Palladium clusters carbon monoxide

Relatively few investigations involving palladium carbonyl clusters have been carried out, partly because palladium per se does not form stable, discrete homometallic carbonyl clusters at room temperature in either solid or solution states.114,917-922 Nevertheless, solution-phase palladium carbonyl complexes have been synthesized with other stabilizing ligands (e.g., phosphines),105,923 and carbon monoxide readily absorbs on palladium surfaces.924 Moreover, gas-phase [Pd3(CO)n]-anions (n = 1-6) have been generated and their binding energies determined via the collision-induced dissociation method.925... 

In 1971 only two complexes of palladium(I) had been identified.65 Although the area has grown significantly, the relative paucity of palladium cluster compounds can be attributed, in part, to the surprising weakness of palladium-carbon monoxide bonds and in particular those where CO is bound terminally. In this chapter the chemistry of palladium(I) and clusters of palladium in other oxidation states will be considered. However, complexes containing organic ligands such as allyl and cyclopentadienyl will not be dealt with as this area has been reviewed recently in a companion volume.66... 

When the cluster [Pd4(C0)4(02CMe)4]-2MeC02H was dissolved in styrene and treated with methanol under an atmosphere of carbon monoxide, methyl cinnamate was formed.516 This reaction was also believed to occur by an alkoxycarbonyl route. The reaction became catalytic when [Pd(OAc)2] (106) was used in presence of NaOAc and a stoichiometric amount of copper(II) as reoxidant for the palladium(O) formed. Stille and co-workers have investigated this reaction, sometimes called carboalkoxylation, in detail. A basic difference between this reaction and the hydroesterification described above is that the oxidative nature of carboalkoxylation permits double functionalization of a double bond. Thus (E)- and (Z)-2-butene react readily with CO and methanol in the presence of a catalytic amount of PdCl2 and a stoichiometric amount of CuCl2 to give methyl 3-methoxy-2-methylbutanoate (equation 124).517,518... 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

Conditions for obtaining unsaturated esters rather than the bicarboxylic ones were the subject of much work, most of which was a matter of patents see, for example. Ref. [12]. Cinnamic methyl ester, previously prepared in low yield from styrenef was obtained preferentially when methanol was added to Moiseev s clusters, a tetrameric palladium complex, previously treated with styrene under carbon monoxide atmosphere at 25 °C (Scheme 9). 

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