Novel polyoxometalate-phosphazene aggregates and their use as catalysts for biphasic oxidations with hydrogen peroxide

Article abstract of DOI:10.1039/c2cc36793a

Polyoxometalate-phosphazene salt aggregates comprising cyclophosphazene cations are highly efficient catalysts for environmentally benign biphasic oxidations with hydrogen peroxide. These catalysts self-assemble in situ simply by mixing commercial Keggin POMs and readily available phosphazenes.

Full text of DOI:10.1039/c2cc36793a

Chemical Communications
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Volume 49 | Number 4 | 14 January 2013 | Pages 317–416
ISSN 1359-7345
COMMUNICATION
Alexander Steiner, Ivan Kozhevnikov et al.
Novel polyoxometalate–phosphazene aggregates and their use as catalysts
for biphasic oxidations with hydrogen peroxide
1359-7345(2013)49:4;1-4

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Novel polyoxometalate–phosphazene aggregates and
their use as catalysts for biphasic oxidations with
hydrogen peroxide†
Cite this: Chem. Commun.,
2013, 49, 349
Received 18th September 2012,
Accepted 24th October 2012
Michael Craven, Rana Yahya, Elena Kozhevnikova, Ramamoorthy Boomishankar,
Craig M. Robertson, Alexander Steiner* and Ivan Kozhevnikov*
DOI: 10.1039/c2cc36793a
www.rsc.org/chemcomm
Polyoxometalate–phosphazene salt aggregates comprising cyclo- multiple hydrogen bonding sites, which makes them versatile
phosphazene cations are highly efficient catalysts for environmentally building blocks for supramolecular assemblies.7 To our knowledge,
benign biphasic oxidations with hydrogen peroxide. These catalysts cyclophosphazenes have not been applied as surfactants in catalysis.
self-assemble in situ simply by mixing commercial Keggin POMs and [(Me2N)6P3N3H]2[Mo6O19] is the only previously reported POM–
readily available phosphazenes.
cyclophosphazene aggregate.8 However, this complex contains a
relatively small dianion and lacks the array of multiple hydrogen
Nanosized metal–oxygen cluster anions – polyoxometalates (POMs) – bonding sites.
form a class of compounds unique in its structural variety and
functional versatility.1 These compounds, which consist of O-sharing
MOx polyhedra (most often M = MoVI, WVI and VV), have found
applications in various disciplines,2 of which catalysis is by far the
most important. This includes large-scale industrial processes such
as the oxidation of methacrolein to methacrylic acid, the hydration
of alkenes to alcohols and the synthesis of ethyl acetate by direct
addition of acetic acid to ethene.2e
Aggregates of RPN {R = isopropyl (iPr), isobutyl (iBu) and benzyl
In the search for highly active catalysts for environmentally (Bz)} and Keggin heteropoly acids H4SiW12O40 (H4SiW), H3PW12O40
benign biphasic oxidations with hydrogen peroxide we have (H3PW) and H3PMo12O40 (H3PMo) form readily upon mixing the
investigated aggregates comprising Keggin type polyanions two components in methanol. The heteropoly acids were used as
[XM12O40]mꢀ {X = PV (m = 3) and SiIV (m = 4)} and lipophilic crystalline hydrates roughly containing 20 H2O per Keggin unit.
cyclophosphazene cations [(RNH)6P3N3Hn]n+ (n = 1 or 2). Cyclic and They were combined with the phosphazenes in POM/RPN molar
polymeric phosphazenes are renowned for their chemical and ratios of 1 : 4 for H4SiW and 1 : 3 and 1 : 6 for H3PMo and H3PW.
thermal robustness. They have been used as high performance The salt aggregates crystallized from methanol upon slow evapora-
elastomers, fire retardants, polymeric electrolytes and also for tion of the solvent. Crystals suitable for X-ray structure determina-
biomedical applications.3 Cyclophospazenes with amino sub- tion were obtained from batches [H4SiW/iBuPN (1 : 4)], [H3PW/
stituents {(RNH)6P3N3, labelled from here RPN} are strong bases; iPrPN (1 : 3)], [H3PW/iBuPN (1 : 3)] and [H3PW/iPrPN (1 : 6)].
