# Florence-Orsay: A Joint Laboratory with Olivier

Dante Gatteschi, Michel Verdaguer

► To cite this version:

Dante Gatteschi, Michel Verdaguer. Florence-Orsay: A Joint Laboratory with Olivier. European Journal of Inorganic Chemistry, 2018, 3-4, pp.215 - 222. 10.1002/ejic.201700985. hal-01761832

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Olivier Kahn

CLUSTER
ISSUE

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# Florence-Orsay: A Joint Laboratory with Olivier

Dante Gatteschi*[a] and Michel Verdaguer*[b]

**Abstract:** The present essay gathers a few anecdotes and slices of the authors' scientific lives, trying to show how Olivier Kahn

occupied a pivotal position in molecular magnetism and why it is possible today to speak of his scientific legacy.

## Introduction

This essay is written by the entangled pens of two researchers who have known Olivier Kahn for a long time, each in a different scientific and human context but with a shared association and friendship. Our stories, implications, and achievements are different, but we were actors in the first rise of our discipline, molecular magnetism, in the seventies/eighties and contributed to its ripening in the nineties in close contact with Olivier in a competitive-collaborative attitude, which we think can be useful to make available to the new generations as an example of synergic efforts to reach important results. The present essay gathers a few anecdotes and slices of our scientific lives, trying to show how Olivier occupied a pivotal place and why it is possible to speak today of his scientific legacy.

Nevertheless, the essence of invisible links that brought us closer to Olivier cannot be traced only to our mutual passion for new materials and Hamiltonians or to our similar ages, but also to our common views on wider social issues. Our childhoods were stamped with the experience of fascism and anti-Semitism in our countries, and then we were engaged in fights against colonial wars, for the development of freedom in the society during the 68 events, for the emancipation of women, for peace, ... We shared with Olivier the ideas of freedom, solidarity, respect for the people, democracy, the rational approach, and the hope in the progress of humankind thanks to science. This was some kind of compost for confident relations and passionate discussions.

## Our Special Relations with Olivier

Both of us have bright memories of Olivier Kahn.

For one of us (D. G.), Olivier was a model to follow as a source of inspiration to improve the quality of the research in Florence. The development of inorganic chemistry was a local development, which started with Sacconi and was continued by Ciampolini, then by Bertini and D. G. It was the time of the ligand field, and D. G. made efforts to develop the treatment for UV/Visible and EPR spectra, essentially in a self-taught way. EPR spectroscopy was D. G.'s battle horse.[1] Of course, sooner or later D. G. was bound to meet Olivier, and the two found a cooperative interaction, which will be highlighted in this contribution.

For the other author of this contribution (M. V.), Olivier was, in the middle of the seventies, a brilliant lecturer whom M. V. hired to teach quantum inorganic chemistry to the top-level students of Ecole Normale Supérieure de Saint-Cloud. He was the charismatic leader of a small team of permanent researchers "Spectrochimie des Eléments de Transition", which he created at Orsay in 1976. M. V. joined the group from October 1977 to September 1988. During these years, M. V. learned a lot, enjoying many passionate discussions. M. V. performed some significant work under Olivier's guidance, and, in this way, they became friends.

## A Common Recognition

The strong interactions between Olivier and D. G. and Olivier and M. V. led later to a long-lasting friendship between D. G. and M. V. The friendship required the development of a common flag, which was the Florence-Orsay group, in short Florsay.

We think it is appropriate here to note that Olivier Kahn is quite often called Olivier and the family name is omitted. A possible explanation is that Olivier was familiar with many people and preferred Christian names, also to give a feeling of familiar friendship.

