Electron-transfer-catalyzed rearrangement of unsymmetrically substituted diiron dithiolate complexes related to the active site of the [FeFe]-hydrogenases

Article abstract of DOI:10.1021/ic701327w

Novel asymmetrically substituted azadithiolate compounds [Fe ₂(CO)₄(κ²-dppe){μ-SCH ₂N(R)CH₂S}] (R = ⁱPr, 1a; CH₂CH ₂OCH₃, 1b; CH₂C₆H₅, 1c) have been synthesized by treatment of [Fe₂(CO)₆(μ-adt)] [adt = SCH₂N(R)CH₂S, with R = ⁱPr, CH ₂CH₂OCH₃, CH₂C₆H ₅] with dppe (dppe = Ph₂PCH₂CH ₂PPh₂) in refluxing toluene in the presence of Me ₃NO. 1a-c have been characterized by single-crystal X-ray diffraction analyses. The electrochemical investigation of 1a-c and of [Fe ₂(CO)₄(κ²-dppe)(μ-pdt)] (1d) [pdt = S(CH₂)₃S] in MeCN- and THF-[NBu₄][PF ₆] has demonstrated that the electrochemical reduction of 1a-d gives rise to an Electron-transfer-catalyzed (ETC) isomerization to the symmetrical isomers 2a-d where the dppe ligand bridges the iron centers. Compounds 2a-d were characterized by IR and NMR spectroscopy, elemental analysis, and X-ray crystallography for 2a.

Full text of DOI:10.1021/ic701327w

Inorg. Chem. 2007, 46, 9863−9872
Electron-Transfer-Catalyzed Rearrangement of Unsymmetrically
Substituted Diiron Dithiolate Complexes Related to the Active Site of
the [FeFe]-Hydrogenases
Salah Ezzaher, Jean-Franc¸ois Capon, Fre´de´ric Gloaguen, Franc¸ois Y. Pe´tillon,
Philippe Schollhammer,* and Jean Talarmin*
UMR CNRS 6521, Chimie, Electrochimie Mole´culaires et Chimie Analytique, UFR Sciences et
Techniques, UniVersite´ de Bretagne Occidentale, CS 93837, 29238 Brest-Cedex 3, France
Received July 4, 2007
Novel asymmetrically substituted azadithiolate compounds [Fe2(CO)4(κ2-dppe){µ-SCH2N(R)CH2S
}
] (R
SCH2N(R)CH2S,
Ph2PCH2CH2PPh2) in refluxing toluene in the presence
c have been characterized by single-crystal X-ray diffraction analyses. The electrochemical investigation
c and of [Fe2(CO)4(κ2-dppe)(
-pdt)] (1d) [pdt S(CH2)3S] in MeCN and THF [NBu4][PF6] has demonstrated
that the electrochemical reduction of 1a d gives rise to an Electron-transfer-catalyzed (ETC) isomerization to the
symmetrical isomers 2a d where the dppe ligand bridges the iron centers. Compounds 2a d were characterized
by IR and NMR spectroscopy, elemental analysis, and X-ray crystallography for 2a.
)
iPr, 1a;
CH2CH2OCH3, 1b; CH2C6H5, 1c) have been synthesized by treatment of [Fe2(CO)6(
with R
iPr, CH2CH2OCH3, CH2C6H5] with dppe (dppe
of Me3NO. 1a
of 1a −
µ-adt)] [adt )
)
)
−
−
µ
)
−
−
−
−
Introduction
subsite of the H-cluster are now known,2,3 and recent efforts
have been directed toward the synthesis of unsymmetrically
substituted diiron dithiolate complexes, with well differenti-
ated metal centers, that would mimic more closely the
biological site.4-9 Quite remarkably, an artificial H-cluster
model of the biological site has been synthesized.10
A new generation of diiron complexes with polydentate
ligands has emerged very recently, which should permit one
to examine the effect of introducing an asymmetrical feature
The discovery1 that the [FeFe]-hydrogenases use an
organometallic diiron dithiolate center with CO and CN-
ligands to efficiently catalyze the H+/H2 conversion is at the
origin of the revival of the chemistry of [Fe2(CO)6-n(L)n(µ-
dithiolate)] complexes.2 A variety of models of the [2Fe]
* To whom correspondence should be addressed. E-mail: jean.talarmin@
univ-brest.fr (J.T.); schollha@univ-brest.fr (P.S.).
(1) (a) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C.
Science 1998, 282, 1853-1858. (b) Nicolet, Y.; Piras, C.; Legrand,
P.; Hatchikian, C. E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13-
23. (c) Nicolet, Y.; de Lacey, A. L.; Vernede, X.; Fernandez, V. M.;
Hatchikian, C. E.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001,
123, 1596-1602. (d) Lemon, B. J.; Peters, J. W. Biochemistry 1999,
38, 12969-12973.
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E. J.; Hall, M. B.; Darensbourg, M. Y. Coord. Chem. ReV. 2003, 238-
239, 255-266. (b) Darensbourg, M. Y.; Lyon, E. J.; Zhao, X.;
Georgakaki, I. P. Proc. Natl Acad. Sci. U.S.A. 2003, 100, 3683-3688.
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(d) Rauchfuss, T. B. Inorg. Chem. 2004, 43, 14-26. (e) Song, L.-C.
Acc. Chem. Res. 2005, 38, 21-28. (f) Artero, V.; Fontecave, M. Coord.
Chem. ReV. 2005, 249, 1518-1535. (g) Best, S. P. Coord. Chem. ReV.
2005, 249, 1536-1554. (h) Bruschi, M.; Zampella, G.; Fantucci, P.;
De Gioia, L. Coord. Chem. ReV. 2005, 249, 1620-1640. (i) Liu, X.;
Ibrahim, S. K.; Tard, C.; Pickett, C. J. Coord. Chem. ReV. 2005, 249,
1641-1652. (j) Sun, L.; Åkermark, B.; Ott, S. Coord. Chem. ReV.
2005, 249, 1653-1663. (k) Capon, J.-F.; Gloaguen, F.; Schollhammer,
P.; Talarmin, J. Coord. Chem. ReV. 2005, 249, 1664-1676. (l) Vignais,
P. Coord. Chem. ReV. 2005, 249, 1677-1690, and references cited
therein.
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D. L.; Barclay, J. E.; Evans, D. J.; Fairhurst, S. A.; Liu, X.; Pickett,
C. J. Dalton Trans. 2003, 586-595. (b) George, S. J.; Cui, Z.; Razavet,
M.; Pickett, C. J. Chem. Eur. J. 2002, 8, 4037-4046. (c) Zampella,
G.; Bruschi, M.; Fantucci, P.; Razavet, M.; Pickett, C. J.; de Gioia, L.
Chem. Eur. J. 2005, 11, 509-520. (d) Lawrence, J. D.; Li, H.;
Rauchfuss, T. B. Chem. Commun. 2001, 1482-1483. (e) Song, L.-
C.; Yang, Z.-Y.; Bian, H.-Z.; Hu, Q.-M. Organometallics, 2004, 23,
3082-3084. (f) Hu, M.-Q.; Ma, C.-B.; Zhang, X.-F.; Chen, F.; Chen,
C.-N.; Liu, Q.-T. Chem. Lett. 2006, 35, 840-841. (g) Windhager, J.;
Rudolph, M.; Bra¨utigam, S.; Go¨rls, H.; Weigand, W. Eur. J. Inorg.
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10.1021/ic701327w CCC: $37.00
Published on Web 10/17/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 23, 2007 9863

Ezzaher et al.
