Mononuclear Organometallic Pd(II), Pd(III), and Pd(IV) Complexes Stabilized by a Pyridinophane Ligand with a C-Donor Group

A series of Pd complexes bearing modified tetradentate pyridinophane ligands ^RN3CH, containing a C-donor phenyl group, were isolated and characterized. The (^RN3CH)Pd^II(OAc)₂ complexes contain a C_ipso-H bond that remains unactivated at the Pd^II stage, even upon heating or addition of excess acetate. These Pd^II(OAc)₂ complexes exhibit catalytic reactivity in the Kharasch radical addition of bromotrichloromethane to methyl methacrylate. Interestingly, novel Pd^III complexes (^RN3C)Pd^IIIBr₂ were isolated from the Kharasch reaction mixture and have been characterized by EPR, UV-vis spectroscopy, and X-ray crystallography, suggesting that activation of the C_ipso-H bond has occurred during the oxidative conditions of the Kharasch radical addition. Inspired by this observation, several (^pMeN3C)Pd^III complexes were synthesized upon oxidation of the Pd^II precursors with PhICl₂ and subsequent halide abstraction with thallium salts. Furthermore, additional one-electron oxidation generates detectable Pd^IV species, including an uncommon tricationic [(^pMeN3C)Pd^IV(MeCN)₃]³⁺ complex. Overall, these initial results show that the ^RN3C(H) ligand system is capable of stabilizing high-valent Pd species that can undergo uncommon oxidative and catalytic reactivity, including C-H bond activation.