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The formation of an FeIII-OOH species by reaction of complex 1 ([(MeN3Py)FeII(CH3CN)2]2+) with H2O2 at room temperature is reported and is studied by a combination of UV/vis absorption, EPR, and resonance Raman spectroscopies. The formation of the FeIII-OOH species, and its subsequent conversion to relatively inert FeIII-O-FeIII species, is shown to be highly dependent on the concentration of water, with excess water favoring the formation of the latter species, which is studied by UV/vis absorption spectroelectrochemistry also. The presence of acetic acid increases the rate and extent of oxidation of 1 to its iron(III) state and inhibits the wasteful decomposition of H2O2 but does not affect significantly the spectroscopic properties of the FeIII-OOH species formed. [ABSTRACT FROM AUTHOR]

Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a NiII precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (H mCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., CH bonds, CC bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic OO bond cleavage of a Ni-H mCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a NiIII-oxyl compound. [ABSTRACT FROM AUTHOR]

We report the characterization and solution chemistry of a series of FeII complexes based on the pentadentate ligands N4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine), MeN4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)ethanamine), and the tetradentate ligand Bn-N3Py (N-benzyl-1,1-di(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine) ligands, i.e., [Fe(N4Py)(CH3CN)](ClO4)2 (1), [Fe(MeN4Py)(CH3CN)](ClO4)2 (2), and [Fe(Bn-N3Py)(CH3CN)2](ClO4)2 (3), respectively. Complexes 2 and 3 are characterized by X-ray crystallography, which indicates that they are low-spin FeII complexes in the solid state. The solution properties of 1-3 are investigated using 1H NMR, UV/vis absorption, and resonance Raman spectroscopies, cyclic voltammetry, and ESI-MS. These data confirm that in acetonitrile the complexes retain their solid-state structure, but in water immediate ligand exchange of the CH3CN ligand(s) for hydroxide or aqua ligands occurs with full dissociation of the polypyridyl ligand at low (<3) and high (>9) pH. pH jumping experiments confirm that over at least several minutes the ligand dissociation observed is fully reversible for complexes 1 and 2. In the pH range between 5 and 8, complexes 1 and 2 show an equilibrium between two different species. Furthermore, the aquated complexes show a spin equilibrium between low- and high-spin states with the equilibrium favoring the high-spin state for 1 but favoring the low-spin state for 2. Complex 3 forms only one species over the pH range 4-8, outside of which ligand dissociation occurs. The speciation analysis and the observation of an equilibrium between spin states in aqueous solution is proposed to be the origin of the effectiveness of complex 1 in cleaving DNA in water with 3O2 as terminal oxidant. [ABSTRACT FROM AUTHOR]

The mono- (1) and dinuclear (2) ruthenium(ii) bis(2,2′-bipyridine) complexes of 2,5-di(pyridin-2-yl)pyrazine (2,5-dpp), for which the UV/Vis absorption and emission as well as electrochemical properties have been described earlier, are reinvestigated here by resonance, surface enhanced and transient resonance Raman spectroscopy together with selective deuteration to determine the location of the lowest lying excited metal to ligand charge transfer (3MLCT) states. The ground state absorption spectrum of both the mono- and dinuclear complexes are characterised by resonance Raman spectroscopy. The effect of deuteration on emission lifetimes together with the absence of characteristic bipy anion radical modes in the transient Raman spectra for both the mono- and dinuclear complexes bridged by the 2,5-dpp ligand confirms that the excited state is 2,5-dpp based; however DFT calculations and the effect of deuteration on emission lifetimes indicate that the bipy based MLCT states contribute to excited state deactivation. Resonance Raman and surface enhanced Raman spectroscopic (SERS) data for 1and 2are compared with that of the heterobimetallic complexes [Ru(bipy)2(2,5-dpp)PdCl2]2+3and [Ru(bipy)2(2,5-dpp)PtCl2]2+4. The SERS data for 1indicates that a heterobimetallic Ru–Au complex forms in situupon addition of 1to a gold colloid. [ABSTRACT FROM AUTHOR]

