Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2026 Jun 1;65(23):13276–13282. doi: 10.1021/acs.inorgchem.6c02005

Manganese-Substituted Polyoxometalates as Functional Mimics of Indole Dioxygenase Enzymes

Adi Herman †, Raanan Carmieli ‡, Ronny Neumann †,*
PMCID: PMC13273813  PMID: 42225055

Abstract

Dioxygenase reactivity, where both oxygen atoms of O2 are incorporated into an organic product, is a relative rarity. Notable is the indoleamine 2,3-dioxygenase enzyme with an iron-heme active site. Biomimetic studies using iron and manganese porphyrins have been successful and mechanistically enriching. We show that inorganic manganese-substituted polyoxometalates are also effective functional mimics of indoleamine 2,3-dioxygenases. Thus, several mono-, di-, and tri-MnII-substituted polyoxometalates were reacted with O2, and the intermediacy of a superoxide species was supported by both EPR spectroscopy using 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide as a spin trap and cyclic voltammetry. Further experiments with 2,3-dimethyl-1H-indole yielded the expected dioxygenated product, N-(2-acetylphenyl)­acetamide, in high yields under O2. Oxygen isotope labeling experiments and the use of triphenylphosphine as a probe supported a stepwise oxygenation process with formation of two suggested manganese-centered intermediates as also shown for porphyrin/heme-based catalysts. Using a combination of UV–vis and EPR measurements, it was concluded that after a first oxygen donation by a Mn­(III)-superoxo species, a second oxygen-donated species, best formulated as a Mn­(III)-oxyl species rather than a Mn­(IV)-oxo species, yielded the final dioxygenated product. The research supports the notion that inorganic manganese polyoxometalates with an oxotungstate framework are true functional models of indoleamine 2,3-dioxygenases.

graphic file with name ic6c02005_0013.jpg

graphic file with name ic6c02005_0011.jpg

Introduction

Research on bioinspired molecular oxygen activation at metal centers can be divided to those having monooxygenase type activity, where one oxygen atom of O2 yields H2O and dioxygenase type activity where both oxygen atoms of O2 are incorporated into the product. Iron-based dioxygenases can be divided into three types: Monoiron nonheme enzymes such as catechol dioxygenases; , the Reiske type dioxygenase that catalyzes the cis-dihydroxylation of arenes in the presence of NADH as a reducing agent; , and enzymes with iron-heme active sites, indoleamine 2,3-dioxygenase (IDO), and tryptophan 2,3-dioxygenase (TDO). The mechanism of the IDO and TDO enzymes have been studied, − and IDO activity has also been studied using biomimetic iron porphyrin catalysts, − and further extended to manganese porphyrins, as well as other functional biomimetic models. −

The activation of molecular oxygen with polyoxometalates is a rather broad topic that includes inner-sphere four-electron reduction to water using phosphovanadomolybdates, autoxidations and reactions with sacrificial reductants, and outer-sphere electron transfer from polyoxotungstates to dioxygen. Typical transformations resulting from these types of activation of molecular oxygen include oxidative dehydrogenations and oxygenations via radical-propagated mechanisms. More recently, iron- and copper-substituted polyoxometalates have been shown to undergo monooxygenase-type reactions reminiscent of iron porphyrins, including the recent electroreductive activation of O2 toward alkane hydroxylation catalyzed by iron- and copper-substituted polyoxometalates. − Dioxygenase activity using ruthenium-substituted polyoxometalates was also reported, albeit reactions were not generally catalytic except for very active substrates such as trans-cyclooctene. ,

Continuing our interest in investigating the use of inorganic polyoxometalates in reactivity functionally similar to those of metalloenzymes and related biomimetic compounds, we report on the use of manganese- and iron-substituted polyoxometalates as functional mimics of indole dioxygenase enzymes. The research is based mostly on Keggin type compounds where monolacunary K8[α-SiW11O39]·13H2O is substituted with M = MnII to yield {α-SiMnII(H2O)­W11O39}6– anions, and trilacunary Na9[β-SiW9O34H]·23H2O is substituted M = MnII, ZnII to yield homo- (MnII) and hetero-substituted (MnII, ZnII) {α-SiMII 3(H2O)3W9O37}10– anions. − The research hypothesis was based on two possible scenarios for indoleamine 2,3-dioxygenase type activation of O2, Figure .

1.

1

Open in a new tab

Possible generic scenarios for molecular oxygen activation for the dioxygenation of 2,3-methyl 1H-indole to N-(2-acetylphenyl)­acetamide. Top pathway - intramolecular activation of O2 at two adjacent MII = Mn, Zn, or Fe sites; bottom pathway −O2 activation at a single MII site. Gray–W; orange–Si, red–O, and light blue −H2O. Other possibilities include two end-on superoxo species instead of species B and also side-on superoxo species as an alternative to compound E.