Scheme 2. Enantiomeric Forms of the Basal-Apical Form of
Complexes 1
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1a-c
1a
1b*
1c#
Fe1-Fe2
Fe1-S1
2.5451(8)
2.2523(11)
2.2551(11)
2.2621(11)
2.2783(12)
2.2314(11)
2.1987(12)
1.452(5)
2.5453(12)
2.2543(16)
2.2571(18)
2.2527(17)
2.2646(17)
2.2318(17)
2.1930(17)
1.435(7)
1.438(7)
1.492(8)
112.2(6)
112.8(6)
110.1(5)
68.77(5)
68.51(5)
87.74(6)
2.5445(11)
2.2471(16)
2.2618(14)
2.2660(15)
2.2793(16)
2.2119(15)
2.1831(16)
1.457(6)
1.456(6)
1.482(6)
111.9(4)
111.7(4)
107.9(4)
68.64(5)
68.16(4)
86.95(6)
Fe1-S2
Fe2-S1
Fe2-S2
Fe1-P1
Figure 1. Variable-temperature 31P{1H} NMR spectra (in CD2Cl2) of [Fe2-
(CO)4(κ2-dppe){µ-SCH2N(R)CH2S}] (R ) CH2CH2OMe, 1b) (* ) impu-
rity).
Fe1-P2
N1-C5
N1-C7 (6)*
N1-C50 (34)#
C5-N1-C7(6)*
1.443(5)
1.493(4)
111.7(3)
Scheme 1
(6)*C7-N1-C50 (34)# 113.9(3)
C5-N1-C50 (34) #
Fe1-S1-Fe2
Fe1-S2-Fe2
P1-Fe1-P2
110.9(3)
68.63(3)
68.31(3)
87.84(4)
and we show that the electrochemical reduction of [Fe2(CO)4-
(κ2-dppe)(µ-dithiolate)] complexes [dithiolate ) pdt or adt;
i
SCH2N(R)CH2S, R ) Pr, CH2CH2OMe, CH2C6H5] gives
rise to an electron-transfer-catalyzed (ETC)11 rearrangement
to the symmetrical isomer, [Fe2(CO)4(µ-dithiolate)(µ-dppe)]
(2).
in diiron systems through chelating ligands with the prospect
of designing more efficient electrocatalysts.4-9 A detailed
understanding of both the reduction processes and the
elementary protonation steps of dithiolate-bridged species
is primordial to shedding light on the hydrogenase process.
To this end, we have reported the synthesis and the
protonation of [Fe2(CO)4(κ2-L)(µ-pdt)] (pdt ) S(CH2)3S)
where L is either a chelating bis-(N-heterocyclic carbene)
or dppe ligand8,9 (dppe ) Ph2P(CH2)2PPh2). We have now
undertaken investigating electrochemically this type of
asymmetrically diphos-substituted compounds where an
azadithiolate (adt) or a propanedithiolate (pdt) bridges the
tetracarbonyl diiron core to assess whether the uneven
substitution at the iron centers induces a specific electron-
transfer chemistry. Herein, we report the synthesis and the
characterization of the novel asymmetrically substituted
azadithiolate diiron complexes [Fe2(CO)4(κ2-dppe){µ-SCH2N-
(R)CH2S}] (R ) iPr, 1a; CH2CH2OCH3, 1b; CH2C6H5, 1c),
Results and Discussion
Synthesis and Characterization of [Fe2(CO)4(κ2-dppe)-
i
{µ-SCH2N(R)CH2S}] (R ) Pr, 1a; CH2CH2OMe, 1b;
CH2C6H5, 1c). Treatment of [Fe2(CO)6(µ-adt)] with dppe
in refluxing toluene for 4 h in the presence of Me3NO
afforded moderate yields of the asymmetric substituted
complexes [Fe2(CO)4(dppe)(µ-adt)] (1) (Scheme 1).
Compounds 1a-c were characterized by elemental analy-
sis and IR and NMR spectroscopy. Strong ν(CO) bands are
observed between 2020 and 1900 cm-1 in the infrared
spectrum of CH2Cl2 solutions of 1a-c. The 31P{1H} NMR
spectra of 1a-c in CD2Cl2 show two singlets between 92
and 76 ppm with a ratio of ca. 8:2. Upon cooling, the more
deshielded signal splits into two ill-resolved AB quartets and
the signal at ca. 76 ppm splits into two singlets (Figure 1).
These observations are in accord with previous studies which
indicate the presence of four isomers at low temperature for
analogous compounds.5,9 On this basis, the former peak was
assigned to a basal-apical isomer and the latter to a basal-
basal form, with two possible orientations of the adt bridge
for each isomer. Interestingly, the nature of the R group borne
by the azadithiolate bridge induces different ratios of the two
forms generated by the motion of the adt bridge. Indeed, for
(8) Morvan, D.; Capon, J.-F.; Gloaguen, F.; Le Goff, A.; Marchivie, M.;
Michaud, F.; Schollhammer, P.; Talarmin, J.; Yaouanc, J.-J.; Pichon,
R.; Kervarec, N. Organometallics 2007, 26, 2042-2052.
(9) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pe´tillon, F.Y.; Schollhammer,
P.; Talarmin, J.; Pichon, R.; Kervarec, N. Inorg. Chem. 2007, 46,
3426-3428.
(10) Tard, C.; Liu, X.; Ibrahim, S. K.; Bruschi, M.; de Gioia, L.; Davies,
S. C.; Yang, X.; Wang, L.-S.; Sawers, G.; Pickett, C. J. Nature 2005,
433, 610-613.
9864 Inorganic Chemistry, Vol. 46, No. 23, 2007

Unsymmetrically Substituted Diiron Thiolate Complexes
Figure 2. Molecular structures of compounds [Fe2(CO)4(κ2-dppe){µ-SCH2N(R)CH2S}] (R ) iPr, 1a; CH2CH2OMe, 1b; CH2C6H5, 1c) with thermal ellipsoids
at 30% probability.
both basal-apical and dibasal isomers, 90:10, 70:30, and
i
60:40 ratios were obtained at 183 K for R ) Pr, CH2CH2-
OCH3, and CH2C6H5, respectively. 1H NMR spectra of 1a-c
display the set of resonances expected for the various R
groups. They confirm the presence of two isomers at 298
K, but full assignment of their resonances could only be
i
proposed for 1a (R ) Pr). As expected, temperature
dependence of the line shapes was observed for all the
complexes by cooling their solutions. For example, at 183
K the 1H NMR spectrum of the major basal-apical form of
1a displays four doublets at 2.83 (1H), 2.40 (1H), 1.55 (1H),
and 1.60 (1H) ppm for the protons of the adt bridge, instead
of two doublets at 2.74 (2H) and 1.75 (2H) ppm at room
temperature. This confirms that the exchange between the
Figure 3. Cyclic voltammetry of [Fe2(CO)4(κ2-dppe){µ-SCH2N(iPr)-
CH2S}] (1a) (1.1 mM). (a) MeCN-[NBu4][PF6] under Ar; (b) the potential
(11) For reviews on electron-transfer-catalyzed (ETC) processes, see: (a)
was held for 10 s at -2.05 V before scan reversal (V ) 0.2 V s-1, vitreous
Chanon, M.; Tobe, M. L. Angew. Chem., Int. Ed. 1982, 21, 1-23. (b)
carbon electrode).
Chanon, M. Acc. Chem. Res. 1987, 20, 214-221. (c) Astruc, D.
Angew. Chem., Int. Ed. 1988, 27, 643-660. (d) Astruc, D. Chem.
ReV. 1988, 88, 1189-1216. (e) Kochi, J. K. J. Organomet. Chem.
two enantiomers (forms A and B) and the adt motion are
slowed down at low temperature (Scheme 2).
1986, 300, 139-166. (f) Hershberger, J. W.; Amatore, C.; Kochi, J.
K. J. Organomet. Chem. 1983, 250, 345-371. (g) Save´ant, J. M. Acc.
Chem. Res. 1980, 13, 323-329. (h) Amatore, C.; Pinson, J.; Save´ant,
J. M.; Thie´bault, A. J. Am. Chem. Soc. 1982, 104, 817-826. (i)
Feldberg, S. W.; Jeftic, L. J. Phys. Chem. 1972, 76, 2439-2446 and
references cited therein.
