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(CH₂^<i>t</i>Bu)][PF₆] (<b>1</b>; dtbpe = 1,2-bis(di-<i>tert</i>-butylphosphino)ethane) sequentially inserts carbon monoxide and ethylene into the Ni−C bond to form isolable products. Upon exposure to carbon monoxide, <b>1</b> inserts 1 equiv of CO to give [(dtbpe)Ni(η²-C(O)CH₂^<i>t</i>Bu)][PF₆] (<b>2</b>). This cationic acyl complex reacts productively with ethylene, resulting in [(dtbpe)Ni(κ²-CH₂CH₂C(O)CH₂^<i>t</i>Bu)][PF₆] (<b>3</b>), in which the oxygen atom is bound to Ni, forming a five-membered metallacycle. Treatment of <b>2</b> with phenyl lithium results in deprotonation at an acidic methylene position adjacent to the carbonyl, affording the neutral ketene complex (dtbpe)Ni(η²-OCCH^tBu) (<b>4</b>). Complexes <b>2</b>, <b>3</b>, and <b>4</b> are stable at 20 °C and have been characterized by NMR and IR spectroscopy as well as single-crystal X-ray diffraction.