Concept of ?magic? number clusters as a new approach to the interpretation of unusual kinetics of the Heck reaction with aryl bromides

Abstract

An increase of reaction rate with decreasing catalyst concentration by performing the Heck reaction with aryl bromides in the presence of several catalytic systems has first been observed. Taking into account palladium colloidal particles formation established experimentally in the Heck reaction with aryl bromides, a new approach to the description of unusual kinetics is suggested. That approach is based on a concept of ''magic'' number clusters.

Electroreductive Palladium‐Catalysed Ullmann Reactions in Ionic Liquids: Scope and Mechanism

“…[20] The unusual kinetic profile for such reactions, where an induction period was also found, was interpreted by Schmidt and Smirnov using the concept of a "cluster magic number". [20,21] We used transmission electron microscopy (TEM) to characterise the particle size at the end of the induction period. Spherical well-dispersed particles were observed (Figure 4, left) with a size distribution of 2.5 AE 0.5 nm.…”

High Catalytic Activity of Palladium(II)‐Exchanged Mesoporous Sodalite and NaA Zeolite for Bulky Aryl Coupling Reactions: Reusability under Aerobic Conditions

“…[11][12][13] The ligand-free system is preferable in heterogeneous catalysis, owing to economic preparation and high stability in air. [14][15][16][17][18] However, the use of ligand-free catalysts is often limited by Pd leaching, and also by changes in the chemical state of the Pd species. [1,19] The limited success of ligand-free, heterogeneous Pd catalysts is attributed to the unique reaction mechanism involving a Pd 0 state.…”

“…There have been numerous works indicating that the catalytic active state is a Pd 0 species in coupling reactions. [6,9,[14][15][16][17][18] In cases where Pd II is used, in situ reduction to Pd 0 is known to occur during the reaction. De Vries and Reetz provided the evidence that monomeric or dimeric Pd 0 species, being in equilibrium with nanoparticulate Pd 0 , is the catalytically active species.…”

“…[14,15] Similar results were also reported by Schmidt at around the same time. [17,18] Notably, it was reported that the reduced Pd 0 species were more easily leached into the reaction media than the Pd II species. [20] The reversible dissolution and redeposition of Pd 0 can result in the agglomeration of Pd 0 species into large Pd nanoparticles or Pd black, causing a loss of metal dispersion and surface area.…”

“…This observation is remarkable, whereas Pd 0 nanoparticles or Pd black (bigger agglomerates) were often detected after reactions in both homogeneous and heterogeneous systems. [6,9,[14][15][16][17][18] Contrary to the result in air, the reaction in N 2 produced Pd 0 nanoparticles. The XANES patterns indicated a change from Pd II to Pd 0 states during the reaction, and a distinct peak corresponding to the Pd-Pd distance occurred at 0.…”