Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts
Samaresh C. Sau, Matthias Schmitz, Chris Burdenski, Marcel Baumert, Patrick W. Antoni, Christoph Kerzig, Max M. Hansmann
Abstract
A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented, and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C–N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N -heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the hybrid photocatalysts reported here feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR, and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state potential of our best-performing catalyst (+2.5 V vs SCE), and they provide evidence for mechanistic key steps and intermediates.