Isolation of a planar π-aromatic Bi5− ring in a cobalt-based inverse-sandwich-type complex

Monocyclic π-aromatic compounds are ubiquitous throughout almost all fields of natural sciences—as synthons in industrial processes, as ligands of metal complexes for catalysis or sensing and as bioactive molecules. Planar organocycles stand out through their specific way of overcoming electron deficiency by a non-localizable set of (4n + 2)π electrons. By contrast, all-metal aromatic monocycles are still rare, as metal atoms prefer to form clusters with multiply bonded atoms instead. This limits the knowledge and potential of corresponding compounds in chemical syntheses or for innovative materials. Here we report the successful generation of Bi₅⁻, the heaviest analogue of (C₅H₅)⁻. Its use as a ligand in [{IMesCo}₂(µ,η⁵:η⁵-Bi₅)] (1) was realized by reacting (TlBi₃)²⁻ with [(IMes)₂CoCl] (where IMes is bis(1,3-(2,4,6-trimethylphenyl))imidazol-2-ylidene) in ortho-difluorobenzene. Compound 1 is mixed-valence Co⁰/Co^I as verified by µ-SQUID measurements and density functional theory, and embeds the planar Bi₅⁻ cycle in an inverse-sandwich-type manner. Capturing Bi₅⁻ represents a landmark in the chemistry of all-metal aromatic molecules and defines a new era for aromatic compounds.