Structural effects upon the durability of hydrogenase-inspired hydrogen-producing electrocatalysts: Variations in the (μ-edt)[Fe 2(CO)6] system

Article abstract of DOI:10.1016/j.jorganchem.2012.12.006

Three variations of the common [FeFe]-Hydrogenase active site mimic μ-(SCH₂CH₂S)-Fe₂(CO)₆, 1, have been studied: μ-(SCH(CH₃)CH₂S)-Fe₂(CO) ₆, 2; μ-(SCH(CH₃)CH(CH₃)S)-Fe ₂(CO)₆, 3; and μ-(SCH₂CH(CH ₂OH)S)-Fe₂(CO)₆, 4. The electrochemistry of these four species is compared, indicating that 3 displays greater durability upon electrochemical cycling. This enhanced stability is ascribed to steric blockage of decomposition routes, rather than any change to the Fe ₂S₂ butterfly structure. Electrocatalysis using a weak acid (4-tert-butylphenol, pK_a = 27.5 in acetonitrile) is observed for each catalyst with similar overpotential. The enhanced stability of the reduction products of 3 leads to a modest increase of the apparent catalytic rate. Compound 4 was tested for an enhancement of the rate of the acid deprotonation step; compound 4 failed to behave in such a manner, providing no improvement in electrochemical catalysis compared to the parent compound.

Full text of DOI:10.1016/j.jorganchem.2012.12.006

Journal of Organometallic Chemistry 726 (2013) 9e13
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Structural effects upon the durability of hydrogenase-inspired
hydrogen-producing electrocatalysts: Variations in the ( -edt)
Fe (CO) ] system
m
[
2
6
a
b
a,
*
Elizabeth S. Donovan , Gary S. Nichol , Greg A.N. Felton
a
Department of Chemistry, 260 Science and Engineering Building, Oakland University, Rochester, MI 48309-4477, USA
School of Chemistry, The University of Edinburgh, Edinburgh EH9 3JJ, UK
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 October 2012
Received in revised form
Three variations of the common [FeFe]-Hydrogenase active site mimic
been studied: -(SCH(CH )CH S)eFe (CO) , 2; -(SCH(CH )CH(CH )S)eFe
S)eFe (CO)
durability upon electrochemical cycling. This enhanced stability is ascribed to steric blockage of decom-
position routes, rather than any change to the Fe butterfly structure. Electrocatalysis using a weak acid (4-
tert-butylphenol, pK
enhanced stability of the reduction products of 3 leads to a modest increase of the apparent catalytic rate.
Compound 4 was tested for an enhancement of the rate of the acid deprotonation step; compound 4 failed to
behave in such a manner, providing no improvement in electrochemical catalysis compared to the parent
compound.
m
e(SCH CH S)eFe (CO) , 1, have
2
2
2
6
m
3
2
2
6
m
3
3
2
(CO)
6
, 3; and
2 2
m-(SCH CH(CH OH)
2
6
, 4. The electrochemistry of these four species is compared, indicating that 3 displays greater
3
December 2012
Accepted 4 December 2012
2 2
S
a
¼ 27.5 in acetonitrile) is observed for each catalyst with similar overpotential. The
Keywords:
Ironesulphur
Hydrogenase model
Electrocatalysis
Hydrogen bonding
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
studied as a H
2
-producing electrocatalyst in the presence of acid
[2,17]. A number of compounds with structural adjustments to the
The compound
m
e(SCH
2
CH
2
S)eFe
2
(CO)
6
, 1 [1], commonly
ethane bridge have been reported, whereby one (or more protons)
have been replaced with alkyl [18] or carboxylic acid groups [19],
or the ethandithiolato ligand is part of a larger cycloalkane/alkene
structure [20].
referred to as “edt” [2] may be considered a simple structural
paradigm for mimicking the [FeFe]-Hydrogenase active site
Figs. 1 and 2) [3]. The attempt to recreate the Hydrogenase
(
enzyme’s ability to reversibly reduce protons to molecular
hydrogen, utilizing a small molecule [4], represents a prominent
scientific endeavour. Such a discovery could lead to a carbon-free
hydrogen-fuelled global economy [5]. The initial observation of
biomimetic hydrogen evolution from diiron dithiolates [6] has
burgeoned into a large field of study [7]. Wherein, catalyst’s
abilities and mechanisms of operation are spectroscopically [8],
computationally [9] and electrochemically [10] interrogated [11].
Compound 1 was first synthesized almost 50 years ago [1], fol-
lowed by an improved synthesis [12] and two X-ray character-
izations [13]. It has been studied as a model of the active site of
the [FeFe]-Hydrogenase enzyme [14], particularly when CO
groups are replaced with phosphine ligands [15]. 1 is observed as
a product of CeS/N bond rearrangement [16], and has been
The nature of the substituents on the ethanedithiolato bridge has
been systematically altered to probe the effect upon reductive elec-
trochemistry and electrocatalytic production of hydrogen (in the
presence of a proton source). The parent compound, 1, is compared
to
-(SCH(CH
Fe (CO) , 4 [22]. (Scheme 1)
m
-(SCH(CH
3
)CH
2
S)eFe
)S)eFe
2
(CO)
6
,
2
[21], the novel compound
m
3
)CH(CH
3
2
(CO)
6
,
3, and -(SCH CH(CH OH)S)e
m
2
2
2
6
2. Results and discussion
2
.1. Synthesis of -(SCH(CH )CH(CH )S)eFe (CO)
m
3 3 2 6
The reaction of iron dodecacarbonyl with 2,3-butanedithiol
yielded the expected -(SCH(CH )CH(CH )S)eFe (CO) , 3, a dime-
m
3
3
2
6
thylated version of the parent compound, 1. Crystallization of this
compound gave a syn:anti isomer ratio of 30:70 (the solution ratio
was 76:24 using NMR) which was not further separated prior to
electrochemical study.
*
Corresponding author. Tel.: þ1 248 370 2349; fax: þ1 248 370 2321.
E-mail address: felton@oakland.edu (G.A.N. Felton).
0
022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.12.006

