Structural and HER studies of diphosphine-monothiolate complexes [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)] and [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-DPEPhos)]

Article abstract of DOI:10.1016/j.ica.2019.119227

Two new compounds [Fe₂(CO)₄(μ-naphthalene-2-thiolate)₂(μ-dppe)] 1 and [Fe₂(CO)₄(μ-naphthalene-2-thiolate)₂(μ-DPEPhos)] 2 with chelating phosphine ligands were synthesized and characterized

Full text of DOI:10.1016/j.ica.2019.119227

Journal Pre-proofs
Structural and HER Studies of Diphosphine-Monothiolate Complexes
[Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)] and [Fe2(CO)4(μ-naphtha-
lene-2-thiolate)2(μ-DPEPhos)]
Hemlata Faujdar, Anke Spannenberg, Sandeep Kaur-Ghumaan
PII:
S0020-1693(19)30769-8
https://doi.org/10.1016/j.ica.2019.119227
ICA 119227
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
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17 October 2019
23 October 2019
Please cite this article as: H. Faujdar, A. Spannenberg, S. Kaur-Ghumaan, Structural and HER Studies of
Diphosphine-Monothiolate Complexes [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)] and [Fe2(CO)4(μ-
naphthalene-2-thiolate)2(μ-DPEPhos)], Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.
2019.119227
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Structural and HER Studies of Diphosphine-Monothiolate
Complexes [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)] and
[Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-DPEPhos)]
Hemlata Faujdar,a Anke Spannenbergb and Sandeep Kaur-Ghumaana,b*
aDepartment of Chemistry, University of Delhi, Delhi – 110007, India
Email: skaur@chemistry.du.ac.in
bLeibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a,
18059 Rostock, Germany

Abstract
Two new compounds [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)]
1 and [Fe2(CO)4(μ-
naphthalene-2-thiolate)2(μ-DPEPhos)]
synthesized and characterized dppe = 1,2-Bis(diphenylphosphino)ethane and DPEPhos =
(Oxydi-2, 1-phenylene)bis(diphenylphosphine) . The bridging coordination mode of the
diphosphine ligand in complex was confirmed by X-ray crystallography. Complexes
and
2 with chelating phosphine ligands were


1
1
2
2
were further evaluated as catalysts for the hydrogen evolution reaction (HER) by
electrochemical investigations. Complex
degraded in presence of acids.
1 showed better stability whereas complex
Keywords: Diphosphine • Electrocatalysis • Hydrogenases • Iron • Proton reduction

1. Introduction
The reversible processes between protons/electrons and molecular hydrogen are known to
be catalyzed by the hydrogenase (H2ase) enzymes [1]. Among the hydrogenase enzymes
known ([NiFe], [FeFe] and Hmd hydrogenase), the hydrogen production capability of the
[FeFe]-H2ase enzyme is 6000-9000 molecules of hydrogen per second at 30 °C and this is
greater than the other two hydrogenases [2, 3]. Inspired by the [Fe2S2] core known to be
present in the structure of the [FeFe]-H2ase active site [4−8], chemists have carried out
intensive studies on aliphatic and aromatic dithiolate-bridged complexes over the past few
decades [9]. Examples reported so far include all carbonyl iron complexes and complexes
in which one or more of the CO ligands have been replaced with electron-donating
ligands. Substitution with basic ligand(s) has been related to the creation of an electron-
rich site for the coordination of the proton(s), which is considered to be an essential step
in the electrocatalytic reduction of protons to H2. Among the electron-donating ligands,
the phosphines have received special attention as they are considered to be good
substitutes for the CN ligands present in the enzyme active site [10].
A number of diiron complexes with bidentate phosphines have been studied as structural
and/or functional mimics [10, 11, 12] since the pioneering work by Tye, Hall and
Darensbourg suggesting that creating asymmetry at the diiron centers in the models was
preferable [11b]. The bidentate phosphines can however, bind in the chelating or bridging
mode depending on the nature of the diphosphine backbone and reaction conditions [11d].
The chelate coordination mode of the phosphines can lead to the formation of different
isomers (dibasal and apical-basal) for the [FeFe] complexes while the bridging mode is
known to form only the cisoid-dibasal isomer [13].

