Asymmetric Hybrid Polyoxometalates: A Platform for Multifunctional Redox-Active Nanomaterials

Article abstract of DOI:10.1002/anie.201912046

Access to asymmetrically functionalized polyoxometalates is a grand challenge as it could lead to new molecular nanomaterials with multiple or modular functionality. Now, a simple one-pot synthetic approach to the isolation of an asymmetrically functionalized organic–inorganic hybrid Wells–Dawson polyoxometalate in good yield is presented. The cluster bears two organophosphonate moieties with contrasting physical properties: a chelating metal-binding group, and a long aliphatic chain that facilitates solvent-dependent self-assembly into soft nanostructures. The orthogonal properties of the modular system are effectively demonstrated by controlled assembly of POM-based redox-active nanoparticles. This simple, high-yielding synthetic method is a promising new approach to the preparation of multi-functional hybrid metal oxide clusters, supermolecular systems, and soft-nanomaterials.

Full text of DOI:10.1002/anie.201912046

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Accepted Article
Title: Asymmetric hybrid polyoxometalates: a new platform for
multifunctional redox-active nanomaterials
Authors: Elizabeth Hampson, Jamie Cameron, Sharad Amin,
Joungman Kyo, Julie Watts, Hiroki Oshio, and Graham N.
Newton
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912046
Angew. Chem. 10.1002/ange.201912046
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10.1002/anie.201912046
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Asymmetric hybrid polyoxometalates: a new platform for
multifunctional redox-active nanomaterials
Elizabeth Hampson,a Jamie M. Cameron,a Sharad Amin,a Joungman Kyo,b Julie A. Watts,c Hiroki
Oshiob,d and Graham N. Newton*a
Abstract: Access to asymmetrically functionalised polyoxometalates
is a grand challenge as it could lead to new molecular nanomaterials
with multiple or modular functionality. Here, a simple “one-pot”
synthetic approach to the isolation of an asymmetrically functionalised
organic-inorganic hybrid Wells-Dawson polyoxometalate in good yield
is presented. The cluster bears two organophosphonate moieties with
contrasting physical properties: a chelating metal-binding group, and
a long aliphatic chain unit that facilitates solvent-dependent self-
assembly into soft nanostructures. The orthogonal properties of the
modular system are effectively demonstrated by controlled assembly
of POM-based redox-active nanoparticles. We propose that this
simple, high-yielding synthetic method is a promising new approach
to the preparation of multi-functional hybrid metal-oxide clusters,
supermolecules and soft-nanomaterials.
in which two identical organic groups are tethered to the inorganic
core. The controlled addition of two different organic groups, to
create an “asymmetric” hybrid system, would open up far greater
opportunities for fine control over the function and
physicochemical properties of such systems, allowing them to be
tailored towards highly specific or advanced applications. A few
key examples of asymmetrically functionalised POM hybrids have
been reported but remain rare due to the significant challenge
posed by their synthesis and purification.18 For instance, Cronin
et al. recently demonstrated the use of HPLC to isolate a
“precursor” asymmetric Mn-Anderson hybrid in which one ligand
terminates in a protecting group.19 Notably, however, this requires
demanding, expensive and time-consuming methods (such as
chromatography or slow fractional crystallisation) followed by
multiple synthetic steps to obtain the asymmetric product.20-22
Reported examples of asymmetric hybrid POMs that exhibit
complex functionalities are even fewer, 21,23 and – with the notable
exception of a series of elegant (though hydrolytically unstable)
organotin hybrid-POMs reported by Lacôte, Thorimbert and
Hasenknopf24 – are limited to the Mn-Anderson cluster type
(which displays limited photo- and redox-activity compared to
many POMs). Conversely, advanced functionalisation strategies
targeting clusters such as the Wells-Dawson anion, with its rich
redox activity and highly tunable organic hybrid structures,25-27 will
provide new pathways for the isolation of advanced, multi-
functional soft-nanostructures28,29 and metal-directed extended
assemblies.30,31 Ultimately, this could allow for the design of
materials with switchable morphologies and functions in a range
of different media, leading to new applications in catalysis, drug-
delivery systems, or encapsulated nano-reactors.32,33