<footnote>[a] Laboratory of Molecular Magnetism (LaMM), Department of Chemistry and INSTM, Università degli Studi di Firenze, Via della Lastruccia, 3, 50019 Sesto Fiorentino, Italy
E-mail: dante.gatteschi@unifi.it
https://www.lamm.unifi.it/vp-191-dante-gatteschi.html</footnote>

<footnote>[b] Institut Parisien de Chimie Moléculaire, UMR CNRS 8232, Department of Chemistry, Université Pierre et Marie Curie, Paris, Case 42, IPCM, UPMC, 75252 Paris Cedex 05, France
E-mail: michel.verdaguer@upmc.fr
http://www.ipcm.fr/VERDAGUER-Michel?lang=fr</footnote>
# The Scientific Frame

## From Magnetochemistry to Molecular Magnetism

The fundamental role of Olivier Kahn in the development of molecular magnetism cannot be overestimated. He was among the first scientists to understand the opportunities of the new field of research. He was successful not only in synthesizing new materials, developing new physical techniques, and introducing new theoretical approaches, which afforded him the opportunities to make an integrated approach to the new field. He was one of the scientists who promoted the switch from magnetochemistry to molecular magnetism.[2]

We recall that magnetochemistry was the use of quantum mechanical approaches to obtain structural information on transition metal compounds from magnetic measurements, while molecular magnetism was the development of a more general approach based on the design, the synthesis, the study, and the use of magnetic molecular systems with expected properties.

## Castiglione, a Cornerstone in Molecular Magnetism

Beyond realizing the importance of the field, Olivier was active in the development of collaborations between the groups active in the field, also taking into consideration the new opportunities coming from European funding sources, which were becoming available. He led several European networks. One of the first masterpieces of Olivier was the publication of a book on the results of a North Atlantic Treatise Organization (NATO) Advanced Study Institute (ASI), which was held in Castiglione della Pescaia, on the Tuscan sea in 1983.[3] ASI meetings were one of the means chosen at this time by the NATO to strengthen the links between American and European scientists, to win the competition against soviet scientists. Not without success at the end. The ASI title was "Magnetostructural Correlations in Exchange-Coupled Systems". D. G. remembers that there were some difficulties for the physicists to understand the meaning of the title, but in the end it worked well. The editors of the book were Roger Willett, Olivier, and D. G.. The theme was ripe to become popular, and the choice made regarding the types of materials was appropriate. It put together for the first time chemists and physicists who wanted to be active on what would be called *molecular magnetism*. Roger was professor in Seattle, Washington, and had a long-standing tradition of collaboration with physicists. The interest for collaboration between physicists and chemists was the interest of the former to have molecules available for investigating low-dimensional magnetic materials. For chemists, it was fundamental to understand what could be useful for the molecules they produced in large quantities. Of course the physicists needed help to understand the obscure structural formulae used by chemists to describe their molecules, and chemists had to overcome the mathematical barrier of heavy physics. Anyway, the efforts made in the two weeks in which the ASI meeting took place were crowned by the improvement of researchers of both disciplines to substantially understand what was needed for the development of a promising research field. In the margins of the

ASI, a passionate controversy (Jos de Jongh, Jill Bonner, Jean-Pierre Renard, ...) arose around a theoretical guess recently conjectured about the ground state of integer spins of antiferromagnetic chains: "Haldane gap", dream or reality? Four years were necessary for Renard to experimentally demonstrate the existence of the gap[4] on a Ni[11] chain published by Olivier in 1982,[5] and it took thirty more years for the physical community to transform F. D. M. Haldane, the conjecture's father and master of the topological phases, into a Nobel Prize winner (2016)!

This 1983 ASI meeting can be considered as one of the cornerstones in the birth of molecular magnetism.

Olivier was the guy capable of managing essentially all the areas of molecular magnetism: chemical synthesis, magnetic properties, theory, and, perhaps last but not least, the advertisement of the investigated systems. His role in the production of the book following the Castiglione meeting was of a fundamental nature, magnifying the relevance of the field. He used hyperbole very naturally, and he covered the scarce propensity of Roger and D. G. to describe the relevance of the systems.

## Visiting Professor in Orsay (D. G.)

The previous analysis evidences a strong interaction between the Orsay and Florence laboratories, a sort of beginning of the Florence-Orsay (Florsay) group. Indeed it was quite unusual to see two competitive researchers being able to collaborate rather than fighting against each other. D. G. covered the EPR technique in particular, which proved to be especially efficient for the investigated systems.

