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Zhang JY, Wang LL, Zhu XQ. Characteristic Activity Parameters of Electron Donors and Electron Acceptors. ACS PHYSICAL CHEMISTRY AU 2023; 3:358-373. [PMID: 37520315 PMCID: PMC10375887 DOI: 10.1021/acsphyschemau.3c00001] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/13/2023] [Revised: 03/01/2023] [Accepted: 03/03/2023] [Indexed: 08/01/2023]
Abstract
It is well-known that for an electron transfer reaction, the electron-donating ability of electron donors and the electron-accepting ability of electron acceptors can be quantitatively described by the oxidation potential of electron donors and the reduction potential of electron acceptors. However, for an electron transfer reaction, the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors cannot be quantitatively described by a characteristic parameter of electron donors and a characteristic parameter of electron acceptors till now. In this paper, a characteristic activity parameter of electron donors and electron acceptors named as their thermo-kinetic parameter is proposed to quantify the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors in electron transfer reactions. At the same time, the thermo-kinetic parameter values of 70 well-known electron donors and the corresponding 70 conjugated electron acceptors in acetonitrile at 298 K are determined. The activation free energies of 4900 typical electron transfer reactions in acetonitrile at 298 K are estimated according to the thermo-kinetic parameter values of 70 electron donors and 70 conjugated electron acceptors, and the estimated results have received good verification of the corresponding independent experimental measurements. The physical meaning of the thermo-kinetic parameter is examined. The relationship of the thermo-kinetic parameter with the corresponding redox potential as well as the relationship of the activation free energy with the corresponding thermodynamic driving force of electron transfer reactions is examined. The results show that the observed relationships between the thermo-kinetic parameters and the redox potentials as well as the observed relationships between the activation free energy and the thermodynamic driving force depend on the choice of electron donors and electron acceptors as well as the electron transfer reactions. The greatest contribution of this paper is to realize the symmetry and unification of kinetic equations and the corresponding thermodynamic equations of electron transfer reactions.
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2
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Fu YH, Zhang Y, Wang F, Zhao L, Shen GB, Zhu XQ. Quantitative evaluation of the actual hydrogen atom donating activities of O-H bonds in phenols: structure-activity relationship. RSC Adv 2023; 13:3295-3305. [PMID: 36756400 PMCID: PMC9869660 DOI: 10.1039/d2ra06877j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2022] [Accepted: 12/31/2022] [Indexed: 01/25/2023] Open
Abstract
The H-donating activity of phenol and the H-abstraction activity of phenol radicals have been extensively studied. In this article, the second-order rate constants of 25 hydrogen atom transfer (HAT) reactions between phenols and PINO and DPPH radicals in acetonitrile at 298 K were studied. Thermo-kinetic parameters ΔG ≠o(XH) were obtained using a kinetic equation [ΔG ≠ XH/Y = ΔG ≠o(XH) + ΔG ≠o(Y)]. Bond dissociation free energies ΔG o(XH) were calculated by the iBonD HM method, whose details are available at https://pka.luoszgroup.com/bde_prediction. Intrinsic resistance energies ΔG ≠ XH/X and ΔG ≠o(X) were determined as ΔG ≠o(XH) and ΔG o(XH) were available. ΔG o(XH), ΔG ≠ XH/X, ΔG ≠o(XH) and ΔG ≠o(X) were used to assess the H-donating abilities of the studied phenols and the H-abstraction abilities of phenol radicals in thermodynamics, kinetics and actual HAT reactions. The effect of structures on these four parameters was discussed. The reliabilities of ΔG ≠o(XH) and ΔG ≠o(X) were examined. The difference between the method of determining ΔG ≠ XH/X mentioned in this study and the dynamic nuclear magnetic method mentioned in the literature was studied. Via this study, not only ΔG o(XH), ΔG ≠ XH/X, ΔG ≠o(XH) and ΔG ≠o(X) of phenols could be quantitatively evaluated, but also the structure-activity relationship of phenols is clearly demonstrated. Moreover, it lays the foundation for designing and synthesizing more antioxidants and radicals.
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Affiliation(s)
- Yan-Hua Fu
- College of Chemistry and Environmental Engineering, Anyang Institute of Technology Anyang Henan 455000 China
| | - Yanwei Zhang
- College of Chemistry and Environmental Engineering, Anyang Institute of Technology Anyang Henan 455000 China
| | - Fang Wang
- College of Chemistry and Environmental Engineering, Anyang Institute of Technology Anyang Henan 455000 China
| | - Ling Zhao
- College of Chemistry and Environmental Engineering, Anyang Institute of Technology Anyang Henan 455000 China
| | - Guang-Bin Shen
- School of Medical Engineering, Jining Medical UniversityJiningShandong272000P. R. China
| | - Xiao-Qing Zhu
- Department of Chemistry, Nankai UniversityTianjin300071China
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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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Xu M, Sun Y, Xu L. Methylated B-type Anderson heteropolymolybdate: synthesis, structure, and magnetic properties. J COORD CHEM 2020. [DOI: 10.1080/00958972.2020.1805443] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Affiliation(s)
- Ming Xu
- Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun, P. R. China
| | - Yu Sun
- Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun, P. R. China
| | - Lin Xu
- Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun, P. R. China
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5
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Bezdek MJ, Pelczer I, Chirik PJ. Coordination-Induced N–H Bond Weakening in a Molybdenum Pyrrolidine Complex: Isotopic Labeling Provides Insight into the Pathway for H 2 Evolution. Organometallics 2020. [DOI: 10.1021/acs.organomet.0c00471] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Affiliation(s)
- Máté J. Bezdek
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - István Pelczer
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Paul J. Chirik
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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6
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Chciuk TV, Anderson WR, Flowers RA. Interplay between Substrate and Proton Donor Coordination in Reductions of Carbonyls by SmI2–Water Through Proton-Coupled Electron-Transfer. J Am Chem Soc 2018; 140:15342-15352. [DOI: 10.1021/jacs.8b08890] [Citation(s) in RCA: 39] [Impact Index Per Article: 6.5] [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 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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7
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Bezdek MJ, Chirik PJ. Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism. J Am Chem Soc 2018; 140:13817-13826. [DOI: 10.1021/jacs.8b08460] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Affiliation(s)
- Máté J. Bezdek
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Paul J. Chirik
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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8
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Mitsuhashi R, Suzuki T, Mikuriya M. Geometric Isomerism and Redox Properties of Ruthenium(II/III) Complexes with 3-Hydroxypicolinamide. CHEM LETT 2018. [DOI: 10.1246/cl.171166] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Affiliation(s)
- Ryoji Mitsuhashi
- Department of Applied Chemistry for Environment, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
| | - Takayoshi Suzuki
- Research Institute for Interdisciplinary Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
| | - Masahiro Mikuriya
- Department of Applied Chemistry for Environment, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
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9
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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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10
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Wilkinson LA, Vincent KB, Meijer AJHM, Patmore NJ. Mechanistic insight into proton-coupled mixed valency. Chem Commun (Camb) 2016; 52:100-3. [DOI: 10.1039/c5cc07101a] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Stabilisation of the mixed-valence state in [Mo2(TiPB)3(HDOP)]2+ (HTiPB = 2,4,6-triisopropylbenzoic acid, H2DOP = 3,6-dihydroxypyridazine) by electron transfer (ET) is related to the proton coordinate of the bridging ligands.
