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

Article abstract of DOI:10.1021/ja904602p

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 (Fe

Full text of DOI:10.1021/ja904602p

Published on Web 10/01/2009
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
James A. Franz,*,† Suh-Jane Lee,† Thomas A. Bowden,† Mikhail S. Alnajjar,†
Aaron M. Appel,† Jerome C. Birnbaum,† Thomas E. Bitterwolf,‡ and Michel Dupuis†
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and
Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343
Received June 10, 2009; E-mail: james.franz@pnl.gov
Abstract: Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)3Fe(µ-
SH)2Fe(CO)3 (Fe2S2H2) and (OC)3Fe(µ-SCH3)(µ-SH)Fe(CO)3 (Fe2S2MeH) by benzyl radicals in benzene.
From the temperature-dependent rate data for Fe2S2H2, ∆Hq and ∆Sq 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, ∆Hq and ∆Sq 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 (∆Hq ) 3.62 ( 0.43 kcal/mol,
∆Sq ) -21.7 ( 1.3 cal/(mol K)) and H2S (∆Hq ) 5.13 ( 0.99 kcal/mol, ∆Sq ) -24.8 ( 3.2 cal/(mol K)),
giving kabs 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 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 dσ-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 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.
Introduction
at sulfur, very few experimental S-H bond strengths are known
for metal hydrosulfides. The only reported experimental S-H
Hydrosulfido complexes of transition metals are of great
importance in catalytic processes such as hydrogenation, hy-
drocracking, hydrodeoxygenation, and hydrodesulfurization. The
Fe2S2 core plays a key role in enzymatic activation of molecular
hydrogen in hydrogenase enzymes and is important in nitroge-
nase enzymes.1,2 The abundance of FeS clusters likely led to
the evolution of critical biochemical functionality on primordial
earth.3 The importance of the reactivity at bridging sulfides and
disulfides is increasingly recognized for metal sulfides used for
hydrogen activation, hydrodesulfurization, hydrogenation of
unsaturated C-C bonds,4 and selective hydrocracking of
alkylaromatic structure.5 In spite of the importance of reactivity
bond strengths for metal hydrosulfides are for dimolybdenum
tetrasulfide complexes,6-8 all but one6 of which are included
in our recently developed extensive family of bond strengths
for Mo(µ-SH)Mo clusters,9 in which the S-H bond dissociation
enthalpies (BDEs) span the remarkable range of 48-72 kcal/
(3) (a) Cody, G. D.; Boctor, N. Z.; Filley, T. R.; Hazen, R. M.; Scott,
J. H.; Sharma, A.; Yoder, H. S., Jr. Science 2000, 289, 1337–1340.
(b) Wa¨chtersha¨user, G. Chem. BiodiVersity 2007, 4, 584–602. (c)
Wa¨chtersha¨user, G. Prog. Biophys. Mol. Biol. 1992, 58, 85.
(4) Kuwata, S.; Hidai, M. Coord. Chem. ReV. 2001, 213, 211–305.
(5) (a) Li, X.; Hu, S.; Jin, L.; Hu, H. Energy Fuels 2008, 22, 1126–1129.
(b) Wei, X. Y.; Ogata, E.; Zong, Z. M.; Niki, E. Energy Fuels 1992,
6, 868–869. (c) Wei, X. Y.; Ogata, E.; Zong, Z. M.; Niki, E. Fuel
1993, 72, 1547–1552.
† Pacific Northwest National Laboratory.
(6) Appel, A. M.; DuBois, D. L.; Rakowski DuBois, M. J. Am. Chem.
Soc. 2005, 127, 12717–12726.
‡ University of Idaho.
(1) Peruzzini, M.; de los Rios, I.; Romerosa, A. Coordination Chemistry
of Transition Metals with Hydrogen Chalcogenido Ligands. In
Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: New York.
2001; Vol. 49, pp 169-443.
(7) Appel, A. M.; Lee, S.-J.; Franz, J. A.; DuBois, D. J.; DuBois, M. R.;
Twamley, B. Organometallics 2009, 28, 749–754.
(8) Appel, A. M.; Lee, S. J.; Franz, J. A.; DuBois, D. L.; Rakowski
DuBois, M.; Birnbaum, J. C.; Twamley, B. J. Am. Chem. Soc. 2008,
130, 8940–8951.
(2) Henderson, R. A. Chem. ReV. 2005, 105, 2365–2438.
9
15212 J. AM. CHEM. SOC. 2009, 131, 15212–15224
10.1021/ja904602p CCC: $40.75  2009 American Chemical Society

Fe(µ2-SH)Fe Bond Strengths and Homolytic Reactivity
A R T I C L E S
Scheme 1. Stepwise Homolytic Hydrogen Transfer from a Clustera
Scheme 2. (Top) Stepwise Homolytic Hydrogen Transfer between
Hydrocarbons13 and (Bottom) Re-initiation of Chain-Transfer
Catalysis18
to an Olefin or Polynuclear Aromatic Initiated by
13
Retro-Disproportionation (kRRD
)
a Clusters may be M ) Mo, Fe (e.g., pyrrhotite, Fe(1-x)S), or mixed metal
clusters.
in hydrogenations and hydroformylations. Norton and co-
workers measured an M-H BDE of 57.9 kcal/mol for the
vanadium hydride HV(CO)4(dppm) (dppm ) Ph2PCH2PPh2) and
measured a re-initiation rate of g1.75 × 10-2 M-1 s-1 at 285
K (bottom of Scheme 2).15 Norton demonstrated a monotonic
increase in rates of hydrogen transfer to styrene with decreasing
metal hydride bond strengths from 62.2 to 54.9 kcal/mol.
Fe(1-x)S catalysts enable selective cleavage of aromatic-alkyl
bonds if the aromatic ring is activated by alkyl substitution or
is otherwise more reactive (to H-atom addition), as in poly-
nuclear aromatic hydrocarbons such as naphthalene.16 Autrey
and co-workers explain the enhanced selectivity for hydro-
genolytic cleavage of the p-CH3C6H4--CH2C6H4CH3 bond over
the methyl-aromatic carbon-carbon bond, p-CH3--
C6H4CH2C6H4CH3, during FeS-catalyzed hydrogenolysis by
invoking an RRD-like mechanism involving rate-determining
reversible transfer of a hydrogen atom from FeS-H groups of
heterogeneous FeS catalyst to all positions of the aromatic rings
of (p-CH3C6H4)2CH2, with hydrogen addition at the methyl and
benzyl ipso positions of the aromatic structure leading to
competing methyl or benzyl scission.17
mol. For analogous iron-sulfide complexes, no comparable
bond strengths or kinetic measures of S-H reactivity exist. FeS-
based heterogeneous catalysts are growing in technological
importance in hydrogenation and hydrocracking, specifically for
large-scale use in direct coal liquefaction. Iron-sulfide based
pyrite- and pyrrhotite-like minerals are solid catalysts for the
selective hydrogenation and hydrocracking of alkyl-substituted
aromatic organic compounds. These catalysts facilitate selective
hydrogenation of aromatic hydrocarbons and hydrocracking of
alkyl side chains of aromatic structures.
While the complex structure of pyrrhotite group minerals has
been explored,10 the structural forms of catalytic intermediates
in hydrogenation and hydrocracking remain mostly speculative.
Early attempts at molecular orbital calculations for iron catalysts
j
revealed the dissociation of H2 to occur on a stepped (1010)
FeS surface, producing a “pseudo-H2S” species.11 The further
evolution of the H2-activated structure and participation in
hydrogen transfer to acceptor molecules was not addressed. The
key hydrogen-transfer step from hydrogen-carrying Fe(1-x)
S
catalysts may occur by direct hydrogen atom transfer (reverse-
radical disproportionation (RRD), or retro-disproportionation)
from the reduced FeS catalyst, as depicted in Schemes 1 and 2.
The retro-disproportionation mechanism is well characterized
for homogeneous hydrogen transfer between organic hydrogen
donors and acceptors and is recognized as a primary initiation
and hydrogen-transfer pathway in hydropyrolysis of aromatic
alkyl structures.12,13 By virtue of the nearly barrierless nature
of the reverse disproportionation steps (kdisp) in Schemes 1 and
2, knowledge of C-H, S-H, and M-H bond strengths allows
estimation of ∆H° (≈ ∆Hq) and prediction of the upper limit
of the forward rates of hydrogen transfer to acceptor molecules.
The retro-disproportionation mechanism is also well established
as a means of hydrogen atom transfer from organometallic
hydrides with weak M-H bonds to reactive olefinic acceptors,14
as in the re-initiation step of chain-transfer catalysis, as well as
The stepwise homolytic hydrogen transfer to olefins from a
hydrogen-containing metal sulfide catalyst (Scheme 1) corre-
sponds directly to the molecule-assisted homolytic transfer of
hydrogen between hydrocarbons and olefinic acceptors (Scheme
2, top).13 The role of M-H bond strengths has been demon-
strated for the homolytic reactivity of the Mo-H bond with
carbon-centered radicals19 and for the activation of the bridging
(15) (a) Choi, J.; Pulling, M. E.; Smith, D. M.; Norton, J. R. J. Am. Chem.
Soc. 2008, 130, 4250–4252. (b) Gridnev, A. A.; Ittel, S. D. Chem.
ReV. 2001, 101, 3611–3659. Heuts, J. P.A.; Roberts, G. E.; Biasutti,
J. D. Aust. J. Chem. 2002, 55, 381–398. (c) Halpern, J. Pure Appl.
Chem. 1986, 58, 575–584. Eisenberg, D. C.; Norton, J. R. Isr. J. Chem.
1991, 31, 55–66.
(16) (a) Walter, T. D.; Klein, M. Energy Fuels 1995, 9, 1058–1061. (b)
Farcasiu, M.; Smith, C. Energy Fuels 1991, 5, 83. (c) Huffman, G. P.;
Ganguly, B.; Zhao, J.; Rao, K. R. P. M.; Shah, N.; Feng, Z.; Taghiei,
M. M.; Lu, F.; Wender, I.; Pradhan, V. R.; Tierney, J. W.; Seehra,
M. S.; Ibrahim, M. M.; Shabtai, J.; Eyring, E. M. Energy Fuels 1993,
7, 285.
(9) Appel, A. M.; Lee, S.-J.; Franz, J. A.; DuBois, D. L.; Rakowski
DuBois, M. J. Am. Chem. Soc. 2009, 131, 5224–5232.
(10) Wang, H.; Salveson, I. Phase Transitions 2005, 78, 547–567.
(11) Ades, H. F.; Companion, A. L.; Subbaswamy, K. R. Energy Fuels
1994, 8, 71–76.
(17) Autrey, T.; Linehan, J. C.; Camaioni, D. M.; Kaune, L. E.; Watrob,
H. M.; Franz, J. A. Catal. Today 1996, 31, 105–111.
(12) McMillen, D. F.; Malhotra, R.; Chang, S. J.; Ogier, W. C.; Nigenda,
S. E.; Fleming, S. H. Fuel 1987, 66, 1611–1620.
(18) (a) Choi, J.; Pulling, M. E.; Smith, D. M.; Norton, J. R. J. Am. Chem.
Soc. 2008, 130, 4250–4252. (b) Gridnev, A. A.; Itel, S. D. Chem.
ReV. 2001, 101, 3611–3659. (c) Heuts, J. P. A.; Roberts, G. E.; Biasutti,
J. D. Aust. J. Chem. 2002, 55, 381–398.
(13) Ru¨chardt, C.; Gerst, M.; Ebenhoch, J. Angew. Chem., Int. Ed. Engl.
1997, 36, 1406.
(14) (a) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990, 112, 6886–
6898. (b) Norton, J. R. Isr. J. Chem. 1991, 31, 55–66.
(19) Franz, J. A.; Linehan, J. C.; Birnbaum, J. C.; Hicks, K. W.; Alnajjar,
M. S. J. Am. Chem. Soc. 1999, 121, 9824–9830.
9
J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009 15213

