Lawrence Que, Jr.

Lewis acid‐bound high valent Mn‐oxo species are of great importance due to their relevance to photosystem II. Here, we report the synthesis of a unique [(BnTPEN)Mn(III)−O−Ce(IV)(NO3)4]+ adduct (2) by the reaction of (BnTPEN)Mn(II) (1) with 4 eq. ceric ammonium nitrate. 2 has been characterized using UV/Vis, NMR, resonance Raman spectroscopy, as well as by mass spectrometry. Treatment of 2 with Sc(III)(OTf)3 results in the formation of (BnTPEN)Mn(IV)−O−Sc(III) (3), while HClO4 addition to 2 forms (BnTPEN)Mn(IV)−OH (4), reverting to 2 upon Ce(III)(NO3)3 addition. 2 can also be prepared by the oxidation of 1 eq. Ce(III)(NO3)3 with [(BnTPEN)Mn(IV)=O]2+ (5). In addition, the EPR spectroscopy revealed the elegant temperature‐dependent equilibria between 2 and Mn(IV) species. The binding of redox‐active Ce(IV) boosts electron transfer efficiency of 2 towards ferrocenes. Remarkably, the newly …

TMC-anti and TMC-syn, the two topological isomers of [FeIV(O)(TMC)(CH3CN)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Me4cyclam), differ in the orientations of their FeIV=O units relative to the four methyl groups of the TMC ligand framework. The FeIV=O unit of TMC-anti points away from the four methyl groups, while that of TMC-syn is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC-syn reacts with HAT (hydrogen atom transfer) substrates at 1.3- to 3-fold faster rates than TMC-anti, but the reactivity difference increases dramatically in oxygen-atom transfer reactions. R2S substrates are oxidized into R2S=O products at rates 2-to-3 orders of magnitude faster by TMC-syn than TMC-anti. Even more remarkably, TMC-syn epoxidizes all the olefin substrates in this study, while TMC-anti reacts only with cis-cyclooctene but at a 100-fold slower rate …

S = 2 FeIV═O centers generated in the active sites of nonheme iron oxygenases cleave substrate C–H bonds at rates significantly faster than most known synthetic FeIV═O complexes. Unlike the majority of the latter, which are S = 1 complexes, [FeIV(O)(tris(2-quinolylmethyl)amine)(MeCN)]2+ (3) is a rare example of a synthetic S = 2 FeIV═O complex that cleaves C–H bonds 1000-fold faster than the related [FeIV(O)(tris(pyridyl-2-methyl)amine)(MeCN)]2+ complex (0). To rationalize this significant difference, a systematic comparison of properties has been carried out on 0 and 3 as well as related complexes 1 and 2 with mixed pyridine (Py)/quinoline (Q) ligation. Interestingly, 2 with a 2-Q-1-Py donor combination cleaves C–H bonds at 233 K with rates approaching those of 3, even though Mössbauer analysis reveals 2 to be S = 1 at 4 K. At 233 K however, 2 becomes S = 2, as shown by its 1H NMR spectrum …

In the postulated catalytic cycle of class Ib Mn2 ribonucleotide reductases (RNRs), a MnII2 core is suggested to react with superoxide (O2·–) to generate peroxido-MnIIMnIII and oxo-MnIIIMnIV entities prior to proton-coupled electron transfer (PCET) oxidation of tyrosine. There is limited experimental support for this mechanism. We demonstrate that [MnII2(BPMP)(OAc)2](ClO4) (1, HBPMP = 2,6-bis[(bis(2 pyridylmethyl)amino)methyl]-4-methylphenol) was converted to peroxido-MnIIMnIII (2) in the presence of superoxide anion that converted to (μ-O)(μ-OH)MnIIIMnIV (3) via the addition of an H+-donor (p-TsOH) or (μ-O)2MnIIIMnIV (4) upon warming to room temperature. The physical properties of 3 and 4 were probed using UV–vis, EPR, X-ray absorption, and IR spectroscopies and mass spectrometry. Compounds 3 and 4 were capable of phenol oxidation to yield a phenoxyl radical via a concerted PCET oxidation …

