Uncoordinated phenoxyl radicals

Since then, chemists have attempted to synthesize model complexes that contain coordinated and uncoordinated phenoxyl radicals. This chapter reviews this literature from 1994 to early 2000. Earlier efforts describing the chemistry of extended radical-metal ion systems such as those derived from ligands shown in Fig. 1 and their metal complexes and oxidation products (3-13) have been reviewed recently by Goldberg and Lippard (14). The coordination chemistry of these interesting ligands remains to be firmly established as does its redox chemistry. 

IV. COORDINATION CHEMISTRY OF IMI IN OX Yl RADICALS A. Complexes with Uncoordinated Phenoxyl Radicals... 

Interesting earlier work on uncoordinated phenoxyl radical complexes shown in Fig. 10 has been reported by Medzhidov and co-workers (138-141) who synthe-... 

In an attempt to establish unequivocally the spectroscopic features of coordinated (vs uncoordinated) phenoxyl radicals a series of phenolato precursor complexes containing a spectroscopically and redox-innocent Ga(III), Sc(III), or Zn(II) central metal ion were synthesized (142-148). In order to avoid metal-ligand bond dissociation in solution, the phenolate or, after one-electron oxidation, phenoxyl moieties were covalently attached to the strongly metal ion binding 1,4,7-triazacy-clononane (149) backbone. Thus a series of phenolate pendent-arm macrocyclic... 

As pointed out earlier, RR spectroscopy is a powerful tool for an unambiguous distinction between coordinated and uncoordinated phenoxyl radicals. Upon excitation in resonance with the n —> ir transition of the phenoxyl, the RR bands originating from the modes ula (-1500 cm-1 C-O stretching) and uSa (-1600 cm-1 C=C stretching) are enhanced and clearly detectable. The exact positions of these bands as well as their RR intensity ratio can be used to distinguish between coor-... 

The most thoroughly studied class of phenoxyl radical complexes was prepared by oxidation of first-row transition metal complexes of the type of macrocyclic phenolate ligands shown in Figure 4. Studies of those compounds that contain Ga , Sc , and have been particularly useful for differentiating and establishing the properties of coordinated vs. uncoordinated phenoxyl radicals. Due to their closed shell nature, these ions are both spectroscopically silent and redox inactive, thus enabling the properties of phenoxyl radicals in their complexes to be seen unmasked from those of the metal ion. Data for oxidized forms of the complexes [L M] ((l)-(4) are illustrative). These compounds undergo three reversible one-electron oxidations at approximately equally spaced... 

Resonance Raman spectroscopy has been shown to a particularly effective tool for differentiating between coordinated and uncoordinated phenoxyl radicals. Excitation into the 400 30 nm 7T 7T transition of metal-phenoxyl radical complexes leads to resonant enhancement of bands at l,500cm and 1,600cm. By analogy to experimental and calculated vibrational spectra for free phenoxyl radicals (e.g., these bands have been assigned as and j/ga modes,... 

A number of ligands comprising protected phenols, exemplified by (30) and (31), have been complexed to metal ions and oxidized in attempts to generate compounds with uncoordinated phenoxyl radicals. In most cases, UV-vis, IR, and/or EPR spectroscopic evidence was provided in support of phenoxyl radical formation, but in few cases were the compounds isolated as pure solids, and no X-ray structural data is available. A survey of these systems has been published. ... 

Here, we will first review the physical organic chemistry and spectroscopic features of the ligand phenoxyl and introduce briefly some well-characterized metalloproteins known to contain tyrosyl radicals and then systematically describe the coordination chemistry of uncoordinated and coordinated phenoxyls. Finally, we will describe the reactivity of coordinated phenoxyls toward some organic substrates. 

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