Retro-Brook rearrangement induces the formation of an octahedral nickel(benzamidinate) complex; synthesis, structure and catalytic activity

The neutral octahedral nickel complex (PhC(NSiMe3)NC(Ph)CHSiMe3)Ni(acac)(TMEDA) (7), has been synthesized and characterized including X-ray diffraction analysis. The complex was formed by the reaction of Ni(acac)2(TMEDA) with the lithium salt of the corresponding β-diketiminate ligand. The formation of the benzamidinate motif from the corresponding β-diketiminate is a consequence of a retro-Brook isomerization that is operative only at the nickel centre. A plausible mechanism for the metal mediated isomerization is proposed. When complex 7 was activated with MAO it showed a good catalytic activity for the addition polymerization of norbornene. Furthermore, this catalytic system has been found to oligomerize ethylene to a mixture of butenes and hexenes with a high turnover frequency, η = 29,300 h−1, when the reaction is performed in dichloromethane.

The β-diketiminate lithium salt [{(SiMe3)NC(Ph)}2CH][Li] reacts with Ni(acac)2(TMEDA) forming the isomerized neutral octahedral complex (PhC(NSiMe3)NC(Ph)CHSiMe3)Ni(acac)(TMEDA) (7). Complex 7 activated by MAO was found to be active in the polymerization of norbornene, producing high molecular weight (Mw > 630,000) polymers. Complex 7 was also found an active catalyst (η = 29,300), in the presence of MAO (methylalumoxane), in either toluene or dichloromethane, for the oligomerization of ethylene producing only dimers (86%) and trimers (14%).Figure optionsDownload as PowerPoint slide