Unveiling Three Interconvertible Redox States of Boraphenalene

Abstract

Neutral 1-boraphenalene displays the isoelectronic structure of the phenalenyl carbocation and is expected to behave as an attractive organoboron multi-redox system. However, the isolation of new redox states have remained elusive even though the preparation of neutral boron(III)-containing phenalene compounds have been extensively studied. Herein, we have adopted an N-heterocyclic carbene ligand stabilization approach to achieve the first isolation of the stable and ambipolar 1-boraphenalenyl radical <b>1</b>^•. The 1-boraphenalenyl cation <b>1</b>^<b>+</b> and anion <b>1</b>^<b>-</b> have also been electrochemically observed and chemically isolated, representing new redox forms of boraphenalene for the study of non-Kekulé polynuclear benzenoid molecules. Experimental and theoretical investigations suggest that the interconvertible three-redox-state species undergo reversible electronic structure modifications, which primarily take place on the polycyclic framework of the molecules, exhibiting atypical behavior compared to known donor-stabilized organoboron compounds. Initial reactivity studies, aromaticity evaluations, and photophysical studies show redox-state-dependent trends. While <b>1</b>^<b>+</b> is luminescent in both the solution and solid states, <b>1</b>^<b>•</b> exhibits boron-centered reactivity and <b>1</b>^<b>-</b> undergoes substitution chemistry on the boraphenalenyl skeleton and serves as a single-electron transfer reductant.