Electrochemical Proton Reduction Catalyzed by [Fe2(CO)6(μ-(TeCH2Te))] Model that Mimics the Structure of the Active Site of [FeFe]-Hydrogenase

Article abstract of DOI:10.1002/zaac.201800070

In this report we discuss the synthesis of [Fe₂(CO)₆(μ-(TeCH₂Te))] (2) using Fe₃(CO)₁₂ and Me₂Sn(CH₂I)₂ in the presence of potassium tellurocyanate, freshly prepared. The resulting complex was characterized by different spectroscopic methods (¹H, ¹³C(¹H) and ¹²⁵Te(¹H) NMR) as well as X-ray diffraction analysis. Moreover, we investigate the redox properties and the catalytic behavior of 2 in the presence of weak and moderate acids, acetic acid (AcOH) and trifluoracetic acid (TFA), respectively, as a source of protons.

Full text of DOI:10.1002/zaac.201800070

Title: Electrochemical Proton Reduction Catalyzed by [Fe2(CO)6{µ-
TeCH2Te)}] Model that Mimics the Structure of the Active Site of
FeFe]-Hydrogense
(
[
Authors: Wolfgang Weigand, Hassan Abul-Futouh, and Helmar Goerls
This manuscript has been accepted after peer review and appears as an
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.201800070
Link to VoR: http://dx.doi.org/10.1002/zaac.201800070

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.201800070
Electrochemical Proton Reduction Catalyzed by [Fe
2
(CO) {µ-
6
(TeCH
2
Te)}] Model that Mimics the Structure of the Active Site of
[FeFe]-Hydrogense
Hassan Abul-Futouh, Helmar Görls and Wolfgang Weigand*
Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena,
Humboldt Str. 8, 07743 Jena, Germany. E-mail: wolfgang.weigand@uni-jena.de
Abstract
In this report we discuss the synthesis of [Fe
Fe (CO)12 and Me Sn(CH I) in the presence of potassium tellurocyanate, freshly
prepared. The resulting complex was characterized by different spectroscopic methods
2
(CO)
6
{µ-(TeCH Te)}] (2) using
2
3
2
2 2
(1
H, C{ H} and Te{ H} NMR) as well as X-ray diffraction analysis. Moreover, we
13
1
125
1
investigate the redox properties and the catalytic behaviour of 2 in the presence of weak
and moderate acids, Acetic acid (AcOH) and trifluoracetic acid (TFA), respectively, as
a source of protons.
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Introduction
The development of hydrogen as an energy resource is of significance for a
competitively sustainable economy.[1a] Nature has developed highly effective enzyme
called [FeFe]-hydrogenase that catalyze the reversible reduction of protons to
molecular hydrogen at an organometallic active site called the H-cluster (Figure 1).[1b]
The latter consists of an [Fe
butterfly [Fe ] subcluster. The [Fe
well as biologically unusual CO and CN ligands.
4
S
4
] cubane attached through a cysteinyl residue to a
2
S
2
2 2
S ] unit features a bridging azadithiolato ligand as
–
[2-5]
Several model complexes
mimicking the active site of the [FeFe]-hydrogenase have been synthesized,
characterized and their catalytic applications have been studied in detail.[6-13] In two
recent papers we described the influence of the presence of the Sn atom in µ-
(
XCH
2
)
2
SnMe
(X = S or Se) linker of the synthetic H-cluster mimics (Figure 1).[14,15]
2
Figure 1. The active H-cluster site of [FeFe]-hydrogenase (left) and synthetic model of
the active site of [FeFe]-hydrogenase (right).
As a result, it has been found that the electron density of the µ-S atoms (and hence that
of Fe-Fe bond) is increased and that protonation of the Fe-Fe bond using the moderately
strong acid CF
efforts in this field, the present work aims to synthesize [Fe
1), through the reaction of in situ generated Me Sn(CH
Unfortunately, complex 1 is not obtained in this reaction and instead we got
Fe (CO) {µ-(TeCH Te)}] (2) as the main product. Indeed, complex 2 was previously
reported by Mathur and co-workers, which they could obtain complex 2 via the reaction
of [Fe (CO) {µ-(Te
)}] and diazomethane.[16] However, the electrochemistry of
3
CO
2
H can be accomplished in the case of X = Se. In continuation of our
(CO) {µ-(TeCH SnMe }]
TeCN) with Fe (CO)12.
