Syntheses, crystal structures, and electrochemical studies of Fe2(Co)6(μ-PPh2)(μ-L) (L = OH, OPPh2, PPh2)

Article abstract of DOI:10.1080/00958972.2014.940925

Reaction of Fe₃(CO)₁₂ and Ph₂PH in the presence of Et₃N in THF at 0 °C immediately forms Fe₂(CO)₆(μ-PPh₂)(μ-OH) (1), Fe₂(CO)₆(μ-PPh₂)(μ-k₂

Full text of DOI:10.1080/00958972.2014.940925

Journal of Coordination Chemistry, 2014
Vol. 67, No. 13, 2330–2343, http://dx.doi.org/10.1080/00958972.2014.940925
Syntheses, crystal structures, and electrochemical studies of
Fe2(CO)6(μ-PPh2)(μ-L) (L = OH, OPPh2, PPh2)
YAO-CHENG SHI*, WEI YANG, YING SHI and DA-CONG CHENG
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, PR China
(Received 31 March 2014; accepted 4 June 2014)
Reaction of Fe3(CO)12 and Ph2PH in the presence of Et3N in THF at 0 °C immediately forms
Fe2(CO)6(μ-PPh2)(μ-OH) (1), Fe2(CO)6(μ-PPh2)(μ-k2O,P-OPPh2) (2), and Fe2(CO)6(μ-PPh2)2 (3) in
yields of 25, 14, and 19%, respectively. Experiments confirm that Et3N shortens the reaction time.
The absence of O2 hinders the formation of 2. The presence of H2O can increase the yield of 1.
Their structures have been determined by X-ray crystallography and the complexes have been com-
pletely characterized by EA, IR, and 1H, 13C, 31P NMR. Electrochemical studies reveal that they
exhibit catalytic H2-producing activities.
Keywords: Iron–iron bond; Bridging hydroxyl group; Carbonyl ligand; Hydrogenase
1. Introduction
Designing catalytic systems based on inexpensive and abundant materials is vital to the
development of a hydrogen energy economy [1–4]. However, few examples of platinum-free
electrode materials capable of catalyzing proton reduction have appeared [5]. Recently, chem-
ists have favored syntheses of organometallic complexes inspired by hydrogenase enzymes
[6–11]. Considerable interest was focused on diiron thiolates [Fe2(μ-SRS)(CO)6-nLn]
*Corresponding author. Email: ycshi@yzu.edu.cn
© 2014 Taylor & Francis

Diiron carbonyl complexes
2331
(R = linking group, L = σ-donating ligand), because of their structural resemblance with the
catalytic site of [Fe–Fe] hydrogenases. Advances in biomimetic modeling were derived from
the studies of substituted compounds (L = cyanide, isocyanide, phosphine, carbene, phenan-
throline), which protonate to produce well-characterized hydrides [12]. Dinuclear iron(I)
complexes with thiolate-bridges were shown to catalyze the electrochemical reduction of pro-
tons to molecular hydrogen in organic solvents [13, 14]. Studies were extended to the possi-
ble role of a heteroatom (N, O, or S) as a proton relay in the bridging ligand [15–20]. Aside
from possibly participating in the heterolytic formation and cleavage of hydrogen, the bridg-
ing ligand plays a key role in controlling the electron-transfer processes [13]. Since proton
reduction catalysis by all CO diiron models (except aza-bridged derivatives) is initiated by a
reduction step [21–23], broadening the range of bridging ligands should include increasing
the chemical reversibility of the reduction step and decreasing the reduction potential of the
diiron complex [24–27]. However, to date, comparatively few electrochemical studies have
examined the effect of replacing sulfur with other donors in (Fe2(μ-NRN) [28, 29], Fe2(μ-
PRP) [18, 30, 31], Fe2(μ-SeRSe) [32–35], and Fe2(μ-TeRTe) [34]). Best and coworkers [30,
31] demonstrated that phosphide-bridged derivatives undergo a reversible single-step two-
electron reduction leading to proton reduction catalysis at rather negative potentials. Seleno-
late-bridged diiron complexes were shown to exhibit mostly irreversible reduction processes
analogous to that observed for related thiolate compounds with bridging ligand having an
alkyl group [32, 33]. With few examples of Fe2(μ-PPh2) complexes [36], we decided to carry
out the syntheses, crystal structures, and electrochemistry of diiron complexes with bridging
PPh2. As the structures differ significantly from the well-characterized Fe2(μ-SRS) motifs,
we felt that they will extend hydrogenase models to catalyze proton reduction.
2. Experimental
2.1. Materials and physical measurements
All reactions were carried out under a prepurified N2 atmosphere with standard Schlenk
techniques. All solvents were dried by refluxing over appropriate drying agents and
stored under an N2 atmosphere. THF was distilled from sodium-benzophenone, petro-
leum ether (60–90 °C), and CH2Cl2 from P2O5. Fe3(CO)12 [37] and Ph2PH [38] were
prepared according to literature procedures. The progress of all reactions was monitored
by TLC (silica gel H, 300-400 mesh). NMR spectra were carried out on a Bruker
Avance 600 spectrometer using TMS as an internal standard or 85% H3PO4 as an
external standard in CDCl3. IR spectra were recorded on a Bruker Tensor 27 spectrom-
eter as KBr disks from 400 to 4000 cm−1. Analyses for C and H were performed on a
Vario EL microanalyzer. Melting points were measured on a Yanagimoto apparatus and
are uncorrected. Electrochemical determinations were carried out on a CHI66OD
potentiostat.
2.2. Reaction of Fe3(CO)12, Ph2PH, S, and Et3N with PhCOCl
A 100-mL Schlenk flask equipped with a stir bar and serum cap was charged with 15 mL
of THF, 0.376 g (2 mM) of Ph2PH, and 0.064 g (2 mM) of elemental sulfur. After 30 min,
to this solution was added 0.28 mL (2 mM) of Et3N and 2 g (4 mM) of Fe3(CO)12. After

