A Computational study of the Ni(II) catalyzed ethylene oligomerization and dimerization with various PP ligands: structure/reactivity/selectivity relationships.

Abstract

The Cossee-Arman mechanism of nickel(II)-catalyzed ethylene oligomerization with various P^ÇP ligands classified as bridge ligands of two different types P(CH₂)nP(Ph)₂ and PNP(NR)₂ was studied using the M06-2X functional with 6-31+G(d) basis sets for the main elements and SDD for the nickel atom. The lowest dimerization energy barrier (Ed = 13.6 kcal/mol) for P(CH₂)nP(Ph)₂ ligands was found to be due to the L2 ligand (at n = 2) in the presence of toluene as a solvent. This dimerization barrier for the other ligands L1 (n = 1), L3, and L4 is higher by about 0.2, 5.6, and 11.5 kcal/mol, respectively. The structures of the L3 and L4 ligands differ from those of the L2 ligand in the presence of nitrogen atoms, the trimerization becomes more active in the L3 case. In the case of PNP(NR)₂ ligands, the lowest free energy barrier (Ed = 11.5 kcal/mol) for the dimerization process is due to the L6 ligand (R = Me), and this barrier is higher by about 0.4, 0.1, and 6.0 for the L5, [(PN(Me)P(Ph)₂], L7 (R = ethyl), L8 (R = isopropyl).