1
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Hatakeyama-Sato K, Oyaizu K. Redox: Organic Robust Radicals and Their Polymers for Energy Conversion/Storage Devices. Chem Rev 2023; 123:11336-11391. [PMID: 37695670 DOI: 10.1021/acs.chemrev.3c00172] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/12/2023]
Abstract
Persistent radicals can hold their unpaired electrons even under conditions where they accumulate, leading to the unique characteristics of radical ensembles with open-shell structures and their molecular properties, such as magneticity, radical trapping, catalysis, charge storage, and electrical conductivity. The molecules also display fast, reversible redox reactions, which have attracted particular attention for energy conversion and storage devices. This paper reviews the electrochemical aspects of persistent radicals and the corresponding macromolecules, radical polymers. Radical structures and their redox reactions are introduced, focusing on redox potentials, bistability, and kinetic constants for electrode reactions and electron self-exchange reactions. Unique charge transport and storage properties are also observed with the accumulated form of redox sites in radical polymers. The radical molecules have potential electrochemical applications, including in rechargeable batteries, redox flow cells, photovoltaics, diodes, and transistors, and in catalysts, which are reviewed in the last part of this paper.
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Affiliation(s)
- Kan Hatakeyama-Sato
- School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku Tokyo 152-8552, Japan
- Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan
| | - Kenichi Oyaizu
- Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan
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2
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Rettig ID, Xu J, Knight EA, Truong PT, Bowring MA. Variable Kinetic Isotope Effect Reveals a Multistep Pathway for Protonolysis of a Pt–Me Bond. Organometallics 2022. [DOI: 10.1021/acs.organomet.2c00504] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Affiliation(s)
- Irving D. Rettig
- Department of Chemistry, Reed College, Portland, Oregon97202, United States
| | - Jingtong Xu
- Department of Chemistry, Reed College, Portland, Oregon97202, United States
| | | | - Phan T. Truong
- Department of Chemistry, Reed College, Portland, Oregon97202, United States
| | - Miriam A. Bowring
- Department of Chemistry, Reed College, Portland, Oregon97202, United States
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3
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Abstract
Coordination-induced bond weakening is a phenomenon wherein ligand X-H bond homolysis occurs in concert with the energetically favorable oxidation of a coordinating metal complex. The coupling of these two processes enables thermodynamically favorable proton-coupled electron transfer reductions to form weak bonds upon formal hydrogen atom transfer to substrates. Moreover, systems utilizing coordination-induced bond weakening have been shown to facilitate the dehydrogenation of feedstock molecules including water, ammonia, and primary alcohols under mild conditions. The formation of exceptionally weak substrate X-H bonds via small molecule homolysis is a powerful strategy in synthesis and has been shown to enable nitrogen fixation under mild conditions. Coordination-induced bond weakening has also been identified as an integral process in biophotosynthesis and has promising applications in renewable chemical fuel storage systems. This review presents a discussion of the advances made in the study of coordination-induced bond weakening to date. Because of the broad range of metal and ligand species implicated in coordination-induced bond weakening, each literature report is discussed individually and ordered by the identity of the low-valent metal. We then offer mechanistic insights into the basis of coordination-induced bond weakening and conclude with a discussion of opportunities for further research into the development and applications of coordination-induced bond weakening systems.
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Affiliation(s)
- Nicholas G Boekell
- Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States
| | - Robert A Flowers
- Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States
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4
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Truong PT, Miller SG, McLaughlin Sta Maria EJ, Bowring MA. Large Isotope Effects in Organometallic Chemistry. Chemistry 2021; 27:14800-14815. [PMID: 34347912 DOI: 10.1002/chem.202102189] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2021] [Indexed: 01/24/2023]
Abstract
The kinetic isotope effect (KIE) is key to understanding reaction mechanisms in many areas of chemistry and chemical biology, including organometallic chemistry. This ratio of rate constants, kH /kD , typically falls between 1-7. However, KIEs up to 105 have been reported, and can even be so large that reactivity with deuterium is unobserved. We collect here examples of large KIEs across organometallic chemistry, in catalytic and stoichiometric reactions, along with their mechanistic interpretations. Large KIEs occur in proton transfer reactions such as protonation of organometallic complexes and clusters, protonolysis of metal-carbon bonds, and dihydrogen reactivity. C-H activation reactions with large KIEs occur with late and early transition metals, photogenerated intermediates, and abstraction by metal-oxo complexes. We categorize the mechanistic interpretations of large KIEs into the following three types: (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step. This comprehensive collection of large KIEs in organometallics provides context for future mechanistic interpretation.
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Affiliation(s)
- Phan T Truong
- Department of Chemistry, Reed College, 3203 SE Woodstock Blvd., Portland, OR 97222
| | - Sophia G Miller
- Department of Chemistry, Reed College, 3203 SE Woodstock Blvd., Portland, OR 97222
| | | | - Miriam A Bowring
- Department of Chemistry, Reed College, 3203 SE Woodstock Blvd., Portland, OR 97222
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5
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Charette BJ, Ziller JW, Heyduk AF. Metal-Ion Influence on Ligand-Centered Hydrogen-Atom Transfer. Inorg Chem 2021; 60:1579-1589. [PMID: 33434022 DOI: 10.1021/acs.inorgchem.0c02981] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The ligand-centered hydrogen-atom-transfer (HAT) reactivity was examined for a family of group 10 metal complexes containing a tridentate pincer ligand derived from bis(2-mercapto-p-tolyl)amine, [SNS]H3. Six new metal complexes of palladium and platinum were synthesized with the [SNS] ligand platform in different redox and protonation states to complete the group 10 series previously reported with nickel. The HAT reactivity was examined for this family of nickel, palladium, and platinum complexes to determine the impact of a metal ion on the ligand-centered reactivity. Thermodynamic measurements revealed that N-H bond dissociation free energies increased by approximately 10 kcal mol-1 along the series Ni < Pd < Pt driven by changes to both the redox potential and pKa of the ligand. Kinetic analyses for all three metal complexes suggest that the barrier to the HAT reactivity is primarily entropic rather than enthalpic for this system.
