Palladium complexes oxygen donors

Palladium(Il) Complexes with Other Chelating Ligands Containing an Oxygen Donor 1115... 

As with many other examples of palladium compounds containing oxygen donors, alkoxide complexes have very rarely been isolated. Two compounds which have been obtained are frfl/is-[Pd(OMe)(CN)(PEt3)2]142 and the methoxide-bridged dimer [Pd2(ju-OMe)2(2,2,6,6-tetramethylheptanedionato)2].143... 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other palladium-catalyzed oxidations of olefins in various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

Note The cationic complex in Figure3.69 features a rare Pd-0 contact that is caused by an unavoidable close proximity between the palladium atom and one of the methoxy groups rather than an affinity of the palladium centre for the oxygen donor. 

Note The cationic palladium(II) carbene complex prefers the weak donor acetonitrile over the chelate carbonyl oxygen donor. 

The literature [101, 102] cites the hydrogenation and isomerization of allylbenzene in the presence of palladium complexes on polymeric supports having oxygen-containing coordination groups like -OPh, -COOH, -OH. It was shown that the activity and selectivity of synthesized polymeric catalytic systems were affected by both the electron-donor properties of graft coordination groups and the solvent type. In the presence of the catalyst, the isomerization of allylbenzene to 1-propenylbenzene mainly yields the trans isomer. 

Metal-solvent interactions can be conveniently considered in terms of the hard-soft acid-base (HSAB) principle (Chapter 1). For example, palladium(II) is a soft metal center and so the hard oxygen donor solvent, diethyl ether, interacts only poorly with it. Simple valence bond models have been presented that adequately explain such soft metal-hard base interactions. In this chapter complexes containing coordinated halocarbons are treated in a separate section (3.7) from those that contain other "hard bases" since such species have only been recognized as well-defined complexes in recent years and their potential for exploitation in coordination chemistry merits special attention. 

Oxygen-containing solvents such as water, alcohols or ethers are such poor donors that few complexes with palladium(II) have been isolated. The most important class of complexes of this type consists of those containing water, which are formed as intermediates in the substitution reactions of palladium(II) when carried out in aqueous solution. In these reactions their formation is in competition with the second order reaction of the complex with the incoming ligand. The aqua complexes can also be formed by reaction of halo complexes with silver salts (e.g. N03, C104, BF4) in water. These complexes are acidic, being in equilibrium with hydroxo complexes in neutral or basic media. 

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