Synthesis and characterisation of hexacarbonyl 2-phenylethene-1,1- dithiolatodiiron

Article abstract of DOI:10.1016/j.ica.2012.06.042

2-Phenylethenethione (1) and its tautomer (1′) were prepared by reaction of ethynyl benzene with S₈. The reaction of the tautomer with triiron dodecacarbonyl in dry toluene led to the formations of a classic triiron cluster, [Fe₃(μ₃-S)₂(CO) ₉] (2), and a novel diiron complex, [Fe₂(μ-S) ₂CCHPh)(CO)₆] (3). In this complex, the bridging linkage, 2-phenylethene-1,1-dithiolate, was formed during the reaction involving both C-S bond formation and cleavage. Complex 3 was fully characterised including X-ray single crystal diffraction analysis. In its solid state, the neighboring phenyl rings are within the distance of π-π stacking interaction. Its electrochemistry was carried out in both dicholormethane (DCM) and acetonitrile. It was found that the reversibility was severely affected by solvent. The complex exhibited improved reversibility in the nonpolar solvent (DCM).

Full text of DOI:10.1016/j.ica.2012.06.042

Inorganica Chimica Acta 392 (2012) 112–117
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Note
Synthesis and characterisation of hexacarbonyl 2-phenylethene-1,1-dithiolatodiiron
Xiufeng Wang a, Zhenhong Wei a, Xiujuan Jiang b, Jia Zhao a, Xiaoming Liu a,b,
⇑
a Department of Chemistry, Nanchang University, Nanchang 330031, China
b College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China
a r t i c l e i n f o
a b s t r a c t
2-Phenylethenethione (1) and its tautomer (10) were prepared by reaction of ethynyl benzene with S8.
The reaction of the tautomer with triiron dodecacarbonyl in dry toluene led to the formations of a classic
triiron cluster, [Fe3(l3-S)2(CO)9] (2), and a novel diiron complex, [Fe2(l-S)2C@CHPh)(CO)6] (3). In this
complex, the bridging linkage, 2-phenylethene-1,1-dithiolate, was formed during the reaction involving
Article history:
Received 3 April 2012
Received in revised form 3 June 2012
Accepted 13 June 2012
Available online 20 July 2012
both C–S bond formation and cleavage. Complex 3 was fully characterised including X-ray single crystal
diffraction analysis. In its solid state, the neighboring phenyl rings are within the distance of p–p stacking
Keywords:
[FeFe]-hydrogenase
Diiron-carbonyl model complexes
Electrochemistry
2-Phenylethene-1,1-dithiolate
interaction. Its electrochemistry was carried out in both dicholormethane (DCM) and acetonitrile. It was
found that the reversibility was severely affected by solvent. The complex exhibited improved reversibil-
ity in the nonpolar solvent (DCM).
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
ond electron transfer [14,15], which is why there appears one
reduction peak in most cases. Occasionally, the second reduction
Diiron carbonyl complexes are a category of classic organome-
tallic compounds. The revelation that the metallic centre of
[FeFe]-hydrogenase resembles structurally the diiron complexes
(Fig. 1) [1,2] have prompted strong interest in the chemistry of
the diiron carbonyl complexes. The stimulation comes from that
the metalloenzyme catalyses rapidly and reversibly proton reduc-
tion and hydrogen oxidation and shows high preference towards
hydrogen evolution [3,4]. Due to the relevance of hydrogen to
future clean energy, the enzyme itself and related mimicking
chemistry have attracted tremendous attentions in the past dec-
ade, and numerous iron-sulphur-carbonyl complexes containing
‘‘Fe2S2’’ core as mimics of the subunit of the enzyme have been
reported [5–7]. In the past years, we followed this fascinating
chemistry and reported a number of diiron complexes to mimic
the dimetallic subunit of the enzyme and investigated into their
substitution reactions, protonation, and electron transfer [8–11].
Since electron transfer is an essential step in catalysing proton
reduction to evolve hydrogen, it is undoubted that understanding
the nature of electron transfer of those models is of fundamental
importance. It is well known that the redox behaviours of the
mimicking complexes of the core ‘‘Fe2(CO)6’’ depend very much
on the nature of the ligands bridging the diiron centre [11–13].
