Palladium PdCl

The first member of the chloro-bridged series di-M-chlorodi-ir-allyldi-palladium, [PdCl(Tr-C3H6)]2 is conveniently prepared by treating allyl alcohol (162, 168, 194) or allyl chloride (121, 122, 123) with palladium... 

Chiral phosphinous amides have been found to act as catalysts in enantio-selective allylic alkylation. Horoi has reported that the palladium-catalyzed reaction of ( )-l,3-diphenyl-2-propenyl acetate with the sodium enolate of dimethyl malonate in the presence of [PdCl(7i-allyl)]2 and the chiral ligands 45 gave 46 in 51-94% yields and up to 97% ee (Scheme 38). It is notorious that when the reaction is carried out with the chiral phosphinous amide (S)-45a, the product is also of (S) configuration, whereas by using (R)-45b the enantiomeric (R) product is obtained [165]. 

Very recently, well-defined complexes with general formula [PdCl(T -Cp) (NHC)] were synthesised and tested for the homocoupling of non-electrodeficient arylboronic acids at room temperature with good results (Scheme 7.7) [51]- This new class of catalysts were synthesised from commercially available NHC palladium(II) chloride dimers and are air-stable. 

Mixed palladium/platinum analogues of these trinuclear clusters have also been reported. The first of these, [PtPd2Cl(PPh2)2(PPh3)3]+, was prepared by the reaction of [PdCl(PPh3)3]+ with [PtCl(PPh3)3]+.538... 

The reaction is catalyzed by palladium complexes either pre-formed, as [Pd(TPPMS)3] [13], or prepared in situ from (usually) [Pd(OAc)2] and various phosphines [21,23-27], TPPTS being one of the most frequently used [14]. Other precursors, e.g. [ PdCl(T -C3H5) 2] and so-caUed ligandless (phosphine-free) Pd-catalysts can also be effective. In fact, in several cases a phosphine inhibition was observed [23]. The solvent can be only slightly aqueous (5 % water in CH3CN, [14]) or neat water [26]. In the latter case a biphasic reaction mixture (e.g. with toluene) facilitates catalyst separation albeit on the expense of the reaction rate. A short selection of the reactions studied in aqueous solvents is shown on Scheme 6.9. 

One of palladiums unique characteristics is its abihty to absorb 900 times its own volume of hydrogen gas. When the surface of the pure metal is exposed to hydrogen gas (H ), the gas molecules break into atomic hydrogen. These hydrogen atoms then seep into the holes in the crystal structure of the metal. The result is a metallic hydride (PdH that changes palladium from an electrical conductor to a semiconductor. The compound palladium dichloride (PdCl ) also has the ability to absorb large quantities of carbon monoxide (CO). These characteristics are useful for many commercial applications. Palladium is the most reactive of all the platinum family of elements (Ru, Rh, Pd, Os, Is, and Pt.)... 

The reaction between aqueous K2PdCl4 and CMe2(OH)CsCCsCCMe2(OH) (L) has been reported to give the palladium(I) complex PdCl(L), which exchanges L for X with salts MX (M = alkali metal, X = SCN, Br, I) and adds py to give PdCl(L)(py). However, it must be said that the dark brown to black solids so formed are not fully characterized by contemporary standards. It was assumed that hydropalladation of one C C triple bond has occurred. ... 

Palladium dichloride forms polymeric carhonyl complexes when the dry chloride is heated in a stream of carhon monoxide charged with methane vapor. Such complexes include [PdCl2(CO)n] and [PdCl(CO)2]n. The reaction also occurs in aqueous phase resulting in decolorization of the solution. 

When ammonia gas is passed through an aqueous solution of PdCL, the product is tetrammine palladium(II) chloride, Pd(NH4)2Cl2. The same product also is obtained in dry state by passing ammonia gas over anhydrous PdCL. 

Palladium forms the largest series of ir-allylic complexes of any metal, and many of these complexes are readily prepared. The majority are chloro-bridged complexes of the type [PdCl(all)]2, where all = an allylic radical. 

PdCl(7r-C3H5)]2 is reported to exist in two forms (163), i.e., the yellow or a-form described above, and a dark green or /3-form prepared by reacting palladium chloride with an acidified aqueous solution of allyl alcohol below 10° at 15° the product is the yellow a-form (163). Isomerization of this type has not been reported for other 7r-allylicpalladium complexes. 

Diphenylacetylene and methylphenylacetylene react with palladium halides to give a very interesting series of compounds some of which are almost certainly ir-allylicpalladium complexes. Thus diphenylacetylene (ChHjo) reacts with palladium(II) chloride in ethanol to give hexaphenyl-benzene and a complex of the composition [PdCl(Ci4Hi0)2(OC2H6)] 148, 149) which we formulate as the ir-allylic complex (XLVI) (see Section... 

The 7r-allylpalladium complex [PdCl(7r-C iH6)]2 on heating decomposes quantitatively into allyl chloride and palladium. The analogous ir-meth-allylpalladium complex gives a 96% yield of methallyl chloride but a little isobutene is also formed 123). 

Treatment of the chloro-bridged complex [PdCl(7r-C3H5)]2 with cyclo-pentadienylsodium gives 7r-al lyl-rr-cyclopentadienylpalladiu m (II) [Pd(ir-C3H6)(7r-C6H6)] as dark red, voltatile crystals, mp 64° (190). Its nmr spectrum (191) confirms the sandwich structure (L) in which the palladium has the electronic configuration of xenon. 

Palladium appears to be especially suitable for forming these complexes. As an example, allyl alcohol reacts with palladium (II) chloride (236) to give bis(allylpalladium chloride), [(C3H6)PdCl]2. Its nmr spectrum shows three peaks (56), in the ratio 1 2 2, indicating the structure shown in Fig. 20. The exact location of the allyl groups with respect to the metal... 

Importantly, the Pd(tppts)3 catalyst, generated in situ by complexation of PdCl2 with tppts in H20 followed by reduction of the resulting [PdCl(tppts)3]+ complex with CO,448 451 was stable under hydrocarboxylation conditions 452 In contrast, the Pd/PPh3 system in organic solvents is not completely stable with respect to palladium metal formation. 

Heterobinuclear complexes have been formed between PdCl(acac-Ci), bipy and Be(acac)2.271 The palladium compound retains an acid character and so replacement of an acac" from the Be(acac)2 gives a product which can be formulated as (50) from MW and NMR studies. 

Fig. 5-8.—The structure of the PdCls crystal. Small circles represent palladium atouws and large circles chlorine atoms.

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