A New Mode of Operation of Pd-NHC Systems Studied in a Catalytic Mizoroki–Heck Reaction

Metal complexes bearing
N-heterocyclic carbene (NHC) ligands are
typically considered the system of choice for homogeneous catalysis
with well-defined molecular active species due to their stable metal–ligand
framework. A detailed study involving 19 different Pd-NHC complexes
with imidazolium, benzimidazolium, and triazolium ligands has been
carried out in the present work and revealed a new mode of operation
of metal-NHC systems. The catalytic activity of the studied Pd-NHC
systems is predominantly determined by the cleavage of the metal–NHC
bond, while the catalyst performance is strongly affected by the stabilization
of in situ formed metal clusters. In the present study, the formation
of Pd nanoparticles was observed from a broad range of metal complexes
with NHC ligands under standard Mizoroki–Heck reaction conditions.
A mechanistic analysis revealed two different pathways to connect
Pd-NHC complexes to “cocktail”-type catalysis: (i) reductive
elimination from a Pd­(II) intermediate and the release of NHC-containing
byproducts and (ii) dissociation of NHC ligands from Pd intermediates.
Metal-NHC systems are ubiquitously applied in modern organic synthesis
and catalysis, while the new mode of operation revealed in the present
study guides catalyst design and opens a variety of novel opportunities.
As shown by experimental studies and theoretical calculations, metal
clusters and nanoparticles can be readily formed from M-NHC complexes
after formation of new M–C or M–H bonds followed by
C–NHC or H–NHC coupling. Thus, a combination of a classical
molecular mode of operation and a novel cocktail-type mode of operation,
described in the present study, may be anticipated as an intrinsic
feature of M-NHC catalytic systems.