Valeriu Mereacre

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.

A Heterometallic FeII–DyIII Single‐Molecule Magnet with a Record Anisotropy Barrier

A record anisotropy barrier (319 cm(-1) ) for all d-f complexes was observed for a unique Fe(II) -Dy(III) -Fe(II) single-molecule magnet (SMM), which possesses two asymmetric and distorted Fe(II) ions and one quasi-D5h Dy(III) ion. The frozen magnetization of the Dy(III) ion leads to the decreased Fe(II) relaxation rates evident in the Mössbauer spectrum. Ab initio calculations suggest that tunneling is interrupted effectively thanks to the exchange doublets.

Combined Magnetic Susceptibility Measurements and 57Fe Mössbauer Spectroscopy on a Ferromagnetic {FeIII4Dy4} Ring

Ferromagnetic interactions in an Fe4Dy4 single-molecule magnet were studied using a combination of magnetic susceptibility measurements (see diagram; inset: cluster core) and 57Fe Mossbauer spectroscopy.

Slow magnetic relaxation in trigonal-planar mononuclear Fe(II) and Co(II) bis(trimethylsilyl)amido complexes. A comparative study

Alternating current magnetic investigations on the trigonal-planar high-spin Co(2+) complexes [Li(15-crown-5)] [Co{N(SiMe3)2}3], [Co{N(SiMe3)2}2(THF)] (THF = tetrahydrofuran), and [Co{N(SiMe3)2}2(PCy3)] (Cy = -C6H13 = cyclohexyl) reveal that all three complexes display slow magnetic relaxation at temperatures below 8 K under applied dc (direct current) fields. The parameters characteristic for their respective relaxation processes such as effective energy barriers Ueff (16.1(2), 17.1(3), and 19.1(7) cm(-1)) and relaxation times τ0 (3.5(3) × 10(-7), 9.3(8) × 10(-8), and 3.0(8) × 10(-7) s) are almost the same, despite distinct differences in the ligand properties. In contrast, the isostructural high-spin Fe(2+) complexes [Li(15-crown-5)] [Fe{N(SiMe3)2}3] and [Fe{N(SiMe3)2}2(THF)] do not show slow relaxation of the magnetization under similar conditions, whereas the phosphine complex [Fe{N(SiMe3)2}2(PCy3)] does, as recently reported by Lin et al. (Lin, P.-H.; Smythe, N. C.; Gorelsky, S. I.; Maguire, S.; Henson, N. J.; Korobkov, I.; Scott, B. L.; Gordon, J. C.; Baker, R. T.; Murugesu, M. J. Am. Chem. Soc. 2011, 135, 15806.) Distinctly differing axial anisotropy D parameters were obtained from fits of the dc magnetic data for both sets of complexes. According to density functional theory (DFT) calculations, all complexes possess spatially nondegenerate ground states. Thus distinct spin-orbit coupling effects, as a main source of magnetic anisotropy, can only be generated by mixing with excited states. This is in line with significant contributions of excited determinants for some of the compounds in complete active space self-consistent field (CASSCF) calculations done for model complexes. Furthermore, the calculated energetic sequence of d orbitals for the cobalt compounds as well as for [Fe{N(SiMe3)2}2(PCy3)] differs significantly from the prediction by crystal field theory. Experimental and calculated (time-dependent DFT) optical spectra display characteristic d-d transitions in the visible to near-infrared region. Energies for lowest transitions range from 0.19 to 0.35 eV; whereas, for [Li(15-crown-5)][Fe{N(SiMe3)2}3] a higher value is found (0.66 eV). Zero-field (57)Fe Mößbauer spectra of the three high-spin iron complexes exhibit a doublet at 3 K with small and similar values of the isomer shifts (δ), ranging between 0.57 and 0.59 mm/s, as well as an unusual small quadrupole splitting (ΔEQ = 0.60 mm/s) in [Li(15-crown-5)][Fe{N(SiMe3)2}3].

