Fluorophenyl-substituted Fe-only hydrogenases active site ADT models: Different electrocatalytic process for proton reduction in HOAc and HBF 4/Et2O

Article abstract of DOI:10.1039/b818012a

A set of fluorophenyl-substituted adt-bridged Fe₂S₂ active site models of Fe-only hydrogenase, [(μ-SCH₂) ₂NR]Fe₂(CO)₆ (1, R = C₆F ₄CF₃-p; 2, R = C₆

Full text of DOI:10.1039/b818012a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Fluorophenyl-substituted Fe-only hydrogenases active site ADT models:
different electrocatalytic process for proton reduction in HOAc and
HBF4/Et2O†
Wen-Guang Wang, Hong-Yan Wang, Gang Si, Chen-Ho Tung and Li-Zhu Wu*
Received 13th October 2008, Accepted 20th January 2009
First published as an Advance Article on the web 18th February 2009
DOI: 10.1039/b818012a
A set of fluorophenyl-substituted adt-bridged Fe2S2 active site models of Fe-only hydrogenase,
[(m-SCH2)2NR]Fe2(CO)6 (1, R = C6F4CF3-p; 2, R = C6H4CF3-p) and [(m-SCH2)2NR]Fe2(CO)5(PPh3)
(3, R = C6F4CF3-p; 4, R = C6H4CF3-p), have been synthesized and well characterized. Spectroscopic
and electrochemical studies demonstrate that the aryl-substituted complexes 1–4 are stable toward a
strong acid HBF4/Et2O, and electrocatalytic process for the hydrogen production is mostly dependent
on the strength of the available proton source. When CH3COOH is used as the proton source, the
electrocatalytic process begins with successively two one-electron reduction processes to produce H2 at
Fe(0)Fe(0) (E2pc); whereas in the presence of strong acid, HBF4/Et2O, the process is initiated by
protonation of a N-bridged atom followed by reduction of the protonated N-bridged atom around
-1.29 V, and then release of H2 at Fe(0)Fe(I) (E1pc). Varying the strength of acid leads to the initial
electron-transfer step from the reduction of a protonated N-bridged atom to the active site of
[Fe(I)Fe(I)].
constructed, albeit replicating the reactivity of enzymes is far more
challenging.
Introduction
Spectroscopic, electrochemical and theoretical studies6–11 have
established that the order of electron and proton uptake by the
model diiron complexes is dependent on the acid strength and
the extent of CO/L substitution. For the pdt-bridged model
complexes, the addition of two electrons to the intact all-CO
complexes of [Fe2(CO)6(m-pdt)] preceded addition of two protons
when weak acid was used as a proton source. Darensbourg
et al.6–7 found that the neutral Fe(I)–Fe(I) bond of [Fe2(CO)4(m-
pdt)(PMe3)L] cannot be protonated by the weak acid CH3COOH,
but the redox level Fe(I)Fe(0) for PMe3 derivatives and Fe(0)Fe(0)
for all-CO substituted model complexes are active, leading to
ECCE or EECC mechanistic possibilities. Rauchfuss et al.8–9
reported that the [Fe2(CO)4(m-pdt)(PMe3)L]z complexes with a
mixed ligand (L = PMe3, z = 0 or L = CN-, z = -1) can catalyze
electrochemical reduction of strong acids like HOTs, HCl and
H2SO4 for hydrogen production. The catalytic mechanism was
proposed to be a CECE mechanism. Pickett et al.11 examined elec-
trocatalytic proton reduction of Fe2(m-pdt)(CO)6 in the presence
of moderately strong acid and found that the acid concentration
dependence of voltammetry was modelled by a mechanism with
two electron/proton additions. On the other hand, the adt-
bridge,12–13 namely the central atom of the dithiolate bridge to
be a secondary amine, is suggested because of the presence of an
amine close to the putative hydrogen binding site of the distal iron
atom would provide a low energy pathway for proton–hydride
combination in the enzymatic hydrogen production. Indeed, the
N heteroatom in the disulfide chelate of the adt-bridge model
complexes has been identified as a potential basic site that could
be protonated by strong acids.14 More interestingly, Sun and Ott15
reported the doubly protonated species, [(m-NH+)Fe(I)Fe(II)-H-],
which carries a proton at the adt N as well as a hydride at
Fe-only hydrogenases have recently aroused much attention due to
their remarkable capability of catalyzing the reversible oxidation
of dihydrogen to protons and electrons: 2H+ + 2e↔H2.1 The high-
resolution X-ray crystallographic structures have established that
Fe-only hydrogenases, isolated from Desulfovibrio desulfuricans2
and Clostridium pasteurianum,3 feature a butterfly Fe2S2 subunit
coordinated by cysteinyl-linked Fe4S4 cluster, carbon monoxide
and cyanide ligands, and by a dithiolate bridging the two iron
centers (Scheme 1). The astonishing efficiency that Fe-only hydro-
genases achieve under mild conditions has stimulated chemists to
mimic the structure and functionality of the active site, aiming at
guiding the rational design of catalyst candidates for biomimetic
hydrogen production and uptake. A number of model complexes4–5
of Fe-only hydrogenases with either a pdt (SCH2CH2CH2S), an
adt (SCH2NHCH2S) or an odt (SCH2OCH2S) bridge have been
Scheme 1 The structure of H cluster (X = CH2, NH or O).
