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

Abstract

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 M 0 and M II 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 M 0 and M II 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 M 0 complexes and the oxidative−reductive transmetalation of M II complexes, including a reaction of highly popular M II /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.

Pd/C as Heterogeneous Catalyst for the Direct Arylation of (Poly)fluorobenzenes

“…This complete inhibition of the catalyst in the presence of mercury is typically expected when heterogeneous species, including colloidal Pd 0 , are active in the catalytic cycle . However, the interaction of Hg 0 inhibiting the activity of soluble molecular palladium complexes has also been observed previously in a few cases …”

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“… 25 , 138 , 218 – 220 Catalytic poisons that capture metals can shift the equilibrium and thus facilitate the M/NHC complexes decomposition. 221 Parameters of the equilibrium and the rates of ligand exchange obviously depend on the relative energies of dissociation of M–NHC and M–L′ bonds ( Scheme 10 ), concentrations, solvent, temperature and the presence of ancillary species.…”

Transfer hydrogenation catalysis in cells

“…[1][2][3][4][5][6] However, in spite of all the precautions, catalysts are often poisoned. [7][8][9][10] On the other hand, nature very carefully uses enzymes with specific metal-ion active sites to catalyze a wide range of intracellular bio-chemical reactions essential for life. 11,12 These metalloenzymes catalyse reactions in living cells in which the conditions and environment are extremely complicated.…”