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 (R)N3CH, containing a C-donor phenyl group, were isolated and characterized. The ((R)N3CH)Pd-II (OAc)(2) complexes contain a C-ipso-H bond that remains unactivated at the Pe 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-II complexes ((R)N3C)(PdBr2)-Br-III 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 ((pMe)N3C)Pd-III complexes were synthesized upon oxidation of the Pd-II precursors with PhICI2 and subsequent halide abstraction with thallium salts. Furthermore, additional one-electron oxidation generates detectable Pd-IV species, including an uncommon tricationic [((p)(Me)N3C)Pd-IV(MeCN)(3)](3+) complex. Overall, these initial results show that the (R)N3C(H) ligand system is capable of stabilizing high-valent Pd-IV species that can undergo uncommon oxidative and catalytic reactivity, including C-H bond activation.