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, the Chinese Academy of
Sciences, Beijing, 100190, P. R. China. E-mail: lzwu@mail.ipc.ac.cn;
Fax: 86-10-82543580; Tel: 86-10-82543580
† CCDC reference numbers 691515 for 1, 691516 for 2, 691517 for 3 and
691518 for 4. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b818012a
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the diiron cores. As a consequence, the molecular hydrogen is
released at less negative potential than that of any other mimic
complexes ever reported. Very recently, Jean Talarmin et al.16
investigated the electrocatalytic mechanism of adt-bridged model
complex in the presence of HBF4/Et2O and HOTs, and realized
the comprehensive catalytic proton reduction processes involved.
Whether the bridge of Fe-only hydrogenases has an N-bridged
atom, the prospect of a proton–hydride interaction has inspired
diverse strategies to promote H2 production in different types of
model complexes. In this context, we are surprised to note that the
aryl-substituted adt models were inactive toward acid17 despite
the fact that the N heteroatom can be protonated to initiate the
catalytic hydrogen production in benzyl and furan– or thiophene–
methylene substituted adt model complexes,14–15 with only a
generated Fe(0)Fe(0) intermediate with high electronic density
rather than the N-bridged atom of the complexes protonated by
the weak acid CH3COOH. The lack of such model systems may
be attributed to the instability of the Fe2S2 model complexes.
To understand the activity of aryl-substituted adt model com-
plexes toward acid, a series of fluorophenyl adt model complexes,
[(m-SCH2)2NR]Fe2(CO)6 (1, R = C6F4CF3-p; 2, R = C6H4CF3-
p) and [(m-SCH2)2NR]Fe2(CO)5(PPh3) (3, R = C6F4CF3-p; 4, R =
C6H4CF3-p), have been designed and synthesized. The selected flu-
orophenyl substituents are known to be electrochemically inactive
toward electrode17c, and the electron-donating tertiary phosphane
ligand is to increase the electron density of the Fe(I)–Fe(I) active
site. In the present work, we report that the designed Fe2S2 model
complexes 1–4 can be prepared in moderate-to-good yields.
X-Ray crystal structural analyses of 1–4 suggest that the model
complexes retain the butterfly structure of Fe2S2 model analogues.
Significantly, electrochemical studies demonstrate that complexes
1–4 with electrochemically inactive fluorophenyl functionality
are stable toward strong acid, and the electrocatalytic hydrogen
production process is mostly dependent on the strength of the
available proton source. Comparison of the electrochemical prop-
erties reveals that the initial electron-transfer step changes from the
reduction of the active site of [Fe(I)Fe(I)] + e-→[Fe(0)Fe(I)] to the
protonated N-bridged atom when CH3COOH and HBF4/Et2O
are used, respectively. Different pathways for proton reduction
can be therefore followed and the electrocatalytic H2 evolution
occurs at different reduction levels. These results indicate that
the aryl-substituted adt model complexes, even complexes 1–4
bearing stronger electron-withdrawing fluorophenyl groups, can
be protonated by strong acid to show greater acid sensitivity if
they can survive. The role of the N-bridged heteroatom not only
provides a basic site for protonation, but also acts as an active site
for electrocatalytic H2 evolution.
Scheme 2 (a) paraformaldehyde, CH2Cl2, room temperature; (b) SOCl2,
ice bath; (c) [(m-LiS)2Fe2(CO)6], THF, -78 ◦C, yield 45% for 1 and 70%
for 2, respectively; (d) PPh3, Me3NO·2H2O, CH3CN, room temperature,
yield: 66% for 3 and 52% for 4, respectively.
mixture was then added to the lithium salt [(m-LiS)2Fe2(CO)6]
{generated in situ from [(m-S)2Fe2(CO)6] and Et3BHLi in THF at
-78 ◦C for 15 min}12 to give 1 or 2 as red solids. Further reaction of
1 or 2 with PPh3 in the presence of 1 equiv decarbonylating agent
Me3NO·2H2O in CH3CN for 3 h at room temperature resulted in
the formation of 3 or 4 in a reasonable yield.