There were very few joint publications, but there were important discussions between us, especially by those who took advantage of the visiting professor positions given by Olivier. D. G. took advantage of the kind offer to visit Orsay in 1985 and used the opportunity to write the first chapter of a book. We cannot avoid mentioning something that highlights well some aspects of Olivier. When invited, D. G. expected to have accommodation in Paris. The idea was to use the free time for some sightseeing. Olivier showed me the accommodation he had reserved for me: an office in the rectorate. As one can imagine, the rectorate was in the campus of Orsay, well separated from the Orsay village, which is not exactly a metropolis. D. G. managed to stay in this residence for one week, and then moved to Paris. Andrea Dei profited by D. G.'s stay to join him in Paris, and the joint visit was very pleasant. The only problem: I (D. G.) was awakened by a click, which continued for a while around six o'clock in the morning. Of course it was Andrea's attempts to light his first cigarette. He has continued to smoke up to now. I think he smokes 30 cigarettes per day, 365 days, which amounts to 10950 cigarettes per year.

One of the favorable properties of the Orsay campus was the presence of groups of physicists who were interested in the properties of low-dimensional magnetic materials (Jacques Friedel, Denis Jérôme, Rémi Julien, Renard, ...). They were therefore interested in molecular systems. Just to provide some information on the materials of interest, we can cite tetramethylammonium manganese chloride (TMMC). It was chosen be-
cause it had a chain structure of manganese ions that was well shielded from neighboring chains, providing a good model for one-dimensional magnets. Although the interest was declining at the time we are referring to, there was still attention directed to molecular systems, which could provide breakthroughs.

The laboratory of Olivier was a top-level structure with excel-
lent people. At the beginning there were senior researchers
who shared the responsibility of the laboratory with Olivier.
Irène Morgenstern, after working in the shade of Olivier for a
while, moved to bioinorganic chemistry, a new field where the
expertise grown in coordination chemistry could be exploited.
A parallel route was followed by Jean-Jacques Girerd, who, after
a thesis dedicated to one-dimensional systems, spent one year
with Richard Holm in the US. The stay marked the change in
direction from molecular magnetism, not before bringing a
relevant contribution to mixed valence theory. In fact, Jean-
Jacques re-examined the ferromagnetic coupling of the two
nickel ions in the mixed-valent [Ninapy]₂Br₃ (napy = 1,8-
naphthyridine), which I (D. G.) had investigated and commented
in a simplistic view, and attributed the ferromagnetic coupling
to antisymmetric exchange.[6,7]

Another researcher was M. V., who developed a friendship
with D. G., which has not diminished for more than 40 years.
Younger was Talal Allah, who would later become the chair
when Olivier moved to Bordeaux. The collaboration with the
physicists in Renard's team was a definite advantage: having a
great expert at walking distance. There was also another advan-
tage of being in contact with Renard. When Roberta Sessoli
graduated, D. G. favored her stay in Orsay. It was a very nice
stay for her, which made her go through a quantum jump that
corresponded to being able to manage chemistry and physics.

The group of Renard, with Pierre Veillet, Claude Chappert, Pierre Beauvillain, and Bernard Lécuyer, was developing intense endeavors to build very sensitive home-made magnetometers up to the point that a small company decided to build and to sell a French SQUID based on their experience. Olivier was the first to buy this machine and advertised it urbi et orbi. Florence bought the second. Unfortunately, the technological solutions, which were so remarkable in the hands of scientists, turned less efficient for mass production in those of engineers, and the dream of a French SQUID died out.

The other Florence members of Florsay, beyond Roberta,
were Alessandro Bencini, who died too early but was a funda-
mental member of the group with a unique capability to go
through synthetic chemistry, experimental techniques, EPR
spectroscopy, and theoretical methods. Other members were
Andrea Caneschi, Claudia Zanchini, who became professor in
Cosenza, and Cristiano Benelli, fond of lanthanides.[8] A charac-
ter is missing: Andrea Dei. Beyond smoking, he had a splendid
intuition to recognize the exciting developments to follow.

