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Organic Ferromagnets. Hydrogen Bonded
Supramolecular Magnetic Organizations Derived from Hydroxylated Phenyl α-Nitronyl Nitroxide Radicals
J. Veciana, J. Cirujeda, C. Rovira, E. Molins, J. Novoa
To cite this version:
J. Veciana, J. Cirujeda, C. Rovira, E. Molins, J. Novoa. Organic Ferromagnets. Hydrogen Bonded Supramolecular Magnetic Organizations Derived from Hydroxylated Phenylα-Nitronyl Nitroxide Rad- icals. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1967-1986. �10.1051/jp1:1996199�. �jpa- 00247292�
Organic Ferromagnets. Hydrogen Bonded Supramolecular Magnetic Organizations Derived from Hydroxylated Phenyl
O.Nitronyl Nitroxide Radicals
J. Veciana (~,*), J. Cirujeda (~), C. Ravira (~), E. Malins (~) and J-J- Novoa (~)
(~ Institut de Ciència de Materials de Barcelona, CSIC, Campus de la U-A-B., 08193-Bellaterra.
Catalunya. Spain
(~ Departament de Quimica Fiqic~, Facultat de Quimica, Universitat de Barcelona, Av. Diagonal 647, 08028-Barcelona, Spain
(Receii.ed 30 April 1996, accepted July1996)
PACS.61.66.Hq Organic compounds PACS.75.30.Gw Magnetic anisotropy
PACS.75.50.Dd Nonmetallic ferromagnetic materials
Abstract. The capacity of hydrogen bonds as crystalline design elements for the prepara- tion of solid state supramolecular organizations with relevant magnetic properties is reviewed.
It will be shown here that hydrogen bonds fuIfiII two distinct purposes. One function is a struc- tural one through which is possible to central up to certain level the relative arrangements of radical molecules. The other rote of hydrogen bonds is a magnetic one smce these bonds intro- duce proper pathways for the transmission of intermolecular magnetic interactions. Using this
methodological approach several molecular magnetic solids have been prepared with hydroxy-
Iated phenyl a-nitronyl nitroxide radicals. These molecular sohds are formed by supramolecular motifs (dimers, chains, and planes) bonded by strong hydrogen bonds which are hnked to each other by other weak forces that form the rest of the crystals. Consequently, the solids show different structural dimensionalities that in some cases do trot agree with trie magnetic ones.
Accordingly with such distinct structural and magnetic dimensionalities, the resulting molecu- Iar solids show a wide variety of magnetic behaviors. Remarkable are the purely organic 2-D
and 3-D ferromagnets reported here. Finally, the most relevant mechanisms explaining inter- moIecuIar magnetic interactions as weII as the most promismg applications expected for purely
orgamc magnets are aise mcluded.
1. introduction
Since the discovery of the first example of a purely organic ferromagnet,, several other interesting organic magnetic materials have been obtained and studied in detail 11,2j. The enormous inter- est that these kind of navet materials motivate in the scientific community lies on the new and unexpected properties that they might exhibit. In this sense, the combination of a macroscopic
magnetic property, like the superpara-, ferri- or ferromagnetism, and the injrinsic properties
of orgamc compounds might open the way tu a new and fascinating world of advanced mate- nais. Some of the foreseeable advantages of organic materais over the traditional inorganic
(* Author for correspondence je-mail: vecianajilicmab,es)
Q Les Éditions de Physique 1996
1968 JOURNAL DE PHYSIQUE I N°12
ones ha~,e beeii discussed elsewhere [3]. They might become superior to traditional magnets
as far as applications iiivolviiig Iight absorption and reflection are concerned. In fact, organic compouiids ai-e usuall» transparent in many spectral regions and coula be in principle obtained
iii opticallj, acti~.e chinai forni~. Thus, they might be used as magneto-optical switches and for trie maiiipulatioii of polarized light iii optical devices if Faraday and Kerr eifects are exploited.
