Iron and Cobalt Complexes of 2,3,7,8-Tetrahydroacridine-4,5(1<i>H</i>,6<i>H</i>)-diimine Sterically Modulated by Substituted Aryl Rings for the Selective Oligomerization to Polymerization of Ethylene

Abstract

A new series of Fe(II) and Co(II) complexes of sterically modulated 2,3,7,8-tetrahydroacridine-4,5(1 H,6 H )-diimine by substituted aryl rings are synthesized. The complex synthesis is achieved through a one-pot methodology by reacting one equivalent of 2,3,7,8-tetrahydroacridine-4,5(1 H,6 H )-dione with two equivalents of required aniline derivatives together with Fe or Co salt in boiling acetic acid. The single-crystal X-ray analyses of Fe and Co complexes bearing a 2,6-diisopropyl-substituted ligand, ( N, N ′ E, N, N ′ E )- N, N ′-(2,3,7,8-tetrahydroacridine-4,5(1 H,6 H )-diylidene)bis(2,6-diisopropylaniline), show a distorted trigonal-bipyramidal geometry for the metal center, a deviation from square-pyramidal geometry due to the opening up of the coordination sphere originating from structural rigidity of the carbonyl carbon atom achieved through cyclization. All the complexes show high activity toward ethylene activation when treated with methylaluminoxane as a cocatalyst. The ortho substituents of the aryl rings and the type of metal have a significant effect on ethylene activation and specifically on product distribution. The Fe complex with a 2,6-diisopropyl-substituted bis(imino)tetrahydroacridyl ligand produced polyethylene of moderate molecular weight (18 000). On changing the ortho substitution of the ligand from 2,6-diisopropyl to 2,6-dimethyl, the Co complexes produce polyethylene waxes and oligomers of predominantly α-olefins simultaneously, obeying Schulz−Flory distribution. The Co complexes bearing ligands with less sterically hindered substituents at the ortho position behave exclusively as dimerization catalysts.