Influence of Radicals on Magnetization Relaxation Dynamics of Pseudo-Octahedral Lanthanide Iminopyridyl Complexes

Controlling quantum tunneling of magnetization (QTM) is a persistent challenge in lanthanide-based single-molecule magnets. As the exchange interaction is one of the key factors in controlling the QTM, we targeted lanthanide complexes with an increased number of radicals around the lanthanide ion. On the basis of our targeted approach, a family of pseudo-octahedral lanthanide/transition-metal complexes were isolated with the general molecular formula of [M­(L^•–)₃] (M = Gd (<b>1</b>), Dy (<b>2</b>), Er (<b>3</b>), Y (<b>4</b>)) using the redox-active iminopyridyl (L^•–) ligand exclusively, which possess the highest ratio of radicals to lanthanide reported for discrete metal complexes. Direct current magnetic susceptibility studies suggest that dominant antiferromagnetic interactions exist between the radical and lanthanide ions in all of the complexes, which is strongly corroborated by magnetic data fitting using a Heisenberg–Dirac–Van Vleck (HDVV) Hamiltonian (−2<i>J</i> Hamiltonian). A good agreement between the fit and the experimental magnetic data obtained using <i>g</i> = 2, <i>J</i>_rad‑rad = −111.9 cm^–1 for <b>4</b> and <i>g</i> = 1.99, <i>J</i>_rad‑rad = −111.9 cm^–1, <i>J</i>_Gd‑rad = −1.85 cm^–1 for <b>1</b>. Complex <b>2</b> shows frequency-dependent slow magnetization relaxation dynamics in the absence of an external magnetic field, while <b>3</b> shows field-induced frequency-dependent χ_M′′ signals. An ideal octahedral geometry around the lanthanide ion is predicted to be unsuitable for the design of a single-molecule magnet (SMM); nevertheless, complex <b>2</b> exhibits slow relaxation of magnetization with a record high anisotropy barrier for a six-coordinate Dy­(III) complex. A rationale for this unusual behavior is detailed and reveals the strength of the synthetic methodology developed

Influence of the Ligand Field on the Slow Relaxation of Magnetization of Unsymmetrical Monomeric Lanthanide Complexes: Synthesis and Theoretical Studies

A series of monomeric lanthanide Schiff base complexes with the molecular formulas [Ce­(HL)₃(NO₃)₃] (<b>1</b>) and [Ln­(HL)₂(NO₃)₃], where Ln^III = Tb (<b>2</b>), Ho (<b>3</b>), Er (<b>4</b>), and Lu (<b>5</b>), were isolated and characterized by single-crystal X-ray diffraction (XRD). Single-crystal XRD reveals that, except for <b>1</b>, all complexes possess two crystallographically distinct molecules within the unit cell. Both of these crystallographically distinct molecules possess the same molecular formula, but the orientation of the coordinating ligand distinctly differs from those in complexes <b>2</b>–<b>5</b>. Alternating-current magnetic susceptibility measurement reveals that complexes <b>1</b>–<b>3</b> exhibit slow relaxation of magnetization in the presence of an optimum external magnetic field. In contrast to <b>1</b>–<b>3</b>, complex <b>4</b> shows a blockade of magnetization in the absence of an external magnetic field, a signature characteristic of a single-ion magnet (SIM). The distinct magnetic behavior observed in <b>4</b> compared to other complexes is correlated to the suitable ligand field around a prolate Er^III ion. Although the ligand field stabilizes an easy axis of anisotropy, quantum tunnelling of magnetization (QTM) is still predominant in <b>4</b> because of the low symmetry of the complex. The combination of low symmetry and an unsuitable ligand-field environment in complexes <b>1</b>–<b>3</b> triggers faster magnetization relaxation; hence, these complexes exhibit field-induced SIM behavior. In order to understand the electronic structures of complexes <b>1</b>–<b>4</b> and the distinct magnetic behavior observed, ab initio calculations were performed. Using the crystal structure of the complexes, magnetic susceptibility data were computed for all of the complexes. The computed susceptibility and magnetization are in good agreement with the experimental magnetic data [χ_M<i>T</i>(<i>T</i>) and <i>M</i>(<i>H</i>)] and this offers confidence on the reliability of the extracted parameters. A tentative mechanism of magnetization relaxation observed in these complexes is also discussed in detail

Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect

Two lanthanide-containing structurally analogous porous coordination polymers (PCPs) have been isolated with the general molecular formula [Ln₂(L₁)₂(H₂O)₄(ox)]_<i>n</i>.4<i>n</i>H₂O (where L₁ = fumarate, ox = oxalate; Ln = Dy (<b>1</b>), Gd (<b>2</b>)). Thermogravimetric analysis (TGA) and TG-MS measurements performed on <b>1</b> and <b>2</b> suggest that not only the solvated water molecules in the crystal lattice but also the four coordinated water molecules on the respective lanthanides in <b>1</b> and <b>2</b> are removed upon activation. Due to the removal of the waters, <b>1</b> and <b>2</b> lost their crystallinity and became amorphous, as confirmed by powder X-ray diffraction (PXRD). We propose the molecular formula [Ln₂(L₁)₂(ox)]_<i>n</i> for the amorphous phase of <b>1</b> and <b>2</b> (where Ln = Dy (<b>1′</b>), Gd (<b>2′</b>)) on the basis of XANES, EXAFS, and other experimental investigations. Magnetization relaxation dynamics probed on <b>1</b> and <b>1′</b> reveal two different relaxation processes with effective energy barriers of 53.5 and 7.0 cm^–1 for <b>1</b> and 45.1 and 6.4 cm^–1 for <b>1′</b>, which have been rationalized by detailed ab initio calculations. For the isotropic lanthanide complexes <b>2</b> and <b>2′</b>, magnetocaloric effect (MCE) efficiency was estimated through detailed magnetization measurements. We have estimated −Δ<i>S</i>_<i>m</i> values of 52.48 and 41.62 J kg^1– K^–1 for <b>2′</b> and <b>2</b>, respectively, which are one of the largest values reported for an extended structure. In addition, a 26% increase in −Δ<i>S</i>_m value in <b>2′</b> in comparison to <b>2</b> is achieved by simply removing the passively contributing (for MCE) solvated water molecule in the lattice and coordinated water molecules