Facile synthesis, X-ray analysis, and spectroscopic studies of di-iron propanedithiolate complexes with tris(aromatic)phosphine ligands

Article abstract of DOI:10.1080/00958972.2014.903329

Three new propanedithiolate-type iron-sulfur complexes containing tris(aromatic)phosphine ligands, [{(-SCH₂)₂CH ₂}Fe₂(CO)₅L] (L = P(PhOMe-p)₃, 1; P(PhMe-p)₃, 2; P(PhF-p)₃, 3), have been prepared through carbonyl substitution in the presence of Me₃NO. The new complexes 1-3 were characterized by elemental analysis, IR, ¹H, ¹³C{¹H}, and ³¹P{H} NMR spectra. The molecular structures of 1-3 were unequivocally determined by single crystal X-ray diffraction, in which the tris(aromatic)phosphine coordinated to Fe resides in an apical position of the pseudo-square-pyramidal geometry. IR spectroscopy and X-ray crystallographic analysis for 1-3 have indicated that the highly electron rich tris(aromatic)phosphine ligands (where the corresponding electron-donating abilities display the following order of P(PhOMe-p)₃ > P(PhMe-p)₃ > P(PhF-p)₃) result in a considerable red shift of the CO-stretching frequencies and a clear change of the Fe-Fe bond distances in 1-3.

Full text of DOI:10.1080/00958972.2014.903329

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Facile synthesis, X-ray analysis,
and spectroscopic studies of di-iron
propanedithiolate complexes with
tris(aromatic)phosphine ligands
Pei-Hua Zhaoa, Xin-Hang Lia, Yun-Feng Liub & Ya-Qing Liua
a Research Center for Engineering Technology of Polymeric
Composites of Shanxi Province, College of Materials Science and
Engineering, North University of China, Taiyuan, PR China
b College of Public Health, Shanxi Medical University, Taiyuan, PR
China
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Accepted author version posted online: 12 Mar 2014.Published
online: 04 Apr 2014.
To cite this article: Pei-Hua Zhao, Xin-Hang Li, Yun-Feng Liu & Ya-Qing Liu (2014) Facile
synthesis, X-ray analysis, and spectroscopic studies of di-iron propanedithiolate complexes
with tris(aromatic)phosphine ligands, Journal of Coordination Chemistry, 67:5, 766-778, DOI:
10.1080/00958972.2014.903329
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 5, 766–778, http://dx.doi.org/10.1080/00958972.2014.903329
Facile synthesis, X-ray analysis, and spectroscopic studies of
di-iron propanedithiolate complexes with tris(aromatic)
phosphine ligands
PEI-HUA ZHAO*†, XIN-HANG LI†, YUN-FENG LIU‡ and YA-QING LIU*†
†Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, College
of Materials Science and Engineering, North University of China, Taiyuan, PR China
‡College of Public Health, Shanxi Medical University, Taiyuan, PR China
(Received 8 January 2014; accepted 10 February 2014)
Three new propanedithiolate-type iron–sulfur complexes containing tris(aromatic)phosphine ligands,
[{(μ-SCH2)2CH2}Fe2(CO)5L] (L = P(PhOMe-p)3, 1; P(PhMe-p)3, 2; P(PhF-p)3, 3), have been
prepared through carbonyl substitution in the presence of Me3NO. The new complexes 1–3 were
characterized by elemental analysis, IR, 1H, 13C{1H}, and 31P{H} NMR spectra. The molecular
structures of 1–3 were unequivocally determined by single crystal X-ray diffraction, in which the
tris(aromatic)phosphine coordinated to Fe resides in an apical position of the pseudo-square-pyrami-
dal geometry. IR spectroscopy and X-ray crystallographic analysis for 1–3 have indicated that
*Corresponding authors. Email: zph2004@163.com (P.-H. Zhao); gczx2012@gmail.com (Y.-Q. Liu)
© 2014 Taylor & Francis

