Magnetic Anisotropy, Magneto-Structural Correlations and Mechanism of Magnetic Relaxation in {DyIII N8 } Complexes: A Theoretical Perspective

Halogen Bond Mediated Self-Assembly of Mononuclear Lanthanide Complexes: Perception of Supramolecular Interactions, Slow Magnetic Relaxation, and Photoluminescence Properties

Five new mononuclear lanthanide complexes, [LnL₂][Et₃NH]·THF/H₂O (Ln = Nd, Tb, Dy) (H₂L^Cl = 2-bis(2-hydroxy-3,5-dichloro benzyl)aminomethyl]pyridine), Ln = Nd (1), Tb (2), and Dy (3), and (H₂L^Br = 2-bis(2-hydroxy-3,5-dibromo benzyl)aminomethyl]pyridine), Ln = Nd (4, H₂O) and Tb (5), were synthesized and structurally characterized by single-crystal X-ray diffraction analyses. Being isostructural in all the five cases, the metal center is octa-coordinated with a triangular dodecahedron (D_2d symmetry) geometry, and it is independent of the halogen substitution (Cl/Br). This close similarity is due to the composite interplay of hydrogen/halogen bond interactions that control the overall crystal packing, yet notable differences in association patterns among the individual ones arise from the subtle stereo-electronic requirement of individual molecules in the three-dimensional (3D) architecture. Hirshfeld surface and density functional theory (DFT) calculations clearly vouch for the importance of the hydrogen bond and halogen bond interactions observed in the structure. Detailed magnetic measurements using direct-current and alternating-current susceptibility measurements show slow magnetic relaxation in 3, a characteristic feature of the single-molecule magnets (SMMs), which is not shown by 1 and 2. Steady-state and time-resolved photoluminescence of Tb(III) complexes shows a strong ligand-to-metal energy transfer that can be modulated by changing the substitution on phenolic ligands. The results from these analyses indicate that it may be advantageous to consider the subtle role of hydrogen bond (HB)/halogen bond (XB) intermolecular interactions judiciously for the design of SMMs and luminescent materials based on halogen-substituted ligands.

Importance of an Axial LnIII-F Bond across the Lanthanide Series and Single-Molecule Magnet Behavior in the Ce and Nd Analogues

The recently reported compound [Dy^IIILF](CF₃SO₃)₂·H₂O (L = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane) displays a strong axial magnetic anisotropy, due to the short axial Dy–F bond, and single-molecule magnet (SMM) behavior. Following our earlier [Dy^IIILF]²⁺ work, herein we report the systematic structural and magnetic study of a family of [Ln^IIILF](CF₃SO₃)₂·H₂O compounds (Ln(III) = 1-Ce, 2-Pr, 3-Nd, 4-Eu, 5-Tb, 6-Ho, 7-Er, 8-Tm, and 9-Yb). From this series, the Ce(III) and Nd(III) analogues show slow relaxation of the magnetization under an applied direct current magnetic field, which is modeled using a Raman process. Complete active space self-consistent field theoretical calculations are employed to understand the relaxation pathways in 1-Ce and 3-Nd and also reveal a large tunnel splitting for 5-Tb. Additional computational studies on model compounds where we remove the axial F^– ligand, or replace F^– with I^–, highlight the importance of the F^– ligand in creating a strong axial crystal field for 1-Ce and 3-Nd and for promoting the SMM behavior. Importantly, this systematic study provides insight into the magnetic properties of these lighter lanthanide ions.

The structural and magnetic properties of a family of [Ln^IIILF](CF₃SO₃)₂·H₂O (L = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane) compounds are reported. In addition to the previously reported Dy(III) analogue, we find that the Ce(III) and Nd(III) analogues show slow relaxation of the magnetization due to the strong axial magnetic anisotropy created by the axial F⁻ ligand. AC magnetic susceptibility data and CASSCF theoretical calculations are employed to understand the single-molecule magnet behavior of 1-Ce and 3-Nd.

The coexistence of long τQTM and high Ueff as a concise criterion for a good single-molecule magnet: a theoretical case study of square antiprism dysprosium single-ion magnets

