Iron(I)-carbonyl clusters tethered to (trifluoromethyl)thiophenolates

Article abstract of DOI:10.1007/s11243-012-9633-0

Two diironhexacarbonyl clusters containing (trifluoromethyl)thiophenolates, as models for the active site of [Fe-Fe] hydrogenase enzyme, have been prepared and characterized. The crystal and electronic structures of the complexes have been probed by X-ray

Full text of DOI:10.1007/s11243-012-9633-0

Transition Met Chem (2012) 37:645–650
DOI 10.1007/s11243-012-9633-0
Iron(I)-carbonyl clusters tethered
to (trifluoromethyl)thiophenolates
•
Charles A. Mebi Joshua J. Trujillo
•
•
Britney L. Rosenthal Robert B. Bowman
•
•
Bruce C. Noll Patrick J. Desrochers
Received: 19 June 2012 / Accepted: 11 July 2012 / Published online: 27 July 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Two diironhexacarbonyl clusters containing
(trifluoromethyl)thiophenolates, as models for the active
site of [Fe–Fe] hydrogenase enzyme, have been prepared
and characterized. The crystal and electronic structures of
the complexes have been probed by X-ray crystallography
and spectroscopic methods. Cyclic voltammetric studies in
the presence of acetic acid show that both compounds
catalyze the electrochemical reduction of acetic acid to
produce hydrogen with favorable overpotentials.
hydrogenase enzyme [3–11]. The structure 1 (X = CH2, NH,
or O) is a description of the active site of [Fe–Fe] hydroge-
nase. It is composed of a diiron center bonded to carbonyls,
cyanides, bridging dithiolate, and a cysteinyl-linked [4Fe4S]
unit [5]. Structure 2 is a generic non-bridging model of 1 with
different thiolate ligands used to tune the electronic, stability,
and electrocatalytic properties of the compounds [12–14].
For example, diironhexacarbonyl complexes coupled to
electron-withdrawing groups have been studied and shown to
have favorable overpotentials (between -0.2 V and -0.8 versus
Fc/Fc?) for the electrocatalytic reduction of acetic acid to
produce hydrogen in acetonitrile [13, 15]. Inspired by these
results, we have prepared two new organometallic complexes 3
and 4 using (trifluoromethyl)thiophenolates in the hope of
obtaining models with mild proton reduction overpotentials.
Herein, we present the synthesis, spectroscopic, and electro-
chemical characterization of compounds 3 and 4.
Introduction
Hydrogen is considered an environmentally benign and
energy dense means of storing energy for use in fuel cell
technology and other applications. To achieve a sustainable
hydrogen economy, it is necessary to produce hydrogen
efficiently and cheaply using abundant and renewable natural
resources. One approach is to develop efficient catalysts that
can generate hydrogen from water [1, 2]. To this end, many
studies have focused on the design, preparation, and char-
acterization of organometallic compounds that are modelsfor
the active site of the efficient hydrogen producing [Fe–Fe]
X
Cys
R
R
S
S
S
S
S
OH2
CO
OC
[4Fe4S]
Fe
Fe
CO
CN
Fe
Fe
CO
CO
NC
OC
C
OC
OC
O
1
C. A. Mebi (&) Á J. J. Trujillo Á B. L. Rosenthal Á
R. B. Bowman
Department of Physical Sciences, College of Natural and Health
Sciences, Arkansas Tech University, 1701 N. Boulder Ave.,
Russellville, AR 72801, USA
2
F3C
CF3
F3C
e-mail: cmebi@atu.edu
CF3
F3C
F3C
S
S
S
S
Fe
OC
B. C. Noll
Bruker AXS Inc., 5465 East Cheryl Parkway, Madison,
WI 53711, USA
CO
CO
OC
OC
OC
Fe
Fe
Fe
OC
CO
CO
CO
CO
OC
P. J. Desrochers
Department of Chemistry, University of Central Arkansas,
201 Donaghey Ave., Conway, AR 72035, USA
3
4
123

646
Transition Met Chem (2012) 37:645–650
Experimental section
Table 1 XRD data collection and structure refinement for 3
3
Synthesis
Empirical formula
Formula weight
Crystal system
Space group
C20H8F6Fe2O6S2
634.08
All reagents were obtained from commercial sources. The
preparations were carried out using standard Schlenk and
vacuum-line techniques. Compounds 3 and 4 were prepared
by the treatment of THF solutions (30 mL) of Fe3(CO)12
(0.39 g, 1.36 mmol) with 3-(trifluoromethyl)thiolbenzene
(1.00 g, 5.61 mmol) and 3,5-bis(trifluoromethyl)thiolben-
zene (1.00 g, 4.06 mmol), respectively, under a nitrogen
atmosphere. The resulting green mixtures were stirred
overnight at room temperature affording orange solutions.
