A tetranuclear iron complex: substitution with triphenylphosphine ligand and investigation into electrocatalytic proton reduction

Article abstract of DOI:10.1007/s12039-018-1529-x

Abstract: The reaction of the tetranuclear iron complex [μ ₄-Sulfido-bis{(μ -2-furylmethanethiolato)bis[tricarbonyliron](Fe—Fe)}] 1 with PPh ₃ was explored. The reaction leads to the formation of the mono-substituted complex [Fe ₂(μ -2-furylmethanethiolate)₂(CO) ₅(PPh ₃)] 2. X-ray crystal structure has been reported for complex 2. Complexes 1 and 2 were found to be active catalysts for proton reduction. Complexes 1 and 2 showed comparable catalytic activity for proton reduction to dihydrogen. Graphical Abstract: SYNOPSIS Monothiolate-bridged, all carbonyl tetranuclear iron complex [μ ₄-Sulfido-bis{(μ -2-furylmethanethiolato)bis[tricarbonyliron](Fe—Fe)}] 1 and phosphine-substituted diiron complex [Fe ₂(μ -furylmethanethiolate)₂(CO) ₅(PPh ₃)] 2 have been investigated for electrocatalytic proton reduction.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-018-1529-x

J. Chem. Sci. (2018) 130:126
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1529-x
REGULAR ARTICLE
A tetranuclear iron complex: substitution with triphenylphosphine
ligand and investigation into electrocatalytic proton reduction
MOOKAN NATARAJAN, VISHAKHA KAIM, NAVEEN KUMAR and
SANDEEP KAUR-GHUMAAN∗
Department of Chemistry, University of Delhi, Delhi 110 007, India
E-mail: skaur@chemistry.du.ac.in
MS received 24 April 2018; revised 8 June 2018; accepted 24 July 2018
Abstract. The reaction of the tetranuclear iron complex [μ4-Sulfido-bis{(μ-2-furylmethanethiolato)
bis[tricarbonyliron](Fe—Fe)}] 1 with PPh3 was explored. The reaction leads to the formation of the mono-
substituted complex [Fe2(μ-2-furylmethanethiolate)2(CO)5(PPh3)] 2. X-ray crystal structure has been reported
for complex 2. Complexes 1 and 2 were found to be active catalysts for proton reduction. Complexes 1 and 2
showed comparable catalytic activity for proton reduction to dihydrogen.
Keywords. Hydrogenases; catalysis; iron carbonyl complexes; proton reduction; monodentate phosphine.
1. Introduction
reported are: [Fe2(μ-napthalene-2-thiolate)2(CO)5L] (L
= CO, PPh3);9a [Fe2(μ-p-toluenethiolate)2(CO)6], [Fe2
Hydrogen is being considered as a potential energy (μ-CH2Ph)2(CO)6];9b
[Fe2(μ-SCH2-o-C6H4OX)2
vector for the future as its combustion produces energy (CO)6] (X = H, Me);10 [Fe2(CO)6(μ-SC6H4-R)2 (R =
and only water as the by-product. Generation of hydro- p-H, o-CH3O, p-CH3O, p-Cl)];11 [Fe2(CO)4(μ-SC6H4
gen is, therefore, a clean alternative to fossil fuels. In -o-CH3O)2(PMe3)2];10
[μ-S-2-RCONHC6H4)2Fe2
nature, the [FeFe] hydrogenase enzyme is capable of (CO)6] (R = Me, CF3, Ph, 4-F-C6H4);12 [Fe2(μ-SEt)2
reversibly and rapidly catalyzing hydrogen evolution (CO)6];13 [Fe2(μ-SEt)2(CO)4(PMe3)2];12 [Fe2(μ-SR)2
and oxidation.1 The active site (H-cluster) of this met- (CO)6](R=2-thienyl, 2-thienylmethyl,2-C4H3SCH2);14
alloenzyme, as revealed by the crystal structure inves- [Fe2(μ-SMe)2(CO)6];15
[Fe2{μ-SCH2CH(OH)CH2
