Preferential Formation of Mono-Metallofullerenes Governed by the Encapsulation Energy of the Metal Elements: A Case Study on Eu@C2n (2n=74-84) Revealing a General Rule

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)-C-74, Eu@C-2v(19138)-C-76, Eu@C-2v(3)-C-78, Eu@C-2v(3)-C-80, and Eu@D-3d(19)-C-84, which are identified by crystallography. Remarkably, Eu@C-2v(19138)-C-76 represents the first Eu-containing EMF with a cage that violates the isolated-pentagon-rule, and Eu@C-2v(3)-C-78 is the first C-78-based EMF stabilized by merely one metal atom.