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

Journal Article · Wed Oct 28 00:00:00 EDT 2009 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja904602p· OSTI ID:973418

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

Absolute rate constants were determined for the abstraction of hydrogen atom from (OC)₃Fe(μ-SH)₂Fe(CO)₃ (Fe₂S₂H₂) and (OC)₃Fe(μ-SCH₃)(μ-SH)Fe(CO)₃ (Fe₂S₂MeH) by benzyl radical in benzene. From the temperature dependent rate data for Fe₂S₂H₂, ΔH^‡ and ΔS^‡ were determined to be 2.03 ± 0.56 kcal/mol and -19.3 ± 1.7 cal/(mol K), respectively, giving k_abs = 1.2 ± 10⁷ M^-1 s^-1 at 25ºC. 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 k_abs = 2.3 ± 10⁷ M^-1 s^-1 at 25ºC. 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 25ºC of (2.5 ± 0.33) x 10⁵ and (4.2 ± 0.51) x 10³ M^-1 s^-1, respectively. DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe₂S₂H₂ and Fe₂S₂MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe₂S₂H₂ and Fe₂S₂MeH isomers. Derived radicals Fe₂S₂H^• and Fe₂S₂Me^• 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 pK_a (16.0 ± 0.4) and the potential for the oxidation of the anion, Fe₂S₂Me^- of -0.26 ± 0.05 V vs. ferrocene in acetonitrile (corrected for dimerization of Fe₂S₂Me^•). 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 (Fe₂S₃MeH) 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 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. This work was supported by the US Department of Energy's Office of Basic Energy Sciences, Chemical Sciences program. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 973418
• Report Number(s): PNNL-SA-66979; JACSAT; KC0302010; TRN: US201006%%364
• Journal Information: Journal of the American Chemical Society, Vol. 131, Issue 42; ISSN 0002-7863
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

08 HYDROGEN
ACETONITRILE
ATOMS
BENZENE
BENZYL RADICALS
CATALYSTS
CHEMICAL SHIFT
DIMERIZATION
DISSOCIATION
ENTHALPY
FERROCENE
FREE ENERGY
HYDROGEN
ISOMERS
OXIDATION
RADICALS
SPIN
SULFUR
THIOPHENOLS