¹³C and ¹³⁹La NMR Studies of La₂@C₈₀: First Evidence for Circular Motion of Metal Atoms in Endohedral Dimetallofullerenes

Abstract

Endohedral metallofullerenes have attracted special interest as spherical molecules with novel properties that are not expected from empty fullerenes.['] The higher fullerenes can even encapsulate two metal atoms inside the carbon cages to form soluble and relatively air-stable endohedral dimetallofullerenes. Because of the difficulty in producing pure samples in large quantities, experimental characterization of these species has been hindered. Important progress was marked by the successful isolation and purification of endohedral dimetallofullerenes such as Sc2 (iC,,12] and La,@C,,[31 in macroscopic quantities. This has made it possible to investigate redox propertiesr3] and reactivity.14] STM''] and TEMI61 studies were also carried out in an attempt to confirm that two metal atoms are inside the fullerene cages.Since there are many isomers for higher fullerenes, determination of cage structures and symmetries has long been of fundamental interest in disclosing the mechanism of growth. This has been a recent subject of theoretical14. ' * and experimental studies. ['-l o ] Since its first observation in 1991[111 La,@C,, has been widely known as a representative and abundant dimetallofullerene. Despite several attempts, its cage structure and symmetry have not yet been experimentally confirmed. In addition, it is currently of increasing interest whether encapsulated metal atoms are rigidly attached to fullerene cages or move about freely. Although rotation of a molecule within a cage is of great help in designing functional molecular devices,['21 very little has been known about the dynamic behavior of metal atoms encapsulated inside carbon cages. Based on 13C and 13'La NMR spectra of La,@CEO, we now report its structural determination and the first experimental evidence for circular motion of two La atoms.For the C,, fullerene there are seven distinct isomers (D,, D,,, Czv, CZv., D,, D,,, and Zh) that satisfy the isolated-pentagon rule.[131 The isomer distribution has been an open question, since C,, was a "missing" fullerene between C,, and C,, . By isolating C,, and analyzing the observed I3C NMR lines, however, it was verified that the D, isomer is most abundantly produced ( > 90 %) .[I4] It seems plausible to assume that two La atoms are encapsulated inside the D, isomer. However, theoretical calculationsl8I have shown that encapsulation of two La atoms inside the most unstable Z, cage (about 52 kcal mol-' less stable than the D, isomer) is most favorable (63 kcalmol-' more stable than encapsulation inside the D, isomer). This is because the I,,-symmetrical C,, has only two electrons in the fourfold degenerate HOMO and can accommodate six more electrons to form the stable, closed-shell electronic state of (La3'),C:;with a large HOMO-LUMO gap. The most stable endohedral structure optimized with the I, cage is shown in Figure 1. It has D,, symmetry; two La atoms are located equivalently on the C, axis facing the hexagonal rings of C,, with a long La-La distance of 3.655 A.r81 The 139La NMR spectrum of La,@C...

Recent advances in the structural determination of endohedral metallofullerenes

“…By analyzing the 13 C and 139 La NMR spectra observed for La @C , however, we have 2 8 0 recently verified that two La atoms are trapped equivalently inside the I cage. 22 This again gives h strong support to the view that an energetically stable structure with a large HOMO᎐LUMO gap is produced and isolated as a major product even for endohedral metallofullerenes, as is so for empty fullerenes. It appears that cage-like structures with Ž .…”

“…Therefore, the 13 C NMR measurements were carried out at lower temperatures. By decreasing temperature to 258 K, broad 13 C NMR signals were observed which are ascribed to the overlap of 13 13 C NMR lines, 22 as expected from the static D structure 2 h Ž . b1 in Fig.…”

Theory and Calculations of Molecules Containing Heavier Main Group Elements and Fullerenes Encaging Transition Metals: Interplay with Experiment

“…100 The rotation of two La 3+ cations should be reflected in the line width of the 139 La NMR peak because it produces a new magnetic field at the positions of La. This fact was also verified by observing the large broadening of the 139 La NMR line width, which is caused by the interaction between nuclear spins and the produced magnetic field (the so-called spinrotation interaction), with increasing temperature from 305 to 363 K. 100 These constituted the first experimental evidence for the circular motion of metals in a fullerene cage. Such an atomic or molecular rotation in a closed cage may be used as an onoff switch to store and retrieve information.…”

Computations on three isomers of La@C74

“…Let us note that in addition to the conventional RRHO treatment in Eq. (1), a further modified approach for the description of the encapsulate motions can be considered [39], following findings [40] that the encapsulated atoms can exercise large-amplitude motions, especially so at elevated temperatures (unless the motions are restricted by cage derivatizations [41]). One can expect that if the encapsulate is relatively free then, at sufficiently high temperatures, its behavior in different cages will bring about the same contribution to the partition functions.…”