner
through extensive hydrogen bonding interactions between the
{P2W18} cluster oxygens and the organic PDCH2/PDCH ligands
of the Yb & Na complexes. The details of these H-bonding
interactions are given in Table S3, ESI†. The formation of 1-D zig-
zag chains from {P2W18} units through bridging cat3+ units and
the inter-connection of these chains in 3-D manner through
As shown in Figure 2, the structure of hybrid
1 can be
thought of as consisting of two major structural sub-units:
{P2W18} polyanions and cat3+ cations. The {P2W18} polyanion
in hybrid
1 exhibits the classical Wells–Dawson structure,
containing two α-A-[PW9O34]9−units originating from the α-
Keggin polyoxoanion by removal of a suite of three corner-
sharing WO6 octahedra. The structure of cat3+ subunit (Figure 2
(B)) consists of two differently coordinated sodium ions. The
central six coordinated sodium ion (Na1) is connected to a Yb
complex ion [Yb(PDCH2)2(PDCH)]2+ on one side and to a sodium
complex (Na(PDCH)(H2O)2) on the other. Additionally, Na1 is
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Scheme 1. Optimized model catalytic reaction: sulfoxidation of 4-
(methylthio)phenol using hybrid 1 as catalyst.
of the constituents of hybrid 1, i.e. Na12[P2W15O56], PDCH2 and
Yb(NO3)3∙5H2O as well as {P2W18} cluster, were also tested as
controls and were found to exhibit less catalytic abilities than
hybrid
1
under the given experimental conditions. These studies
as a whole has better catalytic
revealed that the hybrid
1
efficiency as compared to its constituents taken separately, see
Table S4, ESI† for details. The catalytic oxidation of 4-
(methylthio)phenol using [P2W15O56]12— in combination with
sodium salt of PDCH2 was also tested in order to assess the role
of lanthanide ion in the catalysis. It was noted that a
combination of these starting materials gave poor yields and
Figure 3. Coordination modes of {P2W18} with NA1 atom, (A); Yb complex
[Yb(PDCH)2(PDC)]2+, (B); figure showing {P2W18} unit connected to four cat3+
units, the Yb complex is shown as green polyhedra, (C); zig-zag 1-D chain-like
arrangement of {P2W18} units in hybrid 1, (D). Colour code: WO6 - dark blue
polyhedra, PO4 – pink polyhedra, Na - cyan, Yb – dark green, C - grey, O - red
and N - blue.
selectivity in comparison to hybrid
role of the lanthanide ion in the catalytic property exhibited by
hybrid , see Table S4, ESI†.
After optimization of the reaction conditions, we tested
successfully a variety of sulfides for sulfoxidation using hybrid
as catalyst, see Table 1. The results show that hybrid catalyses
1 confirming the influential
various H-bonding interactions lead to the formation of a 3-D
supramolecular framework structure in the crystal lattice of
1
hybrid
1 as shown in Figure 3(D). Solvent water molecules
1
occupy the void space in the framework structure. The total
void volume of the structure is calculated to be 14.28 %.13
Framework structures based on Keggin or octamolybdate
clusters, PDCH2 based ligands and RE metal ions have been
achieved earlier through hydrothermal synthetic methods.14
However, the formation of organic-inorganic hybrid
supramolecular 3-D framework structures based on plenary
{P2W18} clusters and RE complexes under conventional
reaction conditions is rarely reported in the literature.
1
the sulfoxidation of sulfides bearing varying functional groups -
from electron-withdrawing ester group to electron-donating
hydroxyl group - producing the desired sulfoxides in moderate
to excellent yields with good selectivity. For water insoluble
substrates, ethanol-water mixed solvent system was found to
be useful. The substrates bearing electron-donating functional
groups were found to give better yields than those with electron-
withdrawing groups, see Table 1. It was also noted that hybrid 1 is
useful for the selective oxidation of sulfide functional group in
substrates bearing additional functional groups such as thiophene,
−CH2OH, −NH2 etc. Oxidation of such additional functional groups
along with sulfide moiety was reported earlier in similar catalytic
sulfoxidation reactions.15 However, in the present case, we did not
observe such additional oxidations, see Table 1, entry numbers 15
and 17.
