Palladium electron-deficient

In this section we shall try to review available data on catalytic properties of electron-deficient palladium on supported catalysts. The term electron deficient we shall consider to mean very small clusters of Pd on various supports, and/or palladium ions stabilized by virtue of their presence in an appropriate chemical environment. 

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. 

However, in conclusion of his other work (110), Kohiki states that 

From this discussion one may conclude that, in spite of the present difficulties in interpreting XPS and UPS spectra, small Pd clusters, unsupported or supported (no matter what support is used), would contain fewer d electrons relative to the bulk metal. Next we shall furnish electron spin resonance and infrared evidence for the existence of electron-deficient Pd species in alumina- and zeolite-supported Pd catalysts. 

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Y.Z. Zhang, T.T. Wong, and W.M.H. Sachtler, The effect of Ca + andMg + ions on the formation of electron-deficient palladium-proton adducts in zeolite, J. Catal. 128, 13-22 (1991). 

Z. Karpinski, Catalysis by supported, unsupported and electron-deficient palladium, Adv. Catal. 37, 45-100 (1981). 

Catalysis by Supported, Unsupported, and Electron-Deficient Palladium... 

Even though the versatility of palladium in hydrogenation reactions is recognized, the explanation of its catalytic properties is still far from being satisfactory. The chapter by Z. Karpinski gives a comprehensive survey of Catalysis by Supported, Unsupported, and Electron-Deficient Palladium. ... 

Figueras et al. (105) found some direct evidence for electron-deficient palladium clusters on various cation-exchanged forms of zeolite Y from CO adsorption experiments. In particular, a correlation was observed between the turnover number for benzene hydrogenation and the CO stretching frequency. The shift toward higher frequency with increasing support acidity was considered as evidence for increased electron acceptor properties of the support. Further studies will, however, be required to provide a more detailed understanding of this phenomenon. 

In 2007, the Fagnou group achieved a much more practical and selective Ar-H/ Ar-H cross-coupling [50]. Electron deficient palladium(II) complexes can react via an electrophilic C-H activation mechanism with good selectivity for electron rich arenes. In contrast, Fagnou [51] recently showed that complimentary reactivity to this is displayed by some ArPd(II) complexes that react through a proton-transfer-palladation mechanism, and that they depend on arene C-H acidity rather than arene nucleophilicity (Scheme 31). 

Intermediates which are involved in heterogeneous catalysis could have ionic character, which require an extention of the general treatment of complex reactions. As an example we can consider the catalytic hydrogenation over oxides and sulphides, where intermediates of cationic character were proposed. Ionic intermediates are also possible in catalysis over metals, for instance in the case of neopentane transformations over electron deficient palladium, which occur via formation of carbocations. If we consider olefin hydrogenation over oxides or sulphide with a heterolytic dissociation of hydrogen, the mechanismn of this reaction can be presented in the following form... 

The existence of metal cations close to small metal particles in RU/AI2O3 has been noted," but they are not seen in Rh/AlaOs. They are however commonly seen with palladium catalysts, and electron-deficient palladium and Pd + cations are assigned unusually high catalytic activity in hydrocarbon reactions (Figure 2. lOB). (2) Such species (and similar platinum could arise in acidic... 

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