The identity of the model complexes 1–4 has been characterized
by IR, 1H NMR spectroscopy and satisfactory elemental analysis.
The IR spectra of 1 and 2 show three major absorption bands
around 2000 cm-1 for their terminal carbonyl groups, consistent
with those of other all-CO adt derivatives.17 Incorporation of
the stronger electron-donating ligand PPh3 shifts the stretching
frequencies of CO in 3 and 4 to lower frequencies, suggesting that
PPh3 increases the electron density of the Fe(I)Fe(I) active site
remarkably. Compared with that for 1 (d = 4.03 ppm) and 2 (d =
4.34 ppm), the chemical shifts of 3 and 4 for their CH2 groups on
the adt bridge shift to a relatively higher field. The two respective
doublets at d = 3.68, 2.89 ppm for 3 and d = 4.08, 2.93 ppm for 4 at
higher field reflect that in addition to a considerable delocalization
of the electron density to the adt-bridge the bulky monosubstituted
tertiary phosphane ligand stabilizes one particular conformation
of the Fe2S2 model complexes,7,12 fixes its conformation and makes
the CH2 groups chemically unequivalent.
Structures of model complexes 1–4
The crystal structures of complexes 1–4 were further determined
by X-ray diffraction. The molecular structures are depicted in
Fig. 1 and Fig. 2, and some selected bond lengths and bond angles
are listed in Table 1. For complex 1, the coordination geometry
about each iron center is roughly square pyramidal: the two m-S
atoms, C(4) and C(5) define the base, and C(6), the apical position,
two Fe atoms and two S atoms form a butterfly conformation. The
two Fe(CO)3 units are nearly eclipsed with a Fe–Fe bond distance
Results and discussion
Synthesis and spectroscopic characterization
The designed model complexes were prepared in moderate-to-
good yields according to the reported procedure.12,18 As shown in
Scheme2, 4-aminoheptafluorotolueneor 4-aminobenzotrifluoride
and paraformaldehyde were stirred in dry CH2Cl2 at room temper-
ature for 3.5 h, followed by treatment with excess SOCl2 in an ice
bath to give N,N -bis(chloromethyl)-4-aminoheptafluorotoluene
or N,N -bis(chloromethyl)-4-aminobenzotrifluoride. The resulting
˚
˚
of 2.5138(10) A, 0.1 A shorter than that found in Desulfovibrio
desulfuricans2 and Clostridium pasteurianum (ca. 2.6 A). The
3
˚
sum of the C–N–C angles around the N-bridged atom is 359.7◦,
suggesting that C(7), C(8) and C(9) are nearly in a plane and hence
the p-p electron communication exists between the substituted
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◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for complexes 1–4
1
2
3
4
Fe(1)–Fe(2)
N(1)–C(7)
N(1)–C(8)
N(1)–C(9)
2.5138(10)
1.445(6)
1.442(6)
1.383(6)
2.5012(9)
1.428(5)
1.427(5)
1.401(5)
Fe(1)–Fe(2)
N(1)–C(6)
N(1)–C(7)
N(1)–C(8)
2.5115(4)
1.438(2)
1.442(2)
1.384(2)
Fe(1)–Fe(2)
N(1)–C(24)
N(1)–C(25)
N(1)–C(26)
2.5190(6)
1.428(3)
1.432(3)
1.394(4)
C(9)–N(1)–C(7)
C(9)–N(1)–C(8)
C(8)–N(1)–C(7)
121.5(4)
122.7(4)
115.5(4)
121.8(3)
120.5(3)
113.8(3)
C(8)–N(1)–C(6)
C(8)–N(1)–C(7)
C(7)–N(1)–C(6)
121.47(15)
123.00(14)
115.25(14)
C(26)–N(1)–C(24)
C(26)–N(1)–C(25)
C(25)–N(1)–C(24)
119.3(2)
122.8(2)
113.2(2)
Fig. 1 ORTEP (ellipsoids at 30% probability) diagram of 1 (left) and 2 (right).
Fig. 2 ORTEP (ellipsoids at 30% probability) diagram of 3 (left) and 4 (right).
fluorophenyl ring and the p-orbital of the N-bridged atom.
Notably, the C–N–C angle of 356.1◦ around the N atom in 2 is 3.6◦
that of 1 but somewhat longer than 2. The C–N–C angle around
the N atom is 359.7◦ for 3 and 355.3◦ for 4, respectively.
