Butterfly‐Shaped Pentanuclear Dysprosium Single‐Molecule Magnets

Two new “butterfly‐shaped” pentanuclear dysprosium(III) clusters, [Dy₅(μ₃‐OH)₃(opch)₆(H₂O)₃] ? 3 MeOH ? 9 H₂O ( 1 ) and [Dy₅(μ₃‐OH)₃(Hopch)₂(opch)₄(MeOH)(H₂O)₂] ? (ClO₄)₂ ? 6 MeOH ? 4 H₂O ( 2 ), which were based on the heterodonor‐chelating ligand o‐vanillin pyrazine acylhydrazone (H₂opch), have been successfully synthesized by applying different reaction conditions. Single‐crystal X‐ray diffraction analysis revealed that the butterfly‐shaped cores in both compounds were comparable. However, their magnetic properties were drastically different. Indeed, compound 1 showed dual slow‐relaxation processes with a transition between them that corresponded to energy gaps (Δ) of 8.1 and 37.9 K and pre‐exponential factors (τ₀) of 1.7×10^?5 and 9.7×10^?8 s for the low‐ and high‐temperature domains, respectively, whilst only a single relaxation process was noted for compound 2 (Δ=197 K, τ₀=3.2×10^?9 s). These significant disparities are most likely due to the versatile coordination of the H₂opch ligands with particular keto–enol tautomerism, which alters the strength of the local crystal field and, hence, the nature or direction of the easy axes of anisotropic dysprosium ions.     

Heterodinuclear Lanthanoid‐Containing Polyoxometalates: Stepwise Synthesis and Single‐Molecule Magnet Behavior

Polyoxometalates (POMs) with heterodinuclear lanthanoid cores, TBA₈H₄[{Ln(μ₂‐OH)₂Ln′}(γ‐SiW₁₀O₃₆)₂] ( LnLn′ ; Ln=Gd, Dy; Ln′=Eu, Yb, Lu; TBA=tetra‐n‐butylammonium), were successfully synthesized through the stepwise incorporation of two types of lanthanoid cations into the vacant sites of lacunary [γ‐SiW₁₀O₃₆]^8? units without the use of templating cations. The incorporation of a Ln³⁺ ion into the vacant site between two [γ‐SiW₁₀O₃₆]^8? units afforded mononuclear Ln³⁺‐containing sandwich‐type POMs with vacant sites ( Ln1 ; TBA₈H₅[{Ln(H₂O)₄}(γ‐SiW₁₀O₃₆)₂]; Ln=Dy, Gd, La). The vacant sites in Ln1 were surrounded by coordinating W? O and Ln? O oxygen atoms. On the addition of one equivalent of [Ln′(acac)₃] to solutions of Dy1 or Gd1 in 1,2‐dichloroethane (DCE), heterodinuclear lanthanoid cores with bis(μ₂‐OH) bridging ligands, [Dy(μ₂‐OH)₂Ln′]⁴⁺, were selectively synthesized ( LnLn′ ; Ln=Dy, Gd; Ln′=Eu, Yb, Lu). On the other hand, La1 , which contained the largest lanthanoid cation, could not accommodate a second Ln′³⁺ ion. DyLn′ showed single‐molecule magnet behavior and their energy barriers for magnetization reversal (ΔE/k_B) could be manipulated by adjusting the coordination geometry and anisotropy of the Dy³⁺ ion by tuning the adjacent Ln′³⁺ ion in the heterodinuclear [Dy(μ₂‐OH)₂Ln′]⁴⁺ cores. The energy barriers increased in the order: DyLu (ΔE/k_B=48 K)< DyYb (53 K)< DyDy (66 K)< DyEu (73 K), with an increase in the ionic radii of Ln′³⁺; DyEu showed the highest energy barrier.     

An updated version of the computational package SIMPRE that uses the standard conventions for Stevens crystal field parameters

The crystal field approach used by SIMPRE is analyzed, verifying the exactness of the results concerning energy levels and magnetic properties calculated by the package. To coincide with the prevailing conventions, we reformulate the presentation of the crystal field parameters, so that the results are now, also from a formal point of view, strictly correct. New calculations are presented to test the influence of neglecting the excited J states, a common but critical approximation employed by SIMPRE. For that, we examine the case of Er(trensal) complex (H₃trensal = 2,2′,2″‐tris(salicylideneimino)triethylamine) where the influence of this approximation is found to be minimal. A patched version of the code, SIMPRE 1.1, and an updated version of the user manual are now available. Finally, we comment on “Software package SIMPRE – revisited,” which apparently revisits a software package without inspecting or using the code. © 2014 Wiley Periodicals, Inc.     

