Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole Complex: Hydrogen Tunneling and the Applicability of the Marcus Cross Relation.
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| Title: | Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole Complex: Hydrogen Tunneling and the Applicability of the Marcus Cross Relation. |
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| Authors: | Wu, Adam1, Mayer, James M.1 mayer@chem.washington.edu |
| Source: | Journal of the American Chemical Society. 11/5/2008, Vol. 130 Issue 44, p14745-14754. 10p. 1 Diagram, 2 Charts, 8 Graphs. |
| Subjects: | Hydrogen, Ruthenium, Imidazoles, Charge exchange, Thermochemistry |
| Abstract: | The reaction of RuII(acac)2(py-imH) (RuIIimH) with TEMPO• (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives RuIII(acac)2(py-im) (RuIIIim) 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 k3H = 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 (ΔG°). Deuterium transfer from RuII(acac)2(py-imD) (RuIIimD) to TEMPO• is surprisingly much slower at k3D = 60 ± 7 M-1 s-1, with k3H/k3D = 23± 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, Ea3D - Ea3H = 1.9 + 0.8 kcal mol-1. The large k3H/k3D and ΔEa values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between RuIIimH and RuIIim, measured by ¹H NMR line broadening, occurs with k4H = (3.2 ± 0.3) × 105 M-1 s-1 at 298 K and k4H/k4D = 1.5 ± 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, Iog(A4H/A4D) = -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 RuIIimH(D) + TEMPO that are greater than those observed: k3H,calc/k3H = 31 ± 4 and k3D,caIc/k3D = 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. [ABSTRACT FROM AUTHOR] |
| Copyright of Journal of the American Chemical Society is the property of American Chemical Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.) | |
| Database: | Engineering Source |
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| Items | – Name: Title Label: Title Group: Ti Data: Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole Complex: Hydrogen Tunneling and the Applicability of the Marcus Cross Relation. – Name: Author Label: Authors Group: Au Data: <searchLink fieldCode="AR" term="%22Wu%2C+Adam%22">Wu, Adam</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Mayer%2C+James+M%2E%22">Mayer, James M.</searchLink><relatesTo>1</relatesTo><i> mayer@chem.washington.edu</i> – Name: TitleSource Label: Source Group: Src Data: <searchLink fieldCode="JN" term="%22Journal+of+the+American+Chemical+Society%22">Journal of the American Chemical Society</searchLink>. 11/5/2008, Vol. 130 Issue 44, p14745-14754. 10p. 1 Diagram, 2 Charts, 8 Graphs. – Name: Subject Label: Subjects Group: Su Data: <searchLink fieldCode="DE" term="%22Hydrogen%22">Hydrogen</searchLink><br /><searchLink fieldCode="DE" term="%22Ruthenium%22">Ruthenium</searchLink><br /><searchLink fieldCode="DE" term="%22Imidazoles%22">Imidazoles</searchLink><br /><searchLink fieldCode="DE" term="%22Charge+exchange%22">Charge exchange</searchLink><br /><searchLink fieldCode="DE" term="%22Thermochemistry%22">Thermochemistry</searchLink> – Name: Abstract Label: Abstract Group: Ab Data: The reaction of RuII(acac)2(py-imH) (RuIIimH) with TEMPO• (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives RuIII(acac)2(py-im) (RuIIIim) 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 k3H = 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 (ΔG°). Deuterium transfer from RuII(acac)2(py-imD) (RuIIimD) to TEMPO• is surprisingly much slower at k3D = 60 ± 7 M-1 s-1, with k3H/k3D = 23± 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, Ea3D - Ea3H = 1.9 + 0.8 kcal mol-1. The large k3H/k3D and ΔEa values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between RuIIimH and RuIIim, measured by ¹H NMR line broadening, occurs with k4H = (3.2 ± 0.3) × 105 M-1 s-1 at 298 K and k4H/k4D = 1.5 ± 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, Iog(A4H/A4D) = -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 RuIIimH(D) + TEMPO that are greater than those observed: k3H,calc/k3H = 31 ± 4 and k3D,caIc/k3D = 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. [ABSTRACT FROM AUTHOR] – Name: AbstractSuppliedCopyright Label: Group: Ab Data: <i>Copyright of Journal of the American Chemical Society is the property of American Chemical Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.) |
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| RecordInfo | BibRecord: BibEntity: Identifiers: – Type: doi Value: 10.1021/ja805067h Languages: – Code: eng Text: English PhysicalDescription: Pagination: PageCount: 10 StartPage: 14745 Subjects: – SubjectFull: Hydrogen Type: general – SubjectFull: Ruthenium Type: general – SubjectFull: Imidazoles Type: general – SubjectFull: Charge exchange Type: general – SubjectFull: Thermochemistry Type: general Titles: – TitleFull: Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole Complex: Hydrogen Tunneling and the Applicability of the Marcus Cross Relation. Type: main BibRelationships: HasContributorRelationships: – PersonEntity: Name: NameFull: Wu, Adam – PersonEntity: Name: NameFull: Mayer, James M. IsPartOfRelationships: – BibEntity: Dates: – D: 05 M: 11 Text: 11/5/2008 Type: published Y: 2008 Identifiers: – Type: issn-print Value: 00027863 Numbering: – Type: volume Value: 130 – Type: issue Value: 44 Titles: – TitleFull: Journal of the American Chemical Society Type: main |
| ResultId | 1 |
