Probing the Effect of Binding Site and Metal Centre Variation in Pentadentate Oligopyridylimine‐Bearing Bimetallic (Fe 2 , Co 2 , Ni 2 ) Ethylene Oligomerisation Catalysts

The symmetrical and unsymmetrical pentaaza 2,6‐oligopyridylimines, 6,6″‐[(2,6‐ i Pr 2 C 6 H 3 )N=CMe] 2 ‐2,2′:6′,2″‐C 15 H 9 N 3 (L1) and 6‐[(2,6‐ i Pr 2 C 6 H 3 )N=CMe]‐2,2′:6′,2″:6″,2″′‐C 20 H 13 N 4 (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 MX 2 in n BuOH at elevated temperatures affords the paramagnetic bimetallic complexes [(L1)M 2 X 4 ] [M = Fe, X = Cl ( 1 ); M = Co, X = Cl ( 2a ); M = Co, X = Br ( 2b ); M = Ni, X = Br ( 3 )] and [(L2)M 2 X 4 ] [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 MX 2 units into mixed pyridylimine/dipyridylimine ( 2a ) and pyridylimine/terpyridine [ 5b (NCMe)] binding sites. Unexpectedly during crystallisation of 6 from acetonitrile, the salt [(L2)Ni 2 Br 2 (μ‐Br)(NCMe) 2 ] 2 [NiBr 4 ] ( 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‐ i Pr 2 C 6 H 3 )N=CMe}‐2,2′‐C 10 H 7 N 2 )NiBr 2 ] ( 8 ) are also reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)