PNNL

Abstract

Absolute rate constants were determined for the abstraction of hydrogen atom from (OC)3Fe(?-SH)2Fe(CO)3 (Fe2S2H2) and (OC)3Fe(?-SCH3)(?-SH)Fe(CO)3 (Fe2S2MeH) by benzyl radical in benzene. From the temperature dependent rate data for Fe2S2H2, ?H‡ and ?S‡ were determined to be 2.03 ? 0.56 kcal/mol and 19.3 ? 1.7 cal/mol K, respectively, giving kabs = 1.2 ? 107 M 1 s 1 at 25?C. For Fe2S2MeH, ?H‡ and ?S‡ were determined to be 1.97 ? 0.46 kcal/mol and 18.1 ? 1.5 cal/mol K, respectively, giving kabs = 2.3 ? 107 M 1 s 1 at 25?C. Temperature dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (?H‡ = 3.62 ? 0.43 kcal/mol, ?S‡ = 21.7 ? 1.3 cal/mol K) and H2S (?H‡ = 5.13 ? 0.99 kcal/mol, ?S‡ = 24.8 ? 3.2 cal/mol K), giving kabs at 25?C of 2.5 ? 105 and 4.2 ? 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?-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, 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.