The amphiphile PS-750-M
generates stable, phosphine ligand-free,
and catalytically active ultrasmall Pd(II) nanoparticles (NPs) from
Pd(OAc)
2
, preventing their precipitation, polymerization,
and oxidation state changes. PS-750-M directly interacts with Pd(II)
NP surfaces, as confirmed by high-resolution mass spectrometry and
IR spectroscopy, resulting in their high stability. The Pd cations
in NPs are most likely held together by hydroxides and acetate ions.
The NPs were characterized by HRTEM, revealing their morphology and
particle size distribution, and by HRMS and IR, providing evidence
for NP–amphiphile interaction. The NP catalytic activity was
examined in the context of oxidative Mizoroki–Heck-type couplings
in water at room temperature. Hot filtration, hot extraction, and
three-phase tests indicate heterogeneous catalysis occurring at the
micellar interface rather than homogeneous catalysis occurring in
the solution. NMR studies indicate that the catalytic activity stems
from metal cation−π interactions of the styrene along
with transmetalation by the arylboronic acid, followed by insertion
and β-H elimination to furnish the coupled product along with
the reoxidation of Pd by benzoquinone to complete the catalytic cycle.
This method is very mild and sustainable, both in terms of NP synthesis
and subsequent catalysis, and shows broad substrate scope while circumventing
the need for organic solvents for this important class of couplings.