The first pathway posited an intramolecular activation of O2 at two adjacent metal sites involving formation of two MIV = O oxo or Mn­(III)-oxyl species (Figure , A → B → C) from two MnII centers, followed by oxygenation from equivalent or at least very similar MnIV = O/Mn­(III)-oxyl intermediate species. The second pathway, requiring only a single reactive site, proposed a formation of a MIII-O2 •– superoxide intermediate (Figure , E) as an active species for IDO activity, with co-formation of a MIV = O/Mn­(III)-oxyl species (Figure , F) for the further formation of the final product. As will be shown, the research indicates IDO reactivity at a single metal reaction site by the second pathway.

Results and Discussion

Chronologically speaking, the research was begun to test the possibility of any intramolecular activation of O2 at two adjacent metal sites involving the formation of two MnIV = O oxo species (Figure A → B → C) since such intramolecular activations are rare and have not been documented for polyoxometalates. Thus, tetrahexylammonium (THA) salts of {α-SiMnII 3‑xZnII x (H2O)3W9O37}10– (x = 0, 1, 2) and {α-SiFeII 3(H2O)3W9O37}10– (x = 0, 1, 2) were prepared according to literature procedures. , The characterization of these compounds was by infrared (IR) and high-resolution electrospray ionization mass spectrometry (HR-ESI–MS) for the newly synthesized hetero-substituted (MnII, ZnII) anions, as detailed in the experimental section and the Supporting Information

To examine the reaction of the different manganese compounds with molecular oxygen, EPR spectroscopy was used to detect oxygen-centered radicals through the formation of spin adducts. First solutions of [SiMn3(L3)­W9O37], {SiMn3W9}, in THF or acetonitrile (ACN) in the presence of 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) as a spin trap were exposed to O2. EPR spectra were measured before and after the addition of O2, as shown in Figure .

2.

2

Open in a new tab

EPR spectra of {SiMn3W9} in ACN (left) and THF (right) under N2 (black) and after (red) exposure to O2 for 50 min in the presence of BMPO. Conditions: 2 mM {SiMn3W9}, 5 mg BMPO, 1.5 bar O2 at room temperature.

Under nitrogen in ACN, no formation of a BMPO spin adduct is observed, but in THF, a relatively low intensity spectrum is observed. This spectrum (see insert in Figure ) is associable to a BMPO-carbon radical adduct that in turn can be assigned to an electron transfer between {SiMn3W9} and a coordinated THF molecule to a Mn atom. Such THF ligation to an Fe atom has been previously observed by computation for analogous {SiFe3W9}. After treatment of {SiMn3W9} with O2 in both ACN and THF, spectra clearly associated with the hydroxy spin adduct, BMPO/OH•, are obtained. The formation of BMPO/OH• could be associated with the formation of a hydroxyl radical or a MnIII 2-peroxo or a reactive Mn-oxo intermediate species (Figure , B and C) as originally posited. However, studies with a very nucleophilic substrate such as triphenylphosphine (TPP), presumably reactive toward MnIII 2-peroxo or MnIV = O intermediate species, yielded no product (see also below). Alternatively, it should be noted that superoxo spin adducts such as BMPO/OOH• have a limited lifetime, leading to the formation of BMPO/OH• and that such a transformation can be catalyzed by {SiMn3W9}.

To explore such a BMPO/OOH• → BMPO/OH• transformation versus direct reaction of BMPO with a hydroxyl radical, spin trapping experiments were repeated in the presence of DMSO. Since hydroxyl radicals react faster with DMSO than with BMPO and form methyl radicals, − the absence of a change in intensity in the EPR spectrum upon addition of DMSO and the absence of a BMPO/CH3 • would support the initial formation of BMPO/OOH. This is indeed what is observed, Figure S1. While this experiment precludes the formation of a hydroxyl radical, it does not rule out the formation of MnIII 2-peroxo or MnIV = O intermediate species. To do so, mono-Mn-substituted polyoxometalates, such as THA6[SiMnII(L)­W11O39], {SiMnW11}, and THA8[α2-P2MnII(L)­W17O61], {P2MnW17}, were reacted with O2 in the presence of BMPO. Such reactions have been reported in the past, indicating the reversible formation of MnII–O2 species at room temperature.

In Figure , one can see that the addition of O2 to {SiMnW11}/BMPO shows a spectrum with elements of both BMPO/OOH• and BMPO/OH•. Somewhat differently, the addition of O2 to {P2MnW17}/BMPO showed a spectrum of BMPO/OH• only, while a spectrum carried out in the presence of air showed a low intensity spectrum of BMPO/OOH•. These experiments clarify the intermediacy of a superoxo species and the fast transformation of BMPO/OOH• → BMPO/OH• in the absence of an IDO substrate under experimental conditions.

3.

3

Open in a new tab

EPR spectra of {SiMnW11} (left) and {P2MnW17} (right) in the presence of BMPO. Conditions: 2 mM polyoxometalate, 5 mg BMPO in acetonitrile, under air (black) and after (red) exposure to 1.5 bar O2 for 1 h in the presence of BMPO at room temperature.