10
E.S. Donovan et al. / Journal of Organometallic Chemistry 726 (2013) 9e13
The reverse peak of compound 3 is more prominent than for the
other three compounds, indicating greater electrochemical revers-
ibility, and hence greater chemical stability for any anions formed
from 3. Importantly, this stability is obtained without recourse to
a CO-atmosphere, as required to see such reversibility with
compound 1 [25]. Addition of a methyl group at each of the bridging
carbonsof 1 hasenhanced the durability of theanionsformed from 3,
by preventing the reaction of the cleaved FeeS bond with any species
in solution (such as unreduced 3) [26]. This leads to more FeeS
cleaved dianion, 3 , available for oxidation (and chemical rever-
sion to 3), and hence a larger oxidation peak in Fig. 3. The use of a CO-
atmosphere is unlikely to effect the reduction of these compounds,
but will have an effect upon the products available to undergo
oxidation. The broadoxidationwave forcompound 1 underCO[25] is
replaced by two oxidation peaks under an Ar atmosphere. The
positions of the oxidation waves for these compounds do not follow
a simple electron-density argument, and further study will be
required to definitively assign species to each oxidation wave.
Exhaustive electrolysis of 1 leads to dimerization via the attack of
a thiolate anion(formed viatwo broken FeeS bonds) on the iron ofan
unreduced molecule of 1 [26], a reaction that is retarded by the
additional methyl groups in 3.
Fig. 1. Proposed active-site structure of the [FeFe]-Hydrogenase enzyme.
2ꢀ
2
.2. Electrochemistry of complexes 1e4
Complexes 1e4 were studied using cyclic voltammetry (Fig. 3),
revealing a cathodic shift in the reduction potential as methyl
substituents are added to the two-carbon bridge of the parent
compound,1 (Table 1). A single methyl group yields a 30 mV change
in reduction potential, with a second methyl group yielding
a 20 mV further change. The increase in the number of methyl
2 2
groups leads to modest electron donation to the Fe S butterfly
core, and a subsequently more cathodic reduction. The reduction
potential of 4 is effectively the same as that for 1, a similarity in
electron density readily observed in the CO region of an IR spec-
trum (Fig. 4). Reduction potentials are similarly identical within the
series where R ¼ methyl [23], methoxy or hydroxyl in the three
The study of hydrogen-evolving electrocatalysts utilizing cyclic
voltammetry is well established [27], whereby addition of a proton
source leads to an increase in current at a particular potential [28].
The size of this catalytic peak can be used as an approximate measure
of catalytic rate, while the position of the peak can be used to eval-
2 2 2 6
carbon bridged m-(SCH CHRCH S)eFe (CO) [24].
All four species show a reduction peak that gradually transitions
from a one-electron process (fast scan rates) to a two-electron
process (slow scan rates) (SI-1 and SI-2), indicating a time-
dependent process (breaking of an FeeS bond) that allows
a second reduction to occur, as previously described for 1 [25]. This
previous study clarified the debate around one electron vs. two-
uate overpotential when the pK of the proton source is known [29].
a
Fig. 5 (and SI-6eSI-8) show the cyclic voltammetric response of
compounds 1 and 3 in the presence of a weak acid proton source (4-
tert-butylphenol, pKa ¼ 27.5). A catalytic peak can be seen at
a potential (ca. e2.2 V) where neither the catalysts alone (Fig. 5) nor
the acid are seen to reduce. While the overpotentials for compounds
1 and 3 are similar, at about 0.40 V (Table 1), there is a slight catalytic
rate advantage indicated by the height of the catalytic peak for 3. The
current for this catalytic peak is approximately 20% greater and has
more a “classical” peak shape, indicating fewer competing catalytic
and decomposition mechanisms [17]. A catalytic mechanism is
postulated in SI-13 where a broken FeeS bond is crucial to catalysis,
a bond breakage that has been confirmed spectroelectrochemically
[30] and chemically [31] for similar compounds.
2 2
electron reductions for related three-carbon bridge Fe S -
systems, by studying compound 1 under a CO atmosphere (mini-
mizing CO-loss degradation pathways). The slopes of the plots
presented in Fig. SI-4 are identical, indicating that the rate of FeeS
bond-breakage is unchanged by substitution upon the ethanedi-
thiolato bridge, and that the strength of the FeeS bond is similar in
all four cases. Additionally, the limited variation in the length of the
FeeS bond in each compound (Table 1) indicates similar FeeS bond
strengths.
Fig. 2. The structure of
3 3 2 6
m-(SCH(CH )CH(CH )S)eFe (CO) , 3, showing the major anti isomer (left), the minor syn isomer (right). Both isomers crystallized as overlapping in the same
asymmetric unit, rather than separately as a co-crystal. The refined syn:anti isomer ratio is approximately 30:70. Displacement ellipsoids are at the 50% probability level.

