Iridium Polypyridyl Carboxylates as Excited-State PCET Catalysts for the Functionalization of Unactivated C–H Bonds
Diego A. Granados, Yun E. Du, Shiloh J. Andersson, Avery Cirincione-Lynch, Kai Cui, Adam Reinhold, Philip D. Jeffrey, Gregory D. Scholes, Sharon Hammes‐Schiffer, Robert R. Knowles
Abstract
The design of catalysts capable of functionalizing unactivated C(sp 3 )–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.