Flexible N,N,N-chelates as supports for iron and cobalt chloride complexes; synthesis, structures, DFT calculations and ethylene oligomerisation studies

The aryl-substituted N-picolylethylenediamine and diethylenetriamine ligands, (ArNHCH2CH2){(2-C5H4N)CH2} NH and (ArNHCH2CH2)(2)NH(Ar=2,6-Me2C6H3, 2,4,6-Me3C6H2), have been prepared by employing palladium-catalysed N-C(aryl) coupling reactions of the corresponding primary amines with aryl bromide. Treatment of MCl2 with (ArNHCH2CH2){(2-C5H4N)CH2}NH affords [{(ArNHCH2CH2)((2-C5H4N)CH2)NH}CoCl2] (Ar=2,6-Me2C6H3 1a; 2,4,6-Me3C6H2 1b) and [{(ArNHCH2CH2)((2-C5H4N)CH2)NH}FeCl2](n) (n=1, Ar=2,6-Me2C6H3 2a; n=2, 2,4,6-Me3C6H2 2b) in high yield. The X-ray structures of 1a and 1b are isostructural and reveal the metal centres to adopt distorted trigonal bipyramidal geometries with the N,N,N-chelates adopting fac-structures. A facial coordination mode of the ligand is also observed in bimetallic 2b, however, in 2a the N,N,N-chelate adopts a mer-configuration with the metal centre adopting a geometry best described as square pyramidal. Solution studies indicate that mer-fac isomerisation is a facile process for these systems at room temperature. Quantum mechanical calculations (DFT) have been performed on 1a and 2a, in which the ligands employed are identical, and show the fac- to be marginally more stable than the mer-configuration for cobalt (1a) while for iron (2a) the converse is evident. Reaction of (ArNHCH2CH2)(2)NH with CoCl2 gave the five-coordinate complexes [{(ArNHCH2CH2)(2)NH}CoCl2] (Ar=2,6-Me2C6H3 3a, 2,4,6-Me3C6H2 3b), in which the ligand adopts a mer-configuration; no reaction occurred with FeCl2. All complexes 1-3 act as modest ethylene oligomerisation catalysts on activation with excess methylaluminoxane (MAO); the iron systems giving linear alpha-olefins while the cobalt systems give mixtures of linear and branched products.