Mechanistic Studies of the 1,4-Cis Polymerization of Butadiene According to the π-Allyl Insertion Mechanism. 1. Density Functional Study of the C−C Bond Formation Reaction in Cationic (η³-Allyl)(η²-/η⁴-butadiene)nickel(II) Complexes [Ni(C₃H₅)(C₄H₆)]⁺ and [Ni(C₃H₅)(C₄H₆)(C₂H₄)]⁺

The 1,4-cis polymerization of butadiene according to the π-allyl insertion mechanism has been studied theoretically by density functional theory (DFT) for the ligand free cationic butenylnickel(II) complexes [Ni(C₃H₅)(C₄H₆)]⁺, I and [Ni(C₃H₅)(C₄H₆)(C₂H₄)]⁺, II. DFT energy profiles have been determined for the insertion of s-cis-butadiene into the (η³-butenyl)nickel(II) bond in the supine and prone orientations of the reacting ligands. The primary goal of this study aims to show that the insertion of cis-butadiene into the nickel(II)−allyl bond can occur within the π-coordination of the reacting parts which is characterized by an insertion barrier that should make the process feasible. Due to the lack of coordinative saturation of nickel(II) in the simpler model I, the insertion was calculated to be endothermic, and no clear difference between the supine/prone arrangements was apparent. The influence of the next double bond of the growing polymer chain for an adequate description of the geometrical aspects, as well as reliable energetics of the insertion, was demonstrated by II. The insertion was calculated to be exothermic by 11.6 kcal/mol for supine and 17.3 kcal/mol for prone, while the activation barrier was estimated to be 26.4 kcal/mol for supine and 3.9 kcal/mol for prone. Preference is given to the prone orientation in kinetic and in thermodynamic control.