Enhancement in catalytic proton reduction by an internal base in a diiron pentacarbonyl complex: its synthesis, characterisation, inter-conversion and electrochemical investigation

Article abstract of DOI:10.1039/c6dt04409c

The reaction of a tripodal ligand (H₂L) with a {S₂N} donor-set with tri-iron dodecacarbonyl in toluene leads to the isolation of a diiron pentacarbonyl complex 1 as a model for the sub-site of the [FeFe]-hydrogenase. Protonation of this complex under CO (1 atm.) forms quantitatively the hexacarbonyl complex 2H⁺ with a pendant pyridinium group. Infrared spectroscopic investigations indicate that its pendant pyridinium group dissociates to give hexacarbonyl complex 2 which forms subsequently the pentacarbonyl complex 1. The electrochemistry of these complexes has been investigated. Complex 2H⁺ exhibits electrocatalysis on proton reduction at a potential more positive by over 200 mV compared to that for other neutral diiron hexacarbonyl complexes. This catalysis is enhanced under a CO atmosphere by freeing the bound base group which acts as a proton relay in the catalysis.

Full text of DOI:10.1039/c6dt04409c

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Enhancement in catalytic proton reduction by an
internal base in a diiron pentacarbonyl complex:
its synthesis, characterisation, inter-conversion
and electrochemical investigation†
Cite this: DOI: 10.1039/c6dt04409c
Zhimei Li,a Zhiyin Xiao,b Fenfen Xu,a Xianghua Zengb and Xiaoming Liu*b
The reaction of a tripodal ligand (H2L) with a {S2N} donor-set with tri-iron dodecacarbonyl in toluene
leads to the isolation of a diiron pentacarbonyl complex 1 as a model for the sub-site of the [FeFe]-hydro-
genase. Protonation of this complex under CO (1 atm.) forms quantitatively the hexacarbonyl complex
2H+ with a pendant pyridinium group. Infrared spectroscopic investigations indicate that its pendant pyri-
dinium group dissociates to give hexacarbonyl complex 2 which forms subsequently the pentacarbonyl
complex 1. The electrochemistry of these complexes has been investigated. Complex 2H+ exhibits elec-
trocatalysis on proton reduction at a potential more positive by over 200 mV compared to that for other
neutral diiron hexacarbonyl complexes. This catalysis is enhanced under a CO atmosphere by freeing the
bound base group which acts as a proton relay in the catalysis.
Received 21st November 2016,
Accepted 11th January 2017
DOI: 10.1039/c6dt04409c
www.rsc.org/dalton
finally pinning down the oxidation state of the distal iron in
the diiron sub-site of the enzyme.13 Challenges of this type are
1 Introduction
Over nearly 20 years, chemistry related to [FeFe]-hydrogenase, often encountered in the studies of many other metallo-
which has a diiron centre as its active sub-site, Fig. 1a,1–4 has enzymes.14 The first mimics for the H-cluster framework of the
attracted considerable attention due to its relevance to hydro- [FeFe]-hydrogenase demonstrated that the {Fe4S4} cluster as an
gen production. The research on mimicking the diiron centre electron mediator has electronic communication with the
of the [FeFe]-hydrogenase has made significant progress in the diiron centre as evidenced by the CO stretching band shift of
view point of understanding the catalytic mechanism of the the diiron sub-unit.15 Thorough investigation into the electro-
enzyme and why nature chooses such unique “building chemistry of diiron mimics by a number of groups around the
blocks” to construct its catalytic machinery.5–12 For example, world including our group revealed that the diiron centre may
studies on the model complexes with a {Fe2S3}-core led to undergo two-electron reduction by exhibiting one single redox
peak in the cyclic voltammogram via an ECE mechanism.16–21
The coupled chemical reaction is an isomerisation that
probably is the breaking of one of the four Fe–S bonds, which
generates an intermediate whose reduction potential is likely
no more negative than that of the first reduction (potential
inversion). This observation may have relevance to explain why
the diiron active centre of the enzyme operates without an
over-potential.