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Affiliation(s)
- Luke A. Wilkinson
- Department of Chemical Sciences
- University of Huddersfield
- Huddersfield HD1 3DH
- UK
- Department of Chemistry
| | - Kevin B. Vincent
- Department of Chemical Sciences
- University of Huddersfield
- Huddersfield HD1 3DH
- UK
| | | | - Nathan J. Patmore
- Department of Chemical Sciences
- University of Huddersfield
- Huddersfield HD1 3DH
- UK
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11
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Wang Z, Lu SM, Li J, Wang J, Li C. Unprecedentedly High Formic Acid Dehydrogenation Activity on an Iridium Complex with anN,N′-Diimine Ligand in Water. Chemistry 2015. [DOI: 10.1002/chem.201502086] [Citation(s) in RCA: 102] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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12
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Abstract
An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H(+)) and one electron (e(-)), which is equivalent to transfer of a hydrogen atom (H(•)). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions.
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Affiliation(s)
- Jeffrey J. Warren
- Simon Fraser University, Department of Chemistry, 8888 University Drive, Burnaby BC, Canada V5A 1S6
| | - James M. Mayer
- Yale University, Department of Chemistry, P.O. Box 208107, 225 Prospect Street, New Haven, CT 06520-8107
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13
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Bächle J, Goni F, Grampp G. Influence of the medium's viscosity on the kinetics of hydrogen atom self-exchange for N-hydroxy phthalimide/piperidine-N-oxyl (NHPI/PINO˙) measured by CW-ESR spectroscopy. Phys Chem Chem Phys 2015; 17:27204-9. [DOI: 10.1039/c5cp04921k] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
Rates of hydrogen atom self-exchange measured by CW-ESR line broadening depend linearly on the medium’s viscosity.
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Affiliation(s)
- Josua Bächle
- Institute of Physical and Theoretical Chemistry
- Graz University of Technology
- 8010 Graz
- Austria
| | - Freskida Goni
- Institute of Physical and Theoretical Chemistry
- Graz University of Technology
- 8010 Graz
- Austria
| | - Günter Grampp
- Institute of Physical and Theoretical Chemistry
- Graz University of Technology
- 8010 Graz
- Austria
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14
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Abate A, Hollman DJ, Teuscher J, Pathak S, Avolio R, D’Errico G, Vitiello G, Fantacci S, Snaith HJ. Protic Ionic Liquids as p-Dopant for Organic Hole Transporting Materials and Their Application in High Efficiency Hybrid Solar Cells. J Am Chem Soc 2013; 135:13538-48. [DOI: 10.1021/ja406230f] [Citation(s) in RCA: 138] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Affiliation(s)
- Antonio Abate
- Clarendon Laboratory,
Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
| | - Derek J. Hollman
- Clarendon Laboratory,
Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
| | - Joël Teuscher
- Clarendon Laboratory,
Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
| | - Sandeep Pathak
- Clarendon Laboratory,
Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
| | - Roberto Avolio
- Institute of Polymer Chemistry and Technology (ICTP), National Research Council
of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy
| | - Gerardino D’Errico
- Department
of Chemical
sciences, University of Naples “Federico II”, Via Cinthia, 80126 Napoli, Italy
| | - Giuseppe Vitiello
- Department
of Chemical
sciences, University of Naples “Federico II”, Via Cinthia, 80126 Napoli, Italy
| | - Simona Fantacci
- Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), via Elce di
Sotto 8, I-06213 Perugia, Italy
| | - Henry J. Snaith
- Clarendon Laboratory,
Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
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15
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Dong L, Wang Y, Lv Y, Chen Z, Mei F, Xiong H, Yin G. Lewis-Acid-Promoted Stoichiometric and Catalytic Oxidations by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive Study. Inorg Chem 2013; 52:5418-27. [DOI: 10.1021/ic400361s] [Citation(s) in RCA: 56] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Lei Dong
- School of Chemistry and Chemical Engineering, Hubei
Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
| | - Yujuan Wang
- School of Chemistry and Chemical Engineering, Hubei
Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
| | - Yanzong Lv
- School of Chemistry and Chemical Engineering, Hubei
Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
| | - Zhuqi Chen
- School of Chemistry and Chemical Engineering, Hubei
Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
| | - Fuming Mei
- School of Chemistry and Chemical Engineering, Hubei
Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
| | - Hui Xiong
- School of Chemistry and Chemical Engineering, Hubei
Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
| | - Guochuan Yin
