Strong Magnetic Coupling and Single-Molecule-Magnet Behavior in Lanthanide-TEMPO Radical Chains

Single-Molecule Magnet Behavior in a Tb-Nitronyl Nitroxide Radical Network with [Tb3(NIT)2] Nodes

Two rare two-dimensional Ln-radical networks, namely, [{Ln(tfa)3}3(NIT-4Py)2]n [LnIII = Gd 1 and Tb 2; tfa- = trifluoroacetylacetonato; and NIT-4Py = 2-(4-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], have been successfully constructed and characterized. In these complexes, each NIT-4Py radical functions as a tridentate ligand to ligate three Ln ions, creating a 2D network with linear five-spin [Ln3(NIT)2] nodes. Ferromagnetic Ln-NO interactions govern the characteristic magnetic behavior of a finite spin system. The Tb complex is shown to exhibit SMM behavior in a zero DC field, with an energy barrier for spin flipping of 53 K and hysteretic M-H behavior at 2.3 K, with a coercive field Hcoer = 110 Oe. This complex represents the first example of a Ln-nitronyl nitroxide SMM-based network.

Trigonal Planar Heteroleptic Lanthanide(III) Bis(silyl)amide Complexes Containing Aminoxyl Radicals and Anions

Modulation of the crystal field (CF) in lanthanide (Ln) complexes can enhance optical and magnetic properties, and large CF splitting can be achieved with low coordination numbers in specific geometries. We previously reported that the homoleptic near-linear Sm2+ complex [SmII{N(SiiPr3)2}2] (1-Sm) is oxidized by the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO•) radical to give the heteroleptic, approximately trigonal planar Sm3+ complex, [SmIII{N(SiiPr3)2}2(TEMPO-)] (2-Sm). Here, we report the synthesis of homologous [LnIII{N(SiiPr3)2}2(TEMPO-)] (2-Ln; Ln = Tm, Yb) complexes by the oxidation of the parent [Ln{N(SiiPr3)2}2] (1-Ln; Ln = Tm, Yb) with TEMPO•; complexes 2-Ln all contain TEMPO- anions. The homoleptic bent Ln3+ complexes [LnIII{N(SiiPr3)2}2][B(C6F5)4] (3-Ln; Ln = Sm, Tm, Yb) were also treated with TEMPO• to yield the heteroleptic, approximately trigonal planar Ln3+ complexes [LnIII{N(SiiPr3)2}2(TEMPO•)][B(C6F5)4] (4-Ln; Ln = Sm, Tm, Yb); the cations of 4-Ln all contain TEMPO• radicals. We have compared the electronic structures of the two geometrically similar families of Ln3+ complexes with the TEMPO- anion (2-Ln) or TEMPO• radical (4-Ln) using a combination of UV-vis-NIR and EPR spectroscopy, magnetic measurements, and ab initio calculations. These studies revealed no single-molecule magnet behavior for 2-Yb despite evidence for sizable CF splitting and a high degree of purity of the ground stabilized mJ = |±7/2⟩ state, while the radical TEMPO• in 4-Yb did not significantly improve performance.

Promoting exchange coupling in (CpiPr5)2Gd2X3 complexes

Introducing magnetic coupling between lanthanide ions has been shown to yield better-performing single-molecule magnets (SMMs), as exemplified by the Cp2iPr5Ln2I3 family of compounds (CpiPr5: pentaisopropylcyclopentadienyl, Ln: Gd, Tb, or Dy). This unique coupling is mediated through an unpaired electron hosted in a σ-like orbital, that results from the two 5dz2 Ln ions, and understanding these interactions holds the key to continue advancing the rational design of SMMs. Here, we focus on the Cp2iPr5Gd2I3 spin-only system and apply a recently proposed DFT-based decomposition scheme to assess the chemical and structural factors that affect the magnetic coupling. Based on these, we propose synthetically feasible systems with increased coupling.

Designing Quantum Spaces of Higher Dimensionality from a Tetranuclear Erbium-Based Single-Molecule Magnet

The spin relaxation of an Er3+ tetranuclear single-molecule magnet, [Er(hdcCOT)I]4, (hdcCOT = hexahydrodicyclopentacyclooctatetraenide dianion), is modeled as a near-tetrahedral arrangement of Ising-type spins. Combining evidence from single-crystal X-ray diffraction, magnetometry, and computational techniques, the slow spin relaxation is interpreted as a consequence of symmetry restrictions imposed on quantum tunneling within the cluster core. The union of spin and spatial symmetries describe a ground state spin-spin coupled manifold wherein 16 eigenvectors generate the 3D quantum spin-space described by the vertices of a rhombic dodecahedron. Analysis of the experimental findings in this context reveals a correlation between the magnetic transitions and edges connecting cubic and octahedral subsets of the eigenspace convex hull. Additionally, the model is shown to map to a theoretically proposed quantum Cayley network, indicating an underexplored synergy between mathematical descriptions of molecular spin interactions and quantum computing configuration spaces.