One significant progress was made in the identification of
the nature of the middle atom of the dithiolate bridging the
two iron atoms in the subunit of the enzyme. It turned out
Fig. 1 The diiron units in (a) the H-cluster of the [FeFe]-hydrogenase,
(b) the model complex, 1 and (c) the protonated model complex, 2H+.
that only the diiron mimics bearing azodithiolate can function
after being assembled into the apo-HydA1 protein.22 It is
believed that some proximal base groups around the sub-
aSchool of Chemistry, Nanchang University, Nanchang 330031, China
bCollege of Biological, Chemical Sciences and Engineering, Jiaxing University,
site in the enzyme may have a role to play in proton transfer in
Jiaxing 314001, China. E-mail: xiaoming.liu@mail.zjxu.edu.cn,
catalysis.2,23 Indeed, a pendant basic group in an iron-hydride
xiaoming.liu@ncu.edu.cn; Fax: +86 (0)573 83643937; Tel: +86 (0)573 83643937
complex may relay protons.24 Recently, there have been reports
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6dt04409c
showing that the protonated internal base in model complexes
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could transfer the proton to the Fe–Fe bond forming
hydride.25–28 Rauchfuss and co-workers first reported the syn-
thesis of a structural model to mimic the proposed aza-bridge
head in the diiron sub-unit of the enzyme, in which a base
group was introduced via a ligand, azadithiolate.29 Although
the base group can be protonated, it does not seem to have sig-
nificant improvement in the catalysis of proton reduction
except for the positive shift of the reduction potential.30 This
potential shift could be contributed to the effect of positive
charge introduced by protonation. Due to the same inspi-
ration, a base group was also introduced into model complexes
by simply reacting the diiron hexacarbonyl complex with a free
base, for instance, a pyridine derivative.31,32
Scheme 1 Synthetic route of the ligand H2L and its diiron carbonyl
complex 1.
We reported preliminarily model complexes with
a
{Fe2S2N}-core,33 in which the N-containing functional group is
an integrated part of the entire organic moiety of the bridge
head. One of the reported model complexes possessed a pyri-
dine group, which gives a hexacarbonyl complex upon protona-
tion, Fig. 1b and c. In this complex (1), the hemi-labile Fe–N
bond underwent CO-substitution reaction and this CO substi-
tution was facilitated by protonation by following a “square
scheme”.33 Herein, we further report the synthesis, and full
characterisation of the pyridine containing model complexes,
particularly the electrochemistry, and electrocatalysis of the
hexacarbonyl complex (Fig. 1c) towards proton reduction.
2 Results and discussion
2.1 Synthesis of the tripodal ligand (H2L) and the model
Fig. 2 FTIR spectra of the complexes in dichloromethane: 1 under
N2/CO (1 atm.), 2 and 2H+ under N2.
complex (1)
Ligand H2L was synthesised using a modified literature
method.34 In an autoclave, an aqueous solution of form-
aldehyde (excess) and 2-ethylpyridine was heated for 2–3 days
to give a mixture of products, 2-methyl-2-(pyridin-2-yl)ethanol
and 2-methyl-2-(pyridin-2-yl)propane-1,3-diol, in equilibrium
with their precursor. Distillation for purifying the desired
product 2-methyl-2-(pyridin-2-yl)propane-1,3-diol as employed
in the literature procedure was not adopted due to the high
boiling points of these compounds. Instead, flash chromato-
graphy was used to isolate the diol. This approach improved
both the yield and efficiency. For the rest of the reactions in
the preparations of ligand H2L and its diiron carbonyl
complex, 1, general procedures used for the preparation of
model complexes with a {Fe2S3}-core were followed.13,35 A sche-
matic description of the syntheses is shown in Scheme 1.
Heating a mixture of ligand H2L and Fe3(CO)12 in toluene at
110 °C gave the desired pentacarbonyl products 1 in the yield
of 35% after flash chromatography purification. It should be
being stored in a freezer at −25 °C. Complex 1 in dichloro-
methane shows three major CO stretching bands (Fig. 2) with
a characteristic spectral pattern for model complexes of the
{Fe2S3}-core reported before.7,13 But the FTIR absorption
bands of complex 1 shift to lower frequencies by a few wave-
numbers compared to the SMe containing pentacarbonyl
complex,13 which shows that the pyridine N atom is a slightly
better electron donator than SMe. The electron donating capa-
bility of these ligands is in agreement with their basicity.
