Diironhexacarbonyl clusters with imide and amine ligands: Hydrogen evolution catalysts

Article abstract of DOI:10.1080/00958972.2011.638714

The reaction of Fe₃(CO)₁₂ and N-(4-thiolphenyl)-1,8- naphthalimide afforded a new diironhexacarbonyl complex (3). The integrity and electronic structure of 3 has been determined by elemental analysis and spectroscopy (NMR and infrare

Full text of DOI:10.1080/00958972.2011.638714

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Diironhexacarbonyl clusters with imide and amine
ligands: hydrogen evolution catalysts
Charles A. Mebi , Derek S. Karr & Ruixiao Gao
To cite this article: Charles A. Mebi , Derek S. Karr & Ruixiao Gao (2011) Diironhexacarbonyl
clusters with imide and amine ligands: hydrogen evolution catalysts, Journal of Coordination
Chemistry, 64:24, 4397-4407, DOI: 10.1080/00958972.2011.638714
To link to this article: http://dx.doi.org/10.1080/00958972.2011.638714
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Date: 05 March 2016, At: 00:13

Journal of Coordination Chemistry
Vol. 64, No. 24, 20 December 2011, 4397–4407
Diironhexacarbonyl clusters with imide and amine ligands:
hydrogen evolution catalysts
CHARLES A. MEBI*, DEREK S. KARR and RUIXIAO GAO
Department of Physical Sciences, Arkansas Tech University,
Russellville, AR, USA
(Received 15 September 2011; in final form 21 October 2011)
The reaction of Fe3(CO)12 and N-(4-thiolphenyl)-1,8-naphthalimide afforded
a new
diironhexacarbonyl complex (3). The integrity and electronic structure of 3 has been
determined by elemental analysis and spectroscopy (NMR and infrared). Infrared spec-
trum of 3 shows peaks at 2000, 2040, and 2075 cmꢀ1 ascribed to stretching frequencies
of the terminal metal carbonyls. Compound 4 was obtained from the reaction of Fe3(CO)12 and
4-aminothiolphenol. A comparison of the electronic, electrochemical, and electrocatalytic
properties of 3 and 4 are discussed. Cyclic voltammetric studies show that 3 and 4 catalyze the
reduction of acetic acid to produce hydrogen at ꢀ2.19 V and ꢀ1.88 V versus Fc/Fcþ,
respectively.
Keywords: Coordination compound; Electrocatalysis; Hydrogenase model; Iron carbonyl;
Organometallic
1. Introduction
Syntheses of diironcarbonyl clusters as models for the active site of [Fe–Fe]
hydrogenase enzyme have received great attention in recent years. These iron-clusters
are potential catalysts for the production of hydrogen, a clean alternative to fossil fuels
[1–51]. A representation of the active site of [Fe–Fe] hydrogenase is shown in compound
1 (X ¼ NH, CH2). The active site is a diiron unit linked to an electron transport chain,
[Fe4–S4] clusters [12, 13]. Diironcarbonyl complexes containing electroactive or
electron-withdrawing groups have been reported to possess desirable electrochemical
properties for effective models of 1 [47–51]. For example, Samuel et al. reported a new
*Corresponding author. Email: cmebi@atu.edu
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2011.638714

4398
C.A. Mebi et al.
model containing naphthalene monoimide dithiolate, 2, with one of the most positive
reduction potentials for a diiron carbonyl complex [51].
N
X
O
O
Cys
S
S
S
H2O
CO
CN
[4Fe4S]
Fe
Fe
NC
C
S
S
OC
CO
OC
O
Fe
Fe
OC
CO
CO
OC
1
2
In this study, we present the synthesis, spectroscopic, and electrochemical charac-
terization of a new model; a design containing two naphthalimides linked to a
diironhexacarbonyl unit (3). Naphthalimide, as an electroactive group [52–54], could
mimic the electron transport chain of the hydrogenase enzyme. To investigate
the electronic effect of the naphthalimide moiety on the iron-carbonyl unit in 3,
compound 4 has been synthesized and its electronic and electrochemical properties are
compared to those of 3. In addition, we report on the electrocatalytic reduction of acetic
acid to produce molecular hydrogen by 3 and 4.
CO
OC
S
CO
S
Fe
Fe
O
N
O
O
N
O
CO
OC
CO
3
CO
Fe
OC
CO
NH
H2N
S
S
2
Fe
CO
OC
CO
4

