Aerobic Base-free C–H Bond Activation at a Pd Center in Aqueous Conditions

Recent reports have increasingly documented palladium-catalyzed C–H bond functionalization reactions involving high-valent Pd(III) and Pd(IV) intermediates, yet these transformations often depend on strong oxidants and external bases. Herein, we report a series of Pd(II) complexes [(pRN3CH)PdII(MeCN)](BF4)2, supported by a tetradentate pyridinophane ligand, that undergo aerobic C–H bond activation under mild aqueous conditions to generate isolable Pd(III) complexes. In this transformation, O2 serves as both an oxidant and proton acceptor, eliminating the need for an exogenous base. Kinetic studies confirm a first-order dependence on both [Pd] and [O2] and support a bimolecular, associative electrophilic-type mechanism. Notably, the presence of water in the solvent mixture and slightly elevated temperatures enhance reactivity and lead to quantitative formation of the Pd(III) product, improving its synthetic utility. Mechanistic and computational studies suggest a concerted metalation-deprotonation (CMD) process that is at least partially rate-limiting, and a key Pd–O2 species is proposed to promote the C–H bond activation step. Overall, this study establishes a novel base-free aerobic C–H bond activation pathway at Pd(III) centers, providing key insights into high-valent palladium reactivity and advancing sustainable oxidation strategies.