452
14
Oxomolybdates: From Structures to Functions
in a New Era of Nanochemistry
Achim Müller and Soumyajit Roy
Abstract
From a unique library containing the molybdenum-oxide based building blocks/
fragments under reducing conditions in aqueous solution a huge variety of nanoobjects, allowing specific reactions at well-defined positions, can be generated. This
enables us to perform a new type of nanochemistry. Examples include the wellknown molecular big-wheels of the type {Mo154 } and {Mo176 } and the spherical
big-balls of the type {Mo132 } and their derivatives. These are considered here
in connection with the by far the largest structurally well-characterized cluster
{Mo368 }, which contains besides the 368 molybdenum atoms nearly 2000 nonhydrogen atoms, shows positive and negative curvatures, and has the size of
hemoglobin and the shape of a lemon. A most recent achievement regarding a
unique ‘nanosponge’-behaviour of the spherical clusters showing responsive sensing opens new eras of nanochemistry: nano-spherical-surface, nano-porous-cluster, and
nano-super-supramolecular chemistry. Historically speaking, after solving the mystery
of molybdenum blue solutions, from which the wheel type clusters can be isolated,
it became evident that the acquired knowledge could be useful for advancing the
frontiers of materials science. Some general aspects of the corresponding chemistry are reviewed here.
14.1
Introduction: Similarities between Nanotechnology in Nature and Chemistry?
Der chemische Prozeß ist das ‘Höchste, wozu die unorganische Natur gelangen kann’.
G. W. F. Hegel [1]
Nature plays with patterns. Patterning reaches a pinnacle of elegance and intricacy
in nanoscopic molecular systems, the most relevant being those found in the biosphere: the variety of specific processes of the living cell are directed by an overwhelming number of nanoscaled structures, for example by the proteins which can
act as carriers, fighters, administrators, food engineers, signal receivers and transductors,
The Chemistry of Nanomaterials: Synthesis, Properties and Applications. Edited by C. N. R. Rao, A. Müller,
A. K. Cheetham
Copyright 8 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-30686-2
14.2 Sizes, Shapes, and Complexity of Nano-objects are Determined by the Nature
and transporters. In this context the question arises as to whether it is possible for
chemists/scientists to build up systems with comparable functions that would allow us to speak of machinery en miniature in Nature and technology. Remarkably,
from aqueous solutions of molybdates under reducing conditions, a huge variety of
interesting structures can be obtained which offer a unique substance library and
show really remarkable functionalities, even the phenomenon of cation transport
through pores and of modelling cell response. These nanoscaled systems emerge
by linking well-defined preformed building blocks under the canopy of self-assembly
processes, which are unique regarding the overwhelming variety obtainable. A textbook example is the generation of the tobacco mosaic virus (TMV) in two phases
[2]. In the first phase, the large protein building blocks are generated using encrypted genetic information and these in the second phase become linked to form
the complete viral entity.
14.2
Sizes, Shapes, and Complexity of Nano-objects are Determined by the Nature and
Variety of the Constituent Building Blocks
In inorganic polyoxometalate systems, especially in solutions of molybdates under
reducing conditions, a ‘virtual library’ of well-defined linkable building blocks
exists, which can be connected under suitable conditions to form a plethora of sophisticated nanostructures. For instance, two or three identical well-defined building
blocks or fragments, each comprising 17 molybdenum atoms, become linked to
form the two different nanostructures shown in Figure 14.1 [3a,b]. All such cluster
species, emerging via self-assembly and woven from well-defined building blocks
Fig. 14.1. Two relatively large clusters with different structures
formed by the same {Mo17 } building block: Two or three of
these {Mo17 } building blocks are linked in different modes by
different linkers to form different clusters.
453
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
of molybdenum and oxygen atoms, the most important of which are collected in
Table 14.1, show the following characteristics:
. All listed clusters contain the fundamental pentagonal {(Mo)Mo } building
5
.
block, comprising a central pentagonal bipyramidal {MoO7 } group with six surrounding edge-sharing {MoO6 } octahedra. These can occur in very different
environments and dictate correspondingly the overall size, symmetry, and complexity of the nano-objects.
