3 research outputs found
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 [MÂ(L<sup>•–</sup>)<sub>3</sub>]
(M = Gd (<b>1</b>), Dy (<b>2</b>), Er (<b>3</b>),
Y (<b>4</b>)) using the redox-active iminopyridyl (L<sup>•–</sup>) 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><sub>rad‑rad</sub> = −111.9
cm<sup>–1</sup> for <b>4</b> and <i>g</i> =
1.99, <i>J</i><sub>rad‑rad</sub> = −111.9
cm<sup>–1</sup>, <i>J</i><sub>Gd‑rad</sub> = −1.85 cm<sup>–1</sup> 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<sub>M</sub>′′ 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
DyÂ(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 [CeÂ(HL)<sub>3</sub>(NO<sub>3</sub>)<sub>3</sub>] (<b>1</b>) and [LnÂ(HL)<sub>2</sub>(NO<sub>3</sub>)<sub>3</sub>], where Ln<sup>III</sup> = 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<sup>III</sup> 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<sub>M</sub><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 [Ln<sub>2</sub>(L<sub>1</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>(ox)]<sub><i>n</i></sub>.4<i>n</i>H<sub>2</sub>O (where L<sub>1</sub> = 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
[Ln<sub>2</sub>(L<sub>1</sub>)<sub>2</sub>(ox)]<sub><i>n</i></sub> 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<sup>–1</sup> for <b>1</b> and 45.1 and 6.4 cm<sup>–1</sup> 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><sub><i>m</i></sub> values of 52.48 and 41.62
J kg<sup>1–</sup> K<sup>–1</sup> 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><sub>m</sub> 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