The EPR measurement of K6{SiMnW11} in water gave the typical 6- line EPR spectrum at g = 2 for an octahedral Mn­(II) compound, Figure (left), with 5 oxygen ligands from the polyoxometalate framework and one labile terminal water ligand. However, the EPR spectrum of THA6{SiMnW11} in ACN showed a six-line spectrum at g ∼4 associated with a five-coordinate square pyramidal Mn­(II) compound where the labile oxygen ligand is absent, as also previously observed for another Mn­(II)-substituted polyoxometalate, Figure (right). The vacant coordination site renders the formation of a Mn­(II)–O2 ↔ MnIII–O2 •– very plausible.

4.

4

Open in a new tab

EPR spectra of K6{SiMnW11} under N2 in water at 298 K (left) and THA6{SiMnW11} under N2 in ACN at 120 K (right).

Although direct observation of a MnIII–O2 •– species was not successful at low temperature, such a species was observable for an analogous iron compound, {SiFe3W9}, Thus, under air in THF, {SiFe3W11}, yielded a EPR spectrum consisting of a FeIII signal at g = ∼4, Figure S2, and a less intense signal assignable to a FeIII–O2 •– species, Figure . Reactions in the presence of BMPO gave the spectrum typical for a BMPO/OH• specie, Figure S3, as discussed above.

5.

5

Open in a new tab

EPR spectrum of 2 mM THA6[SiFeII(L)­W11O39] in THF at 15 K after exposure to air.

Another way to examine the interaction of polyoxometalates with O2 is by cyclic voltammetry (CV). Figure S4 shows the CV of O2 with the typical reversible redox transformation, O2 ↔ O2 • at −1.285 V versus Fc/Fc+. In the presence of {SiMn3W9}, Figure , the CV is unchanged indicating a reversible {SiMn3W9} + O2 ↔ {SiMn3W9}+-O2 •– transformation as previously suggested. Upon the addition of ethanol as a proton donor, the reduction of O2 becomes irreversible, supporting the formation of an unstable protonated superoxo species, {SiMn3W9}+-O2H that decomposes.

6.

6

Open in a new tab

CV of (THA)10[SiMn3(L3)­W9O37] in ACN under N2 or air, with and without 50 μL ethanol. Conditions: 2 mM POM, 0.1 M TBABF4 – 5 mL CAN. Working electrode - glassy carbon; counter electrode - Pt wire; reference electrode −Fc/Fc+; and scan rate −50 mV/s.

Inspired by the activity of TDO and IDO enzymes, the oxidation of 2,3-dimethylindole (2,3-DMI) was examined under the presence of air and then under oxygen, Figure . The reactions with 2,3-DMI gave 3 products, Scheme . The main product is the dioxygenated product, N-(2-acetylphenyl)­acetamide. Two additional products are mono-oxygenated products with molecular peaks in their mass spectra at m/z 161. These products, 3,3-dimethylindolin-2-one and 2,2-dimethylindolin-3-one, are suggested to be the result of the rearrangement of the intermediate epoxide that was previously observed in the oxidation of the nitrogen atom-protected analog, N-acetyl-2,3-dimethylindole. The products were identified by GC–MS and quantified by GC-FID, Figures S5 and S6. The activity of the {SiFe3W11} compound was also tested but was significantly less reactive, yielding only ∼6 mol % product within 20 h.

7.

7

Open in a new tab

Oxygenation of 2,3-DMI with different polyoxometalates under 1 bar air or O2 (two right columns). Conditions 2 mM polyoxometalate, 33 mM 2,3-DMI, 50 μL EtOH in 5 mL ACN, t = 24 h. Turquiose - N-(2-acetylphenyl)­acetamide; red–monooxygenates −3,3-dimethylindolin-2-one and 3-methyl-2-methyleneindolin-3-ol.

1. Product Formation through a Proposed Epoxide Intermediate.

1

Open in a new tab

The overall yields of the reactions in air were typically between 75 and 90% with the dioxygenated N-(2-acetylphenyl)­acetamide as the major product. The yield with {SiMn3W9} was somewhat lower. Clearly, monomanganese-substituted catalysts are reactive. The higher activity of {SiMnW11} versus {SiMnZn2W9} and {P2MnW17} may be related to the overall higher negative charge of the latter compounds, resulting in some electrostatic substrate repulsion, as also previously observed in other O2-based reactions. Poly manganese-substituted polyoxometalates are also reactive, where the higher reactivity of {SiMn2ZnW9} versus {SiMn3W9} may be related to a stabilization of the superoxo intermediate by the Lewis acidic Zn center. , This stabilization may also be a factor in the higher activity of {SiMnZn2W9} versus {P2MnW17} that have the same 8– charge. Under 1 bar O2, both the yields and the selectivity to the dioxygenated N-(2-acetylphenyl)­acetamide major product are significantly increased.

