Cookies on this website
We use cookies to ensure that we give you the best experience on our website. If you click 'Continue' we'll assume that you are happy to receive all cookies and you won't see this message again. Click 'Find out more' for information on how to change your cookie settings.

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.

Original publication

DOI

10.1021/ja904602p

Type

Journal article

Journal

J Am Chem Soc

Publication Date

28/10/2009

Volume

131

Pages

15212 - 15224