Search

L’article s’intéresse en premier lieu à la polyphonie énonciative dans Le Jardin des Plantes, plus particulièrement aux énonciateurs que sont Dostoïevski, Proust et Stendhal. Partant de l’avant-texte, on revisite l’importance et le rôle de chacun d’entre eux pour inviter à une relecture du texte avec une curiosité revivifiée. Quelques indices trouvés dans le manuscrit ont permis de découvrir derrière un personnage simonien, celui de la « vieille femme éléphant », un personnage dostoïevskien insoupçonné, demeuré à l’état d’énigme dans la version imprimée. L’approche génétique permet d’en révéler l’origine. The article primarily concerns the enunciative polyphony of Le Jardin des Plantes, more particularly the enunciators Dostoevsky, Proust, and Stendhal. Starting from work preceding the published version we revisit the importance and role of each of these, thus leading to read the text afresh with renewed curiosity. Several hints in the manuscript have made it possible to discover behind one of Simon’s characters, the “old elephant woman”, an unsuspected Dostoevsky figure who remained an enigma in the printed version. The genetic approach has allowed discovery of this origin.

The goal of this paper is to show how from the basic concepts of magnetic orbitals and overlap density between magnetic orbitals, it is possible to predict the nature of the metal ions, and of the terminal and bridging ligands that must be utilized and the geometry that must be achieved to obtain expected magnetic properties. Three aspects are treated, namely the design of ferromagnetically coupled complexes, the possibility to tune the interaction through a given bridging network and the strategy to achieve a strong interaction between metal ions very far away from each other.

[Cu(hyetrz)3](CF3SO3)2·H2O [hyetrz = 4-(2′-hydroxyethyl)-1,2,4-triazole] represents the first structurally characterised ferromagnetically coupled CuII chain compound containing triple N1,N2-1,2,4-triazole bridges. catena-[μ-Tris{4-(2′-hydroxyethyl)-1,2,4-triazole-N1,N2}copper(II)] bis(trifluoromethanesulfonate) hydrate (C14H23F6S2O10CuN9) crystallises in the triclinic space group Pl, a = 13.54(3), b = 14.37(3), c = 15.61(4) A, α = 95.9(1), β = 104.9(1), γ = 106.5(1)°, V = 2763(11) A3, Z = 4 (CuII units). The CuII ions are linked by triple N1,N2-1,2,4-triazole bridges yielding an alternating chain with Cu1−Cu2 = 3.8842(4) A and Cu2−Cu3 = 3.9354(4) A. Analysis of the magnetic data according to a high-temperature series expansion gives a J value of +1.45(3) cm−1. The nature and the magnitude of the ferromagnetic exchange have been discussed on the basis of the structural features. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).

International audience; The spin population distribution in the ferromagnetically coupled hetero-bimetallic chain compound [MnNi(NO(2))(4)(en)(2)] (en = 1,2-ethanediamine) has been investigated by means of polarized neutron diffraction experiments, and the results compared with those from theoretical estimates obtained via calculations based on density functional theory on dinuclear molecular models of the chain. The spin distributions obtained from experiment and from theory are consistent and reflect a larger spin delocalization from the Ni atom due to the more covalent character of the Ni-N bonds compared to the Mn-O ones. Also a nearly isotropic spin distribution is observed for the more ionic d(5) Mn(2+) ion and a clearly anisotropic distribution for the d(8) Ni(2+) ion. The use of dinuclear molecular models for the calculation of the exchange coupling constant between Ni and Mn provide upper and lower limits (+17.6 and -4.2 cm(-)(1)) for the experimentally determined value (+1.3 cm(-)(1)), depending on how the missing part of the chain is simulated, but yield essentially the same spin distribution. The Mn(II)-Ni(II) weak ferromagnetic coupling in the chain is interpreted in a spin delocalization mechanism as resulting from the weakness of the overlap between the magnetic orbitals centered on nickel and those centered on manganese which are only weakly delocalized on the ligands.

We report the synthesis of the compound [Mn(tacn)]2[Mo(CN)7]·5H2O, another addition to the family of MnMo-heptacyanates. It is obtained by capping the Mn(II) with the ligand ‘tacn’ (triazacyclononane) and reacting it with the [MoIII(CN)7]4− precursor. It exhibits ferromagnetic behavior below 90 K.

Electron paramagnetic resonance (EPR) investigations were carried out on two bimetallic molecular magnets (NBu 4 ) 2 Co 2 [Cu(opba)] 3 ·S, where S=3DMSO·3H 2 O and 2H 2 O, known to exhibit long-range magnetic order below 32 and 34 K, respectively, in the temperature range 300–15 K. Temperature dependence of the intensity of the EPR signal arising from Cu 2+ in these compounds showed a rapid decrease in intensity upon cooling below 65 K, which was accompanied by a disappearance of the super-hyperfine structures, observed on the Cu 2+ signal at higher temperatures, pointing to a Co 2+ Cu 2+ spin–spin interaction following the slowing down of Co 2+ spin–lattice relaxation. The appearance of a new signal at zero field (ZFS) below 40 K is attributed to the onset of long-range magnetic ordering in both the compounds.