A mononuclear Co III complex (1) of a bisamide-bisalkoxide donor ligand was synthesized and thoroughly characterized. The reaction of 1 with 0.5 equiv. of m-chloroperbenzoic acid (m-CPBA) in acetonitrile at -40 °C resulted in the formation of a μ-oxodicobalt(IV) complex (2), which was characterized by an array of spectroscopic techniques, including X-ray absorption spectroscopy which revealed a short Co-O oxo distance of 1.67 Å. Reactivity studies of 2 towards oxidation/oxygenation of hydrocarbon C-H bond and triphenylphosphine or thioanisole derivatives have been examined. UV-vis spectroscopy studies showed the appearance of clear isosbestic points during the oxidation of substrates together with a neat transformation of 2 to 1. Detailed kinetic investigations established that 2 follows a Concerted Proton-Electron Transfer (CPET) mechanism for hydrocarbon oxidation and has a weak electrophilic character. Catalytic behavior of 1 was noted towards the oxygen atom transfer reactions. This study showcases the spectroscopic investigation and reactivity studies of a Co IV (μ-O)Co IV moiety. Although the Fe IV analog of such a core has been described before, the study describes the first example with a Co IV center., (© 2025 Wiley-VCH GmbH.)

Understanding the basic structure of the oxygen-evolving complex (OEC) in photosystem II (PS-II) and the water oxidation mechanism can aid in the discovery of more efficient and sustainable catalysts for water oxidation. In this context, we present evidence of the formation of a [(TPA)Mn IV (O)(μ-O)Ce IV (NO 3 ) 3 ] + ( 2 ) complex (TPA = tris(pyridyl-2-methyl)amine) by adding aqueous ceric ammonium nitrate to an acetonitrile solution of the [(TPA)Mn II ] 2+ ( 1 ) complex. This unique intermediate ( 2 ) was analyzed by using various spectroscopic techniques and electrospray ionization mass spectrometry. Remarkably, 2 closely mimics the structure of Mn V (O)(μ-O)Ca II (OH 2 ) proposed in the OEC of PS-II. Notably, 2 reacts effectively with ferrocene derivatives, indicating that redox-active Ce IV binding enhances the electron transfer efficiency. Additionally, 2 demonstrated the ability to perform oxygen atom transfer and hydrogen atom abstraction reactions. The discovery of this reactive [(TPA)Mn IV (O)(μ-O)Ce IV (NO 3 ) 3 ] + species provides exciting opportunities for investigating the structure of the Mn V (O)(μ-O)Ca II (OH 2 ) unit in the OEC.

Haloperoxidase enzymes utilize metal hypohalite species to halogenate aliphatic and aromatic C-H bonds to C-X (X = Cl, Br, I) in nature. In this work, we report the synthesis and spectroscopic characterization of a unique Ru III -OCl species as a structural mimic of haloperoxidase enzymes. The reaction of [(BnTPEN)Ru II (NCCH 3 )] 2+ (BnTPEN = N 1 -benzyl- N 1 , N 2 O mixtures generated a novel [(BnTPEN)Ru N 2 -tris(pyridine-2-ylmethyl)ethane-1,2-diamine) with hypochlorite in the presence of an acid in CH 3 CN : H 2 O mixtures generated a novel [(BnTPEN)Ru III -OCl] 2+ species that persists for 4.5 h at room temperature. This new species was characterized by UV-vis absorption, EPR, and resonance Raman spectroscopic techniques, and ESI-MS. The Ru III -OCl species is capable of performing oxygen atom transfer and hydrogen atom abstraction to various organic substrates.

In this study, a methyl group on the classic tetramethylcyclam (TMC) ligand framework is replaced with a benzylic group to form the metastable [Fe IV (O syn )(Bn3MC)] 2+ (2-syn; Bn3MC=1-benzyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) species at -40 °C. The decay of 2-syn with time at 25 °C allows the unprecedented monitoring of the steps involved in the intramolecular hydroxylation of the ligand phenyl ring to form the major Fe III -OAr product 3. At the same time, the Fe II (Bn3MC) 2+ (1) precursor to 2-syn is re-generated in a 1:2 molar ratio relative to 3, accounting for the first time for all the electrons involved and all the Fe species derived from 2-syn as shown in the following balanced equation: 3 [Fe IV (O)(L Ph )] 2+ (2-syn)→2 [Fe III (L OAr )] 2+ (3)+[Fe II (L Ph )] 2+ (1)+H 2 O. This system thus serves as a paradigm for aryl hydroxylation by Fe IV =O oxidants described thus far. It is also observed that 2-syn can be intercepted by certain hydrocarbon substrates, thereby providing a means to assess the relative energetics of aliphatic and aromatic C-H hydroxylation in this system., (© 2021 Wiley-VCH GmbH.)