E.S. Donovan et al. / Journal of Organometallic Chemistry 726 (2013) 9e13
11
1
2
3
4
Scheme 1. The parent compound
SCH CH(CH OH)S)eFe (CO) , 4.
m
e(SCH
2
CH
2 2 6
S)eFe (CO) , 1, and the other three compounds of this study:
3
m-(SCH(CH )CH
2
S)eFe
2
(CO)
6
, 2;
3 3 2 6
m-(SCH(CH )CH(CH )S)eFe (CO) , 3; m-
(
2
2
2
6
2
.3. Hydrogen bonding
Fig. SI-12) or when comparing 3 with 4 with a moderately strong
acids (p-toluenesulfonic acid, pK
¼ 8.7, Fig. SI-15).
a
The use of a hydroxyl-containing electrocatalyst was explored
using -(SCH CH(CH OH)S)eFe (CO) , 4. A hydroxyl group could
operate as a proton shuttle, or act to enhance the acidity of weak
acids (in solution) via hydrogen bonding. Hydoxy-group containing
Fe S compounds [32] have recently been seen as hydrogenase
2 2
active site mimics [33]. Indeed, a hydroxyl group attached to the
central carbon of a three carbon thiolato bridge [34] has been
m
2
2
2
6
3. Conclusion
The addition of two methyl groups to a two-carbon bridged
thiolato-iron electrocatalyst leads to enhanced electrochemical
reversibility, without the use of a CO atmosphere [25]. A modest
improvement in the ability of compound 3 to evolve hydrogen is
observed (Fig. 5). The suggestion herein is that steric bulk upon the
dithiolate bridge of hydrogenase-inspired catalysts could yield
electrocatalysts with longer lifetimes (greater turnover numbers).
studied, albeit unsuccessfully, for its ability to act as a pK
a
enhancing hydrogen-bonding moiety [35]. Other hydrogenase
active site mimics have been designed with pendant hydroxyl
groups [36] to probe the nature of the exogenous aquo-ligand in the
enzyme resting state [37]. Attachment of the deprotonated oxygen
to an iron (via CO replacement) was not synthetically attempted
with 4 as it was felt this was sterically impossible. The use of
hydroxyl moieties that are sterically unable to bond to iron [38] has
lead to the observation that intramolecular hydrogen bonding to S
of the dianion anodically shifts the reduction potential of the
species [39]. No such hydrogen-bonding is observed herein as no
anodic shift is observed for compound 4, given that the Hammett
parameter for a CH OH group is identical to that of H, and that the
2
reduction potential and IR spectra of compounds 1 and 4 are
effectively identical.
The hydrogen-bonding of the hydroxyl group of 4 to an acid in
solution would be apparent in an enhancement of the catalytic
peak height for a given acid, when compared to a related species
containing no hydroxyl group. No such enhancement was observed
4. Materials and methods
Iron dodecacarbonyl (500 mg, 0.1 mmol) was refluxed under Ar
with the appropriate dithiol (0.1 mmol) in toluene for 90 min,
during which the solution changed from dark green to deep red.
After cooling to room temperature, the solution was filtered; the
filtrate was evaporated to dryness, and applied to a silica prepa-