X-ray analysis of single crystals of complexes 1a-c
obtained from diethyl ether solutions established without any
Inorganic Chemistry, Vol. 46, No. 23, 2007 9865

Ezzaher et al.
lie in an equatorial position and the sum of the C-N-C
angles around N1 is between 336° (1a) and 330° (1c).
Electron Transfer-Catalyzed Rearrangement of [Fe2-
(CO)4(κ2-dppe)(µ-SCH2XCH2S)] (1) (X ) NiPr, 1a;
NCH2CH2OMe, 1b; CH2, 1d). The cyclic voltammetry (CV)
of 1a,b,d in THF- and MeCN-[NBu4][PF6] electrolyte
revealed that the electrochemical reduction of the complexes
gives rise to an ETC11 process. Indeed, whereas the CV in
the oxidation range shows the quasi-reversible or irreversible
oxidation of 1a,b,d (Table 2, Figures 3a and S1, Supporting
Information), the major peaks in the negative domain do not
arise from 1 but from a product generated by its reduction.
The actual reduction of 1 is characterized by the small peak
around -2 V (see arrows in Figures 3a and S1). Experimental
evidence for the occurrence of an ETC process upon
Figure 4. Cyclic voltammetry of [Fe2(CO)4(κ2-dppe)(µ-pdt)] (1d) in
MeCN-[NBu4][PF6] under Ar and under CO (V ) 0.2 V s-1, vitreous
carbon electrode).
electrochemical reduction of 1 is as follows.
red1 13
(1) The current ratio of the minor reduction peak (ip
)
to the major peak at ca. -2.2 V (ipred2) measured from a
MeCN-[NBu4][PF6] solution of 1a,b,d increases with
increasing scan rates (0.04 V s-1 e V e 10 V s-1) as expected
for an ETC process.
(2) Lowering of the temperature results in an increase of
the minor reduction peak at the expense of the peaks situated
at more negative potentials.
(3) The product detected by a new oxidation on the reverse
scan is generated at the potential of the minor reduction peak,
as illustrated in Figure 3b for complex 1a (and for 1d in
Figure S1b, Supporting Information).
(4) Exhaustive electrolysis of a solution of 1 at the
potential of the minor peak produces the symmetrical
complex 2 (see below) with a small charge consumption
(e0.5 F mol-1 1).
The ETC process results from the fast isomerization of
1- to 2- that oxidizes at a potential more negative than the
reduction of 1. Therefore, 2- is oxidized either at the
electrode surface (at the reduction potential of 1) or by 1 in
a homogeneous cross-redox reaction (Scheme 3). As a
consequence of the oxidation of 2-, the reduction peak of 1
is suppressed and 2 is generated by electrolysis of 1 with a
low charge consumed.
Figure 5. 13C[1H] NMR spectrum of [Fe2(CO)4(κ2-dppe)(µ-pdt)] (1d) and
[Fe2(CO)4(µ-pdt)(µ-dppe)] (2d) in the CO region at 298 K.
ambiguity their geometry and the binding mode of the
bidendate diphosphine to a single iron atom in a basal-apical
position (Figure 2 and Table 1). The Fe-Pba (basal position)
bond is slightly shorter than the Fe-Pap (apical position) one
[Fe1-Pba ) 2.2314(11), 2.2318(17), 2.2119(15) Å and Fe1-
Pap ) 2.1987(12), 2.1930(17), 2.1831(16) Å in 1a, 1b, 1c,
respectively]. Other distances and angles are unexceptional.
1 is structurally analogous to other diiron(I) hexacarbonyl
adt-bridged complexes.12 The two {Fe(CO)3} and {Fe(CO)-
P2} units are eclipsed and linked by a direct Fe-Fe bond
and the adt group. The resulting coordination geometry
around each Fe atom is described as a distorted square
pyramid supplemented by the Fe(I)-Fe(I) single bond
required by normal electron counting rules. The NR groups
The CVs in Figure 4 (also see Figure S2, Supporting
Information) are informative as to the mechanism of the
rearrangement since the fact that the reduction peak of 1 is
larger in the presence of CO (note that the onset of the
reduction of 1 under CO is coincident with that under Ar)
demonstrates that the catalytic process is less efficient under
these conditions. This may arise from the reaction of 1-,
2-, or an intermediate between these species with CO. ETC
processes are most frequently associated with substitution
reactions that involve notably diiron dithiolate carbonyl
(13) The parameters ip and Ep are, respectively, the peak current and the
a
c
a
a
peak potential of a redox process; E1/2 ) (Ep + Ep )/2; Ep , ip and
c
c
Ep , ip are, respectively, the potential and the current of the anodic
a
c
and of the cathodic peak of a reversible process; ∆Ep ) Ep - Ep .
CV stands for cyclic voltammetry; V (V s-1) is the scan rate in CV
experiments. An EC process comprises one electron transfer step (E)
followed by a chemical reaction (C).
(12) Vijaikanth, V; Capon, J.-F.; Gloaguen, F.; Pe´tillon, F.Y.; Schollham-
mer, P.; Talarmin, J. J. Organomet. Chem. 2007, 692, 4177-4181
and references cited therein.
9866 Inorganic Chemistry, Vol. 46, No. 23, 2007

Unsymmetrically Substituted Diiron Thiolate Complexes
Table 2. Redox Potentials of [Fe2(CO)4(dppe){µ-dithiolate}] Complexes as Measured by Cyclic Voltammetry (V ) 0.2 V s-1 unless Stated Otherwise;
Vitreous Carbon Electrode)
red
ox
Ep
(V)
Ep
(V)
detection potentials
of products (V)
complex
solvent
MeCN
[Fe2(CO)4(κ2-dppe){µ-SCH2N(R)CH2S}]
1a (R ) iPr)
-2.01
-0.30
-2.12 (E1/2
)
-2.59 (Ep)
ox
0.07 (Ep
)
THF
-2.22
-0.21(E1/2
)
)
-2.32 (E1/2
)
-2.75 (E1/2
)
)
ox
0.10 (Ep
[Fe2(CO)4(µ-dppe){µ-SCH2N(R)CH2S}]
2a (R ) iPr)
MeCN
MeCN
-2.12 (E1/2
)
0.08
-2.59 (Ep)
-2.50 (E1/2
-2.11 (E1/2
-2.49 (E1/2
a
)
[Fe2(CO)4(κ2-dppe){µ-SCH2N(R)CH2S}]
1b (R ) CH2CH2OMe)
-1.98
-0.26
)
)
ox
0.08 (Ep
)
THF
-2.16
-0.25(E1/2
-2.36 (Ep)
-2.78 (E1/2
)
ox
0.13 (Ep
)
[Fe2(CO)4{µ-SCH2N(R)CH2S}(µ-dppe)]
2b (R ) CH2CH2OMe)
[Fe2(CO)4(κ2-dppe) (µ-pdt)]
1d
MeCN
THF
MeCN
-2.10 (E1/2
-2.36 (E1/2
-2.07
)
)
0.08
-2.48 (E1/2
-2.79 (E1/2
-2.22 (Ep)
-2.43 (E1/2
)
)
0.14 (-0.07)b
-0.20
)
ox
0.18 (Ep
)
THF
-2.12
-0.07
-2.37 (Ep)
-2.60 (E1/2
)
)
ox
0.43 (Ep
[Fe2(CO)4(µ-pdt)(µ-dppe)]
2d
MeCN
THF
-2.23
0.17 (-0.04)b
0.41 (0.15)b
-2.43 (E1/2
)
c
-2.12 (E1/2
)
-2.37
-2.59 (E1/2
)
d
-2.32 (E1/2
)
a V ) 2 V s-1
Scheme 3
. . .