To identify the source of the oxygen atoms in the dioxygenated product, an oxygenation reaction with labeled 18O2 was performed. By using GC–MS, Figure S7, we observed that both oxygen atoms in N-(2-acetylphenyl)­acetamide were 18O labeled supporting a dioxygenase pathway. However, as proposed in Scheme , the formation of an epoxide intermediate, a further reactive Mn-oxo intermediate, and some formation by rearrangement to monooxygenated products, Figure , suggests that the two oxygen atoms in N-(2-acetylphenyl)­acetamide product do not necessarily come from the same original O2 molecule. In support of a stepwise reaction, a mixture of 18O2/16O2 with a quantified ratio by GCMS m/z 36/32 = 1.64, was reacted under the conditions in Figure . The GC–MS, Figure S8, shows the prominence of the mixed labeled 18O16O N-(2-acetylphenyl)­acetamide (m/z = 179) versus the monolabeled N-(2-acetylphenyl)­acetamide (m/z 177 and 181) that supports a sequential reaction mechanism.

In previous reports on the mechanism of IDO/TDO, there appears to be a consensus that the second oxygenation reaction occurs through a ferryl-oxo species. ,, By analogy, one could posit that similarly, a Mn­(IV)–O intermediate would be formed, which was also proposed for a Mn-heme catalyzed reaction. The formation of such an oxygen donor intermediate was initially inferred by adding the very reactive nucleophilic triphenylphosphine (TPP) to the reaction mixture. Thus, 2 mM {SiMnW11}, 36 mM 2,3-DMI, 36 mM TPP, and 50 μL EtOH in 2 mL ACN were reacted for 24 h under 1.5 bar O2. Both the IDO product, N-(2-acetylphenyl)­acetamide, and triphenylphosphine oxide (TPPO), 25 μmol and 55 μmol, respectively, were formed. Importantly, a control experiment without 2,3-DMI yielded only traces of TPPO (2.5 μmol), showing that a Mn-oxo intermediate species is formed after the first oxygen transfer to 2,3-DMI as proposed in Scheme (E′ and F′ intermediates). A further control experiment in a reaction of 2,3-DMI and TPP without a catalyst also showed only traces of TPPO formation.

The formation of a proposed Mn-oxo intermediate was observed by UV–vis measurements. The UV–vis spectrum of {SiMnW11} prepared under N2 is noticeably different from the analogous spectrum upon the addition of O2, Figure S9. The change is assignable to the formation of a Mn­(III)-superoxo species. Upon addition of 2,3-DMI, the formation of a further intermediate species λmax = 475 nm is observable with an isosbestic point at λ = 500 nm, Figure .

8.

8

Open in a new tab

UV–vis spectra of 3 mM {SiMnW11} in ACN and O2 upon addition of 10 mM 2,3-DMI.

While it is enticing to assign this new species as a Mn­(IV)-oxo compound as suggested for heme-based catalysts, it should be noted that previous studies have shown that Mn­(IV) substituted into a polyoxometalate are quite basic leading to formation of Mn­(IV)-hydroxide compounds that can be isolated and are not reactive oxygen donors even to TPP. , Thus, in an EPR experiment designed to observe the intermediate species (F′, Scheme ), 33 mM 2,3-DMI was added to a solution of 10 mM {SiMnW11} in acetonitrile under N2. The solution was cooled to 273 K and treated with O2 for 10 min to initiate a dioxygenation reaction and then frozen with liquid nitrogen. The EPR spectrum at 120 K obtained, Figure , shows both a low spin Mn­(III) species that can be attributed to a Mn­(III)-oxyl intermediate (MnIII–O2 •– species was not observed by EPR at 120 K) and the initial Mn­(II) compound D′ formed after dioxygenation.

9.

9

Open in a new tab

EPR spectrum (black) and simulated spectrum (red) of {SiMnW11} in ACN at 120 K. Simulation parameters: (1) Mn­(III) S = 1, g1 = 4.21, A1 = 510 MHz, Iw1 = 3.8 mT, D1 = 250 MHz, D strain = 420 MHz; (2) Mn­(II), S = 5/2, g2 = 3.92, A2 = 510 MHz, Iw2 = 2.8 mT, D2 = 700 MHz, D strain = 420 MHz. Attempts at simulating the spectrum with Mn­(III) S = 2 or Mn­(IV) failed.