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Affiliation(s)
- Bronte J Charette
- Department of Chemistry, University of California at Irvine (UCI), Irvine, California 92677-2025, United States
| | - Joseph W Ziller
- Department of Chemistry, University of California at Irvine (UCI), Irvine, California 92677-2025, United States
| | - Alan F Heyduk
- Department of Chemistry, University of California at Irvine (UCI), Irvine, California 92677-2025, United States
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6
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Zhang L, Lu JR, Waigh TA. Electronics of peptide- and protein-based biomaterials. Adv Colloid Interface Sci 2021; 287:102319. [PMID: 33248339 DOI: 10.1016/j.cis.2020.102319] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2020] [Revised: 11/10/2020] [Accepted: 11/11/2020] [Indexed: 12/22/2022]
Abstract
Biologically inspired peptide- and protein-based materials are at the forefront of organic bioelectronics research due to their inherent conduction properties and excellent biocompatibility. Peptides have the advantages of structural simplicity and ease of synthesis providing credible prospects for mass production, whereas naturally expressed proteins offer inspiration with many examples of high performance evolutionary optimised bioelectronics properties. We review recent advances in the fundamental conduction mechanisms, experimental techniques and exemplar applications for the bioelectronics of self-assembling peptides and proteins. Diverse charge transfer processes, such as tunnelling, hopping and coupled transfer, are found in naturally occurring biological systems with peptides and proteins as the predominant building blocks to enable conduction in biology. Both theory and experiments allow detailed investigation of bioelectronic properties in order to design functionalized peptide- and protein-based biomaterials, e.g. to create biocompatible aqueous electrodes. We also highlight the design of bioelectronics devices based on peptides/proteins including field-effect transistors, piezoelectric energy harvesters and optoelectronics.
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Affiliation(s)
- L Zhang
- Biological Physics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
| | - J R Lu
- Biological Physics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK.
| | - T A Waigh
- Biological Physics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK; Photon Science Institute, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK.
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7
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Zhang J, Yang JD, Cheng JP. Diazaphosphinanes as hydride, hydrogen atom, proton or electron donors under transition-metal-free conditions: thermodynamics, kinetics, and synthetic applications. Chem Sci 2020; 11:3672-3679. [PMID: 34094055 PMCID: PMC8152589 DOI: 10.1039/c9sc05883d] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Exploration of new hydrogen donors is in large demand in hydrogenation chemistry. Herein, we developed a new 1,3,2-diazaphosphinane 1a, which can serve as a hydride, hydrogen atom or proton donor without transition-metal mediation. The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in acetonitrile. It is noteworthy that, the reduction potentials (Ered) of the phosphenium cations 1a-[P]+ and 1b-[P]+ are extremely low, being −1.94 and −2.39 V (vs. Fc+/0), respectively, enabling corresponding phosphinyl radicals to function as neutral super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the hydrogen atom transfer from 1a to the 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore, successful applications of these diverse P–H bond energetic parameters in organic syntheses were exemplified, shedding light on more exploitations of these versatile and powerful diazaphosphinane reagents in organic chemistry. A new 1,3,2-diazaphosphinane, serving as a formal hydride, hydrogen-atom or proton donor without transition-metal mediation was exploited thermodynamically and kinetically. And, its promising potentials in versatile syntheses have been demonstrated.![]()
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Affiliation(s)
- Jingjing Zhang
- Center of Basic Molecular Science, Department of Chemistry, Tsinghua University Beijing 100084 China
| | - Jin-Dong Yang
- Center of Basic Molecular Science, Department of Chemistry, Tsinghua University Beijing 100084 China
| | - Jin-Pei Cheng
- Center of Basic Molecular Science, Department of Chemistry, Tsinghua University Beijing 100084 China .,State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University Tianjin 300071 China
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8
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Sakai K, Oisaki K, Kanai M. Identification of Bond‐Weakening Spirosilane Catalyst for Photoredox α‐C−H Alkylation of Alcohols. Adv Synth Catal 2019. [DOI: 10.1002/adsc.201901253] [Citation(s) in RCA: 23] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Affiliation(s)
- Kentaro Sakai
- Graduate School of Pharmaceutical SciencesThe University of Tokyo 7-3-1, Hongo, Bunkyo-ku Tokyo 113-0033 Japan
| | - Kounosuke Oisaki
- Graduate School of Pharmaceutical SciencesThe University of Tokyo 7-3-1, Hongo, Bunkyo-ku Tokyo 113-0033 Japan
| | - Motomu Kanai
- Graduate School of Pharmaceutical SciencesThe University of Tokyo 7-3-1, Hongo, Bunkyo-ku Tokyo 113-0033 Japan
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9
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Wang Q, Zhao HY, Lo PK, Lam WWY, Lau KC, Lau TC. Proton-Coupled O-Atom Transfer in the Oxidation of HSO3– by the Ruthenium Oxo Complex trans-[RuVI(TMC)(O)2]2+ (TMC = 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane). Inorg Chem 2017; 56:12699-12702. [DOI: 10.1021/acs.inorgchem.7b02509] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Qian Wang
- School of Chemical
Engineering, Shandong University of Technology, Zibo 255049, P. R. China
- Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, China
| | - Hong Yan Zhao
- Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, China
| | - Po-Kam Lo
- Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, China
| | - William W. Y. Lam
- Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, China
| | - Kai-Chung Lau
- Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, China
| | - Tai-Chu Lau
- Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, China
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10
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Hao W, Wu X, Sun JZ, Siu JC, MacMillan SN, Lin S. Radical Redox-Relay Catalysis: Formal [3+2] Cycloaddition of N-Acylaziridines and Alkenes. J Am Chem Soc 2017; 139:12141-12144. [DOI: 10.1021/jacs.7b06723] [Citation(s) in RCA: 95] [Impact Index Per Article: 13.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- Wei Hao
- Department of Chemistry and
Chemical Biology, Cornell University, Ithaca, New York 14853, United States
| | - Xiangyu Wu
- Department of Chemistry and
Chemical Biology, Cornell University, Ithaca, New York 14853, United States
| | - James Z. Sun
- Department of Chemistry and
Chemical Biology, Cornell University, Ithaca, New York 14853, United States
| | - Juno C. Siu
- Department of Chemistry and
Chemical Biology, Cornell University, Ithaca, New York 14853, United States
| | - Samantha N. MacMillan
- Department of Chemistry and
Chemical Biology, Cornell University, Ithaca, New York 14853, United States
| | - Song Lin
- Department of Chemistry and
Chemical Biology, Cornell University, Ithaca, New York 14853, United States
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11
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Morris WD, Mayer JM. Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions. J Am Chem Soc 2017; 139:10312-10319. [PMID: 28671470 DOI: 10.1021/jacs.7b03562] [Citation(s) in RCA: 55] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Multiple-site concerted proton-electron transfer (MS-CPET) reactions were studied in a three-component system. 1-Hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) was oxidized to the stable radical TEMPO by electron transfer to ferrocenium oxidants coupled to proton transfer to various pyridine bases. These MS-CPET reactions contrast with the usual reactivity of TEMPOH by hydrogen atom transfer (HAT) to a single e-/H+ acceptor. The three-component reactions proceed by pre-equilibrium formation of a hydrogen-bonded adduct between TEMPOH and the pyridine base, and the adduct is then oxidized by the ferrocenium in a bimolecular MS-CPET step. The second-order rate constants, measured using stopped-flow kinetic techniques, spanned 4 orders of magnitude. An advantage of this system is that the MS-CPET driving force could be independently varied by changing either the pKa of the base or the reduction potential (E°) of the oxidant. Changes in ΔG°MS-CPET from either source had the same effect on the MS-CPET rate constants, and a combined Brønsted plot of ln(kMS-CPET) vs ln(Keq) was linear with a slope of 0.46. These results imply a synchronous concerted mechanism, in which the proton and electron transfer components of the CPET process make equal contributions to the rate constants. The only outliers to the Brønsted correlation are the reactions with sterically hindered pyridines, which apparently hinder the close approach of proton donor and acceptor that facilitates MS-CPET. These three-component reactions are compared with a related HAT reaction of TEMPOH, with the 2,4,6-tri-tert-butylphenoxyl radical. The MS-CPET and HAT oxidations of TEMPOH at the same driving force occurred with similar rate constants. While this is an imperfect comparison, the data suggest that the separation of the proton and electron to different reagents does not significantly inhibit the proton-coupled electron transfer process.