2
6
2
)
2
2
(
2
2
2
3
[
2
6
2
2
6
2
complex 2 and its reactivity toward protons have not yet been reported. In this
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contribution, we herein characterize complex 2 by employing a variety of analytical
techniques (NMR spectroscopy, elemental analysis, IR spectroscopy, mass
spectrometry and X-ray structure determination). Furthermore, a detailed
electrochemical study of complex 2 in the absence and presence of a weak acid (AcOH)
as well as a moderately stronger acid (TFA) as proton sources has been reported.
Results and Discussion
Synthesis and Characterization
The in situ prepared dimethylbis(tellurocyanatometyl)stannane was obtained via the
[17]
reaction of Me
prepared by heating of tellurium and potassium cyanide in refluxing dimethyl sulfoxide.
The latter solution was then added to a solution of Fe (CO)12 dissolved in THF and
2
Sn(CH
2
I)
2
with twofold excess of potassium tellurocyanate, freshly
3
refluxed for two hours followed by subsequent column chromatography to afford
complex 2 as a major product instead of complex 1 as shown in Scheme 1. In fact, the
reason of not obtaining complex 1 could be explained due to the bigger size of Te atoms
compared to that of S and Se atoms in their corresponding analogues.[14,15] Furthermore,
the steric hindrance or the tension of forming the moiety that contains a Sn atom
together with two Te atoms could avoid the formation of complex 1. Complex 2 was
1
13
1
125
1
characterized by spectroscopic methods ( H NMR, C{ H} NMR, Te{ H} NMR
and IR), mass spectrometry, elemental analysis, and X-ray structure determination. The
mass spectrum of complex 2 shows the parent ion peak at m/z 549 and the consecutive
loss of six CO ligands.
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Scheme 1. Synthetic pathway to complex 2.
In order to understand the influence of the bridging chalcogen atoms on the electronic
properties of complex 2 and its Se/S analogues, we have made comparisons with some
of their spectral as shown in Table 1. As can be seen in Table 1, The IR spectra of
complex 2 and its Se/S analogues exhibited five absorption bands in the ranges of 2054-
-1
1
960, 2071-1980 and 2079-1988 cm , respectively. The increase of these CO stretching
frequencies from complex 2 to its Se/S analogues is obviously due to the decreased π
back-bonding between their Fe atoms and the attached CO groups, caused by the
1
decreasing donor ability from Te to Se/S. The H NMR spectra of complex 2 and its
Se/S analogues showed one singlet peak at δ = 2.20 ppm with 125Te satellites (2J(TeH)
0 Hz) for complex 2, while those for Se/S analogues at δ = 4.02 and 4.90 ppm,
respectively, which are assigned to protons of the methylene moiety. Moreover, the
=
2
1
3
1
C{ H} NMR spectrum of complex 2 displays a signal at δ = 211.8 ppm for the CO
ligands in the Fe(CO)
3
moiety as well as a signal at δ = 22.5 ppm which is assigned to
the methylene carbon atom. The 125Te{ H} NMR spectrum shows one signal for the
1
chemically equivalent Te atoms at δ = 337.9 ppm.
Table 1. Some spectral data of 2 and its Se/S analogues.
-
1
1H NMR (ppm)
(CH
IR (cm )
[Fe
2
(CO)
6
{µ-(XCH X)}]
2
(ν
C≡O
)
2
)
2
054(s), 2025(m),
1998(m), 1986(m),
960(w)
071(m), 2032(vs),
1999(vs), 1992(s),
980(w)
079(m), 2039(s),
008(s), 1999(s), 1988(w)
X = Te
2.20
1
2
X = Se19
X = S18
4.02
4.90
1
2
2
Molecular structure
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Single crystal suitable for X-ray diffraction study was obtained from a saturation
o
solution of 2 in pentane at -20 C. The molecular structure of complex 2 is shown in
Figure 2.