Lanthanide-radical single-molecule magnets: current status and future challenges

In the field of molecular magnetism, the lanthanide-radical (Ln-Rad) method has become one of the most appealing tactics for introducing strong magnetic interactions and has spurred on the booming development of heterospin single-molecule magnets (SMMs). The article is a timely retrospect on the research progress of Ln-Rad heterospin systems and special attention is invested on low dimensional Ln-Rad compounds with SMM behavior, primarily concerning with nitrogen-based radicals, semiquinone and nitroxide radicals. Rational design, molecular structures, magnetic behaviors and magneto-structural correlations are highlighted. Meanwhile, particular attention is focused on the influence of exchange couplings on the dynamic magnetic properties, with the purpose of helping to guide the design of prospective radical-based Ln-SMMs.

Electronic and Magnetic Properties of a Three-Arm Nonconjugated Open-Shell Macromolecule

Nonconjugated radical polymers (i.e., macromolecules with aliphatic backbones that have stable open-shell sites along their pendant groups) have arisen as an intriguing complement to π-conjugated polymers in organic electronic devices and may prove to have superior properties in magneto-responsive applications. To date, however, the design of nonconjugated radical polymers has primarily focused on linear homopolymer, copolymer, and block polymer motifs even though conjugated dendritic macromolecules (i.e., polyradicals) have shown significant promise in terms of their response under applied magnetic fields. Here, we address this gap in creating a nonconjugated, three-arm radical macromolecule with nitroxide open-shell sites using a straightforward, single-step reaction, and we evaluated the electronic and magnetic properties of this material using a combined computational and experimental approach. The synthetic approach employed resulted in a high-purity macromolecule with a well-defined molecular weight and narrow molecular weight distribution. Moreover, epoxide-based units were implemented in the three-arm radical macromolecule design, and this resulted in a nonlinear radical macromolecule with a low (i.e., below room temperature) glass transition temperature and one that was an amorphous material in the solid state. These properties allowed thin films of the three-arm radical macromolecule to have electrical conductivity values on par with many linear radical polymers previously reported, and our computational efforts suggest the potential of higher generation open-shell dendrimers to achieve advanced electronic and magnetic properties. Importantly, the three-arm radical macromolecule also demonstrated antiferromagnetic exchange coupling between spins at temperatures < 10 K. In this way, this effort puts forward key structure-property relationships in nonlinear radical macromolecules and presents a clear path for the creation of next-generation macromolecules of this type.

Strong Antiferromagnetic Interaction in a Gadolinium(III) Complex with Methoxy-TEMPO Radical: A Relation between the Coupling and the Gd-O-N Angle

A new compound [Gd(hfac)₃(MeOTEMPO)(MeOH)] (MeOTEMPO = 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl) was prepared. From the X-ray crystal structure analysis, the Gd-O-N angle is 170.9(3)°. The magnetic study clarified the Gd³⁺-radical interaction with 2J/k_B = -26.6(3) K (in the H = -2JS₁·S₂ convention), which corresponds to one of the strongest antiferromagnetic couplings in the Gd-nitroxide systems. Wider Gd-O-N angles seem to favor stronger antiferromagnetic couplings.

Pseudo-tetrahedral vs. pseudo-octahedral ErIII single molecule magnets and the disruptive role of coordinated TEMPO radical

Tetrahedral Er^III complexes are potential candidates for high-performance single molecule magnets (SMMs).

