Mechanism of Ethylene Migratory Insertion and Dimerization Catalyzed by δ -Alkyl Platinum Complexes-A DFT Study

Abstract

The mechanism of ethylene insertion reactions catalyzed by cationic δ-alkyl platinum complexes has been studied at the B3LYP level of density functional theory. The initial steps of the reactions proceed via the coordination of ethylene to the reactants L2Pt(II)R+, where L2=none, (NH3)2, (CHNH)2; R=H, CH3, C2H5 in which ethylene coordinates strongly to the complexes PtCH+3 and PtC2H+5 (coordination energies (CE) are 296.52 and 229.28 kJ/mol, respectively), while nitrogen-containing ligands decrease the energies: Pt(NH3)2CH+3 (CE: 180.04 kJ/mol), Pt(NH3)2C2H+5 (CE: 97.86 kJ/mol), Pt(CHNH)2CH+3 (CE : 176.31 kJ/mol) and Pt(CHNH)2C2H+5 (CE: 91.00 kJ/mol). Moreover, ethylene insertion into the Pt-alkyl bond, which is the rate-determining step, is endothermic with barrier heights for L2PtCH3(C2H4)+ decreasing in the order: PtCH+3 (164.18 kJ/mol)>(NH3)2 PtCH+3 (129.95 kJ/mol)>(CHNH)2 PtCH+3 (115.27 kJ/mol), which has the same tendency for the ethyl case. The insertion product will continually undergo β-hydride elimination, which is exothermic. On the other hand, the effects of solvent (dichloromethane, THF and benzene) are investigated with PCM method, but the inclusion of the effects in the computations only slightly affects the results. Beside that, a complete catalytic cycle for ethylene dimerization is studied in detail and the calculations agree well with known energetic and recognized tendencies.