Di-iron propanedithiolate complexes
767
the highly electron rich tris(aromatic)phosphine ligands (where the corresponding electron-donating
abilities display the following order of P(PhOMe-p)3 > P(PhMe-p)3 > P(PhF-p)3) result in
a
considerable red shift of the CO-stretching frequencies and a clear change of the Fe–Fe bond
distances in 1–3.
Keywords: Iron–sulfur complexes; Tris(aromatic)phosphine; Carbonyl substitution; X-ray analysis;
Spectroscopic study
1. Introduction
Chemistry of iron–sulfur complexes has attracted attention because of their close resem-
blance in structure to [FeFe]-hydrogenase, which is a type of nature’s most efficient and
inexpensive catalysts for hydrogen production [1–6]. High-resolution single-crystal X-ray
crystallography has shown that [FeFe]-hydrogenase features a butterfly 2Fe2S subunit as its
active site, where the iron centers are coordinated by carbonyl (CO)/cyanide (CN), a bridg-
ing dithiolate and a cysteinyl-S-linked cubic 4Fe4S cluster [7, 8].
The well-established structure of the aforementioned active site has provoked chemists to
design and synthesize a number of iron–sulfur mimics of [FeFe]-hydrogenase. This can be
achieved in two different strategies, one through substitution of CO groups to introduce
electron-donating ligands, such as CN− [9, 10], tertiary phosphine (PR3) [11, 12], isonitrile
[13, 14], and N-heterocyclic carbene [15, 16]; the other is by changing the bridgehead
group or dichalcogenolate to modify the dithiolate linkers, including propanedithiolate
(PDT) [17, 18], azadithiolate [19, 20], oxadithiolate [21, 22], thiodithiolate [23, 24], pro-
panediselenoate [25, 26], azadiselenoate [27, 28], oxadiselenoate [29], thiodiselenoate [30],
and propaneditelluroate [31] bridges. For the first approach, phosphine ligands are prefera-
ble in the [FeFe]-hydrogenase model system as a good substitute for naturally occurring
CN− [32]; for the second strategy, PDT-type iron–sulfur complexes have played an impor-
tant role in the development of biomimetic chemistry of [FeFe]-hydrogenase [33–37].
On the basis of our previous study on PDT-type iron–sulfur complexes [38–40], we
recently investigated the substitution reaction of [{(μ-SCH2)2CH2}Fe2(CO)6] with interest-
ing monophosphine ligands to further extend the iron–sulfur mimics of [FeFe]-hydrogenase,
and have successfully prepared three new tris(aromatic)phosphine-substituted iron–sulfur
complexes [{(μ-SCH2)2CH2}Fe2(CO)5L] (L = P(PhOMe-p)3, 1; P(PhMe-p)3, 2; P(PhF-p)3,
3). Herein, we report the synthesis, spectroscopic characterization, and crystal structures of
the PDT-type iron–sulfur complexes containing tris(aromatic)phosphine ligands.
2. Experimental
2.1. Materials and methods
All reactions and operations were carried out under a dry, oxygen-free nitrogen atmosphere
with standard Schlenk and vacuum-line techniques. MeCN was distilled with CaH2 under
N2. Commercially available materials, Me3NO·2H2O, P(PhOMe-p)3, P(PhMe-p)3, and
P(PhF-p)3, were reagent grade and used as received. [{(μ-SCH2)2CH2}Fe2(CO)6] (A) was
prepared according to the literature [41]. Preparative TLC was carried out on glass plates
(25 cm × 20 cm × 0.25 cm) coated with silica gel G (10–40 mm). IR spectra were recorded
1
on a Nicolet 670 FTIR spectrometer. H, 13C{1H}, and 31P{H} NMR spectra were obtained