But in general, complexes of the core of ‘‘Fe2(CO)n’’ (n = 4–6)
adopt an ECE mechanism often with potential inverse for the sec-
occurs at a potential just marginally more negative than the first
[16–18]. The electrochemical reversibility of those mimicking
complexes is closely associated with the nature of the bridging li-
gands [11,13,14,19]. It has been reported that diiron models bear-
ing conjugate bridging linkage such as benzenedithiolate exhibits
remarkable reversibility due to its capability of diverting electron
density from the diiron centre upon reduction [20,21]. The single-
waved two-electron reduction is coupled with a chemical reac-
tion. At a high scanning rate, the chemical reaction was sup-
pressed and the reduction turned into one-electron process.
Lower the scanning rate, the chemical reaction had enough time
to proceed, and the electron number involved in the reduction
approached to two [15,22,23]. Apparently, the chemical reaction
involved in the process has not destructed the integrity of the
complexes. DFT calculations revealed that the chemical reaction,
in fact, involves the dissociation of one of the four Fe–S bonds
[20,21,24,25].
Under the inspiration of this chemistry, designing and synthes-
ising ligands bearing a conjugate moiety is one of our research
interests. Herein, we report the synthesis and characterisation of
a ligand (2-phenylethenethione, 1) and its reaction with triiron
dodecacarbonyl. In the reaction, a classic triiron cluster, [Fe3(l3
S)2(CO)9] (2), which is widely known in literature [26] and a diiron
complex, [Fe2( -S2C@CHPh)(CO)6] (3), were isolated. This complex
-
l
is analogous to two complexes reported in the 1980s. In the com-
plexes, Si(Me)3 and CO2Me was presented, respectively, rather than
phenyl group. The structures of the two complexes were derived
using NMR analysis [27,28]. The complexes were fully character-
ised using a variety of spectroscopic techniques, including X-ray
⇑
Corresponding author at: College of Biological, Chemical Sciences and Engi-
neering, Jiaxing University, Jiaxing 314001, China. Tel./fax: +86 0573 83643937.
E-mail addresses: xiaoming.liu@mail.zjxu.edu.cn
(X. Liu).
, xiaoming.liu@ncu.edu.cn
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.06.042

X. Wang et al. / Inorganica Chimica Acta 392 (2012) 112–117
113
THF (30 mL) at ice temperature. The reaction mixture was warmed
to 25 °C and stirred overnight. The reaction turned to light brown.
To the mixture was slowly added S8 (1.28 g, 40 mmol) after being
cooled to ice temperature. The reaction was stirred at room tem-
perature for 8 h and then refluxed for further 10 h. After being
cooled in an ice bath, a solution of hydrochloric acid (9 mL,
6 mol Lꢀ1) was dropwise added to the dark-red solution. After
vigorously stirring for 1 h, water was decanted away and remain-
ing solvents were removed under vacuum to give a yellow-orange
residue which was extracted with dichloromethane (20 mL ꢁ 4).
The extracts were combined and concentrated under vacuum to
produce a crude product which was purified using flash chroma-
tography (ethyl acetate:petroleum ether = 2:3) to give a reddish
orange solid (1.7 g, 32%). IR (KBr, cmꢀ1): 1944, 1876, 1798, 1735,
1640, 746, 693; m.p.: 82–84 °C (lit. 79–81 °C) [30]; 1H NMR
(DMSO-d6, d): 7.503–7.230 (m, 5H), 6.816 (s, 1H).
Fig. 1. The diiron subunit of the [FeFe]-hydrogenase (X = NH, CH2 or O).
single crystal diffraction analysis. Complex 3 was also electro-
chemically investigated.