The units are, for example, central to the construction of spherically shaped
entities with positive curvatures. (Note that even the famous so-called bucky-ball
{C60 } fullerene molecule contains 12 pentagonal ‘units’.) However, for the generation of more complex patterns, including negative as well as positive curvatures, that is showing symmetry breaking, a rather large number of negative and
positive vertices with polygons of different kinds, for example including heptagons, is necessary [4].
The highly symmetric [(pent)12 (link)30 ] type spherical clusters, such as the {Mo132 }
type nanospheres (Figure 14.2, right), are comprised of 12 pentagonal {(Mo)Mo5 }
groups (pent) connected by 30 {Mo2 } groups acting as linkers (link) [3]. The
cluster type {Mo132 } also has an interesting stoichiometric aspect in the sense that
it can be formally written as {Mo132 } 1 {Mo11 }12 1 {(Mo)Mo5 Mo5 0 }12 [5] (see below). The number twelve, an emblem of icosahedral symmetry, is significant in the
context of two Platonic solids: the icosahedron and dodecahedron, the latter having
12 faces, the former 12 vertices. Interestingly, the aforementioned {Mo11 } unit, like
its smaller counterpart {(Mo)Mo5 }, also shows a five-fold symmetry axis which is
necessary for the construction of spherical entities with icosahedral symmetry, be it
a {Mo132 } type nanosphere or a spherical virus. The presence of the related 14 and
16 {Mo11 } groups in wheel-shaped {Mo154 } (1 {Mo11 }14 ) and {Mo176 } (1 {Mo11 }16 )
type clusters (Table 14.1), respectively, also reveals the central role of the pentagonal building blocks. The two different types of {Mo11 } building blocks can be distinguished easily as a lower symmetry (that is, Cs ) is observed in the wheel-shaped
clusters resulting from the displacement of one of the {MoO6 } octahedra (see Figures 14.3 and 14.4).
The structure of the {Mo368 } type cluster [4], the largest inorganic cluster known
to date and also obtained from a special type of molybdenum blue solution, presents us with a paradigm shift with respect to symmetry breaking and changing
curvatures in the nanocosmos. The phenomenon of symmetry breaking is visible
at the surface due to the abundance of different large local symmetry areas as well
as the presence of negative curvatures in between positive curvatures (see Figures
14.5 and 14.6). The structure of this cluster can be considered as a hybrid between
the wheel- and ball-type clusters. The hybrid character is, in the terminology [5],
manifested by the occurrence of 24 C5 -type {Mo11 } groups of the ball-type clusters
together with the related 16 Cs -type {Mo10 } groups of the wheel-type clusters. The
{Mo10 } groups correspond to the abundant {Mo11 } groups with only one MoO6
octahedron less (see [4]).
–
14.1
–
14.8
–
14.4
14.9
–
–
14.2
14.2, 14.11
14.5
{Mo36 }
{Mo57 M6 }
{Mo63 M6 }
{Mo146 }
{Mo154 }
{Mo176 }
{Mo248 }
{Mo75 V20 }
{Mo102 }
{Mo132 }
{Mo72 M30 }
{Mo368 }
{Mo17 }2 {Mo1 }2
{Mo17 }3 {Mo2 }3 M6 (M ¼ Fe III , V IV )
{Mo17 }3 {Mo2 }3 {Mo1 }6 M6 *
{Mo8 }16 {Mo2 }9
{Mo8 }14 {Mo2 }14 {Mo1 }14
{Mo8 }16 {Mo2 }16 {Mo1 }16
{Mo8 }16 {Mo2 }16 {Mo1 }16 {Mo36 }2
{(Mo)Mo5 }10 {Mo5 }2 {Mo5=2 }2 V20
{(Mo)Mo5 }12 {Mo1 }30
{(Mo)Mo5 }12 {Mo2 }30
{(Mo)Mo5 }12 M30 (for example with
M ¼ Fe III )
{(Mo)Mo5 } 0 8 {(Mo)Mo5 } 00 32 {Mo2 } 0 16
{Mo2 } 00 8 {Mo2 } 000 8 {Mo1 }64
Formulation Based on Largest
Building Blocks
* Exist with different metal centres.
** Other possible tilings: {Mo17 } ¼ {Mo8 }2 {Mo1 } and
{Mo8 } ¼ {(Mo)Mo5 }{Mo1 }2 .
