1
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Phu PN, Barman SK, Ziller JW, Hendrich MP, Borovik AS. Synthesis, characterization and reactivity of a Mn(III)-hydroxido complex as a biomimetic model for lipoxygenase. J Inorg Biochem 2024; 259:112618. [PMID: 38986289 DOI: 10.1016/j.jinorgbio.2024.112618] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2024] [Revised: 05/19/2024] [Accepted: 05/24/2024] [Indexed: 07/12/2024]
Abstract
Manganese hydroxido (Mn-OH) complexes supported by a tripodal N,N',N″-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido) ([poat]3-) ligand have been synthesized and characterized by spectroscopic techniques including UV-vis and electron paramagnetic resonance (EPR) spectroscopies. X-ray diffraction (XRD) methods were used to confirm the solid-state molecular structures of {Na2[MnIIpoat(OH)]}2 and {Na[MnIIIpoat(OH)]}2 as clusters that are linked by the electrostatic interactions between the sodium counterions and the oxygen atom of the ligated hydroxido unit and the phosphinic (P=O) amide groups of [poat]3-. Both clusters feature two independent monoanionic fragments in which each contains a trigonal bipyramidal Mn center that is comprised of three equatorial deprotonated amide nitrogen atoms, an apical tertiary amine, and an axial hydroxido ligand. XRD analyses of {Na[MnIIIpoat(OH)]}2 also showed an intramolecular hydrogen bonding interaction between the MnIII-OH unit and P=O group of [poat]3-. Crystalline {Na[MnIIIpoat(OH)]}2 remains as clusters with Na+---O interactions in solution and is unreactive toward external substrates. However, conductivity studies indicated that [MnIIIpoat(OH)]- generated in situ is monomeric and reactivity studies found that it is capable of cleaving C-H bonds, illustrating the importance of solution-phase speciation and its direct effect on chemical reactivity. Synopsis: Manganese-hydroxido complexes were synthesized to study the influence of H-bonds in the secondary coordination sphere and their effects on the oxidative cleavage of substrates containing C-H bonds.
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Affiliation(s)
- Phan N Phu
- Department of Chemistry, University of California, Irvine, CA 92697, United States
| | - Suman K Barman
- Department of Chemistry, University of California, Irvine, CA 92697, United States; Department of Chemical Sciences, India Institute of Science Education and Research (IISER) Mohali, Manauli 140306, India
| | - Joseph W Ziller
- Department of Chemistry, University of California, Irvine, CA 92697, United States
| | - Michael P Hendrich
- Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, United States
| | - A S Borovik
- Department of Chemistry, University of California, Irvine, CA 92697, United States.
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2
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Lee Y, Moon D, Cho J. Controlling Redox Potential of a Manganese(III)-Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity. J Am Chem Soc 2024; 146:15796-15805. [PMID: 38829358 DOI: 10.1021/jacs.4c01927] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/05/2024]
Abstract
A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (-0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O-H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol-1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O-H bonds, whereas 2 can activate aliphatic C-H bonds in addition to O-H bonds. The reaction rates increased by 1.4 × 104-fold for the O-H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C-H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)-hydroxo and -aqua complexes through protonation.
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Affiliation(s)
- Yuri Lee
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
| | - Dohyun Moon
- Beamline Department, Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea
| | - Jaeheung Cho
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
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3
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Grotemeyer EN, Parham JD, Jackson TA. Reaction landscape of a mononuclear Mn III-hydroxo complex with hydrogen peroxide. Dalton Trans 2023; 52:14350-14370. [PMID: 37767937 DOI: 10.1039/d3dt02672h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/29/2023]
Abstract
Peroxomanganese species have been proposed as key intermediates in the catalytic cycles of both manganese enzymes and synthetic catalysts. However, many of these intermediates have yet to be observed. Here, we report the formation of a series of intermediates, each generated from the reaction of the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq2Me)]+ with hydrogen peroxide under slightly different conditions. By changing the acidity of the reaction mixture and/or the quantity of hydrogen peroxide added, we are able to control which intermediate forms. Using a combination of electronic absorption, 1H NMR, EPR, and X-ray absorption spectroscopies, as well as density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations, we formulate these intermediates as the bis(μ-oxo)dimanganese(III,IV) complex [MnIIIMnIV(μ-O)2(dpaq2Me)2]+, the MnIII-hydroperoxo complex [MnIII(OOH)(dpaq2Me)]+, and the MnIII-peroxo complex [MnIII(O2)(dpaq2Me)]. The formation of the MnIII-hydroperoxo species from the reaction of a MnIII-hydroxo complex with hydrogen peroxide mimics an elementary reaction proposed for many synthetic manganese catalysts that activate hydrogen peroxide.
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Affiliation(s)
- Elizabeth N Grotemeyer
- The University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, KS 66045, USA.
| | - Joshua D Parham
- The University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, KS 66045, USA.
| | - Timothy A Jackson
- The University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, KS 66045, USA.
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4
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Moore SM, Sun C, Steele JL, Laaker EM, Rheingold AL, Doerrer LH. HAA by the first {Mn(iii)OH} complex with all O-donor ligands. Chem Sci 2023; 14:8187-8195. [PMID: 37538819 PMCID: PMC10395311 DOI: 10.1039/d3sc01971c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2023] [Accepted: 07/05/2023] [Indexed: 08/05/2023] Open
Abstract
There is considerable interest in MnOHx moieties, particularly in the stepwise changes in those O-H bonds in tandem with Mn oxidation state changes. The reactivity of aquo-derived ligands, {MOHx}, is also heavily influenced by the electronic character of the other ligands. Despite the prevalence of oxygen coordination in biological systems, preparation of mononuclear Mn complexes of this type with all O-donors is rare. Herein, we report several Mn complexes with perfluoropinacolate (pinF)2- including the first example of a crystallographically characterized mononuclear {Mn(iii)OH} with all O-donors, K2[Mn(OH)(pinF)2], 3. Complex 3 is prepared via deprotonation of K[Mn(OH2)(pinF)2], 1, the pKa of which is estimated to be 18.3 ± 0.3. Cyclic voltammetry reveals quasi-reversible redox behavior for both 1 and 3 with an unusually large ΔEp, assigned to the Mn(iii/ii) couple. Using the Bordwell method, the bond dissociation free energy (BDFE) of the O-H bond in {Mn(ii)-OH2} is estimated to be 67-70 kcal mol-1. Complex 3 abstracts H-atoms from 1,2-diphenylhydrazine, 2,4,6-TTBP, and TEMPOH, the latter of which supports a PCET mechanism. Under basic conditions in air, the synthesis of 1 results in K2[Mn(OAc)(pinF)2], 2, proposed to result from the oxidation of Et2O to EtOAc by a reactive Mn species, followed by ester hydrolysis. Complex 3 alone does not react with Et2O, but addition of O2 at low temperature effects the formation of a new chromophore proposed to be a Mn(iv) species. The related complexes K(18C6)[Mn(iii)(pinF)2], 4, and (Me4N)2[Mn(ii)(pinF)2], 5, have also been prepared and their properties discussed in relation to complexes 1-3.
