The influence of a peripheral functional group of diiron hexacarbonyl complexes on their electrochemistry and electrocatalytic reduction of proton

Article abstract of DOI:10.1016/j.electacta.2017.07.072

Three diiron hexacarbonyl complexes (2, 3 and 4) with naphthalene-1,8-bis(thiolate) skeleton as their bridging linkages are reported. For comparison, the protonated form of complex 4 was also prepared (4H⁺). They bear respectively a functional group on the naphthalene ring at the position 2, i.e. ?CH₂OH (2), ?COOH (3), ?CH₂N(Et)₂ (4) and ?CH₂NH⁺(Et)₂ (4H⁺). Complex 2 was derived from the direct reduction of its precursor bearing aldehyde group (?CHO, 1) by NaBH₄ while complexes 3 and 4 were routinely synthesized by reacting Fe₃(CO)₁₂ with ligands L₂ and L₃, respectively, which were derived from ligand L₁, naphtho[1,8-cd][1,2]dithiole-3-carbaldehyde. These complexes were fully characterized and complexes 3 and 4 were analyzed using X-ray single crystal diffraction. Electrochemistry of these complexes was also investigated by cyclic voltammetry. The carboxylic acid of complex 3 shows significant influence on the second reduction due to the acid group involving reaction with the reduced species. Both infrared spectral data and the first reduction potentials of the complexes suggest that these functional groups exert hardly electronic influence on the metal center. However, the functional groups which can carry proton (?COOH and ?CH₂N⁺H(Et)₂) can ease the kinetics of the catalysis of proton reduction via probably PCET (proton-coupled electron transfer) mechanism. These proton carriers can also improve catalytic efficiency by acting a proton relay during the catalysis as suggested by the linear plots of peak current against acid concentration for the three complexes.

Full text of DOI:10.1016/j.electacta.2017.07.072

Accepted Manuscript
Title: The influence of a peripheral functional group of diiron
hexacarbonyl complexes on their electrochemistry and
electrocatalytic reduction of proton
Authors: Wei Zhong, Zhiyin Xiao, Guifen Qian, Xiaoming Liu
PII:
S0013-4686(17)31489-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.07.072
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The influence of a peripheral functional group of diiron hexacarbonyl complexes
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The influence of a peripheral functional group of diiron hexacarbonyl complexes on
their electrochemistry and electrocatalytic reduction of proton
Wei Zhong a,b, Zhiyin Xiao a, Guifen Qian c, Xiaoming Liu a,*
a College of Biological, Chemical Sciences and Engineering
Jiaxing University
Jiaxing 314001, China
b China-Australia Institute for Advanced Materials and Manufacturing
Jiaxing University
Jiaxing 314001, China
c School of Chemistry
Nanchang University
Nanchang 330031, China
Email: xiaoming.liu@mail.zjxu.edu.cn; xiaoming.liu@ncu.edu.cn
Tel./Fax: +86 (0)573 83643937
Graphic abstract
1

An internal proton acceptor (base group) can ease the kinetics of the catalysis of
proton reduction via probably PCET (proton-coupled electron transfer) mechanism by
relaying proton during catalysis.
Highlights
 Diiron hexacarbonyl complexes with naphthalene moiety bearing functional
groups.
 The functional groups exert hardly electronic influence on the metal center but
can ease the kinetics of the catalysis of proton reduction.
2

