Dinuclear Benzene Linker Bis(pyrazolyl)amine Cobalt(II) Complexes and their Application as Selective Ethylene Dimerization Catalysts

Reactions of synthons N,N′-(1,4-phenylenebis(methylene))bis(2-(1 H-pyrazol-1-yl)ethan-1-amine) (L1), N,N′-(1,4-phenylenebis(methylene))bis(2-(3,5-dimethyl-1 H-pyrazol-1-yl)ethan-1-amine) (L2), N,N′-(1,4-phenylenebis(methylene))bis(2-(3,5-di-tert-butyl-1 H-pyrazol-1-yl)ethan-1-amine) (L3), and N,N′-(1,3-phenylenebis(methylene))bis(2-(3,5-dimethyl-1 H-pyrazol-1-yl)ethan-1-amine) (L4) with CoCl₂ produce the dinuclear cobalt(II) complexes [Co₂(L1)Cl₄] (Co1), [Co₂(L2)Cl₄] (Co2), [Co₂(L3)Cl₄] (Co3), and [Co₂(L4)Cl₄] (Co4), respectively. These new complexes are characterized using fourier transform-infra red and ultra-violet (UV)–Visible spectroscopies, elemental analysis, and single crystal X-ray crystallography diffraction for complexes Co2 and Co3. The molecular structures of Co2 and Co3 confirm the formation of the dinuclear cobalt species. Activation of the precatalysts Co1–Co4 with either EtAlCl₂ or Me₃Al, affords active (up to 5.26 × 10⁵ g·mol⁻¹Co h⁻¹) and selective ethylene dimerization (>98%) catalysts. The steric parameters of the coordinated ligands, as dictated by the substituents on the pyrazolyl ring, largely controll both the activity and regioselectivity of the resultant catalysts.