Carbonylation Palladium

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... 

Polynuclear platinum and palladium carbonyl clusters containing the bulky tri-ferf-butylphosphine ligand are inherently electron-deficient at the metal centers. The trigonal bipyramidal cluster [Pt3Re2(CO)6(P Bu3)3], as shown in Fig. 11.4.4(a), is electronically unsaturated with a deficit of 10 valence electrons, as it needs 72 valence electrons to satisfy an 18-electron configuration at... 

Palladium carbonyls would produce a Cls XPS signal at 287-288 eV (Barber et al., 1972). This range is not obscured by adsorbed or gas-phase... 

CO, and therefore we exclude the presence of these bulk metal carbonyls. The dissociation of CO by the Boudouard reaction (or the decomposition of palladium carbonyls) would lead to carbon deposition (Kung et al., 2000 McCrea et al., 2001) and should produce a feature at 284.0 eV characteristic of graphite or at 284.4 eV characteristic of amorphous carbon. In the case of carbide species, a feature at lower BE (<283.5 eV) would appear. Even if carbon dissolved in the palladium bulk near the surface region, the escape depth of the Cls electrons (about 2 nm) should have been sufficient to allow its detection. The absence of any carbon-related signals indicates that CO does not dissociate at 400 K and approximately 1 mbar, even over the course of several hours. This result is an important argument in the discussion about the possibility of CO dissociation at high pressure. 

Drozdova, L., Novakova J, Schulz-Ekloff G, Jaeger N. I. Ship-in-bottle synthesis of palladium carbonyl complexes in NaY and NaX zeolites via the... 

Their number, however, is small compared to the rich cluster chemistry found for other transition metals, possibly because of the lower stability of palladium carbonyls. High nuclearity clusters are dealt with in another article (see Polynuclear Organometallic Cluster Complexes). 

Thus the iron atom in (Tr-cyclopentadienvl)iron dicarbonyl bromide forms additional bonds with the halide ligands on palladium. Carbonyl substitution on the generated intermediate (XV) then occurs. In this case the cyclopentadienyl group acts as a bridge. In general, such a mechanism corresponds to an A-type substitution, where the halogens in (7T-tetraphenyl-cyclobutadiene)palladium dichloride function as nucleophiles. 

Zero-valent palladium carbonyls without phosphine ligands have been prepared by two groups of workers by the technique of matrix isolation at low temperatures. Carbonyls of the type Pd(CO) (m = 1 to 4) have been characterized by IR spectra. Diffusion studies indicate that the lower carbonyls react readily with CO to give Pd(CO)4 (46, 167). 

Olefins are usually carbonylated in the presence of metal carbonyls, such as nickel, cobalt, and iron carbonyls under homogeneous conditions, and the mechanism of these carbonylations has been established in several cases. On the other hand, isolation or formation of true palladium carbonyl has not been reported. Since palladium is an efficient and versatile catalyst for various types of the carbonylation mentioned above, the mechanisms of the carbonylation of olefin-palladium chloride complexes and of metallic palladium catalyzed carbonylations seem to be worth investigating. 

While, because of our interest in bimetallic species, most of the work discussed in this account has concentrated upon reactions which involve dppm, the synthetic procedure outlined clearly represents a novel and extremely simple approach to metal carbonyl phosphine complexes in general. Of particular significance, however, are the one-step reactions which lead to heterobimetallic or phosphido-bridged complexes and the ready access to palladium carbonyl complexes. Further research in this area is likely to be extremely productive. 

Luo etal used both HP-IR and HP-NMR in their studies of CO-ethylene co-polymerization. Using a Pd(ll)/dppp/ CF3COOH catalyst system under GO pressure, they detected three palladium-carbonyl absorptions in the IR spectrum, and linked these to three intermediates in the proposed mechanism. In addition, H NMR revealed the presence of coordinated ethylene and a palladium-methylene species (Pd-CH2-). The co-polymerization of CO with propylene was investigated using the unsymmetrical phosphine-phosphite bidentate ligand, (RSl-BINAPHOS. Two major resting states of the catalyst were observed by HP-NMR, namely alkylpalladium and acylpalladium species which are believed to correspond to [Pd(COR)(BINAPHOS)(CO)]+[BF4]- and [Pd(CH2CHCH3(CO)R)(BINAPHOS)]+[BF4]U... 

The chemistry of palladium-carbonyl complexes has experienced extensive recent developments due to an increased interest in the role of palladium in surface catalysis especially in automobile exhaust catalysts. Palladium-carbonyl complexes are nevertheless stiU relatively rare, which is probably due to their relative instability in comparision with that of Ni complexes. The homoleptic Pd(CO)4 only exists at low temperatures (<80 K) in noble gas or CO matrices, in sharp contrast with isoleptic Ni(CO)4, which is stable at ambient temperature. Table 1 compiles some of the known and representative carbonyl complexes of palladium. 

Palladium-carbonyl complexes are typically formed from PdX2 (X = Cl, Br, or I) and CO. For example, Pd2(CO)2Cl4 is obtained by high-pressure carbonylation of PdCl2. Similarly, the anionic M[Pd(CO)X3] are obtained from carbonylation of PdX2 or M2(Pd2X6). While the iodide is relatively unstable, the chloride and bromide could be characterized by X-ray crystallography. 

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