Software package SIMPRE—Revisited

This article elucidates the pitfalls identified in the software package SIMPRE recently developed by Baldoví et al. (J. Comput. Chem. 2013, 34, 1961) for modeling the spectroscopic and magnetic properties of single ion magnets as well as single‐molecule magnets. Analysis of the methodology used therein reveals that the crystal field parameters (CFPs), expressed nominally in the Stevens formalism, exhibit features characteristic for the CFPs expressed in the Wybourne notation. The resemblance of the two types of CFPs introduces a serious confusion that may lead to wrong comparisons of the CFPs taken from various sources. To clarify this confusion, the properties of the CFPs ( , ) associated with the Stevens operators ( X = S , J , or L ), which belong to the class of the tesseral‐tensor operators, are contrasted with those of the CFPs B_kq associated with the Wybourne operators , which belong to the class of the spherical‐tensor operators. Importantly, the confused properties of Stevens and Wybourne operators may bear on reliability of SIMPRE calculations. To consider this question independent calculations are carried out using the complete approach and compared with those of the restricted approach utilized earlier. It appears that the numerical results of the package SIMPRE are formally acceptable, however, the meaning of the CFPs must be properly reformulated. Several other conceptual problems arising from misinterpretations of the crucial notions and the CFP notations identified therein are also discussed and clarified. © 2014 Wiley Periodicals, Inc.     

[2,5‐Bis(2,2′‐bipyridyl‐6‐yl)‐3,4‐diazahexa‐2,4‐diene]dichloridomanganese(II): from a mononuclear compound to a three‐dimensional supramolecular framework through C—H...Cl hydrogen bonds and π–π stacking interactions

The title compound, [MnCl₂(C₂₄H₂₀N₆)], has been synthesized and characterized based on the multifunctional ligand 2,5‐bis(2,2′‐bipyridyl‐6‐yl)‐3,4‐diazahexa‐2,4‐diene (L). The Mn^II centre is five‐coordinate with an approximately square‐pyramidal geometry. The L ligand acts as a tridendate chelating ligand. The mononuclear molecules are bridged into a one‐dimensional chain by two C—H...Cl hydrogen bonds. These chains are assembled into a two‐dimensional layer through π–π stacking interactions between adjacent uncoordinated bipyridyl groups. Furthermore, a three‐dimensional supramolecular framework is attained through π–π stacking interactions between adjacent coordinated bipyridyl groups.     

Magnetic and dielectric response of M-type barium hexaferrite

Barium hexaferrite (BaFe₁₂O₁₉) is a promising candidate for ceramics, microwave devices and numerous applications. Barium hexaferrite was synthesised via the sol-gel auto-combustion technique using glycine fuel. The X-ray diffraction technique confirmed the hexagonal structure of the particles with space group P63/mmc. The morphological analysis was performed using the field-emission scanning microscope, and the images displayed the plate-like particle formation. Transmission electron microscopy was employed to determine the average particle size of the sample, which was estimated to be 155.93 nm. The magnetic studies were taken through the vibrating sample magnetometer (VSM) at 300 K, with which the saturation magnetization (M_s), coercivity (H_c), squareness ratio (M_r/M_s), and energy product (BH_max) was calculated, and the particles were validated to be in single domain arrangement. The dielectric properties were investigated through the LCR meter. Koop and Maxwell-Wagner's model was used to interpret charge conduction and the occurrence of relaxations in the system.     

Analysis of the Magnetic Coupling in a Mn(II)-U(V)-Mn(II) Single Molecule Magnet

[{Mn(TPA)I}{UO2(Mesaldien)}{Mn(TPA)I}]I formula (here TPA=tris(2-pyridylmethyl)amine and Mesaldien=N,N’-(2-aminomethyl)diethylenebis(salicylidene imine)) reported by Mazzanti and coworkers (Chatelain et al. Angew. Chem. Int. Ed. 2014 , 53, 13434) is so far the best Single Molecule Magnet (SMM) in the {3d–5f} class of molecules exhibiting barrier height of magnetization reversal as high as 81.0 K. In this work, we have employed a combination of ab initio CAS and DFT methods to fully characterize this compound and to extract the relevant spin Hamiltonian parameters. We show that the signs of the magnetic coupling and of the g-factors of the monomers are interconnected. The central magnetic unit [U^VO₂]⁺ is described by a Kramers Doublet (KD) with negative g-factors, due to a large orbital contribution. The magnetic coupling for the {Mn(II)-U(V)} pair is modeled by an anisotropic exchange Hamiltonian: all components are ferromagnetic in terms of spin moments, the parallel component J_Z twice larger as the perpendicular one J_⊥. The spin density distribution suggests that spin polarization on the U(V) center favors the ferromagnetic coupling. Further, the J_Z/J_⊥ ratio, which is related to the barrier height, was found to correlate to the corresponding spin contribution of the g-factors of the U(V) center. This correlation established for the first time offers a direct way to estimate this important ratio from the corresponding g^S-values, which can be obtained using traditional ab initio packages and hence has a wider application to other {3d–5f} magnets. It is finally shown that the magnetization barrier height is tuned by the splitting of the [U^VO₂]⁺ 5 f orbitals.     

Undulator with permanent ferrimagnetic magnets

Undulator for terahertz FEL is created of ferrite materials. The length of the undulator period is 9 cm and the number of periods is 27. By means of selection and redistribution of magnetic elements it was succeeded to reduce the spread in amplitudes of the magnetic field down to 7%. Additional windings in magnetic elements were used to compensate for the residual spread. The needed focusing gradient of the magnetic field is obtained by means of relative displacement, along the x-axis, of alternating poles with opposite signs of the magnetic field. The undulator parameters, including the properties of focusing in the horizontal plane, are studied.