Recent progress in the chemistry of endohedral metallofullerenes

Abstract

Putting metal atoms or metallic clusters into fullerenes has generated a new class of hybrid molecules, defined as endohedral metallofullerenes (EMFs), possessing novel structures and fascinating properties which are different from those of empty fullerenes. In particular, it has been revealed that the chemical properties of the cage carbons of EMFs depend strongly on the nature of the internal metallic species, such as their electronic configuration, location and even motion. Since the first report describing the successful derivatization of La@C82 in 1995, great efforts have been devoted to the chemical modification of EMFs during the last two decades. Although earlier studies mainly focused on readily available species such as M@C82, M2@C80 and M3N@C80 and the related results have been systematically summarized in our previous review paper (Chem. Commun., 2011, 47, 5942-5957), recent concerns about some relatively rare EMFs have developed rapidly. Moreover, taking advantage of single crystal X-ray crystallography, we can now clearly demonstrate the mutual influences between the internal metallic species and the chemical behaviours of the surrounding cage carbons, and the addends as well. In this article, we present recent achievements in the chemical functionalization of EMFs, which were mainly published during the last four years. For consistency, we will still pay special attention to the role that the metals play in controlling the properties of the whole EMF molecules. In this review, however, we will not only focus on concrete experimental results such as X-ray crystallographic and NMR spectroscopic data but will also include computational studies which have indeed enhanced our understanding of the chemical properties of EMFs. Applicable materials based on EMFs are also mentioned but are not discussed in detail.

Crystallographic Evidence for Direct Metal–Metal Bonding in a Stable Open‐Shell La2@Ih‐C80 Derivative

“…[2] Surprisingly,o nly one benzyl moiety is attached to the cage,i ns harp contrast to the addition of two benzyl radicals to Sc 3 N@I h -C 80 . [2] Surprisingly,o nly one benzyl moiety is attached to the cage,i ns harp contrast to the addition of two benzyl radicals to Sc 3 N@I h -C 80 .…”

“…[1][2][3][4] Thec harge transfer from the internal unit to the fullerene cage gives rise to the important properties of EMFs.Based on the charge localized on the fullerene cage,two types of EMFs could be identified:o pen-shell EMFs with an unpaired electron residing on the cage or closed-shell EMFs containing only paired electrons.S ignificantly,o pen-shell EMFs have ahigh reactivity toward radical reactions because of their own radical character.F or instance,several insoluble mono-EMFs such as La@C 2n (2n = 72, 74, 80, 82) [5][6][7][8][9] were solubilized during a1 ,2,4-trichlorobenzene extraction process.T he discovery of these insoluble EMFs was achieved because these highly reactive,o pen-shell EMFs can be transformed into stable,c losed-shell derivatives through radical coupling reactions with dichlorophenyl radicals.A dditionally,t he reaction between La@C 82 and toluene [10] also revealed that the open-shell La@C 82 is able to convert into ac losed-shell derivative.S imilarly,a nu nexpected NO 2 radical addition occurred when anthranilic acid and isoamyl nitrite reacted with La@C 82. [1][2][3][4] Thec harge transfer from the internal unit to the fullerene cage gives rise to the important properties of EMFs.Based on the charge localized on the fullerene cage,two types of EMFs could be identified:o pen-shell EMFs with an unpaired electron residing on the cage or closed-shell EMFs containing only paired electrons.S ignificantly,o pen-shell EMFs have ahigh reactivity toward radical reactions because of their own radical character.F or instance,several insoluble mono-EMFs such as La@C 2n (2n = 72, 74, 80, 82) [5][6][7][8][9] were solubilized during a1 ,2,4-trichlorobenzene extraction process.T he discovery of these insoluble EMFs was achieved because these highly reactive,o pen-shell EMFs can be transformed into stable,c losed-shell derivatives through radical coupling reactions with dichlorophenyl radicals.A dditionally,t he reaction between La@C 82 and toluene [10] also revealed that the open-shell La@C 82 is able to convert into ac losed-shell derivative.S imilarly,a nu nexpected NO 2 radical addition occurred when anthranilic acid and isoamyl nitrite reacted with La@C 82.…”

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

“…[3,4] These compounds have generated considerable recent research interest because of their potentiala pplicationsi nt he field of magneticr esonance imaging agents, molecular electronic devices, and solar cells. [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.…”

A systematic study of rare gas atoms encapsulated in small fullerenes using dispersion corrected density functional theory

“…Prime examples are the encapsulation of H 2 O inside C 60 , heavy atoms such as Ba in C 80 or more recently Pb in C 80 , endohedral fullerenes containing neon, argon, or heavier rare gas atoms, metal clusters such as La 2 in C 72 , or even another fullerene cage forming so‐called buckyonions with C 20+2 m @C 20+2 n ( m << n , m ≠ 1) . For a recent comprehensive review on endohedral fullerenes see Popov et al or Lu et al…”