Neutral and cationic methallyl nickel complexes in alkene activation: a combined DFT, ESI-MS and chemometric approach

Abstract

Herein, we report a comparative study of ethylene activation and 1-hexene isomerization carried out with isomeric neutral and cationic methallyl nickel complexes L₁Ni(η³-C₃H₅) and [L₁Ni(η³-C₃H₅)][B(Ar^F)₄] in the presence of borane co-catalysts. To understand the reactivity of the nickel complexes with NacNac ligands, we used chemometric methods to classify different catalysts reported to date. The mechanism of the interaction of [L₁Ni(η³-C₃H₅)][B(Ar^F)₄]/B(C₆F₅)₃ with 1-hexene was studied by ESI-MS which allowed the detection of cationic species formed in situ. Moreover, there is a very small difference in reactivities from combination of nickel complexes and borane co-catalysts used for alkene isomerization, while the reactivity with ethylene of both systems is very different; [L₁Ni(η³-C₃H₅)][B(Ar^F)₄]/B(C₆F₅)₃ produces butene, while L₁Ni(η³-C₃H₅)/B(C₆F₅)₃ forms polyethylene. Furthermore, DFT studies revealed that the origin of the catalytic activity in the cationic and neutral methallyl nickel complexes co-activated by B(C₆F₅)₃ is mainly from direct steric effects of the ligand–nickel center where the conformation of the chelate ring is affected by the catalyst symmetry. This work demonstrates how the cationic or neutral nature of the same system affects its catalytic and structural properties.