Synthesis and characterization of novel chromium complexes based on β-enaminoketonato ligands and their application to ethylene polymerization

• A series of novel chromium(III) complexes were synthesized and characterized.
• The heteroatom in the sidearm coordinated to chromium in non-Cp complexes.
• The sidearm acted as a pendant group in half-sandwich ones.
• Polymerization behaviors were significantly influenced by catalyst structures.

A series of novel non-Cp-based chromium(III) complexes bearing tridentate β-enaminoketonato chelating ligands of type, [R2NC(CF3)C(H)CR1O]CrCl2(thf) (2a: R1 = tBu, R2 = C6H4OMe(o); 2b: R1 = tBu, R2 = C9H6N; 2c: R1 = tBu, R2 = C6H4SMe(o); 2d: R1 = Ph, R2 = C9H6N; 2e: R1 = Ph, R2 = C6H4SMe(o)) have been prepared in high yields (e.g. 63–79%) from CrCl3(thf)3 by treating with 1.0 equiv. of the deprotonated ligands in tetrahydrofuran. These complexes were identified by IR, and mass spectra as well as elemental analysis. Structures for 2b, 2c and 2e were further confirmed by X-ray crystallography. Non-Cp-based chromium(III) complexes have six-coordinate distorted octahedral geometry, and the heteroatom in the sidearm is coordinated to chromium. When activated by modified methylaluminoxane, complexes 2b and 2c exhibited high activities towards ethylene polymerization. These results indicated that both the donor atoms in side arm and the substituents in backbone significantly affected polymerization behaviors. In order to further explore the structure–reactivity relationship of the catalysts, we synthesized some novel half-sandwich chromium(III) complexes, Cp[R2NC(CF3)C(H)CR1O]CrCl (3a: R1 = tBu, R2 = C6H4OMe(o); 3c: R1 = tBu, R2 = C6H4SMe(o); 3e: R1 = Ph, R2 = C6H4SMe(o)) and investigated their potential application in ethylene polymerization under the similar conditions. Complex 3c adopts a three-legged piano stool geometry with a pseudo-octahedral coordination environment, and the heteroatom in the sidearm is not coordinated to chromium. These half-sandwich chromium complexes all displayed high activities for ethylene polymerization under the same conditions. Neither the donor atom in side arm nor the substituent in backbone significantly affected polymerization behaviors, in complete contrast to the results by non-Cp-based chromium catalysts.

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