Molecular Magnets: The Prehistory

Molecular Magnets: The PrehistoryAuthor(s): Peter DaySource: Notes and Records of the Royal Society of London, Vol. 56, No. 1 (Jan., 2002), pp. 95-103Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/532125 .

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Notes Rec. R Soc. Lond. 56 (1), 95-103 (2002)

MOLECULAR MAGNETS: THE PREHISTORY

by

PETER DAY, F.R.S.

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

SUMMARY

The circumstances leading to the present worldwide explosion of interest in molecule-based magnetic materials are summarized. Interactions between developments in inorganic coordination and solid-state chemistry with developments in condensed-matter physics are identified as key factors in the emergence of this new field.

INTERACTIONS: MOLECULES AND MAGNETS

Does the history of science matter to the progress of science? Without going so far as to agree with Henry Ford's famous dictum that 'history is bunk', there is undoubtedly a sense in which the answer must be 'no'. If the point of science is to uncover and then explain the facts of the natural world, and the facts themselves are objective, it is not going to matter much who made a particular observation, or who was the first to expound a particular model. If Einstein had not had the idea of special relativity in 1905, or if Faraday had not discovered the phenomenon of electromagnetic induction in 1831, we can be fairly sure that within a few years somebody else would have done so. The reason that we can be so certain about this lies in the nature of science itself as a cooperative social system. Individual scientists, even those as great as Faraday and Einstein, do not work in isolation from the rest of the scientific community: the sheer volume of Faraday's scientific correspondence is ample testimony to this point. With only the rarest of exceptions, and especially with all the modern means of communication, they are fully (although perhaps only subconsciously) aware of the state of knowledge, not only in their own field but also in cognate fields.

If such a model of science as a complex interacting social system is accepted, then in trying to understand after the event why a certain development took place when (and in the way) that it did, we do need to look at the history, or at least the chronology, of events. In life, generalizations have to be backed up by examples, just as, in science, theories need to be validated by facts. In this brief article I shall take one currently fashionable topic, molecular magnets, and look into its antecedents a

95

( 2002 The Royal Society



Peter Day

little more closely. For example, recent review articles on molecule-based magnets make the following statements:

Molecular-species based ferromagnetic compounds, although postulated in the 1960's, have been realised only within the last decade.1

The first molecular compounds exhibiting a spontaneous magnetisation below a critical temperature were reported during the 1980's.2

Neither statement is correct. For the sake of clarity we can agree that a magnet is a solid that exhibits

spontaneous (i.e. zero-field) magnetization below a critical temperature, and by 'molecule-based' one can construe a solid whose structure consists of recognizable molecular units bound together by ionic, covalent or van der Waals interactions, and usually assembled from solution. What is beyond doubt is that until the 1950s no such materials had been found. In part, at least, that is because nobody had looked, although there is naturally a more profound reason.

INORGANIC CHEMISTRY AND SOLID-STATE PHYSICS

From World War II into the 1960s, inorganic chemistry had bifurcated: there was an explosion of interest in coordination chemistry, symbolized on the one hand by the inauguration of the International Conferences on Coordination Chemistry in 1949 (which still continue), and on the other by what might be called classical solid-state chemistry, which dealt with continuous lattice oxides, halides, chalcogenides, and so on. Crystals formed by coordination complexes are, of course, molecular but the bulk properties of such solids were rarely, if ever, considered. That was because the dominant (and, within its limitations, very successful) theoretical model used to rationalize the structures and electronic properties of coordination complexes, namely ligand field theory, took as its starting point the d-orbitals of a transition metal ion, more or less perturbed by the ligating atoms.3 Thus, attention was thrust on to the individual molecular entity, anything beyond the nearest-neighbour ligands being neglected.

Conversely, much of the impetus behind the solid-state chemistry of oxides during the same period came from technology, in particular microwave communications and information storage. Ferrites, garnets and magnetoplumbates were being thoroughly researched as memory devices, modulators and recording media, which in turn required a better understanding of the microscopic magnetic interaction mechanisms. At the phenomenological level of the molecular field model, this had been accomplished by Neel in his 1948 study of ferrimagnetism,4 but it was not till a few years later that Anderson described a quantum mechanical approach to the interaction between spins localized on neighbouring metal ions mediated by an intervening closed-shell anion such as 02-.5 However, the language of that model was more opaque than most solid-state chemists could handle, and it was only after a

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Molecular magnets: the prehistory

further interval of a few years that Kanamori6 and Goodenough7 translated it into a series of symmetry rules.

