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

Abstract

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 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.