A missing link in the transformation from asymmetric to symmetric metallofullerene cages implies a top-down fullerene formation mechanism

Abstract

Although fullerenes were discovered nearly three decades ago, the mechanism of their formation remains a mystery. Many versions of the classic 'bottom-up' formation mechanism have been advanced, starting with C 2 units that build up to form chains and rings of carbon atoms and ultimately form those well-known isolated fullerenes (for example, I h -C 60 ). In recent years, evidence from laboratory and interstellar observations has emerged to suggest a 'top-down' mechanism, whereby small isolated fullerenes are formed via shrinkage of giant fullerenes generated from graphene sheets. Here, we present molecular structural evidence for this top-down mechanism based on metal carbide metallofullerenes M 2 C 2 @C 1 (51383)-C 84 (M 5 Y, Gd). We propose that the unique asymmetric C 1 (51383)-C 84 cage with destabilizing fused pentagons is a preserved 'missing link' in the top-down mechanism, and in well-established rearrangement steps can form many wellknown, high-symmetry fullerene structures that account for the majority of solvent-extractable metallofullerenes.T he discovery of fullerenes 1 has opened up new vistas in nanoscience, and metallofullerenes have shown great promise in photovoltaic 2 and biomedical 3 applications. However, their formation mechanism remains unclear. Various versions of the 'bottom-up' mechanism have been posited as the main pathway for fullerene formation 4,5 , suggesting that fullerenes are formed by consecutively adding C 2 units to small carbon nanoclusters and cages. In recent years, with the development of graphene 6 , evidence has emerged to suggest a 'top-down' mechanism, whereby fullerene cages are formed via shrinkage of giant fullerene structures generated from graphene. One piece of direct evidence for the topdown mechanism is the reported laboratory transformation from graphene to fullerene 7 . It has also been demonstrated that fullerenes can be pyrolysed and lose carbon atoms to form smaller fullerenes at high temperature in an argon stream 8 . Another strong supporting argument for the top-down mechanism is that fullerenes are formed in the interstellar medium 9 by a photochemical process in which graphene sheets curve and lose C 2 and other fragments 10 . However, evidence for the top-down mechanism has not been demonstrated at the molecular level. Here, we present NMR spectroscopic and X-ray crystallographic structural characterization for M 2 C 2 @C 1 (51383)-C 84 (M ¼ Y, Gd) metallofullerenes. These molecules have a unique asymmetric cage C 1 (51383)-C 84 that represents the first characterized preserved intermediate suggesting top-down formation of fullerenes and metallofullerenes.The proposed top-down mechanism is illustrated in Fig. 1. Under appropriate conditions, graphene sheets can spontaneously roll and warp to form other nanostructures 11,12 , including randomly formed giant closed carbon networks-the primitive fullerene cages 7 . In an important computational study, a 'shrinking hot giant fullerene' mechanism that leads to smaller fullerene structures has been advan...

Interstellar fullerene compounds and diffuse interstellar bands

“…Dunk et al 2012;and Zhang et al 2013, for updated references). The general prominence of even numbers of carbon atoms, N C , and the more or less specific prominence of C 60 , C 70 , and then other "magic" numbers, such as C 50 and C 32 , are explained by stability arguments, especially by minimizing the number of adjacent pentagons in the cage structure (see e.g.…”

Breakdown Products of Gaseous Polycyclic Aromatic Hydrocarbons Investigated With Infrared Ion Spectroscopy

“…Most have invoked formation from carbon atoms or small carbon clusters, chains, or rings, so-called bottom-up mechanisms (Kroto & McKay 1988;Heath 1992;Hunter et al 1994;Dunk et al 2013). But recently a top-down approach has been suggested (Chuvilin et al 2010;Zhang et al 2013). First seen in the aberration-corrected transmission electron microscopy of graphene (Chuvilin et al 2010), these experiments showed directly the loss of edge carbon atoms by the electron beam, reconstruction (i.e., formation of carbon pentagons) at that site, curling of the sheet, and ultimate creation of the fullerene.…”

An atlas of endohedral Sc2S cluster fullerenes

“…Molecular dynamics simulations have shown that hot giant fullerenes can lose or gain carbon in high temperature conditions [45]. This existence of a structural web such as shown in Fig.…”