Platinum, homogeneous hydrocarbon

Homogeneous Hydrocarbon C-H Bond Activation and Functionalization with Platinum... 

HOMOGENEOUS HYDROCARBON C-H BOND ACTIVATION AND FUNCTIONALIZATION WITH PLATINUM... 

Fekl, U. Goldberg, K. I. Homogeneous Hydrocarbon C-H bond Activation and Functionalization with Platinum. In Adv. Inorg. Chem. van Eldik, R., Hubbard, C. D., Eds. Academic Press Amsterdam, 2003 Vol. 54, p 260. 

Hydrogen and a catalyst.2 0 The most common catalysts are platinum and ruthenium, but homogeneous catalysts have also been used.281 Before the discovery of the metal hydrides this was one of the most common ways of effecting this reduction, but it suffers from the fact that C=C, CssC, C=N and C=N bonds are more susceptible to attack than C=0 bonds.282 For aromatic aldehydes and ketones, reduction to the hydrocarbon (9-37) is a side reaction, stemming from hydrogenolysis of the alcohol initially produced (0-78). 

Fie. 29. Relationship between homogeneous, heterogeneous, and enzyme catalysis as inferred from the experimental studies of hydrocarbon catalysis on platinum surfaces. 

Dithianes and gemdithioacetals could be alternatively oxidized indirectly by means of the redox catalysis method. The technique appeared to be particularly mild and mainly avoided inhibition and adsorption phenomena relative to the anode platinum interface. Thus aromatic hydrocarbons (e.g. 9,10-diphenylanthracene) [83] and judiciously substituted triphenylamines [84] afford quite stable cation radicals used homogeneously as oxidants. Their standard potential, E°x, will determine the rate of electron exchange with the concerned sulfur compound. The cleavage of a C—S bond in any dithiane can be regarded as fast enough to draw the redox catalysis process to the indirect oxidation. 

Prior to 1982, Crabtree s report of the reaction of cyclopentane with a solvated IrH2(PPh3)2+ species to give a cyclopentadienyl-iridium product stood as the only well characterized example of a reaction of an alkane with a homogeneous transition metal, in contrast to the widespread reactivity of arenes [2]. Based upon the instability of the platinum methyl hydride complex Pt(PPh3)2(CH3)H, it was believed that alkane oxidative addition might not be a thermodynamically feasible process, and consequently few attempts were made to attempt such a reaction [3]. It was not until the discovery of the formation of stable alkane oxidative addition products in 1982 that it was realized that reactions of hydrocarbons were in fact feasible. 

This salt, which is soluble in acetic acid, is recommended as a homogeneous catalyst for exchange of hydrogen in aromatic hydrocarbons for deuterium.1 The substrate, acetic acid, heavy water, and hydrochloric acid are allowed to react in an evacuated, sealed tube at 25-120°. Aliphatics exchange only slowly by this technique. No dimerization (e.g., benzene — diphenyl) is observed. This reaction is observed with heterogeneous platinum-catalyzed exchange with heavy water. 

The various hydrocarbon oxidation schemes discussed above were believed to proceed at the catalyst surface only. The present concepts accept the occurrence of complex heterogeneous-homogeneous reactions proceeding in part at the solid surface and in part in the gas or liquid phase. Many catalytic oxidation processes considered recently as purely heterogeneous appeared to proceed by the heterogeneous-homogeneous mechanism. Such are the oxidations of hydrogen, methane, ethane, ethylene, propene, and ammonia over platinum at elevated temperatures, as studied by Polyakov et al. (131-136). When hydrocarbons are oxidized over platinum the reaction sets in on the catalyst surface and terminates in the gas phase. 

Deuterium exchange. Garnett et al. have found that rhodium, in addition to platinum and iridium (3, 134), catalyzes deuterium exchange. RhCla functions as a homogeneous catalyst, whereas RhCls reduced with aqueous NaBH4 functions as a heterogeneous catalyst. Rhodium has several advantages, one of which is that deuteration of saturated hydrocarbons e.g., cyclohexane) is possible at reasonable rates. 

In this chapter, we will consider the reactions of C-H compounds, such as alkanes, arenes as well as some others, with platinum complexes containing mainly chloride ligands. The reactions of alkanes with platinum(II) complexes have been the first examples of true homogeneous activation of saturated hydrocarbons in solution. Complexes of Pt(II) exhibit both nucleophilic and electrophilic properties, they do not react with alkanes via a typical oxidative addition mechanism nor can they be regarded as typical oxidants. Due to this, it is reasonable to discuss their reactions in a special chapter which is a bridge between previous chapters (devoted to the low-valent complexes) and further sections of the book that consider mainly complexes in a high oxidation state. Chloride cortplexes of platinum(IV) are oxidants and electrophiles and they will constitute the first subjects in our discussion of processes of electrophilic substitution in arenes and alkanes as well as their oxidation. 

It must be noted that no metalhc platinum is precipitated at all in the PtCU -PtCU system, which oxidizes hydrocarbons at least in the early stages where sufficient amounts of Pt(IV) are present. As will be shown later, both H-D exchange and oxidation involve the same initial step of a reaction of Pt(II) with the hydrocarbon. Thus, there is no doubt at this time that homogeneous Pt(II) solutions react with hydrocarbons. 

The importance of relativistic phenomena both in coordination complexes and in chemisorption has been reviewed. For complexes containing coordinated ethene or other unsaturated hydrocarbons, comparable quantitative information on all the Group 10 metals is extremely hard to come by, but calculations on various ethene and ethyne complexes (Table 4.13) performed by the non-local quasi-relativistic DF method are instructive. For each complex the bond energy is in the sequence Ni > Pt > Pd marked differences in the stabilities and reactivities of complexes of the type M"P2(CH3) (M = Pd, Pt P = PPhs) were also noted. In this context, it is never remarked that nearly all reactions homogeneously catalysed by metal salts or complexes, and metal-mediated reactions, involve elements from the first and second rows, and very rarely a third row element. Ruthenium, rhodium and palladium feature often osmium, iridium and platinum hardly at all. This is because very generally the complexes of the third row elements are too stable to be reactive. 

Experimental and numerical investigation of lean hetero-/ homogeneous propane/air combustion on platinum and numerical studies of hydrocarbon-fueled catalytic microreactors for portable power generation Aristotle University of Thessaloniki, Greece Research Assistant... 

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