Though examples of complexes with dithiolates are known for the bridging coordination
mode of the diphosphines [10, 14], complexes [Fe2(μ-SC6F5)2(µ-dppm)(CO)4] a′ [15],
[Fe2(μ-SR)2(µ-dppm)(CO)4] (R= (CH2)3OH, b'; = SMe, c'; = Ph, d'; = p-tol, e') (dppm =
bis(diphenylphosphino)methane) [12a, 16] with monothiolate ligands have been reported
only as structural mimics (Figure 1). The solely studied complexes are either the all
carbonyl or the monodentate phosphine substituted bis(monothiolate) Fe complexes
[15,17].
Figure 1. Selected thiolate-bridged complexes reported with bridging coordination mode
of diphosphines (a-q studied for HER; a′-e′ are only structural mimics).
Hence, we were interested in studying the reaction of the monothiolate-bridged starting
complex [Fe2(CO)6(μ-naphthalene-2-thiolate)2]
dppe [1,2-bis(diphenylphosphino)ethane]
phenylene)bis(diphenylphosphine)]. The purpose was to study the influence of two
A
with two different diphosphine ligands
and DPEPhos [(Oxydi-2, 1-

different diphosphines (with different bite angles) in forming chelating or bridging
phosphine complexes with monothiolates. Also, complex was designed to explore the
2
possibility of the ‘O’ heteroatom in the DPEPhos ligand to act as a pendant site for proton
attachment during the catalytic process. Though a number of examples of dithiolate-
bridged complexes are known with ligands (mostly -NR) acting as proton relay there is
hardly any report with Fe-monothiolates [13, 18]. The two new complexes [Fe2(CO)4(μ-
naphthalene-2-thiolate)2(μ-dppe)]
1
and
[Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-
DPEPhos)] thus, obtained were further studied as catalysts for the reduction of protons
2
to hydrogen using acetic acid (AA) and trifluoroacetic acid (TFA).
2. Results and Discussion
2.1. Preparation and structural characterization
Complex [Fe2(CO)6(μ-naphthalene-2-thiolate)2]
A
was prepared as reported in the
literature [17f]. During the synthesis of
A, a violet-colored triiron cluster
[Fe3(µ3
formed in higher yields when the reaction was carried out in tetrahydrofuran (THF) but in
toluene only was obtained. Reaction of complex under argon atmosphere with the
chelating diphosphine ligands, dppe in toluene (reflux, 7-8 h) and DPEPhos in acetonitrile
(stirring at room temperature, 8-10 h) led to the production of two new compounds and
, respectively (Scheme 1).The complexes were purified by column/flash chromatography
and re-crystallized from dichloromethane-hexane solution leading to orange-red and red
products, respectively. Complex was obtained in low yields when the reaction was
carried out in refluxing toluene without Me3NO.2H2O or in acetonitrile with
S)2(CO)9] was obtained as a byproduct, also reported earlier [19]. The byproduct
A
A
1
2
1

Me3NO.2H2O. In contrast, the yields for complex
2 were low on carrying out the reaction
in refluxing toluene. The bridging phosphine complex was always the product
irrespective of the reaction conditions used.
Characterization of the new complexes
1
and
2
was carried out by Mass, NMR, FTIR and
and
UV-Vis spectroscopic techniques. The FTIR spectra in CH2Cl2 for complexes
1
2
displayed CO absorption vibrations (corresponding to terminal carbonyls) in the range
1
1990-1880 cm (see supporting information). The values are shifted towards lower
wavenumbers (~85 and 90 cm-1) in comparison to complex
A
(2070-2000 cm-1) (Table 1).
Scheme 1. Schematic diagram for the synthesis of targeted complexes
1 and 2.
Moreover, the CO bands are within the range reported for comparable complexes (Table
1) [10]. The greater shift in the CO frequencies in comparison to complex or other
A
reported complexes with monodentate phosphines [15,17a-i] is due to the attachment of
the diphosphine ligands in the bridging coordination mode.