˚
˚
smaller but the N(1)–C(9) bond length of 1.401(5) A is 0.2 A longer
than those of 1, reflecting that 4-aminoheptafluorotoluene with
stronger electron-withdrawing ability alters the electron density
on the N-bridged atom remarkably. For the tertiary phosphane-
substituted 3 and 4, the PPh3 ligand, trans to the fluorophenyl ring,
is in an apical position of the square-pyramidal geometry of the
Fe atom in order to reduce the steric hindrance between the two
bulky groups. The Fe–Fe distances of 3 and 4 are comparable to
Electrochemical properties of model complexes 1–4
The electrochemical behavior of 1–4 was investigated by cyclic
voltammetry in CH3CN (with 0.1 M n-Bu4NPF6 as electrolyte)
under Ar atmosphere. Similar to the typical redox events of the
adt-bridged models,13e,14a,e,17 the cyclic voltammograms of 1 and 2
display one quasi-reversible reduction process and one irreversible
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Fig. 3 Cyclic voltammograms of 1–4 and their electrochemical response upon addition of HOAc and HBF4/Et2O (0–10 mM), sample concentration
is 1.0 mmol in 0.1 M n-Bu4NPF6/CH3CN, scan rate 100 mV s-1, working electrode: glassy carbon, reference electrode: Ag/Ag+. All potentials versus
Fc/Fc+.
reduction process, while those of 3 and 4 show two irreversible
reduction processes (Fig. 3). For the CO-substituted models of 1
and 2, the first quasi-reversible reduction process (E1pc = -1.54 V)
is attributed to the reduction of Fe(I)Fe(I) to Fe(I)Fe(0), and the
for 4); whereas the first oxidative potential occurs at 0.44 V and
0.41 V for 3 and 4, respectively, and the second oxidation peak
of 3 and 4 is around 0.7 V under identical conditions (Fig. 3c–d).
The incorporation of electron-donating phosphine ligand into the
Fe2S2 model complexes makes the reduction of the Fe–Fe active
site more difficult but the oxidation easier.
The electrochemical behavior of catalytic proton reduction
by 1–4 was studied by cyclic voltammograms in the presence
of HOAc and HBF4/Et2O (0–10 mmol) in CH3CN (Fig. 3).
With the addition of 2 mM of HOAc acid, the current intensity
of the first reduction peak (E1pc) of 1–4 increases slightly and
ceases to grow up further with sequential increments of the
acid concentration. However, the height of the second reduction
peak increases sharply with the acid concentration, which is the
characteristic of proton reduction.7 In contrast, when strong acid,
such as HClO4, CF3COOH and HBF4/Et2O, is used as proton
source, the electrochemical behavior of catalytic proton reduction
by 1–4 is distinctly different from that observed in the presence
of HOAc. Upon addition of HBF4/Et2O, a new reduction peak
second irreversible reduction process (E2 = -2.02 V) is assigned
pc
to the reduction of Fe(I)Fe(0) to Fe(0)Fe(0). With reference to the
previous work on all-CO-substituted and PPh3-substituted Fe2S2
model complexes,13–17 the oxidation potential at 0.61 V for 2 is as-
cribed to the oxidation process of [Fe(I)Fe(I)]→ [Fe(I)Fe(II)] + e-.
The presence of quasi-reversible oxidative peaks around 0.94 V
both for 2 and model compound of 4-aminobenzotrifluoride
implies that the oxidation of the N-bridged atom takes place.
Such phenomenon is also evidenced by a broad oxidative peak
around 0.89 V for 1, in which the oxidation potential of
4-aminoheptafluorotoluene is likely to be overlapped with that of
[Fe(I)Fe(I)]→[Fe(I)Fe(II)] + e-. As CO ligand is substituted by
PPh3, the first reduction potential E1 for 3 and 4 shifts to a
pc
more negative value around -1.73 V by about 200 mV, so does
the second reduction process (E2 at -2.22 V for 3 and -2.09 V
pc
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appears around -1.29 V for 3 and 4, and the current intensities
of all reduction peaks grow up considerably with increasing acid
concentration (Fig. 3a–d).
