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

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 FeIIICl2(Ln) complexes (n = 3, L3 = 1,3-bis­(2′-thiazolylimino)­isoindoline
and n = 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-tert-butylcatechol (H2DBC) and excess triethylamine. These
adducts react with dioxygen in N,N-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 DBC2–. 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.