Chromium complexes bearing pyrrolide-imine N,N-chelate ligands: synthesis, structures and ethylene polymerisation behaviour

摘要

Chromium(II) and chromium(III) complexes bearing two bulky monoanionic N,N-chelating pyrrolide-imine ligands, {ArN=CH(C4H3N-2)}(2)Cr (1) and {ArN=CH(C4H3N-2)}(2)Cr(mu-Cl)(2)Li(THF)(2) (2), have been prepared in high yield by treatment of the sodium and lithium salts of ArN=CH(C4H3NH-2) (Ar = 2,6-i-Pr2C6H3) with CrCl2 (THF) and CrCl3 (THF)(3), respectively. The molecular structure of 1 reveals a square planar geometry about chromium with the pyrrolide-N units disposed mutually trans. Conversely, an octahedral geometry is exhibited by the chromium centre in 2 with mutually cis-oriented pyrrolide-N units and cis-chloride ligands which, in turn, link to a Li(THF)(2) unit. Complex 2 can be alkylated with AlR3 to give the five-coordinate mono-alkyl species {ArN=CH(C4H3N-2)}(2)CrR (3a R = Me; 3b R = Et), in which two pyrrolide-imine ligands occupy the square base of the square pyramidal chromium centre. Complexes 1-3 behave as ethylene polymerisation catalysts upon treatment with an excess of a range of aluminium activators including MAO, Et2AlCl and Me2AlCl, converting ethylene to high molecular weight polyethylene. In all cases the dialkylaluminium chlorides are superior co-catalysts to MAO, affording acitivities up to 120 g mmol(-1) h(-1) bar(-1). The coordinatively unsaturated bis(pyrrolide-imine) chromium alkyl complexes 3a and 3b are inactive for the polymerisation of ethylene in the absence of activator. [References: 61]