Heptadentate, Octadentate, Or Even Nonadentate? Denticity in the Unexpected Formation of an All-Carbon Donor-Atom Ligand in Rh^III(Cp*)(Anthracenyl-NHC) Complexes

Investigations on incorporating an N-flanking anthracenyl moiety to [Rh­(Cp*)­(NHC)­Cl₂] complexes surprisingly led to the formation of an intramolecular C–C bond between the Cp* and anthracenyl moieties, with additional auxiliary interactions between the metal and the anthracenyl ring system. In silico modeling supports a reaction mechanism whereby Rh­(η⁴-tetramethylfulvene) intermediates undergo metallocycloaddition and the abstraction of a chlorido ligand, affording unique cationic complexes that feature Rh centers coordinated by a nonadentate ligand with exclusively carbon donor atoms. Some Rh–C interactions were extremely weak but nevertheless exhibited covalent bonding character. These weak Rh–C interactions were readily displaced by stronger electron donors, and the nonadentate ligand reverted to the heptadentate coordination mode observed in the intermediate. As far as we are aware, this study provides the first conclusive evidence of complexes bearing a single nonadentate κ⁹-coordinating ligand that features only carbon donors bound to a metal center.