 Carboxylic acid and amine groups act as a proton relay and thus improve
catalytic efficiency.
Abstract
Three diiron hexacarbonyl complexes (2, 3 and 4) with naphthalene-1,8-bis(thiolate)
skeleton as their bridging linkages are reported. For comparison, the protonated form
of complex 4 was also prepared (4H). They bear respectively a functional group on
the naphthalene ring at the position 2, i.e. CH2OH (2), COOH (3), CH2N(Et)2 (4)
and CH2NH(Et)2 (4H). Complex 2 was derived from the direct reduction of its
precursor bearing aldehyde group (CHO, 1) by NaBH4 while complexes 3 and 4
were routinely synthesized by reacting Fe3(CO)12 with ligands L2 and L3, respectively,
which were derived from ligand L1, naphtho[1,8-cd][1,2]dithiole-3-carbaldehyde.
These complexes were fully characterized and complexes 3 and 4 were analyzed
using X-ray single crystal diffraction. Electrochemistry of these complexes was also
investigated by cyclic voltammetry. The carboxylic acid of complex 3 shows
significant influence on the second reduction due to the acid group involving reaction
with the reduced species. Both infrared spectral data and the first reduction potentials
of the complexes suggest that these functional groups exert hardly electronic
influence on the metal center. However, the functional groups which can carry proton
(COOH and CH2NH(Et)2) can ease the kinetics of the catalysis of proton
reduction via probably PCET (proton-coupled electron transfer) mechanism. These
proton carriers can also improve catalytic efficiency by acting a proton relay during
the catalysis as suggested by the linear plots of peak current against acid
concentration for the three complexes.
Keywords: [FeFe]-hydrogenase; diiron carbonyl complex; electrocatalysis; proton
reduction; proton-coupled electron transfer (PCET)
3

1. Introduction
Fossil fuels are arguably regarded as limited energy resources and searching for
alternatives has been one of the intensely studied areas. Amongst the many options,
hydrogen has attracted great attention due to its cleanness and sustainability since its
oxidation releases energy and a sole by-product water from which, in principle,
hydrogen can be regenerated via, for example, solar splitting of water. Therefore, the
revelation of the structure of [FeFe]-hydrogenase (Fig. 1, left) [1, 2] initiated great
enthusiasm in mimicking the metal center of the enzyme.
<Fig. 1>
Nearly twenty-year’s effort from the research groups around the world including our
own group has made a great collection of diiron complexes with the form shown in
Fig. 1 (right) as the mimics of the diiron center of the H-cluster of the enzyme [3-7].
The review published by Tard and Pickett summarized well the categories of those
mimics [7]. Although new mimics keep published thereafter [8-14], they do not fall
beyond the categories defined by the review. Either from the enzyme itself or its
mimics, it is clear that a functional group in the secondary coordinating sphere plays
an important role in catalysis. In the enzyme, the middle atom of the dithiolate
bridging linkage is conclusively confirmed as an N atom which plays a critical role as
a proton relay in catalysis [15]. Of the numerous mimics published so far, examples
which demonstrate the essential role of a base in the non-coordinating sphere of a
diiron complex are not unprecedented and confirm the role of such a functional group
in a diiron mimics in proton reduction as a proton relay [11, 16-21]. However, how
the basicity of such a functional group affects significantly the catalysis is not
explored systematically. To this end, further on our previous report on the diiron
mimicking system bearing naphthalene skeleton in the bridging linkage [22, 23], we
exploit
the
reactivity
of
the
aldehyde
group
of
compound
4

naphtho[1,8-cd][1,2]dithiole-3-carbaldehyde (L1) to convert it into alcohol, carboxylic
acid and amine and their corresponding diiron mimics (2, 3 and 4), Scheme 1. These
groups show apparently various proton affinities and are appropriate complexes for
examining how the functional groups exert influence on the catalysis of these mimics.
Herein, we report their synthesis, characterization and electrochemical behaviors. Our
results suggest that a functional group of strong acidity born by a diiron complex may
be beneficial to improving the catalytic efficiency on reduction of proton.
2. Experimental
2.1 General procedures
Reactions were carried out under an atmosphere of argon using standard Schlenk and
high-vacuum techniques. Chemicals were purchased from Aladdin and Alfa Aesar and
used without further purification unless otherwise stated. Most of the organic solvents
in this work were purified using drying agents. Ligand L1 and complexes 1 and 2
were synthesized using the procedure we reported recently [22].
FTIR spectra were recorded on an Agilent 640 spectrometer using a CaF2-cell with a
spacer of 0.1 mm (wavenumber range: 400–4000 cm–1; repetitive scans: 8; resolution:
1
1 cm–1). Elemental analyses were performed on Elementar Vario MICRO. H and 13C
NMR spectra were recorded on Varian 400MHz in CDCl3. Crystallographic data of
complexes 3 and 4 were collected on a Bruker SMART CCD diffractometer with
graphite-monochromated Mo–Kα radiation (λ = 0.71073 Å). The crystal structures
were solved using direct methods in SHELXS program and refined by full-matrix
least-squares routines, based on F2, using the SHELXL package [24]. The
crystallographic data of complexes 3 and 4 were deposited in the Cambridge
Crystallographic Data Center (CCDC 895030 for complex 3 and 1518273 for
complex 4). Electrochemistry was performed in a gas-tighten three-electrode system
in which a vitreous carbon disk ( = 1 mm) was used as a working electrode, a carbon
5