A R T I C L E S
Franz et al.
Chart 1. Abbreviations and Basic Structures for Iron Sulfide
Complexes in This Study
Chart 2. Fe2S2R and Fe2S2RH Isomers in This Study
rate expressions for abstraction of a hydrogen atom by benzyl
radical from H2S and thiophenol are described. Finally, the effect
on S-H bond strengths of replacing the Fe-Fe bond in the
above structures with µ-S or µ-CO bridging structures is
examined.
Results and Discussion
Kinetics of Reaction of Fe2S2H2 and Fe2S2MeH with
Benzyl Radical. For hydrogen atom donors possessing a single
abstractable hydrogen atom, photolysis in the presence of
dibenzyl ketone (DBK) to produce benzyl radical and the
subsequent measurement of the resulting toluene and bibenzyl
concentrations provide a convenient means of measuring rate
constants for hydrogen atom abstraction. Accurate self-termina-
tion rate expressions for benzylic and other alkyl radicals in
common solvents have proven particularly useful in kinetic
determination of competing rate processes.21,23 Using available19
values for the self-termination rate of benzyl radical, kt, in
benzene, the rate constant for abstraction is given by eq 1,
k1t /2 ln(1 - [toluene]/[DH]0)
S-H group in the system [CpMo(µ-SH)(µ-S)]2 to reactivity with
benzyl radical.20 The rate of hydrogen abstraction by benzyl
radical from the Mo(µ2-SH)Mo group was found to be a factor
of 100 faster than that with alkanethiols. The basis of this rate
enhancement, although attenuated by steric effects, was revealed
in our recent finding that the S-H BDE in Cp*Mo(µ2-SH)(µ2-
SCH3)(µ-S)2MoCp* is reduced to 72 kcal/mol compared to those
of H2S (91.2 kcal/mol), simple alkanethiols (87 kcal/mol), and
arenethiols (78-79 kcal/mol).24
Knowledge of FeS-H bond strengths is essential to under-
standing the catalytic reactivity of homogeneous and heteroge-
neous FeS catalysts. We thus present the first measurement of
an S-H homolytic bond strength and kinetic homolytic reactiv-
ity in a prototype Fe(µ-SH)Fe system. The results provide insight
into the reactivity of similar structures expected for heteroge-
neous pyrite/pyrrhotite catalyst systems. This study examines
the homolytic reactivity of the µ2-SH group in low-valent iron
systems (see Chart 1) for hydrogen atom transfer from (OC)3Fe(µ-
SH)2Fe(CO)3 (Fe2S2H2) and (OC)3Fe(µ-SCH3)(µ-SH)2Fe(CO)3
(Fe2S2MeH) to benzyl radical. DFT calculations are applied to
estimate the S-H bond strength of Fe2S2H2 and Fe2S2MeH for
comparison with that of the S-H group of alkane- and
arenethiols, and the experimental homolytic solution bond
dissociation free energy (SBDFE) of Fe2S2MeH is presented.
The kinetic reactivity of benzyl radical with the µ2-SH group
of the di-iron systems is correlated in a Polanyi relationship
with the reactivity predicted from the S-H bond strengths of
Fe2S2H2, Fe2S2MeH, and a series of thiols, and new absolute
kabs ) -
(1)
[bibenzyl]1/2∆t1/2
where [DH]0 is the initial concentration of the hydrogen atom
donor. The steady-state photolysis of DBK produces benzyl and
phenylacetyl radicals (eq 2a, Scheme 3), which is followed by
rapid decarbonylation of the phenylacetyl radical (eq 2b) to form
a second benzyl radical. The generated benzyl radicals abstract
hydrogen atoms from donors to produce toluene (eq 3) or self-
terminate to produce bibenzyl (eq 4).
Equation 1 accounts for declining donor concentration during
photolysis of DBK. The µ2-SH donor Fe2S2H2 exists as a
mixture of three isomers, axial-equatorial (ae ) ea), axial-axial
(aa), and equatorial-equatorial (ee) (Chart 2). When photolysis
of DBK and Fe2S2H2 is carried out to sufficient extent of
reaction to allow observation of products by NMR, the S-H
peaks of the aa, ee, and ae isomers are observed to decrease,
with no evidence of new S-H NMR transitions associated with
(21) See, e.g.: (a) Franz, J. A.; Suleman, N. K.; Alnajjar, M. S. J. Org.
Chem. 1986, 51, 19–25. (b) Autrey, S. T.; Alnajjar, M. S.; Nelson,
D. A.; Franz, J. A. J. Org. Chem. 1991, 56, 2197–2202. (c) Alnajjar,
M. S.; Franz, J. A. J. Am. Chem. Soc. 1992, 114, 1052–1058. (d) Franz,
J. A.; Linehan, J. C.; Birnbaum, J. C.; Hicks, K. W.; Alnajjar, M. S.
J. Am. Chem. Soc. 1999, 121 (42), 9824–9830. (e) Kandanarachchi,
P. H.; Autrey, T.; Franz, J. A. J. Org. Chem. 2002, 67, 7937–7945.
(f) Franz, J. A.; Kolwaite, D. S.; Linehan, J. C.; Rosenberg, E.
Organometallics 2004, 23, 441–445.
(22) von Smoluchowski, M. Z. Z. Phys. Chem. Stoechiom. Verwandtsschaftl.
1917, 92, 129.
(23) Fischer, H.; Paul, H. Acc. Chem. Res. 1987, 20, 200–206.
(24) Appel, A. M.; Lee, S.-J.; Franz, J. A.; DuBois, D. L.; DuBois, M. R.;
Birnbaum, J. C.; Twamley, B. J. Am. Chem. Soc. 2008, 130, 8940–
8951.
(20) Franz, J. A.; Birnbaum, J. C.; Kolwaite, D. S.; Linehan, J. C.;
Camaioni, D. M.; Dupuis, M. J. Am. Chem. Soc. 2004, 126, 6680–
6691.
9
15214 J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009