Removal of the solvent followed by extraction with hexanes
afforded 3 (1.16 g, 65 % yield) and 4 (0.94 g, 60 % yield).
1H NMR (300 MHz, CDCl3): 3; 7.59 (s, 2H), 7.47 (br, 6H)
ppm, 4; 7.66 (s, 2H), 7.72 (br, d, 4H, 4JHH = 1.5 Hz), ppm.
Triclinic
P -1
˚
a (A)
8.1709(11)
10.4132(14)
14.1510(17)
81.644(4)°
85.495(4)°
79.325(4)°
1,169.0(3)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
Volume (A )
Z
Density (calculated) Mg cm-3
Absorption coefficient (m m-1
F(000)
1.801
)
1.503
1
19F NMR (CDCl3): 3; -62.9(d, JCF = 300 Hz) ppm, 4;
628
1
-63.1(d, JCF = 300 Hz) ppm, and 13C NMR (CDCl3): 3;
Reflections collected
11,352
208, 141, 137, 136, 131, 129, 128, 125 ppm, 4; 207, 143, 134,
132, 127, 122 ppm: Elemental Analysis: 3; Calc. C, 37.80;
H, 1.26; S, 10.09 %. Found: C, 38.27; H, 1.64; S, 9.51 %. 4;
Calc. C, 34.30; H, 0.78; S, 8.31 %. Found: C, 34.55; H, 1.07;
S, 7.62 %. Melting points: 3, 139–140 °C; 4, decomposes at
160 °C.
Theta range for data collection
Index ranges
1.46°–25.05°
-9 B h B 9, -12 B k B 12,
-16 B l B 16
Independent reflections
4,084 [R(int) = 0.0476]
Coverage of independent reflections 98.3 %
Absorption correction
Multiscan
-3
˚
0.590 and -0.274 eA
Largest diff. peak and hole
Max. and min. transmission
Final R indices [I [ 2r(I)]
R indices (all data)
Spectroscopy
0.8172 and 0.5848
R1 = 0.0353, wR2 = 0.0878
R1 = 0.0477, wR2 = 0.0939
1.078
The UV–visible spectra of the compounds were measured
using a Shimadzu UV-2501 PC spectrophotometer with a
1 cm quartz cell at room temp. Solution infrared spectra
were recorded on a Nicolet FT/IR Magna spectrophotom-
eter in dichloromethane using a NaCl cell. Multinuclear
(1H, 19F, and 13C) NMR studies were performed on a Jeol
Goodness-of-fit (GOOF) on F2
CCDC Number
876,180
1
groups. In each group, three sites were found for each fluorine
atom. These sites were modeled and refined with restraints on
the C–F distances, and the F–F through-space distances. The
site occupancy factors for these fluorines were modeled as sets
of three and range from 0.20 to 0.40.
ECX-300 NMR spectrometer. H signals are referenced to
residual CHCl3 proton resonances at 7.30 ppm and 13C to
the carbon resonance at 77.00 ppm. 19F NMR spectra were
recorded using [nBu4N]BF4 as internal reference (s,
-152 ppm in CDCl3).
Crystal data collection and refinement
Electrochemistry
The X-ray intensity data for 3 were measured on a Bruker
SMART X2S benchtop diffractometer system equipped with a
doubly curved silicon crystal monochromator and a MoKa
Cyclic voltammetric studies were conducted using an
Epsilon BAS potentiostat with a three-electrode cell under
nitrogen at room temperature. The electrodes used were a
glassy carbon working electrode, platinum auxiliary elec-
trode, and Ag/AgCl reference electrode. The platinum and
glassy carbon electrodes were polished with aluminum
paste and rinsed with water and acetone. A 0.1 M CH3CN
solution of Bu4NPF4 was used as supporting electrolyte.
The concentration of the compounds was 1 mM (10 mL),
˚
microfocus sealed tube (k = 0.70973 A). Complete crystal
data and parameters for data collection and refinement are
presented in Table 1. An orange block-like specimen of 3,
approximate dimensions of 0.14 mm 9 0.14 mm 9 0.40 mm,
was used for the X-ray crystallographic analysis. The structure
was solved and refined using the Bruker SHELXTL Software
Package [16]. The structure exhibited disorder in the CF3
and the scan rate was 100 mV s-1
.