A possible mechanism for the oxidation of sulfides using hybrid 1
as catalyst is given in ESI† Figure S5, which is in accordance with
similar reports.15(a) The interaction of H2O2 with POM cluster
generates an electrophilic intermediate which leads to an
electrophilic attack on the S atom of the sulfide generating the
corresponding sulfoxide, ESI† Figure S5. However, time dependant
product analyses suggest the formation of sulfone with time
indicating the participation of sulfoxide formed in the reaction as a
substrate. The mechanism for the oxidation of sulfoxide to sulfone
Catalytic studies
POMs and their derivatives are known to act as oxidation
catalysts while a few POM hybrids have also been reported as
sulfoxidation catalysts.15 As hybrid
1 is soluble in water, we
decided to explore its homogeneous catalytic activity towards
the selective oxidation of various sulfides in water solvent. The
catalytic experimental conditions were optimized using the
oxidation of 4-(methylthio)phenol by H2O2 as a model reaction,
see Scheme 1. Various optimization studies showed that 0.012
mol % of catalyst at 85 oC reaction temperature was enough to
afford the sulfoxide products in very good yields and selectivity.
In absence of the oxidizing agent H2O2, there was no reaction
whereas the absence of catalyst led to poor yields (29%). These
results showed that both H2O2 and catalyst are indispensable
for the optimum yield of the products. The catalytic capabilities
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involves the formation of a POM-sulfoxide intermediate through the
nucleophilic attack on W atom of the POM unit by the oxygen of the
sulfoxide followed by a nucleophilic attack by H2O2 on the sulfur
atom in POM-sulfoxide intermediate, see see ESI† Figure S5. The
selectivity and efficiency of such reactions are susceptible to
different reaction parameters.15 We believe that the optimized
reaction conditions employed in the current study promotes the
selective formation of sulfoxides over sulfones as presented in Table
1 and in ESI†, Table S4. Moreover, the incorporation of POM
cationic complex units which in turn are inter-connected
through H-bonding interactions leading to the formation of a
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supramolecualr 3-D framework structure. Hybrid
1 was found
to act as a selective oxidation catalyst for a variety of sulfides
including those having additional interfering functional groups
like thiophene, −CH2OH, −NH2 etc. under safe, mild and greener
reaction conditions.
Acknowledgements
Table 1. Selective oxidation of various sulfides under optimized reaction
conditions
C. P. P. thanks SERB, New Delhi for funds under the fast track scheme
(Grant No: SR/FT/CS-58/2011) and IIT Mandi for infrastructural
facilities (AMRC, IIT Mandi) as well as financial support through Seed
Grant.
Notes and references
1
(a) Polyoxometalate Chemistry from Topology via Self-Assembly to
Applications, ed. M. T. Pope and A. Müller, Kluwer Academic
Publishers, Dordrecht, Netherlands, 2001; (b) D.-L. Long, R.
Tsunashima and L. Cronin, Angew.Chem. Int. Ed., 2010, 49, 1736.
(a) B. S. Bassil, M. H. Dickman, I. Römer, B. von der Kammer and U.
Kortz, Angew. Chem. Int. Ed., 2007, 46, 6192; (b) T. Yamase, Chem.
Rev., 1998, 98, 307; (c) C. Ritchie, E. G. Moore, M. Speldrich, P.
Kögerler and C. Boskovic, Angew. Chem. Int. Ed., 2010, 49, 7702.
X. Ma, W. Yang, L. Chen and J. Zhao, CrystEngComm, 2015, 17, 8175.
(a) D.-Y. Du, J.-S. Qin, S.-L. Li, Z.-M. Su and Y.-Q. Lan, Chem. Soc. Rev.,
2014, 43, 4615; (b) M. P. Santoni, G. S. Hanan and B. Hasenknopf,
Coord. Chem. Rev., 2014, 281, 64; (c) X. Feng, Y. Q. Feng, J. J. Chen, S.
W. Ng, L.-Y. Wang and J. -Z. Guo, Dalton Trans., 2015, 44, 804.
X. –J. Feng, H. –Y. Han, Y. –H. Wang, L. –L. Li, Y. –G. Li and E. –B. Wang,
CrystEngComm, 2013, 15, 7267.
