Preferential Formation of Mono‐Metallofullerenes Governed by the Encapsulation Energy of the Metal Elements: A Case Study on Eu@C_2<i>n</i> (2<i>n</i>=74–84) Revealing a General Rule

Abstract

Encapsulating one to three metal atoms or a metallic cluster inside fullerene cages affords endohedral metallofullerenes (EMFs) classified as mono-, di-, tri-, and cluster-EMFs, respectively. Although the coexistence of various EMF species in soot is common for rare-earth metals, we herein report that europium tends to prefer the formation of mono-EMFs. Mass spectroscopy reveals that mono-EMFs (Eu@C_2n ) prevail in the Eu-containing soot. Theoretical calculations demonstrate that the encapsulation energy of the endohedral metal accounts for the selective formation of mono-EMFs and rationalize similar observations for EMFs containing other metals like Ca, Sr, Ba, or Yb. Consistently, all isolated Eu-EMFs are mono-EMFs, including Eu@D_3h (1)-C₇₄ , Eu@C_2v (19138)-C₇₆ , Eu@C_2v (3)-C₇₈ , Eu@C_2v (3)-C₈₀ , and Eu@D_3d (19)-C₈₄ , which are identified by crystallography. Remarkably, Eu@C_2v (19138)-C₇₆ represents the first Eu-containing EMF with a cage that violates the isolated-pentagon-rule, and Eu@C_2v (3)-C₇₈ is the first C₇₈ -based EMF stabilized by merely one metal atom.