Activation of the S-H group in Fe(μ2-SH)Fe clusters

S-H bond strengths and free radical reactivity of the Fe(μ2-SH)Fe cluster

James A. Franz, Suh Jane Lee, Thomas A. Bowden, Mikhail S. Alnajjar, Aaron Appel, Jerome C. Birnbaum, Thomas E. Bitterwolf, Michel Dupuis

Research output: Contribution to journalArticle

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Abstract

Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)3Fe(μ-SH)2Fe(CO)3 (Fe 2S2H2) and (OC)3Fe(μ-SCH 3)(μ-SH)Fe(CO)3 (Fe2S2MeH) by benzyl radicals in benzene. From the temperature-dependent rate data for Fe 2S2H2, ΔH and ΔS were determined to be 2.03 ± 0.56 kcal/mol and -19.3 ± 1.7 cal/(mol K), respectively, giving kabs = (1.2 ± 0.49) × 107 M-1 s-1 at 25°C. For Fe2S2MeH, ΔH and ΔS were determined to be 1.97 ± 0.46 kcal/mol and -18.1 ± 1.5 cal/(mol K), respectively, giving kabs = (2.3 ± 0.23) × 107 M-1 s-1 at 25°C. Temperature-dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (ΔH = 3.62 ± 0.43 kcal/mol, ΔS = -21.7 ± 1.3 cal/(mol K)) and H2S (ΔH = 5.13 ± 0.99 kcal/mol, ΔS = -24.8 ± 3.2 cal/(mol K)), giving k abs at 25°C of (2.5 ± 0.33) × 105 and (4.2 ± 0.51) × 103 M-1 s-1, respectively, both having hydrogen atom abstraction rate constants orders of magnitude slower than those of Fe2S2H2 and Fe2S2MeH. Thus, Fe2S2MeH is 100-fold faster than thiophenol, known as a fast donor. All rate constants are reported per abstractable hydrogen atom (kabs/M-1 s -1/H). DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe2S2H2 and Fe2S 2MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe2S2H 2 and Fe2S2MeH isomers. Derived radicals Fe2S2H and Fe2S 2Me exhibit singly occupied HOMOs with unpaired spin density distributed between the two Fe atoms, a bridging sulfur, and d σ-bonding between Fe centers. The S-H solution bond dissociation free energy (SBDFE) of Fe2S2MeH was found to be 69.4 ± 1.7 kcal/mol by determination of its pKa (16.0 ± 0.4) and the potential for the oxidation of the anion, Fe 2S2Me-, of -0.26 ± 0.05 V vs ferrocene in acetonitrile (corrected for dimerization of Fe2S 2Me). This SBDFE for Fe2S2MeH corresponds to a gas-phase bond dissociation enthalpy (BDE) of 74.2 kcal/mol, in satisfactory agreement with the DFT value of 73.2 kcal/mol. Replacement of the Fe-Fe bond in Fe2S2MeH with bridging μ-S (Fe 2S3MeH) or μ-CO (Fe2S2(CO)MeH) groups leads to (DFT) BDEs of 72.8 and 66.2 kcal/mol, the latter indicating dramatic effects of the choice of bridge structure on S-H bond strengths. These results provide a model for the reactivity of hydrosulfido sites of low-valent heterogeneous FeS catalysts.

Original languageEnglish
Pages (from-to)15212-15224
Number of pages13
JournalJournal of the American Chemical Society
Volume131
Issue number42
DOIs
Publication statusPublished - Oct 28 2009

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Free radicals
Free Radicals
Hydrogen
Carbon Monoxide
Chemical activation
Atoms
Discrete Fourier transforms
Free energy
Rate constants
Temperature
Nuclear magnetic resonance
Dimerization
Benzene
Sulfur
Anions
Chemical shift
Isomers
Gases
Enthalpy
Acetonitrile

ASJC Scopus subject areas

  • Chemistry(all)
  • Catalysis
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Activation of the S-H group in Fe(μ2-SH)Fe clusters : S-H bond strengths and free radical reactivity of the Fe(μ2-SH)Fe cluster. / Franz, James A.; Lee, Suh Jane; Bowden, Thomas A.; Alnajjar, Mikhail S.; Appel, Aaron; Birnbaum, Jerome C.; Bitterwolf, Thomas E.; Dupuis, Michel.

In: Journal of the American Chemical Society, Vol. 131, No. 42, 28.10.2009, p. 15212-15224.

