Characterization of Water Coordination to Ferrous Nitrosyl Complexes with <i>fac</i>-N₂O, <i>cis</i>-N₂O₂, and N₂O₃ Donor Ligands

Electron paramagnetic resonance (EPR) experiments were done on a series of <i>S</i> = ³/₂ ferrous nitrosyl model complexes prepared with chelating ligands that mimic the 2-His-1-carboxylate facial triad iron binding motif of the mononuclear nonheme iron oxidases. These complexes formed a comparative family, {FeNO}⁷(N₂O_<i>x</i>)­(H₂O)_3–<i>x</i> with <i>x</i> = 1–3, where the labile coordination sites for the binding of NO and solvent water were fac for <i>x</i> = 1 and cis for <i>x</i> = 2. The continuous-wave EPR spectra of these three complexes were typical of high-spin <i>S</i> = ³/₂ transition-metal ions with resonances near <i>g</i> = 4 and 2. Orientation-selective hyperfine sublevel correlation (HYSCORE) spectra revealed cross peaks arising from the protons of coordinated water in a clean spectral window from <i>g</i> = 3.0 to 2.3. These cross peaks were absent for the {FeNO}⁷(N₂O₃) complex. HYSCORE spectra were analyzed using a straightforward model for defining the spin Hamiltonian parameters of bound water and showed that, for the {FeNO}⁷(N₂O₂)­(H₂O) complex, a single water conformer with an isotropic hyperfine coupling, <i>A</i>_iso = 0.0 ± 0.3 MHz, and a dipolar coupling of <i>T</i> = 4.8 ± 0.2 MHz could account for the data. For the {FeNO}⁷(N₂O)­(H₂O)₂ complex, the HYSCORE cross peaks assigned to coordinated water showed more frequency dispersion and were analyzed with discrete orientations and hyperfine couplings for the two water molecules that accounted for the observed orientation-selective contour shapes. The use of three-pulse electron spin echo envelope modulation (ESEEM) data to quantify the number of water ligands coordinated to the {FeNO}⁷ centers was explored. For this aspect of the study, HYSCORE spectra were important for defining a spectral window where empirical integration of ESEEM spectra would be the most accurate

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

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 [FeII/III(N2On)(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 d5 FeIIIg = 4.3 signal becomes more symmetrical as the number of carboxylate ligands decreases across the series Fe(N2O3), Fe(N2O2), and Fe(N2O1), 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 FeII–FeIII oxidation–reduction potentials illustrate the fundamental influence of differential carboxylate ligation. The trend towards lower reduction potential for the iron center across the [FeIII(N2O1)Cl3]−, [FeIII(N2O2)Cl2]− and [FeIII(N2O3)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 (FeII(N2O1) \u3c FeII(N2O2) \u3c FeII(N2O3)), 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