Hydrogenolysis of Dinuclear PCN (R) Ligated Pd-II mu-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 mu-hydroxide complexes {[(PCN (R))Pd](2)(mu-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 H-2 to form the analogous dinuclear hydride species {[(PCN (R))Pd](2)(mu-H)}(OTf). The dinuclear mu-hydride complexes were fully characterized, and are rare examples of structurally characterized unsupported singly bridged mu-H Pd-II dimers. The {[(PCNMe)Pd](2)(mu-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](mu-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 H-2 yields the bisphosphine Pd-0 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 Pd-0 complex Pd[(H)PCNMe](2). Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed.