Novel iron(III) complexes of sterically hindered 4N ligands: Regioselectivity in biomimetic extradiol cleavage of catechols

The iron(III) complexes of the 4N ligands 1,4-bis(2-pyridylmethyl)-1,4-diazepane (L1), 1,4-bis(6-methyl-2-pyridylmethyl)1,4-diazepane (L2), and 1,4-bis(2-quinolylmethyl)-1,4-diazepane (L3) have been generated in situ in CH3CN solution, characterized as [Fe(L1)Cl-2](+) 1, [Fe(L2)Cl-2](+) 2, and [Fe(L3)Cl-2](+) 3 by using ESI-MS, absorption and EPR spectral and electrochemical methods and studied as functional models for the extradiol cleaving catechol dioxygenase enzymes. The tetrachlorocatecholate (TCC2-) adducts [Fe(L1)(TCC)](ClO4) 1a, [Fe(L2)(TCC)](ClO4) 2a, and [Fe(L3)(TCC)](ClO4) 3a have been isolated and characterized by elemental analysis, absorption spectral and electrochemical methods. The molecular structure of [Fe(L1)(TCC)](ClO4) la has been successfully determined by single crystal X-ray diffraction. The complex la possesses a distorted octahedral coordination geometry around iron(III). The two tertiary amine (Fe-N-amine, 2.245, 2.145 angstrom) and two pyridyl nitrogen (Fe-N-py, 2.104, 2.249 angstrom) atoms of the tetradentate 4N ligand are coordinated to iron(III) in a cis-beta configuration, and the two catecholate oxygen atoms of TCC2- occupy the remaining cis positions. The Fe-O-cat bond lengths (1.940, 1.967 angstrom) are slightly asymmetric and differ by 0.027 angstrom only. On adding catecholate anion to all the [Fe(L)Cl-2](+) complexes the linear tetradentate ligand rearranges itself to provide cis-coordination positions for bidentate coordination of the catechol. Upon adding 3,5-di-tert-buycatechol (H2DBC) pretreated with 1 equiv of Et3N to 1-3, only one catecholate-to-iron(III) LMCT band (648-800 nm) is observed revealing the formation of [Fe(L)(HDBC)](2+) involving bidentate coordination of the monoanion HDBC-. On the other hand, when H2DBC pretreated with 2 equiv of Et3N or 1 or 2 equiv of piperidine is added to 1-3, two intense catecholate-to-iron(III) LMCT bands appear suggesting the formation of [Fe(L)(DBC)](+) with bidentate coordination of DBC2-. The appearance of the DBSQ/H2DBC couple for [Fe(L)Cl-2](+) at positive potentials (-0.079 to 0.165 V) upon treatment with DBC2- reveals that chelated DBC2- in the former is stabilized toward oxidation more than the uncoordinated H2DBC. It is remarkable that the [Fe(L)(HDBC)](2+) complexes elicit fast regioselective extradiol cleavage (34.6-85.5%) in the presence of O-2 unlike the iron(III) complexes of the analogous linear 4N ligands known so far to yield intradiol cleavage products exclusively. Also, the adduct [Fe(L2)(HDBC)](2+) shows a higher extradiol to intradiol cleavage product selectivity (E/l, 181:1) than the other adducts [Fe(L3)(HDBC)](2+) (E/l, 57:1) and [Fe(L1)(HDBC)](2+) (E/l, 9:1). It is proposed that the coordinated pyridyl nitrogen abstracts the proton from chelated HDBC- in the substrate-bound complex and then gets displaced to facilitate O-2 attack on the iron(III) center to yield the extradiol cleavage product. In contrast, when the cleavage reaction is performed in the presence of a stronger base like piperidine or 2 equiv of Et3N a faster intradiol cleavage is favored over extradiol cleavage suggesting the importance of bidentate coordination of DBC2- in facilitating intradiol cleaage.