Diironhexacarbonyl clusters
4399
2. Experimental
2.1. General
N-(4-Thiolphenyl)-1,8-naphthalimide was prepared following similar literature proce-
dures [52–54]. An N,N-dimethylformamide solution containing equimolar amounts of
1,8-naphthalic anhydride and 4-aminothiolphenol was refluxed overnight. After
cooling, water was added to afford N-(4-thiolphenyl)-1,8-naphthalimide as a pale
yellow precipitate. All other chemicals and dry solvents were purchased from
commercial sources and used without purification. The synthesis and electrochemical
measurements were carried out under dinitrogen. Elemental analyses were performed
by Columbia Analytical Services, Inc., Tucson, AZ, USA. Melting point determination
was conducted using an Electrothermal Melting Point Apparatus.
2.2. Synthesis of 3
A THF solution of N-(4-thiolphenyl)-1,8-naphthalimide (3.03 g, 9.93 mmol) and
Fe3(CO)12 (2.50 g, 4.96 mmol) was purged with nitrogen and stirred overnight at room
temperature. A color change from green to red was observed. The solvent was removed
using a rotary evaporator and the product was separated by column chromatography on
silica gel with dichloromethane as eluent. Compound 3 was obtained as an orange solid in
22% yield (654 mg). IR (CH2Cl2) (cmꢀ1): (ꢀCO) ¼ 2075, 2040, 2000, 1780, 1740, 1710, and
1
1675. H-NMR (300 MHz, CDCl3) (ppm): ꢁ ¼ 7.05–7.30 (m, 4H), 7.40–7.60 (m, 4H),
7.75–7.90 (m, 4H), 8.20–8.40 (m, 4H), 8.60–8.75 (m, 4H). 13C-NMR (75 MHz, CDCl3)
(ppm): ꢁ ¼ 208.0, 164.0, 134.3, 132.4, 132.4, 131.4, 128.7, 128.1, 126.9, 122.5, 115.0.
Elemental Analysis, Found: C: 56.65, H: 2.55, N: 3.29, S: 7.05: Calcd: C: 56.76, H: 2.25,
N: 3.15, S: 7.21; m.p. 179–180ꢁC.
2.3. Synthesis of 4
Compound 4 was prepared and isolated following the procedures described above for 3
using 4-aminothiolphenol (1.0 g, 7.96 mmol) and Fe3(CO)12 (1.0 g, 1.99 mmol).
Compound 4 was obtained as an orange solid (426 mg) in 27% yield (based on
Fe3(CO)12). IR (CH2Cl2) (cmꢀ1): (ꢀ ꢂCO) ¼ 2070, 2035, and 1995. 1H-NMR (300 MHz,
3
3
CDCl3) (ppm): ꢁ ¼ 7.0–7.2 (t, JHH ¼ 7.5 Hz, 4H, ArH), 6.4–6.7 (d, 4H, JHH ¼ 7.5 Hz,
ArH), 3.7 (s, 4H, NH2). 13C-NMR (75 MHz, CDCl3) (ppm): ꢁ ¼ 209.0, 146.5, 135.0,
132.4, 115.0. Elemental Analysis, Found: C: 40.94, H: 2.69, N: 5.16, S: 11.76: Calcd:
C: 40.92, H: 2.27, N: 5.30, S: 12.15; m.p. Dec 102ꢁC.
2.4. Electrochemistry
Electrochemical studies were conducted using an Epsilon BAS potentiostat. Cyclic
voltammograms were obtained in a three-electrode cell under nitrogen at room
temperature. The electrodes used are glassy carbon working electrode, platinum
auxiliary electrode, and Ag/AgCl reference electrode. The potentials are converted to
Fc/Fcþ reference by adding ꢀ0.44 V to the recorded values [1]. The platinum and glassy
carbon electrodes were polished with aluminum paste and rinsed with water and acetone.