tigations a decade ago, consists of one [4Fe4S] cubane (OH)}2(CO)6];16 [Fe2(μ-SPh)2(CO)6];17 [Fe2(μ-CH2
and one [2Fe2S] subunit (Figure 1).2,3 In the [2Fe2S] CH2CH2OH)2(CO)6];18 [Fe2(CO)4(μ-SEt)2(CN)2];19
subunit, the two iron atoms are linked by a dithiolate [Fe2(μ-SC6H2-iPr3-2, 4, 6)2(CO)6];20 [Fe2(μ-SC6H2
ligand, in addition to co-ordination with CO and CN− Me3-2, 4, 6)2(CO)6],19 etc. Most of the above com-
ligands.2–5 Since the revelation of the detailed struc- plexes have been reported as structural models of the
tural information of the enzyme active site, there has H-cluster. A few of the complexes [Fe2(μ-napthalene-2-
been tremendous interest in the study of organometallic thiolate)2(CO)5L] (L = CO, PPh3);9a [Fe2(μ-p-toluene-
model complexes of the H-cluster, in order to develop thiolate)2(CO)6];9b [Fe2(μ-CH2Ph)2(CO)6];9b [Fe2(μ
cheap, efficientandinexpensivecatalystsfortheproduc- -SCH2-o-C6H4OX)2(CO)6] (X = H, Me),10 [Fe2(CO)6
tion of dihydrogen.1,6,7 Dithiolate-bridged complexes of (μ-SC6H4-R)2] (R = p-H, o-CH3O, p-CH3O, p-Cl),11
the type I have been reported with a wide variety of
[Fe2(CO)4(μ-SC6H4-o-OCH3)2(PMe3)2];10 [μ-S-2−
terminal ligand (CO, CN−, PR3, NHCs, etc.) combina- RCONHC6H4)2Fe2(CO)6] (R = Me, CF3, Ph, 4-F-C6
tions (Figure 1).8 Though many complexes of type I H4);12 [Fe2(μ-SEt)2(CO)6];13 [Fe2(μ-SEt)2(CO)4
are known, there are not many reports of monothiolate- (PMe3)2];12 [Fe2(μ-SMe)2(CO)6];15 havebeenreported
bridged complexes i.e., type II. A few selected examples as catalysts for proton reduction in the presence of
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1529-x) contains
supplementary material, which is available to authorized users.
0123456789().: V,-vol

126 Page 2 of 10
J. Chem. Sci. (2018) 130:126
hexafluorophosphate (Sigma-Aldrich, electrochemical grade)
as supporting electrolyte that was dried in vacuum at 383
K. Cyclic voltammetry was carried out using an Autolab
potentiostat with a GPES electrochemical interface. The
working electrode was a glassy carbon disc (diameter 3
mm, freshly polished) for cyclic voltammetry. Platinum
was used as the counter electrode. The reference electrode
was a non-aqueous Ag/Ag+ electrode (CH Instruments,
0.01 M AgNO3 in acetonitrile). All the potentials are quoted
against the ferrocene–ferrocenium couple (Fc/Fc+); fer-
rocene was added as an internal standard at the end of
the experiments. For the electrochemical measurements,
all solutions were prepared from dry acetonitrile (Sigma-
Aldrich, spectroscopic grade, dried with molecular sieves
3 Å).
2.2 Synthesis of [Fe2(μ-2-furylmethanethiolate)2
(CO)5 (P Ph3)] 2
Figure 1. [FeFe] hydrogenase enzyme active site (top) and
model complexes reported (bottom).
The tetranuclear all carbonyl precursor complex 1 was
synthesized as reported in literature.21 A tetrahydrofuran
solution of 1 (600 mg, 0.733 mmol) and the phosphine
ligand (PPh3) (192 mg, 0.733 mmol) were refluxed under
argon atmosphere for 3 h. The resulting solution was evap-
orated to dryness under vacuum and the residue was chro-
acids with low currents and high over potentials.
