# Stabilization of the Benzene Radical Trianion in an Inverse‐Sandwich Yttrium Complex

## TL;DR

Scientists created a new form of benzene with three extra electrons, stabilized by yttrium atoms in a special complex.

## Contribution

The first isolated and proven benzene radical trianion is synthesized and characterized.

## Key findings

- The benzene radical trianion is stabilized by two yttrium cations in an inverse-sandwich complex.
- Spin density of the unpaired electron resides on the benzene trianion and extends toward yttrium ions.
- DFT calculations suggest weak aromaticity in the benzene radical trianion.

## Abstract

Herein, the first report on the isolated and unambiguously proven benzene radical trianion is presented. This unprecedented radical oxidation state of benzene is stabilized through two trivalent rare earth (RE) metal cations each supported by a bis(guanidinate) scaffold. Specifically, the one‐electron chemical reduction of the neutral inverse‐sandwich yttrium complex [[{(Me3Si)2NC(N
i
Pr)2}2Y]2(μ–ƞ
6:ƞ
6–C6H6)] 1, containing a benzene dianion, with potassium graphite (KC8) in the presence of [2.2.2]‐cryptand yielded the title complex [K([2.2.2]‐cryptand)][[{(Me3Si)2NC(N
i
Pr)2}2Y]2(μ–ƞ
6:ƞ
6–C6H6
•)] 2, featuring a benzene radical trianion. Analyses through single‐crystal X‐ray diffraction, EPR and UV–vis spectroscopy, elucidated its molecular structure and revealed strong [YIII–(C6H6)3–•–YIII] metal–radical interactions. Although the Y centers remain in the +3 oxidation state, the spin density of the unpaired electron resides primarily on the benzene trianion moiety and extends toward the YIII ions. Density functional theory (DFT) calculations on 2 corroborate this assignment and further suggest weak aromaticity for the benzene radical trianion.

A new type of benzene anion state is born: The unprecedented benzene radical trianion embedded in an inverse‐sandwich yttrium complex was synthesized by a one‐electron chemical reduction of its benzene dianion precursor using potassium graphite in the presence of [2.2.2]‐cryptand. Notably, the yttrium ions remain in the +3 oxidation state and the spin density of unpaired electron resides at the non‐planar benzene radical trianion and induces weak aromaticity.