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 (CpiPr5Ln)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 CpiPr52Ln2I4 (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-Bzcentroid 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 dxy and dx2-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.