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, (ArNHCH[subscript 2]CH[subscript 2]){(2-C[subscript 5]H[subscript 4]N)CH[subscript 2]}NH and (ArNHCH[subscript 2]CH[subscript 2])[subscript 2]NH (Ar = 2,6-Me[subscript 2]C[subscript 6]H[subscript 3], 2,4,6-Me[subscript 3]C[subscript 6]H[subscript 2]), have been prepared by employing palladium-catalysed N–C(aryl) coupling reactions of the corresponding primary amines with aryl bromide. Treatment of MCl[subscript 2] with (ArNHCH[subscript 2]CH[subscript 2]){(2-C[subscript 5]H[subscript 4]N)CH[subscript 2]}NH affords [{(ArNHCH[subscript 2]CH[subscript 2])((2-C[subscript 5]H[subscript 4]N)CH[subscript 2])NH}CoCl[subscript 2]] (Ar = 2,6-Me[subscript 2]C[subscript 6]H[subscript 3]1a; 2,4,6-Me[subscript 3]C[subscript 6]H[subscript 2]1b) and [{(ArNHCH[subscript 2]CH[subscript 2])((2-C[subscript 5]H[subscript 4]N)CH[subscript 2])NH}FeCl[subscript 2]][subscript n] (n = 1, Ar = 2,6-Me[subscript 2]C[subscript 6]H[subscript 3]2a; n = 2, 2,4,6-Me[subscript 3]C[subscript 6]H[subscript 2]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 (ArNHCH[subscript 2]CH[subscript 2])[subscript 2]NH with CoCl[subscript 2] gave the five-coordinate complexes [{(ArNHCH[subscript 2]CH[subscript 2])[subscript 2]NH}CoCl[subscript 2]] (Ar = 2,6-Me[subscript 2]C[subscript 6]H[subscript 3]3a, 2,4,6-Me[subscript 3]C[subscript 6]H[subscript 2]3b), in which the ligand adopts a mer-configuration; no reaction occurred with FeCl[subscript 2]. All complexes 1–3 act as modest ethylene oligomerisation catalysts on activation with excess methylaluminoxane (MAO); the iron systems giving linear α-olefins while the cobalt systems give mixtures of linear and branched products.