Nickel catalysts bearing bidentate α-aminoaldimines for ethylene polymerization—independent and cooperative structure/reactivity relationship resulting from unsymmetric square planar coordination

Ethylene polymerization catalyzed by Ni(II) complexes that bear new bidentate ligands with a functional hybrid of amine and imine has been studied. A class of new α-aminoaldimines and their nickel complexes [R1R2NCMe2CHN(2,6-R32C6H3)]NiBr2 (R1 = R2 = Me, R3 = Me (Ni-1b); R3 = iPr (Ni-1c); R1 = R2 = Et, R3 = H (Ni-2a); Me (Ni-2b); iPr (Ni-2c); R1 = R2 = nPr, R3 = iPr (Ni-3c); (R1R2) = c-C3H6 R3 = iPr (Ni-4c); (R1R2) = c-C4H8, R3 = iPr (Ni-5c)) were synthesized. The molecular structures of six nickel complexes were determined by X-ray crystallography, showing distorted tetrahedral configurations. The SQUID data of Ni-1c confirms its ground state of triplet spin. Using methylaluminoxanes (MAO) as the activator, the nickel complexes are found to catalyze ethylene polymerization under moderate pressure and ambient temperature. The activity reaches to 106 g PE mol Ni−1 h−1, and increases with the ethylene pressure in the range of 14–28 bar. The highly branching PE products have Mn∼ 105 with PDI < 2. The amine and imine functionalities demonstrate independent control to the polymerization reactions, wherein the activity appears to be facilitated by using the catalysts installed with bulky imino substituents as well as with less sterically hindered amino substituents. This is ascribed to the C2 unsymmetric coordination in the square planar resting state in which the bulky polymer chain prefers cis to the imine and the small ethylene monomer is cis to the amine.