Research output: Contribution to journalArticle

Franz, James A. ; Lee, Suh Jane ; Bowden, Thomas A. ; Alnajjar, Mikhail S. ; Appel, Aaron ; Birnbaum, Jerome C. ; Bitterwolf, Thomas E. ; Dupuis, Michel. / Activation of the S-H group in Fe(μ2-SH)Fe clusters : S-H bond strengths and free radical reactivity of the Fe(μ2-SH)Fe cluster. In: Journal of the American Chemical Society. 2009 ; Vol. 131, No. 42. pp. 15212-15224.
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title = "Activation of the S-H group in Fe(μ2-SH)Fe clusters: S-H bond strengths and free radical reactivity of the Fe(μ2-SH)Fe cluster",
abstract = "Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)3Fe(μ-SH)2Fe(CO)3 (Fe 2S2H2) and (OC)3Fe(μ-SCH 3)(μ-SH)Fe(CO)3 (Fe2S2MeH) by benzyl radicals in benzene. From the temperature-dependent rate data for Fe 2S2H2, ΔH‡ and ΔS‡ were determined to be 2.03 ± 0.56 kcal/mol and -19.3 ± 1.7 cal/(mol K), respectively, giving kabs = (1.2 ± 0.49) × 107 M-1 s-1 at 25°C. For Fe2S2MeH, ΔH‡ and ΔS‡ were determined to be 1.97 ± 0.46 kcal/mol and -18.1 ± 1.5 cal/(mol K), respectively, giving kabs = (2.3 ± 0.23) × 107 M-1 s-1 at 25°C. Temperature-dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (ΔH‡ = 3.62 ± 0.43 kcal/mol, ΔS‡ = -21.7 ± 1.3 cal/(mol K)) and H2S (ΔH‡ = 5.13 ± 0.99 kcal/mol, ΔS‡ = -24.8 ± 3.2 cal/(mol K)), giving k abs at 25°C of (2.5 ± 0.33) × 105 and (4.2 ± 0.51) × 103 M-1 s-1, respectively, both having hydrogen atom abstraction rate constants orders of magnitude slower than those of Fe2S2H2 and Fe2S2MeH. Thus, Fe2S2MeH is 100-fold faster than thiophenol, known as a fast donor. All rate constants are reported per abstractable hydrogen atom (kabs/M-1 s -1/H). DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe2S2H2 and Fe2S 2MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe2S2H 2 and Fe2S2MeH isomers. Derived radicals Fe2S2H• and Fe2S 2Me• exhibit singly occupied HOMOs with unpaired spin density distributed between the two Fe atoms, a bridging sulfur, and d σ-bonding between Fe centers. The S-H solution bond dissociation free energy (SBDFE) of Fe2S2MeH was found to be 69.4 ± 1.7 kcal/mol by determination of its pKa (16.0 ± 0.4) and the potential for the oxidation of the anion, Fe 2S2Me-, of -0.26 ± 0.05 V vs ferrocene in acetonitrile (corrected for dimerization of Fe2S 2Me•). This SBDFE for Fe2S2MeH corresponds to a gas-phase bond dissociation enthalpy (BDE) of 74.2 kcal/mol, in satisfactory agreement with the DFT value of 73.2 kcal/mol. Replacement of the Fe-Fe bond in Fe2S2MeH with bridging μ-S (Fe 2S3MeH) or μ-CO (Fe2S2(CO)MeH) groups leads to (DFT) BDEs of 72.8 and 66.2 kcal/mol, the latter indicating dramatic effects of the choice of bridge structure on S-H bond strengths. These results provide a model for the reactivity of hydrosulfido sites of low-valent heterogeneous FeS catalysts.",
author = "Franz, {James A.} and Lee, {Suh Jane} and Bowden, {Thomas A.} and Alnajjar, {Mikhail S.} and Aaron Appel and Birnbaum, {Jerome C.} and Bitterwolf, {Thomas E.} and Michel Dupuis",
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TY - JOUR

T1 - Activation of the S-H group in Fe(μ2-SH)Fe clusters

T2 - S-H bond strengths and free radical reactivity of the Fe(μ2-SH)Fe cluster

AU - Franz, James A.

AU - Lee, Suh Jane

AU - Bowden, Thomas A.

AU - Alnajjar, Mikhail S.

AU - Appel, Aaron

AU - Birnbaum, Jerome C.

AU - Bitterwolf, Thomas E.

