Palladium complexes oxidation state

Finally, it should be mentioned that two years later, an enantioselective oxidative (boron) Heck-type reaction was reported by Jung and coworkers [32] for a dinuclear NHC-derived CNO-based pincer complex for which for the first time a Pd /Pd" mechanism was suggested to be operative with palladium pincer-type crosscoupling catalysts. Reaction mechanisms without a change of the palladium s oxidation state have never been proposed to be operative for Heck cross-couphng reactions, but have been shown to be operative for xylene-derived selenium-based pincer complexes and related systems in the cross-couphng of vinyl epoxides (and aziridines) with organoboronic acids [24d, 33]. 

A limited chemistry of the +1 oxidation state of palladium and platinum has developed since the 1970s, mainly involving metal-metal bonded dinuclear complexes [61]. 

A wide variety of complexes are formed by both metals in the +2 oxidation state indeed, it is the most important one for palladium. The complexes can be cationic, neutral or anionic. Both Pd2+ and Pt2+ are soft acids so that many stable complexes are formed with S or P as donor atoms but few with O-donors, though there are important ammines. There are pronounced similarities between corresponding palladium and platinum complexes the latter are more studied (and less labile). 

Mononuclear complexes of palladium and platinum in the +3 oxidation state have only recently been unequivocally characterized [157]. The major advance has come in complexes with macrocyclic ligands such as 1,4,7-trithiacyclononane (ttcn) and 1,4,7-triazacyclononane (tacn) (Figure 3.96). 

Organometallic porphyrin complexes containing the late transition elements (from the nickel, copper, or zinc triads) are exceedingly few. In all of the known examples, either the porphyrin has been modified in some way or the metal is coordinated to fewer than four of the pyrrole nitrogens. For nickel, copper, and zinc the 4-2 oxidation state predominates, and the simple M"(Por) complexes are stable and resist oxidation or modification, thus on valence grounds alone it is easy to understand why there are few organometallic examples. The exceptions, which exist for nickel, palladium, and possibly zinc, are outlined below. Little evidence has been reported for stable organometallic porphyrin complexes of the other late transision elements. 

For palladium and platinum, dithiocarbamato complexes with the metal in the oxidation state of -H 2 and -i- 4 are known. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

The studies on palladium-phosphine complexes, especially those with diphosphine ligands, have mainly been focused on the complexes of palladium in lower oxidation state of 0 and 1 84,580,581,583,766,787,788,811-816 suc]1 as [Pd2(dppm)2]2+, [Pd2(dppm)3],815 [Pd3(dppm)3(/i3-... 

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

To circumvent some of the above-mentioned drawbacks of sulfur-based mercury chemodosimeters, a system based on the alkyne oxymercuration of 58 has been developed (Fig. 22) [146]. 58 shows high selectivity, a limit of detection of ca. 8 ppm, resistance against strong oxidants, and a positive reaction even in the presence of cysteine, which is known to form stable mercury complexes and is used for the extraction of mercury from tissue samples. Another metal that is well-known for its catalytic ability is palladium, catalyzing different reactions depending on its oxidation state. Since this metal is toxic, assessment of the maximum allowable concentration of Pd in consumer products such as pharmaceuticals requires highly sensitive and selective detection schemes. For this purpose, indicator 60 was conceived to undergo allylic oxidative insertion to the fluorescein... 

Electronic ligand effects are highly predictable in oxidative addition reactions a-donors strongly promote the formation of high-valence states and thus oxidative additions, e.g. alkylphosphines. Likewise, complexation of halides to palladium(O) increases the electron density and facilitates oxidative addition [11], Phosphites and carbon monoxide, on the other hand, reduce the electron density on the metal and thus the oxidative addition is slower or may not occur at all, because the equilibrium shifts from the high to the low oxidation state. In section 2.5 more details will be disclosed. 

Palladium(0).—Interest in this oxidation state is increasing. Full details have appeared of the preparation of the complexes [Pd(CO)(PPh3)3], [Pd3(CO)3-(PPhglj], and [Pd3(CO)3(PPh3) ], following an earlier communication. The oxidative addition reactions of these complexes are also described,... 

Such key features are strongly interconnected via the complex behavior of supported palladium. It is well known that CH4 combustion activity depends markedly on the oxidation state of palladium. In Figure 12.8, a typical conversion curve obtained in temperature-programmed combustion (TPC) experiments during heating/cooling cycles is plotted. 

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