Magnetic Properties of a Terbium–[1]Ferrocenophane Complex: Analogies between Lanthanide–Ferrocenophane and Lanthanide–Bis‐phthalocyanine Complexes

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

LnIII/MnII-LnIII complexes derived from a salicylic azo dye ligand: synthesis, structures, magnetic and fluorescence properties

Two LnIII complexes Ln(HTMSA)3(H2O)2·5.5H2O (Ln = Dy (1) and Tb (2), H2TMSA = 5-azotriazolyl-3-methoxysalicylaldehyde) and two MnII-LnIII clusters [Mn(H2O)6][MnLn2(TTMSA)4(HTTMSA)2(H2O)6]·4H2O (Ln = Dy (3) and Tb (4), H2TTMSA = 5-azotetrazolyl-3-methoxysalicylaldehyde) have been synthesized and structurally characterized. Single-crystal X-ray diffraction reveals that 1 and 2 are isostructural complexes in which the LnIII ions are surrounded by six oxygen atoms from three chelate HTMSA ligands and two oxygen atoms from two coordinated water molecules forming a distorted square-anti-prismatic geometry. In complexes 3 and 4, the MnII ions adjust two LnIII mononuclear anion clusters into tri-nuclear LnIII-MnII-LnIII anion clusters, with an additional [Mn(H2O)6]2+ as a counter ion to maintain the electroneutrality of the compound. Magnetic studies reveal that all the complexes 1-4 show nonzero out-of-phase signals, indicating single-molecule magnet behavior. The photoluminescence spectra of all the complexes were investigated and are discussed in detail.

Desolvation-Induced Highly Symmetrical Terbium(III) Single-Molecule Magnet Exhibiting Luminescent Self-Monitoring of Temperature

A conjunction of Single-Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read-out of temperature in future SMM-based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII -based emissive SMMs formed in a cyanido-bridged framework whose properties are governed by the reversible structural transformation from [TbIII (H2 O)2 ][CoIII (CN)6 ] ⋅ 2.7H2 O (1) to its dehydrated phase, TbIII [CoIII (CN)6 ] (2). The 8-coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal-prismatic TbIII complexes in 2, showing the SMM features up to 42 K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18) cm-1 (854(26) K), one of the highest among the TbIII -based molecular nanomagnets. Both systems exhibit emission related to the f-f electronic transitions, with the temperature variations resulting in the optical thermometry below 100 K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6 K to 42 K. These functionalities are further enriched after the magnetic dilution. The role of post-synthetic formation of high-symmetry TbIII complexes in achieving the SMM effect and hot-bands-based optical thermometry is discussed.

A Combined Synthetic, Magnetic, and Theoretical Study on Enhancing Ligand-Field Axiality for Dy(III) Single-Molecule Magnets Supported by Ferrocene Diamide Ligands

Molecular design is crucial for improving the performance of single-molecule magnets (SMMs). For dysprosium(III) SMMs, enhancing ligand-field axiality is a well-suited strategy to achieve high-performance SMMs. We synthesized a series of dysprosium(III) complexes, (NN^TIPS)DyBr(THF)₂ (1, NN^TIPS = fc(NSiⁱPr₃)₂; fc = 1,1'-ferrocenediyl, THF = tetrahydrofuran), [(NN^TIPS)Dy(THF)₃][BPh₄] (2), (NN^TIPS)DyI(THF)₂ (3), and [(NN^TBS)Dy(THF)₃][BPh₄] (4, NN^TBS = fc(NSi^tBuMe₂)₂), supported by ferrocene diamide ligands. X-ray crystallography shows that the rigid ferrocene backbone enforces a nearly axial ligand field with weakly coordinating equatorial ligands. Dysprosium(III) complexes 1-4 all exhibit slow magnetic relaxation under zero fields and possess high effective barriers (U_eff) around 1000 K, comparable to previously reported (NN^TBS)DyI(THF)₂ (5). We probed the influences of structural variations on SMM behaviors by theoretical calculations and found that the distribution of negative charges defined by r_q, i.e., the ratio of the charges on the axial ligands to the charges on the equatorial ligands, plays a decisive role. Moreover, theoretical calculations on a series of model complexes 1'-5' without equatorial ligands unveil that the axial crystal-field parameters B₂⁰ are directly proportional to the N-Dy-N angles and support the hypothesis that enhancing the ligand-field axiality could improve SMM performance.

