Novel square pyramidal iron(iii) complexes of linear tetradentate bis(phenolate) ligands as structural and reactive models for intradiol-cleaving 3,4-PCD enzymes: Quinone formation vs. intradiol cleavage

Abstract

The iron(III) complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxy-4-methyl-benzyl)-1,4-diazepane H₂(L1), 1,4-bis(2-hydroxy-4-nitrobenzyl)-1,4-diazepane H₂(L2), 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane H₂(L3) and 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane H₂(L4) have been isolated and studied as structural and functional models for 3,4-PCD enzymes. The complexes [Fe(L1)Cl] 1, [Fe(L2)(H₂O)Cl] 2, [Fe(L3)Cl] 3 and [Fe(L4)Cl] 4 have been characterized using ESI-MS, elemental analysis, and absorption spectral and electrochemical methods. The single crystal X-ray structure of 3 contains the FeN₂O₂Cl chromophore with a novel square pyramidal (τ, 0.20) coordination geometry. The Fe–O–C bond angle (135.5°) and Fe–O bond length (1.855 Å) are very close to the Fe–O–C bond angles (133, 148°) and Fe–O(tyrosinate) bond distances (1.81, 1.91 Å) in 3,4-PCD enzyme. All the complexes exhibit two intense absorption bands in the ranges 335–383 and 493–541 nm, which are assigned respectively to phenolate (pπ) → Fe(III) (dσ*) and phenolate (pπ) → Fe(III) (dπ*) LMCT transitions. The Fe(III)/Fe(II) redox potentials of 1, 3 and 4 (E_1/2, −0.882– −1.010 V) are more negative than that of 2 (E_1/2, −0.577 V) due to the presence of two electron-withdrawing p-nitrophenolate moieties in the latter enhancing the Lewis acidity of the iron(III) center. Upon adding H₂DBC pretreated with two equivalents of Et₃N to the iron(III) complexes, two catecholate-to-iron(III) LMCT bands (656, ε, 1030; 515 nm, ε, 1330 M⁻¹ cm⁻¹) are observed for 2; however, interestingly, an intense catecholate-to-iron(III) LMCT band (530–541 nm) is observed for 1, 3 and 4 apart from a high intensity band in the range 451–462 nm. The adducts [Fe(L)(DBC)]⁻ generated from 1–4in situ in DMF/Et₃N solution react with dioxygen to afford almost exclusively the simple two-electron oxidation product 3,5-di-tert-butylbenzoquinone (DBQ), which is discerned from the appearance and increase in intensity of the electronic spectral band around 400 nm, and smaller amounts of cleavage products. Interestingly, in DMF/piperidine the amount of quinone product decreases and those of the cleavage products increase illustrating that the stronger base piperidine enhances the concentration of the catecholate adduct. The rates of both dioxygenation and quinone formation observed in DMF/Et₃N solution vary in the order 1 > 3 > 4 < 2 suggesting that the ligand steric hindrance to molecular oxygen attack, the Lewis acidity of the iron(III) center and the ability of the complexes to rearrange the Fe–O phenolate bonds to accommodate the catecholate substrate dictate the extent of interaction of the complexes with substrate and hence determine the rates of reactions. This is in line with the observation of DBSQ/H₂DBC reduction wave for the adduct [Fe(L2)(DBC)]⁻ at a potential (E_1/2: −0.285 V) more positive than those for the adducts of 1, 3 and 4 (E_1/2: −0.522 to −0.645 V).