Palladium Black from Decomposition Is a Directly Reusable Catalyst

During cross-coupling reactions, molecular palladium catalysts often decompose into palladium black-precipitated palladium(0) solids that are generally regarded as ill-defined and catalytically inactive. However, their potential for unexpected reactivity is poorly understood, limiting opportunities to optimize reaction rates, selectivity, catalyst loading, cost, and reuse. Here, contrary to the prevailing view of its inactivity, this material was discovered to be viable for reuse in Heck cross-coupling reaction (1.2 mol %, 22 examples, 49-89% yield), with successful demonstration of two recovery-reuse cycles on the gram scale. The regeneration of activity was pinpointed to the surface activity of these solids towards oxidative addition as identified by fluorescence lifetime imaging microscopy (FLIM). A suite of additional mechanistic and characterization studies, including reaction rate analysis, hot-filtration experiments, mercury poisoning tests, atmosphere and substrate exclusion experiments, transmission electron microscopy (TEM), FLIM, and fluorescence correlation spectroscopy (FCS) revealed two mechanistic pathways for catalytic reentry that depend on the identity of the alkene coupling partner. Both pathways involve initial observable oxidative addition of an aryl iodide to palladium nanoparticle surfaces which then generates soluble, solution-phase active catalysts of <20 nm, which are then responsible for turnover. Further, TEM characterized the decomposed solids studied herein as surprisingly regular on the nanoparticle scale given the lack of intentional reaction control and prior contrasting reports. These studies establish the direct surface reactivity of palladium black from decomposition, and show it is an unexpectedly active and reusable catalyst for cross-coupling reactions.