Mononuclear iron(iii) complexes of 3N ligands in organized assemblies: spectral and redox properties and attainment of regioselective extradiol dioxygenase activity

Abstract

Four octahedral iron(III) complexes of the type [Fe(L)Cl₃], where L is a tridentate 3N ligand like N,N-bis(pyrid-2-ylmethyl)amine (bpa, L1), N,N-bis(benzimidazol-2-ylmethyl)amine (bba, L2), 1,4,7-triazacyclononane (tacn, L3) and 2,2′;6′,2′′-terpyridine (terpy, L4), have been isolated and their catechol dioxygenase activity investigated in dichloromethane, water and different aqueous micellar media. The positions of both the catecholato-to-iron(III) LMCT bands observed for the DBC²⁻ (H₂DBC = 3,5-di-tert-butylcatechol) adducts reveal that the adducts are present as cationic [Fe(L)(DBC)(H₂O)]⁺ species, which interact strongly with anionic SDS micelles and dock themselves on the anionic micellar surface, and that they exist in the aqueous phase in CTAB and TX 100 micelles. The Fe(III)/Fe(II) redox potentials of the complexes throw light on the Lewis acidity of the iron(III) center as modified by the ligand donor atoms and hence the interaction of the complexes with different micelles. The DBSQ/DBC²⁻redox potentials in SDS micellar media are more positive than those in aqueous solution confirming the presence of the aqua species [Fe(L)(DBC)(H₂O)]⁺. The DBC²⁻ adducts of the iron(III) complexes of bpa, bba and tacn ligands, all with facial coordination, elicit extradiol (E) cleavage to different extents while the adduct of the terpy complex with meridional coordination of the ligand shows always intradiol (I) cleavage. It is remarkable that the bpa complex shows the highest yield of extradiol product and high product selectivity in aqueous SDS solution (E, 84.0%; E/I, 61.0 : 1) and in SDS : n-hexane reverse micellar medium (E, 93.7%) illustrating that a vacant or solvent coordinated site is essential for observing extradiol cleavage. Interestingly, the rates of dioxygenase reactions in aqueous and aqueous micellar solutions are significantly higher than those in non-aqueous solvents. Also, they diminish in the order, SDS > TX-100 > CTAB, illustrating the facile substitution of coordinated water molecule by molecular oxygen in [Fe(L)(DBC)(H₂O)]⁺ bound to anionic SDS micelles.