2‑Benzimidazol-6-pyrazol-pyridine Chromium(III) Trichlorides: Synthesis, Characterization, and Application for Ethylene Oligomerization and Polymerization

A series
of chromium­(III) complexes, LCrCl3 (Cr1–Cr4: L = 2-(N-R2-benz­imidazol-2-yl)-6-(3,5-R1-pyrazol-1-yl)­pyridine; Cr1: R1 = H,
R2 = H; Cr2: R1 = Me, R2 = H; Cr3: R1 = Me, R2 = Me; Cr4: R1 = Me, R2 = Bn), were synthesized
and characterized by IR spectroscopy and elemental analysis. The studies
of the solid state of Cr2 and Cr3 revealed
distorted octahedral geometries around the chromium centers by X-ray
diffractions. In the presence of MAO, Cr1–Cr4 exhibited high activities toward ethylene oligomerization
(up to 2.17 × 106 g·mol–1(Cr)·h–1) and ethylene polymerization (up to 6.78 × 105 g·mol–1(Cr)·h–1). The oligomers were produced with high selectivity for α-olefins
(>99%), confirmed by FT-IR and 13C NMR, and the distributions
followed the Schulz–Flory equation. Various reaction parameters
including the amount of cocatalyst, reaction temperature, and time
were evaluated in detail, and it was evident that the title complexes
had good thermal stability and the substituents on imidazole-N of
the ligands dramatically impacted the catalytic activities as well
as the distribution of the products.