Nitrogen heterocyclic carbene containing pentacoordinate iron dicarbonyl as a [Fe]-hydrogenase active site model

Article abstract of DOI:10.1039/c5dt02065d

A novel pentacoordinate mono iron dicarbonyl complex bearing a nitrogen heterocyclic carbene ligand was reported as a model of a [Fe]-hydrogenase active site, which exhibits interesting proton coupled CO binding reactivity, electro-catalytic proton reduction and catalytic transfer hydrogenation reactivity.

Full text of DOI:10.1039/c5dt02065d

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DOI: 10.1039/C5DT02065D
Graphical abstract
A novel pentacoordinate mono-iron dicarbonyl was reported as structural and functional model of
Fe]-hydrogenase active site.
[

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Nitrogen Heterocyclic Carbene Containing Pentacoordinate
Iron Dicarbonyl as [Fe]-hydrogenase Active Site Model†
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a,b,
a,b
a,b
a,b,c,
Shuang Jiang, Tianyong Zhang, * Xia Zhang, Guanghui Zhang and Bin Li
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel pentacoordinate mono iron dicarbonyl complex bearing a theoretical investigation have been conducted for the function of
[
11]
nitrogen heterocyclic carbene ligand was reported as model of [Fe]-hydrogenase.
Fe]-hydrogenase active site, which exhibits interesting proton invoke an Fe-H intermediary, Figure 1(a). While DFT calculations
coupled CO bonding reactivity, electro-catalytic proton reduction suggest that hydrogen activation proceeds following the formation
The ternary-complex mechanism does not
[
δ-
δ+
+
and catalytic transfer hydrogenation reactivity.
of a dihydrogen, Fe-H …H -O, bond and substrate (MPT ) triggered
hydride transfer mechanism, Figure 1(b). It is notable that the key
transition state in this calculated mechanism actually bears a six
membered ring of -Fe-H-H-O-C-N-.
Nature has evoked hydrogenase as an expert for efficient proton
reduction and hydrogen oxidation which utilizes abundant first-row
[1]
metals (Fe and Ni).
Typical hydrogenases include [FeFe]-
hydrogenase and [NiFe]-hydrogenase that catalyze reversible
+
- [2]
proton reduction and hydrogen oxidation (H
2
=2H + 2e ) and [Fe]-
5
10
hydrogenase which catalyzes the reversible reduction of N ,N -
+
5
10
methenyl-tetrahydromethanopterin (MPT ) with H
2
to N ,N -
methylene-tetrahydromethanopterin (HMPT) and a proton (Figure
[
3]
1
(a)). Recent study of the [Fe]-hydrogenase active site revealed
that a mono iron center has a pseudo-octahedral coordination with
a cysteine sulfur atom, two cis-CO ligands, a bidentate pyridone
ligand through its nitrogen and acyl carbon atoms, and an open site
[
4]
Figure 1. (a) Structure of the active-site of the [Fe]-hydrogenase and
suggested mechanism (non-hydride); (b) DFT calculated hydride transfer
(or a H
2
O) trans to the acyl group (Figure 1(a)). Some synthetic
structural analogues of [Fe]-hydrogenase have been developed that
2
mechanism for [Fe]-hydrogenase catalytic process; (c) H activation and
[
5,6]
mimic first coordination sphere of the iron in [Fe]-hydrogenase.
catalytic hydrogenation of ketone by Ru complex via a six-member ring
However, due to the lack of suitable substrate, cofactors, similar transition state.
geometric and electronic environment, it is still being a great
challenge to develop functional analogues of the [Fe]-hydrogenase
active site.
The asymmetric hydrogenation of ketones by the
phosphine/diamine Ru complexes via a non-classical metal-ligand
Recently, hydrogen activation and transfer by first-row transition bifunctional catalysis mechanism has been well established (Figure
[12]
metals especially Fe and Ni have received increased interest due to 1(c)). Recently, similar iron complexes were also developed for
[
13]
the high abundance, low price and low toxicity of the metals. the catalytic reduction of C=O or C=N. The ancillary ligand with
Abundant functional [FeFe]-hydrogenase and [NiFe]-hydrogenase NH group played a crucial role in the hydride transfer. Aiming at
[7-10]
analogues were developed.
However, only the biological and developing a potential functional model of [Fe]-hydrogenase, we
aimed to synthesize a pentacoordinate mono iron analogue that
concurrently exhibited both the similar first coordination sphere
a.
School of Chemical Engineering and Technology, Tianjin Key Laboratory of
Applied Catalysis Science and Technology, Tianjin University, Tianjin 300072,
China.
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China.
and the never-reported catalytic reduction functions. In this
communication, we report the synthesis, structural characterization
and the coordination reactivity of a [Fe]-hydrogenase model, which
serves as a bifunctional catalyst for catalytic proton reduction and
transfer hydrogenation.
b.
c.
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
16024, China
Electronic Supplementary Informaꢀon (ESI) available: Experimental procedures,
1
†
Synthesis of complex 1 follows the procedure outlined in Figure
additional spectroscopic details, electrochemistry, X-ray crystallographic data (CIF)
obtained from the structure determination and a full list of metric parameters for
2
(a). Due to the excellent electron donor ability of the nitrogen
complexes 1. CCDC 1040792. For ESI and crystallographic data in CIF or other heterocyclic carbene ligand, complex 1 was isolated successfully as
electronic format see DOI: 10.1039/c000000x/
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an air stable dark crystalline solid in good yield. Unlike similar
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DOI: 10.1039/C5DT02065D
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Figure 2. (a) synthetic route and proton coupled CO bonding activity of
[14]
complex 1, (b) X-ray crystal structure of complex 1, hydrogen atoms are
omitted for clarity. (c) Left: cyclic voltammograms of complex 1 (2.5 mM),
Right: CV of complex 1 with added AcOH (5, 7.5, 10, 12.5, and 15mM) in
phosphine and cyanide derived complexes, the target complex 1
is rather stable and could be handled in air and stored at room
temperature for months. Furthermore, the solution form of 1 in CH CN solution, scan rate = 50mV.
3
methanol (or ethanol) is relatively thermal (50°C) and light stable,
which enables its potential application in catalytic hydrogenation
The reactivity with CO was examined due to the unsaturated
reactions. Figure S1 displays the v(CO) IR spectra (measured in coordination sphere of complex 1 as well as to mimic the [Fe]-
-1
CH
of the two bands suggests that cis-dicarbonyls are about 90° angles tricarbonyl species. Complex 1 showed no reactivity with CO under
2 2
Cl
, 1987 and 1927cm ) of 1 for which the near equal intensity hydrogenase active site which readily binded CO to yield a facial
[
4]
as those of the [Fe]-hydrogenase active site.
