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 planar 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 ReB3- cluster is shown to have a near-pyramidal structure, while ReB4- is found to be a planar pentagonal ring. Chemical bonding analyses show that both ReB4- and ReB4 possess four delocalized π-electrons, including two π-electrons in an orbital of Möbius topology. NICS calculations reveal strong aromatic characters in ReB4- and ReB4, consistent with the 4n electron counting rule for Möbius aromaticity.