σ-Aromaticity in an Unsaturated Ring: Osmapentalene Derivatives Containing a Metallacyclopropene Unit

In general, aromaticity can be clarified as - and sigma-aromaticity according to the type of electrons with major contributions. The traditional -aromaticity generally describes the -conjugation in fully unsaturated rings whereas sigma-aromaticity may stabilize fully saturated rings with delocalization caused by sigma-electron conjugation. Reported herein is an example of sigma-aromaticity in an unsaturated three-membered ring (3MR), which is supported by experimental observations and theoretical calculations. Specifically, when the 3MR in cyclopropaosmapentalene is cleaved by ethane through two isodesmic reactions, both of them are highly endothermic (+29.7 and +35.0kcalmol(-1)). These positive values are in sharp contrast to the expected exothermicity, thus indicating aromaticity in the 3MR. Further nucleus-independent chemical shift and anisotropy of the current-induced density calculations reveal the nature of sigma-aromaticity in the unsaturated 3MR.