End-On and Side-On Peroxo Derivatives of Non-Heme Iron Complexes with Pentadentate Ligands:  Models for Putative Intermediates in Biological Iron/Dioxygen Chemistry

Abstract

Mononuclear iron(III) species with end-on and side-on peroxide have been proposed or identified in the catalytic cycles of the antitumor drug bleomycin and a variety of enzymes, such as cytochrome P450 and Rieske dioxygenases. Only recently have biomimetic analogues of such reactive species been generated and characterized at low temperatures. We report the synthesis and characterization of a series of iron(II) complexes with pentadentate N5 ligands that react with H 2 O 2 to generate transient low-spin Fe III −OOH intermediates. These intermediates have low-spin iron(III) centers exhibiting hydroperoxo-to-iron(III) charge-transfer bands in the 500−600-nm region. Their resonance Raman frequencies, ν O-O , near 800 cm -1 are significantly lower than those observed for high-spin counterparts. The hydroperoxo-to-iron(III) charge-transfer transition blue-shifts and the ν O-O of the Fe−OOH unit decreases as the N5 ligand becomes more electron donating. Thus, increasing electron density at the low-spin Mononuclear iron(III) peroxide species are implicated as intermediates in the mechanisms of oxygen activating biomolecules such as cytochrome P450, 1 heme oxygenase, 2 the antitumor drug bleomycin, 3 and Rieske dioxygenases, 4,5 as well as superoxide reductases from anaerobic bacteria. [6][7][8][9] Experimental evidence for some of these intermediates has

Bispidine Ligand Effects on Iron/Hydrogen Peroxide Chemistry

“…[ , [9,18] are almost precisely as predicted by a simple calculation based on Hooke's law, assuming the hydroperoxo moiety to be a single mass unit. Interestingly, while the n O-O vibrations of the two intermediates are identical, the n Fe-OOH vibration for the L 2 complex is shifted to lower energy by 12 in methanol at À40 8C (2 equiv NaOMe) leads to a blue solution with an absorption maximum at l = 721 nm ( Figure 1); this conversion is easily reversed by back titration with acid (2 equiv HClO 4 ).…”

“…[7][8][9] Deprotonation of these species affords their respective conjugate bases with lower energy charge-transfer bands; spectroscopic studies support their assignment as the corresponding high-spin Fe III side-on peroxo derivatives. To date, no crystal structures are available for any of these Fe III intermediates, so their precise structures remain to a large extent a matter of speculation.…”

“…[11] On the other hand, DFT calculations for [Fe(h 2 -OO)(N4py)] + suggest that the N4py ligand remains pentadentate and that the Fe-N amine bond lengthens from 2.1 to 2.4 to accommodate the side-on peroxo binding; the calculated structure affords iron-ligand bond lengths fully consistent with those derived from EXAFS analysis. [9] The significant difference in properties of the side-on peroxo complexes of L 2 and L 3 argues against the dissociation of the third, additional pyridine donor and leads us to consider seven-coordinate h 2 -O 2 structures. Seven-coordination for transition-metal complexes with bispidine ligands has been observed before [13] and is supported for the high- + reasonably well).…”

Hydrogen Peroxide Dependent cis-Dihydroxylation of Benzoate by Fully Oxidized Benzoate 1,2-Dioxygenase

“…Indeed, one of the first to be recognized in a biological system was found in a ring-cleaving dioxygenase, the product complex of protocatechuate 3,4-dioxygenase (37). Interestingly, another case in which negative zero field splitting has been reported is in inorganic mononuclear Fe(III) chelate complexes that have a bound side-on peroxo moiety reminiscent of the peroxo complex of NDO (29,36). One of the model complexes had D = −1 cm −1 and E/D = 0.11 at 4.2 K, and it yielded a Mössbauer spectrum quite similar in appearance to that of intermediate BZDO P .…”

Probing the properties of the second coordination sphere of biologically relevant metal complexes using amide function.

“…An R value of 0.10 for this compound also indicates a d x 2 -y 2 ground state and a geometry around the Cu close to square pyramidal. We have attempted to convert the end-on hydroperoxo species to a side-on peroxo species as established by Que's group for a Fe(III) model complex (Roelfes, G. et al, 2003). The addition of two equivalents of base to the methanol solution of Cu-OOH did not affect the UV-visible nor the EPR spectra.…”