Table 1. FTIR data for complexes 1 and 2 in CH2Cl2.
Wavenumber [cm−1]
Ref
[Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)] 1
1990, 1948, 1912
Present work
Present work
17f
[Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-DPEPhos)] 2
[Fe2(CO)6(µ-naphthalene-2-thiolate)2] A
[Fe2(CO)5(µ-naphthalene-2-thiolate)2(P(PhOMe-p)3)]
[Fe2(CO)4(µ-dppe)(µ-pdt)]
1985, 1945, 1921, 1887
2075, 2039, 2001
2050, 1990, 1960
17h
10
1990, 1953, 1920,1902
1995, 1965, 1932, 1915
2007, 1981, 1948, 1926
1985, 1955, 1920, 1903
14a
15
[Fe2(CO)4(µ-dppm)(µ-bdt)] n
[Fe2(μ-SC6F5)2(µ-dppm)(CO)4] a′
16a
[Fe2(μ-S(CH2)3OH)2(µ-dppm)(CO)4] b′
1
The H NMR spectrum of
1 in CDCl3 displayed peaks in the aromatic region (7.2−8.3
ppm) for the phenyl rings of dppe and naphthyl ring protons, in addition to peaks between
2.4−2.5 ppm for -CH2- protons of dppe (see supporting information). A singlet at 62.3
31
ppm in the P {1H} NMR spectrum of
1, similar to that observed for the dithiolate-
bridged complex [Fe2(CO)4(µ-dppe)(µ-pdt)] (60.5 ppm) [10] further confirmed the
binding of dppe ligand in the bridging mode. The 1H NMR spectrum of
2
displayed peaks
between 6.3
8.5 ppm for the protons of naphthalene thiol and DPEPhos ligand (see
31
supporting information). Complex
2
displayed two peaks at 62.2 and 60.3 ppm in the P
{1H} NMR spectrum, which could be due to the inequivalence of the two phosphorus
nuclei in solution resulting from the highly flexible diphosphine allowing the two ends to
have slightly different orientations with respect to the two thiolate ligands (see supporting
information) [14h, 17g] .
The UV-Vis spectra of complexes
information). The bands for and
1
and
2 were recorded in acetonitrile (see supporting
1
2
were further red-shifted in comparison to complex A

and the previously reported complex [Fe2(CO)5(µ-naphthalene-2-thiolate)2(P(PhOMe-
p)3)] [17h] because of greater electron densities at both the Fe centres (Table S1).
The low stability of complex
2 in solution was confirmed by UV measurements (see
supporting information). Further, attempts to obtain single crystals for complex
not successful.
2 were
2.2. X-ray crystal structure analysis of complex
Single crystals for complex were obtained from hexane-dichloromethane solution at low
temperature. The molecular structure of complex is illustrated in Figure 2. The
1
1
1
crystallographic details of the complex are given in Table 2. For selected bond distances
(Å) and angles (o) see supporting information.
Figure 2. Molecular structure of complex [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)]
1
. Displacement ellipsoids correspond to 30% probability and hydrogen atoms have been
omitted for clarity.

Complex 1 crystallized in the monoclinic space group P21/c with a butterfly structure of the
Fe2S2 core and the naphthyl groups in anti-position. The diphosphine ligand occupied the
cisoid basal/basal position and bridged the two Fe fragments. A torsion angle P1-Fe1-
Fe2-P2 of -4.29(4)° was observed. The distance between the two Fe centers in complex
1
was 2.5435(7) Å which is in range of the value known for the [FeFe] hydrogenase
enzyme (2.6 Å) [6] and similar to the values reported for analogous complexes (Table S3)
[10, 14f, 14g].
Table 2. Crystallographic details of complex 1.
1
Formula sum
Formula weight
Crystal system
Space group
a (Å)
C50H38Fe2O4P2S2
940.56
Monoclinic
P21/c
18.9602(4)
11.0683(3)
20.8501(5)
90
95.1797(15)
90
b (Å)
c (Å)
α (o)
β (o)
γ (o)
V (Å3)
4357.67(18)
150 (2)
4
T (K)
Z
F000
1936
1.007
1.434
32848
7566 (Rint = 0.085)
5867
493
GOF on F2

calcd (g cm−3)
No. of reflections measured
No. of independent reflections (Rint)
No. of observed reflections [ I > 2σ(I)]
No. of parameters
R1 [I > 2σ(I)]
0.0450
wR2 [all data]
0.1149