Orgaiiic iiiaterials ai-e geiierally electric iiisulators, so they might lead to insulating magnets
iii coiitrast V.ith most of trie traditioiial magnets that are electric conductors being. therefore, good candidates for electroniagiietic shielding purposes. Plasticity, flexibility and solubility in
commoii orgaiiic sol~,eiits ai-e geiieral characteristics of orgamc compounds that confer them
an easy processabilitj.. Hereby, orgaiiic magrets are ideal candidates to obtain magnetically
active thiii films aiid colloidal dispersions with ferrofluid properties. Biocompatibility is an- uther characteristic of orgaiiic compounds that permits to imagine biomedical applications hke for instance trie use of orgaiiic magrets or high-spin molecules for drug addressing and as se- lective coiitrastiiig agents iii nudear magnetic resonance imaging. Finally, trie state-of-trie-art of organic cheniistry techniques permits nowadays to synthesize tailor-made compounds and.
therefore, to perform small structural modifications in order to tune their physical properties.
Such a tunability is unprecedented in most of traditional inorgamc materais and, consequently,
opens a ~vide i-ange of practical opportunities.
From trie perspectives already mentioned it is dear trie enormous interest existing nowadays
for this kind of magnetic materais. However, as we will see latter, trie design and synthesis
of purely organic ferromagnetic materials is net a simple and direct task. Many years are still
required for any practical application. Such materials require three major electronic and struc- tural prerequisites: i) presence of permanent magnetic moments m their building blocks; i,e.,
trie use tf repeating units with electronic open-shell character and high enough persistence;
2) settlement of proper magnetic interactions between these permanent magnetic moments in order to align them parallel with respect to each other; i e., trie establishment of ferromagnetic interactions; 3) arrangement of such interacting permanent magnetic moments over large and three (or two) dimensional regions (domains) of trie material. Trie problems associated with ail these three steps are by no means simple because organic species containing unpaired elec-
trons usually tend to be rather unstable and, further, whenever they are bonded or they came
close enough to interact with each other they tend to pair antiparallel their spins. Neverthe- Iess. several efficient strategies have been already developed in order to tackle bath problems.
Thus, diiferent ways to
mcrease trie chemical and thermal stabilities of open-shell compounds
(free radicals and ion-radicals, etc. bave been reported and. therefore, these persistent species coula be used as open-shell repeating units. Similarly. different strategies that guarantee trie ferromagnetic couplings among neighboring open-shell species bave been reported [4]. Conse- quently, trie most problematic step in trie preparation of an organic magnet is trie extension of trie ferromagnetic couphngs among trie neighbonng open-shell umts iiito trie three (or two)
dimensions of trie materai. In fact, a precise central of connectivities. conformations, and trie relative spatial orientations of trie repeating umts m trie orgamc material is crucial to achieve such a goal. Hence, the challenge to the synthetic chemistry to salve these supramolecular problems is formidable; but we beheve that this field wiII benefit from trie rapta advances of trie Sttpramoiecttlar Chemistry.
Àccordiiig to the nature of the materais and trie type of ferromagnetic coupling mechanisms involved, two diiferent approaches bave beeii followed in order to prepare organic ferromagnets [lj: ii) Poiymenc Mater~als, in which trie magnetic interactions between open-shell monomer
repeating units are transmitted through cavalent skeletons, and iii) Moiecuiar Soiids, in which trie ferromagnetic couphngs among open-shell molecules are transmitted through trie space or
non cavalent bonds.
In the following we will review trie main achievements with trie last kind of crystalline mate- riais paying a special attention to the results obtained with trie family of a-nitronyl nitroxide radicals i in particular with those radicals substituted with hydroxylated phenyl groups. This
family of hydroxylated radicals have permitted to show that hydrogen bonds are able to link
properly the molecules and at the same time to establish proper pathways to transmit the
magnetic interactions yielding relevant macroscopic magnetic properties. This methodologi- cal approach was mspired on the previous work of Àwaga et ai. [5j using Coulombic forces instead of hydrogen bonds. We believe that the methodological approach here deicnbed in detail is representative of present trends that are followed in order to obtain organic/molecular
materais with relevant magnetic properties.