The electrochemical performance of 1–4 in the presence of
strong acid was also investigated in order to understand the
fundamental in the process of H2 evolution. Monitoring by
cyclic voltammetry, the addition of HBF4/Et2O to a solution
of 1–4 shows the typical behavior of proton reduction.7 Since
the N-bridged atom of the complexes can be protonated when
HBF4/Et2O is used as a proton source, the electrocatalytic
process is therefore initiated by protonation of the N-bridged
atom of 1–4. It was previously believed that the existence of the
protonated N-bridged atom would improve the first reduction
potential of Fe(I)Fe(I) to Fe(I)Fe(0) (E1pc) to a less negative
value.14,15 However, the process is unlikely in this case because the
same electrochemical process around -1.29 V could be examined
for reference compounds 4-aminoheptafluorotoluene (5) and
4-aminobenzotrifluoride (6) with comparable current intensity to
that found in complexes 3 and 4 in the presence of HBF4/Et2O
in CH3CN (Fig. 3, Fig. 5). Evidently, the protonation of the N-
bridged atom is followed by reduction occurring at the protonated
(NH)+ species rather than at the Fe–Fe bond, thereby shifting the
reduction peak of 1–2 to less negative potential. This observation is
further strengthened by the fact that the peak at -1.29 V increases
when HBF4/Et2O is added to the solution of PPh3-substituted
complex 3–4 (Fig. 3), of which the two iron active centers are more
nucleophilic with [Fe(I)Fe(I)] + e-→ [Fe(0)Fe(I)] and [Fe(I)Fe(0)]
+ e-→[Fe(0)Fe(0)] around -1.74 V and -2.20 V, respectively. With
only the [Fe(I)Fe(I)] + e-→ [Fe(0)Fe(I)] shifts to more negative
potential, the similar electroreductive behavior around -1.29 V
is observed for proton reduction in the presence of strong acid,
indicating that the N-bridged atom acts as an electron hole and is
easily reduced upon protonation.
Electrochemically catalytic mechanism
To understand the intimate mechanism of catalytic proton re-
duction, detailed 1H NMR and 31P NMR spectrometry were
carried out to probe whether complexes 1–4 could be protonated
by strong acid HBF4/Et2O. As examples of titration, a CD3CN
solution of 1 and 3 titrated with HBF4/Et2O is given in Fig. 4.
It is evident that 1 and 3 exhibit typical triplet signals at d =
5.86 ppm (1JNH = 53 Hz) and d = 6.57 ppm (1JNH = 56 Hz),
respectively, with the addition of HBF4/Et2O. The signals from
the two CH2 groups in 1 and 3 shift downfield and split into
multiple peaks. No signal was detected at d < 0 ppm, indicating
that the protonation on the N-bridged atom takes place rather
than on the Fe(I)–Fe(I) bond to give m-H species,15 which was
further identified by the 31P NMR investigation of 3. As shown in
Fig. 4, three signals were observed at 65.70, 53.80 and 7.07 ppm
with introduction of HBF4/Et2O into CD3CN solution of 3, and
the signals are reversible with addition of excess Et3N. All of the
results demonstrate that the N-bridged atom of the fluorophenyl
substituted adt model complexes are protonated by the strong acid
leading to (NH)+ species formation.12–14 Obviously, complexes 1–4
are stable toward strong acid. The fact that the N-bridged atom
of 1–4 bearing stronger electron-withdrawing fluorophenyl groups
was protonated by strong acid indicates that the N-bridged atom
of the other electron-rich aryl-substituted adt model complexes
ought to be more sensitive toward acid as long as they can survive.
It is pertinent to compare the electrocatalyzed proton reduction
of complexes 1–4 toward strong acid with that in the presence
of weak acid. For the latter, the N atom in the adt-bridge
of the complexes cannot be protonated by the weak acid of
HOAc, even at high concentration. This is consistent with the
relative pKa values of acetic acid (pKa = 22.3 in CH3CN)19
and of the protonated N bridgehead atom of closely related
complexes (7.6 < pKa < 10.6).16c,19 When weak acid is used as
a proton source, the typical process observed at ca. -1.29 V
in the presence of HBF4/Et2O is replaced by the reduction at
-1.54 V for [Fe(I)Fe(I)] + e- → [Fe(0)Fe(I)] as the first reduced
species. Such acid-dependent electrochemical behaviors indicate
that different pathways are followed for proton reduction and
thus the electrochemical catalysis H2 evolution occurs at different
reduction levels.
When strong acid is used as a proton source, the enhanced
current at a potential substantially less negative suggests that the
reduction of the protonated N-bridged atom is most accessible
Fig. 4 1H NMR spectra of protonated (NH)+ species for (a) 1 and (b) 3 in
CD3CN, with 30 equiv. HBF4/Et2O; 31P NMR spectra of (c) 3 in CD3CN,
(d) 3 with 30 equiv. HBF4/Et2O, (e) excess Et3N + d.
Fig. 5 Cyclic voltammograms of 4-amino-heptafluorotoluene (5) (a) and 4-aminobenzotrifluoride (6) (b) in the presence of HBF4/Et2O (0–10 mM)
with the same electrochemical parameters as described in Fig. 3.
2716 | Dalton Trans., 2009, 2712–2720
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Fig. 6 Dependence of current heights of the reduction events (a) for 3 around E1 (ꢀ), - 1.29 V (᭹) and for 4 around - 1.29 V (ꢀ) on concentration of
pc
HBF4/Et2O (0, 2, 4, 6, 8 and 10 mM); (b) for 3 around E2 (᭢) on concentration of HOAc (0, 2, 4, 6, 8 and 10 mM).
pc
and becomes the initial step. The generated [(HN-R)Fe(I)Fe(I)]
species is further reduced to produce [(HN-R)Fe(I)Fe(0)] at E1
which is then protonated by acid leading to [(HN-R)Fe(I)Fe(II)H].
