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, CoE(3) (E = S, Se) was constructed inside the bowl-shaped space of pentaaryl[60]fullerene. X-ray crystallographic analysis of CoS(3)(η(5)-C(60)Ar(5)) revealed that the CoS(3) 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 CoS(3) unit was demonstrated by disruption of 6π-conjugation with abstraction of the sulfur atom, affording a dimer, [CoS(2)(η(5)-C(60)Ar(5))](2). This work provides new insight into the aromaticity of four-membered ring systems and advances the understanding of aromatic organometallic compounds.