Tetraanionic arachno-Carboranyl Ligand Imparts Strong Axiality to Terbium(III) Single-Molecule Magnets

A family of fully sandwiched arachno-lanthanacarborane complexes formulated as {η⁶ -[μ-1,2-[o-C₆ H₄ (CH₂ )₂ ]-1,2-C₂ B₁₀ H₁₀ ]₂ Ln}{Li₅ (THF)₁₀ } (Ln=Tb, Dy, Ho, Er, Y) is successfully synthesized, where the "carbons-adjacent" carboranyl ligand (arachno-R₂ -C₂ B₁₀ H₁₀ ^4- ) bears four negative charges and coordinates to the central lanthanide ions using the hexagonal η⁶ C₂ B₄ face. Thus, the central lanthanide cations are pseudo-twelve-coordinate and have an approximate pseudo-D_6h symmetry or hexagonal-prismatic geometry. As the crystal field effect imparted by this geometry is still unknown, we thoroughly investigated the magnetic properties of this series of complexes and found that the crystal field imposed by this ligand causes a relation of Tb>Dy>Ho>Er for the energy gaps between the ground and the first excited states, which is of striking resemblance to the ferrocenophane and phthalocyanine ligands although the latter two ligands give disparate local coordination geometries. Moreover, the effective energy barrier to magnetization reversal of 445(10) K, the observable hysteresis loop up to 4 K and the relaxation time of the yttrium-diluted sample reaching 193(17) seconds at 2 K under an optimized field for the Tb analogue of this family of arachno-lanthanacarborane complexes, render a new benchmark for Tb³⁺ -based single-molecule magnets.

Blocking like it's hot: a synthetic chemists' path to high-temperature lanthanide single molecule magnets

Progress in the synthesis, design, and characterisation of single-molecule magnets (SMMs) has expanded dramatically from curiosity driven beginnings to molecules that retain magnetization above the boiling point of liquid nitrogen. This is in no small part due to the increasingly collaborative nature of this research where synthetic targets are guided by theoretical design criteria. This article aims to summarize these efforts and progress from the perspective of a synthetic chemist with a focus on how chemistry can modulate physical properties. A simple overview is presented of lanthanide electronic structure in order to contextualize the synthetic advances that have led to drastic improvements in the performance of lanthanide-based SMMs from the early 2000s to the late 2010s.

Trends in trigonal prismatic Ln-[1]ferrocenophane complexes and discovery of a Ho3+ single-molecule magnet

Lanthanide metallocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to transition metal ions. Herein, we present a systematic study of the structural and magnetic properties of the ferrocenophanes, [LnFc₃(THF)₂Li₂]⁻, of the late trivalent lanthanide ions (Ln = Gd (1), Ho (2), Er (3), Tm (4), Yb (5), Lu (6)). One major structural trend within this class of complexes is the increasing diferrocenyl (Fc²⁻) average twist angle with decreasing ionic radius (r_ion) of the central Ln ion, resulting in the largest average Fc²⁻ twist angles for the Lu³⁺ compound 6. Such high sensitivity of the twist angle to changes in r_ion is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 Å). This geometry also allows the non-Kramers ion Ho³⁺ to exhibit slow magnetic relaxation in the absence of applied dc fields, rendering compound 2 a rare example of a Ho-based single-molecule magnet (SMM) with barriers to magnetization reversal (U) of 110–131 cm⁻¹. In contrast, compounds featuring Ln ions with prolate electron density (3–5) don't show slow magnetization dynamics under the same conditions. The observed trends in magnetic properties of 2–5 are supported by state-of-the-art ab initio calculations. Finally, the magneto-structural relationship of the trigonal prismatic Ho-[1]ferrocenophane motif was further investigated by axial ligand (THF in 2) exchange to yield [HoFc₃(THF*)₂Li₂]⁻ (2-THF*) and [HoFc₃(py)₂Li₂]⁻ (2-py) motifs. We find that larger average Fc²⁻ twist angles (in 2-THF* and 2-py as compared to in 2) result in faster magnetic relaxation times at a given temperature.