b Reduction peak (Epred) on the return scan. c V ) 5 V s-1 d V ) 1 V s-1
complete after the passage of 0.1-0.5 F mol-1 1a-d. The
CV of the bright red catholyte showed that 2a-d formed
essentially quantitatively (Scheme 4, Figure S3, Supporting
Information). Electrolyses performed in THF-[NBu4][PF6]
on a preparative scale (0.1-0.2 g of 1) consumed compara-
tively less charge (0.08-0.15 F mol-1) than when smaller
amounts of complex are used, suggesting that homogeneous
redox steps contribute significantly to the formation of the
product. Complexes 2a-d were extracted from the catholyte
and fully characterized by NMR spectroscopy, elemental
analysis, and X-ray crystallography for 2a (see below and
Experimental Section). 1H NMR spectra of these compounds
show the presence of only one isomer in solution with the
expected signals for the R groups. 31P NMR spectra of
complexes 2 display one signal at ca. 60 ppm that accords
with those observed for analogous derivatives.6c,16 The
comparison of their 13C NMR spectra at 298 K with those
of their precursors indicates a characteristic change of the
carbonyl pattern (see Figure 5). A singlet and a triplet (J )
20 Hz) are dectected at 212.7 and 215.3 ppm, respectively,
for the asymmetrically substituted complex [Fe2(CO)4(κ2-
dppe)(µ-pdt)]. These signals are assigned unambigously to
Scheme 4
complexes,14 while examples of ETC isomerization are still
rare15 and are unprecedented for dithiolate-bridged diiron
complexes. An analysis of the mechanism of the catalytic
process will be presented below.
Electron-Transfer Catalyzed Synthesis and Characteri-
zation of [Fe2(CO)4(µ-SCH2XCH2S)(µ-dppe)] (2) (X )
NiPr, 2a; NCH2CH2OMe, 2b; NCH2C6H5, 2c; CH2, 2d).
Controlled-potential electrolyses of brown solutions of 1a-d
carried out at the potential of the minor reduction peak were
(15) (a) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Lemos, M. A. N.
D. A. Coord. Chem. ReV. 2001, 219-221, 53-80. (b) Guedes da Silva,
M. F. C.; Ferreira, C. M. P.; Frau´sto da Silva, J. J. R.; Pombeiro, A.
J. L. J. Chem. Soc., Dalton Trans. 1998, 4139-4145. (c) Geiger, W.
E.; Shaw, M. J.; Wu¨nsch, M.; Barnes, C. E.; Foersterling, F. H. J.
Am. Chem. Soc. 1997, 119, 2804-2811. (d) Mevs, J. M.; Geiger, W.
E. J. Am. Chem. Soc. 1989, 111, 1922-1923. (e) Connelly, N. G.;
Raven, S. J.; Carriedo, G. A.; Riera, V J. Chem. Soc., Chem. Commun.
1986, 992-993. (f) El Kbir, L.; Patin, H.; Darchen, A. Organometallics
1984, 3, 1128-1129. (g) Rieke, R. D.; Kojima, H.; Saji, T.;
Rechberger, P.; O¨ fele, K. Organometallics 1988, 7, 749-755 and
references therein.
(14) (a) Darchen, A.; Mousser, H.; Patin, H. J. Chem. Soc., Chem. Commun.
1988, 968-970. (b) Gloaguen, F.; Morvan, D.; Capon, J.-F.; Scholl-
hammer, P.; Talarmin, J. J. Electroanal. Chem. 2007, 603, 15-20.
(16) Gao, W.; Ekstro¨m, J.; Liu, J.; Chen, C.; Eriksson, L.; Weng, L.;
Åkermark, B.; Sun, L. Inorg. Chem. 2007, 46, 1981-1991.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9867

Ezzaher et al.
Figure 6. Structure of [Fe2(CO)4(µ-SCH2N(iPr)CH2S)(µ-dppe)] (2a) with
thermal ellipsoids at 30% probability (a molecule of CH2Cl2 is omitted).
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 2a
Fe1-Fe2
Fe1-S3
2.5446(11)
2.2519(17)
2.2584(16)
2.2294(17)
1.436(7)
1.495(7)
1.836(6)
1.752(7)
1.775(7)
1.769(7)
1.777(7)
68.79(5)
113.1(5)
Fe2-S3
Fe2-S4
Fe2-P2
C7-N1
C5-S4
2.2519(17)
2.2481(16)
2.2187(17)
1.443(7)
Figure 7. Cyclic voltammetry of [Fe2(CO)4{µ-SCH2N(iPr)CH2S}(µ-dppe)]
(2a) (1 mM) in MeCN-[NBu4][PF6] (a) under Ar and (b) under CO; in (a)
the dashed line shows the CV scan limited to the primary reduction of 2a
(V ) 0.2 V s-1, vitreous carbon electrode).
Fe1-S4
Fe1-P1
C5-N1
C50-N1
C7-S3
1.837(6)
Fe1-C1
C1-O1
1.173(7)
1.145(6)
1.156(6)
1.154(6)
68.76(5)
113.4(5)
The CVs of the fully characterized 2a,b,d in MeCN- or
THF-[NBu4][PF6] confirm that the major reduction peak
observed in the CV of 1a,b,d is due to the symmetrical
isomer (compare Figures 3a and 7a) that undergo reduction
and oxidation steps at potentials respectively more negative
and more positive than those of the unsymmetrical isomers
(Table 2). This indicates a larger HOMO-LUMO separation
in the diphos-bridged vs -chelated isomers. The reduction
of complexes 2a-b is a partially reversible process followed
by chemical reactions (EC mechanism)13,18 whose products
are detected by the presence of a quasi-reversible reduction
around -2.4 (MeCN) or -2.6 V (THF), as well as by several
oxidation peaks on the return scan (Figures 7a and S4,
Supporting Information) that result from the fragmentation
of the reduced species. In agreement with an EC mechanism,
these processes are suppressed upon increasing the scan rate
or lowering the temperature. Comparison of the current
functions of 2a and 2d [ipred(2i)/V1/2] (i ) a or d) at different
scan rates to the current function of the reversible, diffusion-
controlled, one-electron oxidation of [Fe2(CO)4(IMe-CH2-
IMe)(µ-pdt)]8 (IMe ) 1-methylimidazol-2-ylidene) under iden-
tical conditions (Figure 8) clearly shows that the reduction
of the (µ-dppe) complexes is not a simple one-electron
process.19 Because of the similarity of the reductive behavior
Fe1-C2
C2-O2
Fe2-C3
C3-O3
Fe2-C4
C4-O4
Fe1-S3-Fe2
C5-N1-C7
C7-N1-C50
Fe1-S4-Fe2
C5-N1-C50
113.8(5)
the {Fe(CO)3} and {Fe(CO)P2} moieties, respectively. For
2d, two doublets (J ) 21 and 8 Hz) are detected at 215.2
and 214.3 ppm. This pattern accords with the expected
signals for two equivalent {(Fe(CO)2P} moieties with two
equivalent CO in basal position and two other lying in the
apical orientation. Single crystals of 2a and 2d have been
obtained. Only the X-ray analysis of 2a afforded reliable
results, which demonstrates without any ambiguity that the
dppe ligand is symmetrically bridged between the two iron
atoms in a cis dibasal position [Fe-P, 2.2294(17) and
2.2187(17) Å] (Figure 6 and Table 3).
ReductiveElectrochemistryof[Fe2(CO)4(µ-SCH2XCH2S)-
(µ-dppe)] (X ) NiPr), 2a; NCH2CH2OMe, 2b; CH2, 2d)
and a Proposed Mechanism for the ETC Isomerization.
