Observation of Möbius Aromatic Planar Metallaborocycles.

Möbius aromaticity was developed for twisted annulenes with electron counting rules opposite to those of Hückel aromaticity. The introduction of transition metals makes it possible for cyclic systems to exhibit Möbius aromaticity. Here we report the first planar monocyclic metallaboron systems with Möbius aromaticity. The structures and bonding of two rhenium-boride clusters are studied by high-resolution photoelectron imaging and ab initio calculations. The ReB cluster is shown to have a near-pyramidal structure, while ReB is found to be a planar pentagonal ring. Chemical bonding analyses show that both ReB and ReB possess four delocalized π-electrons, including two π-electrons in an orbital of Möbius topology. NICS calculations reveal strong aromatic characters in ReB and ReB, consistent with the 4 electron counting rule for Möbius aromaticity.