strip as counter electrode, and Ag / AgCl reference electrode (inner reference solution:
0.45 mol L−1 [NnBu4]BF4 + 0.05 mol L−1 [NnBu4]Cl in dichloromethane) against
which the potential of ferrocenium / ferrocene couple is 0.55 V in 0.5 mol L−1
[NnBu4]BF4 in dichloromethane as described elsewhere [25, 26]. Ferrocene was
added as an internal standard and all potentials are quoted against ferrocenium /
ferrocene couple (Fc+ / Fc).
2.2 Synthesis
2.2.1 Preparation of 1,8-S2-2-COOHC10H5 (L2)
Compound L1 (179 mg, 0.82 mmol) and KOH (169 mg, 3.02 mmol) were dissolved
in ethanol (40 mL). The mixture was refluxed for 3 hours. After the removal of the
solvent under reduced pressure, water (20 mL) was added and the residue was
extracted by dichloromethane (3 × 20 mL). The aqueous phase was treated by
hydrochloric acid (1:1, wt%) to adjust to pH at 2~3 to form the brown solid L2 (178
mg, 93%). Compound L2 was used for the next step without any further purification.
IR (KBr, cm−1): ν (COOH), 1722.
2.2.2 Preparation of [Fe2(CO)6(1,8-S2-2-COOHC10H5)] (3)
Fe3(CO)12 (199 mg, 0.39 mmol) and L2 (84 mg, 0.36 mmol) were dissolved in dried
THF (15 mL). The mixture was refluxed for 2 hours under argon atmosphere before
the removal of the solvent under reduced pressure. Purification by column
chromatography (elution：dichloromethane / acetic acid = 100 / 1, v / v) yielded
1
complex 3 as red powder (145 mg, 79%). H NMR (d6-DMSO, 400 MHz): δ 13.90
(1H, s, COOH), 8.45 (1H, d, J = 8.0 Hz, NapH), 8.28 (2H, t, J = 8.0 Hz, NapH), 7.65
13
(1H, d, J = 8.0 Hz, NapH), 7.59 (1H, t, J = 8.0 Hz, NapH). C NMR (CDCl3, 400
MHz): δ 207.6, 169.4, 139.4, 134.0, 133.8, 132.8, 132.4, 126.6, 125.7, 124.2, 124.0,
120.0. Microanalysis (C17H6Fe2O8S2, MW = 514.04), calc. (found): C%, 39.72
6