Fe(µ2-SH)Fe Bond Strengths and Homolytic Reactivity
A R T I C L E S
Scheme 3. Hydrogen Atom Abstraction from Fe2S2H2 by Benzyl
Radical
Figure 1. Eyring plot of temperature-dependent rate data for abstraction
of a hydrogen atom by benzyl radical from Fe2S2H2 in benzene. Rate
constants are per abstractable hydrogen atom.
Fe2S2H• is predicted to self-terminate, assuming no steric
retardation, at a rate constant of 2.04 × 109 M-1 s-1, based on
experimental diffusion constants measured for Fe2S2. The
assumption of the absence of steric retardation is supported by
the known formation of stable dimers of the radical Fe2S2R• (R
) Ph). The resulting temperature-dependent rate constants from
numerical integration of eqs 2-5 and 7 for reaction of benzyl
radical with Fe2S2H2 are depicted in Figure 1 and tabulated in
Table 1.
The methylated derivative, Fe2S2MeH, was prepared in order
to avoid the ambiguity created by the presence of two hydro-
sulfides in Fe2S2H2. For Fe2S2MeH, only one hydrosulfide is
present, and so abstracted hydrogen atoms have only one source,
thereby eliminating the need for eqs 5 and 7. Equations 9, 10,
and 11 (Scheme 4) describe reactions that occur as a result of
steady-state photolysis of DBK in the presence of isomers of
Fe2S2MeH, with benzyl radical formation and self-termination
described above in eqs 2a, 2b, and 4. Formation of toluene
occurs only in the abstraction step (eq 9); thus, measurement
of toluene and bibenzyl provides the abstraction rate constant,
kabs, using eq 1. Cross-termination product Fe2S2MeBz (eq 10)
and self-termination product (Fe2S2Me)2 (eq 11) are known
structures. Kinetic data are depicted in Figure 2 and tabulated
in Table 1. Because methyl substitution eliminates cross-
disproportionation for Fe2S2MeH, because Fe2S2H2 was found
to decompose over several days, and because of the need to
model cross-termination and self-termination of the Fe2S2MH
radical, we regard the rate constants for Fe2S2MeH as inherently
more reliable. This is also reflected in the statistical errors in
rate constants, as the error for Fe2S2H2 is approximately twice
that for Fe2S2MeH.
cross-combination product Fe2S2BzH (eq 6) or self-termination
product (Fe2S2H)2 (eq 8). The termination pathways of Fe2S2H•
are thus defined to be cross-disproportionation with benzyl
radical to form toluene and Fe2S2 (eq 5) and self-dispropor-
tionation of two Fe2S2H• radicals to form Fe2S2H2 and Fe2S2
(eq 7). Determination of the ratio of yields of toluene formed
in the primary reaction step (eq 3) versus cross-disproportion-
ation (eq 5) is made possible by the lack of formation of the
cross-combination product (Fe2S2BzH, eq 6), as described in
the Experimental Procedures. Thus, all benzyl radical-derived
products appear as toluene and bibenzyl. This allows numerical
integration of eqs 2, 3, 4, 5, and 7, with kabs varied to reproduce
yields of toluene and bibenzyl. The numerical integration
requires values of kt (eq 4), kct,disp (eq 5), and k′disp (eq 7). Since
only disproportionation appears to occur in reaction of Fe2S2H•
with benzyl radical (eq 5) or in self-reaction (eq 7), the total
diffusion-controlled encounter rates, modified by singlet radical
pair spin selection, can be assigned to these processes. The total
self-termination rate of Fe2S2H• is calculated from the modified
von Smoluchowski equation22 using experimental diffusion
coefficients of Fe2S2 as a model for Fe2S2H•. The estimation of
diffusion- and spin-selected self- and cross-reaction of reactive
free radicals (see Experimental Procedures for details) is
facilitated by the narrow total range of self-reaction rates
exhibited by small free radicals in nonassociating solvents, ca.
(1-8) × 109 M-1 s-1, at the temperatures employed here, unless
the approach of the radical centers is specifically hindered by
bulky substituents.23 The self-termination rate of Fe2S2H• will
be expected to be to comparable to that of small alkyl radicals
and predictable from the viscosity of the solvent and the reaction
distances (see Experimental Procedures for details). Molecular
modeling reveals the bridging sulfide and hydrosulfide in
Fe2S2H• to be sterically unencumbered and readily accessible.
This is in contrast to, for example, slowly dimerizing molyb-
denum- (Cp*Mo(µ-SCH3)(µ-S)2(µ-S•)MoCp*)24 or chromium-
centered radicals (Cp(CO)3Cr•),25 where steric hindrance results
in self-termination rates of 6.5 × 107 and 2.7 × 108 M-1 s-1
for the Mo and Cr radicals, respectively, slower by factors of
30 and 10 than diffusion-controlled rates, clearly due to front
strain in forming S-S and Cr-Cr bonds for these species.
Scheme 4. Hydrogen Atom Abstraction from Fe2S2MeH by Benzyl
Radical
Kinetics of Reaction of Benzyl and H2S. The reactions of
benzyl radical with H2S in benzene under steady-state photolysis
(25) Yao, Q.; Bakac, A.; Espenson, J. H. Organometallics 1993, 12, 2010–
2012.
9
J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009 15215

A R T I C L E S
Franz et al.
Table 1. Rate Expressions for the Reaction of Benzyl Radical with the µ-SH Hydrogen Atom in Fe2S2H2, Fe2S2MeH, a Mo(µ-SH)Mo
Complex, and Thiolsa
f
hydrogen donor
XS-H BDE (kcal/mol)
log A (M-1 s-1
)
∆Sq (eu) (Tm, K)f
-18.1 ( 0.8
Ea (kcal/mol)f
∆Hq (kcal/mol)f
k (M-1 s-1/H) (298 K)
Tm (K_h
temp range (K)
Fe2S2MeH
Fe2S2H2
74.2 (exp)
73.2 (calc)b
73.1 (calc)b
9.29 ( 0.16
2.60 ( 0.23 1.97 ( 0.23 (2.4 ( 0.23) × 107
2.66 ( 0.28 2.03 ( 0.28 (1.2 ( 0.49) × 107
3.62 ( 0.29 2.96 ( 0.29 (2.7 ( 0.35) × 106
4.27 ( 0.22 3.62 ( 0.22 (2.5 ( 0.33) × 105
4.64 ( 0.27 3.96 ( 0.27 (3.1 ( 0.37) × 104
5.77 ( 0.49 5.13 ( 0.50 (4.2 ( 0.51) × 103
316
293-344
9.04 ( 0.19
-19.3 ( 0.8
-19.2 ( 0.9
322
331
299-345
295-372
73.2 (calc)b c
,
[CpMo(µ-S)(µ-SH)]2 70.6 ( 2.2 (expt)g 9.07 ( 0.19
73.2 (calc)
C6H5SH
79.1d
8.52 ( 0.015
7.89 ( 0.18
7.85 ( 0.34
-21.7 ( 0.6
-24.7 ( 0.8
-24.8 ( 0.8
335
341
321
298-366
295-374
293-348
CH3(CH2)7SHc
H2S
87.3d e
91.2e
,
a Errors are 1σ. Rate constants are reported per abstractable hydrogen atom, k (M-1 s-1)/H. b From DFT calculations, this work, see text. c Reference
20. d Bordwell, F. G.; Zhang, X.-M.; Satish, A. V.; Cheng, J. P. J. Am. Chem. Soc. 1994, 116, 6605. e Nicovich, J. M.; Kreutter, K. D.; van Dijk, C. A.;
f
Wine, P. A. J. Phys. Chem. 1992, 96, 2518. A, Arrhenius pre-exponential factor; Ea, Arrhenius exponential constant in k ) A e-E /RT; ∆Sq ) -R(ln(kT
/
a
Tm) - ln(kb/h) + ∆Hq/RTm); ∆H° ) -R(∂ ln(kabs/T)/∂(1/T)); h, Planck’s constant, 6.6260755-34 J s; and kb, Boltzmann’s constant, 1.380658 × 10-23 mJ
K-1 g Reference 9. h Tm, mean temperature of kinetic experiments.
.
contributing insignificantly to the observed toluene due to both
its low concentration and its low abstraction rate. The production
of trace quantities of PhCH2SCH2Ph indicates the operation of
eqs 17 and 18. The reaction of HS• with PhCH2SH (∆H° ) -4
kcal/mol) is expected to occur with rate constants in the range
104-105 M-1 s-1, from comparison of rate constants for
hydrogen atom transfer between hexanethiyl radical and oc-
tanethiol (∆H° ) 0 kcal/mol, kabs(298 K) ) 2.7 × 104 M-1 s-1)
and between thiophenol and octanethiyl radical (∆H° ) -7 kcal/
mol, kabs(298 K) ) 1.3 × 105 M-1 s-1).29 The detection of only
trace quantities of the second-generation products is consistent
with toluene being produced nearly exclusively in eq 13. From
the temperature-dependent rate data for hydrogen abstraction
by benzyl from H2S, ∆Hq ) 5.13 ( 0.99 kcal/mol and ∆Sq )
-24.8 ( 3.2 cal/(mol K), giving kabs ) 4.2 × 103 M-1 s-1 at
25 °C. This rate constant is about 100-fold slower than that for
thiophenol, yielding correspondingly greater magnitudes of ∆Sq
and ∆Hq, fully consistent with expected trends in conventional
bimolecular rate parameters for hydrogen transfer between
carbon and heteroatom centers.
Kinetics of Reaction of Benzyl Radical with Thiophenol.
Temperature-dependent rate data were previously determined30
for the reaction of benzyl radical with thiophenol in hexane:
∆Hq ) 3.13 ( 0.14 kcal/mol and ∆Sq ) -22.8 ( 1.1 cal/(mol
K), giving kabs ) 3.1 × 105 M-1 s-1 at 25 °C. In this work, rate
constants were determined for reaction of benzyl radical with
thiophenol in benzene. The kinetic scheme is directly analogous
to that for the reaction of benzyl radical and Fe2S2MeH, and
eq 1 applies. From the temperature-dependent rate data for
hydrogen abstraction by benzyl from thiophenol in benzene,
∆Hq ) 3.62 ( 0.43 kcal/mol and ∆Sq ) -21.7 ( 1.3 cal/(mol
K), giving kabs ) 2.5 × 105 M-1 s-1 at 25 °C. These activation
parameters agree within experimental error with the earlier
hexane values.
Figure 2. Eyring plot of temperature-dependent rate data for abstraction
of a hydrogen atom by benzyl radical from Fe2S2MeH in benzene.
of DBK are presented in eqs 13-18 (Scheme 5), with benzyl
radical formation and self-termination described above in eqs
2a, 2b, and 4. Toluene is formed by reaction of benzyl radical
with H2S (eq 13). However, if photolysis is continued beyond
a short extent of conversion of H2S (ca. 0.1%), toluene may be
formed by reaction with HSSH (eq 16). Taking the S-H BDE
of HSSH to be 73.6 kcal/molsfrom among the choices of an
older experimental value, 70.0 ( 1.5 kcal/mol,26 an older
computed value of 73.2 kcal/mol,27 and a recent state-of-the-
art calculation, 73.6 kcal/mol28sthe reaction of benzyl radical
with HSSH is predicted to be quite fast, 1.3 × 107 M-1 s-1, on
the basis of the Polanyi expression developed here for abstrac-
tion by benzyl radical from thiols, ln(kabs/M-1 s-1, 298 K) )
-0.465∆H° + 50.64 (see discussion below). At H2S concentra-
tions of ca. 0.1 M with kabs ) 4 × 103 M-1 s-1, concentrations
of HSSH would not have accumulated to greater than about
10-4 M, preventing HSSH from contributing significantly to
the observed toluene.
Experimental Determination of the S-H Bond
Dissociation Free Energy of Fe2S2MeH. The homolytic solution
bond dissociation free energy (SBDFE) of Fe2S2MeH was
determined by using a combination of cyclic voltammetry and
pKa measurements in acetonitrile. The pKa of Fe2S2MeH was
determined to be 16.0 ( 0.4 by equilibration with 2,4,6-
trimethylpyridine (pKa ) 14.98),31 as measured using 1H NMR.
PhCH2SH, formed by cross-termination in eq 14, was
observed at low but detectable concentrations. However, this
hydrogen atom donor will react with benzyl radical (eq 17) with
a rate constant of only about 3 × 104 M-1 s-1, thereby
(26) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33,
493.
(29) Alnajjar, M. S.; Garrossian, M. S.; Autrey, S.T. ; Ferris, K. F.; Franz,
J. A. J. Phys. Chem. 1992, 96, 7037–7043.
(27) DiLabio, G. A.; Pratt, D. A.; LoFaro, A. D.; Wright, J. S. J. Phys.
Chem. 1999, 103, 1653–1661.
(30) Franz, J. A.; Suleman, N. K.; Alnajjar, M. S. J. Org. Chem. 1986, 51,
19.
(28) Grant, D. J.; Dixon, D. A.; Francisco, J. F.; Feller, D.; Peterson, K. A.
J. Phys. Chem. A. 2009, submitted.
(31) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito,
I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028.
9
15216 J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009