Eventually, H2 is released by coupling the H atoms from the N-
bridged atom and the reduced state of the active site in [(HN-
R)Fe(I)Fe(II)H].
the presence of the protonated N-bridged atom indeed permits
proton uptake from a strong acid at the E1pc, Fe(0)Fe(I) level. On
theother hand, CH3COOHis tooweaktoprotonatetheN-bridged
atom of the complexes. The cyclic voltammograms of 3 with
addition of acetic acid and the linear increased current intensity
around E2pc indicates an EECC mechanism for the electrochemical
catalytic hydrogen evolution process (Fig. 6b, Scheme 3, Path
B), as proposed by Darensbourg.7a As a result, the reduction
process either from the protonated N atom or the diiron active
site is determined by the acid strength. The nitrogen heteroatom
not only provides a basic site for protonation, but also acts as
a catalytic site for electrochemical H2 evolution. This allows a
gain of several hundreds of millivolts in the potential for proton
reduction.
,
pc
Fig. 6a shows the acid-dependent increase in the reduction
current intensity around E1 (Scheme 3, Path A), the CEEC
pc
mechanism operates. The slightly increasing current intensity
around -1.29 V indicates an EC mechanism also takes place
(Fig. 6a), in which the hydrogen evolution is either by loss
of the H atom from the [(HN-R)Fe(I)Fe(I)] or a bimolecular
reaction between two [(HN-R)Fe(I)Fe(I)] entities.16 To confirm
electrocatalytic H2 evolution by 1–4 at the reduction potential
of E1 in the presence of HBF4/Et2O, controlled-potential
pc
electrolysis in 0.1 M n-Bu4NPF6/CH3CN were carried out. As
expected, gas generation was clearly observed at E1 (-1.45 V vs
Conclusion
pc
Ag/AgNO3 for 1 and -1.65 V vs Ag/AgNO3 for 3), while in the
absence of the complexes the reduction of HBF4/Et2O is slow
at this potential. Gas chromatography analysis of the reaction
demonstrates that H2 is produced during electrolysis. Therefore,
A set of fluorophenyl substituted adt-bridge Fe2S2 model com-
plexes 1–4 of hydrogenases have been synthesized and well
characterized. X-Ray crystal analysis confirms the butterfly
structure of the Fe2S2 model analogues. Electrochemical studies
demonstrate that complexes 1–4 are stable toward a strong acid
HBF4/Et2O. Comparison of the electrochemical properties reveals
that the electrocatalytic process for the hydrogen production
is mostly dependent on the strength of the available proton
source. When CH3COOH is used as the proton source, the
electrocatalytic process begins with two successively one-electron
reduction processes to produce H2 at E2pc, Fe(0)Fe(0) level;
whereas in the presence of strong acid, HBF4/Et2O, the process
is initiated by protonation of the N-bridged atom followed by
reduction of the protonated N-bridged atom around -1.29 V, and
then release of H2 at E1pc, Fe(0)Fe(I) level. Varying the strength
of the acid leads to the initial electron-transfer step from the
reduction of the protonated N-bridged atom to the active site
of [Fe(I)Fe(I)]. The results indicate that the aryl-substituted adt
model complexes, even complexes 1–4 bearing stronger electron-
withdrawing fluorophenyl groups, can be protonated by strong
acid to show greater acid sensitivity if they can survive. The role of
the N heteroatom not only provides a basic site for protonation,
but also is involved in the catalytic H2 production process. This
feature provides a favorable pathway for H2 evolution in the
presence of proton with considerable acid strength, and makes the
adt-bridge complexes quite unique as compared with the other
Fe2S2 models of Fe-only hydrogeneases.
Scheme 3 Proposed electrocatlytic process for H2 production by 1–4 in
the presence of HBF4 (path A) and HOAc (path B).
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Preparation of [(l-adt)C6F4CF3-p]Fe2(CO)6 (1)
Experimental
A solution of [(m-S)2Fe2(CO)6] (0.17 g, 0.50 mmol) in dry THF
(15 ml) was cooled to -78 ◦C and then treated dropwise with
Et3BHLi (1.00 M in THF, 1.20 mL, 1.20 mmol). The resulting
green solution was stirred for 15 min., N,N -bis(chloromethyl)-
4-amino-heptafluorotoluene [prepared from 4-amino-hepta-
fluorotoluene (0.58 g, 2.50 mmol), paraformaldehyde (0.19 g,
6.00 mmol) and SOCl2 (0.50 ml, 6.90 mmol) in dry CH2Cl2], was
added to cause the THF solution color changed from green to
red. The mixture was stirred and warmed to room temperature
for 3 h. Upon evaporation of the solvent under vacuum, the solid
was eluted with CH2Cl2/petroleum ether (V/V: 1/10) on a silica
gel column. 1 was obtained as red air-stable power (yield 45%).
