Catecholate and Phenolate Iron Complexes as Models for the Dioxygenases

The syntheses and physical properties of a series of Fe(salen)X and Fe(saloph)X complexes where X is phenolate or catecholate are reported. Magnetic susceptibility measurements as well as electronic, infrared, and NMR spectra indicate that the catecholate in Fe(salen)catH behaves very much like a phenolate and is concluded to be monodentate in its coordination to the iron. The abstraction of a proton from Fe(salen)catH results in an anionic complex, [Fe(salen)cat]⁻, with markedly different properties; the catecholate in this complex is chelated. Both monodentate and chelated catecholate complexes are high-spin ferric, demonstrating that catecholate coordination to a bis(phenolato)iron(III) complex does not result in the reduction of the ferric center. This is in agreement with observations made on dioxygenase-substrate complexes. In addition, studies on a series of Fe(salen)X complexes where X is phenolate, thiophenolate, benzoate, and catecholate show that the dominant salen-to-Fe(III) charge-transfer interaction is modulated by the coordination of these ligands. Comparisons with corresponding dioxygenase complexes show that the tyrosinate-to-iron(III) charge-transfer interactions are similarly affected, thus indicating that the salen ligand provides a reasonable approximation of the iron environment in the dioxygenases.