Cooperativity in Highly Active Ethylene Dimerization by Dinuclear Nickel Complexes Bearing a Bifunctional PN Ligand

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-Ni-2 and anti-Ni-2. The Ni centers are successfully locked in relatively close proximity in synNi(2) (6.433(5) angstrom) but are much farther apart in anti-Ni-2 because of the rigid anthracene skeleton. The mononuclear complex [NiBr2(C14H9-N= CH-C6H4-2-PPh2)] (Ni,) was also prepared for control experiments. In the presence of EtAICl(2), syn-Ni(2 )showed a remarkably high activity for ethylene dimerization (>90%) (up to 9.10 x 10(6) g (mol of Ni)(-1) h(-1)), which is approximately 1.5- and 3.3-fold higher, respectively, than those of anti-Ni-2 or of mononuclear Nil . 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-Ni-2 and anti-Ni-2 exhibited higher selectivity for 2-butene in comparison to mononuclear Ni-1 under otherwise identical reaction conditions.

Automated summary

This study focused on the cooperative effects of ligand design on catalytic activity and selectivity in ethylene dimerization. Specifically, dinuclear nickel complexes showed higher activity and selectivity compared to mononuclear complexes, indicating the possible existence of cooperativity between the two metal centers. Although the detailed mechanism is not fully elucidated, cooperative effects favor the isomerization of 1-butene and improve the selectivity for 2-butene in dinuclear complexes.