The use of metal-organic complexes is a potentially fruitful approach for the development of novel enzyme inhibitors. They hold the attractive promise of forming stronger attachments with the target by combining the co-ordination ability of metals with the unique stereoelectronic properties of the ligand. We demonstrated that this approach can be successfully used to inhibit the protease of the human immunodeficiency virus (type 1). Several ligands bearing substituents designed to interact with the catalytic site of the enzyme when complexed to Cu2+ were synthesised. The inhibition pattern of the resulting copper(II) complexes was analysed. We showed that the copper(II) complex of N1-(4-methyl-2-pyridyl)-2,3,6-trimethoxybenzamide (C1) interacts with the active site of the enzyme leading to competitive inhibition. On the other hand, N2-pyridine-amide ligands and oxazinane carboxamide ligand were found to be poor chelators of the cupric ion under the enzymatic assay conditions. In these cases, the observed inhibition was attributed to released cupric ions which react with cysteine residues on the surface of the protease. While unchelated metal cations are not likely to be useful agents, metal chelates such as C1 should be considered as promising lead compounds for the development of targeted drugs.

International audience; Numerous compounds in which a paramagnetic LnIII ion is in an exchange interaction with a second spin carrier, such as a transition metal ion or an organic radical, have been described. However, except for GdIII, very little has been reported about the magnitude of the interactions. Indeed, for these ions both the ligand-field effects and the exchange interactions between the magnetic centers become relevant in the same temperature range; this makes the analysis of the magnetic behavior of such compounds more difficult. In this study, quantitative analyses of the thermal variations of the static isothermal initial magnetic susceptibility measured on powdered samples of the {Ln(NO3)3[organic radical]2} (Ln=DyIII and HoIII) compounds were performed. The ligand-field effects on the Ln ions were taken into account, and the exchange interactions within a molecule were treated exactly within an appropriate Racah formalism. Values of the intramolecular {Ln-aminoxyl radical} exchange parameter have thus been rigorously deduced for both the Dy Kramers and Ho non-Kramers ion-based compounds. Ferromagnetic {Ln-radical} interactions are found for both the Dy and Ho derivatives with J=8 cm−1 and J=4.5 cm−1, respectively.

The synthesis and crystal structure of a novel one-dimensional Cu(II) compound [Cu(1,2-bis(tetrazol-1-yl)ethane)3](ClO4)2 are described. The single-crystal X-ray structure determination was carried out at 298 K. The molecular structure consists of a linear chain in which the Cu(II) ions are linked by three N4,N4' coordinating bis(tetrazole) ligands in syn conformation. The Cu(II) ions are in a Jahn-Teller distorted octahedral environment (Cu(1)-N(11)=2.034(2) A, Cu(1)-N(21)=2.041(2) A and Cu(1)-N(31)=2.391(2) A). The Cu⋯Cu separations are 7.420(3) A.

This paper concerns the chemistry and the magnetic properties of a family of compounds with a fully interlocked structure. The general formula is [Etrad] 2 [M 2 {Cu(opba)} 3 ]·S, with M=Mn 2+ , Co 2+ , Mg 2+ , Ni 2+ , where Etrad stands for a radical cation, and opba stands for ortho -phenylenebis(oxamato). The structure consists of two interpenetrating graphite-like networks with edge sharing hexagons. The magnetic properties of the four compounds have been investigated in detail. The Mg 2+ derivative behaves as a paramagnet with weak ferromagnetic interactions between Cu 2+ and Etrad + ions. The three other compounds are molecule-based magnets with a spontaneous magnetization below 22.8 K for the Mn 2+ derivative, 28 K for the Ni 2+ derivative and 37 K for the Co 2+ derivative. While the Mn 2+ derivative is a soft magnet, the Ni 2+ and Co 2+ derivative display hysteresis loops, with coercive fields higher than 1 kOe depending on the temperature and the size of the particles. The Ni 2+ derivative presents also an inversion of the magnetic poles as the temperature varies under a small external field. For higher field, the magnetization reverses two times. These original results can be properly reproduced using the molecular-field theory adapted to the system and the magnetic anisotropy.

The compounds Ln2[Cu(opba)]3·S (where opba stands for ortho-phenylenebis(oxamato) and S for solvent molecules) with Ln=Tb, Dy, Ho are spin ladder molecular magnets. Specific heat measurements down to 0.1 K show that these compounds undergo three-dimensional magnetic ordering, with TC=0.81 K and TC=0.74 K for the Tb- and the Dy-containing compounds, respectively. For the Ho-containing compound, only the high temperature tail of the λ anomaly is detected by a specific heat measurement. In combination with a low-temperature susceptibility measurement (not presented here), it can be concluded that its ordering temperature is between 0.05 and 0.1 K. These results are compared with the magnetic properties of the isostructural compound Gd2[Cu(opba)]3·S, which is a ferromagnet.

Single-crystal magnetization, heat capacity and neutron diffraction studies of the bimetallic regular chain MnIINiII(NO2)4(en)2 (en = ethylenediamine) are presented. This compound exhibits ferromagnetic interactions between the Mn and Ni ions within the chains. The onset of long-range antiferromagnetic ordering below TN = 2.45 K is associated with weaker antiferromagnetic interactions between neighbouring chains. The magnetization studies reveal an easy axis anisotropy of the susceptibility at low T with a preferred orientation ∥c. This anisotropy results from the low local symmetry around the Mn ions. Below TN, the susceptibility drop observed for H∥c is much larger than for H⊥c, pointing to a moment orientation parallel to the c axis in the ordered state. At 1.8 K, the metamagnetic transition is observed to proceed in two steps for H∥c, with Hc1 = 1.5 kOe and Hc2 = 2.5 kOe. This is associated with the familiar spin flop state at the intermediate field values. For H⊥c, at 1.8 K the ordered state is suppressed only in fields larger than 6 kOe. \mbox{Low-$T$} heat capacity data exhibit a pronounced anomaly at the antiferromagnetic transition. The reduced entropy at TN points to significant short-range ordering effects above TN associated with the ferromagnetic intrachain interactions. The ordered magnetic structure is an antiferromagnetic stacking of ferromagnetic sheets.

