Modeling [FeFe] hydrogenase: Synthesis and protonation of a diiron dithiolate complex containing a phosphine-N-heterocyclic-carbene ligand

Article abstract of DOI:10.1016/j.jorganchem.2009.01.018

The ligand exchange reaction of I_Me-(CH₂)₂-PPh₂ (I_Me = 1-methyimidazol-2-ylidene) and the hexacarbonyl complex [{Fe₂{μ-S(CH₂)₃S}(CO)₆] (1) resulted in the f

Full text of DOI:10.1016/j.jorganchem.2009.01.018

Journal of Organometallic Chemistry 694 (2009) 2801–2807
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Modeling [FeFe] hydrogenase: Synthesis and protonation of a diiron dithiolate
complex containing a phosphine-N-heterocyclic-carbene ligand
a
a
a
Didier Morvan a, Jean-François Capon a, , Frédéric Gloaguen , François Y. Pétillon , Philippe Schollhammer ,
*
Jean Talarmin a, Jean-Jacques Yaouanc a, François Michaud b, Nelly Kervarec c
a Université Européenne de Bretagne, France, Université de Brest, CNRS, UMR 6521 ‘‘Chimie, Electrochimie Moléculaires et Chimie Analytique ”, ISSTB, CS 93837,
29238 Brest-Cedex 3, France
b Service commun d’analyse par diffraction des rayons X, UFR Sciences et techniques, Université de Bretagne Occidentale, 6, av. Le Gorgeu – CS93837 29238 Brest Cedex 3, France
c Service de RMN, UFR Sciences et techniques, Université de Bretagne Occidentale, 6, av. Le Gorgeu – CS93837 29238 Brest Cedex 3, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The ligand exchange reaction of IMe-(CH2)2-PPh2 (IMe = 1-methyimidazol-2-ylidene) and the hexacar-
Received 1 December 2008
Received in revised form 12 January 2009
Accepted 13 January 2009
Available online 20 January 2009
bonyl complex [{Fe2{
l-S(CH2)3S}(CO)6] (1) resulted in the formation of the chelated complex
[{Fe2{ -S(CH2)3S}(CO)4(IMe-(CH2)2-PPh2)] (2). The molecular structure of 2 was confirmed by spectro-
l
scopic and X-ray analyses. This complex catalyzes proton reduction. Low temperature NMR studies on
the protonation of 2 revealed the formation of a terminal hydride intermediate.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Dihydrogen
Hydrogenase
Diiron complexes
N-Heterocyclic carbenes
1. Introduction
vour the formation of a terminal hydride in the protonation process.
Terminal hydrides species may be regarded as key reactive inter-
[FeFe]-hydrogenases are metalloenzymes which very efficiently
catalyze the reversible reduction of protons to dihydrogen
(2H+ + 2eꢀ = H2) [1]. Crystal structure determinations revealed that
the active site, the so-called H-cluster, contains an organometallic
subsite, which consists of a dithiolate bridged diiron core bearing
CO and CNꢀ ligands [2]. As a result, studies on diiron dithiolate com-
plexes as models for [FeFe]-hydrogenases have considerably in-
creased with the goal of understanding the mechanistic steps
involved in the uptake and evolution of dihydrogen by the enzyme
and of replicating hopefully its high activity [3]. Monosubstitution
mediates in the catalytic production of dihydrogen by [FeFe]
hydrogenases [8]. Recently, we showed that nonsymmetric dithio-
lato–diiron complexes undergo protonation at a single metal prior
to forming bridging hydride species [6]. In the case of the chelated
complex [Fe2{
phanylethane)) [6a] the protonation reaction was very slow
at –90 °C, whereas that of the bis-NHC analogue [Fe2{ -S(CH2)3-
l-S(CH2)3S}(CO)4(dppe)] (dppe = bis(diphenylphos-
l
S}(CO)4(IMe-CH2-IMe)] (IMe = 1-methylimidazole-2-ylidene) [6c]
gave a very unstable terminal hydride intermediate. These results
encouraged us to investigate whether a diiron complex containing
a bidentate ligand, with both NHC and phosphine functionalities,
promotes the formation of a stable terminal hydride compound in
the presence of acids. In this paper, we report the synthesis of the
precursor of the carbene–phosphine ligand, as well as its reactivity
of carbonyl groups at both iron centers in complex [Fe2{l-
S(CH2)3S}(CO)6] (1) for better electron-donating ligands, such as
CNꢀ, RNC, PR3 and N-heterocyclic carbene (NHC) ligands, induced
protonation at the diiron site in a bridging position in presence of
strong acids [4]. By using bidentate reagents L2 (L2 = diphosphane,
towards the hexacarbonyl complex [Fe2{l-S(CH2)3S}(CO)6] (1) and
bis-NHC, phenanthroline), unsymmetrical systems [Fe2{
l
-
the reactivity of the chelated complex [{Fe2{l-S(CH2)3S}(CO)4-
S(CH2)3S}(CO)4(L2)], where one iron is chelated by the ligand, are
formed [5,6]. DFT calculations have shown that the presence of
(IMe-(CH2)2-PPh2)] (2) towards protons.
