Molecules and magnets: joining Chemistrywith Physics. The legacy of Olivier KahnPeter Day*

Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BX, UK

Received 27 July 2000; accepted 10 November 2000

Article dedicated to the memory of Olivier Kahn

Abstract – The pioneering work of Olivier Kahn on molecular-based magnetic solids is placed in its conceptual andhistorical context as part of the evolution of coordination chemistry from a molecular to a supramolecular science. Thebackground to the theoretical approaches now current in molecular-based magnetism is surveyed, and those features ofthe long-range ordered magnetic state, which are unique to molecular lattices, are identified. © 2001 Académie dessciences / Éditions scientifiques et médicales Elsevier SAS

molecular-based magnets / Olivier Kahn

Résumé – L’œuvre pionnière d’Olivier Kahn sur les solides cristallins magnétiques basés sur les unités moléculaires estplacée dans son contexte historique et conceptualisée en ce qu’elle constitue une partie de l’évolution de la chimie decoordination. Les approches théoriques couramment utilisées dans le domaine du magnétisme des composés moléculairessont mentionnées et les caractéristiques de l’état magnétique ordonné à longue distance particulières aux réseaux molécu-laires sont identifiées. © 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS

composés magnétiques moléculaires / Olivier Kahn

1. Progress and people

The approach to history symbolised by thephrase ‘Cleopatra’s nose’ has never been an attrac-tive starting point for understanding how scienceprogresses. The notion that, had Cleopatra’s nosebeen half a centimetre shorter or longer, world his-tory would have been different, hardly seems toapply to an endeavour that appears to consist inuncovering objective laws of nature, which presum-ably would be the same irrespective of who firstdiscovered them. Yet we all know that the subjectmatter of science cannot be entirely divorced fromthose who practise it, and that scientific enquirysometimes takes particular directions as a result ofthe interest or advocacy of particular individuals.To exercise such a potent influence on scientific

thinking requires three unusual combinations offactors to be brought together. The first is a combi-nation of background knowledge from fields notpreviously thought of as having much in common;the second is a combination of diverse talents tobring the new perspective to the notice of others,and the third is a combination of circumstances at aparticular moment in the evolution of a scientificdiscipline (perhaps a kind of ‘zeitgeist’) when con-ventional approaches are seen to be inadequate inthe face of new knowledge. All three of these coin-cided in the case of Olivier Kahn, whose life andscience is commemorated by friends and colleaguesin the present volume.

First, Olivier Kahn combined the background ofa preparative coordination chemist with a deep

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knowledge of the theory of chemical bonding; sec-ond he was a fine communicator with highly devel-oped histrionic skills, and an enthusiastic invigora-tor of talented young people; third (and perhapsmost important), he came to his intellectual matu-rity at a key moment in the development of inor-ganic chemistry as applied to condensed matter.

2. Inorganic chemistry: moleculesand solids

Although the classical solid state chemistry ofoxides, halides and pnicnides formed an importantpart of inorganic chemistry in the earlier part of thepresent century, by the 1940’s and 50’s attentionhad largely shifted away from the continuous latticesolid state towards the chemistry of coordinationcomplexes and, later, organometallic compounds[1]. Whether charged or neutral, the fundamentalcharacteristic of these components is that exist asdiscrete molecules. To understand the bonding andstereochemistry in such systems, especially wherethe metallic element involved is from the transitionseries, attention focused on the energy levels aris-ing from the partly filled d-shell, first of all treatingthe surrounding ligands as a perturbation, loweringthe symmetry from spherical, and later by takingaccount of molecular orbital formation from metaland ligand basis functions. The first approach, bor-rowed from the solid state physics of transitionmetal ions in ionic crystals going back to Bethe inthe 1920’s, was called crystal field theory, while thesecond (which was built on the concepts ofmolecular bonding associated primarily with thename of Mulliken), became known as ligand fieldtheory [2].

However, the very success of these twoapproaches in rationalising structural features suchas stereochemical preferences, and physical proper-ties such as paramagnetic susceptibilities and elec-tronic spectra, served to mask one outstandingweakness. They treated the complexes as if theywere totally isolated from one another, without anyinteraction between them, although most com-pounds are prepared and studied as solids. Admit-tedly, the interactions between neighbouring unitsin the crystals of such compounds are often notlarge: to use a famous example from the 1960’s [3],the colour of NiSO4·7 H2O as a crystal is almost thesame as when it is dissolved in water, because bothsolid and solution contains [Ni(H2O)6]

2+. Yet therewere already many well-known examples to thecontrary, some of which were surveyed in the ref-erence just cited. The most spectacular involvedelectron transfer from one metal ion to another,

especially where the metals were in the form oftwo oxidation states of the same element, so-calledmixed valency compounds [4]. On the whole,effects of magnetic exchange interactions betweenneighbouring metal ions in a crystal cause lessspectacular effects on the optical spectra, though itis interesting to note that by the early 1960’s Fergu-son [5] and McClure [6] were studying the intensifi-cation of spin-forbidden ligand field transitions byantiferromagnetic exchange between metal ions influoride and oxide host crystals. Many of thesestudies were driven by a need to understand andmaster the energy transfer processes in the (at thattime) new technology of ruby and related solidstate lasers, and scarcely ruffled the surface of theconventional coordination and organometallicchemistry of the day.

3. Magnetism: from oxides tomolecules

Both experimental and theoretical developmentsin cooperative magnetism of insulating solids(which would later [7] gain significance inmolecular-based magnets) were also being drivenby technological imperatives. The coming of micro-wave devices required new magnetic materials thatwere not metals, i.e. which had only discreteenergy states above the ground state, and the manyfamilies of ferro- and ferrimagnetic perovskites,spinels and magneto-plumbites were the result.Along with these came the theoretical advancesthat were to underpin later developments inmolecular-based magnetism, such as Anderson’smodel [8], based on the Heisenberg–Dirac–VanVleck approach, systematised into the symmetryrules of Kanamori and Goodenough [9, 10]. It wasthe latter, above all, that brought cooperative mag-netism into the realm of solid state chemistry bysetting out clear recipes for predicting (at least inprinciple) how the sign of the magnetic exchangeinteraction, and at least qualitatively its magnitude,depends on the number of electrons occupyingeach orbital on a metal ion and the angle M-X-M’subtended by two neighbouring metals (M, M’) andthe anion X bridging them. The link to molecular-based magnetism becomes clear at once when it isrealised that X can be either mononuclear (O2–), bi-or trinuclear (CN–, NCS–, N3

–), or much moreextended (–O2C–C6H4–CO2

–), etc.).Before advancing further towards the aim of

placing Olivier Kahn’s achievements in their histori-cal and scientific context, a few remarks are inorder about the semantics of the word ‘magnet’,which carries rather different overtones depending

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on whether it is used by a chemist or a physicist.To a physicist, a magnet is a macroscopic objectcontaining a near infinite number of atomicmoments whose orientations are correlated over alength scale comparable to that of the object itself,i.e. effectively an infinite number of crystallographicunit cells. The object therefore shows a spontane-ous (i.e. zero field) magnetisation below a definitephase transition temperature. Chemists, on theother hand, often speak of magnetic ions or mol-ecules, when they really mean ‘paramagnetic’. Thedistinction between the two, and the misunder-standings that have arisen from not taking the dif-ference into account, form an important aspect ofthe collision between the two scientific cultures ofchemistry and physics engendered by OlivierKahn’s work.

On the other hand, the demarcation betweeninfinite and finite spin arrays was already present inthe physics world before chemists started makingnew magnetic objects, through the phenomenon oflow-dimensionality. To a physicist, ‘infinite’ meansjust that – in all three dimensions. But plenty ofinorganic and metal-organic solids exist in the formof layers or chains of strongly interacting atoms ormolecules, with only very weak interactionsbetween them. Thus in principle the possibilityexists of a crystal with atomic moments correlatedover an infinite range in one or two dimensions,but with no three dimensional order.