Previous studies of [Fe2(CO)6(µ-dithiolate)] complexes (dithi-
olate ) pdt or S(C6H4)S, bdt)14 showed that dianionic species
are involved in ETC substitutions of P(OMe)3 for CO (two-
electron-transfer catalysis14a). In order to assess whether this
is also a possibility for the ETC rearrangement of 1, it is
necessary to investigate the electrochemical reduction of both
1 and 2. Unfortunately, the very occurrence of the ETC-
catalyzed process precludes a detailed analysis of the
reduction mechanism of 1, and of the reactivity of 1- toward
CO.17 However, this can be done for 2.
(17) The reduction of complex 1 can be studied at low temperature, that
is, under conditions that prevent the ETC process and restrict a possible
reaction of 1- with CO.
(18) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamen-
tals and Applications; Wiley: New York, 1980; Chapter 11, pp 429-
485. (b) Brown, E. R.; Large, R. F. In Techniques of Chemistry, Vol.
IsPhysical Methods of Chemistry, Part IIA; Weissberger, A., Ed.;
Wiley, 1971; Chapter 6, pp 423-530.
9868 Inorganic Chemistry, Vol. 46, No. 23, 2007

Unsymmetrically Substituted Diiron Thiolate Complexes
Scheme 5
Figure 8. Scan rate dependence of the current function for the reduction
of 1 mM solutions of ([) [Fe2(CO)4{µ-SCH2N(iPr)CH2S}(µ-dppe)] (2a),
(9) [Fe2(CO)4(µ-pdt)(µ-dppe)] (2d), and for the oxidation of a 1 mM
solution of [Fe2(CO)4(µ-pdt)(IMe-CH2-IMe)] (2) in MeCN-[NBu4][PF6].
Scheme 6
of both azadithiolate-bridged derivatives, this should also be
true for 2b. However, differences emerge between the
azadithiolate complexes 2a-b and the propanedithiolate
analogue 2d. As shown in Figure 8, the current function
deviates noticeably from linearity at slow scan rates (V < 1
V s-1) for 2d while it is essentially independent of V over
the range 0.04 V s-1 e V e 2 V s-1 for 2a. This suggests
the possible contribution of an ECE process for 2c that would
be absent for 2a (EC process, see above).18 Furthermore, the
scan rate dependence of [(ip /ip ) (2i)]13 (i ) a, b, or d)
indicates that the reduction of the azadithiolate complexes
2a-b is chemically more reversible than that of the pdt
analogue when identical conditions of scan rate and con-
centration apply. These results confirm that the nature of
the S-to-S link introduces subtle differences in the electro-
chemical behavior of closely related complexes, as we
already noted for the hexacarbonyl precursors of 2a,b,d.28
From the above results it can be concluded that the reduction
of the (µ-dppe) complexes comprises two one-electron steps
(E1/21, E1/22) that cannot be separated by CV at fast scan
rates, in contrast to their hexacarbonyl parents.28 Digital
simulations19b of cyclic voltammograms indicate that the
experimental data are consistent with an EE reduction where
the individual redox steps have similar rate constants ks and
potentials such that E1/22 - E1/21 g 0. As a consequence, the
reduction mechanism of 2a-d (Scheme 5) includes the
disproportionation of the anion. The follow-up reactions,
evidenced by the presence of product peaks in the CVs (see
above), are assumed to consist initially in Fe-S bond(s)
cleavage, but they were not investigated in detail. Further
studies are clearly needed to identify the factors that turn
the electrochemical reduction of closely related [Fe2(CO)4-
(L)2(µ-dithiolate)] complexes (L ) phosphine, phosphite or
(L)2 ) diphosphine) into either a two-electron15a,29,30 or a
one-electron31,32 process.
When CO is present, the reduction of 2 is chemically much
less reversible than under Ar (for identical V), and the quasi-
reversible system around -2.4 (MeCN) or -2.6 V (THF) is
absent (Figures 7b and S4).33 The reaction of 2- (and 22-)
with CO provides a rationale for the inhibiting effect of CO
on the catalytic isomerization of 1, although it does preclude
the possibility that 1- also reacts with CO.17
a
c red
(19) (a) This is based on the reasonable assumption that the diffusion
coefficients of 2 and of [Fe2(CO)4(µ-pdt)(IMe-CH2-IMe)] are similar.
The reduction current of 2a was also compared to the first oxidation
peak current of [Fe2Cp2(CO)2(µ-SMe)2] ([ipox(1e)], reversible one-
electron oxidation20), and to the reduction peak current of [Fe2(CO)6-
(µ-bdt)] ([ipred(2e)], reversible two-electron reduction with E°2 - E°1
> 0 21) at the same concentrations as 2a in MeCN-[NBu4][PF6]. This
confirmed that two electrons are involved in the reduction of 2a, with
[ipred(2a)]/[ipox(1e)] = 2.5 and [ipred(2a)]/[ipred(2e)] = 0.7. Note that
the current function of a 2e--transfer step is 2.83 (23/2) times that of
a 1e--transfer when the second electron transfer is thermodynamically
more favourable than the first one (E°2 - E°1 > 0), and 2.41 times
that of a 1e--transfer when both electrons are transferred at the same
potential (E°2 ) E°1), in case of fast heterogeneous charge-transfer
reactions.18,22-25 (b) Simulations of CV curves using DigiElch (version
26
2 or 3) confirmed that the magnitudes of the peak current and of
the peak-to-peak separation of a two-electron transfer process are
affected by the rate constants of the heterogeneous electron-transfer
(ks) when ∆E° ) E°2 - E°1 > ca 0.1 V, see Supporting Information.
(c) The above method was preferred to coulometry to derive the
number of electrons involved in the reduction of 2 on short time scales
because follow-up reactions (that are detected by CV at slow scan
rate) may be prominent on the longer time scale of bulk electrolyses;
the following chemistry is thus susceptible to affect the coulometrically
measured n value.27 Exhaustive electrolyses of 2a and 2d require that
a charge > 2 F mol-1 starting material be passed and produce
uncharacterized products.
(23) Save´ant, J.-M. Elements of Molecular and Biomolecular Electro-
chemistrysAn Electrochemical Approach to Electron Transfer Chem-
istry; Wiley: New York, 2006; Chapter 1, pp 62-77.
(24) Heinze, J. Angew. Chem., Int. Ed. Engl. 1984, 23, 831-847.
(25) Ryan, M. D. J. Electrochem. Soc. 1978, 125, 547-555.
(26) For detailed information concerning DigiElch, see www.elchsoft.
com and the following papers: (a) Rudolph, M. J. Electroanal. Chem.
2003, 543, 23-29. (b) Rudolph, M. J. Electroanal. Chem. 2004, 571,
289-307. (c) Rudolph, M. J. Comput. Chem. 2005, 26, 619-632.
(d) Rudolph, M. J. Comput. Chem. 2005, 26, 633-641. (e) M.
Rudolph, J. Comput. Chem. 2005, 26, 1193-1204.
(20) (a) Gennett, T.; Geiger, W. E.; Willett, B.; Anson, F. C. J. Electroanal.
Chem. 1987, 222, 151-160. (b) Madec, P.; Muir, K. W.; Pe´tillon,
F.Y.; Rumin, R.; Scaon, Y.; Schollhammer, P.; Talarmin, J. J. Chem.
Soc., Dalton Trans. 1999, 2371-2383.
(21) Capon, J.-F.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. J.
Electroanal. Chem. 2004, 566, 241-247.
(22) Polcyn, D. S.; Shain, I. Anal. Chem. 1966, 38, 366-375.
(27) Pierce, D. T.; Geiger, W. E. J. Am. Chem. Soc. 1992, 114, 6063-
6073.
(28) Capon, J.-F.; Ezzaher, S.; Gloaguen, F.; Pe´tillon, F. Y.; Schollhammer,
P.; Talarmin, J.; Davin, T. J.; McGrady, J. E.; Muir, K. W. New J.
Chem. 2007, DOI 10.1039/b709273c.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9869

Ezzaher et al.
