La–La bonded dimetallofullerenes [La2@C2n]−: species for stabilizing C2n (2n = 92–96) besides La2C2@C2n

Abstract

Recent reports pointed out that the formal La₂C_2n (2n = 92–106) series can exist stably as carbide cluster metallofullerenes (CCMFs) La₂C₂@C_2n−2 with their successful crystallographic characterization. Herein, we suggest that the corresponding dimetallofullerenes (di-EMFs) La₂@C_2n possessing the lowest potential energies are also plausible candidates because of their favorability in statistical thermodynamics. This can be demonstrated in our present theoretical investigations on La₂C₉₄ and previously reported other La₂C_2n (2n = 92, 96–100) series by density functional theory calculations and statistical mechanics analyses. Nevertheless, it was noted that these thermodynamically favorable La₂@C_2n isomers turned out to be kinetically unstable radicals due to the presence of one unpaired electron on the carbon cage, making them missing fullerenes and difficult to be captured in their pristine forms, except for the experimentally obtained La₂@D₅(450)-C₁₀₀ that has no unpaired electron. Such kinetic instability could be modified by electron reduction (the products were denoted as [La₂@C_2n]⁻) or other similar exterior functionalization with ˙CF₃ and benzyl radicals, resulting in La–La bonded and paramagnetic species capable of being captured. On the basis of these approaches, carbon cages D₃(85)-C₉₂, C_s(120)-C₉₄, D₂(186)-C₉₆, and C₂(157)-C₉₆ are predicted to be feasibly captured as both pristine CCMF species and electron reduced di-EMF derivatives.