The single crystals suitable for X-ray diffraction study were grown decomposed in minutes after the addition of strong acid HBF
by slow evaporation of a diethyl ether solution of complex 1 at under N atomsphere. Nevertheless, a proton coupled CO binding
room temperature. The molecular structure is presented in Figure reactivity is observed, Figure 2(a). In the presence of HBF and CO
(b) to emphasize the square pyramidal structure of complex 1 for (0.1MPa), solution of complex 1 in ethanol undergo a color change
room temperature and atmospheric pressure. Moreover 1 will
4
2
4
2
[15]
which analysis according to Addison’s τ value finds complex 1 (τ = from dark brown to orange with concomitant v(CO) spectral
.506, based on α(C1Fe1S1)= 172.12 and β(C2Fe1N3) = 141.77) to changes indicating that mer-[Fe(CO)
be approximated as a square pyramid. The two CO ligands and the (s) and 2031 (m) cm in ethanol solution (Figure S2), formed via
+
0
3
IMes(H-NS)] (2102(w), 2053
-1
IMes ligand reflect their trans influence. The nitrogen and sulfur stoichiometric CO binding. On deprotonation with t-BuOK or Et
ligand is cis to the IMes ligand and the position trans to the latter is CO is released and complex 1 is regenerated, Figure S2.
N,
3
vacant. The C1-Fe1-C2 angle of 91.78(6)° coincided with the IR
spectrum of two cis-CO ligands. The Fe-C3(IMes) bond of voltammetry (CV) in CH
The electrochemistry of complex 1 was studied by cyclic
CN in the presence of Bu NPF as
.9535(12)Å in 1 is shorter than that of 2.002(6)Å in the precursor supporting electrolyte with the goal of evaluating its redox
3
4
6
1
[6]
2 3
of FeI (CO)
IMes , which is consistent with the good stability of 1, properties. Complex 1 exhibits a reversible reductive event at E1/2= -
[
16]
+/0
II
I
and fits well with the other reported Fe-NHC complexes.
selected metric data, bond length and angles are listed in Table S2- according to relative [FeFe]-hydrogenase models.
S3. The 2-amidothiophenolate dianion bidentate ligand is parallel to reduction event at -2.29V was assigned to the couple Fe /Fe , which
the mesityl group flanking the carbene donor, Figure S11.
was not observed under the higher scan rate of 200mV/s, Figure S8.
Upon addition of AcOH (2eq.) to the CH CN solution of complex 1,
The 1.83V vs Fc , which is attributed to the couple Fe /Fe (Figure 2(c))
[17]
The second
I
0
3
the first reduction peak showed an anodic shift by 150mV and the
current intensity did not increase with the incremental addition of
the acid. The second reduction peak at -2.29V resulted in a positive
shift by 130mV. The current of the second reduction peak observed
at -2.16V increased linearly with increasing acid (AcOH)
concentration (2, 3, 4, 5, and 6eq.), Figure S5, which is attributed to
catalytic proton reduction. It is notable that the reduction peak
moved to more positive potentials in the presence of AcOH,
consistent with the protonation of amine group as in other diiron
[18]
hydrogenase models, Figure S4. Based on such series of events,
CEEC (Chemical-Electrochemical-Electrochemical-Chemical)
a
mechanism was probably involved in this catalytic hydrogen
production, Figure S7. In the presence of AcOH, complex 1 is
II
+
+
protonated on the ligand-N atom to form [(Fe )(NH )] ([1(H)] ),
2
I
which undergoes a two-step reduction to form [(Fe )(NH )] ([1(H)])
2
0
-
-
and [(Fe )(NH )] ([1(H)] ) in a stepwise manner. The further
2
-
II
protonation of electron-enriched [1(H)] generate a [(HFe )(NH )]
2
(
[1(H-H)]) intermediate, which can evolve to H2 to fullfill the
catalytic cycle.
The adjustment of electronic properties of the iron center by
protonation of the ligand-N atom leads us to examine the possible
catalytic hydrogenation reactivity of this complex. Inspired by the
Coenzyme Q and vitamin K in nature, which have the quinone or
2
hydroquinone sections in their structures, 1,4-benzoquinone comes
[19]
into our sight. Quinone-hydroquinone couple is a typical organic
redox case and the research on the electrochemical behavior of this
couple has drawn scientists’ attention for nearly one century. The
reported reduction of 1,4-benzoquinone were usually achieved by
typical chemical reductant or catalytic hydrogenation based on Ni,
2
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[20]
Pd or Pt catalyst. Since quinones play key roles in living bodies, verses time fits well with an exponential function, Figure S18. In
many investigations have been made about the redox behavior of contrast, quinone was not reduced under the similar reaction
[
21]
quinone-hydroquinone
the case of coupled electron transfer to the movement of protons, Table 1 shows the transfer hydrogenation of quinone with different
could acts as an inorganic electron donor and quinone as H donors. It is illustrated that the reaction in ethanol proceeds a bit
From the electron transfer chain (ETC) in conditions when no catalyst of complex 1 was adopted, Figure S21.
H
2
[22]
2
carriers. Therefore we used complex 1 to activate H under mild slower than in methanol and the reaction in isopropanol needs
conditions with the assistance of 1,4-benzoquinone in methanol, sufficient time to achieve a low conversion of 20% of quinone. It is
from which we found that 1,4-benzoquinone was reduced to notable that the tertiary butanol could not serve as proton source
hydroquinone with a high conversion rate, which easily mislead us at all.
to assure that H
without a H atomsphere revealed that a transfer hydrogenation hydrogenation of quinone, kinetic studies were conducted. The
process with methanol as the proton source rather than H
2
was activated. While the control experiment
To further elucidate the mechanism of this transfer
2
2
was reactions were carried out under pseudo-first-order conditions of
excess methanol at a fixed molar ratio of catalyst (1.2 mmol% Cat.).
Plots of ln(CA0/CAt) verses time showed good linearity, Figure 3(b),
which demonstrates that the reaction of catalytic transfer
hydrogenation is first order with respect to quinone concentration.
The temperature dependence of observed rate constants for
transfer hydrogenation of quinone are given in Table S4 and
presented graphically as Arrhenius and Eyring plots, Figure S22. The
[
13]
probably involved here.
‡
small values of Ea (64.3kJ/mol) and ΔH (61.8kJ/mol) as well as large
‡
negative value for ΔS (-113.1 J/mol K) are consistent with an
associative pathway.
Table 1 Transfer hydrogenation of quinone with different H donors
Entry H donor Reaction
time (h)
Conversion of Q
Yield of HQ
(%)
95
91
14
20
0
(%)
67
63
5
12
0
1
2
3
4
5
MeOH
EtOH
i-PrOH
i-PrOH
t-BuOH
7
7
7
14
14
Q = quinone; HQ = hydroquinone.
Scheme 1. Proposed pathway for catalytic transfer hydrogenation of quinone.
1
Figure 3. (a)Top: In-situ H NMR monitor of the catalytic transfer
hydrogenation reaction of quinone at 25°C in 0.5 h. Bottom: Control
experiment under the similar conditions without addition of complex 1; (b)