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Affiliation(s)
- Shawn M Moore
- Boston University, Chemistry Department 590 Commonwealth Avenue Boston Massachusetts 02215 USA
| | - Chen Sun
- Boston University, Chemistry Department 590 Commonwealth Avenue Boston Massachusetts 02215 USA
| | - Jennifer L Steele
- Boston University, Chemistry Department 590 Commonwealth Avenue Boston Massachusetts 02215 USA
| | - Ellen M Laaker
- Boston University, Chemistry Department 590 Commonwealth Avenue Boston Massachusetts 02215 USA
| | - Arnold L Rheingold
- University of California, San Diego Department of Chemistry and Biochemistry 9500 Gilman Drive La Jolla California 92093 USA
| | - Linda H Doerrer
- Boston University, Chemistry Department 590 Commonwealth Avenue Boston Massachusetts 02215 USA
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5
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Zhang J, Lee YM, Seo MS, Nilajakar M, Fukuzumi S, Nam W. A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex. Inorg Chem 2022; 61:19735-19747. [PMID: 36445726 DOI: 10.1021/acs.inorgchem.2c02504] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [NiIII(PaPy3*)]2+ (1) in acetone/CH3CN (v/v 19:1). 1 was synthesized by reacting [NiII(PaPy3*)]+ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H2O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H+ to the PaPy3* ligand of 2; the one-electron reduction potential (Ered) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the Ered value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA•+ to 2 due to the binding of H+ to the PaPy3* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.
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Affiliation(s)
- Jisheng Zhang
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Yong-Min Lee
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Mi Sook Seo
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Madhuri Nilajakar
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Shunichi Fukuzumi
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Wonwoo Nam
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
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6
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Lee Y, Tripodi GL, Jeong D, Lee S, Roithova J, Cho J. Aliphatic and Aromatic C–H Bond Oxidation by High-Valent Manganese(IV)-Hydroxo Species. J Am Chem Soc 2022; 144:20752-20762. [DOI: 10.1021/jacs.2c08531] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Affiliation(s)
- Yujeong Lee
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan44919, Republic of Korea
| | - Guilherme L. Tripodi
- Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJNijmegen, Netherlands
| | - Donghyun Jeong
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan44919, Republic of Korea
| | - Sunggi Lee
- Department of Physics and Chemistry, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu42988, Korea
| | - Jana Roithova
- Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJNijmegen, Netherlands
| | - Jaeheung Cho
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan44919, Republic of Korea
- Graduate School of Carbon Neutrality, Ulsan National Institute of Science and Technology (UNIST), Ulsan44919, Republic of Korea
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7
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Massie AA, Kostopoulos N, Grotemeyer EN, Noël J, Jackson TA, Anxolabéhère‐Mallart E. Electrochemical Formation and Reactivity of a Mn‐Peroxo Complex Bearing an Amido N5 Ligand. ChemElectroChem 2022. [DOI: 10.1002/celc.202200112] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Affiliation(s)
- Allyssa A. Massie
- Department of Chemistry University of Kansas 1567 Irving Hill Road Lawrence KS 66045 USA
| | - Nikolaos Kostopoulos
- Université Paris Cité, CNRS, Laboratoire d'Electrochimie Moléculaire F-75013> Paris France
| | | | - Jean‐Marc Noël
- Université Paris Cité, CNRS, ITODYS F-75013 Paris France
| | - Timothy A. Jackson
- Department of Chemistry University of Kansas 1567 Irving Hill Road Lawrence KS 66045 USA
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8
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Park Y, Kim S, Kim K, Shin B, Jang Y, Cho KB, Cho J. Structure and Reactivity of Nonporphyrinic Terminal Manganese(IV)-Hydroxide Complexes in the Oxidative Electrophilic Reaction. Inorg Chem 2022; 61:4292-4301. [PMID: 35226491 DOI: 10.1021/acs.inorgchem.1c03104] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
High-valent transition metal-hydroxide complexes have been proposed as essential intermediates in biological and synthetic catalytic reactions. In this work, we report the single-crystal X-ray structure and spectroscopic characteristics of a mononuclear nonporphyrinic MnIV-(OH) complex, [MnIV(Me3-TPADP)(OH)(OCH2CH3)]2+ (2), using various physicochemical methods. Likewise, [MnIV(Me3-TPADP)(OH)(OCH2CF3)]2+ (3), which is thermally stable at room temperature, was also synthesized and characterized spectroscopically. The MnIV-(OH) adducts are capable of performing oxidation reactions with external organic substrates such as C-H bond activation, sulfoxidation, and epoxidation. Kinetic studies, involving the Hammett correlation and kinetic isotope effect, and product analyses indicate that 2 and 3 exhibit electrophilic oxidative reactivity toward hydrocarbons. Density functional theory calculations support the assigned electronic structure and show that direct C-H bond activation of the MnIV-(OH) species is indeed possible.
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Affiliation(s)
- Younwoo Park
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea.,Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea
| | - Seonghan Kim
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea.,Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea
| | - Kyungmin Kim
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea.,Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea
| | - Bongki Shin
- Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea
| | - Youngchae Jang
- Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea
| | - Kyung-Bin Cho
- Department of Chemistry, Jeonbuk National University, Jeonju 54896, Korea
| | - Jaeheung Cho
- Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea
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9
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Zhang J, Lee YM, Seo MS, Kim Y, Lee E, Fukuzumi S, Nam W. Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes. Inorg Chem Front 2022. [DOI: 10.1039/d2qi00741j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...
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10
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Cook EN, Machan CW. Bioinspired mononuclear Mn complexes for O 2 activation and biologically relevant reactions. Dalton Trans 2021; 50:16871-16886. [PMID: 34730590 DOI: 10.1039/d1dt03178c] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
A general interest in harnessing the oxidizing power of dioxygen (O2) continues to motivate research efforts on bioinspired and biomimetic complexes to better understand how metalloenzymes mediate these reactions. The ubiquity of Fe- and Cu-based enzymes attracts significant attention and has resulted in many noteworthy developments for abiotic systems interested in direct O2 reduction and small molecule activation. However, despite the existence of Mn-based metalloenzymes with important O2-dependent activity, there has been comparatively less focus on the development of these analogues relative to Fe- and Cu-systems. In this Perspective, we summarize important contributions to the development of bioinspired mononuclear Mn complexes for O2 activation and studies on their reactivity, emphasizing important design parameters in the primary and secondary coordination spheres and outlining mechanistic trends.
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Affiliation(s)
- Emma N Cook
- Department of Chemistry, University of Virginia, PO Box 400319, Charlottesville, VA 22904-4319, USA.
| | - Charles W Machan
- Department of Chemistry, University of Virginia, PO Box 400319, Charlottesville, VA 22904-4319, USA.
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11
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Opalade AA, Grotemeyer EN, Jackson TA. Mimicking Elementary Reactions of Manganese Lipoxygenase Using Mn-hydroxo and Mn-alkylperoxo Complexes. Molecules 2021; 26:molecules26237151. [PMID: 34885729 PMCID: PMC8659247 DOI: 10.3390/molecules26237151] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2021] [Revised: 11/20/2021] [Accepted: 11/22/2021] [Indexed: 11/16/2022] Open
Abstract
Manganese lipoxygenase (MnLOX) is an enzyme that converts polyunsaturated fatty acids to alkyl hydroperoxides. In proposed mechanisms for this enzyme, the transfer of a hydrogen atom from a substrate C-H bond to an active-site MnIII-hydroxo center initiates substrate oxidation. In some proposed mechanisms, the active-site MnIII-hydroxo complex is regenerated by the reaction of a MnIII-alkylperoxo intermediate with water by a ligand substitution reaction. In a recent study, we described a pair of MnIII-hydroxo and MnIII-alkylperoxo complexes supported by the same amide-containing pentadentate ligand (6Medpaq). In this present work, we describe the reaction of the MnIII-hydroxo unit in C-H and O-H bond oxidation processes, thus mimicking one of the elementary reactions of the MnLOX enzyme. An analysis of kinetic data shows that the MnIII-hydroxo complex [MnIII(OH)(6Medpaq)]+ oxidizes TEMPOH (2,2′-6,6′-tetramethylpiperidine-1-ol) faster than the majority of previously reported MnIII-hydroxo complexes. Using a combination of cyclic voltammetry and electronic structure computations, we demonstrate that the weak MnIII-N(pyridine) bonds lead to a higher MnIII/II reduction potential, increasing the driving force for substrate oxidation reactions and accounting for the faster reaction rate. In addition, we demonstrate that the MnIII-alkylperoxo complex [MnIII(OOtBu)(6Medpaq)]+ reacts with water to obtain the corresponding MnIII-hydroxo species, thus mimicking the ligand substitution step proposed for MnLOX.
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12
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Opalade AA, Parham JD, Day VW, Jackson TA. Characterization and chemical reactivity of room-temperature-stable Mn III-alkylperoxo complexes. Chem Sci 2021; 12:12564-12575. [PMID: 34703542 PMCID: PMC8494025 DOI: 10.1039/d1sc01976g] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2021] [Accepted: 08/20/2021] [Indexed: 12/30/2022] Open
Abstract
While alkylperoxomanganese(iii) (MnIII-OOR) intermediates are proposed in the catalytic cycles of several manganese-dependent enzymes, their characterization has proven to be a challenge due to their inherent thermal instability. Fundamental understanding of the structural and electronic properties of these important intermediates is limited to a series of complexes with thiolate-containing N4S- ligands. These well-characterized complexes are metastable yet unreactive in the direct oxidation of organic substrates. Because the stability and reactivity of MnIII-OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new MnIII-OOR complexes using a new amide-containing N5 - ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H6Medpaq) ligand, we generated the [MnIII(OO t Bu)(6Medpaq)]OTf and [MnIII(OOCm)(6Medpaq)]OTf complexes through reaction of their MnII or MnIII precursors with t BuOOH and CmOOH, respectively. Both of the new MnIII-OOR complexes are stable at room-temperature (t 1/2 = 5 and 8 days, respectively, at 298 K in CH3CN) and capable of reacting directly with phosphine substrates. The stability of these MnIII-OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [MnIII(OOCm)(6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the MnIII-OOR complexes by O-O bond homolysis. In contrast, direct reaction of [MnIII(OOCm)(6Medpaq)]+ with PPh3 provided evidence of heterolytic cleavage of the O-O bond. These studies reveal that both the stability and chemical reactivity of MnIII-OOR complexes can be tuned by the local coordination sphere.
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Affiliation(s)
- Adedamola A Opalade
- The University of Kansas, Department of Chemistry, Center for Environmentally Beneficial Catalysis 1567 Irving Hill Road Lawrence KS 66045 USA +1-785-864-3968
| | - Joshua D Parham
- The University of Kansas, Department of Chemistry, Center for Environmentally Beneficial Catalysis 1567 Irving Hill Road Lawrence KS 66045 USA +1-785-864-3968
| | - Victor W Day
- The University of Kansas, Department of Chemistry, Center for Environmentally Beneficial Catalysis 1567 Irving Hill Road Lawrence KS 66045 USA +1-785-864-3968
| | - Timothy A Jackson
- The University of Kansas, Department of Chemistry, Center for Environmentally Beneficial Catalysis 1567 Irving Hill Road Lawrence KS 66045 USA +1-785-864-3968
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13
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Opalade AA, Hessefort L, Day VW, Jackson TA. Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond. J Am Chem Soc 2021; 143:15159-15175. [PMID: 34494835 DOI: 10.1021/jacs.1c06199] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
Abstract
The enzymes manganese lipoxygenase (MnLOX) and manganese superoxide dismutase (MnSOD) utilize mononuclear Mn centers to effect their catalytic reactions. In the oxidized MnIII state, the active site of each enzyme contains a hydroxo ligand, and X-ray crystal structures imply a hydrogen bond between this hydroxo ligand and a cis carboxylate ligand. While hydrogen bonding is a common feature of enzyme active sites, the importance of this particular hydroxo-carboxylate interaction is relatively unexplored. In this present study, we examined a pair of MnIII-hydroxo complexes that differ by a single functional group. One of these complexes, [MnIII(OH)(PaPy2N)]+, contains a naphthyridinyl moiety capable of forming an intramolecular hydrogen bond with the hydroxo ligand. The second complex, [MnIII(OH)(PaPy2Q)]+, contains a quinolinyl moiety that does not permit any intramolecular hydrogen bonding. Spectroscopic characterization of these complexes supports a common structure, but with perturbations to [MnIII(OH)(PaPy2N)]+, consistent with a hydrogen bond. Kinetic studies using a variety of substrates with activated O-H bonds, revealed that [MnIII(OH)(PaPy2N)]+ is far more reactive than [MnIII(OH)(PaPy2Q)]+, with rate enhancements of 15-100-fold. A detailed analysis of the thermodynamic contributions to these reactions using DFT computations reveals that the former complex is significantly more basic. This increased basicity counteracts the more negative reduction potential of this complex, leading to a stronger O-H BDFE in the [MnII(OH2)(PaPy2N)]+ product. Thus, the differences in reactivity between [MnIII(OH)(PaPy2Q)]+ and [MnIII(OH)(PaPy2N)]+ can be understood on the basis of thermodynamic considerations, which are strongly influenced by the ability of the latter complex to form an intramolecular hydrogen bond.