Figure 2. Molecular structure (50% probability) of complex 2.
In agreement with the previously reported diiron dithiolato and diselenolato analogues
complexes,[18,19] complex 2 exhibited the [Fe
2
2
S ] core with butterfly conformation. The
geometry around each iron atom can be best described as distorted octahedron in which
the central atom is Fe surrounded by three CO ligands in a facial fashion as well as two
chalcogen atoms that bridge both iron centres. The Fe-Fe bond length in complex 2,
(2.5921(6) Å), is slightly longer than those of diiron dithiolato and diselenolato
analogues complexes.[18,19] This is attributed to the larger van der Waals radii of Te
compared to S and Se atoms. The average Fe-CO bond length in 2, (1.7915 Å), is
comparable to those observed in the corresponding S and Se analogues. The torsion
angle, τ, formed from the intersection between (O)C-Fe-Fe-C(O) of complex 2 is ca.
3
6
3.15°, which is larger than those of corresponding S and Se analogues (9.19° and
.23°, respectively).[18,19]
Electrochemical Investigation
The electrochemical behaviour of complex 2 was investigated by cyclic voltammetry
(
CVs). Figure 3 shows the cyclic voltammetric reduction of 2 in CH
2
Cl
2
-[nBu
4
N][BF ]
4
-1
-1
at various scan rates (0.05 V s ≥ ν ≥ 10 V s ).
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Figure 3. Cyclic voltammetry of 1.0 mM solution of complex 2 in CH
2
Cl -
2
-1
-1
-1
-1
[
nBu
4
N][BF
4
] at (a) 0.05 V s ≥ ν ≥ 1.0 V s and (b) 1.0 V s ≥ ν ≥ 10.0 V s . Glassy
+
carbon disk (d = 1.6 mm). The potential E are given in V and referenced to Fc /Fc
couple. The arrows indicate the scan direction.
On initiating the electrochemical scan in the cathodic direction at a scan rate of 0.2 V
-1
s , a quasi-reversible reduction event at E1/2 = -1.67 V was observed. Increasing the
-1
scan rate of 2 up to 10 V s results in no wave-splitting for the reduction peak of 2
(Figure 3). This reduction peak of complex 2 is due to two-electron reduction in a single
step based on the study of the current function, [I
/c·ν1/2 = (2.69 x 10 )·A·D ·n ,
5
1/2 3/2
p
where I = peak current, c = complex concentration, ν = scan rate, A = electrode area,
p
D = diffusion coefficient and n = number of transferred electron] of 2. It is well
established that the dependence of the current function on the scan rate provides
mechanistic information on the electron transfer chemistry and the number of electrons
n at a given potential.[20-26] In general, when the reduction process does not have any
chemical route (simple E mechanism (E = electron transfere), n = 1), the current
function of this reduction process should remain constant at all scan rates since 2.69 x
5
1/2 3/2
1
0 )·A·D ·n is constant. In contrast, when an ECE mechanism (C = chemical
process) (n = 2) in which a chemical process takes place, the current function of this
reduction process decreases significantly toward that expected for a one electron as the
scan rates increase. This means, at higher scan rates there is no enough time for such a
chemical process to occur and thus the second electron transfer will not take place (i.e.
conversion of the mechanism from ECE to simple E process). Based on that, the value
of the I
/c·ν1/2 of the reduction of 2 decreases with increasing ν towards a value
p
predicted for a one-electron reduction (Figure 4), suggesting an intervening chemical
process in an ECE reduction mechanism.
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Figure 4. The scan rate dependence of the current function of the reduction peak of 2
in 0.1 M CH Cl -[nBu N][BF
] solution. Glassy carbon electrode (A = 0.0206 cm2).
The dashed line represents the current function expected for a one-electron process
2
2
4
4
-6
2
-1
assuming D = 9 x 10 cm ·s , a value calculated for various [FeFe]-hydrogenase
models.
Catalytic reduction of protons
We have probed by cyclic voltammetry the plausible catalytic behaviour of complex 2
toward proton reduction using weak and moderate acids, AcOH and TFA, respectively.