This article details the local spin density determination in the cyano-bridged, two-dimensional, molecular based magnet K 2 Mn 3 (H 2 O) 6 [Mo(CN) 7 ] 2 ·6 H 2 O ( T c = 39 K). The crystal structure, determined at room temperature by X-ray diffraction, was redetermined at 50 K using unpolarised neutron diffraction. The importance of intermolecular hydrogen-bonding interactions is clearly demonstrated in this study, previously characterised with X-rays. The local spin density was determined from polarised neutron diffraction data at 4 K with an applied field of 3 T. Positive spin densities were observed on the manganese sites, consistent with high-spin d 5 ions in octahedral fields, whilst a negative spin density was found on the molybdenum sites, signifying delocalisation onto the cyano ligands. The alternating sign of the spin populations on the metal sites, suggests that the primary Mo III –Mn II interactions are antiferromagnetic in nature and the delocalisation onto the cyano-bridges clearly demonstrates the role of the ligand bridges in the magnetic exchange pathway.

A series of novel polymeric compounds of formula [M(btzb)3][ClO4]2 (Mll = Fe, Ni or Cu) with btzb = 1,4-bis-(tetrazol-1-yl)butane have been prepared and their physical properties investigated. The btzb ligand has been prepared and its crystal structure determined, together with a tentative crystal structure of the 3-D compound [Fe(btzb)3][ClO4]2. The model of the latter shows two symmetry-related, interpenetrating Fe-btzb networks in which the iron(II) ions approach each other as close as 8.3 and 9.1 A. This supramolecular catenane undergoes a sharp thermal spin transition around 160 K with hysteresis (20 K) along with a pronounced thermochromic effect. The spin crossover behaviour has been followed by magnetic, DSC, optical spectroscopy and 57Fe Mossbauer spectroscopy measurements. Irradiation with green light at low temperature leads to population of the metastable high-spin state for the thermally active iron(ll) ions. The nature of the spin crossover behaviour has been discussed in detail.

The compounds of formula {Ln2[Ni(opba)]3}·S (opba being o-phenelynebis(oxamato), Ln lanthanideIII ions, and S solvents molecules) have been structurally and magnetically characterized for Ln = La, ...

Several authors or practitioners have remarked that Q-switched lasers have an immediate blackening effect on mineral pigments such as vermilion (HgS), minium (Pb 3 O 4 ), white lead [2PbCO 3 . Pb(OH) 2 ], malachite [Cu 2 CO 3 .(OH) 2 ] and yellow ochre (FeOH 2 ). A similar blackening effect can also be obtained on veined Carrara marble. This stone contains several trace elements, particularly iron and manganese. The latter is responsible for a particular response in electronic paramagnetic resonance spectroscopy (EPRS). We investigated the effect of a Q-switched Nd:YAG laser irradiation on the EPRS trace of Carrara marble. Further experiments were performed with the same analytical tool on synthetic calcite powders that had been doped with iron or manganese. It appeared that no significant change of EPRS trace could be put into evidence after laser irradiation of both bulk Carrara marble or doped calcite powders, although some samples had actually blackened. It is concluded that the blackening effect is not due to an oxidation of manganese ions. Colour measurements were also performed within the framework of this investigation. They showed that the colour shift to black is proportional to the fluence applied to the stone surface

A series of coordination compounds, [M(PM-L)2(NCS)2], with large aromatic ligands (PM-L) and a divalent metal ion (MII=Mn, Fe, Co, Ni and Zn) have been synthesized. The magnetic data are consistent with divalent metal ions in an octahedral ligand field environment. Hyper-Rayleigh scattering measurements show an enhancement of the molecular hyperpolarizabilities (β) from Ni compounds (with two unpaired electrons) to Mn compounds (five unpaired electrons). In the case of the Fe(II) metal ion, the influence of the ligand on the β values is reported. This work provides attractive information for the design of switchable materials based on the spin-crossover phenomenon.

The spin crossover (SC) compounds [Fe(PM-AzA)2(NCX)2] and [Fe(PM-FIA)2(NCX)2] (with PM-AzA = N-2'-pyridylmethylene-4-(phe-nylazo)aniline, PM-FIA = N-2'-pyridylmethylene-4-(2-amino)fluorene, and X = S, Se) have been prepared. The SC regimes have been deduced from variable-temperature magnetic susceptibility data. The enthalpy and entropy changes associated with the SC have been evaluated from DSC measurements. A cooperativity factor, C, has been defined, and its values for the different compounds have been deduced from the spin crossover curves. At 10 K, the light-induced excited spin state trapping (LIESST) effect has been observed within the cavity of the SQUID magnetometer. The critical temperatures Tc(LIESST) have been determined for [Fe(PM-AzA)2(NCS)2] and [Fe(PM-F1A)2(NCX)2], and the role of cooperativity has been analyzed. A linear correlation has been found between the Tc(LIESST) and C values. The kinetics of HS-->LS relaxation have been investigated; a thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures have been found. Finally, the magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed the occurrence of the light-induced thermal hysteresis (LITH) effect.