Conclusions

The substitution of MnII and FeII cations into lacunary positions of polyoxotungstates led to species that upon reaction with O2 led to the initial formation of superoxo species, MnIII–O2 –• or FeIII–O2 –•. For MnIII–O2 –•, the assignment was supported by EPR using 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide as a spin trap. The intermediacy of a hydroxyl radical was ruled out by the addition of DMSO as a radical trap. Using 2,3-dimethylindole as a substrate, the formation of N-(2-acetylphenyl)­acetamide via a dioxygenase reaction was carried out. In the presence of 1 bar O2, the reaction was almost quantitative and mostly selective. Minor amounts of monooxygenated compounds were also obtained and identified. Reaction yields were negatively affected by higher anionic charges of the polyoxometalate catalyst but positively affected by the presence of Lewis acidic Zn­(II) that can stabilize intermediate superoxo species. Use of labeled 18O2 supported the dioxygenase pathway with a sequential reaction pathway of successive oxygen transfer reactions. An overview of the entire reaction mechanism as supported by various UV–vis, CV, and EPR experiments presented in Scheme indicates that a MnIII–O2 –• species is the first oxygen donor species, while the second oxygen donor species points to a Mn­(III)-oxyl intermediate as a most likely conclusion. The research supports the finding that transition metal substituted into polyoxometalate frameworks are surprising functional mimics of heme oxygenases as also previously observed for polyoxometalates showing monooxygenase activity. −

Experimental Part

Polyoxometalate Syntheses

Lacunary POMs K8[α-SiW11O39]·13H2O, Na9[β-SiW9O34H]·23H2O, and K10[α2-P2W17O61]·20H2O were synthesized according to the known literature procedures. K6[α-SiMn­(H2O)­W11O39] was synthesized by reacting manganese sulfate, MnSO4·H2O, with K8[α-SiW11O39]·13H2O according to a known literature procedure. K10[α-Si­{Mn­(H2O)}3W9O37] was synthesized by reacting manganese acetate, Mn­(CH3CO2)2·4H2O, with Na10[α-SiW9O34] according to a known literature procedure. K8[α2-P2 MnII(H2O)]­W17 O61} was synthesized by reacting manganese chloride, MnCl2·4H2O, with K10[α2-P2W17O61] according to a known literature procedure. Cs10[β-SiW9O37{Mn2Zn­(H2O)}3] and Cs10[β-SiW9O37{MnZn2(H2O)}3] were synthesized by reacting “triple” salts, [Mn2ZnO­(MeCO2)6(H2O)3] and [MnZn2O­(MeCO2)6(H2O)3], with Na9[β-SiW9O34H]·23H2O as previously outlined in the literature. Thus, [Mn2ZnO­(MeCO2)6(H2O)3] and [MnZn2O­(MeCO2)6(H2O)3] were prepared by adding a solution of NaOAc·H2O (0.31 mol) in water (70 mL) to a filtered, stirred solution of manganese nitrate (0.02 or 0.01 mol) and zinc nitrate (0.01 or 0.02 mol) in water (70 mL). This resulted in a colored solution, which was evaporated and dried under vacuum. Na9[β-SiW9O34H]·23H2O (1.5 mmol) was added in small amounts with vigorous stirring to a solution of [Mn2ZnO­(MeCO2)6(H2O)3] or [MnZn2O­(MeCO2)6(H2O)3] (11.7 mM) in NaOAc (pH 6.5, 150 mL, 0.25 M), and the mixture was then heated to 50 °C for 1 h. The colored solution that was formed was cooled and treated with a solution of CsCl (0.33 g/mL) until the formation of precipitates of Cs10[β-SiW9O37{Mn2Zn­(H2O)}3] and Cs10[β-SiW9O37{MnZn2(H2O)}3]. These new compounds were characterized by high resolution electrospray ionization mass spectrometry and IR spectrometry as previously reported, Figures S10 and S11. Metathetical exchange of the alkali cations with tetrahexylammonium (THA) was carried out in dichloromethane or toluene using stoichiometric amounts of tetrahexylammonium bromide (THABr) to synthesize polyoxometalates soluble in organic solvents. Unless otherwise noted, the polyoxometalates were used as THA compounds.

Electrochemistry

All experiments were carried out using a BioLogic Science VSP-201 potentiostat. CV was performed in an 18 mL airtight Pyrex vial with a 5 mL solution. The conditions were 2 mM (THA)10[SiMn3(L3)­W9O37] 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as a supporting electrolyte, a glassy carbon disk as a working electrode, Pt wire as a counter electrode, and Fc/Fc + as a reference electrode. All solutions were prepared under N2 in a glovebox, and the experiments were performed under a N2 or air/O2 mixture.

Product Analysis

Combined GC-FID and GC-MSD measurements were carried out to quantify and identify reaction liquid phase products, respectively, using a HP 6890 instrument with a flame ionization detector and a HP 5973 instrument with a mass selective detector. The separations were carried out using a 30 m column (Restek 5 MS, 0.32 mm internal diameter) that has a 5% phenylmethyl silicone coating of 0.25 μm; helium was the carrier gas. Product identification was also verified using ACD/MS Fragmenter with Spectrus Processor Software.

