Ligand-centered radicals phenolate ligands

Thianthrene radical cation is also an excellent one-electron oxidant of iron porphyrin complexes. Such oxidation of Fem(0Cl03)(TPP), where TPP is meso-tetraphenylporphyrin, provides the corresponding porphyrin 7r-cation radical analytically pure [32]. Similar oxidation of the AT-methyl porphyrin complex (N-MeTPP)FenCl, where AT-MeTPP is AT-methyl-meso-tetraphenylporphyrin, afforded [N-MeTPPFemCl]+ which was not further oxidized [33]. Thus thianthrene radical cation selectively oxidized the aromatic porphyrin ligand in one case and the metal center in the other. Ligand oxidation at a phenolic moiety has also been reported [34] on treatment of a 1,4,7-triazacyclononane appended with one or two phenol moieties ligated to Cu(II) complex with thianthrene radical cation. 

In 1955, Terent ev and Mogilyanskii reported the catalytic oxidative coupling of aniline to azobenzene with a yield of 88%, mediated by copper chloride in pyridine (which acts as both a metal ligand and solvent) in the presence of molecular oxygen [1]. This system was subsequently used for the generation of various conjugated and nonconjugated main-chain aromatic azo polymers from primary aromatic diamines [2]. In contrast, the use of CUCI-O2 with phenol produces tars [3] as a result of the inherent properties of this reactant. Indeed, while the metal salt produces iV-centered aniline radicals that dimerize to form azobenzene... 

The character of the coordinated catechols has been studied by Mossbauer spectroscopy [38]. Since the value of A is a measure of the hyperfine interaction of the iron electron with Fe, it reflects the covalency of the metal-ligand bands. The value of A/gnPn for the catechol complex is -18.9 T, that is the smallest observed for a dioxygenase complex -20.8 T for the phenol complex and -20.0 T for the thiophenol complex. The lowest values are explained by a greater delocalization of unpaired spin density away from the ferric center onto the catechol, generating a radical character of the catechol. The unpaired spin density delocalization is consistent with the paramagnetic shifts of protons of the catecholate ligand observed by NMR. 

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