Metal-Ligand Redox Cooperativity in Cerium Amine-/Amido-Phenolate-Type Complexes

Abstract

The synthesis and characterization of two cerium complexes of redox-active amine/amido-phenolate-type ligands are reported. A tripodal framework comprising the tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-phenyl) amine (H₆Clamp) proligand was synthesized for comparison of its cerium complex with a potassium-cerium heterobimetallic complex of the 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (^dippap) proligand. Structural studies indicate differences in the cerium(III) cation coordination spheres, where Ce^III(CH₃CN)_1.5(H₃Clamp) (1-Ce(H₃Clamp)) exhibits shorter Ce-O distances and longer Ce-N bond distances compared to the analogous distances in K₃(THF)₃Ce^III(^dippap)₃ (2-Ce(ap)), due to the gross structural differences between the systems. Differences are also evident in the temperature-dependent magnetic properties, where smaller χT products were observed for 2-Ce(ap) compared to 1-Ce(H₃Clamp). Solution electrochemical studies for the complexes were interpreted based on ligand- and metal-based oxidation events, and the cerium(III) oxidation of 2-Ce(ap) was observed to be more facile than that of 1-Ce(H₃Clamp), behavior that was cautiously attributed to the rigidity of the encrypted 1-Ce(H₃Clamp) complex compared to the heterobimetallic framework of 2-Ce(ap). These results contribute to the understanding of how ligand designs can promote facile redox cycling for cerium complexes of redox-active ligands, given the large contraction of cerium-ligand bonds upon oxidation.