Despite
widespread use of heterogeneous Pd catalysts in Suzuki–Miyaura
coupling reactions, detailed roles of Pd, especially the nature of
its active species, are still a topic of controversial debate. While
some studies showed an active surface of Pd nanoparticles or nanoclusters
acting heterogeneously, others claimed soluble Pd species leached
from the metallic Pd to be active species which are homogeneous in
nature. Besides, within the homogeneous mechanism, how the Pd leaches
and promotes the cross-coupling reaction is then another question
that needs to be addressed. It could be envisioned that if the soluble
Pd species and solid-phase Pd are physically separated, the mechanism
of Suzuki–Miyaura coupling could then be confirmed through
examining the catalytic activity in different reaction regions. Herein
we use microporous Stöber silica as a membrane to separate
the soluble Pd species from solid Pd and conduct size-selective reactions
which allow the passage of leaching Pd species, but not of reactants
or products larger than the membrane aperture. With this strategy,
we have been able to differentiate the surface reaction from the solution
cross-coupling. We find that the leached Pd species are the only genuine
catalytic intermediate in the cross-coupling reactions. We also confirm
that oxidative addition of aryl halides to the solid Pd leads to leaching
of the soluble Pd species which is necessary to promote Suzuki–Miyaura
reactions.