4400
C.A. Mebi et al.
A 0.1 mol Lꢀ1 CH3CN solution of Bu4NPF4 was used as supporting electrolyte. The
concentration of compound used was 1 mmol Lꢀ1 (10 mL) and the scan rate was
100 mV sꢀ1. Glacial acetic acid was used as proton source.
2.5. Spectroscopy
Infrared spectra were recorded on a Nicolet FT/IR Magna spectrophotometer using
NaCl IR solution cell. Proton and carbon-13 NMR spectroscopic measurements were
performed on a Bruker 300 MHz spectrometer. NMR spectra were referenced to
residual solvent relative to TMS.
3. Results and discussion
3.1. Synthesis and spectroscopy
Preparation of 3 and 4 are described in Schemes 1 and 2, respectively. Reaction of N-(4-
thiolphenyl)-1,8-naphthalimide and Fe3(CO)12 carried out under nitrogen using
Schlenk line techniques afforded 3 in 22% yield. A mixture of N-(4-thiolphenyl)-1,
8-naphthalimide and Fe3(CO)12 was stirred at room temperature overnight until a color
change from green to red was observed. The product was purified by column
chromatography on silica gel using dichloromethane as eluting solvent. The product
was collected and, after the removal of solvent, afforded 3 as an orange powder. A
similar procedure was utilized for the preparation, isolation, and purification of 4
(scheme 2). Four equivalents of 4-aminothiolphenol and one equivalent of Fe3(CO)12
O
N
O
SH
Fe3(CO)12
THF, RT
CO
Fe
OC
S
CO
S
O
N
O
O
N
O
Fe
CO
OC
CO
Scheme 1. Synthesis of 3.

Diironhexacarbonyl clusters
4401
were dissolved in THF and stirred under nitrogen overnight. Compound 4 was obtained
as an orange solid in 27% yield after chromatographic separation on silica gel using
dichloromethane as solvent.
1
Compound 3 has been characterized by IR and NMR spectroscopy. The H-NMR
spectrum of 3 shows five sets of multiplets between 7.0 and 8.8 ppm. The peaks are of
equal integrated ratios corresponding to the five groups of aromatic protons on the
naphthalimides. The spectrum is rather complex, an indication of the presence of
isomers of 3 in solution. As reported for similar diironhexacarbonyl complexes [1, 55],
the isomeric forms of 3 are based on the spatial orientation (axial or equatorial) of the
1
naphthalimide groups (scheme 3). H-NMR spectrum of 4 recorded in chloroform
contains three sets of peaks; a singlet at 3.7 ppm for the NH2 protons and multiplets at
6.5 and 7.1 ppm assigned to the aromatic protons. 13C-NMR spectra of 3 and 4 in
chloroform contain, among other peaks, a peak at 208 ppm ascribed to the CO ligands.
H2N
SH
Fe (CO)
3
12
THF, RT
CO
Fe
OC
S
CO
S
NH2
H2N
Fe
CO
OC
CO
Scheme 2. Synthesis of 4.
N
N
N
N
S
S
S
S
CO
OC
OC
CO
Fe
Fe
Fe
CO
Fe
OC
OC
CO
OC
CO
CO
OC
Axial-Equatorial
Equatorial-Equatorial
Scheme 3. Possible isomers of 3 (N ¼ naphthalimide).