Therefore, with an aim to further understand the influ-
ence of monothiolate ligands in the model complexes,
the reported tetra iron precursor complex μ4 -Sulfido- matographed on a silica gel column. Elution with a mixture
of hexane/dichloromethane (4:1, v/v) afforded an orange-red
solution. Complex 2 was obtained as an air-stable orange-red
powder after removal of the solvent. The complex was then
recrystallized from hexane-dichloromethane solution by slow
evaporation at – 4 ◦C. 1: IR (CH2Cl2, cm−1): 2070 (s), 2027
(s), 1968 (br).
bis {(μ-2-furylmethanethiolato) bis[tricarbonyliron]
(Fe–Fe)} 1 was used to synthesize the monodentate
phosphine substituted complex [Fe2(μ-2-furylmethane-
thiolate)2(CO)5(PPh3)] 2 (L = PPh3).21 The catalytic
aspects of both the complexes (1 and 2) were investi-
gated in the presence of acids of varying strengths by
electrochemical investigations.
2.2a Complex 2: [Fe2(μ-2-furylmethanethiolate)2
(CO)5(PPh3)]: Yield: 79.2% (0.772 g); IR (CH2Cl2,
cm−1): C=O 2041 (s), 1979 (br), 1960 (s), 1926 (s); 1H NMR
ν
2. Experimental
(400 MHz, 298 K, ppm in CDCl3): 7.70–7.30 (m, 15H, PPh3),
6.22–5.78 (6H, furyl-H) 3.43–2.81 (4H, -CH2-). 31P NMR
(161.8 MHz, 298 K, ppm in CDCl3): 59.8 ppm (s, PPh3).
2.1 Materials and physical measurements
All the experiments were carried out under an inert
atmosphere using Schlenk line techniques. The reagents Fe3
(CO)12, 2-furylmethanethiolate, triphenylphosphine, deuter-
2.3 X-ray crystal structure analysis
ated (CDCl3) and anhydrous solvents (dichloromethane, Single crystals for complexes 1 and 2 were grown by slow
tetrahydrofuran, acetonitrile) were obtained from Sigma- evaporation of hexane/dichloromethane solutions. X-ray data
Aldrich and used without further purification. The precur- of the complexes were collected on an Oxford Xcalibur
sor complex μ4-Sulfido-bis{(μ-2-furylmethanethiolato)bis CCD single-crystal X-ray diffractometer at 293 K, equipped
[tricarbonyliron](Fe—Fe)} 1 was prepared according to the with graphite monochromatic MoKα radiation ( 0.71073
λ
procedure reported in literature.21 The 1H and 31P {1H} Å). Significant crystallographic parameters and refinement
NMR spectra were recorded at room temperature in CDCl3 details for complex 2 are listed in Table 1. For details of
solution with Jeol 400 MHz NMR spectrometer. The FTIR complex 1, see Supplementary Information. The crystal struc-
spectra were recorded from dichloromethane and acetonitrile tures of the complexes were solved by direct methods using
solutions of the samples over the range 400−4000 cm−1 SIR-92 and refined by full-matrix least squares refinement
on a Perkin Elmer FTIR Spectrometer. The UV-Vis spectra techniques on F2 using SHELXL-97.22,23 The structure was
for complexes were recorded on a Perkin-Elmer Lambda- solved and refined by standard procedures. The multi-scan
25 spectrophotometer. Electrochemical measurements were absorption correction was applied. The coordinates of non-
conducted in acetonitrile with 0.1 M tetrabutylammonium hydrogen atoms were refined anisotropically using SHELXL-

J. Chem. Sci. (2018) 130:126
Page 3 of 10 126
97.24 All calculations were done using the Wingx software distance in complex 2 is 2.5207(4) Å which is close
package.25 For the molecular graphics, the program ORTEP-
to that reported for the [FeFe] hydrogenase enzyme
3 was used.26
active site (2.6 Å).2,3,27 The Fe–Fe bond distances in
the reported complex 1 are Fe(1)–Fe(2), 2.5263(10)
and Fe(3)–Fe(4), 2.5377(10) Å, which are only slightly
higher than the Fe–Fe bond distance in complex 2 proba-
bly due to the bridging sulphide between the iron centers
(see Supplementary Information).21 The replacement of
one CO ligand with the phosphine ligand in complex 2
has only a small effect on the Fe–Fe bond distance. The
phosphine ligand occupies the apical position in 2 and
the [2Fe2S] site is in butterfly conformation, with the
equatorial-axial spatial orientation of the furylthiolate
groups tethered to the iron centers.9 The equatorial-
axial spatial orientation is due to the presence of a
monodentate phosphine ligand on one of the iron cen-
ters. This orientation has also been observed for similar
complexes reported earlier.9 The crystal packing along
a axis showed two molecules present together (Figure
S2, Supplementary Information). Intermolecular bond-
ing for the complex was seen between C29–H29· · ·O1
and C20–H20· · ·O7 with bond distances of 2.551 and
2.612 Å, respectively (Figure S3, Supplementary Infor-
mation).