(39.47), H%, 1.18 (1.27). IR (dichloromethane, cm−1): ν (CO), 2075, 2039, 2001, ν
(COOH), 1722.
2.2.3 Preparation of 1,8-S2-2-CH2N(Et)2C10H5 (L3)
Diethylamine (352 mg, 4.8 mmol) and L1 (218 mg, 1.0 mmol) were mixed in
1,2-dichloroethane (10 mL) and then treated with sodium triacetoxyborohydride (532
mg, 2.5 mmol). The mixture was stirred at room temperature under argon atmosphere
for 12 h. The reaction mixture was quenched by adding aqueous saturated NaHCO3,
and the product was extracted with ethyl acetate. The organic phase was dried
(MgSO4), and the solvent was evaporated under reduced pressure to give crude
product. Purification on a silica-gel column using a mixture of dichloromethane /
1
petroleum ether (1 / 2) as eluent to produce a yellow solid L3 (236 mg, 86%). H
NMR (CDCl3, 400 MHz): δ 8.42 (1H, d, J = 8.0 Hz, NapH), 8.13 (1H, d, J = 8.0 Hz,
NapH), 7.79 (1H, d, J = 8.0 Hz, NapH), 7.44 (1H, d, J = 8.0 Hz, NapH), 7.08 (1H, d,
J = 8.0 Hz, NapH), 3.71 (2H, s, CH2N), 2.69 (4H, m, 2×CH2), 1.12 (6H, t, J = 8.0 Hz,
13
2×CH3). C NMR (CDCl3, 100 MHz): δ 130.8, 127.9, 127.6, 126.9, 126.6, 122.2,
121.8, 118.8, 116.0, 105.0, 55.9, 45.8, 10.3.
2.2.3 Preparation of [Fe2(CO)6(1,8-S2-2-CH2N(Et)2C10H5)] (4)
Fe3(CO)12 (452 mg, 0.90 mmol) and L3 (220 mg, 0.80 mmol) were dissolved in dry
THF (30 mL). The mixture was refluxed for 2 hours under argon atmosphere before
removal of the solvent under reduced pressure. Purification by column
chromatography (elution：petroleum ether / ethyl acetate = 10 / 1, v / v) yielded
complex 4 as red powder (393 mg, 89%). 1H NMR (CDCl3, 400 MHz): δ 8.24 (1H, d,
J = 8.0 Hz, NapH), 7.96 (1H, d, J = 8.0 Hz, NapH), 7.90 (1H, d, J = 8.0 Hz, NapH),
7.70 (1H, d, J = 8.0 Hz, NapH), 7.36 (1H, t, J = 8.0 Hz, NapH), 4.18 (2H, s, CH2N),
2.61 (4H, m, 2×CH2), 1.09 (6H, t, J = 8.0 Hz, 2×CH3). 13C NMR (CDCl3, 100 MHz):
δ 207.8, 144.8, 133.5, 132.8, 132.0, 130.6, 128.8, 127.1, 124.9, 124.7, 123.6, 56.6,
7

46.9, 11.8. Microanalysis (C21H17Fe2NO6S2, MW = 555.18), calc. (found): C%, 45.43
(45.71), H%, 3.09 (3.03), N%, 2.52 (2.36). IR (dichloromethane, cm−1): ν (CO), 2074,
2039, 1999.
2.2.4 Preparation of [Fe2(CO)6(1,8-S2-2-CH2NH(Et)2C10H5)BF4] (4H+)
To a solution of complex 4 (278 mg, 0.5 mmol) in dichloromethane (10 mL) was
added HBF4·Et2O (89 mg, 0.55 mmol) to form a red-brown precipitate immediately.
The mixture was stirred at room temperature for 1 hour under N2 atmosphere.
Removal of the solvent gave a red-brown residual which was washed with dry hexane
1
(3 × 20 mL) and then dried under vacuo (297 mg, 92%). H NMR (CD3CN, 400
MHz): δ 8.40 (1H, d, J = 8.0 Hz, NapH), 8.20 (2H, t, J = 8.0 Hz, NapH), 7.66 (1H, d,
J = 8.0 Hz, NapH), 7.57 (1H, d, J = 8.0 Hz, NapH), 6.99 (1H, s, N+H(Et)2), 4.95 (2H,
13
d, J = 4.0 Hz, CH2N), 3.28 (4H, m, 2 × CH2), 1.39 (6H, t, J = 8.0 Hz, 2 × CH3). C
NMR (CD3CN, 100 MHz): δ 207.5, 134.9, 134.2, 132.8, 132.7, 129.2, 127.1, 127.0,
126.9, 125.2, 118.3, 54.8, 48.5, 8.3. HRMS (ESI, ): m / z 555.9217 (calcd. for 4H:

555.9274); (ESI, ): m/z 87.0059 (calcd. for BF4 : 87.0035). Microanalysis for
4H+·H2O (C21H20Fe2NO7S2BF4, MW = 661.01), calc. (found): C%, 38.16 (37.79),
H%, 3.05 (2.86), N%, 2.12 (1.95). IR (dichloromethane, cm−1): ν (CO), 2078, 2043,
2005.
3. Results and discussion
3.1 Synthesis and characterization of naphthalene-1,8-dithiolate diiron
hexacarbonyl complexes
Naphthalene-1,8-disulfide ligands L1–L3 and their diiron hexacarbonyl complexes
1–4 were synthesized following the routes shown in Scheme 1. Ligand L1 was
prepared using modified literature procedure [27] and reported in our previous work
[22]. Ligands L2 and L3 were derived from L1 via Cannizzaro reaction and reductive
amination, respectively. Reaction of ligands L1–L3 with triiron dodecacarbonyl led to
8