The Rational Design of New Materials

The role of Olivier in the development of molecular magnetism
has been expanded at different levels. The first level was that
of research, where he started many new fields based on coordi-
nation chemistry, molecular magnets, and spin transitions.

The first lesson to be grasped from Olivier is his constant will
to theoretically predict the magnetic properties of new mol-
ecules to be created and to place experimental results on a firm
theoretical basis. Two main directions (and dreams) are present
on the path of Olivier: transforming molecules into magnets,
and finding systems presenting molecular bistability.

A Heitler–London Model of Exchange

Olivier designed, with Bernard Briat, a theoretical Heitler-
London model of exchange, summarized by a simple formula,
understandable by any chemist, which connects the coupling
constant *J* to fundamental integrals (overlap *S*, resonance *β*, or
exchange *k*) (*J* = 2*k* + 4*β*S). The singlet–triplet physical observ-
able *J* is the sum of two terms: one positive (*k*) favoring ferro-
magnetic (F) interaction, the other one negative (*β*S), pushing
towards antiferromagnetic (AF) interaction. This boring defini-
tion became, during the lectures of Olivier, with raised arms, an
unforgettable fight between the ferro and the antiferro gladia-
tors.

Simple Molecular Models

Olivier convinced his co-workers at Orsay to synthesize simple
molecules to check the validity of the theoretical model: (1)
how the orthogonality of magnetic orbitals can give rise to
ferromagnetism in a bimetallic CuII–VOII (d⁹–d¹) complex – illus-
trated in the table of contents graphic of this essay;[⁹,¹⁰] (2) how
orbital overlap between centers having different spins leads to
antiferromagnetic interaction and to nonzero total spin, that is,
molecular ferrimagnetism, in bimetallic CuII–FeIII and CuII–MnII
pairs[¹¹] (and then in chains); (3) how overlap between magnetic
orbitals generates antiferromagnetic interaction [copper(II) di-
nuclear complexes];[¹²] (4) how to tune the antiferromagnetic
interaction by changing the bridge or by tuning the symme-
try.[¹³,¹⁴]

From Simple to Complex: From Ferrimagnetic Chains to Magnets

The basic idea, or the dream, was to go from the isolated molecule to the molecular magnet with long-range magnetic ordering, thus moving from simple to complex. A possible route to change from zero-dimensional to three-dimensional systems (from 0D to 3D) is to go through 1D. The first bimetallic ferromagnetic chain was synthesized by Alain Gleizes in Toulouse, clearly following the ideas developed by Olivier at this time.[15] The following years knew a burst of results of bimetallic ferromagnetic chains, thanks to the molecular design of suitable bridging ligands – based on the oxamide or oxamate bridges that are more flexible than oxalate or dithiooxalate. Most of them showed a 3D antiferromagnetic transition, among them catena MnCu(pba)(H₂O)₃·2H₂O [Figure 1c; pba = 1,3-propylenebis(oxamate)]. Thanks to a simple spin model (Figure 1a, b) and subtle but clever modifications of the bridging ligand suggested by Olivier [going from pba to pba-OH = 2-hydroxy-1,3-propylenebis(oxamate)], Yu Pei was able to obtain a ferrimag-
netic chain by molecular design (Figure 1c), ferrimagnetic planes by crystal engineering of hydrogen bonds (Figure 1d), which transformed into a molecular magnet, ordering at 4.6 K.[16] Olivier later summarized these supramolecular approaches.[17]

<figure><img src="image_4.png" /></figure>

Figure 1. Ordering ferrimagnetic chains: (a) Spin strategy: trivial antiferromagnetic (af) interaction due to an antiferromagnetic interaction between large spins in neighboring chains. (b) Spin strategy: ferrimagnetic arrangement by displacing one spin chain to create antiferromagnetic coupling between the large spins on one chain and the small spins on the neighboring chains (displacement shown by empty red arrow). Crystal engineering from (c) MnCu(pba)(H₂O)₃·2H₂O, (antiferromagnetic interaction between neighboring ferrimagnetic chains) to (d) MnCu(pba-OH)(H₂O)₃·2H₂O (ferrimagnetic planes). The blue lines schematize the dominant antiferromagnetic interactions, and the red arrows point out the displacement of the chains in the crystal thanks to the hydrogen-bond network (adapted from ref.[18]).

