Self-assembled monolayer electrode of a diiron complex with a phenoxo-based dinucleating ligand: Observation of molecular oxygen adsorption/desorption in aqueous media

Article abstract of DOI:10.1039/b711802c

The phenoxo-based dinucleating ligand, 2,6-bis[bis(6-pivalamido-2- pyridylmethyl)amino-methyl]-4-aminophenol (1), and its Fe₂(ii) complex, [Fe₂(ii)(1)(PhCOO)₂](CF₃SO ₃) (2), were prepared and 2 deposited on the Au surface (2/Au) is much more stable than in solution and exhibits redox behavior in aqueous media as well as reversible adsorption/desorption of oxygen at room temperature. The Royal Society of Chemistry.

Full text of DOI:10.1039/b711802c

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Self-assembled monolayer electrode of a diiron complex with a
phenoxo-based dinucleating ligand: observation of molecular oxygen
adsorption/desorption in aqueous media{{
Tomohiko Inomata,*a Kazuma Shinozaki,a Yuya Hayashi,a Hidekazu Arii,b Yasuhiro Funahashi,a
Tomohiro Ozawaa and Hideki Masuda*a
Received (in Cambridge, UK) 2nd August 2007, Accepted 31st October 2007
First published as an Advance Article on the web 9th November 2007
DOI: 10.1039/b711802c
ligand. The newly synthesized dinucleating ligand was deposited
onto the electrode surface. Investigations indicate that the Fe2 core
displays redox behavior in aqueous media at room temperature
with reversible adsorption/desorption of O2.
The phenoxo-based dinucleating ligand, 2,6-bis[bis(6-pivala-
mido-2-pyridylmethyl)amino-methyl]-4-aminophenol (1), and
its Fe2(II) complex, [Fe2(II)(1)(PhCOO)2](CF3SO3) (2), were
prepared and 2 deposited on the Au surface (2/Au) is much
more stable than in solution and exhibits redox behavior in
aqueous media as well as reversible adsorption/desorption of
oxygen at room temperature.
The ligand 1 which includes a terminal amino group was
prepared using a modification of previous methods (see ESI{).7
After stirring a CH2Cl2–MeOH solution containing 1 (0.10 mmol),
sodium benzoate bridging reagent (0.20 mmol), Fe(CF3SO3)2
(0.20 mmol) and Et3N (0.10 mmol) for a few hours in a glove box,
the target Fe2 complex 2 was obtained as yellow prismatic crystals.
The product was characterized by IR and ESI-TOF MS spectro-
scopy and elemental analysis.§ The X-ray structure of 2 is indicated
in Fig. S1{ and exhibits two Fe centers bridged by two benzoate
groups and one phenoxo group." The formal charge of each Fe
The capture, release and activation of molecular oxygen are critical
processes which must be considered for the development of
applications such as oxygen sensors and fuel cell cathodes.1,2
Various metal-containing proteins contribute to these processes in
biological systems. In particular, iron-containing proteins play
important roles in the storage, transport and activation of O2.3
Since investigations of functional model complexes aid in the
elucidation of the mechanisms of biological systems, numerous
studies of functional model complexes as models for the active
centers of heme and non-heme enzymes have been reported.4,5
Examples of immobilization of these model complexes onto
substrates such as electrode surfaces have also been studied.6 In
these studies, the complexes are stabilized and in some cases display
unique reactivity that is not observed in homogeneous solution.
We have previously reported Fe2(II) non-heme enzyme model
2
center is 2+ due to the presence of a CF3SO3 counter anion. A
hydrogen bond exists between the pivalamide group N–H site and
the oxygen atom of the bridging benzoate.
complexes with
a dinucleating ligand that includes bulky
pivalamide groups on its pyridine rings.7 These Fe2 complexes
react irreversibly with O2 to form stable m-1,2-peroxo complexes at
low temperatures. It is expected that the complexes deposited on
the electrode will gain increased stability and react with O2 at room
temperature in aqueous media, while this electrode can provide
electrons to these complexes and activate the bound dioxygen. Our
goal is to use these non-heme functional model complexes in
aqueous media for the development of devices capable of
capturing and activating O2.
