Maria G. F. Vaz

A Cobalt Pyrenylnitronylnitroxide Single‐Chain Magnet with High Coercivity and Record Blocking Temperature

Coordination of a [Co(hfac)2] moiety (hfac = hexafluoroacetylacetonate) with a nitronylnitroxide radical linked to bulky, rigid pyrene (PyrNN) gives a helical 1:1 chain complex, in which both oxygen atoms of the radical NO(·) groups are bonded to Co(II) ions with strong antiferromagnetic exchange. The complex shows single-chain magnet (SCM) behavior with frequency-dependent magnetic susceptibility, field-cooled and zero-field-cooled susceptibility divergence with a high blocking temperature of around 14 K (a record among SCMs), and hysteresis with a very large coercivity of 32 kOe at 8 K. The magnetic behavior is partly related to good chain isolation induced by the large pyrene units. Two magnetic relaxation processes have been observed, a slower one attributable to longer, and a faster one attributable to short chains. No evidence of magnetic ordering has been found.

New Families of Hetero-tri-spin 2p−3d−4f Complexes: Synthesis, Crystal Structures, and Magnetic Properties

In this work we report the synthesis, crystal structures, and magnetic behavior of 2p-3d-4f heterospin systems containing the nitroxide radical 4-azido-2,2,6,6-tetramethylpiperidine-1-oxyl radical (N3tempo). These compounds were synthesized through a one-pot reaction by using [Cu(hfac)2], [Ln(hfac)3] (hfac = hexafluoroacetylacetonate, Ln = Dy(III), Tb(III) or Gd(III)), and the N3tempo radical. Depending on the stoichiometric ratio used, the synthesis leads to penta- or trimetallic compounds, with molecular formulas [Cu3Ln2(hfac)8(OH)4(N3tempo)] (Ln = Gd, Tb, Dy) and [CuLn2(hfac)8(N3tempo)2(H2O)2] (Ln = Gd, Dy). The magnetic properties of all compounds were investigated by direct current (dc) and alternating current (ac) measurements. The ac magnetic susceptibility measurements of Tb(III)- and Dy(III)-containing compounds of both families revealed slow relaxation of the magnetization, with magnetic quantum tunneling in zero field.

Structure and morphology of spinel MFe2O4 (M=Fe, Co, Ni) nanoparticles chemically synthesized from heterometallic complexes.

We synthesized magnetic spinel ferrites from trimetallic single-source precursors. Fe(II), Co(II), and Ni(II) ferrite nanoparticles in the range of 9-25 nm were synthesized by solvothermal decomposition of trimetallic acetate complex precursors in benzyl ether in the presence of oleic acid and oleylamine, using 1,2-dodecanediol as the reducing agent. For comparison, spinel ferrite nanoparticles were synthesized by stoichiometric mixtures of metal acetate or acetylacetonate salts. The nanoparticles (NP) were characterized by TEM, DLS, powder XRD, and Raman spectroscopy; and their magnetic properties were characterized by ZFC-FC and M(H) measurements. The ferrite-NP were more homogeneous and had a narrower size distribution when trimetallic complexes were used as precursors. As a consequence, the magnetic properties of these ferrite-NP are closer to the aimed room temperature superparamagnetic behavior, than are those of other ferrites obtained by a mixture of salts.

A single-chain magnet with a very high blocking temperature and a strong coercive field.

Two isostructural 1D complexes, [M(hfac)2NaphNN]n [M = Mn(II) (1) or Co(II) (2); NaphNN = 1-naphthyl nitronylnitroxide], were synthesized and exhibit very strong antiferromagnetic metal-radical exchange coupling. Compound 2 shows slow magnetic relaxation behavior with a high blocking temperature (TB ≈ 13.2 K) and a very high coercive field of 49 kOe at 4.0 K.

Crystal Structure and Magnetic Properties of a New Chiral Manganese(II) Three-Dimensional Framework: Na3[Mn3(HCOO)9]

A structural and magnetic characterization of a trinuclear chiral Mn(II) formate three-dimensional framework exhibiting a triangular arrangement is presented. Compound Na(3)[Mn(3)(HCOO)(9)] was obtained by solvothermal synthesis and crystallizes in the chiral cubic space group P2(1)3 and is well described by a Delta conformation. The structure displays triangular Mn(3) building blocks, in which the metal centers are bonded by formate ligands in a syn-anti mode (Mn-Mn 5.697(1) A). The coordination sphere of manganese(II) is completed by six oxygen atoms from six formate ligands, resulting in an octahedral geometry. Magnetic susceptibility measurements showed antiferromagnetic interactions at high temperature and a strongly field dependent magnetic behavior below 40 K. At fields higher than 1.0 kOe only the antiferromagnetic interactions can be observed. At applied fields lower than 1.0 kOe magnetic susceptibility becomes irreversible with maxima observed at 22 and 34 K. These maxima suggest a weak ferromagnetic behavior because of spin canting, allowed by the presence of the noncentrosymmetric syn-anti HCOO bridges linking the Mn sites. This non-collinear antiferromagnetism and irreversible behavior can be due to the existence of a high degree of frustration in this unique lattice composed of linked triangular arrangements of interacting magnetic centers.

