Zirconium Carbide Clusters Stabilized within Carbon Cages

Zirconium-based endohedral metallofullerenes (EMFs) were first detected in the gas phase in 1992, but their bulk synthesis has remained elusive over the past three decades. Herein, we present the first successful synthesis of Zr-based EMFs, addressing this longstanding challenge in fullerene chemistry. By utilizing an optimized synthesis strategy with zirconium carbide as the metal source, we obtained three Zr-based carbide clusterfullerenes, namely, Zr₂C₂@D_3h(5)-C₇₈, Zr₂C₂@I_h(7)-C₈₀, and Zr₃C₂@I_h(7)-C₈₀. Single-crystal X-ray diffraction analyses revealed that the Zr₂C₂ clusters adopt a planar rhombic configuration bridged by a (C≡C)^2- unit, while the Zr₃C₂ cluster exhibits a rare trifoliate geometry with a bridging (C-C)^6- unit. The coordination modes in these Zr-based clusters are different from those observed in conventional organozirconium complexes, highlighting the unique coordination environments within fullerene cages. Combined experimental and theoretical studies further reveal the electronic structures of the Zr-based EMFs as (Zr₂C₂)⁶⁺@(C_2n)^6- (2n = 78, 80) and (Zr₃C₂)⁶⁺@(I_h(7)-C₈₀)^6-. This breakthrough in the synthesis of Zr-based EMFs not only offers new insights into the mechanisms of arc-discharge reaction and the formation of EMFs but also provides a substantial guideline for the encapsulation of new elements into fullerene cages, establishing a foundation for the design and synthesis of novel EMFs.