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 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 ( R N3C)Pd III Br 2  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 PhICl 2  and subsequent halide abstraction with thallium salts. Furthermore, additional one-electron oxidation generates detectable Pd IV  species, including an uncommon tricationic [( pMe 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 species that can undergo uncommon oxidative and catalytic reactivity, including C-H bond activation.