Stabilization of the Benzene Radical Trianion in an Inverse-Sandwich Yttrium Complex

Herein, the first report on the isolated and unambiguously proven benzene radical trianion is presented. This unprecedented radical oxidation state of benzene is stabilized through two trivalent rare earth (RE) metal cations each supported by a bis(guanidinate) scaffold. Specifically, the one-electron chemical reduction of the neutral inverse-sandwich yttrium complex [[{(Me₃Si)₂NC(NⁱPr)₂}₂Y]₂(μ-ƞ⁶:ƞ⁶-C₆H₆)] 1, containing a benzene dianion, with potassium graphite (KC₈) in the presence of [2.2.2]-cryptand yielded the title complex [K([2.2.2]-cryptand)][[{(Me₃Si)₂NC(NⁱPr)₂}₂Y]₂(μ-ƞ⁶:ƞ⁶-C₆H₆ ^•)] 2, featuring a benzene radical trianion. Analyses through single-crystal X-ray diffraction, EPR and UV-vis spectroscopy, elucidated its molecular structure and revealed strong [Y^III-(C₆H₆)^3-•-Y^III] metal-radical interactions. Although the Y centers remain in the +3 oxidation state, the spin density of the unpaired electron resides primarily on the benzene trianion moiety and extends toward the Y^III ions. Density functional theory (DFT) calculations on 2 corroborate this assignment and further suggest weak aromaticity for the benzene radical trianion.