C-H Bond Activation via Concerted Metalation-Deprotonation at a Palladium(III) Center

Herein we report the direct observation of C-H bond activation at an isolated mononuclear Pd(III) center. The oxidation of the Pd(II) complex (MeN4)PdII(neophyl)Cl (neophyl = -CH2C(CH3)2Ph; MeN4 = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF6) yields the stable Pd(III) complex [(MeN4)PdIII(neophyl)Cl]PF6. Upon the addition of an acetate source, [(MeN4)PdIII(neophyl)Cl]PF6 undergoes Csp2-H bond activation to yield the cyclometalated product [(MeN4)PdIII(cycloneophyl)]PF6. This metallacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd(III) complexes. The C-H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest the C-H activation likely occurs after the rate determining step. Density functional theory calculations support that molecular isomerization/ligand rearrangement is rate limiting and proceeds through a κ3 ligand geometry. Overall, this study represents the first example of discrete C-H bond activation occurring at a Pd(III) center through a concerted metalation-deprotonation mechanism, akin to that observed for Pd(II) and Pd(IV) centers.