Table 4. Crystallographic Data for Complexes 1a-c and 2a
1a
1b
1c
2a
empirical formula
fw
C35H35 Fe2NO4P2S2
771.40
C35H35 Fe2NO5P2S2
787.40
C39H35 Fe2NO4P2S2
819.44
C36H37Cl2 Fe2NO4P2S2
856.33
T/K
170(2)
295(2)
170(2)
170(2)
cryst syst
space group
a(Å)
b(Å)
c(Å)
monoclinic
P21/n
10.9362 (8)
10.7765(8)
30.1258(16)
90
93.908(5)
90
3542.2(4)
4
1.446
1.066
0.24 × 0.18 × 0.05
2.66-28.85
21 064
monoclinic
C2/c
34.495 (5)
11.8409(9)
20.873(3)
90
123.853(19)
90
7080.3(15)
8
1.477
1.070
0.13 × 0.09 × 0.08
3.29-23.26
19 820
monoclinic
P21/c
11.0438(6)
22.4200(14)
14.8519(9)
90
96.859(6)
90
3651.0(4)
4
1.491
1.039
0.16 × 0.09 × 0.04
2.84-23.26
14 883
monoclinic
P21/c
11.1813(7)
22.9345(15)
15.1949(10)
90
98.029(6)
90
3858
R(°)
â(°)
γ(°)
V(Å3)
Z
4
F
calcd (mg mm-3
)
1.474
1.120
0.22 × 0.07 × 0.05
3.24-23.26
9335
µ (mm-1
)
crystal size (mm3)
range of θ (°)
reflns measured
Rint
0.0427
0.0683
0.0782
0.0519
unique data/params
R1 [I > 2σ(I)]
R1 (all data)
7100/417
0.0489
0.0921
4767/425
0.0546
0.1036
4968/451
0.0538
0.1096
5389/444
0.0541
0.0954
wR2 (all data)
0.1225
1.081
0.761, -0.616
0.1047
1.019
0.271, -0.446
0.0799
0.958
0.382,-0.360
0.1150
0.965
0.456,-0.455
GOF on F2
∆Fmax,∆Fmin/e Å-3
Scheme 7. X ) N(R) (1a/2a and 1b/2b) or CH2 (1d/2d); the Species in Braces Are Postulated
From these results, the reactions in Scheme 6 are proposed
to account for the ETC rearrangement of 1 under an inert
atmosphere (eqs 1-7) and for its inhibition by CO (eq 8).
As noted above, the reduction of 1 could not be investigated
electrochemically under conditions that allow the ETC-
catalyzed isomerization to occur,17 so whether or not 12- is
generated and contributes to the catalytic isomerization
cannot be established.34 The contribution of 22- is shown in
Scheme 6 (eqs 5 and 7) due to the potential disproportion-
ation of 2- (eq 3, Scheme 6).28
models of the [FeFe]-H2ase active site. DFT calculations have
shown that the presence of donor ligands in unsymmetrically
substituted [Fe2(CO)4L2(µ-dithiolate)] complexes favors the
“rotated” geometry where a CO ligand lies beneath the Fe-
Fe bond.4 DFT analysis of the substitution of PMe3 for CO
in complexes closely related to 1, namely [Fe2(CO)4(dppv)-
(µ-dithiolate)] (dppv ) cis-Ph2PCHdCHPPh2), pointed out
that the electron-richness of the Fe(CO)(dppv) fragment
assists the rotation of the Fe(CO)3 group and the formation
of a bridging CO in the transition state.5 It is conceivable
that the rotation of the Fe(CO)3 fragment could also be
facilitated by the weakening of the Fe-Fe bond in the one-
electron-reduced form of the complex (1-), where the CO
in a semi-bridging position would contribute to relief of the
The mechanism of the isomerization step (eq 2, Scheme
6) that initiates the catalytic process is of particular interest
in connection with the design and reactivity of unsymmetrical
(33) (a) The overall two-electron reduction of 2 is likely to involve structural
change, such as Fe-S bond cleavage, at some stage of the process.33b
The characteristics of the redox process suggest that the structure
change is reversible under Ar while a reduced species is irreversibly
trapped by CO, making the overall reduction irreversible. This is quite
similar to the CO effect on the electrochemical reduction of [Fe2-
(CO)5(µ-pdt)(IMe)].28 (b) For a discussion of structural consequences
of electron-transfer steps, see: (i) Geiger, W. E. Prog. Inorg. Chem.
1985, 33, 275-352. (ii) Evans, D. H.; O’Connell, K. M. In Elec-
troanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York,
1986; Vol. 14, pp 113-207. (iii) Pe´tillon, F. Y.; Schollhammer, P.;
Talarmin, J. In Encyclopedia of Electrochemistry; Bard, A. J.,
Stratman, M., Eds.; Wiley: New York, 2006; Vol. 7b, pp 565-590.
(iv) Uhrhammer, D.; Schultz, F. A. J. Phys. Chem. A 2002, 106,
11630-11636, and references therein.
(29) Mathieu, R.; Poilblanc, R.; Lemoine, P.; Gross, M. J. Organomet.
Chem. 1979, 165, 243-252.
(30) Gloaguen, F.; Morvan, D.; Capon, J.-F.; Schollhammer, P.; Talarmin,
J. Eur. J. Inorg. Chem. 2007, submitted for publication.
(31) (a) Gao, W.; Ekstro¨m, J.; Liu, J.; Eriksson, L.; Weng, L.; Åkermark,
B.; Sun, L. Inorg. Chem. 2007, 46, 1981-1991. (b) Dong, W.; Wang,
M.; Liu, T.; Liu, X.; Jin, K.; Sun, L. J. Inorg. Biochem. 2007, 101,
506-513. (c) Na, Y.; Wang, M.; Jin, K.; Zhang, R.; Sun, L. J.
Organomet. Chem. 2006, 691, 5045-5051. (d) Li, P.; Wang, M.; He,
C.; Li, G.; Liu, X.; Chen, C.; Åkermark, B.; Sun, L. Eur. J. Inorg.
Chem. 2005, 2506-2513.
(32) (a) Mejia-Rodriguez, R.; Chong, D.; Reibenspies, J. H.; Soriaga, M.
P.; Darensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004-12014.
(b) Chong, D.; Georgakaki, I. P.; Mejia-Rodriguez, R.; Sanabria-
Chinchilla, J.; Soriaga, M. P.; Darensbourg, M. Y. Dalton Trans. 2003,
4158-4163.
(34) If 12- is generated upon reduction of 1, the isomerization step may
involve anionic (1- f 2-) and/or dianionic (12- f 22-) species.
9870 Inorganic Chemistry, Vol. 46, No. 23, 2007

Unsymmetrically Substituted Diiron Thiolate Complexes
electronic strain on the disubstituted iron center.35 Whether
the full migration of this CO from one iron center to the
other causes, results from, or is concerted with, the deco-
ordination of one end of the diphosphine chelate (Scheme
7) is not known. In the proposed mechanism, the rearrange-
ment of the anionic complex is completed by the binding of
the dangling phosphorus atom to the coordinatively unsatur-
ated iron center. This latter step might understandably be
hindered under a CO atmosphere due to competitive CO
binding to the exposed coordination site, which is a possible
deactivation pathway of the ETC isomerization in addition
to the reaction of 2- (and/or 22-) with CO.33 A rotated
geometry was also recently proposed to result from the
electrochemical reduction of a phosphine singly substituted
diiron dithiolate complex.36 However, DFT calculations on
this model complex suggested that one Fe-S bond was
cleaved in the µ-CO species.37 In the case of our complexes,
the contribution of an intermediate with a terminally bound
S ligand to the 1- f 2- isomerization process is also a
possibility.
Electronic factors that were reported to favor the rotated
geometry in diiron dithiolate complexes may thus be
important to the occurrence of the ETC isomerization.