Polyoxometalates (POMs) are anionic nanoscale metal-oxide
clusters comprising early transition metals in their highest
oxidation states. They continue to attract significant interest due
to their vast structural diversity, excellent stability and versatile
chemical properties, making them ideal building blocks for
functional materials.1-3 Their capacity to undergo reversible multi-
electron redox processes is of particular interest, and renders
them promising photo/electro-catalysts in a variety of systems.4,5
The covalent integration of organic moieties and POMs is an
exciting and rapidly expanding area within the field.6 Through the
design of the individual components the intrinsic properties of the
resulting hybrid systems can be fine-tuned. This allows hybrid
POMs to exhibit diverse functionality7-9 and facilitates their
assembly into a range of supramolecular nanostructures for use
in devices or catalytic systems.10-17 The development of simple
synthetic procedures that maximise control in the tailoring of multi-
functionality in hybrid systems is an ongoing challenge. Almost all
reported bi-functionalised hybrid POMs are “symmetric” systems,
a)[*] E. Hampson, Dr. J. M. Cameron, S. Amin, and Dr. G. N. Newton
GSK Carbon Neutral Laboratory for Sustainable Chemistry
University of Nottingham, Nottingham, NG7 2GA, UK
E-mail: graham.newton@nottingham.ac.uk
b)
c)
d)
Prof. H. Oshio and J. Kyo
Graduate School of Pure and Applied Sciences, University of
Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571, Japan
Dr. J. A. Watts
Nanoscale and Microscale Research Centre, University of
Nottingham, Nottingham, NG7 2RD, UK
Prof. H. Oshio
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, 2 Linggong Rd., Dalian, 116024 P.R. China
For detailed experimental details including all synthetic methods,
NMR and mass spectroscopic analysis, dynamic light scattering and
cyclic voltammetry experiments, please see Supporting Information.
Scheme 1. An asymmetric hybrid POM bearing two different organic moieties
(A and B) and its tuneable solvent and cation-dependent self-assembly.
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Figure 1. Synthesis and purification of the asymmetric hybrid-POM, 1, and the symmetric by-products, 2 and 3, illustrating the purification process. 31P NMR of the
reaction mixture in DMSO-d6 is shown on the right, taken after each purification step (top: crude; middle 1 + 2; bottom: 1 only). Chemical shifts corresponding to
each product are colour coded as shown (note that for simplicity, only the positive chemical shift region for the organophosphonate 31P nuclei is presented here).
Colour code: blue polyhedra = {WO6}, pink polyhedra = {PO4}, red spheres = oxygen. Cations and solvent molecules are omitted for clarity.
Here, we present a simple, inexpensive and high-yielding
synthetic approach to isolate the first reported example of an
asymmetric bifunctional Wells-Dawson hybrid POM, obtained by
exploiting the different solubilities of three hybrid POMs produced
in the crude mixture. The cluster bears a chelating metal-binding
group, and a long aliphatic chain unit that allows solvent-
dependent self-assembly into soft nanostructures (Scheme 1).
The simple nature of this approach (requiring no specialist lab
equipment) will allow its application in the development of a whole
new class of asymmetric hybrid POMs.
of 1 the signals for the covalently-bound ligands, TPY and C18,
appear at 13.31 and 16.77 ppm respectively, while the
corresponding peaks appear at 14.10 and 15.96 ppm in the
spectra of the respective symmetric structures, 2 and 3. Isolation
of the desired product, 1, was achieved as follows. A large excess
of ether was added to the reaction mixture upon which the solution
turned cloudy. On centrifugation, a mixture of orange-brown and
green solid was collected and the yellow filtrate decanted.
Removing the solvent from the filtrate leaves an orange-brown
crystalline solid that by 31P NMR is identified as 3. The remaining
crude solid, a mixture of 2 and 1, was re-dissolved in acetonitrile,
in which 2 is sparingly soluble and can be separated by
centrifugation. Finally, an excess of ether was used to re-
precipitate 1 while leaving any traces of 3 in solution. The process
of dissolution in minimum acetonitrile to remove any insoluble 2,
followed by re-precipitating 1 from the filtrate with ether, can be
repeated as necessary until no signals corresponding to 2 or 3
are visible by 31P or 1H NMR (Figure 1). 1 can be reliably isolated
in good yield (typically 10-25 total yield %) and excellent purity.
