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

• Published in: Chemistry : a European journal 2019-07, Vol.25 (42), p.9920-9929
• Main Authors: Bailey, Wilson D., Phearman, Alexander S., Luconi, Lapo, Rossin, Andrea, Yakhvarov, Dmitry G., D'Accolti, Lucia, Flowers, Sarah E., Kaminsky, Werner, Kemp, Richard A., Giambastiani, Giuliano, Goldberg, Karen I.
• Format: Article
• Language: English
• Subjects: Aromatic compounds; Atomic properties; Chemistry; density functional calculations; Dimers; Hydrides; hydrogen; Hydrogenolysis; Hydroxides; Ligands; palladium; Phosphorus; pincer complexes; Pyrazole; Reaction mechanisms; Rollover
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201900507

The hydrogenolysis of mono‐ and dinuclear PdII hydroxides was investigated both experimentally and computationally. It was found that the dinuclear μ‐hydroxide complexes {[(PCNR)Pd]2(μ‐OH)}(OTf) (PCNH=1‐[3‐[(di‐tert‐butylphosphino)methyl]phenyl]‐1H‐pyrazole; PCNMe=1‐[3‐[(di‐tert‐butylphosphino)methyl]phenyl]‐5‐methyl‐1H‐pyrazole) react with H2 to form the analogous dinuclear hydride species {[(PCNR)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 PdII dimers. The {[(PCNMe)Pd]2(μ‐OH)}(OTf) hydrogenolysis mechanism was investigated through experiments and computations. The hydrogenolysis of the mononuclear complex (PCNH)Pd‐OH resulted in a mixed ligand dinuclear species [(PCNH)Pd](μ‐H)[(PCC)Pd] (PCC=a dianionic version of PCNH 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)PCNH]2. When the ligand was protected at the pyrazole 5‐position in the (PCNMe)Pd−OH complex, no hydride formed under the same conditions; the reaction proceeded directly to the bisphosphine Pd0 complex Pd[(H)PCNMe]2. Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed. Unsymmetrical pincers: The hydrogenolysis of mono‐ and dinuclear PdII hydroxides supported by unsymmetrical pyrazole‐based PCNR pincer ligands has been examined experimentally and computationally. New methods of H2 interaction and activation at late‐transition‐metal centers are discovered, supported by both experiment and theory (see scheme).