three diiron hexacarbonyl complexes 1, 3 and 4, respectively. The robustness of the
diiron core in complex 1 allowed reducing the formyl group by NaBH4 into
hydroxymethyl group at low temperature, which afforded complex 2 in a moderate
yield [23].
<Scheme 1>
Complexes 3 and 4 were fully characterized by NMR, FTIR spectroscopies, elemental
1
analysis, and X-ray crystallography. H NMR spectra showed the expected aromatic
signals in the range of 7.30~8.50 ppm and the characteristic –COOH signal of
complex 3 with a broad singlet at 13.90 ppm. 13C NMR spectra contain a
characteristic peak around 208 ppm, which is assigned to the carbonyl group of the
ligands. In dichloromethane, complexes 2–4 exhibit similar infrared spectral pattern,
three absorption bands between 2080 and 2000 cm–1 (Fig. 2), which are analogous to
the reported diiron hexacarbonyl complex bearing a bridging linkage of
naphthalene-1,8-dithiolate [23, 28-30]. Due to that these peripheral groups are
three-atom away from the diiron center, their electronic effect on the metal center is
weak and thus only subtle differences are caused.
The crystallographic details and selected bonding parameters of complexes 3 and 4
are tabulated in Tables S1 and S2, respectively. As shown in Fig. 3, the molecular
structures reveal the characteristic butterfly Fe2S2 core of complexes 3 and 4 featuring
an Fe−Fe bond length of 2.5224(17) Å and 2.5120(11) Å, respectively, which are
analogous to previously reported diiron dithiolato complexes [23, 28-30].
<Fig. 2>
<Fig. 3>
9

3.2 Electrochemistry of complexes 2–4
Due to the poor solubility of complex 3 in common organic solvents such as
acetonitrile, tetrahydrofuran and dichloromethane, electrochemistry of complexes 2–4
was performed in dimethylformamide (DMF). In the cyclic voltammograms (CVs) of
complexes 2 and 4, similar electrochemical profile to what we reported previously is
exhibited [22, 23]. Different from the complexes bearing benzenedithiolate bridging
linkage which exhibit a perfect two-electron process [31-36], these complexes show
two reduction processes, Fig. 4. The first one is at around –1.40 V and the second one
appears at a potential more negative by approximately from 400 mV to 600 mV (vs.
Fc+ / Fc, sic passim). The first reduction for complexes 2 and 4 shows decent
reversibility at the time scale of cyclic voltammetry whereas the second reduction is
completely irreversible due to a fast reaction coupled to the second electron transfer
[5, 22, 31, 34, 35, 37-40]. Since these complexes are highly similarly in structure with
the complex we reported previously [22], their electron transfer process may adopt the
same mechanism.
As mentioned above, the variation of the functional group on the naphthalene moiety
does not change significantly their infrared spectra and their first reduction process.
However, the functional group exerts much stronger influence on the second
reduction process as shown in Fig. 4. Compared to complex 4, complex 2 has more
negative potential for the second reduction (Fig. 4). As we discussed before [22], this
suggests that 2 is thermodynamically more stable than 4. For complex 3, however,
both reductions are irreversible. The presence of the carboxylic acid group not only
destabilizes thermodynamically the monoanion 3, but also affects the reversibility of
the first redox process. There has been report that diiron carbonyl complexes bearing
a carboxylic group are usually instable [41]. This effect stems certainly from the
reactivity of the carboxylic group. As shown in Fig. 4 (bottom), if the scanning did not
go through the second reduction process, the first redox process exhibits decent
reversibility whereas scanning further negatively to cover the second reduction, in its
10

returning scanning, the reversibility of the first process losts completely. Very likely,
the proton of the carboxylic acid reacts with both the monoanion (3) and the dianion
2

