Rodolphe Clérac

Dinuclear Dysprosium(III) Single-Molecule Magnets with a Large Anisotropic Barrier†

Due to the large intrinsic magnetic anisotropy of the lanthanide ions, rare-earth metal systems, and in particular dysprosium (Dy) based materials, have sparked increasing interest in the area of molecular magnetism. In a molecular complex, when such a unique property is combined with a high-spin ground state (S), slow relaxation of the magnetization can be obtained as seen for single-molecule magnets (SMMs). Although, a number of mixed transition-metal/ lanthanide SMMs have been reported, pure lanthanide SMMs are relatively scarce. The latter molecules are rare owing to the difficulty in promoting magnetic interactions in these systems. These interactions are attained by the overlap of bridging ligand orbitals with the 4f orbitals of the lanthanide ions. Thus, ligand design is one of the key components for achieving such interactions in pure lanthanide-based systems. To induce significant magnetic interaction between the lanthanide ions and synthesize high-energy-barrier SMMs, we have been investigating the use of (2-hydroxy-3-methoxyphenyl)methylene (isonicotino)hydrazine (H2hmi) as a rigid chelate in lanthanide chemistry. Such a linear ligand provides O,N,O,O-based multichelating sites that are especially favorable for lanthanide ion complex formation. They can form dinuclear systems using the bridging phenoxide oxygen atom, and the pyridine group promotes the formation of extended networks that can control the organization of the SMM units in the three-dimensional structure. Herein we report the use of the H2hmi ligand to design materials based on ferromagnetically coupled dinuclear dysprosium(III) SMMs with large relaxation barriers. [Dy2(hmi)2(NO3)2(MeOH)2] (1) and [Dy2(hmi)2(NO3)2 (MeOH)2]1·MeCN (2·MeCN) were obtained from a suspension of Dy(NO3)3·5H2O / H2hmi in methanol (treated with triethylamine) and in a 3:1 mixture of acetonitrile and methanol (treated with pyridine), respectively. After two days, pale orange single crystals were obtained, which were kept in contact with the mother liquor to prevent deterioration. Complexes 1 (Figure 1) and 2 (Figure 2) crystallize in monoclinic P21/c and orthorhombic Pbca space groups, respectively. Both complexes have similar dinuclear

Pentanuclear Dysprosium Hydroxy Cluster Showing Single-Molecule-Magnet Behavior

A pentanuclear dysprosium hydroxy cluster of composition [Dy 5(mu 4-OH)(mu 3-OH) 4(mu-eta (2)-Ph 2acac) 4(eta (2)-Ph 2acac) 6] ( 1; Ph 2acac = dibenzoylmethanide) was prepared starting from [DyCl 3.6H 2O] and dibenzoylmethane. Both static (dc) and dynamic (ac) magnetic properties of 1 have been studied. Below 3 K, the appearance of slow relaxation of the magnetization typical for single-molecule magnets is seen, even if no hysteresis effects on the M vs H data are observed above 1.8 K.

Heterometallic [Mn5‐Ln4] Single‐Molecule Magnets with High Anisotropy Barriers

The reaction of [Mn6O2(Piv)(10)(4-Me-py)(2.5)(PivH)(1.5)] (1) (py: pyridine, Piv: pivilate) with N-methyldiethanolamine (mdeaH2) and Ln(NO3)3 x 6 H2O in MeCN leads to a series of nonanuclear compounds [Mn5Ln4(O)6(mdea)2(mdeaH)2(Piv)6(NO3)4(H2O)2]2 MeCN (Ln=Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5)). Single-crystal X-ray diffraction shows that compounds 2-5 are isostructural, with the central core composed of two distorted {Mn(IV)Mn(III)Ln2O4} cubanes sharing a Mn(IV) vertex, representing a new heterometallic 3d-4f motif for this class of ligand. The four new compounds display single-molecule magnet (SMM) behaviour, which is modulated by the lanthanide ion used. Moreover, the values found for Delta(eff) and tau(o) for 3 of 38.6 K and 3.0 x 10(-9) s respectively reveal that the complex 3 exhibits the highest energy barrier recorded so far for 3d-4f SMMs. The slow relaxation of the magnetisation for 3 was confirmed by mu-SQUID measurements on an oriented single crystal and the observation of M versus H hysteresis loops below 1.9 K.

Trinuclear Heterobimetallic Ni2Ln complexes [L2Ni2Ln][ClO4] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er; LH3 = (S)P[N(Me)N═CH−C6H3-2-OH-3-OMe]3): From Simple Paramagnetic Complexes to Single-Molecule Magnet Behavior

The reaction of LH3 with Ni(ClO4)(2).6H 2O and lanthanide salts in a 2:2:1 ratio in the presence of triethylamine leads to the formation of the trinuclear complexes [L2Ni2Ln][ClO4] (Ln=La (2), Ce (3), Pr (4), Nd (5), Sm (6), Eu (7), Gd (8), Tb (9), Dy (10), Ho (11) and Er (12) and L: (S)P[N(Me)NCH-C6H3-2-O-3-OMe]3). The cationic portion of these complexes consists of three metal ions that are arranged in a linear manner. The two terminal nickel(II) ions are coordinated by imino and phenolate oxygen atoms (3N, 3O), whereas the central lanthanide ion is bound to the phenolate and methoxy oxygen atoms (12O). The Ni-Ni separations in these complexes range from 6.84 to 6.48 A. The Ni-Ni, Ni-Ln and Ln-O phenolate bond distances in 2-12 show a gradual reduction proceeding from 2 to 12 in accordance with lanthanide contraction. Whereas all of the compounds (2-12) are paramagnetic systems, 8 displays a remarkable ST=(11)/2 ground state induced by an intramolecular Ni. . .Gd ferromagnetic interaction, and 10 is a new mixed metal 3d/4f single-molecule magnet generated by the high-spin ground state of the complex and the magnetic anisotropy brought by the dysprosium(III) metal ion.

