Linear Inverse Sandwich Complexes of Tetraanionic Benzene Stabilized by Covalent δ-Bonding with Late Lanthanides

A series of dilanthanide benzene inverse sandwich complexes of the type (Cp^iPr5Ln)₂(μ–η⁶:η⁶-C₆H₆) (1-Ln) (Ln = Y, Gd, Tb, Dy, Tm) are reported. These compounds are synthesized by reduction of the respective trivalent dimers Cp^iPr5₂Ln₂I₄ (Ln = Y, Gd, Tb, Dy, Tm) in diethyl ether with potassium graphite in the presence of benzene, and they feature an unusual linear coordination geometry with a highly planar benzene bridge as verified by single-crystal X-ray diffraction. The Ln–Bz_centroid distances of 1-Ln are the shortest distances observed to date, ranging from 1.943(1) Å for 1-Tm to 2.039(6) Å for 1-Gd. Structural, spectroscopic, and magnetic analyses together with density functional theory calculations support the presence of a rare, unsubstituted tetraanionic benzene in each compound, which is stabilized by strong covalent δ bonding interactions involving the filled π* orbitals of (C₆H₆)^4– and vacant d_xy and d_x²–y² orbitals of the Ln³⁺ ions. Notably, 1-Ln are the first examples of compounds of the later lanthanides to feature an unsubstituted tetraanionic benzene.