(3 ). This suggests that the reactivity of the monoanion in this case is rather limited
and mainly the dianion reacts with the proton of the carboxylic group. The reaction
stops the isomerization which ensures the system reversible [22]. The
electrochemistry of complex 3 in acidic medium supports this argument (vide infra,
Fig. 5).
<Fig. 4>
3.3 Catalysis on proton reduction by the complexes
<Fig. 5>
The electrochemistry or catalysis on proton reduction of the three complexes in acidic
medium was examined. The electrochemistry was performed in DMF and
trifluoroacetic acid (TFA) was used as the proton source. Their responses upon the
addition of the acid are shown in Fig. 5. Detailed comparison with / without the
presence of the complexes were shown in Fig. S1. In general, all the complexes
exhibit rather similar behaviors in electrochemistry. The first reduction shows rather
weak catalysis on proton reduction and that the major catalysis occurs at 1.9 V.
However, further looking into the CVs in details could reveal that the influence of the
various functional groups is quite different on their catalytic behaviors. For complex 2
bearing a hydroxylmethyl group, it shows much clean electrochemistry, a major
catalytic process at about 1.9 V right before the second reduction. Most likely, this
can be attributed to the reduction of the protonated monoanion 2H which is the
product of protonation of the monoanion 2. Its further reduction to 2H and catalysis
on proton reduction ought to occur at a potential more positive than that for the
reduction of 2. The first reduction of complex 3 shows slightly better catalysis
11

compared to complex 2 which may be due to the proton-relay effect of the carboxylic
group. For the second catalysis, its onset potential is about 100 mV more positive than
that for complex 2. Again, the proton-relay effect of the carboxylic group plays
probably a role in this enhancement. It is particularly noteworthy that after 6 eq. of the
acid was added, the catalysis shifts to a potential more negative by about 50 mV.
Whether this shift is caused by a new process or not is not clear.
Due to the presence of CH2N(Et)2 group, complex 4 shows two distinct features. At
the early stage of the addition of the acid, the first reduction potential shifts positively
by about 100 mV. We believe that this shift is caused by the positive charge due to
protonation of the amine group to form the cation, 4H To confirm this hypothesis,
protonated form (4H) of complex 4 was prepared. Its FTIR spectrum and CV are
shown in Figs. 2 and 4, respectively. Since the positively charged group is far away
from the metal center, the absorption bands of the bound CO shifted to higher
frequencies by a few wavenumbers. Although all characterization data including its
FTIR spectral profile suggest its genuineness, its electrochemical response looks
complicated compared to other CVs as shown in Fig. 4. The complication can be
attributed to complex 4H undergoes dissociation which is not unprecedented [42].
When the solvent is absolutely dry, this dissociation may be neglected. For solvent
such as DMF which is notorious for its difficulty to get rid of trace water. Therefore,
the dissociation might not be avoidable. The dissociation led to complex 4 in addition
to 4H,as indicated by the FTIR spectra variation in Fig. 6. Comparing the CVs of
complex 4 with the presence of the acid which generates in-situ 4H and the isolated
complex 4H indicates clearly the presence of the two species in solution, Fig. 7 and
Fig. S2. The second feature is that it exhibits much more positive onset potential (at
ca. 1.5 V) after the first process. This is about 400 mV more positive than that for
the other two complexes. This enhancement is presumably achieved by the cationic
nature of the protonated species (4H) which eases kinetically the electron transfer
process. Like complex 3, after a certain concentration of the acid, the second catalysis
shifts to a more negative potential and again exact cause for this is not clear.
12

<Fig. 6>
<Fig. 7>
Based on above discussion and the mechanism we proposed before [22], simplified
mechanisms for the proton reduction catalyzed by these complexes are depicted in
Scheme 2ac. As we proposed in our previous report [22], the first process is a mixed
2

one comprising a two-electron reduction to produce an isomerized dianion (X’ , X 
2, 3, 4), Scheme 2. The weak catalysis observed in the first process should be
attributed to this species, which support further the mechanistic analysis that in the
first process, the two-electron process with potential inversion is not dominant. As
shown in Fig. 5, both complexes 3 and 4 catalyze proton reduction at on-set potentials
more positive than complex 2 by about 100 mV and 200 mV, respectively. As
indicated by their infrared spectral absorption bands, electronically, the influence of
these functional groups on the metal center shows no much difference. Therefore,
such a difference does not stem from any alternation caused by these functional
groups in electron density of the metal center. But looking into the three functional
groups, CH2OH (2), COOH or COO (3) and CH2N(Et)2 (4), we can easily figure out
that the group, CH2OH, does not have the capability of relaying proton. In other word,
it is the key that the functional group involves in transferring proton in the catalysis.
As shown in Scheme 2, for the two complexes, when the species after first
one-electron reduction (3 or 4H) accepts the second electron, the group, COOH or
CH2NH(Et)2 can transfer its proton to the metal center and accept readily another
proton. This is of the characteristic feature of PCET (proton-coupled electron transfer)
which is a mechanism widely observed in both laboratory and nature [43-47]. The
concerted proton coupling can greatly reduce the reaction energy barrier. This may
explain why the second catalysis for both complexes 3 and 4 starts at a more positive
potential than complex 2. Further examining the catalytic peak current against the
13