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Affiliation(s)
- Wesley D Morris
- Department of Chemistry, Yale University , New Haven, Connecticut 06511, United States
| | - James M Mayer
- Department of Chemistry, Yale University , New Haven, Connecticut 06511, United States
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12
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Kuijpers PF, Tiekink MJ, Breukelaar WB, Broere DLJ, van Leest NP, van der Vlugt JI, Reek JNH, de Bruin B. Cobalt-Porphyrin-Catalysed Intramolecular Ring-Closing C-H Amination of Aliphatic Azides: A Nitrene-Radical Approach to Saturated Heterocycles. Chemistry 2017; 23:7945-7952. [PMID: 28332743 PMCID: PMC5488222 DOI: 10.1002/chem.201700358] [Citation(s) in RCA: 109] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2017] [Indexed: 12/22/2022]
Abstract
Cobalt-porphyrin-catalysed intramolecular ring-closing C-H bond amination enables direct synthesis of various N-heterocycles from aliphatic azides. Pyrrolidines, oxazolidines, imidazolidines, isoindolines and tetrahydroisoquinoline can be obtained in good to excellent yields in a single reaction step with an air- and moisture-stable catalyst. Kinetic studies of the reaction in combination with DFT calculations reveal a metallo-radical-type mechanism involving rate-limiting azide activation to form the key cobalt(III)-nitrene radical intermediate. A subsequent low barrier intramolecular hydrogen-atom transfer from a benzylic C-H bond to the nitrene-radical intermediate followed by a radical rebound step leads to formation of the desired N-heterocyclic ring products. Kinetic isotope competition experiments are in agreement with a radical-type C-H bond-activation step (intramolecular KIE=7), which occurs after the rate-limiting azide activation step. The use of di-tert-butyldicarbonate (Boc2 O) significantly enhances the reaction rate by preventing competitive binding of the formed amine product. Under these conditions, the reaction shows clean first-order kinetics in both the [catalyst] and the [azide substrate], and is zero-order in [Boc2 O]. Modest enantioselectivities (29-46 % ee in the temperature range of 100-80 °C) could be achieved in the ring closure of (4-azidobutyl)benzene using a new chiral cobalt-porphyrin catalyst equipped with four (1S)-(-)-camphanic-ester groups.
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Affiliation(s)
- Petrus F. Kuijpers
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
| | - Martijn J. Tiekink
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
| | - Willem B. Breukelaar
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
| | - Daniël L. J. Broere
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
| | - Nicolaas P. van Leest
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
| | - Jarl Ivar van der Vlugt
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
| | - Joost N. H. Reek
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
| | - Bas de Bruin
- Van ‘t Hoff Institute for Molecular Sciences (HIMS)University of Amsterdam (UvA)Science Park 9041098 XHAmsterdamThe Netherlands
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13
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Pannwitz A, Prescimone A, Wenger OS. Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents. Eur J Inorg Chem 2017. [DOI: 10.1002/ejic.201601403] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Affiliation(s)
- Andrea Pannwitz
- Department of Chemistry; University of Basel; St. Johanns-Ring 19 and Spitalstrasse 51 4056 Basel Switzerland
| | - Alessandro Prescimone
- Department of Chemistry; University of Basel; St. Johanns-Ring 19 and Spitalstrasse 51 4056 Basel Switzerland
| | - Oliver S. Wenger
- Department of Chemistry; University of Basel; St. Johanns-Ring 19 and Spitalstrasse 51 4056 Basel Switzerland
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14
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Bao JL, Truhlar DG. Variational transition state theory: theoretical framework and recent developments. Chem Soc Rev 2017; 46:7548-7596. [DOI: 10.1039/c7cs00602k] [Citation(s) in RCA: 207] [Impact Index Per Article: 29.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
This article reviews the fundamentals of variational transition state theory (VTST), its recent theoretical development, and some modern applications.
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Affiliation(s)
- Junwei Lucas Bao
- Department of Chemistry
- Chemical Theory Center, and Minnesota Supercomputing Institute
- University of Minnesota
- Minneapolis
- USA
| | - Donald G. Truhlar
- Department of Chemistry
- Chemical Theory Center, and Minnesota Supercomputing Institute
- University of Minnesota
- Minneapolis
- USA
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15
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Mitome H, Ishizuka T, Kotani H, Shiota Y, Yoshizawa K, Kojima T. Mechanistic Insights into C–H Oxidations by Ruthenium(III)-Pterin Complexes: Impact of Basicity of the Pterin Ligand and Electron Acceptability of the Metal Center on the Transition States. J Am Chem Soc 2016; 138:9508-20. [DOI: 10.1021/jacs.6b03785] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Hiroumi Mitome
- Department
of Chemistry, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
| | - Tomoya Ishizuka
- Department
of Chemistry, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
| | - Hiroaki Kotani
- Department
of Chemistry, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
| | - Yoshihito Shiota
- Institute
for Materials Chemistry and Engineering, Kyushu University, Motooka, Nishi-Ku, Fukuoka 819-0395, Japan
| | - Kazunari Yoshizawa
- Institute
for Materials Chemistry and Engineering, Kyushu University, Motooka, Nishi-Ku, Fukuoka 819-0395, Japan
| | - Takahiko Kojima
- Department
of Chemistry, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
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16
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Chciuk TV, Anderson WR, Flowers RA. Proton-Coupled Electron Transfer in the Reduction of Carbonyls by Samarium Diiodide–Water Complexes. J Am Chem Soc 2016; 138:8738-41. [DOI: 10.1021/jacs.6b05879] [Citation(s) in RCA: 60] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Affiliation(s)
- Tesia V. Chciuk
- Department of Chemistry, Lehigh University, 6 E. Packer Avenue, Bethlehem, Pennsylvania 18015, United States
| | - William R. Anderson
- Department of Chemistry, Lehigh University, 6 E. Packer Avenue, Bethlehem, Pennsylvania 18015, United States
| | - Robert A. Flowers
- Department of Chemistry, Lehigh University, 6 E. Packer Avenue, Bethlehem, Pennsylvania 18015, United States
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17
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Miller DC, Tarantino KT, Knowles RR. Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities. Top Curr Chem (Cham) 2016; 374:30. [PMID: 27573270 PMCID: PMC5107260 DOI: 10.1007/s41061-016-0030-6] [Citation(s) in RCA: 78] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2015] [Accepted: 04/21/2016] [Indexed: 10/21/2022]
Abstract
Proton-coupled electron transfers (PCETs) are unconventional redox processes in which both protons and electrons are exchanged, often in a concerted elementary step. While PCET is now recognized to play a central a role in biological redox catalysis and inorganic energy conversion technologies, its applications in organic synthesis are only beginning to be explored. In this chapter, we aim to highlight the origins, development, and evolution of the PCET processes most relevant to applications in organic synthesis. Particular emphasis is given to the ability of PCET to serve as a non-classical mechanism for homolytic bond activation that is complimentary to more traditional hydrogen atom transfer processes, enabling the direct generation of valuable organic radical intermediates directly from their native functional group precursors under comparatively mild catalytic conditions. The synthetically advantageous features of PCET reactivity are described in detail, along with examples from the literature describing the PCET activation of common organic functional groups.
