Inverse-Sandwich Rare Earth Metal Complexes Comprising a Planar Toluene Dianion

We report the first inverse-sandwich complexes containing rare earth (RE^III) metal ions that captured a toluene dianion between them. Toluene-bridged complexes [{(Me₃Si)₂NC(NⁱPr)₂}₂RE]₂(μ-η⁶:η⁶-C₆H₅Me) (RE = Y (1), Dy (2), and Er (3)) were synthesized via chemical reductions of chloride-bridged RE complexes in which each tripositive metal is stabilized by two guanidinate ligands. Compounds 1-3 were unambiguously characterized by crystallography, NMR, UV-vis, and IR spectroscopy, magnetometry, and computations. The bond metrics from single-crystal X-ray diffraction analysis revealed a planar, cyclohexadienediide-like structure for the ligated arene, indicative of a dianionic toluene. The ¹H NMR spectrum of 1 exhibits upfield-shifted resonances representing increased shielding from excess electrons, further validating its dianionic nature. DFT calculations afforded similar bond metrics, and natural bond orbital (NBO) analysis uncovered ionic bonding interactions between the bridging toluene and the yttrium centers, supporting the assignment of a -2 charge to the toluene. UV-vis spectroscopy highlighted that the electronic excitations primarily stem from toluene- and guanidinate-based orbitals. The Dy and Er congeners were further probed by SQUID magnetometry, with 3 revealing weak magnetic exchange coupling between the Er^III centers. These findings highlight the ability of reduced arenes to serve as bridging ligands in multimetallic rare earth architectures.