[FeFe]-Hydrogenase Mimic Employing κ2-C,N-Pyridine Bridgehead Catalyzes Proton Reduction at Mild Overpotential

Article abstract of DOI:10.1002/ejic.201900405

Two novel κ²-C,N-pyridine bridged [FeFe]-H₂ase mimics (1 and 2) have been prepared and are shown to function as efficient molecular catalysts for electrocatalytic proton reduction. The elemental and structural composition of the complexes are confirmed by NMR and IR spectroscopy, high-resolution mass spectrometry and single-crystal X-ray diffraction. Electrochemical investigations reveal that the complexes reduce protons at their first reduction potential, resulting in the lowest overpotential (120 mV) ever reported for [FeFe]-H₂ase mimics in proton reduction catalysis when mild acid (phenol) is used as proton source.

Full text of DOI:10.1002/ejic.201900405

A Journal of
Accepted Article
Title: [FeFe]–hydrogenase mimic employing κ2–C,N–pyridine
bridgehead catalyzes proton reduction at mild overpotential
Authors: Esther Schipper, Sandra Nurttila, Jean-Pierre Oudsen,
Moniek Tromp, Wojciech Dzik, Jarl Ivar van der Vlugt, and
Joost Reek
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900405
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10.1002/ejic.201900405
European Journal of Inorganic Chemistry
FULL PAPER
[FeFe]–hydrogenase mimic employing κ2–C,N–pyridine
bridgehead catalyzes proton reduction at mild overpotential
Esther C. F. Schippers+,[a] Sandra S. Nurttila+,[a] Jean–Pierre H. Oudsen,[b] Moniek Tromp,[b] Wojciech I.
Dzik,[a] Jarl Ivar van der Vlugt,[a] and Joost N. H. Reek*[a]
Abstract: Two novel κ2–C,N–pyridine bridged [FeFe]–H2ase mimics
(1 and 2) have been prepared and are shown to function as efficient
molecular catalysts for electrocatalytic proton reduction. The
elemental and structural composition of the complexes are confirmed
by NMR and IR spectroscopy, high–resolution mass spectrometry and
single crystal X–ray diffraction. Electrochemical investigations reveal
that the complexes reduce protons at their first reduction potential,
resulting in the lowest overpotential (120 mV) ever reported for
[FeFe]–H2ase mimics in proton reduction catalysis when mild acid
(phenol) is used as proton source.
In most [FeFe]–H2ase mimics the two iron atoms are
connected via a bridging dithiolate fragment and each iron atom
is coordinated by three terminal carbonyl ligands. Along these
lines, the first class of mimics was based on structures in which a
propanedithiolato (μ–pdt) fragment bridged the two iron atoms,
and this group of mimics has been studied in detail by several
groups including Pickett, Darensbourg and Rauchfuss (Figure
1).[8] The next generation of mimics focused on analogues with
the biologically relevant aza–dithiolato (μ–adt) bridge, wherein the
basic amine functionality can act as ‘proton relay’.[13,14] Among the
many other dithiolato bridges, the more rigid benzenedithiolato
(μ–bdt) bridge has been studied in some detail and found to give
rather active proton reduction catalysts.[15,16] Most parent
hexacarbonyl complexes can undergo substitution of one or more
of the six carbonyls by a great variety of ligands, which by now
has resulted in a library of hundreds of reported complexes.
Typically, these substitutions lead to an increase in the complex’
overall basicity, resulting in higher reduction potentials. Given the
inverse relationship between overall basicity and redox potential,
such terminal ligand substitutions generally do not lead to
catalysts that operate with high rates at a mild overpotential.
Introduction
To facilitate large–scale production of cheap renewable energy
that can be stored cost–effectively, there is a great demand for
catalysts that can efficiently produce dihydrogen from water and
are preferably made from earth–abundant transition metals. The
[FeFe]–hydrogenase ([FeFe]–H2ase) enzymes catalyze the
reversible reduction of protons at ambient conditions with a low
overpotential.[2] It is envisioned that synthetic mimics of the active
site of the [FeFe]–H2ase enzyme can serve as efficient catalysts
for proton reduction in renewable fuel applications.[2],[3] Ever since
the structure of the active site of the [FeFe]–H2ase enzyme was
elucidated,[4–7] a large number of synthetic mimics has been
developed.[8–11] It has been shown that it is possible to develop
mimics that operate at similar and even higher rates than the
natural enzyme.[12] Next to a high rate it is important to develop
catalytic systems that operate at a low overpotential, as it is
essential to reduce the loss of energy in the overall conversion of
electrical energy to chemical energy. Despite intensive
investigations, the development of synthetic [FeFe]–H2ase
mimics that operate at a mild overpotential remains a challenge
to be solved.
