UN@C2(13)-C84: a diatomic cluster with a UN triple bond inside a C84 fullerene cage†

A novel monometallic nitride clusterfullerene, UN@C₈₄, has been successfully synthesized and characterized. Single-crystal X-ray diffraction analysis reveals that a rare diatomic UN cluster, one of the smallest known encapsulated clusters, is encapsulated within a carbon cage of C₂(13)-C₈₄, featuring a UN triple bond with a very short bond length of 1.763(10) Å. This bond length remains nearly unchanged compared to previously reported UN@C₈₂ isomers, suggesting that the intrinsic nature of the encapsulated UN triple bond was not affected by cage size. Furthermore, combined with the previous studies about the metal position and the symmetry plane of the carbon cage, we find that in monometallic endohedral clusterfullerenes, the encapsulated metal atom preferentially aligns not only with the symmetry plane but also with the symmetry axis of the fullerene cage, namely the symmetry elements of the carbon cage, thereby preserving the overall molecular symmetry. These results deepen our understanding of positional preference for encapsulated metal atoms and provide valuable guidance for the crystallographic refinement of EMFs, offering new insights into the structural analysis, prediction, and further exploration of novel EMFs.