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

Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)(3)Fe(mu-SH)(2)Fe(CO)(3) (Fe(2)S(2)H(2)) and (OC)(3)Fe(mu-SCH(3))(mu-SH)Fe(CO)(3) (Fe(2)S(2)MeH) by benzyl radicals in benzene. From the temperature-dependent rate data for Fe(2)S(2)H(2), DeltaH(++) and DeltaS(++) were determined to be 2.03 +/- 0.56 kcal/mol and -19.3 +/- 1.7 cal/(mol K), respectively, giving k(abs) = (1.2 +/- 0.49) x 10(7) M(-1) s(-1) at 25 degrees C. For Fe(2)S(2)MeH, DeltaH(++) and DeltaS(++) were determined to be 1.97 +/- 0.46 kcal/mol and -18.1 +/- 1.5 cal/(mol K), respectively, giving k(abs) = (2.3 +/- 0.23) x 10(7) M(-1) s(-1) at 25 degrees C. Temperature-dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (DeltaH(++) = 3.62 +/- 0.43 kcal/mol, DeltaS(++) = -21.7 +/- 1.3 cal/(mol K)) and H(2)S (DeltaH(++) = 5.13 +/- 0.99 kcal/mol, DeltaS(++) = -24.8 +/- 3.2 cal/(mol K)), giving k(abs) at 25 degrees C of (2.5 +/- 0.33) x 10(5) and (4.2 +/- 0.51) x 10(3) M(-1) s(-1), respectively, both having hydrogen atom abstraction rate constants orders of magnitude slower than those of Fe(2)S(2)H(2) and Fe(2)S(2)MeH. Thus, Fe(2)S(2)MeH is 100-fold faster than thiophenol, known as a fast donor. All rate constants are reported per abstractable hydrogen atom (k(abs)/M(-1) s(-1)/H). DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe(2)S(2)H(2) and Fe(2)S(2)MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe(2)S(2)H(2) and Fe(2)S(2)MeH isomers. Derived radicals Fe(2)S(2)H(*) and Fe(2)S(2)Me(*) exhibit singly occupied HOMOs with unpaired spin density distributed between the two Fe atoms, a bridging sulfur, and d(sigma)-bonding between Fe centers. The S-H solution bond dissociation free energy (SBDFE) of Fe(2)S(2)MeH 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, Fe(2)S(2)Me(-), of -0.26 +/- 0.05 V vs ferrocene in acetonitrile (corrected for dimerization of Fe(2)S(2)Me(*)). This SBDFE for Fe(2)S(2)MeH 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 Fe(2)S(2)MeH with bridging mu-S (Fe(2)S(3)MeH) or mu-CO (Fe(2)S(2)(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.