Photo‐Chromium: Sensitizer for Visible‐Light‐Induced Oxidative C−H Bond Functionalization—Electron or Energy Transfer?

Abstract The chromium(III) sensitizer Cr(ddpd) 2 3+ (ddpd= N , N ′‐dimethyl‐ N , N ′‐dipyridine‐2‐ylpyridine‐2,6‐diamine), based on an earth‐abundant metal centre, possesses a unique excited‐state potential‐energy landscape. The very large energy gap between the redox‐active and substitutionally labile 4 T 2 state and the long‐lived low‐energy 2 E spin‐flip state enables a selective, efficient sensitization of triplet dioxygen to give singlet dioxygen. Ultrafast intersystem crossing after the Franck Condon point from the 4 T 2 to the 2 E excited state within 3.5 ps precludes intermolecular electron‐transfer pathways from the ultrashort‐lived excited 4 T 2 state. This specific excited‐state reactivity enables a selective visible‐light‐induced C−H bond activation of tertiary amines with 1 O 2 and subsequent trapping with cyanide to yield α‐aminonitriles in good to excellent yields.