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Transition Met Chem (2012) 37:645–650
647
Scheme 1 Synthesis of
compounds 3 and 4
(RT, solvent: THF)
CF3
F3C
CF3
CF3
F3C
HS
CF3
HS
CF3
F3C
F3C
S
S
S
S
CO
OC
CO
OC
Fe3(CO)12
Fe
Fe
CO
CO
Fe
OC
Fe
CO
CO
OC
OC
OC
4
3
Results and discussion
Synthesis
Compounds 3 and 4 were prepared following the procedure
described in Scheme 1. This involves the reaction of
Fe3(CO)12 with 3-(trifluoromethyl)thiolbenzene and 3,5-
bis(trifluoromethyl)thiolbenzene, respectively, at room
temperature. A color change from green to red was
observed, and the products were isolated by extracting with
hexanes. The compounds were recrystallized from hexane
solutions at -20 °C affording block orange crystals.
Fig. 2 Infrared spectra of 3 and 4 in dichloromethane
Spectroscopy
The UV–visible spectra of 3 and 4 were recorded in
dichloromethane. The spectra are similar and that of
compound 4 is depicted in Fig. 1. One peak at 338 nm
(e = 1.4 9 104 M-1 cm-1) that tails to 500 nm was
observed for both compounds. The peak at 338 nm may be
tentatively assigned to p ? p* and/or charge transfer
transitions. The electronic structure of 3 and 4 was also
probed using IR spectroscopy. Three intense vibrational
signals corresponding to the stretching frequencies of ter-
minal metal carbonyls (tCO) were found between 2,000 and
2,100 cm-1 (Fig. 2). As observed in Fig. 2, the tCO peaks
for 4 are at slightly higher wavenumbers compared to those
of 3 due to the electron-withdrawing effect of the addi-
tional trifluoromethyl groups in 4.
hydrogen atoms of the thiolate ligands. The carbon-13
NMR spectra of 3 and 4 show among others a peak at 208
and 207 ppm, respectively. These peaks are assigned to the
carbonyl ligands. 19F-NMR spectral analyses of the com-
pounds reveal a doublet at -62.9 ppm for 3 and
-63.1 ppm for 4 consistent with reported data for CF3
groups [17].
Crystal structure
The spectroscopic data presented above suggest the for-
mation of the desired products (3 and 4) described in
Scheme 1. Additional confirmation was obtained by ele-
mental analyses and X-ray crystallographic study. Orange
crystals of 3 and 4 were obtained from hexane solutions of
the compounds at -20 °C. Block-like crystals of 3 suitable
for X-ray diffraction were examined and the molecular
structure is depicted in Fig. 3. Crystallographic data and
selected bond parameters for 3 are presented in Tables 1
and 2, respectively. The butterfly-shaped structure of
compound 3 is shown in Fig. 3 containing two iron atoms
each bonded to three terminal CO ligands. The (trifluoro-
methyl)thiophenolate ligands are tethered to the iron atoms
and exhibit equatorial-axial spatial orientation. Signifi-
Further characterization of the compounds was accom-
plished by multinuclear (1H, 19F, and 13C) NMR spectro-
scopic studies. Proton NMR spectra of 3 and 4 have peaks
between 7.3 and 7.8 ppm corresponding to the aromatic
˚
cantly, the Fe–Fe bond length that is 2.508 A is close to the
values reported for the active site of the hydrogenase
˚
enzyme (2.6 A) and similar models [4, 5]. The crystal
packing diagram of 3 shows a ring-to-ring distance of
˚
*3.87 A suggesting a weak intermolecular p–p interaction
between benzene rings of adjacent complexes (Fig. 4) [18].
Fig. 1 UV–visible spectrum of 4 in dichloromethane (5.1 9 10-5 M)
123

648
Transition Met Chem (2012) 37:645–650
o
Table 2 Selected bond lengths (A) and bond angles ( ) for 3
˚
Fe1–Fe2
Fe1–S1
2.5080(7)
2.2609(9)
2.2594(9)
2.2660(9)
2.2719(9)
67.29(3)
S1–Fe1–Fe2
S1–Fe2–Fe1
S2–Fe2–Fe1
S2–Fe1–Fe2
S1–Fe1–S2
S1–Fe2–S2
56.45(2)
56.26(2)
56.16(2)
56.63(2)
80.55(3)
80.17(3)
Fe1–S2
Fe2–S1
Fe2–S2
Fe2–S1–Fe1
Fe2–S2–Fe1
67.21(3)
Fig. 5 Cyclic voltammograms of 3 (1 mM) in 0.1 M n-Bu4NPF6/
MeCN at a scan rate of 100 mV s-1
Table 3 Electrochemical data for 3 and 4
Compound Epc(1) (V) Epc(2) (V) Epa (V) Ecat (V) g/(V)a
3
4
-1.39
-1.31
-2.15
-1.97
?0.79
?1.01
-2.14
-2.07
-0.68
-0.61
The values are referenced to Fc/Fc?