added acid concentration reveals that these functional groups enhance also catalytic
efficiency, Fig. 8, since the internal base can efficiently transfer proton during the
catalysis.
<Scheme 2>
<Fig. 8>
4. Conclusions
In summary, three diiron complexes bearing naphthalene moiety as their bridging
linkage were synthesized and fully characterized. The naphthalene moiety of the three
complexes possesses a functional group, CH2OH (2), COOH (3) and CH2N(Et)2 (4),
respectively. The structures of complexes 3 and 4 were crystallographically
determined. These functional groups exert no much electronic influence on the metal
center as suggested by their infrared spectral absorption bands and their redox
potentials for their first process. However, when the functional group is a carboxylic
acid (3), it alters significantly the second redox process due to that the proton of the
acid group de-stabilizes the monoanion 3 via probably internal protonation. In the
catalysis of proton reduction, both groups, COOH (3) and CH2N(Et)2 (4), enhance the
catalysis occurred after the first reduction by shifting their on-set potentials by about
100 mV and 200 mV, respectively. The enhancement is probably achieved by PCET
mechanism. For complex 4, the cationic nature after protonation can further drive
positively the reduction potential. The peripheral internal base can also improve the
catalytic efficiency as the gradients of the linear plots suggested (Fig. 8).
14

Acknowledgements
We thank the Natural Science Foundation of China (Grant Nos. 21571083, 21171073),
the provincial government of Zhejiang for Qianjiang Professorship (X Liu) and
Jiaxing University (Summit Program of Jiaxing University for Leading Talents) for
financial supports.
15

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Figures and Schemes
Fig. 1 Schematic view of the H-cluster of [FeFe]-hydrogenase and general form of a
mimic in which the thiolates could be derived from monothiol or dithiol (the dash line)
and L could be a CO molecule or a non-CO ligand.
Fig. 2 FTIR spectra of complexes 2–4 and 4H+ in CH2Cl2.
21

Fig. 3 Crystal structures of complexes 3 (top) and 4 (bottom) with 30% displacement
ellipsoids. All H atoms are omitted for clarity.
22

23

1
Fig. 4 Cyclic voltammograms of complexes 2–4 and 4H+ (c = 2.86 mmol L ) in 0.25
1
1
mol L [NnBu4]BF4 / DMF solution at a scanning rate of 0.1 V s (298 K) (top),
cyclic voltammograms of complex 3 at various potential ranges (middle) and cyclic
voltammograms of complex 4H+ at various potential ranges (bottom).
24

25

Fig. 5 Cyclic voltammograms of complexes 2 (top), 3 (middle) and 4 (bottom) (c =
1
1
2.86 mmol L ) in 0.1 mol L [NnBu4]BF4 / DMF solution upon successive addition
of TFA (scanning rate = 0.1 V s–1): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 mol equiv.
Fig. 6 FTIR spectra of the complexes. Black and Red: complexes 4 and 4H+ in dry
DMF, respectively; Blue: complex 4H+ in open air in DMF in which complex 4H+
26

dissociates partially into complex 4. Green: removing the contribution of complex
4H+ from the blue spectrum gives the green one (complex 4 generated from
dissociation).
27

Fig. 7 Cyclic voltammograms and differential pulse voltammograms of 4 (top), 4H+
(middle) and the comparisons of differential pulse voltammograms of 4 and 4H+
1
1
(bottom) (c = 2.86 mmol L ) in 0.1 mol L [NnBu4]BF4 / DMF solution upon the
addition of TFA (scanning rate = 0.1 V s–1).
28

Fig. 8 Plots of peak current against the addition of the acid.
29