Recent Advances in the Field of Phenoxyl Radical–Metal Complexes

Abstract

The phenolate ligand is well-known as one of the non-innocent ligands. It is typically shown in biological systems as the ligand of a single copper enzyme galactose oxidase, forming a relatively stable Cu(II)-phenoxyl radical intermediate. On the other hand, some of the one-electron oxidized metal-phenolate complexes are revealed to be high valent metal-phenolate complexes, which are isoelectronic with the metal-phenoxyl radical complexes. Therefore, characterization of the one-electron oxidized metal-phenolate complexes has recently been the subject of intense studies aiming at understanding their oxidation states under the given conditions. This review focuses on metal-phenolate complexes and their one-electron oxidized species and compares the properties of the metal-phenoxyl and high-valent metal-phenolate complexes.

π–π Stacking Interaction of Metal Phenoxyl Radical Complexes

“…The π orbital is assigned to the SOMO on the phenoxyl radical as whole; therefore, a SOMO–SOMO interaction can be considered in general [ 63 ]. Such a SOMO–SOMO interaction could not be observed in the crystals of metal phenoxyl radical complexes except alkylthio-phenoxyl radical species [ 39 , 40 , 64 , 65 , 66 ]. In many cases, the phenoxyl radical moiety interacted only with the counter anion weakly [ 39 , 40 , 64 ].…”

“…Such a SOMO–SOMO interaction could not be observed in the crystals of metal phenoxyl radical complexes except alkylthio-phenoxyl radical species [ 39 , 40 , 64 , 65 , 66 ]. In many cases, the phenoxyl radical moiety interacted only with the counter anion weakly [ 39 , 40 , 64 ]. On the other hand, the alkylthio-phenoxyl radical, which is similar to the phenoxyl radical in the active form of GO, showed a significant interaction with the other phenoxyl radical by π–π stacking interaction in the solid state [ 65 ].…”

“…A number of model studies of GO have been reported with emphasis on the characteristic electronic structure of copper(II)-phenoxyl radical and its reactivity [ 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ]. Copper(II)-phenoxyl radical complexes have quite different geometric and electronic structures compared with the copper(III)-phenolate species, which is the same one-electron oxidized form of the copper(II)-phenolate species [ 48 , 49 , 50 , 51 , 52 , 53 ].…”

The Study of Anti-/Pro-Oxidant, Lipophilic, Microbial and Spectroscopic Properties of New Alkali Metal Salts of 5-O-Caffeoylquinic Acid

“…The first stable phenoxyl radical complex—iron(III) phenolate radical—was obtained by Hockert et al, in the form of a powder which was stable for over one year under dry air in the room temperature [ 25 ]. Since that, many crystal structures of metal-phenoxyl radicals were described [ 26 ]. The uncoordinated phenoxyl radicals are generally unstable and immediately change to non-radical molecules by polymerization as well as enzymatic and non-enzymatic reduction.…”

Catalytic oxidation and conversion of kraft lignin into phenolic products using zeolite‐encapsulated Cu(II) [H4]salen and [H2]salen complexes

“…In the oxidation of kraft lignin with Cu(II) [H 4 ]salen and [H 2 ]salen complexes, the complexes (compound 1) were oxidized by the formation of a dioxygen species (compounds 2 and 3) as shown in Scheme 2. The reaction was initiated by the hydrogen abstraction from phenolic group by the superoxo complex (LACuAOAO) initially formed [29]. The species formed under these conditions have been spectrally observed in our previous studies [14].…”