Unveiling Three Interconvertible Redox States of Boraphenalene

• Published in: Journal of the American Chemical Society Vol. 146; no. 9; pp. 6145 - 6156
• Main Authors: Deng, Chun-Lin, Hollister, Kimberly K., Molino, Andrew, Tra, Bi Youan E., Dickie, Diane A., Wilson, David J. D., Gilliard, Robert J.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.03.2024
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.3c13726

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 1 •. The 1-boraphenalenyl cation 1 + and anion 1 – 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 1 + is luminescent in both the solution and solid states, 1 • exhibits boron-centered reactivity and 1 – undergoes substitution chemistry on the boraphenalenyl skeleton and serves as a single-electron transfer reductant.