Figure 1. Common [FeFe]–H2ase mimics employing different dithiolate–based
bridges.
The beneficial effect on the catalytic overpotential when
transitioning from a μ–pdt to a μ–adt bridge suggests that
modification of the bridging ligand could be key to lowering the
overpotential. The class of hydrogenase mimics based on a
bridging pyridine–monothiolate ligand is relatively unexplored.
The synthesis of such complexes has been described from the
corresponding thioester[17] and to the best of our knowledge there
is only one earlier report on the catalytic activity in the presence
of acetic acid.[18]
With the aim to study the effect of changing the dithiolate
bridge for a pyridine bridge on the catalytic overpotential, we
report novel [FeFe]–H2ase mimics 1 and 2, in which the iron
atoms are connected by a pyridine bridge in a κ2–C,N fashion
(Figure 2). The pyridine ring of 1 is substituted with a thioisopropyl
group and that of 2 with a dimethylamine group, which may serve
as a ‘proton relay’ due to its basic nature. The catalysts are
capable of reducing protons from acids that are weaker than
acetic acid at their first reduction potential, resulting in a catalytic
[a]
E. C. F. Schippers,+ Dr. S. S. Nurttila,+ Dr. W. I. Dzik, Dr. Ir. J. I. van
der Vlugt, Prof. Dr. J. N. H. Reek
Homogeneous, Supramolecular and Bio–Inspired Catalysis, Van ‘t
Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904, 1098 XH Amsterdam (The Netherlands)
E–mail: j.n.h.reek@uva.nl
Homepage: homkat.nl
[b]
[+]
J. P. H. Oudsen, Prof. Dr. M. Tromp
Sustainable Materials Characterization, Van ‘t Hoff Institute for
Molecular Sciences, University of Amsterdam
Science Park 904, 1098 XH Amsterdam (The Netherlands)
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201900405
European Journal of Inorganic Chemistry
FULL PAPER
1
overpotential that is up to 240 mV smaller than that of [Fe2(μ–
information, Figure S1). The doublets in the H NMR spectrum
bdt)(CO)6].
indicate that the CH3 groups are diastereotopic. X-ray analysis of
1 confirms that it exists as two enantiomers, making the methyl
groups diastereotopic (Supporting information, Figure S8). The
13C NMR spectrum of 1 shows a signal at 193.5 ppm, which is
typical for a species with a Fe–C bond (Supporting information,
Figure S2).[22] Complex 2 shows similar features in its 1H and 13
C
NMR spectrum (Supporting information, Figures S3–S4).
Figure 2. Structures and relative overpotentials of the novel κ2–C,N pyridine di–
iron complexes.
Table 1. IR stretching frequencies of 1 and 2 compared to [Fe2(μ–bdt)(CO)6]
and [Fe2(μ–pdt)(CO)6].
Catalyst
IR stretches of three
Solvent
main bands [cm–1
2065, 2024, 1988
2062, 2022, 1986
2072, 2032, 1988
2079, 2044, 2004
]
Results and Discussion
Complex 1
Pentane
Pentane
Toluene
Hexane
Synthesis and characterization
Complex 2
Ligand L1 is obtained in one step according to a literature
procedure, by reacting 2,3–dichloropyridine with an excess of
sodium isopropylthiolate at 85 °C in an SNAr reaction.[19] Ligand
L2 is prepared in two steps starting from 3–amino–2–
chloropyridine. First, the free aminopyridine is methylated using
formaldehyde and formic acid in an Eschweiler–Clarke reaction.
Subsequently, the chloride substituent is displaced by
isopropylthiolate in an SN2 reaction. Complex 1 is obtained in 27%
yield by heating a mixture of L1 and iron precursor Fe2(CO)9 (2
equiv.) in toluene at 100 °C for 15 min under an inert atmosphere
(Scheme 1). Complex 2 (17% yield) is prepared using an identical
procedure in the presence of ligand L2 (for the full synthetic
protocol of 1 and 2, see Supporting information, Section 2).
