Iron(III) complexes of linear 3N ligands as functional models for catechol dioxygenases: ligand modification...

Four new iron(III) complexes of the type [Fe(L)Cl₃] 1-4, where L is N,N-(bispyridin-2-ylmethyl)-N-(thiophene-2-methyl)amine (L1), N,N-(bispyridin-2-ylmethyl)-N-(thiophene-2-ethyl)amine (L2), N-(pyridin-2-ylmethyl)-N-(1-methylimidazole-2-methyl)-N-(thiophene-2-methyl)amine (L3) and N-(pyridin-2-ylmethyl)-N-(1-methylimidazole-2-methyl)-N-(thiophene-2-ethyl)amine (L4), have been synthesized and studied as functional models for catechol dioxygenases. In the X-ray structures of [Fe(L1)Cl₃] 1 and [Fe(L3)Cl₃] 3, the 3N ligands are facially coordinated to Fe(III) in a distorted octahedral geometry. DFT studies reveal that L1 is meridionally coordinated in the 3,5-di-tert-butylcatecholate (DBC^2-) adduct [Fe(L1)(DBC)Cl] 1A, whereas L2-L4 are facially coordinated in [Fe(L2-L4)(DBC)Cl] 2A-4A. In O₂-saturated methanol solution, 1A undergoes catechol cleavage with the highest rate constant for the second-order reaction (k_O2: 1 (11.38 ± 0.02 × 10^-3 M^-1 s^-1) > 3 (4.45 ± 0.02 × 10^-3 M^-1 s^-1) > 4 (1.58 ± 0.02 × 10^-3 M^-1 s^-1) > 2 (1.28 ± 0.02 × 10^-3 M^-1 s^-1)) to yield major amounts of extradiol cleavage products (1, 94%; 2, 90%; 3, 52%; 4, 80%), with the highest extradiol-to-intradiol product selectivity (E/I: 1, 94/1; 2, 18/1; 3, 1/1; 4, 4/1). Upon replacing a pyridyl nitrogen in 1A with an imidazolyl nitrogen to obtain 3A, and extending the thiophenylmethyl arm in 1A and 3A to obtain 2A and 4A, respectively, both the rate and extradiol cleavage yield decrease. DFT studies reveal that the intermediate [Fe(L1)(DBSQ)(O₂)]⁺ contains meridionally coordinated L1 and antiferromagnetic interaction between (DBSQ)Fe^III and O²˙^-, which facilitates efficient extradiol cleavage.