Platinum, homogeneous hydrocarbon complexes

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. 

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. 

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. 

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. 

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