A New Tripodal Iron(III) Monophenolate Complex: Effects of Ligand Basicity, Steric Hindrance, and Solvent on Regioselective Extradiol Cleavage

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 FeIII/FeII 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
DBC2-. 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
DBC2- 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.