Lanthanide ferrocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to iron ions.

The duality of electron localization and covalency in lanthanide and actinide metallocenes

Previous magnetic, spectroscopic, and theoretical studies of cerocene, Ce(C8H8)2, have provided evidence for non-negligible 4f-electron density on Ce and implied that charge transfer from the ligands occurs as a result of covalent bonding. Strong correlations of the localized 4f-electrons to the delocalized ligand π-system result in emergence of Kondo-like behavior and other quantum chemical phenomena that are rarely observed in molecular systems. In this study, Ce(C8H8)2 is analyzed experimentally using carbon K-edge and cerium M5,4-edge X-ray absorption spectroscopies (XAS), and computationally using configuration interaction (CI) calculations and density functional theory (DFT) as well as time-dependent DFT (TDDFT). Both spectroscopic approaches provide strong evidence for ligand → metal electron transfer as a result of Ce 4f and 5d mixing with the occupied C 2p orbitals of the C8H8 2- ligands. Specifically, the Ce M5,4-edge XAS and CI calculations show that the contribution of the 4f1, or Ce3+, configuration to the ground state of Ce(C8H8)2 is similar to strongly correlated materials such as CeRh3 and significantly larger than observed for other formally Ce4+ compounds including CeO2 and CeCl6 2-. Pre-edge features in the experimental and TDDFT-simulated C K-edge XAS provide unequivocal evidence for C 2p and Ce 4f covalent orbital mixing in the δ-antibonding orbitals of e2u symmetry, which are the unoccupied counterparts to the occupied, ligand-based δ-bonding e2u orbitals. The C K-edge peak intensities, which can be compared directly to the C 2p and Ce 4f orbital mixing coefficients determined by DFT, show that covalency in Ce(C8H8)2 is comparable in magnitude to values reported previously for U(C8H8)2. An intuitive model is presented to show how similar covalent contributions to the ground state can have different impacts on the overall stability of f-element metallocenes.

Main Group Chemistry at the Interface with Molecular Magnetism

Innovative synthetic coordination and, increasingly, organometallic chemistry are at the heart of advances in molecular magnetism. Smart ligand design is essential for implementing controlled modifications to the electronic structure and magnetic properties of transition metal and f-element compounds, and many important recent developments use nontraditional ligands based on low-coordinate main group elements to drive the field forward. This review charts progress in molecular magnetism from the perspective of ligands in which the donor atoms range from low-coordinate 2p elements-particularly carbon but also boron and nitrogen-to the heavier p-block elements such as phosphorus, arsenic, antimony, and even bismuth. Emphasis is placed on the role played by novel main group ligands in addressing magnetic anisotropy of transition metal and f-element compounds, which underpins the development of single-molecule magnets (SMMs), a family of magnetic materials that can retain magnetization in the absence of a magnetic field below a blocking temperature. Nontraditional p-block donor atoms, with their relatively diffuse valence orbitals and more diverse bonding characteristics, also introduce scope for tuning the spin-orbit coupling properties and metal-ligand covalency in molecular magnets, which has implications in areas such as magnetic exchange coupling and spin crossover phenomena. The chemistry encompasses transition metals, lanthanides, and actinides and describes recently discovered molecular magnets that can be regarded, currently, as defining the state of the art. This review identifies that main group chemistry at the interface molecular magnetism is an area with huge potential to deliver new types of molecular magnets with previously unseen properties and applications.

Three Dy(iii) single-ion magnets bearing the tropolone ligand: structure, magnetic properties and theoretical elucidation

Three Dy(iii) SIMs bearing the tropolone ligand are reported here with the tuning effect on their performances by ancillary ligands revealed via experimental magnetic investigations and theoretical calculations.

High-temperature magnetic blocking and magneto-structural correlations in a series of dysprosium(iii) metallocenium single-molecule magnets

A series of dysprosium(iii) metallocenium salts, [Dy(CpiPr4R)2][B(C6F5)4] (R = H (1), Me (2), Et (3), iPr (4)), was synthesized by reaction of DyI3 with the corresponding known NaCpiPr4R (R = H, iPr) and novel NaCpiPr4R (R = Me, Et) salts at high temperature, followed by iodide abstraction with [H(SiEt3)2][B(C6F5)4]. Variation of the substituents in this series results in substantial changes in molecular structure, with more sterically-encumbering cyclopentadienyl ligands promoting longer Dy-C distances and larger Cp-Dy-Cp angles. Dc and ac magnetic susceptibility data reveal that these structural changes have a considerable impact on the magnetic relaxation behavior and operating temperature of each compound. In particular, the magnetic relaxation barrier increases as the Dy-C distance decreases and the Cp-Dy-Cp angle increases. An overall 45 K increase in the magnetic blocking temperature is observed across the series, with compounds 2-4 exhibiting the highest 100 s blocking temperatures yet reported for a single-molecule magnet. Compound 2 possesses the highest operating temperature of the series with a 100 s blocking temperature of 62 K. Concomitant increases in the effective relaxation barrier and the maximum magnetic hysteresis temperature are observed, with 2 displaying a barrier of 1468 cm-1 and open magnetic hysteresis as high as 72 K at a sweep rate of 3.1 mT s-1. Magneto-structural correlations are discussed with the goal of guiding the synthesis of future high operating temperature DyIII metallocenium single-molecule magnets.

A new 3-D coordination polymer as a precursor for CuI-based thermoelectric composites

Two complexes, [Cu6I6(L1)3]n (I) and [Cu4I4(L2)2]n (II) (L1 = 1,4-bis(phenylthio)but-2-yne; L2 = 1,4-bis(phenylthio)butane), as precursors for thermoelectric composites were prepared using a literature procedure. During the preparation of I, an unexpected 3-D polymorph [Cu4I4(L1)2]n (1) with a triclinic space group and an infinite [CuI]n staircase structure was obtained. This new polymorph (1) exhibited the same structure at both room temperature and 173 K. Complexes 1 and II were therefore pyrolysed to composites 2 and 3, respectively, at 400 °C under a nitrogen gas flow. Composite 3 was pale in color with a low carbon content (0.05 wt%) and easily disassembled during handling. By comparison, the high carbon containing (10.2 wt%) composite 2 can be compressed into a robust, light pellet (density 3.58 g cm-3), which showed a moderate to high Seebeck coefficient (543-1308 μV K-1) over the temperature range 70-240 °C.

Slow magnetic relaxation in a Tb(iii)-based coordination polymer

A Tb(iii)-based coordination polymer (polyTb) was synthesized by complexation of Tb(NO3)3·(6H2O) and 4',4''''-[1,1'-biphenyl]-4,4'-diylbis[6,6''-bis(ethoxycarbonyl)2':6',2''-terpyridine](L). The polymer structure was determined by Job's plots, DFT calculation, and X-ray absorption fine structure (XAFS) measurement. Job's plots indicated that the mole ratio (Tb ion : L) is 1 : 1. The optimized model structures suggested a La model: the LaN6(O[double bond, length as m-dash]C)2 model. The bond distances of La-O and La-N are ∼2.80 Å and 2.60 Å, respectively. The EXAFS fitting indicated that the bond distances of Tb-O and Tb-N are 2.65 Å and 2.95 Å, respectively. polyTb shows field-induced magnetic relaxation in the solid and solution state. The luminescence of polyTb, originating from an f-f transition, was observed (φ = 6.9%). polyTb formed a porous structure on a Si substrate, whereas a fibrous complex structure was formed on glass. polyTb chains are orientated on glass, which were determined by XRD.