2. Experimental
2.1. Materials and methods
2.3. Reaction of compound 1 with triiron dodecacarbonyl
Ethynyl benzene was purchased from J & K Chemical. Triiron
dodecacarbonyl was prepared by following literature procedures
[29]. Other chemicals were purchased locally. All chemicals were
used as received. All solvents were freshly distilled by using appro-
priate agents under Ar prior to use except for those used in flash
chromatography. FTIR measurements were performed on Varian
Scimitar 2000. 1H NMR and 13C NMR experiments were collected
on Bruker Avance III 600 and microanalysis was carried out on Ele-
mentar vario EL III. Cyclic voltammetry was carried out under Ar
atmosphere using Echo Chemie Autolab PGESTAT 30 in 0.1 mol Lꢀ1
[NBu4][BF4]/acetonitrile, 0.5 mol Lꢀ1 [NBu4]BF4/dichloromethane,
respectively. A conventional three-electrode system was employed,
in which vitreous carbon disk (/ = 1 mm) was used as working elec-
trode, vitreous carbon strip as counter electrode and Ag/AgCl (inner
reference solution: 0.45 mol Lꢀ1 [NBu4]BF4 + 0.05 mol Lꢀ1 [NBu4]Cl
in dichloromethane) as reference electrode. All potentials were
quoted against ferrocenium/ferrocene couple.
A solution of compound 1 (2 g, 14.9 mmol) and Fe3(CO)12
(3.98 g, 7.9 mmol) in dry toluene (80 mL) were refluxed under stir-
ring for 5 h. Removal of the solvent gave a residue which was
purified by flash chromatography using hexanes as eluent. The first
purple band was collected and solvent was removed under vacuum
to give a dark-red solid (complex 2, 0.16 g, 2%). Recrystallisation
from dichloromethane gave single crystals suitable for X-ray
diffraction.
Collection of the second orange band and removal of the solvent
produced complex 3 which was recrystallised from diethyl ether to
give dark-red crystals suitable for X-ray diffraction (0.25 g, 4%).
FTIR (MeCN, cmꢀ1): 2080.9, 2044.5, 2004.4. m.p.: 97–98 °C. Micro-
analysis for C14H6Fe2O6S2 (FW = 446.01), Calc.: C, 37.70; H, 1.36; S,
14.38. Found: C, 37.71; H, 1.25; S, 14.31%. 1H NMR (CDCl3): 7.30
(br, s, 5H), 6.44 (s, 1H); 13C NMR (CDCl3): 207.7, 134.5, 129.7,
128.7, 125.3 (Figs. S3 and S4).
2.2. Preparation of 2-phenylethenethione (1)
2.4. X-ray single crystal diffraction analysis
nBuLi (2.5 mol Lꢀ1 hexane solution, 16 mL, 40 mmol) was drop-
wise added into a solution of ethynylbenzene (4.4 mL, 40 mmol) in
Crystallographic data for complex 3 was collected on a SMART
APEX CCD diffractometer with Mo K
a radiation (k = 0.71073 Å).
Scheme 1. Synthesis of compound 1, complexes 2, and 3.

114
X. Wang et al. / Inorganica Chimica Acta 392 (2012) 112–117
Fig. 2. The IR spectra of complex 3 (solid line) and [Fe2(EPT)(CO)6] (EPT = 1,2-
ethanedithiolate) (dash line) in MeCN.
Table 1
Crystal data and refinement details for complex 3.
Molecular formula
Formula weight (g molꢀ1
Crystal size (mm)
Crystal form
Crystal colour
Crystal system
Space group
Radiation
C14H6O6S2
446.03
)
Fig. 3. The crystal structure of complex 3 (the thermal ellipsoids are drawn at 50%
probability level).
0.34 ꢁ 0.30 ꢁ 0.22
block
red
monoclinic
P21/c
Table 2
Mo K
a
Selected bond lengths (Å) and angles (°) for complex 3.
k (Å)
0.71073
a (Å)
b (Å)
c (Å)
13.2281(7)
11.6337(6)
11.1937(6)
90.00
101.197(1)
90.00
1689.83(15)
4
1.989
888
2.35–28.36
15608
4236 (Rint = 0.0192)
1.006
Bond lengths (Å)
Fe1–Fe2
Fe1–S1
Fe2–S1
S1–C7
2.4859(4)
2.2691(6)
2.2735(6)
1.803(3)
C7–C8
Fe1–S2
Fe2–S2
S2–C7
1.311(3)
2.2825(6)
2.2733(6)
1.788(2)
a
(°)
b (°)
c
(°)
Bond angles (°)
S1–Fe2–Fe1
Fe1–S1–Fe2
C8–C7–S1
C7–S1–Fe1
C7–S2–Fe2
C7–C8–C9
V (Å3)
56.735(17)
66.357(18)
127.5(2)
85.36(8)
86.26(8)
126.0(2)
S2–Fe1–Fe2
Fe2–S2–Fe1
C8–C7–S2
C7–S1–Fe2
C7–S2–Fe1
S1–C7–S2
56.757(16)
66.139(17)
133.6(2)
85.92(8)
85.29(8)
Z
l
(mmꢀ1
)
F(000)
2h Range (°)
Reflections measured
98.83(12)
Reflections unique
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r(I)]
R1 = 0.0288, wR2 = 0.0764
R1 = 0.0368, wR2 = 0.0831
singlet peak at 6.816 ppm at approximate ratio of 5:0.36. The for-
mer signals were tentatively assigned to the hydrogen atoms on
the benzene ring and the singlet to the ethenyl proton. It was re-
ported that hydroxyacetylenes and ketenes are tautomers [37].
