No abstract

Fullerenes are carbon cages assembled from fused hexagons andpentagons that have closed networks and conjugated π systems. The curve of the fullerene structure requires that the constituent carbon atoms take on a pyramidal shape and produces extra strain energy. However, the highly symmetrical geometry of the fullerene decreases the surface tension in these structures, so highly symmetrical fullerenes are usually very stable. For example, C60 with icosahedral symmetry (Ih) is the most stable fullerene molecule. However, another highly symmetrical fullerene, Ih-C80, is extremely unstable. The reason for this difference is the open-shell electronic structure of Ih-C80, which has a 4-fold degenerate HOMO occupied by only two electrons. Predictably, once the degenerate HOMO of Ih-C80 accepts six more electrons, it forms a closed-shell electronic structure similar to Ih-C60 and with comparable stability. Because the hollow structure of fullerenes can encapsulate metal atoms and those internal metals can transfer electrons to the fullerene cage, the encapsulation of metal clusters may provide an ideal technique for the stabilization of the Ih-C80 fullerenes. In this Account, we focus on the molecular structures and paramagnetic properties of spherical Ih-C80 endohedral fullerenes encaging a variety of metal moieties, such as metal atoms (Mn), metal nitride (M3N), metal carbide (MnC2), metal carbonitride (M3CN), and metal oxides (M4Om). We introduce several types of endohedral metallofullerenes such as Sc4C2@Ih-C80, which exhibits a Russian-doll-like structure, and Sc3CN@Ih-C80, which encapsulates a planar metal carbonitride cluster. In addition, we emphasize the paramagnetic properties of Ih-C80-based metallofullerenes, such as Sc3C2@Ih-C80, Y2@C79N, and M3N@Ih-C80, to show how those spin-active species can present a controllable paramagnetism. This Account highlights an inspiring molecular world within the spherical Ih-C80 cages of various metallofullerenes.

Fullerenes have attracted a great deal of attention since their discovery [1] because of their unique physical and chemical properties and potential applications.[2] Many applications require the deposition of fullerenes onto a variety of surfaces including metals [3][4][5][6] or semiconductors. [7] The fabrication of ordered arrays of fullerenes on solid surfaces is of interest from both scientific and technological viewpoints. It has been demonstrated that fullerenes can form well-ordered arrays on metal or semiconductor surfaces under ultrahigh vacuum (UHV) conditions at low temperatures. [8][9][10][11][12] The arrangement is determined by the size and symmetry of the fullerenes and rigidly maintains its shape. On the other hand, C 60 is highly mobile at room temperature (RT) even when adsorbed on metal surfaces, and has no preferential orientation unless cooled to low temperatures. [13][14][15][16][17][18][19][20] Therefore, a well-decorated surface, which serves as a molecular template and provides binding sites, is important and necessary for the fabrication of highly ordered arrays of fullerenes at RT. Open porous networks obtained by metal-organic coordination, [21] the formation of hydrogen bonds, [8] or even van der Waals interactions [22][23][24] have been successfully used to direct the formation of ordered fullerene arrays. In this way the interfullerene distance and symmetry of the arrangement are solely determined by the molecular template and can be changed by adjusting its structure. More importantly, interactions between fullerenes can also be modified with this strategy.We have designed and synthesized a tetraacidic azobenzene molecule NN4A (Figure 1a) that exclusively forms KagomØ open networks with two types of cavities that have different size and symmetry at the liquid-solid interface. These cavities are capable of accommodating fullerene molecules as guest species. Herein, we examine the site selectivity of the networks for different fullerenes, a feature which has not been observed for other systems at liquid-solid interfaces. Azobenzene derivatives are typical photochromic compounds that have a wide range of potential applications, including for optical switching, holographic storage, light harvesting, long-term energy storage, and nonlinear optical materials. [25,26] The attractive photosensitive properties of azobenzene compounds have resulted in them being utilized as photoswitching units to control the structure and function of supramolecular systems. [27][28][29][30][31] Thus, the use of azobenzene units in the host matrix could allow for further control of the host-guest architectures.When deposited onto a graphite surface, NN4A forms a well-ordered open network with a KagomØ structure which is correlated to molecular geometry and network symmetry. [32,33]