Although there is no pKa value available for ligand H2L, it
would be sensible to refer this ligand to 2-methylpyridine
whose pKa is about 6 in aqueous solution.37 Compared to the
pKa value of thioether, −6.8,38 ligand H2L is a much stronger
base than SMe.
2.2 Structural analysis
noticed that, after re-examination of the reaction of dithiol Complex 1 and its protonated hexacarbonyl 2H+ were crystallo-
(H2L) with Fe3(CO)12, another interesting diiron pentacarbonyl graphically characterized and preliminarily reported, Fig. 3.33
complex ([Fe2{(SCH2)2C(CH3)(2-C5H8N)}(CO)5]) with the pryidi- The crystal data and experimental details are summarized in
nyl ring of the ligand hydrogenated partially should be Table S1.† Selected bonds and bond angles of some of the
obtained, as we reported otherwise.36 Single crystals of complexes were tabulated for comparison in Table 1, in which
complex 1 suitable for X-ray diffraction were grown from aceto- the Fe1 is designated to the Fe atom proximal to the pyridine/
nitrile solution through either slow solvent evaporation or pyridinium ring. Compared to other complexes containing the
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2.3 Protonation of complex 1
Under a CO (1 atm.) atmosphere, addition of HBF4·Et2O to a
solution of complex 1 in organic solvent yielded quantitatively
the hexacarbonyl complex 2H+ with a pendant pyridinium
group, Fig. 1c. However, under a N2 atmosphere, adding one
equivalent of HBF4·Et2O to complex 1 formed a mixture of
complex 1 and complex 2H+, Fig. S7.† Increasing one more
equivalent acid led to the disappearance of complex 1 and for-
mation of complex 2H+. No other carbonyl complex was observed
as illustrated by their IR spectra of the protonation reactions. The
addition of an excess base (triethylamine) could fully deprotonate
complex 2H+ to complex 1 with a recovery of about 70%. At a low
temperature (<10 °C), the addition of one equivalent of acid
HBF4·Et2O to the solution of complex 1 in dichloromethane gen-
erates a mixture of the bridging hydride complex 1H+ and the
hexacarbonyl complex 2H+, Fig. 4. The spectrum of the bridg-
ing hydride 1H+ is similar to that of the amine containing
complex ([Fe2(μ-H){(SCH2)2C(CH3)(2-CH2NH2)}(CO)5]+, Table S2†)
reported previously but with less stability.33 This bridging
hydride decays to hexacarbonyl complex 2H+ by “cannibalising”
CO when raising the temperature.
It has been known that metal hydrides undergo intra-
molecular transfer.43 This intramolecular hydride transfer
strongly depends on the basicity of the hydride and its accep-
tor. Metal hydride is commonly regarded as a base in
Brønsted/Lowry acid–base theory.44 However, acidic metal
hydrides are observed and a typical example of this kind is the
classic iron carbonyl hydride, H2Fe(CO)4 with pKa = 4.4.45
Indeed, the bridging hydride 1H+ reacts with Et3N to restore
its parent complex 1, which shows its acidity. In the conversion
of the bridging hydride 1H+ to 2H+, the hydride may undergo
intramolecular transfer. As we reported before, the transfer
may adopt two pathways as suggested by DFT calculations.36
In either pathway, the bridging hydride in the species 1H+
switches to the terminal hydride first and this terminal
hydride eventually transfers to the nearby base. One of the
Fig. 3 Crystallographic structures of complexes 1 (a) and 2H+ (b).