*** A. Müller, S. Q. N. Shah, H. Bögge et al., Thirty Electrons
‘‘Trapped’’ in a Spherical Matrix: A Molybdenum Oxide-Based
Nanostructured Keplerate Reduced by 36 Electrons, Angew. Chem. Int.
Ed. Engl. 2000, 39, 1614–1616.
Figure
Cluster
{Mo17 } ¼ {(Mo)Mo5 }2 {Mo1 }5 **
{Mo17 } ¼ {(Mo)Mo5 }2 {Mo1 }5 **
{Mo17 } ¼ {(Mo)Mo5 }2 {Mo1 }5 **
{Mo8 } ¼ {(Mo)Mo5 }{Mo1 }2
{Mo8 } ¼ {(Mo)Mo5 }{Mo1 }2
{Mo8 } ¼ {(Mo)Mo5 }{Mo1 }2
{Mo36 } ¼ {Mo8 }2 {Mo8 } 0 2 {Mo2 }2
{Mo5=2 } ¼ 5{Mox } (x ¼ 0:2–0.8)
Smaller Building Blocks of the
Larger Groups
SO4 2 , 10 Naþ
CH3 COO
CH3 COO
CH3 COO
48 SO4 2
10
12
30
12
16 SO4 2 , 16 Kþ
Associated Counterions/Ligands
4
7
3
3
10
3a, 3b, 6
3a, 6
3a, 12a
3a
***
3
3, 14
References
Tab. 14.1. The nanocosmos of molybdenum oxide clusters formulated according to metal-based building blocks especially {(Mo)Mo5 }. This important unit
which is found in all molybdenum-based nanoarchitectures (Figure 14.3), has a pentagonal {MoO7 } bipyramid in the centre while the Mo atoms in all other
building blocks, like {Mo1 } or {Mo2 }, have Mo in an octahedral environment. Building blocks, like {Mo1 }, can show different ligand dispositions. (References
are based mainly on our earlier reviews.)
14.2 Sizes, Shapes, and Complexity of Nano-objects are Determined by the Nature
455
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
‘‘Sizing’’ the nanospheres is
possible. Top: polyhedral representation
of the cluster type [(pent)12 (link)30 ] 1
{(Mo)Mo5 }12 {link}30 (linkers: {MoV 2 } and
{FeIII } type units, indicated by arrows) showing a size-comparison of the two clusters
{Mo72 Fe30 } (a) and {Mo132 } (b). Middle: sizecomparison of the two clusters via wire-frame
representation of the metal skeletons. Note in
(a) the size-‘shrinking’ as the dinuclear {MoV 2 g
Fig. 14.2.
of (b) is replaced by the mononuclear {FeIII }
linker and the increase in ‘pore size’ on
moving from a {Mo3 Fe3 O6 } ring of the
{Mo72 Fe30 } cluster (a) to an {Mo9 O9 } ring of
the {Mo132 } cluster (b). Bottom: structures
only formed by the linkers: the icosidodecahedron with 12 pentagons and 20 triangles
formed by {Fe30 } (a) and the (distorted)
truncated icosahedron with 12 pentagons
and 20 hexagons formed by {MoV 2 }30 (b).
An imminent question in this context might be: what leads to the development
of such positive and negative curvatures in the nanoscopic landscape of a cluster?
Without doubt, the presence of a large number of building blocks in the reaction
medium is a reason for the emergence of such complex architectures showing
14.3 Nanoscaled Clusters with Unusual Form–Function Relationships
Fig. 14.3. Two ‘‘pentagonal’’ {Mo11 } units
less symmetric (Cs ) (left) with one shifted
with the central {(Mo)Mo5 } building block.
{MoO6 } octahedron (light grey) in the wheel
One finds the more symmetric (C5 ) unit (right) type clusters (cf. Table 14.1 and Figure 14.4).
in the spherical type cluster {Mo132 } and the
symmetry breaking. But still the question remains: how can such a structure be
produced or how can a large number of building blocks be generated in solution?
