Platinum complexes oxidation catalysts

Aromatic nitro compounds can be converted to the corresponding 4-fluoroanilines if the 4-position carries a hydrogen atom. Thus, nitrobenzene can be converted to 4-fluoroaniline with 100% conversion and 95% selectivity by heating with hydrogen under pressure [platinum(IV) oxide catalyst] in the presence of boron trifluoride-diethyl ether complex at 42°C for 12.5 hours. 3-Chloro-l-nitrobenzene can be similarly hydrogenated-fluorinated to 3-chloro-4-fluoroaniline.25,26... 

At present, the main industrial catalyst of ammonia oxidation is platinum and its alloys with aluminium and rhodium. Taking into account the deficit and high cost of platinum metals, the dcCTcasing of the consumption and losses of platinum metals is an urgent problem. Therefore, several compositions of complex oxide catalysts have been developed with iron (111), cobalt and chromium oxides as an active component. Complex oxides with perovskite structure are used as new catalysts they provide selective oxidation of ammonia with an yield not less than 90 %. The authors of [33] proposed to use perovskite powders LaMeOj, where Me=Fe, Co, Ni, Cr, Mn, and La,.,Sr,Me03, where Me=Co, Mn and x=0.25-0.75. To prepare these compounds, they used the precipitation by tetraethyl ammonia from diluted nitrate solutions taken at necessary ratios. The powders as prepared are poorly molded as in the form of honeycomb stractures as well as in the form of simple granules. 

Palazzi, C., Pinna, F. and Strukul, G. (2000). Polymer-Anchored Platinum Complexes as Catalysts for the Baeyer-Villiger Oxidation of Ketones preparation and Catalytic Properties, J. Mol. 

Activation of hydrogen peroxide has been achieved by the use of methyltrioxorhe-nium (MTO) [322]. Strukul and coworkers employed cationic platinum complexes as catalysts and hydrogen peroxide as the oxidant in the conversion of cyclohexanones into caprolactones [323]. A niobiocene complex has been applied giving esters with a regioselectivity opposite to that generally observed [324]. Some supported platinum [325], nickel [326] and methyltrioxorhenium [327] catalysts have also been used in reactions with hydrogen peroxide. 

Salts of neodecanoic acid have been used in the preparation of supported catalysts, such as silver neodecanoate for the preparation of ethylene oxide catalysts (119), and the nickel soap in the preparation of a hydrogenation catalyst (120). Metal neodecanoates, such as magnesium, lead, calcium, and zinc, are used to improve the adherence of plasticized poly(vinyl butyral) sheet to safety glass in car windshields (121). Platinum complexes using neodecanoic acid have been studied for antitumor activity (122). Neodecanoic acid and its esters are used in cosmetics as emoUients, emulsifiers, and solubilizers (77,123,124). Zinc or copper salts of neoacids are used as preservatives for wood (125). 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

Balthis and Bailar6 obtained tris (ethylenediamine) chromium-(III) complexes by the oxidation of chromium(II) solutions, using a procedure somewhat similar to that used for the synthesis of cobalt (III) com plexes. Mori7 described the preparation of hexaamminechromium(III) salts from the oxidation of chromium (II) salts in the presence of ammonia. The results obtained in both syntheses have been erratic.8,9 Berman noted that the foregoing syntheses are rendered dependable by the use of a catalyst of activated platinum on asbestos. Schaeffer,100 in a subsequent study, independently used colloidal platinum as a catalyst but reported some difficulty in separating it from the product.106 The procedures recommended and described here are based on the use of platinized asbestos as the catalyst. 

Working with polymethyl methacrylate, Sirdesai and Wilkie (36) have shown that certain phosphine-platinum complexes undergo oxidative insertion reactions and thus catalyze crosslinking leading to flame retardance. This catalyst is expensive and not particularly efficient, but serves as a lead. 

This cycle, often referred to as the Shilov-cycle converts methane into methanol and chloromethane in homogeneous aqueous solution at mild temperatures of 100-120 °C (11). However, while Pt(II) (added to the reaction as PtCl ) serves as the catalyst, the system also requires Pt(IV) (in the form of PtCle-) as a stoichiometric oxidant. Clearly, this system impressively demonstrates functionalization of methane under mild homogeneous conditions, but is impractical due to the high cost of the stoichiometric oxidant used. A recent development by Catalytica Advanced Technology Inc., often referred to as the Catalytica system used platinum(II) complexes as catalysts to convert methane into methyl-bisulfate (12). The stoichiometric oxidant in this case is S03, dissolved in concentrated H2S04 solvent. This cycle is depicted in Scheme 3. 

Since tetranuclear platinum-blues are oxidized by 02 to Pt(III) dinuclear complexes and are reversively reduced to the platinum-blues and further to the Pt(II) dinuclear complexes, an attempt was made to use these complexes as catalysts for olefin oxidation to ketones and epoxides. The catalysts used were a-pyrrolidonato-bridged Pt-tan [Pt4(NH3)8(C4H6N0)4](N03)6 -2H20 (19), pivalamidato-bridged Pt-blue [Pt4(NH3)8(C5H10NO)4](NO3)5 (57), a-pyrrolidonato-tan [Pt4(NH3)8... 

The analogous platinum complexes could be synthesized by a new synthetic route and have been structurally characterized [58], but can not be used as catalysts, since they immediately decompose in trifluoroacetic acid under formation of platinum black, whereas compoimds 18-22 form clear yellow solutions in the same solvens, which even after 20 hours do not show signs of decomposition according to a NMR analysis. Reprotonation of the carbene ligands to the corresponding bisimidazoUum salts can be excluded. The complexes are also stable against the addition of strong oxidants and no precipitation of palladium(II) salts was observed. 

Although homogeneously catalyzed reactions of platinum complexes are mostly concerned with hydrogenation, hydroformylation, isomerization and hydrosilylation reactions, the complexes trans-PtHX(PPh3)2 (X = C1, Br, I) have been used used as catalysts for the oxidative chlorination of n-pentane. H2PtCl6 and K2PtCl are used as oxidants.201... 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

---