Crystallographic characterization of Er3N@C2n (2n = 80, 82, 84, 88): the importance of a planar Er3N cluster

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.

Structure and single-molecule conductance of two endohedral metallofullerenes with large C88 cage

Endohedral metallofullerenes are capable of holding peculiar metal clusters inside the carbon cage. Additionally, these display many chemical and physical properties originating from the complexation between the metal clusters and carbon cages, which could be acquired for wide applications. In this study, two metallofullerenes (Ce2O@C88 and Ce3N@C88) with an identical large C88-D2(35) cage, and their molecular structures and single-molecule conductance properties were investigated comparatively. Characterizations of UV-vis-NIR absorption spectroscopy, Raman spectroscopy, and DFT calculations were employed to determine the geometries and electronic structures of Ce2O@C88 and Ce3N@C88. These molecules revealed varied energy gaps, structural parameters, vibrational modes, and molecular frontier orbitals. Although the two metallofullerenes have an identical cage isomer of C88-D2(35), their different endohedral clusters can influence their structures and physicochemical properties. Furthermore, the single-molecule conductance properties were measured using the scanning tunneling microscopy break junction technique (STM-BJ). The experimental results revealed that Ce2O@C88 has a higher conductance than Ce3N@C88 and C60. This revealed the cluster-dependent electron transportation as well as the significant research value of metallofullerenes with large carbon cages. These results provide guidance for fabricating single-molecule electronic devices.

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.

A spider hanging inside a carbon cage: off-center shift and pyramidalization of Sc3N clusters inside C84 and C86 fullerene cages

Structural analysis shows that, in Sc3N@Cs(51365)-C84 and Sc3N@D3(19)-C86, the Sc3N clusters are shifted to one side of the cages and unexpectedly pyramidalized inside the large cages of C84 and C86, which resembles a spider attached to a web.

A non-isolated pentagon rule C82 cage stabilized by a stretched Sc3N cluster

A novel cluster fullerene, Sc3N@Cs(39 663)-C82, has been synthesized and characterized. Crystallograpic charaterization unambiguously determines the non-IPR cage structure of Cs(39 663)-C82. Structural analyses further reveal that the Sc3N cluster is notably stretched to facilitate the interaction with the non-IPR fullerene cage.

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

Trimetallic carbide clusterfullerenes (TCCFs) encapsulating a quinary M3C2 cluster represent a special family of endohedral fullerenes with an open-shell electronic configuration. Herein, a novel TCCF based on a medium-sized rare earth metal, dysprosium (Dy), is synthesized for the first time. The molecular structure of Dy3C2@I h(7)-C80 determined by single crystal X-ray diffraction shows that the encapsulated Dy3C2 cluster adopts a bat ray configuration, in which the acetylide unit C2 is elevated above the Dy3 plane by ∼1.66 Å, while Dy-Dy distances are ∼3.4 Å. DFT computational analysis of the electronic structure reveals that the endohedral cluster has an unusual formal charge distribution of (Dy3)8+(C2)2-@C80 6- and features an unprecedented three-center single-electron Dy-Dy-Dy bond, which has never been reported for lanthanide compounds. Moreover, this electronic structure is different from that of the analogous Sc3C2@I h(7)-C80 with a (Sc3)9+(C2)3-@C80 6- charge distribution and no metal-metal bonding.

Crystallographic evidence and spin activation for the Russian-doll-type metallofullerene Sc4C2@C80

We report unambiguous crystallographic evidence for the Russian-doll-type metallofullerene Sc4C2@Ih-C80. 45Sc NMR further demonstrates the tetrahedron arrangement of the Sc4C2 cluster. Moreover, the electrochemical test reveals the stable oxidation state of Sc4C2@C80. Hence, the Sc4C2@C80 cation radical was studied by electron spin resonance spectroscopy. These results provide better understanding for the previously less-explored Sc4C2@C80.

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.