~~jÀ~ç~-OR
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2. Magnetic Interactions in Molecular Solids
Generally speaking, a macroscopic physical property of an orgaiiic molecular solid, like mag-
netism, is always related to trie relative arrangement of its constituent building blocks smce such layouts central the intermolecular electronic interactions among these umts which are the ultimately responsible for their inacroscopic magnetic properties. Àccordingly, there is a great need to central these structural characteristics iii order to rule the magnetic behavior of the material. It is well kiio~v.n that trie relative niulecular arrangements in crystalline organic
molecular solids are a consequence of ~everal stnictural factors and numerous subtle inter- moIecuIar forces [6]. Coulombic attractive forces. hj,drogeii bonds iii, ~ ~ stacking and van der Waals forces are the niost often used pulliiig foi-ces in trie attempts to direct the packing
of a given purely orgaiiic iiiolecular niaterial. Nevertheless. there is still a long way to go in order to produce tailor-tirade iuolecular solids: i.e. designed crystalline materais with prede- fined structures aria propert,ies. Trie iuain reason for this dilliculty is the fact that the way molecules arrange iii a crystal is trot ai ail easy to predict [8]. Only in the most favorable cases
some crystalline feitures or motifs eau be concluded a priori from trie molecular geometry and connecti~;ity.. Trie overall crystal packing remams a Iargely unknown variable until it is
experimentallj, deternnned. Iii coiiseqiience, neither the crystal packing nor the electronic in- termolecular interactions of a lier. materai can at ail be predicted a priori. Thus, it is obvions that trie establishnient of nec; methodologies for designmg crystal structures is indispensable
m this field.
Purely orgaiiic iuagnetic materais, and in particular substituted o-nitronyl nitroxide radicals i, bave been Mudied dunng the last decade almost exdusively m the solid state [9]. Most probably, trie drivmg force for most of such studies was the discovery of the first example of purely organic ferroinagnet [2] and the relevant magnetic properties that have several of
their metal transition complexes [loi. Nevertheless, this family of radicals exhibit many other
interesting properties that are net restricted to the solid state and that were extensively studied
smce Ullman and co~v.orkers synthesized for trie first time an a-nitronyl nitroxide radical in 1968 iii].
1970 JOURNAL DE PHYSIQUE I N°12
The geometry constraints for magnetically active organic molecules, like free radicals or radical ions, are by fat more complex than the highly syinmetric structures of inorganic metal
ions. In consequence, trie relationship between relative molecular arrangements and magnetic
interactions is less evident, and probably can net be established as dearly as it can for inorganic compounds. Nevertheless, we believe that there is a methodological approach for designing crjrstal structures showing controlled magnetic properties. This methodological approach can
be divided into trie following two steps: The first one implies the use of a sttpramolecttlar design
eiement or tari capable to link molecules together in a predetermined manner. This trot should be a smtable driving force, that forces trie molecules to lay eut in a highly directional way with respect to each other. Coulombic forces m charged molecules, hydrogen bonds, the intelligent
use of stenc hindrances [12], or trie combination of these or other trois may fulfill trie desired target providing excellent opportumties for controlling the relative arrangement of molecules.
The second step is to test if the achieved relative arrangements between molecules are able to induce a ferra- or an antiferromagnetic interaction and to determine trie intensity of such
a magnetic exchange. In order to accomplish bath steps, a series of compounds with several structural and electromc reqmrements should be synthesized. This series of compounds will permit magneto-structural correlations, and from these, new tailor-made compounds might be synthesized.
Before going ahead with this methodological approach. a brief review of the most widely accepted mechanism that explains the magnetic interactions in magnetic molecular solids, the
so known Mcconnell I mechanism, is given here [13]. This inechanism was proposed in trie earher sixties by Mcconnell and permits the prediction of trie sign of trie exchange interaction between two free electrons belonging to two iieighbonng open-shell molecules arranged in
a given way. Àll trie atoms of a free radical bear a certain amount of trie so called spm
density that primanly reflects trie distribution of trie unpaired electron ail over trie molecule.
