Immobilization of a non-heme diiron complex encapsulated in an ammonium-type ionic liquid layer modified on an Au electrode: Reactivity of the electrode for O2 reduction

Article abstract of DOI:10.1039/c5cc10263d

An unstable diiron(ii,ii) complex possessing O₂ binding ability at low temperature was encapsulated and stabilized in an ammonium-type ionic liquid layer polymerized on an electrode. The encapsulated complex revealed catalytic reactivity for four-electron reduction of O₂ at an ambient temperature in aqueous solution.

Full text of DOI:10.1039/c5cc10263d

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Immobilization of a non-heme diiron complex
encapsulated in an ammonium-type ionic liquid
layer modified on an Au electrode: reactivity of
the electrode for O2 reduction†
Cite this:DOI: 10.1039/c5cc10263d
Received 14th December 2015,
Accepted 19th February 2016
Tatsuya Kitagawa,a Junpei Nishino,a Tomohiko Inomata,*b Tomohiro Ozawa,a
DOI: 10.1039/c5cc10263d
Yasuhiro Funahashic and Hideki Masuda*ad
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An unstable diiron(II,II) complex possessing O2 binding ability at low diiron complex (Fe2) modified on an Au electrode.4 In the same
temperature was encapsulated and stabilized in an ammonium-type study, we reported that Fe2, a non-heme enzyme model complex
ionic liquid layer polymerized on an electrode. The encapsulated with a dinucleating ligand bearing bulky pivalamide groups on
complex revealed catalytic reactivity for four-electron reduction of its pyridine rings, binds O2 in acetone at low temperatures. The
O2 at an ambient temperature in aqueous solution.
Fe2 complex is fixed to the Au electrode. The diiron complex is
substituted with alkanethiol to provide a self-assembled monolayer
Four electron reduction of dioxygen is an important cathode (SAM).4 However, the redox potential of the diiron complex in the
reaction in fuel cells. At the present time, Pt nanoparticles are Fe2-modified electrode is shifted towards a negative direction relative
typically used as a cathode catalyst in fuel cells, because they to that of Fe2 under homogenous conditions. The SAM method often
promote four electron reduction of dioxygen.1 However, Pt is very alters the chemical properties of the molecules immobilized on the
expensive and thus restricts widespread commercial development surface.5 As a result, the Fe2-modified Au electrode would be
of fuel cells. Therefore, many researchers have been motivated to incapable of activating dioxygen efficiently.
develop novel electrode catalysts such as Ag or Fe alloy nano-
To resolve this problem, we investigated a new method to
particles as substitutes for Pt.2 In addition, metal-complex-based entrap a functional compound on the surface as an alternative
molecular catalysts have been investigated.3 Miyazato et al. to the use of SAMs. We focused on using an ionic liquid (IL) to
succeeded in promoting the four electron reduction of dioxygen provide a reaction field on the Au surface.6 Generally an IL, an
using dinuclear cobalt complexes.3 Lei et al. also developed a Co ionic compound in the liquid state at room temperature,
complex polymer and deposited it onto a glassy carbon electrode exhibits properties such as ionic conductivity, incombustibility,
in an effort to develop an electrode-based catalyst for dioxygen low vapor pressure and a wide-potential window. These properties
reduction.3 Ogo et al. recently reported a Ru complex for use as are quite useful for the development of lithium batteries, fuel
both the anode and cathode catalysts of a fuel cell.3 However, Co cells, dye-sensitized solar cells, and other related applications.7
and Ru are also expensive materials. There are few reports on the Previously, Chi et al. entrapped a negatively-charged metal
study using iron complexes. We have studied inexpensive iron complex on an imidazolium-type IL SAM modified-electrode.8
complexes as molecular catalysts for dioxygen reduction and Unfortunately, a positively-charged complex could not be entrapped
reported two electron reduction of dioxygen using a non-heme on the IL modified-electrode because of the electrostatic repulsion
between them. Thus, an electrostatic interaction was found to be
a Department of Frontier Materials, Graduate School of Engineering, Nagoya
Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan
b Department of Material Science and Engineering, Graduate School of Engineering,
Nagoya Institute of Technology, Japan. E-mail: tino@nitech.ac.jp;
Fax: +81 52 735 5673; Tel: +81 52 735 5673
important for entrapping the complex. On the other hand, in our
previous report, we modified an Au electrode with a bulky
phosphonium-type IL,6 and have succeeded in entrapping three
types of metal complexes into the vacant space generated among
the IL molecules, regardless of the net charge of the complexes and
without generating any chemical bonds. We applied this method to
