Decisive role of non-rare earth metals in high-regioselectivity addition of μ3-carbido clusterfullerene

Abstract

Endohedral clusterfullerenes featuring encapsulation of metal clusters which transfer electrons to the outer fullerene cages show intriguing chemical properties different from empty fullerenes. Despite the extensive studies on the chemical properties of empty fullerenes, especially C₆₀, chemical functionalization of endohedral clusterfullerenes has been less explored, and previous reports are primarily limited to the well-known metal nitride and carbide clusterfullerenes. Herein, we report the first chemical functionalization of an emerging endohedral clusterfullerene μ₃-carbido clusterfullerene (abbreviated as μ₃-CCF) bearing central μ₃-C and Ti(IV) atoms forming a TiC double bond. A μ₃-CCF Dy₂TiC@I_h-C₈₀ is synthesized, and its molecular structure is unambiguously determined by single-crystal X-ray diffraction for the first time. A photochemical cycloaddition reaction of Dy₂TiC@I_h-C₈₀ with 2-adamantane-2,3-[3H]-diazirine (abbreviated as AdN₂) is then carried out and only one monoadduct Dy₂TiC@I_h-C₈₀-Ad is obtained, indicating its high regioselectivity. According to the X-ray single-crystal structure of Dy₂TiC@I_h-C₈₀-Ad, the Ad moiety selectively attacks the [6,6]-bond (conjunction of two fused hexagons), which is adjacent to the Ti⁴⁺ ion instead of the two Dy³⁺ ions, affording a [6,6]-open addition pattern. Theoretical calculations unveil that the Ti(IV) ion plays a decisive role in high regioselectivity, and the formation of [6,6]-open Dy₂TiC@I_h-C₈₀-Ad is thermodynamically preferred. Contrarily, a similar reaction of a Ti(III)-containing nitride clusterfullerene Y₂TiN@C₈₀ with AdN₂ is predicted to generate a different type of adduct with the addition sites adjacent to the Y³⁺ ion instead of the Ti³⁺ ion. This reveals the peculiarity of the chemical property of μ₃-CCF resulting from the existence of the non-rare earth metal Ti with a high oxidation state.