Palladium Bisphosphine Monoxide Complexes: Synthesis, Scope, Mechanism, and Catalytic Relevance

Recent studies in transition metal catalysis employing chelating phosphines have suggested a role for partial ligand oxidation in formation of the catalytically active species, with potentially widespread relevance in a number of catalytic systems. We examine the internal redox reaction of Pd^II(bisphosphine)X₂ (X = Cl, OAc, etc.) complexes to reveal previously underexplored aspects of bisphosphine monoxides (BPMOs), including evaluation of ligand structure and development of general reaction conditions to access a collection of structurally diverse BPMO precatalysts based on organopalladium oxidative addition complexes. In particular, a series of Pd^II(BPMO)(R)(X) (R = aryl, alkyl; X = I, Br) oxidative addition complexes bearing 24 different BPMO ligands were characterized by NMR and X-ray crystallography. Comparison of the catalytic performance of the oxidative addition complexes of bisphosphine versus bisphosphine monoxides as precatalysts is demonstrated to be an enabling diagnostic tool in Pd catalytic reaction development. Finally, the differences in catalytic behavior between bisphosphine and bisphosphine monoxide complexes were rationalized through solid-state parametrization and stoichiometric experiments.