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Figure 1 From theory to reality.a,A planar representation of aKagomé lattice,and b,a space-filling representation of the latticesynthesized by Zaworotko andcolleagues6 by self-assemblingcopper(II) cations with 1,3-benzenedicaboxylate anions.Copper atoms are shown inorange,carboxylate groups in redand benzene rings in grey.

KAGOMÉ LATTICE

A molecular toolkit for magnetismKagomé lattices are the most geometrically frustrated magnetic systems. But their magnetic properties remain poorly characterized because they are difficult to synthesize. A new versatile synthetic route to Kagomé lattices yields a spin-frustrated material from paramagnetic building blocks.

JERRY L.ATWOOD is at the Department ofChemistry,University of Missouri-Columbia,Columbia,Missouri 65211,USA. e-mail: atwoodj@missouri.edu

Synthetic chemists have recently demonstrated thatcrystal engineering1,which exploits the biologicalconcept of self-assembly,can be used to design

novel framework topologies and nanostructures that arebased on simple geometric principles.For example,platonic and archimedean solids have been used asdesign targets,and molecular versions of these pseudo-spherical structures have been created from simple building blocks — molecular polygons — by linkingthem together at vertices2 or edges3–5.A team led byMichael Zaworotko at the University of South Florida has now reported6 a variant of this approach.They use molecular squares to generate a nanoscale version of atopology that has been much sought after by physicistsand materials scientists7: the two-dimensional Kagomélattice (Fig.1a).The nanoscale Kagomé lattice built byZaworotko and colleagues has interesting magneticproperties at room temperature,confirming theoreticalexpectations about its behaviour.

The Kagomé lattice has been the subject of muchtheoretical study,but there are few examples of molecularKagomé lattices,and nanoscale versions have not beenreadily available to synthetic chemists until now6,8.In terms of building new magnetic materials,Kagomélattices are attractive because they naturally lead to spin-frustration when the system containsantiferromagnetic interactions.Spin frustration can beunderstood by comparing the behaviour of magneticspins located at the vertices of a square lattice with thoseon a triangular lattice.In antiferromagnetic materials,magnetic spins tend to align antiparallel with theirneighbours,which is possible if the spins are in a squarelattice,but impossible for a triangular lattice because oneof the spins in each triangle cannot align simultaneouslywith both its neighbours.Similarly,in a Kagomé latticesome of the spins remain frustrated and the material candevelop weak long-range ferromagnetic order,especiallywhen the lattice is constructed at the nanoscale.

The new material described by Zaworotko,Hariharan and colleagues is based upon simple chemicalbuilding blocks:paramagnetic dicopper tetracarboxylateunits9 linked together by 1,3-benzenedicarboxylate (bdc)anions (Fig.1b).The molecular units have anapproximately square geometry,and can self-assemble attheir vertices to form the triangular building blocknecessary for generation of a Kagomé lattice (Fig.2a).

But the molecular squares can also assemble to form asquare lattice,which the authors exploit to create a closelyrelated isomer with the same chemical formula,but aclearly different structure (Fig.2b).

The most interesting feature of the new nanoscaleKagomé lattice is the report of remnant magnetization attemperatures up to 300 K.The magnetic properties ofthe dicopper tetracarboxylate unit have been well-studied10 and it is surprising that magnetic hysteresis isobserved in this material.However,the authors arguethat the special topological arrangement of these units inthe Kagomé lattice is entirely responsible for thisbehaviour because antiferromagnetic order is precluded.This claim is strongly supported by directly comparingthe behaviour of the Kagomé lattice to that of thestructural isomer with a square topology.This materialhas an identical composition,yet does not develophysteresis,even at low temperatures.Because the squarelattice is not spin frustrated, it exhibits more traditionalparamagnetic behaviour .

This result demonstrates the advantages of crystalengineering in the design of materials with specificbulk properties,particularly molecular magnetism11.It also highlights the critical importance of topology indesigning functional materials.When one considersstructure–function relationships, the crystal structureis often of paramount importance,perhaps more sothan the molecular structure.Composition does notalways rule.Another advantage of this approach is theuse of well-known air- and water-stable compounds,

a b

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facile synthetic routes, and inexpensive chemicalbuilding blocks.

Because of the inherent nanoporosity of thisnanoscale lattice it will be possible to incorporate gueststhat impart different functionality to the magneticproperties of the Kagomé lattice framework.In particular,those in the field envision that metallicconductors,organic conductors,or organicchromophores could be incorporated into the lattice.Another feature of this system that appeals to crystalengineers is modularity.Many of the components areeasily interchangeable.For example,most transitionmetals can be used to form the dimetal tetracarboxylateunits.The bdc ligand can also be derivatized at any of fourpositions without changing its ability to sustain a Kagomélattice,or other bridging ligands can be used,in particulardicarboxylates with similar angles to that formed by bdc.

In summary,the structural and chemical features ofthis nanoscale Kagomé lattice make it a good prototypefor a wide range of related structures,and its nanoscalechannels mean that it should be possible to generatemultifunctional properties that are rationally tied tostructure and composition.With such a toolkit,thepotential structures are limited only by the imagination.

References1 Moulton, B. & Zaworotko, M. J. Chem. Rev. 101, 1629–1658 (2001).

2. Leininger, S., Olenyuk, B. & Stang, P. J. Chem. Rev. 100, 853–907 (2000).

3. MacGillivray, L. R. & Atwood, J. L. Nature 389, 469–472 (1997).

4. Orr, G. W., Barbour, L. J. & Atwood, J. L. Science 285, 1049–1052 (1999).

5. Fujita, M. et al. Chem. Commun. 509–518 (2001).

6. Moulton, B., Lu, J., Hajndl, R., Hariharan, S. & Zaworotko, M. J. Angew. Chem.

Int. Ed. Engl. 41, 2821–2824 (2002).

7. Ramirez, A. P. Annu. Rev. Mater. Sci. 24, 453–480 (1994).

8. Paul, G., Choudhury, A. & Rao, C. N. R. Chem. Commun. 1904–1905 (2002).

9. Eddaoudi, M. et al. Acc. Chem. Res. 34, 319–330 (2001).

10. Jotham, R. W., Marks, J. A. & Kettle, S. F. A. Dalton Trans. 428–438 (1972).

11. Kahn, O. Molecular Magnetism (VCH, Weinheim, Germany, 1993).

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Figure 2Two structural isomerscan be obtained by linking thevertices of molecular squares.a,A triangular Kagomé lattice,and b,a square lattice.Becauseonly the first one exhibits spinfrustration, the magneticproperties of the two lattices arevery different.

a b MATERIAL WITNESS

Old conflicts of interest

Government in the pocket ofbig-business paymasters.Academic integrity compromised by commercialconflicts of interest. No, this

is not a story about the oil or tobaccoindustries, but about the European materials economy in the sixteenth century.

The era of Leonardo da Vinci, Erasmus and Copernicus was atime of shifting power structures: the feudal hierarchy of theMiddle Ages was giving way to the modern system of mercan-tile capitalism.Hans Fugger,a textile merchant of Augsburg,established a thriving firm in the late fourteenth century,andhis son Jakob expanded the business by acquiring a share ofthe silver-mining industry in the Tyrol.But it was his son,also named Jakob,who turned this profitable trade into aninstrument of power and influence.

Jakob the younger began to lend money to princes andlords, whose finances were stretched by the constant warswaged in Europe in the late fifteenth century. In 1487,Jakob Fugger effectively gained control of all silver productionin the Tyrol. He widened the family’s mining interests to Spain and Hungary, and set up banks all across Europe formoney lending.

His most eminent client was Maximilian I, the Holy RomanEmperor, whose empire was vast, unwieldy and under threatfrom France, the Turks, the Italian states and the Lutheranswithin. When Maximilian died in 1519, his grandson Charles Iof Castile came to the Fuggers for a loan that allowed him tobribe his way to becoming the next Emperor.

Jakob Fugger later reminded Charles silkily of his indebtedness: ‘it is also well known and clear as day thatyour Imperial Majesty could not have acquired the RomanCrown without my help.’ Now business called the tune evenwith the head of Christendom.

The financial power of the Fuggers gave them control overacademia too.Using their influence over Charles, they acquireda monopoly on imports of guaiac wood from the New World,considered a miracle cure for the dreaded disease of syphilis.The guaiac decoctions were actually rather ineffective,butwhen the Swiss physician Paracelsus tried to say so in 1530,his book was suppressed by the Nuremberg publishers.

They had sought the advice of an independent referee,Heinrich Stromer of the University of Leipzig, who recom-mended that the book be banned. What Stromer neglectedto mention was that he owned shares in the Fugger’s guaiacimport business.

The social and political contexts of the sixteenth centuryof course bear little relation to those today, and it would bedisingenuous to suggest that we have much to learn aboutthe ethics of research or business from the tale of theFuggers. Except, perhaps, to say that the tension betweenthe free market, freedom of research and democracy isapparently a fundamental one and not the product of anyspecial set of circumstances.

PHILIP BALL

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