Steering Single-Electron Metal-Metal Bonds and Hyperfine Coupling between a Transition Metal-Lanthanide Heteronuclear Bimetal Confined in Carbon Cages

Anomalous Magnetic Hysteresis up to 20 K in a Non-Kramers Light-Lanthanide Single-Molecule Magnet

Light-lanthanides (Ln), despite their widespread uses in commercial permanent magnets, are among the least explored metal elements as building blocks of single-molecule magnets (SMMs) due to their smaller magnetic moments as well as weaker spin-orbit couplings than those of the heavy-Ln counterparts, and so far, only a neodymium (Nd) complex has been reported showing a small magnetic hysteresis at 2 K. Here, we report a low-coordination praseodymium (Pr) complex, namely, Pr@C81N, featuring a non-Kramers trivalent Pr ion entrapped within an azafullerene cage and behaving as the first Pr-based SMM. Although the weak nonaxial ligand field imposed only by the carbon cage on the integer-spin Pr(III) elicits nondegenerate spin states, Pr@C81N shows an anomalous magnetic hysteresis up to 20 K using a field sweep rate of 20 Oe/s, which is the highest temperature among all reported light-Ln SMMs. In addition to the ideally minimized spin-phonon coupling in the low coordination, the origin of such an unexpected magnetic bistability for the non-Kramers complex likely lies in nuclear sublevels arising from a large Pr hyperfine coupling, which feature distinct electronic magnetizations within the highly mixed ground electronic state.

Evaluation of Intermolecular Weak Binding Through the Zero-Field Splitting of an Endofullerene High-Spin Probe

Weak binding between molecules is becoming a renowned aspect of molecular engineering, offering an alternative to the tuning of conventional chemical bonds. However, the subtle nature of these interactions poses challenges for characterization using standard techniques. Herein, an efficient method of weak binding characterization based on the zero-field splitting of a high-spin probe by electron paramagnetic resonance is presented. Using endofullerene as the high spin center, we achieved direct comparison of the weak binding strengths in F-MTPP (M = H2, Zn, Ni, and Pd) and 2F-3MTPP (M = H2, Ni, Pd, and Pt) through the zero-field splitting parameters. The high-spin resonance spectrum is sensitive to the small variation in the supramolecular structure. By examining fullerene-porphyrin co-crystals with various metal centers and packing arrangements, the relation between intermolecular weak binding and microstructural strain on the fullerene cage is elucidated.

Fluoride Clusterfullerenes: Tuning Metal-Metal Bonding and Magnetic Properties via Single Fluorine Atom Doping

Endohedral fullerenes are known for their exceptional ability to host metal clusters that contain unique bonding motifs. In this study, we report a facile strategy to synthesize a new family of clusterfullerenes, fluoride clusterfullerenes (FCFs). This work demonstrates that actinides and rare earth metals as well as alkaline earth metals can be encapsulated within a variety of fullerene cages, and these fullerenes can be obtained in their pristine form without additional functionalization methods. Notably, Th2F@Ih(7)-C80 and CaScF@Cs(6)-C82 were isolated and their molecular structures and magnetic properties were characterized by X-ray single-crystal diffraction and multiple spectroscopic techniques as well as DFT calculations. These findings reveal that the unique internal addition of a single fluorine atom significantly alters the metal-metal bonding interactions of Th-Th and Ca-Sc. While Th2@Ih(7)-C80 hosts a σ2 Th-Th bond, an unprecedented actinide-actinide (Th-Th) single electron metal-metal bond is formed inside Th2F@Ih(7)-C80 upon the internal addition of fluoride. Similarly, while a Ca-Sc single electron bond exists in CaSc@Cs(6)-C82, which exhibits excellent molecular qubit properties, the addition of fluoride transforms the compound into a singlet. The present study not only highlights the successful synthesis of a novel family of FCFs, which will likely be an extensive family, it also shows that fluorine doping can induce novel metal-metal bonding motifs leading to potentially intriguing magnetic properties.