The two Fe

P bond distances (Fe1 P1 = 2.2187(10) and Fe2-P2 = 2.2098(9) Å) and the

P-Fe-Fe bond angles (P1

Fe1Fe2 = 104.19(3) and P2Fe2
Fe1 = 108.79(3)o).
However, the slight difference in the Fe-S/Fe-P bond distances and Fe-Fe-P bond angles
in complex compared to reported dppe-dithiolate complexes [Fe2(CO)4(µ-
1
SCH2N(C6H4CO2Me-p)CH2S)(µ-dppe)] [14f], [Fe2(CO)4(µ-SCH2N(iPr)CH2S)(µ-dppe)]
[14g] and [Fe2(CO)4(µ-pdt)(µ-dppe] [10] could be due to less steric hindrance in the
monothiolate complex
1 (Table S3).
2.3. Electrochemical and electrocatalytic studies of complexes
1 and 2
The cyclic voltammograms (CVs) for complexes 1 and 2 were studied in acetonitrile under argon
n
with Bu4NPF6as the supporting electrolyte (Table 3). The CVs for 1 and 2 exhibited two
irreversible one-electron reduction waves at Epc = 1.96, –2.33 and 1.74, 1.94 V, respectively
(see Figure 3 and supporting information). Oxidations peaks were observed at 0.35, 0.73 (1) and
0.36, 1.13 V (2). The reductions for complex 1 and 2 occurred at more negative potentials than A
[17f] or the monodentate phosphine substituted complexes due to removal of two CO ligands
[15, 17a, 17f-i]. Chemical steps or rearrangements might be involved as the reductions in the
absence of acid were irreversible. CVs for complex 1 were also recorded at different scan rates
1
(0.0251 Vs ). Linear plots for ip (current of the reduction peak) vs. square root of scan rate
indicate that the reduction processes were diffusion-controlled (see supporting information) [20,
21]. Complexes 1 and 2 were further probed as electrocatalysts for the reduction of protons to
molecular hydrogen in the presence of acids of varying strengths acetic acid (AA) and
trifluoroacetic acid (TFA) (Figures 4 and 5) (Table 3).

Figure 3.
Cyclic voltammogram for complex 1 (0.638 mM) in acetonitrile (scan rate 0.1
Vs−1).
With the addition of AA complex 1 showed two new peaks at ‒1.88 and ‒2.28 Vat lower acid
concentrations (Figure 4). The first peak is anodically shifted while the second peak shifted
cathodically with increase in current height on subsequent addition of acid. The current height
for the first peak at -1.88 V did not increase on further acid additions indicating the formation of
an intermediate. The currents stabilized with the addition of upto101 mM of acetic acid (AA).
Electrochemical investigations for complex 1 with TFA showed different results (Figures S9 and
5). On initial addition of acid (1.2 mM) a new peak appeared at -1.74 V (Figure S9). With further
acid additions, the complex reduction peaks disappeared with two new peaks (–1.82 and –2.05
V) on addition of 17 mM of acid (Figure 5). At higher acid concentrations, the peaks shifted
cathodically, and levelled off with 78.26 mM of acid. Furthermore, the peak-type catalytic
currents and the appearance of two catalytic peaks have been associated with disturbances in the
diffusion layer and homoconjugation processes involving TFA [22].