However, they are not decisive since such a catalytic
rearrangement was not detected for [Fe2(CO)4(IMe-CH2-
IMe)(µ-pdt)], which is another type of unsymmetrically
disubstituted model complex that reduces at a potential more
negative than 1.8 Besides the electronic properties of the
chelating ligand, its nature is clearly a central aspect of the
resistance of the unsymmetrical structure to isomerization
upon reduction, and further studies with other diphosphine
ligands are underway to explore this issue.
Experimental Section
Methods and Materials. All the experiments were carried out
under an inert atmosphere, using Schlenk techniques for the
syntheses. Tetrahydrofuran (THF) was purified as described previ-
ously.38 Acetonitrile (Merck, HPLC grade) was used as received.
[Fe2(CO)4(κ2-dppe)(µ-S(CH2)3S)]9 (1d) and [Fe2(CO)6{µ-SCH2N-
(R)CH2S}] (R ) iPr,28 CH2CH2OMe,28 CH2C6H539) were prepared
according to the reported procedure. The preparation and the
purification of the supporting electrolyte [NBu4][PF6] were de-
scribed previously.38 The electrochemical equipment consisted either
in a GCU potentiostat (Tacussel/Radiometer) driven by a PAR 175
Universal Programmer (CV traces were recorded with a SEFRAM
TGM 164 X-Y recorder) or a µ-AUTOLAB (Type III) driven by
a GPES software. Controlled-potential electrolyses were performed
using a GCU potentiostat and an IG5-N (Tacussel/Radiometer)
integrator. The cell and electrodes were as described previously.38
All the potentials (text, tables, figures) are quoted against the
ferrocene-ferrocenium couple; ferrocene was added as an internal
standard at the end of the experiments. The NMR spectra (1H, 13C,
31P) were recorded at room temperature in CD2Cl2, CDCl3, or C6D6
solutions with a Bruker AMX 400 or AC300 spectrometer and were
referenced to SiMe4(1H) and H3PO4 (31P). 1H-13C experiments were
carried out on a Bruker DRX 500 spectrometer. The infrared spectra
were recorded on a Nicolet Nexus Fourier transform spectrometer.
Chemical analyses were made by the Service de Microanalyses
I.C.S.N., Gif sur Yvette, France or the Centre de Microanalyses
du CNRS, Vernaison (France). Crystal data (Table 4) for com-
pounds 1a-c and 2a were collected on a Oxford Diffraction
X-Calibur-2 CCD diffractometer, equipped with a jet cooler device
and graphite-monochromated Mo KR radiation (λ ) 0.71073 Å).
The structures were solved and refined by standard procedures.40
Synthesis of [Fe2(CO)4(κ2-dppe){µ-SCH2N(R)CH2S}] (R )
iPr, 1a; CH2CH2OMe, 1b; CH2C6H5, 1c). In a typical procedure,
a solution of the azadithiolate hexacarbonyl derivative [Fe2(CO)6-
i
{µ-SCH2N(R)CH2S}] (0.2 g, 0.47 mmol, R ) Pr; 0.3 g, 0.675
mmol, R ) CH2CH2OMe; 0.24 g, 0.507 mmol, R ) CH2C6H5)
and 2 equiv of dppe (0.37 g, 0.932 mmol; 0.54 g, 1.35 mmol; 0.40
g, 1.014 mmol, respectively) in THF (100 mL) was heated in the
presence of 2 equiv of Me3NO,2H2O (0.16 g, 0.978 mmol; 0.23 g,
1.42 mmol; 0.18 g, 1.065 mmol, respectively) at 70 °C for 4 h.
The initially orange solution became brownish. After evaporation
of the solvent, the residue was chromatographed on a silica gel
column. Elution with hexane-dichloromethane mixtures afforded
a brownish solution of 1 which was evaporated under vacuum.
Compounds 1a-c were washed with pentane and obtained as
brownish air-stable powders (1a, 0.16 g, 39% yield; 1b, 0.22 g,
41% yield; 1c, 0.20 g, 47% yield). Crystals of 1a-c, suitable for
X-ray analysis, were formed by crystallization at room temperature
from CH2Cl2/Et2O solutions. 1a. IR (CH2Cl2, cm-1): ν(CO) 2019-
Conclusion
In this paper we report the first example of an ETC
isomerization of four [Fe2(CO)4(κ2-dppe)(µ-dithiolate)] com-
plexes to the symmetrical (µ-dppe) isomer. Although the
electron-releasing properties of the chelating ligand are
probably important as to the occurrence of the catalytic
isomerization, electrochemical studies of unsymmetrical
complexes are still too rare to precisely identify the factors
that control the fate of their reduced forms. Further studies
of unsymmetrically disubstituted diphosphine complexes are
now underway in our laboratory since this kind of compound,
with well-differentiated iron centers, bear more resemblance
with the [2Fe] subsite of the H-cluster than their symmetrical
isomers, and thus are more attractive models of the [FeFe]-
hydrogenases.
1
(s), 1953(s), 1941(s), 1903(w). H NMR (CD2Cl2, 25 °C): major
isomer, 7.95-7.20 (m, 20H, C6H5), 2.94 (m, 2H, PCH2CH2P), 2.74
(d, 2H, J ) 10.8 Hz, (µ-SCH2)2N), 2.70 (m, 2H, PCH2CH2P), 2.29
(spt, 1H, J ) 6.6 Hz, iPr), 1.75 (d, 2H, J ) 10.8 Hz, (µ-SCH2)2N),
i
(35) For discussions on the contribution of semi-bridging CO in redressing
the charge imbalance between two metal centres, see: (a) Cotton, F.
A.; Troup, J. M. J. Am. Chem. Soc. 1974, 96, 1233-1234. (b) Cotton,
F. A. Prog. Inorg. Chem. 1976, 21, 1. (c) Bino, A.; Cotton, F. A.;
Lahuerta, P.; Puebla, P.; Uson, R. Inorg. Chem. 1980, 19, 2357-
2359. (d) Crabtree, R. H.; Lavin, M. Inorg. Chem. 1986, 25, 805-
812. (e) Baxter, R. J.; Knox, G. R.; Pauson, P. L.; Spicer, M. D.
Organometallics 1999, 18, 215-218, and references therein.
(36) Hou, J.; Peng, X.; Zhou, Z.; Sun, S.; Zhao, X.; Gao, S. J. Organomet.
Chem. 2006, 691, 4633-4640.
0.47 ppm (d, 6H, J ) 6.6 Hz, Pr); minor isomer, 7.95-7.20 (m,
20H, C6H5), 3.35 (d, J ) 12.5 Hz, (µ-SCH2)2N), 3.15 (d, 2H, J )
(38) Cabon, J.-Y.; Le Roy, C.; Muir, K. W.; Pe´tillon, F. Y.; Quentel, F.;
Schollhammer, P.; Talarmin, J. Chem.-Eur. J. 2000, 6, 3033-3042.
(39) Lawrence, J. D.; Li, H.; Rauchfuss, T. B. Chem. Commun. 2001,
1482-1483.
(40) Programs used: (a) Sheldrick, G. M. SHELX97; University of
Go¨ttingen: Go¨ttingen, Germany, 1998. (b) WinGX - A Windows
Program for Crystal Structure Analysis (Farrugia, L. J. J. Appl.
Crystallogr. 1999, 32, 837).
(37) Greco, C.; Bruschi, M.; Fantucci, P.; de Gioia, L. Eur. J. Inorg. Chem.
2007, 1835-1843.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9871

Ezzaher et al.