3. Results and Discussion
3.1 Synthesis and characterization
The tetranuclear all carbonyl iron precursor complex
μ4-Sulfido-bis {(μ-2-furylmethanethiolato)bis[tricarb-
onyliron](Fe–Fe)} 1 was synthesized as reported in liter-
ature.21 The reaction of precursor complex 1 with PPh3
in refluxing THF afforded the target complex [Fe2(μ-
2-furylmethanethiolate)2(CO)5(PPh3)] 2 in 79.2% yield
(Scheme1). Recrystallizationfromhexaneanddichloro-
methane solutions at low temperature afforded orange-
red crystals for complex 2.
3.2 Structure of the complexes
3.2a Crystal structure of 2: The molecular structure
for 2 was confirmed by single crystal X-ray diffraction
analysis (Figure 2, Tables 1 and 2). The Fe–Fe bond
Table 1. Crystallographic parameters and refinement details for complex 2.
Empirical formula
Formula weight
Crystal size (mm)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C33H25Fe2O7PS2
740.36
0.270 × 0.260 × 0.250
293(2)
0.71073
Triclinic
¯
P1
9.0095(4)
b (Å)
11.8668(8)
c (Å)
16.8481(10)
70.847(6)
84.436(4)
76.716(5)
α (o)
β (◦)
γ (o)
V (Å3)
1655.60(18)
Z
2
ρcalcd (g cm−3
)
1.485
1.096
Absorption coefficient (mm−1
)
Two theta range for data collection (o)
6.42 to 58.82
Refinement method
Full-matrix least-squares on F2
No. of unique reflns (Rint
Index ranges
F000
)
8022 (0.0257)
−12 ≤ h ≤ 11, −16 ≤ k ≤ 15, −22 ≤ l ≤ 23
1010
0.0373
R1[I > 2σ(I)] (on F for obsd reflns)a
wR2[I > 2σ(I)] (on F2 for all reflns)b
Goodness-of-fit on F2
0.0866
1.019
Largest diff. peak and hole
0.334 and −0.273 e.Å−3
a
R1 = ꢀI IFoI − IFc I I/ꢀIFoI.
b wR2 = {ꢀ[w(Fo2 − Fc2)2]/ꢀw(Fo2)2}1/2
.

126 Page 4 of 10
J. Chem. Sci. (2018) 130:126
Scheme 1. Synthetic scheme for complexes 1 and 2.
3.3 FTIR spectral studies
The complexes were characterized by different
spectroscopic techniques. The FTIR spectra for the
complexes were recorded in both dichloromethane and
acetonitrile (Table 3 and Figures S4–S5, Supplementary
Information). For the tetranuclear iron complex, the CO
stretching frequencies were observed between 2070 and
1970 cm−1. The peaks correspond to the terminal car-
bonyl ligands. On the other hand, the phosphine substi-
tuted complex 2 displayed peaks at lower wavenumbers
(shifted by 30 cm−1) in comparison to complex 1. The
attachment of electron-donating triphenylphosphine lig-
and at the apical position on one of the iron centers leads
to lower terminal CO stretching frequencies.
Figure 2. Molecular
structure
of
3.4 NMR spectroscopy
[Fe2(μ-2-furylmethanethiolate)2(CO)5(PPh3)] 2. Hydrogen
atoms have been omitted for clarity.