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Affiliation(s)
- Adedamola A Opalade
- The University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
| | - Logan Hessefort
- The University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
| | - Victor W Day
- The University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
| | - Timothy A Jackson
- The University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
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14
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Synthesis, structural and physicochemical properties of a series of manganese(II) complexes with a novel N5 tripodal-amidate ligand and their potential use as water oxidation catalysts. Polyhedron 2021. [DOI: 10.1016/j.poly.2021.115260] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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15
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Mukherjee G, Satpathy JK, Bagha UK, Mubarak MQE, Sastri CV, de Visser SP. Inspiration from Nature: Influence of Engineered Ligand Scaffolds and Auxiliary Factors on the Reactivity of Biomimetic Oxidants. ACS Catal 2021. [DOI: 10.1021/acscatal.1c01993] [Citation(s) in RCA: 24] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Affiliation(s)
- Gourab Mukherjee
- Department of Chemistry, Indian Institute of Technology Guwahati, 781039, Assam, India
| | - Jagnyesh K. Satpathy
- Department of Chemistry, Indian Institute of Technology Guwahati, 781039, Assam, India
| | - Umesh K. Bagha
- Department of Chemistry, Indian Institute of Technology Guwahati, 781039, Assam, India
| | - M. Qadri E. Mubarak
- Manchester Institute of Biotechnology, The University of Manchester, 131 Princess Street, Manchester M1 7DN, United Kingdom
- Fakulti Sains dan Teknologi, Universiti Sains Islam Malaysia, Bandar Baru Nilai, 71800 Nilai, Negeri Sembilan Malaysia
| | - Chivukula V. Sastri
- Department of Chemistry, Indian Institute of Technology Guwahati, 781039, Assam, India
| | - Sam P. de Visser
- Department of Chemistry, Indian Institute of Technology Guwahati, 781039, Assam, India
- Manchester Institute of Biotechnology, The University of Manchester, 131 Princess Street, Manchester M1 7DN, United Kingdom
- Department of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
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16
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Crossland PM, Guo Y, Que L. Spontaneous Formation of an Fe/Mn Diamond Core: Models for the Fe/Mn Sites in Class 1c Ribonucleotide Reductases. Inorg Chem 2021; 60:8710-8721. [PMID: 34110143 PMCID: PMC8997264 DOI: 10.1021/acs.inorgchem.1c00684] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
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 compounds can be analyzed by EPR, revealing S = 1/2 signals that reflect antiferromagnetic coupling between the high-spin Fe(III) and Mn(III) units of heterobimetallic centers. These compounds undergo electron transfer with various ferrocenes, linear compounds being capable of oxidizing diacetyl ferrocene, a weak reductant, and diamond core compounds being capable of oxidizing acetyl ferrocene. Diamond core compounds can also perform HAT reactions from substrates with X-H bonds with bond dissociation free energies (BDFEs) up to 75 kcal/mol and are capable of oxidizing TEMPO-H at rates of 0.32-0.37 M-1 s-1, which are comparable to those reported for some mononuclear FeIII-OH and MnIII-OH compounds. However, such reactivity is not observed for the corresponding diiron compounds, a difference that Nature may have taken advantage of in evolving enzymes with heterobimetallic active sites.
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Affiliation(s)
- Patrick M. Crossland
- Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United States
| | - Yisong Guo
- Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
| | - Lawrence Que
- Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United States
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17
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Wong KH, Cheung WM, Pham HL, So YM, Sung HHY, Williams ID, Leung WH. Oxidizing Cerium(IV) Alkoxide Complexes Supported by the Kläui Ligand [Co(η 5-C 5H 5){P(O)(OEt) 2} 3] -: Synthesis, Structure, and Redox Reactivity. Inorg Chem 2021; 60:2261-2270. [PMID: 33499604 DOI: 10.1021/acs.inorgchem.0c03105] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Tetravalent cerium alkoxide complexes supported by the Kläui tripodal ligand [Co(η5-C5H5){P(O)(OEt)2}3]- (LOEt-) have been synthesized, and their nucleophilic and redox reactivity have been studied. Treatment of the Ce(IV) oxo complex [CeIV(LOEt)2(O)(H2O)]·MeCONH2 (1) with iPrOH or reaction of [CeIV(LOEt)2Cl2] (2) with Ag2O in iPrOH afforded the Ce(IV) dialkoxide complex [CeIV(LOEt)2(OiPr)2] (3-iPr). The methoxide and ethoxide analogues [CeIV(LOEt)2(OR)2] (R = Me (3-Me), Et (3-Et)) have been prepared similarly from 2 and Ag2O in ROH. Reaction of 3-iPr with an equimolar amount of 2 yielded a new Ce(IV) complex that was formulated as the chloro-alkoxide complex [CeIV(LOEt)2(OiPr)Cl] (4). Treatment of 3-iPr with HX and methyl triflate (MeOTf) afforded [Ce(LOEt)2X2] (X- = Cl-, NO3-, PhO-) and [CeIV(LOEt)2(OTf)2], respectively, whereas treatment with excess CO2 in hexane led to isolation of the Ce(IV) carbonate [CeIV(LOEt)2(CO3)]. 3-iPr reacted with water in hexane to give a Ce(III) complex and a Ce(IV) species, presumably the reported tetranuclear oxo cluster [CeIV4(LOEt)4(O)5(OH)2]. The Ce(IV) alkoxide complexes are capable of oxidizing substituted phenols, possibly via a proton-coupled electron transfer pathway. Treatment of 3-iPr with ArOH afforded the Ce(III) aryloxide complexes [CeIII(LOEt)2(OAr)] (Ar = 2,4,6-tri-tert-butylphenyl (5), 2,6-diphenylphenyl (6)). On the other hand, a Ce(III) complex containing a monodeprotonated 2,2'-biphenol ligand, [CeIII(LOEt)2(tBu4C12H4O2H)] (7) (tBu4C12H4O2H2 = 4,4',6,6'-tetra-tert-butyl-2,2'-biphenol), was isolated from the reaction of 3-iPr with 2,4-di-tert-butylphenol. The crystal structures of complexes 3-iPr, 3-Me, 3-Et, and 5-7 have been determined.
