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10.1002/chem.202001468
Chemistry - A European Journal
FULL PAPER
36-nuclearity organophosphonate-functionalized
polyoxomolybdates: Synthesis, Characterization and Selective
catalytic oxidation of sulfides
Qiaofei Xu,[a] Xinmiao Liang,[b] Baijie Xu,[a] Jiawei Wang,[a] Peipei He,[a] Pengtao Ma,[a] Jiwen Feng,[b]
Jingping Wang*[a] and Jingyang Niu*[a]
Abstract:
The
crown-shaped
36-molybdate
cluster
with fascinating structures have been reported in this way, but to
some extent, structural diversity is still limited, primarily due to
the lack of controllable structural units.[9]
organophosphonate-functionalized polyoxomolybdates with the
highest nuclearity in organophosphonate-based polyoxometalates
chemistry, (NH4)19Na7H10[Cu(H2O)TeMo6O21{N(CH2PO3)3}]6·31H2O,
has been reported for the first time. The synthesized 36-molybdate
cluster was characterized by routine techniques and employed as a
heterogeneous catalyst for selective oxidation of sulfides with
impressive catalytic and selective performances after heat treatment.
High efficiency (TON=15333) was achieved in the selective oxidation
of sulfides to sulfoxides, caused by the synergic effect between
copper and polyoxomolybdates and the generation of the cuprous
species during the heat treatment.
In
the
year
of
2013,
our
group
reported
Na5[H7{N(CH2PO3)3}Mo6O16(OH)(H2O)4]4·18H2O, the highest
nuclearity of metal atoms in the field of organophosphonate-
based polyoxometalates at that time,[10] prepared from amino
trimethylene phosphonic acid (ATMP), multidentate and flexible
triphosphates ligand. It opened the pathways that ATMP could
be
a suitable ligand to construct novel polyoxometalate
structures. Compared to ‘‘Bottom-up’’ approach, ‘‘Top-down’’
method emerged quickly in the synthesis of POMs: larger oxo-
clusters could disassemble into smaller oligomers with highly
reactive in solution, which can be reassembled with the
compatible ligands or heteroatoms to construct original
architectures.
Introduction
Selective oxidation of sulfides to sulfoxides is a challenging
reaction not only for the organic synthesis but also for the
decontamination of chemical warfare agents,[11] where POMs
have shown superior catalytic performance.[12] Here, the
integration of ‘‘Top-down’’ strategy (Scheme 1) and
triphosphates ligand were applied to explore the possibility of
making a breakthrough, where the disassembly of ammonium
heptamolybdate and reassembly of dissociated products, ATMP,
heteroatom and copper. As we report in this paper, the original
Polyoxometalates (POMs), have attracted enormous interests
for their highly structural diversity and cumulative applications in
material science,[1] optics,[2] and also tremendous impact in
catalysis.[3] Over the past decades, organophosphonate-
functionalized polyoxometalates (POMs) as a subclass of POMs
have received much attention. Within this field, considerable
research efforts on organophosphonate-functionalized POMs
including
polyoxotungstates,
polyoxovanadates
and
polyoxomolybdates have been presented by Pope,[4] Zubieta,[5]
Mialane,[6] Kortz[7] and et. al.,[8] which lead to a significant
development in the class of organophosphonates-based POMs.
However, much more unknowns deserve explorations: a)
diphosphonates and monophosphonates have occupied the
dominant position in this field; b) traditional synthesis of
organophosphonates-based POMs is based on ‘‘Bottom-up’’
condensation reaction from the monomeric metal–oxo units to
ever-larger molecular oxo-clusters. A large number of POMs
crown-shaped
36-molybdate
cluster
(NH4)19Na7H10[Cu(H2O)TeMo6O21{N(CH2PO3)3}]6·31H2O (1) was
synthesized, which has exceeded previous 24-molybdate cluster
and represents the largest organophosphonate-based POM so
far.[13] Interestingly, POM
1
exhibited decent catalytic
performance in selective oxidation of sulfides to sulfoxides under
mild conditions. Notably, boosted catalytic performance
(TON=15333) and good recyclability of 1 after heat treatment at
100 oC (1-HT) was observed, which may be attributed to the
generation of Cu(Ⅰ) in the polyanions under the heat treatment.
It is important to highlight that the catalytic selectivity to
sulfoxides of 1-HT are well reserved even excessive oxidant
employed, which is rare to be found in the oxidation reactions of
sulfides. By comparison, good catalytic activity but low selectivity
towards the generation of sulfoxides were observed when
ammonium heptamolybdate was used as the catalyst. Obviously,
copper in the POM playd an inportant role in the catalytic
efficiency, especially the reaction selectivity in this oxidative
process.
[a]
Q. Xu, B. Xu, J. Wang, P. He, Dr. P. Ma, Pro. J. Niu, Pro. J. Wang
Henan Key Laboratory of Polyoxometalate Chemistry
Institution Institute of Molecular and Crystal Engineering, College
of Chemistry and Chemical Engineering, Henan University Kaifeng,
Henan 475004 (P.R. China)
[b]
X. Liang, Pro. J. Feng
State Key Laboratory of Magnetic Resonance and Atomic and
Molecular Physics, National Center for Magnetic Resonance in
Wuhan
Institute of Physics and Mathematics, Chinese Academy of
Sciences, Wuhan 430071, P. R. China.
Supporting information for this article is given via a link at the end
of the document.
Results and Discussion
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