Synthesis, crystal structure and electrocatalysis of 1,2-ene dithiolate bridged diiron carbonyl complexes in relevance to the active site of [FeFe]-hydrogenases

Article abstract of DOI:10.1016/j.jorganchem.2012.01.021

A series of binuclear Fe^IFe^I complexes have been prepared by the treatment of N-heterocyclic 1,2-dithiols, such as, quinoxaline-6,7-dithiol (H₂6,7-qdt), 2,3-diphenyl-6,7-quinoxaline dithiol (H₂diph-6,7-qdt) and

Full text of DOI:10.1016/j.jorganchem.2012.01.021

Journal of Organometallic Chemistry 706-707 (2012) 37e45
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Journal of Organometallic Chemistry
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Synthesis, crystal structure and electrocatalysis of 1,2-ene dithiolate bridged
diiron carbonyl complexes in relevance to the active site of [FeFe]-hydrogenases
*
Gummadi Durgaprasad, Ramababu Bolligarla, Samar K. Das
School of Chemistry, University of Hyderabad, Hyderabad 500046, Andhra Pradesh, India
a r t i c l e i n f o
a b s t r a c t
A series of binuclear FeIFeI complexes have been prepared by the treatment of Neheterocyclic 1,2-
dithiols, such as, quinoxaline-6,7-dithiol (H26,7-qdt), 2,3-diphenyl-6,7-quinoxaline dithiol (H2diph-6,7-
Article history:
Received 5 November 2011
Received in revised form
16 January 2012
qdt) and 2,1,3-benzothiadiazole-5,6-dithiol (H2btdt) with Fe2(CO)9 resulting in the formation of [Fe2{
m-
6,7-qdt}(CO)6] (1), [Fe2{ -diph-6,7-qdt}(CO)6] (2) and [Fe2{ -btdt}(CO)6] (3) respectively, that serve as
m
m
Accepted 17 January 2012
model systems for the active site of [FeFe]-hydrogenase. These new complexes 1e3 have been charac-
terized by IR, 1H, 13C, and 31P{1H} NMR, mass spectroscopy, elemental analysis and single-crystal X-ray
structure analysis. The electrochemistry of 1e3 was performed by cyclic voltammetry and the electro-
catalytic activities of model complexes 2 and 3 toward proton reduction of a strong acid peHOTs have
been described.
Keywords:
[FeFe]hydrogenase
1,2-ene dithiolate bridged diiron carbonyl
complex
Ó 2012 Elsevier B.V. All rights reserved.
Bioinorganic chemistry
e
ꢀ- withdrawing effect
Electrochemistry
1. Introduction
number of biomimetic models for the active site of [FeFe] hydrog-
enase has been prepared and structurally characterized [20e26].
The [FeFe] hydrogenases are vital enzymes, which are well
known to catalyze the reduction of protons to molecular hydrogen
in numerous metallo-enzyme systems with extremely high effi-
ciency which is, therefore, of great interest in the field of renewable
energies [1e3]. [FeFe]-hydrogenases, isolated from Desulfovibrio
desulfuricans and Clostridium pasteurianum [4,5], feature a butterfly
2Fe2S subunit with CO and CNꢀ ligands with one of the Fe (Fep)
atoms linked to a 4Fe4S cluster by the sulfur atom of a cysteinyl
ligand (Fig. 1(i)). Eventually, such a structure has inspired the
organometallic chemists to do an extensive work on developing
simple and robust structural analogues to mimic the [FeFe]-
hydrogenase active site. Several review articles have been pub-
lished in recent times that are associated with hydrogenase model
complexes and their catalytic activities [6e10]. The catalytic prop-
erty for the hydrogen generation, mediated by [FeFe]H2ase model
complexes, can be tuned by the substitution of the CO ligands of
diiron carbonyl complexes. Thus, displacement of one or two CO
ligands from [FeFe]H2ase model complexes by various good ligands
such as CNꢀ, phosphine, phosphite, and carbene has been reported
[11e19]. These carbonyl substituted complexes also serve as
synthetic models of the active site of [FeFe]H2ases. To date, a large
Among such models, we are particularly interested in 1,2-ethylene-
dithiol bridged 2Fe2S core, such as, benzene dithiolate (bdt2e
)
bridged diiron core (representation A, shown below in Fig. 1(ii))
type models [27e30].