[Fe2(μ–pdt)(CO)6][23]
[Fe2(μ–bdt)(CO)6][24]
Complex 1 was crystallized by layering a dichloromethane
solution with pentane. Single crystals of 2 were obtained by slow
evaporation of a pentane solution under argon at 5 °C. The crystal
structures are similar to previously reported κ2–C,N–pyridine
bridged di–iron compounds (Figure 3).[17,18] The Fe1–Fe2 bond
lengths of 1 and 2 are 2.5741(5) and 2.5726(5) Å, respectively.
These bond lengths are slightly larger than the Fe–Fe bond length
in [Fe2(μ–bdt)(CO)6] (2.480(2) Å[20]
) and [Fe2(μ–pdt)(CO)6]
(2.5103(11) Å[25]). The Fe2–C7 bond length is 1.996(3) Å for 1 and
1.992(2) Å for 2 and this is comparable to the Fe–C bond length
in in similar κ2–C,N bridged hydrogenase mimics.[17] The Fe1–N1
bond is 1.984(2) for 1 and a little shorter for 2 (1.9744(19) Å).
Scheme 1. Synthesis of novel [FeFe]–H2ase mimics 1 and 2.
1 and 2 are fully characterized by IR and NMR spectroscopy,
high–resolution mass spectrometry and single crystal X–ray
diffraction (for full characterization, see Supporting information,
Section 2). The IR spectra of both complexes display the typical
fingerprint of di–iron hexacarbonyl complexes (Table 1 and
Supporting information, Figures S5–S7).[20,21] The bands of 2
appear at a lower stretching frequencies compared to those of 1,
in line with more electron–rich iron centers of 2 due to the
electron–donating amino substituent. The 1H NMR spectrum of 1
shows two signals with a doublet splitting pattern for the CH3
groups of the bridging isopropyl substituent (Supporting
Figure 3. X–ray crystal structures of 1 (left) and 2 (right) with displacement
ellipsoids drawn at 50% probability. Hydrogen atoms have been omitted for
clarity. The CCDC number of 1 is 1893259 and that of 2 is 1893262. Color code:
C, grey; N, blue; O, red; sulfur, yellow; iron, orange.
Redox behavior of 1 and 2 in the absence of acid
Cyclic voltammetry of 1 in acetonitrile reveals an irreversible
reduction wave with a cathodic peak potential of around –1.8 V
(vs. Fc0/+; all potentials are reported against this redox couple),
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10.1002/ejic.201900405
European Journal of Inorganic Chemistry
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followed by at least one anodically shifted re–oxidation wave at
around –0.6 V (Figure 4). Complex 2 displays a similar redox
behavior as 1, but with a 50 mV cathodic shift in both the reduction
and oxidation event. This shift is caused by the more electron–
donating nature of the dimethylamine substituent of 2 as
compared to the isopropylthiol substituent of 1, and this is in line
with the observations in the IR measurements (vide supra). Both
1 and 2 are more difficult to reduce than known [FeFe]–H2ase
mimics [Fe2(μ–bdt)(CO)6] and [Fe2(μ–pdt)(CO)6] (Table 2 and
Supporting information, Figures S13–S14). For both 1 and 2 the
peak current of the reduction wave varies linearly with the square
root of the scan rate, indicative of a solution–based redox event
(Supporting information, Figures S9–S12).[26]
1730 cm–1, which is characteristic for reduced diiron compounds
with a bridging carbonyl ligand.[27,28] Complex 2 shows similar
bands in its IR spectrum upon reduction (Figure 5b).
Figure 5. IR spectroscopic changes observed during the reduction of 2 mM 1
(a) and 2 (b) in CH3CN containing 0.2 M NBu4PF6 (0.001 V s–1).
The semi–integral convolution plot of 1 in the presence of
an equimolar amount of ferrocene suggests a one–electron
reduction process, assuming that the diffusion constant of 1 and
ferrocene are similar (Supporting information, Figure S15).