2332
Y.-C. Shi et al.
30 min, to this solution was added 0.23 mL (2 mM) of PhCOCl. After stirring for 24 h,
the solvent was removed under reduced pressure and the resulting residue was chromato-
graphed by TLC on silica gel. Elution with petroleum ether/CH2Cl2 (v/v, 1 : 2) afforded a
red solid of 1 in 26% yield (0.247 g).
2.3. Reaction of Fe3(CO)12 and Ph2PH in the presence of Et3N
A 100-mL Schlenk flask equipped with a stir bar and serum cap was charged with 15 mL
of THF, 0.94 g (5 mM) of Ph2PH, 0.7 mL (5 mM) of Et3N, and cooled to 0 °C. To this stir-
red solution was added 2 g (4 mM) of Fe3(CO)12. After 1 min of stirring, the solvent was
removed in vacuo and the resulting residue was chromatographed by TLC on silica gel.
Elution with petroleum ether/acetone (v/v, 1 : 12) gave yellow, orange-yellow and orange-
red bands in the order of decreasing Rf values. The orange-red band afforded red crystals of
1 after recrystallization from deoxygenated petroleum ether (60–90 °C) and CH2Cl2, m.p.
121–123 °C, in 25% yield (0.61 g). The orange-yellow band offered orange-red crystals of
2 after recrystallization from deoxygenated petroleum ether (60–90 °C) and CH2Cl2, m.p.
132–134 °C, in 14% yield (0.228 g). The yellow band provided yellow crystals of 3 after
recrystallization from deoxygenated petroleum ether (60–90 °C) and CH2Cl2, m.p. 188.3–
188.9 °C (lit.: 178–179 °C [39]; 186–188 (dec.) °C [40]), in 19% yield (0.307 g).
In 1, Anal. Calcd for C18H11Fe2O7P (%): C, 44.86; H, 2.30; found: C, 44.65; H, 2.47. IR
1
(KBr disk): ν(OH) 3544.90 (m); ν(C≡O) 2066.51 (s), 2029.67 (vs), 1966.48 (vs) cm−1. H
3
NMR (600 MHz, CDCl3, TMS): δ −2.931 (d, JH-P = 11.4 Hz, 1H, OH), 7.365–7.408,
7.408–7.444, 7.477–7.509, 7.696–7.727 (4m, 5H, 1H, 2H, 2H, 2C6H5). 13C NMR
(150.928 MHz, CDCl3, TMS): δ 128.522, 128.592, 128.927, 128.992, 130.302 (d, J = 3.17
Hz), 130.661 (d, J = 2.87 Hz), 133.496, 133.555, 134.591, 134.644 (2C6H5), 209.392
3
(s, 6C≡O). 31P NMR (242.98 MHz, CDCl3, 85% H3PO4): δ 122.181 (d, JP-H = 10.69 Hz).
In 2, Anal. Calcd for C30H20Fe2O7P2 (%): C, 54.09; H, 3.03; found: C, 54.21; H,
3.25. IR (KBr disk): ν(C≡O) 2065.70 (s), 2020.63 (vs), 1995.55 (s), 1958.83 (s) cm−1.
1H NMR (600 MHz, CDCl3, TMS): δ 6.690–6.721, 6.779–6.809, 6.861–6.913, 6.978–
7.001, 7.290–7.375, 7.461–7.479, 7.582–7.613, 7.819–7.848 (8 m, 2H, 2H, 3H, 1H, 6H,
2H, 2H, 2H, 4C6H5). 13C NMR (150.928 MHz, CDCl3, TMS): δ 125.934 (d, J = 10.87
Hz), 126.438 (d, J = 14.41 Hz), 127.069, 127.128, 127.193, 127.299, 127.601 (d, J =
9.21 Hz), 128.426, 128.879 (d, J = 14.64 Hz), 132.141 (d, J = 7.39 Hz), 132.963 (d, J =
6.79 Hz), 134.932, 135.106, 139.434, 139.673, 141.517, 141.798, 142.312, 142.644
(4C6H5), 208.148, 208.663, 208.782, 209.709, 209.992, 211.228 (6C≡O). 31P NMR
2
(242.98 MHz, CDCl3, 85% H3PO4): δ 178.037 (d, JP-P = 57.10 Hz, PPh2), 100.731
2
(d, JP-P = 57.10 Hz, OPPh2).
In 3, Anal. Calcd for C30H20Fe2O6P2 (%): C, 55.42; H, 3.10; found: C, 55.33; H, 3.07.
1
IR (KBr disk): ν(C≡O) 2055.63 (s), 2016.85 (s), 1983.18 (s), 1961.21 (vs) cm−1. H NMR
(600 MHz, CDCl3, TMS): δ 6.684–6.709, 6.797–6.821, 7.092–7.124, 7.214–7.270, 7.560–
7.589 (t, t, q, m, q, 4H, 2H, 4H, 6H, 4H, 4C6H5). 13C NMR (150.928 MHz, CDCl3, TMS):
δ 127.985 (d, J = 4.98 Hz), 128.019 (d, J = 5.28 Hz), 128.407 (d, J = 3.47 Hz), 128.439 (d,
J = 4.83 Hz), 128.854 (s), 129.679 (s), 133.096 (d, J = 3.62 Hz), 133.121 (d, J = 3.77 Hz),
2
133.751 (d, J = 4.08 Hz), 133.777 (d, J = 3.77 Hz) (4C6H5), 212.903 (t, JC-P = 4.53 Hz,
6C≡O). 31P NMR (242.98 MHz, CDCl3, 85% H3PO4): δ 142.441(s).

Diiron carbonyl complexes
2.4. Reaction of Fe3(CO)12 and Ph2PH
2333
A 100-mL Schlenk flask equipped with a stir bar and serum cap was charged with 15 mL
of THF, 0.376 g (2 mM) of Ph2PH, and 1.007 g (2 mM) of Fe3(CO)12. After 24 h of stirring,
the solvent was removed under reduced pressure; the resulting residue was chromato-
graphed by TLC on silica gel. Elution with petroleum ether/acetone (v/v, 1 : 20) gave an
orange-red band from which red crystals of 1 were obtained in 45% yield (0.445 g) after
recrystallization from deoxygenated petroleum ether (60–90 °C) and CH2Cl2.
2.5. Influence of O2 and H2O on the reaction of Fe3(CO)12 and Ph2PH
After THF standing in air for 1 h and 0.5 mL of H2O added, the reaction of Fe3(CO)12
and Ph2PH afforded 1 in 15% yield with trace amounts of 2 and 3. Using deoxygenated
THF, H2O (0.5 mL) was added, the reaction of Fe3(CO)12 and Ph2PH offered 1 in 24%
yield, with trace amounts of 2 and 3. Using deoxygenated THF with 0.1 mL of H2O
added, the reaction of Fe3(CO)12 and Ph2PH gave 1 in 49% yield with trace amounts of
2 and 3.
2.6. X-ray structure determinations of complexes
Single crystals of 1–3 suitable for X-ray diffraction analyses were grown by slow evapo-
ration of the CH2Cl2-petroleum ether solutions of 1–3 at 0–4 °C. For each complex, a
selected single crystal was mounted on a Bruker APEX II CCD diffractometer using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 296 K. Data collection and
reduction were performed using SAINT software. An empirical absorption correction was
applied using SADABS. The structures were solved by direct methods using the SIR-
2004 software and refined by full-matrix least-squares on F2 with anisotropic thermal
parameters for all non-hydrogen atoms using the SHELXTL package [41, 42]. All
hydrogens attached to carbon in 1–3 were placed at geometrically idealized positions
and subsequently treated as riding with C–H = 0.93 (aromatic) Å and Uiso(H) values of
1.2Ueq(C). For 1, hydrogen bonded to oxygen was located from the difference Fourier
map and refined isotropically with Uiso(H) of 1.5Ueq(O). Platon views of complexes
were drawn using PWT software [43]. Details of crystal data, data collections, and struc-
ture refinements are summarized in table 1.
2.7. Electrochemical determinations of complexes
Cyclic voltammograms (CV) were obtained by using a 3-electrode cell with a glassy carbon
3 mm diameter working electrode, platinum plate counter electrode, and SCE reference
n
electrode. The electrolyte solution for all experiments was 0.100 M Bu4NPF6 in MeCN.
The potentials (E) at the working electrode in all CV’s are reported with respect to the fer-
rocenium/ferrocene couple in electrolyte solution. The ferrocenium/ferrocene coupling data
were collected at the end of each experiment. All CV’s reported are background-corrected,
i.e. the scan with only electrolyte present was subtracted from the raw data. All background
scans confirmed sufficient removal of O2 as seen by the absence of a reduction peak at ca.
−1.2 V. CV data were collected under flow of N2 gas.