Capon and co-workers first reported the compound A as [FeFe]-
hydrogenase active site model system [31] and further Lichten-
berger, Evans, Glass and their co-workers have successfully
demonstrated electrocatalytic hydrogen generation from weak acid
(HOAc) by using the same system A. They proposed a novel
mechanism, deduced from both electrochemical and theoretical
studies on complex A [32,33]. Sascha Ott and his co-workers
worked on arene dithiolate bridged diiron complexes extensively
[34e38]. For the system A, the benzene dithiolate (bdt2e) ligand
has a special ability; it assists to modulate the redox reactions of the
catalyst by lowering the potential difference between the succes-
sive reduction states. This has been explained by an interaction of
the metal d-orbitals with a combination of the filled sulfur pp
orbitals and the arene pp orbitals, which acts to shield the change in
electron density at the iron center as the oxidation state is changed,
thus minimizing the changes in electron energies upon reduction.
The importance of 1,2-ethylene-dithiolate in such model complexes
can also be realized by the fact that “Nature” has used this ligand in
the active site of Mo/W-oxotransferase redox enzymes because of
its unique electronic property [39,40].
* Corresponding author.
E-mail address: skdsc@uohyd.ernet.in (S.K. Das).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.01.021

38
G. Durgaprasad et al. / Journal of Organometallic Chemistry 706-707 (2012) 37e45
reaction between the 4,5-bis(thiocyanato)-benzene-1,2-diamine
(i)
X
(ii)
and benzil by using iodine as a catalyst in acetonitrile solution at
room temperature for 1 h affording a quantitative yield. Finally
H2diph-6,7-qdt was synthesized by treating compound B with Na2S
followed by acidification, which gives a good yield (90%) as shown
in Scheme 2. Compound B and ligand H2diph-6,7-qdt were char-
acterized by regular IR, NMR, LC-MS spectral and elemental
analysis.
Cys
S
S
S
L
S
S
[4S4Fe]
Fe
Fe
CO
CO
OC
OC
CO
Fe
CO
Fe
CO
C
O
CN
H2btdt. H2btdt was synthesized by a reported literature proce-
dure [43].
CN
CO
L=H2O, H or Vacant X= C, N, O or S.
(A)
2.2. Synthesis and characterization of model complexes [Fe2{m-6,7-
qdt}(CO)6] (1), [Fe2{m-diph-6,7-qdt}(CO)6] (2) and [Fe2{m-
Fig. 1. (i) Schematic view of the active site of [FeFe]hydrogenase. (ii) A represents
benzene dithiol bridged diiron complex as a model of [FeFe] hydrogenase active site.
btdt}(CO)6] (3)
We have synthesized [FeFe]-hydrogenase model compounds
[Fe2{
m
-6,7-qdt}(CO)6] (1), [Fe2{
m-diph-6,7-qdt}(CO)6] (2) and
Thus, the rational design and syntheses of 1,2-ethylene-dithio-
late-coordinated 2Fe2S complexes are challenging tasks for
synthetic chemists in modeling the active sites of [FeFe]H2ases. In
view of this, recently we have reported pyrazine dithiolate bridged
diiron complexes as [FeFe]hydrogenase active site model
complexes [41]. In the present contribution, we preferred and
synthesized the following Neheterocyclic ene-1,2-dithiol (benzene
ring attached o-dithiols) ligands, such as, quinoxaline-6,7-dithiol
(H26,7-qdt), 2,3-diphenyl-6,7-quinoxaline dithiol (H2diph-6,7-
qdt) and 2,1,3-benzothiadiazole-5,6-dithiol (H2btdt) as shown in
Scheme 1.
[Fe2{m
-btdt}(CO)6] (3) by treating 6,7-quinoxaline-dithiol (H26,7-
qdt), diphenyl-6,7-quinoxaline dithiol (H2diph-6,7-qdt) and 2,1,3-
benzothiadiazole-5,6-dithiol (H2btdt) respectively with Fe2(CO)9
in THF at 50 ꢁC as shown in Scheme 3. Complexes 1, 2 and 3 are air
stable red solids and purified directly by column chromatography,
which have been fully characterized by IR, 1H, and 13C NMR spec-
troscopy and elemental analysis. The IR spectra of complexes 1, 2
and 3 show four to five strong absorption bands in the region of
2080e1950 cmꢀ1, that are assigned to the terminal carbonyl
groups. The 1H NMR spectrum (in CDCl3 solution at 298 K) of
complex 1 displays two singlets at
2 shows one singlet at 7.94 and one multiplet for two phenyl
7.43e7.29 ppm whereas for compound 3, only one
7.85 ppm is observed in their respective NMR spectra.
