Studies of iron(ii) and iron(iii) complexes with fac-N2O, cis-N2O2 and N2O3 donor ligands: models for the 2-His 1-carboxylate motif of non-heme iron monooxygenases

Abstract

Enzymes in the oxygen-activating class of mononuclear non-heme iron oxygenases (MNOs) contain a highly conserved iron center facially ligated by two histidine nitrogen atoms and one carboxylate oxygen atom that leave one face of the metal center (three binding sites) open for coordination to cofactor, substrate, and/or dioxygen. A comparative family of [Fe(II/III)(N(2)O(n))(L)(4-n))](±x), n = 1-3, L = solvent or Cl(-), model complexes, based on a ligand series that supports a facially ligated N,N,O core that is then modified to contain either one or two additional carboxylate chelate arms, has been structurally and spectroscopically characterized. EPR studies demonstrate that the high-spin d(5) Fe(III)g = 4.3 signal becomes more symmetrical as the number of carboxylate ligands decreases across the series Fe(N(2)O(3)), Fe(N(2)O(2)), and Fe(N(2)O(1)), reflecting an increase in the E/D strain of these complexes as the number of exchangeable/solvent coordination sites increases, paralleling the enhanced distribution of electronic structures that contribute to the spectral line shape. The observed systematic variations in the Fe(II)-Fe(III) oxidation-reduction potentials illustrate the fundamental influence of differential carboxylate ligation. The trend towards lower reduction potential for the iron center across the [Fe(III)(N(2)O(1))Cl(3)](-), [Fe(III)(N(2)O(2))Cl(2)](-) and [Fe(III)(N(2)O(3))Cl](-) series is consistent with replacement of the chloride anions with the more strongly donating anionic O-donor carboxylate ligands that are expected to stabilize the oxidized ferric state. This electrochemical trend parallels the observed dioxygen sensitivity of the three ferrous complexes (Fe(II)(N(2)O(1)) < Fe(II)(N(2)O(2)) < Fe(II)(N(2)O(3))), which form μ-oxo bridged ferric species upon exposure to air or oxygen atom donor (OAD) molecules. The observed oxygen sensitivity is particularly interesting and discussed in the context of α-ketoglutarate-dependent MNO enzyme mechanisms.

Crystal structure of dichlorido{2-[(2-hydroxyethyl)(pyridin-2-ylmethyl)amino]ethanolato-κ4N,N′,O,O′}iron(III) dihydrate from synchrotron data

“…The average Fe-X Hpmide (X = N, O) bond length is 2.10 Å and the Fe-Cl bond lengths are 2.2773 (5) (equatorial) and 2.3581 (7) (axial) Å . Both the average Fe-N (2.182 Å ) and Fe-O (2.010 Å ) distances in (I) are comparable to those found in related N 2 O 2 -chelated highspin Fe III complexes (Shin et al, 2014;Cappillino et al, 2012). The bite angles of the five-membered chelate rings in (I) range from 76.59 (5) to 81.45 (4) .…”

Mimicry of the 2‐His‐1‐Carboxylate Facial Triad Using Bulky N,N,O‐Ligands: Non‐Heme Iron Complexes Featuring a Single Facial Ligand and Easily Exchangeable Co‐Ligands

“…[4,5] To date, the activity of non-heme iron enzymes is mimicked most efficiently by iron complexes derived from N 4 -type ligands such as BPMEN, Me 2 PyTACN, PDP, and TPA ( Figure 1). [1,[6][7][8][9][10][11][12][13][14][15][16][17] Highly active and selective catalysts for the epoxidation of olefins with turnover numbers over 120 and high stereoselectivity (97 % ee) [18] been described. [10,[19][20][21][22][23][24][25] Most of these complexes are structurally distinct from the typical non-heme enzyme active site in the sense that these lack a carboxylate, or in general an Odonor moiety and that the N-donors often stem from a pyridine ring instead of imidazole rings.…”

The Reactivity of FeII and CoII Disulfide Compounds with Dihydrogen Peroxide

“…The IR spectrum of 2 shows two intense absorption bands at around 1022 and 1146 cm –1 likely corresponding to the symmetric and asymmetric S=O bond stretching frequencies . The EPR spectrum of 2 dissolved in dimethyl sulfoxide shows a rather broad, rhombic spectrum with a g value of around 4.25, typical for an iron(III) center in a high‐spin state ( S = 5/2; Figure S2) . The magnetic susceptibility of 2 was estimated using Evans' method in dimethyl sulfoxide solution at 20 °C, revealing a µ eff of 5.28 µ B [a value of 5.92 µ B is expected for an S = 5/2 iron(III) center] .…”