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Affiliation(s)
- Kai-Hong Wong
- Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
| | - Wai-Man Cheung
- Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
| | - Hoang-Long Pham
- Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
| | - Yat-Ming So
- Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
| | - Herman H-Y Sung
- Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
| | - Ian D Williams
- Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
| | - Wa-Hung Leung
- Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
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18
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Mayfield JR, Grotemeyer EN, Jackson TA. Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes. Chem Commun (Camb) 2020; 56:9238-9255. [PMID: 32578605 PMCID: PMC7429365 DOI: 10.1039/d0cc01201g] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.
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Affiliation(s)
- Jaycee R Mayfield
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, KS 66045, USA.
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19
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Rice DB, Grotemeyer EN, Donovan AM, Jackson TA. Effect of Lewis Acids on the Structure and Reactivity of a Mononuclear Hydroxomanganese(III) Complex. Inorg Chem 2020; 59:2689-2700. [PMID: 32045220 DOI: 10.1021/acs.inorgchem.9b02980] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
The addition of Sc(OTf)3 and Al(OTf)3 to the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq)]+ (1) gives rise to new intermediates with spectroscopic properties and chemical reactivity distinct from those of [MnIII(OH)(dpaq)]+. The electronic absorption spectra of [MnIII(OH)(dpaq)]+ in the presence of Sc(OTf)3 (1-ScIII) and Al(OTf)3 (1-AlIII) show modest perturbations in electronic transition energies, consistent with moderate changes in the MnIII geometry. A comparison of 1H NMR data for 1 and 1-ScIII confirm this conclusion, as the 1H NMR spectrum of 1-ScIII shows the same number of hyperfine-shifted peaks as the 1H NMR spectrum of 1. These 1H NMR spectra, and that of 1-AlIII, share a similar chemical-shift pattern, providing firm evidence that these Lewis acids do not cause gross distortions to the structure of 1. Mn K-edge X-ray absorption data for 1-ScIII provide evidence of elongation of the axial Mn-OH and Mn-N(amide) bonds relative to those of 1. In contrast to these modest spectroscopic perturbations, 1-ScIII and 1-AlIII show greatly enhanced reactivity toward hydrocarbons. While 1 is unreactive toward 9,10-dihydroanthracene (DHA), 1-ScIII and 1-AlIII react rapidly with DHA (k2 = 0.16(1) and 0.25(2) M-1 s-1 at 50 °C, respectively). The 1-ScIII species is capable of attacking the much stronger C-H bond of ethylbenzene. The basis for these perturbations to the spectroscopic properties and reactivity of 1 in the presence of these Lewis acids was elucidated by comparing properties of 1-ScIII and 1-AlIII with the recently reported MnIII-aqua complex [MnIII(OH2)(dpaq)]2+ ( J. Am. Chem. Soc. 2018, 140, 12695-12699). Because 1-ScIII and 1-AlIII show 1H NMR spectra essentially identical to that of [MnIII(OH2)(dpaq)]2+, the primary effect of these Lewis acids on 1 is protonation of the hydroxo ligand caused by an increase in the Brønsted acidity of the solution.
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Affiliation(s)
- Derek B Rice
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, The University of Kansas, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
| | - Elizabeth N Grotemeyer
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, The University of Kansas, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
| | - Anna M Donovan
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, The University of Kansas, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
| | - Timothy A Jackson
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, The University of Kansas, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States
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20
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Zaragoza JPT, Cummins DC, Mubarak MQE, Siegler MA, de Visser SP, Goldberg DP. Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies. Inorg Chem 2019; 58:16761-16770. [PMID: 31804814 DOI: 10.1021/acs.inorgchem.9b02923] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the FeIV(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of FeIV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex 1 reacts with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants k2 = 3.6(1)-1.21(3) × 104 M-1 s-1 and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O-H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of 1 to the analogous Mn complex, MnIV(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope for 1. The O-H bond dissociation energy (BDE) of the MIII(HO-H) complexes were estimated from a kinetic analysis to be 85 and 89 kcal mol-1 for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are discussed in the context of how they influence the overall H atom transfer reactivity.
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Affiliation(s)
- Jan Paulo T Zaragoza
- Department of Chemistry , The Johns Hopkins University , 3400 North Charles Street , Baltimore , Maryland 21218 , United States
| | - Daniel C Cummins
- Department of Chemistry , The Johns Hopkins University , 3400 North Charles Street , Baltimore , Maryland 21218 , United States
| | - M Qadri E Mubarak
- The Manchester Institute of Biotechnology and Department of Chemical Engineering and Analytical Science , The University of Manchester , 131 Princess Street , Manchester M1 7DN , United Kingdom
| | - Maxime A Siegler
- Department of Chemistry , The Johns Hopkins University , 3400 North Charles Street , Baltimore , Maryland 21218 , United States
| | - Sam P de Visser
- The Manchester Institute of Biotechnology and Department of Chemical Engineering and Analytical Science , The University of Manchester , 131 Princess Street , Manchester M1 7DN , United Kingdom
| | - David P Goldberg
- Department of Chemistry , The Johns Hopkins University , 3400 North Charles Street , Baltimore , Maryland 21218 , United States
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21
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Parham JD, Wijeratne GB, Mayfield JR, Jackson TA. Steric control of dioxygen activation pathways for Mn II complexes supported by pentadentate, amide-containing ligands. Dalton Trans 2019; 48:13034-13045. [PMID: 31406966 PMCID: PMC6733413 DOI: 10.1039/c9dt02682g] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
Dioxygen activation at manganese centers is well known in nature, but synthetic manganese systems capable of utilizing O2 as an oxidant are relatively uncommon. These present investigations probe the dioxygen activation pathways of two mononuclear MnII complexes supported by pentacoordinate amide-containing ligands, [MnII(dpaq)](OTf) and the sterically modified [MnII(dpaq2Me)](OTf). Dioxygen titration experiments demonstrate that [MnII(dpaq)](OTf) reacts with O2 to form [MnIII(OH)(dpaq)](OTf) according to a 4 : 1 Mn : O2 stoichiometry. This stoichiometry is consistent with a pathway involving comproportionation between a MnIV-oxo species and residual MnII complex to form a (μ-oxo)dimanganese(iii,iii) species that is hydrolyzed by water to give the MnIII-hydroxo product. In contrast, the sterically modified [MnII(dpaq2Me)](OTf) complex was found to react with O2 according to a 2 : 1 Mn : O2 stoichiometry. This stoichiometry is indicative of a pathway in which a MnIV-oxo intermediate abstracts a hydrogen atom from solvent instead of undergoing comproportionation with the MnII starting complex. Isotopic labeling experiments, in which the oxygenation of the MnII complexes was carried out in deuterated solvent, supported this change in pathway. The oxygenation of [MnII(dpaq)](OTf) did not result in any deuterium incorporation in the MnIII-hydroxo product, while the oxygenation of [MnII(dpaq2Me)](OTf) in d3-MeCN showed [MnIII(OD)(dpaq2Me)]+ formation. Taken together, these observations highlight the use of steric effects as a means to select which intermediates form along dioxygen activation pathways.