Spin probe for dynamics of the internal cluster in endohedral metallofullerenes

Endohedral metallofullerenes (EMFs) are constructed by fullerene cages encapsulating various metal atoms or metal clusters, which usually exhibit some motion. However, due to the fact that the elusive endohedral dynamics are related to many factors, it remains a challenge to image the motion of internal species. Recently, the electron spin was found to be a sensitive probe to detect the motion of internal species in EMFs. Moreover, this technique can be widely applied for many metallofullerenes, i.e., for paramagnetic EMFs, the unpaired electron spin is a natural probe for the endohedral dynamics, and for diamagnetic EMFs, an electron can be introduced to produce spin-active EMF molecules. Based on the analysis of hyperfine coupling constants (hfcc), g-factors, and line patterns of the ESR spectra of EMFs, the spin centers and endohedral dynamics can be deduced. It has been revealed that the spin probes can provide unexpected information about the dynamics of the internal clusters in EMFs. Through changing the temperature, exohedral modification of the EMF, and supramolecular assembly, the motion of the internal species in EMFs can be manipulated, as clearly reflected by the spin probe. These studies revealed that the spin in EMFs exhibits promising applications in quantum sensing and molecular machine technology. In this review, we will address the use of the spin probe in EMFs and attempt to understand the effects in the detection of the endohedral dynamics.

Advancements in endohedral metallofullerenes: novel metal-cage interactions driving new phenomena and emerging applications

Since the discovery of La@C82, a wide array of endohedral metallofullerenes (EMFs) have been synthesized and documented. Various metals, including lanthanides, transition metals, alkali metals, alkaline earth metals and actinides, have been successfully incorporated into the inert fullerene cavities. The interaction between these encapsulated metal species and the fullerene cage isomers plays a crucial role in determining distinct molecular structures and imparting versatile chemical behaviors to these compounds. In particular, recent advancements in EMFs with medium-sized carbon cages, which are among the most versatile categories of EMFs, have marked a significant breakthrough in fundamental coordination chemistry and opened up a wide range of potential applications. The formation of various abnormal metal clusters, possessing unique chemical bonding character and geometric conformations, has been shown to be influenced by novel electron transfer mechanisms between the metal atoms and the carbon cage. Moreover, these specialized metal-cage interactions have also facilitated the stabilization of giant fullerene families and promoted the exploration of these structures in greater detail, particularly with respect to the unanticipated metallofullertubes. Therefore, this review aims to highlight the new phenomena arising from these novel metal-cage interactions in the fundamental study of pristine EMFs. On this basis, we also discussed innovative applications of EMF-based supramolecular complexes that stem from their unique host-guest association.

CaY@C2n: Exploring Molecular Qubits with Ca-Y Metal-Metal Bonds

Metal-metal bonding is crucial in chemistry for advancing our understanding of the fundamental aspects of chemical bonds. Metal-metal bonds based on alkaline-earth (Ae) elements, especially the heavier Ae elements (Ca, Sr, and Ba), are rarely reported due to their high electropositivity. Herein, we report two heteronuclear di-EMFs CaY@Cs(6)-C82 and CaY@C2v(5)-C80, which contain unprecedented single-electron Ca-Y metal-metal bonds. These compounds were characterized by single-crystal X-ray crystallography, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations. The crystallographic study of CaY@Cs(6)-C82 shows that Ca and Y are successfully encapsulated into the carbon cage with a Ca-Y distance of 3.691 Å. The CW-EPR study of both CaY@Cs(6)-C82 and CaY@C2v(5)-C80 exhibits a doublet, suggesting the presence of an unpaired electron located between Ca and Y. The combined experimental and theoretical results confirm the presence of a Ca-Y single-electron metal-metal bond with substantial covalent interaction, attributed to significant overlap between the 4s4p orbitals of Ca and the 5s5p4d orbitals of Y. Furthermore, pulse EPR spectroscopy was used to investigate the quantum coherence of the electron spin within this bond. The unpaired electron, characterized by its s orbital nature, is effectively protected by the carbon cage, resulting in efficient suppression of both spin-lattice relaxation and decoherence. CaY@Cs(6)-C82 behaves as an electron spin qubit, displaying a maximum decoherence time of 7.74 μs at 40 K. This study reveals an unprecedented Ae-rare-earth metal-metal bond stabilized by the fullerene cages and elucidates the molecular qubit properties stemming from their unique bonding character, highlighting their potential in quantum information processing applications.

Torsion Effects Beyond the δ Bond and the Role of π Metal-Ligand Interactions

Previous studies on bimetallic paddlewheel compounds have established a direct correlation between metal-metal distance and ligand torsion angles, leading to the rule that higher torsion results in longer metal-metal bond distances. Here, the new discovery based on diarylformamidinate Ru₂⁵⁺ paddlewheel compounds [Ru2Cl(DArF)4] that show an opposite behavior is reported: higher torsions lead to shorter metal-metal distances. This discovery challenges the assumption that internal rotation solely impacts the δ bond. By combining experimental and theoretical techniques, it is demostrated that this trend is associated with previously overlooked π metal-ligand interactions. These π metal-ligand interactions are a direct consequence of the paddlewheel structure and the conjugated nature of the bidentate ligands. This findings offer far-reaching insights into the influence of equatorial ligands and their π-conjugation characteristics on the electronic properties of paddlewheel complexes. That this effect is not exclusive of diruthenium compounds but also occurs in other bimetallic cores such as ditungsten or dirhodium is demonstrated, and with other ligands showing allyl type conjugation. These results provide a novel approach for fine-tuning the properties of these compounds with significant implications for materials design.

Unveiling the Dynamic Electrocatalytic Activity of Online Synthesized Bimetallic Nanocatalysts via Electrochemiluminescence Microscopy

Effective bimetallic nanoelectrocatalysis demands precise control of composition, structure, and understanding catalytic mechanisms. To address these challenges, we employ a two-in-one approach, integrating online synthesis with real-time imaging of bimetallic Au@Metal core-shell nanoparticles (Au@M NPs) via electrochemiluminescence microscopy (ECLM). Within 120 s, online electrodeposition and in situ catalytic activity screening alternate. ECLM captures transient faradaic processes during potential switches, visualizes electrochemical processes in real-time, and tracks catalytic activity dynamics at the single-particle level. Analysis using ECL photon flux density eliminates size effects and yields quantitative electrocatalytic activity results. Notably, a nonlinear activity trend corresponding to the shell metal to Au surface atomic ratio is discerned, quantifying the optimal surface component ratio of Au@M NPs. This approach offers a comprehensive understanding of catalytic behavior during the deposition process with high spatiotemporal resolution, which is crucial for tailoring efficient bimetallic nanocatalysts for diverse applications.

Endohedral metallofullerene molecular nanomagnets

Magnetic lanthanide (Ln) metal complexes exhibiting magnetic bistability can behave as molecular nanomagnets, also known as single-molecule magnets (SMMs), suitable for storing magnetic information at the molecular level, thus attracting extensive interest in the quest for high-density information storage and quantum information technologies. Upon encapsulating Ln ion(s) into fullerene cages, endohedral metallofullerenes (EMFs) have been proven as a promising and versatile platform to realize chemically robust SMMs, in which the magnetic properties are able to be readily tailored by altering the configurations of the encapsulated species and the host cages. In this review, we present critical discussions on the molecular structures and magnetic characterizations of EMF-SMMs, with the focus on their peculiar molecular and electronic structures and on the intriguing molecular magnetism arising from such structural uniqueness. In this context, different families of magnetic EMFs are summarized, including mononuclear EMF-SMMs wherein single-ion anisotropy is decisive, dinuclear clusterfullerenes whose magnetism is governed by intramolecular magnetic interaction, and radical-bridged dimetallic EMFs with high-spin ground states that arise from the strong ferromagnetic coupling. We then discuss how molecular assemblies of SMMs can be constructed, in a way that the original SMM behavior is either retained or altered in a controlled manner, thanks to the chemical robustness of EMFs. Finally, on the basis of understanding the structure-magnetic property correlation, we propose design strategies for high-performance EMF-SMMs by engineering ligand fields, electronic structures, magnetic interactions, and molecular vibrations that can couple to the spin states.

Nd─Nd Bond in Ih and D5h Cage Isomers of Nd2 @C80 Stabilized by Electrophilic CF3 Addition

Synthesis of molecular compounds with metal-metal bonds between 4f elements is recognized as one of the fascinating milestones in lanthanide metallochemistry. The main focus of such studies is on heavy lanthanides due to the interest in their magnetism, while bonding between light lanthanides remains unexplored. In this work, the Nd─Nd bonding in Nd-dimetallofullerenes as a case study of metal-metal bonding between early lanthanides is demonstrated. Combined experimental and computational study proves that pristine Nd2 @C80 has an open shell structure with a single electron occupying the Nd─Nd bonding orbital. Nd2 @C80 is stabilized by a one-electron reduction and further by the electrophilic CF3 addition to [Nd2 @C80 ]- . Single-crystal X-ray diffraction reveals the formation of two Nd2 @C80 (CF3 ) isomers with D5h -C80 and Ih -C80 carbon cages, both featuring a single-electron Nd─Nd bond with the length of 3.78-3.79 Å. The mutual influence of the exohedral CF3 group and endohedral metal dimer in determining the molecular structure of the adducts is analyzed. Unlike Tb or Dy analogs, which are strong single-molecule magnets with high blocking temperature of magnetization, the slow relaxation of magnetization in Nd2 @Ih -C80 (CF3 ) is detectable via out-of-phase magnetic susceptibility only below 3 K and in the presence of magnetic field.