Synthesis of Cationic, Dimeric α-Diimine Nickel Hydride Complexes and Relevance to the Polymerization of Olefins

Well-defined nickel hydride complexes bearing aryl-substituted α-diimine (iPrDI = N,N′-bis(2,6-diisopropylphenyl)-2,3-butanediimine or MesDI = N,N′-bis(2,4,6-trimethylphenyl)-2,3-butanediimine) ligands have been synthesized and studied for the oligomerization of linear internal olefins. Neutral diimine nickel hydride dimers, [(iPrDI)Ni(μ2-H)]2 (1) and [(MesDI)Ni(μ2-H)]2 (2), were explored as synthetic entries to monomeric cationic hydride complexes; however, the dimeric structure was preserved upon oxidation with ferrocenium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([Cp2Fe][BArF24]). The monocationic nickel hydride dimers, [(iPrDI)Ni(μ2-H)]2[BArF24] (3) and [(MesDI)Ni(μ2-H)]2[BArF24] (4), were characterized by X-ray diffraction, electron paramagnetic resonance, and nuclear magnetic resonance spectroscopies that support a mixed-valent nickel(I)-nickel(II) assignment. The catalytic activity of 3 and 4 for the oligomerization of 1-hexene, trans-2-hexene, and trans-3-hexene was determined and showed decreased activity as compared to the corresponding nickel dibromide precatalyst activated by methylaluminoxane.