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Spin Transition compounds exhibit an outstanding bistability (magnetic and optical), that makes them usable in memory devices. The origin of the hysteresis is under investigation; the macroscopic cooperative effect has already been well documented, but the phenomenon could also occur at the molecular scale, so that a model is proposed to take that point into account.

The Mn(II) ion reacts on Cu(II) dianionic precursors containing an oxarnido and two caboxylato groups to afford Mn(II)Cu(lI) compounds. The structures and magnetic properties of these compounds present an exceptional diversity. The following situations were encountered: uncoupled carboxylato-bridged binuclear compound, antiferromagnetically coupled oxamido-bridged binuclear compound, alternating bimetallic chain behaving magnetically as binuclear species, alternating bimetallic chain exhibiting the characteristic one-dimensional ferrimagnetic behavior, finally molecular-based compound exhibiting a spontaneous magnetization below a critical temperature Tc. In each case, the theoretical models appropriate to interpret the magnetic properties are presented, and the mechanism of the interactions between Mn(II) and Cu(II) ions is discussed. In the course of this work, quite an unusual polymerization reaction in the solid state has been discovered.

The antiferromagnetic structure of the purely organic free-radical 4,5-dimethyl-1,2,4-triazole-nitronyl-nitroxide ( T N =0.33 K) was determined by neutron diffraction. The observed spin arrangement gives important information about the competing ferro- and antiferromagnetic exchange pathways in this compound.

In this short review, we summarize the various approaches we have in mind to design molecular-based compounds exhibiting a spontaneous magnetization, and we present some of the achievements. Both the orthogonality of the magnetic orbitals and the double exchange lead to a stabilization of the state of highest spin. The energy involved in the latter case, however, is one order of magnitude larger than in the former. The double exchange results from an electron delcalization between magnetic centers. The ferrimagnetic approach at the molecular scale leads to a ground state of intermediate spin; we succeeded in assembling ferrimagnetic chains within the crystal lattice in a ferromagnetic fashion and ferromagnetic transitions up to 30 K were observed. The weak ferromagnetic approach requires a canting of the local spins superimposing to dominant antiferromagnetic interactions. A rare example of molecular weak ferromagnet is reported. Finally, the spin polarization is evoked.

The first part of the discussion has been devoted to the various strategies which have already been explored to design high-spin molecules and molecular-based ferromagnets. Those strategies may be sumed up as follows: The topological degeneracy. This approach proved to be very efficient, in particular in polycarbene chemistry. A S = 5 pentacarbene has been reported during the Conference. The main strength of this strategy is that it does not require symmetry conditions which are difficult to control. However, so far it did not lead to bulk ferromagnetism. The chemical stabilization of the polycarbenes also remains a difficult problem. This topological degenaracy approach could be extended to non-alternant systems as well as to heterocycles.

The LIESST effect has been observed for an iron(II) compound organized in Langmuir−Blodgett (LB) architecture. Both the LIESST population and relaxation are faster for the LB film than for the powder. These results can be related to the high optical quality of the LB films and the intensity pressure effect induced by the surroundings.

The compounds Gd 2 (ox)[Cu(pba)] 3 [Cu(H 2 O) 5 ] · 20H 2 O ( 1 ) and Gd 2 [Cu(opba)] 3 · 6DMSO · 4H 2 O ( 2 ) are spin ladder-based molecular ferromagnets. Low-temperature specific heat measurements show that both compounds undergo three-dimensional magnetic ordering with T c = 1.05 K for ( 1 ) and T c = 1.78 K for ( 2 ). These materials represent the unique examples of long-range ordering, reported so far, for gadolinium and copper-based molecular magnets.

A Prussian-blue-like magnet exhibiting complicated behaviour has been designed from first principles. This first ‘magnet by design’ switches North and South poles twice as the temperature changes.