ratory TLC plate. A red solid was recovered as the first band when
eluted with pentane, and then purified by recrystallization from
diethyl ether.
1
m
e(SCH
match to reference 2a.
-(SCH(CH )CH S)eFe
and IR match to reference 21.
-(SCH(CH )CH(CH )S)eFe
NMR (CDCl ):
CO ¼ 208.80,
H NMR (400 MHz, CDCl ) :
d, 6H, cis isomer, CH
2
CH
2
S)eFe
2
(CO)
6
, 1: Yield: 141 mg (55%). H NMR and IR
3
2
2 6
(CO)
, 2: Yield: 106 mg (40%). 1H NMR
m
m
3
3
2
(CO)
alkyl ¼ 19.27, 20.74, 49.37, 55.20 ppm.
¼ 1.32 (d, 6H, trans isomer, CH ), 1.19
, 3: Yield: 138 mg (49%). 13C
6
when comparing 2 with 4 with a weak acid (acetic acid, pK
a
¼ 22.3,
3
d
d
d
1
3
3
(
3
), 2.21 (m, 2H, trans isomer), 2.92 (m, 2H, cis
Table 1
Reduction potentials, carbonyl IR frequencies, X-ray derived bond lengths and
overpotentials for catalysis (with 4-tert-butylphenol).
1
2
3
4
E
pc (V)a
ꢀ1.626
ꢀ1.656
2077
2037
2008
1994
1984
2.520d
2.238d
e
ꢀ1.676
2076
2036
2008
1992
1982
2.513
2.256
0.40
ꢀ1.634
2078
2039
2009
1996
1985
2.499e
2.239e
0.75
IRCO (cm 1)b
ꢀ
2078
2
2
1
1
038
008
996
985
ꢀ
f
c
c
FeeFe (A)
FeeS (A)
2.502
2.244
ꢀ
g
Eover (V) 4-t-BuPhOH
0.36/0.56
a
b
c
d
e
f
þ
ꢀ1
.
Potential of reduction peak vs Fc/Fc at 0.100 V s
In hexanes.
Taken from reference [13]b.
Taken from reference [21].
Taken from reference [22].
Fig. 3. Cyclic voltammograms (CV) of 1.00 mM of 1 (green), 2 (red), 3 (blue), and 4
purple) in 0.100 M Bu NPF in acetonitrile at 0.100 V/s. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
For asymmetrically substituted compounds, 2 and 4, FeeS refers to the average
(
4
6
FeeS bond length.
g
Two catalytic peaks are observed for compound 1 in the presence of 10 mM 4-
this article.)
tert-butylphenol (Fig. SI-6).

12
E.S. Donovan et al. / Journal of Organometallic Chemistry 726 (2013) 9e13
resistance from the electrolyte solution in acetonitrile was
compensated for by applying an automated voltage proportional to
a resistance of 180
U
.
Acknowledgements
The authors acknowledge the financial support of the Oakland
University Kenny Biomedical Science Fellowship (ED), Provost
Research award (ED) and faculty start-up grant (GF). The National
Science Foundation (NSF) instrumentation award CHE-0821487
(
(
NMR) is acknowledged. GN acknowledges CHE-0741837
diffractometer).
Appendix A. Supplementary data
Fig. 4. Overlaid IR spectra of compounds 1e4 in hexane highlight the subtle change in
CO vibration frequency (see also Table 1). Compound 4 is at a lower concentration due
to low solubility in hexane.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2012.12.006.
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2
Cl
2
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n
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.
The red solid was dissolved in diethyl ether and allowed to evap-
orate at 4 C, yielding a large red crystal suitable for X-ray
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