Probing the effect of binding site and metal centre variation in pentadentate oligopyridylimine-bearing bimetallic (Fe2, Co 2, Ni2) ethylene oligomerisation catalysts

Abstract

The symmetrical and unsymmetrical pentaaza 2,6-oligopyridylimines, 6,6″-[(2,6-iPr2C6H3)N=CMe]2-2,2′: 6′,2″-C15H9N3 (L1) and 6-[(2,6-iPr2C6H3)N=CMe]-2,2′:6′, 2″:6″,2‴-C20H13N4 (L2), have been prepared in good yield using a combination of palladium(0)-mediated cross-coupling and condensation strategies. Treatment of L1 or L2 with two equivalents of MX2 in nBuOH at elevated temperatures affords the paramagnetic bimetallic complexes [(L1)-M2X4] [M = Fe, X = Cl (1); M = Co, X = Cl (2a); M = Co, X = Br (2b); M = Ni, X = Br (3)] and [(L2)M2X4] [M = Fe, X = Cl (4); M = Co, X = Cl (5a); M = Co, X = Br (5b); M = Ni, X = Br (6)] in high yield, respectively. The molecular structures of 2a along with the acetonitrile adduct of 5b, [5b(NCMe)], have been determined and reveal that L1 and L2 compartmentalise the MX2 units into mixed pyridylimine/dipyridylimine (2a) and pyridylimine/terpyridine [5b(NCMe)] binding sites. Unexpectedly during crystallisation of 6 from acetonitrile, the salt [(L2)Ni2Br2(μ-Br)(NCMe) 2]2[NiBr4] (7) was obtained as the only crystalline product. On activation with MAO (methylaluminoxane), 4-6 show only low activities for ethylene oligomerisation (6/MAO > 5/MAO) or are inactive (4/MAO). On the other hand, 1-3 are considerably more active (3/MAO > 2/MAO > 1/MAO) with the most productive system, di-nickel-based 3/MAO (450 gmmol-1h-1bar-1), yielding methyl-branched waxes composed of mostly internal unsaturation along with lower levels of α-olefins; conversely the di-iron (1/MAO) and dicobalt (2/MAO) systems give uniquely linear α-olefins. For purposes of comparison the synthesis, structure and catalytic activity of mono-nickel [(6-{(2,6-iPr2C 6H3)N=CMe}-2,2′-C10H7N 2)NiBr2] (8) are also reported. © Wiley-VCH Verlag GmbH & Co. KGaA, 2008.