EPR Measurements

EPR measurements were carried out at room temperature by using a Bruker ELEXYS E500 spectrometer operating at X-band frequencies (9.5 GHz) and a Bruker ER4119HS resonator. Samples were loaded into Vitrocom quartz capillaries, CV1012-Q-100, with a 1 mm inner diameter. The experimental conditions of the EPR measurements were as follows: microwave power of 20 mW, 1 G modulation amplitude, and 100 kHz modulation frequency. For measurements with BMPO, the sweep range was 200 gauss and the spectra consisted of 400 points. For manganese measurement, the sweep range was 1500 G and the spectra consisted of 750 points.

Low-temperature CW EPR measurements were carried out at 15 K or 120 K on a Bruker Elexsys E580 spectrometer operating at 9.5 GHz and outfitted with an X-band resonator (EN-4118X-MD4). The temperature was controlled by an Oxford Instruments CF935 continuous-flow cryostat using liquid He or N2. The experimental conditions of the EPR measurements were as follows: microwave power of 20 mW, 1 G modulation amplitude, and 100 kHz modulation frequency. 2000 G and the spectra consisted of 1000 points.

Supplementary Material

ic6c02005_si_001.pdf (772.4KB, pdf)

Acknowledgments

The research was supported by a research grant from the Yotam project and the Weizmann Institute sustainability and energy research initiative.

Glossary

Abbreviations:

ACN

acetonitrile

BMPO

5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide

2,3-DMI

2,3-dimethyl indole

IDO

indole dioxygenase

TDO

tryptophan dioxygenase

THA

tetrahexylammonium

TPP

triphenyl phosphine

TPPO

triphenylphosphine oxide

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.6c02005.

  • Additional EPR spectra; UV–vis spectra, CV scans; GC-FID chromatograms; mass spectra from GC-MSD chromatograms; peaks from HR-ESI-MS of new compounds; IR spectra of new compounds (PDF)

The manuscript was written through the contributions of all authors. All authors have given their approval to the final version of the manuscript.

The authors declare no competing financial interest.

