Re©B8- and Re©B9-: New Members of the Transition-Metal-Centered Borometallic Molecular Wheel Family.

Transition-metal-centered monocyclic boron wheel clusters (M©B n q) represent a family of interesting borometallic compounds with double aromaticity. A variety of early and late transition metal atoms have been found to form such structures with high symmetries and various B n ring sizes. Here we report a combined photoelectron spectroscopy and quantum-chemistry theoretical study of two M©B n- clusters from the middle of the transition metal series: Re©B8- and Re©B9-. Global minimum structure searches revealed that ReB8- adopts a pseudo- C8 v structure while ReB9- is a perfectly planar D9 h molecular wheel. Chemical bonding analyses showed that both clusters exhibit σ and π double aromaticity and obey the electronic design principle for metal-centered borometallic molecular wheels. The central Re atoms are found to possess unusually low oxidation states of +I in Re©B8- and +II in Re©B9-, i.e., the Re atom behaves similarly to late transition metal elements (Ru, Fe, Co, Rh, Ir) in the M©B n- molecular wheels. These two clusters become new members of the family of transition-metal-centered monocyclic borometallic molecular wheels, which may be viable for chemical syntheses with appropriate ligands.