4402
C.A. Mebi et al.
The observed position of the 13C-signal for CO in 3 and 4 is similar to the values
reported for other Fe–CO compounds [1–52].
Infrared spectroscopy is a very useful tool in ascertaining the structure and probing
the electronic properties of iron-carbonyl complexes. The infrared spectra of 3 and 4
recorded in dichloromethane are represented in figure 1. Relevant IR data for 3 and 4
alongside those of 2 are presented in table 1. For 3, we observe peaks at 2000, 2040, and
2075 cmꢀ1 ascribed to stretching frequencies of terminal metal carbonyls. The spectrum
also contains four peaks at 1780, 1740, 1710, and 1675 cmꢀ1 attributed to
naphthalimide [54]. The IR spectrum of 4 exhibits ꢀ(CO) peaks at 2070, 1995, and
2035 cmꢀ1 (table 1). These peaks are ꢂ5 cmꢀ1 lower than those observed for 3. This
small shift suggests the naphthalimides do not significantly influence the electronic
properties of the Fe–CO core in 3. Compound 2 was reported to have ꢀ(CO) values at
2005, 2043, and 2078 cmꢀ1 [51]. The IR results confirm the structures and integrity of 3
and 4, further supported by elemental analysis data (vide supra).
3.2. Electrochemistry
To investigate the electrochemical and electrocatalytic properties of 3, cyclic
voltammetric studies were performed on 3 in acetonitrile under nitrogen. The cyclic
Figure 1. IR spectra of 3 and 4 in dichloromethane.
Table 1. Characteristic IR-frequencies for 2–4.
Compound
ꢀ(CO) (cmꢀ1
)
Other peaks
2a
3
2005, 2043, 2078
2000, 2040, 2075
1995, 2035, 2070
Not reported
1780, 1740, 1710, 1675
1621, 1595
4
aValues obtained from ref. [51].

Diironhexacarbonyl clusters
4403
voltammogram of 3 is presented in figure 2. Two quasi-irreversible redox events are
observed at E1/2 ¼ ꢀ1.60 V (Ipa/Ipc ¼ 0.5, Epa ¼ ꢀ1.59 V, Epc ¼ ꢀ1.61 V) and E1/
¼ ꢀ1.75 V (Ipa/Ipc ¼ 0.4, Epa ¼ ꢀ1.76 V and Epc ¼ ꢀ1.74 V) versus Fc/Fcþ. A tentative
2
description of these electrochemical events is depicted in scheme 4. The event at ꢀ1.60 V
can be assigned to FeIFeI/FeIFe0 couple and that at ꢀ1.75 V to the FeIFe0/Fe0Fe0
couple. An irreversible oxidation peak (FeIFeI/FeIIFeI) at þ0.47 V versus Fc/Fcþ was
also observed. Compound 4 undergoes irreversible reduction (FeIFeI/FeIFe0) at
ꢀ1.53 V and oxidation (FeIFeI/FeIIFeI) at þ0.46 V versus Fc/Fcþ. A comparative
examination of the cyclic voltammetric data for 2–4 is contained in table 2. Compounds
3 and 4 have similar iron-centered redox potentials attesting to the similarity in the
electronic nature of the diiron cores. This is further confined by IR spectroscopy (vide
supra).
Models with similar structures to 3 and 4 have been evaluated as electrocatalysts for
the production of hydrogen atoms. Table 3 contains electrochemical data for four such
Figure 2. Cyclic voltammogram of 3 (1 mmol Lꢀ1) recorded in CH3CN, 0.1 mol Lꢀ1 Bu4NPF6, scan rate
100 mV sꢀ1
.
I
I
–
I
0
[Fe –Fe ] + e
[Fe –Fe ]
E
E
= –1.60V
= –1.75V
1/2
I
0
–
0
0
[Fe –Fe ] + e
[Fe –Fe ]
1/2
I
I
II
I
–
[Fe –Fe ]
[Fe –Fe ] + e
E
= +0.47V
pa
Scheme 4. Electrochemical processes observed for 3.
Table 2. Electrochemical data for 2–4.a
Compound
Epa (V)
E1/2 (V)
Ecat (V)
Overpotential
2b
3
–
ꢀ1.13, ꢀ1.49
ꢀ1.60, ꢀ1.75
ꢀ1.53c
–
–
þ0.47
ꢀ2.19
ꢀ0.73
4
þ0.46
ꢀ1.88
ꢀ0.42
aPotentials referenced to Fc/Fcþ.
bValues are obtained from ref. [51] and are reported vs. SCE. We converted the
values to Fc/Fcþ as described in ref. [1].
cEpc value.