d 8.66 and 7.86 ppm. Compound
We have synthesized model complexes [Fe2{
m-6,7-qdt}(CO)6]
d
(1), [Fe2{ -diph-6,7-qdt}(CO)6] (2) and [Fe2{ -btdt}(CO)6] (3) using
m
m
groups at
singlet at
d
these N-heterocyclic 1,2-dithiol ligands (Scheme 1). Here we
describe the syntheses, spectroscopic characterization, X-ray crys-
tallography and electrochemical studies of complexes 1e3. We
have demonstrated the electrocatalytic proton reduction of a strong
acid peHOTs mediated by compounds 2 and 3 and plausible cata-
lytic mechanisms for the production of molecular hydrogen have
been proposed.
d
The 13C NMR spectra of complexes 1, 2 and 3 exhibit only one peak
at
d
207 ppm for their terminal carbonyl group.
The molecular structures of 1, 2 and 3 are unambiguously
confirmed by X-ray diffraction technique and thermal ellipsoidal
plots are shown in Fig. 2aec. Table 1 lists their selected bond
lengths and bond angles. The wine-red colored crystals of
complexes 1, 2 and 3 are grown by cooling respective CH2Cl2
solutions at ꢀ10 ꢁC. The crystal structure analyses show that the
complexes 1, 2 and 3 crystallize in monoclinic space groups P21/c,
C2/c and P21/c respectively. Z0 ¼ 1 for complexes 1, 2 and 3 and
bonding dimensions within the complexes are unexceptional as
shown in Fig. 2. The central 2Fe2S structures of all three diiron 1,2-
ene dithiolate bridged complexes (1, 2 and 3) are in the butterfly
conformation and each iron atom is coordinated with a pseudo
squareepyramidal geometry as in previously reported relevant
models [11,17,46e50]. In addition, the Fe/Fe separation in the
crystal structures of complexes 1, 2 and 3 are 2.4900, 2.5023 and
2.4907 Å respectively, that are somewhat longer than that of
complex A (2.480(2) Å) (Fig. 1) [28] and relatively shorter than
2. Results and discussion
2.1. Synthesis and characterization of Neheterocyclic 1,2-dithiol
ligands
H26,7-qdt. H26,7-qdt was prepared by literature procedure [42].
H2diph-6,7-qdt. The starting precursor for this synthesis is 4,5-
bis(thiocyanato)-benzene-1,2-diamine, which was prepared from
benzene-1,2-diamine and KSCN/Br2 by following reported litera-
ture procedure [43]. 2,3-Diphenyl-6,7-bis-thiocyanato-quinoxaline
(B) (see Scheme below) was prepared according to modified liter-
ature procedure [44,45], which was obtained by a condensation
N
N
SH
SH
N
N
SH
SH
SH
SH
N
N
S
H26,7-qdt
H2diph-6,7-qdt
H2btdt
Scheme 1. Neheterocyclic 1,2-dithiol ligands.

G. Durgaprasad et al. / Journal of Organometallic Chemistry 706-707 (2012) 37e45
39
Ph
N
N
SH
SH
H N
SCN
SCN
2
Ph
Ph
N
N
SCN
SCN
Benzil, I (10 mol%)
Na S
2
2
o
water, 70-80 C
CH CN, RT
3
Ph
H N
2
(B)
H2diph-6,7-qdt
Scheme 2. Synthetic route of H2diph-6,7-qdt.
those found in D. desulfuricans [5] and Clostridium pasteurianum [4]
(ca. 2.6 Å).
process that occurs at E1/2 ¼ ꢀ1.27 V [32,34,52]. This reflects that
the present ligand system Neheterocyclic 1,2-ene dithiolates
(Scheme 1) is of more electron-withdrawing character than
benzene dithiolate (bdt2e) in complex A, thereby, shifting the
reduction potentials of complexes 1, 2 and 3 to relatively more
anodic side by 40, 30 and 20 mV, respectively (see Table 2) than
complex A with benzene dithiolate (bdt2e) ligand. As a conse-
quence, complexes 1, 2 and 3 are thermodynamically more facile
than that of complex A as far as reduction is concerned. The
reduction potential of the related model compound, quinoxaline
Interestingly, complex 3 shows non-covalent interactions, such
as, weak S/N and S/S supramolecular contacts. The S/N contact
distance is 3.0701(1), which is less than the sum of their van-der
Waals radii (1.55 Å for nitrogen and 1.80 Å for sulphur) [51]. The
non-covalent S/S interactions are described by a distance of
3.641(1) Å. These combined weak S/N and S/S non-covalent
interactions result in the formation of one dimensional chainlike
arrangement as shown in Fig. 2d.