2334
Y.-C. Shi et al.
Table 1. Crystal data, data collections, and structure refinements for 1–3.
1
2
3
Formula
Mr
C18H11Fe2O7P
481.94
C60H40Fe4O14P4·CH2Cl2
1417.13
C30H20Fe2O6P2·CH2Cl2
735.03
Cryst system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
P1
Triclinic
P1
Monoclinic
P21/m
11.3318(12)
12.7182(14)
11.8256(13)
90
106.161(4)
90
1637.0(3)
2
ꢀ
ꢀ
9.9129(12)
9.9866(12)
12.2975(14)
96.5898(10)
106.860(2)
118.474(3)
976.7(2)
2
11.8204(15)
16.9649(11)
17.1472(14)
65.974(3)
89.2813(12)
84.8664(16)
3127.0(5)
2
α (°)
β (°)
γ (°)
V (Å3)
Z
DCalcd (g cm−3
)
1.639
1.604
484
1.505
1.160
1436
1.491
1.188
744
μ (mm−1
F (0 0 0)
)
Index ranges
−13 ≤ h ≤ 12
−13 ≤ k ≤ 12
−16 ≤ l ≤ 15
20,090
−15 ≤ h ≤ 15
−22 ≤ k ≤ 17
−22 ≤ l ≤ 22
33,564
−14 ≤ h ≤ 14
−16 ≤ k ≤ 16
−15 ≤ l ≤ 15
25,268
Reflections measured
Unique reflections
4563
14,091
3925
Reflections (I > 2σ(I))
3264
7795
2844
Rint
0.0741
0.0532
0.0516
2θmax (°)
55.8
55.2
55.0
R
0.0459
0.0625
0.0479
Rw
0.1204
0.1465
0.1273
GooF
1.02
0.62/−0.58
1.02
0.57/−0.50
1.04
1.08/−0.89
Largest diff. peak and hole (e Ǻ−3
)
3. Results and discussion
3.1. Syntheses of complexes
In order to develop the synthetic methodologies of Fe/S cluster complexes [44–49], we
have explored the reaction of Fe3(CO)12 and Ph2PSH in situ generated from Ph2PH and ele-
mental sulfur in the presence of Et3N with PhCOCl. This reaction afforded 1, [Fe2(CO)6(μ-
PPh2)(μ-OH)], rather than an expected complex, [Fe2(CO)6(μ-k2P,S-Ph2PS)(μ-COPh)]. In
view of scarce μ-OH-containing complexes [50] and the important function of the μ-OH
ligand in hydrogenases [51], we have carried out the reaction of Fe3(CO)12, Ph2PH, and
Et3N in the absence of PhCOCl. This reaction offered Fe2(CO)6(μ-PPh2)(μ-L) (1, L = OH;
2, L = OPPh2; 3, L = PPh2) (scheme 1). THF was not deoxygenated by bubbling N2 for
15 min before use and Et3N was used as received. As mentioned below, if oxygen and
water were strictly excluded, 1 and 2 could not be obtained.
In order to increase the yield of 1 and to understand the role of Et3N, we have performed
the following experiments: (1) the reaction of Fe3(CO)12 and Ph2PH in the presence of
H2O; (2) the reaction of Fe3(CO)12, Ph2PH, and H2O in the presence of Et3N; and (3) the
above reactions under strict O2-free conditions [52]. These experiments confirm that Et3N
as a Lewis base shortens the reaction time from 24 h to 1 min. The absence of O2 can hin-
der formation of 2. However, excess O2 can lead to disappointing results. The presence of
H2O (1–5 eq) can increase the yield of 1 up to 49%.
Particularly, so far, three examples with the μ-OH ligand have appeared in the
literature [50]: Fe2(CO)6[P(p-C6H4CH3)2]OH was generated from deprotonation of

