Ni(ii) and Fe(ii) complexes based on bis(imino)aryl pincer ligands: synthesis, structural characterization and catalytic activities

Abstract

Bis(imino)aryl NCN pincer Ni(II) complexes 2,6-(ArNCH)₂C₆H₃NiBr (1: Ar = 2,6-Me₂C₆H₃; 2: Ar = 2,6-Et₂C₆H₃; 3: Ar = 2,6-ⁱPr₂C₆H₃) were prepared via the oxidative-addition of Ni(0)(Ph₃P)₄ with bis(N-aryl)-2-bromoisophthalaldimine. These nickel complexes were characterized by NMR and elemental analyses. Their solid molecular structures were established by X-ray diffraction analyses. The nickel metal centers adopt distorted square planar geometries with the bromine atoms acting as one coordinate ligands. The NCN pincer Fe(II) complexes 2,6-(ArNCH)₂C₆H₃Fe(μ-Cl)₂Li(THF)₂ (4: Ar = 2,6-Me₂C₆H₃; 5: Ar = 2,6-Et₂C₆H₃; 6: Ar = 2,6-ⁱPr₂C₆H₃) were synthesized by lithium salt metathesis reactions of the ligand lithium salts with FeCl₂. X-ray structure analyses of 4 and 5 revealed that the Fe(II) complexes are hetero-dinuclear with the iron atoms in trigonal bipyramidal environments. When activated with MAO, the nickel complexes are active for norbornene vinyl polymerization but are inert for butadiene polymerization. The Fe(II) complexes show moderate activities in butadiene polymerization when activated with alkylaluminium, affording the cis-1,4 enriched polymer.