Platinum ethylene dimerization catalysts: Diphosphine vs. diimine ancillary ligand effects

摘要

Kinetic and mechanistic studies are presented for the (dfepe)Pt(Me)(NC5F5)(+) (dfepe = (C2F5)(2)PCH2CH2P (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(eta(3)-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. (C) 2020 Elsevier Ltd. All rights reserved.