Top: Dependence of conversion of quinone and yield of hydroquinone on
time at 25°C, 1.2 mol% complex 1. Bottom: Natural log plots of
concentration of quinone verse time at various temperatures, 1.2mol%
complex 1.
1
In-situ H NMR study was utilized to examine such a transfer
hydrogenation process. Quinone and 1.2mol% complex 1 was
dissolved in MeOD and it was observed that about 60% of 1,4-
benzoquinone (chemical shift 6.78ppm) transformed to
hydroquinone (chemical shift 6.61ppm) in 2.5h, Figure 3(a). In
contrast, the control experiment without 1 as catalyst showed no
obvious change in the concentration of quinone under the same
conditions, Figure 3(a). The slight peak of hydroquinone at 6.61ppm
was attributed to the impurity of standard commercial quinone
reagent with comparison, Figure S12(b).
Furthermore, the transfer hydrogenation reaction of quinone
catalyzed by 1.2mol% complex 1 in methanol at room temperature
gave conversion of quinone over 95% and yield of hydroquinone ca.
Taking into consideration of our current findings and previous
[13,23]
67% in 7h, Figure 3(b). The dependency of quinone concentration studies,
we propose a mechanism of transfer hydrogenation
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shown in Scheme 1. The amine-methanol specie 2 is first generated 5 D. F. Chen, R. Scopelliti and X. L. Hu, J Am Chem Soc, 2010, 132,
from complex 1 and methanol. Polarization of methanol by the iron
center and amine ancillary would generate a transition state species
ts-[1], then the C-H and O-H bonds were fractured respectively,
with a drop of methanol. With quinone conjuct to the proton and
hydride simultaneously, a six-membered ring of -Fe-H-C-O-H-N-
formed in ts-[2]. The dipole of quinone and easy formation of O-H-
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[23]
N hydrogen bond should respond to this transformation. Then
the hydride transfer from iron center to quinone carbon and the H-
N bond split, generating hydroquinone finally. The species ts-[2] is
similar to the DFT calculated [Fe]-hydrogenase catalytic process
[
11]
transition state demonstrated by Hall,
and coincided with the 6 B. Li, T. Liu, C. V. Popescu, A. Bilko and M. Y. Darensbourg, Inorg
[13,23]
metal-ligand bifunctional catalytic system.
Chem, 2009, 48, 11283; T. B. Liu, B. Li, C. V. Popescu, A. Bilko, L.
M. Perez, M. B. Hall and M. Y. Darensbourg, Chem-Eur J, 2010,
Conclusions
1
6, 3083.
In summary, we have provided a nitrogen heterocyclic carbene
containing model of [Fe]-hydrogenase active site. Complex 1 not
only reproduced the first coordination sphere, but also exhibited
proton coupled CO bonding reactivity with the unsaturated
coordinate open site. Moreover this complex catalyzed
electrochemical catalytic proton reduction and quinone redcution
7
8
R. P. Happe, W. Roseboom, A. J. Pierik, S. P. J. Albracht and K. A.
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The hydrogen transfer via this molecular catalyst platform also
provides insight into the function of [Fe]-hydrogenase. Based on
insights obtained regarding the effect of amine auxiliary and C=O
dipole substrate, the design and development of mono-iron
catalysts with more efficient activity and better substrate
adaptability are ongoing in our laboratory.
2
6.
1
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Nitrogen Heterocyclic Carbene Containing Pentacoordinate Iron
Dicarbonyl as [Fe]-hydrogenase Active Site Model
a,b
a,b,*
a,b
a,b
a,b,c,*
Shuang Jiang , Tianyong Zhang
, Xia Zhang , Guanghui Zhang , and Bin Li
a
Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of Chemical Engineering
and Technology, Tianjin University, Tianjin 300072, China
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
c
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
Experimental Section
Materials and Instruments
The synthetic reactions and operations were carried out on a double manifold Schlenk
vacuum line under a N atmosphere with rigorous exclusion of light. The solvents were
2
dried and distilled prior to use according to the standard methods. The purified solvents
were stored with molecular sieves under a nitrogen atmosphere for no more than 1 week
before use. Complexes FeI (CO) and FeI (CO) IMes were prepared according to the
2
4
2
3
1
ꢀ2
literature procedures. The following materials were of reagent grade and available
commercially from SigmaꢀAldrich: 1,3ꢀbis(2,4,6ꢀtrimethylphenyl) imidazolium chloride,
potassium tertꢀbutoxide, 2ꢀaminothiophenol and nBu NPF . The Fe(CO) was obtained
4
6
5
freely from Jiangsu Tianyi Ultraꢀfine Metal Powder Co., Ltd (China).
The NMR spectra were measured on a Bruker AVANCE III 400MHz NMR
1
13
spectrometer. H and C NMR shifts were referenced to residual solvent resonances
according to the literature values. Solution IR spectra were recorded on a Shimadzu
FTIRꢀ8400 infrared spectrometer using 0.1mm KBr sealed cells. Quinone and
hydroquinone samples were analyzed by a reversed phase high performance liquid
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chromatography (C , Ф150×4.6mm) using the external standard method on an Agilent
1
8
1
100 spectrometer. The mobile phase was CH CN/water (30/70, v/v), and the flow rate
3
was 1.0mL/min. The measurement was performed at the wavelength of 298nm.
Elemental analysis was carried out on a Heraeus CHNꢀRapid, fully automatic elemental
analyzer with TCD detection, type: TMT CHN, BESTELLꢀNR 2215001.
Synthetic Procedures.
FeI (CO) and FeI (CO) IMes are both light and heat sensitive. Therefore, preparations,
2
4
2
3
isolations, and manipulations were conducted in the dark. The isolated complex
Fe(CO) IMes(NS) (1) was rather stable to light and heat (50°C), and could be handled in
2
air as a solid and in the form of solution for more than 10h.
2
Equiv. of sodium tertꢀbutoxide (112mg, 1.0mmol) in MeOH （10mL）was added
dropwise to a MeOH (10ml) solution of 2ꢀaminothiophenol (54ꢁL, 0.5mmol) in a N2
atmosphere, followed by vigorous stirring for an hour. The solvent was removed under
vacuum and the residual dark solids were washed with diethyl ether (20mL). The residue
was dried in vacuo, dissolved in methanol (10mL), then added into a THF (10mL)
solution of FeI (CO) IMes (279mg, 0.4mmol) under a N atmosphere. The reaction was
2
3
2
monitored by IR to completion. The mixed solvent was removed in vacuo and the
residual dark solids were extracted with diethyl ether (30mL). The ether was removed
under vacuum to yield a dark red powder. Yield: 155mg (71.5%). The single crystals
suitable for Xꢀray diffraction analysis were grown by slow evaporation of a diethyl ether
1
solution of Complex 1 at ꢀ15°C. H NMR (ppm, CDCl ): δ = 8.99 (s, 1H, FeNH), 7.65(s,
3
1
H, NCH), 6.97(s, 1H, NCH), 6.80(s, 4H, mꢀMes), 2.27 (s, 6H, pꢀMes), 2.01 (s, 12H,
1
3
oꢀMes). C NMR (ppm, CDCl ): δ = 216.48 (CO), 184.14, 163.96, 140.65, 139.33,
3
2

