Capturing the Hybrid Palladium(I)-Radical Pair Relevant to Photoexcited Palladium Catalysis.

Understanding and controlling one-electron chemistry in palladium coordination compounds can unlock reactivity that is inaccessible to ground-state Pd chemistry. One burgeoning area involves the photochemical excitation of a Pd0 catalyst to activate alkyl halides, substrates that are traditionally challenging to activate thermally, and Pd compounds supported by wide bite-angle diphosphines such as Xantphos are privileged catalysts in this realm. Mononuclear PdI halide compounds supported by such ligands are proposed as key intermediates formed during the photoexcitation of Pd0 species; however, no examples of isolated mononuclear PdI halide compounds have been reported to date. Herein, we report that a Xantphos ligand (tBuXantphos) with tert-butyl substituents on the phosphorus atoms enables the isolation and full spectroscopic characterization of neutral, three-coordinate PdI-chloride and -bromide compounds as crystalline solids. Furthermore, the corresponding Pd0 complex is capable of cleaving C-X bonds in alkyl halides upon photoexcitation with visible light, forming the respective PdI halide species. This provides definitive experimental evidence for this elementary step nearly 30 years after the original proposal by Suzuki and Miyaura. In addition, a PdII monomethyl compound supported by the same ligand framework undergoes Pd-C bond homolysis under visible light irradiation to form the same PdI halide species. Finally, the reactivity difference between the bulky tBuXantphos and less sterically hindered diarylether-based diphosphines was examined in model stoichiometric and catalytic reactions. Overall, these findings provide unambiguous experimental evidence for the role of PdI compounds as critical intermediates in photochemical palladium chemistry.