A Redox-Active Ligand as a Reservoir for Protons and Electrons: O2 Reduction at Zirconium(IV)



The [ONO] ligand {(ONOcat)3– = bis(3,5-di-tert-butyl-2-phenoxy)amide} can be installed onto zirconium(IV) in two different protonation states. The reaction of (ONOcat)3– with ZrCl4 afforded the octahedral complex (ONOcat)ZrCl(THF)2 (1). This complex, characterized in the solid state by single-crystal X-ray diffraction and in solution by NMR,UV/Vis, and IR spectroscopy, is assigned as a zirconium(IV) complex coordinated to the fully reduced form of the [ONO] redox-active ligand. When (ONOcat)H3 was doubly deprotonated and treated with ZrCl4, the complex (ONHO)ZrCl2(THF) (2) was isolated, in which one ligand proton remains on the [ONO] ligand. Exposure of 2 to dry O2 resulted in a four-electron, two-proton reaction to form the bis(hydroxo) complex [(ONOq)ZrCl2(μ-OH)]2 (3), which shows that the [ONO] ligand platform can serve as both an electron and proton reservoir in small-molecule reactions.