Diiron(IV)-oxo species are proposed to effect the cleavage of strong C-H bonds by nonheme diiron enzymes such as soluble methane monooxygenase (sMMO) and fatty acid desaturases. However, synthetic mimics of such diiron(IV) oxidants are rare. Herein we report the reaction of (TPA*)Fe II (1) (TPA*=tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) in CH 3 CN with 4 equiv CAN and 200 equiv HClO 4 at 20 °C to form a complex with an [Fe IV 2 (μ-O) 2 ] 4+ core. CAN and HClO 4 play essential roles in this unprecedented transformation, in which the comproportionation of Fe III -O-Ce IV and Fe IV =O/Ce 4+ species is proposed to be involved in the assembly of the [Fe IV 2 (μ-O) 2 ] 4+ core., (© 2020 Wiley-VCH GmbH.)

Soluble methane monooxygenase (sMMO) carries out methane oxidation at 4 °C and under ambient pressure in a catalytic cycle involving the formation of a peroxodiiron(III) intermediate ( P ) from the oxygenation of the diiron(II) enzyme and its subsequent conversion to Q , the diiron(IV) oxidant that hydroxylates methane. Synthetic diiron(IV) complexes that can serve as models for Q are rare and have not been generated by a reaction sequence analogous to that of sMMO. In this work, we show that [Fe II (Me 3 NTB)(CH 3 CN)](CF 3 SO 3 ) 2 (Me 3 NTB = tris((1-methyl-1 H -benzo[d]imidazol-2-yl)methyl)amine) ( 1 ) reacts with O 2 in the presence of base, generating a (μ-1,2-peroxo)diiron(III) adduct with a low O-O stretching frequency of 825 cm -1 and a short Fe···Fe distance of 3.07 Å. Even more interesting is the observation that the peroxodiiron(III) complex undergoes O-O bond cleavage upon treatment with the Lewis acid Sc 3+ and transforms into a bis(μ-oxo)diiron(IV) complex, thus providing a synthetic precedent for the analogous conversion of P to Q in the catalytic cycle of sMMO.

Protons play essential roles in natural systems in controlling O-O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O-O bond cleavage for a nonheme S = 1/2 Fe III -OOH species [(BnTPEN)Fe III (OOH)] 2+ ( 2 , BnTPEN = N -benzyl- N,N',N' -tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having p K a CN results in O-O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with p 3 values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the Fe K -OOH intermediate at -40 °C. These results implicate the generation of a highly reactive Fe a values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the Fe III -OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.V ═O species via proton-assisted O-O bond heterolysis of the Fe III -OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.

The [Fe IV (O)(Me 3 NTB)] 2+ (Me 3 NTB=tris[(1-methyl-benzimidazol-2-yl)methyl]amine) complex 1 has been shown by Mössbauer spectroscopy to have an S=1 ground state at 4 K, but is proposed to become an S=2 trigonal-bipyramidal species at higher temperatures based on a DFT model to rationalize its very high C-H bond-cleavage reactivity. In this work, 1 H NMR spectroscopy was used to determine that 1 does not have C 3 -symmetry in solution and is not an S=2 species. Our results show that 1 is unique among nonheme Fe IV =O complexes in retaining its S=1 spin state and high reactivity at 193 K, providing evidence that S=1 Fe IV =O complexes can be as reactive as their S=2 counterparts. This result emphasizes the need to identify factors besides the ground spin state of the Fe IV =O center to rationalize nonheme oxoiron(IV) reactivity., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The oxidation of the C-H and C=C bonds of hydrocarbons with H 2 O 2 catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive Fe IV =O species such as [(N4Py)Fe IV =O] 2+ formed by homolytic cleavage of the O-O bond of an Fe III -OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)- N , N -bis(pyridin-2-ylmethyl)methanamine). We show here that at low H 2 O 2 concentrations the Fe IV =O species formed is detectable in methanol. Furthermore, we show that the decomposition of H 2 O 2 to water and O 2 is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H 2 O 2 are present. Although independently prepared [(N4Py)Fe IV =O] 2+ oxidizes stoichiometric H 2 O 2 rapidly, the rate of formation of Fe IV =O from the Fe III -OOH intermediate is too low to account for the rate of H 2 O 2 decomposition observed under catalytic conditions. Indeed, with excess H 2 O 2 , disproportionation to O 2 and H 2 O is due to reaction with the Fe III -OOH intermediate and thereby prevents formation of the Fe IV =O species. These data rationalize that the activity of these catalysts with respect to hydrocarbon/alkene oxidation is maximized by maintaining sub-/near-stoichiometric steady-state concentrations of H 2 O 2 , which ensure that the rate of the H 2 O 2 oxidation by the Fe III -OOH intermediate is less than the rate of the O-O bond homolysis and the subsequent reaction of the Fe IV =O species with a substrate., Competing Interests: The authors declare no competing financial interest.

Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non-heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N-methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [Fe IV (O)(L)] 2+ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe-N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k 2 ' values for O- and H-atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N-Fe=O arrangement by 10°., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

[Fe(β-BPMCN)(CH 3 CN) 2 ] 2+ (1, BPMCN = N,N' -bis(pyridyl-2-methyl)- N,N' -dimethyl- trans-1,2-diaminocyclo-hexane) is a relatively poor catalyst for cyclohexane oxidation by H 2 O 2 and cannot perform benzene hydroxylation. However, addition of Sc 3+ activates the 1/H 2 O 2 reaction mixture to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 Fe III -(η 1 -OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addition of Sc 3+ at rates independent of [substrate] but linearly dependent on [Sc 3+ ]. HClO 4 elicits comparable reactivity as Sc 3+ at the same concentration. We thus postulate that these additives both facilitate O-O bond heterolysis of 2 to form a common highly electrophilic Fe V ═O oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date.

A novel hydroperoxoiron(III) species [Fe III (OOH)(MeCN)(PyNMe 3 )] 2+ (3) has been generated by reaction of its ferrous precursor [Fe II (CF 3 SO 3 ) 2 (PyNMe 3 )] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low-spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C-H bonds. However, triflic acid addition to 3 causes its transformation into a much more reactive compound towards organic substrates that is capable of oxidizing unactivated C-H bonds with high stereospecificity. Stopped-flow kinetic analyses and theoretical studies provide a rationale for the observed chemistry, a triflic-acid-assisted heterolytic cleavage of the O-O bond to form a putative strongly oxidizing oxoiron(V) species. This mechanism is reminiscent to that observed in heme systems, where protonation of the hydroperoxo intermediate leads to the formation of the high-valent [(Porph . )Fe IV (O)] (Compound I)., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower pK a for the bound H 2 O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex [Fe IV (O)(TMC)(OH 2 )] 2+ (1-OH 2 ; TMC=1,4,8,11-tetramethylcyclam) by determining the pK a of the H 2 O ligand. We establish that 1-OH 2 has a pK a of 6.9±0.5, a value that falls in between those found for [Fe III (OH 2 ) 6 ] 3+ and [Fe II (OH 2 ) 6 ] 2+ . This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1-OH 2 , anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1-OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of [(PyNMe 3 )Fe II (CF 3 SO 3 ) 2 ], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe 3 (an electron-enriched version of PyNMe 3 ) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe 3 )Fe V (O)(OC(O)R)] 2+ (∼75%) with some Fe IV (O)( • OC(O)R) and Fe III (OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe III (OC(O)R) ferric acyl peroxides and Fe IV (O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm -1 ) relative to the 903 cm -1 value predicted for the hypothetical [(MeO-PyNMe 3 )Fe V (O)(NCMe)] 3+ complex.

Non-heme (L)Fe III and (L)Fe III -O-Fe III (L) complexes (L=1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)ethan-1-amine) underwent reduction under irradiation to the Fe II state with concomitant oxidation of methanol to methanal, without the need for a secondary photosensitizer. Spectroscopic and DFT studies support a mechanism in which irradiation results in charge-transfer excitation of a Fe III -μ-O-Fe III complex to generate [(L)Fe IV =O] 2+ (observed transiently during irradiation in acetonitrile), and an equivalent of (L)Fe II . Under aerobic conditions, irradiation accelerates reoxidation from the Fe II to the Fe III state with O 2 , thus closing the cycle of methanol oxidation to methanal., (© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Near-UV excitation of non-heme Fe IV [double bond, length as m-dash]O complexes results in light intensity dependent increase in reaction rates for the oxidation of C-H bonds even at low temperature (-30 °C). The enhancement of activity is ascribed to the ligand-to-[Fe IV [double bond, length as m-dash]O] charge transfer character of the near-UV bands to generate a highly reactive [(L + ) Fe III -O*] species. The enhancement is not observed with visible/NIR excitation of the d-d absorption bands.