Table 1 Selected bond lengths (Å) and angles (°) for complexes 1 and
2H+
Bond length/angle
1
2H+
Fe1–Fe2
Fe1–S1
Fe2–S1
Fe1–S2
Fe2–S2
2.5325(3)
2.2538(5)
2.2602(5)
2.2461(5)
2.2572(5)
2.0099(14)
68.255(14)
68.438(14)
93.10(4)
2.4990(4)
2.2814(5)
2.2595(5)
2.2509(6)
2.2573(5)
Fe1–N
Fe1–S1–Fe2
Fe1–S2–Fe2
S1–Fe1–N
S2–Fe1–N
66.778(15)
67.330(15)
93.96(4)
{Fe2S3}-core,13,35,39 the Fe1–Fe2 bond length (2.5325(3) Å) in
complex 1 is slightly longer. This bond lengthening is attribu-
ted to the π-interaction between the metals and the pyridine
group. The π-back bonding interaction between the Fe1 atom
and the pyridine ring strengthens the Fe1–N bonding. When
the π-back bonding interaction does not exist in complex 2H+,
the Fe1–Fe2 bond length is 2.4990(4) Å, comparable to the
bond lengths of this type found in most diiron hexacarbonyl
complexes.40–42
This π-back bonding interaction is also evidenced by the
UV/vis spectrum of complex 1, Fig. S6.† An absorption band
for complex 1 shows a red-shift down to 376 nm compared to
370 nm for other pentacarbonyl complexes.39 This red-shift
renders the complex dark green colour instead of the normally
observed dark brown for other pentacarbonyl complexes.13
This π-back bonding interaction is further demonstrated by
the improvement in the reversibility of the reduction of
complex 1 (vide infra).
For both pentacarbonyl,13,35,39 including complex 1, and
hexacarbonyl complexes,40–42 the four Fe–S bonds which
bridge the two iron atoms are nearly identical in bond length
with a standard deviation less than 0.007 Å. But for complex
2H+, the Fe1–S2 bond length at the same side as the title pyri-
dinium group is at least 0.02 Å longer than the other three
(Table 1). This bond lengthening is probably due to the steric
effect caused by the pyridinium group. Compared to the orien-
tation of the bound pyridine in complex 1, the pyridinium
group in complex 2H+ turns nearly by 90°.
Fig. 4 Infrared spectrum of complex 1 in dichloromethane at low
temperature after the addition of one equivalent of the acid, HBF4·Et2O,
which comprises the contributions from both 1H+ (inset) and 2H+.
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Scheme 3 Equilibrations among complexes established based on the
infrared spectrum of complex 2H+ in dichloromethane.
Scheme 2 Proposed pathway for the conversion of 1H+ to 2H+.
possible pathways is shown in Scheme 2.36 In the end, the pyri-
dine group is protonated and cleaved as pyridinium salt.
Consequently, a vacant site is exposed at the proximal
{Fe(CO)2} unit for CO binding.
In an absolutely dry solvent, the spectrum of complex 2H+
shows a characteristic spectral pattern for the hexacarbonyl
diiron complex. However, any trace amount of water in a
solvent in which its solution is prepared will significantly alter
its infrared spectrum, Fig. 5 (black). The multiple bands
between 2100 and 1900 cm−1 indicate multiple species in the
solution of hexacarbonyl 2H+. Further examination of this
spectrum and de-convolution show that there are spectral fea-
tures belonging to complexes 2 (green), 1 (blue) and 2H+ (red),
Fig. 5, respectively. The relationship of these species is
depicted in Scheme 3. The equilibrium between complexes 1
and 2 was observed when the solution of complex 1 was satu-
rated with CO,33 Fig. S8.†
Fig. 6 Cyclic voltammograms of complex 1 (3.7 mmol L−1) under an
Ar/CO atmosphere in dichloromethane at the scanning rate of 0.1 V s−1
at 298 K.
−1.83 V and one irreversible oxidation process at 0.22 V
(Fig. S9†). The reduction process of complex 1 is essentially
similar to those complexes containing the {Fe2S3}-core except
for enhanced reversibility.13 This is attributed to the moderate
π-accepting nature of the pyridine group as discussed above.
Under a CO atmosphere, the electrochemistry of complex 1
changed dramatically. Its quasi-reversibility under an Ar atmo-
sphere was lost. The reduction potential shifted positively with
a slight increase in current intensity. To understand this sig-
nificant change in electrochemistry, following observations
should be bore in mind. Complex 1 undergoes conversion into
complex 2 under a CO atmosphere (Scheme 3) and reduction
of hexacarbonyl complexes occurs at a potential more positive
than that of pentacarbonyl by over 100 mV.46 Thus, most
likely, the reduction potential shift could be attributed to the
formation of complex 2 under a CO atmosphere. This conver-
sion can be facilitated by the reduction of complex 2 whose
reduction potential is more positive than that of complex 1.
