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 Pd^I halide compounds supported by such ligands are proposed as key intermediates formed during the photoexcitation of Pd⁰ species; however, no examples of isolated mononuclear Pd^I 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 Pd^I-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 Pd^I 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 Pd^II monomethyl compound supported by the same ligand framework undergoes Pd-C bond homolysis under visible light irradiation to form the same Pd^I 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 Pd^I compounds as critical intermediates in photochemical palladium chemistry.