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1
36.35, 135.62, 129.09, 128.07, 124.97, 121.80, 119.39, 118.62, 21.30, 18.05. IR
ꢀ
1
(
CH Cl , cm ): 1987 (s), 1927 (s). Elemental analysis, calc. for C H O N SFe: C, 64.56;
2 2 29 29 2 3
H, 5.38; N, 7.79; S, 5.94. Found: C, 64.73; H,5.35; N, 7.78; S, 5.89.
CO Uptake of Complex 1 in the Presence of HBF4
In a CO (0.1 MPa) atmosphere, complex 1 (15mg, 0.028mmol) dissolved in 10 mL
ethanol was treated by the dropwise addition of HBF . The reaction was monitored by IR
4
+
-
ꢀ1
to indicate the generation of [1(CO)-H] BF (2102(w), 2053(s) and 2031(m) cm )
4
accompanied by a color change from dark brown to orange within 30 min. The treatment
+
-
of [1(CO)-H] BF with tꢀBuOK led to the liberation of CO and stoichiometric
4
regeneration of complex 1, as monitored by IR. (see Figure S2)
X-ray Structure Determination of Complex Fe(CO) IMes(NS) (1)
2
The singleꢀcrystal of 1 was mounted on a Rigaku MMꢀ007 diffractometer equipped
with a Saturn 724CCD. Data were collected at 113K by using a confocal monochromator
with MoꢀKα radiation (λ=0.71073 Å). Data collection, reduction and absorption
3
correction were performed with the CRYSTALCLEAR program. The structures were
4
solved by direct methods using the SHELXSꢀ97 program and refined by the fullꢀmatrix
5
2
leastꢀsquares techniques (SHELXLꢀ97) on F . Hydrogen atoms were located by
geometrical calculation. Details of crystal data, data collections and structure refinements
are summarized in Table S2 and Table S3.
3

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Cyclic Voltammetry
Cyclic voltammograms were obtained in a threeꢀelectrode cell using a CHI 660B
electrochemical workstation. A solution of 0.1 M nBu NPF in CH CN was used as the
4
6
3
supporting electrolyte. The electrolyte solution was degassed by bubbling dry N through
2
for 10 min before measurements. The working electrode was a glassy carbon disc with a
diameter of 3 mm polished with 3 and 1 ꢁm diamond pastes and sonicated in ionꢀfree
water for 20 min and washed by CH CN prior to use. The reference electrode was a
3
+
nonꢀaqueous Ag/Ag (0.01M AgNO in CH CN) electrode and the counter electrode was
3
3
platinum wire. Ferrocene was used as an external standard under the same measuring
+
/0
conditions and all potentials are referenced to the Cp Fe couple at 0 V. During the
2
electrocatalytic experiments under N , increments of glacial HOAc (chromatographic
2
grade,S99.8%, water 0.15%, by the Karl Fischer method) were added by microsyringe.
Kinetic Measurements
Pseudoꢀfirstꢀorder reaction conditions were employed for all kinetic studies with 1.2
mol% catalyst of complex 1. HPLC was used to monitor the concentration of quinone
during the transfer hydrogenation reaction. In a typical experiment, 50 mg (0.46 mmol)
quinone and 3 mg (0.0056 mmol) complex 1 was dissolved in 12.5 mL methanol. While
being magnetically stirred, samples were taken at 8 to 40 min intervals typically through
four reaction halfꢀlives. Rates of the reactions were measured by following the decrease
of concentration of quinone. Rate constants were calculated from plots of ln(A /A )
0
t
4

