Theoretical Insight into Sc2O@C84: Interplay between Small Cluster and Large Carbon Cage

Ho2O@C84: Crystallographic Evidence Showing Linear Metallic Oxide Cluster Encapsulated in IPR Fullerene Cage of D2d(51591)-C84

Fullerene C₈₄ is the third-most-abundant species after C₆₀ and C₇₀. In the past decade, a variety of C₈₄-based clusterfullerenes have been well-studied experimentally, which otherwise do not include oxide clusterfullerenes (OCFs). In this work, we report a comprehensive inspection of Ho₂O@C₈₄, including its mass, spectroscopic, crystallographic, electrochemical (EC), and density functional theory (DFT) studies. Importantly, crystallographic data reveal an IPR cage of D_2d(51591)-C₈₄ with a linear endohedral Ho-O-Ho cluster, indicating that the compression effect of the C₈₄ cage is less pronounced compared to that of a smaller cage. The experimentally observed structure is confirmed by DFT computations, which also verify its superior stability. Further studies suggest that Ho₂O@C₈₄ has reduced EC and HOMO-LUMO gaps compared to those of empty species, again demonstrating the effect of cluster encapsulation.

Trapping Metallic Oxide Clusters inside Fullerene Cages

The sub-nanometer sized void inside a fullerene cage permits the accommodation of a single atom, atomic cluster, or even small molecule, resulting in the formation of endohedral fullerenes. Particularly, clusterfullerenes can be formed by encapsulating multiple metallic ions in most cases along with nonmetal ions (i.e., N^3-, C₂^2-, S^2-, O^2-) inside the fullerene cage. Such an association makes clusterfullerene more functional than empty fullerenes and conventional mono-metallofullerenes. To date, a variety of clusterfullerenes have been reported, including metal nitrides, carbides, oxides, sulfides, cyanides, and so on. Among them, oxide clusterfullerenes (OCFs) can contain variable oxide clusters (i.e., M₄O₂, M₄O₃, M₃O, and M₂O; M = Sc or other metal), yielding one of the most versatile families. Thus, OCFs may provide a more convenient platform for developing new functional molecules and for studying previously less-explored topics such as formation mechanisms of clusterfullerenes. In this Account, we review recent progress in the field of OCFs, including their synthesis, isolation, and structural and electrochemical studies as well as the preliminary exploration into their potential functions and applications. Thanks to the concrete crystallographic results of an OCF series, we can track the transition of endohedral cluster and fullerene cage. It is suggested that the configuration and internal dynamics of the oxide cluster are highly dependent on not only the cage size but also cage structure. On the other hand, based on the experimental observations, two competitive transformation pathways are established for the majority of OCFs, verifying the bottom-up or top-down formation mechanism. It is also found that the redox behaviors of OCFs are more or less comparable to their isoelectronic species with common cage structure and similar cluster geometry but varied greatly with the cluster variety (i.e., Sc₂O vs Sc₄O_2-3). The mechanism behind such phenomena has been discussed. In addition, the potential of Dy-based OCFs as single molecular magnets (SMMs) is presented theoretically. Nevertheless, experimental advance remains to be achieved.

Crystallographic characterization of Lu2C2n (2n = 76-90): cluster selection by cage size

The successful isolation and unambiguous crystallographic assignment of a series of lutetium-containing endohedral metallofullerenes (EMFs), Lu2C2n (2n = 76, 78, 80, 84, 86, 88, 90), reveal an unrecognized decisive effect of the cage size on the configuration of the encapsulated clusters. The molecular structures of these compounds are unambiguously assigned as Lu2@T d(2)-C76, Lu2@D 3h(5)-C78, Lu2@C 2v(5)-C80, Lu2@C 2v(7)-C84, Lu2@C s(8)-C86, Lu2@C s(15)-C86, Lu2@C 1(26)-C88, Lu2C2@C 2v(9)-C86, Lu2C2@C s(32)-C88 and Lu2C2@D 2(35)-C88. Specifically, when the cage is relatively small, Lu2@C2n (2n = 76-86) are all dimetallofullerenes (di-EMFs) and a Lu-Lu single bond could be formed between the two lutetium ions inside the cages. However, when the cage expands further, the valence electrons forming the possible Lu-Lu bond donate to a readily inserted C2-unit, resulting in the formation of carbide EMFs, Lu2C2@C2n (2n = 86, 88). Consistently, our theoretical results reveal that all these EMFs are thermodynamically favorable isomers. Thus the comprehensive characterization of the series of Lu2C76-90 isomers and the overall agreement between the experimental and theoretical results reveal for the first time that the exact configuration of the internal metallic cluster is determined by the cage size, taking a solid step towards the controlled synthesis of novel hybrid molecules which may have potential applications as building blocks of single molecule devices.

Quantum Chemical Insight into La2C96: Metal Carbide Fullerene La2C2@C94 versus Dimetallofullerene La2@C96

A family of dilanthanum-containing endohedral metallofullerene La₂C_2n (n = 46-51) was synthesized recently. In the present work, a systematical investigation on La₂C₉₆ series including the carbide clusterfullerene form La₂C₂@C₉₄ and the conventional dimetallofullerene form La₂@C₉₆ was implemented by density functional theory, combined with statistical mechanics. Three isomers, i.e., La₂@D₂(191838)-C₉₆, La₂C₂@C_s(153479)-C₉₄, and La₂C₂@C₁(153491)-C₉₄ were disclosed to be thermodynamically stable at the temperature region of endohedral metallofullerene formation. La₂@D₂(191838)-C₉₆ is the prevailing isomer at low temperature, while La₂C₂@C_s(153479)-C₉₄ and La₂C₂@C₁(153491)-C₉₄ are the most and second-most abundant isomers at high temperature. Interestingly, the highest occupied molecular orbital (HOMO) of La₂C₂@C₁(153491)-C₉₄ is distributed on one pole of the cage, and the lowest unoccupied molecular orbital (LUMO) of this isomer is mainly located on the equator of the cage, which can facilitate synthesis of regioselective derivatives. This work will provide useful information for further experimental identification and application of La₂C₉₆.