electron-donating ligands at one iron centre in complexes [Fe2{
l-
a
2. Results and discussion
S(CH2)3S}(CO)4(L2)] might facilitate transition state with
a
‘‘rotated” structure similar to the one observed in the enzyme [7].
The existence of this inverted pyramid at one iron centre could fa-
2.1. Synthesis
The phosphine imidazolium salt 3-[2-diphenylphosphino-
ethyl]-1-methylimidazolium iodide A was easily prepared accord-
ing to the procedure (i) depicted in Scheme 1, i.e. by addition of
* Corresponding author.
E-mail address: jean-francois.capon@univ-brest.fr (J.-F. Capon).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.01.018

2802
D. Morvan et al. / Journal of Organometallic Chemistry 694 (2009) 2801–2807
+
+
I-
I-
N
N
N
N
PPh2
1) Ph2PH
2) NaH
CH3I
N
N
(i)
A
O
+
-
K+
I-
N
N
N
PPh2
O
PPh2
O
Ph2P(CH2)2-Cl
CH3I
CH3SiCl2H/EtOH
(ii)
A
N
N
N
+
Br-
+
Br
Br-
N
N
PPh2
Br
Ph2PH, KtBuO, DMSO
N
N
N
Br
(iii)
N
Scheme 1.
diphenylphosphine to 3-methyl-1-vinylimidazolium iodide in
presence of NaH. This synthetic procedure is more convenient than
the previous methodology based on the use of phosphine oxide
followed by reduction by CH3SiCl2H/EtOH (Scheme 1, procedure
(ii)) [9]. A similar strategy was reported for the synthesis of
3-[2-diphenylphosphino-ethyl]-1-mesitylimidazolium salts [10].
Syntheses of phosphine-imidazolium chloride, bromide, and hexa-
fluorophosphate compounds have been also described on reacting
1-methylimidazole with 1,2-dihalogenoethane to give 3-(2-halo-
genoethyl)-1-methylimidazolium compounds, further addition of
potassium diphenylphosphine in dimethylsulfoxide affords the de-
sired phosphine-imidazolium salt (Scheme 1, procedure (iii)) [11].
It should be noted that in this procedure (iii), long reaction periods
are required to avoid the formation of bis-imidazolium side-prod-
ucts, whereas according to path (i) the reaction goes to completion
in less than 2 h.
mass spectrometry and X-ray crystallography. Three strong
bands at 2006, 1933 and 1882 cmꢀ1 are observed in the IR spec-
trum. The (CO) values of 2 are between those of the chelated
bis-NHC complex [Fe2{ -S(CH2)3S}(CO)4(IMe-(CH2)2-IMe)] (1994,
m(CO)
m
l
1921, 1869 cmꢀ1) [6c] and those of the dppe analogue [Fe2{
l-
S(CH2)3S}(CO)4(dppe)] (2019, 1949, 1904 cmꢀ1) [6a], which is
consistent with the stronger electron-donating ability of the NHC
ligand relative to that of the phosphine ligand. The large difference
between the highest and the lowest
of 2 is consistent with a phosphine–carbene ligand chelating a
single iron atom as observed in [Fe2{ -S(CH2)2S}(CO)4(dppv)]
(dppv = cis Ph2PCH@CHPPh2) [5e], which displays (CO) bands at
2023, 1953, 1915 cmꢀ1 CO = 108 cmꢀ1). For comparison the va-
lue of
CO = 90 cmꢀ1 is smaller for the disubstituted derivative
[Fe(CO)2(PMe3){ -S(CH2)3S}Fe(CO)2(IMes)] (IMes = 1,3-bis(2,4,6,-
m(CO) band (Dm )
CO = 124 cmꢀ1
l
m
(Dm
D
m
l
trimethylphenyl)imidazol-2-ylidene) [12] than in complex 2.
