Taming the Chlorine Radical: Selective C-H Activation using Photogenerated Chlorine-Atom Complexes

Selective C–H activation remains one of the most intensely studied reactions across many fields of chemistry, due to its applications in both industrial and fine chemicals synthesis. Although chlorine radicals have been established as potent hydrogen-atom abstraction agents in free-radical chlorination reactions and, more recently, in photoredox catalysis, the high reactivity of these intermediates severely limits their selectivity. We demonstrate that the secondary coordination sphere of a metal complex can confine photoeliminated chlorine radicals and impart steric control over their reactivity. Specifically, a series of pyridinediimine iron(iii) chloride complexes exhibit activity for photochemical alkane C–H functionalization with selectivity for primary and secondary C–H bonds, overriding thermodynamic preference for weaker tertiary C–H bonds. Photocrystallography and time-resolved transient absorption spectroscopy experiments confirm that this site-selective reactivity arises from the generation of an arene–Cl• complex within the steric environment enforced by the iron secondary coordination sphere.