M. Góral, T. McCormac, E. Dempsey, D.-L. Long, L. Cronin and A. M.
Bond, Dalton Trans., 2009, 6727.
Y. –Q. Jiao, C. Qin, H. –Y. Zang, W. –C. Chen, C. –G. Wang, T. –T. Zheng,
K. –Z. Shao and Z. –M. Su, CrystEngComm, 2015, 17, 2176.
C.-J. Li and T.-H. Chan, Comprehensive Organic Reactions in Aqueous
Media, Wiley, New York, 2007.
(a) I. Fernández and N. Khiar, Chem. Rev., 2003, 103, 3651; (b) (c) J. E.
Backvall, Modern Oxidation Methods, 2nd ed. Wiley-VCH, Weinheim,
Germany, 2010.
2
3
4
5
6
7
8
9
units into porous framework structures are known to increase
their catalytic performances due to the improvement in their
specific surface area, stability under catalytic conditions etc.16
Probably, this could be one of the reasons behind the improved
10 (a) N. K. Jana and J. G. Verkade, Org. Lett., 2003,
5, 3787; (b) R. S.
Varma, R. K. Saini and H. M. Meshram, Tetrahedron Lett., 1997, 38
,
6525; (c) N. Fukuda and T. Ikemoto, J. Org. Chem., 2010, 75, 4629; (d)
B. Yu, A.-H. Liu, L.-N. He, B. Li, Z.-F. Diao and Y.-N. Li, Green Chem.,
2012, 34, 957.
catalytic performance of hybrid
POM building units. The catalytic recyclability of hybrid
tested using the oxidation of 4-(methylthio)phenol as a model
reaction. These studies suggested that hybrid could be reused
1
as compared to the individual
1
was
11 R. Noyori, M. Aoki and K. Sato, Chem. Commun., 2003, 1977.
12 E. G. Moore, J. Grilj, E. Vauthey and P. Ceroni, Dalton Trans., 2013, 42
,
2075.
1
13 T. L. Spek, Acta Cryst., 1990, A46, c34.
for at least three times without significant reduction in catalytic
activity, see ESI† Table S5, Figure S6.
14 (a) M. Mirzaei, H. Eshtiagh-Hosseini, N. Lotfian, A. Salimi, A. Bauzá, R.
V. Deun, R. Decadt, M. Barceló-Oliver and A. Frontera, Dalton Trans.,
2014, 43, 1906; (b) J. Lu, E. Shen, Y. Li, D. Xiao, E. Wang and L. Xu,
Cryst. Growth & Des., 2005, 5, 65.
15 (a) R. Frenzel, Á. G. Sathicq, M. N. Blanco, G. P. Romanelli and L. R.
Pizzio, J. Mol. Catal. A: Chem., 2015, 403, 27; (b) M. Amini, H.
Naslhajian, S. M. F. Farnia and M. Hołyńska Eur. J. Inorg. Chem., 2015,
3873; (c) E. Rafiee and F. Mirnezami, J. Mol. Liq., 2014, 199, 156; (d)
F. Jalilian, B. Yadollahi, M. R. Farsani, S. Tangestaninejad, H. A. Rudbari
and R. Habibi, Catal. Commun., 2015, 66, 107; (e) A. Rezaeifard, R.
Haddad, M. Jafarpour and M. Hakimi, ACS Sustainable Chem. Eng.,
Conclusions
Herein, we have reported a simple and flexible synthetic
procedure to develop
a new organic-inorganic hybrid
supramolecular 3-D framework RECP based on Yb & Na cationic
complex units of pyridine 2,6-dicarboxylic acid and plenary
Wells-Dawson type cluster anions [P2W18O62]6−. In the
framework structure, [P2W18O62]6− cluster building units are
connected together in a zig-zag 1-D chain fashion by bridging
2014, 2, 942.
16 X.-L. Hao, Y.-Y. Ma, H.-Y. Zang, Y.-H. Wang, Y.-G. Li and E.-B. Wang,
Chem. Eur. J., 2015, 21, 3778.
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Table of Content
A Yb containing polyoxometalate organic hybrid framework material has been achieved which catalyzes
the selective oxidation of a variety sulfides under green reaction conditions.