The room temperature 13C–{1H} NMR spectrum of compound 2
shows two low field doublets at 224.0 ppm (2JP–CO = 14 Hz) and
190.6 ppm (2JP–Ccarbene = 33 Hz) due respectively, to the carbonyl
and the CCarbene of the {Fe(CO)(IMe-(CH2)2-PPh2)} moiety, while a
singlet at 214.4 ppm is noted for the carbonyls of the {Fe(CO)3}
moiety. The room-temperature 31P–{1H} NMR spectrum of 2 con-
sists of one singlet at 66.3 ppm; upon lowering the temperature
to –60 °C, three signals are observed at 68.0, 66.1 and 64.2 ppm
in a 10:2:1 ratio. These signals are tentatively attributed to a mix-
The precursor imidazolium salt A was treated with the strong
base KtBuO to generate the free NHC, IMe-(CH2)2-PPh2, before reac-
tion with the hexacarbonyl complex [{Fe2{
Heating a toluene solution of 1 with an excess of free NHC for
1.5 h at 80 °C afforded [Fe2{ -S(CH2)3S}(CO)4(IMe-(CH2)2-PPh2)]
l-S(CH2)3S}(CO)6] (1).
l
(2), isolated as a red complex, after column chromatography
(Scheme 2).
The desired chelated complex 2 was obtained in 20% yield; it
was characterized by IR and NMR spectroscopy, elemental analysis,
+
O
O
S
N
N
C
S
I-
C
Me
+ KtBuO
Ph
PPh2
Ph
Fe
Fe
P
C
[Fe2{µ−S(CH2)3S}(CO)6]
O
Me
N
C
N
1
O
2
Scheme 2.

D. Morvan et al. / Journal of Organometallic Chemistry 694 (2009) 2801–2807
2803
Ph
Fe
Ph
O
Me
Fe
O
Me
N
O
N
C
O
C
S
S
C
S
C
S
P
S
N
S
Fe
Fe
N
N
Fe
Fe
P
C
C
O
Ph
N
P
C
O
C
O
C
C
Me
C
O
C
Ph
Ph
O
O
O
O
Ph
basal-basal isomer
basal(P)-axial(carbene) isomer
basal(carbene)-axial(P) isomer
Fig. 1. Structures of various isomers discussed.
Fig. 2. ORTEP view of 2. The thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°):
C1–O1 = 1.164(3), C1–Fe1 = 1.736(3), C3–O3 = 1.158(3), C3–Fe2 = 1.762(3), C4–O4 = 1.146(3), C4–Fe2 = 1.800(3), C5–O5 = 1.149(3), C5–Fe2 = 1.775(3), C9–Fe1 = 1.961(3),
P1–Fe1 = 2.2320(8), S1–Fe1 = 2.2471(7), S1–Fe2 = 2.2639(8), S2–Fe1 = 2.2412(8), S2–Fe2 = 2.2622(8), Fe1–Fe2 = 2.5995(5), S2–Fe2–Fe1 = 54.37(2), S1-Fe2–Fe1 = 54.51(2),
S2–Fe1–Fe2 = 55.12(2), S1–Fe1–Fe2 = 55.12(2), C9–Fe1–P1 = 87.80(8).
ture of basal–basal, basal(P)–axial(carbene) and basal(carbene)–
axial(P) isomers (Fig. 1).
increased electron density at the diiron center and, in the case of
monosubstituted compounds at both iron centers, e.g. [Fe2{
S(CH2)3S}(CO)4(L)(L0)], the protonation reactions afforded bridging
hydride species [Fe2( -H){
-S(CH2)3S}(CO)4(L)(L0)]+ [3]. 1H NMR
spectra of these compounds display typical high field signals in
the range (–12 to ꢀ20 ppm) due to the bridging hydride. In the
family of diiron dithiolate compounds, only one terminal hydride
l-
Crystals suitable for X-ray diffraction studies were grown from
dichloromethane/diethylether mixture (60/40) at 4 °C. The molec-
ular structure of the complex 2 (Fig. 2) features the typical butter-
fly Fe2S2 skeleton found in related diiron derivatives [3], and the
binding mode of the bidentate phosphine–carbene ligand to a
single iron atom in a basal–basal position like other Fe2S2 models
containing chelating bis-NHC [6c] or carbene-pyridine [13] ligands.