It is known that the largest cluster in acidified molybdate solution is, in the absence of a reducing agent, the ‘‘rather small’’ {Mo36 } type anion [3a, 4, 6, 7] which
starts growing under reducing conditions. (Note that this is even larger than the
largest discrete silicate ion.) An increase in negative charge, due to reducing conditions, not only prevents uncontrolled linking, but also initiates further protonation which is a prerequisite for further growth [4]. The increase in negative
charge in the present case is also supported by the abundance of SO4 2 ligands
coordinated to the intermediates and the final cluster species. Another important
aspect is the related coordination ability: SO4 2 is a moderately strong ligand and
therefore favors an increase in the number of building blocks which might be
coordinated or non-coordinated. In the presence of much weaker coordinating ligands, such as Cl or ClO4 ions, in cases where HClO4 or HCl is used as acidification agent instead of H2 SO4 , the {Mo368 } cluster is not formed, ‘‘only’’ pure
molybdenum oxide-based {Mo154 } or {Mo176 } wheel-type species without Cl or
ClO4 ligands. For the present purpose it is important to note that the coordination ability of the SO4 2 ligand is ideal, as it is not too strong (as would be the case
for PO4 3 where all units would be coordinated) and not too weak [4].
14.3
Nanoscaled Clusters with Unusual Form–Function Relationships
Let us continue our tour of sophisticated nanostructures by giving some striking
examples of the multifaceted form–functionality correlation principles involved.
The following parent cluster types will be considered (here we refer to earlier published reviews):
457
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Molecular giant wheels and balls show a remarkably
similar (formal) {Mo11 }n stoichiometry : comparison of the
structures of the {Mo11 }12 (top) and {Mo11 }16 clusters (middle
and bottom, with side and top view, respectively). The {Mo11 }
unit is highlighted in each case.
Fig. 14.4.
14.3 Nanoscaled Clusters with Unusual Form–Function Relationships
Fig. 14.5. An exciting molecular blue ‘nanolemon’: the
[Hx Mo368 O1032 (H2 O)240 (SO4 )48 ] 48 cluster anion in polyhedral
representation showing large areas of different local symmetry
based on the different abundant basic building blocks {Mo1 },
{Mo2 }, and {(Mo)Mo5 }.
1. Wheel-shaped clusters of the type {Mo154 } and {Mo176 } [3, 6] with large central
cavities that can be easily modified by means of simple well-known chemical
reactions (see Figure 14.4).
2. Spherical-shaped clusters of the type [(pent)12 (link)30 ] [3, 5, 8] with nanoscaled
tunable cavities and large tunable pores (see Figure 14.2).
3. Torus-shaped clusters of the type {Mo57 M6 } [3a] with potential functional indentations on the cluster surface (see Figure 14.1 right).
The giant wheel molybdenum-oxide based anions can be considered as objects which
show a variety of nanoscale structural functionalities (‘‘nanostructured landscape’’)
not only allowing reactions at a variety of well-defined centres/groups but also
offering the option of acting as potential nano-reactors or -containers ([6, 9–12]).
Some relevant reactions are:
. The substitution of H O ligands coordinated to the molybdenum centres of
2
{Mo154 } by ambivalent and/or multivalent ligands, for example the amino acid
cystine can be coordinated to the inner wall of the cluster cavity and hence creates hydrophobic and hydrophilic surroundings with different functionalities
(see Figure 14.7) [9].
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Forms with positively and negatively curved
surfaces: the sphere (top) and the outer part of a torus
(bottom left) are examples of surfaces exhibiting positive
curvatures, the saddle (bottom right) and the inner part of the
torus (bottom left) representing negative curvatures.
Fig. 14.6.
Endohedral functionalization of the inner surface of
a wheel-shaped {Mo154 } type nanocluster by cystine ligands
(polyhedral representation, carboxylate group in ball-and-stick).
Fig. 14.7.
14.3 Nanoscaled Clusters with Unusual Form–Function Relationships
Fig. 14.8. Left: structure of [MoVI 114 MoV 32 O429 (H2 O)50 (KSO4 )16 ] 30 cluster anion and
separately of the integrated {KSO4 }16 ring on a
smaller scale in the centre (top view) in crystals of K14 Na16 [MoVI 114 MoV 32 O429 (H2 O)50 (KSO4 )16 ]ca. 500 H2 O in ball-and-stick
representation showing the KO11 units in
polyhedral representation to demonstrate the
overall K environment (see [10] for further
details). Right ((b), (d)): demonstration of the
multitude of specific adjacent receptors
(forming a zigzag ring) for 16 Kþ ({Mo6 O6 }
. The (larger) hexadecameric {Mo
.