0
-100
-200
-300
-400
-500
-600
-700
Increasing acid concentration
-2.5
-2.0
-1.5
-1.0
-0.5
E / V / Fc / Fc+
Figure 4. Cyclic voltammograms for complex 1 (0.638 mM) (- - -) and complex 1 with AA (—)
(6.95, 13.86, 20.75, 27.62, 34.45, 48.05 mM) at 0.1 Vs−1.
In comparison to complex 1, complex 2 did not show catalysis with AA but showed small
current increase on adding upto 23 mM TFA. Hence, detailed catalytic studies were not
performed for complex 2.
Table 3. Electrochemical data of complexes 1, 2 and similar reported complexes in CH3CN
(scan rate, 0.1 Vs-1).
a
Complex
HER
Epc/ V
Epa/ V
Acid
AA
Ecat/ V
Ref
OP /
V
[Fe2(CO)4(μ-naphthalene-2-
thiolate)2(μ-dppe)] 1
Active
-1.96
-2.33
0.35
0.73
-2.28
-0.82
Present
work
Active
-
-
-
TFA
-
-1.98
-
-1.09
-
Present
work
-1.74
-1.94
[Fe2(CO)4(μ-naphthalene-2-
thiolate)2(μ-DPEPhos)] 2
0.36
1.13
Present
work
[Fe2(CO)6(µ-naphthalene-2-thiolate)2]
A
Active
Active
- 1.33
0.61
AA
- 2.00
-0.54
17f
[Fe2(CO)5(µ-naphthalene-2-
thiolate)2(P(PhOMe-p)3)]
-1.66
-2.37
0.29
1.00
AA
-2.02
-0.56
17h
aOverpotential ()

Also curve crossing was seen for the reverse scans that can be related to plausible
electrodeposition and nucleation of the complex on the glassy carbon surface, however, it may
not always be the case [23]. CVs measured for only solvent, solvent with acid and solvent with
acid and complexes indicate that the enhanced currents were due to catalysis (see supporting
information).
The plots of icat/ip vs. [acid] for complex 1 with AA and TFA are shown in supporting
information. For the icat vs. [acid] and icat vs. scan rate plots see supporting information. The
linear icat vs. [catalyst] plot indicates first order reaction with respect to [catalyst] (see supporting
information). The catalytic efficiency (C.E.) for complex 1 in AA (0.55 to 0.21) and TFA (0.58
to 0.34) was determined using the formula described by Lichtenberger et. al. [24]. TOFs (s-1) for
complex 1 were calculated using the icat/ip values at the maximum [acid] used (78 mM TFA,
icat/ip = 42.6, TOF = 352 and 101 mM AA, icat/ip = 35.7, TOF = 247) [25].
0
-200
-400
-600
-800
Increasing acid concentration
-1000
-1200
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
E / V vs. Fc/Fc+
Figure 5. Cyclic voltammograms for complex 1 (0.638 mM) (- - -) and complex 1 with TFA (—
) (1.2, 2.39, 3.58, 4.76, 5.94, 7.11, 8.28, 10.61, 17.48, 23.08, 28.57, 33.96, 44.44, 54.54, 64.28,
78.26 mM) at 0.1 Vs−1. For clarity the reverse scans have been neglected.

Since the catalytic peaks with acetic acid are more negative than the neutral complex reduction
peaks, the first step in the catalytic cycle is assumed to be reduction (electrochemical, E) of the
neutral complex. This is followed by the protonation step (chemical, C) leading to a FeH
intermediate. The second subsequent reduction (electrochemical, E) and protonation (chemical,
C) is followed by hydrogen elimination [14k]. With TFA, the initial anodic shift of the peaks in
comparison to the complex reduction peaks indicates a chemical first step (formation of hydride)
[9c, 14c]. Also, since the current becomes independent after addition of certain amounts of acid,
it can be assumed that elimination of hydrogen is the rate-limiting step [26]. The mechanism for
HER has been speculated based on previously reported systems [9c, 14c]. The catalytic currents
observed for complex 1 were higher than known examples [14b, 14m, 17h], indicating that the
bridging dppe ligand provided additional stability to the complex.
3.
Experimental Section
3.1. General Procedure
All the reactions were carried out under an oxygen-free argon environment using Schlenk
techniques unless otherwise specified. Complex [Fe2(CO)6(μ-naphthalene-2-thiolate)2] A was
prepared according to the procedure reported in literature[17f]. All the anhydrous solvents
(dichloromethane, acetonitrile, toluene), deuterated solvent (CDCl3), n-Bu4NPF6 and starting
1
materials were obtained from Sigma–Aldrich and used without further purification. The H and
31P NMR spectra were recorded on a JEOL 400 MHz NMR Spectrometer at room temperature in
CDCl3. The FTIR spectra of the samples were recorded on a Perkin-Elmer FTIR Spectrometer in
CH2Cl2 over the range 400-4000 cm–1, the UV-Vis spectra on an Avantes spectrometer system

