Strong Equatorial Crystal Field Enhances the Axial Anisotropy and Energy Barrier for Spin Reversal Process in Yb2 Single Molecule Magnets

Epitome of polyoxotungstate-coordinated lanthanide-based single-molecule magnets

Single-molecule magnets (SMMs) have garnered significant interest due to their possible use in high-density data storage devices and quantum computing. SMMs based on lanthanide-encapsulated polyoxotungstate (POT) have emerged due to their stability under ambient conditions and potential application in magnetic devices. The POTs can stabilize various anisotropic lanthanide ions (Ln(III)) with a range of strong to moderate ligand fields. The systematic combination of Ln(III) ions with POTs leads to significant perturbation in the electronic structure of the Ln(III) ion, which notably influences their magnetic properties. The magnetic properties of Ln(III)-POT clusters are highly dependent on the first coordination geometry and ligand field strength around the Ln(III) ions. Moreover, the bulky and diamagnetic POTs efficiently minimize the intermolecular magnetic interactions. As a result, the under-barrier magnetic relaxation is suppressed, and magnetic performance is enhanced. Over the years, a diverse array of Ln-POT SMMs have enriched the literature containing vacant POTs as building blocks (such as Keggin, Lindqvist, Wells-Dawson, and Preyssler types). In this review, we have discussed and summarized the effects of structural and bonding diversities of POTs on the SMM behaviour of Ln-POT clusters. This review aims to provide future direction and exploration of the challenging and compelling field of POT-based SMMs.

Square planar Pd(II)/oximate complexes as 'metalloligands' for the directional assembly of 4f-metal ions: a new family of {Ln2Pd} (Ln = lanthanide) clusters exhibiting slow magnetization relaxation

A relatively unexplored approach in heterometallic chemistry of transition metals and lanthanides has been developed toward the controlled synthesis of a new family of linear heterotrinuclear Ln(III)-Pd(II)-Ln(III) complexes with the general formula [Ln2Pd(pao)2(NO3)6(MeOH)2(H2O)2]·[Pd(pao)2]4, where LnIII = DyIII (2), GdIII (3), ErIII (4) and YbIII (5). This strategy was based on the diamagnetic 'metalloligand' [Pd(pao)2] (1), where pao- is the anion of 2-pyridinealdoxime, containing two dangling oximate O-atoms which were trans to each other and available for binding with oxophilic lanthanide ions. Because of their trans-configuration, the [Pd(pao)2] 'metalloligand' was able to direct the binding of two {Ln(NO3)3(MeOH)(H2O)} units on opposite sites, thus yielding the reported trinuclear {Ln-Pd-Ln} clusters. Complexes 2-5 constitute a new family of trinuclear heterometallic {Ln2Pd} species, and they represent the first examples of a directional assembly approach towards the coordination of 4f-metal ions. Compounds 2 and 5 exhibit out-of-phase signals under applied dc fields of 300 and 2000 Oe, respectively, characteristics of the slow magnetization relaxation, albeit with very small energy barriers for the magnetization reversal. This was due to the combined onset of fast quantum tunneling and the weak crystal field effects induced by the coordinated ligands. The combined results highlight the potential of using the 'metal complexes as ligands' method to deliberately prepare heterometallic PdII-LnIII complexes with unique structural and interesting physicochemical (magnetic, optical, catalytic) properties.

Novel stable ytterbium acetylacetonate-quinaldinate complexes as single-molecule magnets and surprisingly efficient luminophores

The first Yb complexes comprising a quinoline-2-carboxylate (quinaldinate, Q-) ligand, namely 1D-polymeric [Yb(acac)2(Q)]n (1, acac- is the acetylacetonate (pentane-2,4-dionate) anion) and mononuclear [Yb(acac)2(Q)(Phen)] (2, Phen is 1,10-phenanthroline), are reported. The bifunctionality of both complexes as field-induced single-molecule magnets (SMMs) and near IR luminophores has been revealed. The SMM properties of 1 and 2 have been discussed in terms of the geometry and composition of the coordination environment. Also, 1 is the first example of 1D-polymeric SMMs with the capped octahedral surrounding of Yb3+. The photoluminescence quantum yields (PLQYs) of 1 and 2 are 2 and 4%, respectively. The origins of this difference are discussed. Surprisingly, the PLQY value of 2 is high for compounds comprising a lot of C-H vibrational quenchers, being the highest one for reliably characterized Yb β-diketonate complexes, and surpassing those for complexes with a broad range of anionic ligands. In this respect, the role of the Phen ligand is to tune the coordination mode of Q- thereby decreasing the energy of coordinating C-O oscillators rather than to act as a typical antenna ligand. These results can give rise to an alternative route to elaborate efficient Yb-based luminophores via the substitution of the β-diketonate ligands controlled by the introduction of appropriate neutral ligands.