- School of Chemistry and Chemical Engineering, Hubei
Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
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16
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Das D, Lee YM, Ohkubo K, Nam W, Karlin KD, Fukuzumi S. Temperature-independent catalytic two-electron reduction of dioxygen by ferrocenes with a copper(II) tris[2-(2-pyridyl)ethyl]amine catalyst in the presence of perchloric acid. J Am Chem Soc 2013; 135:2825-34. [PMID: 23394287 DOI: 10.1021/ja312523u] [Citation(s) in RCA: 54] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
Selective two-electron plus two-proton (2e(-)/2H(+)) reduction of O(2) to hydrogen peroxide by ferrocene (Fc) or 1,1'-dimethylferrocene (Me(2)Fc) in the presence of perchloric acid is catalyzed efficiently by a mononuclear copper(II) complex, [Cu(II)(tepa)](2+) (1; tepa = tris[2-(2-pyridyl)ethyl]amine) in acetone. The E(1/2) value for [Cu(II)(tepa)](2+) as measured by cyclic voltammetry is 0.07 V vs Fc/Fc(+) in acetone, being significantly positive, which makes it possible to use relatively weak one-electron reductants such as Fc and Me(2)Fc for the overall two-electron reduction of O(2). Fast electron transfer from Fc or Me(2)Fc to 1 affords the corresponding Cu(I) complex [Cu(I)(tepa)](+) (2), which reacts at low temperature (193 K) with O(2), however only in the presence of HClO(4), to afford the hydroperoxo complex [Cu(II)(tepa)(OOH)](+) (3). A detailed kinetic study on the homogeneous catalytic system reveals the rate-determining step to be the O(2)-binding process in the presence of HClO(4) at lower temperature as well as at room temperature. The O(2)-binding kinetics in the presence of HClO(4) were studied, demonstrating that the rate of formation of the hydroperoxo complex 3 as well as the overall catalytic reaction remained virtually the same with changing temperature. The apparent lack of activation energy for the catalytic two-electron reduction of O(2) is shown to result from the existence of a pre-equilibrium between 2 and O(2) prior to the formation of the hydroperoxo complex 3. No further reduction of [Cu(II)(tepa)(OOH)](+) (3) by Fc or Me(2)Fc occurred, and instead 3 is protonated by HClO(4) to yield H(2)O(2) accompanied by regeneration of 1, thus completing the catalytic cycle for the two-electron reduction of O(2) by Fc or Me(2)Fc.
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Affiliation(s)
- Dipanwita Das
- Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
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17
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Schrauben JN, Cattaneo M, Day TC, Tenderholt AL, Mayer JM. Multiple-site concerted proton-electron transfer reactions of hydrogen-bonded phenols are nonadiabatic and well described by semiclassical Marcus theory. J Am Chem Soc 2012; 134:16635-45. [PMID: 22974135 PMCID: PMC3476473 DOI: 10.1021/ja305668h] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
Photo-oxidations of hydrogen-bonded phenols using excited-state polyarenes are described to derive fundamental understanding of multiple-site concerted proton-electron transfer reactions (MS-CPET). Experiments have examined phenol bases having -CPh(2)NH(2), -Py, and -CH(2)Py groups ortho to the phenol hydroxyl group and tert-butyl groups in the 4,6-positions for stability (HOAr-NH(2), HOAr-Py, and HOAr-CH(2)Py, respectively; Py = pyridyl; Ph = phenyl). The photo-oxidations proceed by intramolecular proton transfer from the phenol to the pendent base concerted with electron transfer to the excited polyarene. For comparison, 2,4,6-(t)Bu(3)C(6)H(2)OH, a phenol without a pendent base and tert-butyl groups in the 2,4,6-positions, has also been examined. Many of these bimolecular reactions are fast, with rate constants near the diffusion limit. Combining the photochemical k(CPET) values with those from prior thermal stopped-flow kinetic studies gives data sets for the oxidations of HOAr-NH(2) and HOAr-CH(2)Py that span over 10(7) in k(CPET) and nearly 0.9 eV in driving force (ΔG(o)'). Plots of log(k(CPET)) vs ΔG(o)', including both excited-state anthracenes and ground state aminium radical cations, define a single Marcus parabola in each case. These two data sets are thus well described by semiclassical Marcus theory, providing a strong validation of the use of this theory for MS-CPET. The parabolas give λ(CPET) ≅ 1.15-1.2 eV and H(ab) ≅ 20-30 cm(-1). These experiments represent the most direct measurements of H(ab) for MS-CPET reactions to date. Although rate constants are available only up to the diffusion limit, the parabolas clearly peak well below the adiabatic limit of ca. 6 × 10(12) s(-1). Thus, this is a very clear demonstration that the reactions are nonadiabatic. The nonadiabatic character slows the reactions by a factor of ~45. Results for the oxidation of HOAr-Py, in which the phenol and base are conjugated, and for oxidation of 2,4,6-(t)Bu(3)C(6)H(2)OH, which lacks a base, show that both have substantially lower λ and larger pre-exponential terms. The implications of these results for MS-CPET reactions are discussed.
