Palladium Phosphine oxides, nickel complexes with

In a Kumada-Corriu reaction, an aryl halide is oxidatively coupled with a homogeneous nickel(ll)-phosphine catalyst [2], This species reacts with a Grignard reagent to give biaryl or alkylaryl compounds. Later, palladium-phosphine complexes were also successfully applied. By this means, stereospecific transformations were achieved. 

The carboxylation reaction shown in reaction (11) is catalyzed by both nickel and palladium phosphine complexes. For example, Ni(dppe)Cl2 (where dppe is l,2-bis(diphenylphosphino)ethane) and Pd(PPh3)2Cl2 both catalyze reaction (11) [84-86]. Mechanistic studies have been carried out on these two systems, and the results indicate that two different mechanisms are involved. In the case of the Ni complex, the first step is the reduction of Ni(dppe)Cl2 to a transient Ni(dppe) species [85]. This process occurs in two one-electron steps (reaction 12). Bromobenzene then oxidatively adds to Ni(dppe) to form Ni(dppe)(Br)(Ph), reaction (13). The resulting Ni(II) aryl species is reduced in a one-electron process to form Ni(dppe)(Ph), which reacts rapidly with CO2 to form a Ni—CO2 intermediate as shown in reaction (14). The rate-determining step for the overall catalytic reaction is the insertion of CO2 into the Ni-aryl bond, reaction (15) step 1. This reaction is followed by a final one-electron reduction to regenerate Ni(dppe), the true catalyst in the cycle (reaction 15, step 2). 

The pale yellow [Ni(PEt3)4] is also tetrahedral but with some distortion. In sharp contrast to nickel, palladium forms no simple carbonyl, Pt(CO)4 is prepared only by matrix isolation at very low temperatures and reports of K4[M(CN)4] (M = Pd, Pt) may well refer to hydrido complexes in any event they are very unstable. The chemistry of these two metals in the zero oxidation state is in fact essentially that of their phosphine and arsine complexes and was initiated by L. Malatesta and his school in the 1950s. Compounds of the type [M(PR3)4], of which [Pt(PPh3)4] has been most thoroughly studied, are in general yellow, air-stable solids or liquids obtained by reducing complexes in H2O or H20/EtOH solutions with hydrazine or sodium borohydride. They are tetrahedral molecules whose most important property is their readiness to dissociate in solution to form... 

Nickel catalysis has also been used in the formation of biaryls, such as (51), by substitution of the methoxy group in 1-methoxynaphthalene by tolylmagnesium bromide. It is also reported that the reaction of aryl or heteroaryl tosylates with phenylmag-nesium bromide to give biaryl derivatives is catalysed by palladium complexed with heteroatom-substituted secondary phosphine oxide ligands. 

In these and related reactions, the ether and alcohol ligands dissociate readily. The coordinated THF dissociates from the zirconium methyl complex to allow olefin polymerization, albeit more slowly than in the absence of THF. The ether dissociates from palladium and nickel to allow olefin to bind to the cationic palladium and nickel species, and alcohol and water are easily displaced from related Pt(II) complexesby hydrocarbons prior to C-H activation processes. Likewise, the THF and phosphine oxide ligands reversibly dissociate from the zirconium knido complex (Equation 2.18) prior to [2+2] additions with alkynes. - ... 

There are many carbonyl complexes of nickel, palladium, and platinum containing phosphines (L). Nickel compounds of the type [Ni(CO)4 j,Lj,] are readily formed in substitution reactions of [Ni(CO)4]. Palladium and platinum phosphine carbonyls are prepared by reactions of compounds of these metals with carbon monoxide in the presence of phosphines. The following complexes are known [M(CO)L3], [M3(C0)3L3], [M3(C0)3L4], [Pt(CO)2L2] and [M4(CO)5L4] (M = Pd, Pt). Trinuclear platinum compounds resist oxidation. 

The arylation of activated alkenes with aryl halides in the presence of base was discovered by R. F. Heck in 1971 and is now one of the standard methods for C—C bond formation. The catalysts are mostly palladium or nickel phosphine complexes, which react via a succession of oxidative addition and insertion reactions, as shown in the following simplified cycle ... 

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