Lanthanide Single-molecule Magnets: Synthetic Strategy, Structures, Properties and Recent Advances

Single-molecule magnets (SMMs) show wide potential applications in the field of ultrahigh-density storage materials, quantum computing, spintronics, and so on. Lanthanide (Ln) SMMs, as an important category of SMMs, open up a promising prospect due to their large magnetic moments and huge magnetic anisotropy. However, the construction of high performance for Ln SMMs remains an enormous challenge. Although remarkable advances are focused on the topic of Ln SMMs, the research on Ln SMMs with different nuclear numbers is still deficient. Therefore, this review summarizes the design strategies for the construction of Ln SMMs, as well as the metal skeleton types. Furthermore, we collect reported Ln SMMs with mononuclearity, dinuclearity, and multinuclearity (three or more Ln spin centers) and the SMM properties including energy barrier (U_eff ) and pre-exponential factor (τ₀ ) are described. Finally, Ln SMMs with low-nuclearity SMMs, especially for single-ion magnets (SIMs), are highlighted to understand the correlations between structures and magnetic behavior of the detail SMM properties are described. We expect the review can shed light on the future developments of high-performance Ln SMMs.

Structurally Defined Water-Soluble Metallofullerene Derivatives towards Biomedical Applications

Endohedral metallofullerenes (EMFs) are excellent carriers of rare-earth element (REE) ions in biomedical applications because they preclude the release of toxic metal ions. However, existing approaches to synthesize water-soluble EMF derivatives yield mixtures that inhibit precise drug design. Here we report the synthesis of metallobuckytrio (MBT), a three-buckyball system, as a modular platform to develop structurally defined water-soluble EMF derivatives with ligands by choice. Demonstrated with PEG ligands, the resulting water-soluble MBTs show superb biocompatibility. The Gd MBTs exhibit superior T₁ relaxivity than typical Gd complexes, potentially superseding current clinical MRI contrast agents in both safety and efficiency. The Lu MBTs generated reactive oxygen species upon light irradiation, showing promise as photosensitizers. With their modular nature to incorporate other ligands, we anticipate the MBT platform to open new paths towards bio-specific REE drugs.

Unexpected Formation of Metallofulleroids from Multicomponent Reactions, with Crystallographic and Computational Studies of the Cluster Motion

New multicomponent reactions involving an isocyanide, terminal or internal alkynes, and endohedral metallofullerene (EMF) Lu₃ N@C₈₀ yield metallofulleroids which are characterized by mass-spectrometry, HPLC, and multiple 1D and 2D NMR techniques. Single crystal studies revealed one ketenimine metallofulleroid has ordered Lu₃ N cluster which is unusual for EMF monoadducts. Computational analysis, based on crystallographic data, confirm that the endohedral cluster motion is controlled by the position of the exohedral organic appendants. Our findings provide a new functionalization reaction for EMFs, and a potential facile approach to freeze the endohedral cluster motion at relatively high temperatures.

Substrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2 ScN@C80 on Metals and Insulators

Magnetic hysteresis is demonstrated for monolayers of the single-molecule magnet (SMM) Dy2 ScN@C80 deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy2 ScN@C80 adsorbed on Au(111) were studied by STM. X-ray magnetic CD studies show that the Dy2 ScN@C80 monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy2 ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene-surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene-SMMs on conducting substrates and facilitates their application in molecular spintronics.

Endohedral Metallofullerenes: New Structures and Unseen Phenomena

Endohedral metallofullerenes (EMFs), namely fullerenes with metallic species encapsulated inside, represent an ideal platform to investigate metal-metal or metal-carbon interactions at the sub-nanometer scale by means of single-crystal X-ray diffraction (XRD) crystallography. Herein, recent progress in the identification of new structures and unprecedented properties are discussed according to the categories of monometallofullerenes, dimetallofullerenes, carbide clusterfullerenes, and nitride clusterfullerenes. In particular, the dimerization and the cage-isomer dependent oxidation state of the inner metal atom are summarized in terms of pristine monometallofullerenes. Metal-metal bonds involving lanthanide-lanthanides or actinide-actinides are discussed based on both experimental and theoretical studies. The cluster-cage matching and/or mutual selections, as well as the rarely seen M=C double bonds, are discovered in M₂ C₂ @C_2n , U₂ C@C₈₀ , M₂ TiC@C₈₀ , and Ti₃ C₃ @C₈₀ . Subsequently, the geometries of different M₃ N clusters in various cages are discussed, revealing size-matching between the internal M₃ N cluster and the outer cage induced by the planarity of the cluster. Finally, an outlook regarding the future developments of the molecular structures and applications of EMFs is presented.

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.

Isolation and Crystallographic Characterization of Two, Nonisolated Pentagon Endohedral Fullerenes: Ho3 N@C2 (22010)-C78 and Tb3 N@C2 (22010)-C78

Purified samples of Ho₃ N@C₂ (22010)-C₇₈ and Tb₃ N@C₂ (22010)-C₇₈ have been isolated by two distinct processes from the rich array of fullerenes and endohedral fullerenes present in carbon soot from graphite rods doped with Ho₂ O₃ or Tb₄ O₇ . Crystallographic analysis of the endohedral fullerenes as cocrystals with Ni(OEP) (in which OEP is the dianion of octaethylporphyrin) shows that both molecules contain the chiral C₂ (22010)-C₇₈ cage. This cage does not obey the isolated pentagon rule (IPR) but has two sites where two pentagons share a common C-C bond. These pentalene units bind two of the metal ions, whereas the third metal resides near a hexagon of the cage. Inside the cages, the Ho₃ N or Tb₃ N unit is planar. Ho₃ N@C₂ (22010)-C₇₈ and Tb₃ N@C₂ (22010)-C₇₈ use the same cage previously found for Gd₃ N@C₂ (22010)-C₇₈ rather than the IPR-obeying cage found in Sc₃ N@D_3h -C₇₈ .

Crystallographic and Theoretical Investigations of Er2 @C2 n (2 n=82, 84, 86): Indication of Distance-Dependent Metal-Metal Bonding Nature

Successful isolation and characterization of a series of Er-based dimetallofullerenes present valuable insights into the realm of metal-metal bonding. These species are crystallographically identified as Er₂ @C_s (6)-C₈₂ , Er₂ @C_3v (8)-C₈₂ , Er₂ @C₁ (12)-C₈₄ , and Er₂ @C_2v (9)-C₈₆ , in which the structure of the C₁ (12)-C₈₄ cage is unambiguously characterized for the first time by single-crystal X-ray diffraction. Interestingly, natural bond orbital analysis demonstrates that the two Er atoms in Er₂ @C_s (6)-C₈₂ , Er₂ @C_3v (8)-C₈₂ , and Er₂ @C_2v (9)-C₈₆ form a two-electron-two-center Er-Er bond. However, for Er₂ @C₁ (12)-C₈₄ , with the longest Er⋅⋅⋅Er distance, a one-electron-two-center Er-Er bond may exist. Thus, the difference in the Er⋅⋅⋅Er separation indicates distinct metal bonding natures, suggesting a distance-dependent bonding behavior for the internal dimetallic cluster. Additionally, electrochemical studies suggest that Er₂ @C_82-86 are good electron donors instead of electron acceptors. Hence, this finding initiates a connection between metal-metal bonding chemistry and fullerene chemistry.