Ceric ammonium nitrate (CAN) or Ce IV (NH 4 ) 2 (NO 3 ) 6 is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe III -O-Ce IV (OH 2 )(NO 3 ) 4 ] + (3), a complex obtained from the reaction of [(N4Py)Fe II (NCMe)] 2+ with 2 equiv CAN or [(N4Py)Fe IV =O] 2+ (2) with Ce III (NO 3 ) 3 in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe IV and Ce IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 Fe IV in 2 to S=5/2 in 3, which is found to be facile despite the formal spin-forbidden nature of this process. This observation suggests that Fe IV =O complexes may avail of reaction pathways involving multiple spin states having little or no barrier., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In this report we compare the geometric and electronic structures and reactivities of [Fe V (O)] - and [Fe IV (O)] 2- species supported by the same ancillary nonheme biuret tetraamido macrocyclic ligand (bTAML). Resonance Raman studies show that the Fe═O vibration of the [Fe IV (O)] 2- complex 2 is at 798 cm -1 , compared to 862 cm -1 for the corresponding [Fe V (O)] - species 3, a 64 cm -1 frequency difference reasonably reproduced by density functional theory calculations. These values are, respectively, the lowest and the highest frequencies observed thus far for nonheme high-valent Fe═O complexes. Extended X-ray absorption fine structure analysis of 3 reveals an Fe═O bond length of 1.59 Å, which is 0.05 Å shorter than that found in complex 2. The redox potentials of 2 and 3 are 0.44 V (measured at pH 12) and 1.19 V (measured at pH 7) versus normal hydrogen electrode, respectively, corresponding to the [Fe IV (O)] 2- /[Fe III (OH)] 2- and [Fe V (O)] - /[Fe IV (O)] 2- couples. Consistent with its higher potential (even after correcting for the pH difference), 3 oxidizes benzyl alcohol at pH 7 with a second-order rate constant that is 2500-fold bigger than that for 2 at pH 12. Furthermore, 2 exhibits a classical kinteic isotope effect (KIE) of 3 in the oxidation of benzyl alcohol to benzaldehyde versus a nonclassical KIE of 12 for 3, emphasizing the reactivity differences between 2 and 3.

Angiogenesis is a key aspect of the dynamic changes occurring during the normal ovarian cycle. Hyperplasia and hypervascularity of the ovarian theca interna and stroma are also prominent features of the polycystic ovary syndrome (PCOS), a leading cause of infertility. Compelling evidence indicated that vascular endothelial growth factor (VEGF) is a key mediator of the cyclical corpus luteum angiogenesis. However, the nature of the factor(s) that mediate angiogenesis in PCOS is less clearly understood. Endocrine gland-derived (EG)-VEGF has been recently identified as an endothelial cell mitogen with selectivity for the endothelium of steroidogenic glands and is expressed in normal human ovaries. In the present study, we compared the expression of EG-VEGF and VEGF mRNA in a series of 13 human PCOS and 13 normal ovary specimens by in situ hybridization. EG-VEGF expression in normal ovaries is dynamic and generally complementary to VEGF expression in both follicles and corpora lutea. A particularly high expression of EG-VEGF was detected in the Leydig-like hilus cells found in the highly vascularized ovarian hilus. In PCOS ovaries, we found strong expression of EG-VEGF mRNA in theca interna and stroma in most of the specimens examined, thus spatially related to the new blood vessels. In contrast, VEGF mRNA expression was most consistently associated with the granulosa cell layer and sometimes the theca, but rarely with the stroma. These findings indicate that both EG-VEGF and VEGF are expressed in PCOS ovaries, but in different cell types at different stages of differentiation, thus suggesting complementary functions for the two factors in angiogenesis and possibly cyst formation.