The synthesis of structural and functional models of the active site of the non-heme iron enzyme cysteine dioxygenase (CDO) is reported. A bis(imino)pyridine ligand scaffold was employed to synthesize a mononuclear ferrous complex, Fe II (LN 3 S)(OTf) (1), which contains 3 neutral nitrogen and one anionic thiolato donor. Complex 1 is a good structural model of the Cys-bound active site of CDO. Reaction of 1 with O 2 results in oxygenation of the thiolato sulfur, affording the sulfonato complex Fe II (LN 3 SO 3 )(OTf) (2) under mild conditions. Isotope labeling studies show that O 2 is the sole source of O atoms in the product, and that the reaction proceeds via a dioxygenase-type mechanism for two out of three O atoms added, analogous to the dioxygenase reaction of CDO. The zinc(II) analog, Zn(LN 3 S)(OTf) (4), was prepared and found to be completely unreactive toward O 2 , suggesting a critical role for Fe II in the oxygenation chemistry observed for 1. To our knowledge, S-oxygenation mediated by an Fe II -SR complex and O 2 is unprecedented.The utilization of O 2 for the oxidation of organic substrates is a critical process carried out by metalloenzymes, and a highly desirable one for synthetic chemists to replicate. Cysteine dioxygenase (CDO) is a mononuclear non-heme iron enzyme that catalyzes the Soxygenation of cysteine to cysteine sulfinic acid with O 2 as oxidant (Figure 1).1 Loss of CDO function has been correlated with Alzheimer's, Parkinson's, and other neurological disorders. CDO contains a mononuclear Fe II center bound by 3 His ligands, in contrast to the 2-His-1-carboxylate "facial triad" that is the canonical motif for non-heme Fe oxygenases. This unexpected structural variation suggests that the ligation of three neutral N donors may be important for CDO function.1h X-ray crystal structures of the native iron(II) CDO,1b a Cys-bound complex,1c and an intriguing Cys-persulfenate species1f have been determined (Figure 1). Little is known regarding the mechanism of CDO, although the persulfenate structure suggests an Fe-O 2 intermediate may be important.Herein we describe the first structural and functional synthetic models of CDO. To obtain biologically relevant models, we targeted polydentate ligand platforms that would 1) provide 3 neutral N donors, 2) stabilize Fe II , 3) allow for the facile incorporation of a thiolate donorCorrespondence to: David P. Goldberg, dpg@jhu.edu. Supporting Information Available: Experimental details, spectra, and crystallographic data for complexes 1, 3, and 4. This material is available free of charge via the Internet at htpp://pubs.acs.org. Attempts to crystallographically characterize 2 after O 2 addition were unsuccessful. However, demetalation and acid hydrolysis (1 M HCl), followed by quantitative reversephase HPLC (H 2 O/CH 3 CN 95/5, 0.1% TFA) shows that the expected oxygenated organic fragment 2-H 2 N-C 6 H 4 SO 3 H is formed in good yield (60%). These data confirm that Soxygenation occurs upon reaction of O 2 with 1. EPR spectra at 15 K of mixtures...

Iron peroxide species have been identified as important intermediates in a number of non-heme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N 4 S(thiolate))Fe II complexes that react with tert-butyl hydroperoxide (tBuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N 4 S(thiolate)Fe III -OOR) at low temperature. These complexes were designed specifically to mimic the non-heme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the Fe II complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the Fe III -OOR species, lowtemperature UV-vis spectra reveal intense peaks between 500 -550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the Fe III -OOR unit comes from resonance Raman (RR) spectroscopy, which shows ν(Fe-O) modes between 600 -635 cm −1 and ν(O-O) bands near 800 cm −1 . These Fe-O stretching frequencies are significantly lower than those found in other low-spin Fe III -OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and DFT calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme.