one or two ring N-centres are protonated in the presence of
The crystals obtained from [H4SiW/iBuPN (1 : 4)] yielded a
structure of composition [iBuPNH]4[SiW]ꢁ2CH3OH. Although
2+
Brønsted acids yielding mono and dications, RPNH+ and RPNH2
,
respectively.4 These compounds are easy to prepare on a large scale H-atoms could not be located from difference maps, the bond
in one-step reactions from commercially available hexachloro cyclo- length alternation within the P3N3 ring is a reliable indicator that
triphosphazene and a wide range of primary amines.5 Their phos- protonation at ring N sites has occurred. While the ring bonds of
phazene backbone is remarkably inert even towards concentrated neutral RPN are in the region of 1.60 Å,5 cations RPNH+ show
acids and bases at elevated temperatures.6 These compounds are substantial elongation of bonds around the protonated ring N site
highly soluble in non-polar solvents, but at the same time provide and shortening of bonds around the non-protonated sites.9 This
pattern is also observed in [iBuPNH]4[SiW]. Furthermore, the ring
N atom connected via the long ring bonds (averaging 1.66 Å) is also
Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK.
in hydrogen bonding distance to the polyanion. Fig. 1a shows how
E-mail: a.steiner@liv.ac.uk, kozhev@liverpool.ac.uk
four [iBuPNH]+ ions surround one [SiW]4ꢀ ion. Remarkably, this
† Electronic supplementary information (ESI) available. CCDC 892678–892680.
assembly features twenty NHꢁꢁꢁO interactions to which every
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc36793a
phosphazene ligand contributes one ring NH and four exocyclic
c
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of these crystals is [iBuPNH]3[PW]ꢁ3iBuPNꢁx solvent containing
both cationic and neutral phosphazene ligands.
The crystal structures of the POM–RPN aggregates show that
the phosphazene ligands are able to encapsulate the large
polyanions effectively via multiple H-bonding interactions.
They reveal polyprotic ligand behaviour that utilizes not only the
monocationic ligand [RPNH]+ but also the neutral and dicationic
systems. Furthermore, the POM–phosphazene aggregates exhibit
high thermal stability. Thermogravimetric analyses show decom-
position onsets ranging from 260 (PW-iPr) to 340 1C (SiW-iBu). The
UV-Vis and FTIR spectra of the aggregates correspond well to the
spectra of POM acid hydrates Hm[XM12O40]ꢁxH2O, which indicates
that the presence of the highly redox-stable phosphazenes has
little effect on the electronic structure of the POM clusters (see ESI†
for details).
Catalytic studies showed that POM–phosphazene aggregates
serve as highly efficient catalysts for biphasic oxidations with
hydrogen peroxide. We looked at two reactions of significant
practical importance – oxidative desulfurization10 and olefin
epoxidation.11 Both are catalyzed by POMs in homogeneous or
two-phase systems and are the subject of much current interest.
There is compelling evidence that Keggin polyanions are trans-
formed by excess H2O2 in solution to form peroxo polyoxometalate
species, e.g. {PO4[WO(O2)2]4}3ꢀ. These have been suggested to be the
active intermediates in oxidations with H2O2.11 Tungsten POMs are
usually more active than the corresponding molybdenum deriva-
tives. Biphasic oxidation in a water–organic system is particularly
attractive, as it can facilitate product separation. But it requires
an efficient phase-transfer agent to effectively move the peroxo
polyanions into the organic phase. Commonly, POM aggregates
containing quaternary ammonium cations with C8–C18 alkyl
groups are used; these, however, are often cumbersome to
prepare. We therefore anticipated that the POM–phosphazene
aggregates would function as effective and easily applicable
peroxide carriers between the phases.
Oxidative desulfurization catalyzed by tungsten POMs using
30% H2O2 as a ‘‘green’’ oxidant has been developed as a
promising method for deep desulfurization of transportation
fuel.10c The oxidation of dibenzothiophene (DBT) (eqn (1)) is
typically employed as a model reaction for catalyst testing. We
found that POM–RPN aggregates are highly active catalysts for
DBT oxidation with H2O2 in a biphasic system, e.g. toluene–
water (Table 1), yielding DBT sulfone as the sole product. The
POM–RPN aggregates can be introduced in the reaction system
as pre-synthesized compounds or, if preferred, POM and RPN
components can simply be added separately to form the active
catalyst in situ. Catalyst activity was found to increase with the
size of the R group in line with increasing phase-transfer
efficiency of RPN: iPr o iBu o Bz (entries 1–3). The activity
also increased with the [BzPN]/[POM] molar ratio, levelling off
at a ratio of 6 : 1 (entries 4–6). Unexpectedly, PMo exhibited a
higher activity than PW (entries 5 and 7), whereas SiW showed no
activity at all. The latter is in agreement with the well-known stability
of SiW to degradation in solution2e and its resistance to form peroxo
species. It is important to note that practically no decomposition of
H2O2 to molecular oxygen took place in this system, giving >99%
efficiency for H2O2 utilization. The oxidation of DBT in the presence
Fig. 1 Crystal structure of [iBuPNH]4[SiW]ꢁ2CH3OH: (a) [iBuPNH]4[SiW] unit, (b)
crystal packing. WOx polyhedra, green; SiO4, turquoise; P, purple; N, blue; H, red;
alkyl groups, grey; hydrogen bonds are drawn as dotted lines.
NH sites. Further NHꢁ ꢁ ꢁO contacts involving the two remaining
exocyclic NH sites result in a 3D networked structure (Fig. 1b).
It is interesting to note that 1 : 3 mixtures of triprotic H3PW,
which is a stronger Brønsted acid than H4SiW,2e and RPN
yielded crystals which contained only two phosphazene ions
for every polyoxometalate ion. However, close examination of
the crystal structures revealed that the phosphazenes exist as
mono- and dications, [RPNH]+ and [RPNH2]2+, respectively,
while the polyoxometalate ion carries the expected ꢀ3 charge.
The crystals exhibit compositions [iPrPNH][iPrPNH2][PW]ꢁ
6CH3OH and [iBuPNH][iBuPNH2][PW]ꢁ3H2OꢁCH3OH. In both cases
the extra proton is disordered between two symmetrically equiva-
lent cations across a hydrogen bonded chain of solvent molecules.
Fig. 2 illustrates the disorder of [iPrPNH][iPrPNH2][PW]ꢁ6CH3OH
where the two phosphazene rings are connected via H-bonds across
two methanol molecules (Fig. 2a). This is also indicated by alter-
nations in the bond lengths of the P3N3 rings, which vary between
long (around the fully protonated site N1), short (around the non-
protonated site N3) and medium (around the partially protonated
site N2, which binds the disordered proton).
Crystals were also obtained from 1 : 6 mixtures of H3PW and
iBuPN. However, their poor quality only permitted the location of W
and P atoms, which confirmed that the 1 : 6 ratio is retained in the
crystal structure. The six phosphazene ligands surround the poly-
oxometalate ion in an octahedral fashion (Fig. 2b, see also ESI†).
Considering the presence of [PW]3ꢀ ions the most likely composition
Fig. 2 (a) Crystal structure of [iPrPNH][iPrPNH2][PW]ꢁ6CH3OH highlighting the
disorder of the proton (see Fig. 1 for colour coding, O1S and O1Sa are part of
methanol molecules). Note that there is a centre of inversion; therefore the two
phosphazenes are symmetrically equivalent. As a result, the proton is bonded to
either N2 or N2a. Changing between the two states involves shifting H-atoms
along the H-bonding chain N2ꢁ ꢁ ꢁO1Sꢁ ꢁ ꢁO1Saꢁ ꢁ ꢁN2a. Methanol H-atoms were not
considered in the refinement, but were added here to illustrate the connectivity
of the H-bonding chain. (b) Octahedral assembly of [iBuPN]3[iBuPNH]3[PW]
modelled at the positions of W and P atoms.
c
350 Chem. Commun., 2013, 49, 349--351
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Table 1 Oxidation of DBT by H2O2 in toluene–H2O two-phase system in the
presence of POM–phosphazene catalystsa
Table 2 Biphasic epoxidation of cyclooctene by 10% H2O2 catalyzed
by PW–BzPNa
Entry POM RPN
Temp. [1C] Conversion [%] H2O2 efficiency [%] Temperature
[1C]
[BzPN]/[PW]
[mol/mol]
Time
[h]
Conversion
[%]
Yieldb
[%]
1
2
3
4
5
6
7
PWb iPrPN 40
38
89
99
99
99
99
99
99
99
PWb iBuPN 40
PWb BzPN 40
PWc BzPN 60
PW BzPN 60
PWd BzPN 60
PMo BzPN 60
25
50
50
6
6
4
3.0
0.5
1.5
>99
>99
>99
>99
>99
>99
94
81
98
100
100
a
Toluene (10 mL), cyclooctene (7.0 mmol, 1.0 mL), aqueous 10% H2O2
b
(1 mmol, 0.3 mL), H3PW12O40ꢁ20H2O (5.53 mmol, 0.018 g). Yield based
on the initial amount of H2O2.
a
Toluene (10 mL), DBT (0.50 mmol, 1 wt%), aqueous 30% H2O2; molar
ratios: [DBT]/[POM] = 90 : 1, [DBT]/[H2O2] = 1 : 3, [RPN]/[POM] = 4 : 1;
b
30 min time. DBT sulfone was the only reaction product. [H2O2]/
Besides, our reaction system is safer since it only requires dilute
10% rather than 30% H2O2.
c
d
[POM] = 20 : 1. [BzPN]/[POM] = 2 : 1. [BzPN]/[POM] = 6 : 1.
of PW–BzPN was found to be first order in DBT (Fig. S7, ESI†).
