Functional Models for Catechol 1,2-Dioxygenase. Synthesis, Structure, Spectra, and Catalytic Activity of Certain Tripodal Iron(III) Complexes

A series of iron(III) complexes of tetradentate tripodal ligands [L1−H 2 (L7); L1 = tris(pyrid-2-ylmethyl)amine, L2 = bis(pyrid-2-ylmethyl)(pyrid-2-ylethyl)amine, H(L3) = 2-bis(pyrid-2-ylmethyl)aminomethyl-4-nitrophenol, H 2 (L4) = N, N -bis(2-hydroxy-5-nitrobenzyl)aminomethylpyridine, L5 = tris(benzimidazol-2-ylmethyl)amine, H(L6) = 2-bis(benzimidazol-2-yl)methylaminomethyl-4-nitrophenol, and H 2 (L7) = N, N -bis(2-hydroxy-5-nitrobenzyl)aminomethylbenzimidazole] have been synthesized. These complexes have been characterized using UV−vis spectral and electrochemical techniques. The single-crystal X-ray structure of [Fe( L3 )Cl 2 ] has been determined by standard procedures and refined by least squares to a conventional R factor of 0.054. The violet crystals belong to the triclinic space group P 1̄ with Z = 2 and unit cell dimensions a = 7.018(4) Å, b = 9.087 Å, and c = 13.962(5) Å. The iron(III) complex has a pseudo-octahedral N 3 OCl 2 donor set with two chloride ions occupying cis positions; the Fe−O(phenolate) bond is trans to one of the Fe−N py bonds. The interaction of all the Fe(III) complexes with a variety of monodentate and bidentate heterocyclic bases as well as phenols has been investigated using electronic spectra. The interactions with catecholate anions reveal changes in the phenolate-to-iron(III) charge transfer band, similar to catechol dioxygenase−substrate complexes. The redox behavior of the complexes and their 1:1 adducts with 3,5-di- tert -butylcatechol (H 2 DBC) has been investigated. All the complexes catalyze the oxidative cleavage of H 2 DBC by molecular oxygen to yield cis, cis -muconic anhydride. The catalytic activity could be illustrated on the basis of the Lewis acidity of the complex−DBC adducts, derived from the CAT 2 - /DBC 2- → Fe(III) CT band energy and the E 1/2 of the DBSQ/DBC couple, and steric hindrance to substrate binding and by assuming that the product release is the rate-determining phase of the reaction.