Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes of an Aliphatic PNP Pincer Ligand

Abstract

AbstractTo investigate metal–ligand cooperativity as a strategy for promoting nickel‐catalyzed alkene hydrogenation, cationic and neutral nickel(II) hydride complexes of the aliphatic pincer ligand PNHPCy {PNHPCy = HN[CH2CH2P(Cy)2]2} have been synthesized and characterized. Cationic hydride complex [(PNHPCy)Ni(H)]BPh4 (2) catalyzed the hydrogenation of styrene and 1‐octene under mild conditions. Only low conversion was observed in the hydrogenation of 3,5‐dimethoxybenzaldehyde using 2. The neutral hydride complex (PNPCy)Ni(H) (3) was also found to be an alkene hydrogenation catalyst. Mechanistic experiments suggest that for catalyst 2, the hydrogenation reaction proceeds through a pathway involving initial insertion of the alkene into the Ni–H bond. Contrary to the initial hypothesis, reactivity comparisons with the methyl‐substituted hydride complex [(PNMePCy)Ni(H)]BPh4 {PNMePCy = (CH3)N[CH2CH2P(Cy)2]2} suggest that metal–ligand cooperativity is not involved in these rare examples of mild and homogeneous nickel hydrogenation catalysis.