Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a Cr(III) Polypyridine Complex and Their Use in Photoredox Catalysis

[Image: see text] The photoredox activity of well-known Ru(II) complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. Cr(III) polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a Cr(III) complex is very similar to that for a Ru(II) polypyridine, and thereby validate the concept of estimating the redox potential of d(3) spin-flip excited states in analogous manner as for the MLCT states of d(6) compounds. Building on this insight, we use our Cr(III) complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.