## From 1D to 3D

After this notable result and another NATO meeting, organized by D. G. in Il Ciocco where Renard recalled Landau's result "long-range order is forbidden in 1D at T ≠ 0 K", several groups began to synthesize 2D and 3D materials directly (Gregory Girolami in the US, M. V. in Paris, ...). The results of Yu and Olivier paved the way to the direct rational synthesis of high-T~C~

molecular magnets, including room-temperature ones, with the Prussian blue analogue V[Cr(CN)₆]₀.₈₆·2.8H₂O.[19,20] In a "news and views" in Nature, Olivier pointed out that "... remarkably it was obtained in a rational way, rather than as a result of serendipity. V[Cr(CN)₆]₀.₈₆·2.8H₂O is an interesting material in its own right and an excellent example on which to learn (or to teach) the basic concepts of molecular magnetism". This result is evidently part of the ferrimagnetic legacy of Olivier.[21] At the end of the nineties, combining exchange and anisotropy, Olivier developed many 3D molecular magnets combining high T~C~ values and record coercitivities.

## High Nuclearity Spin Clusters and High Spin Molecules

Another trend impulsed by Olivier was the synthesis of molecules with large spins as building blocks for molecule-based magnets. This chemistry developed on both sides of the Atlantic Ocean (at Orsay and particularly in the US with David Hendrickson and George Christou and in Florence with Andrea, Roberta, and D. G.) either with high nuclearity spin clusters[22] or through a metal-radical approach, in collaboration with Paul Rey in Grenoble.[23,24] It is in this area that something abruptly changed in molecular magnetism: the discovery of dynamic magnetic properties and slow relaxation of the magnetization of some high spin molecules presenting at the same time axial anisotropy.[25] The molecular bistability, long sought by Olivier with spin crossover materials, was finally achieved through these objects quickly surnamed "single molecule magnets". See below the discussion on the lively vocabulary and smart names.

## Spin Crossover Complexes, the Search for Molecular Bistability

Olivier explored for years the possibility to achieve molecular bistability with spin crossover complexes, both theoretically and with new materials. This subfield was associated with the investigation of devices for information storage. Olivier exploited the fact that iron(II) complexes such as Fe(phen)₂(NCS)₂ (with phen = o-phenanthroline) were stable in the diamagnetic state at low temperature, while high temperature stabilized the high spin state. With a one-dimensional system made from Fe<sup>II</sup> ions bridged by triazole molecules, he engineered "molecular alloys" presenting a large hysteresis loop around room temperature. He realized that this transition could be exploited for new types of phone cards, which allowed reading the level of credit still available, thanks to a nice red to white color change. He established contact with a company, and some operational prototypes were made.[26] But the time of use never came, because more efficient technologies became available. Just to keep the reader informed, Florence had discovered another class of spin crossover transition involving high spin and low spin Co<sup>II</sup>. We only toyed with the concept as an exercise, but were never
really involved in research. However, in Orsay the group of Jacqueline Zarembowitch was developing the field. During his visitor's stay, D. G. had the opportunity to interact with Jacqueline, taking advantage of the experience developed in Florence. No joint publication, but a fruitful exchange of ideas and the hope that molecules could be used as bistable systems, grew out of the discussions.

With this smart card spin crossover saga, Olivier had in mind the necessity for molecular magnetism to provide examples of devices useful in everyday life applications. Last but not least, Olivier has been named by a journalist "a chiseler of molecules" since he always tried to design elegant molecules or smart frameworks. One of the best examples is the beautiful fully interlocked three-dimensional magnet, bearing three different spin bearers including an organic radical,[27,28,17] immediately surnamed "necklace" by Olivier (Figure 2).