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Affiliation(s)
- Joshua D Parham
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, USA.
| | - Gayan B Wijeratne
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, USA.
| | - Jaycee R Mayfield
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, USA.
| | - Timothy A Jackson
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, USA.
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22
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Banerjee A, Tolla AS, Stjepanovic S, Sevilla MD, Goodsell JL, Angerhofer A, Brennessel WW, Loloee R, Chavez FA. Structural, Spectroscopic, Electrochemical, and Magnetic Properties for Manganese(II) Triazamacrocyclic Complexes. Inorganica Chim Acta 2019; 486:546-555. [PMID: 33981118 PMCID: PMC8112617 DOI: 10.1016/j.ica.2018.11.013] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
We report the synthesis of [Mn(tacud)2](OTf)2 (1) (tacud = 1,4,8-triazacycloundecane), [Mn(tacd)2](OTf)2 (2) (tacd = 1,4,7-triazacyclodecane), and [Mn(tacn)2](OTf)2 (3) (tacn = 1,4,7-triazacyclononane). Electrochemical measurements on the MnIII/II redox couple show that complex 1 has the largest anodic potential of the set (E 1/2 = 1.16 V vs NHE, ΔE p = 106 mV) compared to 2 (E 1/2 = 0.95 V, ΔE p = 108 mV) and 3 (E 1/2 = 0.93 V, ΔE p = 96 mV). This is due to the fact that 1 has the fewest 5-membered chelate rings and thus is least stabilized. Magnetic studies of 1-3 revealed that all complexes remain high spin throughout the temperature range investigated (2 - 300 K). X-band EPR investigations in methanol glass indicated that the manganese(II) centers for 2 and 3 resided in a more distorted octahedral geometric configuration compared to 1. To ease spectral interpretation and extract ZFS parameters, we performed high-frequency high-field EPR (HFEPR) at frequencies above 200 GHz and a field of 7.5 T. Simulation of the spectral data yielded g = 2.0013 and D = -0.031 cm-1 for 1, g = 2.0008, D = -0.0824 cm-1, |E/D| = 0.12 for 2, and g = 2.00028, D = -0.0884 cm-1 for 3. These results are consistent with 3 possessing the most distorted geometry. Calculations (PBE0/6-31G(d)) were performed on 1-3. Results show that 1 has the largest HOMO-LUMO gap energy (6.37 eV) compared to 2 (6.12 eV) and 3 (6.26 eV). Complex 1 also has the lowest HOMO energies indicating higher stability.
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Affiliation(s)
- Atanu Banerjee
- Department of Chemistry, Oakland University, Rochester, MI 48309-4477, USA
| | - Azam S Tolla
- Department of Chemistry, Oakland University, Rochester, MI 48309-4477, USA
| | | | - Michael D Sevilla
- Department of Chemistry, Oakland University, Rochester, MI 48309-4477, USA
| | - Justin L Goodsell
- Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
| | | | | | - Reza Loloee
- Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-1322, USA
| | - Ferman A Chavez
- Department of Chemistry, Oakland University, Rochester, MI 48309-4477, USA
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23
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Liu Y, Lau TC. Activation of Metal Oxo and Nitrido Complexes by Lewis Acids. J Am Chem Soc 2019; 141:3755-3766. [DOI: 10.1021/jacs.8b13100] [Citation(s) in RCA: 51] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Affiliation(s)
- Yingying Liu
- Department of Chemistry and Institute of Molecular Functional Materials, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong 999077, China
| | - Tai-Chu Lau
- Department of Chemistry and Institute of Molecular Functional Materials, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong 999077, China
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24
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Rice DB, Munasinghe A, Grotemeyer EN, Burr AD, Day VW, Jackson TA. Structure and Reactivity of (μ-Oxo)dimanganese(III,III) and Mononuclear Hydroxomanganese(III) Adducts Supported by Derivatives of an Amide-Containing Pentadentate Ligand. Inorg Chem 2019; 58:622-636. [PMID: 30525518 DOI: 10.1021/acs.inorgchem.8b02794] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Mononuclear MnIII-hydroxo and dinuclear (μ-oxo)dimanganese(III,III) complexes were prepared using derivatives of the pentadentate, amide-containing dpaq ligand (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate). Each of these ligand derivatives (referred to as dpaq5R) contained a substituent R (where R = OMe, Cl, and NO2) at the 5-position of the quinolinyl group. Generation of the MnIII complexes was achieved by either O2 oxidation of MnII precursors (for [MnII(dpaq5OMe)]+ and [MnII(dpaq5Cl)]+ or PhIO oxidation (for [MnII(dpaq5NO2)]+). For each oxidized complex, 1H NMR experiments provided evidence of a water-dependent equilibrium between paramagnetic [MnIII(OH)(dpaq5R)]+ and an antiferromagnetically coupled [MnIIIMnIII(μ-O)(dpaq5R)2]2+ species in acetonitrile, with the addition of water favoring the MnIII-hydroxo species. This conversion could also be monitored by electronic absorption spectroscopy. Solid-state X-ray crystal structures for each [MnIIIMnIII(μ-O)(dpaq5R)2](OTf)2 complex revealed a nearly linear Mn-O-Mn core (angle of ca. 177°), with short Mn-O distances near 1.79 Å, and a Mn···Mn separation of 3.58 Å. X-ray crystallographic information was also obtained for the mononuclear [MnIII(OH)(dpaq5Cl)](OTf) complex, which has a short Mn-O(H) distance of 1.810(2) Å. The influence of the 5-substituted quinolinyl moiety on the electronic properties of the [MnIII(OH)(dpaq5R)]+ complexes was demonstrated through shifts in a number of 1H NMR resonances, as well as a steady increase in the MnIII/II cyclic voltammetry peak potential in the order [MnIII(OH)(dpaq5OMe)]+ < [MnIII(OH)(dpaq)]+ < [MnIII(OH)(dpaq5Cl)]+ < [MnIII(OH)(dpaq5NO2)]+. These changes in oxidizing power of the MnIII-hydroxo adducts translated to only modest rate enhancements for TEMPOH oxidation by the [MnIII(OH)(dpaq5R)]+ complexes, with the most reactive [MnIII(OH)(dpaq5NO2)]+ complex showing a second-order rate constant only 9-fold larger than that of the least reactive [MnIII(OH)(dpaq5OMe)]+ complex. These modest rate changes were understood on the basis of density functional theory (DFT)-computed p Ka values for the corresponding [MnII(OH2)(dpaq5R)]+ complexes. Collectively, the experimental and DFT results reveal that the 5-substituted quinolinyl groups have an inverse influence on electron and proton affinity for the MnIII-hydroxo unit.