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Affiliation(s)
| | | | - Thomas C. Day
- Department of Chemistry, University of Washington, Seattle WA 98195
| | | | - James M. Mayer
- Department of Chemistry, University of Washington, Seattle WA 98195
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18
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Fukuzumi S, Hasobe T, Ohkubo K, Crossley MJ, Kamat PV, Imahori H. π-Complex formation in electron-transfer reactions of porphyrins. J PORPHYR PHTHALOCYA 2012. [DOI: 10.1142/s1088424604000180] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
π-Complex formation between porphyrins and their radical cations plays an important role in self-exchange electron transfer between neutral porphyrins and the radical cations, leading to negative activation enthalpies when the stabilization energy of the π-complex is larger than the activation energy for the intracomplex electron transfer in the π-complex. A number of porphyrin molecules are self-organized on three-dimensional gold nanoclusters to form monolayer-protected gold nanoclusters (MPCs) that act as an efficient photocatalyst for the uphill reduction of HV2+ by BNAH to produce 1-benzylnicotinamidinium ion (BNA+) and hexyl viologen radical cation (HV·+). Such three-dimensional architectures of porphyrin MPCs with large surface area allow supramolecular π-complexation between MPCs and HV2+, resulting in fast electron transfer from the singlet excited state of porphyrin to HV2+ on MPCs. The π-π interaction is exploited to develop efficient photovoltaic devices consisting of porphyrin and fullerene assemblies which have an enhanced light-harvesting efficiency throughout the solar spectrum together with a highly efficient conversion of the harvested light into electrical energy.
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Affiliation(s)
- Shunichi Fukuzumi
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
| | - Taku Hasobe
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
| | - Kei Ohkubo
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
| | | | - Prashant V. Kamat
- Radiation Laboratory and Department of Chemical & Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA
| | - Hiroshi Imahori
- Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, PRESTO, Japan Science and Technology Agency (JST), Sakyo-ku, Kyoto 606-8501, Japan
- Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-Nishihiraki-cho, Nishikyo-ku, Kyoto 615-8510, Japan
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19
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Fukuzumi S, Hasobe T, Endo Y, Ohkubo K, Imahori H. Fast self-exchange electron transfer and delocalization of unpaired electron between zinc porphyrin radical cations and zinc porphyrins. J PORPHYR PHTHALOCYA 2012. [DOI: 10.1142/s1088424603000422] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Self-exchange electron transfer rates between π-radical cations of zinc porphyrins and the neutral metalloporphyrins have been determined from the line-width broadening in the ESR spectra in different solvents at various temperatures. Fine tuning of the substituent on the porphyrin ring and the proper choice of the solvent have enabled us to observe negative activation enthalpies for the self-exchange electron transfer reactions. The observation of negative activation enthalpies indicates that the self-exchange electron transfer occurs via the charge-transfer π-complexes formed between zinc porphyrin radical cations and the neutral zinc porphyrins. The complete delocalization of the unpaired electron over two porphyrin moieties is observed in the radical cation of a zinc porphyrin dimer, 5,5'-bis(10,20-bis(3,5-di-tert-butylphenyl)porphyrinatozinc(II)). This is regarded as the extreme limit of the rapid self-exchange electron transfer between zinc porphyrin radical cation and the neutral form.
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Affiliation(s)
- Shunichi Fukuzumi
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, JAPAN Science and Technology Corporation, Suita, Osaka 565-0871, Japan
| | - Taku Hasobe
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, JAPAN Science and Technology Corporation, Suita, Osaka 565-0871, Japan
| | - Yoshito Endo
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, JAPAN Science and Technology Corporation, Suita, Osaka 565-0871, Japan
| | - Kei Ohkubo
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, JAPAN Science and Technology Corporation, Suita, Osaka 565-0871, Japan
| | - Hiroshi Imahori
- Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, PRESTO, JAPAN Science and Technology Corporation (JST), Nishikyo-ku, Kyoto 615-8510, Japan and Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan
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Simultaneous true, gated, and coupled electron-transfer reactions and energetics of protein rearrangement. J Inorg Biochem 2012; 106:143-50. [DOI: 10.1016/j.jinorgbio.2011.09.017] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2011] [Revised: 09/06/2011] [Accepted: 09/09/2011] [Indexed: 11/19/2022]
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21
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Ohzu S, Ishizuka T, Hirai Y, Jiang H, Sakaguchi M, Ogura T, Fukuzumi S, Kojima T. Mechanistic insight into catalytic oxidations of organic compounds by ruthenium(iv)-oxo complexes with pyridylamine ligands. Chem Sci 2012. [DOI: 10.1039/c2sc21195e] [Citation(s) in RCA: 66] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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22
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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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23
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Brala CJ, Pilepić V, Sajenko I, Karković A, Uršić S. Ions Can Move a Proton-Coupled Electron-Transfer Reaction into Tunneling Regime. Helv Chim Acta 2011. [DOI: 10.1002/hlca.201100035] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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24
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Proton-Coupled Electron Transfer Originating from Excited States of Luminescent Transition-Metal Complexes. Chemistry 2011; 17:11692-702. [DOI: 10.1002/chem.201102011] [Citation(s) in RCA: 69] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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25
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Horikoshi S, Sato T, Sakamoto K, Abe M, Serpone N. Microwave discharge electrodeless lamps (MDEL). Part VII. Photo-isomerization of trans-urocanic acid in aqueous media driven by UV light from a novel Hg-free Dewar-like microwave discharge thermally-insulated electrodeless lamp (MDTIEL). Performance evaluation. Photochem Photobiol Sci 2011; 10:1239-48. [PMID: 21523270 DOI: 10.1039/c1pp05059a] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A novel mercury-free Dewar-like (double-walled structure) microwave discharge thermally-insulated electrodeless lamp (MDTIEL) was fabricated and its performance evaluated using the photo-isomerization of trans-urocanic acid (trans-UA) in aqueous media as a test process driven by the emitted UV light when ignited with microwave radiation. The photo-isomerization processes trans-UA → cis-UA and cis-UA → trans-UA were re-visited using light emitted from a conventional high-pressure Hg light source and examined for the influence of UV light irradiance and solution temperature; the temperature dependence of the trans → cis process displayed a negative activation energy, E(a) = -1.3 cal mol(-1). To control the photo-isomerization of urocanic acid from the heat usually dissipated by a microwave discharge electrodeless lamp (single-walled MDEL), it was necessary to suppress the microwave-initiated heat. For comparison, the gas-fill in the MDEL lamp, which typically consists of a mixture of Hg and Ar, was changed to the more eco-friendly N(2) gas in the novel MDTIEL device. The dynamics of the photo-isomerization of urocanic acid driven by the UV wavelengths of the N(2)-MDTIEL light source were compared to those from the more conventional single-walled N(2)-MDEL and Hg/Ar-MDEL light sources, and with those from the Hg lamp used to irradiate, via a fiber optic, the photoreactor located in the wave-guide of the microwave apparatus. The heating efficiency of a solution with the double-walled N(2)-MDTIEL was compared to the efficiency from the single-walled N(2)-MDEL device. Advantages of N(2)-MDTIEL are described from a comparison of the dynamics of the trans-UA → cis-UA process on the basis of unit surface area of the lamp and unit power consumption. The considerably lower temperature on the external surface of the N(2)-MDTIEL light source should make it attractive in carrying out photochemical reactions that may be heat-sensitive such as the photothermochromic urocanic acid system.
