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.