The1HNMRspectrumofcomplex 2inCDCl3 displayed
peaks in the range 7.70–7.30 ppm corresponding to the
phenyl ring protons of the PPh3 ligand (Figure S6, Sup- rings (3 protons in each ring). The peaks between
plementary Information). The peaks between 6.22 and 3.43 and 2.81 ppm correspond to four protons from
5.78 ppm were due to the protons present in the two furyl the -CH2- a bridge between sulphur and the furyl ring.

J. Chem. Sci. (2018) 130:126
Page 5 of 10 126
Table 2. Selected bond lengths (Å) and angles (o) for
complex 2.
50
40
30
20
10
0
Bond lengths
Bond angles
Fe(1)–Fe(2) 2.5207(4) Fe(1)–S(2)–Fe(2) 67.600 (19)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(2)–S(1)
Fe(2)–S(2)
Fe(1)–P(1)
Fe(1)–C(4)
Fe(1)–C(5)
Fe(2)–C(1)
Fe(2)–C(2)
Fe(2)–C(3)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
C(4)–O(4)
C(5)–O(5)
2.2518(6) Fe(1)–S(1)–Fe(2) 67.772 (18)
2.2646(6) S(2)–Fe(2)–Fe(1) 56.162 (17)
2.2691(6) S(2)–Fe(1)–Fe(2) 56.238 (17)
2.2666(6) S(1)–Fe(2)–Fe(1) 55.788 (16)
2.2344(6) S(1)–Fe(1)–Fe(2) 56.440 (17)
1.772(2)
1.760(2)
1.801(3)
1.787(3)
1.767(3)
1.139(3)
S(2)–Fe(2)–S(1)
S(1)–Fe(1)–S(2)
C(4)–Fe(1)–S(1)
C(4)–Fe(1)–S(2)
C(5)–Fe(1)–S(1)
C(5)–Fe(1)–S(2)
79.90 (2)
80.32 (2)
158.89 (8)
87.00 (8)
91.94 (7)
157.27(8)
300
400
500
600
Wavelength / nm
Figure 3. UV-Vis spectra for complexes 1 (—-) and 2 (---)
in acetonitrile.
1.131(3) C(5)–Fe(1)–Fe(2) 101.72 (8)
1.136(3)
1.137(3)
1.146(3)
C(5)–Fe(1)–C(4)
C(4)–Fe(1)–P(1)
C(5)–Fe(1)–P(1)
P(1)–Fe(1)–S(1)
P(1)–Fe(1)–S(2)
P(1)–Fe(1)–Fe(2)
C(1)–Fe(2)–S(1)
93.40 (10)
95.92 (8)
96.53 (8)
103.74 (2)
106.04 (2)
153.12 (2)
105.92 (9)
measured UV-Vis absorption spectra in acetonitrile
(Figure 3). The spectra of both the complexes displayed
two absorption bands at 332 (46,000), 468 (6200) and
372 (17,000), 505 (2400) nm (M−1 cm−1) for 1 and 2,
respectively. The peaks in the UV region were due to
C(1)–Fe(2)–Fe(1) 149.94 (8)
C(2)–Fe(2)–Fe(1) 96.85 (9)
C(3)–Fe(2)–Fe(1) 105.16 (10)
π
π∗
−
electronic transitions while those in the visible
C(1)–Fe(2)–S(2)
C(2)–Fe(2)–S(2)
100.07 (9)
87.91 (8)
region can be assigned as d-d transitions. The significant
similarity between the UV–Vis spectra of complexes 1
and 2 suggests that there is very little difference of the
influence of the monothiolate and phosphine ligands on
the resultant Fe(CO)6 and Fe(CO)5 cores.
C(3)–Fe(2)–S(2) 160.91 (10)
C(2)–Fe(2)–C(1) 100.59 (12)
C(3)–Fe(2)–C(1)
C(3)–Fe(2)–C(2)
98.88 (13)
90.91 (13)
3.6 Electrochemistry
The 31P { H} NMR spectrum of complex 2 displayed a
peak at 59.8 ppm due to the presence of the PPh3 ligand
at one of the iron centers. In addition, a small peak at
53.98 ppm is probably due to the presence of an isomer
for complex 2 (Figure S7, Supplementary Information).