The six membered metallacycle (Fe(1)P(1)C(14)C(13)N(2)C(9))
l
l
complex,
[Fe(H)(PMe3)2(l-CO){l-S(CH2)2S}Fe(CO)(PMe3)2](PF6),
has been structurally characterized: it exhibits a 1H NMR signal
at –4.6 ppm, that was assigned to the terminal hydrido ligand
[14]. Terminal hydrides were previously found in diphosphido-
bridged diiron compounds [15].
adopts
a
boat-like conformation. The Fe(1)-S distances
(2.2471(7)–2.2412(8) Å) are about 0.02 Å shorter than those to
the Fe(CO)3 unit. The Fe-Fe distance of 2.5995(5) Å is longer than
the average Fe–Fe distance (2.54 0.03 Å) [2e] found in Fe2S2 mod-
els but is close to that observed in other basal–basal diphosphane
complexes [3k].
The protonation of 2 with HBF4 ꢁ Et2O in CH2Cl2 at room temper-
ature afforded the bridging hydride [Fe2{l-S(CH2)3S}(l-H)(CO)4-
(IMe-(CH2)2-PPh2)](BF4) (3) (Fig. 5) which was isolated as a brown
powder after addition of diethylether. The presence of a high field
2
signal that appears as a doublet at –15.9 ppm (d, JPH = 21 Hz) in
2.2. Protonation studies
the 1H NMR spectrum confirms the existence of the bridging hy-
dride. The 2JPH coupling constant of 21 Hz accords with a basal po-
sition of the phosphine ligand in 3 [16]. Its IR spectrum displays
The substitution of carbonyl ligands in [Fe2{l-S(CH2)3S}(CO)6]
by better electron-donating ligands afforded compounds with an
m
(CO) at 2092, 2039, 1972 cmꢀ1 which are as expected shifted to

2804
D. Morvan et al. / Journal of Organometallic Chemistry 694 (2009) 2801–2807
Fig. 3. 13C–{1H} NMR spectrum of the terminal hydride 4 in the low field region recorded at –90 °C in CD2Cl2.
higher wavenumbers (by about 90 cmꢀ1) as compared to the neu-
tral complex 2 (
(CO): 2006, 1933 and 1882 cmꢀ1) [17]. The 31P–
The Ccarbene atom was observed as a doublet (2JPC = 22 Hz) at
172.4 ppm in the 13C NMR spectrum. A doublet (2JPC = 12 Hz) at
210.6 ppm was attributed to the fourth carbonyl ligand. These data
provide strong support for the presence of a distal terminal
m
{1H} NMR spectrum shows a singlet at 53.7 ppm. 31P and 1H
NMR spectra revealed also the existence of non-identified prod-
ucts, and therefore, no analytical data are available for this product.
Monitoring of the protonation reaction of a CD2Cl2 solution of 2
with HBF4 ꢁ Et2O by 1H and 31P NMR spectroscopy at ꢀ90 °C re-
vealed the clean formation of a new hydride intermediate as indi-
cated by the appearance of a singlet at ꢀ5.2 ppm in the 1H NMR
spectrum and of a singlet at 32.4 ppm in the 31P–{1H} NMR spec-
trum. The 13C–{1H} NMR spectrum (Fig. 3) showed three low field
signals at 232.3, 203.9 and 201.1 ppm due to carbonyl ligands. A
1H–13C HMBC 2D NMR experiment indicated correlations between
the hydride signal at ꢀ5.2 ppm and these three carbonyl signals
(Fig. 4).
hydride [Fe(H)(CO)3{
(Fig. 5).
l-S(CH2)3S}Fe(CO)(IMe-(CH2)2-PPh2)](BF4) (4)
Protonation at the distal iron center was also observed in tetra-
carbonyl complexes [Fe2{ -S(CH2)3S}(CO)4(L2)] for L2 = phenan-
l
throline [6d], bis(dimethylphosphanylethane) (dmpe) [6e] and
dppe [6a] below ꢀ50 °C. An increase of the temperature to
ꢀ40 °C led to the appearance of the doublet at ꢀ15.9 ppm in the
1H NMR spectrum, which corresponds to the bridging hydride 3.
