Palladium Complexes Containing Metal Ligands

SilylpaUadium complexes have attracted attention due to their relevance in the mechanism of Pd complex catalyzed reactions such as hydrosilylation of alkenes and dienes, bis-silylation of dienes and alkynes, carbosilylation of alkynes, cross-coupling of organic halides with disilanes, and ring-opening oligomerization and polymerization of cyclic disilanes and polysilanes. 

A variety of organosilyl-hydridopalladium complexes have been assumed to be generated in situ from hydrosilanes through oxidative addition of the silicon-hydrogen bonds onto palladium complexes. However, few studies have been reported on the isolation and characterization of silyl-hydrido complexes because of their instability. 

In contrast, bis(organosilyl)palladium(II) complexes were easily prepared through oxidative addition of the Si—Si bonds onto low-valent palladium complexes. 

Handbook of Organopalladium Chemistry for Organic Synthesis, Edited by Ei-ichi Negishi ISBN 0-471-31506-0 2002 John Wiley Sons, Inc. 

An X-ray analysis of the air-sensitive aystal of the bis( 1-adamantyl isocyanide)pal-ladacycle complex 3b revealed a square planar stracture with two coordinated isocyanide ligands on the palladium atom. An addition of phosphine ligands to a solution of 3 induced reductive elimination of the Si—Si bond to give 1,2-disilacyclopentane 2. Thus, tert-alkyl isocyanide ligand plays a critical role to stabilize bis(organosilyl)palladium(II) complexes. 

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Platinum and palladium complexes containing metal-carbon o bonds and metal-bipyridine coordination bonds have been incorporated into dendrimeric materials. " Polymers with both platinum and palladium coordinated to bipyridine ligands (83) have been synthesized by Puddephatt and coworkers. These dendrimers were prepared by oxidative addition of a C Br bond to a platinum complex to give die core molecule. Further reaction with the metal complex resulted in homo- or heterometallic materials. 

The first examples of highly active olefin polymerization catalysts based on late transition metals were nickel and palladium complexes containing bulky diimine ligands.310 312 For example, complex (120) was found to polymerize ethylene with an activity of ll,000gmmol h bar A range of PE materials with molecular weights up to 106 and... 

Catalytic enantioselective synthesis of 4,4-dimethyl-l-phenyl-l,2-pentadiene from 4,4-dimethyl-1,2-pentadiene and iodobenzene using 0.4 to 1 mol % of palladium complexes containing chiral phosphane ligands as the catalyst for the enantioselective cross coupling134 is the only example of substoichiometric transition metal catalyzed enantioselective allene synthesis. 

Dicarboxylic acids form monomeric complexes with palladium(II), K2[Pd(X2)2] (X2 = oxalate, malonate, etc,).153154 They may be prepared by warming a suspension of palladium(II) chloride with a concentrated solution of the alkali metal dicarboxylate or by using other palladium complexes containing readily substituted ligands such as [Pd(OH)2], [Pd(N03)2(H20)2] or [Pd(02CMe)2]3-155 These complexes are claimed to have useful antitumour properties.155 Complexes [Pd(X2)L2] (X2 = dicarboxylate L = amine or L2 = diamine) may be prepared by reaction of the dichloro complex with a carboxylate salt.156,128... 

Better catalytic behavior was observed in the copolymerization of CO and norbornadiene with palladium complexes containing a hemilabile pyridyl-carbene ligand [23]. The choice of a pyridine-functionalized carbene can be explained by the fact that the hemilabile arm in such a ligand is capable of reversible dissociation from the metal center, leading to vacant coordination sites for the complexation of the substrates while the strong donor moiety remains bound to the metal. 

The carbonylation of aryl halides with alcohols and amines catalysed by palladium complexes with triphenylphosphine ligand is the convergent and direct route to the synthesis of aromatic esters as well as aromatic amides. Even though these palladium complexes are widely employed as the best catalytic system, those catalysts are difficult to separate and reuse for the reaction without further processing. The major drawbacks are oxidation of triphenylphosphine to phosphine oxide, reduction of palladium complex to metal and termination of the catalytic cycle. The phosphine-free, thermally stable and air resistant catalyst (1) containing a carbon-palladium covalent bond (Figure 12.3) has been found to be a highly selective and efficient catalyst for the carbonylation of aryl iodides.[1]... 

Alkynyl complexes contain metal-carbon bonds in which the metal is bound to the sp-hybridized carbon at the terminus of a metal-carbon triple bond. The materials properties of these complexes have been investigated extensively. The properties of these complexes include luminescence, optical nonlinearity, electrical conductivity, and liquid crystallinity. These properties derive largely from the extensive overlap of the metal orbitals with the ir-orbitals on the alkynyl ligand. The M-C bonds in alkynyl complexes appear to be considerably stronger than those in methyl, phenyl, or vinyl complexes. Alkynyl complexes are sometimes prepared from acetylide anions generated from terminal alkynes and lithium bases (e.g., method A in Equation 3.42), but the acidity of alkynyl C-H bonds, particularly after coordination of the alkyne to the transition metal, makes it possible to form alkynyl complexes from alkynes and relatively weak bases (e.g., method B in Equation 3.42). Alkynyl copper complexes are easily prepared and often used to make alkynylnickel, -palladium, or -platinum complexes by transmetallation (Equation 3.43). This reaction is a step in the preparation of Ni, Pd, or Pt alkynyl complexes from an alkyne, base, and a catalytic amoimt of Cul (Equation 3.44). This protocol for... 

Indeed, direct measurements of the rates of insertion of CO and ethylene into alkyl-metal olefin and acylmetal olefin complexes show that the insertion of ethylene into the metal-acyl linkage is faster than the insertion of ethylene into the metal-alkyl linkage. Comparisons of these rates for insertions into cationic palladium complexes containing phenanthroline and bis-diphenylphosphinopropane as ancillary ligand have been made by Brookhart and co-workers. These reactions are shown in Equations 9.69 and 9.70. A summary of the barriers for insertion is provided in Table 9.2. The rate of insertion of ethylene into the metal-acyl bond is orders of magnitude faster than the rate of insertion of ethylene into the metal-alkyl bond. - - ... 

A. L. Balch, Adv. Chem. Ser, 1982, 196, 243-255. Novel Reactions of Dinuclear Metal Complexes. Reactions and Structures of Palladium Complexes Containing Is(diphenylphosphino)methane as a Bridging Ligand. 

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