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

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

Abstract

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[CH₂CH₂P(Cy)₂]₂} have been synthesized and characterized. Cationic hydride complex [(PNHP^Cy)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 (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)]BPh₄ {PNMeP^Cy = (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.

Cationic and neutral nickel(II) hydride complexes of the aliphatic pincer ligand PNHP^Cy {PNHP^Cy = HN[CH₂CH₂P(Cy)₂]₂} have been prepared and were found to be active alkene hydrogenation catalysts. Mechanistic experiments suggest that for cationic catalyst 2, hydrogenation may proceed through a pathway involving initial insertion of the alkene into the Ni–H bond.