This could be due to the axial/equatorial or axial/axial
orientation of the two furyl rings.
1
The cyclic voltammograms (CVs) for complexes 1 and
2 were measured in acetonitrile under an argon atmo-
sphere (Figure 4 and Table 4). The CVs for 1 displayed
two one-electron irreversible reduction waves (Epc
=
−1.80 and –2.53 V vs. Fc/Fc+) that can be assigned
as FeIFeI → FeIFe0 and FeIFe0 → Fe0Fe0 redox pro-
cesses (Figure 4). One-electron irreversible oxidation
for 1 was observed at 0.70 V which can be ascribed
as FeIFeI → FeIFeII. The reduction of compound 2
occurred at Epc = −1.92, and its oxidation at 0.36 V.
3.5 UV-visible absorption spectroscopy
For complexes 1 and 2, the electronic effect of the Substitution of one CO ligand in 1 with PPh3 led to
monothiolate ligands and the monodentate phosphine a more electron-rich Fe center in 2, hence leading to
ligand on the iron centers was elucidated from the more negative first reduction potential and less
Table 3. FTIR data for complexes 1 and 2.
Complex
Wavenumber/cm−1
Dichloromethane
Acetonitrile
1
2
2070 (s), 2027 (s), 1968 (br)
2041 (s), 1979 (m), 1960 (s), 1926 (s)
2069 (s), 2036 (s), 1988 (br)
2033 (s), 1980 (m), 1953 (s), 1918 (s)

126 Page 6 of 10
J. Chem. Sci. (2018) 130:126
0
-300
0
-50
1
2
-600
-100
-150
-200
-900
Increasing acid concentration
-1200
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
+
-3
-2
-1
0
E / V vs Fc/Fc
+
E / V vs Fc / Fc
Figure 4. Cyclic voltammograms for 1 mM of complexes
1 (—) and 2 (----) in acetonitrile at a scan rate of 0.1 Vs−1
.
Figure 5. Cyclic voltammograms for complex 2 (1 mM)
in CH3CN in the absence (---) and presence of increasing
amounts (6.95, 34.45, 61.53, 88.19, 133.84, 196.81, 257.78,
316.36, 372.86, 453.91 mM) of acetic acid (—) at a scan rate
of 0.1 Vs−1
.
positive oxidation for complex 2. Both the complexes
were examined as electrocatalysts for the reduction of
protons to molecular hydrogen in the presence of acetic of increasing amounts of acid. This additional peak was
and trifluoroacetic (TFA) acids. The complexes were absent for complex 2.
found to be more efficient catalysts in presence of TFA
The background current due to direct reduction of
which is a moderately strong acid in comparison to protons at the glassy carbon electrode without cata-
acetic acid. The complexes were also found to be active lyst in the presence of the two different acids were
catalysts in the presence of perchloric acid, however, the found to be negligible for potentials in the range
catalytic currents were lower and diminished with the –1.0 to –2.5 V.29 Hence, the acid-induced current in
addition of about 15 mM of acid.
the presence of catalysts can be attributed to cat-
CVs of 1 and 2 in the presence of acetic acid showed alytic turnover 1 and 2 showed comparable electrocat-
new peaks at −2.33 and −2.18 V, respectively, vs. alytic activity. The catalytic currents were, however,
Fc/Fc+ which shifted to more negative potentials with much higher than the recently reported complexes
an increase in the amount of acid (Figure 5 and Fig- [Fe2(μ-p-toluenethiolate)2(CO)6],
[Fe2(μ-CH2Ph)2
9b
α
ure S8, Supplementary Information). The increase of (CO)6]and[μ4-S(μ2-( -toluenethiolate)Fe2(CO)6)2].
current at this reduction potential with the increase in Also the electrocatalytic proton reduction for these
the amount of acid can be attributed to the reduction reported complexes was studied in dichloromethane.
of protons to molecular hydrogen.28 For complex 1, an The CVs for 1 in the presence of TFA initially dis-
additional reduction peak was observed at –1.48 V the played a peak at –1.47 V (Figure S9, Supplementary
peak current for which did not increase with the addition Information). On addition of 48 mM of acid two
Table 4. Electrochemical data for complexes 1 and 2 in acetonitrile.