768
P.-H. Zhao et al.
on a Bruker Avance 400 MHz spectrometer. Elemental analyses were performed on a
Perkin-Elmer 240C analyzer. Melting points were determined on a YRT-3 apparatus and are
uncorrected.
2.2. Synthesis of [{(μ-SCH2)2CH2}Fe2(CO)5P(PhOMe-p)3] (1)
A mixture of [{(μ-SCH2)2CH2}Fe2(CO)6] (0.193 g, 0.5 mM), P(PhOMe-p)3 (0.211 g,
0.6 mM), and Me3NO·2H2O (0.056 g, 0.5 mM) was dissolved in MeCN (15 mL) and was
stirred at room temperature for 2 h to give a black-red solution. The solvent was removed
on a rotary evaporator and the residue was subjected to preparative TLC separation using
CH2Cl2/petroleum ether (v/v = 1 : 5) as eluent. From the main red band, 1 (0.241 g, 68%)
was obtained as a red solid. M.p.: 180–181 °C. Anal. Calcd for C29H27Fe2O8PS2: C, 49.04;
H, 3.83%. Found: C, 48.93; H, 3.99%. IR (KBr disk, cm−1): νC≡O 2041 (vs), 1985 (vs),
1
3
1979 (vs), 1958 (vs), 1930 (vs). H NMR (400 MHz, CDCl3, TMS, ppm): 7.58 (t, JHH
= 3JHP = 7.6 Hz, 6H, PhH), 6.93 (d, JHH = 7.6 Hz, 6H, PhH), 3.84 (s, 9H, OCH3), 1.78–
3
1.72 (m, 2H, SCHaHe), 1.54–1.48 (m, 4H, SCHaHe and CH2). 13C{1H} NMR (100.6 MHz,
2
CDCl3, TMS, ppm): 213.92 (d, JPC = 12.0 Hz, PFeCO), 209.83 (s, FeCO), 160.97 (s, ipso-
PhCOMe), 134.97 (d, JPC = 12.3 Hz, o-PhCH), 127.55 (d, JPC = 44.8 Hz, ipso-PhCP),
2
1
3
114.00 (d, JPC = 10.5 Hz, m-PhCH), 55.37 (s, OCH3), 30.02 (s, CH2), 22.29 (s, SCH2).
31P{1H} NMR (161.9 MHz, CDCl3, 85% H3PO4, ppm): 60.26 (s).
2.3. Synthesis of [{(μ-SCH2)2CH2}Fe2(CO)5P(PhMe-p)3] (2)
The procedure was similar to that of 1 except P(PhMe-p)3 (0.183 g, 0.6 mM) was used
instead of P(PhOMe-p)3 (0.211 g, 0.6 mM). Complex 2 (0.172 g, 52%) was obtained as a
red solid. M.p.: 199–200 °C. Anal. Calcd for C29H27Fe2O5PS2: C, 52.59; H, 4.11%. Found:
C, 52.69; H, 4.03%. IR (KBr disk, cm−1): νC≡O 2043 (vs), 1989 (vs), 1974 (vs), 1959
1
3
(vs), 1928 (vs). H NMR (400 MHz, CDCl3, TMS, ppm): 7.55 (t, JHH = 3JHP = 8.4 Hz, 6H,
3
PhH), 7.21 (d, JHH = 8.4 Hz, 6H, PhH), 2.38 (s, 9H, PhCH3), 1.75–1.69 (m, 2H, SCHaHe),
1.54–1.39 (m, 4H, SCHaHe and CH2). 13C{1H} NMR (100.6 MHz, CDCl3, TMS, ppm):
2
213.82 (d, JPC = 10.7 Hz, PFeCO), 209.79 (s, FeCO), 140.30 (s, ipso-PhCMe), 133.48
2
1
3
(d, JPC = 11.4 Hz, o-PhCH), 132.86 (d, JPC = 40.7 Hz, ipso-PhCP), 129.24 (d, JPC
=
9.7 Hz, m-PhCH), 30.05 (s, CH2), 22.21 (s, SCH2), 21.44 (s, CH3). 31P{1H} NMR
(161.9 MHz, CDCl3, 85% H3PO4, ppm): 62.38 (s).
2.4. Synthesis of [{(μ-SCH2)2CH2}Fe2(CO)5P(PhF-p)3] (3)
The procedure was similar to that of 1 except P(PhF-p)3 (0.190 g, 0.6 mM) was used instead
of P(PhOMe-p)3 (0.211 g, 0.6 mM). Complex 3 (0.108 g, 32%) was obtained as a red solid.
M.p.: 198–199 °C. Anal. Calcd for C26H18F3Fe2O5PS2: C, 46.32; H, 2.69%. Found: C,
46.20; H, 2.88%. IR (KBr disk, cm−1): νC≡O 2048 (vs), 1979 (vs), 1972 (vs), 1954 (vs),
1932 (vs). 1H NMR (400 MHz, CDCl3, TMS, ppm): 7.63 (m, 6H, PhH), 7.15
3
(t, JHH = 3JHF = 8.0 Hz, 6H, PhH), 1.83–1.78 (m, 2H, SCHaHe), 1.54–1.48 (m, 4H,
2
SCHaHe and CH2). 13C{1H} NMR (100.6 MHz, CDCl3, TMS, ppm): 213.41 (d, JPC
=
1
10.5 Hz, PFeCO), 209.24 (s, FeCO), 164.00 (d, JFC = 253.2 Hz, ipso-PhCF), 135.51
2
3
1
(dd, JPC = 12.5 Hz, JFC = 8.2 Hz, o-PhCH), 131.41 (d, JPC = 40.8 Hz, ipso-PhCP), 116.07

