Synthesis, structure, spectra and reactivity of iron(III) complexes of facially coordinating and sterically hindering 3N ligands as models for catechol dioxygenases

Abstract

A series of 1:1 iron(III) complexes of sterically hindered and systematically modified tridentate 3N donor ligands have been isolated and studied as functional models for extradiol-cleaving catechol dioxygenases. All of them are of the type [Fe(L)Cl₃], where L is N-methyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L1), N-ethyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L2), N-benzyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L3), N,N-dimethyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L4), N'-methyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L5), N'-ethyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L6) and N'-benzyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L7). They have been characterized by elemental analysis and spectral and electrochemical methods. The X-ray crystal structures of the complexes [Fe(L2)Cl₃] 2, [Fe(L3)Cl₃] 3 and [Fe(L7)Cl₃] 7 have been successfully determined. All the three complexes possess a distorted octahedral coordination geometry in which the ligand is facially coordinated to iron(III) and the chloride ions occupy the remaining coordination sites. Upon replacing the N-ethyl group on the terminal nitrogen donor in 2 by the bulky N-benzyl group as in 3, the terminal Fe-N bond distance increases slightly from 2.229(5) Å to 2.244(5) Å. Upon incorporating the sterically demanding N-benzyl group on the central nitrogen donor in 4 to obtain 7, the central Fe-N_amine bond distance increases from 2.181(5) Å to 2.299(2) Å. The catecholate adducts [Fe(L)(DBC)(Cl)] and [Fe(L)(DBC)(Sol)]⁺, where H₂DBC is 3,5-di-tert-butylcatechol and Sol = solvent (H₂O/DMF), have been generated in situ and their spectral and redox properties and dioxygenase activities have been studied in N,N-dimethylformamide and dichloromethane solutions. The adducts [Fe(L)(DBC)(Sol)]⁺ undergo cleavage of DBC^2- in the presence of molecular oxygen to afford both intra- and extradiol cleavage products. The extradiol products are higher in dichloromethane than in DMF solution and the extradiol to intradiol product selectivity (E/I, 7.2 : 1-18.5:1) observed decreases upon increasing the steric bulk of N-alkyl substituent on the terminal nitrogen atom and upon incorporating an N-alkyl substituent on the central nitrogen atom. The plot of log(k_O2) vs. energy of the low energy catecholate-iron(III) LMCT band is linear, which is consistent with the proposal that the LMCT band energy corresponds to the energy needed for a spin-inversion process at the iron center upon dioxygen attack. Also, the rate of oxygenation is dictated by the solvent as well as the Lewis acidity of the iron(III) center, as shown by the linear plot of log(k_O2) vs.E_1/2 of the Fe(III)/Fe(II) redox potentials of the [Fe(L)(Sol)₃]³⁺ complexes.