Remarkable Energy Barrier for Magnetization Reversal in 3D and 2D Dysprosium-Chloranilate-Based Coordination Polymers

Herein, two coordination polymers (CPs) [{Dy(Cl₂ An)_1.5 (CH₃ OH)}⋅4.5 H₂ O]_n (1) and [Dy(Cl₂ An)_1.5 (DMF)₂ ]_n (2), in which Cl₂ An is chloranilate (2,5-dihydroxy-1,4-benzoquinone dianion), exhibiting field-induced single-molecule magnet behavior with moderate barrier of magnetization reversal are reported. Detailed structural and topological analysis disclosed that 1 has a 3D network, whereas 2 has a 2D layered-type structure. In both CPs, magnetic measurements showed weak antiferromagnetic exchange interaction between the dysprosium centers and field-induced slow magnetic relaxation with barriers of 175(9)K and 145(7)K for 1 and 2, respectively. Notably, the energy barriers of magnetization reversal of 1 and 2 are remarkable for metal-chloranilate-based 3D (1) and 2D (2) CPs. The temperature and field dependence of relaxation time indicate the presence of multiple relaxation pathways, such as direct, quantum tunneling of magnetization, Raman, and Orbach processes, in both CPs. Ab initio theoretical calculations reinforced the experimentally observed higher energy barrier in 1 as compared with 2 due to the presence of large transverse anisotropy in the ground state in the latter. The average transition magnetic moment between the computed low-lying spin-orbit states also rationalized the relaxation as Orbach and Raman processes through the first excited state. BS-DFT calculations were carried out for both CPs to provide more insight into the exchange interaction.

Manipulating On/Off Single-Molecule Magnet Behavior in a Dy(III)-Based Photochromic Complex

Exploitation of room temperature (RT) photochromism and photomagnetism to induce single-molecule magnet (SMM) behavior has potential applications toward optical switches and magnetic memories, and remains a tremendous challenge in the development of new bulk magnets. Herein, a series of chain complexes [Ln₃(H-HEDP)₃(H₂-HEDP)₃]·2H₃-TPT·H₄-HEDP·10H₂O (QDU-1; Ln = Dy (QDU-1(Dy)), Gd (QDU-1(Gd)), and Y (QDU-1(Y)); HEDP = hydroxyethylidene diphosphonate; TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine) were synthesized by solvothermal reactions. All the compounds exhibited reversible photochromic and photomagnetic behaviors via UV light irradiation at RT, induced by the photogenerated radicals via a photoinduced electron transfer (PET) mechanism. More importantly, the PET process induced significant variations in magnetic interactions for the Dy(III) congener. Strong ferromagnetic coupling with remarkably slow magnetic relaxation without applied dc fields was observed between Dy^III ions and photogenerated O^• radicals, showing SMM behavior after RT illumination. For the first time, we observed the reversible RT photochromism and photomagnetism in the lanthanide-based materials. This work realized the radicals-actuated on/off SMM behavior via RT light irradiation, providing a new strategy for constructing the light-induced SMMs.

Modulating the structural topologies and magnetic relaxation behaviour of the Mn–Dy compounds by using different auxiliary organic ligands

A MnIII4Dy^III complex and a one-dimensional chain containing MnIII2Dy^III units have been obtained by using different combinations of organic ligands, and a slow magnetic relaxation behavior was observed for both complexes.

Activation of lignin by selective oxidation: An emerging strategy for boosting lignin depolymerization to aromatics

Lignin is the most abundant, renewable aromatic resource on earth and holds great potential for the production of value-added chemicals. The efficient valorization of lignin requires to deal with several formidable challenges, especially to prevent it from re-condensation reactions during its depolymerization. Recently, a strategy involving the activation of lignin side chains by selective oxidation of the benzylic alcohol in β-O-4 linkages to facilitate lignin degradation to aromatic monomers has become very popular. This strategy provides great advantages for lignin selective degradation to high yields of aromatics under mild conditions, but requires an additional pre-oxidation step. The purpose of this review is to provide the latest cutting-edge innovations of this novel approach. Various catalytic systems, including those using chemo-catalytic methods, physio-chemo catalytic methods, and/or bio-catalytic methods, for the oxidative activation of lignin side chains are summarized. By analyzing the current situation of lignin depolymerization, certain promising directions are emphasized.

Kinetic Stabilization of Carbazole Nitroxides by Inclusion in a Macrocage and Their Electron Spin Resonance Characterization

Some nitroxides, for example, tetramethylpyridiniumoxide, are known as stable radicals; however, carbazole nitroxide is less stable. The kinetic stabilization of labile radicals by introduction of bulky substituents is usually effective to investigate intrinsic properties of the molecule because of small electronic perturbation induced by the substituents. In this study, macrocage molecules with a carbazole nitroxide connected by covalent bonds were newly designed as kinetically stabilized carbazole nitroxides. The nitroxides were prepared and characterized by electron spin resonance spectroscopy. The caged nitroxides presented long half-lives (∼50 h) by kinetic analysis.