Varying the scanning rate increased steadily the peak current
intensity of the returning wave, Fig. S10.† The plot of the
reduction peak current against the square-root of the scanning
rate shows good linearity (R = 0.998), Fig. S10† (inset),
suggesting the diffusion-controlled nature of the reduction.
The electrochemistry of complex 2H+ in dichloromethane is
fundamentally different from other neutral hexacarbonyl com-
plexes, for instance [Fe2(pdt)(CO)6] (pdt = propane-1,3-dithio-
late).46 As shown in Fig. 7, there are two reduction processes,
2.4 Electrochemistry and electrocatalysis towards proton
reduction of complexes 1 and 2H+
Complex
1 is electrochemically investigated in dichloro-
methane. As shown in Fig. 6, it has one reduction process at
Fig. 5 FTIR spectrum of the solution of complex 2H+ (black) in “wet” di-
chloromethane which, in fact, comprises the spectra of complexes 1
(blue), 2H+ (red) and 2 (green), respectively.
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Fig. 7 Cyclic voltammogram of complex 2H+ (3.4 mmol L−1) under an
Ar atmosphere in dichloromethane at the scanning rate of 0.1 V s−1 at
298 K.
Fig. 8 Cyclic voltammograms of complex 1 (3.96 mmol L−1) with the
addition of lutidinium under an Ar atmosphere in dichloromethane at
the scanning rate of 0.1 V s−1 at 298 K.
one at −1.44 V and the other at −1.84 V. The peak current of
the latter process is about 1.6 fold stronger than that of the
first one. The peak potential separation between the two
processes is about 400 mV. As discussed above, complex 2H+
dissociates into 2 which forms further complex 1 in the absence
of external CO. It is known that protonation of a pendant base
of a diiron model complex can bring the reduction to more posi-
tive potential by a few hundred mV.47,48 Therefore, the first
reduction at −1.44 V is assigned to the reduction of complex
2H+ and the second process to that of complex 1.
In the oxidation potential range, complex 2H+ shows a
major irreversible peak at 0.86 V and a minor peak at 0.20 V,
Fig. S11.† The potential of the small peak is comparable to
that of the oxidation process of complex 1, Fig. S9.† Thus,
these oxidations are assigned to complexes 2H+ and 1, respect-
ively. This is further experimental evidence showing that the
major reduction of complex 2H+ has the origin of complex 1.
Upon addition of a weak acid, for example lutidinium,
complex 1 shows electrocatalysis on proton reduction and the
catalytic current increases steadily with the concentration of
the added acid, Fig. 8. Compared to the potential (ca. 1.9 V) for
Fig. 9 Electrochemical responses of complex 2H+ (3.4 mmol L−1) upon
addition of lutidinium under an Ar atmosphere in dichloromethane at
the scanning rate of 0.1 V s−1 at 298 K.
proton reduction in the absence of complex 1, the improve- the equilibrium concentration of this protonated complex 2H+.
ment in over-potential is not substantial. Since complex 1 Whatever the origin of this increase is, the first reduction does
cannot be protonated with this acid prior to reduction, the cat- not effectively catalyse proton reduction when this weak acid
alysis should be attributed to the species of the reduced was employed.
complex 1.