One of the main challenges in the field of the molecular materials is the design of molecular-based ferromagnets. Our basic strategy along this line consists of assembling ferrimagnetic chains within the crystal lattice in a ferromagnetic fashion. This can be achieved owing to the (almost) limitless flexibility of the molecular chemistry. The chains may be either regular or alternating. Examples of both situations are presented. MnCu(pbaOH)(H2O)3 with pbaOH=2-hydroxy-1,3-propylenebis(oxamato) is a regular chain compound ordering ferromagnetically at TC=4.6 K, and MnCu(obbz).1H2O with obbz=oxamido-N,N′-bis(2-benzoato) is an alternating chain compound exhibiting a spontaneous magnetization below TC=14 K. To get information on the mechanism of the magnetic ordering, a broad spectrum of physical techniques is utilized, including magnetic susceptibility and magnetization measurements, EPR spectroscopy and heat capacity data. The perspectives in this new field are outlined.

One of the main challenges in the field of the molecular materials is the design of molecular-based ferromagnets. Our basic strategy along this line consists of assembling ferrimagnetic chains within the crystal lattice in a ferromagnetic fashion. This can be achieved owing to the (almost) limitless flexibility of the molecular chemistry. The chains may be either regular or alternating. Examples of both situations are presented. MnCu(pbaOH)(H2O)3 with pbaOH=2-hydroxy-1,3-propyleneb1s(oxamato) Is a regular chain compound ordering ferromagnetically at Tc=4.6 K, and MnCu(obbz).1H2O with obbz=oxamido-N,N’-bis(2-benzoato) is an alternating chain compound exhibiting a spontaneous magnetization below Tc=14 K. To get information on the mechanism of the magnetic ordering, a broad spectrum of physical techniques is utilized, including magnetic susceptibility and magnetization measurements, and EPR spectroscopy. The perspectives in this new field are briefly outlined.

Slow diffusion of aqueous solutions containing K4[MoIII(CN)7]·2H2O and a MnII salt, respectively, affords well shaped single crystals of two cyano-bridged bimetallic phases ordering ferromagnetically with a pronounced magnetic anisotropy in the magnetically-ordered phase.

Das vollstandige Manuskript dieser Zuschrift erscheint in: Angew. Chem. Suppl. 1982, 953. DOI: 10.1002/ange.198209530

The first example of a spin-crossover iron(II) compound with a large negative third-order polarizability, associated with an instantaneous electronic response, is reported; magnetic measurements show that this compound, cis-bis(thio-cyanato)bis[N-(2-pyridylmethylene)aminobip henyl]iron(II), characterized by X-ray structural data is in the high-spin form at room temperature.

[Fe(hyetrz) 3 ](anion) 2 ·3H 2 O [hyetrz=4-(2′-hydroxyethyl)-1,2,4-triazole, anion=3-nitrophenylsulfonate] is a novel linear polynuclear Fe II spin-crossover compound. The low-spin to high-spin transition accompanied by a pronounced thermochromic effect occurs at 370 K in a very abrupt way. Just before this temperature, the three non-coordinated water molecules are removed. The dehydrated high-spin form remains stable down to ca. 100 K, where it transforms into a new low-spin form, implying that this material shows an apparent thermal hysteresis width of 270 K. Applications of this material are discussed.

RE 2 (ox)⦜Cu(pba)] 3 ⦜Cu(H 2 O) 5 ] · 20H 2 O, RE = Gd, La, are molecular materials with layered crystal structures. Low-temperature specific heat measurements show magnetic ordering on both systems. Two-dimensional behaviour is observed in the Gd compound, but the supression of the Cu-RE-Cu interactions for RE = La avoids this effect.

The synthesis and characterization of the new spin crossover mononuclear complex [FeII(DPEA)(bim)](ClO4)2·0.5 H2O, where DPEA = (2-aminoethyl)bis(2-pyridylmethyl)amine and bim = 2,2-bisimidazole, are reported. Variable-temperature magnetic susceptibility measurements (77−295 K) reveal the occurrence of a two-step spin transition. Two steps on the magnetic curve are separated by an inflection point at 200 K, corresponding to about 50% of the complexes that have undergone a thermal spin transition. The first step is centered at 171 K and the second one at 218 K. Mossbauer spectroscopy and X-ray analysis show that the profile of the magnetic curve is a consequence of the presence of two inequivalent iron(II) molecules in the crystal lattice. The crystal structure was resolved at 293 K (high-spin form) and at 123 K (low-spin form). Both spin-state isomers belong to the monoclinic space group P21/c (Z = 4). The main differences between high-spin and low-spin isomers are found in the geometry of the [FeN6] core, the shorter Fe−N distances are seen at lower temperatures. Two inequivalent HS molecules at lattice sites 1 and 2 successively undergo a thermal spin transition. The analysis of the mean Fe−N distances for functionally different nitrogen donor atoms show that the two steps of the spin transition can be assigned to the inequivalent lattice sites. The gradual character of the spin transition at both lattice sites is accounted for in terms of intermolecular H-bonding via the perchlorate ions. At 10 K the light-induced excited spin state trapping (LIESST) effect is observed within the SQUID magnetometer cavity. Two critical temperatures Tc(LIESST) were recorded (36 K and 21 K) and are attributed to the lattice sites 1 and 2.