Fe(µ2-SH)Fe Bond Strengths and Homolytic Reactivity
A R T I C L E S
Cyclic voltammetry (CV) in acetonitrile revealed an irreversible
oxidation wave for the anion, Fe2S2Me-, for scan rates from
0.05 to 100 V/s. The cause of this irreversibility is assigned to
rapid dimerization of Fe2S2Me• to form (Fe2S2Me)2, supported
by the known dimer formation in the phenyl analogue.32
Assuming a diffusion- and spin-selected self-termination rate
(2 × 109 M-1 s-1), E1/2 can be estimated from the irreversible
peak potentials at each scan rate by use of CV simulations
(DigiSim).33 Using the corrected potentials for each of the scan
rates, E1/2 was estimated to be -0.26 ( 0.05 V vs ferrocene.
By combining the pKa and E1/2 using eq 12, the homolytic S-H
SBDFE for Fe2S2MeH was determined to be 69.4 ( 1.7 kcal/
mol in acetonitrile, corresponding to a gas-phase BDE of 74.2
kcal/mol.34,35
SBDFE ) 23.06E1/2 + 1.37pKa + 53.6
(12)
Scheme 5. Hydrogen Atom Abstraction from H2S by Benzyl
Radical
Figure 3. Alpha orbital density of the SOMO of Fe2S2H•. Spin density is
primarily located about the two Fe atoms and the bridging S atoms.
for parent thiols with the CEP-121G basis set and ECP
pseudopotential for Fe and the 6-311++G(2d,2p) basis set for
C, H, S, and O atoms.41 Ground states were confirmed by the
absence of imaginary frequencies. Tight SCF convergence
criteria were used. Vibrational frequencies were left unscaled
in the enthalpy and free energy calculations, as their application
leads to less than 0.2 kcal/mol difference in energies for the
small scaling factors of DFT methods. The performance and
shortcomings of B3LYP have been reviewed for organic
molecules.42 Truhlar and co-workers have compared B3LYP
with more recently developed approaches for metallic com-
pounds.43 The prediction of organic S-H bond strengths is quite
satisfactory with B3LYP at the level of theory used here. The
present work provides a comparison of experimental data with
results of the B3LYP and ECP density functional methods for
organometallic S-H bond strengths.
Computational Studies. Electronic structure calculations (us-
ing DFT and TDDFT methods) have been recently reported for
Fe2(S2C3H6)(CO)6, an analogue of the aa isomer of Fe2S2H2 in
which the S-H groups have been replaced by a bridging
propanedithiolate, SCH2CH2CH2S.36 Earlier work characterized
the molecular orbitals of Fe2S2 and Fe2S2.37 Recently, Silaghi-
Dumitrescu, Bitterwolf, and King examined the electronic
structures, geometries, and excited states of Fe2S2 and the
diradicals derived from Fe2S2 and related phosphorus ana-
logues.38 Thus, the ground and excited states of the parent Fe2S2
cluster and of the diradicals have been characterized, but not
those of the related radical systems.
The orbital spin density of the SOMO of Fe2S2H• is depicted
in Figure 3. Geometries of Fe2S2MeH (ae) and of the radical
Fe2S2Me• are depicted in Figure 4 and Table 2. The butterfly
geometry of the parent structures Fe2S2MeH and Fe2S2H2 is
retained in the product radicals Fe2S2Me• and Fe2S2H•, but with
extension and weakening of the Fe-Fe bond. Electron spin
densities for Fe2S2Me• and Fe2S2H• are largely shared between
the two iron centers and the bridging µ-S bridge.
In the current work, electronic structure calculations were
carried out using Gaussian 03.39 Geometries, frequencies, and
electronic energies were determined using DFT in the restricted
open-shell reference, ROB3LYP,40 for radicals and RB3LYP
DFT bond lengths calculated by the method used here
typically give bond lengths 0-3% longer than X-ray crystal
structure bond lengths. This is illustrated in Table 3, which
presents X-ray data for Fe2S2Et2 and Fe2S2 (Figure 5) to
compare with geometric parameters obtained using the B3LYP
functional and CEP-121G on Fe and 6-311++G(2d,2p) on C,
44
45
(32) Seyferth, D.; Kiwan, A. M.; Sinn, E. J. Organomet. Chem. 1985, 281,
111–118.
(33) Rudolph, M.; Feldberg, S. W. DigiSim, version 3.03b; Bioanalytical
Systems. 2004.
(40) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. 1988, B37, 785.
(34) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287–
294.
(41) (a) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem.
Phys. 1980, 72, 650. (b) Blaudeau, J.-P.; McGrath, M. P.; Curtiss,
L. A.; Radom, L. J. Chem. Phys. 1997, 107, 5016. (c) Clark, T.;
Chandrasekhar, J.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294.
(42) Tirado-Rives, J.; Jorgensen, W. L. J. Chem. Theory Comput. 2008, 4,
297–306.
(35) Ellis, W. W.; Raebiger, J. W.; Curtis, C. J.; Bruno, J. W.; DuBois,
D. L. J. Am. Chem. Soc. 2004, 126, 2738–2743.
(36) Bertini, L.; Greco, C.; De Gioia, L.; Fantucci, P. J. Phys. Chem. A
2009, 113, 5657–5670.
(37) DeKock, R. L.; Baerends, E. J.; Hengelmolen, R. Organometallics
1984, 3, 289–292.
(43) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Phys. 2005, 123,
161103.
(38) Silaghi-Dumitrescu, I.; Bitterwolf, T. E.; King, R. B. J. Am. Chem.
Soc. 2006, 128, 5342–5343.
(44) Seyferth, D.; Kiwan, A. M. J. Organomet. Chem. 1985, 281, 111–
118.
(39) Frisch, M. J.; et al. Gaussian 03, Revision D.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(45) Wei, C. H.; Dahl, L. F. Inorg. Chem. 1965, 4, 1–11.
9
J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009 15217

