SMM Behaviour of the Butterfly {CrIII 2 DyIII 2 } Pivalate Complex and Magneto-structurally Correlated Relaxation Thermal Barrier.

We are reporting the synthesis, single-crystal X-ray structure characterization, and magnetic property investigations of the pivalate butterfly {CrIII 2 LnIII 2 } complexes with Ln= Gd and Dy and the isostructural Y(III) sample. We found an anti-ferromagnetic Cr(III)-Ln(III) exchange interaction, which, as previously observed in related Cr(III)/Ln(III) systems, plays a key role in suppressing quantum tunnelling of magnetization and enhances the SMM performance in the Dy(III) complex. In fact, a pure Orbach relaxation mechanism, with absence of QT regime, is observed with a thermal barrier of 50 cm-1 , leading to magnetization hysteresis opening, measured with a commercial magnetometer, up to 3.6 K with a coercive field of 2.9 T. Analysis of SMM behaviour in literature-known butterfly {CrIII 2 DyIII 2 } complexes, reveals the existence of a magneto-structural correlation between Ueff , the thermal barrier size, and the mean Cr-Dy bond distances. Moreover, a clear correlation is found for the thermal barrier magnitude and the maximum temperature hysteresis opening and coercive field.

SMM features of a large lanthanide family of butterfly CrIII2LnIII2 pivalate complexes (Ln = Gd, Tb, Dy, Ho, Er, Tm and Yb).

In this work we report the synthesis, structural characterization and magnetic properties of a family of butterfly complexes {Cr2Ln2} with Ln = Tb (4), Ho (5), Er (6), Tm (7) and Yb (8), extending the family of previously reported isostructural compounds with Gd (1), Dy (2) and Y (3). As in 1 and 2, an anti-ferromagnetic Cr(III)-Ln(III) exchange interaction is found. For oblate ions 4 and 5, SMM behavior with a purely Orbach relaxation mechanism is observed with thermal barriers of 38 cm-1 (4) and 32 cm-1 (5). Complex 4 displays hysteresis opening up to 2.4 K with loss of magnetization at zero field. The prolate complex ions 6 and 7 are field-induced SMM's with dominant Orbach, direct and QTM relaxation mechanisms. Compound 8 did not show SMM properties.

A dinuclear Co(III)/Co(II) complex based on the H2pmide ligand showing field-induced SMM behaviour.

We report the synthesis, structural characterization and SMM behaviour of a new mixed valence Co(II)/Co(III) dinuclear complex bearing the H2pmide ligand. Well defined molecule pairs are observed in the crystal structure, bound through H-bond interactions directed by aqua ligands. From DC magnetometry data, a spin-only Hamiltonian approach including an axial zero-field splitting term seems to be enough for reasonable modelling, with a sizeable D parameter close to 40 cm-1. The first order orbital contribution is extensively quenched due to strong distortion from octahedral symmetry of the Co(II) site. Quantum computation at the CASSCF level supports this interpretation. To model low temperature magnetization data, the H-bond intermolecular exchange interaction is required, with a magnitude close to -1 cm-1, well supported by broken-symmetry DFT computation. This exchange is highly anisotropic due to the existence of a well isolated Kramers doublet at the Co(II) site. AC magnetic susceptibility shows field-induced SMM behaviour with competing Orbach and Raman relaxation pathways as well as a quantum tunnelling process at the lowest probed temperatures. The Orbach thermal barrier agrees with the expected one from combined experimental and quantum computed DC magnetometry analysis.

Synthesis, structural characterization, and magnetic property study of {Cr3Ln3}, Ln = Gd and Dy complexes.

We have successfully prepared and structurally characterized triangle-in-triangle {Cr3Ln3} complexes with Ln = Gd and Dy employing alcohol-amine, N-methyldiethanolamine (H2mdea) and pivalate ligands. These complexes and the Yttrium analogue proved to be isostructural and crystallized in a P1 triclinic cell. DC and AC magnetic measurements were carried out and supported by quantum computations at DFT and CASSCF levels. DC magnetic data are dominated by the Cr(III)-Ln(III) antiferromagnetic interaction and by single-ion anisotropy in the case of the Dy(III) complex. Ln(III)-Ln(III) magnetic interactions are negligible, as well as Cr(III)-Cr(III) ones. From AC data, slow relaxation of the magnetization is observed at 0 DC applied magnetic field in the case of the Dy(III) complex below 4 K. From temperature and field dependence data, possible Raman and Orbach relaxation mechanisms are established in the absence of quantum tunnelling pathways, suggesting a successful suppression of the latter due to the Cr(III)-Dy(III) exchange interaction.

Anisotropic exchange interaction and field-induced SMM behaviour in a mixed valence {CoCo} complex.

We are reporting the synthesis and structural characterization of a new hexanuclear Co(ii)/Co(iii) complex starting from a versatile pivalate cobalt precursor and the racemic mixture of a chelating Schiff base type ligand. The main [CoII4CoIII2(µ3-OH)2(µ-OR)2(µ-OR')2(µ-OR'')2]6+ core is unprecedented and exhibits an inversion center that affords only two unique Co(ii) sites. We performed DC and AC magnetic measurements and analysed them in terms of the anisotropic exchange of ground Kramers doublets at each Co(ii) site due to their unquenched angular orbital contribution to the magnetic moment. Quantum computations support the experimental data treatment. The interplay of dominant antiferromagnetic exchange, inversion symmetry and a non-collinear main quantization axis affords an exchange energy spectrum with mostly non-magnetic states. Nevertheless, field induced SMM behaviour is observed at 1500 Oe and below 3 K which might be explained by the relaxation of the first excited magnetic state (which is populated enough) through the next closest excited state. The Orbach and/or Raman mechanism could be operative from the experimental and quantum computed results.