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Affiliation(s)
- David C Miller
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Kyle T Tarantino
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA.
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18
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Nguyen LQ, Knowles RR. Catalytic C–N Bond-Forming Reactions Enabled by Proton-Coupled Electron Transfer Activation of Amide N–H Bonds. ACS Catal 2016. [DOI: 10.1021/acscatal.6b00486] [Citation(s) in RCA: 93] [Impact Index Per Article: 11.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- Lucas Q. Nguyen
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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19
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Shen GB, Xia K, Li XT, Li JL, Fu YH, Yuan L, Zhu XQ. Prediction of Kinetic Isotope Effects for Various Hydride Transfer Reactions Using a New Kinetic Model. J Phys Chem A 2016; 120:1779-99. [DOI: 10.1021/acs.jpca.5b10135] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Guang-Bin Shen
- Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry and the ‡Collaborative Innovation Center
of Chemical Science and Engineering, Nankai University, Tianjin 300071, China
| | - Ke Xia
- Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry and the ‡Collaborative Innovation Center
of Chemical Science and Engineering, Nankai University, Tianjin 300071, China
| | - Xiu-Tao Li
- Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry and the ‡Collaborative Innovation Center
of Chemical Science and Engineering, Nankai University, Tianjin 300071, China
| | - Jun-Ling Li
- Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry and the ‡Collaborative Innovation Center
of Chemical Science and Engineering, Nankai University, Tianjin 300071, China
| | - Yan-Hua Fu
- Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry and the ‡Collaborative Innovation Center
of Chemical Science and Engineering, Nankai University, Tianjin 300071, China
| | - Lin Yuan
- Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry and the ‡Collaborative Innovation Center
of Chemical Science and Engineering, Nankai University, Tianjin 300071, China
| | - Xiao-Qing Zhu
- Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry and the ‡Collaborative Innovation Center
of Chemical Science and Engineering, Nankai University, Tianjin 300071, China
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20
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Pannwitz A, Wenger OS. Proton coupled electron transfer from the excited state of a ruthenium(ii) pyridylimidazole complex. Phys Chem Chem Phys 2016; 18:11374-82. [DOI: 10.1039/c6cp00437g] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Transfer of one electron and one proton from [Ru(bpy)2pyimH]2+ to monoquat (MQ+) upon photoexcitation, corresponding to net transfer of a hydrogen atom.
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Affiliation(s)
- Andrea Pannwitz
- Department of Chemistry
- University of Basel
- 4056 Basel
- Switzerland
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21
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Chciuk TV, Flowers RA. Proton-Coupled Electron Transfer in the Reduction of Arenes by SmI2–Water Complexes. J Am Chem Soc 2015; 137:11526-31. [DOI: 10.1021/jacs.5b07518] [Citation(s) in RCA: 63] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Affiliation(s)
- Tesia V. Chciuk
- Department of Chemistry, Lehigh University, 6 E. Packer
Ave. Bethlehem, Pennsylvania 18015, United States
| | - Robert A. Flowers
- Department of Chemistry, Lehigh University, 6 E. Packer
Ave. Bethlehem, Pennsylvania 18015, United States
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22
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Tarantino KT, Miller DC, Callon TA, Knowles RR. Bond-weakening catalysis: conjugate aminations enabled by the soft homolysis of strong N-H bonds. J Am Chem Soc 2015; 137:6440-3. [PMID: 25945955 PMCID: PMC4657442 DOI: 10.1021/jacs.5b03428] [Citation(s) in RCA: 78] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The ability of redox-active metal centers to weaken the bonds in associated ligands is well precedented, but has rarely been utilized as a mechanism of substrate activation in catalysis. Here we describe a catalytic bond-weakening protocol for conjugate amination wherein the strong N-H bonds in N-aryl amides (N-H bond dissociation free energies ∼100 kcal/mol) are destabilized by ∼33 kcal/mol upon by coordination to a reducing titanocene complex, enabling their abstraction by the weak H-atom acceptor TEMPO through a proton-coupled electron transfer process. Significantly, this soft homolysis mechanism provides a method to generate closed-shell, metalated nucleophiles under neutral conditions in the absence of a Brønsted base.
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Affiliation(s)
- Kyle T. Tarantino
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - David C. Miller
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Ted A. Callon
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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23
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Migliore A, Polizzi NF, Therien M, Beratan DN. Biochemistry and theory of proton-coupled electron transfer. Chem Rev 2014; 114:3381-465. [PMID: 24684625 PMCID: PMC4317057 DOI: 10.1021/cr4006654] [Citation(s) in RCA: 345] [Impact Index Per Article: 34.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2013] [Indexed: 02/01/2023]
Affiliation(s)
- Agostino Migliore
- Department
of Chemistry, Department of Biochemistry, and Department of Physics, Duke University, Durham, North Carolina 27708, United States
| | - Nicholas F. Polizzi
- Department
of Chemistry, Department of Biochemistry, and Department of Physics, Duke University, Durham, North Carolina 27708, United States
| | - Michael
J. Therien
- Department
of Chemistry, Department of Biochemistry, and Department of Physics, Duke University, Durham, North Carolina 27708, United States
| | - David N. Beratan
- Department
of Chemistry, Department of Biochemistry, and Department of Physics, Duke University, Durham, North Carolina 27708, United States
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24
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Park J, Morimoto Y, Lee YM, Nam W, Fukuzumi S. Unified view of oxidative C-H bond cleavage and sulfoxidation by a nonheme iron(IV)-oxo complex via Lewis acid-promoted electron transfer. Inorg Chem 2014; 53:3618-28. [PMID: 24605985 DOI: 10.1021/ic403124u] [Citation(s) in RCA: 103] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), were remarkably enhanced by the presence of triflic acid (HOTf) and Sc(OTf)3 in acetonitrile at 298 K. All the logarithms of the observed second-order rate constants of both the oxidative C-H bond cleavage and sulfoxidation reactions exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes between PCET and MCET were taken into account, respectively. Thus, the mechanisms of both the oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by [(N4Py)Fe(IV)(O)](2+) in the presence of HOTf and Sc(OTf)3 have been unified as the rate-determining electron transfer, which is coupled with binding of [(N4Py)Fe(IV)(O)](2+) by proton (PCET) and Sc(OTf)3 (MCET). There was no deuterium kinetic isotope effect (KIE) on the oxidative C-H bond cleavage of toluene via the PCET pathway, whereas a large KIE value was observed with Sc(OTf)3, which exhibited no acceleration of the oxidative C-H bond cleavage of toluene. When HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [Ru(II)(bpy)3](2+) (bpy =2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+). The PCET and MCET reactivities of [(N4Py)Fe(IV)(O)](2+) with Brønsted acids and various metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity.
