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Keywords:

  • Homogeneous catalysis;
  • Nickel;
  • Hydride ligands;
  • Hydrogenation;
  • Earth-abundant metals;
  • Metal–ligand cooperativity

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 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.