Hydrogenolysis of Palladium(II) Hydroxide, Phenoxide, and Alkoxide Complexes

A series of pincer (tBuPCP)Pd(II)–OR complexes (tBuPCP = 2,6-bis(CH2PtBu2)C6H3, R = H, CH3, C6H5, CH2C(CH3)3, CH2CH2F, CH2CHF2, CH2CF3) were synthesized to explore the generality of hydrogenolysis reactions of palladium–oxygen bonds. Hydrogenolysis of the Pd hydroxide complex to generate the Pd hydride complex and water was shown to be inhibited by formation of a water-bridged, hydrogen-bonded Pd(II) hydroxide dimer. The Pd alkoxide and aryloxide complexes exhibited more diverse reactivity. Depending on the characteristics of the −OR ligand (steric bulk, electron-donating ability, and/or the presence of β-hydrogen atoms), hydrogenolysis was complicated by hydrolysis by adventitious water, a lack of reactivity with hydrogen, or a competing dissociative β-hydride abstraction reaction pathway. Full selectivity for hydrogenolysis was observed with the partially fluorinated Pd(II) 2-fluoroethoxide complex. The wide range of Pd–OR substrates examined helps to clarify the variety of reaction pathways available to late-transition-metal alkoxides as well as the conditions necessary to tune the reactivity to hydrogenolysis, hydrolysis, or dissociative β-hydride abstraction.