Dongmin Kang, Minseok Kim, Heenang Choi, Jeong-Yoon Hwang, Gayeong Lim, Jaelim Kim, Youngsuk Kim*
Cite this article as: J. Am. Chem. Soc. 2025, 147, 35829.
DOI: https://doi.org/10.1021/jacs.5c12377
This study reports the unprecedented synthesis of exceptionally stable, open-shell sulfide radical cations supported by cyclic (alkyl)(amino)carbenes (CAACs), demonstrating extreme resistance to air, water, and harsh chemicals due to extensive electronic delocalization and steric shielding.
Exceptional Environmental Stability: CAAC-supported sulfide radical cations ([2a–c•+][PF6–] ) were synthesized under ambient conditions. They exhibited no significant degradation in a methanol/water mixture even after 15 days of exposure to open air.
Robustness Against Harsh Chemicals: The bulky Dipp-substituted radical (2a•+) survives extreme chemical environments, showing a remarkable <1% absorbance decay in 0.1 M H₂SO₄ and only 3.6% decay in 0.1 M H₂O₂ over 12 hours.
The Crucial Role of Steric Protection: Steric bulk is the key to survival. Compared to the highly stable Dipp variant, the less sterically hindered Mes-substituted analogue (2c•+) degrades rapidly in peroxides (73% decay in 12h) and decomposes immediately upon contact with strong bases.
Electronic Delocalization: Density functional theory (DFT) and EPR spectroscopy confirm that while the unpaired electron (S = 1/2) has its highest spin density on the sulfur atom, it is significantly delocalized across the N–C–C–S π-network, providing fundamental thermodynamic stability.
Stable organic radicals have attracted significant attention owing to their key role in understanding reactive intermediates and their diverse applications as functional materials. Sulfide radical cations, in particular, serve as crucial intermediates in various fields such as organic chemistry, biochemistry, and polymer/materials science. However, the development of air- and water-stable sulfide radical cations has remained a significant challenge. In this study, highly stable sulfide radical cations supported by two cyclic (alkyl)(amino)carbene (CAAC) moieties have been synthesized. In addition to demonstrating high air- and water stability, these radical cations can withstand harsh conditions, including exposure to reactive chemicals such as sulfuric acid and hydrogen peroxide, as well as elevated temperatures. Combined experimental and computational studies reveal that the sulfur-centered unpaired electron is stabilized through electronic delocalization and steric protection provided by the CAAC substituents. These findings establish a new class of exceptionally stable sulfur-based radicals and pave the way for their potential applications as functional materials.
Sulfide radical cations (R₂S•+) are incredibly important reactive intermediates in organic synthesis, biochemistry (e.g., methionine oxidation), and materials science (organic conductors). However, capturing and utilizing them as functional materials has been a massive challenge because they are notoriously unstable and readily decompose in the presence of air or moisture.
To overcome this intrinsic instability, we employed cyclic (alkyl)(amino)carbenes (CAACs) as a stabilizing scaffold. While CAACs are known to stabilize main-group radicals, applying them to synthesize stable sulfur-centered radical cations is a groundbreaking leap. The unique geometry of the 1,2,4-trithiolane-derived scaffold wraps the highly reactive sulfur atom in a robust steric "jacket" (especially with bulky Dipp groups), physically blocking reactive molecules like water or peroxides from attacking the radical center.
This research proves that with strategic molecular design, even the most fleeting reactive intermediates can be tamed into bench-stable powders. Furthermore, these radical cations undergo clean, reversible one-electron reduction to form neutral sulfides (2a⁰). This combination of extreme environmental stability, highly localized spin density, and clean redox activity paves the way for their direct application in next-generation molecular optoelectronics and redox-active smart materials.
2025-jacs-sulfide-radical.pdf