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Affiliation(s)
- Satoshi Horikoshi
- Department of Material & Life Science, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, Japan.
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26
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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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27
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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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28
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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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29
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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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30
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Li J, Widlicka DW, Fichter K, Reed DP, Weisman GR, Wong EH, DiPasquale A, Heroux KJ, Golen JA, Rheingold AL. Comparative structural coordination chemistry of two tricyclic bisamidines. Inorganica Chim Acta 2010. [DOI: 10.1016/j.ica.2010.08.015] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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31
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Mader EA, Mayer JM. The importance of precursor and successor complex formation in a bimolecular proton-electron transfer reaction. Inorg Chem 2010; 49:3685-7. [PMID: 20302273 DOI: 10.1021/ic100143s] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
The transfer of a proton and an electron from the hydroxylamine 1-hydroxyl-2,2,6,6-tetramethylpiperidine (TEMPOH) to [Co(III)(Hbim)(H(2)bim)(2)](2+) (H(2)bim = 2,2'-biimidazoline) has an overall driving force of DeltaG degrees = -3.0 +/- 0.4 kcal mol(-1) and an activation barrier of DeltaG(degrees) = 21.9 +/- 0.2 kcal mol(-1). Kinetic studies implicate a hydrogen-bonded "precursor complex" at high [TEMPOH], prior to proton-electron (hydrogen-atom) transfer. In the reverse direction, [Co(II)(H(2)bim)(3)](2+) + TEMPO, a similar "successor complex" was not observed, but upper and lower limits on its formation have been estimated. The energetics of formation of these encounter complexes are the dominant contributors to the overall energetics in this system: DeltaG degrees ' for the proton-electron transfer step is only -0.3 +/- 0.9 kcal mol(-1). Thus, formation of the precursor and successor complexes can be a significant component of the thermochemistry for intermolecular proton-electron transfer, particularly in the low-driving-force regime, and should be included in quantitative analyses.
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Affiliation(s)
- Elizabeth A Mader
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
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32
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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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33
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Ni Z, McDaniel AM, Shores MP. Ambient temperature anion-dependent spin state switching observed in “mostly low spin” heteroleptic iron(ii) diimine complexes. Chem Sci 2010. [DOI: 10.1039/c0sc00303d] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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34
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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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The reaction of copper(II) and 2-hydrazino-2-imidazoline leading to the in situ formation of bisimidazoline (biz). Crystal structure and vibrational spectroscopy of mixed-valence [Cu(biz)2][Cu2Br4] complex. Polyhedron 2009. [DOI: 10.1016/j.poly.2009.03.003] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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36
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Lord RL, Schultz FA, Baik MH. Spin Crossover-Coupled Electron Transfer of [M(tacn)2]3+/2+ Complexes (tacn = 1,4,7-Triazacyclononane; M = Cr, Mn, Fe, Co, Ni). J Am Chem Soc 2009; 131:6189-97. [DOI: 10.1021/ja809552p] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Richard L. Lord
- Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, and Department of Chemistry and Chemical Biology, Indiana University Purdue University Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46402
| | - Franklin A. Schultz
- Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, and Department of Chemistry and Chemical Biology, Indiana University Purdue University Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46402
| | - Mu-Hyun Baik
- Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, and Department of Chemistry and Chemical Biology, Indiana University Purdue University Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46402
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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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Mader EA, Manner VW, Markle TF, Wu A, Franz JA, Mayer JM. Trends in ground-state entropies for transition metal based hydrogen atom transfer reactions. J Am Chem Soc 2009; 131:4335-45. [PMID: 19275235 PMCID: PMC2723939 DOI: 10.1021/ja8081846] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors M(II)LH and oxyl radicals. [Fe(II)(H(2)bip)(3)](2+), [Fe(II)(H(2)bim)(3)](2+), [Co(II)(H(2)bim)(3)](2+), and Ru(II)(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or (t)Bu(3)PhO(*) (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M(III)L and TEMPOH or (t)Bu(3)PhOH. Solution equilibrium measurements for the reaction of [Co(II)(H(2)bim)(3)](2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K(-1). This is even more negative than the DeltaS(o)(HAT) = -30 +/- 2 cal mol(-1) K(-1) for the two iron complexes and the DeltaS(o)(HAT) for Ru(II)(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K(-1), as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe(II)(H(2)bip)(3)](2+) + TEMPO, thus also confirming DeltaS(o)(HAT). Calorimetry on TEMPOH + (t)Bu(3)PhO(*) gives DeltaH(o)(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (t)Bu(3)PhOH supports the common assumption that DeltaS(o)(HAT) approximately 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |DeltaS(o)(HAT)| for reactions with TEMPO, Ru(II)(acac)(2)(py-imH) << [Fe(II)(H(2)bip)(3)](2+) = [Fe(II)(H(2)bim)(3)](2+) < [Co(II)(H(2)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, DeltaS(o)(ET), in aprotic solvents. This is because both DeltaS(o)(ET) and DeltaS(o)(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between DeltaS(o)(HAT) and DeltaS(o)(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.
