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) N‐Aryl‐ and bis(N‐aryl)pentaaza 2,6‐oligopyridylimine‐type ligands have been employed as effective scaffolds to generate bimetallic Fe2, Co2 and Ni2 complexes in which the metal centres are compartmentalised into inequivalent binding sites; depending on oligopyridylimine and metal centre their activation with MAO can lead to active catalysts for the oligomerisation of ethylene.