Paramagnetic complexes phenolate ligands

A reaction mechanism for the above reactions was proposed which consists of initial formation of the copper precursor complexes of Fig. 3 (without coordinated phenolate), coordination of phenolate, electron transfer from phenolate to Cu2+ and subsequent reduction to Cu1+ with formation of a phenoxy radical, and reoxidation of Cu1+ to Cu2+ with oxygen. Various copper(II) catalysts having different stereochemistries (octahedral or tetrahedral coordination) due to coordination of amines like pyridine (Py) or acetate (OAc) groups in different ligand sites were observed by NMR and electron paramagnetic resonance techniques. 

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. 

---