Iron(III) complexes of pyrrolidine and piperidine appended tridentate 3N donor ligands as models for catechol dioxygenase enzymes

Abstract

Six new mononuclear iron(III) complexes [Fe(L1)Cl3]-[Fe(L6)Cl3] bearing tridentate all-nitrogen donor ligands (where L1 = N-(pyridin-2-ylmethyl)-2-(pyrrolidin-1-yl)ethanamine, L2 = 2-(piperidin-1-yl)-N-(pyridin-2-ylmethyl)ethanamine, L3 = 1-(piperidin-2-yl)-N-(pyridin-2-ylmethyl)methanamine, L4 = N-((1-methyl-1H-imidazol-2-yl)methyl)-2-(pyrrolidin-1-yl)ethan-1-amine, L5 = N-((1-methyl-1H-imidazol-2-yl)methyl)-2-(piperidin-1-yl)ethan-1-amine), L6 = 1-(1-methyl-1H-imidazol-2-yl)-N-(piperidin-2-ylmethyl)methanamine) have been prepared, characterized and evaluated as the functional models for the carbon–carbon bond-cleaving catechol dioxygenase enzymes. The molecular structure of complexes 1 and 3 revealed a distorted octahedral geometry around iron(III) center with facial coordination of ligands via three nitrogen atoms and three chloride ions occupy the remaining sites of the octahedron. Iron(III) catecholate adducts were generated in situ by adding one equivalent of various catechols pretreated with two equivalent of triethylamine. The 3,5-di-tert-butylcatecholate adduct of the complexes 1–6 show two intense bands in visible region (540–900 nm) as the result of DBC2--to-iron(III) ligand-to-metal charge-transfer transitions in acetonitrile. During electrochemical analysis, all the complexes display FeIII→FeII redox couple in the similar potential range (−0.260 to −0.336 V). The dioxygenase activity of present complexes produces major amount of extradiol cleavage products (75–92%) over the intradiol cleavage products (4–14%) at moderate rate (0.561–1.744 × 10−2 M−1 s−1). Interestingly, the iron(III) complexes 3 and 6 with facially coordinating tridentate ligands with heterocyclic piperidine amine (NH) nitrogen donor moiety produce major amounts of extradiol-cleavage products (3, 89%; 6, 75%) along with only minor amounts of other product (10–24%) without any intradiol-cleavage products.