Transition Metal/Lanthanide-Nitrogen Double Bonds Co-stabilized in a Carbon Cage

Metal-nitrogen double bonds have been commonly reported for conventional metal complexes, but the coexistence of both transition metal-nitrogen and lanthanide-nitrogen double bonds bridged by nitrogen within one compound has never been reported. Herein, by encapsulating a ternary transition metal-lanthanide heteronuclear dimetallic nitride into a C₈₄ fullerene cage, transition metal-nitrogen and lanthanide-nitrogen double bonds are costabilized simultaneously within the as-formed clusterfullerene TiCeN@C₁(12)-C₈₄, which is a representative heteronuclear dimetallic nitride clusterfullerene. Its molecular structure was unambiguously determined by single-crystal X-ray diffraction, revealing a slightly bent μ₂-bridged nitride cluster with short Ti-N (1.761 Å) and Ce-N (2.109 Å) bond lengths, which are comparable to the corresponding Ti=N and Ce=N double bonds of reported metal complexes and consistent with the theoretically predicted values, confirming their coexistence within TiCeN@C₁(12)-C₈₄. Density functional theory (DFT) calculations unveil three-center two-electron (3c-2e) bonds delocalized over the entire TiCeN cluster, which are responsible for costabilization of Ti=N and Ce=N double bonds. An electronic configuration of Ti⁴⁺Ce³⁺N^3-@C₈₄ ^4- is proposed featuring an intramolecular four-electron transfer, drastically different from the analogous actinide dimetallic nitride clusterfullerene (U₂)⁹⁺N^3-@C₈₀ ^6- and trimetallic nitride clusterfullerene (Sc₂)⁶⁺Ti³⁺N^3-@C₈₀ ^6-, indicating the peculiarity of 4-fold negatively charged fullerene cage in stabilizing the heteronuclear dimetallic nitride cluster.