Di-iron propanedithiolate complexes
769
3
2
(d, JPC = 10.6 Hz, JFC = 21.1 Hz, m-PhCH), 30.10 (s, CH2), 22.37 (s, SCH2). 31P{1H}
NMR (161.9 MHz, CDCl3, 85% H3PO4, ppm): 63.14 (s).
2.5. X-ray structure determination
Single crystals of 1–3 suitable for X-ray diffraction analysis were grown by slow evapora-
tion of the CH2Cl2/hexane solution at 5 °C. Single crystals of 1–3 were mounted on a Riga-
ku MM-007 CCD diffractometer. Data were collected at 293(2) K for 1, 2, and 3 using a
graphite monochromator with Mo Kα radiation (λ = 0.71073 Å) in the ω–φ scanning mode.
Data collection, reduction, and absorption correction were performed by CRYSTALCLEAR
[42]. The structure was solved by direct methods using SHELXS-97 [43] and refined by
full-matrix least-squares (SHELXL-97) on F2 [44]. Hydrogens were located using the
geometric method.
3. Results and discussion
3.1. Synthesis and spectroscopic studies
The complexes 1–3 were readily prepared in moderate yields by reaction of
[{(μ-SCH2)2CH2}Fe2(CO)6] (A) with different tris(aromatic)phosphine ligands [namely,
P(PhOMe-p)3, P(PhMe-p)3, or P(PhF-p)3] using the decarbonylating agent Me3NO in a 1 :
1.2 : 1 M ratio in MeCN at room temperature (scheme 1). The tris(aromatic)phosphine-
S
S
S
OC
P(PhOMe-p)3
P(PhOMe-p)3
P(PhMe-p)3
Fe
Fe
OC
OC
CO
CO
1
2
S
S
S
OC
CO
Me3NO
MeCN
P(PhMe-p)3
OC
Fe
Fe
OC
Fe
Fe
OC
OC
CO
CO
OC
CO
CO
A
P(PhF-p)3
S
S
OC
P(PhF-p)3
Fe
Fe
OC
CO
OC
CO
3
Scheme 1. Preparation of 1–3.