modify an Au surface with Fe2. Interestingly, the redox potential of
Fe2 for O2 activation appears in a more positive potential region
relative to that of the SAM method.4,6 Moreover, the O2 activation
process catalyzed by the Fe2 complex entrapped on the IL-modified
Au surface includes a four electron reduction process in addition to
the two electron reduction process. The O2 reduction activity was
c Department of Chemistry, Graduate School of Science, Osaka University, 1-1,
Machikaneyama, Toyonaka, Osaka 560-0043, Japan
d Department of Applied Chemistry, Aichi Institute of Technology, 1247 Yakusa,
Yakusa-cho, Toyota 470-0392, Japan
† Electronic supplementary information (ESI) available: Details of the preparation
of 1, IR spectrum and CV for 1/Au electrodes, IR spectra, CVs, and relationships
between peak current densities and scan rates for Fe2@1/Au and Fe2@poly-1/Au, CVs
and LSVs of Fe2@1/Au and Fe2@poly-1/Au under O2 condition, Koutecky–Levich plots
for Fe2@1/Au and Fe2@poly-1/Au. See DOI: 10.1039/c5cc10263d
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Fig. 1 Schematic view of the ammonium-type IL containing a disulphide group and six terminal olefins, 1. 1 can be constructed on an Au surface using a
self-assembled monolayer method to give 1/Au. 1/Au can entrap an external metal complex inside its IL monolayer, Fe2@1/Au. This IL monolayer can be
polymerized in an olefin metathesis reaction and encapsulate the entrapped metal complex in order to prevent its decomposition and release from the
monolayer, Fe2@poly-1/Au.
apparently enhanced. However, the reactivity of the Fe2 complex CH2Cl2 and acetone, Fe2@1/Au was successfully obtained. Introduc-
entrapped on the IL-modified Au surface might decrease through tion of Fe2 onto 1/Au was confirmed by IR-RAS measurements and
the O2 activation process because the entrapped Fe2 complex may various electrochemical measurements (Fig. S4 and S5a, ESI†).
be gradually released from the IL-modified Au surface.
The cyclic voltammogram of Fe2@1/Au in 0.1 M NaClO4 (scan rate =
Herein, we propose an improved method as shown in Fig. 1. 0.1 V sÀ1) has two redox waves at 0.13 V vs. Ag/AgCl. The redox waves
This method includes the step of introduction of an original are assignable to two-electron redox waves of Fe2(II,II/III,III), as
ammonium-type IL having a disulphide group at the terminus deduced from a comparison with the previous data.4,6 In order to
and six terminal olefins in the IL-head groups, 1. Fe2 was evaluate the situation of Fe2 in 1/Au, the peak current densities of Fe2
encapsulated in the IL SAM modified on the Au electrode by vs. the scan rates were plotted. Interestingly, a linear relationship
performing a cross-linking reaction among the olefins of IL-head (Fig. S5b, ESI†) was obtained, obviously indicating that Fe2 is
groups in order to protect Fe2 on the Au surface from external entrapped in SAMs of 1/Au.
materials. Fe2 was stabilized on the Au surface and was found to
promote effective O2 activation.
Finally, we performed a polymerization of 1 by conducting
an olefin metathesis reaction with 0.4 mg mLÀ1 Grubbs 2nd
The ammonium-type IL, 1, was synthesized by a modified generation catalyst solution to obtain a new device, Fe2@poly-1/
version of the previously published method.6 1 is in a liquid state Au, that was encapsulated with Fe2 in the SAMs of 1/Au. The
at room temperature, and was characterized by spectroscopic obtained Fe2@poly-1/Au would prevent Fe2 from being released
measurements (Schemes S1 and S2, ESI†). It was determined from the SAMs. The IR-RAS spectrum of the resultant surface
that 1 was able to construct its self-assembled monolayers has a peak at 1647 cmÀ1, which is assignable to the CQC
(SAMs) on an Au electrode by forming Au–S bonds and also by stretching frequency of terminal olefins of 1. This peak dis-
cross-linking among molecules of 1 on the Au electrode in an appears after the olefin metathesis reaction, indicating that
olefin metathesis reaction. The Au electrode modified with 1, Fe2@poly-1/Au is generated (Fig. S4, ESI†). The peaks near
1/Au, was prepared by directly casting a few drops of 1 onto the 1450 cmÀ1 and a peak at 718 cmÀ1, which are assignable to the
Au disk electrode of a rotating ring-disk electrode (RRDE) over pyridyl backbone of Fe2 and the C–H bending vibration of benzoate
the course of a few days. After washing with EtOH and CH3Cl, related to Fe2, respectively, were still observed in the spectrum of
1/Au was successfully obtained. Modification of the Au disk Fe2@poly-1/Au. Furthermore, the peaks assignable to CQC bond
electrode with 1 as a SAM was confirmed by IR-reflection- stretching were not found in the spectrum after polymerization of
absorption spectroscopy (IR-RAS) measurements and various 1. On the basis of the selection rule for IR-RAS, this implies that the
electrochemical measurements (Fig. S1 and S2, ESI†). The surface CQC bond is parallel to the Au electrode surface.