When a strong acid is used, for example HBF4·Et2O, the
Catalytic responses of complex 2H+ toward this weak acid electrochemistry of both complexes 1 and 2H+ converges to
are also investigated, Fig. 9. For the second reduction, the cata- that of complex 2H+ in that the protonation of complex 1
lysis is rather similar to that observed for complex 1 (Fig. 8). forms effectively complex 2H+. As shown in Fig. 10 and 11, the
Therefore, the second catalytic process is attributed to the protonated hexacarbonyl complex catalyses proton reduction
in situ generated complex 1. For the first reduction, a slight under both Ar and CO atmospheres. The catalytic peak
increase in current was observed but it rapidly levelled off. current against the acid concentration shows good linearity
This minor increase may be due to kinetically controlled cata- with coefficients over 0.99 in both cases, Fig. S14.† Contrary to
lytic reduction of protons. It may also comprise contribution CO-poisoning encountered by precious metal catalysts, the cat-
from the suppression of the dissociation of complex 2H+ in alysis is enhanced rather than suppressed under a CO atmo-
the presence of the acid, Scheme 3, which effectively increased sphere. Furthermore, the catalysis under a CO atmosphere pro-
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increase of acid concentration. But this did not occur for the
second process under either a CO or Ar atmosphere, Fig. 10
and 11. Instead, the peak potential steadily shifted positively.
This is not completely surprising since in acidic medium, the
catalytic species ought to be the species derived from the
reduction of complex 1 and subsequent protonation rather
than the directly reduced complex. These species catalyse
proton reduction at more positive potentials.
3 Experimental section
All chemicals were used as purchased, unless otherwise stated.
Freshly distilled solvents were used when necessary and
manipulations under an inert atmosphere using Schlenk
technique were employed in synthesis when appropriate.
Electrochemistry was carried out under an argon atmosphere
in dry solvent on Autolab Potentiostat 30. A conventional
three-electrode system was employed in which a vitreous
carbon disk (ϕ = 1 mm) was used as a working electrode, a
vitreous carbon strip as a counter electrode and AgCl/Ag
(Metrohm) as a reference electrode whose inner reference solu-
tion is composed of 0.05 mol L−1 [NBu4]Cl and 0.45 mol L−1
[NBu4]BF4. In dichloromethane, the electrolyte concentration
is 0.5 mol L−1 [NBu4]BF4. All the potentials have been cali-
brated versus the Fc+/Fc redox couple (0.55 V versus AgCl/Ag
electrode in dichloromethane). Solution FTIR spectra were
recorded on a Varian Scimitar 2000 using a solution cell with a
spacer of 0.1 mm. UV/Vis measurements were performed on
an Agilent 8453. NMR spectra were obtained on a Bruker
Avance DRX 400 MHz. Micro analysis service was provided by
Nanjing University (China). Procedures for organic synthesis
are included in the ESI.†
Fig. 10 Catalysing proton reduction by the associated species in the di-
chloromethane solution of complex 2H+ (3.7 mmol L−1) in the presence
of the acid HBF4·Et2O varying from 1 to 6 equivalents under an Ar atmo-
sphere at the scanning rate of 0.1 V s−1 at 298 K.
3.1 [Fe2{(SCH2)2C(CH3)(2-Py)}(CO)5], 1
To a solution of Fe3(CO)12 (0.216 g, 0.4 mmol) in dry toluene
(5 mL) was added a solution of 2-methyl-2-(pyridin-2-yl)
propane-1,3-dithiol (0.079 g, 0.4 mmol) in dry toluene (10 mL)
under N2. The reaction was heated at 110 °C for 4 hours
under stirring. After removing the solvent, the reaction residues
were purified using flash chromatography (eluting system: a
mixture of ethyl acetate and n-hexane at a ratio of 1 : 4). A green-
Fig. 11 Catalysing proton reduction by the associated species in the di-
chloromethane solution of complex 2H+ (4.0 mmol L−1, in situ gener-
ated by the addition of one equivalent of the acid) in the presence of the
acid HBF4·Et2O (from 1 to 6 equivalents) under a CO atmosphere at the
scanning rate of 0.1 V s−1 at 298 K.
ceeded more cleanly as indicated by the cyclic voltammo- ish brown band was collected. The removal of the solvents
grams, Fig. 11. The CO-enhanced catalytic behaviour is prob- under vacuum yielded a black-green solid. The obtained
ably due to the fact that the dissociation of complex 2H+ is product was re-dissolved in CH3CN (10 mL) and insoluble
suppressed by CO, Scheme 3, on the one hand. On the other materials were filtered off under N2. Both slow evaporation and
hand, the presence of CO may also suppress any processes storing at a low temperature (−20 °C) of the filtrate gave single
involving CO-loss which may occur for iron-carbonyl com- crystals suitable for X-ray diffraction structural determination.