.
receptors (light grey, indicated by an arrow in
(a) and (b)) and 16 SO4 2 ions (sites between
{Mo5 O6 } compartments schematically emphasized in gray, indicated by an arrow and
{O4 } in (d)); furthermore, the change from the
‘‘classical’’ {Mo2 Mo8 Mo1 }n type ((b), (d)) [6]
to the {Mo2 }nx {Mo8 }n type ((a), (c)) ring
structure with the change of the {Mo5 O6 } [6]
to the {Mo4 O6 } unit by a release of {Mo1 } is
demonstrated ({Mo1 } dark gray octahedra in
(b); SO4 2 light gray tetrahedra, Kþ emphasized in (a) as large gray spheres).
176 } type cluster also shows a similar variety of
functionalities and even a manifold of (16 þ 16) non-equivalent receptor sites for
cations as well as anions allowing, for example, the formation of a novel supramolecular host–guest system while integrating 16 Kþ and 16 SO4 2 ions, thus
leading to the formation of an unusual encapsulated 64-membered zigzag ring
system (see Figure 14.8) [10].
Paramagnetic metal centres like Cu 2þ ions can be incorporated into the cavities
spanned by four O atoms and having the appropriate size, for example of the
{Mo154 }-type rings. This type of reaction can lead to the formation of nanoobjects with unique neutral magnetic properties [11].
The ‘‘addition’’ of two neutral {Mo36 }-type ‘hub-caps’ to the surface inside the
cavity of the {Mo176 }-type wheel leads to the formation of a {Mo248 } cluster and
corresponds to a nucleation process under boundary conditions [2a] (see Figure
14.9). This reaction can be regarded as a model for both crystal growth, especially
for the related initial nucleation process, which is still not clearly understood, and
also for metal-centre uptake processes in biological systems. In particular, such
461
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Addition of two {Mo36 } ‘hub-caps’ to the inner
surface of the {Mo176 } type wheel leading to the formation of
the {Mo248 } cluster.
Fig. 14.9.
.
growth processes might be allied to biomineralization processes in compartments.
In this context the molybdenum-atom uptake of the Mo storage protein of the
N2 -fixing microorganism A. vinelandii is worth mentioning. According to our
preliminary results the inorganic nucleus corresponds to a rather large polyoxomolybdate.
The nucleophilic {Mo6 O6 } type hexagonal ring-shaped sites act as receptors for
cations, for example protonated urea guests [12b], a comparable situation to the
above-mentioned case of the integrated 16 Kþ [10]. In the urea case this can be
used to decrease the negative charge thereby nicely facilitating chain formation
[12b], a fact which shows perspectives for related nano-, supramolecular-, and
solid-state chemistry.
In the case of the spherical nano-objects (Table 14.1), formed by 12 pentagonal
{(Mo)Mo5 } units linked by 30 mono- or dinuclear linkers, there is, for each species,
a nanoscaled capsule cavity and 20 size-tunable specific nanopores of the type
{Mn On } (for example with n ¼ 6 or 9), which can act as substrate specific receptors
[8]. The cavity can even house smaller polyoxometalates, like different Keggin ions,
to form a supramolecular nanocomposite of a unique kind (see Figure 14.10) [13].
Some other interesting reactions are:
. The replacement of the 30 diamagnetic {Mo }-type linkers of the spherical
2
.
{Mo132 }-type clusters by 30 paramagnetic centres like Fe III leads to the emergence of unusual molecular nanoantiferromagnets of the type {Mo72 Fe30 } (see
Figures 14.2 and 14.11) [14].
