Iridium Polypyridyl Carboxylates as Excited-State PCET Catalysts for the Functionalization of Unactivated C-H Bonds.

The design of catalysts capable of functionalizing unactivated C(sp³)-H bonds remains a significant goal in synthetic organic chemistry. Herein, we present a novel set of iridium polypyridyl complexes bearing pendent Brønsted basic carboxylates that become potent hydrogen atom abstraction catalysts upon visible light irradiation. Thermochemical and spectroscopic characterization reveal that these excited-state complexes exhibit bond dissociation free energies (BDFEs) of up to 105 kcal mol^-1 with long excited-state lifetimes. We demonstrate that these complexes can catalyze C-H alkylation reactions in which the Ir carboxylate mediates both C-H abstraction and formation steps. Mechanistic, spectroscopic, and computational studies are consistent with C-H abstraction proceeding through an excited-state proton-coupled electron transfer (PCET) step. The modular nature of these Ir polypyridyl complexes establishes a foundation for designing tunable and efficient C-H functionalization catalysts based on covalent tethering of excited-state oxidants and bases.