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

Abstract

The aryl-substituted N-picolylethylenediamine and diethylenetriamine ligands, (ArNHCH₂CH₂){(2-C₅H₄N)CH₂}NH and (ArNHCH₂CH₂)₂NH (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-Me₂C₆H₃1a; 2,4,6-Me₃C₆H₂1b) and [{(ArNHCH₂CH₂)((2-C₅H₄N)CH₂)NH}FeCl₂]_n (n = 1, Ar = 2,6-Me₂C₆H₃2a; n = 2, 2,4,6-Me₃C₆H₂2b) 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)₂NH with CoCl₂ gave the five-coordinate complexes [{(ArNHCH₂CH₂)₂NH}CoCl₂] (Ar = 2,6-Me₂C₆H₃3a, 2,4,6-Me₃C₆H₂3b), 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 α-olefins while the cobalt systems give mixtures of linear and branched products.