Controlled potential coulometry (–1.9 V) of a solution of 1 confirms
the passage of one electron per molecule (Supporting information,
Figure S16). Moreover, the IR spectrum of 1 in the presence of
1.6 equiv. of the reducing agent decamethylcobaltocene (E1/2 = –
1.91 V in MeCN)[29] shows complete reduction to 1– (Supporting
information, Figure S17). On the contrary, the IR spectrum of
[Fe2(μ–bdt)(CO)6], which is known to undergo disproportionation
and therefore has a two–electron reduction at E1/2 = –1.32 V,
shows a mixture of neutral and [FeFe]2– species in the presence
of the same amount of reductant (Supporting information, Figure
S18). Based on these experiments we prudently conclude that 1
undergoes a one–electron reduction, and 2 is expected to display
the same electrochemical behavior, albeit at slightly different
potential.
Figure 4. Cyclic voltammetry (0.1 V s–1) of 1.0 mM 1 and 2 in CH3CN containing
0.1 M NBu4PF6 on a glassy carbon working electrode.
Table 2. Reduction potentials of 1 and 2 compared to [Fe2(μ–bdt)(CO)6] and
[Fe2(μ–pdt)(CO)6] (0.1 V s–1, 0.1 M NBu4PF6 in CH3CN).
DFT calculations and XAS analysis on the reduced species 1–
The irreversible redox behavior of 1 suggests that the complex
displays follow–up chemistry upon reduction. More detailed
insight into the structure of the mono–reduced species comes
from DFT calculations (Supporting information, Section 6).
Computations were performed on the monoanionic 1– and anionic
12– species, both with all terminal carbonyl ligands and with one
bridging carbonyl ligand. A comparison of the computed IR
spectra with the experimental spectra reveals that mono–anion B,
which contains a bridging carbonyl ligand, shows the best fit
(Figure 6b). This is consistent with the spectroelectrochemical
measurements that indicate the presence of a bridging carbonyl
ligand upon reduction of 1. The significant difference in the
calculated and experimental wavenumber for the bridging
carbonyl ligand is likely due to its position being greatly affected
by the solvent.[30] The Fe–N bond and one of the Fe–S bonds are
broken in the mono–reduced species, allowing for the structural
rearrangement into a bridging carbonyl species (Figure 6a). Such
Catalyst
Reduction potential [V vs. Fc/Fc+]
Complex 1
–1.82
–1.87
–1.65
–1.32
Complex 2
[Fe2(μ–pdt)(CO)6]
[Fe2(μ–bdt)(CO)6]
Spectroelectrochemical studies provided more insight into
the structures of the species that are formed upon reduction of 1
and 2. Linear sweep voltammetry of 1, while probing the IR
spectrum, reveals bleaching of IR signals associated to the
neutral complex, concomitant with the appearance of new red–
shifted bands assigned to the reduced species 1– (Figure 5a). The
absorption-difference spectra show a small band growing in at
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10.1002/ejic.201900405
European Journal of Inorganic Chemistry
FULL PAPER
a rearrangement accounts for the irreversibility of the reduction
wave observed in the electrogram and is reminiscent of chemistry
observed with benzene dithiolate analogs.[15,16]
Extended X–ray absorption fine structure (EXAFS) analysis
of 1– shows the first Fe – C shell at a distance of 1.83(1) Å with
an overall coordination number of 2.5 and C/N bond distances of
2.10(1) Å with a coordination number of 2 (for full description of
EXAFS analysis, see Supporting Information, Section 5).
Additionally, the Fe – Fe contribution is fitted with an elongated
bond distance of 2.69(2) Å compared to the original value of
2.60(1) Å. This analysis confirms the breakage of the Fe – N bond
and one Fe – S bond and the formation of a bridged CO ligand,
as suggested by the IR analysis and DFT computations.
Figure 7. Electrocatalytic reduction using AcOH as proton source. (a) Cyclic
voltammetry (0.1 V s–1) of 1.0 mM 1 (red trace) and [Fe2(μ–bdt)(CO)6] (black
trace) in CH3CN containing NBu4PF6 and 0–60 equiv. AcOH. The inset shows
the cyclic voltammogram of 1 in the presence of 0 and 1 equiv. AcOH. (b) Cyclic
voltammetry (0.1 V s–1) of 1.0 mM 2 (red trace) and [Fe2(μ–bdt)(CO)6] (black
trace) in CH3CN containing NBu4PF6 and 0–60 equiv. AcOH. The inset shows
the cyclic voltammogram of 2 in the presence of 0 and 1 equiv. AcOH.
