Luca Pardi

Large clusters of metal ions: The transition from molecular to bulk magnets

Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles.

Spontaneous Symmetry Breaking in the Formation of a Dinuclear Gadolinium Semiquinonato Complex: Synthesis, High-Field EPR Studies, and Magnetic Properties

The synthesis and characterisation of an asymmetric dinuclear gadolinium(III) semiquinonato complex, namely [Gd2(HBPz3)2(dtbsq)4] CHCl3 (1; HBPz3 = hydrotris(pyrazolyl)borate, dtbsq = 3,5-di-tert-butyl-O-semiquinone), is reported. The crystal structure of 1 was determined at room temperature. It crystallises in the triclinic system P1, with a = 16.735(5) A, b = 17.705(5) A, c = 19.553(5) A, alpha = 99.680(5) degrees, beta = 109.960(5), gamma = 107.350(5) degrees, Z = 2 and R = 9.96. The structure of 1 consists of a dinuclear asymmetric unit in which the two gadolinium(III) ions have coordination numbers of eight and nine. Three of the dioxolene molecules act as asymmetric bridging ligands, while the fourth molecule behaves as a bidentate ligand towards a single metal ion. The magnetic properties of 1 were investigated by means of susceptibility measurements and high-field electron paramagnetic resonance (HF-EPR) spectroscopy. They revealed an S = 0 ground spin state with excited states of higher spin very close in energy and a small negative zero-field splitting with a transverse anisotropy term for a S = 7 state.

Single‐Ion versus Dipolar Origin of the Magnetic Anisotropy in Iron(III)‐Oxo Clusters: A Case Study

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)6ClO4 (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(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. 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 Z are 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 DFe parameter 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.

The redox behaviour of ferrocene derivatives: VI. Benzylferrocenes. The crystal structure of decabenzylferrocenium tetrafluoroborate☆

The electrochemical behaviour of the benzyl-substituted ferrocenes [(η5-C5Bz5)2Fe] and [(η5-C5Bz5)Fe(η5- C5H5)] in non-aqueous solutions has been examined. As expected, they undergo reversible one-electron removal more easily than ferrocene itself, but with significantly more difficulty than [(η5-C5R5)2Fe] (R = Me or Et). In dichloromethane solution, the formal electrode potentials (vs. SCE) are as follows: E°′([(C5Bz5)2Fe]+/0) = +0.38 V, E°′([(C5Me5)2Fe] +/0) = −0.10 V, E°′([(C5Et5)2Fe]+/0) = −0.06 V, E°′([(C5H5)2Fe]+/0) = +0.45 V. The electrochemical properties suggest that the [(C5Bz5)2Fe]+/0 redox change should be accompanied by minor, but detectable geometrical strains. In order to define more quantitatively the geometrical reorganization accompanying such electron removal processes, the X-ray structure of [(C5Bz5)2Fe][BF4] has been determined. Comparison with the previously determined structure of neutral [(C5Bz5)2Fe] shows an elongation of about 0.05 A of the FeC(cyclopentadienyl) distances, which is a common feature of all ferrocene/ferrocenium couples, but this is also accompanied by reorganization of the peripheral benzyl substituents. In the neutral precursor, the methylene fragments are tilted away from the plane of the cyclopentadienyl rings towards the iron atom, whereas in the monocation they become coplanar.

Redox potentials and charge transfer spectra of catecholate and semiquinone adducts of a cobalt-tetraazamacrocyclic complex

Abstract Catecholate and semiquinone complexes of cobalt(II) and cobalt(III) with the macrocyclic ligand DL -5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane have been synthesized and characterized. The dioxolene ligands are 3,5-di-t-butylcatechol and tetrachlorocatechol. Evidence is presented that oxidation of cobalt(II) to cobalt(III) occurs before oxidation of catecholate to semiquinone and quinone. The energies of the charge transfer bands either LMCT and MLCT are discussed in terms of electrochemical potentials.

High-field/ high-frequency EPR study on stable free radicals formed in sucrose by gamma-irradiation

The EPR spectrum of sucrose irradiated by high-energy radiation is complex due to the presence of more than one radical species. In order to decompose the spectrum and elucidate the radical magnetic parameters a high-field (HF(-)EPR) study on stable free radicals in γ-irradiated polycrystalline sucrose (table sugar) was performed at three different high frequencies—94, 190 and 285 GHz as well as at the conventional X-band. We suggest a presence of three stable radicals R1, R2 and R3 as the main radical species. Due to the increase of g-factor resolution at high fields the g-tensors of these radicals could be extracted by accurate simulations. The moderate g-anisotropy suggests that all three radicals are carbon-centred. Results from an earlier ENDOR study on X-irradiated sucrose single crystals (Vanhaelewyn et al., Appl Radiat Isot, 52, 1221 (2000)) were used for analyzing of the spectra in more details. It was confirmed that the strongest hyperfine interaction has a relatively small anisotropy, which indicates either the absence of α-protons or a strongly distorted geometry of the radicals.

Evaluating the magnetic anisotropy in molecular rare earth compounds. Gadolinium derivatives with semiquinone radical and diamagnetic analogues

The magnetic anisotropy of two Gd(III) derivatives has been investigated by multifrequency HF-EPR spectroscopy. The reliability of the method of diamagnetic substitution for the estimation of crystal field effects in rare earth compounds is discussed on the basis of the experimental results and of the estimation of the dipolar contribution to the anisotropy.

Millimeter band EPR spectra reveal large zero-field splittings in copper(II)—semiquinonato complexes

Abstract The magnetic interaction between copper(II) and ortho-semiquinones has been investigated by the EPR technique using a spectrometer operating in the millimeter-wavelength range. The polycrystalline-powder EPR spectrum of [Cu(Me 3 [12]N 3 )-(DTBSQ)]ClO 4 (Me 3 [12]N 3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene; DTBSQ = 3,5-di- t -butyl- o -semiquinonato) recorded at 4.2 with an exciting frequency of 8.17 cm −1 , is typical of a spin triplet with a large zero-field splitting (ZFS). The ZFS parameters have been obtained directly from the spectrum using the Wasserman equations. Both dipolar and exchange contributions to the ZFS are discussed within a molecular-orbital framework.

How and why the characterization of magnetic materials can give directions in the methodological development in high field-high frequency EPR

The experimental problems linked to the use of the ultra-wide band high field-high frequency Electron Paramagnetic Resonance (HF2-EPR) spectrometers are illustrated with some key examples. The single pass technique is critically analyzed and compared with other techniques. Propagation and magnetic field dependent phenomena are illustrated with the aid of three key examples and discussed. The possible actions to be taken in order to minimize and possibly eliminate these extrinsic effects affecting the spectra obtained with the single pass technique are proposed. Future instrumental developments are discussed in the light of the experience acquired in this study.

Magnetic interactions involving rare earth ions

Abstract The bulk properties of magnetic materials containing rare earths are essentially determined by the anisotropy of the ground state of 4f ions and by the nature of lanthanide-lanthanide and/or lanthanide-transition metal interactions. In an attempt to clarify the role of these mechanisms we tried to synthesize simple molecular compounds where rare earth ions were present together with other paramagnetic centers. The analysis of the magnetic behavior of some of these systems lead us to propose a simple model based on a spin polarization mechanism to justify the observed exchange interactions. Low dimensional systems containing 4f ions have been synthesized: the unexpected magnetic properties of these systems have been analyzed assuming the presence of next-nearest-neighbors competing interactions.