Formation and reactivity of a persistent radical in a dinuclear molybdenum complex

Aaron Appel, Sun Jane Lee, James A. Franz, Daniel L DuBois, M. Rakowski DuBois, Jerome C. Birnbaum, Brendan Twamley

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

The reactivity of the S-H bond in Cp*Mo(μ-S)2(μ-SMe) (μ-SH)MoCp* (S4MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(μ-S) 3(μ-SMe)MoCp* (S4Me•), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, ΔH and ΔS were determined to be 1.54 ± 0.25 kcal/mol and -25.5 ± 0.8 cal/mol K, respectively, giving kabs = 1.3 × 106 M-1 s-1 at 25°C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo2S4 core was found to be the benzyl cross-termination product, Cp*Mo(μ-S) 2(μ-SMe)(μ-SBz)MoCp* (S4MeBz), consistent with the Fischer-Ingold persistent radical effect. S4Me• was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(μ-S)2(μ-SMe)MoCp*) 2(μ-S2) ((S4Me)2). The radical-dimer equilibrium constant has been determined to be 5.7 × 10 4 ± 2.1 × 104 M-1 from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 × 103 s-1 at 25°C, based on variable temperature 1H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 × 107 M-1 s-1 in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for (S 4Me)2, S4MeBz, and the cationic Cp*Mo(μ-S2)(μ-S)(μ-SMe)MoCp*(OTf) (S 4Me+), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S4 core is presented.

Original languageEnglish
Pages (from-to)8940-8951
Number of pages12
JournalJournal of the American Chemical Society
Volume130
Issue number28
DOIs
Publication statusPublished - Jul 16 2008

Fingerprint

Molybdenum
Dimers
Hydrogen
Dimerization
Rate constants
Temperature
Equilibrium constants
Toluene
Disulfides
Atoms
Kinetics
Paramagnetic resonance
Nuclear magnetic resonance
Derivatives
Experiments

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Formation and reactivity of a persistent radical in a dinuclear molybdenum complex. / Appel, Aaron; Lee, Sun Jane; Franz, James A.; DuBois, Daniel L; DuBois, M. Rakowski; Birnbaum, Jerome C.; Twamley, Brendan.

In: Journal of the American Chemical Society, Vol. 130, No. 28, 16.07.2008, p. 8940-8951.

Research output: Contribution to journalArticle

Appel, Aaron ; Lee, Sun Jane ; Franz, James A. ; DuBois, Daniel L ; DuBois, M. Rakowski ; Birnbaum, Jerome C. ; Twamley, Brendan. / Formation and reactivity of a persistent radical in a dinuclear molybdenum complex. In: Journal of the American Chemical Society. 2008 ; Vol. 130, No. 28. pp. 8940-8951.
@article{4bf09cc61bf04fc0b085018800053064,
title = "Formation and reactivity of a persistent radical in a dinuclear molybdenum complex",
abstract = "The reactivity of the S-H bond in Cp*Mo(μ-S)2(μ-SMe) (μ-SH)MoCp* (S4MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(μ-S) 3(μ-SMe)MoCp* (S4Me•), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, ΔH‡ and ΔS‡ were determined to be 1.54 ± 0.25 kcal/mol and -25.5 ± 0.8 cal/mol K, respectively, giving kabs = 1.3 × 106 M-1 s-1 at 25°C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo2S4 core was found to be the benzyl cross-termination product, Cp*Mo(μ-S) 2(μ-SMe)(μ-SBz)MoCp* (S4MeBz), consistent with the Fischer-Ingold persistent radical effect. S4Me• was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(μ-S)2(μ-SMe)MoCp*) 2(μ-S2) ((S4Me)2). The radical-dimer equilibrium constant has been determined to be 5.7 × 10 4 ± 2.1 × 104 M-1 from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 × 103 s-1 at 25°C, based on variable temperature 1H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 × 107 M-1 s-1 in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for (S 4Me)2, S4MeBz, and the cationic Cp*Mo(μ-S2)(μ-S)(μ-SMe)MoCp*(OTf) (S 4Me+), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S4 core is presented.",
author = "Aaron Appel and Lee, {Sun Jane} and Franz, {James A.} and DuBois, {Daniel L} and DuBois, {M. Rakowski} and Birnbaum, {Jerome C.} and Brendan Twamley",
year = "2008",
month = "7",
day = "16",
doi = "10.1021/ja078115r",
language = "English",
volume = "130",
pages = "8940--8951",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
number = "28",

}

TY - JOUR

T1 - Formation and reactivity of a persistent radical in a dinuclear molybdenum complex

AU - Appel, Aaron

AU - Lee, Sun Jane

AU - Franz, James A.

AU - DuBois, Daniel L

AU - DuBois, M. Rakowski

AU - Birnbaum, Jerome C.

AU - Twamley, Brendan

PY - 2008/7/16

Y1 - 2008/7/16

N2 - The reactivity of the S-H bond in Cp*Mo(μ-S)2(μ-SMe) (μ-SH)MoCp* (S4MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(μ-S) 3(μ-SMe)MoCp* (S4Me•), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, ΔH‡ and ΔS‡ were determined to be 1.54 ± 0.25 kcal/mol and -25.5 ± 0.8 cal/mol K, respectively, giving kabs = 1.3 × 106 M-1 s-1 at 25°C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo2S4 core was found to be the benzyl cross-termination product, Cp*Mo(μ-S) 2(μ-SMe)(μ-SBz)MoCp* (S4MeBz), consistent with the Fischer-Ingold persistent radical effect. S4Me• was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(μ-S)2(μ-SMe)MoCp*) 2(μ-S2) ((S4Me)2). The radical-dimer equilibrium constant has been determined to be 5.7 × 10 4 ± 2.1 × 104 M-1 from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 × 103 s-1 at 25°C, based on variable temperature 1H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 × 107 M-1 s-1 in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for (S 4Me)2, S4MeBz, and the cationic Cp*Mo(μ-S2)(μ-S)(μ-SMe)MoCp*(OTf) (S 4Me+), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S4 core is presented.

AB - The reactivity of the S-H bond in Cp*Mo(μ-S)2(μ-SMe) (μ-SH)MoCp* (S4MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(μ-S) 3(μ-SMe)MoCp* (S4Me•), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, ΔH‡ and ΔS‡ were determined to be 1.54 ± 0.25 kcal/mol and -25.5 ± 0.8 cal/mol K, respectively, giving kabs = 1.3 × 106 M-1 s-1 at 25°C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo2S4 core was found to be the benzyl cross-termination product, Cp*Mo(μ-S) 2(μ-SMe)(μ-SBz)MoCp* (S4MeBz), consistent with the Fischer-Ingold persistent radical effect. S4Me• was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(μ-S)2(μ-SMe)MoCp*) 2(μ-S2) ((S4Me)2). The radical-dimer equilibrium constant has been determined to be 5.7 × 10 4 ± 2.1 × 104 M-1 from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 × 103 s-1 at 25°C, based on variable temperature 1H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 × 107 M-1 s-1 in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for (S 4Me)2, S4MeBz, and the cationic Cp*Mo(μ-S2)(μ-S)(μ-SMe)MoCp*(OTf) (S 4Me+), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S4 core is presented.

UR - http://www.scopus.com/inward/record.url?scp=48949118180&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=48949118180&partnerID=8YFLogxK

U2 - 10.1021/ja078115r

DO - 10.1021/ja078115r

M3 - Article

VL - 130

SP - 8940

EP - 8951

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 28

ER -