Search

A comparative analysis of the magnetic properties of iron oxide nanoparticles grown in the cavity of the DNA‐binding protein from starved cells of the bacterium Listeria innocua, LiDps, and of its triple‐mutant lacking the catalytic ferroxidase centre, LiDps‐tm, is presented. TEM images and static and dynamic magnetic and electron magnetic resonance (EMR) measurements reveal that, under the applied preparation conditions, namely alkaline pH, high temperature (65 °C), exclusion of oxygen, and the presence of hydrogen peroxide, maghemite and/or magnetite nanoparticles with an average diameter of about 3 nm are mineralised inside the cavities of both LiDpsand LiDps‐tm. The magnetic nanoparticles (MNPs) thus formed show similar magnetic properties, with superparamagnetic behaviour above 4.5 K and a large magnetic anisotropy. Interestingly, in the EMR spectra an absorption at half‐field is observed, which can be considered as a manifestation of the quantum behaviour of the MNPs. These results indicate that Dpsproteins can be advantageously used for the production of nanomagnets at the interface between molecular clusters and traditional MNPs and that the presence of the ferroxidase centre, though increasing the efficiency of nanoparticle formation, does not affect the nature and fine structure of the MNPs. Importantly, the self‐organisation of MNP‐containing Dpson HRTEM grids suggests that Dps‐enclosed MNPs can be deposited on surfaces in an ordered fashion.

[{Dy(hfac)3}2{Fe(bpca)2}]⋅CHCl3([Dy2Fe]) and [{Dy(hfac)3}2{Ni(bpca)2}]⋅CHCl3([Dy2Ni]) (in which hfac−=1,1,1,5,5,5‐hexafluoroacetylacetonate and bpca−=bis(2‐pyridylcarbonyl)amine anion) were synthesized and characterized. Single‐crystal X‐ray diffraction shows that [Dy2Fe] and [Dy2Ni] are linear trinuclear complexes. Static magnetic susceptibility measurements reveal a weak ferromagnetic exchange interaction between NiIIand DyIIIions in [Dy2Ni], whereas the use of the diamagnetic FeIIion leads to the absence of magnetic exchange interaction in [Dy2Fe]. Dynamic susceptibility measurements show a thermally activated behavior with the energy barrier of 9.7 and 4.9 K for the [Dy2Fe] and [Dy2Ni] complexes, respectively. A surprising negative effect of the ferromagnetic exchange interaction has been found and has been attributed to the structural conformation of these trinuclear complexes.

A new tetragonal (P4212) crystalline form of [Cr8F8Piv16] (HPiv=pivalic acid, trimethyl acetic acid) is reported. The ring-shaped molecules, which are aligned in a parallel fashion in the unit cell, form almost perfectly planar, regular octagons. The interaction between the Cr^III ions is antiferromagnetic (J=12 cm⁻¹) which results in a S=0 spin ground state. The low-lying spin excited states were investigated by cantilever torque magnetometry (CTM) and high-frequency EPR (HFEPR). The compound shows hard-axis anisotropy. The axial zero-field splitting (ZFS) parameters of the first two spin excited states (S=1 and S=2, respectively) are D1=1.59(3) cm⁻¹ or 1.63 cm⁻¹ (from CTM and HFEPR, respectively) and D2=0.37 cm⁻¹ (from HFEPR). The dipolar contributions to the ZFS of the S=1 and S=2 spin states were calculated with the point dipolar approximation. These contributions proved to be less than the combined single-ion contributions. Angular overlap model calculations that used parameters obtained from the electronic absorption spectrum, showed that the unique axis of the single-ion ZFS is at an angle of 19.3(1)° with respect to the ring axis. The excellent agreement between the experimental and the theoretical results show the validity of the used methods for the analysis of the magnetic anisotropy in antiferromagnetic Cr^III rings.

A multitechnique approach has allowed the first experimental determination of single‐ion anisotropies in a large iron(III)‐oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6]ClO4(1) in which Hpmdbm=1,3‐bis(4‐methoxyphenyl)‐1,3‐propanedione. High‐frequency EPR (HF‐EPR), bulk susceptibility measurements, and high‐field cantilever torque magnetometry (HF‐CTM) have been applied to iron‐doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12(pmdbm)6]ClO4, whose synthesis and X‐ray structure are also presented. HF‐EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard‐axis type anisotropy with DFe=0.43(1) cm−1and EFe=0.066(3) cm−1in the zero‐field splitting (ZFS) Hamiltonian H=DFe[Sz2−S(S+ 1)/3] + EFe[Sx2−Sy2]. HF‐CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z.The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Zare negative, DFe(ZZ)=−0.164(4) cm−1. Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard‐axis molecular anisotropy in 1, as experimentally observed. By adding point‐dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1=4.70(9) cm−1, is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1=4.57(2) cm−1. Iron‐doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm=dibenzoylmethane), have also been investigated by HF‐EPR at 525 GHz. The single‐ion anisotropy is of the hard‐axis type as well, but the DFeparameter is significantly larger [DFe=0.770(3) cm−1, EFe=0.090(3) cm−1]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single‐ion terms can contribute significantly to the magnetic anisotropy of iron(III)‐oxo clusters, which are currently investigated as single‐molecule magnets.