References

  1. Que L. Jr., Ho R. Y. N.. Dioxygen Activation by Enzymes with Mononuclear Non-heme Iron Active Sites. Chem. Rev. 1996;96:2607–2624. doi: 10.1021/cr960039f. [DOI] [PubMed] [Google Scholar]
  2. Bugg T. D. H., Ramaswamy S.. Non-heme Iron-dependent Dioxygenases: Unraveling Catalytic Mechanisms for Complex Enzymatic Oxidations. Curr. Opin. Chem. Biol. 2008;12:134–140. doi: 10.1016/j.cbpa.2007.12.007. [DOI] [PubMed] [Google Scholar]
  3. Efimov I., Basran J., Thackray S. J., Handa S., Mowat C. G., Raven E. L.. Structure and Reaction Mechanism in the Heme Dioxygenases. Biochemistry. 2011;50:2717–2724. doi: 10.1021/bi101732n. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Chauhan N., Thackray S. J., Rafice S. A., Eaton G., Lee M., Efimov I., Basran J., Jenkins P. R., Mowat C. G., Chapman S. K., Raven E. L.. Reassessment of the Reaction Mechanism in the Heme Dioxygenases. J. Am. Chem. Soc. 2009;131:4186–4187. doi: 10.1021/ja808326g. [DOI] [PubMed] [Google Scholar]
  5. Shin I., Ambler B. R., Wherritt D., Griffith W. P., Maldonado A. C., Altman R. A., Liu A.. Stepwise O-Atom Transfer in Heme-Based Tryptophan Dioxygenase: Role of Substrate Ammonium in Epoxide Ring Opening. J. Am. Chem. Soc. 2018;140:4372–4379. doi: 10.1021/jacs.8b00262. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Lewis-Ballester A., Batabyal D., Egawa T., Lu C., Lin Y., Marti M. A., Capece L., Estrin D. A., Yeh S.-R.. Evidence for a Ferryl Intermediate in a Heme-Based Dioxygenase. Proc. Natl. Acad. Sci. U.S.A. 2009;106:17371–17376. doi: 10.1073/pnas.0906655106. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Sacramento J. J. D., Goldberg D. P.. Oxidation of an Indole Substrate by Porphyrin iron­(III) Superoxide: Relevance to Indoleamine and Tryptophan 2,3-dioxygenases. Chem. Commun. 2020;56:3089–3092. doi: 10.1039/C9CC10019A. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Mondal P., Wijeratne G. B.. Modeling Tryptophan/Indoleamine 2,3-dioxygenase with Heme Superoxide Mimics: is Ferryl the Key Intermediate? J. Am. Chem. Soc. 2020;142:1846–1856. doi: 10.1021/jacs.9b10498. [DOI] [PubMed] [Google Scholar]
  9. Samanta S., Sengupta S., Biswas S., Ghosh S., Barman S., Dey A.. Iron Dioxygen Adduct Formed during Electrochemical Oxygen Reduction by Iron Porphyrins Shows Catalytic Heme Dioxygenase Reactivity. J. Am. Chem. Soc. 2023;145:26477–26486. doi: 10.1021/jacs.3c10980. [DOI] [PubMed] [Google Scholar]
  10. Ohkubo K., Sagawa T., Ishida H.. Catalytic and Stereoselective Activities of Manganese Achiral and Chiral Porphyrins in Dioxygenation of Tryptophan Derivatives. Inorg. Chem. 1992;31:2682–2688. doi: 10.1021/ic00039a006. [DOI] [Google Scholar]
  11. Dufour M. N., Crumbliss A. L., Johnston G., Gaudemer A.. Reaction of indoles with molecular oxygen catalyzed by metalloporphyrins. J. Mol. Catal. 1980;7:277–287. doi: 10.1016/0304-5102(80)85025-5. [DOI] [Google Scholar]
  12. Leone L., D’Alonzo D., Maglio O., Pavone V., Nastri F., Lombardi A.. Highly Selective Indole Oxidation Catalyzed by a Mn-Containing Artificial Mini-Enzyme. ACS Catal. 2021;11:9407–9417. doi: 10.1021/acscatal.1c01985. [DOI] [Google Scholar]
  13. Weinstock I. A., Schreiber R. E., Neumann R.. Dioxygen and Its Role in Polyoxometalate Mediated Reactions. Chem. Rev. 2018;118:2680–2717. doi: 10.1021/acs.chemrev.7b00444. [DOI] [PubMed] [Google Scholar]
  14. Bugnola M., Carmieli R., Neumann R.. Aerobic Electrochemical Oxygenation of Light Hydrocarbons. ACS Catal. 2018;8:3232–3236. doi: 10.1021/acscatal.8b00477. [DOI] [Google Scholar]
  15. Bugnola M., Shen K., Haviv E., Neumann R.. Reductive Electrochemical Activation of Molecular Oxygen Catalyzed by an Iron-Tungstate Oxide Capsule: Reactivity Studies and Support for an Fe­(V)-oxo Active Intermediate. ACS Catal. 2020;10:4227–4237. doi: 10.1021/acscatal.0c00897. [DOI] [Google Scholar]
  16. Kaufman Y., Neumann R.. Copper Substituted Polyoxotungstates as Catalysts for the Electrocatalytic Oxygenation of Light Alkanes. Inorg. Chem. 2025;64:18365–18375. doi: 10.1021/acs.inorgchem.5c02816. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Dahan M., Neumann R.. Molecular Oxygen Activation: A Ruthenium Substituted Polyoxometalate as an Inorganic Dioxygenase Catalyst. Nature. 1997;388:353–355. doi: 10.1038/41039. [DOI] [Google Scholar]
  18. Dahan M., Neumann R.. Molecular Oxygen Activation by a Ruthenium Substituted “Sandwich” Type Polyoxometalate. J. Am. Chem. Soc. 1998;120:11969–11976. doi: 10.1021/ja974243b. [DOI] [Google Scholar]
  19. Liu J., Ortega F., Sethuraman F., Katsoulis D. E., Costello C. E., Pope M. T.. Trimetallo derivatives of lacunary 9-tungstosilicate heteropolyanions. Part 1. Synthesis and characterization. J. Chem. Soc., Dalton Trans. 1992:1901–1906. doi: 10.1039/DT9920001901. [DOI] [Google Scholar]
  20. Tourné C. M., Tourné G. F., Malik S. A., Weakley T. J. R.. Triheteropolyanions containing copper­(II), manganese­(II), or manganese­(III) J. Inorg. Nucl. Chem. 1970;32:3875–3890. doi: 10.1016/0022-1902(70)80566-8. [DOI] [Google Scholar]
