π-Aromatic Bi5– – The Heaviest Analogue of Cyclopentadienide (C5H5)– Finally Captured in the Mixed-Valent Compound [{IMesCo}2(μ,η5:η5-Bi5)]

Abstract 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. They stand out through their specific way of overcoming electron deficiency as planar organocycles comprising a non-localizable set of (4n+2)π electrons.1 By contrast, all-metal aromatic monocycles with all C atoms replaced by metal atoms are still rare, as metal atoms prefer to form clusters instead of cycles with multiply-bonded atoms, making them a huge synthetic challenge.2–4 This limits the knowledge and potential of corresponding compounds in chemical syntheses or for innovative materials. Here, we show the first successful generation of Bi5–, the heaviest analogue of the aromatic ligand (C5H5)–. Its use as ligand in [{IMesCo}2(µ,η5:η5-Bi5)] (1) was realized by reacting (TlBi3)2– with [(IMes)2CoCl] (IMes: bis(1,3-(2,4,6-trimethylphenyl))imidazol-2-ylidene) in ortho-difluorobenzene. Compound 1 is mixed-valent (Co0/CoI) as verified by µ-SQUID measurements and DFT, and perfectly 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. Immediate implications include the systematic development of new complexes featuring Bi5– and its derivatives as innovative bismuth-based materials.