We describe our latest results concerning the molecular-based compounds exhibiting a spontaneous magnetization. The ordering temperature of one of our compounds is Tc = 30 K, which presently seems to be a record in this field.

Numerous molecular compounds in which a paramagnetic Ln(III) ion is in interaction with a second spin carrier such as a transition metal ion or an organic radical have been described. However, except for the isotropic Gd(III) ion, very little is known concerning the nature (ferro- or antiferromagnetic) and mechanism of the interaction involving a Ln(III) ion. This study addresses the problem of the interaction between the Ln(III) ions displaying spin−orbit coupling and an organic radical. The magnetic properties of a series of isostructural compounds, {Ln(organic radical)2(NO3)3}, comprising a Ln(III) ion (Ln = Ce to Dy) surrounded by two N,O-chelating aminoxyl radicals (3-nitronyl nitroxide-4,5-dimethyltriazole) have been investigated. The experimental approach used to get insight into the {Ln-Radical} coupling occurring within these compounds is based on the knowledge of the intrinsic paramagnetic contribution of the metal ion. This contribution has been deduced from the corresponding {Ln(Nitrone)2(NO3)...

The magnetic properties of metallic single crystals of D[Ni(dmit)2]2 were determined at ambient pressure down to 1.7 K with a SQUID extraction technique. The magnetic field (0.1 T) was perpendicular to the plane of the dimerized platelets for D = NMe4, and NHMe3, and parallel to the a axis of the monoclinic structure of the D = TTF salt. Substitution of radical Me by H increases the absolute value of the observed diamagnetic susceptibility χ, in agreement with the increasing conductivity. The molecular diamagnetism derived from Pascal rules is negligible. As temperature decreases, the diamagnetism of the compounds increases, which indicates its orbital origin. X rays diffusion experiments show none structural instabilities in the NMe4 compound. “Metal-insulator” transitions are observed around 100 K and 50 K in these salts. Hence, it is suggested they arise from localization of the carriers induced by the freezing of the degrees of freedom of the Me groups. The TTF salt shows a huge χ Pauli above 40 K. Below this temperature, the observed χ susceptibility increases in two steps : this is probably related to charge density waves condensations which affect the LUMO and then the HOMO bands of the Ni(dmit)2 slabs. Below 4K, χ decreases by a factor 2, which could be related to an increase of the diamagnetic properties of the salt. This is in agreement with the 2D hole pocket which remains at low temperature. Magnetization measurements at 1.6 K under magnetic field up to 8 T will be presented.

The reaction of the 2,6-diformyl-4-methylphenol with a primary amine in the presence of gadoliniurn(III) affords a complex in which gadolinium(III) is coordinated to two mono-Schiff bases through their carbonyl and phenolic oxygen atoms.

Magnetic and EPR data concerning the molecular-based compound Cu(ta)(MeOH)·1/2H2O (ta = terephthalato), are consistent with triplet pair states coupling ferromagnetically at the scale of the crystal lattice, with a critical temperature around 13 K.

Le compose du titre cristallise dans le systeme orthorhombique, groupe Pnma. La structure est constituee de chaines bimetalliques ordonnees avec les ions Mn(II) en coordination octaedrique et les ions Cu(II) en coordination pyramidale a base carree, relies par des groupes oxamato. Importantes interactions antiferromagnetiques intrachaine et tres faibles interactions antiferro-interchaine

Ein Komplex, der zwei oder mehr paramagnetische ubergangsmetall-Ionen enthalt, kann vollig andere magnetische Eigenschaften haben als Einkernkomplexe dieser Ionen. Die Eigenschaften von Zweikernkomplexen hangen von Art und Starke der Metall-Metall-Wechselwirkungen uber die verbruckenden Liganden ab. Weisen beide Ionen je ein ungepaartes Elektron auf (z. B. Cu2+-Ionen), dann ist der energetische Grundzustand des Molekuls entweder ein Spin-Singulett oder ein Spin-Triplett. Im ersten Fall spricht man von antiferromagnetischer, im zweiten von ferromagnetischer Wechselwirkung. Art und Starke der Wechselwirkung konnen durch die Wahl der Metall-Ionen sowie der verbruckenden und der terminalen Liganden und damit durch die Symmetrie und die Delokalisierung der metallzentrierten Orbitale, die von den ungepaarten Elektronen besetzt sind (magnetische Orbitale), gesteuert werden. Dies gelang zuerst bei einem rein ferromagnetischen Cu2+VO2+-Komplex. Die gleiche Strategie konnte zur Herstellung molekularer Ferromagnete genutzt werden, die eine der grosten Herausforderungen auf dem Feld molekularer Werkstoffe ist. Man kann das Ausmas der Wechselwirkung in einer vorgegebenen verbruckten Struktureinheit durch Variation der terminalen Liganden, die die Rolle von „Justierschrauben” ubernehmen, genau abstimmen. Durch die sorgfaltige Wahl sowohl der verbruckenden als auch der terminalen Liganden kann eine sehr starke antiferromagnetische Wechselwirkung erreicht werden, sogar im Falle weit voneinander entfernter Metall-Ionen. Einige schwefelhaltige Brucken sind dafur besonders geeignet.

Exchange interaction in ring chains of N transition ions with n unpaired electrons per ion is investigated. The study is focused on the case where N becomes very great, i.e. on the case of one-dimensional magnetic compounds. The main results are the following: (i) An orbital justification to the use of the phenomenological spin hamiltonian of the Heisenberg-Dirac-Van Vleck model is given. (ii) The exchange interaction parameter J is expressed as a function of orbital integrals related to the real mechanism of the phenomenon. The key expression is in which Sμμ is the overlap integral between two magnetic orbitals of the same symmetry centred on nearest-neighbour transition ions, Δμ is the width of the molecular orbital band built from the magnetic orbitals of symmetry μ for the spectroscopic term of highest spin multiplicity. Finally J μν is the two-electron exchange integral between magnetic orbitals of symmetry μ and ν respectively, centred on nearest-neighbour ions.

The two approaches defined in the title, NMO and OMO, are compared and their advantages discussed. In the OMO approach, the stabilization of the spin singlet arising apparently from the ground configuration-charge-transfer configuration coupling is in fact a black box which implicitly contains a first-order contribution independent of the energy gap between the two configurations.

A previously described model for exchange interaction is applied to the study of antiferromagnetic coupling in binuclear oxo-bridged iron (III) complexes. The energies of the magnetic orbitals (singly occupied d orbitals of each transition ion, partially delocalized towards the ligands surrounding this ion), and of the m.o.s. built from these magnetic orbitals are calculated, using the Angular Overlap Model. The wavefunctions associated with the low lying terms are expressed in the form of H.L. functions and the relative energies of these terms are calculated from a one electron matrix, which couples the magnetic orbitals in such a way that its eigenfunctions an eigenvalues are the m.o.s. and their energies. An interpretation of the relative magnitudes of antiferromagnetic coupling in FeIII—O—FeIII and CrIII—O—CrIII dimers is proposed.