The neutral spherical nano-objects of the type {Mo72 Fe30 } can be crosslinked in
a solid-state reaction, even at room temperature (!), to form layers by following
an elementary inorganic condensation reaction, known already for the formation of polycations like those of Fe III in aqueous solution. The rapid loss of
water molecules from the freshly filtered crystals, comprising discrete spheri-
14.3 Nanoscaled Clusters with Unusual Form–Function Relationships
Fig. 14.10. Reaction scheme showing the
decomposition of a part of the Keggin anions
{PMo12 O40 } 3 (left, polyhedral representation)
in the presence of Fe 3þ ions, leading finally to
the formation of the [{(Mo)Mo5 }12 {Fe}30 ] type
cage (wireframe representation of the capsule’s
.
Mo atoms in dark gray, Fe atoms light gray)
which has one of the remaining nondecomposed Keggin anions encapsulated
(polyhedral representation) thereby forming
the unusual supramolecular species with core–
shell topology : guest H [(pent)12 (link)30 ].
cal clusters, leads to their approximation which is a condition for their covalent linking (see Figure 14.12) [15]. The possibility of forming a related onedimensional chain (paramagnetic Keplerate ‘‘necklaces’’) from similar spherical
nanospheres in a room-temperature reaction also exists [16].
It is even possible to ‘kick out’, like soccer balls, the neutral giant molybdenum
oxide spheres, for example the {Mo72 Fe30 } ones, into the gas phase using all
matrix-assisted laser desorption and ionization (MALDI) methods. It is important that the clusters range from dimers to pentamers (see Figure 14.13) [17].
A further fascinating point refers to the possibility of performing different types
of reactions inside the cavities of such spherical clusters at well-defined sites,
which can be hydrophilic or hydrophobic. Here we describe for the first time
an example, the details of which have, as yet, not been published: the reaction
of [{(Mo VI )Mo VI 5 }12 {L}30 ] 42 (L ¼ {Mo2 O4 (CH3 COO)}þ ) with phosphate and molybdate in aqueous solution at different pH values demonstrates that, after replacement of the acetate ligands, the cluster shows in its cavity a pH-dependent
nucleation process in the presence of molybdate. This leads to the formation of
new PaOaMo bonds at low pH, a situation comparable to the formation of the
[PMo12 O40 ] 3 Keggin anion at low pH values [18] (see also Figure 14.14).
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Fig. 14.11. Nanomagnet as a classical magnet en miniature on the cover-page of
ChemPhysChem. The {(Mo)Mo5 }12 {Fe}30
cluster (see also Figure 14.2) in two representations, including one showing the orien-
tation of the spins at the FeIII 3 triangles was,
because of its unusual properties, studied at
very low temperature (there it still behaves
as a classical antiferromagnet) and extremely
high magnetic field strength.
14.4 Perspectives for Materials Science and Nanotechnology
Fig. 14.12. Crosslinking neutral spherical nanoclusters of the
type [{(Mo)Mo5 }12 {FeIII }30 ], indicated by two FeO6 octahedra
sharing corners, leads to the formation of a layered structure
based on an unusual solid-state reaction at room temperature.
All these spherical clusters can be described, as mentioned above, as
[{(Mo)Mo5 }12 (link)30 ]. It was suggested to call them ‘Keplerates’ because of their
striking similarity to a fragment of Kepler’s early speculative model of the universe, as described in his opus Mysterium Cosmographicum (see Figure 14.15). The
related Keplerate type chemistry is especially interesting for mathematicians
[3c]. Furthermore, these clusters also show, topologically speaking, a similarity to
spherical viruses (Figure 14.16).
14.4
Perspectives for Materials Science and Nanotechnology: En Route to SphericalSurface, Nanoporous-Cluster, and Super-Supramolecular Chemistry including the
Option of Modelling Cell Response
The clusters described above are interesting for materials science and nanotechnology aspects because of their stability, size, solubility (in several solvents), giant
465
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Fig. 14.13. Unique nanocluster aggregates in the gas phase:
MALDI mass spectra of the neutral clusters of the type
{Mo72 Fe30 } and {Mo102 } and their oligomers.
Fig. 14.14. pH controlled nucleation in
nanoscaled cluster cavities: the anion
[{(MoVI )MoVI 5 }12 {L}30 ] with L ¼ phosphate
(gray tetrahedra left) shows in the presence of
molybdate in solution and by lowering the pH
to nearly 2 a ‘nucleation’ process inside the
cluster cavities with the formation of PaOaMo
bonds ({MoO3 } units gray spheres).
14.4 Perspectives for Materials Science and Nanotechnology
Fig. 14.15. Nano- and macrocosmos/universe
similarities: schematic representation of the
{Mo132 } type cluster on the cover-page of
Angew. Chem., highlighting its similarity to
Kepler’s early model of the universe and
‘‘abundance’’ of related icosahedra in both
cases. Important: it was the starting point for
the consideration of Keplerate type nanochemistry from a topological point of view (see [3c]).
467
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Fig. 14.16. Nanoscaled systems of the
biosphere and of chemistry show the same
topology : schematic representation of the
icosahedron spanned by the 12 centres of the
{(Mo)Mo5 } units of the {Mo132 } type cluster
(a) and of the satellite tobacco necrosis virus
(STNV) with the triangulation number T ¼ 1
(see for example [3a]) highlighting one of the
60 protomers (dark gray) (b). Two {Mo11 }
units in (a), each formed by the {(Mo)Mo5 }
groups and the five related Mo centres of the
five neighbouring {MoV 2 O4 } 2þ linkers, are
highlighted.
cavities, ‘nanosized’ changeable pores, unique surfaces, unusual electronic structures, and
last but not least abundance of magnetic centres in a variety of topologies. Moreover, most of the clusters, especially the spherical ones, can be manipulated whilst
keeping the robust nanoscopic oxomolybdate skeleton intact. Encapsulation of the
giant clusters with surfactant molecules allows one to produce LB films and monolayers. Some of these even form a new type of vesicle with the option of getting
information about the structure of liquid water. A summary of these and other
relevant aspects is presented in Table 14.2; for further details see the collection of
papers in [19].
A recent challenge in nanotechnology is to mimic the material constructions and
molecular-recognition-type responsive sensing of biological systems. Intracellular
response to extracellular signals involves signal transduction via molecular recognition of the specific receptors on the cell surface, which can trigger enzymatic reactions in the cell. Such a ‘‘cognitive isomorphism’’ on a molecular level, a rare
phenomenon in classical chemistry, is, for example, of immense importance for
understanding the complex dynamics of ‘uptake and release’ of substrates in
general.
The nanoball of the type {Mo132 } shows such molecule-response activity. It can
be compared to a ‘nanosponge’ with a multitude of open pores accessible for a
specific substrate, for example, guanidinium cation guests in the case of {Mo132 }
14.4 Perspectives for Materials Science and Nanotechnology
Tab. 14.2.
Nanoscaled polyoxometalate clusters for materials science problems.
Topics/Keywords
Characteristics
References
LB films
Based on encapsulation of {Mo132 } with cationic
surfactants
Based on encapsulation of {Mo132 } with cationic
surfactants
Based on encapsulation of {Mo154 } with cationic
surfactants
Based on integration of {Mo154 } and {Mo132 }
clusters
19c
Ball-shaped clusters with pores are highly selective
for cations and allow the incorporation of clusters
and important aggregates, for example of water
molecules including those with electrolytes
Wheel-shaped clusters show functionalities and
especially nucleation processes inside their central
nanoscaled cavity
Giant clusters show hierarchic patterning in solution
and lead to a new type of vesicles
8, 13, 21*
Monolayers
Liquid crystals
Hybrid materials
with silica and
carbon tubes
Nanoporous
clusters with
cavities
Nanoreactors
Soft matter:
aggregates,
vesicles, colloids
Clusters as magnets
{Mo75 V IV 20 } with 10 V IV triangles linked
{Mo72 Fe III 30 } is a model for a classical magnet
{V IV 15 } shows a layered magnetic structure
19c
19g
19f, 19h
3a**
19i, 19k, 19l
3a, 8c, 14***
* A. Müller, B. Botar, H. Bögge et al., A Potassium Selective
‘Nanosponge’ with Well Defined Pores, Chem. Commun. 2002, 2944–
2945.
** A. Müller, S. Q. N. Shah, H. Bögge et al., Molecular Growth from a
Mo176 to a Mo248 cluster, Nature 1999, 397, 48–50.
