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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry