Stabilizing a three-center single-electron metal-metal bond in a fullerene cage

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.

Stability and Electronic Properties of Mixed Rare-Earth Tri-Metallofullerenes YxDy3-x@C80 (x = 1 or 2)

Tri-metallofullerenes, specifically M3@C80 where M denotes rare-earth metal elements, are molecules that possess intriguing magnetic properties. Typically, only one metal element is involved in a given tri-metallofullerene molecule. However, mixed tri-metallofullerenes, denoted as M1xM23-x@C80 (x = 1 or 2, M1 and M2 denote different metal elements), have not been previously discovered. The investigation of such mixed tri-metallofullerenes is of interest due to the potential introduction of distinct properties resulting from the interaction between different metal atoms. This paper presents the preparation and theoretical analysis of mixed rare-earth tri-metallofullerenes, specifically YxDy3-x@C80 (x = 1 or 2). Through chemical oxidation of the arc-discharge produced soot, the formation of tri-metallofullerene cations, namely Y2Dy@C80+ and YDy2@C80+, has been observed. Density functional theory (DFT) calculations have revealed that the tri-metallofullerenes YxDy3-x@C80 (x = 1 or 2) exhibit a low oxidation potential, significantly lower than other fullerenes such as C60 and C70. This low oxidation potential can be attributed to the relatively high energy level of a singly occupied orbital. Additionally, the oxidized species demonstrate a large HOMO-LUMO gap similar to that of YxDy3-xN@C80, underscoring their high chemical stability. Theoretical investigations have uncovered the presence of a three-center two-electron metal-metal bond at the center of Y2DY@C80+ and YDy2@C80+. This unique multi-center bond assists in alleviating the electrostatic repulsion between the metal ions, thereby contributing to the overall stability of the cations. These mixed rare-earth tri-metallofullerenes hold promise as potential candidates for single-molecule magnets.

Recent advances in supramolecular fullerene chemistry

Fullerene chemistry has come a long way since 1990, when the first bulk production of C60 was reported. In the past decade, progress in supramolecular chemistry has opened some remarkable and previously unexpected opportunities regarding the selective (multiple) functionalization of fullerenes and their (self)assembly into larger structures and frameworks. The purpose of this review article is to provide a comprehensive overview of these recent developments. We describe how macrocycles and cages that bind strongly to C60 can be used to block undesired addition patterns and thus allow the selective preparation of single-isomer addition products. We also discuss how the emergence of highly shape-persistent macrocycles has opened opportunities for the study of photoactive fullerene dyads and triads as well as the preparation of mechanically interlocked compounds. The preparation of two- or three-dimensional fullerene materials is another research area that has seen remarkable progress over the past few years. Due to the rapidly decreasing price of C60 and C70, we believe that these achievements will translate into all fields where fullerenes have traditionally (third-generation solar cells) and more recently been applied (catalysis, spintronics).

A stabilization rule for metal carbido cluster bearing μ3-carbido single-atom-ligand encapsulated in carbon cage

Metal carbido complexes bearing single-carbon-atom ligand such as nitrogenase provide ideal models of adsorbed carbon atoms in heterogeneous catalysis. Trimetallic μ3-carbido clusterfullerenes found recently represent the simplest metal carbido complexes with the ligands being only carbon atoms, but only few are crystallographically characterized, and its formation prerequisite is unclear. Herein, we synthesize and isolate three vanadium-based μ3-CCFs featuring V = C double bonds and high valence state of V (+4), including VSc2C@Ih(7)-C80, VSc2C@D5h(6)-C80 and VSc2C@D3h(5)-C78. Based on a systematic theoretical study of all reported μ3-carbido clusterfullerenes, we further propose a supplemental Octet Rule, i.e., an eight-electron configuration of the μ3-carbido ligand is needed for stabilization of metal carbido clusters within μ3-carbido clusterfullerenes. Distinct from the classic Effective Atomic Number rule based on valence electron count of metal proposed in the 1920s, this rule counts the valence electrons of the single-carbon-atom ligand, and offers a general rule governing the stabilities of μ3-carbido clusterfullerenes.

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.

Bandgap Engineering of Erbium-Metallofullerenes toward Switchable Photoluminescence

Encapsulating photoluminescent lanthanide ions like erbium (Er) into fullerene cages affords photoluminescent endohedral metallofullerenes (EMFs). Few reported photoluminescent Er-EMFs are all based on encapsulation of multiple (two to three) metal atoms, whereas mono-Er-EMFs exemplified by Er@C82 are not photoluminescent due to its narrow optical bandgap. Herein, by entrapping an Er-cyanide cluster into various C82 cages to form novel Er-monometallic cyanide clusterfullerenes (CYCFs), ErCN@C82 (C2 (5), Cs (6), and C2 v (9)), the photoluminescent properties of CYCFs are investigated, and obvious near-infrared (NIR) photoluminescence only is observed for ErCN@C2 (5)-C82 . Combined with a comparative photoluminescence study of three medium-bandgap di-Er-EMFs, including Er2 @Cs (6)-C82 , Er2 O@Cs (6)-C82 , and Er2 C2 @Cs (6)-C82 , this study proposes that the optical bandgap can be used as a simple criterion for switching the photoluminescence of Er-EMFs, and the bandgap threshold is determined to be between 0.83 and 0.74 eV. Furthermore, the photoluminescent patterns of these three di-Er-EMFs differ dramatically. It is found that the location of the Er atom within the same Cs (6)-C82 cage is almost fixed and independent on the endo-unit; thus the previous statement on the key role of metal position in photoluminescence of di-Er-EMFs seems erroneous, and the geometric configuration of the endo-unit, especially the bridging mode of two Er ions, is decisive instead.

USc2C2 and USc2NC Clusters with U-C Triple Bond Character Stabilized Inside Fullerene Cages

The chemistry of f-block metal-carbon multiple bonds is underdeveloped compared to well-established carbene complexes of the d-block transition metals. Herein, we report two new actinide-rare earth mixed metal carbides and nitrogen carbide cluster fullerenes, USc2C2@D5h(6)-C80 and USc2NC@D5h(6)-C80, which contain U-C bonds with triple bond character and were successfully synthesized and characterized by mass spectrometry, UV-vis-NIR spectroscopy, Fourier transform infrared spectroscopy, single crystal X-ray diffraction, and DFT calculations. Crystallographic studies show that the two previously unreported clusters, USc2C2 and USc2NC, are stabilized in the D5h(6)-C80 carbon cage and adopt unique trifoliate configurations, in which C2/NC units are almost vertically inserted into the plane defined by the U and two Sc atoms. Combined experimental and theoretical studies further reveal the bonding structure of USc2C2 and USc2NC, which contain C═U(VI)═C and C═U(V)═N bonding motifs. The electronic structures of the two compounds are determined as U6+(Sc2)6+(C4-)2@D5h(6)-C804- and U5+(Sc2)6+(N)3-(C)4-@D5h(6)-C804-, respectively. Quantum-chemical studies confirm that the U-C bonds in both molecules show unprecedented multicenter triple-bond character. The discovery of this unique U-C multiple bond offers a deeper understanding of the fundamentals of uranium chemistry.

Locating the hydrogen atoms in endohedral clusterfullerenes by density functional theory

There is a unique type of endohedral clusterfullerene containing a hydrogen atom inside the carbon cage (hydrogen-containing clusterfullerenes, HCFs). Unfortunately, the precise positions of the H atoms cannot be determined by powerful single-crystal X-ray diffraction, and thus, the reported internal cluster structures of HCFs are ambiguous. In this study, HCFs were investigated using density functional theory calculations. Various internal cluster structures were obtained for Sc₄CNH@I_h(7)-C₈₀ and then carefully inspected to summarize all the favorable H locations in the HCFs. Encouragingly, following these structural characteristics, a new Sc₄C₂H@I_h(7)-C₈₀ isomer with a μ₃-H coordination to three Sc atoms was found to be 12.6 kcal mol^-1 more stable than a previously reported isomer. It also holds a much larger SOMO-LUMO gap energy (3.57 vs. 2.36 eV). Its increased stability was further understood by the formation of multicenter bonds (three-center one-electron, three-center two-electron, and even four-center two-electron bonds) and electron density topology analyses. The changed H position may lead to rather different electronic structures, bonding states, and relative stability, indicating its critical role in HCFs. The simulated infrared and Raman spectra based on the new structure also agree fairly well with the experimental observations. Our work not only successfully locates the unpredictable H atom inside HCFs but also demonstrates a practical strategy to quickly determine the internal cluster configurations for more complex clusterfullerenes.

Capturing the Long-Sought Dy@C2v(5)-C80 via Benzyl Radical Stabilization

Endohedral metallofullerenes (EMFs) are one type of intriguing metal/carbon hybrid molecule with the molecule configuration of sphere cavity-encapsulating metal ions/metal clusters due to their unique physicochemical properties and corresponding application in the fields of biological materials, single molecule magnet materials and energy conversion materials. Although the EMF family is growing, and versatile EMFs have been successfully synthesized and confirmed using crystal structures, some expected EMF members have not been observed using the conventional fullerene separation and purify strategy. These missing EMFs raise an interesting scientific issue as to whether this is due to the difficulty in separating them from the in situ formed carbon soot. Herein, we successfully captured a long-sought dysprosium-based EMF bearing a C_2v(5)-C₈₀ cage (Dy@C_2v(5)-C₈₀) in the form of Dy@C_2v(5)-C₈₀(CH₂Ph)(Ph = -C₆H₅) from carbon soot containing versatile EMFs using simple benzyl radical functionalization and unambiguously confirmed the molecule structure using single crystal X-ray diffraction characterization. Meanwhile, the crystal structure of Dy@C_2v(5)-C₈₀(CH₂Ph) showed that a single benzyl group was grafted onto the (5,6,6)-carbon, suggesting the open-shell electronic configuration of Dy@C_2v(5)-C₈₀. The theoretical calculations unveiled that the benzyl radical addition enables the modulation of the electronic configuration of Dy@C_2v(5)-C₈₀ and the corresponding stabilization of Dy@C_2v(5)-C₈₀ in conventional organic solvents. This facile stabilization strategy via benzyl radical addition exhibits the considerable capability to capture these missing EMFs, with the benefit of enriching the endohedral fullerene family.

Cluster-Geometry-Associated Metal-Metal Bonding in Trimetallic Carbide Clusterfullerenes

Geometry configurations of the metallic clusters play a significant role in the involved bonding nature. Herein, we report the crystallographic characterization of unprecedented erbium-based trimetallic clusterfullerenes, namely, Er₃C₂@I_h(7)-C₈₀, in which the inner Er₃C₂ cluster presents a lifted bat ray configuration with the C₂ unit elevated by ∼1.62 Å above the Er₃ plane. Within the plane, the Er···Er distances for Er1···Er2, Er1···Er2A, and Er2···Er2A are 3.4051(15), 3.4051(15), and 3.3178(15) Å, respectively, falling into the range of the metal-metal bonding. Density functional theory calculations unveil the three-center-one-electron Er-Er-Er bond in Er₃C₂@I_h(7)-C₈₀ with one electron shared by three metals, and thus, its exceptional electronic structure can be expressed as (Er₃)⁸⁺(C₂)^2-@C₈₀^6-. Interestingly, with the further observation on the geometry configurations of the encapsulated clusters in M₃C₂@C_2n (M = Sc, Y, and Lu) series, we find that the lifted bat ray configuration of the inner cluster is explicitly associated with the formation of the bonding interactions between the inner metals. This finding provides insights into the nature of metal-metal bonding and gives guidelines for the design of the single-molecule magnet.

Overlooked Effects of La-4f Orbitals in Endohedral Metallofullerenes

For endohedral metallofullerenes (EMFs), a central issue is how to correctly describe the intracluster and metal-cage interactions, which are critical for understanding their structures, stabilities, and various properties. In this work, density functional theory calculations were carried out for 13 La-based EMFs covering all four reported types and a rather wide cage size range (C₃₂-C₁₀₄). The results reveal that the usually core-like lanthanide 4f subshell may play a critical role in the structural characteristics, energetic stabilities, frontier orbital energy levels, metal charges, and chemical reactivities of these endofullerenes. Regardless of the encapsulated forms, the La-4f contributions to the chemical bonding and structural stability increase with the reduced cage sizes because of the gradually enhanced cage confinement. The combination of metal-to-nonmetal charge transfer and compression of the cage cavity exposes and effectively activates the otherwise chemically inert 4f orbitals. By disclosing the important role of long-neglected metal orbitals inside fullerenes, the current work not only deepens our understanding of EMFs, but also provides new insights into the chemical bondings in general confined spaces at the subnanometer scale.

Ln3@C80+ (Ln = lanthanide): a new class of stable metallofullerene cations with multicenter metal-metal bonding in sub-nanometer confined space

Among the large number of members in the metallofullerene family, the nitride clusterfullerene M3N@C80 (M = trivalent metal) is a special one with unordinary high stability. It is generally thought...

Metal–metal bond in lanthanide single-molecule magnets

This review surveys recent critical advances in lanthanide SMMs, highlighting the influences of metal–metal bonds on the magnetization dynamics.

Decisive role of non-rare earth metals in high-regioselectivity addition of μ3-carbido clusterfullerene

The reaction of μ3-CCF Dy2TiC@Ih-C80 with AdN2 affords only one [6,6]-open monoadduct along with the addition sites adjacent to the Ti4+ ion instead of the two Dy3+ ions, revealing the decisive role of the non-rare earth metal Ti(IV).

Electrophilic Trifluoromethylation of Dimetallofullerene Anions en Route to Air-Stable Single-Molecule Magnets with High Blocking Temperature of Magnetization

Lanthanide dimetallofullerenes with single-electron M-M bonds are an important class of single molecular magnets and qubit candidates, but stabilization of their unique electronic and spin structure in the form of a neutral molecule requires functionalization of the fullerene cage with a single radical group. The lack of selectivity of the currently available procedure results in a complicated and tedious separation process. Here we demonstrate that electrophilic trifluoromethylation of a mixture of metallofullerene anions with Umemoto reagent II is highly selective toward M₂@C₈₀^- (M = Tb, Y) anions, yielding M₂@C₈₀(CF₃) monoadducts as the main reaction product. Single-crystal X-ray diffraction study proved attachment of the CF₃ group to the pentagon/hexagon/hexagon junction and revealed that positions of metal atoms inside the fullerene cage in the cocrystal with NiOEP are strongly related to the position of the porphyrin moieties. Magnetic characterization of Tb₂@C₈₀(CF₃) showed that it is a robust single-molecule magnet with broad magnetic hysteresis, 100 s blocking temperature of 25 K, and the relaxation barrier of 801(4) K, corresponding to the flipping of the Tb magnetic moment in the strongly ferromagnetically coupled [Tb³⁺-e-Tb³⁺] spin system.

Molecular structures and magnetic properties of endohedral metallofullerenes

Endohedral metallofullerenes have fascinating core-shell structures, with metal atoms or metal clusters encaged in fullerene cages, and they display various chemical, optical and magnetic properties derived from different types of fullerene cages and metal moieties. Fullerene cages can act as carriers to stabilize unusual cluster moieties. Many bizarre species that are hard to produce via synthetic methods survive well under the protection of a fullerene cage, making metallofullerenes ideal platforms for generating new clusters and bonds. Fullerene cages can also be carriers to hold active unpaired electrons. Some metallofullerenes possess electron spin and show intriguing magnetic properties, making them applicable for use in quantum computing, high density information storage and magnetoreception systems. The exploration of new metallofullerenes is still ongoing, while function-oriented studies are also promoted for the future application of metallofullerenes. Herein, we highlight the recent progress in the synthesis, electron spin characteristics and magnetic properties of metallofullerenes. Discussions and an outlook on the future development of metallofullerenes are also stated.