A comparison of the catalytic performance of 1 and 2 to that
of [Fe2(μ–bdt)(CO)6] reveals that these complexes behave
differently (Figure 7a–b). While 1 and 2 reduce protons from
AcOH at their first reduction potential, [Fe2(μ–bdt)(CO)6] displays
catalysis at a considerably more negative potential than its first
reduction potential. However, [Fe2(μ–bdt)(CO)6] is a faster
catalyst, as is evident from its sharper catalytic waves along with
a higher current (Figure 7a–b, black traces). The catalytic
parameters are determined as previously reported[33–37] and
summarized in Table 3 (For a detailed description of the
determination of the catalytic parameters, see Supporting
information, Section 7). Complex 2 operates with a three times
higher rate than 1, but it is still 250 times slower than [Fe2(μ–
bdt)(CO)6] (Table 3, Entries 1–3). The calculated overpotential (η)
for both 1 and 2 is 0.57 V, which is an 170 mV lower than that of
[Fe2(μ–bdt)(CO)6]. This large decrease in overpotential is a result
of 1 and 2 performing catalysis at the potential of their first
reduction.
Figure 6. (a) DFT calculated (BP86, def2–TZVP) structure (top) and chemical
structure (bottom) of species 1– (mono–anion B). (b) DFT calculated IR
spectrum of 1– (blue columns) overlaid with the experimental spectrum of 1–.
The calculated spectrum is scaled by νCO (scaled) = 1.023 × νCO (calc.) – 24.6.[30]
Electrocatalytic proton reduction
To investigate the effect of the κ2–C,N–pyridine bridge on the
catalytic performance of complexes 1 and 2, cyclic voltammetric
studies were undertaken in the presence of various weak acids
and the reactivity was compared to that of [Fe2(μ–bdt)(CO)6]. In
the presence of one equiv. acetic acid (AcOH; pKa = 22.3 in
CH3CN) the peak potential of 1 undergoes an anodic shift of
around 15 mV, indicating an electrochemical event followed by
protonation (Figure 7a, inset).[31,32] A slightly larger potential shift
(around 25 mV) in the presence of one equiv. of AcOH is
observed for 2, revealing that protonation of 2– is
thermodynamically more favorable than protonation of 1– (Figure
7b, inset).[31] The currents of the reduction waves of 1 and 2
increase as a function of the acid concentration, confirming
catalytic proton reduction at the first reduction potential for both
complexes (Figure 7a–b).
Table 3. Catalytic parameters of 1, 2 and [Fe2(μ–bdt)(CO)6] in the presence
of different acids (0.1 M NBu4PF6 in CH3CN).
Entry
Catalyst
Proton
source
Catalytic
E1/2
η [V]
kcat
[M–1s–1
[b]
]
(V vs.
Fc/Fc+)
1
2
3
4
5
6
7
1
AcOH
–1.77
–1.77
–1.94
–1.76
–1.81
–1.81
–1.79
0.57
0.57
0.74
0.12
0.17
0.88
0.86
136[b]
391[b]
2
AcOH
[Fe2(μ–bdt)(CO)6]
AcOH
1×105 [38]
1.4[b]
1
2
1
2
PhOH
PhOH
ClAcOH
ClAcOH
6.9[b]
566[b]
659[b]
[a] EHA/H2 = –0.028 –0.05916 × pKa; –1.64 V for PhOH and –0.93 V for
ClCH2COOH. For AcOH the effect of homoconjungation has been
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10.1002/ejic.201900405
European Journal of Inorganic Chemistry
FULL PAPER
described[34] and by taking this into account a value of –1.2 V is obtained
and applied as the thermodynamic potential.
similar behavior as 2, whereas [Fe2(μ–bdt)(CO)6] displays yet
again a higher catalytic rate but also a higher overpotential than
both 1 and 2 as clear from the higher current of the catalytic wave
as well as its more negative potential (Figures 9a and 9c).
[b] Calculated using Dubois’ formula as described in the Supporting
information, Section 7.