Complex
Epc/V
E
cat/V AcOH
E
cat/V TFA
Overpotential/V
AcOH
TFA
1
−1.80
−2.53
−1.48
−1.86
−1.92
−1.44
−1.33
−1.49
−2.33
−1.96
−1.47
−0.87
−0.58
a,b
α
[μ4-S(μ2-( -toluenethiolate)Fe2(CO)6)2]
–
–
–
2
−2.18
−2.26
−2.00
−1.97
−1.70
−0.72
−0.80
−0.54
−0.51
−0.81
[Fe2(μ-SPh)2(CO)6]c
–
–
–
–
–
–
d
[Fe2(μ-napthalene-2-thiolate)2(CO)6]
d
[Fe2(μ-napthalene-2-thiolate)2(CO)5PPh3]
aIn CH2Cl2 vs. Ag/AgCl; bRef.9b; cRef.11; dRef.9a

J. Chem. Sci. (2018) 130:126
Page 7 of 10 126
0
25
20
15
10
5
-300
-600
-900
Increasing acid concentration
0
-1200
-3
-2
-1
0
100
200
300
400
500
+
E / V vs Fc / Fc
Acid concentration (mM)
Figure 6. Cyclic voltammograms for complex 2 (1 mM)
in CH3CN in the absence (---) and presence of increasing
amounts of TFA (4.76, 10.61, 15.20, 23.08, 33.96, 44.44,
Figure 7. Plot of icat/ip vs. acid concentration for com-
plexes 1 (ꢀ) and 2 (ꢁ) (1 mM) in presence of acetic acid
(black); and for the second reduction peak of complexes 1 ( )
and 2 ( ) (1 mM) in presence of TFA (blue) at a scan rate of
54.54, 64.28, 78.26 mM) (—) at a scan rate of 0.1 Vs−1
.
0.1 Vs−1
.
peaks were observed at –1.56 and –1.71 V. These two
peaks shifted cathodically and the current height for
all the peaks increased with increase in the amount
2, it was observed that kobs increased linearly with acid
concentration, which suggests a first-order dependence
of the catalytic rate on acid concentration (Figures S11–
S12, Supplementary Information).39–42
α
of acid. A tetra-nuclear iron complex [μ4-S(μ2-( -
toluenethiolate)2Fe2(CO)6] similar to complex 1 has
been reported recently as a catalyst for proton reduction
in dichloromethane (Table 4).9b A few other tetranuclear
complexes reported earlier with di- and tetra-thiolate
linkers displayed lower catalytic currents.30–38 How-
ever, the catalytic currents for the reported complex
The catalytic efficiency (C.E.) for complexes 1
and 2 in AcOH and TFA was calculated using the
method defined by Felton and co-workers,28 C.E. =
(icat/id)/(CHA/Ccat)(icat = catalytic current, id = cur-
α
[μ4-S(μ2-( -toluenethiolate)2Fe2(CO)6] were lower
rent for reduction of catalyst in absence of acid, CHA
=
than complex 1.9b On the other hand, complex 2 dis-
played a peak at –1.70 V on the addition of TFA. A
second peak was observed at –1.43 V, after addition
of about 10 mM of acid (Figure 6). These two peaks
shifted cathodically and the current height for all the
peaks increased with increase in the amount of acid.
The plots of icat/ip vs. acid concentration for complexes
1 and 2 in presence of two different acids are shown in
Figure 7 (Figure S10, Supplementary Information). The
overpotential for 2 in acetic acid was found to be lower
than those of complexes 1 and [Fe2(μ-SPh)2(CO)6]
while it was higher than complex 1 in presence of TFA
(Table 4).11,28 The catalytic rate of proton reduction was
investigated by using the following equation:
concentration of acid, Ccat = concentration of catalyst).
The values of C.E. of 1 and 2 vary in the range 0.34
to 0.11 (for 1) and 0.25 to 0.19 (for 2) in acetic acid,
and 0.77 to 0.54 (for 1) and 0.71 to 0.51 (for 2) in TFA,
decreasing with increasing acid concentration.