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‡
‡
versus time; activation parameters ꢂH , ꢂS were obtained from Eyring plots; activation
energies, E , were obtained from Arrhenius plots.
a
References
(
(
(
1) B. Li, T. B. Liu, C. V. Popescu, A. Bilko, M. Y. Darensbourg, Inorg Chem 2009, 48, 11283.
2) W. Hieber, G. Bader, Ber. Dtsch. Chem. Ges. 1928, 61, 1717.
3) CRYSTALCLEAR 1.3.6. Rigaku and Rigaku/MSC (2005). 9009 New Trail Dr. The Woodlands
TX 77381 USA.
4) Sheldrick, G. M. SHELXSꢀ97, A Program for Crystal Structure Solution; University of
Göttingen: Germany, 1997.
5) Sheldrick, G. M. SHELXLꢀ97, A Program for Crystal Structure Refinement; University of
Göttingen: Germany, 1997.
(
(
5

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Table S1. Comparisons of v(CO) absorption of Fe(CO) IMes(NS) and some five
2
II
coordinate Fe complexes derivatives (reported in CH Cl solution unless otherwise
2
2
noted; NS = aminothiophenylate).
ꢀ1
a
Complexes
v(CO) (cm )
Ref.
Fe(CO) IMes(NS) (1)
1987(s), 1927(s)
1985(s), 1927(s)
2002(s), 1942(s)
2012(s), 1956(s)
2000(s), 1936(s)
2011(s), 1944(s)
2031(s), 1972(s)
2074(s), 2020(s), 1981(s)
this work
(1)
(1)
(1)
(2)
(3)
(3)
(3)
2
Fe(CO) PCy (NS)
2
3
Fe(CO) PPh (NS)
2
3
Fe(CO) P(OEt) (NS)
2
3
ꢀ
[
Fe(CO) (CN)(NS)]
2
Hmd
Extracted cofactor
COꢀinhibited Hmd
+
[
[
[
Fe(CO) IMes(NS)H ]
2102(w), 2053(s), 2031(m) (in EtOH) this work
2102(w), 2046(vs)
2102(w), 2052(vs)
3
+
Fe(CO) PCy (NS)H ]
(1)
(1)
3
3
FeI (CO) P(OEt) ]
2
3
3
a
Ref.:
(
(
(
1) T. B. Liu, B. Li, C. V. Popescu, A. Bilko, L. M. Perez, M. B. Hall, M. Y. Darensbourg, ChemꢀEur J
010, 16, 3083.
2) W. F. Liaw, N. H. Lee, C. H. Chen, C. M. Lee, G. H. Lee, S. M. Peng, J Am Chem Soc 2000, 122,
88.
2
4
3) E. J. Lyon, S. Shima, R. Boecher, R. K. Thauer, F. W. Grevels, E. Bill, W. Roseboom, S. P. J.
Albracht, J Am Chem Soc 2004, 126, 14239.
Figure S1. FTꢀIR spectrum of complex Fe(CO) IMes(NS) recorded in CH Cl .
2
2
2
6

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(a)
(b)
Figure S2. (a) Reversible CO uptake/release of complex 1 in the presence of acid/base. (b) IR
monitor of the protonꢀcoupled, CO binding reactivity.
7

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Figure S3. Cyclic voltammograms of complex 1 (2.5 mM) (black line) with AcOH (2, 3, 4, 5 and 6
+
equivalents) (colored lines) in 10 mL CH CN. All potentials are reported vs Fc /Fc (0.1 M
3
n
[
Bu N][PF ] , scan rate = 50 mV/s, 22 °C).
4 6
Figure S4. Cyclic voltammograms of complex 1 (2.5 mM) (black line) with AcOH (0.2, 0.3, 0.4, 0.5,
+
0
.6, 0.7 and 0.8 equivalents) in 10 mL CH CN. All potentials are reported vs Fc /Fc (0.1 M
3
n
[
Bu N][PF ] , scan rate = 50 mV/s, 22 °C).
4
6
8