Functionalisation of polyoxotungstates with aryl-phosphonate
groups enhances their photochemical properties and can be
employed to direct their self-assembly into micellar
superstructures.29,34-36 Here, we aimed to introduce two distinct,
orthogonal functionalities via asymmetric functionalisation and
designed two suitable aryl phosphonates accordingly:
(PO3C21H16N3) (TPY),
a
terpyridine-based ligand, and
(PO4C24H43) (C18), an aliphatic chain group (see SI). Hybridisation
was then conducted based on the previously reported method.34
In a “one-pot” acid-catalysed condensation reaction, one molar
equivalent each of TPY and C18, were reacted with one molar
equivalent of the potassium salt of the mono-lacunary Dawson-
type anion, [P2W17O61]10- (see SI). A 1:1 solvent mixture of N,N′-
dimethylformamide (DMF) and acetonitrile was used in order to
1
The composition and purity of 1 was confirmed by H NMR,
31P NMR, ESI-MS, elemental (CHN) analysis, thermogravimetric
(TGA) analysis and FT-IR (see SI). 1H NMR confirms the
presence of both the aliphatic carbon chain and the aromatic TPY
group in a 1:1 stoichiometric ratio. Interestingly, slight shielding of
the TPY phosphorus and deshielding of the C18 phosphorus
relative to their 31P chemical shifts in the respective symmetric
hybrids suggests a degree of electronic communication via the
POM core of the asymmetric structure.37
adequately solubilise both ligands. After reaction completion, 31
P
NMR spectroscopy indicated the presence of three species
(Figure 1, S1), later fully characterised as a racemic mixture of the
two
enantiomers
of
the
asymmetric
hybrid,
K4(C2H8N)2[P2W17O57(PO3C21H14N3)(PO4C24H41)] (1) and the
symmetric hybrids, (C2H8N)6[P2W17O57(PO3C21H14N3)2] (2), and
K2(C2H8N)4[P2W17O57(PO4C24H41)2] (3). In the 31P NMR spectrum
Following successful isolation of 1, we explored the unique
multifunctionality of the hybrid POM system derived from its
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Figure 2. a) Cryo-TEM imaging of micellar assemblies of 1 formed in 1.4 mM
water-acetonitrile (9:1 v/v) solution; b) expanded view of a single micelle; c)
proposed structure of the micellar assemblies of compound 1. Colour code: blue
spheres = {P2W17} units, grey rods = C18 units, green rings = TPY units.
Figure 3. a) Molecular structure of the dimeric hybrid-POM complex Fe-1; b)
Cryo-TEM imaging of assemblies of Fe-1 formed in 1.4 mM water-acetonitrile
(9:1 v/v) solution; c) expanded view of a single dimeric molecule.
asymmetric structure. First, we examined the self-assembly of 1
enabled by the aliphatic chain ligand, C18, which imparts
amphiphilic character to the hybrid POM (the POM core and TPY
group acting as a polar head-group). 1 was first dissolved in
acetonitrile and then 9 eq. water added to facilitate spontaneous
self-assembly. Dynamic Light Scattering (DLS) experiments on a
1.4 mM solution confirmed the formation of nanoscale assemblies
electrochemical devices. This behaviour also corresponds closely
to that seen for our previously reported symmetric surfactant
hybrid system,35 and for the symmetric product 3 (Figure S21).
Post-functionalisation of 1 was investigated by reacting the
hybrid-POM with 0.5 molar equivalents of FeCl2. After stirring the
reactants at RT overnight in acetonitrile, the product,
of low dispersity with
a hydrodynamic diameter (Dh) of
K7(C2H8N)3{Fe[P2W17O57(PO3C21H14N3)(PO4C24H41)]2}
Fe-1
approximately 6 nm (Figure S18). In agreement with this, Cryo-
TEM analysis of a solution of 1 in water-acetonitrile (9:1 v/v)
deposited and frozen on a graphene-oxide and holey carbon-
supported Cu grid showed spherical structures with diameters of
(Figure 3a), was collected by removal of the solvent. Mass
spectrometry confirmed the formation of a POM-Fe-POM dimeric
structure, and all aromatic peaks in 1H NMR were shifted,
indicating the presence of a single new species. Downfield peak
shifts of both organophosphoryl signals in the 31P NMR spectrum
of Fe-1 demonstrate a degree of electronic conjugation across the
whole asymmetric system. The dark purple colour of the product
is characteristic of a low spin d6 Fe2+ centre, and UV/Vis
absorption spectroscopy confirms the presence of a peak at 576
nm corresponding to a Fe2+-TPY metal-to-ligand-charge-transfer
(MLCT) band (Figure S13). CV studies in DMF reveal a reversible
Fe2+/3+ redox couple (E1/2 = 0.543V vs Fc/Fc+) alongside the 4
quasi-reversible POM-centred processes (Figure S16).