Diiron carbonyl complexes
2335
Fe(CO)3
(1)
O
H
Ph2P
Ph2P
Ph2P
Fe(CO)3
Fe3(CO)12
+
+
Fe(CO)3
O
THF
HPPh2
(2)
(3)
PPh2
Fe(CO)3
+
Et3N
+
Fe(CO)3
PPh2
Fe(CO)3
Scheme 1. Syntheses of 1–3.
Fe(CO)4[P(p-C6H4CH3)2]H with BuLi and subsequently treated with Fe2(CO)9 [53];
Fe2(CO)6[μ-PHCH(SiMe3)2](μ-OH) was obtained as one of the products in the reaction of
[NEt4]2[Fe2(CO)8] with Cl2PCH(SiMe3)2 [54]; Fe2(CO)4(μ-PPh2)(μ-dppm)(μ-OH) resulted
from slow oxidation of Fe2(CO)4(μ-PPh2)(μ-PPh2CH2PPh2)(μ-CO)(μ-H) on alumina [55].
The direct route to μ-OH-containing diiron complexes has been discovered by us
(Equation (1)).
Fe3(CO)12
Fe(CO)3
H2O/Et3N
(1)
O
H
+
Ph2P
THF
Fe(CO)3
HPPh2
Synthesis of 1.
To our knowledge, 2 with the μ-OPPh2 ligand is unprecedented although Pd, Pt, and Re
complexes with μ-OPPh2, μ-OPCy2, and μ-OPBut2 ligands have been reported [56–59].
Complex 3 was first prepared by Job et al. [39] from pentacarbonyl iron and tetraphenyl-
phosphine at 220 °C, which cannot be conveniently performed on a large scale. Collman
et al. [40] then developed an alternative synthesis of 3 via diphenylchlorophosphine and
disodium octacarbonyldiferrate. Ballinas-López et al. [60] reported a third synthesis via
diphenylphosphine and dodecacarbonyltriiron at 120 °C in a sealed ampoule for 24 h.
Strangely, 3 has not been completely characterized by these authors.
3.2. X-ray structures of 1–3
Structures of the above complexes have been determined by X-ray crystallography. Selected
geometric parameters have been listed in table 2. The X-ray crystallographic study of 1

