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

Area of Science:

• Organometallic Chemistry
• Lanthanide Chemistry
• Coordination Chemistry

Background:

• Lanthanide complexes with organic ligands are crucial in catalysis and materials science.
• Inverse sandwich complexes offer unique electronic and structural properties.
• The stabilization of low-valent organic ligands by lanthanides is an area of active research.

Purpose of the Study:

• To synthesize and characterize novel dilanthanide benzene inverse sandwich complexes.
• To investigate the bonding and electronic structure of these unique compounds.
• To explore the possibility of a tetraanionic benzene ligand stabilized by lanthanides.

Main Methods:

• Synthesis via reduction of trivalent lanthanide dimers with potassium graphite in the presence of benzene.
• Single-crystal X-ray diffraction for structural determination.
• Spectroscopic (NMR, UV-Vis) and magnetic susceptibility measurements.
• Density Functional Theory (DFT) calculations for electronic structure analysis.

Main Results:

• Successful synthesis of dilanthanide benzene inverse sandwich complexes (Cp^iPr5Ln)₂(μ-η⁶:η⁶-C₆H₆) for Ln = Y, Gd, Tb, Dy, Tm.
• X-ray diffraction confirmed unusual linear coordination geometry and a planar benzene bridge.
• Observed shortest Ln-Bz_centroid distances to date (1.943(1) Å for Tm to 2.039(6) Å for Gd).
• Evidence for a rare, unsubstituted tetraanionic benzene ligand stabilized by strong covalent δ bonding.

Conclusions:

• The reported compounds represent the first examples of later lanthanide complexes featuring an unsubstituted tetraanionic benzene.
• Strong covalent interactions between lanthanide f orbitals and the benzene π* orbitals are key to stabilizing the tetraanionic ligand.
• These findings expand the understanding of lanthanide-organic ligand interactions and open new avenues in organometallic chemistry.