The transformation of 4 into 3 was completed above ꢀ20 °C. Com-
pound 4 did not evolve dihydrogen in the presence of an excess of
HBF4. When the protonation reaction was performed at ꢀ50 °C,
only signals due to hydride compounds 3 and 4 were observed,
no other terminal hydride complex was detected. For example,
there was no evidence of the formation of proximal hydride
compounds (Fig. 5), that were reported to be obtained as minor
species in the protonation of the chelated complexes [{Fe2{l-
S(CH2)3S}(CO)4(L2)] (L2 = dppe [6a] or bis(diphenylphosphanylpro-
pane) (dppp) [18]). When the chelating bidentate ligand (L2)
adopts a basal–basal conformation as the major isomer, proton-
ation is favored at the Fe(CO)3 moiety [3k]. Protonation at the Fe(-
CO)(L2) moiety was observed only in the case of the complexes
[Fe2{l-S(CH2)3S}(CO)4(L2)], where the basal–apical isomer is pre-
dominant [3k]. Accordingly, we propose that the major isomer ob-
served in the 31P–{1H} NMR spectrum (ꢀ60 °C) of 2 at 68.0 ppm
may be the basal–basal isomer 2. It should be noted that the termi-
nal hydride compound 4 is more stable towards isomerization into
the bridging form than its dppe analogue. It has been reported re-
cently that the protonation of a symmetrical tetrasubstituted com-
plex [Fe2{l-S(CH2)3S}(CO)2(L2)2] (L2 = dppv, cis Ph2PCH@CHPPh2)
afforded a terminal hydride which is more stable than compound
4 [19]. This difference of stability could be explained by stronger
basicity of the metal site in the tetrasubstituted complex with re-
spect to that in 2. Recent results [3k] suggest that the reactivity of
unsymmetrical complexes [Fe2{l-S(CH2)3S}(CO)4(L2)] depends on
its ability to generate a transition state having a rotated structure
bearing a vacant site at one iron center. For these systems,
the inversion of the Fe(CO)3 moiety may be easier than that of
the Fe(CO)L2 one; this explains that the initial site of protonation
is the Fe(CO)3 moiety.
Fig. 4. Partial contour plot of the 1H–13C HMBC 2D NMR experiment in the hydride
region, recorded at –90 °C in CD2Cl2, for the terminal hydride 4.
-
-
+
BF4
+
BF4
-
+
BF4
O
O
Me
N
Me
N
O
Me
N
S
H
O
C
S
C
S
H
S
C
S
S
C
C
Fe
Fe
Ph
Fe
Fe
Fe
Fe
N
N
N
C
C
C
H
P
O
P
O
O
P
C
C
C
C
O
O
O
O
O
Ph
Ph
Ph
Distal terminal hydride 4
Fig. 5. Structures of various hydride species discussed.
Ph
Ph
Bridging hydride 3
Proximal terminal hydride : not observed

D. Morvan et al. / Journal of Organometallic Chemistry 694 (2009) 2801–2807
2805
0,00
-0,02
-0,04
-0,06
-0,08
-0,10
-0,12
12
1 eq. H+
10
8
6
3,3 eq. H+
6,6 eq. H+
11 eq. H+
1 eq. H+
4
2
0
-2400-2200 -2000 -1800-1600 -1400-1200 -1000 -800 -600 -400 -200
0
0
2
4
6
8
10
12
eq H+
E mV/Fc
Fig. 6. Dependence of current heights of electrocatalytic waves for 2 in the presence of 1–11 equiv. of HBF4 ꢁ Et2O in N2 purged CH2Cl2 solution + Bu4NPF6 electrolyte and plot
of the catalytic peak current (Ip/Ip1, where Ip1 is the intensity of the current after addition of one equivalent of HBF4 ꢁ Et2O) as a function of the acid concentration.