2336
Y.-C. Shi et al.

Diiron carbonyl complexes
2337
Figure 1. X-ray crystal structure of 1.
reveals that it exists as a discrete dinuclear molecule in which the irons are linked by PPh2
and OH groups (figure 1).
The Fe–Fe bond distance of 2.5127(7) Å is close to that found (2.511(2) Å) in
Fe2(CO)6[μ-P(p-C6H4CH3)2](μ-OH) [53] but markedly shorter than those in 2–3 (see
below). Also, the Fe–P bond distances of 2.2263(9) and 2.2338(9) Å are very close to those
observed (2.236(3) and 2.239(3) Å) in the reported complex. The OH group bridges the two
Fe ions symmetrically, with the Fe–O distances of 1.963(2) and 1.964(2) Å, and Fe–O–Fe
angle of 79.56(9)°. The H–O···P angle of 170(3)° indicates that 1 exists as an equatorial
rather than an axial isomer [45, 49].
ꢀ
Unlike 1, 2 with two different molecules (2a and 2b) crystallizes in the P1 space group
(figure 2). In 2a and 2b, the OPPh2 via O and P links two iron ions in a σ,n-mode, with
bond distances of 2.6542(7) (Fe1–Fe2), 2.2625(11) (Fe2–P2), 2.022(2) (Fe1–O7), and
1.555(2) (P2–O7) Å for 2a and 2.6445(8) (Fe3–Fe4), 2.2598(12) (Fe4–P4), 2.035(2) (Fe3–
O14), and 1.534(3) (P4–O14) Å for 2b. The P–O bond distances are close to 1.536(4) Å in
[Pd(μ-OPPh2)(N(Me)C(O)N(Me)PPh2)]2 and 1.556(4) Å in (PHCy2)2Pt2(μ-PCy2)(μ-OPCy2),

2338
Y.-C. Shi et al.
(a)
(b)
Figure 2. X-ray crystal structure of 2 (up, 2a; down, 2b). Dashed lines represent intramolecular Fe···P contacts.
in agreement with a P–O single bond [57, 58]. Noteworthy is that the distances of 2.8074
(10) (Fe1–P2) and 2.8101(12) (Fe3–P4) Å indicate the presence of strong intramolecular
contacts between Fe and P in 2, because these are significantly shorter than the sum of the
van der Waals radii of iron and phosphorus [R(Fe) + R(P) = 3.91 Å].
As reported by Ballinas-López et al. [60], 3 with solvent CH2Cl2 crystallizes in the
P21/m space group (figure 3). The molecule of 3 lies across a mirror plane. Two PPh2
ligands bridge two Fe(CO)3 units to form a dinuclear complex, in which the Fe–Fe distance
is 2.6047(9) Å and almost equal to 2.6071(6) Å reported by Ballinas-López et al. The P2–
Fe1–Fe1i–P1 ring adopts a butterfly arrangement with the P-donor in the wingtip positions
[code: (i) x, −y + 1/2, z]. The dihedral angle between the two Fe–P–Fe planes is 75.03(6)°.

Diiron carbonyl complexes
2339
Figure 3. X-ray crystal structure of 3 (symmetry code: (i) x, −y + 1/2, z).
The Fe–P bond distances and Fe–P–Fe bond angles are comparable to those (2.2250(9),
2.2200(8) Å; 71.73(3)°, 71.91(3)°) observed by Ballinas-López et al.
3.3. Spectroscopic analysis of 1–3
The above complexes have also been characterized by elemental analyses and spectrosco-
pies. As indicated in figures 1–3, their IR spectra show characteristic absorption bands at
1958–2067 cm−1 for their terminal CO ligands. Additionally, the OH group appears at
3545 cm−1 for 1, indicating that it does not act as a hydrogen bonding donor which is in
agreement with the X-ray diffraction analysis of 1.
1
The H NMR spectra of all complexes display the corresponding signals for their phenyl
groups. Particularly, for 1, the OH group which is confirmed by D2O exchange, exhibits a
3
doublet with a JH-P coupling constant of 11.40 Hz at −2.931 ppm. The 31P NMR spectra
3
show a doublet with a JH-P coupling constant of 10.69 Hz at 122.181 ppm for 1, two dou-
2
blets with a JP-P coupling constant of 57.10 Hz at 178.037 ppm (PPh2) and 100.731 ppm
(OPPh2) for 2 and a singlet at 142.441 ppm for 3. In the 13C{1H} NMR spectra, 1 shows
only a singlet at 209.128 ppm corresponding to terminal carbonyl C, suggesting that the
carbonyl ligands are undergoing rapid exchange between two Fe atoms on the NMR time
scale at room temperature [44–49]. Terminal carbonyls of 2 are six singlets at 208.148,
208.663, 208.782, 209.709, 209.992, and 211.228 ppm. However, 3 exhibits an expected
2
triplet (with a JC-P coupling constant of 4.53 Hz at 212.903 ppm). Therefore, except for
fluxionality of CO site-exchanges in their solutions, spectroscopic data are in accordance
with the above X-ray diffraction analyses.

