Mononuclear Palladium(I) Halides Stabilized by Bis(di-tert-butylphosphino)ferrocene Ligand

Many radical-involving palladium-catalyzed organic transformations are proposed to proceed via mononuclear palladium(I) halide intermediates. However, isolable palladium(I) complexes of this type remain elusive, casting doubt on these mechanistic hypotheses. In this work, we demonstrate that using the bulky bisphosphine ligand bis(di-tert-butylphosphino)ferrocene (dtbpf) enables the one-electron reduction of palladium(II) halide complexes Pd(dtbpf)X₂ (X = Cl, Br, I) to yield isolable mononuclear palladium(I) halides [Pd(dtbpf)X] (X = Cl, Br, I) in high yields. These palladium(I) complexes have been characterized by paramagnetic ¹H NMR spectroscopy, single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy, and Pd K-edge X-ray absorption spectroscopy. Theoretical studies revealed that [Pd(dtbpf)X] (X = Cl, Br, I) in their doublet ground states have their single-occupied molecular orbitals in antibonding character and that their Pd-X bonds are polar covalent bonds. Furthermore, speciation studies confirmed the formation of palladium(I) halide species in reactions of the palladium(0) complex [Pd(dtbpf)(norbornene)] with benzyl halides and perfluoroalkyl iodide, as well as in the thermal and photoirradiated decomposition of the aryl palladium(II) complex [Pd(dtbpf)(C₆H₄-p-Me)]I. These findings provide compelling evidence for the involvement of palladium(I) halide intermediates in relevant palladium-catalyzed transformations.