Metal Coordinated Phenoxyl Radicals

Abstract

The term "phenoxyl" radical was first introduced in 1914 by Pummerer 1 to designate species involved in the oxidation of naphthols and phenanthroles. It was not until the 1960s that the existence of phenoxyls was demonstrated by Electron Paramagnetic Resonance (EPR). The term "stable" is commonly used to designate radicals such as nitroxides, trityls, and so on. Although the term "stable" had been associated with some phenoxyl radicals in 1967, 2 it must be realized that phenoxyl radicals exhibiting stabilities comparable to those of nitroxides were rather rare at this time. During the 1970s, Reichard et al . 3 demonstrated that radicals related to phenoxyls could also arise from mono-electronic oxidation of the phenolic side chain of tyrosines in some proteins, and thus could be involved in biological processes. The new term "tyrosyl" was then introduced to designate these residues, and many other biological systems involving such radicals have been described. 4,5 One of the more important recent advances in tyrosyl radical history was achieved in the 1990s with the characterization of the galactose oxidase (GO) active site. 6 -8 For the first time it was demonstrated that tyrosyls could exist coordinated to a metal ion. To better understand this association, chemists have subsequently developed many complexes involving coordinated phenoxyl radicals. 9 -13 Elucidation of the properties of coordination compounds involving redox active ligands, especially those involving sterically hindered phenolates, is one of the most recent and fascinating topics of interest in bioinorganic chemistry. The challenge in this kind of chemistry is the right description of the electronic structure of the M n+ -OPh entity (where OPh represents a phenolate group) once it has been oxidized by one electron. In principle, either the M (n+1)+ -OPh, that is the oxidation is metal based, or the M n+ -• OPh form, that is the oxidation is ligand based, could be obtained. The right description is thus not obvious and, as will be shown below, it strongly depends on the nature of the metal ion, the denticity of the ligand, the substituents of the phenolate precursor and even the temperature. Coordination is also found to improve the radical stability, and exerts an extraordinary control on their magnetic properties and reactivity (regio-and stereoselective oxidations are promoted by a radical). Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron CompoundsEdited by Robin G. Hicks

Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes

“…The τ 4 index is somewhat larger (0.31), disclosing significant tetrahedral distortions at the metal center, which are confirmed by O1–Cu–N2 and O2–Cu–N1 angles of 158°. Finally, the O1–C1 and O2–C13 bond distances in both 3 and 4 are 1.302–1.309(3) Å, as observed in related copper-phenolate complexes. ,,,,,− ,− …”

“…Both the increased metal charge and the orthogonality between the peripheral rings likely contribute to the localization of the radical on one arm of the ligand, in contrast to the above copper-phenoxyl complexes. A further consequence is a lack of specific NIR signature for the radical complex, whose vis-NIR spectrum compares with that of isolated phenoxyl radicals …”

“…Manganese and cobalt complexes are for example famous catalysts for the epoxidation of alkenes − and hydrolytic kinetic resolution (HKR) of epoxides , or copolymerization of CO 2 /epoxides, − respectively, while the vanadium complexes can be used for the asymmetric synthesis of cyanohydrins. − First row salen complexes are also good candidates for direct CO 2 reduction and were regarded for catalyzing the water splitting reaction . Apart from these catalytic applications and medicinal purposes, salen complexes of the first row metal ions are also excellent candidates for electrochemical immunosensing, photoelectrochemical energy conversion, , as well as molecular magnetism. , Bulky tert -butyl (or any other electron-rich) groups are often introduced in ortho and para positions of the salicylidene moieties to improve the enantioselectivity during turnover. − ,− In some instances these groups can alternatively confer a redox noninnocence to the salen ligand − and stable radicals can be isolated and characterized. In these latter cases the site of oxidation and subsequently the properties of the salen complex are governed by several interdependent factors such as the electronics of the ligand, − the nature of the metal ion, − the solvent, ,− ,− the temperature, ,,, and others. − Among the pro-phenoxyl (sterically hindered) salen complexes, the copper derivatives ,,− …”

“…Apart from these catalytic applications and medicinal purposes, salen complexes of the first row metal ions are also excellent candidates for electrochemical immunosensing, photoelectrochemical energy conversion, , as well as molecular magnetism. , Bulky tert -butyl (or any other electron-rich) groups are often introduced in ortho and para positions of the salicylidene moieties to improve the enantioselectivity during turnover. − ,− In some instances these groups can alternatively confer a redox noninnocence to the salen ligand − and stable radicals can be isolated and characterized. In these latter cases the site of oxidation and subsequently the properties of the salen complex are governed by several interdependent factors such as the electronics of the ligand, − the nature of the metal ion, − the solvent, ,− ,− the temperature, ,,, and others. − Among the pro-phenoxyl (sterically hindered) salen complexes, the copper derivatives ,,− have attracted particular interest due to their ability to mimic the active site of the radical enzyme galactose oxidase. − The one-electron oxidized copper complexes of the N,N’ -bis­(3- tert -butyl, 5-R-salicylidene)-1,2-cyclohexanediamine ([Cu­(Sal R )] + , Scheme ) are both models of galactose oxidase and prototypical examples of the subtle balance between the possible valence tautomers: For [Cu­(Sal tBu )] + the Cu­(II)-radical valence tautomer is favored at high temperatures, whereas the high-valent Cu­(III) complex predominates at low temperature …”

π–π Stacking Interaction of Metal Phenoxyl Radical Complexes

“…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 ].…”