Probing the Effect of Binding Site and Metal Centre Variation in Pentadentate Oligopyridylimine-Bearing Bimetallic (Fe2, Co2, Ni2) Ethylene Oligomerisation Catalysts

The symmetrical and unsymmetrical pentaaza 2,6-oligopyridylimines, 6,6 ''-[(2,6-iPr(2)C(6)H(3))N=CMe](2)-2,2':6',2 ''-C15H9N3 (L1) and 6-[(2,6 -iPr(2)C(6)H(3))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 Ll or L2 with two equivalents of MX2 in nBuOH at elevated temperatures affords the paramagnetic bimetallic complexes [(Ll)-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 pyridyhniine/dipyridylimine (2a) and pyridylimine/terpyridine [5b(NCMe)] binding sites. Unexpectedly during crystallisation of 6 from acetonitrile, the salt [(L2)Ni2Br2(mu-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, dinickel-based 3/MAO (450 gmmol(-1)h(-1) bar(-1)), yielding methyl-branched waxes composed of mostly internal unsaturation along with lower levels of alpha-olefins; conversely the diiron (1/MAO) and dicobalt (2/MAO) systems give uniquely linear a-olefins. For purposes of comparison the synthesis, structure and catalytic activity of mono-nickel [(6-{(2,6-iPr(2)C(6)H(3))N=CMe}-2,2'-C10H7N2)NiBr2] (8) are also reported. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)