Synthesis, ESR and Electrochemical Characterization of Dioxygen Binding to Dirhodium Complexes with 2-anilinopyridinato Bridging Ligand

摘要

The Rh2(ap)4(2,2-trans) isomer (ap = 2-anilinopyridinate), which has two anilino nitrogens and two pyridyl nitrogens bound to each rhodium ion trans to their own kind, shows activation towards the one electron reduction of dioxygen at -0.40 V vs SCE. The ESR spectrum taken at 123 K proves the formation of a [Rh2(ap)4(O2)] ion with oxygen axially bound to one rhodium ion and the complex is at a Rh￥±2 oxidation state. The complex will form [Rh2(ap)4(O2)(CH3CN)]- in presence of CH3CN/CH2Cl2 mixture without breaking the Rh-O2- bond. When oxidized at -0.25 and 0.55 V, [Rh2(ap)4(O2)] will undergo two one electron oxidations to form Rh2(ap)4(O2)[Rh2(ap)4(O2)]+. Both species have an axially bound superoxide ion but the former is at Rh￥±Rh￥2 and the later at Rh￥22 oxidation states. The ESR spetra and CH3CN addition study, on the other hand, show that the later complex is better described as [Rh￥±Rh￥2(ap)4(O2)]+ with the odd electron localized on rhodium ion and the complex has an axially coordinated molecular oxygen. The electrochemical and ESR studies also show that the degree of dioxygen activation is a function of electrochemical redox potential.