<figure><img src="image_5.png" /><figcaption>Figure 2. Fully interlocked structure of (Etrad)₂Mn₂[Cu(opba)]₃·solvent emphasizing the two interlocked networks A (red) and B (blue) [Etrad⁺ = ethylpyridiniumnitronyl nitroxide radical; opba = o-phenylenebis(oxamato)]. Reprinted with permission from ref.[17] Copyright 2000 American Chemical Society. Color image: Courtesy of L. Ouahab.</figcaption></figure>

## A Proper Lively Language

Olivier realized early that a proper language had to be developed, to be understood by synthetic chemists, physical chemists, physicists, etc. In Orsay, for example, he organized seminars with Friedel and the Solid State Physics Laboratory to gather chemists and physicists engaged in the synthesis and studies of molecular materials [(super)conducting, magnetic, ...]. The goal was to strengthen the scientific links and favor a common culture and a common language. But let us clarify what we mean by giving an example, here the participation of Olivier is not explicit, but it must have been inspired by him.

One of the most important steps in the field of molecular magnetism was the discovery of a cluster containing 12 manganese ions, which was found to behave like a macroscopic specimen but comprised only 12 metallic ions. The first choice for the name was left to D. G., who did not find anything better than "High Nuclearity Spin Cluster" (HNSC).[29] The idea was that of producing a generic name of the organometallic clusters that were intensively investigated by the Milan school of Chini. In that environment high nuclearity was currently used, and the expression had a meaning. But the term HNSC was obscure.

The right proposal was made by Hendrickson, one of the discoverers of the new clusters, who first suggested "high spin

molecules" (HSMs)[22,30] and then "single molecule magnets" (SMMs).[31] Perfect! It transmitted all the needed information in three words. Olivier could not do any better, and indeed the term SMM was a big success, and is still widely used, followed by the offspring "single chain magnets" (SCMs) and "single ion magnets" (SIM).[32] We are sure that the input of professionals in finding the proper words will increase in the next years.

In our times of fashionable communication, one can think of establishing an international competition to improve the appeal of the words defining scientific concepts. What about high density electrons, or sausage rolls?

Seriously, it may be a nice exercise.

Another example is a series of compounds that could reversibly accept and reject water molecules. This chemical structural transition is accompanied by a magnetic transition. The paper was called "molecular magnetic sponges" and it is a textbook example of how the magnetic properties can be tuned to increase the spin dimensionality, a requisite needed to attach bulk magnetism to the material.[33] Not a bad name in this context, provided the adjective molecular is always used with the term magnetic sponge. See, for example, a new publication on magnetic materials.[34] Exploring the web, one finds a few magnetic sponges referring to devices that are used to clean surfaces, a property that does not apply to the molecular counterparts.

Olivier was indeed skilled in this matter: he coined a dynamic "molecular magnetism" to describe the whole discipline, which is now recognized worldwide, instead of the antiquated "magnetochemistry" or "molecule-based magnets". Olivier popularized visual expressions such as "magnetic orbitals" instead of singly occupied molecular orbitals (SOMO), or invented expressions such as "orbital reversal" or "irregular spin state structure" and "molecular bistability", which have known various fates later.

## Common Work, Collaboration, and Controversy

As we mentioned before, there were few collaborations between Orsay and Florence but there were important discussions.

### EPR Spectroscopy of Dinuclear Copper(II) Complexes

An example was the paper we wrote together on the unexpected presence of a dinuclear "impurity" in the crystal of a dinuclear copper complex.[35]

In order to establish structural magnetic correlations, the Orsay people synthesized [(tmen)Cu(ox)Cu(dien)(H₂O)₂]²⁺, (dien = diethylenetriamine; ox = oxalate; tmen = tetramethyl ethanediamine), whose structure is shown in Figure 3a. The two copper(II) ions are bridged by one oxalate group.