2,3-dithiol (H2qdt) bridged diiron complex [Fe2{m-qdt}(CO)6], is
comparable (E1/2 ¼ ꢀ1.22 V) [34,41] to those of complexes 1, 2 and
3. In all these cases, the electron-withdrawing nature of the
ligands decreases the electron density around the iron centers,
making the reduction process easier [34].
2.3. Electrochemistry of model complexes 1e3
The electrochemical properties of complexes 1e3 were inves-
tigated by cyclic voltammetry (CV) studies in MeCN (with 0.1 M
neBu4NClO4 as supporting electrolyte,
n
¼ 0.1 V/s) solution under
an atmosphere of Ar. Table 2 lists the electrochemical data and
Fig. 3 shows the cyclic voltammograms of the complexes 1e3.
They were initiated from the open circuit and scanned in the
cathodic direction as indicated in Fig. 3. It is shown that
complexes 1, 2 and 3 display one quasiereversible two electron
reduction process at E1/2 ¼ ꢀ1.23, ꢀ1.24 and ꢀ1.25 V (vs. Fc/Fcþ)
respectively, as shown in Fig. 3. The electrochemical behavior of
the complex A (Fig. 1 (ii)) was already welleestablished and it
shows that the first reduction potential undergoes a two-electron
2.4. Electrocatalytic hydrogen formation from p-toluenesulfonic
acid (p-HOTs) by complexes 2 and 3
The behavior of electrocatalytic proton reductions by
compounds 2 and 3 were performed by cyclic voltammograms in
the presence of a strong acid p-toluenesulfonic acid (p-HOTs). The
Eop-HOTs for p-HOTs is ꢀ0.65 V vs. Fc/Fcþ in acetonitrile [53]. This
indicates that the least negative potential of the catalyst, that will
allow reduction of p-HOTs to dihydrogen, should be more negative
R
N
R
N
N
R
R
SH
SH
1
R = H
2 R = -C6H5
N
50 0C THF, 3 h
S
S
CO
OC
OC
Fe
Fe
CO
OC
N
OC
Fe2(CO)9
S
N
SH
S
N
S
SH
3
50 0C THF, 3 h
S
S
CO
CO
OC
Fe
OC
Fe
OC
CO
Scheme 3. Synthesis of complexes 1e3.

40
G. Durgaprasad et al. / Journal of Organometallic Chemistry 706-707 (2012) 37e45
Fig. 2. Molecular structures of compounds (a) 1, (b) 2 and (c) 3 with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity. (d) The molecular packing
diagram of compound 3, characterized by NꢂꢂꢂS and SꢂꢂꢂS weak interactions, when viewed down to the crystallographic c axis.
Table 1
Selected bond lengths [Å] and angles [ꢁ] for complexes 1, 2 and 3.
1
2
3
Fe(1)eS(1)
Fe(2)eFe(1)
O(1)eC(1)
C(9)eN(1)
2.2658 (7)
2.4740 (5)
1.136 (3)
1.366 (3)
1.807 (3)
56.96 (2)
56.819 (19)
66.15 (2)
2.2632 (16)
2.4772 (11)
1.128 (8)
1.359 (6)
1.801 (7)
56.96 (4)
56.56 (4)
66.28 (4)
2.267 (2)
2.4907 (18)
1.147 (9)
1.357 (8)
1.785 (9)
56.52 (6)
56.60 (6)
66.73 (6)
Fe(1)eC(1)
S(1)eFe(1)eFe(2)
S(2)eFe(1)eFe(2)
Fe(1)eS(1)eFe(2)
Table 2
Cyclic voltammetric data (vs. Fc/Fcþ) for complexes 1e3 in MeCNa.
1
2
3
Ep1cðVÞ
ꢀ1.34
ꢀ1.12
ꢀ1.23
ꢀ1.34
ꢀ1.13
ꢀ1.24
ꢀ1.35
ꢀ1.15
ꢀ1.25
Ep1aðVÞ
red
E
ðVÞ
1=2
a
0.1 M n-Bu4NClO4 in CH3CN; scan rate 100 mV sꢀ1; working electrode: glassy
carbon electrode of diameter 3 mm; reference electrode: non-aqueous Ag/Agþ
electrode (0.01 M AgNO3 in CH3CN); counter electrode: platinum wire.