plexes upon reduction. Most importantly, as shown in FTIR (CH3CN) νCO: 2048.6, 1978.2, 1914.1 cm−1
;
1H NMR
Scheme 3, under a CO atmosphere, the bound internal base (CDCl3, ppm): 1.49 (s, 3H), 1.88 (d, 2H, J = 12.0 Hz), 2.04 (d, 2H,
pyridinyl group becomes pendant. The pendant pyridinyl J = 11.8 Hz), 7.15 (broad, 1H), 7.42 (broad, 1H), 7.70 (broad, 1H),
group can be protonated and hence relay protons between the 9.51 (broad, 1H). 13C NMR (CDCl3, ppm): 31.4, 31.7, 48.1, 77.3,
metal centre and the proton in bulk solution. It is noteworthy 122.6, 123.6, 137.9, 159.2, 166.6, 208.6, 210.9. MS (ES): (M + 1)/z =
that the peak potential shifts for the second process. As contri- 450.8, (M
− CO + 1)/z = 422.3. Microanalysis (%) for
bution of the ir-drop increases with current intensity, the peak C14H11NO5S2Fe2 (449.07), calculated (found): C, 37.44 (37.40);
potential for reduction ought to shift negatively with the H, 2.47 (2.50); N, 3.12 (3.15).
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3.2 [Fe2{(SCH2)2C(CH3)(2-HPy)}(CO)6]+, 2H+
References
Complex 1 (13.7 mg, 3.05 mmol) was dissolved in dry toluene
(10 mL). Then the solution was saturated with CO and 1 eq.
HBF4·Et2O (4.28 µl) was added under CO on stirring at room
temperature and a reddish precipitate appeared instantly to
give nearly quantitative conversion of 1 to the protonated
complex 2H+. The solid was collected, washed with dry
toluene and dried under vacuum. A solution of the red precipi-
tate in dichloromethane at a low temperature (−20 °C) pro-
duced red single crystals suitable for X-ray diffraction analysis.
FTIR (DCM with HBF4·Et2O) νCO: 2079.5, 2038.9, 2011.5,
2001.8 cm−1. Due to its instability, the complex was not
further characterised except for electrochemistry.
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4 Conclusion remarks
The synthesis, characterisation, inter-conversion, and electro-
chemical and electrocatalytic investigations of two diiron com-
plexes 1 and 2H+ which possess an integrated pyridine/pyridi-
nium group were reported. The inter-conversion of the two
complexes proceeds via the intermediate 2, the deprotonated
form of complex 2H+. Upon protonation, the inter-conversion
equilibrium strongly shifts to the formation of complex 2H+
even under an Ar/N2 atmosphere, in which the need for the CO
molecule is met by “cannibalisation”. In complex 1, the bond
Fe–N (pyridine) is hemi-labile and subject to protonation.
Although no hexacarbonyl species (2) can be detected in the
solution of complex 1 under an Ar/N2 atmosphere, the content
of complex 2 could be over 5% in the presence of CO.33 Under
a CO atmosphere, complex 1 shows unusual electrochemistry
which is attributed to the existence of the equilibrium between
complexes 1 and 2. Complex 2H+ exhibits two reductions
rather than one and the first one occurs at a more positive
potential due to its positive charge. Furthermore, the second
reduction is much stronger than the first one. This reduction
is assigned to the reduction processes of complex 1 generated
in situ. Complex 2H+ shows catalysis towards proton reduction
in the presence of HBF4·Et2O at its first reduction, which is
enhanced in the presence of CO due to mainly the presence of
CO which helps free the bound pyridinyl group. The freed base
group acts as a proton relay to enhance the catalysis. Thus, our
results and reports in the literature24–26 suggest strongly that
an internal base has a role to play in improving the catalysis of
proton reduction.
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Acknowledgements
We thank the Natural Science Foundation of China (Grant No. 21 X. Zeng, Z. Li, Z. Xiao, Y. Wang and X. Liu, Electrochem.
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Professor Pickett (University of East Anglia, UK).
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