Mechanism of the reaction of an NHC-coordinated palladium(II)-hydride with O2 in acetonitrile

• Published in: Polyhedron Vol. 182; p. 114501
• Main Authors: Konnick, Michael M., Knapp, Spring M.M., Stahl, Shannon S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.05.2020
• Subjects: Hydride; N-heterocyclic carbene; Oxidation; Oxygen; Palladium; N-heterocyclic carbene; Oxygen; Palladium; Hydride; Oxidation
• ISSN: 0277-5387
• DOI: 10.1016/j.poly.2020.114501

The PdII-hydride complex trans-[(IMes)2Pd(H)(OBz)] generates the solvent-coordinated cationic complex trans-[(IMes)2Pd(H)(NCMe)][OBz] in acetonitrile. Subsequent reaction with O2 generates the corresponding PdII-hydroperoxide trans-[(IMes)2Pd(OOH)(NCCD3)][OBz], initiated by rate-limiting deprotonation of the hydride. The kinetic behavior of this process exhibits substantial differences from the analogous reaction in benzene, which proceeds via neutral species. [Display omitted] PdII-hydride species are important intermediates in many Pd-catalyzed aerobic oxidation reactions, and their reaction with molecular oxygen has been the subject of considerable previous study. This investigation probes the reactivity of trans-[(IMes)2Pd(H)(OBz)] (IMes = 1,3-dimesitylimidazol-2-ylidene) with O2 in acetonitrile, a polar coordinating solvent that leads to substantial changes in the kinetic behavior of the reaction relative the previously reported reaction in benzene and other non-coordinating solvents. In acetonitrile, the benzoate ligand dissociates to form the solvent-coordinated complex trans-[(IMes)2Pd(H)(NCMe)][OBz]. Upon exposure to O2, this cationic PdII–H complex reacts to form the corresponding PdII-hydroperoxide complex trans-[(IMes)2Pd(OOH)(NCCD3)][OBz]. Kinetic studies of this reaction revealed a complex rate law, rate = k1k2[3][OBz]/(k−1[CD3CN] + k2[OBz]) + k3[3][OBz], which is rationalized by a mechanism involving two parallel pathways for rate-limiting deprotonation of the PdII–H species to generate the Pd0 complex, Pd(IMes)2. The latter complex undergoes rapid (kinetically invisible) reaction with O2 and BzOH to afford the PdII-hydroperoxide product. The results of this study are compared to observations from the previously reported reaction in benzene and discussed in the context of catalytic reactivity.