EVERYONE is probably familiar with theintense blue colour obtained when acidifiedsolutions of molybdate (MoVI), or asuspension of molybdenum oxide (MoO3)in water, are treated with a reducing agentsuch as stannous (SnII) ions, sulfur dioxideor hydrazine (N2H4). It is used as the basisfor sensitive colorimetric tests for phosphateand arsenate ions and tin. For at least twocenturies, since ‘molybdenum blue’ was firststudied by Carl Scheele and Jöns Berzelius,chemists have puzzled over the nature ofthe molybdenum species that the solutioncontains. It is a substance or group ofsubstances about which there has beenmuch discussion.1 As a leading researcherin synthesising and characterising solublemetal oxides and sulfides, ProfessorMüller was also fascinated by molybdenumblue solutions. By probing its remarkablestructure, he has shown that it unlocks thedoor to a new inorganic world parallelingthat of biology.

The complex world of molybdenumThe chemistry of molybdenum oxides andthe soluble molybdates has long beenrecognised as extremely complex – as aquick look in any inorganic textbook willattest. The formation of distinctmolybdenum-oxygen species is stronglydependent on environmental conditions.One of the most important basic buildingblocks are MoO6 octahedra which can

803This journa l is © The Roya l Soc iety of Chemistry 2003 CHEM. COMMUN., 2003

Bringing inorganicchemistry to life

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Achim Müller

Fig. 1

The structure of the first published ‘big wheel’ containing154 Mo atoms – half in polyhedral representation with

different building units (Mo8 blue, Mo2 red, Mo1 yellow),and half in ball and stick representation (Mo blue, O red)

In recent years, the development of advanced materialshas been stimulated by the combinatorial strategies thatNature employs to construct assemblies of complexmolecules with a specific form and function. ProfessorAchim Müller of the University of Bielefeld, Germany hasextended this approach into the realm of inorganicchemistry in an extraordinarily imaginative way. He hasbeen exploring the intricate and versatile chemicalbehaviour of molybdenum and its oxides, creating self-assembling systems of molybdates in solution that can beregarded as inorganic ‘nanomodels’ for biological activityat the cellular level.

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CHEM. COMMUN., 2003 This journa l is © The Roya l Soc iety of Chemistry 2003804

share the oxygen corners or edges.2

Studies showed that molybdenum blueappeared to contain clusters of thesebuilding blocks as well as clusterassemblies large enough to scatter lightfrom a laser.3

In the early 1990s, Müller’s groupworked on characterising molybdenumblue and optimising its preparation. Theywere able to obtain a single crystal of thematerial and, using a range of analyticaltechniques, reveal that it was constitutedfrom the most breathtaking molecularassemblies – giant cyclic polyoxo-molybdates, 4 nanometres wide (Fig. 1).Each molecule was made up of a total of154 molybdenum atoms embedded in anetwork of oxygen atoms. Dubbed the‘Bielefeld giant wheel’, it was at that timethe largest inorganic molecular cluster evercharacterised.4,5 Since then, wheel-shapedclusters with 176 and 248 molybdenumatoms have also been obtained.6

Not surprisingly, the paper on theBielefeld giant wheel, published in 1995,attracted considerable attention. At thesame time, Müller saw the possibledirections in which the research could go.“I realised,” says Müller, “that this

aqueous solution of molybdate was uniquein the sense that it pointed the way to adynamical library of complex structures.”Linking the molybdenum-oxygen buildingblocks in different ways would lead to a“combinatorial explosion of materials”.The subunits can be formally compared toamino acids which link to form proteins,adds Müller.7 Two units can be joined viaa condensation process – analogous to thatin peptide formation. Protonation is thecondition for linking and the size of thecluster is controlled by the pH. Müllerpoints out that such structures can be madeto self-assemble in aqueous solution in acontrollable and tunable way “undervarying but well-defined reducingconditions”.

Müller and his team went on to createthe most marvellous highly symmetricball-shaped structures, incorporatingpentagonal bipyramidal components(MoO7) as the directing units (Fig. 2).8

While the octahedral units look like twoEgyptian pyramids with their bases stucktogether, the pentagonal bipyramidalstructures have five edges around thecentre instead of four. These can shareedges with five MoO6 octahedra to createpentagonal molybdenum-based units. This is obviously significant in building upfootball-shaped structures, and usefulanalogies can be drawn with the carbonfullerenes which are also based onpentagonal units. The first of these

molybdenum superfullerenes, or‘Keplerates’ as Müller calls them, wasmade in 1998. (The ball-shaped speciescontain fragments formally comparable tothe early cosmos of Johannes Kepler.) Itwas composed of a hollow sphere of 132molybdenum atoms with an inner clusterof 60 molybdenum atoms arranged like theC60 bucky-ball. Müller points out that thearrangements of the subunits in suchstructures are reminiscent of the packingof protein molecules on the surface of aspherical virus.