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Figure S5. Dependence of current heights of electrocatalytic waves for complex 1 (2.5 mM) on acid
concentration (5, 7.5, 10, 12.5, and 15 mM) in CH CN.
3
Figure S6. The cyclic voltammograms of AcOH (10, 20 and 30 mM) in MeCN.
9

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S
FeII
N
H
ꢀ
H2
+ H+
1
H2
+
H
N
S
S
FeII
FeII
N
H
H
[1(H)]+
[1(H )]
2
+
e
CEEC
H
H
N
H
N
S
S
FeII
FeI
H
H
[1(H-H)]
[1(H)]
+
e
+
H+
ꢀ
H
N
S
Fe0
H
[
1(H)]-
Figure S7. Proposed CEEC mechanism for H produciton from AcOH reduction by complex 1 as
2
electrochemical catalyst.
1
0

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Figure S8. Cyclic voltammograms of complex 1 (2.5 mM) (Top) and complex 1 (2.5 mM) with
added increments of AcOH (2, 4, 8,14 and 20 equivalents) (Bottom) in CH CN. All potentials are
3
+
n
reported vs Fc /Fc (0.1M [ Bu N][PF ] , scan rate = 200 mV/s, 22 °C)
4
6
11

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1
13
Figure S9.
H NMR (a) and C NMR (b) spectra of complex 1.
1
2

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When τ = 0, the geometry is defined as square pyramid. When τ = 1, the geometry is
defined as trigonal bipyramid. For the largest LꢀFeꢀL’ angles, the following is obtained.
Complex 1: Fe(CO) IMes(NS)
2
τ = (C1Fe1S1ꢀC2Fe1N3)/60 = (172.12 ꢀ 141.77 )/60 =0.506
Complex
τ
Fe(CO) IMes(NS)
0.506
2
a
Fe(CO) PCy (NS)
0.53
2
3
a
Fe(CO) PPh (NS)
0.192
2
3
a
Ref.
(1) T. B. Liu, B. Li, C. V. Popescu, A. Bilko, L. M. Perez, M. B. Hall, M. Y. Darensbourg, ChemꢀEur J
2
010, 16, 3083.
Figure S10. Defining the geometry of complex 1.
1
3

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Figure S11. Stick drawing (a) and spaceꢀfilling (b) models of complex 1 given in three
perspectives.
1
4

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Table S2. Crystallographic data and processing parameters for complex 1
Complex
1
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
space group
a (Å)
b (Å)
c (Å)
α (deg)
C H FeN O S
539.46
113(2) K
0.71075 A
Monoclinic
P 21/n
10.2200(12)
19.200(2)
13.3650(17)
90
29
29
3
2
β (deg)
94.569(8)
90
2614.2(5)
4
γ (deg)
3
V (Å )
Z
3
Ρcalcd (Mg/m )
Crystal size (mm )
θ min/max (deg)
Reflections collected/unique
Parameters refined
1.371
3
0.24 × 0.22 × 0.20
1.861/33.340
43635/10075
335
2
GOF on F
1.112
R1 [I > 2 σ(I)]
wR2
Residual electron density (e Å )
0.0427
0.1022
0.448, ꢀ0.534
ꢀ3
1
5

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Table S3. Selected bond length (Å) and angles (deg) for complex 1
Fe(1)ꢀC(2)
Fe(1)ꢀC(1)
Fe(1)ꢀN(3)
Fe(1)ꢀC(3)
Fe(1)ꢀS(1)
S(1)ꢀC(25)
O(1)ꢀC(1)
O(2)ꢀC(2)
N(1)ꢀC(3)
N(1)ꢀC(4)
N(1)ꢀC(6)
N(2)ꢀC(3)
N(2)ꢀC(5)
N(3)ꢀC(24)
C(4)ꢀC(5)
1.7676(14)
1.7755(14)
1.8545(11)
1.9535(12)
2.2330(5)
1.7213(14)
1.1456(17)
1.1456(17)
1.3626(16)
1.3827(17)
1.4356(16)
1.3559(16)
1.3870(17)
1.3675(16)
1.339(2)
C(24)ꢀC(29)
C(24)ꢀC(25)
C(25)ꢀC(26)
C(26)ꢀC(27)
C(27)ꢀC(28)
C(28)ꢀC(29)
C(2)ꢀFe(1)ꢀC(1)
C(2)ꢀFe(1)ꢀN(3)
C(1)ꢀFe(1)ꢀN(3)
C(2)ꢀFe(1)ꢀC(3)
C(1)ꢀFe(1)ꢀC(3)
N(3)ꢀFe(1)ꢀC(3)
C(2)ꢀFe(1)ꢀS(1)
C(1)ꢀFe(1)ꢀS(1)
N(3)ꢀFe(1)ꢀS(1)
C(3)ꢀFe(1)ꢀS(1)
C(25)ꢀS(1)ꢀFe(1)
C(24)ꢀN(3)ꢀFe(1)
O(1)ꢀC(1)ꢀFe(1)
O(2)ꢀC(2)ꢀFe(1)
N(2)ꢀC(3)ꢀN(1)
N(2)ꢀC(3)ꢀFe(1)
N(1)ꢀC(3)ꢀFe(1)
C(26)ꢀC(25)ꢀS(1)
C(24)ꢀC(25)ꢀS(1)
1.4067(18)
1.4068(18)
1.401(2)
1.371(2)
1.399(2)
1.375(2)
91.78(6)
141.77(6)
90.17(5)
100.29(6)
95.77(6)
117.49(5)
88.37(5)
172.12(4)
84.87(4)
91.95(4)
99.25(5)
123.78(9)
177.22(12)
170.79(12)
103.37(10)
131.89(9)
124.74(9)
124.73(11)
115.71(10)
1
6