Fig. 3. Cyclic voltammograms of complexes 1, 2 and 3 (1.0 mM) in 0.1 M n-Bu4NClO4/
MeCN at a scan rate of 100 mV sꢀ1. Potential are vs. Fc/Fcþ
.

G. Durgaprasad et al. / Journal of Organometallic Chemistry 706-707 (2012) 37e45
41
H+
[(FeIFeI)(N N)]
H2
C
[(Fe0Fe0)(NH NH)]
[(FeIFeI)(NH N)]+
e-
C
E
H+
E
[(Fe0Fe0)(NH N)]-
[(FeIFe0)(NH N)]
e-
Fig. 7. A schematic representation of plausible CEEC mechanism for electrocatalytic
proton reduction in the presence of peHOTs for compound 2.
Fig. 4. Cyclic voltammogram of complex 2 with 1.0 mM complex and 0e12 mM p-
HOTs in CH3CN solution (0.1 M n-Bu4NClO4) at a scan rate of 100 mV sꢀ1
than ꢀ0.65 V vs. Fc/Fcþ. As described in preceding section, the E
values obtained from the electrochemical reductive responses of
compounds 2 and 3 qualify the requirement of reducing p-HOTs to
.
dihydrogen. For compound [Fe2{m-diph-6,7-qdt}(CO)6] (2), upon
the addition of 2 mmol of p-HOTs, a new peak appeared at
Epc ¼ ꢀ0.53 V and the current intensity of the reduction at
Epc ¼ ꢀ1.35 V increased with a cathodic shift by 40 mV. The height
of this reduction peak at Epc ¼ ꢀ1.35 V continuously grew as the
acid concentrations increase from 2 to 12 mM with final cathodic
shift by 260 mV (Fig. 4). On the other hand, the current intensity of
the anodic shifted peak Epc ¼ ꢀ0.53 V (appeared on addition of
2 mmol of p-HOTs) did not grow up further with sequential
increments of the acid concentrations (4e12 mM). This observation
indicates the protonation of one of the ring nitrogens present in
the ligand system of compound 2. This is supported by
electronic absorption spectral changes, occurred on addition of
10e60
mL p-HOTs to the acetonitrile solution of complex 2 that
shows a clean isosbestic point at 395 nm (Fig. 5). Protonation of the
ring nitrogens in the complex 2 is also observed on energies of the
IR bands as shown in the FTIR spectrum (Fig. 6) of compound 2
solution (CH3CN) treated with p-HOTs. As shown in Fig. 6, the IR
spectrum exhibited significant changes in the carbonyl region.
When protonation occurs, there is a considerable shift of the CO
bands to higher frequencies (energy) region. The IR peaks at 2076,
2037, 2028, 2003, 1973 cmꢀ1 (compound 2) shift to 2081,
Fig. 5. Changes in the absorption spectra of compound 2, (5.0 ꢃ 10ꢀ5 M) upon the
addition of 10e60 mL aliquots of dilute peHOTs (0.1 mM) in CH3CN.
Fig. 6. FTIR spectrum of acetonitrile solution of compound 2 treated with peHOTs.
5 mg of p-HOTs was added to a CH3CN solution (2 mL) of complex 2 (10 mg).
Fig. 8. Cyclic voltammogram of complex 3 with 1.0 mM complex and 0e10 mM p-
HOTs in CH3CN solution (0.1 M n-Bu4NClO4) at a scan rate of 100 mV s-1
.

42
G. Durgaprasad et al. / Journal of Organometallic Chemistry 706-707 (2012) 37e45
2049, 2012 cmꢀ1 on protonation. The NꢀH bond, formed on
protonation, displays a strong and broad absorption band at
3388 cmꢀ1 region because of NꢀH stretching vibrations (Fig. 6). For
comparison, the solution IR spectrum of compound 2 (without
adding p-HOTs) is shown in the last section of Supporting
Information.
