Effects of metal ions on physicochemical properties and redox reactivity of phenolates and phenoxyl radicals: Mechanistic insight into hydrogen atom abstraction by phenoxyl radical-metal complexes

Phenolate and phenoxyl radical complexes of a series of alkaline earth metal ions as well as monovalent cations such as Na⁺ and K⁺ have been prepared by using 2,4-di-tert-butyl-6-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-ylmethyl) phenol (L1H) and 2,4-di-tert-butyl-6-(1,4,7,10,13-pentaoxa-16-aza-cyclo-octadec-16-ylmethyl) phenol (L2H) to examine the effects of the cations on the structure, physicochemical properties and redox reactivity of the phenolate and phenoxyl radical complexes. Crystal structures of the Mg²⁺- and Ca²⁺-complexes of L^1- as well as the Ca²⁺- and Sr²⁺-complexes of L2^- were determined by X-ray crystallographic analysis, showing that the crown ether rings in the Ca²⁺-complexes are significantly distorted from planarity, whereas those in the Mg²⁺- and Sr²⁺-complexes are fairly flat. The spectral features (UV-vis) as well as the redox potentials of the phenolate complexes are also influenced by the metal ions, depending on the Lewis acidity of the metal ions. The phenoxyl radical complexes are successfully generated in situ by the oxidation of the phenolate complexes with (NH₄)₂[Ce⁴⁺(NO₃)₆] (CAN). They exhibited strong absorption bands around 400 nm together with a broad one around 600-900 nm, the latter of which is also affected by the metal ions. The phenoxyl radical-metal complexes are characterized by resonance Raman, ESI-MS, and ESR spectra, and the metal ion effects on those spectroscopic features are also discussed. Stability and reactivity of the phenoxyl radical-metal complexes are significantly different, depending on the type of metal ions. The disproportionation of the phenoxyl radicals is significantly retarded by the electronic repulsion between the metal cation and a generated organic cation (Ln⁺), leading to stabilization of the radicals. On the other hand, divalent cations decelerate the rate of hydrogen atom abstraction from 10-methyl-9,10-dihydroacridine (AcrH₂) and its 9-substituted derivatives (AcrHR) by the phenoxyl radicals. On the basis of primary kinetic deuterium isotope effects and energetic consideration of the electron-transfer step from AcrH₂ to the phenoxyl radical-metal complexes, we propose that the hydrogen atom abstraction by the phenoxyl radical-alkaline earth metal complexes proceeds via electron transfer followed by proton transfer.