Ultra microanalysis of ferric ion using triazene and capillary electrophoresis

Article abstract of DOI:10.1246/cl.2006.1418

The triazene compound {2,2′-[3-(4-methoxyphenyl)triaz-2-ene-1,1-diyl] diethanol} was utilized for the high sensitive and quantitative detection of ferric ion. Copyright

Full text of DOI:10.1246/cl.2006.1418

1418
Chemistry Letters Vol.35, No.12 (2006)
Ultra Microanalysis of Ferric Ion Using Triazene and Capillary Electrophoresis
Keiji Nishiwaki,ꢀ1 Atsushi Taga,2 Masako Yamaya,1 Yusuke Morita,1 Yukiko Suzuki,1
Susumu Honda,2 and Keizo Matsuoꢀ1
1Department of Pharmaceutical Sciences, Kinki University, 3-4-1 Kowakae Higashiosaka, Osaka 577-8502
2Department of Pharmacy, School of Pharmacy, Kinki University, 3-4-1 Kowakae Higashiosaka, Osaka 577-8502
(Received September 19, 2006; CL-061082; E-mail: k-nishi@phar.kindai.ac.jp)
The triazene compound {2,20-[3-(4-methoxyphenyl)triaz-2-
ene-1,1-diyl]diethanol} was utilized for the high sensitive and
quantitative detection of ferric ion.
5
4
3
2
1
3 + Fe3+ (2 eq)
3 + Fe3+ (1 eq)
3
Fe3+
Microanalyses of transition-metal ions are desired in diverse
areas including biochemistry, environmental pollution, etc.1 Re-
cently, the selective detection of ferric ion is being intensively
studied because it plays an important role in many biochemical
processes in the cell2 and is also well known as a fluorescence
quencher due to its paramagnetic nature. Therefore, we tried
to develop a simple and precise analysis for it using UV–vis
spectroscopy. The triazenes possesses unique structure and three
successive nitrogen atoms, and they are utilized in widespread
areas including organic and polymer synthesis, and medicinal
chemistry.3 Consequently, we focused on 1-aryl-3,3-dialkyltri-
azenes as new indicators. We designed and synthesized triazenes
that specifically reacted with ferric ion. On the other hand, the
field-amplified sample stacking technique in capillary zone
electrophoresis, investigated by Mikkers et al., was a powerful
microanalysis method.4 We assessed the properties of triazenes
by measuring UV–vis spectra and capillary electrophoretic
analysis to achieve ultra microanalysis of ferric ion.
The triazene compounds were synthesized according to the
literature (Scheme 1).5 Thus, 3,3-bis(2-methoxyethyl)-1-phenyl-
triazene (1) was obtained in 94% yield employing aniline as the
arylamine and bis(2-methoxyethyl)amine as the sondary amine.
With the same procedure, 2,20-(3-phenyltriaz-2-ene-1,1-diyl)-
diethanol (2) was synthesized from aniline and diethanolamine
in 96% yield. Similarly 2,20-[3-(4-methoxyphenyl)triaz-2-ene-
1,1-diyl]diethanol (3) and 2,20-[3-(4-nitrophenyl)-2-triazedi-
ethanol (4) were obtained in 63 and 64% yields, respectively.
To evaluate the functionality of ferric ion and other metal
ions, the UV–vis spectra of the mixed methanol solution of the
synthesized triazenes (1–4) and some multivalent metal ions
were measured. Among them, only 3 reacted with ferric ion,
and the mixtue of other compounds and other metal ions did
not give any new absorption maxima (Figure 1).
0
190
300
400
500
600
700
800
Wavelength (nm)
Figure 1. UV–vis spectra of triazene 3, ferric ion, and their
mixture.
complex could not be observed in the long-wavelength region.
From the above results, this triazene could be useful to detect
trace ferric ion. In this stage, it was thought that the mixture
was transformed to different compounds possessing strong ab-
sorbancy, for example, a diazonium ion. Therefore, we per-
formed the same reaction in large scale to confirm our specula-
tion. The ferric nitrate solution was added to a methanol solution
of 3, and then phenol was added to the mixture to form 4-(4-me-
thoxyphenylazo)phenol (5) in 47% yield. This result indicated
that the reaction progressed via the diazonium cation as a reac-
tion intermediate. We proposed a possible mechanism of diazo-
nium ion generation from 3 as follows (Scheme 2). The ferric
ion coordinated with the 3-nitrogen and the two oxygens of
the hydroxylethylamino units of 3 and behaved as a Lewis acid.
The following cleavage of 2- and 3-nitrogen bond occurred to
produce a stable diazonium ion.
Furthermore, we synthesized stable diazonium salt: 4-me-
thoxybenzene diazonium tetrafluoroborate (6) from p-anisidine,
sodium nitrite, and tetrafluoroboric acid6 and its absorption spec-
trum was measured. Peak of spectrum was observed at 311 nm.
The wavelength and molar absorptivity were almost the same as
in the mixture of 3 and ferric ion (Table 1).
We tried to apply this system to the microscale determina-
tion of ferric ion using capillary electrophoresis, which was per-
formed by Applied Biosystems model 270A system equipped
After adding 1.0 equiv. of ferric ion to 3 in methanol, new
absorption was observed (blue line). Surprisingly, an additional
1.0 equiv. of ferric ion resulted in the appearance of absorption
maxima at 280 and 330 nm that increased nonlinearly (green
line). The new absorption that might arise from a charge-transfer
H
O
OH
OH
H
Fe
N
Fe(NO3)3
O
N
N
N
N
N
3
MeO
MeO
OH
OH
N
OR
N
N
NH2
N
N
N
1) NaNO2 / HCl
N
OR
MeO
5
MeO
2) HN(CH2CH2OR)2
/ K2CO3
1 - 4
X
X
Scheme 2. Possible reaction mechanism for formation of diazo-
nium cation from triazene (3) and ferric nitrate.
Scheme 1. Triazene preparation.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.12 (2006)
1419
Table 1. Comparison of molar extinction coefficient
c/mol Lꢃ1 Abs ꢀmax/nm "max/Mꢃ1 cmꢃ1
Vacuum
Introduction
(30 s)
Electrophoretic Introduction
(120 s)
Compounds
(30 s)
(420 s)
6
1:70 ꢂ 10ꢃ5 0.38
311
313
2:23 ꢂ 104
1 µM
100 nM
5.6 ppb
10 nM
0.56 ppb
1 nM
56 ppt
3 + Fe3þ (1 equiv.) 1:70 ꢂ 10ꢃ5 0.38
2:24 ꢂ 104
with a capillary oven thermostated by circulating air. Separa-
tions were performed in an untreated fused-silica capillary
(0.05-mm i.d.), which had a total length of 72 cm and a separa-
tion length of 50 cm. The electrophoretic solution was a 50 mM
phosphate buffer (pH 6.8), and the applied voltage was set at
þ20 kV. Detection was carried out at 280 nm, and capillary tem-
perature was maintained at 30 ꢁC. Samples were prepared by
addition of ferric ion solution of several concentrations to
5 mM triazene solution and then introduced by vacuum (5-inch
Hg) injection for 1.5 s. The molar ratio of ferric ion and triazene
was 1:1. Ferric nitrate was used as a ferric ion provider in the
UV–vis spectra measurements. The migration time of 3 was
ca. 6.2 min, and ferric ion was not detected at the used wave-
length. The mixture of the aqueous solutions of ferric nitrate
and 3 gave single peak at 3.6 min, indicating a quick and quan-
titative reaction (data not shown). The increased positive-charg-
ed material was produced from the triazene compound. We pro-
posed a novel quantization method for ferric ion using the same
manner.
0
5
0
5
0
5
0
5
Migration time (min)
Figure 2. Introduction-method detection limit of diazonium
an identification limit of 56 ppt employing an on-line condensa-
tion technique was developed. This method will be easy to pre-
pare the reagent and the sample solution and to set up the device.
It is considered that these results can be applied to biochemical
analysis, clinical laboratory tests, and water examination, etc.
Next, we tried to make a calibration curve of the ferric ion
as diazonium ion in the triazene solution. The peak responses
of diazonium salt were plotted against ferric ion concentration.
The curve was given by following equation:
This research was supported in part by Grant-in-Aid for
Encouragement for Young Scientists, Kinki University.
y ¼ 6:46 ꢂ 104x þ 5:77 ꢂ 103
And good linearity was obtained with correlation coeffi-
cients over 0.99 in a range of 10 mM–2 mM, calculated to
56 ppb–112 ppm. These results indicated the potential of this
system for quantitative analyses of ferric ion.
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7
8
9
In conclusion, we synthesized coordinatable and easily re-
actable triazene with ferric ion and assessed it for analysis.
The triazene reacted quantitatively with the solution containing
ferric ion to afford diazonium salt. By using capillary electro-
phoresis, a highly sensitive analytical method of ferric ion with
Published on the web (Advance View) November 18, 2006; doi:10.1246/cl.2006.1418