Figure 5. Cyclic voltammetry of 1.0 mM solution of complex 2 in CH
2
Cl
2
-
-1
[
nBu
4
N][BF
4
] at 0.2 V s in the presence of a) various equivalents of AcOH and b)
various equivalents of TFA. Glassy carbon disk (d = 1.6 mm). The potential E are given
+
in V and referenced to Fc /Fc couple. The arrows indicate the scan direction.
Figure 5a shows the cyclic voltammetry of complex 2 in CH
Cl
2 2
-[nBu
4
N][BF ] in the
4
presence of various equivalents of AcOH. The presence of one equiv. AcOH shifts the
primary reduction peak of 2 to less negative potential (ca. 32 mV), which is typical
observation when protonation of a reduced species takes place.[15,27,28] However, when
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the acid concentration was increased, the current of this process remains unaffected. At
more reducing potentials in the area of the vicinity of -1.80 V to -2.10 V for complex
2
, the voltammetric profile of 2 shows a reduction peak (Epc = -1.98 V) with a current
that increases in response to the systematic increase in the acid concentration, indicating
a catalytic proton reduction.
On the other hand, the addition of a moderate acid, TFA, to a solution of 2 in CH
nBu N][BF ], shifts the primary reduction peak of 2 to less negative potential (ca. 60
mV) and the current of this process evidently grew with the sequential addition of TFA
Figure 5b). This observation could be explained as arising from a catalytic cycle that
evolved H (process I, Figure 5b). Additionally, another reduction peak was also
2
Cl -
2
[
4
4
(
2
observed at more negative potential (Epc = -1.96 V) and its current significantly
increases with consecutive addition of TFA indicating that a catalysis process is also
occurring at this potential (process II. Figure 5b).
Conclusion
In this paper we have reported the synthesis of [Fe
2
(CO)
6
{µ-(TeCH Te)}] (2) by the
2
reaction of in situ generated dimethylbis(tellurocyanatometyl)stannane with Fe
3
(CO)12
1
under reflux conditions. The resulting complex was fully characterized by use of H,
1
3
1
125
1
C{ H} and Te{ H} NMR as well as IR spectroscopy, mass spectrometry, elemental
analysis and X-ray crystallography. Moreover, the proton reduction catalyzed by
complex 2 was also investigated by cyclic voltammetry using weak and moderate acids
such as AcOH and TFA, respectively. As a result, complex 2 shows a catalytic
behaviour via an EECC electrocatalysis mechanism for H
2
production in the case of
AcOH, while in the case of TFA shows a catalytic behaviour though two processes via
an EECEC mechanism (process I) and EECC mechanism (process II) for H production.
2
Experimental Section
All reactions were performed using standard Schlenk and vacuum-line techniques
1
13
1
125
1
under an inert gas (nitrogen). The H, C{ H} and Te{ H} NMR spectra were
recorded with a Bruker Avance 400 MHz spectrometer. Chemical shifts are given in
1
13
. The 125Te
Te.
parts per million with references to internal SiMe
4
( H, C) or CHCl
3
chemical shift was measured vs. external PhTeTePh and converted to that of Me
2
The mass spectrum was recorded with Finnigan MAT SSQ 710 instrument. The IR
spectra were recorded with a Bruker Equinox 55 spectrometer equipped with an ATR
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unit. Elemental analysis was performed with a Leco CHNS-932 apparatus. TLC was
performed by using Merck TLC aluminum sheets (Silica gel 60 F254). Solvents from
Fisher Scientific and other chemicals from Acros and Aldrich were used without further
purification. All solvents were dried and distilled prior to use according to standard
methods. Me
2
Sn(CH
2
I)
was synthesized according to the known literature methods.17
2
Electrochemistry: Corrections for the iR drop were performed for all experiments.
