Isolation of Planar Four-Membered Aromatic Systems by Using Confined Spaces of Cobalt Pentaaryl[60]fullerene Complexes

As a new class of neutral closed-shell aromatic four-membered ring systems, CoE3 (E = S, Se) was constructed inside the bowl-shaped space of pentaaryl[60]fullerene. X-ray crystallographic analysis of CoS3(η5-C60Ar5) revealed that the CoS3 unit is planar, and DFT calculations suggested an aromatic 6π-electron system. Steric protection by the pentaaryl[60]fullerene ligand is critical for isolation and characterization of the aromatic hetero cobaltacyclobutane. Unique reactivity of the CoS3 unit was demonstrated by disruption of 6π-conjugation with abstraction of the sulfur atom, affording a dimer, [CoS2(η5-C60Ar5)]2. This work provides new insight into the aromaticity of four-membered ring systems and advances the understanding of aromatic organometallic compounds.