(diode-array spectrometer) and the elemental analysis was carried out with a Perkin-Elmer 240 C
elemental analyzer.
3.2. Synthesis of [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)] 1
Complex A (200 mg, 0.334 mmol), dppe (133.20 mg, 0.334 mmol) and trimethylamine N-oxide
dihydrate (Me3NO.2H2O) (74.24 mg, 0.668 mmol) were refluxed in toluene (argon) for 7-8 h.
The solvent was then removed by rotary evaporation. The residue was purified using a silica gel
column. Elution with a mixture of hexane/dichloromethane (6:4 v/v) gave a light orange-red
colored solution. Complex 1 was obtained as an air-stable light orange-red powder (65 mg, 20
1
% yield) on removing the solvent. H NMR (400 MHz, CDCl3, 298 K): δ = 7.2−8.2 (m, 34H,
31
aromatic protons), 2.45 ppm (m, 4H, PCH2CH2P). P NMR (161.8 MHz, CDCl3, 298 K): δ=
62.3 ppm (s, dppe). FTIR (CH2Cl2): ṽ = 1990 (s), 1948 (s) and 1912 (s) cm–1. C50H38Fe2O4P2S2
(940.6 gmol−1): Calcd. C 63.85, H 4.07, S 6.82; found C 63.57, H 4.13, S 6.78.ES-MS: m/z 941
[M+ H] (calcd 941).
3.3. Synthesis of[Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-DPEPhos)] 2
Precursor complex A (200 mg, 0.334 mmol), phosphine ligand DPEPhos (180.05 mg, 0.334
mmol) and decarbonylating agent trimethylamine N-oxide dihydrate (Me3NO.2H2O) (74.24 mg,
0.668 mmol) were stirredin acetonitrile at room temperature(argon) for 8-10 h. The solvent was
then removed by rotary evaporation and the residue was purified using a silica gel column by
flash chromatography. Elution with a mixture of hexane/dichloromethane (3:7 v/v) gave a red-
colored solution. Complex 2 was obtained as a light-red powder (60 mg, 17 % yield) after
removal of solvent. 1H NMR (400 MHz, CDCl3, 298 K): δ = 6.3-8.5 (m, 42H, aromatic protons)

ppm. 31P NMR (161.8 MHz, CDCl3, 298 K): δ = 62.2 and 60.3 ppm (DPEPhos). FTIR (CH2Cl2):
ṽ = 1985 (s), 1945 (s), 1921 (s) and 1887 (s) cm–1.C60H42Fe2O5P2S2 (1080.74 gmol−1): Calcd. C
66.68, H 3.92, S 5.93; found C 66.52, H 4.25, S 5.87.
3.4. X-ray crystallography
Slow evaporation of hexane/dichloromethane solutions at low temperature resulted in single
crystals for1. Diffraction data were collected on a Bruker Kappa APEX II Duo diffractometer
using CuKα radiation. The structure was solved by direct methods (SHELXS-97) [27a] and
refined by full-matrix least-squares procedures on F2 (SHELXL-2014) [27b]. XP (Bruker AXS)
was used for graphical representation. CCDC 1904985 contains the supplementary
crystallographic data for this paper. These data are provided free of charge by The Cambridge
Crystallographic Data Centre.
3.5. Electrochemistry
All
the
electrochemical
measurements
were
carried
out
in
CH3CN.
Tetrabutylammoniumhexafluorophosphate (0.1 M) (n-Bu4NPF6) was used as the supporting
electrolyte. Cyclic voltammetry was performed using an Autolab potentiostat with a GPES
electrochemical interface (Eco Chemie). A threeelectrode cell was used with glassy carbon disc
(diameter 3 mm, freshly polished) as working electrode, platinum wire as the counter electrode
and a non-aqueous Ag/Ag+ electrode for reference (CH Instruments, 0.010 M AgNO3 in
acetonitrile). All the potentials were calibrated with the ferrocene-ferrocenium couple (Fc/Fc+);
ferrocene was added as an internal standard at the end of the experiments.