Starting from the principles of structural inorganicchemistry, in the 1970’s a broad correlation ofstructure and properties in low-dimensional solidsemerged, based on the presence of single or mixedvalency, and of ligands bridging the neighbouringmetal centres in the lattice [11]. In the same period,low-dimensional magnetism began to interestphysicists, especially from the viewpoint of statisti-cal thermodynamics of phase transitions and criticalexponents of the various order parameters. To vali-date their models they measured properties of vari-ous prototypes taken from the rich library of inor-ganic and metal-organic solids, a development thatbrought them into mutually fruitful contact withsynthetic chemists. Examples from this period arethe hexagonal perovskite halides with organic cat-ions, N(CH3)4MX3 [12] and the layer perovskitehalides with n-alkylammonium cations, (RNH3)2MX4

[13]. The latter, in particular, furnished the firstseries of solids containing molecular units to berecognised by physicists as magnets, in the sensethat the layers of ferromagnetically-correlated spinswere in turn coupled ferromagnetically to give bulkferromagnets. The M = Cu compounds have sharpphase transitions to long range order 6–10 K [13],

while the M = Cr ions, having S = 2 rather thanS = ½, have Curie temperatures between 35 and50 K, depending on X and on the organic cation[14].

All of which begs the question as to what reallyconstitutes a molecular, or molecule-based magnet.Here the above examples of chain and layer per-ovskites provide an interesting contrast. In theMX3

– chains in the hexagonal perovskites the M–Xbond lengths are equal, so the chain is like aninfinite polymer. In the layer perovskite halides ofCu and Cr, in contrast, a strong static Jahn–Tellerdistortion around the metal ions produces an anti-ferrodistortive ordering of the X in the basal planeso that, to the eye of a coordination chemist, theMX4

2– layer appears to consist of well-definedMX4

2– planar molecular ions with neighbouringmolecular planes all orthogonal. This cooperativeJahn–Teller ordering results in a lattice in whichneighbouring M orbitals carrying the unpaired elec-trons responsible for the magnetic moment are con-strained to be orthogonal. That is the classicKanamori–Goodenough rule for ferromagneticexchange.

It was one of Olivier Kahn’s early successes toengineer a combination of d1 ion (VO2+) and a d9

one (Cu2+) into a dimeric unit using the specificligating capabilities of a Schiff base anion in such away that xy and x2 – y2 orbitals on the two metalswere also orthogonal, inducing a very large ferro-magnetic exchange interaction between them [15].Nevertheless, such a dimer is not a magnet. It isalso an inconvenient fact that more exchange path-ways between localised atomic moments throughbridging ligands lead to antiferromagnetic exchangethan ferromagnetic. To engineer infinite latticescontaining bridging ligands, Kahn therefore turnedto ferrimagnetism, using metal ions (Cu2+ andMn2+) having the largest difference in moment pos-sible for 3d ions [16]. Further ingenious modifica-tion of ligand substituents by adding –OH groupsled to ferrimagnetic chains, interacting to give acrystal with 3D ferromagnetic order [17, 18].Unusual lattice topologies also emerged from bime-tallic combinations of the same elements, includinginterlocked orthogonal hexagonal networks, which(as Kahn said more than once with a typical rhe-torical flourish) reminded him of a necklace that hehad given to his wife [19].

4. A scientific legacy

Many more examples could be cited of Kahn’sfocussed imagination in devising new solid statecoordination compounds with unusual structures

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and properties. High coercivity magnets, forexample, based on strikingly elaborate structures[19] containing Mo(CN)7

4– [20], and interlocked net-works [21] or spin-crossover compounds tailoredbeautifully to exhibit sharp hysteretic transitionsstraddling room temperature and therefore, as aresult of the colour different between the spinstates, able to act as thermochromics [22]. But,above all, what could be claimed as Kahn’s scien-tific legacy is the growth and increasing maturity ofthe whole field of molecule-based magnets.

With the enormous number of conventional mag-netic materials available to technology as the out-come of a century’s progress, it is legitimate to askwhat fundamentally new features the molecularsolid state introduces into the field. In my viewthere are three. First, and by no means least, is theissue of processing. Until the coming of molecule-based magnets all known magnetic materials had tobe synthesised at high temperatures by ceramic ormetallurgical methods. Now there is a serious pros-pect of soluble magnets, or at least materials thatcan be deposited in this films by dip or spin-coating. Second, in contrast to continuous latticemagnetic materials, the vast majority of which areeither metals or small band gap semi-conductors,molecular-based magnets are insulators whose low-est energy excited states are discrete and localised.This opens up the possibility of spectacular photo-