770
P.-H. Zhao et al.
monosubstituted products are soluble in polar organic solvents, such as CH2Cl2, EtOAc,
THF, MeCN, etc. All the complexes are air stable in solution and in the solid state.
The complexes 1–3 are characterized by elemental analysis, IR, 1H, 13C{1H}, and
31P{H} NMR spectroscopy. The elemental analyses for 1–3 are in agreement with the
respective compositions. The IR spectra of 1–3 in KBr exhibit three infrared absorptions
from 2048–1928 cm−1 for terminally coordinated CO. IR data of ν(CO) bands are usually
considered as a useful indicator for detecting variation in the electron density of Fe in di-iron
CO complexes and evaluating the electron-donating abilities of the ligands coordinated to
Fe. IR data of ν(CO) for 1–3 are compared to that of the parent complex (A) (table 1). The
values of the highest ν(CO) bands for 1–3 are red shifted by 33, 31, and 27 cm−1 to lower
frequencies as compared to that of the all-CO A, respectively, indicating that introduction of
the highly electron rich tris(aromatic)phosphine ligands has a considerable enhancement of
the electron density on the Fe cores. The order of red shifts of the highest ν(CO) bands for
1–3 indicate that the electron-donating abilities of three tris(aromatic)phosphine ligands
exhibit the order, P(PhOMe-p)3 > P(PhMe-p)3 > P(PhF-p)3, which can be attributed to the
structural and electronic variations of the substituents of the aromatic rings in the corre-
sponding phosphine ligands.
1H NMR spectra of 1–3 display two multiplets at 1.83–1.69 and 1.54–1.39 ppm, which
are assigned to resonances of the methylene protons in the PDT bridge. A sharp singlet at
1
3.84 ppm for the methoxy protons is observed in the H NMR spectrum of 1, while a strong
singlet at 2.38 ppm for the methyl protons is present in the corresponding 2. Furthermore,
two types of peaks at 7.58 and 6.93 ppm for 1, 7.55 and 7.21 ppm for 2, as well as 7.63 and
7.15 ppm for 3 are ascribed to phenyl protons, indicating that OMe, Me, and F substituents
in the tris(aromatic)phosphine ligands are located in the para position of the phenyls.
In the 31C{1H} NMR spectra, two peaks for bridging propane carbons are present at ca.
30/22 pm for 1–3. The spectra of 1–3 display four peaks for phenyl carbons from 164 to
1
114 ppm, consistent with the different peaks for the phenyl protons in their H NMR spec-
2
tra. Furthermore, the spectra of 1–3 exhibit a doublet at 213 ppm with a JPC coupling con-
stant (ca. 11 Hz) for the coordinated COs in the PFe(CO)2 unit and a singlet at 209 ppm for
the terminal COs of Fe(CO)3 [40, 50]. In addition, the 31P{1H} NMR spectra of 1–3 show
a sharp singlet at 60.26, 62.38, and 63.14 ppm for phosphorus of the coordinated tris(aro-
matic)phosphine ligands.
3.2. X-ray crystallographic analysis
The molecular structures of 1–3 have been further confirmed by X-ray crystallography
(figures 1–3). The crystallographic parameters, data collection, and structure refinement of
1–3 are summarized in table 2. Selected bond lengths and angles are listed in table 3.
Table 1. A comparison of the ν(CO) bands for [{(μ-SCH2)2CH2}Fe2(CO)5L] (L = CO, A; P(PhOMe-p)3,
1; P(PhMe-p)3, 2; P(PhF-p)3, 3).
Complex
L
ν(CO) (cm−1
)
Δν(CO)highesta (cm−1
)
Note
1
2
3
A
P(PhOMe-p)3
P(PhMe-p)3
P(PhF-p)3
CO
2041, 1985, 1979, 1958, 1930
2043, 1989, 1974, 1959, 1928
2048, 1979, 1972, 1954, 1932
2074, 2036, 1995
−33
−31
−26
–
This work
This work
This work
Ref. [47]
aΔν = ν(CO)substituted-CO − ν(CO)all-CO
.

Di-iron propanedithiolate complexes
771
Figure 1. Molecular structure of 1 with thermal ellipsoid at 30% probability.
Figure 2. Molecular structure of 2 with thermal ellipsoid at 30% probability.