The phosphazene was stable under the reaction conditions,
which was confirmed by monitoring the reactions with 31P
NMR. In the presence of PW–BzPN (1 : 6) and PMo–BzPN (1 : 4)
the oxidation of DBT proceeded with 100% conversion at 60 1C
(Table 1). Moreover, the catalyst could be recovered and reused
without loss of activity (see ESI†). This compares well with the
best results reported so far. Jiang et al.10b used amphiphilic
decatungstate [(CH3)3NC16H33]4W10O32 as the catalyst under
similar conditions and observed similar DBT conversions
(99.6%), but without giving the H2O2 efficiency. Similar results
ð2Þ
In conclusion, we have synthesized and structurally char-
acterised novel POM–phosphazene salt aggregates and demon-
strated their high efficiency as amphiphilic catalysts for
environmentally benign oxidations with H2O2 in biphasic sys-
tems. These catalysts self-assemble in situ simply by mixing
commercial Keggin POMs and readily available phosphazenes.
The phosphazene ligands provide large arrays of hydrogen
bonding sites enabling the effective encapsulation of poly-
anions. Further studies directed to analogous reactions and
their mechanistic understanding are underway.
have also been reported for the [(C18H37)2N(CH3)2]3[PW12O40
]
catalyst.10c These catalysts, however, have to be pre-synthesized
via a cumbersome procedure,10b which is not required for the
POM–RPN system. The other advantage of our system is that
molybdenum based systems can be used instead of tungsten,
which reduces the catalyst weight by 25–30%.
We thank EPSRC for financial support.
Notes and references
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(e) I. V. Kozhevnikov, Catalysis by Polyoxometalates, Wiley, Chichester,
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3 Polyphosphazenes: A World Insight, ed. M. Gleria and R. De Jaeger,
Nova Science Publishers, New York, 2004.
ð1Þ
We have also examined the epoxidation of cyclooctene
(eqn (2)), the standard screening test for epoxidation catalysts
(Table 2). The reaction was carried out in a biphasic toluene–
water system at 25–50 1C using 10% H2O2 as the oxidant and a
mixture of PW and BzPN (1 : 4–1 : 6) as the catalyst. This
catalyst was very efficient even at room temperature, yielding
>99% epoxycyclooctane with >99% efficiency of H2O2 utiliza-
tion in 3 h reaction time. Again, the decomposition of H2O2 was
negligible, and the catalyst could be recovered and reused. Our
catalyst can be compared favourably with one of the best POM
epoxidation catalysts – the lacunary silicotungstate (nBu4N)4
[g-SiW10O34(H2O)2] reported by Kamata et al.11c This catalyst
epoxidizes cyclooctene with 30% H2O2 in homogeneous MeCN
4 (a) H. R. Allcock, Chem. Rev., 1972, 72, 315; (b) pKa values of alkyl
derivatives of RPN range from 7.9 to 8.4 for [RPNH+/RPN] and ꢀ1.3
to ꢀ1.8 for [RPNH22+/RPNH+], see: D. Feakins, W. A. Last and
R. A. Shaw, J. Chem. Soc., 1964, 4464.
5 J. F. Bickley, R. Bonar-Law, G. T. Lawson, P. I. Richards, F. Rivals,
A. Steiner and S. Zacchini, Dalton Trans., 2003, 1235.
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21, 515; (b) J. Ledger, R. Boomishankar and A. Steiner, Inorg. Chem.,
2010, 49, 3896.
7 A. Steiner, in Polyphosphazenes for Biomedical Applications, ed.
A. K. Adrianov, Wiley, New Jersey, 2009, pp. 411–453.
8 H. R. Allcock, E. C. Bissell and E. T. Shawl, Inorg. Chem., 1973, 12, 2963.
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10 (a) F. M. Collins, A. R. Lucy and C. Sharp, J. Mol. Catal. A, 1997, 117, 397;
(b) X. Jiang, H. Li, W. Zhu, L. He, H. Shu and J. Lu, Fuel, 2009, 88, 431;
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48, 3831; (b) Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida
and M. Ogawa, J. Org. Chem., 1988, 53, 3587; (c) K. Kamata,
K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi and N. Mizuno,
giving also 99% epoxide yield with >99% H2O2 efficiency. But
in contrast to the straightforward preparation of PW–BzPN, the
synthesis of lacunary silicotungstate is an elaborate procedure.
Science, 2003, 300, 964.
c
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Chem. Commun., 2013, 49, 349--351 351