Hydrogenolysis of Dinuclear PCN^R Ligated Pd^II μ-Hydroxides and Their Mononuclear Pd^II 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.