Fig. S2{ shows the UV–vis spectrum of 2 in acetone under Ar
and under saturated O2 conditions. The spectrum of 2 drastically
changed under saturated O2 at 230 uC. The band observed
around 600 nm is typical of phenoxo-bridged dinuclear Fe
complexes reacting with molecular oxygen.5 Two LMCT bands,
which are assignable to ‘peroxo to Fe(III) ion’ and ‘phenoxo to
Fe(III) ion’, overlapped in this region.8 The resonance Raman
spectrum of a similar Fe2 complex, which has a CH3-group instead
of an NH2-group at the p-position of phenolate, showed bands at
890 cm21 for 16O2 and at 840 cm21 for 16O2.7 These values are
typical of cis-m-1,2-peroxo Fe2 complexes.5 However, this peroxo
complex decomposes as the temperature increases to room
temperature. A similar Fe2(II) complex which does not contain
pivalamide groups does not form a peroxo intermediate.9 In these
cases, Fe2 complexes are only oxidized from the Fe2(II,II) state to
the Fe2(II,III) state. This clearly indicates that the pivalamide
groups contribute to the mechanism of the reaction with O2. As
was mentioned above, these pivalamide groups form hydrogen
We report herein the construction of a self-assembled monolayer
(SAM) of diiron complexes with a phenoxo-based dinucleating
aDepartment of Applied Chemistry, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan.
E-mail: masuda.hideki@nitech.ac.jp; Fax: +81-52-735-5209;
Tel: +81-52-735-5228
bDepartment of Chemistry, Gakushuin University, Mejiro, Toshima-ku,
Tokyo, 171-8588, Japan
{ Electronic supplementary information (ESI) available: Preparation of 1,
X-ray crystal structure of 2, UV–vis spectrum and CV of 2, Table for the
redox potential of 2/Au. See DOI: 10.1039/b711802c
{ The HTML version of this article has been enhanced with colour
images.
392 | Chem. Commun., 2008, 392–394
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bonds and allow the release of the bridging benzoate group.
Consequently, 2 generates the peroxo complex under saturated O2
conditions.
direction. Although the cause of these negative shifts is not
currently understood, Wackerbarth et al. reported a similar
significantly negative shift of SAMs of Co2 oxygen complexes
which have a similar dinuclear ligand.11 It was pointed out that the
proximity of the peroxide ligand to the Au surface and the
translocation of the surface charge to the peroxo group could
provide an explanation for the negative shift. When the bridging
benzoate in 2/Au was substituted with p-MeO-benzoate or p-NC-
benzoate, the redox potentials of each of the SAMs were altered in
a manner that depends upon the substituent effect of the bridging
benzoate group (Table S1{). Thus, we assign the redox waves I, II
and III to Fe2(II,II/III,III), Fe2(II,III/III,III) and Fe2(II,II/II,III),
respectively.**
In cyclic voltammetry, one anodic wave and two cathodic peaks
were observed at Epa = 0.43 V and Epc = 20.16, 0.32 V vs. Ag/Ag+
(Fig. S3{). The anodic waves were compared twice to two other
cathodic peaks. These waves are assignable to Fe2(II,II/III,III),
Fe2(II,II/II,III) and Fe2(II,III/III,III), respectively, which correspond
well with those of previously reported Fe2 complexes with similar
structures.7 These previously reported complexes had a terminal
–CH3 group instead of a terminal –NH2 group at the p-position of
phenolate. The redox behavior of Fe2 complexes with pivalamide
groups is complex and not as clearly defined as the redox behavior
of the previously reported Fe2 complexes. This complex electro-
chemical behavior of 2 is probably due to the pivalamide groups
influencing the weakening of the Fe–O bond of bridging benzoate
groups and causing loss of coordination of one of the bridging
benzoate groups from the Fe center. Thus, several species coexist
in each oxidation state of 2 and complicate the redox behavior.