772
P.-H. Zhao et al.
Figure 3. Molecular structure of 3 with thermal ellipsoid at 30% probability.
Single-crystal X-ray diffraction analysis reveals that the 2Fe2S skeleton of 1–3 has the
expected butterfly conformation and each Fe displays distorted square-pyramidal
coordination geometry, in accord with reported PDT-type iron–sulfur mimics of [FeFe]-
hydrogenase [45–57]. Moreover, both 31P{H}-NMR and X-ray crystallographic analyses of
1–3 suggest that one CO-displacement by the tris(aromatic)phosphine ligands in the all-CO
parent complex (A) affords only an apical isomer (figures 1–3). The corresponding ligands
[i.e. P(PhOMe-p)3, P(PhMe-p)3 and P(PhF-p)3] in 1–3 are coordinated to an apical site on
one Fe and roughly trans to the Fe–Fe bond, consistent with those in the common configu-
rations of the PR3-monosubstituted complexes [{(μ-SCH2)2CH2}Fe2(CO)5L] (L = PPh3
[45], P(OEt)3 [45], PhPMe2 [45], PPh2NH(2-NH2Ph) [48], PPh2NH(CH2)2NMe2 [48],
PPh2(2-NMe2CH2Ph) [48], PPh2(CH2CO2H) [49], PPh2(2-NHPy) [50], PPh2Fc [51], and
PPh2(H)C60 [52]) but different from basal and cis conformations of the previously
reported analogs [{(μ-SCH2)2CH2}Fe2(CO)5L] (L = PTA [53], P(NC4H8)3 [54], and
P(CH2CH2CO2H)3 [55]).
The Fe–Fe distances in 1 (2.5166(7) Å), 2 (2.5144(10) Å), and 3 (2.5029(8) Å) are slightly
longer or shorter than that of the parent [{(μ-SCH2)2CH2}Fe2(CO)6] (A) (2.5103(11) Å)
[56], but these distances are shorter than that found in the natural [FeFe]-hydrogenase
enzymes (2.55–2.60 Å) [7, 8]. It should be noted that substitution of CO with different tris
(aromatic)phosphines which lie in an apical position of Fe(1) or Fe(2), enables Fe…Fe sepa-
rations to be lengthened in 1 and 2, but shortened in 3 as compared to that of the all-CO A.
This may be because the electron-donating capabilities of the tris(aromatic)phosphines

Di-iron propanedithiolate complexes
Table 2. Crystal data and structural refinement details for 1–3.
773
Complex
1
2
3
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C29H27Fe2O8PS2
710.29
293 (2)
0.71073
Triclinic
C29H27Fe2O5PS2
662.30
293 (2)
0.71073
Monoclinic
P2(1)/n
14.1021 (10)
12.7469 (7)
16.9925 (12)
90
106.983 (7)
90
2921.3 (3)
4
1.506
C26H18F3Fe2O5PS2
674.19
293 (2)
0.71073
Monoclinic
P(2)1/n
9.1082 (4)
8.5451 (5)
34.6865 (15)
90
91.864 (4)
90
2698.2 (2)
4
1.660
P-1
10.9434 (5)
11.5081 (6)
13.2598 (7)
87.436 (4)
86.194 (4)
67.398 (5)
1537.93 (14)
2
γ (°)
V (Å3)
Z
DCalcd (g cm−3
)
1.532
1.179
726
μ (mm−1
F (0 0 0)
)
1.228
1360
1.346
1360
Crystal size (mm)
min, θmax (°)
Reflections collected/unique
0.30 × 0.20 × 0.20
3.08, 26.37
11,241/6291
0.0343
0.30 × 0.22 × 0.18
2.97, 26.37
14,368/5924
0.0609
0.26 × 0.20 × 0.03
2.96, 26.37
12,359/5518
0.0386
θ
Rint
hkl range
−13 ≤ h ≤ 11
−13 ≤ k ≤ 14
−16 ≤ l ≤ 15
99.8
6291/6/377
1.033
0.0508/0.0981
0.0807/0.1149
0.937/−0.691
−16 ≤ h ≤ 17
−15 ≤ k ≤ 13
−17 ≤ l ≤ 21
99.1
5924/0/355
1.024
0.0621/0.0805
0.1225/0.1011
0.624/−0.394
−11 ≤ h ≤ 11
−9 ≤ k ≤ 10
−34 ≤ l ≤ 43
99.8
5518/4/362
1.109
0.0537/0.0927
0.0805/0.1022
0.416/−0.386
Completeness to θmax (%)
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
R1/wR2 [I > 2σ(I)]
R1/wR2 (all data)
Largest difference peak/hole (e A−3
)
relative to the CO ligands display a resulting order of P(PhOMe-p)3 > P(PhMe-p)3 >
P(PhF-p)3 as observed in IR spectra of 1–3. Meanwhile, the average Fe–S distances [1:
(2.2590 Å); 2: (2.2618 Å); 3: (2.2675 Å)] and the Fe–P distances [1: (2.2482(11) Å); 2:
(2.2372(14) Å); 3: (2.2425(11) Å)] are comparable to each other and to the corresponding
lengths reported for PR3-coordinated iron–sulfur analogs [51, 52]. In the case of 1–3, the
average Fe–CCO distances of the coordinated-Fe center [1: (1.760 Å); 2: (1.762 Å); 3: (1.763
Å)] are notably shorter than the corresponding lengths of the uncoordinated-Fe [1: (1.789
Å); 2: (1.786 Å); 3: (1.785 Å)]. Accordingly, the average C–O distances in the Fe(CO)2L
unit [1: (1.142 Å); 2: (1.148 Å); 3: (1.141 Å)] are a bit longer than the corresponding lengths
in the Fe(CO)3 unit [1: (1.137 Å); 2: (1.141 Å); 3: (1.137 Å)]. A reasonable explanation is
that the higher electron density of the coordinated-Fe leads to stronger electron back-dona-
tion from the coordinated-Fe to CO in Fe(CO)2L relative to the back-donation from the
uncoordinated-Fe to CO in Fe(CO)3 [26, 57].
The dihedral angles, defined by S(1)–Fe(1)–Fe(2) and S(2)–Fe(1)–Fe(2) planes, are
108.1, 72.1, and 107.1 in 1–3, respectively, and different from each other, implying that
self-regulation of the central 2Fe2S skeleton is accompanied by variation of the electron-
densities on the Fe core for molecular stability. The angles of P(1)–Fe(1)–Fe(2) are 8.47,
4.75, and 7.56, larger than the ones of C(5)–Fe(2)–Fe(1), C(4)–Fe(2)–Fe(1), and C(5)–
Fe(2)–Fe(1) in 1–3, respectively, demonstrating that steric effects of the tris(aromatic)

