Pd and Pt Catalyst Poisoning in the Study of Reaction Mechanisms: What Does the Mercury Test Mean for Catalysis?

The mercury test is a rapid and widely
used method for distinguishing truly homogeneous molecular catalysis
from nanoparticle metal catalysis. In the current work, using various
M0 and MII complexes of palladium and platinum
that are often used in homogeneous catalysis as examples, we demonstrated
that the mercury test is generally inadequate as a method for distinguishing
between homogeneous and cluster/nanoparticle catalysis mechanisms
for the following reasons: (i) the general and facile reactivity of
both molecular M0 and MII complexes toward metallic
mercury and (ii) the very high and often unpredictable dependence
of the test results on the operational conditions and the inability
to develop universal quantitatively defined operational parameters.
Two main types or mercury-induced transformations, the cleavage of
M0 complexes and the oxidative–reductive transmetalation
of MII complexes, including a reaction of highly popular
MII/NHC complexes, were elucidated using NMR, ESI-MS, and
EDXRF techniques. A mechanistic picture of the reactions involving
metal complexes was revealed with mercury, and representative metal
species were isolated and characterized. Even in an attempt to not
overstate the results, one must note that the use of the mercury tests
often leads to inaccurate conclusions and complicates the mechanistic
studies of these catalytic systems. As a general concept, distinguishing
reaction mechanisms (homogeneous vs cluster/nanoparticle) by using
catalyst poisoning requires careful rethinking in the case of dynamic
catalytic systems.