Iron(III) complexes of tridentate N3 ligands as models for catechol dioxygenases: Stereoelectronic effects of pyrazole coordination

The iron(III) complexes of the tridentate N3 ligands pyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L1), 3,5-dimethylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L2), 3-iso-propylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L3) and (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethylamine (L4) have been isolated and studied as functional models for catechol dioxygenases. They have been characterized by elemental analysis and spectral and electrochemical methods. The X-ray crystal structure of the complex [Fe(L1)Cl3] 1 has been successfully determined. The complex possesses a distorted octahedral coordination geometry in which the tridentate ligand facially engages iron(III) and the Cl− ions occupy the remaining coordination sites. The Fe–Npz bond distance (2.126(5) Å) is shorter than the Fe–Npy bond (2.199(5) Å). The systematic variation in the ligand donor substituent significantly influences the Lewis acidity of the iron(III) center and hence the interaction of the present complexes with a series of catechols. The catecholate adducts [Fe(L)(DBC)Cl], where H2DBC = 3,5-di-tert-butylcatechol, have been generated in situ and their spectral and redox properties and dioxygenase activities have been studied in N,N-dimethylformamide solution. The adducts [Fe(L)(DBC)Cl] undergo cleavage of DBC2− in the presence of dioxygen to afford major amounts of intradiol and smaller amounts extradiol cleavage products. In dichloromethane solution the [Fe(L)(DBC)Cl] adducts afford higher amounts of extradiol products (64.1–22.2%; extradiol-to-intradiol product selectivity E/I, 2.6:1–4.5:1) than in DMF (2.5–6.6%; E/I, 0.1:1–0.4:1). The results are in line with the recent understanding of the function of intra- and extradiol-cleaving catechol dioxygenases.