1
4-7 nm (Figure 2). Corresponding H and 31P NMR experiments
showed broadening/disappearance of peaks in a D2O-CD3CN
(9:1 v/v) mixture, indicative of reduced T2 transverse relaxation
time arising from the slow tumbling of nanostructures (Fig. S21).
Cyclic voltammetry (CV) of 1 in a 0.1 M DMF solution of
TBA.PF6 revealed four distinct quasi-reversible redox processes
in the potential range of –0.5 to –2.25 V vs. Fc/Fc+ (Figure S10),
all of which are positively shifted relative to the “parent” Wells-
Dawson anion, [P2W18O62]6-, as is typical of organophosphonate
hybrid POMs.37 Furthermore, E1/2 potentials of 1 were found to be
intermediate to those of the respective symmetric hybrids, 2 and
3, with redox processes in 2 more positively shifted by an average
of 5 mV and those of 3 more negative by an average of 20 mV,
relative to the equivalent redox couples of 1 (see SI for details).
The combination of metal-coordination and self-assembly of 1
was then investigated by dissolving Fe-1 in a water-acetonitrile
(9:1 v/v) mixture at RT. DLS studies indicated the formation of
nanoscale aggregates (Figure S20) with Dh(avg) = 4 nm, and NMR
studies again showed the disappearance of the peaks in D2O-
CD3CN (9:1 v/v) solution (Figure S22). CryoTEM analysis
confirmed the presence of the assemblies (Figure 3b), but unlike
the corresponding studies on 1, the aggregates were of variable
shape and were not hollow. This is reasonable, given that Fe-1
lacks the head/tail structure of 1, and is therefore unlikely to form
typical micellar assemblies. Indeed, the assemblies appear to
resemble those recently reported by Izzet and co-workers, who
demonstrated the organic solvent and transition metal-dependant
assembly of symmetric terpyridine-functionalised POMs into a
range of nanoscale architectures.38,39 It is also apparent that in
our experiment many of the POMs (dark areas on the micrograph)
appear in ‘pairs’ (Figure 3c), consistent with the expected
molecular structure of Fe-1, in which the POM lobes should be
Recently, we showed that self-assembled micellar
nanostructures formed from hybrid-POM species exhibit different
redox properties to their constituent molecular species.29,35 CV
experiments were conducted on
1 dissolved in a 0.1 M
H2SO4:CH3CN (9:1, v/v) mixture, allowing for spontaneous
assembly into micellar species as described above. Here, just two
redox processes were observed between –0.5 and 0.5 V (vs.
Ag/AgCl), in contrast to the three seen in pure DMF (Figure S15),
as appears typical of micellar species of this type.29,35 Notably,
characteristic molecular redox behaviour can be rapidly
recovered by addition of DMF to the electrolyte solution, indicating
the dynamic nature of this system and offering interesting
potential for the development of responsive or switchable
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Wells-Dawson hybrid POM using a simple and inexpensive
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Acknowledgements
EH thanks the Low Dimensional Materials and Interfaces Doctoral
Training Programme at the University of Nottingham for support.
SA thanks the EPSRC for funding through the Centre for Doctoral
Training in Sustainable Chemistry (EP/L015633/1). JMC and GN
thank the Leverhulme Trust (RPG-2016-442) and the University
of Nottingham Propulsion Futures Beacon of Excellence. The
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Keywords: • Hybrid Materials • Polyoxometalates •
Supramolecular Chemistry • Redox Properties • Clusters
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COMMUNICATION
Elizabeth Hampson, Jamie M.
Cameron, Sharad Amin,
Joungman Kyo, Julie A. Watts,
Hiroki Oshio, and Graham N.
Newton*
Page No. – Page No.
Asymmetric hybrid
polyoxometalates: a new
platform for multifunctional
redox-active nanomaterials
A “one-pot” synthetic approach to the isolation of an asymmetrically functionalised
organic-inorganic hybrid Wells-Dawson polyoxometalate is presented. The cluster
bears two organophosphonate moieties with contrasting physical properties: a
chelating metal-binding group, and a long aliphatic chain unit that facilitates solvent-
dependent self-assembly into soft redox-active nanostructures.
This article is protected by copyright. All rights reserved.