They defined the sign of the magnetic exchange interaction between two metal ions M and M' separated by an anion X according to the numbers of unpaired electrons occupying T(tzg) and o(eg) orbitals and the angle M-X-M', with the

limiting cases of 90° and 180°. Most tellingly, they noted that if orbitals carrying the unpaired electrons on the two metal ions are orthogonal to one another the interaction is ferromagnetic; this was in fact an extension of Hund's rule. That put a predictive weapon into the hands of the experimentalists, with the result that an enormous number of complex oxides and sulphides were synthesized and magnetically characterized, as summarized, for example, in Goodenough's magisterial book Magnetism and the chemical bond.8

MAGNETS AND COORDINATION COMPOUNDS

However, where were the coordination chemists while all this was going on? The only coordination compound in the 1950s whose unusual magnetic properties were shown conclusively to arise from interaction between metal ions was dimeric copper(II) acetate, the diamagnetization of which at low temperature was quantitatively explained in terms of antiferromagnetism by the physicists Bleaney and Bowers.9 The same group at the Clarendon Laboratory, Oxford, also uncovered the first case of magnetic exchange interaction between two metal ions not directly connected by bridging ligands when they found that the electron paramagnetic resonance spectrum of IrC12- doped in the diamagnetic host lattice of K2PtCl6 varied with doping level, because pairs of anions formed at higher concentrations.10 The pure compound K2IrC16 becomes antiferromagnetically ordered at very low temperature. Although an ionic lattice, the IrC12- can hardly be called anything but molecular.

However, those examples are antiferromagnets; what of ferromagnets? As the quotation at the beginning of this article avers, it was indeed in the 1960s that a potential mechanism for ferromagnetic exchange between localized moments in an insulating molecular crystal was put forward.11 Nevertheless, it was a decade before then that the first genuine example of bulk ferromagnetism in molecule-based crystals came to light at Bell Laboratories. It is true that the paper reporting this seminal result is only half a page long, but the abstract is unequivocal; I quote, in full: 'Certain of the complex cyanides of elements of the 3d transition group appear to be ferromagnetic at very low temperatures'.12 As the satirical magazine Private Eye might say, '...er, that's it'! The evidence adduced was twofold: a maximum in the susceptibility and a remanence. The compounds in question were transition-metal cyanide complexes of the Prussian Blue type and the physicist authors were suitably reticent about their precise chemical formulae. The role of the theoretical physicist P.W. Anderson, the author of the superexchange model mentioned above, as a co- author of this brief experimental paper is certainly worthy of note.

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Peter Day

PRUSSIAN BLUE: FERROMAGNET

Prussian Blue itself, with the stoichiometric formula Fe[IIFeII(CN)61 3 14H20 (figure la) was first subjected to study by neutron diffraction, the definitive technique for verifying the order of magnetic moments in solids, in 1973 (figure lb), and by polarized neutron diffraction only a few years later.13 Unlike the other bimetallic hexacyano salts MI[M"I(CN)6]3 or MII[M"II(CN)6]2 in which both metal sites carry unpaired spins, the fact that Prussian Blue itself is a ferromagnet seems rather strange at first sight because the Fe" is low spin and therefore diamagnetic. Moreover, in the simple cubic lattice of alternating FeII and FeIII, bridged by CN groups, nearest-neighbour Fe"' with S = are separated by no fewer than five closed-shell atoms (Fe"I-NC-FeI-CN-Fe11) at a distance of 10.2 A. Admittedly the Curie temperature is low (5.5 K), but the surprise is that it orders at all.

Prussian Blue is, of course, the grand-daddy of all mixed-valency compounds, a subject that had attracted my own attention in the 1960s,14 and in 1975, with a graduate student Bryan Mayoh, I published an article with the title 'Contribution of mixed valency to the ferromagnetism of Prussian Blue'.15 In it we showed quantitatively how an admixture of the low-lying intervalence FeII--FeIII excited state with the ground state stabilizes a parallel rather than an antiparallel arrangement of the Fe"' spins. Much more recently, a systematic synthesis of other mixed-valency cyanides with Prussian Blue structures has driven the Tc values of this compound type even higher, so that they are now well above room temperature.16

Argument of a somewhat semantic nature has turned on what really is a 'molecular' magnet; is it a solid constructed from molecular building blocks (what the late Olivier Kahn called 'bricks') or should one restrict the phase to solids in which there are no direct bridging groups between the magnetic units? Prussian Blue would be excluded by the latter criterion because it consists of an infinite three- dimensional network of coordinate links FelI-C and Fe"II-N. It seems clear that the first strictly molecular solid definitively verified as sustaining ferromagnetic order is

(a) FeII cNT (b) I -

-z-C--N-, / + _

_III ·.8- Fe N C- C N

~c ~ ~ .J^N--^ >^*~ .6-

.4-

.2-

o0 I i I t t 0 2 4 6

temperature K

Figure 1. Prussian Blue: (a) the crystal structure; (b) temperature dependence of the magnetic contribution to the neutron powder diffraction, showing Tc - 5.5 K. (Reproduced courtesy of Helvetica Chimica Acta

(see note 13).)