2.3. Electrochemical study
and then they were deoxygenated and dried using conventional
methods. Deuterated dichloromethane was stored under argon
over molecular sieves before use. NMR spectra were recorded on
a Bruker AMX 400 or a Bruker DRX 500 spectrometers with chem-
ical shifts reported in d values relative to residual protonated sol-
vents for 1H NMR spectra, and to the solvent CD2Cl2 and CD3OD
for 13C NMR spectra. Infrared spectra were recorded with a FT-IR
NEXUS NICOLET spectrometer. Chemical analyses were carried
out by the ‘‘Service de microanalyse I.C.S.N.” Gif-sur-Yvette
(France), or the ‘‘Centre de microanalyse du CNRS”, Vernaison
(France). Cyclic voltammetry experiments were carried out as de-
Compound
2 displays a partially reversible reduction at
red
E
¼ ꢀ2:25 V versus Fc+/0 in CH3CN and a reversible oxidation
1=2
ox
at E ¼ ꢀ0:40 V. These potentials are in good agreement with
1=2
the donor ability of the phosphine-N-heterocyclic-carbene ligand
respective to dppe and IMe-CH2-IMe observed in the complexes
[Fe2{
l-S(CH2)3S}(CO)4(IMe-CH2-IMe)]
ðErped ¼ ꢀ2:42 V; Eox
¼
1=2
ꢀ0:41 VÞ
[6c] and [Fe2{
l
-S(CH2)3S}(CO)4(dppe)]
ðErped
¼
ox
ꢀ2:07 V; E ¼ ꢀ0:20 VÞ [6b]. The reversible oxidation of 2 at
1=2
ox
E
¼ ꢀ0:40 V suggests that 2 could be oxidized by ferricinium
1=2
salts as observed in [Fe(CO)2(PMe3){
[12a], [Fe(CO)2(PMe3){ -S(CH2CMe2CH2)S}Fe(CO)2(PMe3)] [20]
and [Fe(CO)2(PMe3){ -S(CH2)2S}Fe(CO)(dppv)] [21]. In the pres-
l
-S(CH2)3S}Fe(CO)2(IMes)]
scribed elsewhere [23]. The starting complex [Fe2{l-S(CH2)3-
l
S}(CO)6] (1) was prepared according to literature methods [24].
The imidazolium salt, 3-methyl-1-vinylimidazolium iodide, was
prepared according to a modified procedure previously reported
[25].
l
ence of acid HBF4 ꢁ Et2O, the cyclic voltammogram of 2 exhibits a
new reduction wave at about ꢀ1.45 V, most certainly correspond-
ing to the reduction of a bridging hydride. This reduction increases
in intensity upon increasing the acid concentration, consistent
with catalytic proton reduction (Fig. 6). Closer inspection of the
CV reveals however that the catalytic wave is composed of several
peaks depending on the acid concentration, which suggests that
several mechanisms might be involved [22].
4.2. Preparation of 3-methyl-1-vinylimidazolium iodide
A 100 mL round bottomed flask was charged with vinylimidaz-
ole (1.50 g, 15.9 mmol) in 2 mL of iodomethane. The resulting mix-
ture was stirred for 2 h at 70 °C. The volatile compounds were
removed in vacuo and the brown residue was washed with dichlo-
romethane until it was no longer sticky. The product 3-methyl-1-
vinylimidazolium iodide (3.15 g, 83%) was obtained as an orange
powder.
3. Conclusion
We have synthesised
a nonsymmetric complex [Fe2{l-
1H NMR (CD3OD): d 9.25 (s, 1H, HIMe), 7.98 (s, 1H, N–CH = CH–
N), 7.68 (s, 1H, N–CH@CH–N), 7.25 (dd, JHH = 15.6 Hz, JHH = 8.6 Hz,
1H, N–CH@CH2), 5.90 (dd, JHH = 15.6 Hz, JHH = 2.7 Hz, 1H, N–
CH@CHH), 5.43 (dd, JHH = 8.6 Hz, JHH = 2.7 Hz, 1H, N–CH@CHH),
3.98 (s, 3H, N–CH3). 13C–{1H} NMR (CD3OD): d 138.0 (NCHN),
132.4 (N–CH@CH2), 111.1 (N–CH@CH2) 127.3 (N–CH@CH–N),
122.0 (N–CH@CH–N), 39.3 (N–CH3).
