Synthesis, structure, spectra and reactivity of iron(III) complexes of imidazole and pyrazole containing ligands as functional models for catechol dioxygenases

Abstract

A series of new 1:1 iron(III) complexes of the type [Fe(L)Cl₃], where L is a tridentate 3N donor ligand, has been isolated and studied as functional models for catechol dioxygenases. The ligands (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethyl-amine (L1), N,N-dimethyl-N'-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L2) and N-(1-methyl-1H-imidazol-2-ylmethyl)-N'-phenylethane-1,2-diamine (L3) are linear while the ligands tris(1-pyrazolyl)methane (L4), tris(3,5-dimethyl-1-pyrazolyl)methane (L5) and tris(3-iso-propylpyrazolyl)methane (L6) are tripodal ones. All the complexes have been characterized by spectral and electrochemical methods. The X-ray crystal structure of the dinuclear catecholate adduct [Fe(L2)(TCC)]₂O, where TCC^2- is a tetrachlorocatecholate dianion, has been successfully determined. In this complex both the iron(III) atoms are bridged by a μ-oxo group and each iron(III) center possesses a distorted octahedral coordination geometry in which the ligand L2 is facially coordinated and the remaining coordination sites are occupied by the TCC^2- dianion. Spectral studies suggest that addition of a base like Et₃N induces the mononuclear complex species [Fe(L2)(TCC)Cl] to dimerize forming a μ-oxo-bridged complex. The spectral and electrochemical properties of the catecholate adducts of the complexes generated in situ reveal that a systematic variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(III) center and hence the interaction of the complexes with simple and substituted catechols. The 3,5-di-tert-butylcatecholate (DBC^2-) adducts of the type [Fe(L)(DBC)Cl], where L is a linear tridentate ligand (L1-L3), undergo mainly oxidative intradiol cleavage of the catechol in the presence of dioxygen. Also, the extradiol-to-intradiol product selectivity (E:I) is enhanced upon removal of the coordinated chloride ion in these adducts to obtain [Fe(L)(DBC)(Sol)]⁺ and upon incorporating coordinated N-methylimidazolyl nitrogen in them. In contrast to the iron(III) complexes of imidazole-based ligands, those of the tripodal pyrazole-based ligands L4-L6 yield major amounts of the oxidized product benzoquinone and small amounts of both intra- and extradiol products. One of the pyrazole arms coordinated in the equatorial plane of these sterically constrained complexes is substituted by a solvent molecule upon adduct formation with DBC^2-, which encourages molecular oxygen to attack this site leading to benzoquinone formation. The DBSQ/DBC^2- redox potentials of both the imidazole- and pyrazole-based complexes fall in the narrow range of -0.186 to -0.214 V supporting this proposal.