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the rather unlikely compound diethyldithiocarbamato-FeIIIC. How it came to light is itself a fascinating instance of serendipity.

THE FIRST MOLECULAR FERROMAGNET

During the 1960s there was much interest in making coordination complexes that would mimic the active sites in metalloenzymes, among them the Fe-S proteins such as ferredoxin. Fe complexes of dithiocarbamates (figure 2b) made for this purpose proved to have some quite strange and unlooked for properties, such as intermediate spin states of Fe"' between S = - (low spin) and S = - (high spin),17 and also temperature-dependent spin crossover from one spin state to the other.18 The latter has burgeoned into a subject in its own right, with implications for data storage devices.19 Over the same period, M6ssbauer spectroscopy was coming into its own as a tool for identifying electronic ground states, and thus it was that one (and only one) of the dialkyldithiocarbamates of Fe"' proved to exhibit magnetic hyperfine splitting at low temperature, indicating that there was an internal magnetic field within the lattice (figure 2a).20 Zero-field a.c. susceptibility identified Tc as 2.5 K. The crystal structure is uncompromisingly molecular, with no intermolecular contacts shorter than van der Waals radii. Subsequent very detailed measurements of single crystal susceptibility fully confirmed the ferromagnetic order and the easy axis of magnetization. Curiously, changing the alkyl group and the halide did not produce any more magnets of this kind.

Not long afterwards, another purely molecular solid with spontaneous magnetization at low temperature came to light in the form of Mn phthalocyanine.21 The phthalocyanines containing a divalent transition metal form an isomorphous series in which the planar molecules form stacks, arrayed in a herringbone fashion so that an N atom on one molecule lies above the metal atom at the centre of the next (figure 3a). Except for the Mn compounds they were antiferromagnets at low temperature but the latter proved to have an appreciable saturation magnetization, indicating that the moments were substantially canted, giving rise to weak ferromagnetism (figure 3b).

LOW-DIMENSIONAL MOLECULE-BASED MAGNETS

During the 1970s further examples of ferromagnetic exchange between simple coordination complexes were synthesized and, indeed, constructed by following the Kanamori-Goodenough recipe of maintaining orthogonality between magnetic orbitals on neighbouring metal centres. The first cases of this type were the so-called layer perovskite halide salts of Cu"I, whose critical behaviour was closely studied by the Leiden group as test cases for the statistical thermodynamics of the S = 2 square- planar Heisenberg ferromagnetic Hamiltonian.22 In Oxford, we realized that the first strictly molecular solid definitively verified as sustaining ferromagnetic order is

99



100 Peter Day

(a) 100.00

94.40 -

91.40-

89.40 -

100.00 o i g>8 C 3.0°k

98.00 -

100.00 j

99600 ` Mf A 2.50k

98.00-

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97.58-

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98.80- 2.3k

9760 -2 .

96.40-

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96.40-

94.60-

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|H WoO F

'-^s 00c N410 CH,

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I CH3

Figure 2. Bis-diethyldithiocarbamato-FelIC: (a) magnetic hyperfine splitting in the Mossbauer spectrum, showing the internal magnetic field; (b) the molecular structure.

(Reproduced courtesy of Physical Review (see note 20).)




180 240 300

T/K

Figure 3. Mn phthalocyanine: (a) molecular packing in the crystals; (b) temperature dependence of magnetic moment. (Reproduced courtesy of the Journal of Chemical Physics (see note 21).)

the orbital orthogonality induced by cooperative Jahn-Teller distortion of the coordination environment, and Tc values up to 50 K were found (figure 4a).23 In the Cr series there was the added bonus of extraordinary optical properties, whereby the ligand field transitions changed intensity by several orders of magnitude from the paramagnetic to ferromagnetic state, thus causing a colour change visible to the naked eye.24 These compounds, like the cyanides already described, and the bimetallic oxamido-ferrimagnets synthesized by Kahn and his colleagues in the 1980s25 somewhat beg the question as to what is 'molecular' because the planar MX4- units are organized into layers with neighbouring planes orthogonal via weak intermolecular M...X contacts (figure 4b). However, they are all prepared from solutions of their molecular constituents.

(b) 8.0 -

7.0 -

S Ei 0

t 0

5.0 -

0

~0

y

-V

3.0

0 60 120

101



Peter Day

(a) 1.0-

(b) 0.5-

rN~~oo d ~0°~ 0--- - 0 I I ,o I I o

TIK

Figure 4. Bis-alkylammonium tetrachlorochromates(II): (a) magnetization; (b) schematic crystal structure. (Reprodiced courtesy of the Journal of the Chemical Society-Dalton Transactions (see note 24).)