S(CH2)3S}(CO)4(IMe-(CH2)2-PPh2)] (2) with a chelating phosphine-
N-heterocyclic-carbene ligand on a single iron atom. The study of
this compound in acidic media supports that the kinetic proton-
ation site is the Fe(CO)3 unit and confirms that a terminal hydride
is accessible as an intermediate upon protonation of diiron dithiol-
ate complexes. Such a terminally bound hydride species is sup-
posed to be involved in the protonation process of [FeFe]
hydrogenases. Complex 2 catalyzes proton reduction at ꢀ1.45 V
versus Fc+/0. The nonsymmetric complex 2 is an interesting plat-
form for further substitution of carbonyl ligands to obtain trisub-
stituted or tetrasubstituted diiron species.
4.3. Preparation of 3-[2-diphenylphosphino)-ethyl]-1-
methylimidazolium iodide [PPh2-(CH2)2-(HIMe)](I) (A)
A 250 mL round bottomed flask was charged with 3-methyl-1-
vinylimidazolium iodide (1.40 g, 6.00 mmol) under nitrogen. An
excess of diphenylphosphine used as solvent (3 mL) and NaH
(0.24 g, 6.00 mmol) were then added under intense stirring. The
mixture was stirred for 3 h at 80 °C and left to precipitate at room
temperature. The solution was removed through a cannula under
nitrogen. The crude product was dissolved in dichloromethane
4. Experimental
4.1. General procedures
Reactions were carried out under an atmosphere of argon using
standard Schlenck techniques. Solvents were distilled prior to use

2806
D. Morvan et al. / Journal of Organometallic Chemistry 694 (2009) 2801–2807
Table 1
Crystal data for 2.
(3 mL) and addition of ether afforded a yellow precipitate. This
procedure was repeated once again, in similar conditions, giving
1.95 g of A (76% yield) as a yellow powder.
Asymmetric unit
Formula weight
Wavelength (Å)
Crystal system
Space group
a (Å)
C25H25Fe2N2O4PS2
624.26
0.71073
Monoclinic
P21/n
12.2377(5)
14.5514(4)
14.9746(4)
99.965(3)
1H NMR (CD3OD): d 8.60 (s, 1H, HIMe), 7.34–7.45 (m, 10H,
C6H5), 7.58 (s, 1H, N–CH@CH–N), 7.39 (s, 1H, N–CH@CH–N), 4.39
(dt, JHH = 7 Hz, JPH = 10.3 Hz, 2H, N–CH2), 3.80 (s, 3H, N–CH3) 2.75
(t, JHH,PH = 7 Hz, 2H, Ph2P–CH2). 31P NMR (CD3OD): d –17.0 (s,
PPh2) ppm. 13C–{1H} NMR (CD3OD): d 131–139 (m, PPh2), 137.1
(NCHN), 125.3 (N–CH@CH–N), 126.4 (N–CH@CH–N), 50.0 (N–
CH2), 37.0 (N–CH3), 31.1(d, JCP = 15 Hz, CH2–P) ppm. ESIMS (m/z),
Calc. for C18H20N2P (M+): 295.1364; found: 295.1366.
b (Å)
c (Å)
b (°)
V (Å3)
2626.38(15)
4
1.579
Z
qcalc (g cmꢀ3
)
Crystal size (mm)
Absorption coefficient (mmꢀ1
F(000)
0.15 ꢂ 0.12 ꢂ 0.04
4.4. Preparation of [Fe2{l-S(CH2)3S}(CO)4(IMe-(CH2)2-PPh2)] (2)
)
1.359
1280
h Range for data collection (°)
Limiting indices
2.74–26.37
Potassium tert-butoxide (0.715 g, 6.37 mmol) and THF (100 mL)
were added to A (2.30 g, 5.40 mmol). After 3 h of vigorous stirring
at room temperature, the reaction mixture was filtered and the
yellow solution collected was concentrated to 5 mL in a round bot-
tom flask. 50 mL of toluene was then transferred to this solution.