  21. Azaiza-Dabbah D., Wang F., Haddad E., Solé-Daura A., Carmieli R., Poblet J. M., Vogt C., Neumann R.. Heterometallic Transition Metal Oxides Containing Lewis Acids as Molecular Catalysts for the Reduction of Carbon Dioxide to Carbon Monoxide with Bimodal Reactivity. J. Am. Chem. Soc. 2024;146:27871–27885. doi: 10.1021/jacs.4c10412. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Tzaguy A., Masip Sanchéz A., Avram L., Solé-Daura A., López X., Poblet J. M., Neumann R.. Electrocatalytic Reduction of Dinitrogen to Ammonia with Water as Proton and Electron Donor Catalyzed by a Combination of a Tri-ironoxotungstate and an Alkali Metal Cation. J. Am. Chem. Soc. 2023;145:19912–19924. doi: 10.1021/jacs.3c06167. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Chang J., Taylor R. D., Davidson R. A., Sharmah A., Guo T.. Electron Paramagnetic Resonance Spectroscopy Investigation of Radical Production by Gold Nanoparticles in Aqueous Solutions Under X-ray Irradiation. J. Phys. Chem. A. 2016;120:2815–2823. doi: 10.1021/acs.jpca.6b01755. [DOI] [PubMed] [Google Scholar]
  24. Zhao H., Joseph J., Zhang H., Karoui H., Kalyanaraman B.. Synthesis and biochemical applications of a solid cyclic nitrone spin trap: a relatively superior trap for detecting superoxide anions and glutathiyl radicals. Free Radical Biol. Med. 2001;31:599–606. doi: 10.1016/s0891-5849(01)00619-0. [DOI] [PubMed] [Google Scholar]
  25. Keszler A., Kayanaraman B., Hogg N.. Comparative investigation of superoxide trapping by cyclic nitrone spin traps: the use of singular value decomposition and multiple linear regression analysis. Free Radical Biol. Med. 2003;35:1149–1157. doi: 10.1016/s0891-5849(03)00497-0. [DOI] [PubMed] [Google Scholar]
  26. https://www.bruker.com/fileadmin/user_upload/8-PDF-Docs/MagneticResonance/EPR_brochures/superoxide.pdf (accessed June 25, 2025).
  27. Katsoulis D. E., Pope M. T.. Reactions of Heteropolyanions in Non-polar solvents. Part 3. Activation of Dioxygen by Manganese­(II) Centres in Polytungstates. Oxidation of Hindered Phenols. J. Chem. Soc., Dalton Trans. 1989:1483–1489. doi: 10.1039/dt9890001483. [DOI] [Google Scholar]
  28. Gara M., Neumann R.. The Manganese-Containing Polyoxometalate, [WZnMnII 2(ZnW9O34)2]12-, as a Remarkably Effective Catalyst for Hydrogen Peroxide Mediated Oxidation. J. Am. Chem. Soc. 1995;117:5066–5074. doi: 10.1021/ja00123a008. [DOI] [Google Scholar]
  29. Sobanska K., Krasowska A., Mazur T., Podolska-Serafin K., Pietrzyk P., Sojka Z.. Diagnostic Features of EPR Spectra of Superoxide Intermediates on Catalytic Surfaces and Molecular Interpretation of Their g and A Tensors. Topics Catal. 2015;58:796–810. doi: 10.1007/s11244-015-0420-y. [DOI] [Google Scholar]
  30. Zhang X., Foote C. S.. Dimethyldioxirane Oxidation of Indole Derivatives. Formation of Novel Indole-2,3-epoxides and a Versatile Synthetic Route to Indolinones and Indolines. J. Am. Chem. Soc. 1993;115:8867–8868. doi: 10.1021/ja00072a061. [DOI] [Google Scholar]
  31. Tolbert G. B., Jayawardana S. B., Lee Y., Sun J., Qu F., Whitt L. M., Shafaat H. S., Wijeratne G. B.. Secondary Sphere Lewis Acid Activated Heme Superoxo Adducts Mimic Crucial Non-Covalent Interactions in IDO/TDO Heme Dioxygenases. Chem. Eur. J. 2024;30:e202402310. doi: 10.1002/chem.202402310. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Zhang X.-Y., Pope M. T., Chance M. R., Jameson G. B.. High-valent manganese in polyoxotungstates 1. Manganese­(IV) keggin derivatives. Polyhedron. 1995;14:1381–1392. doi: 10.1016/0277-5387(94)00387-T. [DOI] [Google Scholar]
  33. Schreiber R. E., Cohen H., Leitus G., Wolf S. G., Zhou A., Que L. Jr, Neumann R.. Reactivity and O2 Formation by Mn­(IV)- and Mn­(V)-Hydroxo Species Stabilized within a Polyfluoroxometalate Framework. J. Am. Chem. Soc. 2015;137:8738–8748. doi: 10.1021/jacs.5b03456. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Ginzburg, A. P. Inorganic Synthesis; Wiley; 1990,vol 27, p 107. [Google Scholar]
  35. Tourné C. M., Tourné G. F., Malik S. A., Weakley T. J. R.. Triheteropolyanions Containing Copper­(II), Manganese­(II), or Manganese­(III) J. Inorg. Nucl. Chem. 1970;32:3875–3890. doi: 10.1016/0022-1902(70)80566-8. [DOI] [Google Scholar]
  36. Liu J., Ortega F., Sethuraman F., Katsoulis D. E., Costello C. E., Pope M. T.. Trimetallo Derivatives of Lacunary 9-Tungstosilicate Heteropolyanions. Part 1. Synthesis and Characterization. J. Chem. Soc., Dalton Trans. 1992:1901–1906. doi: 10.1039/DT9920001901. [DOI] [Google Scholar]
  37. Randall W. J., Lyon D. K., DeMille P. J., Finke R. G.. Transition Metal Complexes of the Lacunary Heteropolytungstate, [P2W17O61]10– . Inorg. Synth. 1998;32:242–268. doi: 10.1002/9780470132630.ch40. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ic6c02005_si_001.pdf (772.4KB, pdf)

Articles from Inorganic Chemistry are provided here courtesy of American Chemical Society

RESOURCES