Sequential Insertion Reactions of Carbon Monoxide and Ethylene into the Ni−C Bond of a Cationic Nickel(II) Alkyl Complex

The cationic nickel(II) neopentyl complex [(dtbpe)Ni(CH2tBu)][PF6] (1; dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) sequentially inserts carbon monoxide and ethylene into the Ni−C bond to form isolable products. Upon exposure to carbon monoxide, 1 inserts 1 equiv of CO to give [(dtbpe)Ni(η2-C(O)CH2tBu)][PF6] (2). This cationic acyl complex reacts productively with ethylene, resulting in [(dtbpe)Ni(κ2-CH2CH2C(O)CH2tBu)][PF6] (3), in which the oxygen atom is bound to Ni, forming a five-membered metallacycle. Treatment of 2 with phenyl lithium results in deprotonation at an acidic methylene position adjacent to the carbonyl, affording the neutral ketene complex (dtbpe)Ni(η2-OCCHtBu) (4). Complexes 2, 3, and 4 are stable at 20 °C and have been characterized by NMR and IR spectroscopy as well as single-crystal X-ray diffraction.