Ionic palladium species

The influence of different classes of palladium compounds was also investigated (Table 2). Pd(II)- and Pd(0)-complexes, neutral and ionic palladium species as well as mono- and dinuclear palladium compounds were applied as catalyst precursors. 

Ott, L. S. Cline, M. L. Deetlefs, M. et al. Nanoclusters in ionic liquids evidence for A-heterocyclic carbene formation from imidazolium-based ionic liquids detected by H-2 NMR, J. Am. Chem. Soc., 2005, 127(16), 5758-5759 Hamill, N. A., Hardacre, C. McMath, S. E. J. In situ XAES investigation of palladium species present during the Heck reaction in room temperature ionic liquids. Green Chem., 2002, 4(2), 139-142. 

It has been suggested that active sites in heterogeneous Heck reactions are actually dissolved palladium species.1901 Such a hypothesis is in agreement with a mechanism where palladium nanoparticles suspended in an ionic liquid are suggested to act as reservoir to a molecular catalyst, which is formed upon oxidative addition, as shown in Figure 6.4.1311... 

The electrodeposition of palladium, Pd, has been studied in a EMICI-AICI3 ionic liquid [61, 62]. Palladium dichloride, PdCl2, is soluble in the basic ionic liquid but less soluble in the neutral and acidic ionic liquids. In the basic ionic liquid, the divalent palladium species is considered to be PdCl, which is reducible to metallic Pd ... 

Palladium-catalysed processes typically utilise only 1-5 mol% of the catalyst and proceed through small concentrations of transient palladium species there is a sequence of steps, each with an organopalladium intermediate, and it is important to become familiar with these basic organopalladium processes in order to rationalise the overall conversion. Concerted, rather than ionic, mechanisms are the rule, so it is misleading to compare them too closely with apparently similar classical organic mechanisms, however curly arrows can be used as a memory aid (in the same way as one may use them for cycloaddition reactions), and this is the way in which palladium-catalysed reactions are explained in the following discussion. (For convenience, an organometallic component can be referred to as the nucleophilic partner and the halide as the electrophilic partner, but this should not necessarily be taken to imply reactivity as defined in classical chemistry. Also, references to the halide should be understood to include all related substrates, such as triflates.)... 

Using pal1adium(II)-complexes, yields of the 6-lactone in the range of 35 to 45 % were obtained, whereas Pd(0)-compounds afforded yields lower than 10 %. Ionic palladium complexes and dimeric palladium compounds gave almost the same results as the neutral monometallic pal-ladium(II) species. 

The Mizoroki-Heck reaction is usually performed in polar solvents, and salt additives such as tetrabutylammonium chloride have been shown to activate and stabihze the catalytically active palladium species [19]. Furthermore, the reactions in ionic hquids perform differently in terms of thermodynamic and kinetic properties of the reaction system. Additionally, ionic liquids allow a facile recovery of catalyst and substrates, as well as an easy product separation. Here, another beneficial effect might be used by combination of solvent mixtures for example, of ionic liquids and SCFs. SCFs and ionic liquids have a mixing gap which allows working in two-phase systems, and results in a straightforward phase separation [20]. 

Yokoyama and coworkers described the use of palladium catalysts (Pd(NH3)4)Cl2 supported on Si02 for Mizoroki-Heck reactions in [BMImlPFe [43]. In the selective couphng of iodobenzene and ethyl acrylate to (F)-ethylcinnamate, the activities of palladium(O) and palladium(II) on Si02 and on charcoal were compared. Palladium(II) on Si02 was the preferred catalyst for this coupling reaction. Reuse of the ionic liquid resulted in similar conversion, indicating that the reaction was catalysed by a palladium species dissolved in the ionic liquid. 

In spite of the difficulties mentioned above, Brookhart and co-workers succeeded in measuring the barrier for ethene insertion into (dppp)PdC(0)CH3+ at 160 K, starting from the ethene adduct, generated at still lower temperatures, in the absence of CO. The barrier measured (AG ) amounted to only 51.4 kJ/mol, i.e. the reaction is faster than the insertion of CO in an ionic alkylpalladium complex. The barrier of insertion of ethene into a palladium methyl species or palladium ethyl species was higher, at 67 kJ/mol at 233 K. As for the CO insertion described above, these values concern the barriers in preformed ethene adducts at higher temperatures the overall barrier will be higher, because alkene coordination will be disfavoured by entropy and competition with CO and solvent. Formation of CO adducts will also be less favourable at higher temperatures. 

Ionic liquids have been used for the selenium- or palladium-catalyzed carbonylation of primary amines to form carbamates or ureas.After completion of the carbonylation, addition of water induced the precipitation of desired products, which were isolated by fdtration and separated from the ionic liquid, containing the catalytic species. Then, the catalyst could be reused after removal of residual methanol and water by distillation. Although the conversion of the reaction slightly decreased after the second run, the catalytic activity was considerably improved (from 70% to 99 %) by the addition of a small amount of the fresh catalyst. " ... 

We were interested in the change in the oxidation state of Pd (II), incorporated in the zeolite, during heat treatment in oxygen or in vacuo. Hydrogen and carbon monoxide interactions were also studied. The experiments involved two techniques ESR, which provides direct identification of palladium in an ionic state, and IR spectroscopy, which gives information on the superficial structure of the exchanged zeolite and on the adsorbed species. 

Surface reactions (1) and (2] have been widely used to prepare silica supported metal complex catalysts [6]. The stabilization of ionic forms of palladium was strurigly increased by the presence of -OLi moiety. Simultaneous presence of -OLi and (-D) Pd surface species resulted in a very active hydrodehalogenation and reduction catalyst. 

---