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

To 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 PNHP Cy {PNHP Cy = HN[CH 2 CH 2 P(Cy) 2 ] 2 } have been synthesized and characterized. Cationic hydride complex [(PNHP Cy )Ni(H)]BPh 4 ( 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 (PNP Cy )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 [(PNMeP Cy )Ni(H)]BPh 4 {PNMeP Cy = (CH 3 )N[CH 2 CH 2 P(Cy) 2 ] 2 } suggest that metal–ligand cooperativity is not involved in these rare examples of mild and homogeneous nickel hydrogenation catalysis.