A R T I C L E S
Franz et al.
Figure 4. DFT butterfly geometries of (left) Fe2S2MeH (ae) and (right) Fe2S2Me• (a).
Table 2. DFT Geometries of Fe2S2MeH (ae) and Fe2S2Me• (a)a
in Table 4 (hartree/molecule ) 627.5095 kcal/mol). For the three
isomers of (OC)3Fe(µ-SH)2Fe(CO)3, the relative free energies
compared to the ae () ea) isomer are shown in Chart 3. Using
the free energy values from Table 4, the equilibrium ratio
Fe2S2H2 (ae ) ea):Fe2S2H2 (ee):Fe2S2H2 (aa) is calculated to
be 1.0:0.21:0.13 in the gas phase. Experimentally, the ratio is
found to be 1.0:0.29:0.31 in benzene-d6 and 1.0: 0.09: 0.1 in
CDCl3. Seyferth et al. reported a ratio of ae:ee:aa of 14:2:1.47
To match the experimental isomer populations observed by
NMR with the correct isomers, gauge-independent atomic orbital
NMR calculations were carried out with the same level of DFT
theory and basis functions utilized for energy calculations.
Isotropic chemical shifts were found to be σiso ) 35.0714 and
33.1963 ppm for Fe2S2H2 (ae), σiso ) 35.450 ppm for Fe2S2H2
(aa), and σiso ) 33.7467 ppm for Fe2S2H2 (ee), compared to
σiso ) 31.6022 ppm for methane. Using 0.18 ppm for the
Fe2S2MeH (ae)
Fe2S2Me• (a)
Bond Distances, Å
Fe(1)-Fe(8)
Fe(8)-S(15)
Fe(1)-S(16)
Fe(8)-C(9)
Fe(8)-C(10)
Fe(8)-C(11)
S(15)-C(17)
S(16)-H(21)
2.53
2.31
2.32
1.808
1.805
1.809
1.838
1.345
2.66
2.32
2.23
1.834
1.82
1.806
1.834
Angles, deg
66.38
Fe(1)-S(16)-Fe(8)
Fe(1)-S(15)-Fe(8)
Fe(1)-Fe(8)-C(10)
C(17)-S(15)-Fe(1)
C(9)-Fe(8)-C(10)
C(10)-Fe(8)-C(11)
72.99
70
110.46
109.47
93.92
97.06
65.98
101.76
113.23
92.18
100.22
1
methane H shift reference predicts the equatorial and axial
a See Figure 4.
protons of Fe2S2H2 (ae) to appear at -1.77 and -3.65 ppm,
respectively, corresponding to experimental chemical shifts of
equal intensity peaks at -0.61 and -2.85 ppm in benzene-d6.
Similarly, the equatorial protons of Fe2S2H2 (ee) and axial
protons of Fe2S2H2 (aa) are calculated to appear at -2.33 and
-4.03 ppm, respectively, corresponding to experimental chemi-
cal shifts of -1.134 and -3.20 ppm in benzene-d6. The
calculations consistently show the equatorial protons to be
downfield from the axial protons for all isomers. The GIAO
NMR calculations, while exhibiting an absolute error of about
-1 ppm, correctly predict the ranking of SH chemical shifts
and allow the identity of the isomers to be established.
a
Table 3. DFT Geometries of Fe2S2Et2 (ae) and Fe2S2
Fe2S2Et2 (ae)
length (Å) or angle (°)
X-ray23
DFT
Fe(1)-Fe(8)
2.537(10)
2.259(7)
1.81(3)
68.3(3)
81.0(3)
2.522
2.315
1.854
65.916
81.22
Fe(1)-S(15)
S(15)-C(17)
Fe(1)-S(15)-Fe(8)
S(15)-Fe(1)-S(16)
Fe2S2
The calculations of the (gas-phase) relative populations of
the isomers of Fe2S2H2 from theoretical free energies are much
closer to the (solution, benzene-d6) experimental populations
observed in this work than to the populations reported by
Seyferth and co-workers. However, it is not known whether
the isomers are at equilibrium in solution in Seyferth’s early
report. Chen et al. reported that ea, ae, and ee isomers of
unsymmetric dialkyl derivatives of the iron complexes (e.g.,
Fe2S2RR′) are observed and do equilibrate to a constant ratio,
with the exception of the aa isomer, which is not observed.48
For the S-H-containing complexes, acid- or base-catalyzed
length (Å) or angle (°)
X-ray24
DFT
Fe(1)-Fe(8)
2.552(2)
2.228(2)
2.007(5)
69.9(1)
2.574
2.284
2.073
68.6
Fe(1)-S(15)
S(15)-C(16)
Fe(1)-S(15)-Fe(8)
S(15)-Fe(1)-S(16)
53.5(1)
53.96
a See Figure 5.
H, S, and O atoms. The largest discrepancy appears with the
S-S bond of Fe2S2, overestimated by 0.066 Å.
The relative stabilities of ae, ea, aa, and ee isomers of FeS
clusters were determined using free energy calculations, and
the isomers were assigned using GIAO theory in Gaussian03,
as described below.46 The electronic energies, and enthalpy and
free energy corrections, for the species of interest are presented
(46) (a) London, F. J. Phys. Radium 1937, 8, 397. (b) McWeeny, R Phys.
ReV. 1962, 126, 1028. (c) Ditchfield, R. Mol. Phys. 1974, 27, 789. (d)
Dodds, J. L.; McWeeny, R.; Sadlej, A. J. Mol. Phys. 1980, 41, 1419.
(e) Wolinski, K.; Hilton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990,
112, 8251.
(47) Seyferth, D.; Henderson, R. S.; Song, L. J. Organomet. Chem. 1980,
192, C1–C5.
9
15218 J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009

Fe(µ2-SH)Fe Bond Strengths and Homolytic Reactivity
A R T I C L E S
Figure 5. Optimized geometries for (left) Fe2S2Et2 (ae) and (right) Fe2S2.
Table 4. Electronic Energies and Enthalpy Corrections (298 K) for Fe2S2 Cluster Systems Fe2S2H2 and Fe2S2MeH and Radicals Fe2S2H•
and Fe2S2Me• (hartree/molecule ) 627.5095 kcal/mol)
species
electronic energy (hartree/molecule)
enthalpy [free energy] correction (hartree/molecule)
Fe2S2
-1723.37532
0.075547
Fe2S2H2 (ae ) ea)
Fe2S2H2 (aa)
Fe2S2H2 (ee)
-1724.73186
0.093783[0.024199]
0.093677[0.023970]
0.093718[0.024021]
0.084343[0.013710]
0.084473[0.013944]
0.124785[0.051375]
0.124654[0.050852]
0.124718[0.051336]
0.124762[0.051321]
0.115280[0.040752]
0.115281[0.040760]
0.128037[0.052042]
0.118969[0.042669]
0.118979[0.042698]
0.134666[0.056249]
0.125423[0.047232]
0.125510[0.047402]
0.00236
-1724.72968
-1724.73019
Fe2S2H• (a)
-1724.10645
Fe2S2H• (e)
-1724.10843
Fe2S2MeH (ae)
Fe2S2MeH (aa)
Fe2S2MeH (ea)
Fe2S2MeH (ee)
Fe2S2Me• (a)
Fe2S2Me• (e)
Fe2S3MeH (ae)
Fe2S3Me• (a)
-1764.05411
-1764.0501139
-1764.055151
-1764.052825
-1763.43009
-1763.43172
-2162.29137
-2161.66861 (UB3LYP, 〈S2 〉 ) 0.757)
-2161.66734 (ROB3LYP)
-1877.39713
Fe2S2(CO)MeH (ae)
Fe2S2(CO)Me• (a)
-1876.78469 (UB3LYP, 〈S2〉 ) 0.799)
-1876.78477 (ROB3LYP)
-0.49982 (-0.5)
H•
protonation/deprotonation of the S-H protons is expected to
lead to more rapid equilibration. For the four isomers of
(OC)3Fe(µ-SH)(µ-SCH3)Fe(CO)3, the gas-phase equilibrium
ratio is calculated to be 1:0.32:0.09:0.01 for Fe2S2MeH (ea):
Fe2S2MeH (ae):Fe2S2MeH (ee):Fe2S2MeH (aa). The solution
(benzene-d6) ratio measured was Fe2S2MeH (ea):Fe2S2MeH
(ae) in the ratio 1.0:0.20, while the ee and aa isomers were not
observed. The predicted gas-phase ratios are in reasonable
agreement with the solution values, consistent with equivalent
free energies of solvation of the various isomers.
Effect of Substitution of µ-S and µ-CO Bridges for the
Fe-Fe Bond on SH Bond Strengths. To further address the
question of the importance of the Fe-Fe bond in stabilization
of the radical on the FeS cluster, we examined the effect of
replacing the Fe-Fe bond with µ-S and µ-CO bridges. The latter
structure is a familiar feature of part of the core structure of
hydrogenase model systems. The H-cluster of FeFe hydroge-
nases contains an Fe4S4 cubane active site bridged to an Fe2S2
binuclear cluster, the latter of which contains a CO bridge
between two Fe atoms,49 reminiscent of the complexes studied
here. The geometries are found in Tables 6 and 7 and Figures
6 and 7, and the resulting BDEs are shown in Table 5.
Replacement of the Fe-Fe bond with the µ-S bond has no effect
on the S-H BDE compared to that of the Fe2S2MeH system.
However, insertion of the µ-CO bridge forces a contraction of
the Fe-Fe internuclear distance, as well as Fe-C-Fe and
Fe-S-Fe angles, in the radical compared to the thiol, resulting
in a substantial reduction in BDE from 74 to 66 kcal/mol. While
the source of the reduced bond strength is not readily deduced,
the dramatic effect on the S-H BDE suggests that design of
highly reactive thiols can be envisioned for heterogeneous and
homogeneous hydrogen transfer. Finally, from the data of Table
4, the thermochemical cycle for reaction of hydrogen with the
Fe2S2 system is estimated (Scheme 6). Loss of a single hydrogen
atom from Fe2S2H2 occurs with ∆H° ) 73 kcal/mol to form
Fe2S2H•. Loss of the second hydrogen atom is predicted to occur
(48) Lu, X. Q.; Zhu, C. Y.; Song, L. C.; Chen, Y. T. Inorg. Chim. Acta
1988, 143, 55–57.
(49) Peters, W. J.; Lanzillota, W. N.; Lemon, B. J.; Seefeld, L. C. Science
1998, 282, 1853.
9
J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009 15219

