Cationic α-Diimine Nickel and Palladium Complexes Incorporating Phenanthrene Substituents: Highly Active Ethylene Polymerization Catalysts and Mechanistic Studies of <i>syn</i>/<i>anti</i> Isomerization
Quan H. Tran, Xiqu Wang, Maurice Brookhart, Olafs Daugulis
Abstract
α-Diimine palladium complexes incorporating phenanthryl- and 6,7-dimethylphenanthrylimino groups have been synthesized and characterized. The (diimine)PdMeCl complexes prepared from 2,3-butanedione and acenaphthenequinone bearing the unsubstituted phenanthrylimino groups, 12a and 14a, respectively, exist as a mixtures of syn and anti isomers in a ca. 1:1 ratio. Separation and X-ray diffraction analysis of 14a-syn and 14a-anti isomers confirms the syn/anti assignments. The barrier to interconversion of 14a-syn and 14a-anti via ligand rotation, ΔG⧧, was found to be 25.5 kcal/mol. The corresponding (diimine)PdMeCl complex prepared from acenaphthenequinone and incorporating the 6,7-dimethylphenanthrylimino group exists solely as the anti isomer, 14b, due to steric crowding which destabilizes the syn isomer. Analogous (diimine)NiBr2 complexes were prepared from 2,3-butanedione incorporating the phenanthrylimino group, 16a, and the 6,7-dimethylphenanthrylimino group, 16b. Nickel-catalyzed polymerizations of ethylene were carried out by activation of the dibromide complexes 16a,b using various aluminum alkyl activators. Complex 16a yields a bimodal distribution polymer, the low-molecular-weight fraction originating from the syn isomer and the high-molecular-weight fraction arising from the anti isomer. Polymerizations carried out by 16b yield only high-molecular-weight polymers with monomodal distributions due to the existence of a single isomer (anti) as the active catalyst. All polymers are linear or nearly so. All catalysts are highly active, but catalysts derived from 16b are somewhat more active than 16a and exhibit turnover frequencies generally over 106 and up to 5 × 106 per hour (40 °C, 27.2 atm ethylene, 15 min). Active palladium ethylene oligomerization catalysts were generated by conversion of the neutral methyl chloride complexes 14a,b to the cationic nitrile complexes 15a,b via halide abstraction.