Balancing high thermal stability with high activity in diaryliminoacenaphthene-nickel(II) catalysts for ethylene polymerization

The N,N-diaryliminoacenaphthenes, 1,2-[2,4-((4-FC6H4)2CH)2-6-MeC6H4N]2-C2C10H6 (L1) and 1-[2,4-((4-FC6H4)2CH)2-6-MeC6H4N]-2-(ArN)C2C10H6 (Ar=2,6-Me2C6H3 L2, 2,6-Et2C6H3 L3, 2,6-i-Pr2C6H3 L4, 2,4,6-Me3C6H2 L5, 2,6-Et2-4-MeC6H2 L6), incorporating at least one N-2,4-bis(difluoro benzhydryl)-6-methylphenyl group, have been synthesized and fully characterized. Interaction of L1-L6 with (DME)NiBr2 (DME=1,2-dimethoxyethane) generates the corresponding nickel(II) bromide N,N-chelates, LNiBr2 (1-6), in high yield. The molecular structures of 3 and 6 reveal distorted tetrahedral geometries at nickel with the ortho-substituted difluorobenzhydryl group providing enhanced steric protection to only one side of the metal center. On activation with various aluminum alkyl co-catalysts, such as methylaluminoxane (MAO) or Et2AlCl, 1-6 displayed outstanding activity toward ethylene polymerization (up to 1.02 × 107 g of PE (mol of Ni)-1 h-1). Notably 1, bearing equivalent fluorobenzhydryl-substituted N-aryl groups, was able in the presence of Et2AlCl to couple high activity with exceptional thermal stability generating high molecular weight branched polyethylenes at temperatures as high as 100 °C.