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Affiliation(s)
- Elizabeth A Mader
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA
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Ni Z, Shores MP. Magnetic Observation of Anion Binding in Iron Coordination Complexes: Toward Spin-Switching Chemosensors. J Am Chem Soc 2008; 131:32-3. [DOI: 10.1021/ja807379a] [Citation(s) in RCA: 78] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Zhaoping Ni
- Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
| | - Matthew P. Shores
- Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
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Wu A, Mayer JM. Hydrogen atom transfer reactions of a ruthenium imidazole complex: hydrogen tunneling and the applicability of the Marcus cross relation. J Am Chem Soc 2008; 130:14745-54. [PMID: 18841973 PMCID: PMC2633126 DOI: 10.1021/ja805067h] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives Ru(III)(acac)2(py-im) (Ru(III)im) and the hydroxylamine TEMPO-H by transfer of H(*) (H(+) + e(-)) (acac = 2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k(3H) = 1400 +/- 100 M(-1) s(-1) at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (Delta G(o)). Deuterium transfer from Ru(II)(acac)2(py-imD) (Ru(II)imD) to TEMPO(*) is surprisingly much slower at k(3D) = 60 +/- 7 M(-1) s(-1), with k(3H)/k(3D) = 23 +/- 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E(a3D) - E(a3H) = 1.9 +/- 0.8 kcal mol(-1). The large k(3H)/k(3D) and DeltaE(a) values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru(II)imH and Ru(III)im, measured by (1)H NMR line broadening, occurs with k(4H) = (3.2 +/- 0.3) x 10(5) M(-1) s(-1) at 298 K and k(4H)/k(4D) = 1.5 +/- 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A(4H)/A(4D)) = -0.5 +/- 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru(II)imH(D) + TEMPO(*) that are greater than those observed: k(3H,calc)/k(3H) = 31 +/- 4 and k(3D,calc)/k(3D) = 140 +/- 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.
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Affiliation(s)
- Adam Wu
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
| | - James M. Mayer
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
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Wu A, Masland J, Swartz RD, Kaminsky W, Mayer JM. Synthesis and characterization of ruthenium bis(beta-diketonato) pyridine-imidazole complexes for hydrogen atom transfer. Inorg Chem 2007; 46:11190-201. [PMID: 18052056 PMCID: PMC2596074 DOI: 10.1021/ic7015726] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Ruthenium bis(beta-diketonato) complexes have been prepared at both the RuII and RuIII oxidation levels and with protonated and deprotonated pyridine-imidazole ligands. RuII(acac)2(py-imH) (1), [RuIII(acac)2(py-imH)]OTf (2), RuIII(acac)2(py-im) (3), RuII(hfac)2(py-imH) (4), and [DBU-H][RuII(hfac)2(py-im)] (5) have been fully characterized, including X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2, cyclic voltammetry in MeCN shows the RuIII/II reduction potential (E1/2) to be -0.64 V versus Cp2Fe+/0. E1/2 for the deprotonated imidazolate complex 3 (-1.00 V) is 0.36 V more negative. The RuII bis-hfac analogues 4 and 5 show the same DeltaE1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 and -0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pKa values of 22.1 and 19.3 in MeCN for 1 and 4, respectively. From the E1/2 and pKa values, the bond dissociation free energies (BDFEs) of the N-H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol(-1) in MeCN - a remarkable difference of 17.6 kcal mol(-1) for such structurally similar compounds. Consistent with these values, there is a facile net hydrogen atom transfer from 1 to TEMPO* (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The DeltaG degrees for this reaction is -4.5 kcal mol(-1). 4 is not oxidized by TEMPO* (DeltaG degrees = +13.1 kcal mol(-1)), but in the reverse direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im) (6). A RuII-imidazoline analogue of 1, RuII(acac)2(py-imnH) (7), reacts with 3 equiv of TEMPO* to give the imidazolate 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.
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Affiliation(s)
- Adam Wu
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
| | - Joshua Masland
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
| | | | | | - James M. Mayer
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
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Manner VW, Markle TF, Freudenthal JH, Roth JP, Mayer JM. The first crystal structure of a monomeric phenoxyl radical: 2,4,6-tri-tert-butylphenoxyl radical. Chem Commun (Camb) 2007:256-8. [PMID: 18092105 DOI: 10.1039/b712872j] [Citation(s) in RCA: 122] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Crystals of the 2,4,6-tri-tert-butylphenoxyl radical have been isolated and characterized by X-ray diffraction, and calculations have been performed that give the distribution of spin density in the radical.
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Affiliation(s)
- My Hang V Huynh
- DE-1: High Explosive Science and Technology Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
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Halcrow MA. The spin-states and spin-transitions of mononuclear iron(II) complexes of nitrogen-donor ligands. Polyhedron 2007. [DOI: 10.1016/j.poly.2007.03.033] [Citation(s) in RCA: 270] [Impact Index Per Article: 15.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Zhu XQ, Zhang JY, Cheng JP. Negative kinetic temperature effect on the hydride transfer from NADH analogue BNAH to the radical cation of N-benzylphenothiazine in acetonitrile. J Org Chem 2007; 71:7007-15. [PMID: 16930056 DOI: 10.1021/jo061145c] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The reaction rates of 1-(p-substituted benzyl)-1,4-dihydronicotinamide (G-BNAH) with N-benzylphenothiazine radical cation (PTZ(*+)) in acetonitrile were determined. The results show that the reaction rates (k(obs)) decreased from 2.80 x 10(7) to 2.16 x 10(7) M(-1) s(-1) for G = H as the reaction temperature increased from 298 to 318 K. The activation enthalpies of the reactions were estimated according to Eyring equation to give negative values (-3.4 to -2.9 kcal/mol). Investigation of the reaction intermediate shows that the charge-transfer complex (CT-complex) between G-BNAH and PTZ(*+) was formed in front of the hydride transfer from G-BNAH to PTZ(*+). The formation enthalpy of the CT-complex was estimated by using the Benesi-Hildebrand equation to give the values from -6.4 to -6.0 kcal/mol when the substituent G in G-BNAH changes from CH(3)O to Br. Detailed thermodynamic analyses on each elementary step in the possible reaction pathways suggest that the hydride transfer from G-BNAH to PTZ(*+) occurs by a concerted hydride transfer via a CT-complex. The effective charge distribution on the pyridine ring in G-BNAH at the various stages-the reactant G-BNAH, the charge-transfer complex, the transition-state, and the product G-BNA(+)-was estimated by using the method of Hammett-type linear free energy analysis, and the results show that the pyridine ring carries relative effective positive charges of 0.35 in the CT-complex and 0.45 in the transition state, respectively, which indicates that the concerted hydride transfer from G-BNAH to PTZ(*+) was practically performed by the initial charge (-0.35) transfer from G-BNAH to PTZ(*+) and then followed by the transfer of hydrogen atom with partial negative charge (-0.65). It is evident that the present work would be helpful in understanding the nature of the negative temperature effect, especially on the reaction of NADH coenzyme with the drug phenothiazine in vivo.
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Affiliation(s)
- Xiao-Qing Zhu
- Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China.
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Mader EA, Davidson ER, Mayer JM. Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. J Am Chem Soc 2007; 129:5153-66. [PMID: 17402735 PMCID: PMC2628630 DOI: 10.1021/ja0686918] [Citation(s) in RCA: 116] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.
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Affiliation(s)
- Elizabeth A. Mader
- University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA, E-mail:
| | - Ernest R. Davidson
- University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA, E-mail:
| | - James M. Mayer
- University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA, E-mail:
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Madhiri N, Finklea HO. Potential-, pH-, and isotope-dependence of proton-coupled electron transfer of an osmium aquo complex attached to an electrode. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2006; 22:10643-51. [PMID: 17129042 DOI: 10.1021/la061103j] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
An osmium complex, [OsII(bpy)2(4-aminomethylpyridine)(H2O)]2+, is attached to a mixed self-assembled monolayer on a gold electrode. The complex exhibits 1-electron, 1-proton redox chemistry (OsIII(OH)/OsII(H2O)) at pHs and potentials that are experimentally accessible with gold electrodes in aqueous electrolytes. The thermodynamic behavior and kinetic behavior of the system are investigated as a function of pH in both H2O and D2O. The two formal potentials and two pKa values are relatively constant for two chain lengths in H2O and in D2O. The standard rate constants at all pHs are strongly and uniformly affected by chain length, indicating that electronic coupling is the dominant factor controlling the rate of electron transfer. In both H2O and D2O, the standard rate constant is weakly dependent on the pH, exhibiting a minimum value midway between the pKa values. The kinetic isotope effect is small; standard rate constants decrease by roughly a factor of 2 in D2O over a wide range of pHs, but not at the more acidic pHs. The Tafel plots and plots of the transfer coefficient vs overpotential are asymmetrical at all pHs. These results are interpreted in terms of a larger reorganization energy for the OsII species and a smaller reorganization energy for the OsIII species. The OsIII reorganization energy is constant at all pHs in both H2O and D2O. The pH dependence of the OsII reorganization energy accounts for some or all of the pH dependence of the standard rate constant in H2O and D2O. The data deviate substantially from predictions of the stepwise proton-coupled electron-transfer mechanism. The observation of a kinetic isotope effect supports the concerted mechanism.
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Affiliation(s)
- Nicholas Madhiri
- C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, USA
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Czap A, Neuman NI, Swaddle TW. Electrochemistry and Homogeneous Self-Exchange Kinetics of the Aqueous 12-Tungstoaluminate(5−/6−) Couple. Inorg Chem 2006; 45:9518-30. [PMID: 17083254 DOI: 10.1021/ic060527y] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The effect of alkali metal (M) chloride or triflate supporting electrolytes (0.1-1.0 mol L(-1)) on the midpoint potential E(m) of the aqueous AlW12O40(5-/6-) couple in cyclic voltammetry, after correction (E(corr)) for liquid junction potentials, can be represented in terms of ionic strength according to the extended Debye-Hückel equation. However, unrealistically short AlW12O40(5-/6-)-cation closest-approach distances are required to accommodate the specific effects of M+, and the infinite-dilution potential E(corr)(0) values are not quite consistent from one M+ to another. The pressure dependence of Em is qualitatively consistent with expectations based on the Born-Drude-Nernst theory. The strong accelerating effects of supporting electrolytes on the standard electrode reaction rate constant k(el) at pH 3 as measured by alternating current voltammetry (ACV), and on the homogeneous self-exchange rate constant k(ex) at pH 3-7 as measured by 27Al line broadening, depend specifically on the identity and concentration of M+ (Li+ < Na+ < K+ < Rb+) rather than on the ionic strength, whereas the effect of the nature of the supporting anion (Cl- or CF3SO3-) is negligible. Extrapolation of k(el) and k(ex) to zero [M+] indicates that the uncatalyzed electron transfer rate is negligibly small relative to the M+ catalyzed rates. The kinetic effects of M+ show no evidence of the saturation expected had they been due primarily to ion pairing with AlW12O40(5-/6-). The catalytic effect of M+ operates primarily through lowering the enthalpy of activation, which is partially offset by a strongly negative entropy of activation and, for the homogeneous exchange catalyzed by K+ or Rb+, becomes mildly negative; thus, the catalytic effect of M(+) is enthalpy-driven but entropy-limited. For the electrode reaction, the volume of activation averages +4.5 +/- 0.2 cm(3) mol(-1) for all M+ and [M+], in contrast to the negative value predicted theoretically for the uncatalyzed reaction. These results are consistent with a reaction mechanism, previously proposed for other anion-anion electron-transfer reactions, in which anion-anion electron transfer is facilitated by partially dehydrated M+.
