Platinum Ethylene Dimerization Catalysts: Diphosphine vs. Diimine Ancillary Ligand Effects

Abstract Kinetic and mechanistic studies are presented for the (dfepe)Pt(Me)(NC5F5)+ (dfepe = (C2F5)2PCH2CH2P(C2F5)2) ethylene dimerization catalyst system. New labile complexes (dfepe)PtMe(L)+ (L = NC5F5, C6F5CN, C6F5NH2, C6F5NO2) have been prepared. A general extension to a variety of other chelating diphosphine analogues (PP)Pt(Me)(C2H4)+ has been accessed by methyl abstraction from donor (PP)PtMe2 precursors with Ph3C+B(C6F5)4− in the presence of ethylene to cleanly afford (PP)Pt(Me)(C2H4)+ products. Catalysis studies for these more electron-rich diphosphine systems demonstrate moderate dimerization activity which is uniformly higher than reported for (diimine)Pt(Me)(C2H4)+. In several cases allylic catalyst decomposition products (PP)Pt(η3-C3H4Me)+ have been identified. A DFT study of insertion barriers for diimine and diphosphine systems is presented which suggests that weakening of Pt-ethylene ground state binding by strong-field diphosphine ligands is a major contributing factor to the lower ethylene insertion barriers for PP systems.