Synthesis, structure, magnetic and catalytic properties of new dinuclear chromium(iii) complexes with oxazoline alcoholate ligands

Abstract

The dinuclear chromium complexes [Cr₂(N,O)₃Cl₃] (6) (N,O = 4,4-dimethyl-2-oxazolylmethanolate), [Cr₂(N,O^Me2)₂(EtOH)₂Cl₄] (7) and [Cr₂(N,O^Me2)₂(H₂O)₂Cl₄] (8) (N,O^Me2 = 4,4-dimethyl-2-oxazolyldimethylmethanolate) have been prepared and characterized, including by single-crystal X-ray diffraction. Complex 6 is unsymmetrical, with two chloride ligands terminally bound to one Cr atom, whereas 7 and 8 (in 8·C₄H₈O) which contain two molecules of coordinated ethanol or water, respectively, are centrosymmetric. These chromium complexes are paramagnetic, and the magnetic properties of 6 and 7 in the solid state correspond to antiferromagnetic behaviour, which was confirmed by DFT calculations of their electronic structures. Complexes 6–8 were evaluated in the catalytic oligomerization and/or polymerization of ethylene with different aluminium-based cocatalysts, and MMAO proved to be the most effective one. In the presence of MMAO, the influence of different reaction parameters, such as the Al/Cr molar ratio, reaction temperature and ethylene pressure, was investigated. Complex 7 showed the highest activity for ethylene polymerization at both 1 atm and 10 atm of ethylene pressure, up to 620 000 g mol⁻¹(Cr) h⁻¹ in the latter case.