Hydrogenolysis of Dinuclear PCNR Ligated PdII μ-Hydroxides and Their Mononuclear PdII Hydroxide Analogues

The hydrogenolysis of mono- and dinuclear Pd^II hydroxides was investigated both experimentally and computationally. It was found that the dinuclear μ-hydroxide complexes {(PCN^R)Pd]₂(μ-OH)}(OTf) (PCN^H=1-3-(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole; PCN^Me=1-3-(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) react with H₂ to form the analogous dinuclear hydride species {(PCN^R)Pd]₂(μ-H)}(OTf). The dinuclear μ-hydride complexes were fully characterized, and are rare examples of structurally characterized unsupported singly bridged μ-H Pd^II dimers. The {(PCN^Me)Pd]₂(μ-OH)}(OTf) hydrogenolysis mechanism was investigated through experiments and computations. The hydrogenolysis of the mononuclear complex (PCN^H)Pd-OH resulted in a mixed ligand dinuclear species (PCN^H)Pd](μ-H)(PCC)Pd] (PCC=a dianionic version of PCN^H bound through phosphorus P, aryl C, and pyrazole C atoms) generated from initial ligand “rollover” C−H activation. Further exposure to H₂ yields the bisphosphine Pd⁰ complex Pd(H)PCN^H]₂. When the ligand was protected at the pyrazole 5-position in the (PCN^Me)Pd−OH complex, no hydride formed under the same conditions; the reaction proceeded directly to the bisphosphine Pd⁰ complex Pd(H)PCN^Me]₂. Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed.