Linear Inverse Sandwich\nComplexes of Tetraanionic\nBenzene Stabilized by Covalent δ‑Bonding with Late Lanthanides

A series of dilanthanide benzene inverse sandwich complexes\nof\nthe type (Cp<sup>iPr5</sup>Ln)<sub>2</sub>(μ–η<sup>6</sup>:η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>) (<b>1-Ln</b>) (Ln = Y, Gd, Tb, Dy, Tm) are reported. These compounds are synthesized\nby reduction of the respective trivalent dimers Cp<sup>iPr5</sup><sub>2</sub>Ln<sub>2</sub>I<sub>4</sub> (Ln = Y, Gd, Tb, Dy, Tm) in diethyl\nether with potassium graphite in the presence of benzene, and they\nfeature an unusual linear coordination geometry with a highly planar\nbenzene bridge as verified by single-crystal X-ray diffraction. The\nLn–Bz<sub>centroid</sub> distances of <b>1-Ln</b> are\nthe shortest distances observed to date, ranging from 1.943(1) Å\nfor <b>1-Tm</b> to 2.039(6) Å for <b>1-Gd</b>. Structural,\nspectroscopic, and magnetic analyses together with density functional\ntheory calculations support the presence of a rare, unsubstituted\ntetraanionic benzene in each compound, which is stabilized by strong\ncovalent δ bonding interactions involving the filled π*\norbitals of (C<sub>6</sub>H<sub>6</sub>)<sup>4–</sup> and vacant\nd<sub><i>xy</i></sub> and d<sub><i>x</i><sup>2</sup><i>–y</i><sup>2</sup></sub> orbitals of the Ln<sup>3+</sup> ions. Notably, <b>1-Ln</b> are the first examples\nof compounds of the later lanthanides to feature an unsubstituted\ntetraanionic benzene.