A realistic model for heme-containing catalases and peroxidases: the X-ray structural characterisation of a non-porphyrin iron(III) macrocyclic complex, and the mechanism of its peroxidation of aromatic substrates

Abstract

In acidic buffered aqueous solution the complex dichloro{meso-2,12-dimethyl-3,7,11,17-tetra-azabicyclo[11.3.1] heptadeca-1 (17),13,15-triene}iron(III) tetrafluoroborate, [Fe(meso-L′)Cl₂]BF₄, exhibits both catalase- and peroxidase-like activity. The predominant cation in aqueous buffered solution at pH 4.65 is the mixed species [Fe(meso-L′)(OH)(H₂O)]²⁺. The catalase- and peroxidase-like activity is proposed to occur via a high oxidation state intermediate rather than through involvement of free hydroxyl radicals. Quantitative compliance with the algebraic forms of theoretical rate laws fails to distinguish between these possibilities. However, the kinetics of dioxygen evolution in the presence of hydroxyl radical traps leads to the elimination of the hydroxyl radical model. In addition, the peroxidase-like reactivity of substituted benzenes toward the [Fe(meso-L′)(OH)(H₂O)]²⁺–H₂O₂ model system parallels that expected for an electrophilic oxidant, and not that of free OH radicals. Parallel experiments with Fenton's reagent support this view. An X-ray structural determination on the dichloro complex indicates that the macrocycle adopts a folded conformation allowing the two chloride ligands to occupy cis positions in the co-ordination sphere. This stereochemistry is proposed to be retained in aqueous solution, and may allow bidentate co-ordination of hydrogen peroxide, a structural feature that may be critical to the catalase- and peroxidase-like activity. The iron(III) complex crystallises in the orthorhombic system, a= 10.046(2), b= 13.322(2), c= 15.262(3)Å, space group Pnma, with four molecules per unit cell. Final residuals had values of 0.042 and 0.043, for R and R′, respectively, upon convergence for 1 525 observed reflections. Both the cationic, macrocyclic complex and the BF₄ anion display crystallographically imposed mirror symmetry. The iron(III) ion displays an approximately octahedral geometry, with co-ordination angles ranging from 77 to 95°. An analysis of associated torsion angles suggests that the folded conformation of the macrocycle is almost strain free.