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Affiliation(s)
- Jiyun Park
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA , Japan Science and Technology Agency (JST) , Suita, Osaka 565-0871, Japan
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25
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Lewandowska-Andralojc A, Grills DC, Zhang J, Bullock RM, Miyazawa A, Kawanishi Y, Fujita E. Kinetic and Mechanistic Studies of Carbon-to-Metal Hydrogen Atom Transfer Involving Os-Centered Radicals: Evidence for Tunneling. J Am Chem Soc 2014; 136:3572-8. [DOI: 10.1021/ja4123076] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
| | - David C. Grills
- Chemistry
Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States
| | - Jie Zhang
- Chemistry
Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States
| | - R. Morris Bullock
- Physical
Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, United States
| | - Akira Miyazawa
- National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake,
Miyagino, Sendai, Miyagi 983-8551, Japan
| | - Yuji Kawanishi
- National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki, 305-8565, Japan
| | - Etsuko Fujita
- Chemistry
Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States
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26
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Kryvohuz M. Calculation of Kinetic Isotope Effects for Intramolecular Hydrogen Shift Reactions Using Semiclassical Instanton Approach. J Phys Chem A 2014; 118:535-44. [DOI: 10.1021/jp4099073] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Maksym Kryvohuz
- Chemical Sciences and Engineering
Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
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27
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Abstract
Proton-coupled electron transfer (PCET) plays a crucial role in many enzymatic reactions and is relevant for a variety of processes including water oxidation, nitrogen fixation, and carbon dioxide reduction. Much of the research on PCET has focused on transfers between molecules in their electronic ground states, but increasingly researchers are investigating PCET between photoexcited reactants. This Account describes recent studies of excited-state PCET with d(6) metal complexes emphasizing work performed in my laboratory. Upon photoexcitation, some complexes release an electron and a proton to benzoquinone reaction partners. Others act as combined electron-proton acceptors in the presence of phenols. As a result, we can investigate photoinduced PCET involving electron and proton transfer in a given direction, a process that resembles hydrogen-atom transfer (HAT). In other studies, the photoexcited metal complexes merely serve as electron donors or electron acceptors because the proton donating and accepting sites are located on other parts of the molecular PCET ensemble. We and others have used this multisite design to explore so-called bidirectional PCET which occurs in many enzymes. A central question in all of these studies is whether concerted proton-electron transfer (CPET) can compete kinetically with sequential electron and proton transfer steps. Short laser pulses can trigger excited-state PCET, making it possible to investigate rapid reactions. Luminescence spectroscopy is a convenient tool for monitoring PCET, but unambiguous identification of reaction products can require a combination of luminescence spectroscopy and transient absorption spectroscopy. Nevertheless, in some cases, distinguishing between PCET photoproducts and reaction products formed by simple photoinduced electron transfer (ET) (reactions that don't include proton transfer) is tricky. Some of the studies presented here deal directly with this important problem. In one case study we employed a cyclometalated iridium(III) complex. Our other studies with ruthenium(II) complexes and phenols focused on systematic variations of the reaction free energies for the CPET, ET, and proton transfer (PT) steps to explore their influence on the overall PCET reaction. Still other work with rhenium(I) complexes concentrated on the question of how the electronic structure of the metal-to-ligand charge transfer (MLCT) excited states affects PCET. We used covalent rhenium(I)-phenol dyads to explore the influence of the electron donor-electron acceptor distance on bidirectional PCET. In covalent triarylamine-Ru(bpy)₃²⁺/Os(bpy)₃²⁺-anthraquinone triads (bpy = 2,2'-bipyridine), hydrogen-bond donating solvents significantly lengthened the lifetimes of photogenerated electron/hole pairs because of hydrogen-bonding to the quinone radical anion. Until now, comparatively few researchers have investigated this variation of PCET: the strengthening of H-bonds upon photoreduction.
