Alkali-metal coating: an effective method to inject electrons into cage molecules and achieve direct metal–metal bonds and spherical aromaticity for endohedral metallofullerenes

Abstract

Perfluorocubane (C₈F₈) has a hollow cage structure which can store an electron upon electrochemical reduction. Similarly, fullerenes can encapsulate various metallic species inside their cages to form endohedral metallofullerenes (EMFs), and the possible existence of internal metal–metal bonds is important for understanding the nature of the chemical bond. Besides, although the spherical aromaticity for fullerenes has been proposed for decades, it is seldom reported for EMFs. Herein, density functional theory calculations reveal that coating the cage surface with some alkali-metal atoms (e.g., Li, Na, and K) can effectively inject electrons into the cavities of both C₈F₈ fluorocubane and La₂@I_h(7)-C₈₀ dimetallofullerene. Depending on the coating amount, the number of trapped electrons can be easily controlled. One to two electrons are encased by C₈F₈, whereas one to four electrons form a half, a single and even an unprecedented LaLa double bond inside La₂@I_h(7)-C₈₀. Interestingly, Li₁₂&La₂@I_h(7)-C₈₀ and Li₄₂&La₂@I_h(7)-C₈₀ with all the pentagons or rings coated exhibit strong spherical aromaticity complying with Hirsch's 2(N + 1)² (N = 6, 7) electron counting rule. Our work demonstrates that alkali-metal coating is a “one stone three birds” strategy: (1) it can transfer and confine outer alkali-metal electrons into molecular cages; (2) it helps form direct metal–metal bonds of tunable strengths inside fullerenes; and (3) it can be used to achieve rather rare spherically aromatic EMFs with fully closed electronic shells.