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Table 3. Selected bond lengths (Å) and angles (°) for 1–3.
Complex
1
2
3
Fe(1)–Fe(2)
Fe(1)–S(2)
Fe(1)–S(1)
Fe(2)–S(1)
Fe(2)–S(2)
O(1)–C(1)
2.5166 (7)
2.2590 (11)
2.2630 (11)
2.2564 (12)
2.2576 (12)
1.139 (5)
2.5144 (10)
2.2757 (13)
2.2579 (14)
2.2574 (13)
2.2563 (14)
1.147 (5)
2.5029 (8)
2.2780 (11)
2.2652 (12)
2.2661 (11)
2.2606 (10)
1.145 (4)
O(2)–C(2)
1.145 (5)
1.149 (5)
1.137 (5)
O(3)–C(3)
1.137 (5)
1.136 (5)
1.138 (5)
S(2)–Fe(1)–S(1)
S(2)–Fe(1)–Fe(2)
S(1)–Fe(1)–Fe(2)
Fe(2)–S(1)–Fe(1)
C(2)–Fe(1)–Fe(2)
C(1)–Fe(1)–Fe(2)
P(1)–Fe(1)–Fe(2)
C(6)–S(1)–Fe(2)
C(6)–S(1)–Fe(1)
Fe(1)–P(1)
84.41 (4)
56.11 (3)
56.04 (3)
67.67 (3)
100.39 (13)
99.72 (13)
154.80 (4)
110.04 (17)
115.22 (17)
2.2482 (11)
1.757 (4)
84.24 (5)
55.94 (4)
56.15 (4)
67.68 (4)
98.72 (18)
102.30 (17)
155.64 (5)
112.79 (17)
114.05 (18)
2.2372 (14)
1.757 (5)
84.09 (4)
56.20 (3)
56.49 (3)
67.06 (3)
101.49 (13)
96.66 (13)
158.83 (4)
111.21 (14)
114.80 (16)
2.2425 (11)
1.762 (4)
Fe(1)–C(2)
Fe(1)–C(1)
1.763 (4)
1.767 (5)
1.763 (5)
Fe(2)–C(4)
1.786 (6)
1.776 (6)
1.797 (5)
Fe(2)–C(3)
1.788 (5)
1.787 (5)
1.780 (4)
Fe(2)–C(5)
1.794 (5)
1.794 (6)
1.779 (5)
O(4)–C(4)
1.138 (6)
1.137 (5)
1.141 (5)
O(5)–C(5)
1.137 (5)
1.150 (5)
1.133 (5)
S(1)–Fe(2)–S(2)
S(1)–Fe(2)–Fe(1)
S(2)–Fe(2)–Fe(1)
Fe(2)–S(2)–Fe(1)
C(4)–Fe(2)–Fe(1)
C(3)–Fe(2)–Fe(1)
C(5)–Fe(2)–Fe(1)
C(8)–S(2)–Fe(2)
C(8)–S(2)–Fe(1)
84.60 (4)
56.29 (3)
56.16 (3)
67.72 (3)
103.31 (14)
100.82 (15)
146.33 (14)
111.70 (16)
114.93 (15)
84.70 (5)
56.17 (4)
56.67 (4)
67.39 (4)
150.89 (16)
95.66 (16)
106.3 (2)
109.69 (18)
115.54 (17)
84.47 (4)
56.45 (3)
56.87 (3)
66.94 (3)
99.81 (15)
102.45 (14)
151.27 (13)
110.75 (16)
116.79 (15)
phosphine ligands on the structures of 1–3 exhibit the following order: P(PhOMe-p)3 >
P(PhF-p)3 > P(PhMe-p)3. The differences between [C(6)–S(1)–Fe(1) versus C(6)–S(1)–
Fe(2)/C(8)–S(2)–Fe(1) versus C(8)–S(2)–Fe(2) in 1–3] are 5.18/3.23, 1.26/5.85, and
3.59/6.04, respectively. This indicates that the FeS2C3 six-member ring of 1,3-PDT in 1–3
is pushed away from the site occupied by an apical P(PhOMe-p)3, P(PhMe-p)3, or
P(PhF-p)3 owing to the steric effect [46], which results in tilt of the iron-dithiacyclohexane
ring towards the Fe(CO)3 site.
It can be concluded from the above X-ray crystallographic discussions that coordination
geometry of the PDT-type iron–sulfur complexes is mainly determined by electron-donating
abilities and steric effects of the coordinated ligands, which are bound to the Fe core in the
butterfly 2Fe2S structure.
Solid-state structures of 1–3 were stabilized by van der Waals’ interactions and π–π
stacking observed in their crystal packing diagrams (figures 4–6).