The continuous growth of the reduction peak at Epc ¼ ꢀ1.35 V
with final cathodic shift of 260 mV (on addition of 2e12 mM p-
HOTs) is characteristic of an electrochemical catalytic reduction of
p-HOTs to hydrogen. The catalyzed reduction of p-HOTs occurs at
about ꢀ1.61 V vs. Fc/Fcþ with overpotential of 0.96 V. The cyclic
voltammograms (Fig. 4) and the spectroscopic evidences
(Figs. 5 and 6) suggest a CEEC (chemical-electrochemical-electro-
chemical-chemical) mechanism (Fig. 7) for the electrocatalytic
proton reduction by diph-6,7-qdt-bridged all-carbonyl {Fe1Fe1}
compound 2. A possible CECE (chemical-electrochemical-chem-
ical-electrochemical) mechanism can be ruled out by the fact that
the catalyzed reduction of p-HOTs occurs (present study) with
cathodic shift instead of anodic shift; earlier report of the ADT-
Fig. 9. Changes in the absorption spectra of compound 3, (5.0 ꢃ 10ꢀ5 M) upon the
addition of 10e60
m
L aliquots of dilute peHOTs (0.1 mM) in CH3CN.
diiron complex [{(m-SCH2)2N(4eNO2C6H4)Fe2(CO)6] catalyzed
acid reduction has been described by a CECE mechanism, for
which the catalyzed acid reduction occurs with an anodic shift
[54].
2e-
[(FeIFeI)(N N)]
The hexa carbonyl compound [Fe2{m-btdt}(CO)6] (3) exhibits
H2
the electrocatalytic proton reduction with p-HOTs as shown in
Fig. 8. In this case, protonation of oxidized {FeIFeI} species does not
occur as shown by electronic absorption spectral scans, recorded
E
on addition of 10e60
mL p-HOTs to the acetonitrile solution of
[(Fe0Fe0)(N N)]2-
[(Fe0Fe0)(NH NH)]
complex 3 that do not show any spectral changes (Fig. 9). The
current height of the reduction potential at Epc ¼ ꢀ1.35 V
continuously increased with increasing acid concentration
(2e12 mM p-HOTs) with final cathodic shift of 290 mV indicating
a catalytic proton reduction. The concerned cyclic voltammograms
(Fig. 8) and unchanged electronic spectra (Fig. 9) on acid addition
suggest an EC (electrochemical-chemical) mechanism as shown in
Fig. 10.
C
2H+
Fig. 10. A schematic representation of plausible EC mechanism for electrocatalytic
proton reduction in the presence of peHOTs for compound 3.
Table 3
Crystal data and structural refinement for complexes 1e3.
1
2
3
Empirical formula
Formula weight
Temperature (K)
Crystal size (mm)
Crystal system
Space group
Z
C14H4Fe2N2O6S2
472.01
298 (2)
0.24 ꢃ 0.18 ꢃ 0.04
Monoclinic
P21/c
C27H14Fe2N2O6S2Cl2
709.12
100 (2)
0.50 ꢃ 0.42 ꢃ 0.38
Monoclinic
C2/c
C12H2Fe2N2O6S3
478.04
298 (2)
0.33 ꢃ 0.25 ꢃ 0.19
Monoclinic
P21/c
4
8
4
Wavelength (Å)
a [Å]
b [Å]
0.71073
0.71073
0.71073
22.247 (8)
6.877 (3)
14.146 (5)
90.0
94.152 (7)
90.0
18.0645 (13)
6.8573 (5)
13.2585 (10)
90.0
96.8580 (10)
90.0
36.976 (6)
7.5657 (11)
25.353 (4)
90.0
124.119 (5)
90.0
c [Å]
a
b
g
(ꢁ)
[ꢁ]
(ꢁ)
Volume [Å3]
1630.6 (2)
1.923
5871.8 (15)
1.604
2151.7 (14)
1.472
Calculated density (Mg/mꢀ3
)
Reflections collected/unique
R(int)
6956/3139
0.0228
23565/5175
0.0511
19265/3741
0.1160
F(000)
936
2848
944
Max. and min. transmission
Q
Refinement method
0.9217 and0.6363
1.14 to 25.98
Full-matrix Least-squares on F2
3139/0/235
1.053
0.0313/0.0764
0.0353/0.0785
0.531 and-0.319
0.6268 and0.5504
1.33 to 25.06
Full-matrix Least-squares on F2
5175/0/370
1.069
0.0647/0.01567
0.0789/0.1655
1.193 and-0.952
0.7434 and 0.6105
1.84 to 24.98
Full-matrix Least-squares on F2
3741/0/226
1.037
0.0746/0.1559
0.1372/0.1782
0.652 and ꢀ0.513
range for data collection (ꢁ)
Data/restraints/parameters
Goodness-of-fit on F2
R1/wR2 [I > 2sigma(I)]
R1/wR2 (all data)
Largest diff. peak and hole [e.Åꢀ3
]

G. Durgaprasad et al. / Journal of Organometallic Chemistry 706-707 (2012) 37e45
43
3. Conclusion
detector distance 60 mm, collimator 0.5 mm. Data reduction per-
formed by SAINTPLUS [55]. Empirical absorption corrections using
equivalent reflections performed program SADABS [55]. Absorption
We have described the synthesis, characterization and electro-
chemistry of three N-heterocyclic cis-1,2 dithiolato bridged diiron