a
g = overpotential
redox power of the complexes. To investigate the effect of
these groups on the redox properties of the [Fe2(CO)6] unit,
cyclic voltammetric analyses were performed on com-
pounds 3 and 4. The results are presented in Fig. 5 and
Table 3. The redox chemistry of 3 is similar to that of 4
Fig. 3 An ORTEP drawing of 3. Thermal ellipsoids drawn at the
50 % probability level. All hydrogen atoms are omitted for clarity
with both showing two irreversible reduction peaks Epc(1)
:
-1.39 V (3) and -1.31 V (4); and Epc(2): -2.15 V (3) and
-1.97 V (4). Similar peaks have been reported for other
hydrogenase models and assigned to the FeI–FeI/Fe0–FeI
and FeI–FeI/Fe0–Fe0 couples, respectively [19]. Irreversible
oxidation peaks at ?0.79 V (3) and ?1.01 V (4) versus
Fc/Fc? were also observed. Since the peak currents of the
oxidation peaks are close to those of the first reduction
peaks, we have tentatively assigned the oxidation processes
to the FeI–FeI/FeII–FeI couple. The first reduction potentials
of complexes 3 and 4 exhibit large anodic shifts (310–390 mV)
in comparison with that of [(l-SCH2CH2CH2S)Fe2(CO)6]
[13, 20] and smaller positive shifts (50–130 mV) with that
of the analogous complex [(l-SPh)2Fe2(CO)6] [13, 21]. As
expected, compound 4 is more easily reduced and less
easily oxidized than 3. This can be explained by the elec-
tron-withdrawing effect of the additional trifluoromethyl
groups in 4, consistent with the IR spectroscopy results. In
addition to the peaks mentioned above, some small irre-
versible oxidation peaks were observed, which can be
attributed to unidentified species generated from the
decomposition of the parent compounds during the catho-
dic sweep.
Fig. 4 Packing drawing of 3 showing p–p stacking interaction
(purple dashed line). (Color figure online)
Electrochemistry
In the design of compounds
3 and 4, (trifluoro-
methyl)thiophenolate groups were employed because of
their electron-withdrawing properties to modulate the
Cyclic voltammograms of 3 and 4 in the absence of acid
and upon addition of increasing amounts of acetic acid
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Transition Met Chem (2012) 37:645–650
649
the absence of catalyst, the reduction of acetic acid to
produce hydrogen has been recorded at -2.3 V using a
glassy carbon electrode. Therefore, the new complexes, 3
and 4, lower the reduction electropotential of acetic acid by
0.16 V (3) and 0.23 V (4).
Considering the electrochemical results presented
above, we propose a tentative mechanism for the reduction
in acid by 3 and 4. This EECC (electrochemical–electro-
chemical–chemical–chemical) mechanism (Scheme 2) is
in accordance with that suggested by Song et al. [19]. As
depicted in Scheme 2, 3 first undergoes successive one-
electron reductions to produce the intermediate 3=, which
upon protonation at the Fe-center affords H32. Subsequent
protonation of H32 regenerates 3 and produces hydrogen.
Fig. 6 Cyclic voltammograms of 3 (10 mL, 1 mM) in the presence
of acetic acid (0, 7, 28, 49, 70, 91, 112 mM) in 0.1 M n-Bu4NPF6/
MeCN at a scan rate of 100 mV s-1
Conclusions
were recorded. The results for compound 3 are presented in
Fig. 6. For both complexes, the catalytic waves corre-
sponding to the reduction of protons to molecular hydrogen
were observed between -2.05 and -2.15 V (versus
Fc/Fc?). As depicted in Fig. 6 for compound 3, the current
of catalytic reduction increased with increasing acid con-
centration. However, compound 4 behaved differently.
Initially, we observed a catalytic peak current in the pres-
ence of 14 mM glacial acetic acid. The catalytic peak
current decreased by 65 % when more acid (28 mM) was
added and remained unchanged with subsequent increase
in acid concentration. This is probably due to significant
decomposition of 4. The overpotential (g), the difference
between the standard potential for the reduction of acetic
acid (-1.46 V versus Fc/Fc? in acetonitrile, pKa = 22.3)
and the catalytic proton reduction potential for 3 and 4 was
found to be between -0.60 and -0.70 V (Table 3) [22]. In
Two new diiron(I) carbonyl clusters coupled to (trifluoro-
methyl)thiophenolate ligands have been prepared in
60–65 % yield. The electron-withdrawing (fluorometh-
yl)thiophenolate groups were used to modulate the redox
power of the Fe2(CO)6 core. The results indicate that both
compounds catalyze the electrogeneration of hydrogen by
proton reduction with overpotentials between -0.6 and
-0.7 V. These overpotentials are comparable to those
reported for similar models.
Acknowledgments The authors gratefully acknowledge support for
this work from Arkansas Tech University. P. J. D. gratefully acknowl-
edges the National Science Foundation (Grant CCLI 0125711) for
funding the NMR used for this work.
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