The antiferromagnetic contribution JAF to the magnetic exchange interaction parameter J defined by the HDVV spin Hamiltonian is calculated in linear chains AMCl3 (M = V, Mn, and Ni; A = Cs or N(CH3)4) using a model described in a previous paper. Each of the N coupled transition ions possesses n unpaired electrons and its ground term is an orbital singlet. JAF is then expressed by JAF = — where Sμμ is the overlap integral between two magnetic orbitals of the same symmetry μ centred on nearest-neighbour transition ions and Δμ is the width of the molecular orbital band built from these magnetic orbitals. Δμ is obtained by an extended Huckel calculation with charge iteration and Madelung correction for the potential. Sμμ is calculated by applying the Wolfsberg-Helmholz approximation to the magnetic orbitals. The calculated values of JAF are compared to the experimental J values. They have the good order of magnitude and their relative order agree with the experimental data. Nous avons calcule la contribution antiferromagnetique JAF a la constante d'interaction magnetique d'echange J definie par l'hamiltonien de spin HDVV: — dans des chaines lineaires AMCl3 (M = V, Mn et Ni; A = Cs ou N(CH3)4). Dans le modele decrit precedemment, chacun des N ions de transition couples possede n electrons celibataires et son terme fondamental est un singulet orbital. Dans ces conditions JAF s'exprime par JAF = — est l'integrale de recouvrement entre deux orbitales magnetiques de měme symetrie centrees sur deux ions voisins et Δμ la largeur de la bande d'orbitales moleculaires construites a partir de ces orbitales magnetiques. Δμ est determine par un calcul de Huckel generalise avec iteration sur la charge et correction de Madelung pour le potentiel. Sμμ est calcule en appliquant la relation de Wolfsberg-Helmholz aux orbitales magnetiques. Les valeurs calculees de JAF sont du měme ordre de grandeur que les valeurs experimentales de J et, pour les composes etudies, les valeurs de JAF se classent dans le měme ordre que celles de J.

Single-crystal EPR spectra of the heterobimetallic alternating double-chain compound MnCu(obp) (H2O)3·H2O (obp=oxamido bis (N,N′-propionato)) were recorded in the 300-20 K range. Analysis of the spectra indicate a substantially dipolar-determined linewidth with enhancement of the secular term of the second moment due to spin diffusion effects. The anisotropic shifts in the resonance field observed in low-temperature spectra revealed that interchain interactions are relevant in determining the preferred spin orientations.

(1) cristallise dans P2 1 /c avec Z=4; affinement jusqu'a R=0,049. (2) cristallise dans Pbca avec Z=8; affinement jusqu'a R=0,065. Proprietes magnetiques de 4,2 a 300 K

[(C 6 H 16 N 2 )(CH 3 OH)Cu] 2 (C 6 O 4 I 2 )(ClO 4 ) 2 cristallise dans le systeme monoclinique, groupe d'espace P2 1 /n et sa structure est affinee jusqu'a R=0,044. Coordination carree pyramidale autour des Cu

Dans le cas des complexes dinucleaires Cu(II) (μ-oxalato), la difference d'energie S-T peut etre controlee par la nature des ligands terminaux lies aux ions Cu(II). Etude structurale de trois nouveaux complexes

When two paramagnetic transition metal ions are present in the same molecular entity, the magnetic properties can be totally different from the sum of the magnetic properties of each ion surrounded by its nearest neighbors. These new properties depend on the nature and the magnitude of the interaction between the metal ions through the bridging ligands. If both ions have an unpaired electron (e.g. Cu2+ ions), then the molecular state of lowest energy is either a spin singlet or a spin triplet. In the former case, the interaction is said to be antiferromagnetic, in the latter case ferromagnetic. The nature and the order of magnitude of the interaction can be engineered by judiciously choosing the interacting metal ions and the bridging and terminal ligands, and, thus, by the symmetry and the delocalization of the orbitals centered on the metal ions and occupied by the unpaired electrons (magnetic orbitals). The first success in this “molecular engineering” of bimetallic compounds was in the synthesis of a Cu2+VO2+ heterobimetallic complex in which the interaction is purely ferro-magnetic. The same strategy could be utilized for designing molecular ferromagnets, one of the major challenges in the area of molecular materials. Another striking result is the possibility of tuning the magnitude of the interaction through a given bridging network by modifying the nature of the terminal ligands, which, in some way, play the role of “adjusting screws”. By careful selection of the bridging and terminal ligands, a very large antiferro-magnetic interaction can be achieved, even if the metal ions are far away from each other. Some sulfur-containing bridges are especially suitable in this respect.