Di-iron propanedithiolate complexes
775
Figure 4. Crystal packing diagram of 1 along the a-axis.
Figure 5. Crystal packing diagram of 2 along the b-axis.

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Figure 6. Crystal packing diagram of 3 along the b-axis.
4. Conclusion
We have synthesized four new tris(aromatic)phosphine-substituted PDT-type iron–sulfur
complexes, 1–3, which are regarded as iron–sulfur mimics of [FeFe]-hydrogenase. All the
complexes are fully characterized by IR and NMR spectroscopic analyses and also by sin-
gle-crystal X-ray structures. IR spectroscopic and X-ray crystallographic studies indicated
electron-donating abilities of the tris(aromatic)phosphine ligands in the order P(PhOMe-p)3
> P(PhMe-p)3 > P(PhF-p)3 lead to increased electron density at the iron centers and further
cause considerable red shift of the CO-stretching frequencies, and change in the Fe–Fe bond
distances in 1–3.
Supplementary material
CCDC-952267 (1), CCDC-952266 (2), and CCDC-952264 (3) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Center, 12 Union Road Cambridge CB2 1EZ, UK; Fax: (+44) 1223-336-033; or E-mail:
deposit@ccdc.cam.ac.uk.
Funding
This work was financially supported by the National Natural Science Foundation of China [grant num-
ber 21301160] and the Natural Science Foundation for Young Scholars of Shanxi Province [grant
number 2012021007-4].

Di-iron propanedithiolate complexes
777
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