Chiral iron(II) and cobalt(II) complexes with biphenyl-bridged bis(pyridylimine) ligands : syntheses, structures and reactivities

Abstract

A new class of iron(II) and cobalt(II) complexes with neutral, tetradentate bis(pyridylimine) chelate ligands bridged by a chiral 6,6′-dimethylbiphenyl backbone has been synthesized. Their dihalogen derivatives gave spectral, magnetic and X-ray diffraction results in agreement with a C₂-symmetric, approximately octahedral geometry with cis-positioned halide ligands and high-spin d-electron configuration. ¹H NMR signals of these paramagnetic complexes were assigned by correlating line widths with reciprocal proton-metal distances. Methyl substitution in 6-position of the pyridyl rings lead to a different, presumably square-pyramidal coordination geometry under dissociation of one chloride ion. FeII dimethyl and diisocyanide derivatives were shown to have low-spin configuration and C₁-symmetric geometry. With respect to MAO-activated olefin polymerization, significant catalytic activities were reproducibly obtained only under conditions, where the original complex structure was degraded.