complexes as potential models for the active sites of the [FeFe]
hydrogenase. N-heterocyclic cis-1,2 dithiols belong to an important
ligand system as far as the active sites of many metalloenzymes are
concerned. Molybdopterin, awell characterized co-factor associated
the active sites molybdenum hydroxylases, contains N-heterocyclic
cis-1,2 dithiol as a ligand coordinated to the metal center and this
class of enzymes catalyses many essential oxidationereduction
reactions. It was proposed that cis-1,2 dithiolato (non-innocent)
ligand plays an important role in these redox/electron transfer
reactions. The metallo-enzyme of the present context, [FeFe]
hydrogenaseis alsoknownas an important redoxenzyme catalyzing
an acid to dihydrogen. Therefore, complexes that contains N-
heterocyclic cis-1,2 dithiolato bridged diiron centers would serve as
potential modelsfor [FeFe]hydrogenases. Eventhrough, the number
of reports of modeling the active sites of iron only hydrogenases is
not anymore limited, the number of diiron model systems that
contains bridged N-heterocyclic cis-1,2 dithiolato ligand is very
limited. All three model complexes 1e3, reported in the present
study, are N-heterocyclic cis-1,2 dithiolato bridged diiron
complexes. Interestingly, each of the three carbonyl complexes 1, 2
and 3 display one quasiereversible two electron reduction at rela-
tively very low potentials (E1/2 ¼ ꢀ1.23, ꢀ1.24 and ꢀ1.25 V vs. Fc/Fcþ
for 1, 2 and 3 respectively) compared to other related model
compounds. We have demonstrated the electrocatalytic proton
reduction of a strong acid p-toluenesulfonic acid mediated by
compounds 2 and 3. For the electrocatalytic proton reduction by the
corrections using multi
J-scans were applied. The structures were
solved by direct methods and least-square refinement on F2 for all
the compounds 1e3 by using SHELXS-97 [56]. All the non hydrogen
atoms were refined anisotropically. Hydrogen atoms on the
aromatic rings were introduced on calculated positions and
included in the refinement riding on their respective parent atoms.
The crystallographic parameters, data collection and structure
refinement of the compounds 1ꢀ3 are summarized in Table 3.
4.3. Electrochemical studies of complexes 1e3
Acetonitrile (Finar, HPLC grade) used for performance of elec-
trochemistry was dried with molecular sieve (4 Å) and then freshly
distilled from CaH2 under N2. A solution of 0.1 M [nBu4N][ClO4]
(TBAP) (Across, electrochemical grade) in CH3CN was used as
supporting electrolyte. Electrochemical measurements were
recorded by using a BAS electrochemical analyzer. The electrolyte
solution was degassed by bubbling with dry Argon for 10 min
before measurement. Cyclic voltammograms were obtained in
a three-electrode cell under argon. The working electrode was
a glassy carbon disc (diameter 3 mm) successively polished with
0.3-mm alumina paste and sonicated in an ionꢀfree water for
10 min; it was then washed with ethanol and finally rinsed with
acetone followed by air drying before usage. The home-built
reference electrode was a silver wire in contact with a solution of
0.1 M neBu4NClO4 and 0.01 M AgNO3 in acetonitrile. The reference
electrode was separated from the contents of the cell by means of
a porous Vycor frit. When not in use, the reference electrode
assembly was kept immersed in 0.1 M neBu4NClO4/acetonitrile to
prevent drying of the frit. The auxiliary electrode was a platinum
wire. The potentials, reported here, are quoted against the ferro-
cene/ferrocenium (Fc/Fcþ) couple.
all-carbonyl compound [Fe2{m-diph-6,7-qdt}(CO)6] (2), we have
proposed CEEC mechanism. The hexa carbonyl compound [Fe2{
m-
btdt}(CO)6] (3) does not undergo protonation with p-toluene-
sulfonic acid in its {FeIFeI} oxidation state, probably due to the
involvement/delocalization of lone pair of electrons of ring nitrogen
with ring sulfur pi-electrons. Accordingly an EC electrocatalytic
reduction mechanism is proposed for compound 3.