4.
Conclusions
The reaction of the monothiolate-bridged precursor [Fe2(CO)6(µ-naphthalene-2-thiolato)2] A has
been investigated with diphosphine ligands. Previously, Schollhammer has shown that the
chelating phosphine complex [Fe2(CO)4(2-dppe)(μ-pdt)] is formed upon reaction of
[Fe2(CO)6(μ-pdt)] and dppe in acetonitrile in the presence of decarbonylating agent
Me3NO.2H2O [11d,f]. On the other hand, Hogarth reported that in refluxing toluene in the
absence of Me3NO. 2H2O, the bridging phosphine complex [Fe2(CO)4(μ-dppe)(μ-pdt)] is the
major product [14h, 11f]. However, in the present work, we have shown that the formation of the
bridged phosphine complexes 1 and 2 was independent of the reaction conditions used (refluxing
toluene with or without Me3NO.2H2O or acetonitrile/Me3NO.2H2O/RT). Complexes with the
phosphines binding through one phosphorus atom or in the chelating mode were not obtained
contrary to the earlier reported examples with dithiolates [11(d,f),12b,14(a-d, g, l-m, p), 17]. The
reduction potentials for neutral 1 and 2 were less negative in comparison to their reported
dithiolate counterparts [14a-c, g, i, k-n]. The catalytic properties of both the complexes were
studied in acetonitrile in the presence of acids. Complex 1 successfully electrocatalyzed the
reduction of acids to dihydrogen with low overpotentials [0.82 (AA) and 1.09 (TFA) V]. Also,
high icat/ip values for complex 1 suggest faster catalytic processes [14m, 25, 28]. As 2 is unstable
in solution in air we have been unable to access the HER activity. Based on the high catalytic
currents and icat/ip value obtained for complex 1, it would be interesting to expand this study in
future by designing bis(monothiolate) complexes with PNP-type diphosphine ligands.
Acknowledgements
We thank the Council of Scientific & Industrial Research (CSIR), India (01(2957)/18/EMR-II),
New Delhi for financial support. SK-G is thankful to University Science Instrumentation Center

(USIC), University of Delhi for providing the research facilities. HF is grateful to the University
Grants Commission (UGC) for Research Fellowship.
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Highlights:
. Design, synthesis and characterization of diphosphine-bridged
monothiolate {FeFe}-complexes [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-
dppe)] 1 and [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-DPEPhos)] 2.
. X-ray crystal structure analysis for complex 1.
. Studies on homogeneous electrocatalysis by the {FeFe}-monothiolate
complexes in presence of acids.

Structural and HER Studies of Diphosphine-Monothiolate
Complexes [Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-dppe)] and
[Fe2(CO)4(μ-naphthalene-2-thiolate)2(μ-DPEPhos)]
Hemlata Faujdar,a Anke Spannenbergb and Sandeep Kaur-Ghumaana,b*
aDepartment of Chemistry, University of Delhi, Delhi – 110007, India
Email: skaur@chemistry.du.ac.in
bLeibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a,
18059 Rostock, Germany
Diphosphine-bridged iron carbonyl complex 1 has been reported as bioinspired mimic of the
[FeFe] hydrogenase enzyme. The new complex 1 with bis(monothiolate) linker assisted in
electrocatalytic proton reduction with acetic and trifluoroacetic acid in CH3CN.

Declaration of interests
☐ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: