Csp³-H bond activation mediated by a Pd(ii) complex under mild conditions

In this work, we disclose a new pentadentate pyridinophane ligand, N-methyl-N′-(2-methylpyryl)-2,11-diaza[3.3](2,6)pyridinophane (^PicCH3N4), and its Pd(ii) complexes. The reaction of the Pd precursor [Pd^II(MeCN)₄]²⁺ with ^PicCH3N4 leads to the formation of the palladacycle compound [(^PicCH2N4)Pd^II]⁺via an uncommon room-temperature Csp³-H bond activation at a Pd(ii) metal center. The isolated complex was characterized by single-crystal X-ray diffraction, NMR spectroscopy, and cyclic voltammetry. Furthermore, various experimental investigations, including additive studies, kinetic isotope effect measurements, and Eyring analysis were carried out to probe the reaction kinetics and mechanism of the Csp³-H bond activation by the Pd(ii) center. A combined experimental and theoretical mechanistic analysis suggests that acetate-assisted Csp³-H bond activation is preferred at high temperature, while both acetate-assisted and acetate-free Csp³-H bond activation pathways are competitive at room temperature.