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Affiliation(s)
- Oliver S. Wenger
- Departement Chemie, Universität Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland
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28
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Park J, Lee YM, Nam W, Fukuzumi S. Brønsted Acid-Promoted C–H Bond Cleavage via Electron Transfer from Toluene Derivatives to a Protonated Nonheme Iron(IV)-Oxo Complex with No Kinetic Isotope Effect. J Am Chem Soc 2013; 135:5052-61. [DOI: 10.1021/ja311662w] [Citation(s) in RCA: 87] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- Jiyun Park
- Department
of Material and Life
Science, Graduate School of Engineering, ALCA, Japan Science and Technology
Agency, Osaka University, Suita, Osaka
565-0871, Japan
| | - Yong-Min Lee
- Department
of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
| | - Wonwoo Nam
- Department
of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
| | - Shunichi Fukuzumi
- Department
of Material and Life
Science, Graduate School of Engineering, ALCA, Japan Science and Technology
Agency, Osaka University, Suita, Osaka
565-0871, Japan
- Department
of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
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29
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Kryvohuz M, Marcus RA. Semiclassical evaluation of kinetic isotope effects in 13-atomic system. J Chem Phys 2012; 137:134107. [DOI: 10.1063/1.4754660] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
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30
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Taketoshi A, Beh XN, Kuwabara J, Koizumi TA, Kanbara T. Aerobic oxidative dehydrogenation of benzyl alcohols to benzaldehydes by using a cyclometalated ruthenium catalyst. Tetrahedron Lett 2012. [DOI: 10.1016/j.tetlet.2012.05.001] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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31
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Weinberg DR, Gagliardi CJ, Hull JF, Murphy CF, Kent CA, Westlake BC, Paul A, Ess DH, McCafferty DG, Meyer TJ. Proton-Coupled Electron Transfer. Chem Rev 2012; 112:4016-93. [DOI: 10.1021/cr200177j] [Citation(s) in RCA: 1125] [Impact Index Per Article: 93.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Affiliation(s)
- David R. Weinberg
- Department
of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290,
United States
- Department of Physical and Environmental
Sciences, Colorado Mesa University, 1100 North Avenue, Grand Junction,
Colorado 81501-3122, United States
| | - Christopher J. Gagliardi
- Department
of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290,
United States
| | - Jonathan F. Hull
- Department
of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290,
United States
| | - Christine Fecenko Murphy
- Department
of Chemistry, B219
Levine Science Research Center, Box 90354, Duke University, Durham,
North Carolina 27708-0354, United States
| | - Caleb A. Kent
- Department
of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290,
United States
| | - Brittany C. Westlake
- The American Chemical Society,
1155 Sixteenth Street NW, Washington, District of Columbia 20036,
United States
| | - Amit Paul
- Department
of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290,
United States
| | - Daniel H. Ess
- Department
of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290,
United States
| | - Dewey Granville McCafferty
- Department
of Chemistry, B219
Levine Science Research Center, Box 90354, Duke University, Durham,
North Carolina 27708-0354, United States
| | - Thomas J. Meyer
- Department
of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290,
United States
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32
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Wang Y, Shi S, Wang H, Zhu D, Yin G. Kinetics of hydrogen abstraction by active metal hydroxo and oxo intermediates: revealing their unexpected similarities in the transition state. Chem Commun (Camb) 2012; 48:7832-4. [DOI: 10.1039/c2cc33615d] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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33
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Manner VW, Lindsay AD, Mader EA, Harvey JN, Mayer JM. Spin-forbidden hydrogen atom transfer reactions in a cobalt biimidazoline system. Chem Sci 2012. [DOI: 10.1039/c1sc00387a] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
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34
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Kojima T, Nakayama K, Ikemura K, Ogura T, Fukuzumi S. Formation of a Ruthenium(IV)-Oxo Complex by Electron-Transfer Oxidation of a Coordinatively Saturated Ruthenium(II) Complex and Detection of Oxygen-Rebound Intermediates in C–H Bond Oxygenation. J Am Chem Soc 2011; 133:11692-700. [DOI: 10.1021/ja2037645] [Citation(s) in RCA: 87] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Takahiko Kojima
- Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennoudai, Tsukuba, Ibaraki 305-8571
| | - Kazuya Nakayama
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, Japan
| | - Kenichiro Ikemura
- Graduate School of Life Science, University of Hyogo, Kouto, Hyogo 678-1297, Japan
| | - Takashi Ogura
- Graduate School of Life Science, University of Hyogo, Kouto, Hyogo 678-1297, Japan
| | - Shunichi Fukuzumi
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, Japan
- ALCA, Japan Science and Technology Agency (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
- Department of Bioinspired Science, Ewha Womans University, Seoul, South Korea
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35
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Dzik WI, Fuente Arruga L, Siegler MA, Spek AL, Reek JNH, de Bruin B. Open-Shell Organometallic [MII(dbcot(bislutidylamine)]2+ Complexes (M = Rh, Ir): Unexpected Base-Assisted Reduction of the Metal Instead of Amine Ligand Deprotonation. Organometallics 2011. [DOI: 10.1021/om101157r] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Wojciech I. Dzik
- Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
| | - Luis Fuente Arruga
- Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
| | - Maxime A. Siegler
- Bijvoet Center for Biomolecular Research, Utrecht University, Utrecht, The Netherlands
| | - Anthony L. Spek
- Bijvoet Center for Biomolecular Research, Utrecht University, Utrecht, The Netherlands
| | - Joost N. H. Reek
- Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
| | - Bas de Bruin
- Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
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36
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Mayer JM. Understanding hydrogen atom transfer: from bond strengths to Marcus theory. Acc Chem Res 2011; 44:36-46. [PMID: 20977224 DOI: 10.1021/ar100093z] [Citation(s) in RCA: 606] [Impact Index Per Article: 46.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Hydrogen atom transfer (HAT), a key step in many chemical, environmental, and biological processes, is one of the fundamental chemical reactions: A-H + B → A + H-B. Traditional HAT involves p-block radicals such as tert-BuO(•) abstracting H(•) from organic molecules. More recently, the recognition that transition metal species undergo HAT has led to a broader perspective, with HAT viewed as a type of proton-coupled electron transfer (PCET). When transition metal complexes oxidize substrates by removing H(•) (e(-) + H(+)), typically the electron transfers to the metal and the proton to a ligand. Examples with iron-imidazolinate, vanadium-oxo, and many other complexes are discussed. Although these complexes may not "look like" main group radicals, they have the same pattern of reactivity. For instance, their HAT rate constants parallel the A-H bond strengths within a series of similar reactions. Like main group radicals, they abstract H(•) much faster from O-H bonds than from C-H bonds of the same strength, showing that driving force is not the only determinant of reactivity. This Account describes our development of a conceptual framework for HAT with a Marcus theory approach. In the simplest model, the cross relation uses the self-exchange rate constants (k(AH/A) for AH + A) and the equilibrium constant to predict the rate constant for AH + B: k(AH/B) = (k(AH/A)k(BH/B)K(eq)f)(1/2). For a variety of transition metal oxidants, k(AH/B) is predicted within one or two orders of magnitude with only a few exceptions. For 36 organic reactions of oxyl radicals, k(AH/B) is predicted with an average deviation of a factor of 3.8, and within a factor of 5 for all but six of the reactions. These reactions involve both O-H or C-H bonds, occur in either water or organic solvents, and occur over a range of 10(28) in K(eq) and 10(13) in k(AH/B). The treatment of organic reactions includes the well-established kinetic solvent effect on HAT reactions. This is one of a number of secondary effects that the simple cross relation does not include, such as hydrogen tunneling and the involvement of precursor and successor complexes. This Account includes a number of case studies to illustrate these and various other issues. The success of the cross relation, despite its simplicity, shows that the Marcus approach based on free energies and intrinsic barriers captures much of the essential chemistry of HAT reactions. Among the insights derived from the analysis is that reactions correlate with free energies, not with bond enthalpies. Moreover, the radical character or spin state of an oxidant is not a primary determinant of HAT abstracting ability. The intrinsic barriers for HAT reactions can be understood, at least in part, as Marcus-type inner-sphere reorganization energies. The intrinsic barriers for diverse cross reactions are accurately obtained from the HAT self-exchange rate constants, a remarkable and unprecedented result for any type of chemical reaction other than electron transfer. The Marcus cross relation thus provides a valuable new framework for understanding and predicting HAT reactivity.
