C-H Bond Activation 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 (N4)Pd(neophyl)Cl (neophyl = -CHC(CH)Ph; N4 = ,'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF) yields the stable Pd(iii) complex [(N4)Pd(neophyl)Cl]PF. Upon the addition of an acetate source, [(N4)Pd(neophyl)Cl]PF undergoes Csp-H bond activation to yield the cyclometalated product [(N4)Pd(cycloneophyl)]PF. 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 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.