A R T I C L E S
Franz et al.
Chart 3. Relative Stabilities, (Enthalpies, 298 K), [Free Energies, 298 K] of Isomers of Fe2S2H2 and Fe2S2MeH and Radicals Fe2S2H• and
Fe2S2Me•, All in kcal/mol
Table 5. Computed Bond Dissociation Energies [∆H°(298 K),
kcal/mol] for Geometries Optimized at ROB3LYP/CEP-121G ECP
on Fe and 6-311++G(2d,2p) on C, H, S, and O
Table 7. DFT Geometries for Fe2S2(CO)MeH and Fe2S2(CO)Me•
Fe2S2(CO)MeH
Fe2S2(CO)Me•
Bond Distances, Å
2.991
reaction
XS-H f XS• + H•
Fe(7)-Fe(1)
Fe(7)-C(15)
Fe(7)-S(16)
Fe(7)-S(14)
S(14)-C(18)
2.78
2.051
2.35
2.395
1.831
Fe2S2H2 (ae) f Fe2S2H• (a) + H•
73.1
73.2
72.8
66.2
2.028
2.419
2.392
1.835
Fe2S2MeH (ea) f Fe2S2Me• (e) + H•
Fe2S3MeH (ae) f Fe2S3Me• (a) + H•
Fe2S2(CO)MeH (ae) f Fe2S2(CO)Me• (a) + H•
Angles, deg
95.01
Fe(7)-C(15)-Fe(1)
Fe(7)-S(16)-Fe(1)
Fe(7)-S(14)-Fe(1)
O(22)-C(15)-Fe(7)
85.3
72.5
70.9
Table 6. DFT Geometries of Fe2S3MeH (ea) and Fe2S3Me• (e)
76.36
77.39
137.3
Fe2S3MeH (ea)
Fe2S3Me•
132.5
Bond Distances, Å
Fe(7)-Fe(1)
Fe(7)-S(16)
Fe(7)-S(14)
Fe(7)-S(15)
S(14)-C(18)
S(15)-H(22)
S(14)-S(16)
S(14)-S(15)
3.06
2.356
2.39
2.389
1.832
1.348
3.221
3.099
3.157
2.339
2.381
2.334
1.835
) 71 kcal/mol), for which the hydrogen atom abstraction rate
is a factor of 10 slower than that for Fe2S2MeH (BDE )74
kcal/mol). This must reflect steric retardation of the abstraction
reaction by [Cp*Mo(µ-SH)(µ-SCH3)(µ-S)2]2, as the benzyl
radical is constrained to enter a channel between the Cp* rings
to abstract the S-H atom. This Polanyi relation predicts that
Fe2S2(CO)MeH (S-H BDE ) 66 kcal/mol) will donate a
hydrogen atom to the benzyl radical with a bimolecular rate
constant of 4.5 × 108 M-1 s-1 at 298 K, a factor of 4 slower
than diffusion control. Thus, activation of the hydrosulfido group
in the Fe(µ-SH)Fe structure, reminiscent of the Mo(µ-SH)Mo
cluster, can be designed and modified to produce bond strengths
near the range of organometallic M-H bond strengths, with
reaction rates with carbon-centered radicals approaching diffu-
sion control, and capable of donating S-H hydrogen atoms to
π-acceptors at appreciable rates near ambient temperature.
3.213
3.111
Angles, deg
Fe(7)-S(16)-Fe(1)
Fe(7)-S(15)-Fe(1)
Fe(7)-S(14)-Fe(11)
81
79.7
79.7
84.9
85.1
83.1
with ∆H° ) 47 kcal/mol. Overall dihydrogen addition to Fe2S2
occurs with ∆H° ) -16 kcal/mol.
Correlation of Benzyl Radical Abstraction Rates and
BDEs. With BDEs in hand, the relationship between ln(kabs) and
the S-H BDE for the thiols and hydrosulfides in Table 1 may
be explored in the form of a Polanyi correlation, as shown in
Figure 8. The Polanyi correlations are useful for predicting rates
from enthalpy changes for closely related series of hydrogen-
transfer reactions, in the absence of significant changes in polar
and steric effects across a series of donors. Donors with BDEs
varying from 91.2 kcal/mol (H2S) to 74.2 kcal/mol (the
Fe2S2MeH thiol) appear reasonably well correlated. However,
a notable outlier is Cp*Mo(µ-S)2(µ-SH)(µ-SCH3)MoCp* (BDE
Summary and Conclusions
The S-H bond strength for (OC)3Fe(µ-SCH3)(µ-SH)Fe(CO)3
has been determined; it is the first measured for Fe(µ-SH)Fe
structures. The first absolute rate expressions have been
determined for reaction of a carbon-centered benzyl radical with
the activated hydrosulfido group in these iron sulfide complexes.
9
15220 J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009

Fe(µ2-SH)Fe Bond Strengths and Homolytic Reactivity
A R T I C L E S
Figure 6. DFT structures of (left) the parent hydrosulfide Fe2S3MeH (ea) and (right) the resulting radical Fe2S3Me• (e).
Figure 7. DFT structures of (left) the parent hydrosulfide Fe2S2(CO)MeH and (right) the resulting radical Fe2S2(CO)Me•.
Scheme 6. Thermochemical Cycle for the Addition of Dihydrogen
a
and Subsequent Hydrogen Atom Losses for Fe2S2
Figure 8. Polanyi50 plot of ln(kabs/M-1 s-1) versus S-H bond strength
(kcal/mol) for reaction of benzyl radical with thiol donors in benzene, 298
K, ln(kabs/M-1 s-1) ) (-0.465 ( 0.07)(BDE) + 50.6 ( 5.0 (1σ error, R2
) 0.95), ∆H° is the S-H bond strength of the thiol. The data are
equivalently plotted as ln(kabs/M-1 s-1) ) (-0.465)(∆BDE) + 9.2, where
∆BDE is the difference in BDE between toluene, 89 kcal/mol, and the thiol.
(a) Fe2S2H2, (b) Fe2S2MeH, (c) Cp*Mo(µ-SCH3)(µ-SH)(µ-S)2Cp* (], not
included in correlation), (d) PhSH, (e) octanethiol, and (f) H2S.
a Note that the sum of the energies of the steps is equal to the homolytic
bond strength for dihydrogen.
Activation of the hydrosulfido groups leads to enhancement in
hydrogen-donating rates of 102-103-fold over those of al-
kanethiols and H2S, and with suitable substitution, a predicted
(Polanyi) enhancement of hydrogen abstraction rates up to 105-
fold over that of H2S is achievable. As in the case of Mo(µ-
SH)Mo systems, S-H bond strengths in Fe(µ-SH)Fe systems
span a range overlapping metal-hydride bond strengths. For
the present iron systems, the (RO)B3LYP method employed
here appears to give quite satisfactory results in predicting S-H
bond strengths, thermodynamic populations, and NMR proper-
ties of the Fe(µ-SH)Fe structures. Finally, the present results
illustrate that Fe(µ-SH)Fe bond strengths can be designed to
9
J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009 15221

A R T I C L E S
Franz et al.
to dark green, indicating the formation of Fe2S2Me-. After the
reaction was stirred for 30 min, 35 µL of benzyl bromide was added,
and the mixture was slowly warmed to room temperature. The
solution changed from green to reddish-brown at about 10 °C. The
mixture was stirred at room temperature for an additional 30 min,
and the solvent was removed under vacuum, leaving a red-orange
oil. The crude product was extracted with pentane to yield 53 mg
of product (41% yield). 1H NMR (C6D6, 500 MHz): δ 1.41 (s, 0.9
H, CH3, isomer 1), 1.55 (s, 2.1 H, CH3, isomer 2), 2.73 (s, 0.63 H,
CH2, isomer 1), 2.99 (s, 1.37 H, CH2, isomer 2), 7.00 (m, 5 H,
C6H5, both isomers).
Kinetics of Reaction of Benzyl Radical and Fe2S2H2. Stock
solutions of 1 × 10-4 M Fe2S2H2 and tert-butylbenzene (GC
standard) and 0.02 M dibenzyl ketone were dissolved in degassed
benzene, and 100-µL samples were freeze-thaw degassed and
sealed in Pyrex tubes. The samples were thermostatted in a
temperature-controlled hexadecane bath equipped with a quartz
optical window. Photolysis was carried out with the diffuse light
of a 1000-W Hanovia high-pressure xenon lamp filtered through a
water filter, over periods of 0.5 s, with less than 5% conversion of
Fe2S2H2, by means of a computer-controlled electronic shutter,
Uniblitz model 225L0A0T522952. Kinetics were measured from
299 to 345 K. Samples were opened, and toluene and bibenzyl
concentrations were determined by capillary gas chromatography.
Concentrations of products and Fe2S2H2 were corrected for tem-
perature using the relation FT ) 1.192 - 1.059 × 10-3 K g/mL for
the temperature dependence of the density of benzene.54
Self-Diffusion Coefficients of Fe2S2 in Benzene. Self-diffusion
coefficients for Fe2S2 in benzene were measured as a model for
the self-reaction rates of radical Fe2S2H• by the Taylor method,55
over the range 20-53 °C. The apparatus consisted of a Waters
HPLC pump delivering flow through a 20-cm normal-phase silica
column to dampen pressure oscillations, followed by a Waters
HPLC injector attached to a 31.7-m long, internal radius 0.052 cm,
PEEK column thermostatted in a Neslab RTE-211 water bath.
Analysis of the effluent was carried out with a Waters model 2410
refractometer. The variance (σ2) of the eluted peak for Fe2S2 was
determined by measuring the full-width at half height, Γ ) 2.345σ.
The relationship between the internal radius of the coiled tube, r,
the retention volume t, σ2, and the diffusion coefficient, D, is given
by D ) r2t/24σ2. Laminar flow was confirmed by reducing the
column flow rate until measured values of D were constant,
achieved at a flow rate of 0.3 mL/min. The resulting diffusion
coefficients for Fe2S2 in benzene were (10-5 cm2/s): 20 °C, 1.338,
1.282; 28 °C, 1.535, 1.526; 40 °C, 1.87, 1.95; 53 °C, 2.327, 2.301.
be sufficiently low to make possible the participation of the S-H
bond in selective hydrogenation of acceptor π systems, con-
sistent with the retro-disproportionation mode of hydrogen
transfer from Fe(µ-SH)Fe to aromatic π acceptors, and the bond
strengths of designed Fe(µ-SH)Fe systems may be low enough
to enable re-initiation in controlled polymerization.
Experimental Procedures
Instrumentation. Electrochemical data were collected in 0.3 M
NEt4BF4 in acetonitrile using a CH Instruments model 660C
computer-aided three-electrode potentiostat. The working electrode
was either a glassy carbon or platinum disk, the counter electrode
was a glassy carbon rod, and a silver chloride coated silver wire
was used as a pseudo-reference electrode and was separated from
the main compartment by a Vycor disk (1/8 in. diameter) obtained
from Bioanalytical Systems, Inc. Ferrocene or decamethylferrocene
was used as an internal reference, with all potentials reported versus
the ferricenium/ferrocene couple.
Materials. Reagents were purchased commercially and used
without further purification unless otherwise specified. All reactions,
syntheses, and manipulations of Fe2S2 complexes were carried out
under nitrogen using standard Schlenk techniques or in a glovebox.
Acetonitrile was dried by activated alumina column in an Innovative
Technology, Inc. PureSolv system. CD3CN was dried over activated
sieves, degassed, and stored in a glovebox. Benzene for photolysis
experiments was triply fractionally distilled to reduce trace toluene
to ∼10-7 M. Dibenzyl ketone (DBK) was recrystallized from
methanol.
Preparation of Fe2S2H2. Following the procedure of Seyferth
et al.,51 Fe2S2H2 was prepared from Fe2S2. The crude product was
dissolved in warm hexane, insoluble particulates were removed by
filtration, and the soluble fraction was recrystallized under an inert
atmosphere. The 1H NMR spectrum revealed a mixture of
axial-equatorial, axial-axial, and equatorial isomers similar to the
ratio reported. Fe2S2H2 was found to have a shelf life of a few
days at room temperature; it was stored in a freezer and recrystal-
lized from hexane as necessary, and its thiol content was assayed
1
by H NMR immediately prior to use.
Preparation of Fe2S2MeH. Following the method of Seyferth
and co-workers,52 methyllithum (1.8 mL of 1.6 M in Et2O, 2.9
mmol) was added via syringe into a red solution of 2.91 mmol of
Fe2S2 in THF at -78 °C. The red solution became green upon
formation of the anion, Fe2S2Me-. The green mixture was stirred
for 1 h. Addition of trifluoroacetic acid (0.22 mL, 2.9 mmol) to
the mixture at -78 °C restored a deep red color. The mixture was
continuously stirred for an additional 1 h and slowly warmed to
room temperature. The solvent was removed, and the red solid was
dried under vacuum. The product was recrystallized from pentane
to give a highly pure mixture of two isomers in 75% yield in a 5:1
Estimation of Self-Termination Rates for Fe2S2H• and
Cross-Termination Rates for the Reaction of Benzyl Radical
with Fe2S2H•. The above-measured values of self-diffusion of Fe2S2
were employed as a model for diffusion of Fe2S2H•, using the
classical von Smoluchowski equation for self-termination, 2kt )
(8π/1000)σFNDAB, where σ ) 1/4, the fraction of encounter pairs
undergoing reaction, assuming slow intersystem crossing of singlet
and triplet cage radical pairs of Fe2S2H•, F is the reaction diameter,
1
ratio. H NMR (C6D6, 500 MHz): δ 1.40 and 1.39 (s, 3.0 H total,
SCH3, isomers A and B), δ -1.16 (s, 0.12 H, SH, isomer B), -2.86
(s, 0.73 H, SH, isomer A).
Preparation of Fe2S2MeBz. Methyllithium (0.2 mL of 1.6 M
in Et2O, 3.2 mmol) was added via syringe into a solution of Fe2S2
(0.100 g, 0.29 mmol) in THF (20 mL) at -78 °C following the
procedure of Seyferth.53 The reaction mixture changed from orange
(7.53
(
0.25)
×
10-8 cm, estimated for Fe2S2H• using
Spernol-Wirtz,56 van der Waals,57 and LeBas58 methods (or from
the molecular volume of Fe2S2 estimated by PCModel59), N ) 6.022
× 1023 (Avogadro’s number), and DAB are experimental diffusion
coefficients of Fe2S2, the model for Fe2S2H•. This results in an
expression for the total self-termination rate for Fe2S2H•:
(50) (a) Gardner, K. A.; Kuehnert, L. A.; Mayer, J. M. Inorg. Chem. 1997,
36, 2069–2078. (b) Ingold, K. U. In Free Radicals; Kochi, J. K., Ed.;
Wiley: New York, 1973; Vol. 1, Chapter 2, p 69. (c) Russell, G. A.
In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 1,
Chapter 7, pp 283-293. (d) Tedder, J. M. Angew. Chem., Int. Ed.
Engl. 1982, 21, 401–410. (e) Korzekwa, K. R.; Jones, J. P.; Gillette,
J. R. J. Am. Chem. Soc. 1990, 112, 7042–7046.
J. Organomet. Chem. 1985, 292, 9–17. (c) Song, L.-C. Acc. Chem.
Res. 2005, 38, 21–28.
(54) Reid, R. C.; Prausnitz, J. M. Sherwood, T. K. The Properties of Gas
and Liquids and Gases; McGraw-Hill: New York, 1977.
(55) Taylor, G. Proc. R. Soc. London, Ser. A 1954, 235, 473.
(56) Spernol, A.; Wirtz, K. Z. Naturforsch. 1953, 8a, 522.
(57) Edward, J. T. J. Chem. Educ. 1970, 47, 261.
(51) Seyferth, D.; Womack, G. B.; Hendersen, R. S. Organometallics 1986,
5, 1568–1575.
(52) Seyferth, D.; Henderson, R. S.; Song, L.; Womack, G. B. J.
Organomet. Chem. 1985, 292, 9–17.
(53) (a) Seyferth, D.; Henderson, R. S. J. Am. Chem. Soc. 1979, 101, 508–
509. (b) Seyferth, D.; Henderson, R. S.; Song, L.-C.; Womack, G. B.
(58) Ghai, R. L.; Dullien, F. A. L. J. Phys. Chem. 1974, 78, 2283.
(59) PCMODEL, version 9.1; Serena Software: Bloomington, IN, 2002.
9
15222 J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009

Fe(µ2-SH)Fe Bond Strengths and Homolytic Reactivity
A R T I C L E S
ln(2kt′/M-1 s-1) ) 27.8 - 3359.6/RT (R ) 1.9872 and T, K)
Kinetics of Reaction of Benzyl Radical with Fe2S2MeH
and Calculation of Rate Constants. One-hundred-microliter
samples of a solution of Fe2S2MeH (3.2 × 10-5 M), dibenzyl
ketone (0.02 M), and tert-butylbenzene (5.2 × 10-4 M, GC
standard) in benzene were freeze-thaw degassed, sealed in
5-mm-o.d. × 5-cm Pyrex tubes, and photolyzed for 1 s with
diffuse light from a 1-kW high-pressure Xe lamp and computer-
controlled optical shutter as described above. Under these
conditions, the photolysis of dibenzyl ketone occurs at ca. 1.5
× 10-5 M/L · s. Rate constants were calculated according to eq
8 from the concentrations of toluene and bibenzyl, photolysis
time, and kt values determined from the expression ln(2kt/M-1
s-1) ) 27.23 - 2952.5/RT. Results are given in Table 1.
To estimate the cross-termination rate constants for reaction of
benzyl radical and Fe2S2H•, rate constants for self-termination of
benzyl radical in benzene, given by ln(2kt/M-1 s-1) ) 27.23 -
2952.5/RT, were combined with the above expression for self-
termination of Fe2S2H•, utilizing the relationship 2kct ) 2(ktkt′)1/2
.
The basis for using twice the geometric mean of self-termination
rate constants for estimation of cross-termination constants has been
addressed in detail.60 The resulting rate expression is given by
ln(2kct/M-1 s-1) ) 28.21 - 3156.1/RT
Photolysis of Fe2S2MeH with DBK in C6D6 for Product
Analysis. A stock solution was prepared to be 2.7 mM of
Fe2S2MeH and 0.2 M of DBK in C6D6. The stock solution was
diluted by a factor of 10 and divided into 10 NMR tubes for
photolysis. Each tube was photolized for 2 s, and then all 10
samples were combined and condensed by evaporation to 1 mL
for analysis by 1H NMR. The two methylene resonances for the
Calculation of Rate Constants for Reaction of Fe2S2H2 and
Benzyl Radical. In the case of Fe2S2H2, kinetic modeling
suggests that a significant portion (ca. one-third) of the total
toluene was formed from Fe2S2H• by disproportionative cross-
termination (eq 5). Photolysis of DBK in the presence of Fe2S2H2
revealed (by NMR) no formation of combination cross-termina-
tion products Fe2S2BzH or the self-termination combination
product (Fe2S2H)2. The lack of Fe2S2BzH or (Fe2S2H)2 suggests
that disproportionation pathways dominate self-termination reac-
tions. The portion of toluene formed in disproportionation, eq
5, can be calculated by numerical integration of eqs 2a-5 and
7 with neglect of eqs 6 and 8 using termination rate constants
available from the von Smoluchowski treatment of the self-
diffusion of Fe2S2H•, as illustrated above. Thus, the rate constants
kabs (eq 2) were determined by numerical integration of eqs 2-5
and 7 with kabs as the sole variable. Inputs for the numerical
integration and optimization were as follows: For eqs 2a and
2b, the rate of photolysis of DBK is given by ([toluene]/2 +
[bibenzyl])/∆t, where ∆t is the duration of photolysis, 0.5 s.
The value of kabs was the optimized variable in the numerical
integration. For eq 4, kt was calculated from the expression ln(2kt/
M-1 s-1) ) 27.23 - 2952.5/RT. For eq 5, based on the lack of
cross-combination product Fe2S2BzH, all cross-termination is
assigned to eq 5, with kct given by ln(2kct/M-1 s-1) ) 28.21 -
3156.1/RT, and eq 6 is neglected. From the absence of product
(Fe2S2H)2, the self-termination of Fe2S2H• is assigned entirely
to eq 7, and eq 8 is neglected. The disproportionative self-
termination rate of Fe2S2H• is estimated from the experimental
self-diffusion coefficients of Fe2S2, as described above, and is
given by ln(2kt′/M-1 s-1) ) 27.8 - 3359.6/RT. For each
experiment, experimental inputs for numerical integration include
the observed concentrations of toluene and bibenzyl, the initial
concentration of Fe2S2H2, the rate of photolysis of DBK
() ([toluene]/2 + [bibenzyl])/∆t, typically (1.3 ( 0.5) × 10-5
M s-1, and the duration of photolysis, ∆t ) 0.5 s. Equations
2-6 are numerically integrated with optimization of kabs using
the Berkeley Madonna61 program while holding constant the
values of kt, kt′, and kct from the expressions developed above.
A fourth-order Runge-Kutta integration routine was used with
time increments of 1 × 10-7 s. Values of kabs are varied in the
optimization to fit experimental bibenzyl and toluene concentra-
tions. Under the present conditions, it was found that values of
kabs are reduced by ca. 35% by taking cross-termination into
account compared to the neglect of cross-termination of benzyl
radical with Fe2S2H• (eq 5). A sensitivity test of the error in kabs
as a function of error in kt, kct, and kt′ was carried out; an error
of 100% in either of the latter two termination rate constants
1
benzylthiolate groups observed in the H NMR spectrum match
the resonances of the authentic Fe2S2MeBz at 2.73 and 2.99 ppm.
The methanethiolate resonances, however, could not be cleanly
observed due to overlap with the signals for the starting material,
Fe2S2MeH.
Kinetics of Reaction of Benzyl Radical and H2S. On a vacuum
line, ca. 15 mL samples of gaseous H2S (1 atm) were condensed
into 200-µL samples of a benzene-d6 solution of dibenzyl ketone
(0.02 M), tert-butylbenzene (0.001 M), and hexamethylbenzene
(0.02 M) in 3-mm Pyrex tubes, freeze-thaw degassed, and
sealed. After sealing, the samples, having 300 µL of head space
above the liquid phase, were placed in standard 5-mm NMR
tubes and equilibrated at the desired photolysis temperature to
constant H2S concentrations, as determined by careful integration
of the H2S peak relative to tert-butylbenzene and hexamethyl-
benzene internal standards. H2S exhibits long T1 (ca. 30 s) values,
necessitating long recycle times: 45° 1H pulse widths and 120-s
delays were employed. The samples required up to 4 h for H2S
concentrations to fully equilibrate. The samples were placed into
a thermostatted hexadecane photolysis bath and photolyzed for
1-15 s. Rate constants were calculated according to eq 1.
Determination of pKa by NMR. In a typical experiment,
Fe2S2MeH and 2,4,6-trimethylpyridine (10.8 and 9.2 mmol,
respectively) were mixed in 1.0 mL of CD3CN. Due to rapid
proton exchange, the chemical shifts observed for each species
were the weighted averages of the chemical shifts for the
protonated and deprotonated species. Using the known pKa of
2,4,6-trimethylpyridine (14.98),31 the ratio of 2,4,6-trimethylpy-
ridine to 2,4,6-trimethylpyridinium from the chemical shift (3.29:
1), and the 1:3.55 ratio of Fe2S2Me- to Fe2S2MeH (the anion
to the neutral S-H) gives a pKa of 16.0 ( 0.4 from four
independent equilibrium determinations (the listed error is 2σ
+ 0.2 to account for the reproducibility and any error in the
reference base). Details of the pKa determination are shown in
the Supporting Information.
Cyclic Voltammetry. Solutions of Fe2S2MeH were prepared
at 0.5 and 1.5 mM in 0.3 M NEt4BF4 in acetonitrile. The anion,
Fe2S2Me-, was generated in situ by adding 1 equiv of triethy-
lamine. The anodic peak potential (Epa) for the resulting
irreversible oxidation was observed between -0.39 and -0.29
V vs FeCp2 over the range 0.05-100 V/s. Simulations of the
CVs were run using DigiSim33 with a one-electron oxidation,
followed by dimerization with a diffusion-controlled rate constant
of 2 × 109 M-1 s-1, based upon the previously reported dimer
synthesis for the phenyl analogue.32 For each concentration and
scan rate, the difference between the E1/2 and the resulting
irreversible Epa was used to correct the experimental data to give
an average E1/2 of -0.26 ( 0.05 V vs ferrocene (where the listed
error is 2σ + 10 mV to account for the reproducibility of the
results in less than 10% error in kabs
.
(60) (a) Mu¨nger, K.; Fischer, H. Int. J. Chem. Kinet 1984, 16, 1213. (b)
Paul, H.; Segaud, C. Int. J. Chem. Kinet. 1980, 7, 637. (c) Fischer,
H.; Paul, H. Acc. Chem. Res. 1987, 20, 200.
(61) Macey, R. I.; Oster, G. F. Berkeley Madonna, version 8.0.1; 2000;
http://www.berkeleymadonna.com/.
9
J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009 15223

A R T I C L E S
Franz et al.
measurement and correction of the potential, as well as any error
in the reference).
U.S. DOE, is gratefully acknowledged. This paper is dedicated
to Keith U. Ingold on the occasion of his 80th birthday.
Acknowledgment. Work at Pacific Northwest National Labo-
ratory (PNNL) was supported by the Division of Chemical
Sciences, Office of Basic Energy Sciences, U.S. Department of
Energy (DOE). Battelle operates PNNL for DOE. Provision of
computational resources at the National Energy Research
Scientific Computing Facility (NERSC) by the Office of Science,
Supporting Information Available: Tables of the kinetic data,
detailed summaries of DFT calculations, details of the pKa
determination, and complete ref 39. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA904602P
9
15224 J. AM. CHEM. SOC. VOL. 131, NO. 42, 2009