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

To investigate metalligand 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 NiH 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 metalligand cooperativity is not involved in these rare examples of mild and homogeneous nickel hydrogenation catalysis.