A systematic theoretical study is performed on a group of 16 square antiprism dysprosium single-ion magnets. Based on ab initio calculations, the quantum tunneling of magnetization (QTM) time, i.e., τ_QTM, and effective barrier of magnetic reversal, U_eff, are theoretically predicted. The theoretical τ_QTM is able to identify the ones with the longest QTM time with small numerical deviations. Similar results occur with respect to U_eff too. The systems possessing the best single-molecule magnet (SMM) properties here are just the ones having both the longest τ_QTM and the highest U_eff, from either experiment or theory. Thus, our results suggest the coexistence of long τ_QTM and high U_eff to be a criterion for high-performance SMMs. Although having its own limits, this criterion is easy to be applied in a large number of systems since both τ_QTM and U_eff could be predicted by theory with satisfactory efficiency and reliability. Therefore, this concise criterion could provide screened candidates for high-performance SMMs quickly and, hence, ease the burden of further exploration aiming for a higher degree of precision. This screening is important since the further exploration could easily demand tens or even hundreds of ab initio calculations for a single SMM. A semi-quantitative crystal field (CF) analysis is performed and shown here to be capable of indicating the general trends in a more chemically intuitive way. This analysis could help to identify the most important coordinating atoms for both diagonal and non-diagonal CF components. Thus, it could give some direct clues for improving the SMM properties: reducing the distance of the axial atom to the central ion, rotating the axial atom closer to the easy axis or increasing the amount of its negative charge. Correspondingly, opposite operations on the equatorial atom could give the same result.

The interpretation and prediction of lanthanide single-ion magnet from ab initio electronic structure calculation: The capability and limit

Single-molecule magnet (SMM) is a fascinating system holding the potential of being revolutionary micro-electronic device in information technology. However current SMMs are still far away from real-life application due to...

Role of Ab Initio Calculations in the Design and Development of Organometallic Lanthanide-Based Single-Molecule Magnets

Single-molecule magnets based on lanthanides are very attractive due to their potential applications proposed in the area of microelectronic devices. Very recent advances in this area are due to the blend of conventional lanthanide chemistry with organometallic ligands, and several breakthrough achievements are attained with this combination. Ab initio methods based on multi-reference CASSCF calculations are playing a vital role in the design and development of such molecules. In this minireview, we aim to appraise various contributions in the area of organometallic lanthanide complexes (those containing lanthanide-carbon bonds) and describe how these robust wavefunction-based methods have played a constructive role not only in rationalizing the observed magnetic properties but also proven to be a potential predictive tool with some selected examples.

New examples of triangular terbium(iii) and holmium(iii) and hexagonal dysprosium(iii) single molecule toroics

The structural, magnetic and theoretical aspects are described for three triangular lanthanide complexes, [Tb(OH)(teaH₂)₃(paa)₃]Cl₂ (1), [Dy(OH)(teaH₂)₃(paa)₃]Cl₂ (2) and [Ho(OH)(teaH₂)₃(paa)₃]Cl₂ (3), and a hexanuclear wheel of formula [Dy(pdeaH)₆(NO₃)₆] (4) [teaH₃ = triethanolamine, paaH = N-(2-pyridyl)-acetoacetamide and pdeaH₃ = 3-[bis(2-hydroxyethyl)amino]propan-1-ol]. Each complex displays single molecule toroidal behaviour as rationalised using high-level ab initio calculations. Complexes 2 and 3 are the first examples of mixed moment single molecule toroidal complexes featuring non-Kramers ions.

Multireference Ab Initio Studies of Magnetic Properties of Terbium-Based Single-Molecule Magnets

We investigate how different chemical environments influence magnetic properties of terbium(III) (Tb)-based single-molecule magnets (SMMs), using first-principles relativistic multireference methods. Recent experiments showed that Tb-based SMMs can have exceptionally large magnetic anisotropy and that they can be used for experimental realization of quantum information applications, with a judicious choice of chemical environment. Here, we perform complete active space self-consistent field calculations including relativistic spin-orbit interaction for representative Tb-based SMMs such as TbPc₂ and TbPcNc in three charge states. We calculate the low-energy electronic structure from which we compute the Tb crystal-field (CF) parameters and construct an effective pseudospin Hamiltonian. Our calculations show that the ligand type and fine points of molecular geometry do not affect the gap between the ground-state and first-excited doublets, whereas the latter varies weakly with oxidation number. On the other hand, higher-energy levels have a strong dependence on all these characteristics. For neutral TbPc₂ and TbPcNc molecules, the Tb magnetic moment and ligand spin are parallel to each other and the coupling strength between them does not depend much on the ligand type and details of the atomic structure. However, ligand distortion and molecular symmetry play a crucial role in transverse CF parameters which lead to tunnel splitting. The tunnel splitting induces quantum tunneling of magnetization by itself or by combining with other processes. Our results provide insights into the mechanisms of magnetization relaxation in the representative Tb-based SMMs.

Boosting axiality in stable high-coordinate Dy(iii) single-molecule magnets

A new nine-coordinate, air-stable Dy(iii) single-ion magnet has been successfully isolated. Our in silico studies demonstrate that through carefully modulating the ligand electronics, the axiality can be boosted to generate Ucal barriers of over 600 K.

Experimental and theoretical exploration of magnetic exchange interactions and single-molecule magnetic behaviour of bis(η1:η2:μ2-carboxylate)GdIII2/DyIII2 systems

This investigation demonstrates differences in SMM properties and nature of magnetic exchange in closely related DyIII2/GdIII2 compounds.