Heterogeneous, platinum-catalyzed hydrogenation of (diolefin)dialkylplatinum(II) complexes: kinetics

(Diolefin)dialkylplatinum(II) complexes ((Ol/sub 2/)PtR/sub 2/) are rapidly reduced by dihydrogen in the presence of platinum black catalyst: the organic groups are converted to alkanes, and the platinum(II) to platinum(0). This platinum(0) is incorporated into the surface of the platinum black catalyst. The reaction is a heterogeneous process catalyzed by platinum(0). Its rate is strongly influenced by mass transport and by the surface area of the catalyst. It is poisoned by dineopentylmercury, di-n-octyl sulfide, and tri-tert-butylphosphine. Because the catalyst surface is constantly renewed by deposition of platinum, the reaction is, however, less sensitive to poisoning than more familiar platinum-catalyzed reactions such as hydrogenation of olefins. The form of the kinetic rate law observed for reduction of (1,5-cyclooctadiene)dimethylplatinum(II) depends on the reaction conditions. Two limiting kinetic regimes are observed. In one, mass transport of dihydrogen to the catalyst surface is rate-limiting; in the second, a reaction occurring on the platinum surface is rate-limiting. The activation energy for reaction in the mass transport limited regime is approx. 3 kcalmol and that in the reaction rate limited regime is approx. 15 kcalmol. In neither regime is there a kinetic isotope effect: nu/sub H/sub 2//nu/sub D/sub 2// approx. = 1.0. Examination of the relative rates of reaction of a series of (diolefin)dialkylplatinum(II) complexes indicates that the structure of the diolefin has a greater influence on the rate of reaction than does that of the alkyl group. Competitive experiments show that rates are influenced by adsorption of the (Ol/sub 2/)PtR/sub 2/ complex to the catalyst.