ꢀ14 6 h 6 14, ꢀ18 6 k 6 18,
ꢀ18 6 l 6 18
17749/4991 [Rint = 0.0473]
92.9
Empirical (SCALE3 ABSPACK)
0.9476 and 0.8537
Full-matrix least-squares on F2
4991/0/326
Reflections collected/unique
Completeness to h = 26.37° (%)
Absorption correction
Maximum and minimum transmission
Refinement method
The complex [Fe2{l-S(CH2)3S}(CO)6] (1) (0.615 g, 1.59 mmol) was
dissolved in 60 mL of toluene and added dropwise to a yellow solu-
tion of the free NHC. The mixture was heated at 80 °C for 1.5 h, and
then evaporated to dryness under vacuum. The resulting red solid
was dissolved in a minimal amount of CH2Cl2 and applied to a sil-
ica gel column. Elution with a dichloromethane-hexane (40/60)
mixture afforded a red solution which was evaporated under vac-
uum. The residue was washed with hexane (3 ꢂ 10 mL). The red
powder obtained was dissolved in a dichloromethane/hexane
(10/90) mixture; slow evaporation of the solvents to about 10 mL
gave a red powder of compound 2 (100 mg, yield 20%) after filtra-
tion of the solvents. No other product was neither isolated nor
characterised.
Data/restraints/parameters
Goodness-of-fit on F2
0.919
Final R indices [I > 2
r
(I)]
R1 = 0.0296, wR2 = 0.0554
R1 = 0.0618, wR2 = 0.0624
0.395 and ꢀ0.295
R indices (all data)
Largest difference in peak and hole
(e Åꢀ3
)
4.6. X-ray crystallography
Crystal data for compound 2 were collected with an Oxford Dif-
fraction X-Calibur-2 CCD diffractometer equipped with a jet cooler
device and graphite-monochromated Mo
0.71073 Å), and are detailed in Table 1. The structure was solved
Ka radiation (k =
IR (CH2Cl2): mCO 2006(vs), 1933(vs), 1882(s) cmꢀ1 1H NMR
.
(CD2Cl2): d 7.50–7.21 (m, 10H, 2 C6H5), 6.98 (d, 1H, JHH = 7 Hz, N–
CH@CH–N), 6.94 (d, 1H, JHH = 7 Hz, N–CH@CH–N), 4.70 (m, 1H,
N–CHH), 4.39 (m,1H, N–CHH), 4.16 (s, 3H, N–CH3) 2.81 (m, 1H,
P–CHH), 2.41 (m, 1H, PCHH), 2.03–1.08 (m, 6H, S–(CH2)3–S). 13C–
{1H} NMR (CD2Cl2): d 224.2 (d, JPC = 14 Hz, FeL2(CO)), 214.4 (broad,
Fe(CO)3), 190.6 (d, JPC = 33 Hz, Ccarbene), 142.3, 142.5,133.0, 132.1,
129.5, 129.1, 128.5 and 128.3 (C6H5), 123.7 and 122.6 (N–
CH@CH–N), 48.3 (N–CH2), 39.1 (N–CH3), 30.1, 25.8 and 25.3 (S–
(CH2)3–S), 26.7 (d, JPC = 21 Hz, PCH2). 31P NMR (CD2Cl2, 25 °C): d
66.3 (s). 31P NMR (CD2Cl2, ꢀ60 °C): d 68.0 (s) 66.1 (s), 64.2 (s).
LSIMS (m/z), Calc. for C25H25Fe2N2O4S2P (M+): 623.9692; found:
623.9725. Anal. Calc. for C25H25Fe2N2O4S2P: C, 48.10; H, 4.04; N,
4.49; P, 4.96. Found: C, 47.75; H, 4.04; N, 4.23; P, 4.89%.
and refined by standard procedures [26].
5 Supplementary material
CCDC 706317 contains the supplementary crystallographic
data for compound 2. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
This work was supported by ANR (Agence Nationale de la
Recherche, through the programme ‘‘CatH2”), CNRS (Centre Natio-
nale de la Recherche Scientifique) and Université de Bretagne
Occidentale.
4.5. Protonation of 2 in situ
A slight excess of HBF4 ꢁ Et2O was added to a CD2Cl2 solution of
2 (7.0 mg, 1.1 ꢂ 10ꢀ5 mol) in a NMR tube at ꢀ95 °C and the spec-
trum was recorded at ꢀ90 °C.
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