The low overpotential of 1 and 2 in the catalytic proton
reduction of the weak acid AcOH encouraged further studies with
the even weaker acid phenol (PhOH; pKa = 27.2 in MeCN). Cyclic
voltammetry of 1 or 2 in the presence of 500 equiv. PhOH as the
proton source reveals a catalytic wave at the first reduction
potential of the catalysts (Figure 8a–b). Complex 2 operates with
a five times higher rate than 1 and again is significantly slower
than [Fe2(μ–bdt)(CO)6], in line with studies performed with AcOH
(Figure 8c and Table 3, entries 4–5). Interestingly, the catalytic
overpotentials of both 1 and 2 in the reduction of PhOH are
remarkably low, with 1 operating at an overpotential of only 120
mV, which is the lowest overpotential ever reported for a [FeFe]–
H2ase mimic, to the best of our knowledge.
Figure 9. Electrocatalytic reduction of ClCH2COOH. (a) Cyclic voltammetry (0.1
V s–1) of 1.0 mM 1 in CH3CN containing 0.1 M NBu4PF6 and 0–50 equiv.
ClCH2COOH. (b) Cyclic voltammetry (0.1 V s–1) of 1.0 mM 2 in CH3CN
containing 0.1 M NBu4PF6 and 0–50 equiv. ClCH2COOH. (c) Cyclic voltammetry
(0.1 V s–1) of 1.0 mM [Fe2(μ–bdt)(CO)6] in CH3CN containing 0.1 M NBu4PF6
and 0–50 equiv. ClCH2COOH.
With the method reported by Artero and Savéant, a Tafel
plot is constructed for each catalyst–substrate combination from
TOFmax=2ꢀkcat[H+] (TOFmax
= maximum turnover frequency;
extrapolated for a 1ꢀM concentration of substrate).[39] The Tafel
plot for AcOH clearly demonstrates that 2 displays a higher
catalytic rate than 1 and [Fe2(μ–bdt)(CO)6] when operating at an
overpotential below 0.6 V. Above this threshold value, [Fe2(μ–
bdt)(CO)6] shows a significantly higher rate than 1 and 2 (Figure
10a). In the reduction of PhOH and ClCH2COOH 1 and 2 show
similar efficiency (Figure 10b). In the case of PhOH the
overpotentials of 1 and 2 are similar, but 2 operates with a higher
rate. For ClCH2COOH, 2 operates with both a higher rate and
lower overpotential than 1.
Figure 8. Electrocatalytic reduction of PhOH. (a) Cyclic voltammetry (0.1 V s–1
)
of 1.0 mM 1 in CH3CN containing 0.1 M NBu4PF6 and 0 or 500 equiv. PhOH. (b)
Cyclic voltammetry (0.1 V s–1) of 1.0 mM 2 in CH3CN containing 0.1 M NBu4PF6
and 0 or 500 equiv. PhOH. (c) Cyclic voltammetry (0.1 V s–1) of 1.0 mM [Fe2(μ–
bdt)(CO)6] in CH3CN containing 0.1 M NBu4PF6 and 0–0.09 M PhOH.
To investigate whether the dimethylamine substituent of 2
can be applied as a ‘proton relay’ and thereby also affect the
catalytic overpotential, catalytic studies were undertaken using
the slightly stronger acid, chloroacetic acid (ClCH2COOH; pKa =
15.3). Cyclic voltammetry of 2 in the presence of ClCH2COOH
reveals that the acid is not strong enough to protonate the
complex prior to reduction, as evident from the lack of an anodic
shift in the reduction wave (Figure 9b). A significant anodic shift
of the reduction wave of the catalyst would be expected if the
complex was protonated prior to reduction. Complex 1 shows
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10.1002/ejic.201900405
European Journal of Inorganic Chemistry
FULL PAPER
Figure 10. (a) Tafel plots of 1, 2 and [Fe2(μ–bdt)(CO)6] in the presence of AcOH.
(b) Tafel plots of 1 and 2 in the presence of PhOH and ClCH2COOH. The value
of TOFmax is extrapolated for a 1 M concentration of protons.
hexane/CH2Cl2 80:20). The thus obtained pure compound was dissolved
in 25 mL pentane after which the volatiles were removed under vacuum to
afford complex 1 as an orange powder (27% yield with respect to L1).
Single crystals suitable for X–ray diffraction analysis were obtained by
liquid–liquid diffusion of pentane into a solution of 1 in dichloromethane.