4.3.1. Preparationof 2,3-Diphenyl-6,7-bis-thiocyanato-quinoxaline(B)
4. Experimental section
A mixture containing 1,2-diaminobenzene-bis(thiocyanate)
(300 mg, 1.35 mmol), benzil (316 mg, 1.50 mmol) and iodine
(10 mol%) in CH3CN (10.0 mL) was stirred for 1 h. The resulting
yellow precipitate was separated by filtration, washed with little
CH3CN, and dried in vaccuo. Yield: 0.45 g (84.0%); Yellow solid;
LCeMS: m/z ¼ 395 [M ꢀ H]þ. IR (KBr pellet, cmꢀ1): 3067(CeH Str,
Ar), 2160 (C^N str), 1653, 1597, 1537, 1435, 1390, 1338, 1253,
1194, 1057, 1022, 952, 893, 871, 769, 723, 698, 597, 543, 493. 1H
4.1. General procedures
All the reactions were carried out using standard Schlenk and
vacuum-line technique under an atmosphere of nitrogen. Dichloro-
methane was distilled from CaH2, THF from sodium/benzophenone
ketyl, and MeOH, EtOH from Mg powder under a N2 atmosphere.1,2-
diamino benzene (from Loba), Fe2(CO)9 and Me3NO were purchased
from Aldrich and directly used without further purification. Micro
analytical (C, H, N) data were obtained with a FLASH EA 1112 series
CHNS Analyzer. The IR spectra (with KBr pellet) were recorded in the
range of 400e4000 cmꢀ1 on a JASCO FT/IR-5300 spectrometer.
Electronic spectra were obtained on a Cary 100 Bio UVeVisible
spectrophotometer. 1H, 13C and 31P{1H} NMR spectra were recorded
on Bruker DRXe400 spectrometer using Si(CH3)4 (TMS) as an
internal standard. Solution mass spectra (LCeMS) were obtained on
a LCeMS-2010A Shimadzu spectrometer. Melting points were
measured in open capillary tubes and are uncorrected.
NMR (400 MHz, CDCl3)
d
: 8.66 (s, 2H), 7.55 (d, 4H), 7.36e7.46 (m,
156.32, 141.47, 137.75, 134.35,
6H) ppm. 13C NMR (CDCl3):
d
129.95, 129.88, 128.53, 126.72, 107.91 ppm. Anal. Calc. for
C22H12N4S2: C, 66.64; H, 3.05; N, 14.13%. Found: C, 66.48; H, 3.14;
N, 14.25%.
4.3.2. Preparation of 2,3-diphenyl-quinoxaline-6,7-dithiol (H2diph-
6,7-qdt)
2,3-diphenyl-6,7-bis-thiocyanato-quinoxaline(0.3 g, 0.757 mmol)
was added as a solid to a solution of Na2S (0.3 g, 3.85 mmol) in 100 mL
of degassed water and the mixture was heated to 70e80 ꢁC for 24 h to
produce a clear, orange-red solution. The mixture was cooled to room
temperature, and 20 mL of 10% HCl was added drop-wise to afford
a heavy, brown precipitate. The precipitate was filtered off, washed
with water, and air-dried. Thus dithiol ligand was obtained. Yield:
0.180 g (68.6%). LCeMS: m/z ¼ 347 [M ꢀ H]þ. IR (KBr pellet, cmꢀ1):
3057 (CeH Str, Ar), 2530 (SeH str),1581,1533,1433,1388,1338,1253,
1188, 1059, 1022, 960, 869, 765, 727, 694, 596, 543. 13C NMR (DMSO-
4.2. Single crystal structure determination of complexes 1e3
Single crystals suitable for facile structural determination for the
compounds 1, 2 and 3 were measured on a three circle Bruker
SMART APEX CCD, area detector system under
) ¼ 0.7103 Å], graphite monochromator X-ray beam, 2400 frames
were recorded with an
scan width of 0.3ꢁ, each for 8 s, crystal-
[
l
(Mo
Ka
u
d6):
d 145.2,140, 136.5, 134, 131.0, 129, 128.5, 127.0 ppm. Anal. Calc. for

44
G. Durgaprasad et al. / Journal of Organometallic Chemistry 706-707 (2012) 37e45
C20H14N2S2: C, 69.33; H, 4.07; N, 8.09%. Found: C, 69.86; H, 3.92; N,
7.82%.
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Government of India (Project No. SR/S1/ICe23/2007) and Centre for
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The National X-ray Diffractometer facility at the University of
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infrastructure facility at University of Hyderabad under a UPE grant.
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