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Affiliation(s)
- Almut Czap
- Department of Chemistry, University of Calgary, Calgary, Alberta, Canada
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Wik BJ, Lersch M, Krivokapic A, Tilset M. Adding a new dimension to the investigation of platinum-mediated arene C-H activation reactions using 2D NMR exchange spectroscopy. Dynamics of Pt(II) phenyl/benzene site exchange. J Am Chem Soc 2006; 128:2682-96. [PMID: 16492055 DOI: 10.1021/ja056694z] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Protonation of (N-N)PtPh(2) (1; N-N = diimine ArN=CMe-CMe=NAr with Ar = 2,6-Me(2)C(6)H(3) (a), 2,4,6-Me(3)C(6)H(2) (b), 4-Br-2,6-Me(2)C(6)H(2) (c), 3,5-Me(2)C(6)H(3) (d), and 4-CF(3)C(6)H(4) (e)) in the presence of MeCN at ambient temperature generates (N-N)Pt(Ph)(NCMe)(+) (2). At -78 degrees C, protonation of 1a yielded (N-N)PtPh(2)(H)(NCMe)(+) (3a), which produced benzene and 2a at ca. -40 degrees C. Protonation of 1a-e in CD(2)Cl(2)/Et(2)O-d(10) furnished (N-N)Pt(C(6)H(5))(eta(2)-C(6)H(6))(+) (4a-e). The pi-benzene complexes 4a-c, sterically protected at Pt, eliminate benzene at ca. 0 degree C. The sterically less protected 4d-e lose benzene already at -30 degrees C. SST and 2D EXSY NMR demonstrate that phenyl and pi-benzene ligand protons undergo exchange with concomitant symmetrization of the diimine ligand, most likely via oxidative insertion of Pt into a C-H bond of coordinated benzene. The kinetics of the exchange processes for 4a-c were probed by quantitative EXSY spectroscopy, resulting in DeltaH() of 70-72 kJ mol(-1) and DeltaS of 37-48 J K(-1) mol(-1). A large, strongly temperature-dependent H/D kinetic isotope effect (9.7 at -34 degrees C; 6.9 at -19 degrees C) was measured for the dynamic behavior of 4a versus 4a-d(10), consistent with the proposed pi-benzene C-H bond cleavage. The fact that the pi-benzene complex 4a is thermally more robust in the absence of MeCN than is the Pt(IV) hydridodiphenyl complex 3a in the presence of MeCN agrees with the notion that arene elimination from Pt(IV) hydridoaryl complexes occurs via Pt(II) pi-arene intermediates that eliminate the hydrocarbon associatively, in this case, promoted by MeCN. Compounds 1a, 1b, 1d, 2a, and 2b have been crystallographically characterized.
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Ding BB, Weng YQ, Mao ZW, Lam CK, Chen XM, Ye BH. Pillared-Layer Microporous Metal−Organic Frameworks Constructed by Robust Hydrogen Bonds. Synthesis, Characterization, and Magnetic and Adsorption Properties of 2,2‘-Biimidazole and Carboxylate Complexes. Inorg Chem 2005; 44:8836-45. [PMID: 16296838 DOI: 10.1021/ic051195k] [Citation(s) in RCA: 137] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Two new isostructural complexes [M(H2biim)3][M(btc)(Hbiim)].2H2O (M = Co, (1); M = Ni, (2)) (btc = 1,3,5-benzenetricarboxylate; H2biim = 2,2'-biimidazole) have been synthesized and characterized by single-crystal X-ray diffraction. They present a unique structure consisting of two distinct units: the monomeric cations [M(H2biim)3]2+ and the two-dimensional (2D) anionic polymer [M(Hbiim)(btc)]2-. In the anionic moiety, the Hbiim- monoanion is simultaneously coordinated to one metal atom in a bidentate mode and further to another metal atom in a monodentate mode. The imidazolate groups bridge the two adjacent metal ions into a helical chain which is further arranged in left- and right-handed manners. These chains are bridged by btc ligands into a 2D brick wall structure. The most interesting aspect is that the [M(H2biim)3]2+ cations act as pillars and link the anionic layers via robust heteromeric hydrogen-bonded synthons (9) and (7) formed by the uncoordinated oxygen atoms of carboxylate groups and the H2biim ligands, resulting in a microporous metal-organic framework with one-dimensional (1D) channels (ca. 11.85 angstroms x 11.85 angstroms for 1 and 11.43 angstroms x 11.43 angstroms for 2). Magnetic properties of these two complexes have also been studied in the temperature range of 2-300 K, and their magnetic susceptibilities obey the Curie-Weiss law in the temperature range of 20-300 K (for 1) and 2-300 K (for 2), respectively, showing anti-ferromagnetic coupling through imidazolate bridging. Taking into consideration the Heisenberg infinite chain model as well as the possibility of chain-to-chain and chain-to-cation interactions, the anti-ferromagnetic exchange of 2 is analyzed via a correction for the molecular field, giving the values of g(cat) = 2.296, g(Ni) = 2.564, J = -13.30 cm(-1), and zJ' = -0.017 cm(-1). The microporous frameworks are stable at ca. 350 degrees C. They do not collapse after removal of the guest water molecules in the channels, and they adsorb methanol molecules selectively.
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Affiliation(s)
- Bing-Bing Ding
- School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
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