Magnetic properties of a novel molecule-based ferrimagnet exhibiting multiple magnetic pole reversal

The magnetic properties of a new molecule-based ferrimagnet containing three different spin sources have been studied. Below the magnetic ordering temperature, the magnetic poles reverse once in a small external field as the temperature varies. At a higher field, they reverse twice at temperatures that depend on the measurement process. A double thermal hysteresis is observed in a constant field. These experimental results can be rationalized by a combination of the molecular-field theory (MFT) and magnetic anisotropy.

New Synthetic Route toward Heterometallic 3d–3d′ and 3d–4f Single-Molecule Magnets. The First CoII–MnIII Heterometallic Complex

Four tetranuclear heterometallic complexes, [Co(II)2Mn2(III)(dpm)4(MeO)6] (1) and [Ln(III)2Mn(III)2(dpm)6(MeO)6(MeOH)n], where Ln = Gd (2, n = 2), Tb (3, n = 2), and Dy (4, n = 0), have been obtained following the same general synthetic route, namely, the one-pot reaction between 2,2,6,6-tetrametil-3,5-heptanodione (Hdpm), MnCl2 and CoCl2 or Ln(NO3)3 in the presence of sodium methoxide. Within the four compounds, the metal ions bridged by methoxide ligands display a defect-diheterocubane core. Compounds 1, 3, and 4 show slow relaxation of the magnetization below 4 K.

New copper(II)-radical one dimensional chain: Synthesis, crystal structure, EPR, magnetic properties and DFT calculations

The novel chain compound [Cu(Phtfac)(2)(NITpPy)](n) (where NITpPy = 4-pyridyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and Phtfac = 4,4,4-trifluoro-1-phenylbutane-1,3-dione) was synthesized and characterized structurally, magnetically and by EPR. The compound contains two non equivalent Cu(II) ions, Cu1 and Cu2, located at inversion centers and bridged by a NITpPy ligand coordinating Cu1 through the pyridine donor atom, and Cu2 through a N-O group, resulting in a head-to-head chain structure. The chain exhibits an unusual spin topology with two alternating pairs of magnetic coupling constants. The magnetic behavior was modeled considering a 16-membered ring with alternating exchange couplings. The best fit parameters indicate a ferromagnetic (J(1) = 29.4 cm(-1)), and antiferromagnetic (J(2) = -4.6 cm(-1)) couplings and an average g = 2.05, corresponding to a ground state with three parallel and one anti-parallel spin for each Cu(2)NITpPy(2) unit. DFT calculations allowed assigning the ferromagnetic coupling to Cu-O-NITpPy and the antiferromagnetic coupling to Cu-N(Py)-NITpPy. Single crystal EPR spectra display only one resonance for most field orientations, as a consequence of the collapse of the signals of the different spins produced by the exchange interactions. The observed g-tensor of this resonance is related to those expected for the Cu(II) and radical ions. Comparison of this compound with other Cu-NIT radicals chains bearing different substituents in the organic radicals, highlights that the beta-diketonate ligand plays an important role in determining the final architecture. Moreover, we show how a knowledge of the spin density distribution in the initial building blocks is essential to rationalize the magnetic behavior of the resulting product.

New properties of the [3-MeRad]2[Mn2{Cu(opba)}3] system [3-MeRad+=radical cation, opba=ortho-phenylenebis(oxamato)]: a molecule-based magnet with two-dimensional behavior

Abstract Three compounds have been synthesized, the formulae of which are [3-MeRad] 2 [Cu(opba)]·4H 2 O ( 1 ), [3-MeRad] 2 [Mn 2 {Cu(opba)} 3 ]·6DMSO·8H 2 O ( 2 ), and [3-MeRad] 2 [Mn 2 {Cu(opba)} 3 ]·3DMSO·2.5H 2 O ( 3 ), where opba is ortho -phenylenebis(oxamato) and 3-MeRad + is 2-(3- N -methyl-pyridinium)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. Their magnetic properties have been investigated in the 1.8–300 K temperature range. By reaction of 1 and MnCl 2 ·3H 2 O in DMSO, small crystals of 2 were obtained. Single crystal X-ray analysis was employed to determine the lattice parameters for 2 : a =15.9(2); b =17.8(2); c =18.0(3) A; α = γ =90(1); β =114(1)°. The temperature dependence of the magnetic susceptibility in the form of the χ M T versus T plot reveals a typical ferrimagnetic behavior for 2 , although the low field FC and ZFC curves show an antiferromagnetic-like interaction below 13 K. Above 200 Oe this interaction disappears as in weak metamagnets. With a thermal-vacuum treatment applied to 2 , solvent molecules are removed affording 3 which presents long-range order around 15 K. The results indicate that the bimetallic compounds present a structure consisting of two-dimensional honeycomb-like networks without interlocking of the M 6 Cu 6 hexagons.

HF-EPR to monitor electron transfer in mixed valence dioxolene metal complexes

Abstract The reaction of Mn(II) salts with N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine yields a dinuclear Mn2L3 complex, which on the basis of its structural and magnetic characterisation can be formulated as a Mn(IV) derivative of both dinegative bis-iminosemiquinonato and mixed valence trinegative iminosemiquinonato–iminocatecholato forms of the reacting ligand. HF-EPR spectra at various frequencies suggest that the electron transfer process in this mixed valence species is frozen on the time scale of EPR at high frequency, whereas is still dynamic at lower frequencies. This spectral behaviour therefore indicates a thermally activated character of the electron transfer process, as expected in a class II mixed valence system.