magnetic and magneto-optical effects. Combiningthe first two of these attributes, one of my owngraduate students 20 years ago said of such a com-pound he had made that it was a green ferromag-netic soap! [23] The third attribute of molecular-based magnets, potentially the most fruitful in thelong term, is the opportunity it gives to harness thewhole variety of synthetic molecular chemistry,organic as well as inorganic, to devise latticescapable of supporting properties (or combinationsof properties) not attainable with simpler continu-ous lattice solids. Properties based on a combina-tion of magnetic order and structural chirality cometo mind, or new lattice topologies, of which theKagome (whose critical behaviour is intensely inter-esting to statistical thermodynamics) is only onesimple example.

Seen from this point of view, molecular-basedmagnetism falls into place as one of many fruitfuloutcomes of the evolution from molecular tosupramolecular chemistry. It is a flourishing depart-ment in d- and f-block coordination chemistry, andholds the promise of eventual technological out-comes in data storage and display. Considerablecredit for that state of affairs rests firmly withOlivier Kahn, whose combination of talents gavethe field the impetus to develop so strongly. WasCleopatra’s nose the right length? In this case theanswer must be an unequivocal yes!

References

[1] Cotton F.A., Wilkinson G., For an eloquent justification of thisgeneralisation, note the relative amounts of space devoted tomolecular and non-molecular species in, Inorganic Chemistry,1st ed., John Wiley, New York, 1963.

[2] Ballhausen C.J., Introduction to Ligand Fields, McGraw Hill, ???1965.

[3] Day P., Inorg. Chim. Acta. Rev. 3 (1969) 81.

[4] Robin M.B., Day P., Adv. Inorg. Chem. Radiochem. 10 (1967)248.

[5] Ferguson J., Guggenheim H.J., Tanabe Y., J. Phys. Soc. Jpn 21(1966) 692.

[6] McClure D.S., in: Seitz F., Turnbull D. (Eds.), Solid State Phys-ics, vol. 9, John Wiley, New York, 1959.

[7] Kahn O., Molecular Magnetism, VCH, 1995.

[8] Anderson P.W., in: Seitz F., Turnbull D. (Eds.), Solid State Phys-ics, vol. 14, John Wiley, New York, 1963, p. 99.

[9] Kanamori J., J. Phys. Chem. Solids 10 (1959) 87.

[10] Goodenough J.B., Magnetism and the Chemical Bond, WileyInterscience, New York, 1963.

[11] Day P., Ann. N.Y. Acad. Sci. 313 (1978) 9.

[12] Hutchings M.T., Shirane G., Birgeneau R.J., Holt S.L., Phys.Rev. B 5 (1972) 1999.

[13] De Jongh L., Miedema A.R., Adv. Phys. 23 (1974) 1.

[14] Bellitto C., Day P., J. Chem. Soc. Chem. Commun. (1976) 870.

[15] Kahn O., Galy J., Journaux Y., Jaudard J., Morgenstern-Badarau I., J. Am. Chem. Soc. 104 (1982) 2165.

[16] Pei Y., Verdaguer M., Kahn O., Sletten J., Regnard J.P., Inorg.Chem. 26 (1987) 138.

[17] Pei Y., Verdaguer M., Kahn O., Sletten J., Regnard J.P., J. Am.Chem. Soc. 108 (1986) 7428.

[18] Kahn O., Pei Y., Verdaguer M., Regnard J.P., Sletten J., J. Am.Chem. Soc. 110 (1988) 782.

[19] Stumpf H.O., Ouahab L., Pei Y., Grandjean D., Kahn O., Sci-ence 261 (1993) 447.

[20] Larrionova J., Clerac R., Sanchez J., Kahn O., Golhen S., Oua-hab L., J. Am. Chem. Soc. 120 (1998) 13088.

[21] Vaz M.G.F., Pinheiros L.M.M., Stumpf H.O., Alcanatara A.F.C.,Golhen S., Ouahab L., Cador O., Mathonière C., Kahn O.,Chem. Eur. J. 5 (1999) 1486.

[22] Kahn O., Chem. Br. 27 (1999) 00.

[23] Stead M.J., Day P., J. Chem. Soc. Dalton Trans. (1982) 1081.

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