The spin transition of Fe2+ ions in the mononuclear compound cis-bis(thiocyanato)-bis(N-2'-pyridylmethylene)-4-(aminobiphenyl)-iron(II) is studied by electron paramagnetic resonance (EPR) of Mn2+. In cooling the compound down to the temperature range 174-168 K, the Fe2+ ions undergo a complete transition from the high spin (HS, S = 2) to the low spin (LS, S = 0) state, occurring with a narrow, ~5 K and unusually sharp hysteresis loop. The temperature dependence of the unit cell parameters is almost linear on both sides of the spin transition; the variation of the unit cell parameters at the spin transition is very anisotropic. The EPR spectra, typical of the Mn2+ ion, only gradually change with temperature in the two spin states of Fe2+ but undergo a striking transformation in the spin transition range. This shows that a considerable cooperativity exists between the metal ions. Computer simulations using a laboratory-developed simulation program indicate significant changes in the zero-field splitting (zfs) parameters in the course of the spin transition. Lower-than-axial symmetry of the environment of Mn2+ persists in both spin states of Fe2+; however, a stronger axial distortion arises in the HS state. The temperature variations of the zfs parameter D are related to transformations of the crystal structure using the Newman superposition model amended for contributions of thermal expansion of the crystal lattice and lattice vibrations. Computer fits show a reduction of the model parameter - power law exponent - in the LS state, t2 = 4, in comparison with the HS state, t2 = 8. Such a tendency is consistent with the decrease of the Fe-N bond lengths in the HS to LS transition.

Heat capacities of the metal-assembled ferromagnet MnCu(obbz)·H2O (obbz = oxamidobis(benzoato)) have been measured in the temperature region between 0.1 and 301 K by adiabatic calorimetry. A broad phase transition due to the three-dimensional ferromagnetic ordering was observed around 17 K. Above the transition temperature, a heat capacity tail arising from the short-range order characteristic of a low-dimensional magnet was found. The enthalpy and entropy gained by the phase transition are 227 J mol-1 and 12.7 J K-1 mol-1, respectively. This entropy acquisition is very close to the value Rln 5 ( = 13.4 J K-1 mol-1) expected for the spin manifold of the spin quantum number S = 2 ground state, where R stands for the gas constant. The thermal anomaly above the transition temperature can be accounted for in terms of the S = 2 two-dimensional ferromagnetic Heisenberg model rather than the one- or three-dimensional models. This suggests that this complex can be regarded as a two-dimensional ferrimagnet, in whi...

The crystal structures of two compounds have been determined, namely [Ce(NO 3 ) 3 {Cu(salen)} 2 ] ( 1 ) and [Gd(NO 3 ) 3 Cu(salen)] ( 2 ), salen being N,N′-ethylenebis(salicylideneaminato). The compounds [Ln(NO 3 ) 3 Cu(salen)] with Ln=Tb ( 3 ), Dy ( 4 ), Ho ( 5 ) and Er ( 6 ), which are isomorphous with 2 , have also been synthesized. 1 crystallizes in the monoclinic system, space group C 2/ c , with a =10.925(2) A, b =20.406(5) A, c =15.618(5) A, α =90°, β =94.73(2)°, γ =90°, Z =8. 2 crystallizes in the monoclinic system, space group P 2 1 / c , with a =12.123(3) A, b =10.321(3) A, c =18.248(2) A, α =90.04°, β =102.93(2)°, γ =90.04°, Z =4. The structure of 1 consists of neutral trinuclear units in which the Ce atom is linked to the two Cu atoms via the oxygen atoms of the salen ligand. The structure of 2 consists of neutral binuclear units in which the metal atoms are linked by the oxygen atoms of the salen ligand. In 1 and 2 the bridging network LnO 2 Cu has a butterfly shape. The magnetic properties of compounds 1 – 6 have been investigated. As expected, the Gd III Cu II magnetic interaction for 2 has been found to be ferromagnetic. The energy gap between ground S =4 and excited S =3 states has been determined as 4 J =+17.32 cm –1 . This value has been compared to what has been reported for other Gd III Cu II binuclear compounds. The magnetic properties for 1 , 3 , 4 , 5 and 6 are dominated by the crystal field effect on the Ln III site, masking the magnetic interaction between the paramagnetic centers. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS copper/ crystal structure/ heterobinuclear complexes/ lanthanide/ magnetic properties

The synthesis of a trans coordinated 1-D μ1,3-azido-Mn(II) chain compound of the formula [Mn(L)(N3)PF6]n (L=2,13-dimethyl-3,6,9,12,18-pentaazabicyclo{12.3.1}octadeca-1,(18),2,12,14,16-pentaene) is reported. Its crystal structure consists of 1-D arrays of Mn(II) units linked by single End-to-End azido bridges in trans position. Magnetic susceptibility measurements indicate long range antiferromagnetic correlation between the Mn(II) centres below 30 K (J=−4.8 cm−1, H=−J∑ SiSi+1). The solid state EPR behaviour confirms that [Mn(L)(N3)PF6]n consists of 1-D Mn(II) arrays magnetically well isolated from the neighbouring chains.