A symmetry-breaking spin-state transition in iron(III)

Stepping up: A two-step magnetic spin transition with accompanying structural phase transitions is reported for the first time for Fe III. The transitions are observed at 187 K and 90 K on cooling with a hysteretic transition recorded upon heating during the first crossover at 106 K. The intermediate phase persists over 97 K and contains an unprecedented [HS-HS-LS] motif with tripling of the unit cell. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Family of heterometallic semicircular Mn(III)2Ln(III)3 strands.

A series of heterometallic complexes of formula [Mn(III)(2)Ln(III)(3)(bdeaH)(3)(bdea)(2)(piv)(8)] x MeCN, where bdeaH(2) = N-butyldiethanolamine, piv = pivalate, and Ln = Y (1), Tb (2), Dy (3), Ho (4), and Er (5), has been prepared. 1-5 are isostructural, with the metal centers forming a novel semicircular Ln(III)-Mn(III)-Ln(III)-Mn(III)-Ln(III) strand. Only 2 and 3 exhibit slow relaxation of the magnetization, suggesting that the highly anisotropic Ln(III) ions (Tb(III) and Dy(III)) are essential for an energy barrier to spin reversal.

Tridecanuclear [MnIII5LnIII8] Complexes Derived from N-tButyl-diethanolamine: Synthesis, Structures, and Magnetic Properties

The synthesis, structures and magnetic properties of a family of heterometallic [Mn(III)(5)Ln(III)(8)(mu(3)-OH)(12)(L(2))(4)(piv)(12)(NO(3))(4)(OAc)(4)](-) (Ln = Pr, 2; Nd, 3; Sm, 4; Gd, 5; Tb, 6) aggregates are reported. The complexes were obtained from the direct reaction of N-(t)butyldiethanolamine (H(2)L(2)) with Mn(OAc)(2) x 4 H(2)O and Ln(NO(3))(3) x 6 H(2)O in the presence of pivalic acid (pivH) in MeCN under ambient conditions. Compounds 2-6 are isomorphous and crystallize in the monoclinic space group P2(1)/n with four molecules in the unit cell. The complexes have a centrosymmetric tridecanuclear anionic core consisting of two distorted inner heterometallic [Mn(III)Ln(III)(3)(mu(3)-OH)(4)](8+) cubane subunits sharing a common Mn vertex flanked by four edge-sharing heterometallic [Mn(III)Ln(III)(2)(mu(3)-OH)(4)](5+) defect cubane units. Complexes 2-6 are the first high-nuclearity 3d-4f aggregates reported to date using (t)Bu-deaH(2) as ligand. These compounds show no evidence of slow relaxation behavior above 1.8 K, which appears to be the consequence of the very weak or non-existent magnetic interactions between the Mn(III) and Ln(III) ions resulting from the particular angles at the bridging oxygens.

Probing Lanthanide Anisotropy in Fe–Ln Aggregates by Using Magnetic Susceptibility Measurements and 57Fe Mössbauer Spectroscopy