coverage of 1 was estimated to be 2.7 Â 10À10 mol cmÀ2 from
The cyclic voltammogram of Fe2@poly-1/Au in 0.1 M NaClO4
charges for a reductive desorption wave of 1 in linear sweep aqueous solution (scan rate = 0.1 V sÀ1) has two redox waves at
voltammetry measurements of 1/Au (Fig. S3, ESI†). The resulting 0.12 V vs. Ag/AgCl (Fig. 2a), which are assigned to two-electron
coverage is greater than that of a previous SAM on an Au redox processes of Fe2(II,II/III,III) based on a comparison with
electrode which was generated using a phosphonium-type IL those of Fe2@1/Au. In order to evaluate Fe2@poly-1/Au, the
(9.1 Â 10À11 mol cmÀ2).6 The coverage of 1 was found to be three peak current densities of Fe2 were plotted against the scan
times greater than that of the previous SAM. This may be rates. The resulting linear relationship, which is similar to that
explained in terms of the atomic sizes of the cation of the of Fe2@1/Au, indicates that Fe2 remains entrapped in SAMs of
modified ILs. The ammonium-type IL is smaller than the 1/Au (Fig. 2b). Additionally, the surface coverage of Fe2 in
phosphonium-type ion. Therefore, the electrostatic repulsion Fe2@poly-1/Au was estimated to be 4.4 Â 10À12 mol cmÀ2 as
between cations of 1 is smaller than that of the phosphonium judged from its redox waves. These electrochemical data are
type SAM.
The soaking of Fe2 into the IL SAMs of 1/Au, to obtain Fe2@1/Au,
discussed in comparison with those of Fe2@1/Au.
The redox potential of Fe2@poly-1/Au was found to be
was carried out in MeOH solution (1 mM). After washing with similar to that of Fe2@1/Au (Table S1, ESI†), indicating that
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were performed in O2 saturated aqueous solution using RRDE.
Fig. S10 (ESI†) shows that the O2 molecules bound to Fe2@poly-1/
Au on the disk electrode are converted to the 2-electron reduced
species, such as H2O2 or O22À, at around 0 V vs. Ag/AgCl. The same
behaviour was also observed in Fe2@1/Au (Fig. S8, ESI†). These
findings indicate that both Fe2@1/Au and Fe2@poly-1/Au can
reduce O2 to O22À. We estimated the number of electrons related
to O2 reduction in these systems by generating a Koutecky–Levich
plot.9 In the Koutecky–Levich equation, iDÀ1 (iD: the disk current) is
proportional to oÀ1/2 (o1/2: square root of the rotation rate) when
the electron transfer rate is sufficiently slow (i.e. an irreversible
system). For Fe2@poly-1/Au, a linear relationship was observed
Fig. 2 (a) Cyclic voltammogram of Fe2@poly-1/Au in 0.1 M NaClO4
aqueous solution (scan rate = 0.1 V sÀ1). (b) The plots of the peak current
densities of Fe2@poly-1/Au towards the scan rates of the CV measurement
for Fe2@poly-1/Au.
À1
between iD and oÀ1/2 only with slower rotation rates (100–1600
rpm) (Fig. S12, ESI†). Fe2@1/Au also showed almost the same
the electrochemical conditions of Fe2 on their respective electrodes behaviour as Fe2@poly-1/Au. The apparent number of electrons
are similar and that the polymerization of 1 on the electrode does (napp) related to the O2 reduction process promoted by Fe2@poly-1/
not affect the electrochemical conditions of Fe2 on the electrode. In Au was estimated to be 3.4 from the plots. The resulting napp in
addition, stabilities of Fe2 on their respective electrodes are similar Fe2@poly-1/Au indicates that two reduction processes for the O2
to each other, because the redox waves assignable to Fe2 were molecule, two- and four-electron reductions, coexist in the system.