1H NMR (400 MHz, CDCl3, d ppm): 4.03 (s, 4H, 2CH2). 13C
Reagents and instruments
All reactions and operations were carried out under a dry argon
atmosphere with standard Schlenk techniques. All solvents were
dried and distilled prior to use according to the standard methods.
Fe(CO)5 was purchased from Lanzhou Institute of Chemistry and
Physics, CAS. Paraformaldehyde, perchloric acid, HBF4/Et2O
solution, 4-aminobenzotrifluoride, 4-aminoheptafluorotoluene,
triphenylphosphine and super hydride were purchased from
Aldrich and used as received. [(m-S)2Fe2(CO)6] was prepared
according to the literature procedures.18 Infrared spectra were
recorded on a Nicolet NEXUS 670 FT-IR spectrophotometer.
NMR spectra were recorded on a Bruker Avance dpx 400 MHz
instrument using TMS as internal standard. Elemental analyses
were performed on a FLASH EA1112 elemental analyzer. Voltam-
metric experiments were manipulated on a Princeton Applied
Research Potentionstat-gavanostat model 283.
Acetonitrile (Fisher Chemicals, HPLC grade) used to perform
electrochemistry was distilled under an argon atmosphere once
from CaH2, once from P2O5, and CaH2 again. 0.1 M of n-Bu4NPF6
in CH3CN was used as the electrolyte in all the cyclic voltammetric
measurements. The electrolyte solution was degassed by bubbling
with argon for 10 min before measurement. A three electrode
system is used for measurement and bulk electrolysis, with a
3 mm glassy carbon working electrode, a platinum wire counter
electrode, and a non-aqueous Ag/Ag+ reference electrode. The
working electrode was polished with a 0.05 mm alumina paste
and sonicated for 15 min before use. Gas chromatograms were
recorded on a GC-14B (SHIMADZU), and nitrogen was selected
as carrier gas.
NMR (100 MHz, CDCl3, d ppm): 51.9, 103.2 (m), 121.2 (q, JF-C
=
271.6 Hz), 130.5, 139.0, 141.4, 145.5; 207.1. 19F NMR (376 MHz,
CDCl3, d ppm): -55.7 (s, 3F, CF3), -140.2 (s, 2F, ArF), -146.2
(s, 2F, ArF). IR (CH2Cl2): nmax/cm-1 (CO) 2083, 2031, and 1994.
Calc. for C15H4F7Fe2NO6S2: C, 29.88; H, 0.67; N, 2.32, Found C,
30.03; H, 0.30; N, 2.68.
Preparation of [(l-adt)C6H4CF3-p]Fe2(CO)6 (2)
Complex 2 was synthesized by the procedure similar to that
for 1, except that 4-aminobenzotrifluoride (0.40 g, 2.50 mmol)
was used in place of 4-amino-heptafluorotoluene. The mixture
was stirred and warmed to room temperature for 3 h. Upon
evaporation of the solvent under vacuum, the residue was eluted
with CH2Cl2/petroleum ether (V/V: 1/10) on a silica gel column
to afford 2 (yield 70%). 1H NMR (400 MHz, CDCl3, d ppm): 7.57
(d, 2H, J = 8.52 Hz), 6.80 (d, 2H, J = 8.52 Hz), 4.34 (s, 4H; 2CH2).
Table 2 X-Ray structure determination of 1–4
Complex
Formula
1
2
3
4
C15H4F7Fe2NO6S2
603.01
C15H8F3Fe2NO6S2
531.04
C32H19F7Fe2NO5PS2
837.27
C32H23F3Fe2NO5PS2
765.30
M
w
Crystal system
Space group
monoclinic
P2(1)/c
21.464(4)
6.8442(14)
14.102(3)
90.00
106.12(3)
90.00
1990.2(7)
4
0.50 ¥ 0.35 ¥ 0.20
2.013
1.767
1184
293 (2)
10769
3496
0.0475
triclinic
P-1
triclinic
P-1
triclinic
P-1
˚
a/A
8.118(2)
10.938(3)
11.434(4)
104.100(4)
90.395(5)
98.663(5)
972.5(5)
2
0.22 ¥ 0.18 ¥ 0.12
1.814
1.767
528
9.2202(8)
10.2088(10)
17.6686(16)
100.153(3)
97.735(4)
90.871(2)
1620.9(3)
2
0.32 ¥ 0.28 ¥ 0.20
1.716
1.157
840
9.4981(13)
10.2902(15)
17.554(2)
73.294(2)
79.295(2)
88.532(2)
1614.0(4)
2
0.26 ¥ 0.24 ¥ 0.22
1.575
1.137
776
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
V/A
Z
3
˚
Crystal Size [mm]
D
caled/g cm-3
m/mm-1
F (000)
T [K]
294 (2)
5039
3400
0.0223
113 (2)
20503
7695
0.0339
294 (2)
8398
5667
0.0165
Reflns collected
Reflns unique
R(int)
Parameters
299
1.022
0.0381
0.0783
290
1.010
0.0321
0.0908
451
1.031
0.0294
0.0681
443
1.039
0.0307
0.0706
GOF on F2
a
R1 [I > 2s(I)]
b
wR2 [I > 2s(I)]
-3
˚
Max. peak/hole [e A ]
0.620/- 0.498
0.526/- 0.337
0.401/- 0.330
0.355/- 0.255
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = ꢀF0|-|Fcꢀ/ |F0|. b wR2 = [ (|F0 - Fce|)2/ (wF02)2]1/2
.
