Why Is the Nickel(II) Diphenyldiimine Complex the Best Catalyst for Polymerization of Ethylene in Three Kinds of Cationic Nickel(II) Complexes, [Ni(CH₃)L]⁺ (L = Diphenyldiimine, 2,2‘-Bipyridine, or 1,2-Diphosphinoethane)? A Theoretical Study

Abstract

Ligand effects in the insertion of ethylene into the Ni(II)−CH 3 bond of [Ni(CH 3 )L] + {L = diphenyldiimine (PhNCH CHNPh; dpdiim), diimine (HNCH CHNH; diim), 2,2‘-bipyridine (bpy), 1,2-bis(dimethylphosphino)ethane (Me 2 PCH 2 CH 2 PMe 2; dmdipe), or 1,2-diphosphinoethane (H 2 PCH 2 CH 2 PH 2; dipe)} were theoretically investigated with the DFT method. Ethylene is easily inserted into the Ni(II)−CH 3 bond with a moderate activation barrier with L = diim, dpdiim, dipe, and dmdipe but with a larger activation barrier with L = bpy. This is reasonably interpreted in terms that the coordinate bond of bpy is less flexible than those of the others because of its well-conjugated electron system. Next, coordination of ethylene with the nickel(II) propyl complex [Ni(CH 2 CH 2 CH 3 )L] + involving β-H agostic interaction takes place with a considerably large exothermicity with L = dpdiim, diim, bpy, and dipe but with much less exothermicity with L = dmdipe. Since the propagation process needs the next coordination of ethylene, the nickel(II) dmdipe complex is less favorable for the propagation than the nickel(II) dpdiim complex. These results suggest that the nickel(II) dpdiim complex is the best catalyst for polymerization of ethylene and that the nickel(II) dmdipe complex is less useful as a catalyst for polymerization of ethylene. However, we wish to propose that the chelate diphosphine complex would become useful as a polymerization catalyst if its ability for ethylene coordination was improved, since the ethylene insertion into the Ni(II)−CH 3 bond easily occurs with a similar E a value in both the dmdipe and the dpdiim complexes. One of the worthwhile modifications is to introduce an electron-withdrawing group to chelate phosphine to lower the lone pair orbital in energy.