Trie spin density on a certain atom cari be positive or negative depending on the particular
electromc structure of the studied molecule. Specifically, as a result of the spin polarization
mechamsm, negative spin density can appear on some atoms. Simply speaking, trie Mcconnell I mechamsm states that if the atoms of both molecules that are closest one to each other, having
trie most significant overlap, bear the same sign of trie spin density the resulting magnetic
interaction between bath molecules will be antiferromagnetic. By contrast, trie interaction will be ferromagnetic if the sign of trie overlapping spin densities is diiferent. It can be imagmed
that in the regions m which there is a direct overlap of the molecular orbitals, the same
phenomenon that in trie formation of a covalent bond occurs; i.e., trie spiiis of trie two involved
non paired electrons m trie spatial areas with most overlap should be of opposite sign. Trie
validity of this qmte simple model was expenmentally confirmed several years later by Iwamura et ai. [14] using trie three diiferent isomers of trie bis(phenylmethylenyl)[2.2]paracyclophane.
EPR spectroscopy revealed that trie quintet state was the ground state of the psettdo-ortho
and pse~do-para isomers of these biscarbene denvatives, while trie singlet state was trie ground
state for trie pseudo-mena isomer. Results that are m accordance with theoretical predictions
based on the signs of the overlapping spin densities of the two interacting carbene môieties of each isomer.
In addition to the mentioned mechamsm it is important to stress another iiiteresting situation that is also based on trie Pauh exclusion pnnciple and has also important consequences in
Moiecttiar Magnetism. Thus, it is predicted that when trie overlap between trie orbitals carrymg trie Ione electron is strong and adopt an orthogonal disposition, a ferroinagnetic interaction is
most favored lis]. In short, such ferromagnetic interactions appear when a maximum overlap between the spin carrying orbitals occurs and at the same time the orbital overlap integral
should tend to be almost zero. Under such conditions the electronic repulsion tend to align
the spms in a parallel fashion.
3. Hydrogen Bonds as Structural and Magnetic Design Elements
It is very well known that hydrogen bonds act as a very directional force giving rise to well defined supramolecular pattems of molecules [16]. Consequently, these kind of bonds are often used in order to control the relative disposition of neighboring molecules in the solid state as well as in solution. À lot of work has been done in the study of the hydrogen bond itself by
means of surveys and correlations with the Cambridge Structural Database il7j or also in the
study of the formation of aggregates of molecules linked together through hydrogen bonds [18j.
The extreme importance of the studies on these intermolecular forces can easily be understood
by realizing that most of the biological macromolecules (peptides, enzymes, etc. and processes
(DNÀ replication, enzyme activity, etc.) are mainly controlled by hydrogen bonds.
Hydrogen bonds are known to be a non-covalent bonding interaction being basically elec- trostatic in its nature. For this reason, an explanation of its characteristics can reasonable be achieved by studying the eiectrostatic potentiai maps of the involved molecules. For this
purpose, two eqmpotential surfaces are drawn around the isolated molecule, one with positive and the other with negative potentials. The analysis of the shape and the self-complementary of both equipotential surfaces provides an idea about the expected relative spatial orienta- tion of hydrogen bonded molecules. In the frame of Molecttlar Magnetism, hydrogen bonds
linking together spm carrying units, either metal ions or radical molecules, have some char- acteristics that make them very interesting in order to control the intermolecular magnetic
interactions. On one hand, the relative disposition of hydrogen bonded molecules is expected
to have a restricted degree of freedom due to the high directionality of the hydrogen bonds. On
the other hand, hydrogen bonds usually result in very short intermolecular distances between the two interacting sites of neighboring molecules. In consequence, specific magnetic inter- actions can appear between the hydrogen bonded spin carrying units being furthermore such intermolecular magnetic interactions controllable with organic chemistry synthetic techniques.
Some examples are found in the literature where hydrogen bonds have been used and studied
as transmitters of magnetic interactions m metal ions salts [19]. In these compounds water molecules act as a bonding unit between dilferent met al ions and have actually proved to trans- mit magnetic interactions eifectively between the metal centers. By contrast, no systematic study has been carried out concerning the rote of hydrogen bonds m the magnetic transmission in purely organic magnetic materials.