4. Conclusions
In summary, a new diiron carbonyl complex 2 was
prepared from the tetranuclear precursor complex 1
using the monodentate phosphine ligand (PPh3). The
electrochemical investigations of complexes 1 and 2
were performed in acetonitrile in the presence of acetic
acid and TFA. Complexes 1 and 2 were found to show
comparable catalytic activity for proton reduction to
ꢀ
icat
ip
n
RT kobs
=
0.446
Fv
where icat is the catalytic current, ip is the peak dihydrogen. The reduction potential for complex 1 in
current measured in the absence of acid, n is the num- the presence of TFA appeared at more negative poten-
ber of electrons involved in the catalytic reaction, kobs tial than that observed for 2. However, in the presence
is the observed first-order rate constant, R is the ideal of acetic acid, the reduction potentials were compara-
gas constant, T is the temperature in Kelvin, F is Fara- ble. Also, the complexes were more efficient catalysts
ν
day’s constant, is the scan rate. For complexes 1 and in presence of stronger acids.

126 Page 8 of 10
J. Chem. Sci. (2018) 130:126
Supplementary Information (SI)
synthetic models: the role of metal hydrides Chem. Rev.
116 8693; (c) Li Y and Rauchfuss T B 2016 Synthesis
of diiron(I) dithiolato carbonyl complexes Chem. Rev.
116 7043; (d) Wang N, Wang M, Chen L and Sun L
2013 Reactions of [FeFe]-hydrogenase models involv-
ing the formation of hydrides related to proton reduction
and hydrogen oxidation Dalton Trans. 42 12059; (e)
Tschierlei S, Ott S and Lomoth R 2011 Spectroscopically
characterized intermediates of catalytic H2 formation
by [FeFe] hydrogenase models Energy Environ. Sci. 4
2340; (f) Simmons T R, Berggren G, Bacchi M, Fonte-
cave M and Artero V 2014 Mimicking hydrogenases:
from biomimetics to artificial enzymes Coord. Chem.
Rev. 270–271 127; (g) Zhao P-H, Ma Z-Y, Hu M-Y,
He J, Wang Y-Z, Jing X-B, Chen H-Y Wang Z and Li
Y-L 2018 PNP-chelated and -bridged diiron dithiolate
complexes Fe2(μ-pdt)(CO)4{(Ph2P)2NR} together with
related monophosphine complexes for the [2Fe]H subsite
of [FeFe]-hydrogenases: preparation, structure, and elec-
trocatalysis Organometallics 37 1280; (h) Rauchfuss T
B 2015 Diiron azadithiolates as models for the [FeFe]-
hydrogenase active site and paradigm for the role of the
second coordination sphere Acc. Chem. Res. 48 2107;
(i) Berggren G, Adamska A, Lambertz C, Simmons T
R, Esselborn J, Atta M, Gambarelli S, Mouesca J M,
Reijerse E, Lubitz W, Happe T, Artero V and Fonte-
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[FeFe]-hydrogenases Nature 499 66; (j) Weber K, Wey-
hermüller T, Bill E, Erdem Ö F and Lubitz W 2015
Design and characterization of phosphine iron hydrides:
toward hydrogen-producing catalysts Inorg. Chem. 54
6928
The supplementary information includes X-ray, FTIR, NMR
and electrochemical data. CCDC reference numbers 1519040
(for 1) and 1442928 (for 2) contain the supplementary crystal-
lographic data for this paper. Copies of this information are
available on request at free of charge from CCDC, Union
Road, Cambridge, CB21EZ, UK (fax: +44-1223-336-033;
E-mail: deposit@ccdc.ac.uk or http://www.ccdc.cam.ac.uk).
Supplementary Information is available at www.ias.ac.in/
chemsci.
Acknowledgements
Financial support from the Department of Science &
Technology (DST), India (SR/S1/IC-28/2011) is gratefully
acknowledged. SKG is thankful to the University of Delhi,
India for providing R&D grant. MN is grateful to Coun-
cil of Scientific & Industrial Research (CSIR) and VK and
NK are grateful to University Grant Commissions (UGC) for
fellowship.
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