A novel three-dimensional copper(II) compound of the formula [Cu3(btre)5(H2O)2](ClO4)6·H2O (btre = 1,2-bis(1,2,4-triazol-4-yl)ethane) has been synthesized. The structure has been solved at 150 K by single-crystal X-ray analysis. The compound [C30H44Cu3N30O2·6(ClO4)·H2O] crystallizes in the triclinic space group P1bar with a = 12.488(1), b = 13.195(1), c = 19.756(2) A, α = 78.959(8), β = 84.553(8), γ = 63.869(7)°, V = 2868.4(5) A3, Z = 2 (trinuclear CuII units), 6484 reflections [I > 4σ(I)], RF = 0.0581, wR2 = 0.1214. The structure of this coordination polymer consists of a three-dimensional network built up from linear trinuclear CuII cations of formula [Cu3(btre)5(H2O)2]6+. This crystallographically independent unit comprises a linear array of three CuII ions linked by triple N1,N2-1,2,4-triazole bridges, yielding Cu1–Cu2 = 3.893(1) A and Cu2–Cu3 = 3.889(1) A. These linear trinuclear CuII units are linked by various pathways involving tridentate and tetradentate bridging btre ligands, which yields an unprecedented three-dimensional network. The CuII ions are weakly antiferromagnetically coupled with J = –1.01(2) cm–1 (based on the Hamiltonian: H = –J[SCu1·SCu2 + SCu2·SCu3] using the expression for the molar magnetic susceptibility S = 1/2 of a linear trinuclear system). The nature and the magnitude of the antiferromagnetic exchange have been discussed on the basis of the structural features.

Two FeII-based molybdenum polycyanides, Fe2[MoIII(CN)7]·8H2O and [Fe2(H2O)4][MoIV(CN)8]·4H2O, have been synthesized. The compound [Fe2(H2O)4][MoIV(CN)8]·4H2O crystallizes in the tetragonal system, space group I422. There are two molybdenum sites in a distorted square antiprism arrangement, each site being surrounded by eight CN–Fe linkages. The distorted octahedral iron site is formed by four NC–Mo linkages and two water molecules in apical positions. The structure is three-dimensional and highly symmetrical. The magnetic characteristics of these two compounds were compared. Fe2[MoIII(CN)7]·8H2O orders below 65 K as a ferrimagnet, but [Fe2(H2O)4][MoIV(CN)8]·4H2O shows no evidence of long-range magnetic order, obviously due to the contribution of the diamagnetic MoIV which suppresses the propagation of magnetic interaction between adjacent iron(II) ions through CN bridges. The results are discussed in the light of the single-crystal structural features of [Fe2(H2O)4][MoIV(CN)8]·4H2O.

A one-dimensional chain [MnII2(L)2(H2O)][MoIV(CN)8]·5H2O 2 (L = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene, macrocyclic ligand) has been prepared from the reaction of K4[MoIV(CN)8]·2H2O 1 with the complex [MnIIL(H2O)2]Cl2·4H2O and its structure and photomagnetic properties have been studied. The structure of 2 consists of a polymeric zigzag chain of alternating Mn(L) and Mo(CN)8 units with another Mn(L) unit linked to the Mo as a pendant arm. The paramagnetic compound 2 shows important modifications of its magnetic properties under UV-light irradiation, leading to the formation of ferrimagnetic chains. This non-reversible photomagnetic effect is thought to be caused by an internal photo-oxidation of MoIV (diamagnetic) to MoV (paramagnetic) as observed in the precursor 1. The crystallographic data seem to indicate that the formation of hydrogen bonds between water molecules and nitrogen atoms of non-bridging cyano groups is a key factor to observe the molybdenum photo-oxidation.

Reversible release or reabsorption of water like sponges, a process which is accompanied by a dramatic change of structure and magnetic properties, can be achieved by some molecular magnetic materials. When the sponges are hydrated, or wet, the structure consists of isolated units with a nonmagnetic ground state. When the sponges are dehydrated, or dry, the units are linked to each other, affording polymeric molecule-based magnets (see picture).

The compound 2-{3-[4-methyl-1,2,4-triazolyl]}-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide, abbreviated as Metrz-Nit, crystallizes in the non-centrosymmetric space group P2 1 2 1 2 1 . The investigation of the magnetic properties has revealed the occurrence of intermolecular ferromagnetic interactions. The crystal structure has been refined by neutron diffraction at 11 K. The spin density distribution has been determined from polarized neutron diffraction experiments carried out at 1.5 K under a magnetic field of 5 T. As expected, the main contributions of the spin distribution have been observed in the 2p π orbitals of the nitrogen and oxygen atoms of the two NO groups, and a significant negative spin population has been detected on the sp 2 carbon atom of the nitronyl nitroxide moiety. The spin distribution is slightly dissymmetrical, so that the sp 3 carbon atoms in α-position of the nitro nitrogen atoms carry spin populations of opposite signs. Concerning the triazole ring, the main spin population, of negative sign, has been found on the nitrogen atom occupying the 3-position. The carbon atom of the methyl group attached to the 4-position has been also found to carry a significant negative spin population. The spin populations on the hydrogen atoms have been determined. These experimental data have been compared to the results of LSD calculations performed on an isolated molecule. The role of intermolecular interactions in the spin distribution has been discussed.

Palladium(II) and ruthenium(II) complexes containing aminoxyl radical-substituted triphenylphosphine ligands are reported. The novel spin-labeled phosphine ligands [(p(4,4,5,5,-tetramethylimidazolinyl-1-oxyl-3-oxide)phenyl)diphenylphosphine] (1) and [(p-(Noxyl-tert-butylamino-2-)phenyl)diphenylphosphine] (2) react with PdCl2 to yield transPd[1]2Cl2 and trans-Pd[2]2Cl2 (complexes 3 and 4, respectively), which show exchange-coupled EPR spectra. This through-space, intramolecular coupling provides a method of determining the number of radical phosphines coordinated to the Pd(II) center. Systems with only one coordinated phosphine show EPR spectra typical of the ligand. Phosphine 2 reacts with [( 3 C3H5)PdCl]2 and [( 6 -p-cymene)RuCl2]2 to form the mononuclear metal phosphine adducts ( 3 -C3H5)Pd[2]Cl (5) and ( 6 -p-cymene)Ru[2]Cl2 (6). All complexes show broad, shifted 1 H NMR spectra, and magnetic susceptibility measurements confirm the presence of one (complexes 5, 6) or two (complexes 3, 4) radicals per complex accordingly. The X-ray crystal structures of Pd[2]2Cl2 and ( 3 -C3H5)Pd[2]Cl are also reported. Complexes 5 and 6 are unusual examples of stable organometallic systems with peripheral free radicals, i.e., spin-labeled organometallics.