Spin-state energetics and magnetic anisotropy in penta-coordinated Fe(III) complexes with different axial and equatorial ligand environments

The penta-coordinated trigonal-bi-pyramidal (TBP) Fe(III) complex (PMe₂Ph)₂FeCl₃ shows a reduced magnetic anisotropy in its intermediate-spin (IS) state as compared to its methyl-analog (PMe₃)₂Fe(III)Cl₃. In this work, the ligand environment in (PMe₂Ph)₂FeCl₃ is systematically altered by replacing the axial -P with -N and -As, the equatorial -Cl with other halides, and the axial methyl group with an acetyl group. This has resulted in a series of Fe(III) TBP complexes modelled in their IS and high-spin (HS) states. Lighter ligands -N and -F stabilize the complex in the HS state, while the magnetically anisotropic IS state is stabilized by -P and -As at the axial site, and -Cl, -Br, and -I at the equatorial site. Larger magnetic anisotropies appear for complexes with nearly degenerate ground electronic states that are well separated from the higher excited states. This requirement, largely controlled by the d-orbital splitting pattern due to the changing ligand field, is achieved with a certain combination of axial and equatorial ligands, such as -P and -Br, -As and -Br, and -As and -I. In most cases, the acetyl group at the axial site enhances the magnetic anisotropy compared to its methyl counterpart. In contrast, the presence of -I at the equatorial site compromises the uniaxial type of anisotropy of the Fe(III) complex leading to an enhanced rate of quantum tunneling of magnetization.

Effect of an axial coordination environment on quantum tunnelling of magnetization for dysprosium single-ion magnets with theoretical insight

Herein, we report two mononuclear dysprosium complexes [Dy(H₄L){B(OMe)₂(Ph)₂}₂](Cl)·MeOH (1) and [Dy(H₄L){MeOH)₂(NCS)₂}](Cl) (2) [where H₄L = 2,2'-(pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))bis(N-phenylhydrazinecarboxamide)] with different axial coordination environments. The structural analysis revealed that the pentadentate H₄L ligand binds through the equatorial position in both complexes. In complex 1, the axial positions are occupied by bidentate dimethoxydiphenyleborate [B(OMe)₂(Ph)₂]^-. On the other hand, in complex 2, one axial position is occupied by two NCS^- and one MeOH molecule while another MeOH molecule is coordinated to the other axial position. Magnetic measurements disclose the presence of field-induced slow relaxation of magnetization with an energy barrier of U_eff = 30 K for 1 whereas no such effective barrier was observed in complex 2. Detailed analysis of field and temperature dependence of the relaxation time confirms the major role of Raman, QTM, and direct processes rather than the Orbach process in complex 1. It was observed that [B(OMe)₂(Ph)₂]^- provides higher axial anisotropy which slows down the QTM process (relaxation time for the QTM process is 2.70 × 10^-5 s) in 1 as compared to NCS anions and MeOH molecules in 2 (1.03 × 10^-8 s), and is responsible for the absence of an effective energy barrier in the latter complex as confirmed by ab initio calculations. The calculations also show that the presence of a large bidentate dimethoxydiphenyleborate ligand in axial positions may result in high-performance Dy-based single-ion magnets.

A remarkable energy barrier for spin reversal in a field induced dinuclear ytterbium single molecule magnet

A dinuclear ytterbium complex has been designed with a strong ligand field in equatorial positions. Magnetic studies reveal the presence of easy-axis anisotropy and field induced slow relaxation of magnetization with a remarkable energy barrier, U_eff = 53.58 cm^-1, the highest value reported for any Yb-based SMMs to date. Furthermore, the ab initio calculations disclose the importance of a weak axial ligand field to design high-performance Yb-based SMMs.

Assembly of tetra-nuclear YbIII-containing selenotungstate clusters: synthesis, structures, and magnetic properties

Two tetra-nuclear Yb^III-incorporated selenotungstate clusters, Keggin (C₂H₈N)₆Na₁₄[Yb₄Se₆W₄₄O₁₆₀(H₂O)₁₂]·40H₂O (1) and Wells-Dawson (C₂H₈N)₄Na₁₄[Yb₄Se₆W₄₅O₁₅₉(OH)₆(H₂O)₁₁]·38H₂O (2), have been isolated through a pH-controlled assembly, which exhibit the first Yb^III-containing polyoxotungstates with selenium heteroatoms. Their assemblies rely on the structure-directing effects of SeO₃^2- anion templates to give rise to available Se-containing Keggin-/Wells-Dawson-type motifs. Both compounds were characterized by single-crystal X-ray diffraction, IR spectroscopy, power X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) as well as electrospray ionization mass spectrometry (ESI-MS). Furthermore, systematic magnetic studies revealed that 1 exhibits field-induced single-molecule magnetic behavior with a pre-exponential factor of τ₀ = 6.60(7) × 10^-8 s and a relaxation energy barrier of ΔE/k_B = 39.44(2) K, while 2 only displays antiferromagnetic interactions between the ytterbium centers.