Be atoms in fullerenes: strong metal-cage covalency with formation of Be-Be banana bonds, multicenter bonds and trapped superatoms

A genuine Be-Be bond does not exist in the Be2 dimer due to the formal zero bond order predicted by the molecular orbital theory. Endohedral metallofullerenes (EMFs) have recently been used to obtain some elusive metal-metal bonds involving rare-earth and actinide elements via the cage encapsulation, but their potential in stabilizing the Be-Be bond still remains unexplored thus far. In this work, density functional theory calculations were carried out to investigate various fullerenes with different numbers of Be atoms inside, including Be@C2n (2n = 24, 32, 60, 74) mono-EMFs, Be2@C2n (2n = 60, 74) di-EMFs and Bex@C60 (x = 3-14) multi-EMFs. They mostly could experimentally form due to the large encapsulation energies, sizable HOMO-LUMO gaps and feasible metal insertion energy barriers. The obvious intramolecular charge transfer makes their metal-cage interactions mainly ionic but with substantial covalency even comparable to the actinide EMFs. Interestingly, rare two-center two-electron (2c-2e) Be-Be bonds with banana characteristics form inside the di-EMFs, and the bond length and bond shape can be flexibly regulated by simply changing the cage sizes. When multiple Be atoms are encased, they could form distinct multicenter bonds and simultaneously exhibit obvious superatomic characteristics, thus significantly enhancing the system stability. By finely tuning the electron number on internal metals, entrapped magic clusters such as the Be13 unit in (Be13@C60)4- anion obeying the jellium shell model (1S21P61D10) can be readily obtained. Our study unprecedentedly unveils the intriguing metal-cage and metal-metal interactions in the long overlooked Be-based EMFs. It not only demonstrates the great potential of fullerenes for achieving unusual and tailorable chemical bonds, but also introduces them as a novel platform to obtain more superatom clusters with new structures and properties.