The light-induced crossover in a series of iron(II) compounds has been investigated by irradiating the sample at 10 K with a Kr + laser coupled to an optical fiber within a SQUID cavity. The temperature dependence of the molar fraction of the light-induced metastable HS state has been recorded for 22 compounds. The critical LIESST temperature, T c (LIESST), has been defined as the temperature for which the light-induced HS information was erased in the SQUID cavity. The dependence of T c (LIESST) as a function of the thermal spin crossover temperature, T 1/2 , has been discussed. The effect of cooperation has been analyzed and the influence of horizontal and vertical displacements of the LS and HS potential wells has been discussed on the basis of the non-adiabatic multiphonon theory.

The goal of this paper is to introduce the dimension ‘magnetic anisotropy’ in the field of molecule–based magnets. For that, we have focused on two aspects, namely the design of a hard magnet through the incorporation of magnetically anisotropic spin carriers, and the determination of the magnetic phase diagrams of two strongly anisotropic ferromagnets. The hard magnet contains three kinds of spin carriers, Co II and Cu II ions as well as radical cations. Its structure is very peculiar; it consists of two perpendicular honeycomb–like networks which interpenetrate in such a way that each hexagon belonging to one of the networks is interlocked with an hexagon belonging to the perpendicular network. The coercive field depends on the grain size. It can be as large as 25 kOe at 5 K. The two strongly anisotropic ferromagnets are cyano–bridged Mn II Mo III compounds synthesized from the [Mo III (CN) 7 ] 4– precursor. Mo III has a low–spin configuration, with a local spin S Mo = 1/2, and a strongly anisotropic g tensor. One of the compounds, Mn 2 (H 2 O) 5 Mo(CN) 7 · 4H 2 O, has a three–dimensional structure. The other one, K 2 Mn 3 (H 2 O) 6 [Mo(CN) 7 ] 2 · 6H 2 O, has a two–dimensional structure. For both compounds, we have succeeded to grow well–shaped single crystals suitable for magnetic anisotropy measurements, and we have investigated the magnetic properties as follows: first, we have determined the magnetic axes by looking for the extremes of the magnetization in the three crystallographic planes, ab , bc , and ac . Then, we have measured the temperature and field dependences of the magnetization in the DC mode along the three magnetic axes. These measurements have revealed the existence of several magnetically ordered phases for the three–dimensional compound, and of field–induced spin reorientations for both compounds. For the very first time in the field of molecular magnetism, we believe we have been able to determine the magnetic phase diagrams.

The goal of this paper is to obtain further information on the magnetic phase diagram of the cyano-bridged bimetallic ferromagnet of formula Mn2(H2O)5Mo(CN)7·4H2O (α phase). For that, the temperature dependence of the heat capacity has been measured. A λ peak has been observed at Tc = 50.5 K, corresponding to the transition between the ferromagnetic and paramagnetic states. The temperature dependencies of the alternating current (ac) in-phase and out-of-phase magnetic susceptibilities have also been measured on single crystals oriented along their magnetic axes, a, b, and c*. The effect of the frequency and of the value of the static field have been investigated. These data confirm the magnetic phase diagram determined previously from single-crystal direct current (dc) magnetic measurements. The effect of pressure on the magnetic properties has also been investigated. Tc was found to vary linearly as a function of pressure, with a positive dTc/dP slope. All these results have been discussed.

The title compound, [Fe(btr)(3)](ClO(4))(2), has been synthesized. The investigation of its magnetic properties has revealed a low-spin--high-spin conversion occurring in two steps, each step involving 50% of the Fe(2+) ions. The low-temperature step is very abrupt and occurs with a thermal hysteresis whose width is about 3 K around T(1) = 184 K. The high-temperature step, centered around T(2) = 222 K, is rather gradual. Differential scanning calorimetric measurements have confirmed the occurrence of a two-step spin conversion. The enthalpy and entropy variations associated with the two steps have been found as DeltaH(1) = 5.7 kJ mol(-)(1) and DeltaS(1) = 30.1 J mol(-)(1) K(-)(1), and DeltaH(2) = 6.5 kJ mol(-)(1) and DeltaS(2) = 28.6 J mol(-)(1) K(-)(1), respectively. The crystal structure of [Fe(btr)(3)](ClO(4))(2) has been solved at three temperatures, namely, above the high-temperature step (260 K), between the two steps (190 K), and below the low-temperature step (150 K). The compound crystallizes in the trigonal system, space group Rthremacr;, at the three temperatures. The structure is three-dimensional. There are two Fe(2+) sites, denoted Fe1 and Fe2. Each of them is located on a 3-fold symmetry axis and an inversion center and is surrounded by six btr ligands through the nitrogen atoms occupying the 1- or 1'-positions. Each btr ligand bridges an Fe1 and an Fe2 site, with an Fe1-Fe2 separation of 8.67 Å at 260 K. The perchlorate anions are located in the voids of the three-dimensional architecture and are hydrogen bonded to the triazole rings of the btr ligands. These anions do not interact with the Fe1 and Fe2 sites exactly in the same way. At 260 K, both the Fe1 and Fe2 sites are high-spin (HS) with Fe-N bond lengths of 2.161(3) and 2.164(3) Å, respectively. At 190 K, the Fe1 site remains HS while the Fe2 site is low-spin (LS) with Fe-N bond lengths of 2.007(3) Å. Finally, at 150 K, both the Fe1 and Fe2 sites are LS with Fe-N bond lengths of 1.987(5) and 1.994(5) Å, respectively. It turns out that the two-step spin conversion is associated with the presence of two slightly different Fe(2+) sites. The spin conversion regime has also been followed by Mössbauer spectroscopy. These findings have been discussed and compared to the previously reported cases of two-step spin conversions.

