Diatomic C₂/NC Ligand Induced Conformation Variation of Trimetallic Clusters within a Carbon Cage

Understanding the interplay between conformation and electronic properties of metal complexes at the atomic level is the key for rational design of new functional molecules. Trimetallic clusterfullerenes (TMCFs) encapsulating quinary M₃C₂ carbide or M₃NC carbonitride clusters offer an ideal model system for elucidating conformation-electronic property correlation due to its unusual conformational versatility. Herein, we synthesize and isolate two novel lanthanide-transition metal heteronuclear TMCFs, namely, CeTi₂C₂@I(7)-C₈₀ and CeTi₂NC@I(7)-C₈₀. Crystallographic studies reveal singly bonded C₂/NC ligands coordinating vertically to the CeTi₂ trimetallic plane within the cluster, drastically different from the bat-ray conformations reported for all homonuclear TMCFs. Such bonding characters give rise to variable electronic structures of [Ce⁴⁺(Ti₂)⁸⁺(C₂)^6-]⁶⁺@C₈₀^6- and [Ce³⁺(Ti₂)⁸⁺(NC)^5-]⁶⁺@C₈₀^6-, featuring a fixed tetravalent oxidation state of Ti and tunable oxidation states of Ce (Ce⁴⁺/Ce³⁺), as confirmed by X-ray absorption spectroscopy and magnetic measurements in combination with theoretical studies. Furthermore, scrutinizing all reported TMCFs bearing M₃C₂/M₃NC clusters, we find that the conformations of trimetallic carbide/carbonitride clusters are precisely dictated by the bonding nature of the diatomic C₂/NC ligand so as to suffice strong coordination interactions with the encapsulated metals. Following such a general rule governing conformational variation, a very strong ligand field is achieved on Ce³⁺ in the presence of a singly bonded NC ligand in CeTi₂NC@C₈₀, giving rise to the first Ce-based single-molecule magnet that shows an open magnetic hysteresis at 2 K.