Palladium carbonyl, structure

The palladium and platinum metals also form carbonyl compounds. Of the expected compounds Pd(CO)4, Pt(CO)4, Ru(CO)5, Os (CO) 5, Mo-(CO)e, and W(CO)6 only Mo(CO)e has been prepared, although some unsaturated ruthenium carbonyls have been prepared. The compounds Pd(CO)2Cl2, Pt(CO)2Cl2, K[PtCOCl3], etc., show the stability of the four dsp2 bonds. It would be interesting to determine whether or not each CO is bonded to two metal atoms in compounds such as [Pt(CO)Cl2]2, whose structure is predicted to be... 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

Another way to produce acetic acid is based on a carbonylation of methanol in the so called Monsanto process, which is the dominant technology for the production of acetic acid today [15]. Acetic acid then is converted to VAM by addition of ethylene to acetic acid in the gas phase using heterogeneous catalysts usually based on palladium, cadmium, gold and its alloys (vinylation reaction 3 in Fig. 2) [16] supported on silica structures. 

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

Furukawa and co-workers (368,369) succeeded in applying the softer dicationic Pd-BINAP 260 as a catalyst for the 1,3-dipolar cycloaddition between 225 and 241a (Scheme 12.82). In most cases, mixtures of endo-243 and exo-243 were obtained, however, enantioselectives of up to 93% ee were observed for reactions of some derivatives of 225. A transition state structure has been proposed to account for the high selectivities obtained for some of the substrates (368). In the structure shown in Scheme 12.82, the two phosphorous atoms of the Tol-BINAP ligand and the two carbonyl oxygens of the crotonoyl oxazolidinone are arranged in a square-planar fashion around the palladium center. This leaves the ii-face of the alkene available for the cycloaddition reaction, while the re-face is shielded by one of the Tol-BINAP tolyl groups. 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

A structurally unusual 3-blocker that uses a second molecule of itself as the substituent on nitrogen is included here in spite of the ubiquity of this class of compounds. Exhaustive hydrogenation of the chromone (13-1) leads to a reduction of both the double bond and the carbonyl group, as in the case of (11-2). The car-boxyhc acid is then reduced to an aldehyde (13-2) by means of diisobutylaluminum hydride. Reaction of that intermediate with the ylide from trimethylsulfonium iodide gives the oxirane (13-3) via the addition-displacement process discussed earlier (see Chapters 3 and 8). Treatment of an excess of that epoxide with benzylamine leads to the addition of two equivalents of that compound with each basic nitrogen (13-4). The product is then debenzylated by catalytic reduction over palladium to afford nebivolol (13-5) [16]. The presence of four chiral centers in the product predicts the existence of 16 chiral pairs. 

Comparing these results for CO bonded to Ni(lOO) and Pd(lOO) to the structure of metal carbonyl clusters, one finds that the multiply-coordinated CO on palladium has... 

In connection with the structure of carbonyl metal complexes, these bands seem to be the result of the symmetric and the antisymmetric stretching vibrations of two CO molecules bonded linearly with the same Pd(II) ion. Imelik et al. (23) have shown that palladium ions are trigonally coordinated in Si, sites (d Om-Pd = 2 A). Because of chemisorbed CO, the palladium ions acquire a trigonal bypyramidal coordination. 

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