HETEROGENEOUS REDUCTIONS OF (HOMOHYPOSTROPHENE)DIALKYLPLATINUM(II) COMPLEXES PROVIDE A USEFUL SYSTEM FOR THE STUDY OF INTERMEDIATE SURFACE ALKYLS ON PLATINUM

This paper reports a comparison of the ethyl moieties generated on the surface of platinum black during the heterogeneous, platinum-catalyzed hydrogenations of (1,5-cyclooctadiene)diethylplatinum(II) ((COD)PtEt2), of (homohypostrophene)diethylplatinum(II) ((HOP)PtEt2), and of ethylene using D2 in ethyl alcohol-d. In reductions of the platinum complexes, the pi-olefin and sigma-alkyl orangic ligands are converted to alkanes by reaction of the intermediate surface alkyls with surface hydrides, and the platinum(II) becomes part of the platinum(0) surface. These reductions are characterized by two kinetic regimes: one in which the rate of reaction is limited by the mass transport of hydrogen to the surface of the catalyst (the mass transport limited regime, MTL), and one in which the rate is limited by a reaction on the surface of the catalyst (the reaction rate limited regime, RRL). In the RRL regime, the isotopic compositions of the ethanes-d(n) produced from the reductions of the platinum complexes and of ethylene suggest that the surface ethyls (Et *) generated from each precursor have similar relative rates of isotopic exchange (and thus of C-H bond activation) and of reductive elimination as ethane. In the MTL regime, the Et * derived from each precursor have different reactivities: the Et * generated from (HOP)PtEt2 have reactivities that are more similar to the Et * generated from ethylene than do the Et * generated from (COD)PtEt2.