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Affiliation(s)
- James M. Mayer
- Department of Chemistry, Campus Box 351700, University of Washington, Seattle, Washington 98195-1700, United States
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37
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Mayer JM. A Simple Marcus-Theory Type Model for Hydrogen Atom Transfer/Proton-Coupled Electron Transfer. J Phys Chem Lett 2011; 2:1481-1489. [PMID: 21686056 PMCID: PMC3115700 DOI: 10.1021/jz200021y] [Citation(s) in RCA: 68] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
Hydrogen atom transfer reactions are the simplest class of proton-coupled electron transfer (PCET) processes. These reactions involve transfer of one electron and one proton from one reagent to another, in the same kinetic step: XH + Y → X + HY. A predictive model for these reactions based on the Marcus cross relation is described. The model predicts rate constants within one or two orders of magnitude in most cases, over a very wide range of reactants and solvents. This remarkable result implies a surprising generality of the additivity postulate for the reaction intrinsic barriers, and a smaller role for the quantum mechanical details of the proton and electron transfers.
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Affiliation(s)
- James M Mayer
- Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195-1700
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38
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Warren JJ, Tronic TA, Mayer JM. Thermochemistry of proton-coupled electron transfer reagents and its implications. Chem Rev 2010; 110:6961-7001. [PMID: 20925411 PMCID: PMC3006073 DOI: 10.1021/cr100085k] [Citation(s) in RCA: 1193] [Impact Index Per Article: 85.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Affiliation(s)
- Jeffrey J. Warren
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195-1700
| | - Tristan A. Tronic
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195-1700
| | - James M. Mayer
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195-1700
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39
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Warren JJ, Mayer JM. Tuning of the thermochemical and kinetic properties of ascorbate by its local environment: solution chemistry and biochemical implications. J Am Chem Soc 2010; 132:7784-93. [PMID: 20476757 DOI: 10.1021/ja102337n] [Citation(s) in RCA: 86] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Ascorbate (vitamin C) is a ubiquitous biological cofactor. While its aqueous solution chemistry has long been studied, many in vivo reactions of ascorbate occur in enzyme active sites or at membrane interfaces, which have varying local environments. This report shows that the rate and driving force of oxidations of two ascorbate derivatives by the TEMPO radical (2,2',6,6'-tetramethylpiperidin-1-oxyl) in acetonitrile are very sensitive to the presence of various additives. These reactions proceed by the transfer of a proton and an electron (a hydrogen atom), as is typical of biological ascorbate reactions. The measured rate and equilibrium constants vary substantially with added water or other polar solutes in acetonitrile solutions, indicating large shifts in the reducing power of ascorbate. The correlation of rate and equilibrium constants indicates that this effect has a thermochemical origin rather than being a purely kinetic effect. This contrasts with previous examples of solvent effects on hydrogen atom transfer reactions. Potential biological implications of this apparently unique effect are discussed.
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Affiliation(s)
- Jeffrey J Warren
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
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40
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Lancaster KM, Gerken JB, Durrell AC, Palmer JH, Gray HB. Electronic structures, photophysical properties, and electrochemistry of ruthenium(II)(bpy)2 pyridylimidazole complexes. Coord Chem Rev 2010. [DOI: 10.1016/j.ccr.2010.04.005] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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41
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Warren JJ, Mayer JM. Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation. Proc Natl Acad Sci U S A 2010; 107:5282-7. [PMID: 20215463 PMCID: PMC2851756 DOI: 10.1073/pnas.0910347107] [Citation(s) in RCA: 92] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO(*)) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments are shown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2'-6,6'-tetramethyl-piperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and alpha-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.
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Affiliation(s)
- Jeffrey J. Warren
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98107-1700
| | - James M. Mayer
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98107-1700
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Taketoshi A, Tsujimoto A, Maeda S, Koizumi T, Kanbara T. Aerobic Oxidative Dehydrogenation of 2-Substituted Imidazolines Promoted by a Cyclometalated Ruthenium Catalyst. ChemCatChem 2010. [DOI: 10.1002/cctc.200900251] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Koizumi TA, Kanbara T. Oxidative Dehydrogenation Promoted by Cyclometalated Ruthenium Complexes. ACTA ACUST UNITED AC 2010. [DOI: 10.4019/bjscc.56.14] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
Affiliation(s)
| | - Takaki Kanbara
- Graduate School of Pure and Applied Sciences, University of Tsukuba
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Miyazaki S, Kojima T, Mayer JM, Fukuzumi S. Proton-coupled electron transfer of ruthenium(III)-pterin complexes: a mechanistic insight. J Am Chem Soc 2009; 131:11615-24. [PMID: 19722655 DOI: 10.1021/ja904386r] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK(a) and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru(II)(Hdmp)(TPA)](ClO(4))(2) (1), [Ru(II)(Hdmdmp)(TPA)](ClO(4))(2) (2), [Ru(II)(dmp(-))(TPA)]ClO(4) (3), and [Ru(II)(dmdmp(-))(TPA)]ClO(4) (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA = tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru(III)(dmp(-))(TPA)](2+), was determined to be 89 kcal mol(-1), which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol(-1)) to [Ru(III)(dmp(-))(TPA)](2+), the activation parameters were determined to be DeltaH(double dagger) = 1.6 +/- 0.2 kcal mol(-1) and DeltaS(double dagger) = -36 +/- 2 cal K(-1) mol(-1). This small activation enthalpy suggests a hydrogen-bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.
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Affiliation(s)
- Soushi Miyazaki
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
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Wu A, Mader EA, Datta A, Hrovat DA, Borden WT, Mayer JM. Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: tunneling in hydrogen atom transfer. J Am Chem Soc 2009; 131:11985-97. [PMID: 19618933 PMCID: PMC2775461 DOI: 10.1021/ja904400d] [Citation(s) in RCA: 65] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO(*) (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ((t)Bu(2)NO(*)), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (t)Bu(2)NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |DeltaG (o)| < or = 1.4 kcal mol(-1) and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO(*) + TEMPO-H --> 4-oxo-TEMPO-H + TEMPO(*) occurs with k(2H,MeCN) = 10 +/- 1 M(-1) s(-1) in MeCN at 298 K (K(2H,MeCN) = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k(2D,MeCN) = 0.44 +/- 0.05 M(-1) s(-1) with k(2H,MeCN)/k(2D,MeCN) = 23 +/- 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH(2)Cl(2), 23 +/- 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO(*) with 4-MeO-TEMPO-H(D) also has a large KIE, k(3H)/k(3D) = 21 +/- 3 in MeCN. For these three reactions, the E(aD) - E(aH) values, between 0.3 +/- 0.6 and 1.3 +/- 0.6 kcal mol(-1), and the log(A(H)/A(D)) values, between 0.5 +/- 0.7 and 1.1 +/- 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of (t)Bu(2)NO(*) + TEMPO-H(D) in MeCN has a large KIE, 16 +/- 3 in MeCN, and very unusual isotopic activation parameters, E(aD) - E(aH) = -2.6 +/- 0.4 and log(A(H)/A(D)) = 3.1 +/- 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH(3))(2)NO(*) + (CH(3))(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO((*)/H), (t)Bu(2)NO((*)/H), and Ph(2)NO((*)/H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.