*** Several sophisticated physical studies have been published of the
V15 cluster have been published, for example: I. Chiorescu, W. Wernsdorfer, A. Müller et al., Butterfly Hysteresis Loop and Dissipative Spin
Reversal in the S ¼ 1=2, V15 Molecular Complex, Phys. Rev. Lett. 2000,
84, 3454–3457; B. Barbara, I. Chiorescu, W. Wernsdorfer et al., The
V15 Molecule, a Multi-Spin Two-Level System: Adiabatic LZS Transition with or without Dissipation and Kramers Theorem, Progr. Theor.
Phys. Suppl. 2002, 145, 357–369.
(Figure 14.17). The suction of substrates into this ‘nanosponge’, while simultaneously closing the pores, essentially triggers a restructuring of the incarcerated
solvent molecules, a situation comparable to the mentioned intracellular response
to an extracellular stimulus. Moreover, as different types of substrates can interact
with different types of pores on a specific spherical nanosurface, the situation
might be noted as the beginning of an era of nanoscaled spherical-surface chemistry.
Additionally, since the pore sizes of the clusters can be varied and show corre-
469
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Fig. 14.17. The 20 pores of a spherical
nanocluster of the type {Mo132 } can be closed
through substrate uptake. Right: one {Mo9 O9 }
pore in ball-and-stick representation (PO2 H2 /
SO4 2 ligands shown as tetrahedra). Left:
space-filling model of the cluster with inserted
guanidinium cations (only CN3 part shown).
Closing the pores leads to a structural
reorganization of the encapsulated water
molecules.
spondingly different reactivities, we can also refer to a nanoporous-cluster chemistry
and, because of the large number of receptors involved (20 in the case of icosahedral spheres), to a super-supramolecular chemistry. In this connection recent unpublished 1 H NMR investigations have revealed that the uptake and release of
‘guanidinium’ substrates in solution by the ‘nanosponge’ shows an interesting
temperature dependence corresponding to an open-and-shut process [20].
An aspect of particular relevance refers to the exciting problem of the structure
of liquid water. The closing of the cluster’s ‘nanowindow’ leads, in the case of the
{Mo132 } cluster, to the reorganization of the internal H2 O molecules into a giant
and structurally well-defined (H2 O)100 cluster which can be considered as a snapshot of liquid water (see [8, 21] and Figures 14.18 and 14.19). (The endohedral
clusterization of water molecules upon substrate uptake into the pores formally
corresponds to the cellular response to an extracellular stimulus on the cell surface.) Our special endohedral clusterization of water molecules would certainly
have pleased both Plato and Archimedes as the water cluster can be formally (!)
arranged in shells showing a larger and smaller dodecahedron, each composed of a
(H2 O)20 segment and one rhombicosidodecahedron (H2 O)60 [8, 21]. This example
shows that with the present nanocontainers, available for the encapsulation of
segments of different kinds of (macroscopic) substances and even rather large
molecular systems, problems of tremendous importance for contemporary science
can be solved. Even the liquid water problem which has been characterized as ‘‘The
most important problem in science that hardly anyone wants to solve’’ [22]. Now we
deny this statement.
14.4 Perspectives for Materials Science and Nanotechnology
Fig. 14.18. A nanodrop of water found in the
cavity of a {Mo132 } cluster (top) and the
hierarchical structure of the (H2 O)100 assembly
showing two Platonic (a larger and a smaller
(H2 O)20 dodecahedron) and one (H2 O)100
Archimedean solid in a formal consideration
(middle). One tetrahedrally coordinated O
atom as in liquid water is also emphasized
(bottom).
471
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
Fig. 14.19. A result of modern nanochemistry :
Plato and Archimedes look pleased viewing
the nanodrop of water {H2 O}100 inside a
molybdenum-oxide ‘nanosponge’ occurring
(formally) in the form of shells, that is two
(H2 O)20 dodecahedra and a rhombicosidodecahedron (H2 O)60 . Note, as in liquid water
all O O distances are of the order of 2.7 Å.
References
Acknowledgments
We thank Dr. H. Bögge (Bielefeld) for his collaboration. The financial support of
the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, the
Volkswagenstiftung, and the European Commission is gratefully acknowledged.
S. R. thanks the Graduiertenkolleg ‘Strukturbildungsprozesse’, Bielefeld University,
for a fellowship.
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14 Oxomolybdates: From Structures to Functions in a New Era of Nanochemistry
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