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-H2(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, H2(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 H2(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)Cl2] 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↑ 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 N3OCl2 donor set with two chloride ions occupying cis positions; the Fe-O(phenolate) bond is trans to one of the Fe-Npy 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 (H2DBC) has been investigated. All the complexes catalyze the oxidative cleavage of H2DBC 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 CAT2-/DBC2- → Fe(III) CT band energy and the E1/2 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