Cooperativity in Highly Active Ethylene Dimerization by Dinuclear Nickel Complexes Bearing a Bifunctional PN Ligand
Chunyu Feng, Shengmei Zhou, Danbo Wang, Yingjie Zhao, Shaofeng Liu, Zhibo Li, Pierre Braunstein
Abstract
In order to examine the possibility to promote cooperative effects on catalytic activity and selectivity in ethylene dimerization through ligand design, the bisphosphino-iminato ligands syn-L and anti-L were prepared to support the dinuclear nickel complexes syn-Ni2 and anti-Ni2. The Ni centers are successfully locked in relatively close proximity in syn-Ni2 (6.433(5) Å) but are much farther apart in anti-Ni2 because of the rigid anthracene skeleton. The mononuclear complex [NiBr2(C14H9-N═CH-C6H4-2-PPh2)] (Ni1) was also prepared for control experiments. In the presence of EtAlCl2, syn-Ni2 showed a remarkably high activity for ethylene dimerization (>90%) (up to 9.10 × 106 g (mol of Ni)−1 h–1), which is approximately 1.5- and 3.3-fold higher, respectively, than those of anti-Ni2 or of mononuclear Ni1. The redox properties of dinuclear complexes were studied by cyclic voltammetry (CV) and their comparison with those of the mononuclear complex indicates the possible existence of cooperativity between the two metal centers in the dinuclear structures. Although a detailed mechanism has not been elucidated, cooperative effects favor the isomerization of 1-butene, and dinuclear syn-Ni2 and anti-Ni2 exhibited higher selectivity for 2-butene in comparison to mononuclear Ni1 under otherwise identical reaction conditions.