The hydroxylation of C–H bonds can be carried out by the high-valent CoIII,IV2(µ-O)2 complex 2a supported by the tetradentate tris(2-pyridylmethyl)amine ligand via a CoIII2(µ-O)(µ-OH) intermediate (3a). Complex 3a can be independently generated either by H-atom transfer (HAT) in the reaction of 2a with phenols as the H-atom donor or protonation of its conjugate base, the CoIII2(µ-O)2 complex 1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm−1 associated with the symmetric Co–O–Co stretching mode of the Co2O2 diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for 1a and 2a by Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their “diamond core” structural assignments. The independent generation of 3a allows us to investigate HAT reactions of 2a with …

Leigh Aldous opened the discussion of the paper by Carole Duboc: Since the CoFe has a lower overpotential for H2 production, the current will be much lower than those of NiFe and FeFe (even if they have the same kinetic parameters). So, was there any evidence for electrocatalytic ability, eg even at slow scan rates? This Co–Fe complex was in theory very promising compared to Ni–Fe and Fe–Fe because of the more positive reduction potential, but in my mind it needs to have much better parameters to have any current due to the lower overpotential. So, have you tried hour-long reduction reactions, eg by reducing with cobaltocene? Does it have any activity at all, or is no H2 ever produced?Carole Duboc answered: The CoFe complex can be reduced by one electron and its reduced form has been characterized. We have experiments showing that it cannot directly react with H+ and needs to be further reduced …

Shyamalava Mazumdar opened the discussion of the paper by Abhishek Dey: Thank you for the excellent talk. You talked about the possibilities of different mechanisms in model systems. What is the mechanism in the case of real proteins, such as peroxidase or oxygenases? How does the asymmetric protein environment with very well de ned channels etc. affect the pathways?Abhishek Dey answered: The whole point of including the second sphere residues is not only to mimic the function of the enzymes but also to mimic the mechanism of the enzyme. We have reported iron porphyrins with pendant pyridine and amine groups which form compound I from peroxides at rates faster than that of HRPs (see ref. 1). Accordingly, these were found to be functional models of HRP and in some cases show a ping-pong mechanism with comparable Km and Kcal (see ref. 2). Apart from the second sphere residues …

Methanotrophic bacteria utilize methane monooxygenase (MMO) to carry out the first step in metabolizing methane. The soluble enzymes employ a hydroxylase component (sMMOH) with a nonheme diiron active site that activates O2 and generates a powerful oxidant capable of converting methane to methanol. It is proposed that the diiron(II) center in the reduced enzyme reacts with O2 to generate a diferric-peroxo intermediate called P that then undergoes O–O cleavage to convert into a diiron(IV) derivative called Q, which carries out methane hydroxylation. Most (but not all) of the spectroscopic data of Q accumulated by various groups to date favor the presence of an FeIV2(μ-O)2 unit with a diamond core. The Que lab has had a long-term interest in making synthetic analogs of iron enzyme intermediates. To this end, the first crystal structure of a complex with a FeIIIFeIV(μ-O)2 diamond core was reported in 1999 …

This special issue celebrates the scientific achievements of Alison Butler on the occasion of her receiving the American Chemical Society Alfred Bader Award in Bioorganic or Bioinorganic Chemistry at the 2018 American Chemical Society Spring National Meeting, held in New Orleans. Butler’s Bader Award recognizes her research group’s many significant contributions elucidating the bioinorganic chemistry of the marine environment, including the chemistry of siderophores and vanadium haloperoxidases. The unusual transition metal composition of surface seawater captured her interest early on [1]. She has investigated the chemistry of vanadium haloperoxidases from marine seaweeds and shown that the stereoselective bromocyclization of terpenes catalyzed by these enzymes is important in the biosynthesis of halogenated marine natural products [2]. While vanadium is the second-most abundant transition …

Herein are described substrate oxidations with H2O2 catalyzed by [FeII(IndH)(CH3CN)3](ClO4)2 [IndH = 1,3-bis(2′-pyridylimino)isoindoline], involving a spectroscopically characterized (μ-oxo)(μ-1,2-peroxo)diiron(III) intermediate (2) that is capable of olefin epoxidation and alkane hydroxylation including cyclohexane. Species 2 also converts ketones to lactones with a decay rate dependent on [ketone], suggesting direct nucleophilic attack of the substrate carbonyl group by the peroxo species. In contrast, peroxo decay is unaffected by the addition of olefins or alkanes, but the label from H218O is incorporated into the the epoxide and alcohol products, implicating a high-valent iron–oxo oxidant that derives from O–O bond cleavage of the peroxo intermediate. These results demonstrate an ambiphilic diferric–peroxo intermediate that mimics the range of oxidative reactivities associated with O2-activating nonheme …

Reactivities of non‐heme iron(IV)‐oxo complexes are mostly controlled by the ligands. Complexes with tetradentate ligands such as [(TPA)FeO]2+ (TPA=tris(2‐pyridylmethyl)amine) belong to the most reactive ones. Here, we show a fine‐tuning of the reactivity of [(TPA)FeO]2+ by an additional ligand X (X=CH3CN, CF3SO3−, ArI, and ArIO; ArI=2‐(tBuSO2)C6H4I) attached in solution and reveal a thus far unknown role of the ArIO oxidant. The HAT reactivity of [(TPA)FeO(X)]+/2+ decreases in the order of X: ArIO > MeCN > ArI ≈ TfO−. Hence, ArIO is not just a mere oxidant of the iron(II) complex, but it can also increase the reactivity of the iron(IV)‐oxo complex as a labile ligand. The detected HAT reactivities of the [(TPA)FeO(X)]+/2+ complexes correlate with the Fe=O and FeO−H stretching vibrations of the reactants and the respective products as determined by infrared photodissociation spectroscopy. Hence, the most …

In this study, a methyl group on the classic tetramethylcyclam (TMC) ligand framework is replaced with a benzylic group to form the metastable [FeIV(Osyn)(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 FeIII−OAr product 3. At the same time, the FeII(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 [FeIV(O)(LPh)]2+ (2‐syn)→2 [FeIII(LOAr)]2+ (3)+[FeII(LPh)]2+ (1)+H2O. This system thus serves as a paradigm for aryl hydroxylation by FeIV=O oxidants described thus far. It is also observed that 2‐syn …

Streszczenie w języku oryginału Comprehensive Coordination Chemistry III, Nine Volume Set describes the fundamentals of metal-ligand interactions, provides an overview of the systematic chemistry of this class of compounds, and details their importance in life processes, medicine, industry and materials science. This new edition spans across 9 volumes, 185 entries and 6600 printed pages. Comprehensive Coordination Chemistry III is not just an update of the second edition, it includes a significant amount of new content. In the descriptive sections 3-6, emphasis is placed upon material that has appeared in primary and secondary review literature since the previous edition published. The material in other sections is newly written, with an emphasis on modern aspects of coordination chemistry and the latest developments. The metal-ligand interaction is the link between the award of the 1913 Nobel Prize in …

Bio-coordination chemistry: An introduction — Experts@Minnesota Skip to main navigation Skip to search Skip to main content Experts@Minnesota Home Experts@Minnesota Logo Home Profiles Research units University Assets Projects and Grants Research output Press/Media Datasets Activities Fellowships, Honors, and Prizes Search by expertise, name or affiliation Bio-coordination chemistry: An introduction Lawrence Que, Yi Lu Chemistry (Twin Cities) Research output: Chapter in Book/Report/Conference proceeding › Chapter Overview Original language English (US) Title of host publication Comprehensive Coordination Chemistry III Publisher Elsevier Pages 1-2 Number of pages 2 Volume 1-9 ISBN (Electronic) 9780081026885 ISBN (Print) 9780081026892 DOIs https://doi.org/10.1016/B978-0-08-102688-5.00118-5 State Published - Jul 21 2021 Publisher link 10.1016/B978-0-08-102688-5.00118-5 Find It …

Synthetic nonheme high-valent iron-oxo complexes structures and oxidative function — Experts@Minnesota Skip to main navigation Skip to search Skip to main content Experts@Minnesota Home Experts@Minnesota Logo Home Profiles Research units University Assets Projects and Grants Research output Press/Media Datasets Activities Fellowships, Honors, and Prizes Search by expertise, name or affiliation Synthetic nonheme high-valent iron-oxo complexes structures and oxidative function Chase S Abelson, Ahmed M. Aboelenen, Waqas Rasheed, Lawrence Que Chemistry (Twin Cities) Research output: Chapter in Book/Report/Conference proceeding › Chapter 2 Scopus citations Overview Original language English (US) Title of host publication Comprehensive Coordination Chemistry III Publisher Elsevier Pages 412-454 Number of pages 43 Volume 1-9 ISBN (Electronic) 9780081026885 ISBN (Print) …