2718 | Dalton Trans., 2009, 2712–2720
This journal is
The Royal Society of Chemistry 2009
©

View Online
13C NMR (100 MHz, CDCl3, d ppm): 49.5, 115.2, 122.2 (q, JF-C
=
Notes and references
40.2 Hz), 124.5 (q, JF-C = 261.6 Hz), 127.4, 147.2, 206.9. 19F
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NMR (376 MHz, CDCl3, d ppm): -61.7 (s, 3F, CF3). IR (CH2Cl2):
n
max/cm-1 (CO) 2076, 2033, and 2008. Calc. for C15H8F3Fe2NO6S2:
C, 33.93; H, 1.52; N, 2.64, Found C, 33.65; H, 1.65; N, 2.95.
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at room temperature for 3 h. The color of the initially red solution
became brownish. After evaporation of the solvent, the residue
was eluted with CH2Cl2/petroleum ether (V/V: 1/5) on a silica
gel column to afford 3 as brown air-stable solids (yield 66%). 1H
NMR (400 MHz, CDCl3, d ppm): 7.46–7.70 (m, 15H, 3C6H5),
3.68 (d, J = 11.4 Hz, 2H, CH2), 2.89 (s, 2H, CH2). 19F NMR
(376 MHz, CDCl3, d ppm): -55.6 (s, 3F, CF3), -146.8 (s, 2F, ArF),
-146.2 (s, 2F, ArF). IR (CH2Cl2): nmax/cm-1 (CO) 2048, 1986, and
1937. Calc. for C32H19F7Fe2NO5PS2: C, 46.50; H, 2.60; N, 1.64,
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A red mixture of 2 (0.106 g, 0.2 mmol), PPh3 (0.052 g, 0.2 mmol),
Me3NO·2H2O (0.022 g, 0.2 mmol) in MeCN (15 mL) was stirred
at room temperature for 3 h. After evaporation of the solvent, the
residue was eluted with CH2Cl2/petroleum ether (V/V: 1/5) on a
silica gel column to give 4 as brown air-stable solids (yield 52%).
1H NMR (400 MHz, CDCl3, d ppm): 7.46–7.74 (m, 17H), 6.61
(s, 2H), 4.08 (d, J = 10.24 Hz, 2H, CH2), 2.93 (s, 2H, CH2). 19F
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n
max/cm-1 (CO) 2047, 1985 and 1925. Found C, 50.32; H, 3.48; N,
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X-Ray crystal structure determination of complexes 1–4
Single crystals suitable for X-ray analysis of complexes 1–4 were
obtained by immersing hexane into CH2Cl2. Accurate unit cell
parameters were determined by least-squares fit of 2q values,
intensity data sets were measured on a Bruker Smart 1000 CCD
˚
diffractometer with Mo-Ka radiation (l = 0.71073 A). The
intensities were corrected for Lorentz and polarization effects.
All structures were solved by direct methods. The nonhydrogen
atoms were located in successive difference Fourier synthesis.
The final refinement was performed by full-matrix least-squares
methods with anisotropic thermal parameters for non-hydrogen
atoms on F2. The hydrogen atoms were added theoretically and
treated as riding on the concerted atoms. Crystallography data
and experimental details for structure analyse were summarized
in Table 2.
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We thank the National Science Foundation of China (No.
20732007, 20728506, 20672122), the Ministry of Science and
Technology of China (No. 2006CB806105, 2007CB808004
2007CB936001 and 2009CB22008), and the Bureau for Basic
Research of the Chinese Academy of Sciences for financial
support.
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2720 | Dalton Trans., 2009, 2712–2720
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The Royal Society of Chemistry 2009
©