More recently, the Bielefeld teamprepared a huge cluster which they call a ‘nano-hedgehog’ because of its shapeand as it has an outer layer of oxygenatoms pointing outwards (Fig. 3). Itcontains 368 molybdenum atoms and isactually the size of a protein.7 It has aninternal cavity 2.5 nanometres wide and 4 nanometres long which encapsulatesabout 400 water molecules. The electron-rich structure is deep blue in colour – dueto delocalised electrons. Müller believesthat such structures could be used asselective catalysts, similar to zeolites, or as nanoreactors.

Müller’s aim has thus been to explorehow the functionality of such materialscould be explored through tuning thestructure by varying the reactionconditions and reagents. One combinationof conditions created spherical clusterswith an exotic host-guest topology

Fig. 3 The Mo368-typenano-hedgehog (Moblue, O red, S yellow)

Fig. 2 The firstpublished Keplerate,

an inorganicsuperfullerene built up

by 12 pentagonal Mobased units (blue) and

30 different MoV2

-based linkers (red)

Fig. 3

Fig. 2

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805This journa l is © The Roya l Soc iety of Chemistry 2003 CHEM. COMMUN., 2003

suggesting potential use in molecularelectronics. The starting material was asolution of the well-known Keggin anionsof the type [PMo12O40]3-, formed fromphosphate and molybdate. These arenormally stable, but in the presence ofiron(III) ions (and acetate), they decomposeto form giant icosahedral clusters composedof pentagonal molybdate building blockslinked by hydrated iron(III) groups.Remaining Keggin anions are just the rightsize to be encapsulated inside the clustershell (Fig. 4).9 Müller notes that the highersymmetry of the icosahedral cluster winsout over the lower symmetry of theindependent Keggin ions in a Darwiniancompetition to form a supramolecularstructure somewhat analogous to the firstspherical protocells that may have formedon Earth.

The final result is a bizarre electronicstructure consisting of a polyhedralmolybdenum-iron magnetic dot – renderedparamagnetic by the presence of 30 ironions, hosting a noncovalently-bondedKeggin ion guest which behaves like aquantum dot in that it possesses discreteelectronic energy levels. In the solid state,the nanosized-clusters crosslink via iron-oxygen bonds to form a two-dimensionalnetwork of magnetic dots which representthe strongest paramagnetic molecularmaterial known today – each cluster has150 unpaired electrons. Derivatives ofsuch systems could act as elements ofmagnetic or electronic storage devices.Müller envisages that this type of ‘crystalengineering’ will lead to novel compositescontaining arrays of clusters whoseelectronic or other properties can be tuneddepending on the constituents used. This isespecially valid as not only extendedstructures but also molecular assemblies,i.e. oligomers of the single neutral clustercan be obtained.10

Inorganic receptorsA current challenge in nanotechnology isto mimic molecular recognition thatcharacterises biological regulation. Thehighly soluble Keplerates offer an idealinorganic model for investigating theassociated structural and dynamicalchanges. Recently, Müller and his teamhave synthesised giant molybdenumoxide-based nanoclusters with tailor-madepores.11,12 These ‘nanosponges’ are createdfrom the basic pentagonal units connectedwith different linkers in the form|(pent)12(link)30|, the pore as well as theoverall size depending on the length andcharge of the linker. Spherical systemscontaining 20 pores corresponding toMo9O9 rings with a diameter between 0.6and 0.8 nanometres have been prepared.11

The system, with its 12 pentagonal unitsand Mo-Mo linkers generated from twoedge-shared MoO6 octahedra (i.e. linkersare made from a total of 60 octahedra), isstructurally the inorganic analogue of thesimplest icosahedral virus.

Such systems show behaviour analogousto simple cell responses. The pores can beclosed by cations of the appropriate sizeand shape, in this case guanidiniumcations (Fig. 5a). Guanidine is of interestas it is used in a variety of industrial andpharmaceutical processes. Its functionalgroup occurs in an amino acid and can bealso regarded as a useful anion receptor.Other clusters containing mononuclearlinkers, for example, iron(III) orvanadium(IV) generate smaller porescreated from Mo6O6 rings. These are theright size to capture potassium ions (Fig. 5b), in a manner corresponding to thesupramolecular behaviour shown bycrownethers – although repeated 20 timesover!12

What is particularly remarkable is thatclosing the pores induces changes in thestructure of the droplet of water trappedinside the cluster. The water moleculesbecome arranged in highly symmetricalendohedral clusters (Fig. 6). “This effect canbe compared with signal transduction by

BiographyProfessor Müller first studied chemistry andphysics at the University of Göttingen,becoming Associate Professor at theUniversity of Dortmund in 1971, and later in1977, Professor of Inorganic Chemistry at theUniversity of Bielefeld. He maintains anextremely broad range of interests, rangingfrom studies into soluble metal oxides andsulfides through spectroscopic techniques tobiological nitrogen fixation.

Fig. 4

Fig. 4 A Keggin-cluster encapsulated in aKeplerate (wire-frame representation): the30 FeIII centres act as linkers (yellow) forthe 12 Mo-based pentagonal units (blue)

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which a cell converts an extracellular signalinto a response,” says Müller.Furthermore, studies of encapsulatedassemblies of water molecules could offera useful tool in exploring water’s complexphases. The structure of water inbiological systems, in particular, is still notwell understood but known to besignificant in mediating proteinconformation, for example.

The molybdate clusters, derived fromstudies of molybdenum blue, have turnedout to exhibit an extraordinarily richbehaviour which has a bearing on manyaspects of chemical research today:coordination chemistry, catalysis,advanced material design, molecularelectronics, crystal engineering, single-molecule chemistry (perhaps?),nanochemistry in the real sense, surfacemodelling and supramolecular chemistry.13

However, what is unique about thesecomplex inorganic systems is that not onlydo they form an almost limitless variety ofstructures in which form and function canbe closely correlated, but most important,they are stable, flexible macromoleculeswhich are highly soluble in water, andwhich maintain their structure andfunctionality while in a mobile state sothat chemical reactions can be performedat specific sites. This means that they aresingularly equipped to mimic biologicalprocesses. As Professor Müller says: “Withthese materials we are bringing inorganicchemistry to life.”

Nina HallThe author thanks Professor Müller for his help with this article.

R E F E R E N C E S1. N. V. Sidgwick, The Chemical Elements

and their Compounds, vol. II, Clarendon,Oxford, 1962, p. 1046.

2. M. T. Pope, Heteropoly and IsopolyOxometalates, Springer, Berlin, 1983.

3. A. Müller, E. Diemann, C. Kuhlmann, W. Eimer, C. Serain, T. Tak, A. Knöchel andP. K. Pranzas, Chem. Commun., 2001, 1928.

4. A. Müller and C. Serain, Acc. Chem.Res., 2000, 33, 2.

5. A. Müller, P. Kögerler and C. Kuhlmann,Chem. Commun., 1999, 1347.

6. A. Müller, Syed Q. N. Shah, H. Böggeand M. Schmidtmann, Nature (London),1999, 397, 48.

7. A.Müller, E. Beckmann, H. Bögge, M. Schmidtmann and A. Dress, Angew.Chem., Int. Ed., 2002, 41, 1162.

8. A. Müller, E. Krickemeyer, H. Bögge, M. Schmidtmann and F. Peters, Angew.Chem., Int. Ed., 1998, 37, 3360.

9. A. Müller, S. K. Das, H. Bögge, M. Schmidtmann, A. Botar and A. Patrut,Chem. Commun., 2001, 657; A. Müller, S. K. Das, P. Kögerler, H. Bögge, M. Schmidtmann, A. X. Trautwein, V. Schünemann, E. Krickemeyer and W. Preetz, Angew. Chem., Int. Ed., 2000,39, 3413; ed. S. Hurtley, Editors’ Choice –Highlights, Science (Washington D.C.),2000, 290, 411.

10. A. Müller, E. Diemann, S. Q. N. Shah, C. Kuhlmann and M. C. Letzel, Chem.Commun., 2002, 440.

11. A. Müller, E. Krickemeyer, H. Bögge, M. Schmidtmann and S. Roy, A. Berkle,Angew. Chem., Int. Ed., 2002, 41, 3604.

12. A. Müller, B. Botar, H. Bögge, P. Kögerlerand A. Berkle, Chem. Commun., 2002, 2944.

13. The New Chemistry, ed. N. Hall, CambridgeUniversity Press, Cambridge, 2000.

Fig. 5b

Fig. 6

Fig. 5 Structures of two ‘nanosponges’ ofthe type (pent)12(link)30 (space-filling

representation). The 20 pores havedifferent sizes and illustrate the ‘lock andkey’ principle between the sponge/host

and its 20 guests: K+ (a) and guanidiniumcations (b) (Mo blue, O red, N green, C

black, K yellow, linker in (b) green)

Fig. 6 A possible snapshot of liquid water:a nanodrop of water with well-defined

structure encapsulated in the Keplerateof Fig. 2 (only O atoms shown)

Fig. 5a

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