Mechanistic study of alkene hydrogenation catalyzed by cationic Ni(II)-hydride PNHP complexes – Ni(II/IV) vs. Ni(II) cycle

The hydrogenation mechanisms of alkenes catalyzed by square-planar cationic Ni(II)-PNHP hydride complexes have been computed (M06L-GD3-SCRF). It is found that the reaction prefers a non-bifunctional inner-sphere mechanism, the Ni(II)/Ni(IV) cycle involving H2 oxidative addition forming unusual Ni(IV) oxidation state and reductive elimination is accessible and has been proven to be superior or competitive with the widely accepted Ni(II) cycle undergoing concerted H2 σ-bond metathesis in formal Ni(II) oxidation state. Systematic investigations into the electronic and geometric effects of phosphine ligand substituents uncover their significant influence on the Gibbs free energy barrier of elementary steps, leading to differences in catalytic activity. Analysis into these effects demonstrates a strong correlation between the apparent free energy barrier and frontier orbital energy, natural charge of Ni-center and buried volume ratio of the catalyst. These findings offer valuable insights for the design of future catalysts.