However, multiple cyclic voltammetry scans were essentially
identical, indicating that decomposition of 2 in solution does not
occur.
The surface coverage of the Fe2 units on the Au electrode
surface, which was estimated from the area of the redox wave, was
calculated to be ca. 3–4 6 10211 mol cm22. Considering the ideal
coverage calculated from the projection area of 2 (4.0 6 10211 mol
cm22, estimated from the X-ray crystallography of 2), the complex
is quite densely packed on the electrode surface. 2 is difficult to
dissolve in H2O and is easily oxidized which causes decomposition
in solution in the presence of even trace amounts of oxygen and/or
moisture. The high stability of the 2/Au complex is most likely
caused by the dense packing and hydrophobic interactions of the
Fe2 complex in 2/Au. These effects partially protect the Fe2
complex from attack by H2O and O2. Consequently, the Fe2 units
in 2/Au are very stable with regard to reactivity with O2. The
difference in the lengths of DTSC also contributes to the stability
of 2. A multi-cycled CV measurement indicated that 2/Au formed
with DTSU, which has a nine-carbon chain, is more stable than
that formed with DTSP, which has only an ethylene chain. This
stability most likely originates from the contribution of van der
Waals interactions among the alkyl chains of DTSC.
As described in the experimental section, 2 was deposited onto
the Au electrode surface stepwise (Fig. 1). The Au electrode was
dipped into dithiobis(succinimidyl carboxylate) (DTSC) solution
(Fig. 1a). The resulting monolayer of TSC/Au (TSC, thiobis-
(succinimidyl carboxylate) was immersed into the solution of 2 to
produce the target Fe2 complex-modified Au electrode, 2/Au
(Fig. 1b).I Two types of DTSC were used (DTSP and DTSU
containing propionate and undecanoate linkers, respectively).
It is notable that the Fe2 core of 2/Au is remarkably stable in
aqueous media. Fig. 2a shows a cyclic voltammogram of 2/Au in
0.1 M aq. NaCF3SO3. The linear relationship between each scan
rate and peak current indicates that these waves are derived from
the species attached to the electrode surface.10 The redox behavior
is similar to that of 2 in homogeneous solution. However, all of the
redox potentials of 2/Au are markedly shifted in the negative
Under saturated O2 conditions, the CV of 2/Au is drastically
altered in aqueous solution, indicating the occurrence of binding of
O2 (Fig. 2b). The areas of waves IV and V under saturated O2 were
almost the same as those of I, II under Ar. Thus, the waves IV
Fig. 1 Schematic views of the process of surface modification with 2. (a)
Self-assembly of the TSC monolayer on the bare Au electrode by
spontaneous splitting of the DTSC. (b) Cross-linking of 2 with the TSC
monolayer.
Fig. 2 Cyclic voltammograms of 2/Au (constructed with DTSU) in
aqueous media (pH 6.0) under (a) Ar and (b) saturated O2 conditions,
respectively. Scan rate is 50 mV s21
.
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Notes and references
§ Selective data for 2: IR (KBr, cm21): 3436 (nN–H), 3341 (nN–H), 1692
(nCLO), 1614 (nCOO), 1415 (nCOO), 1279 (nC–F), 1258 (nC–F), 1161 (dS–O),
1032 (nS–O), 639 (nCLS). m/z (ESI-TOF MS) [M 2 CF3SO32]+: 1280.6.
Found: C, 55.97; H, 5.21; N, 10.57%. Calcd. for 2?MeOH: C, 55.86; H,
5.65; N, 10.54%.
