Palladium electronic properties

Palladium chemistry involving heterocycles has its unique characteristics stemming from the heterocycles inherently different structural and electronic properties in comparison to the corresponding carbocyclic aryl compounds. One example illustrating the striking difference in reactivity between a heteroarene and a carbocyclic arene is the heteroaryl Heck reaction (vide infra, see Section 1.4). We define a heteroaryl Heck reaction as an intermolecular or an intramolecular Heck reaction occurring onto a heteroaryl recipient. Intermolecular Heck reactions of carbocyclic arenes as the recipients are rare [12a-d], whereas heterocycles including thiophenes, furans, thiazoles, oxazoles, imidazoles, pyrroles and indoles, etc. are excellent substrates. For instance, the heteroaryl Heck reaction of 2-chloro-3,6-diethylpyrazine (1) and benzoxazole occurred at the C(2) position of benzoxazole to elaborate pyrazinylbenzoxazole 2 [12e]. 

The push-spectator stabilization system enables one to employ various alkyl groups with different types of steric environment, which differentiate amino(alkyl) carbenes dramatically from the NHCs as ligands. Taking advantage of their steric and electronic properties, Bertrand et al. nicely demonstrated the utility of CAACs as ligands in the palladium catalyzed a-arylation of ketones. Depending on the nature of the aryl chloride used, dramatic differences were observed in the catalytic activity of Pd-complexes with CAACs featuring different types of steric environment [36]. 

Palkovits et al. took a different approach to the telomerization of 1,3-butadiene with glycerol, screening various phosphine ligands and running the reaction without solvent. The importance of the steric and electronic properties of the phosphine ligand on activity and selectivity of the palladium catalyst has been highlighted in... 

Of course, such a chemical probing would be possible only if we can prove, by means of other (mainly physical) techniques, that the existence of electron-deficient palladium in supported palladium is possible. Therefore, the organization of this section is as follows First, we discuss the results of XPS studies of electronic properties of small Pd particles deposited on various supports. Then we examine other evidence for the existence of positively charged Pd species using other techniques, such as electron spin resonance (ESR) and infrared (IR) spectroscopy of adsorbed CO. Finally, catalytic consequences of the appearance of positively charged species in the Pd/support catalysts will be demonstrated. 

Baetzold used extended Hiickel and complete neglect of differential overlap (CNDO) procedures for computing electronic properties of Pd clusters (102, 103). It appeared that Pd aggregates up to 10 atoms have electronic properties that are different than those of bulk palladium. d-Holes are present in small-size clusters such as Pd2 (atomic configuration 4dw) because the diffuse s atomic orbitals overlap strongly and form a low-energy symmetric orbital. In consequence, electrons occupy this molecular orbital, leaving a vacant d orbital. For a catalytic chemist the most important aspect of these theoretical studies is that the electron affinity calculated for a 10-atom Pd cluster is 8.1 eV. This value, compared to the experimental work function of bulk Pd (4.5 eV), means that small Pd clusters would be better than bulk metal as electron acceptors. 

Another indication that electronic properties of Pd may be important in hydrogenation reactions originates from the work of Carturan et al. (167), who investigated palladium supported on vitreous materials in hydrogenation of phenylacetylene. A relatively better catalytic activity of the catalyst with smaller alkaline content (Na20) suggests that an electron transfer from Pd to the support is smaller in the case of less acidic (containing more alkaline) supports. Similar metal particle sizes (2.8-3.4 nm) exhibited by all the catalysts rule out an explanation that takes into account a surface sensitivity of this reaction. 

Another aspect of evolution of Pd catalysts during a reaction is the change in the electronic properties of palladium. Namely, both in the reaction of ethylene dimerization (in the absence of H2 in the gas phase) and in the reaction of CO with H2, drastic changes in the activity and selectivity during initial stage of the reaction were correlated with the fact of formation of electron-deficient Pd species. 

Arylamines display electronic properties that are favorable for materials science. In particular, arylamines are readily oxidized to the aminium form, and this leads to conductivity in polyanilines, hole-transport properties in triarylamines, stable polyradicals with low energy or ground-state, high-spin structures, and the potential to conduct electrochemical sensing. The high yields of the palladium-catalyzed formation of di- and triarylamines has allowed for ready access to these materials as both small molecules and discrete oligomeric or polymeric macromolecules. 

Two studies have been conducted that outline the effects of ligand steric and electronic properties on the relative rates for reductive elimination of amine and P-hydrogen elimination from amides. One study focused on the amination chemistry catalyzed by P(o-C6H4Me)3 palladium complexes [111], while the second focused on the chemistry catalyzed by complexes containing chelating ligands [88]. 

Because the reductive elimination from DPPF-ligated palladium does not involve geometric rearrangements or changes in coordination number before the rate-determining step, the DPPF complexes allowed an assessment to be made of the electronic properties of the transition state in this reaction. The relative rates for elimination from amido groups were found... 

This paper describes the use of polydentate ligands to optimize the performance of palladium catalysts for CO2 reduction and to probe mechanistic aspects of catalytic reactions. Polydentate ligands can be used to precisely control coordination environments, electronic properties, and specific steric interactions that can lead to new insights into the relationship between catalyst structure and activity. 

The catalytic activity of Pti c polymer-protected bimetallics has been found to vary strongly with composition [75]. As shown in Figure 11, such catalysts have been studied by Pt NMR. However, because the palladium NMR has not been observed, the average surface electronic properties can be determined only indirectly and tentatively. The Pt NMR in Figure 11 has shown that the interior of the alloy particles is bulklike. In the bulk alloys the f-LDOS on both Pt and Pd sites varies rapidly with composition around x = 0.8 [54]. It is supposed, but not proven, that on the surfaces of the alloy particles the f-LDOS changes strongly with composition as well and that this explains the variation in catalytic activity. 

Palladium chemistry involving heterocycles has many unique characteristics stemming from the inherently different structural and electronic properties of heterocyclic molecules in comparison to the corresponding aromatic carbocycles. One salient feature of heterocycles is the marked activation at positions a and y to the heteroatom. For N-containing heterocycles, the presence of the N-atom polarizes the aromatic ring, thereby activating the ex and y positions, making them more prone to nucleophilic attack. For example, the order of S Ar displacement of heteroaryl hahdes with EtO is [3] ... 

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