In fact, in the crystal grown by Miguel Julve, we had two dinuclear Cu¹¹ AF complexes. Most of the crystal was made of the asymmetric [(tmen)Cu(ox)Cu(dien)]²⁺, difficult to synthesize
<figure><img src="image_6.png" /><figcaption>Figure 3. Perspective view of (a) [(tmen)Cu(ox)Cu(dien)(H₂O)₂]²⁺; (b) [(dien)Cu(ox)Cu(dien)]²⁺. Hydrogen atoms are omitted for clarity. (c) Single-crystal EPR spectrum of [(tmen)Cu(ox)Cu(dien)(H₂O)₂](ClO₄)₂ at X-band frequency and 4.2 K, recorded with the static magnetic field parallel to the z axis. Adapted and reprinted with permission from ref.[35] Copyright 1986 American Chemical Society.</figcaption></figure>

(Figure 3a) and presenting a *J* value around −75 cm⁻¹. The second species was the symmetric [(dien)Cu(ox)Cu(dien)(H₂O)₂]²⁺ (Figure 3b), as an impurity of a few percent, not detectable by X-ray diffraction. It can be synthesized independently and presents a *J* value around −13 cm⁻¹. At low temperature (4 K), the complex [(tmen)Cu(ox)Cu(dien)(H₂O)₂], strongly antiferromagnetically coupled, is present as a singlet. It is EPR-silent. The only signal detectable comes from the triplet of the less coupled minority component [(dien)Cu(ox)Cu(dien)(H₂O)₂], well isolated in a diamagnetic matrix, with a beautiful EPR signal obtained by Bencini on a single crystal (Figure 3c).

Of course, the orbital interpretation of the antiferromagnetic interactions, a characteristic mark, followed the model of Olivier (Figure 4).

This example evidenced how flexible the lattice of a coordination compound can be and how its properties can be modulated by a doping procedure. For us today, it illustrates the way Olivier was trying to see in depth, to explore, and to understand an unexpected observation (which would have been described as a trivial "Curie tail" at low temperature by many others at this time).

<figure><img src="image_7.png" /><figcaption>Figure 4. Orbital interpretation of the tunable exchange in three complexes. Schematic representation of one of the molecular orbitals built from the two magnetic orbitals: (a) [(tmen)Cu(ox)Cu(tmen)]²⁺; (b) [(dien)Cu(ox)Cu(dien)]²⁺; (c) [(tmen)Cu(ox)Cu(dien)]²⁺. Shown in red are the metal d orbitals, represented down the z direction. In blue are the p orbitals of the oxygen atoms of the oxalate bridge emphasizing the orbital overlap *s* on one side of the bridge. On the right, comparison of the experimental data and the theoretical model conceived by Olivier. Adapted with permission from ref.[36] Copyright 1990 American Chemical Society.</figcaption></figure>

## EPR Spectroscopy of Bimetallic Ferrimagnetic Chains

An important feature of Olivier's scientific activity is his interest in the detailed properties of the systems he created. A clear indication of this was the study of the EPR spectra of a series of copper(II) manganese(II) bimetallic chains, which were originally synthesized in the frame of exploring the nature of the coupling in molecular magnets (See Figure 1). During D. G.'s stay in Orsay, there was a discussion on the opportunity to investigate the EPR spectra to further characterize the magnetic properties, usually explored by magnetic susceptibility, and to combine spectral and thermodynamic measurements. The discussion took place in the presence of John Ammetter of the University of Zurich. He shared D. G.'s interest in EPR spectroscopy, and as a result they decided to study the single crystals of the CuMn ferromagnetic chains described in Figure 1 by EPR spectroscopy.

