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‐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, dinickel‐based 3/MAO (450 g mmol–1 h–1 bar–1), yielding methyl‐branched waxes composed of mostly internal unsaturation along with lower levels of α‐olefins; conversely the diiron (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‐iPr2C6H3)N=CMe}‐2,2′‐C10H7N2)NiBr2] (8) are also reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)