Chloroplatinic acid, catalysis

Catalysis. Platinum-catalyzed hydrosilation is used for cross-linking siUcone polymers and for the preparation of functionally substituted silane monomers (250). The most widely used catalyst is chloroplatinic acid (Spier s catalyst), H2PtCl3. Other compounds that catalyze the reaction include Pt(Il)... 

The proposed mechanism for the hydrosilylation of olefins catalyzed by chloroplatinic acid is outlined in Fig. 6. Catalysis by square-planar or trigonal bipyramidal rf complexes can be similarly described (54, 55, 105). 

Chloroplatinic acid is used in preparing most platinum salts and complexes. It also is used as an electroplating bath for plating and coating of platinum. Other applications are in catalysis. 

Instead of adding two hydrogen atoms to an alkynyl silane we could add H and SiMe3 to a simple alkyne by hydrosilylation (addition of hydrogen and silicon). This is a cis addition process catalysed by transition metals and leads to a tram (E-) vinyl silane. One of the best catalysts is chloroplatinic acid (H2PtCl6) as in this formation of the E-vinyl silane from phenylacetylene. In this case photochemical isomerization to the Z-isomer makes both available. Other than the need for catalysis, this reaction should remind you of the hydroboration reactions earlier in the chapter. The silicon atom is the electrophilic end of the Si-H bond and is transferred to the less substituted end of the alkyne. 

The very soluble hexachloroplatinic acid, H2 [PtCl ] nH20, is the most useful precursor for synthetic and catalytic work. For the hydrosilation of unsaturated substrates, the catalyst of choice is chloroplatinic acid, because of its very high activity. There is currently some doubt as to whether the true catalyst is homogeneous or colloidal metal and therefore heterogeneous see Hydrosilation Catalysis). Nearly all halides andpseudohahdes (= X) form salts of the [PtXe] ion and of the [PtX4(NH3)2] type. 

Bel skaya, O.B., Kaiymova, R.Kh., Kochubey, D.L, Duplyakin, V.K (2008). Genesis of the Active-Compwnent Precursor in the Synthesis of Pt/ AI2Q5 Catalysts I. Transformation of the [HCU] Complex in the Interaction between Chloroplatinic Acid and the y-A12Q3 Surface. Kinetics and Catalysis, Vol.49, No.5, (September 2008), pp. 720-728, ISSN 0023-1584... 

Mechanism and Side Reactions. Many reviews and articles have been published on the mechanism of hydrosilylation catalysis by late transition metal complexes proposed in 1965 by Chalk and Harrod. This mechanism, originally derived from the studies of chloroplatinic acid as a precursor (Pt catalyst), provided a qualitative rational generalization of the role of catalyst of other transition metal complexes (37). 

Early concepts regarding the extremely efficient hydrosilylation catalysis by chloroplatinic acid involved the reduction of the complex to colloidal platinum, which was believed to be the real catalyst. However, the reports by Lewis and co-workers (e.g., (131,132)) have shown that the processes leading to colloid formation can account for the high activity of a hydrosilylation catalyst based on the complexes of Pt(0) with olefins and dienes such as divinyltetramethyldisiloxane (Karstedt s catalyst) and cyclooctadiene. The reactivity order of platinum group... 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

---