AU - Dupuis, Michel

PY - 2009/10/28

Y1 - 2009/10/28

N2 - Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)3Fe(μ-SH)2Fe(CO)3 (Fe 2S2H2) and (OC)3Fe(μ-SCH 3)(μ-SH)Fe(CO)3 (Fe2S2MeH) by benzyl radicals in benzene. From the temperature-dependent rate data for Fe 2S2H2, ΔH‡ and ΔS‡ were determined to be 2.03 ± 0.56 kcal/mol and -19.3 ± 1.7 cal/(mol K), respectively, giving kabs = (1.2 ± 0.49) × 107 M-1 s-1 at 25°C. For Fe2S2MeH, ΔH‡ and ΔS‡ were determined to be 1.97 ± 0.46 kcal/mol and -18.1 ± 1.5 cal/(mol K), respectively, giving kabs = (2.3 ± 0.23) × 107 M-1 s-1 at 25°C. Temperature-dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (ΔH‡ = 3.62 ± 0.43 kcal/mol, ΔS‡ = -21.7 ± 1.3 cal/(mol K)) and H2S (ΔH‡ = 5.13 ± 0.99 kcal/mol, ΔS‡ = -24.8 ± 3.2 cal/(mol K)), giving k abs at 25°C of (2.5 ± 0.33) × 105 and (4.2 ± 0.51) × 103 M-1 s-1, respectively, both having hydrogen atom abstraction rate constants orders of magnitude slower than those of Fe2S2H2 and Fe2S2MeH. Thus, Fe2S2MeH is 100-fold faster than thiophenol, known as a fast donor. All rate constants are reported per abstractable hydrogen atom (kabs/M-1 s -1/H). DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe2S2H2 and Fe2S 2MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe2S2H 2 and Fe2S2MeH isomers. Derived radicals Fe2S2H• and Fe2S 2Me• exhibit singly occupied HOMOs with unpaired spin density distributed between the two Fe atoms, a bridging sulfur, and d σ-bonding between Fe centers. The S-H solution bond dissociation free energy (SBDFE) of Fe2S2MeH was found to be 69.4 ± 1.7 kcal/mol by determination of its pKa (16.0 ± 0.4) and the potential for the oxidation of the anion, Fe 2S2Me-, of -0.26 ± 0.05 V vs ferrocene in acetonitrile (corrected for dimerization of Fe2S 2Me•). This SBDFE for Fe2S2MeH corresponds to a gas-phase bond dissociation enthalpy (BDE) of 74.2 kcal/mol, in satisfactory agreement with the DFT value of 73.2 kcal/mol. Replacement of the Fe-Fe bond in Fe2S2MeH with bridging μ-S (Fe 2S3MeH) or μ-CO (Fe2S2(CO)MeH) groups leads to (DFT) BDEs of 72.8 and 66.2 kcal/mol, the latter indicating dramatic effects of the choice of bridge structure on S-H bond strengths. These results provide a model for the reactivity of hydrosulfido sites of low-valent heterogeneous FeS catalysts.

AB - Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)3Fe(μ-SH)2Fe(CO)3 (Fe 2S2H2) and (OC)3Fe(μ-SCH 3)(μ-SH)Fe(CO)3 (Fe2S2MeH) by benzyl radicals in benzene. From the temperature-dependent rate data for Fe 2S2H2, ΔH‡ and ΔS‡ were determined to be 2.03 ± 0.56 kcal/mol and -19.3 ± 1.7 cal/(mol K), respectively, giving kabs = (1.2 ± 0.49) × 107 M-1 s-1 at 25°C. For Fe2S2MeH, ΔH‡ and ΔS‡ were determined to be 1.97 ± 0.46 kcal/mol and -18.1 ± 1.5 cal/(mol K), respectively, giving kabs = (2.3 ± 0.23) × 107 M-1 s-1 at 25°C. Temperature-dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (ΔH‡ = 3.62 ± 0.43 kcal/mol, ΔS‡ = -21.7 ± 1.3 cal/(mol K)) and H2S (ΔH‡ = 5.13 ± 0.99 kcal/mol, ΔS‡ = -24.8 ± 3.2 cal/(mol K)), giving k abs at 25°C of (2.5 ± 0.33) × 105 and (4.2 ± 0.51) × 103 M-1 s-1, respectively, both having hydrogen atom abstraction rate constants orders of magnitude slower than those of Fe2S2H2 and Fe2S2MeH. Thus, Fe2S2MeH is 100-fold faster than thiophenol, known as a fast donor. All rate constants are reported per abstractable hydrogen atom (kabs/M-1 s -1/H). DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe2S2H2 and Fe2S 2MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe2S2H 2 and Fe2S2MeH isomers. Derived radicals Fe2S2H• and Fe2S 2Me• exhibit singly occupied HOMOs with unpaired spin density distributed between the two Fe atoms, a bridging sulfur, and d σ-bonding between Fe centers. The S-H solution bond dissociation free energy (SBDFE) of Fe2S2MeH was found to be 69.4 ± 1.7 kcal/mol by determination of its pKa (16.0 ± 0.4) and the potential for the oxidation of the anion, Fe 2S2Me-, of -0.26 ± 0.05 V vs ferrocene in acetonitrile (corrected for dimerization of Fe2S 2Me•). This SBDFE for Fe2S2MeH corresponds to a gas-phase bond dissociation enthalpy (BDE) of 74.2 kcal/mol, in satisfactory agreement with the DFT value of 73.2 kcal/mol. Replacement of the Fe-Fe bond in Fe2S2MeH with bridging μ-S (Fe 2S3MeH) or μ-CO (Fe2S2(CO)MeH) groups leads to (DFT) BDEs of 72.8 and 66.2 kcal/mol, the latter indicating dramatic effects of the choice of bridge structure on S-H bond strengths. These results provide a model for the reactivity of hydrosulfido sites of low-valent heterogeneous FeS catalysts.

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