We were interested at that time in the detailed interpretation of spectral parameters. It was a field that was essentially virgin for molecular magnets, although a few experiments had been reported. The Florsay group decided to tackle the problem, and it was a heavy concentration of strong actors: Olivier, Yu, M. V., and Olivier Guillou on the French side, Claudia, Roberta, and D. G. on the Italian team. I (D. G.) take the opportunity to comment on Olivier "le petit" who spent more than one year as pre-doc in Florence. He made good progress in science but was a complete disaster in Italian. The only word he was familiar was "graccie", which should have been "grazie" ("merci" in French).

The results obtained by Guillou provided an example showing how EPR spectroscopy could produce important information.[37] More precisely, the angular dependence of the EPR line widths measured in the temperature range 4–300 K on the two CuMn chains shown in Figure 1c, d was discussed in relation to dipolar interactions between the paramagnetic centers. Short-range-order effects were evidenced, and the EPR spectroscopic
study enabled the elucidation of the spin distributions in both compounds.

These conclusions could not be obtained with other methods. The analysis of the properties of the chains was therefore very interesting, and D. G. would have appreciated the opportunity to investigate much more, but the results obtained in different fields drew attention away to the investigation of other sophisticated properties.

## Scientific Controversy (D. G.)

One of the common scientific playgrounds in Florence and Orsay, and then Bordeaux, was the magnetic properties of molecular compounds in which transition metal ions interact with lanthanides. Having strong characters, Olivier and D. G. happened to have different views on relevant problems. An example was the mechanism of exchange coupling in copper(II)-gadolinium(III) systems. We in Florence reported some examples where a ferromagnetic coupling between the two ions was observed. It was a surprising result, because one expected that antiferromagnetic coupling should dominate due to the presence of seven unpaired electrons on gadolinium, which should have nonzero overlap with copper orbitals. In order to rationalize the ferromagnetic coupling, we tried to explore an alternative model taking into account spin polarization effects through empty gadolinium orbitals. I (D. G.) tried to use my favorite theoretical model using fingers for tracing the overlap integrals. The emerging model suggested the involvement of 7s empty orbitals. The model suggested by Olivier suggested the 5d orbitals instead. The two alternative models were gently discussed in a theoretical paper with state-of-the-art quantum chemical calculations (CASSCF, CASPT2, MS-CASPT2).[38] Both suggestions were revealed to be valid, even though the 5d hypothesis appears to be a closer estimate on the basis of the computations.

## Conclusion

We were unable, in the context of this brief essay, to quote all the actors of the saga, young and less young, especially when Olivier moved from Orsay to Bordeaux to create the Laboratory of Molecular Sciences within the Institute of Chemistry of Condensed Matter of Bordeaux (with Corinne Mathonière, Jean-François Létard, Jean Pascal Sutter and others ...). The seeds sown by Olivier in Orsay, Bordeaux, and many other places are still active and going strong. The tradition of cooperation in the French community, promoted by Olivier, continues with a CNRS "Groupement de Recherche Magnétisme et Commutation Moléculaires", comprising several tens of teams, led by newcomers, Guillaume Chastanet (Bordeaux), Boris Le Guennic (Rennes), and Sébastien Pillet (Nancy).

All throughout his life as leading scientist, in his laboratory, in professional societies, in the French Academy of Sciences, Olivier supported talented young researchers. After Olivier's too early demise in 1999, D. G., as a leader of the Magmanet European Network of Excellence, suggested the creation of an Olivier Kahn International Award to be presented to promising

young researchers. M. V. served up to now as secretary of the Award with Andrea. Since 2006, the laureates of the International Award bearing Olivier's name, Wolfgang Wernsdorfer (Grenoble, Karlsruhe), José Ramon Galán-Mascarós (Tarragona), Marta Mas (Barcelona), Stefano Carreta (Parma), Lapo Bogani (Oxford), and Matteo Mannini (Florence) are the active witnesses and actors of the future of molecular magnetism.

We are very happy to have had the opportunity to write some memories of a great scientist and a great person, who was influential to a large number of people of his generation. We feel that it was the right time to tell the great Olivier story from our point of view.

**Keywords:** Magnetic properties · Spin crossover · Molecular magnetism · History of science

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Received: August 15, 2017