" CCDC 656303. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b711802c
˚
I Preparation of deposited 2/Au: Au film with a thickness of 1000 A was
21
˚
deposited onto a cleaved mica substrate (14 6 14 mm) at 1.0 A s by a
JIS-300AK vacuum coater (Sinku Technology Co., Ltd.). The Au film was
annealed with a hydrogen gas flame before it was dipped into each sample
solution. TSC/Au was prepared by immersing the Au film into a 2 mM
solution of DTSC in DMSO for 5 min. After washing with DMSO and
H2O, the resulting TSC/Au preparation was dipped into 1 mM 2 in H2O
and left to stand for 1 day.
** Electrochemical measurements were performed by using an ALS-600
electrochemical analyzer (BAS Inc.). The cyclic voltammograms were
recorded using a glassy carbon electrode or each SAM as a working
electrode, Pt wire as a counter electrode and Ag/Ag+ (0.01 M AgNO3 in
acetonitrile) or Ag/AgCl (3 M NaCl) as a reference electrode. A 0.1 M
solution of TBA(CF3SO3) in CH2Cl2 or a 0.1 M NaClO4 aqueous solution
was used as an electrolyte. Rotating ring-disk electrode measurements were
performed using a Hokutodenko HR-201 electrode motor and a
Hokutodenko HR-202 rotation controller with a Hokutodenko HZ-5000
electrochemical analyzer. A ring-disk electrode (Au disk (ø 5.0 mm) and Pt
ring (ø 6.8 mm with 1.2 mm width), available from Hokutodenko) was
modified by deposition of 2 in the manner described above. All
measurements were performed under a rigorous Ar atmosphere.
Fig. 3 Plot of the redox potential of Fe2(II,II/III,III) on 2/Au (constructed
with DTSP). The wave designated ‘IV’ was obtained after a flash of O2 gas
for 20 s. The wave designated ‘I’ was obtained after bubbling with Ar for
10 min.
(2e2) and V (1e2) were assigned to the waves of m-1,2-peroxo
complexes, Fe2(O2)(II,II/III,III) and Fe2(O2)(II,III/III,III), respec-
tively. It is very important that this voltammogram showed
reversible changes. Fig. 3 shows the plot of the redox potentials of
I and IV, which indicate the redox wave of Fe2(II,II/III,III) under Ar
and saturated O2 conditions. From this plot it can be seen that the
redox waves observed under Ar shift to the negative region under
saturated O2 conditions. The redox wave shift under saturated O2
occurs after bubbling of Ar. This finding indicates that reversible
O2 adsorption/desorption occurs on 2/Au in aqueous media at
room temperature.
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22
binding to 2/Au on the disk electrode is detected as H2O2 or O2
at the ring electrode. Thus, 2 has the potential to bind molecular
O2 and to reduce it electrocatalytically. Unfortunately the redox
potential of the wave VI was quite negative. In this potential
region, a bare Au electrode can also reduce molecular O2 to H2O2.
Because the starting potential of the catalytic wave of 2/Au
depends upon the redox potential of 2, the observed catalytic wave
originates from the 2/Au electrode. This result indicates that 2/Au
can activate molecular oxygen to at least O222. Since the redox
potential of this catalytic wave is quite negative, 2/Au is not
currently applicable as an O2 activation method. We are
continuing to develop useful Fe2 complexes that activate O2 at
more positive potentials.
In summary, we developed a Au electrode modified by
deposition of a non-heme functional model Fe2 complex with a
phenoxo-based dinucleating ligand and observed a rare example of
reversible oxygen adsorption/desorption in aqueous media at room
temperature. Immobilization of the complex onto the electrode
surface was found to greatly stabilize 2.
This work was supported partly by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports, and
Culture of Japan and partly by a grant from the NITECH 21st
Century COE Program, to which our thanks are due.
11 H. Wackerbarth, F. B. Larsen, A. G. Hansen, C. J. McKenzie and
J. Ulstrup, Dalton Trans., 2006, 3438–3444.
394 | Chem. Commun., 2008, 392–394
This journal is ß The Royal Society of Chemistry 2008