Abstract
The aryl-substituted N-picolylethylenediamine and diethylenetriamine ligands, (ArNHCH(2)CH(2))[(2-C(5)H(4)N)CH(2)]NH and (ArNHCH(2)CH(2))(2)NH (Ar = 2,6-Me(2)C(6)H(3), 2,4,6-Me(3)C(6)H(2)), have been prepared by employing palladium-catalysed N-C(aryl) coupling reactions of the corresponding primary amines with aryl bromide. Treatment of MCl(2) with (ArNHCH(2)CH(2))[(2-C(5)H(4)N)CH(2)]NH affords [[(ArNHCH(2)CH(2))((2-C(5)H(4)N)CH(2))NH]CoCl(2)](Ar = 2,6-Me(2)C(6)H(3) 1a; 2,4,6-Me(3)C(6)H(2)) 1b and [[(ArNHCH(2)CH(2))((2-C(5)H(4)N)CH(2))NH]FeCl(2)](n)(n= 1, Ar = 2,6-Me(2)C(6)H(3) 2a; n= 2, 2,4,6-Me(3)C(6)H(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(2)CH(2))(2)NH with CoCl(2) gave the five-coordinate complexes [[(ArNHCH(2)CH(2))(2)NH]CoCl(2)](Ar = 2,6-Me(2)C(6)H(3) 3a, 2,4,6-Me(3)C(6)H(2) 3b), in which the ligand adopts a mer-configuration; no reaction occurred with FeCl(2). 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.
Highlights
The application of tridentate nitrogen donor ligand sets as supports for transition metal-based olefin polymerisation catalysts has been the source of considerable interest in recent years
Several points emerge; firstly, these results show that for both cobalt and iron species, fac- and mer-conformations are very close in energy
A series of iron and cobalt chloride complexes (1, 2) containing aryl-substituted N-picolylen ligands have been prepared and fully characterised. Both the experimental results and calculations indicate that fac- and mer-structures are possible for this ligand type with the energy difference between each minimal
Summary
The application of tridentate nitrogen donor ligand sets as supports for transition metal-based olefin polymerisation catalysts has been the source of considerable interest in recent years. The optimised M–ligand bond lengths (especially the M–N ones) are slightly longer that the experimental ones for both the fac-1a and mer-2a, with discrepancies less than 0.1 Å; discrepancies of this magnitude are sometimes observed with high spin systems studied by B3LYP.[14] Further comparison of the first coordination sphere of the metal atoms reveals that some reorganisation of the ligands occurs between experimental and theoretical structures. A series of iron and cobalt chloride complexes (1, 2) containing aryl-substituted N-picolylen ligands have been prepared and fully characterised Both the experimental results and calculations indicate that fac- and mer-structures are possible for this ligand type with the energy difference between each minimal. The conformation of the tridentate nitrogen donor ligands in the catalytically active species is uncertain and will be the subject of further studies
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