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

摘要

The aryl-substituted N-picolylethylenediamine and diethylenetriamine ligands,(ArNHCH_2CH_2){(2-C_5H_4N)CH_2}NH and(ArNHCH_2CH_2)_2NH(Ar = 2,6-Me_2C_6H_3,2,4,6-Me_3C_6H_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_2CH_2){(2-C_5H_4N)CH_2}NH affords [{(ArNHCH_2CH_2)((2-C_5H_4N)CH_2)NH}CoCl_2](Ar = 2,6-Me_2C_6H_3 la;2,4,6-Me_3C_6H_2 Ib)and [{(ArNHCH_2CH_2)((2-C_5H_4N)CH_2)NH}FeCl_2]_n(n = 1,Ar = 2,6-Me_2C_6H_3 2a;n = 2,2,4,6-Me_3C_6H_2 2b)in high yield.The X-ray structures of la and Ib are isostructural and reveal the metal centres to adopt distorted trigonal bipyramidal geometries with the N,N,N-chelates adoptingyac-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 la and 2a,in which the ligands employed are identical,and show the fac-to be marginally more stable than the mer-configuration for cobalt(la)while for iron(2a)the converse is evident.Reaction of(ArNHCH_2CH_2)_2NH with CoCl_2 gave the five-coordinate complexes [{(ArNHCH_2CH_2)_2NH}CoCl_2](Ar = 2,6-Me_2C_6H_3 3a,2,4,6-Me_3C_6H_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.