Crowded diphosphinomethane ligands in catalysis: [(R2PCH2PR′2-κ2P)-NiR″]+ cations for ethylene polymerization without activators

The preparation of a series of nickel dichloride complexes with bulky diphosphinomethane chelate ligands R 2PCH 2PR′ 2 is reported. Reaction with the appropriate Grignard reagent leads to the corresponding dimethyl and dibenzyl complexes. Cationic monomethyl and mono-ν 3-benzyl complexes are generated from these dialkyl complexes by protonation with [H(OEt 2) 2] +[B(3,5-(CF 3) 2C 6H 3) 4] -, while the complex [(dtbpm-κ 2P) Ni(ν 3-CH(CH 2Ph)Ph] +[B(3,5-(CF 3) 2C 6H 3) 4] - is obtained from protonation of the Ni(0) olefin complex (dtbpm- κ 2P)Ni(ν 2-trans-stilbene). Crystal structures of examples of dichlorides, dimethyl, dibenzyl, cationic methyl, and cationic ν 3-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 ν 3-benzyl cation complexes are more stable and somewhat less active but still very efficient in C 2H 4 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 ν 3 to ν 3 haptotropic shift processes of the [(R 2PCH 2PR′ 2)Ni] fragment and the ν 3-ν 1 change of the coordination mode of the benzyl group required for polymerization in those cations. © 2011 American Chemical Society.