observed during several cycles of the CV measurements of the In contrast, napp for Fe2@1/Au was estimated to be 2.6, indicating
respective electrodes. On the other hand, the surface coverage of that the two-electron reduction process is dominant over the four-
Fe2 on Fe2@poly-1/Au is approximately 3 times less than that of electron reduction, as compared with Fe2@poly-1/Au. Furthermore,
Fe2@1/Au. The decrease in Fe2@poly-1/Au might be an indication the capture rate (N) of H2O2 generated on the disk electrode is also
of a decrease in the space generated among 1 molecules, where Fe2 estimated from the equation N = iR/iD (iR: ring current). The
should be entrapped on the surface. The polymerization of 1 may resulting Ns in Fe2@poly-1/Au and Fe2@1/Au were found to be
have shortened the distances between neighbouring molecules of 0.36 and 0.43, respectively. These findings clearly indicate that the
1. Approximately 60% of Fe2 that was entrapped in Fe2@1/Au was four-electron reduction process of O2 in Fe2@poly-1/Au is more
released outside of the SAMs after the polymerization. This means prominent.
that the electrochemical characteristics of Fe2 entrapped on an Au
Fe2@poly-1/Au and Fe2@1/Au can at least activate O2 to
electrode, Fe2@poly-1/Au, are similar to its characteristics in reduce to O22À. The four-electron reduction process of O2 in
CH2Cl2 solution. Two major differences were observed between Fe2@poly-1/Au is dominant. This indicates that Fe2@poly-1/Au
the electrochemical behaviour of Fe2 in Fe2@poly-1/Au and that of has the potential to be developed as a cathodic electrode for a
Fe2 in CH2Cl2. The first difference is in the number of redox waves fuel cell. Previously, Islam et al. reported that an ionic liquid is
and the second difference is in redox potential values. CVs of capable of lowering the overpotential for O2 reduction relative
Fe2@poly-1/Au (Fig. 2a) and Fe2@1/Au (Fig. S5a, ESI†) both exhibit to an organic solvent.10 Therefore, we can conclude that the IL
two redox waves related to Fe2. However, the CV of Fe2 in CH2Cl2 contributes to the activation of the reduced species of O2. The
has three reduction waves and one oxidation wave (Fig. S7, ESI†). four-electron reduction process of O2 in our system has been
The decrease in the number of the reduction waves may be due to achieved by a synergistic effect provided by Fe2 and the ionic
the restriction in the structural change of Fe2 during reduction on liquid 1 immobilized on the surface. Herein, why is the napp
the electrode surface. In our previous work,4 we also reported that value improved after the polymerization of the IL layer on the
Fe2 undergoes such a structural change during the electrochemical surface? We believe that the improved napp value in Fe2@poly-1/
reduction process in solution; Fe2 possesses two reduction pro- Au is due to the more stabilized intermediate of the complex
cesses after one electron reduction of Fe2(II,III/III,III). The Fe2(II,II/II,III) binding a dioxygen than that in Fe2@1/Au. We have suggested
reduction process is accompanied by a structural change and that the hydrophobic space constructed with the alkyl chains of
the Fe2(II,II/II,III) reduction process occurs without any structural the ILs in Fe2@poly-1/Au prevents access of water molecules to
change (Scheme S3, ESI†). This is because Fe2 is restricted from the intermediate more than that in Fe2@1/Au. As a result, the
undergoing structural changes due to space constraints when it is polymerized IL layer in Fe2@poly-1/Au inhibits a disproportional
located in the IL SAMs of Fe2@1/Au and Fe2@poly-1/Au, although it reaction of the O2 activated by Fe2 to H2O2 or O22À. Therefore,
can transform freely in a homogeneous solution. In addition, the four-electron reduction of O2 has been promoted in the poly-
electron transfer rate between Fe2 and the electrode on the Au merized IL layer. This is reason why the napp value has been
surface is faster than that in bulk solution. Therefore, it is possible improved after the polymerization of the IL layer. Finally, we
that a two electron reduction process of Fe2 entrapped on the have concluded that the encapsulation of Fe2 on the surface by
surface proceeds immediately and then only two redox waves are polymerization of 1/Au enhances the synergetic effect.
observed in the CVs.
In summary, we succeeded in promoting four-electron
To evaluate the catalytic activity of Fe2 for O2 reduction in reduction of O2 using Fe2@1/Au (an entrapped and immobilized
Fe2@poly-1/Au, linear sweep voltammetry (LSV) measurements complex) and Fe2@poly-1/Au (an entrapped and immobilized
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We gratefully acknowledge support for this work by a Grant-in-
Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan and by ‘‘Program for
Advancing Strategic International Networks to Accelerate the
Circulation of Talented Researchers’’ from Japan Society for the
Promotion of Science.
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