Crowded diphosphinomethane ligands in catalysis: [(R2PCH 2PR′2-κ2P)-NiR″]+ cations for ethylene polymerization without activators

Article abstract of DOI:10.1021/om200715w

The preparation of a series of nickel dichloride complexes with bulky diphosphinomethane chelate ligands R₂PCH₂PR′ ₂ is reported. Reaction with the appropriate Grignard reagent leads to the corresponding dimethyl and dibenzyl complexes. Cationic monomethyl and mono-ν³-benzyl complexes are generated from these dialkyl complexes by protonation with [H(OEt₂)₂] ⁺[B(3,5-(CF₃)₂C₆H₃) ₄]^-, while the complex [(dtbpm-κ²P) Ni(ν³-CH(CH₂Ph)Ph]⁺[B(3,5-(CF ₃)₂C₆H₃)₄]^- is obtained from protonation of the Ni(0) olefin complex (dtbpm- κ²P)Ni(ν²-trans-stilbene). Crystal structures of examples of dichlorides, dimethyl, dibenzyl, cationic methyl, and cationic ν³-benzyl complexes are reported. Solutions of the cations polymerize ethylene under mild conditions and without the necessity of an activating agent, to form polyethylene having high molecular weights and low degrees of chain branching. In comparison to the Ni methyl cations, the ν³-benzyl cation complexes are more stable and somewhat less active but still very efficient in C₂H₄ polymerization. The effect on the resulting polyethylene of varying the substituents R, R′ on the phosphine ligand has been examined, and a clear trend for longer chain PE with less branching in the presence of more bulky substituents on the diphosphine has been found. Density functional calculations have been used to examine the rapid suprafacial ν³ to ν³ haptotropic shift processes of the [(R₂PCH₂PR′₂)Ni] fragment and the ν³-ν¹ change of the coordination mode of the benzyl group required for polymerization in those cations.