4404
C.A. Mebi et al.
CO
OC
CO
S
Fe
X
S
X
Fe
CO
OC
CO
Table 3. Electrochemical data for 3, 4, and similar compounds.a
X
Epc (V)
Epa (V)
Ecat (V)
F
Cl
H
MeO
Naphthalimide (3)
NH2 (4)
ꢀ1.19
ꢀ1.35
ꢀ1.44
ꢀ1.57
ꢀ1.60b
ꢀ1.53
þ0.77
þ0.79
þ0.81
þ0.93
þ0.47
þ0.46
ꢀ1.7
ꢀ0.73
Not reported
Not reported
ꢀ2.19
ꢀ1.88
aValues obtained from ref. [1] except for 3 and 4 (this work). The values are
referenced to Fc/Fcþ.
bE1/2 of first reduction process.
Figure 3. Cyclic voltammogram of 3 (1 mmol Lꢀ1, 10 mL) recorded in CH3CN, 0.1 mol Lꢀ1 Bu4NPF6, scan
rate 100 mV sꢀ1; acetic acid added: 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, and 70 mmol Lꢀ1
.
models alongside those of 3 and 4 for comparison. The reduction potentials ascribed to
the [FeI–FeI]/[FeI–Fe0] couple for 3 and 4 (X ¼ naphthalimide and NH2, respectively)
are close to the value recorded for the model with X being methoxy. These reduction
potentials are more negative than those observed for models with X: F, Cl, or H,
suggesting that naphthalimide has less electron-withdrawing effect than fluorine and
chlorine.
3.3. Electrocatalytic production of hydrogen
Compound 3 was examined as electrocatalyst for the reduction of proton to molecular
hydrogen. The results are contained in figure 3. Cyclic voltammograms of 3 in the
presence of acetic acid show a catalytic peak at ꢀ2.19 V with an overpotential of

Diironhexacarbonyl clusters
4405
Figure 4. Plot of current vs. concentration of acetic acid added in the electrocatalytic reduction of proton to
hydrogen atoms by 3.
Figure 5. Cyclic voltammograms of 4 (1 mmol Lꢀ1, 10 mL) in the presence of acetic acid. Recorded in
CH3CN, 0.1 mol Lꢀ1 Bu4NPF6, scan rate 100 mV sꢀ1; acetic acid added: 0, 4, 7, 9, 11, 14, 25, and
30 mmol Lꢀ1
.
ꢀ0.73 V versus Fc/Fcþ. The overpotential was calculated as described by Evans et al.
[1]. The catalytic peak increases with increasing amount of acid added, as shown in
figure 4. Figure 5 contains cyclic voltammograms of 4 in the presence of increasing
concentration of acetic acid. A reduction peak at ꢀ1.88 V versus Fc/Fcþ was observed.
The current at this potential increases with acid concentration and is due to the
reduction of proton to hydrogen [1]. The overpotential was calculated to be ꢀ0.42 V
versus Fc/Fcþ, 0.31 V smaller than the value obtained for 3.
4. Conclusions
Two new diironhexacarbonyl complexes (3 and 4) containing imide and amine ligands
have been synthesized. The structures, electronic, and electrochemical properties of the
compounds have been probed with spectroscopic and electrochemical techniques. Both
3 and 4 are revealed by cyclic voltammetric studies in the presence of acetic acid to
catalyze the reduction of proton to molecular hydrogen at relatively modest potentials
(ꢀ2.19 V and ꢀ1.88 V vs. Fc/Fcþ, respectively).
Acknowledgments
Acknowledgement is made to the Donors of the American Chemical Society Petroleum
Research Fund for support of this research.

4406
C.A. Mebi et al.
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