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Affiliation(s)
- Adam Wu
- Department of Chemistry, Campus Box 351700, University of Washington, Seattle, Washington 98195-1700, USA
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Mandal TK, Pati SK, Datta A. Degenerate intermolecular and intramolecular proton-transfer reactions: electronic structure of the transition states. J Phys Chem A 2009; 113:8147-51. [PMID: 19606892 DOI: 10.1021/jp9044513] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Density functional theory (DFT) calculations are performed on a series of double and single proton-transfer reactions to study the variation in polarizations in complexes during the dynamics of proton transfer from one isoenergetic, hydrogen-bonded ground-state structure to the other. The isotropic average polarizability (alpha(av)) shows an interesting single-humped profile with a maxima coinciding with the transition state of the reaction. Similar profiles are also computed at Nd:YaG frequencies. The origin of the maximal polarizability at the transition state is traced to maximal charge separation and large D (donor)-A (acceptor) distances. Maximal polarizability for the transition state suggests an interesting, novel, and less memory extensive computational tool to locate the transition state for hydrogen-transfer reactions in hydrogen-bonded complexes.
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Sreenath K, Suneesh CV, Gopidas KR, Flowers RA. Generation of Triarylamine Radical Cations through Reaction of Triarylamines with Cu(II) in Acetonitrile. A Kinetic Investigation of the Electron-Transfer Reaction. J Phys Chem A 2009; 113:6477-83. [DOI: 10.1021/jp9027222] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Affiliation(s)
- Kesavapillai Sreenath
- Photosciences and Photonics, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), Council of Scientific and Industrial Research (CSIR), Trivandrum 695 019, India, and Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015-3172
| | - Chettiyam Veettil Suneesh
- Photosciences and Photonics, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), Council of Scientific and Industrial Research (CSIR), Trivandrum 695 019, India, and Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015-3172
| | - Karical R. Gopidas
- Photosciences and Photonics, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), Council of Scientific and Industrial Research (CSIR), Trivandrum 695 019, India, and Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015-3172
| | - Robert A. Flowers
- Photosciences and Photonics, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), Council of Scientific and Industrial Research (CSIR), Trivandrum 695 019, India, and Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015-3172
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Scepaniak J, Young J, Bontchev R, Smith J. Formation of Ammonia from an Iron Nitrido Complex. Angew Chem Int Ed Engl 2009. [DOI: 10.1002/ange.200900381] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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Scepaniak J, Young J, Bontchev R, Smith J. Formation of Ammonia from an Iron Nitrido Complex. Angew Chem Int Ed Engl 2009; 48:3158-60. [DOI: 10.1002/anie.200900381] [Citation(s) in RCA: 196] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Waidmann CR, Zhou X, Tsai EA, Kaminsky W, Hrovat DA, Borden WT, Mayer JM. Slow hydrogen atom transfer reactions of oxo- and hydroxo-vanadium compounds: the importance of intrinsic barriers. J Am Chem Soc 2009; 131:4729-43. [PMID: 19292442 PMCID: PMC2735118 DOI: 10.1021/ja808698x] [Citation(s) in RCA: 63] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Reactions are described that interconvert vanadium(IV) oxo-hydroxo complexes [V(IV)O(OH)(R(2)bpy)(2)]BF(4) (1a-c) and vanadium(V) dioxo complexes [V(V)O(2)(R(2)bpy)(2)]BF(4) (2a-c) [R(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine ((t)Bu(2)bpy), a; 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy), b; 2,2'-bipyridine (bpy), c]. These are rare examples of pairs of isolated, sterically unencumbered, first-row metal-oxo/hydroxo complexes that differ by a hydrogen atom (H(+) + e(-)). The V(IV)-(t)Bu(2)bpy derivative 1a has a useful (1)H NMR spectrum, despite being paramagnetic. Complex 2a abstracts H(*) from organic substrates with weak O-H and C-H bonds, converting 2,6-(t)Bu(2)-4-MeO-C(6)H(2)OH (ArOH) and 2,2,6,6-tetramethyl-N-hydroxypiperidine (TEMPOH) to their corresponding radicals ArO(*) and TEMPO, hydroquinone to benzoquinone, and dihydroanthracene to anthracene. The equilibrium constant for 2a + ArOH <==> 1a + ArO(*) is (4 +/- 2) x 10(-3), implying that the VO-H bond dissociation free energy (BDFE) is 70.6 +/- 1.2 kcal mol(-1). Consistent with this value, 1a is oxidized by 2,4,6-(t)Bu(3)C(6)H(2)O(*). All of these reactions are surprisingly slow, typically occurring over hours at ambient temperatures. The net hydrogen-atom pseudo-self-exchange 1a + 2b <==> 2a + 1b, using the (t)Bu- and Me-bpy substituents as labels, also occurs slowly, with k(se) = 1.3 x 10(-2) M(-1) s(-1) at 298 K, DeltaH(double dagger) = 15 +/- 2 kcal mol(-1), and DeltaS(double dagger) = 16 +/- 5 cal mol(-1) K. Using this k(se) and the BDFE, the vanadium reactions are shown to follow the Marcus cross relation moderately well, with calculated rate constants within 10(2) of the observed values. The vanadium self-exchange reaction is ca. 10(6) slower than that for the related Ru(IV)O(py)(bpy)(2)(2+)/Ru(III)OH(py)(bpy)(2)(2+) self-exchange. The origin of this dramatic difference has been probed with DFT calculations on the self-exchange reactions of 1c + 2c and on monocationic ruthenium complexes with pyrrolate or fluoride in place of the py ligands. The calculations reproduce the difference in barrier heights and show that transfer of a hydrogen atom involves more structural reorganization for vanadium than the Ru analogues. The vanadium complexes have larger changes in the metal-oxo and metal-hydroxo bond lengths, which is traced to the difference in d-orbital occupancy in the two systems. This study thus highlights the importance of intrinsic barriers in the transfer of a hydrogen atom, in addition to the thermochemical (bond strength) factors that have been previously emphasized.
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Affiliation(s)
- Christopher R. Waidmann
- Department of Chemistry, Campus Box 351700, University of Washington, Seattle, WA, 98195-1700
| | - Xin Zhou
- Department of Chemistry, University of North Texas, P.O. Box 305070, Denton, TX 76203-5070
| | - Erin A. Tsai
- Department of Chemistry, Campus Box 351700, University of Washington, Seattle, WA, 98195-1700
| | - Werner Kaminsky
- Department of Chemistry, Campus Box 351700, University of Washington, Seattle, WA, 98195-1700
- UW crystallographic facility
| | - David A. Hrovat
- Department of Chemistry, University of North Texas, P.O. Box 305070, Denton, TX 76203-5070
| | - Weston Thatcher Borden
- Department of Chemistry, University of North Texas, P.O. Box 305070, Denton, TX 76203-5070
| | - James M. Mayer
- Department of Chemistry, Campus Box 351700, University of Washington, Seattle, WA, 98195-1700
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