Iron(III) Complexes with Meridional Ligands as Functional Models of Intradiol-Cleaving Catechol Dioxygenases

Abstract

Six dichloroiron­(III) complexes of 1,3-bis­(2′-arylimino)­isoindoline (BAIH) with various N-donor aryl groups have been characterized by spectroscopy (infrared, UV–vis), electrochemistry (cyclic voltammetry), microanalysis, and in two cases X-ray crystallography. The structurally characterized Fe^IIICl₂(L^<i>n</i>) complexes (<i>n</i> = 3, L³ = 1,3-bis­(2′-thiazolylimino)­isoindoline and <i>n</i> = 5, L⁵ = 1,3-bis­(4-methyl-2′-piridylimino)­isoindoline) are five-coordinate, trigonal bipyramidal with the isoindoline ligands occupying the two axial and one equatorial positions meridionally. These compounds served as precursors for catechol dioxygenase models that were formed in solution upon addition of 3,5-di-<i>tert</i>-butylcatechol (H₂DBC) and excess triethylamine. These adducts react with dioxygen in <i>N</i>,<i>N</i>-dimethylformamide, and the analysis of the products by chromatography and mass spectrometry showed high intradiol over extradiol selectivity (the intradiol/extradiol product ratios varied between 46.5 and 6.5). Kinetic measurements were performed by following the change in the intensity of the catecholate to iron ligand-to-metal charge transfer (LMCT) band, the energy of which is influenced by the isoindolinate-ligand (827–960 nm). In combination with electrochemical investigations the kinetic studies revealed an inverse trend between reaction rates and oxidation potentials associated with the coordinated DBC^2–. On the basis of these results, a substrate activation mechanism is suggested for this system in which the geometry of the peroxide-bridged intermediate may be of key importance in regioselectivity