Iron(III) complexes of tripodal monophenolate ligands as models for non-heme catechol dioxygenase enzymes: correlation of dioxygenase activity with ligand stereoelectronic properties

Abstract

The iron(III) complexes [Fe(L)Cl₂] 1-6 of the tripodal monophenolate ligands N,N-bis(2-pyridylmethyl)-N'-(2-hydroxybenzyl)amine H(L1), [(1-methylimidazol-2-ylmethyl)-(pyrid-2-ylmethyl)aminomethyl]-phenol H(L2), 2,4-dimethyl-6-[(1-methylimidazol-2-ylmethyl)(pyrid-2-ylmethyl)aminomethyl]phenol H(L3), N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxybenzyl)ethylenediamine H(L4), N,N-dimethyl-N'-(1-methylimidazol-2-ylmethyl)-N'-(2-hydroxybenzyl)ethylenediamine H(L5), and N,N-dimethyl-N'-(1-methylimidazol-2-ylmethyl)-N'-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine H(L6) have been isolated and studied as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The complexes have been characterized using elemental analysis, electrospray ionization mass spectrometry, and absorption spectral and electrochemical methods. The single crystal X-ray structures of [Fe(L3)Cl₂] 3 and [Fe(L6)Cl₂] 6 have been successfully determined, and the rhombically distorted octahedral coordination geometry around iron(III) in them are constituted by the phenolate oxygen and pyridyl/-NMe₂, and N-methylimidazolyl and tertiary amine nitrogens of the tripodal tetradentate ligands H(L3)/H(L6) and two cis-coordinated chloride ions. The sterically demanding -NMe₂ group as in 6 imposes an Fe-O-C bond angle (139.8° ) and Fe-O bond length (1.852 Å), which are very close to those (Fe-O-C, 133, 148° ; Fe-O(tyrosinate), 1.81, 1.91 Å) of 3,4-PCD enzymes. The Fe-O-C bond angle observed for 6 is higher than that for 3 (125.1° ), and the Fe-O(phenolate) bond distance in 6 is shorter than that in 3 (1.905 Å). In methanol solution all the complexes exhibit two phenolate-to-Fe(III) ligand-to-metal charge transfer (LMCT) bands in the ranges 536-622 and 329-339 nm. Further, when 3,5-di-tert-butylcatechol (H₂DBC) pretreated with two moles of Et₃N is added to 1-6, two new intense DBC^2--to-iron(III) LMCT bands (466-489, 676-758 nm) are observed, which are similar to those observed for 3,4-PCD enzyme-substrate complex. All the complexes elicit oxidative intradiol cleavage of H₂DBC in the presence of O₂. Interestingly, among the present complexes, 3 containing coordinated N-methylimidazolyl nitrogen shows the highest rate of intradiol cleavage, which correlates with the highest energy of DBC^2--to-iron(III) LMCT band and the most negative DBSQ/DBC^2- redox potential. Also, the catecholate adducts of complexes 4 and 5, both containing a -NMe₂ donor group, react faster and produce higher amounts of intradiol cleavage products (4: 55.3; 5, 50.6%) than the analogous complexes 1 (43.2%) and 2 (32.7%), both containing a pyridyl nitrogen donor, which is consistent with the more negative DBSQ/DBC^2- redox potentials for 4 and 5. The increase in rate of catechol dioxygenation with increase in the DBC^2--to-iron(III) LMCT band energy and decrease in DBSQ/DBC^2- redox potential is illustrated by invoking a facile α-electron transfer from iron(III) to catecholate-bound molecular oxygen in the substrate activation mechanism proposed for the intradiol-cleaving catechol dioxygenases. Also, when the substituents on the phenolate arm are varied to tune the Lewis acidity of iron(III) center, the reaction rate decreases with decrease in Lewis acidity and, interestingly, extradiol cleavage is also observed when the Lewis acidity is decreased further by incorporating a 3,5-dimethylphenolate arm as in 6.