Palladium oxidation states

The present study obviously demonstrates the significance of the promotion effect in oxidation with a O2/H2. There is deficient in direct information on the mechanism of this phenomenon. The role of promoter can be speculatively proposed as (i) facilitating oxygen transfer from activator (palladium species) to organic substrate and/or (ii) stabilizing the proper palladium oxidation state. In the latter case oxidized palladium is suggested to participate dioxygen activation. 

In this work, the mechanism of methane oxidation over PCI/AI2O3 catalyst is investigated, the palladium oxidation state under stream reaction is identified and the reactive form of oxygen is determined. XPS, thermal gravimetric and surface potential measurements are performed on this catalyst under various dynamic gaseous treatments. 

The supported palladium catalyst known to be the most active for total methane oxidation was the subject of considerable amount of research [1-9]. However, no agreement about the mechanism reaction was observed in the literature [1-8]. The Langmulr-Hinshelwood [1-4], the Eley-Rideal [5-7], and the Mars-Van Krevelen [8], mechanisms were proposed for the total oxidation of methane on the supported palladium catalysts. This diversity is explained by the variation of the active surface in each case. Indeed, according to Burch et al.[9], the active sites can be modified by the pre-treatment conditions, by the particle size, by the support nature and by the presence of some poisons such as chlorides. Others difficulties result from the fact that it is not confirmed if the active site is a partial or a total oxidized palladium particle. In addition, little is known about the reactive oxygen form. Indeed, it is not yet established if the reactive oxygen is a chemisorbed molecular or ionic form or a lattice oxygen ion. The aim of this paper is to identify the palladium oxidation state under catalytic stream, to study the reactive form of oxygen and to propose a mechanism of the reaction. 

As noted in the Introduction, the hydride-based cycle involves an increase of palladium oxidation state (from 0 to II or, possibly, according to recent suggestions from II to IV), while the alkoxycarbonyl-based cycle treated in this section implies a decrease or the preservation of the oxidation state (II). 

The process of choice for acetaldehyde production is ethylene oxidation according to the so-called Wacker-Hoechst process [route (c) in Topic 5.3.2]. The reaction proceeds by homogeneous catalysis in an aqueous solution of HQ in the presence of palladium and copper chloride complexes. The oxidation of ethylene occurs in a stoichiometric reaction of PdQ2 with ethylene and water that affords acetaldehyde, metallic palladium (oxidation state 0), and HQ [step (a) in Scheme 5.3.5). The elemental Pd is reoxidized in the process by Cu(II) chloride that converts in this step into Cu(I) chloride [step (b) in Scheme 5.3.5). The Cu(II) chloride is regenerated by oxidation with air to finally close the catalytic cycle [step (c) in Scheme 5.3.5). 

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

Table 27.2 Oxidation states and stereochemistries of compounds of nickel, palladium and platinum...

The mechanism by which this low oxidation state is stabilized for this triad has been the subject of some debate. That it is not straightforward is clear from the fact that, in contrast to nickel, palladium and platinum require the presence of phosphines for the formation of stable carbonyls. For most transition metals the TT-acceptor properties of the ligand are thought to be of considerable importance and there is... 

A third access to isocorroles was found7 when a tetrapyrrole 11 having an acrylaldehyde side chain was cyclized in presence of copper(II) or cobalt(II) salts. In this case isocorrole-9-carb-aldehydes 12 are formed with copper and cobalt in the oxidation state + III. The copper compound can easily be demetaled by hydrochloric acid to yield the metal-free isocorrole. In contrast, the cyclization of the tetrapyrrole in the presence of palladium(II) gives the isopor-phycene (see Section 1.7.1.). 

A few isocyanides of palladium and platinum are known in the zerovalent oxidation state. The best characterized compounds involve triangular M3 clusters with M-M bonds. 

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). 

Some data have been obtained on the activity of the catalyst in a reduced state [for nickel (141,143,144), palladium (144°), and molybdenum (145, 145a). In the case of nickel catalysts the formation of nickel in the zero oxidation state takes place during the reduction of the surface organometallic compound by H2. The infrared spectrum shows the total restoration of the concentration of Si—OH groups (139), so the reduction proceeds according to the scheme ... 

The chemistry of ruthenium, osmium, rhodium, iridium, palladium and platinum in the higher oxidation states. D. J. Gulliver and W. Levason, Coord. Chem. Rev., 1982,46,1-127 (1131). 

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. 

Each of the following reactions can be accomplished with a palladium reagent or catalyst. Write a detailed mechanism for each reaction. The number of equivalents of each reagent is given in parentheses. Specify the oxidation state of Pd in the intermediates. Be sure your mechanism accounts for the regeneration of catalytically active species in those reactions that are catalytic in palladium. 

Sherer et al.32 carried out systematic DFT calculations (DFT = density functional calculations) on the compound in question and on pertinent simpler compounds. They optimized the geometric structures and compared the computed bond lengths and bond orders. These authors, too, chose the oxidation state II for the central palladium atom. 

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