2340
Y.-C. Shi et al.
100
0
-100
-200
-300
-400
-500
-600
-700
-800
0 mM CF3COOH
2 mM CF3COOH
4 mM CF3COOH
6 mM CF3COOH
8 mM CF3COOH
10 mM CF3COOH
12 mM CF3COOH
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5
0.0
E/V vs. Fc
Figure 4. CV of 1 (1.0 mM) with CF3CO2H (0–12 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV s−1
.
40
20
0
-20
-40
0 mM CF3COOH
-60
2 mM CF3COOH
-80
4 mM CF3COOH
-100
-120
-140
-160
-180
6 mM CF3COOH
8 mM CF3COOH
10 mM CF3COOH
12 mM CF3COOH
-3
-2
-1
0
1
E/V vs. Fc
Figure 5. CV of 2 (1.0 mM) with CF3CO2H (0–12 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV s−1
.
3.4. Electrochemistry of complexes
In order to examine whether the above complexes could act as electrocatalysts for proton
reduction to hydrogen, we have investigated their electrochemical properties in the presence
or absence of CF3CO2H under CV conditions. CV was carried out in MeCN/Bu4NPF6 solu-
tions to identify the electrochemically induced oxidation and reduction processes. In MeCN,
CV of 1 shows two irreversible reduction waves at −1.837 and −2.690 V (figure 4). For
each step, the peak current varies as the square root of the scan rate in the measured range,
consistent with a fast diffusion limited electron transfer (figure S13, see online supplemental
material at http://dx.doi.org/10.1080/00958972.2014.940925).
CV of 2 displays two irreversible oxidation waves at 0.432 and −1.358 V and two irre-
versible reduction waves at −1.483 and −2.133 V (figure 5). For each step, the peak current
varies as the square root of the scan rate in the measured range, consistent with a fast diffu-
sion limited electron transfer (figure S15).

Diiron carbonyl complexes
2341
200
0
-200
-400
-600
-800
-1000
0 mM CF3COOH
2 mM CF3COOH
4 mM CF3COOH
6 mM CF3COOH
8 mM CF3COOH
10 mM CF3COOH
12 mM CF3COOH
-3
-2
-1
0
1
E/V vs. Fc
Figure 6. CV of 3 (1.0 mM) with CF3CO2H (0–12 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV s−1
.
Table 3. Redox potentials (V) of 1–3 referenced against Fc+/Fc in MeCN.
ox1
ox2
red1
red2
Complex
Ep
Ep
Ep
Ep
1
2
3
−1.837
−1.483 (0.14)
−1.496 (0.13)
−2.690 (0.38)†
−2.133
−1.982
0.432
−1.358
†CE = (icat/id)/(CHA/Ccat): W for 0 < CE < 0.25, M for 0.25 < CE < 0.75 and S for CE > 0.75.
As in 1, 3 exhibits two irreversible reduction waves at −1.496 and −1.982 V (figures 6
and S16).
According to a criterion of catalytic efficiency (CE = (icat/id)/(CHA/Ccat), where icat is the
catalytic current, id is the current for reduction of the catalyst in the absence of acid, CHA is
the acid concentration and Ccat is the catalyst concentration) proposed by Lichtenberger
et al. [14], as listed in table 3, 1 shows medium CE at the second reduction wave, whereas
2 and 3 exhibit weak CE at the first reduction wave [61, 62]. Therefore, this type of com-
plex containing the bridging hydroxyl group will be of interest to further investigate
hydrogenase models.
4. Conclusion
Three complexes, Fe2(CO)6(μ-PPh2)(μ-L) (1, L = OH; 2, L = OPPh2; 3, L = PPh2), have
been synthesized by reaction of Fe3(CO)12 and Ph2PH in the presence of Et3N. The reaction
conditions generating 1 have been optimized. Et3N acting as a Lewis base can largely
shorten the reaction time. The presence of H2O can increase the yield of 1. Electrochemical
studies confirm that they show catalytic H2-producing activities.
Supplementary material
CCDC 978089–978092 contain the supplementary crystallographic data (including structure
factors) for this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

2342
Y.-C. Shi et al.
Funding
We are grateful to the Mao Zedong Foundation of China, Project funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions (PAPD), Natural Science Foundation of China [grant number
20572091] and Natural Science Foundation of Jiangsu Province [grant number 05KJB150151] for financial support
of this work.
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