Nickel(II) complexes with mono(imino)pyrrole ligands: preparation, structure, DFT calculation and catalytic behavior

Abstract

In recent years, a variety of novel late transition metal catalysts have been used for polymerization of methyl methacrylate (MMA); in order to study the effects of fine-tuning of catalyst structure on polymerization of MMA, we synthesized a series of mono(imine)pyrrole ligands (L1–L5) with different substituents in the same position using microwave reactions. Then, the ligands were directly coordinated with NiCl₂·6H₂O through liquid-phase reaction method to prepare the Ni(II) complexes (1–4). All the compounds were characterized by spectroscopic methods, and compounds L1, L3, L5 and 2 were characterized by X-ray crystallography. The results showed that the structures of the three ligands were mono(imine)pyrrole Schiff base structure. Complex 2 contains two molecular ligands’ structure and a Ni(II) atom to form a standard square plane geometry. The total molecular energy and other data for each Ni(II) complex were obtained by Gaussian 09 calculation program. The negative total molecular energy indicated the structures of complexes were relatively stable. Additionally, the bond parameters were in good agreement with X-ray single-crystal diffraction data. With the aid of azodiisobutyronitrile (AIBN), the synthesized Ni(II) complexes were used in the polymerization of MMA. By comparing the Ni(II) catalysts with different structures, we found catalytic activities were in a sequence of 4 > 1 > 2 > 3, which indicated the stronger electron-withdrawing ability of the substituent, the better catalytic activity of catalyst for MMA. We also studied the effects of polymerization temperature and time on catalytic performance, the optimum activity (11.71 × 10⁴ g mol⁻¹·h⁻¹⁾ was obtained at 100℃ and 10 h, and the polymer with relatively narrow molecular weight distribution (PDI = 1.91) and large molecular weight (M_n = 36.10 × 10⁴ g mol⁻¹) was obtained under the same condition.