Analogous tautomerisation might also exist between 2-phenyle-
thenethione (1) and 2-phenylethynethiol (10), Scheme 1. The ratio
of the signal integration deviated significantly from 5:1 and com-
plication in its low field signals (Fig. S1) may be supportive of this
tautomerisation. Although no signal for SH was found in the 1H
NMR spectrum possibly due to broadening, in its infrared spectrum
The data were collected for Lorentz-polarization factors as well as
for absorption. Structures were solved by direct methods and
refined by full-matrix least-squares methods on F2 with the
SHELX-97 program [31,32]. All non-hydrogen atoms were refined
anisotropically while hydrogen atoms were placed in positions
geometrically calculated.
3. Results and discussion
(Fig. S2) were found two tiny bands at 2520 and 2164 cmꢀ1
,
3.1. Synthesis
respectively. The former may be assigned to SH band whereas
the latter to the carbon triple bond of compound 10.
It has been well established that reaction of carbanion and sul-
phur at a low temperature afforded thiols [33,34]. It was reported
that ethynylbenzene could be deprotonated by strong base and
then reacted with sulphur to afford 2-phenylethynethiolate
[35,36]. We attempted to prepare 2-phenylethynethiol using this
strategy. Acidification after deprotonation and reaction with S8, a
smelly and reddish orange solid was separated. In its 1H NMR spec-
trum, multiple signals located between 7.50–7.23 ppm and one
It has been well-established that the reaction of one equivalent
triiron dodecacarbonyl and two equivalents mercaptan leads to
diiron hexacarbonyl complexes with two thiolates bridging to the
two iron atoms, respectively. We intended to prepare a ligand as
mentioned above, 2-phenylethynethiol (PhC„CSH), and react it
with triiron dodecacarbonyl to synthesise a diiron hexacarbonyl
complex, [Fe2(l-SC„CPh)2(CO)6]. But the isolated product might
be tautomers (1 and 10, Scheme 1). Its reaction with triiron

X. Wang et al. / Inorganica Chimica Acta 392 (2012) 112–117
115
Fig. 4. The packing diagram of complex 3, dash lines showed the p–p interaction of two benzene rings.
dodecacarbonyl in toluene led to the isolation of a triiron cluster
(2) and a novel complex 3, Scheme 1. The isolation of the cluster
is not quite surprising since it is well known that reactions of
dodecacarbonyl triiron with compounds containing sulphur
including thiols could lead to its formation through an oxidative
process [26,38]. The formation of complex 3 was not straightfor-
ward either. As the formation of complex 2, C–S bond cleavages/
formations were involved in the synthesis of complex 3. It is
known that the reaction of triiron dodecacarbonyl with com-
pounds containing sulphur is notoriously complicated. It is impos-
sible to fully understand their exact formation mechanisms of
those complexes without referring to theoretic calculations which
was not further explored in the current work.