1H NMR (400 MHz, CD2Cl2) δ (ppm) = 7.44 (d, J = 5.4 Hz, 1H), 7.10 (d, J
= 7.9 Hz, 1H), 6.67 (dd, J = 7.9, 5.4 Hz, 1H), 3.33 (septet, J = 6.6 Hz, 1H),
2.61 (septet, J = 6.4 Hz, 1H), 1.49 (d, J = 6.4 Hz, 3H), 1.48 (d, J = 6.8 Hz,
3H), 1.33 (d, J = 6.5 Hz, 3H) 1.32 (d, J = 6.5 Hz, 3H). 13C NMR (101 MHz,
CD2Cl2) δ (ppm) = 213.60, 211.55, 211.15, 193.54, 150.86, 147.29, 132.49,
Conclusions
In this work we describe the electrocatalytic performance of two
novel well–defined and structurally characterized κ2–C,N–
pyridine–bridged [FeFe]–H2ase mimics (1 and 2) in proton
reduction catalysis. The effect of the pyridine bridge on the
catalytic properties of 1 and 2 is evaluated by comparing the
parameters with the known complex [Fe2(μ–bdt)(CO)6]. The novel
complexes are shown to reduce protons at their first reduction
potential, whereas [Fe2(μ–bdt)(CO)6] requires a more negative
potential to drive catalysis. As a consequence, proton reduction
catalysis is demonstrated with the lowest overpotential (120 mV)
ever reported for [FeFe]–H2ase mimics. The impact of the pyridine
bridge of 1 and 2 on their overpotential is remarkable, and the
effect of modifying the bridging fragment of di–iron hydrogenase
mimics is interesting to study further, as the development of a
system that operates with a mild overpotential is the key challenge
to efficient storage of electrical energy in chemical bonds.
120.75, 44.49, 37.95, 30.27, 27.31, 26.88, 22.99. FT–IR (pentane) cm–1
=
2065, 2026, 1994, 1985, 1972, 1970. HRMS (FD) calc. for [1]+
(C17H17Fe2NO6S2+) 506.91961, found: 506.92126.
Crystallographic details
1: C17H17Fe2NO6S2, Fw = 507.14, orange block, 0.630×0.403×0.200
mm, monoclinic, P21/n (No: 14)), a = 13.9060(12), b = 9.3308(8), c =
17.1082(14) Å, β = 9.3308(8) o, V = 2089.0(3) Å3, Z = 4, Dx = 1.612 g/cm3,
µ = 1.621 mm–1. 21812 Reflections were measured up to a resolution of
(sin q/l)max = 0.84 Å–1. 3693 Reflections were unique (Rint = 0.0417), of
which 3240 were observed [I>2s(I)]. 257 Parameters were refined with 0
restraints. R1/wR2 [I > 2s(I)]: 0.0308 / 0.1043. R1/wR2 [all refl.]: 0.0390 /
0.1216. S = 1.026. Residual electron density between –0.483 and 0.711
e/Å3. CCDC 1893259.
Synthesis of complex 2
Experimental Section
An oven–dried Schlenk flask was charged with Fe2(CO)9 (82 mg, 0.23
mmol) and equipped with a gas bubbler filled with oil via a needle through
the septum of the Schlenk flask. In a separate Schlenk flask L2 (40 mg,
0.2 mmol) was dissolved in 8 mL toluene and transferred to the iron
precursor and the resulting mixture was heated to 100 °C in a preheated
oil bath. The reaction progress was monitored by IR spectroscopy. After
15 minutes the dark red mixture was allowed to cool down to room
temperature. The volatiles, including the side product Fe(CO)5, were
carefully removed under vacuum. The crude product was purified by
column chromatography (silica, eluent: gradient from hexane to 0.5–1%
trimethylamine in hexane) to yield 2 as an orange solid in 17% yield. Single
crystals suitable for X–ray diffraction analysis were obtained by slow
evaporation of a pentane solution of 2 at 5 °C. 1H NMR (500 MHz, CD2Cl2)
δ (ppm) = 7.39 (d, J = 5.3 Hz, 1H), 7.02 – 6.91 (d, J = 8.0 Hz, 1H), 6.67
(dd, J = 8.0, 5.4 Hz, 1H), 2.63 (s, 6H), 2.57 (septet, J = 6.7 Hz, 1H), 1.48
(d, J = 6.8 Hz, 3H), 1.46 (d, J = 6.8 Hz, 3H). 13C NMR (126 MHz, CD2Cl2)
δ 193.54, 159.05, 150.15, 125.73, 120.83, 45.28, 44.28, 27.22, 26.89. FT–
IR (pentane) cm–1 =2067, 2063, 2024, 2000, 1991, 1984, 1969, 1963.
