Palladium aqua complex

The palladium chloride process for oxidizing olefins to aldehydes in aqueous solution (Wacker process) apparendy involves an intermediate anionic complex such as dichloro(ethylene)hydroxopalladate(II) or else a neutral aqua complex PdCl2 (CH2=CH2)(H2 0). The coordinated PdCl2 is reduced to Pd during the olefin oxidation and is reoxidized by the cupric—cuprous chloride couple, which in turn is reoxidized by oxygen, and the net reaction for any olefin (RCH=CH2) is then... 

Several cationic palladium(II) aqua complexes, [Pd(H20)4], cis-[PdL(H20)2] (L = en, methionine methyl ester, l,5-dithiacycloocta-3-ol), and [Pd(dien)(H20)], serve as the active catalyst for the selective hydration of various nitriles to the corresponding carboxamides, e.g., CHCI2CN was hydrated to CHCl2C(0)NH2 in the presence of... 

Selective cleavage of peptides and proteins is an important procedure in biochemistry and molecular biology. The half-life for the uncatalyzed hydrolysis of amide bonds is 350 500 years at room temperature and pH 4 8. Clearly, efficient methods of cleavage are needed. Despite their great catalytic power and selectivity to sequence, proteinases have some disadvantages. Peptides 420,423,424,426 an(j proteins428 429 can be hydrolytically cleaved near histidine and methionine residues with several palladium(II) aqua complexes, often with catalytic turnover. 

Kostic et al. reported the use of various palladium(II) aqua complexes as catalysts for the hydration and alcoholysis of nitriles,435,456 decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.457 Labile aqua or other solvent ligands can be displaced by a substrate. In many cases, the coordinated substrate thus becomes activated toward nucleophilic addition of water or alcohols. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

L. I. Elding, and L. F. Olsson, Electronic absorption spectra of square-planar chloro-aqua and bromo-aqua complexes of palladium (11) and platinum (11), J. Phys. Cherru 82(1), 69—74 (1978)... 

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. 

W-(2-Aminoethyl)-l,2-diaminoethane (dien) yields complexes [PdX(dien)]X (X = Cl or I) 204,20s a range of solvated complexes may be obtained by reaction of [Pdl(dien)] with AgC104 in the appropriate solvent. As for the platinum analogue, the aqua complex [Pd(dien)(H20)](C104)2 reacts with tetraphenylborate to yield [PdPh(dien)](BPh4). Reaction of the hydroxo complex with carbon monoxide yields C02 and palladium metal is deposited.206... 

Palladium(IV) complexes are rather sparse and much less stable than those of Pt. The best known are the hexahalogeno complexes [PdXe] - (X = F, Cl, Br) of which [PdCle], formed when the metal is dissolved in aqua regia, is the most familiar. In all of these the Pd is readily reducible to Pd°. In water, [PdFe] hydrolyses immediately to PdOa-xHaO while the chloro and bromo complexes give [PdX ] plus Xa- An organometallic chemistry of Pd is developing (p. 1167). 

Methyl-group transfer of a different kind has been observed in the reaction of methylcobalamin with the [PdCy ion. Although a fast reaction occurs to give a palladium(ii) complex in which methylcobalamin is co-ordinated to the 5,6-dimethyl-benzimazole nitrogen, this complex is inactive towards methyl-group transfer. However, a much slower direct reaction occurs to give Pd metal, MeCl, and aqua-and chloro-cobalamins. ... 

To access useful arylamines derivatives, the palladium catalyst is usually prepared as a cationic aqua or diaqua palladium(ll) complex or other organopalladium complex. In the Scheme 6.23, a putative catalytic cycle for the Pd-catalyzed asymmetric addition reaction is shown. The hydroxyl-Pd species (A) is formed in the presence of a base or MS that undergo transmettJlation with an organometallic reagent such as ArM to produce the corresponding aryl palladium complex (B). Insertion of an imine substrate into the Ar-Pd bond, followed by hydrolysis, affords the desired product [29b]. 

Van Koten et al. have found that NCN-palladium complexes, for example, [(NCN)Pd-(H20)][BF4], (146) are active catalysts in various C-C bond formation reactions, such as aldol condensation and the Michael reaction. Recently, they have developed the attachment of palladium(II) complexes (146) to Cgo (Scheme 16.38) [45]. The corresponding cationic aqua complexes were evaluated as Lewis acid catalysts for the Michael addition reaction of (49a) and (64), and aldol condensation of (la) and methyl isocyanoacetate (64). In the case of Michael reactions catalyzed by methanofullerenes (18)-(20) are significantly slower than the reaction catalyzed by (146). On the other hand, no such deactivation process was observed using... 

Several processes are available for the recovery of platinum and palladium from spent automotive or petroleum industry catalysts. These include the following. (/) Selective dissolution of the PGM from the ceramic support in aqua regia. Soluble chloro complexes of Pt, Pd, and Rh are formed, and reduction of these gives cmde PGM for further refining. (2) Dissolution of the catalyst support in sulfuric acid, in which platinum is insoluble. This... 

Noble metals, such as gold, platinum, palladium, rhodium and iridium are not attacked by nitric acid at ordinary temperatures. These metals, however, dissolve in aqua regia (3 1 HCl— HNOs mixture). Nitric acid in aqua regia oxidizes gold to Au , which readily combines with Cl to form soluble chloro-complex, AuClF. 

Palladium was discovered in 1803 by W.H. Wollaston during refining and purification oP platinum metal. This new metal was found in the aqua regia extract of native platinum and was detected in solution after platinum was precipitated. It was removed as ammonium chloroplatinate. Treating this solution with mercurous cyanide precipitated a yellow palladium complex salt. The precipitate was washed and ignited to form palladium metal. Wollaston named the element palladium after the newly discovered asteroid Pallas. 

---