[ReCl4(CN)2]2-: A High Magnetic Anisotropy Building Unit Giving Rise to the Single-Chain Magnets (DMF)4MReCl4(CN)2 (M = Mn, Fe, Co, Ni)

An S = 3/2, high-anisotropy building unit, trans-[ReCl(4)(CN)(2)](2-), representing the first paramagnetic complex with a mixture of just cyanide and halide ligands, has been synthesized through the reaction of (Bu(4)N)CN with ReCl(4)(THF)(2). This species is characterized in detail and employed in directing the formation of a series of one-dimensional coordination solids of formula (DMF)(4)MReCl(4)(CN)(2) (M = Mn (2), Fe (3), Co (4), Ni (5)). Variable-temperature dc magnetic susceptibility measurements demonstrate the presence of intrachain antiferromagnetic (2) and ferromagnetic (3-5) exchange coupling within these solids. In addition, probing the ac magnetic susceptibility as a function of both temperature and frequency reveals that all of the chain compounds exhibit slow relaxation of the magnetization. The relaxation time is shown to be thermally activated, with energy barriers to relaxation of Delta(tau) = 31, 56, 17, and 20 cm(-1) for 2-5, respectively. Notably, the field-dependent magnetization of the iron congener exhibits a significant hysteresis effect at low temperature, with a coercive field of H(C) = 1.0 T, thus demonstrating magnetlike behavior in this one-dimensional system. Finally, the magnetization dynamics of all solids occur within the finite-size regime, where the magnetic domain growth is limited due to physical defects along the chains within the crystals.

Synthesis, structure, and magnetism of heterobimetallic trinuclear complexes {[L2Co2Ln][X]} [Ln = Eu, X = Cl; Ln = Tb, Dy, Ho, X = NO3; LH3 = (S)P[N(Me)N=CH-C6H3-2-OH-3-OMe]3]: A 3d-4f family of single-molecule magnets.

Sequential reaction of LH3 (LH3 = (S)P[N(Me)N=CH-C6H3-2-OH-3-OMe]3) with Co(OAc)2 x 4 H2O followed by reaction with lanthanide salts afforded trinuclear heterobimetalllic compounds {[L2Co2Ln][X]} [Ln = Eu (1), X = Cl; Ln = Tb (2), Dy (3), Ho (4), X = NO3] in excellent yields. These compounds retain their integrity in solution as determined by electrospray ionization mass spectrometry studies. The molecular structures of 1-4 were confirmed by a single-crystal X-ray structural study and reveal that these are isostructural. In all of the compounds, the three metal ions are arranged in a perfectly linear manner and are held together by two trianionic ligands, L3-. The two terminal Co(II) ions contain a facial coordination environment (3N, 3O) comprising three imino nitrogen atoms and three phenolate oxygen atoms. The coordination geometry about the cobalt atom is severely distorted. An all-oxygen coordination environment (12O) is present around the central lanthanide ion, which is present in a distorted icosahedral geometry. The coordination sphere around the lanthanide ion is achieved by utilizing three phenolate oxygen atoms and three methoxy oxygen atoms of each ligand. In all of these trinuclear complexes (1-4), the Co-Ln distances are around 3.3 A, while the Co-Co distances range from 6.54 to 6.60 A. The screw-type coordination mode imposed by the ligand induces chirality in the molecular structure, although all of the complexes crystallize as racemates. Magnetic properties of 1-4 have been studied in detail using dc and ac susceptibility measurements. Dynamic measurements reveal that 2-4 display a single-molecule magnet behavior, while the Co2Eu (1) analogue does not show any out-of-phase ac susceptibility.

A One-Pot, High-Yield Synthesis of a Paramagnetic Nickel Square from Divergent Precursors by Anion Template Assembly**

A paramagnetic Ni-containing molecular square has been synthesized in high yield from divergent precursors. An X-ray structure (shown in the picture) reveals the presence of an encapsulated tetrafluoroborate anion, which appears to be a requisite condition for the formation of the cyclic oligomer.

Reversible Thermally and Photoinduced Electron Transfer in a Cyano‐Bridged {Fe2Co2} Square Complex

Flip to be square: Structural, spectroscopic, magnetic, and photomagnetic studies conclusively demonstrate that a tetranuclear cyanometalate {Fe2Co2} complex undergoes reversible thermally and light-induced changes in optical and magnetic properties. This instability is induced by an intramolecular electron transfer, as oberved in three-dimensional Co/Fe Prussian blue compounds (see picture).

Glauber dynamics in a single-chain magnet: From theory to real systems

The Glauber dynamics is studied in a single-chain magnet. As predicted, a single relaxation mode of the magnetization is found. Above 2.7 K, the thermally activated relaxation time is mainly governed by the effect of magnetic correlations and the energy barrier experienced by each magnetic unit. This result is in perfect agreement with independent thermodynamical measurements. Below 2.7 K, a crossover towards a relaxation regime is observed that is interpreted as the manifestation of finite-size effects. The temperature dependences of the relaxation time and of the magnetic susceptibility reveal the importance of the boundary conditions.

Enantiomerically Pure Chiral {Fe28} Wheels

Wheels of steel: Two enantiomerically pure chiral {Fe(28)} wheel-like aggregates have been synthesized from the acetate buffer solution containing ferric ions and chiral tartrate ligands (see picture). These compounds are the largest chiral ferric aggregates isolated to date.