Preferential encapsulation and stability of La(3)N cluster in 80 atom cages: experimental synthesis and computational investigation of La(3)N@C(79)N

Abstract

We report the synthesis and electronic stabilization of La(3)N@C(79)N. Unsuccessful efforts to encapsulate bulky La(3)N clusters in small C(80) cages have been attributed to large ionic radii. The preferred species for La(3)N clusters in all-carbon cages is La(3)N@C(96). A surprising finding is the synthesis of La(3)N@C(79)N, a new metallofullerene present in higher abundance than La(3)N@C(96). This reduction in cage size from 96 to 80 atoms reflects the significance and role of electronic effects. To understand the geometric and electronic properties of this first metallic nitride azafullerene (M(3)N@C(79)N, M = La), density functional theory (DFT) investigations were performed on a number of isomers. Results indicate a preferred N-substitution at the 665 junction site on the cage in lieu of a 666 substitution. The relative stabilities of different isomers can be well reproduced by using the minimum distance between the metal atom and the nitrogen atom of the cage (R(N'M)(min)). Long R(N'M)(min) values indicate distant contacts between six atoms that bear significantly large positive charges: the three metal atoms and the three carbon atoms bonded with the nitrogen atom in the cage, which are favored. These results suggest a dominant electronic effect on the stabilities of metalloazafullerenes. Interestingly, spin densities of the 665 substitution isomers of La(3)N@C(79)N are located predominantly in the metal cluster, while spin densities of the 666 substitution isomers are primarily on the cage.

Figure 1

Overview of preferred cages for M₃N clusters with C₈₀ (blue), C₈₈ (green), and C₉₆ (yellow). The preferred 80 atom cage for La₃N@C₇₉N is circled.

Figure 2

MALDI mass spectral data of soot extract obtained under CAPTEAR conditions.

Figure 3

Isosurface representations of spin densities of C₇₉N isomers 1–2 and La₃N@C₇₉N isomers 3–8 in A–H, respectively (contour values = ± 0.004 au). Atom color schemes are as follows: cyan – C, blue – N, and red – La.

Figure 4

A. Predicted stabilities from eqn 1 vs. DFT results. B. Computed relative stabilities using high level methods vs. those from low-level calculations.

Figure 5

Isosurface representations (A–F) of HOMOs in La₃N@C₇₉N isomers 3–8, respectively (contour values = ± 0.04 au).