C–H bond activation via concerted metalation–deprotonation at a palladium(iii) center

Abstract

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)Pd^II(neophyl)Cl (neophyl = –CH₂C(CH₃)₂Ph; ^MeN4 = N,N′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF₆) yields the stable Pd(III) complex [(^MeN4)Pd^III(neophyl)Cl]PF₆. Upon the addition of an acetate source, [(^MeN4)Pd^III(neophyl)Cl]PF₆ undergoes Csp²–H bond activation to yield the cyclometalated product [(^MeN4)Pd^III(cycloneophyl)]PF₆. 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 κ³ 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.