A new tripodal iron(III) monophenolate complex: effects of ligand basicity, steric hindrance, and solvent on regioselective extradiol cleavage

Abstract

The new iron(III) complex [Fe(L3)Cl₂], where H(L3) is the tripodal monophenolate ligand N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine, has been isolated and studied as a structural and functional model for catechol dioxygenase enzymes. The complex possesses a distorted octahedral iron(III) coordination geometry constituted by the phenolate oxygen, pyridine nitrogen and two amine nitrogens of the tetradentate ligand, and two cis-coordinated chloride ions. The Fe-O-C bond angle (134.0° ) and Fe-O bond length (1.889 Å) are very close to those (Fe-O-C, 133° and 148° , Fe-O(tyrosinate), 1.81 and 1.91 Å) of protocatechuate 3,4-dioxygenase enzymes. When the complex is treated with AgNO₃, the ligand-to-metal charge transfer (LMCT) band around 650 nm (ε , 2390 M^-1 cm^-1) is red shifted to 665 nm with an increase in absorptivity (ε , 2630 M^-1 cm^-1) and the Fe^III/Fe^II redox couple is shifted to a slightly more positive potential (-0.329 to -0.276 V), suggesting an increase in the Lewis acidity of the iron(III) center upon the removal of coordinated chloride ions. Furthermore, when 3,5-di-tert-butylcatechol (H₂DBC) pretreated with 2 mol of Et₃N is added to the complex [Fe(L3)Cl₂] treated with 2 equiv of AgNO₃, two intense catecholate-to-iron(III) LMCT bands (719 nm, ε , 3150 M^-1 cm^-1; 494 nm, ε , 3510 M^-1 cm^-1) are observed. Similar observations are made when H₂DBC pretreated with 2 mol of piperidine is added to [Fe(L3)Cl₂], suggesting the formation of [Fe(L3)(DBC)] with bidentate coordination of DBC^2-. On the other hand, when H₂DBC pretreated with 2 mol of Et₃N is added to [Fe(L3)Cl₂], only one catecholate-to-iron(III) LMCT band (617 nm; ε , 4380 M^-1 cm^-1) is observed, revealing the formation of [Fe(L3)(HDBC)(Cl)] involving monodentate coordination of the catecholate. The appearance of the DBSQ/H₂DBC couple for [Fe(L3)(DBC)] at a potential (-0.083 V) more positive than that (-0.125 V) for [Fe(L3)(HDBC)(Cl)] reveals that chelated DBC^2- in the former is stabilized toward oxidation more than the coordinated HDBC^-. It is remarkable that the complex [Fe(L3)(HDBC)(Cl)] undergoes slow selective extradiol cleavage (17.3%) of H₂DBC in the presence of O₂, unlike the iron(III)-phenolate complexes known to yield only intradiol products. It is probable that the weakly coordinated (2.310 Å) -NMe₂ group rather than chloride in the substrate-bound complex is displaced, facilitating O₂ attack on the iron(III) center and, hence, the extradiol cleavage. In contrast, when the cleavage reaction was performed in the presence of a stronger base-like piperidine before and after the removal of the coordinated chloride ions, a faster intradiol cleavage was favored over extradiol cleavage, suggesting the importance of the bidentate coordination of the catecholate substrate in facilitating intradiol cleavage. Also, intradiol cleavage is favored in dimethylformamide and acetonitrile solvents, with enhanced intradiol cleavage yields of 94 and 40%, respectively.