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Affiliation(s)
- Derek B Rice
- Department of Chemistry , University of Kansas , Lawrence , Kansas 66045 , United States
| | - Aruna Munasinghe
- Department of Chemistry , University of Kansas , Lawrence , Kansas 66045 , United States
| | - Elizabeth N Grotemeyer
- Department of Chemistry , University of Kansas , Lawrence , Kansas 66045 , United States
| | - Andrew D Burr
- Department of Chemistry , University of Kansas , Lawrence , Kansas 66045 , United States
| | - Victor W Day
- Department of Chemistry , University of Kansas , Lawrence , Kansas 66045 , United States
| | - Timothy A Jackson
- Department of Chemistry , University of Kansas , Lawrence , Kansas 66045 , United States
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25
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Sankaralingam M, Lee YM, Karmalkar DG, Nam W, Fukuzumi S. A Mononuclear Non-heme Manganese(III)–Aqua Complex as a New Active Oxidant in Hydrogen Atom Transfer Reactions. J Am Chem Soc 2018; 140:12695-12699. [DOI: 10.1021/jacs.8b07772] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Affiliation(s)
| | - Yong-Min Lee
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Deepika G. Karmalkar
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Wonwoo Nam
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
| | - Shunichi Fukuzumi
- Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
- Faculty of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502, Japan
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26
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Jeong D, Yan JJ, Noh H, Hedman B, Hodgson KO, Solomon EI, Cho J. Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid. Angew Chem Int Ed Engl 2018. [DOI: 10.1002/ange.201802641] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Affiliation(s)
- Donghyun Jeong
- Department of Emerging Materials Science DGIST Daegu 42988 Korea
| | - James J. Yan
- Department of Chemistry Stanford University Stanford CA 94305 USA
| | - Hyeonju Noh
- Department of Emerging Materials Science DGIST Daegu 42988 Korea
| | - Britt Hedman
- Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Menlo Park CA 94025 USA
| | - Keith O. Hodgson
- Department of Chemistry Stanford University Stanford CA 94305 USA
- Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Menlo Park CA 94025 USA
| | - Edward I. Solomon
- Department of Chemistry Stanford University Stanford CA 94305 USA
- Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Menlo Park CA 94025 USA
| | - Jaeheung Cho
- Department of Emerging Materials Science DGIST Daegu 42988 Korea
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Jeong D, Yan JJ, Noh H, Hedman B, Hodgson KO, Solomon EI, Cho J. Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid. Angew Chem Int Ed Engl 2018; 57:7764-7768. [PMID: 29701293 PMCID: PMC6013404 DOI: 10.1002/anie.201802641] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2018] [Revised: 04/05/2018] [Indexed: 11/06/2022]
Abstract
Naphthalene oxidation with metal-oxygen intermediates is a difficult reaction in environmental and biological chemistry. Herein, we report that a MnIV bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as ESI-MS, UV/Vis, and EPR analysis, X-ray diffraction, and XAS, can be employed for the oxidation of naphthalene in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the MnIV bis(hydroxo) complex gave a one-electron reduction potential of 1.09 V, which is the most positive potential for all reported nonheme MnIV bis(hydroxo) species as well as MnIV oxo analogues. Kinetic studies, including kinetic isotope effect analysis, suggest that the naphthalene oxidation occurs through a rate-determining electron transfer process.
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Affiliation(s)
- Donghyun Jeong
- Department of Emerging Materials Science, DGIST, Daegu, 42988, Korea
| | - James J Yan
- Department of Chemistry, Stanford University, Stanford, CA, 94305, USA
| | - Hyeonju Noh
- Department of Emerging Materials Science, DGIST, Daegu, 42988, Korea
| | - Britt Hedman
- Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA, 94025, USA
| | - Keith O Hodgson
- Department of Chemistry, Stanford University, Stanford, CA, 94305, USA
- Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA, 94025, USA
| | - Edward I Solomon
- Department of Chemistry, Stanford University, Stanford, CA, 94305, USA
- Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA, 94025, USA
| | - Jaeheung Cho
- Department of Emerging Materials Science, DGIST, Daegu, 42988, Korea
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28
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Rice DB, Jones SD, Douglas JT, Jackson TA. NMR Studies of a MnIII-hydroxo Adduct Reveal an Equilibrium between MnIII-hydroxo and μ-Oxodimanganese(III,III) Species. Inorg Chem 2018; 57:7825-7837. [DOI: 10.1021/acs.inorgchem.8b00917] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- Derek B. Rice
- Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States
| | - Shannon D. Jones
- Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States
| | - Justin T. Douglas
- Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States
| | - Timothy A. Jackson
- Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States
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29
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Parham JD, Wijeratne GB, Rice DB, Jackson TA. Spectroscopic and Structural Characterization of Mn(III)-Alkylperoxo Complexes Supported by Pentadentate Amide-Containing Ligands. Inorg Chem 2018; 57:2489-2502. [DOI: 10.1021/acs.inorgchem.7b02793] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Joshua D. Parham
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
| | - Gayan B. Wijeratne
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
| | - Derek B. Rice
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
| | - Timothy A. Jackson
- Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
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30
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Affiliation(s)
- Paolo Pirovano
- School of Chemistry and CRANN/AMBER Nanoscience Institute; Trinity College Dublin; The University of Dublin; College Green 2 Dublin Ireland
| | - Aidan R. McDonald
- School of Chemistry and CRANN/AMBER Nanoscience Institute; Trinity College Dublin; The University of Dublin; College Green 2 Dublin Ireland
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31