HRMS (FD) calc. for [2]+ (C16H16Fe2N2O6S+) 475.94279, found: 475.94134.
General procedures
All reactions were carried out under an atmosphere of argon using
standard Schlenk techniques. Solvents used for synthesis and analysis
were degassed and dried using suitable drying agents. Purification that
involves extraction or column chromatography was performed in air with
solvents used as received. The iron compounds were protected from light
as much as possible. Commercial chemicals were used without further
purification. The supporting electrolyte NBu4PF6 was prepared from
saturated solutions of NBu4Br and KPF6 in water and recrystallized several
times from hot methanol and dried overnight in a vacuum oven. Sodium
isopropylthiolate was obtained by stirring an excess of thiol and small
pieces of sodium in Et2O in a Schlenk flask connected to a gas bubbler at
room temperature until all the metallic sodium had reacted. All NMR
spectra were recorded on a Bruker Avance 400 (400 MHz) or a Bruker
DRX 500 (500 MHz) spectrometer and referenced internally to the residual
solvent signal of CD2Cl2: 1H (5.32 ppm) and 13C (54.00 ppm). IR
measurements were conducted on a Thermo Nicolet Nexus FT–IR
spectrometer. Mass spectra were collected on a JMS–T100GCV mass
spectrometer using field desorption (FD), or a JEOL AccuTOF LC, JMS–
T100LP mass spectrometer using electron–spray ionization (ESI).
Crystallographic details
2: C16H16Fe2N2O6S, Fw = 476.07, dark yellow block, 0.128×0.380×0.506
mm, monoclinic, P21/c (No: 14)), a = 14.2199(9), b = 14.2199(9), c =
17.2070(11) Å, β =109.504(2)o, V = 3959.7(4) Å3, Z = 8, Dx = 1.597 g/cm3,
µ = 1.604 mm–1. 114346 Reflections were measured up to a resolution of
(sin q/l)max = 0.84 Å–1. 6953 Reflections were unique (Rint = 0.0423), of
which 6052 were observed [I>2s(I)]. 495 Parameters were refined with 0
restraints. R1/wR2 [I > 2s(I)]: 0.0310/0.01046. R1/wR2 [all refl.]: 0.00391/
0.1171. S = 0.989. Residual electron density between –0.374 and 0.332
e/Å3. CCDC 1893262.
Synthesis of complex 1
An oven–dried Schlenk flask was charged with Fe2(CO)9 (1.34 g, 3.68
mmol) and equipped with a gas bubbler filled with oil via a needle through
the septum of the Schlenk flask. In a separate Schlenk flask, L1 (0.42 g,
1.85 mmol) was dissolved in 25 mL toluene. The solution was transferred
to the iron precursor and the mixture was heated to 100 °C in a preheated
oil bath. The reaction progress was monitored by IR spectroscopy. After a
reaction time of 15 minutes the dark red mixture was allowed to cool to
room temperature. The volatiles, including the side–product Fe(CO)5, were
carefully removed under vacuum. The crude product was purified by
column chromatography (silica, eluent: gradient from hexane to
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10.1002/ejic.201900405
European Journal of Inorganic Chemistry
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10.1002/ejic.201900405
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Entry for the Table of Contents (Please choose one layout)
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Esther C. F. Schippers, Sandra S.
Nurttila, Jean–Pierre H. Oudsen, Moniek
Tromp, Wojciech I. Dzik, Jarl Ivar van
der Vlugt, and Joost N. H. Reek*
Page No. – Page No.
[FeFe]–Hydrogenase Mimic
Employing κ2–C,N–pyridine
Bridgehead Catalyzes Proton
Reduction at Mild Overpotential
The synthesis, characterization and electrocatalytic activity of two novel κ2–C,N–
pyridine bridged [FeFe]–H2ase mimics is described. The complexes are shown to
function as efficient molecular catalysts for electrocatalytic proton reduction, resulting
in the lowest overpotential for [FeFe]–H2ase mimics reported to date.
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