Eine ferromagnetische Fernordnung bei 3 K weist die Titelverbindung auf, die sowohl als eine vollstandig lokalisiert gemischtvalente Spezies (Mo3+ und Mo4+) wie auch als eine Spezies mit gemischten Spins (Low-spin- und High-spin-Mn2+-Ionen) beschrieben werden kann. Ihre zweidimensionale Struktur besteht aus herzformigen, 48gliedrigen Ringen mit jeweils 16 Metallzentren (siehe Bild).

The synthesis of new paramagnetic phosphorus-containing compounds, (para-RAD-phenyl)diphenylphosphine and phosphine oxide (RAD = nitronylnitroxide. tBuNO), is described. Complexation of these phosphines with diamagnetic metal centres yields, for example, intermolecular ferromagnetic interactions with {Mo(CO)5(phosphine)}, antiferromagnetic interactions with {AuCl(phosphine)} and {PdCl2(phosphine)2}, a complex with multiple paramagnetic ligands. Ligation of nitronylnitroxide-containing phosphines to paramagnetic metals is hindered by catalytic nitrogen to phosphorus oxo-transfer to form the corresponding iminonitroxide-substituted phosphine oxide. A molecular square containing two nitronylnitroxide-substituted phosphine oxide ligands and two Mn(hfac)2 units is also described.

The compounds K2Mn3(H2O)6[Mo(CN)7]2·6H2Oand Mn2(H2O)5Mo(CN)7·nH2O (α phase, n = 4; β phase, n = 4.75) have been synthesized in the form of well shaped single crystals by slow diffusion of aqueous solutions containing [Mo(CN)7]K2·2H2O and [Mn(H2O)6](NO3), respectively. The first compound has been obtained in the presence of an excess of K± ions. The crystal structures have been determined. K2Mn3(H2O)6[Mo(CN)7]2·6H2O is two-dimensional, and Mn2(H2O)5Mo(CN)7 ·nH2O (α and β phases) are three-dimensional. The magnetic properties have been studied in great detail through single crystal magnetic measurements, and the magnetic phase diagrams have been determined. The three compounds exhibit a long-range ferromagnetic ordering along with a field-induced spin reorientation. Furtuthermore, the last two compounds show an additional magnetically ordered state which disappears when applying a suffeciently large magnetic field. The magnetic properties have been found to be dramatically modified when the non-coo...

The chemical tuning of the bulk magnetic properties of an organic radical has been envisaged in a series of nitronyl nitroxide and imino nitroxide substituted triazole derivatives.

Dehydration of single crystals of the non-magnetic molecular compound CoCu(obp)(H2O)3.H2O yields a compound CoCu(obp) which exhibits long range magnetic order. However, the magnetic transition temperature is rather low (viz. 7 K) and its M(H) plot shows no hysterisis width, which is unusual for a compound derived from Co(II)Cu(II) bimetallic chains. The origin of this magnetic behaviour appears to lie in the absence of the first-order angular momentum of the parent compound CoCu(obp)(H2O)3.H2O

Three new complexes involving nitronyl nitroxide and PhCOO- or N3- ligands have been synthesized and structurally and magnetically characterized. These compounds are formulated as Mn(L)4(X)2·nH2O, L = 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PNN) or 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (PN) and X = PhCOO- or N3-. Compound A [Mn(PNN)2(PhCOO)2(H2O)2] presents the following structural parameters: triclinic, space group P1 (No. 2), a = 6.859(1) A, b = 11.271(2) A, c = 13.978(6) A, α = 88.88(2)°, β = 89.38(2)°, γ = 78.28(2)°, Z = 1. The complex has a centrosymmetric distorted octahedral geometry in which the manganese ion is bound to two radical ligands through the nitrogen atom of the pyridine rings, two benzoate groups, and two water molecules. The two compounds B [Mn(PNN)4(N3)2] and C [Mn(PN)4(N3)2] are isostructural with the following structural parameters: B; triclinic, space group P1 (No. 2), a = 7.177(4) A, b = 13.767(3) A, c = 13.928(4) A, α = 90.20(2)°, β = 102.94...

Heat capacities of the molecule-based antiferromagnet MnCu(obbz)·5H2O (obbz = oxamidobis(benzoato)) have been measured at temperatures from 0.1 to 300 K by means of adiabatic calorimetry. A phase transition due to three-dimensional antiferromagnetic ordering was observed at 2.18 K, above which a heat capacity hump appeared, arising from the short-range ordering characteristic of a low-dimensional magnet. The enthalpy and entropy gained at the magnetic phase transition were evaluated to be 48.6 J mol−1 and 12.1 J K−1 mol−1, respectively. This entropy gain is close to the value R ln 5 ( = 13.4J K−1 mol−1) expected for the spin-manifold of the S = 2 ground state. The heat capacity hump above the magnetic transition temperature is explained well by the S = 2 one-dimensional ferromagnetic Heisenberg model with the exchange interaction J/kB = 0.75 K, where kB is the Boltzmann constant. These results confirm that the present complex behaves as a one-dimensional ferrimagnet, in which strong antiferromagnetic coup...