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Rice DB, Massie AA, Jackson TA. Manganese-Oxygen Intermediates in O-O Bond Activation and Hydrogen-Atom Transfer Reactions. Acc Chem Res 2017; 50:2706-2717. [PMID: 29064667 DOI: 10.1021/acs.accounts.7b00343] [Citation(s) in RCA: 46] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Biological systems capitalize on the redox versatility of manganese to perform reactions involving dioxygen and its derivatives superoxide, hydrogen peroxide, and water. The reactions of manganese enzymes influence both human health and the global energy cycle. Important examples include the detoxification of reactive oxygen species by manganese superoxide dismutase, biosynthesis by manganese ribonucleotide reductase and manganese lipoxygenase, and water splitting by the oxygen-evolving complex of photosystem II. Although these enzymes perform very different reactions and employ structurally distinct active sites, manganese intermediates with peroxo, hydroxo, and oxo ligation are commonly proposed in catalytic mechanisms. These intermediates are also postulated in mechanisms of synthetic manganese oxidation catalysts, which are of interest due to the earth abundance of manganese. In this Account, we describe our recent efforts toward understanding O-O bond activation pathways of MnIII-peroxo adducts and hydrogen-atom transfer reactivity of MnIV-oxo and MnIII-hydroxo complexes. In biological and synthetic catalysts, peroxomanganese intermediates are commonly proposed to decay by either Mn-O or O-O cleavage pathways, although it is often unclear how the local coordination environment influences the decay mechanism. To address this matter, we generated a variety of MnIII-peroxo adducts with varied ligand environments. Using parallel-mode EPR and Mn K-edge X-ray absorption techniques, the decay pathway of one MnIII-peroxo complex bearing a bulky macrocylic ligand was investigated. Unlike many MnIII-peroxo model complexes that decay to oxo-bridged-MnIIIMnIV dimers, decay of this MnIII-peroxo adduct yielded mononuclear MnIII-hydroxo and MnIV-oxo products, potentially resulting from O-O bond activation of the MnIII-peroxo unit. These results highlight the role of ligand sterics in promoting the formation of mononuclear products and mark an important step in designing MnIII-peroxo complexes that convert cleanly to high-valent Mn-oxo species. Although some synthetic MnIV-oxo complexes show great potential for oxidizing substrates with strong C-H bonds, most MnIV-oxo species are sluggish oxidants. Both two-state reactivity and thermodynamic arguments have been put forth to explain these observations. To address these issues, we generated a series of MnIV-oxo complexes supported by neutral, pentadentate ligands with systematically perturbed equatorial donation. Kinetic investigations of these complexes revealed a correlation between equatorial ligand-field strength and hydrogen-atom and oxygen-atom transfer reactivity. While this trend can be understood on the basis of the two-state reactivity model, the reactivity trend also correlates with variations in MnIII/IV reduction potential caused by changes in the ligand field. This work demonstrates the dramatic influence simple ligand perturbations can have on reactivity but also illustrates the difficulties in understanding the precise basis for a change in reactivity. In the enzyme manganese lipoxygenase, an active-site MnIII-hydroxo adduct initiates substrate oxidation by abstracting a hydrogen atom from a C-H bond. Precedent for this chemistry from synthetic MnIII-hydroxo centers is rare. To better understand hydrogen-atom transfer by MnIII centers, we developed a pair of MnIII-hydroxo complexes, formed in high yield from dioxygen oxidation of MnII precursors, capable of attacking weak O-H and C-H bonds. Kinetic and computational studies show a delicate interplay between thermodynamic and steric influences in hydrogen-atom transfer reactivity, underscoring the potential of MnIII-hydroxo units as mild oxidants.
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Affiliation(s)
- Derek B. Rice
- Department of Chemistry and
Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
| | - Allyssa A. Massie
- Department of Chemistry and
Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
| | - Timothy A. Jackson
- Department of Chemistry and
Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
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32
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Mn K-edge X-ray absorption studies of mononuclear Mn(III)–hydroxo complexes. J Biol Inorg Chem 2017; 22:1281-1293. [DOI: 10.1007/s00775-017-1501-0] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2017] [Accepted: 10/13/2017] [Indexed: 11/25/2022]
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33
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Ching WM, Zhou A, Klein JEMN, Fan R, Knizia G, Cramer CJ, Guo Y, Que L. Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand. Inorg Chem 2017; 56:11129-11140. [DOI: 10.1021/acs.inorgchem.7b01459] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
| | | | | | - Ruixi Fan
- Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
| | - Gerald Knizia
- Department
of Chemistry, Pennsylvania State University, 401A Chemistry Bldg; University Park, Pennsylvania 16802, United States
| | | | - Yisong Guo
- Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
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34
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Biological functions controlled by manganese redox changes in mononuclear Mn-dependent enzymes. Essays Biochem 2017; 61:259-270. [PMID: 28487402 DOI: 10.1042/ebc20160070] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2016] [Revised: 03/05/2017] [Accepted: 03/17/2017] [Indexed: 02/06/2023]
Abstract
Remarkably few enzymes are known to employ a mononuclear manganese ion that undergoes changes in redox state during catalysis. Many questions remain to be answered about the role of substrate binding and/or protein environment in modulating the redox properties of enzyme-bound Mn(II), the nature of the dioxygen species involved in the catalytic mechanism, and how these enzymes acquire Mn(II) given that many other metal ions in the cell form more stable protein complexes. Here, we summarize current knowledge concerning the structure and mechanism of five mononuclear manganese-dependent enzymes: superoxide dismutase, oxalate oxidase (OxOx), oxalate decarboxylase (OxDC), homoprotocatechuate 3,4-dioxygenase, and lipoxygenase (LOX). Spectroscopic measurements and/or computational studies suggest that Mn(III)/Mn(II) are the catalytically active oxidation states of the metal, and the importance of 'second-shell' hydrogen bonding interactions with metal ligands has been demonstrated for a number of examples. The ability of these enzymes to modulate the redox properties of the Mn(III)/Mn(II) couple, thereby allowing them to generate substrate-based radicals, appears essential for accessing diverse chemistries of fundamental importance to organisms in all branches of life.
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35
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Gao H, Groves JT. Fast Hydrogen Atom Abstraction by a Hydroxo Iron(III) Porphyrazine. J Am Chem Soc 2017; 139:3938-3941. [PMID: 28245648 DOI: 10.1021/jacs.6b13091] [Citation(s) in RCA: 44] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
A reactive hydroxoferric porphyrazine complex, [(PyPz)FeIII(OH) (OH2)]4+ (1, PyPz = tetramethyl-2,3-pyridino porphyrazine), has been prepared via one-electron oxidation of the corresponding ferrous species [(PyPz)FeII(OH2)2]4+ (2). Electrochemical analysis revealed a pH-dependent and remarkably high FeIII-OH/FeII-OH2 reduction potential of 680 mV vs Ag/AgCl at pH 5.2. Nernstian behavior from pH 2 to pH 8 indicates a one-proton, one-electron interconversion throughout that range. The O-H bond dissociation energy of the FeII-OH2 complex was estimated to be 84 kcal mol-1. Accordingly, 1 reacts rapidly with a panel of substrates via C-H hydrogen atom transfer (HAT), reducing 1 to [(PyPz)FeII(OH2)2]4+ (2). The second-order rate constant for the reaction of [(PyPz)FeIII(OH) (OH2)]4+ with xanthene was 2.22 × 103 M-1 s-1, 5-6 orders of magnitude faster than other reported FeIII-OH complexes and faster than many ferryl complexes.
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Affiliation(s)
- Hongxin Gao
- Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States
| | - John T Groves
- Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States
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