Complexes 2 and 3 were separated on a silicon gel column (hex-
ane as eluent). The crystals of complex 2 were obtained by evapo-
rating its dichloromethane solution in open air. Both the IR spectral
data and the crystal structural analysis establish unambiguously
the identity of the cluster, [Fe3(l3-S)2(CO)9] [26]. Since it is a clus-
ter widely reported in literature [26,29,38], no further character-
isation was performed. Complex
3 shows a group of CO
absorption bands at 2081, 2045, 2004 cmꢀ1, respectively, in MeCN
(Fig. 2), which is of the characteristic spectral pattern of diiron
hexacarbonyl complexes [39,40]. Compared to the diiron com-
plexes reported in literature, for example, [Fe2(l-S2C2H4)(CO)6]
(2077, 2037, 1997 cmꢀ1 in MeCN) [41], the infrared absorption
bands of complex 3 is slightly higher by a few wavenumbers. Its
structural analysis (vide infra) confirmed that the bridging linkage
in complex 3 is a dithiolate, the deprotonated form of dithiol,
(HS)2C@CHPh. Thus, this ‘‘blue’’ shift could be attributed to the fact
that this organic moiety of the bridging linkage should be more
electron-withdrawing compared to alkyl groups. The 1H NMR
Fig. 5. Cyclic voltammograms of complex 3 in 0.5 mol Lꢀ1 [NBu4]BF4/DCM (top) and
0.1 mol Lꢀ1 [NBu4]BF4/MeCN (bottom) (scanning rate = 0.1 V sꢀ1, room tempera-
ture, the concentration of the complex is 8.20 and 7.62 mmol Lꢀ1, respectively).

116
X. Wang et al. / Inorganica Chimica Acta 392 (2012) 112–117
spectrum of complex 3 showed a single signal of one proton inte-
gration at 6.44 ppm, which is assigned to the ethylenyl proton. The
broad singlet signal at the low field is certainly attributed to the
aromatic ones.
well known triiron cluster (2) in addition to complex 3. The extre-
mely low yields for both complexes imply a complication of the
reaction. This is not only because of the intrinsic diversity of the
reaction of a thiol with triiron dodecacarbonyl, but also that the li-
gand is a pair of tautomers. Complex 3 was fully characterised
using a variety of spectroscopic techniques and crystallography.
In its solid state, the phenyl rings of every two neighbouring com-
3.2. Structural analysis
plexes are at the distance of p–p stacking interaction. The complex
Complex 3 is soluble in many organic solvents, for example,
hexane, acetonitrile, and dichloromethane. Single crystals suitable
for structural analysis were recrystallised from diethyl ether at
ꢀ25 °C. Its crystallographic detail is tabulated in Table 1 and struc-
ture is shown in Fig. 3. Selected bonding parameters are shown in
Table 2. The complex possesses a ‘‘Fe2S2(CO)6’’ motif as other diiron
analogues widely reported in literature [7,11,39,42]. The closest
exhibits a quasi-reversible reduction process in both DCM and ace-
tonitrile. But the reversibility deteriorated in more polar solvent
(acetonitrile).
Acknowledgements
The authors acknowledge NSF of China (Grant Nos.: 20871064,
21171073), Ministry of Science and Technology of China (973 pro-
gram, 2009CB220009) and the Government of Zhejiang Province
(Qianjiang Professorship, XL) for financial supports.
analogue is [Fe2(l-S)2C@CHC(O)C6H4F(CO)6] [43]. The two have
rather similar metal–metal distance at 2.4859(4) and 2.4872(3) Å,
respectively. As shown in the structure, the Fe–Fe unit was almost
symmetrically bridged by the two
l-sulphur atoms. The distance
of C7–C8 1.311(3) Å is in agreement with a C–C double bond dis-
tance which is generally at 1.34 Å. This bond length is also close
Appendix A. Supplementary material
to that (1.328(2) Å) found in the reported complex, [Fe2(l-
CCDC 872053 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.ca-
m.ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.ica.2012.06.042.
S)2C@CHC(O)C6H4F(CO)6]. The Fe–S distance, \SFeS angle, and
\FeCO angles are all within the range of those defined by analo-
gous complexes reported in the literature.[43] As revealed by its
structure, the bridging linkage in complex 3 is a dithiolate whose
protonated form corresponds to an unusual dithiol, (HS)2C@CHPh.
It is highly likely that the dithiol may not exist in the reaction. In-
stead, the dithiolate was generated during the reaction involving
the cleavage and formation of C–S bonds, which is not unprece-
dented in an analogous reaction [41]. Reaction of the dianion,
References
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analysis.
The packing diagram of complex 3 is shown in Fig. 4. It is obser-
vable that every two neighbouring phenyl rings are in a parallel
position. The distance of two benzene ring centroids is
4.0219(16) Å with perpendicular distance of the two rings is
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