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 ((Me)N4)Pd(II)(neophyl)Cl (neophyl = –CH(2)C(CH(3))(2)Ph; (Me)N4 = N,N′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF(6)) yields the stable Pd(iii) complex [((Me)N4)Pd(III)(neophyl)Cl]PF(6). Upon the addition of an acetate source, [((Me)N4)Pd(III)(neophyl)Cl]PF(6) undergoes Csp(2)–H bond activation to yield the cyclometalated product [((Me)N4)Pd(III)(cycloneophyl)]PF(6). This metalacycle 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 that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ(3) ligand coordination and that the flexible ligand structure is important for this transformation. 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.