The synthesis of a mononuclear, five-coordinate ferrous complex [([15]aneN4)FeII(SPh)](BF4) (1) is reported. This complex is a new model of the reduced active site of the enzyme superoxide reductase (SOR), which is comprised of a [(NHis)4(Scys)FeII] center. Complex 1 reacts with alkylhydroperoxides (tBuOOH, cumenylOOH) at low temperature to give a metastable, dark red intermediate (2a: R = tBu; 2b: R = cumenyl) that has been characterized by UV-vis, EPR, and resonance Raman spectroscopy. The UV-vis spectrum (-80 degrees C) reveals a 526 nm absorbance (epsilon = 2150 M-1 cm-1) for 2a and a 527 nm absorbance (epsilon = 1650 M-1 cm-1) for 2b, indicative of alkylperoxo-to-iron(III) LMCT transitions, and the EPR data (77 K) show that both intermediates are low-spin iron(III) complexes (g = 2.20 and 1.97). Definitive identification of the Fe(III)-OOR species comes from RR spectra, which give nu(Fe-O) = 612 (2a) and 615 (2b) cm-1, and nu(O-O) = 803 (2a) and 795 (2b) cm-1. The assignments for 2a were confirmed by 18O substitution (tBu18O18OH), resulting in a 28 cm-1 downshift for nu(Fe-18O), and a 46 cm-1 downshift for nu(18O-18O). These data show that 2a and 2b are low-spin FeIII-OOR species with weak Fe-O bonds and suggest that a low-spin intermediate may occur in SOR, as opposed to previous proposals invoking high-spin intermediates.

A new 5-coordinate, (N 4 S(thiolate))Fe II complex, containing tertiary amine donors, [Fe II (Me 4 ([15] aneN 4 )(SAr)(OOR)] + , which contain secondary amine donors. Importantly, alkylation at nitrogen leads to a change from low-spin (S = ½) to high -spin (S = 5/2) of the iron(III) center. The resonance Raman data reveal that this change in spin-state has a large effect on the ν(Fe-O) and ν(O-O) vibrations, and a comparison between 2a and the non-thiolate-ligated complex 3a shows that axial ligation has an additional significant impact on these vibrations. In addition, to our knowledge this study is the first in which the influence of a ligand trans to a peroxo moiety has been evaluated for a structurally equivalent pair of high-spin/low-spin peroxo-iron(III) complexes. The implications of spin state and thiolate ligation are discussed with regard to the functioning of SOR.

The reaction of a series of thiolate-ligated iron(II) complexes [FeII([15]aneN4)(SC6H5)]BF4 (1), [FeII([15]aneN4)(SC6H4-p-Cl)]BF4 (2), and [FeII([15]aneN4)(SC6H4-p-NO2)]BF4 (3) with alkylhydroperoxides at low temperature (−78 °C or −40 °C) leads to the metastable alkylperoxo-iron(III) species [FeIII([15]aneN4)(SC6H5)(OOtBu)]BF4 (1a), [FeIII([15]aneN4)(SC6H4-p-Cl)(OOtBu)]BF4 (2a), and [FeIII([15]aneN4)(SC6H4-p-NO2)(OOtBu)]BF4 (3a), respectively. X-ray absorption spectroscopic studies (XAS) were conducted on the FeIII-OOR complexes and their iron(II) precursors. The edge energy for the iron(II) complexes (~7118 eV) shifts to higher energy upon oxidation by ROOH, and the resulting edge energies for the FeIII-OOR species range from 7121 – 7125 eV and correlate with the nature of the thiolate donor. EXAFS analysis of the iron(II) complexes 1 – 3 in CH2Cl2 show that their solid state structures remain intact in solution. The EXAFS data on 1a – 3a confirm their proposed structures as mononuclear, 6-coordinate FeIII-OOR complexes with 4N and 1S donors completing the coordination sphere. The Fe-O bond distances obtained from EXAFS for 1a – 3a are 1.82 – 1.85 Å, significantly longer than other low-spin FeIII-OOR complexes. The Fe-O distances correlate with the nature of the thiolate donor, in agreement with the previous trends observed for ν(Fe-O) from resonance Raman (RR) spectroscopy, and supported by optimized geometries obtained from density functional theory (DFT) calculations. Reactivity and kinetic studies on 1a – 3a show an important influence of the thiolate donor.