To date, most of the studies dealing with the magnetic properties of 4f-3d compounds have been limited to the case in which the 4f ion was Gd(III), with a pure spin ground state. For the lanthanide(III) ions with a first-order orbital momentum, the determination of the nature of the 4f-3d interaction is still a challenge. This paper addresses this problem. The magnetic properties of the compounds of formula {Ln(2)[M(opba)](3)}.S (abbreviated hereafter as Ln(2)M(3)) have been investigated; Ln stands for a lanthanide element, M for Cu or Zn, opba for ortho-phenylenebis(oxamato), and S for solvent molecules. All of these compounds have similar one-dimensional structures consisting of ladder-type motifs. Our approach consisted of comparing the magnetic properties of Ln(2)Cu(3) and Ln(2)Zn(3) for each Ln(III) ion. The former are governed by both the thermal population of the Stark components of Ln(III) and the Ln(III)-Cu(II) interaction; the latter are only governed by the thermal population of the Ln(III) Stark components. It has been confirmed that the Gd(III)-Cu(II) interaction was ferromagnetic, and it was found that the Tb(III)-Cu(II) and Dy(III)-Cu(II) interactions were ferromagnetic as well. The Tm(III)-Cu(II) interaction might also be ferromagnetic; the situation, however, is uncertain. On the other hand, in all other cases the Ln(III)-Cu(II) interaction is not ferromagnetic; it is either not detectable by the magnetic technique or antiferromagnetic. The difference between Dy(III)-Cu(II) (ferromagnetic) and Ho(III)-Cu(II) (not ferromagnetic) is particularly striking. These findings have been discussed.

The optical absorption spectroscopy for the trinuclear compounds Mn(CuL)2·xB with L standing for N-(4-methyl-6-oxo-3-azahept-4-enyl)oxamato and B = (CH3)2SO (x = 2) (1) or H2O (x = 5) (2) has been investigated in the 4−300 K temperature range. The crystal structure of 1 was known; it consists of isolated and neutral trinuclear units in which the central MnII ion is linked to two CuL complex ligands and two (CH3)2SO molecules in a cis configuration. The Cu−Mn−Cu linkage is bent with an angle of 127°. The spectra for 1 and 2 exhibit narrow and intense MnII spin-forbidden transitions in the 24000−28000 cm-1 range, activated by an exchange mechanism. The temperature dependences of the main feature corresponding to the 6A1 → 4A1, 4E(G) MnII transitions have been recorded. Using a model proposed first by Tanabe and co-workers and adapted to the CuMnCu topology, a theoretical expression for the temperature dependence of the intensity of the transition has been derived and compared to the experimental data. The p...

The title compound, K2Mn3(H2O)6[Mo(CN)7]2·6H2O, has been synthesized by slow diffusion of aqueous solutions saturated in KNO3 and containing K4[Mo(CN)7]·2H2O and [Mn(H2O)6](NO3)2, respectively. The compound crystallizes in the monoclinic system, space group C2. The Mo site is surrounded by six −C−N−Mn linkages and one terminal cyano group in a strongly distorted pentagonal bipyramid fashion. There are two distorted octahedral Mn sites, both with four −N−C−Mo linkages and two water molecules in trans conformation. The structure is two-dimensional with anionic layers perpendicular to the a axis and K+ ions located between the layers. Each layer is constituted of two parallel gridlike sheets made of edge-sharing Mo2Mn2 lozenge motifs, connected by Mn(CN)4(H2O)2 units situated between the sheets. Thorough magnetic investigations have been carried out. First, it has been demonstrated that the magnetic axes are collinear with the a, b, and c* crystallographic directions. The temperature and field dependences of...

The compounds of formula Ln2[M(opba)]3·xH2O·yDMSO containing 4f LnIII and 3d MII ions have been synthesized; opba stands for ortho-phenylenebis(oxamato). These compounds crystallize poorly, and hence structural information has been obtained from the wide angle X-ray scattering (WAXS) technique. It has been shown that the compounds present the same ladder-like motif as that previously described for Tm2[Cu(opba)]3·4H2O·10DMF. Changing the LnIII and/or MII ions results in more or less pronounced distortions of the ladders. For a given MII ion, changing the LnIII ion from LaIII to LuIII results in a decrease of the Ln–M distance parallel to the lanthanide contraction. In the same way, for a given rare earth, changing the MII ion from CuII to NiII and ZnII ions also results in the decrease of the Ln–M distance in accord with the decreasing radii of the MII ions. The structural details have been analyzed and discussed.

EPR studies of (NBu4)2Mn2[Cu(opba)]3·6DMSO·1H2O, in the 10–300 K range with in-situ illumination by copper vapour laser radiation (510.5 and 578.2 nm) have given evidence for the following: (i) The EPR results obtained on these compounds as a function of temperature are not predominantly due to demagnetization effects, (ii) the magnetic order is affected by light most likely due to the decoupling of spins followed by electronic excitation. This enables the possibility of photo-induced switching of magnetic order.

Coupled cobalt and nickel sandwich complexes with one positive charge are paramagnetic mixed-valence species, in which the charge is delocalized in solution yet localized in the solid state. When the mono- and dications (figure) have more than one unpaired electron, antiferromagnetic interaction across the ligand bridge is observed. The pathway to the neighboring sandwich complex preferentially consists of particular ligand MOs, such that the spin resides on selected ligand nuclei.