A handful of oxygen-activating enzymes has recently been found to contain Fe/Mn active sites, like Class 1c ribonucleotide reductases and R2-like ligand-binding oxidase, which are closely related to their better characterized diiron cousins. These enzymes are proposed to form high-valent intermediates with Fe–O–Mn cores. Herein, we report the first examples of synthetic Fe/Mn complexes that mimic doubly bridged intermediates proposed for enzymatic oxygen activation. Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis has been used to characterize the structures of each of these compounds. Linear compounds accurately model the Fe···Mn distances found in Fe/Mn proteins in their resting states, and doubly bridged diamond core compounds accurately model the distances found in high-valent biological intermediates. Unlike their diiron analogues, the paramagnetic nature of Fe/Mn …

Nonheme iron enzymes generate powerful and versatile oxidants that perform a wide range of oxidation reactions, including the functionalization of inert C−H bonds, which is a major challenge for chemists. The oxidative abilities of these enzymes have inspired bioinorganic chemists to design synthetic models to mimic their ability to perform some of the most difficult oxidation reactions and study the mechanisms of such transformations. Iron‐oxygen intermediates like iron(III)‐hydroperoxo and high‐valent iron‐oxo species have been trapped and identified in investigations of these bio‐inspired catalytic systems, with the latter proposed to be the active oxidant for most of these systems. In this Review, we highlight the recent spectroscopic and mechanistic advances that have shed light on the various pathways that can be accessed by bio‐inspired nonheme iron systems to form the high‐valent iron‐oxo intermediates.

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 [FeII(Me3NTB)(CH3CN)](CF3SO3)2 (Me3NTB = tris((1-methyl-1H-benzo[d]imidazol-2-yl)methyl)amine) (1) reacts with O2 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 …

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*)FeII (1) (TPA*=tris(3,5‐dimethyl‐4‐methoxypyridyl‐2‐methyl)amine) in CH3CN with 4 equiv CAN and 200 equiv HClO4 at 20 °C to form a complex with an [FeIV2(μ‐O)2]4+ core. CAN and HClO4 play essential roles in this unprecedented transformation, in which the comproportionation of FeIII‐O‐CeIV and FeIV=O/Ce4+ species is proposed to be involved in the assembly of the [FeIV2(μ‐O)2]4+ core.

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 [Feᴵᴵ (Me₃NTB)(CH₃CN)](CF₃SO₃) ₂ (Me₃NTB= tris ((1-methyl-1H-benzo [d] imidazol-2-yl) methyl) amine)(1) reacts with O₂ 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 Sc³⁺ 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.

A one-step synthesis of enones from olefins is described. The reaction was performed under visible-light irradiation in the presence of molecular oxygen and a photocatalyst. The reaction proceeded with various types of trisubstituted olefins to give enones in good yields with high regioselectivity. In particular, oxygen- and nitrogen-containing functional groups, heteroaromatic rings, and cyclopropanes were tolerated. Mechanistic studies and previous reports indicated that the active oxygen species generated in the reaction system is singlet oxygen.

Nicht‐Häm‐Eisenenzyme generieren leistungsstarke und vielfältige Oxidantien, die eine Vielzahl an Oxidationsreaktionen vermitteln, einschließlich der Funktionalisierung von inerten C‐H‐Bindungen, die eine anspruchsvolle Aufgabe für Chemiker darstellt. Die oxidativen Fähigkeiten dieser Enzyme inspirieren Bioorganiker zur Entwicklung synthetischer Modelle, um die Fähigkeit solcher Enzyme zur Vermittlung schwierigster Oxidationsreaktionen nachzuahmen und die Mechanismen derartiger Transformationen zu erforschen. Eisen‐Sauerstoff‐Intermediate wie Eisen(III)‐Hydroperoxo‐Spezies und hochvalente Eisen‐Sauerstoff‐Spezies wurden bei Untersuchungen dieser bioinspirierten katalytischen Systemen abgefangen und identifiziert, wobei Letztgenannte als aktives Oxidationsmittel für die meisten dieser Systeme vorgeschlagen wurden. In diesem Aufsatz heben wir die jüngsten spektroskopischen und …