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

## TL;DR

Scientists created new rare earth metal complexes with a toluene dianion acting as a bridge between metals.

## Contribution

This is the first report of inverse-sandwich rare earth complexes with a dianionic toluene ligand.

## Key findings

- Toluene dianion is planar and stabilized between two rare earth metals.
- NMR and DFT confirm the dianionic nature of toluene in the complexes.
- Weak magnetic coupling is observed in the erbium complex.

## Abstract

We report the first inverse-sandwich complexes containing
rare
earth (REIII) 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 ErIII centers. These findings
highlight the ability of reduced arenes to serve as bridging ligands
in multimetallic rare earth architectures.