Stability and structures of La, Y, and Sc endohedral metalloazafullerenes: the role of cage topology, N-doping site, and metal

Abstract

Endohedral metalloazafullerenes (EMAFs) are a distinctive class of fullerene derivatives characterized by the encapsulation of metal atoms or clusters within azafullerene cages. These intriguing nanomaterials exhibit unique properties with potential applications in quantum computing, molecular magnets, and optoelectronics. However, due to the experimental characterization limitations and the structural complexity that complicates computational studies, reliably identifying the molecular structures of EMAFs remains a challenging task. Moreover, the factors influencing their stability, such as cage topology, nitrogen doping sites, and encapsulated metal species, are not yet well understood. In this study, we employ density functional theory (as high as BP86/Def2-QZVP) to systematically investigate the stability and structures of monometallic EMAFs, M@C_2n–1N (M = La, Y, Sc; 2n = 82, 84, 80, 72), focusing on the interplay between cage size and isomerism, nitrogen substitution, and metal encapsulation. We demonstrate that the experimentally observed La@C₈₁N-C_3v(8) structure corresponds to the most thermodynamically stable isomer. We further predict that all EMAFs studied exhibit significantly negative formation free energies, suggesting they are promising synthetic targets, particularly La@C₈₃N-D_2d(23) and La@C₇₉N-D_5h(6). Our results show that larger cage sizes, La encapsulation, and nitrogen substitution at pentagon-rich sites enhance the stability of monometallic EMAFs. These observations can be explained using simple electrostatic models and the topological charge stabilization rule. Our findings not only deepen the understanding of EMAF chemistry but also provide valuable insights for the design of EMAF-based functional materials with engineered electronic and magnetic properties.