Chromium(III) complexes bearing 2-benzazole-1,10-phenanthrolines: synthesis, molecular structures and ethylene oligomerization and polymerization

Abstract

The 2-benzazole-1,10-phenanthrolines and their chromium(III) complexes were synthesized and characterized by elemental and spectroscopic analyses as well as by single-crystal X-ray crystallography. X-Ray crystallographic analyses of C6 and C8 reveal an octahedral geometry around both chromium centers. The electronic spectra of the chromium complexes exhibit d–d transitions typically of a pseudo-octahedral coordinated d³ ion, falling into the region ν₁⁴A_2g→⁴T_2g 600–700 nm,ν₂⁴A_2g→⁴T_1g(F) 430–470 nm, and the 10Dq values between 15674 and 16529 cm⁻¹. The variation of the heteroatoms (X = N, O, S) in benzazole group shows regular influence on the 10Dq values as 10Dq (O) > 10Dq (N) > 10Dq (S), which reflects the σ-donor/π-acceptor properties of these benzazole ligands. Upon activation with modified methylaluminoxane (MMAO), these complexes exhibited high activities for ethylene oligomerization (up to 7.36 × 10⁶ g mol⁻¹ (Cr) h⁻¹) and ethylene polymerization (up to 1.28 × 10⁶ g mol⁻¹ (Cr) h⁻¹). The effects of non-coordinated N, O or S heteroatoms in the ligands have been examined, and interestingly catalytic activities also strictly adhered to the trend O > N > S under various ethylene pressures. Reaction conditions significantly influenced the catalytic properties of the complexes.