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

Abstract

Chromium(II) and chromium(III) complexes bearing two bulky monoanionic N,N-chelating pyrrolide-imine ligands, {ArNCH(C₄H₃N-2)}₂Cr (1) and {ArNCH(C₄H₃N-2)}₂Cr(µ-Cl)₂Li(THF)₂ (2), have been prepared in high yield by treatment of the sodium and lithium salts of ArNCH(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 {ArNCH(C₄H₃N-2)}₂CrR (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.