A MATURE DISCIPLINE EMERGES

Some years later, the 1980s saw a further step forward with the synthesis of the organometallic charge transfer complex decamethylferrocinium-tetracyanoethylene, which becomes fully ferromagnetic at 4.8 K and is entirely molecular, with the moment on the tetracyanoethylene anion mediating magnetic exchange between the Fe moments.26 Early in the following decade came an even more exciting departure in the field, the first molecule-based ferromagnet that is purely organic, in other words in which the unpaired spins are p rather than d or f.27 Thus we come to the present day, when hundreds of coordination, organometallic and even organic chemists throughout the world are engaged in the search for new molecule-based magnets with higher Tc values, other properties characteristic of the molecular solid state such as mesomorphism and chirality and with the first technological applications being close. The European Science Foundation sponsors a Programme on Molecular Magnets, the European Commission funds three networks on different aspects of the field and a biennial international conference attracts hundreds of participants. It is to be hoped that this brief account of the antecedents leading up to the present burst of activity in this interdisciplinary field will be of interest in highlighting some of the influences that shaped it.

REFERENCES

1 J.S. Miller and A.J. Epstein, Angew. Chem. Int. Edn 33, 385 (1994). 2 0. Kahn, Accts Chem. Res. 33, 647 (2000). 3 See, for example, C.J. Ballhausen, Introduction to ligandfield theory (New York, McGraw-

Hill, 1962).

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4 L. Neel, Ann. Phys. (Paris) 3, 137 (1948). 5 For a review, see PW. Anderson, Solid state physics (ed. F Seitz and D. Turbull), vol. 14,

p. 99 (New York, J. Wiley, 1963). 6 J. Kanamori, J Phys. Chem. Solids 10, 87 (1959). 7 J.B. Goodenough, Phys. Rev. 100, 564 (1955). 8 J.B. Goodenough, Magnetism and the chemical bond (New York, Interscience, 1963). 9 B. Bleaney and K.D. Bowers, Proc. R. Soc. Lond. A 214, 451 (1952). 10 J.H.E. Griffiths, J. Owen, J.G. Park and M.F. Partridge, Phys. Rev. 108, 1345 (1957). 11 H.M. McConnell, J Chem. Phys. 39, 1920 (1963); idem, Proc. R.A. Welch Found. Chem.

Res. 11, 144 (1967). 12 A.N. Holden, B.T. Matthias, P.W. Anderson and H.W. Lewis, Phys. Rev. 102, 1463 (1956). 13 H.J. Buser, P. Fischer, T. Studach and B.W. Dale, Z. Phys. Chem. 92, 354 (1974); P. Day,

F. Herren, A. Ludi, H.U. Giidel, F Hulliger and D. Givord, Helv. Chim. Acta 63, 148 (1980).

14 M.B. Robin and P. Day, Adv. Inorg. Chem. Radiochem. 10, 249 (1967). 15 B. Mayoh and P. Day, J Chem. Soc. Dalton Trans., 1483 (1976). 16 M. Verdaguer, A. Bleuzen, C. Train, R. Garde, F Fabrizi di Biani and C. Desplanches, in

Metal-organic and organic molecular magnets (ed. P. Day and A.E. Underhill), p.105 (Royal Society of Chemistry, London, 2000).

17 B.F Hoskins, R.L. Martin and A.H. White, Nature 211, 627 (1966). 18 A.H. Ewald, R.L. Martin, I.G. Ross and A.H. White, Proc. R. Soc. Lond. A 280, 235 (1964). 19 See, for example, 0. Kahn, Chemy. Br. 35, 24 (1999). 20 H.H. Wickman, A.M. Trozzolo, H.J. Williams, G.W. Hull and FR. Merritt, Phys. Rev. 155,

563 (1967). 21 C.C. Barraclough, R.L. Martin, S. Mitra and R.C. Sherwood, J Chem. Phys. 53, 1638

(1970). 22 L.J. de Jongh and A.R. Miedenna, Adv. Phys. 23, 1 (1974). 23 C. Bellitto and P. Day, J Chem. Soc. Chem. Commun., 870 (1976). 24 C. Bellitto and P. Day, J Chem. Soc. Dalton Trans., 1207 (1978). 25 0. Kahn, Y. Pei, M. Verdaguer, J.P. Rennard and J. Sletten, J Am. Chem. Soc. 110, 782

(1988). 26 J.S. Miller, J.C. Calabrese, H. Rommelman, S.R. Chittipedi, J.H. Zang, WM. Reiff and

A.J. Epstein, J Am. Chem. Soc. 109, 769 (1987). 27 Y. Nakazawa, M. Tamura, N. Shirakawa, D. Shiomi, M. Takahashi, M. Kinoshita and

M. Ishikawa, Phys. Rev. B 46, 8906 (1992).

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Documents

Molecular Magnets: The Prehistory