How Radical Are “Radical” Photocatalysts? A Closed-Shell Meisenheimer Complex Is Identified as a Super-Reducing Photoreagent

• Published in: Journal of the American Chemical Society Vol. 143; no. 35; pp. 14352 - 14359
• Main Authors: Rieth, Adam J, Gonzalez, Miguel I, Kudisch, Bryan, Nava, Matthew, Nocera, Daniel G
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 08-09-2021
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.1c06844

Super-reducing excited states have the potential to activate strong bonds, leading to unprecedented photoreactivity. Excited states of radical anions, accessed via reduction of a precatalyst followed by light absorption, have been proposed to drive photoredox transformations under super-reducing conditions. Here, we investigate the radical anion of naphthalene monoimide as a photoreductant and find that the radical doublet excited state has a lifetime of 24 ps, which is too short to facilitate photoredox activity. To account for the apparent photoreactivity of the radical anion, we identify an emissive two-electron reduced Meisenheimer complex of naphthalene monoimide, [NMI­(H)]−. The singlet excited state of [NMI­(H)]− is a potent reductant (−3.08 V vs Fc/Fc+), is long-lived (20 ns), and its emission can be dynamically quenched by chloroarenes to drive a radical photochemistry, establishing that it is this emissive excited state that is competent for reported C–C and C–P coupling reactivity. These results provide a mechanistic basis for photoreactivity at highly reducing potentials via singlet excited state manifolds and lays out a clear path for the development of exceptionally reducing photoreagents derived from electron-rich closed-shell anions.