Oxidative Addition of Aryl Bromides at Palladium(I) to form Palladium(III) Complexes

Herein, we report the first systematic study of the oxidative addition of aryl bromides to a Pd^I center to generate organometallic Pd^III complexes. These isolable Pd^III complexes stabilized by tetradentate macrocyclic pyridinophane ligands exhibit distinct UV–vis and EPR spectroscopic signatures that allowed for the monitoring of their generation in situ. These ligand scaffolds were sterically and electronically tuned using a modular synthetic approach to probe the kinetic properties and activation parameters of the oxidative addition reaction, and a combination of UV–vis and cryo stopped-flow spectroscopic studies reveal a rapid oxidative addition step occurring at a Pd^I center. In addition, these results are in strong agreement with our recent reactivity studies, which demonstrated that mononuclear Pd^I systems are competent catalysts in Kumada cross-coupling reactions, and thus set the stage for an improved understanding of potential catalytic applications for odd-electron Pd systems.