4. Structural and Electronic Characteristics of Hydroxylated Phenyl a-Nitronyl
Nitroxide Radicals
À family of open-shell compounds suitable to perform such a study with hydrogen bonds is the hydroxylated phenyl a-mtronyl nitroxide radicals. The aforementioned radicals have in
common the following structural and electronic charactenstics. On one hand, they have an
heterocyclic imidazolyl-1-oxy-3-oxide ring the a-mtronyl nitroxide ring in which most of the spin density is located. On the other hand, they are built up by a second ring, in this
case an aromatic phenyl ring, that can be substituted in diiferent positions by one, two, or more hydroxy groups. Moreover, it is known that a certain amount of spm density is situated
on this second aromatic ring due to a spin polarization mechamsm that transmits some spin density and at the same time induces an alternation of their signs on these aromatic atoms.
Different experimental results have confirmed such electromc charactenstics [20]. But by far
1972 JOURNAL DE PHYSIQLTE I N°12
Fig. 1. Schematic view of the spin density distribution of phenyl a-mtronyl nitroxide radical.
the most interesting feature of this family of compounds is the simultaneous presence of OH and NO groups in each molecule. Semi-empincal and ab-initia computations performed with these radicals show that trie mentioned groups are suitable to act as hydrogen bond donor and acceptor groups, respectively [21]. Àccordingly, these functional groups are expected to favor trie establishment of strong hydrogen bonds of trie O-H O-N type inducing interesting close contacts between certain spatial regions of trie involved molecules. Iii a normal hydrogen bond trie O O distance lies around 2.5 2.9 À [22], and, therefore, these quite short intermolecular
distances can be expected to bave a relevant magnetic importance because one of trie involved groups (trie NO moiety) carries a large amount of trie spin density.
À doser structural analysis shows that the magnetic interactions that are established be-
tween hydrogen bonded compounds can be explained by using the above mentioned Mcconnell
I mechamsm. For this purpose, it is convenient to draw and discuss briefly the spin density
distribution map of phenyl o-nitronyl nitroxide radical, depicted in Figure i. This map de- picts trie main features of trie distribution of trie Ione electron around this nitronyl mtroxide
radical, as extracted from polarized neutron diffraction experiments [23j, NMR data [24], and semi-empincal and ab-imtio calculations [25]. No significant differences m trie spm density distribution are observed depending on trie aromatic substitution, as revealed by solution EPR spectra of different radicals [26]. Thus, for trie present family of compounds trie main spin density lies equally distributed on both NO groups, while the bridgehead carbon atom bears a quite large negative spin density. In the aromatic ring, the spin densities alternate m sign being positive on trie ipso and mena carbon atoms and negative on trie ortho and para ones. Trie spin densities on trie aromatic hydrogen atoms and trie oxygen atoms bave opposite sign than that found on trie carbon atoms they are Iinked to. It is expected that trie hydroxylic hydrogen atom hears an extremely small spin density that can not even be detected by EPR but, following
the alternation rule, its sign is supposed to be trie contrary to those found on its neighbonng
oxygen atom. Spin densities on hydrogen atoms of trie CH3 groups were experimentally found to bave a small but always negative values [21, 24,26].
In consequence, this simple picture predicts that in case of an intermolecular O-H O-N
hydrogen bond involving trie ortho or para hydroxy groups, that bave a negative spin density
on their hydrogen atom, a weak ferromagnetic couphng, if any, Will be favored due to trie close contact of nudei with opposite spm density signs. On the other hard, m the case of an O-H O-N hydrogen bond involving trie mena hydroxy group, t~v.o atoms of trie same sign of spm density will be very close and one would expect weak antiferromagnetic interactions.
Finally, close contacts produced by weak C-H O-N hydrogen bonds between CH3 and NO groups are also expected to mduce intermolecular ferromagnetic interactions due to trie op- posite signs of trie spm densities on trie mvolved nudei. In addition, hydrogen bonds would