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1
Figure S12. H NMR spectrum of standard (a) hydroquinone and (b) quinone (with tiny amount of
hydroquinone as impurity) in MeOD.
1
7

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1
Figure S13. Inꢀsitu H NMR spectrum monitor of the transfer hydrogenation of quinone to
hydroquinone in MeOD at 25°C (a) with complex 1 as catalyst; (b) no complex 1 was added
1
8

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(a)
0.12
0.10
0.08
0.06
0.04
y
= 3.23Eꢀ07x ꢀ 0.0078
2
R = 0.9993
100000
150000
200000
250000
300000
350000
400000
Peak area
(b)
0.14
0.12
0.10
0.08
0.06
y
= 5.18Eꢀ8
x
+0.001
2
R = 0.9997
1000000
1500000
2000000
2500000
Peak area
Figure S14. HPLC calibration curve of quinone (a) and hydroquinone (b).
1
9

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0
.04
.035
.03
.025
.02
.015
.01
Quinone
0
0
0
0
Hydroquinone
ꢀ
0.0242x
0
y = 0.0404e
2
R = 0.9846
0
0
.005
0
0
20
40
60
80
t/min
100
120
140
100
Quinone
9
0
Hydroquinone
80
70
60
50
40
30
20
10
0
0
20
40
60
80
t/min
100
120
140
Figure S15. Top: Dependence of the concentration of quinone and hydroquinone on time at 40°C in
methanol (1.2 mol% complex 1). Bottom: Dependence of the conversion of quinone and the yield of
hydroquinone on time at 40°C in methanol (1.2mol% complex 1)
2
0

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0
.04
.035
.03
.025
.02
.015
.01
Quinone
0
0
0
0
ꢀ
0.0159x
y = 0.042e
Hydroquinone
0
2
R = 0.9855
0
0
.005
0
0
50
100
150
200
250
300
t/min
100
Quinone
90
80
70
60
50
40
30
20
10
0
Hydroquinone
0
50
100
150
200
250
300
t/min
Figure S16. Top: Dependence of the concentration of quinone and hydroquinone on time at 35°C in
methanol (1.2 mol% complex 1). Bottom: Dependence of the conversion of quinone and the yield of
hydroquinone on time at 35°C in methanol (1.2 mol% complex 1)
2
1

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0
.04
.035
.03
.025
.02
.015
.01
Quinone
0
0
0
0
Hydroquinone
0
ꢀ
0.0102x
y = 0.0335e
2
R = 0.984
0
0
.005
0
0
50
100
150
200
250
300
t/min
100
Quinone
9
0
0
0
0
0
0
0
0
0
0
Hydroquinone
8
7
6
5
4
3
2
1
0
50
100
150
200
250
300
t/min
Figure S17. Top: Dependence of the concentration of quinone and hydroquinone on time at 30°C in
methanol (1.2 mol% complex 1). Bottom: Dependence of the conversion of quinone and the yield of
hydroquinone on time at 30°C in methanol (1.2 mol% complex 1)
2
2

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0
0
0
0
.04
.03
.02
.01
0
Quinone
Hydroquinone
ꢀ
0.0073x
y = 0.0422e
2
R = 0.9974
0
100
200
300
400
t/min
1
00
Quinone
Hydroquinone
8
6
4
2
0
0
0
0
0
0
100
200
300
400
t/min
Figure S18. Top: Dependence of the concentration of quinone and hydroquinone on time at 25°C in
methanol (1.2 mol% complex 1). Bottom: Dependence of the conversion of quinone and the yield of
hydroquinone on time at 25°C in methanol (1.2 mol% complex 1)
2
3