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