Cyclic voltammetric measurements were conducted in three-electrode technique
+
[
glassy carbon disk (diameter d = 1.6 mm) as working electrode, Ag/Ag in MeCN as
reference electrode, Pt wire as counter electrode] using a Reference 600 Potentiostat
(Gamry Instruments). All experiments were performed in CH
2
Cl
2
solutions
(
concentration of the complex 1.0 mM) containing 0.1 M [n-Bu N][BF
4
4
] at room
temperature. Solutions were deaerated by N
2
purge for 5-10 min, and a blanket of N
2
was maintained over the solutions during the measurements. The vitreous carbon disk
was polished on a felt tissue with alumina before each measurement. All potential
values reported in this paper are referenced to the potential of the ferrocenium/ferrocene
+
(
Fc /Fc) couple.
Crystal structure determination of complex 2:
The intensity data were collected on a Nonius KappaCCD diffractometer, using

graphite-monochromated Mo-K radiation. Data were corrected for Lorentz and
polarization effects; absorption was taken into account on a semi-empirical basis using
multiple-scans.[29-31] The structure was solved by direct methods (SHELXS)[32] and
refined by full-matrix least squares techniques against Fo (SHELXL-97).[32] Both
hydrogen atoms were located by difference Fourier synthesis and refined isotropically.
MERCURY[33] was used for structure representations.
2
-1
Crystal Data for 2: C
7
H Fe
2 2
O
6
Te , Mr = 548.99 gmol , red-brown prism, size 0.088 x
2
3
0
1
.082 x 0.078 mm , monoclinic, space group P 2
1
/c, a = 6.8003(1), b = 13.7428(3), c =
3
4.0199(4) Å,  = 104.035(1)°, V = 1271.12(5) Å , T= -140 °C, Z = 4, calcd. = 2.869
-3
-1

gcm , µ (Mo-K ) = 67.9 cm , multi-scan, transmin: 0.5704, transmax: 0.7456, F(000)
=
992, 16724 reflections in h(-8/8), k(-17/17), l(-18/18), measured in the range 3.09° 
 27.47°, completeness max = 99.6%, 2900 independent reflections, Rint = 0.0348,

2
693 reflections with F
o
> 4(F
), 162 parameters, 0 restraints, R1obs = 0.0206, wR2obs
o
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10.1002/zaac.201800070
=
0.0422, R1all = 0.0233, wR2all = 0.0434, GOOF = 1.070, largest difference peak and
-3
hole: 0.554 / -0.655 e Å .
General Procedure for the Synthesis of Complex 2: A mixture of Te (1.86 mmol)
and KCN (1.86 mmol) was dissolved in dimethyl sulfoxide (10 mL) and refluxed for
one hour. The resulting deep-red solution was cooled down to room temperature before
the addition of Me
2
Sn(CH
2
I)
2
(0.93 mmol) and then stirred overnight. Subsequently, a
solution of Fe (CO)12 (1.90 mmol) in THF (25 mL) was added and the mixture was
3
stirred under reflux for two hours. The resulting reddish solution was cooled down and
the solvent removed by rotary evaporator. The residue was introduced into column
chromatography using 100% hexane as eluent. Complex 2 was collected from the first
band as red powder.
1
[
(
Fe
2
(CO)
6
{µ-(TeCH
2
Te)}], 2: Yield 33%. H NMR (400 MHz, CHCl
3
, ppm): δ = 2.20
). C{ H} NMR (100.6 MHz, CHCl
, ppm): δ =
s, with 125Te satellites, (2J(TeH) = 20 Hz, 2H, CH
13
1
,
2
3
), 211.8 (s, CO). 125T{ H} NMR (126 MHz, CHCl
1
ppm): δ = 22.5 (s, CH
2
3
-1
3
37.9. IR (νCO, cm ): 2054 (s), 2025 (m), 1998 (m), 1986 (m) and 1960 (w). DEI-MS:
+
m/z = 549 [M] . C
7
H
2
Fe O
2 6
Te : C 15.61 (calcd. 15.32), H 0.60 (0.37), S 13.45 (13.70).
2
Supporting Information Available: Crystallographic data deposited at the Cambridge
Crystallographic Data Centre under CCDC-1825815 for 2 contain the supplementary
crystallographic data excluding structure factors; this data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1
223-336-033; or deposit@ccdc.cam.ac.uk).
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