1
12.5 Hz, (µ-SCH2)2N), 2.94 (m, 2H, PCH2CH2P), 2.60 (m, 2H,
PCH2CH2P), 2.76 (spt, 1H, J ) 6.6 Hz, iPr), 0.94 ppm (d, 6H, J )
6.6 Hz, iPr). 31P{1H} NMR (CD2Cl2, 25 °C): 92.1 (s) (major isomer,
80%), 78.1 ppm (s) (minor isomer, 20%). Anal. Calcd (%) for
C35H35Fe2NO4P2S2: C, 54.49; H, 4.57; N, 1.82; P, 8.03. Found:
C, 53.65; H, 4.61; N, 1.74; P, 7.82. 1b. IR (CH2Cl2, cm-1): ν(CO)
(w). H NMR (CD2Cl2, 25 °C): 7.95-7.20 (m, 20H, C6H5), 3.56
(m, 2H, PCH2CH2P), 3.34 (m, 2H, PCH2CH2P), 3.08 (spt, 1H, J )
i
6.5 Hz, Pr), 2.33 (m, 4H, µ-SCH2)2N), 1.07 ppm (d, 6H, J ) 6.5
Hz, iPr). 31P{1H} NMR (CD2Cl2, 25 °C): 61.3 ppm (s). Anal. Calcd
(%) for C35H35Fe2NO4P2S2: C, 54.49; H, 4.57; N, 1.82. Found:
C, 54.33; H, 4.97; N, 1.51. 2b. IR (CH2Cl2, cm-1): ν(CO) 1989-
1
1
2019(s), 1949(s), 1941(sh), 1905(w). H NMR (CD2Cl2, 25 °C):
(s), 1952(s), 1920(s), 1901(w). H NMR (CD2Cl2, 25 °C): 7.80-
major isomer, 7.95-7.20 (m, 20H, C6H5), 3.03 (s, 3H, OCH3), 2.81
(t, J ) 5.5 Hz, 2H, NCH2CH2O), 2.78 (m, 2H, PCH2CH2P), 2.75
(d, 2H, J ) 11.3 Hz, (µ-SCH2)2N), 2.67 (m, 2H, PCH2CH2P), 2.30
(d, J ) 11.3 Hz, 2H, (µ-SCH2)2N), 2.11 ppm (t, J ) 5.5 Hz, 2H,
NCH2CH2O); 31P{1H} NMR (CD2Cl2, 25 °C): 91.1 (s) (major
isomer, 78%), 75.2 ppm (s) (minor isomer, 22%). Anal. Calcd (%)
for C35H35Fe2NO5P2S2: C, 53.39; H, 4.48; N, 1.78; P, 7.86.
Found: C, 53.96; H, 4.32; N, 1.58; P, 8.34. 1c. IR (CH2Cl2, cm-1):
7.45 (m, 20H, C6H5), 4.03 (m, 2H, PCH2CH2P), 3.75 (m, 2H,
PCH2CH2P), 3.38 (t, J ) 5.5 Hz, 2H, NCH2CH2O), 3.32 (s, 3H,
OCH3), 3.26 (t, J ) 5.5 Hz, 2H, NCH2CH2O), 2.33 ppm (m, 4H,
(µ-SCH2)2N). 31P{1H} NMR (CD2Cl2, 25 °C): 60.8 ppm (s). Anal.
Calcd (%) for C35H35Fe2NO5P2S2: C, 53.39; H, 4.48; N, 1.78.
Found: C, 53.85; H, 5.24; N, 1.23. 2c. IR (CH2Cl2, cm-1): ν(CO)
1990(s), 1955(s), 1922(s), 1902(w). 1H NMR (CDCl3, 25 °C):
7.74-7.26 (m, 25H, C6H5), 4.04 (s, 2H, NCH2C6H5), 3.60 (m, 4H,
PCH2CH2P), 2.30 ppm (m, 4H, (µ-SCH2)2N). 31P{1H} NMR
(CDCl3, 25 °C): 61.2 (s) ppm. 2d. IR (CH2Cl2, cm-1): ν(CO) 1989-
1
ν(CO) 2020(s), 1951(s), 1941(s), 1905(w). H NMR (CD2Cl2,
25 °C): major isomer, 7.85-7.70 (m, 25H, C6H5), 2.80 (m, 2H,
PCH2CH2P), 2.72 (m, 2H, PCH2CH2P), 2.68 (d, 2H, J ) 11.1 Hz,
(µ-SCH2)2N), 3.01 (s, 2H, NCH2C6H5), 2.13 ppm (d, J ) 11.1 Hz,
(µ-SCH2)2N). 31P{1H} NMR (CD2Cl2, 25 °C): 92.0 (s) (major
isomer, 80%), 77.8 ppm (s) (minor isomer, 20%). Anal. Calcd (%)
for C39H35Fe2NO4P2S2·H2O: C, 55.93; H, 4.45; N, 1.67; P, 7.52.
Found: C, 55.62; H, 4.29; N, 1.53; P, 7.40.
1
(s), 1953(s), 1920(s), 1901(w). H NMR (CDCl3, 25 °C): 7.74-
7.26 (m, 20H, C6H5), 2.35 (m, 2H, PCH2CH2P), 2.11 (m, 2H,
PCH2CH2P), 1.56 (m, 2H, µ-pdt), 1.39 (m, 2H, µ-pdt), 1.08 ppm
(m,2H, µ-pdt). 31P{1H} NMR (CDCl3, 25 °C): 60.3 ppm (s). Anal.
Calcd (%) for C33H30Fe2O4P2S2: C, 54.42; H, 4.15; P, 8.52.
Found: C, 54.64; H, 4.33; P, 7.67.
Electrosynthesis of [Fe2(CO)4{µ-SCH2N(R)CH2S}(µ-dppe)]
(R ) iPr, 2a; CH2CH2OMe, 2b; CH2Ph, 2c) and of [Fe2(CO)4-
(µ-pdt)(µ-dppe)] (2d). In a typical experiment, 0.1 g (1.4 10-4 mol)
of complex 1d was dissolved in 25 mL THF-[NBu4][PF6] in the
electrochemical cell. The potential of the graphite cathode was set
at -2.1 V and the electrolysis was started. The cell current decayed
to the background level after the passage of 1.6 C (0.12 F mol-1
1d). The electrolysis was stopped, and CV of the electrolyzed
solution showed that 2d was formed almost quantitatively (ca 90%).
The catholyte was cannulated in a Schlenk flask under dinitrogen.
The solution was then taken down to dryness and the solid residue
extracted by 3 × 50 mL of diethyl ether in order to separate the
electrolysis product from the supporting electrolyte. After evapora-
tion of the solvent, the solid was dried under vacuum. This afforded
0.073 g (73%) of 2d. A similar procedure was followed to obtain
2a (0.072 g, 36% yield), 2b (0.135 g, 96% yield), and 2c (0.54 g,
54% yield) from 1a (0.2 g), 1b (0.14 g), and 1c (0.1 g), respectively.
Crystals of 2a, suitable for X-ray analysis, were formed by
crystallization at room temperature from CH2Cl2/Et2O solutions.
2a. IR (CH2Cl2, cm-1): ν(CO) 1989(s), 1954(s), 1921(s), 1902-
Acknowledgment. This work was supported by CNRS
(Programme “Energie”, PRI-4.1) and ANR (Programme
“PhotoBioH2”). The Ministe`re de l’Education Nationale, de
l’Enseignement Supe´rieur et de la Recherche is acknowl-
edged for providing a studentship to S.E., and the Universite´
de Bretagne Occidentale is acknowledged for financial
support.
Supporting Information Available: Crystallographic informa-
tion files (CIF) for 1a-c and 2a, and Figure S1 (CV of 1d in
MeCN-[NBu4][PF6]), Figure S2 (CV of 1b in THF-[NBu4][PF6]
under Ar and under CO), Figure S3 (CV of 1b before and after
controlled-potential electrolysis in MeCN-[NBu4][PF6]), Figure S4
(CV of 2d in THF-[NBu4][PF6] under Ar and under CO) and
digital simulations of 2e--transfer processes as pdf files. This
material is available free of charge via the Internet at http://
pubs.acs.org.
IC701327W
9872 Inorganic Chemistry, Vol. 46, No. 23, 2007