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.