Trimetallic Nitride Template Endohedral Metallofullerenes: Discovery, Structural Characterization, Reactivity, and Applications

Abstract

Shortly after the discovery of the carbon fullerene allotrope, C₆₀, researchers recognized that the hollow spheroidal shape could accommodate metal atoms, or clusters, which quickly led to the discovery of endohedral metallofullerenes (EMFs). In the past 2 decades, the unique features of EMFs have attracted broad interest in many fields, including inorganic chemistry, organic chemistry, materials chemistry, and biomedical chemistry. Some EMFs produce new metallic clusters that do not exist outside of a fullerene cage, and some other EMFs can boost the efficiency of magnetic resonance (MR) imaging 10-50-fold, in comparison with commercial contrast agents. In 1999, the Dorn laboratory discovered the trimetallic nitride template (TNT) EMFs, which consist of a trimetallic nitride cluster and a host fullerene cage. The TNT-EMFs (A₃N@C2n, n = 34-55, A = Sc, Y, or lanthanides) are typically formed in relatively high yields (sometimes only exceeded by empty-cage C₆₀ and C₇₀, but yields may decrease with increasing TNT cluster size), and exhibit high chemical and thermal stability. In this Account, we give an overview of TNT-EMF research, starting with the discovery of these structures and then describing their synthesis and applications. First, we describe our serendipitous discovery of the first member of this class, Sc₃N@Ih-C₈₀. Second, we discuss the methodology for the synthesis of several TNT-EMFs. These results emphasize the importance of chemically adjusting plasma temperature, energy, and reactivity (CAPTEAR) to optimize the type and yield of TNT-EMFs produced. Third, we review the approaches that are used to separate and purify pristine TNT-EMF molecules from their corresponding product mixtures. Although we used high-performance liquid chromatography (HPLC) to separate TNT-EMFs in early studies, we have more recently achieved facile separation based on the reduced chemical reactivity of the TNT-EMFs. These improved production yields and separation protocols have allowed industrial researchers to scale up the production of TNT-EMFs for commercial use. Fourth, we summarize the structural features of individual members of the TNT-EMF class, including cage structures, cluster arrangement, and dynamics. Fifth, we illustrate typical functionalization reactions of the TNT-EMFs, particularly cycloadditions and radical reactions, and describe the characterization of their derivatives. Finally, we illustrate the unique magnetic and electronic properties of specific TNT-EMFs for biomedicine and molecular device applications.

An atlas of endohedral Sc2S cluster fullerenes

“…To date, more than two hundred EMFs have been characterized, including mono-metallofullerenes, dimetallofullerenes, trimetallofullerenes, and fullerenes incorporating clusters various kinds [4]. Amongst the EMFs, those in which the encapsuland is a metallic cluster have attracted attention because of their novel geometries and properties [5,6]. However, as far as we are aware, the mechanism of formation of EMFs has not yet been clarified, even though so many EMFs have been reported.…”

Sc2O@Td(19151)‐C76: Hindered Cluster Motion inside a Tetrahedral Carbon Cage Probed by Crystallographic and Computational Studies

“…[5][6][7][8][9] Since the discovery of Sc 3 N@C 80 in 1999, [10] cluster fullerenes (CFs) have been of particular interest in the field of endohedral fullerenes as ar esult of their exceptionally high yield and excellent stabilitya nd the unique structural variety of both clusters and cages, which resultsi nt heir tunable physical and chemical properties. [11][12][13][14] To date,avariety of cluster fullerenes have been reported, including nitride (NCFs), [15][16][17] carbide( CCFs), [18,19] hydrocarbide (HCCFs), [20] oxide (OCFs), [21,22] sulfide( SCFs), [23][24][25] carbonnitride (CNCFs), [26,27] and di/trimetallic clusterf ullerenes. [28,29] In particular,t he OCF family has demonstratede xceptional structuralv ersatility of the encapsulated oxide clusters, whichh ave includedS c 2 O, [21] Sc 4 O 2 , [22] and Sc 4 O 3 [30] structures.I ncontrast, the reported cage structures of this family have been limited to I h (7)-C 80 , C s (6)-C 82 ,a nd the very recent C 2 (7892)-C 70 cages.…”

Bottom-up formation of endohedral mono-metallofullerenes is directed by charge transfer

“…It is generally established that a plasma serves as the starting point for synthesis by the electric arc discharge and laser vapourization techniques 7,10,12 . In fact, a distinct carbon-metal vapour (plasma) plume is physically observable during graphite evaporation 13 . Notably, one of the most vigorously exploited EMFs, M@C 82 (M ¼ metal), has been macroscopically synthesized by the use of both methods since the dawn of metallofullerene science 12,14 .…”