The use of lanthanides to modulate the magnetic properties of transition-metal single-molecule magnets has become more common in recent years, mainly as a result of the magnetic anisotropy of some lanthanides that can increase the blocking temperature for reversal of magnetization. The main technique used to probe the reorientation of the magnetisation is ac magnetic susceptibility, but this technique yields only averaged magnetic moment information. Another more sensitive technique is Fe Mçssbauer spectroscopy, which can give information about oxidation levels and spin states, local moments at the iron nuclei, and spin-relaxation dynamics, and, more importantly, about the anisotropy not only of the studied isotope, but also of elements interacting with this isotope. Herein, the magnetic properties of complexes containing iron and lanthanide (or rareearth) cations have been probed by a combination of both Mçssbauer spectroscopy and magnetic susceptibility measurements. Reaction between FeCl2·4 H2O, Ln ACHTUNGTRENNUNG(NO3)3·6H2O, pivalic acid, H4edte (N,N,N’,N’-tetrakis-(2-hydroxyethyl)ethylenediamine) and phenol in a 1:0.5:5:1:1 molar ratio in MeCN/ CH2Cl2 gave a red solution from which light red crystals of isomorphous compounds [FeIII4Ln2ACHTUNGTRENNUNG(m4-O)2ACHTUNGTRENNUNG(NO3)2ACHTUNGTRENNUNG(piv)6ACHTUNGTRENNUNG(Hedte)2]·x CH3CN·y CH2Cl2·z C6H5OH, (Ln =Y (1, x= 2.3, y=1.7, z=0), Gd (2, x=1.8, y= 2.2, z=0), Dy (3, x= 4, y=0, z=1)) deposited after 24 h. The structure of the Dy compound 3 is described here. The central core of the aggregate possesses a [FeIII4Dy2ACHTUNGTRENNUNG(m4-O)2]14+ architecture with the four Fe ions arranged in what is often termed a “butterfly” shape, although here the Fe centers are strictly coplanar. Each Fe3 triangle is connected to the capping Dy through m4-O bridges, which are thus displaced out of such a triangle comprising one wingand two body-iron atoms, for example, Fe(2), Fe(1), and Fe(1’) bridged to Dy(1) through O(1) (Figure 1). Peripheral ligation is provided by one nitrato and one pivalato ligand chelating Dy(1), four m-pivalato ligands in their common syn,syn bridging mode between body-iron atoms and Dy centers, and two triply deprotonated (Hedte) ions. Each Hedte ligand chelates Fe(2) through its two nitrogens and provides three m-alkoxo oxygen bridges from Fe(2) to Fe(1), Fe(1’), and Dy(1), respectively. The fourth protonated arm of each Hedte ligand does not coordinate, but forms an intermolecular hydrogen bond to a chelating pivalato oxygen atom of a neighboring molecule, forming 1D supramolecular chains. Taking the structural information of Fe4Y2 (1) into account, the general spin-Hamiltonian describing the isotropic [a] M. N. Akhtar, Dr. V. Mereacre, Dr. G. Novitchi, Dr. C. E. Anson, Prof. Dr. A. K. Powell Institut f r Anorganische Chemie, Universit t Karlsruhe Engesserstrasse 15, 76128 Karlsruhe (Germany) Fax: (+49) 721-608-8142 E-mail : powell@aoc.uni-karlsruhe.de [b] Prof. Dr. J.-P. Tuchagues Laboratoire de Chimie de Coordination du CNRS, UPR 8241 205 Route de Narbonne, 31077 Toulouse Cedex 04 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200900758. Figure 1. Molecular structure of 3 (molecule 1). Solvent molecules, disordered atoms, and organic H atoms have been omitted for clarity.

Effect of ligand substitution on the interaction between anisotropic Dy(III) ions and 57Fe nuclei in Fe2Dy2 coordination clusters.

A series of [Fe(2)Dy(2)(OH)(2)(teaH)(2)(RC(6)H(4)COO)(6)] compounds has been synthesized and studied using Mössbauer spectroscopy. It is suggested that the local crystal field of the Dy(III) centers and the external magnetic field can control their shape anisotropy and thus the interactions between the dysprosium and iron centers.

Family of MnIII2Ln2(μ4-O) Compounds: Syntheses, Structures, and Magnetic Properties

An isostructural family of tetranuclear aggregates [Mn(III)(2)Ln(2)(O)(Piv)(2)(hep)(4)(NO(3))(4)].MeCN (where Ln = Y(III) (1), Pr(III) (2), Nd(III) (3), Gd(III) (4), Tb(III) (5), Dy(III) (6), Ho(III) (7), and Yb(III) (8)) is reported. They were obtained from the reactions of 2-(2-hydroxyethyl)pyridine (hepH) with a preformed hexanuclear manganese complex, [Mn(6)], and the respective lanthanide salt. The complexes are isomorphous and represent a new heterometallic 3d-4f complex type for this class of ligand. The structural core of 1-8 consists of a distorted Mn(2)Ln(2) tetrahedron with the four metal centers linking through a mu(4)-O(2-) bridging atom. The magnetic properties of all complexes were investigated by variable temperature magnetic susceptibility and magnetization measurements. The magnetic data of all compounds suggest that antiferromagnetic interactions are present between adjacent paramagnetic centers. Complexes 5-7 containing highly anisotropic lanthanide ions (Tb, Dy, and Ho) show slow relaxation of their magnetization.