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

Abstract

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.

New avenues for ligand-mediated processes – expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

“…74 Heyduk et al explored the use of an (phenol)aminophenol ONO ligand as both electron-and proton-reservoir for the reduction of molecular oxygen with zirconium(IV). 75 Similar to the Ta mentioned in Section 2, the zirconium(IV) ion coordinates the triply deprotonated ONO trianion to form 104. However, selective deprotonation of only the phenol-groups proved facile, as the pK a values for the O-H and N-H fragments are quite different.…”

Homoleptic Nickel(II) Complexes of Redox-Tunable Pincer-type Ligands

“…These electrochemical results also parallel those from a recent report by the Heyduk group demonstrating that it was possible to tune the redox potential of tungsten(V) complexes of a trianionic triamido ligand over a 270 mV range by changing groups along the ligand periphery without greatly altering the structures or nitrene transfer reactivity of the complexes. 11a The separation between the two oxidation potentials of the 12 Ni(X,Y) 2 complexes ranges between 200 and about 300 mV. Accordingly, the equilibrium constant for comproportionation ( K com , eq 1) varies between 10 4 and 10 6 depending on the complex, but without any obvious trend.…”

Radical‐Type Reactivity and Catalysis by Single‐Electron Transfer to or from Redox‐Active Ligands

“…[57] However,s trategies to effectively bridge redox non-innocence and such chemical non-innocence have been only scarcely identified to date. [58][59][60][61][62] However,i ft his selective ligand-centered hydrogen-based chemistry could be exploited alongside the established electron-based reactivity of such ligands, this might allow for interesting crossover studies between previously disparate ligand-based reactivity patterns. Ultimately,t his offers the foresight of selective and efficient proton-coupled electron transfer [63] in catalysis.I ti st herefore deemed only am atter of Scheme10.…”