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[Display omitted] The set of heterogeneous Pd catalysts containing different forms of Pd (PdCl 4 2– or Pd0) was prepared by chemical modification and laser electrodispersion using two types of + supports, namely, SiO 2 modified by ionic liquid and γ-Al 2 O 3. Testing of the synthesized catalysts in the Suzuki–Miyaura reaction with aryl bromides pointed out the possibility to achieve the prominent TOF and TON values. The dependencies of TOF on the catalyst loading indicate that only a fraction of loaded Pd was involved in the catalysis. [ABSTRACT FROM AUTHOR]

• Endogenous halide and/or base anions present in active Pd complexes. • Pd complexes activating aryl halide are anionic and contain halide anions. • Base- and/or hydroxo-containing Pd complexes activate arylboronic acid. Despite extensive studies regarding Pd intermediates reacting in the transmetalation step of the Suzuki–Miyaura reaction (SMR), direct evidence obtained under real turnover conditions supporting the widely accepted assumptions regarding the activity of Pd hydroxo complexes is lacking. Herein, results concerning the major reactivity of Pd-OH- and/or Pd-OR-containing complexes (RO− is the base anion) in the transmetalation step with arylboronic acid under "ligand-free" turnover conditions are presented. By contrast, aryl chloride activation was mainly realized through halide-containing Pd complexes. Data regarding the catalytic involvement of particular Pd species were obtained under real catalytic conditions by measuring differential selectivity of the competing reactions. In the reaction with demanding aryl chlorides catalyzed by "ligand-free" catalytic systems that is optimal from the viewpoints of environment, cost, and availability, the composition of active Pd complexes in the transmetalation step was similar to that for the reaction using more reactive aryl bromides and iodides. [Display omitted] [ABSTRACT FROM AUTHOR]

• Endogenous halide and/or base anions present in active Pd complexes. • Anionic character of Pd complexes activating both aryl halide and alkene was revealed. • Anionic Pd-PR 3 complexes considerably contribute to the substrates conversion. • Anionic Pd "ate" complexes are directly involved in alkene insertion in the Pd-C bond. The role of ionic Pd species in the catalytic systems used for cross-coupling reactions has been revised in recent years, mainly utilizing in situ analytical techniques to detect catalyst species in the reaction mixture. The possible inclusion of phosphine molecules in the active Pd complexes is still debated. We have carried out a comparative study on the differential selectivity of the Mizoroki–Heck reaction with competition between two aryl halides or two alkenes to elucidate the active Pd species in different types of catalytic systems, including those favoring the formation of ionic complexes. The sensitivity of the reaction differential selectivity of two competing aryl halides to the additives of tertiary phosphine and halide salts points to the catalytic activity of anionic Pd(0) complexes containing at least one phosphine ligand when the phosphine-containing catalytic systems are used. The changes of the reaction differential selectivity of two competing alkenes to phosphine and halide salt allow to propose the entrance of both phosphine and halide ions into the coordination sphere of active Pd(II) complexes possessing anionic character. In addition, evidence for the effect of phosphine ligand and salt additives on the composition of the active Pd complexes that determine the regioselectivity of the reaction is presented. The data obtained indicate the decisive role of anionic Pd species in the Mizoroki–Heck reaction using both phosphine-free and phosphine-containing catalytic systems. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Differential selectivity of two arylboronic acids is independent of aryl halide type. • The results cannot be explained by "textbook" mechanism of Suzuki-Miyaura reaction. • Aryl halide and arylboronic acid are activated by two distinct Pd species. • Cooperative non-linear mechanism can be proposed for Suzuki-Miyaura reaction. Herein, the independent differential selectivity in the Suzuki-Miyaura reaction of two competing arylboronic acids with regard to the nature of the aryl halide (both halide and aryl moieties) was established. This pattern cannot be realized via the "textbook" mechanism of the reaction, which suggests the interaction of boron-containing species with an ArPdX intermediate (product of the aryl halide catalytic activation). The data obtained indicate the participation of two distinct palladium species in the aryl halide and arylboronic acid activations in the catalytic cycle of biaryl formation, which are in accordance with the so-called cooperative mechanism. [ABSTRACT FROM AUTHOR]

• The reversible character of the oxidative addition of ArCl to Pd was demonstrated. • The degree of reversibility depended on the ArCl and ArB(OH) 2 used. • Reversibility of the oxidative addition indicated that it was not the RDS. The retarded oxidative addition of aryl chloride to Pd(0) is believed, by most scientists, to be the main hindrance in achieving effective conversion in the Suzuki–Miyaura reaction and other cross-coupling reactions of aryl chlorides. Herein, we have demonstrated by competing experiments, using two aryl chlorides under "ligand-free" catalytic conditions (absence of strong ligands; high ratio of substrate to catalyst), that the elementary step of oxidative addition is substantially reversible. This implies that the hypothesis on the rate-determining character of the oxidative addition step is incorrect, and the existing problems with aryl chloride conversion in the Suzuki–Miyaura reaction are caused by some other reasons that need to be investigated. The reversibility of the oxidative addition of aryl chlorides to Pd(0) has been established in the Suzuki–Miyaura catalytic cycle. Therefore, the hypothesis on the rate-determining oxidative addition can be completely rebutted. Image, graphical abstract [ABSTRACT FROM AUTHOR]