Synthesis, molecular structure, and solution-dependent behavior of nickel complexes chelating anilido-imine donors and their catalytic activity toward olefin polymerization

A series of novel nickel complexes (1-4) bearing anilido-imine ligands, [((ArN)-N-1=CHC6H4NAr2)NiBr](2) (Ar-1 = Ar-2 = 2,6-dimethylphenyl, 1; Ar-1 = 2,6-dimethylphenyl, Ar-2 = 2,6-diisopropylphenyl, 2; Ar-1 = Ar-2 = 2,6-diisopropylphenyl, 3; Ar-1 = 2,6-diisopropylphenyl, Ar-2 = 2,6-dimethylphenyl, 4), have been synthesized and characterized. The solid-state structures of the complexes 1, 2, and 4 were confirmed by X-ray single-crystal analyses to be in the form of a dinuclear and bromine-bridged structure. However, there is an equilibrium that shifts between the monomer and dimer in solution, which has been monitored using H-1 NMR and UV-vis spectrophotometry. The themodynamic parameters for the equilibriums were calculated to be DeltaH = +13.68 kJ/mol and DeltaS = 40.32 J/(mol K) for 1 and DeltaH = +8.35 kJ/mol and DeltaS = 15.21 J/(mol K) for 3. All nickel complexes show low activities for ethylene oligomerization with MAO as cocatalyst but high catalytic activities for norbornene polymerization in the presence of MAO.