Preparations of low-spin tetra cyano iron complexes of N-benzyl-1,2-ethanediamine and its oxidative dehydrogenation: Isolation of the intermediate with monoimine ligand

Article abstract of DOI:10.1246/bcsj.77.987

The oxidation of tetracyano(N-benzyl-1,2-ethanediamine) with one equivalent oxidant, hydrogen peroxide or peroxodisulfate, under neutral or basic conditions yielded tetracyano(2-benzylimino-ethylamine)ferrate(II), owing to an acceleration of disproportion

Full text of DOI:10.1246/bcsj.77.987

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 987–991 (2004)
987
Preparations of Low-Spin Tetra Cyano Iron Complexes
of N-Benzyl-1,2-ethanediamine and Its Oxidative Dehydrogenation:
Isolation of the Intermediate with Monoimine Ligand
ꢀ
Masafumi Goto, Yasuhiko Ohse, Sayuri Koga, Yuka Kudo, Rinko Kukihara, and Hiromasa Kurosaki
Graduate School of Pharmaceutical Sciences, Kumamoto University, Oe-honmachi, Kumamoto 862-0973
Received November 10, 2003; E-mail: gmphiwin@gpo.kumamoto-u.ac.jp
The oxidation of tetracyano(N-benzyl-1,2-ethanediamine) with one equivalent oxidant, hydrogen peroxide or per-
oxodisulfate, under neutral or basic conditions yielded tetracyano(2-benzylimino-ethylamine)ferrate(II), owing to an ac-
celeration of disproportionation by the presence of N-benzyl substituents. Further oxidation yielded the corresponding
conjugated 1,2-diimineferrate(II) complex.
The metal-assisted oxidation of peptides is one of the causes
of damage to proteins, and is pointed out as being a plausible
Experimental
cause of aging.1 The participation of iron and the lysine side
chain and proline was suggested.1 The metal-assisted oxidative
dehydrogenation of amines has been extensively studied using
group-18 triad, Fe,2–7 Ru,8–13 and Os.14,15 The general mecha-
nism involves the oxidation of metal ions to the high-valent
state and the base-promoted disproportionation of the latter,
shown in Scheme 1.4,10,16 The amines so far employed encom-
pass from simple amine to diamines and macrocyclic tetra-
amines. The secondary amine site is more susceptible to dispro-
portionation than primary amine sites by 3000 times.2
Materials. Ferrous perchlorate hexahydrate (Alfa) was used
without further purification. Diethylenetriamine (dien) and N-ben-
zyl-1,2-ethanediamine (Been) was obtained from Wako. All other
materials were commercially purchased and used without further
purification.
For iron complexes, there is a strong tendency to form a con-
jugated 1,2-diimine complex by the dehydrogenation of 1,2-di-
amine because of a high reactivity in the disproportionation of
the presumed intermediate monoimine complex. We have re-
ported that N,N0-dimethyl-1,2-ethanediamine, when coordinat-
ed to the tetracyanoferrate moiety, is subject about 3000 times
to dehydrogenation,4 but we could not detect or isolate the cor-
responding monoimine complex. In this study, N-benzyl-1,2-
ethanediamine was employed as a ligand and in the preparation
of the iron complex, 1. Its oxidation to yield a dehydrogenated
ligand including a monoimine stage, 2, and a diimine stage, 3,
shown in Scheme 2, is described, including their isolation.
Scheme 1.
Scheme 2.
Published on the web May 6, 2004; DOI 10.1246/bcsj.77.987

988 Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
Dehydrogenation of [Fe(CN)4(1,2-diamine)]2ꢄ
Na2[Fe(CN)4(Been)] (1). To a vigorously stirred solution of
Fe(ClO4)2 6H2O (14.5 g, 40 mmol) in 80 mL of anhydrous meth-
an optical cell in a spectrophotometer kept at constant temperature
(25 ꢂC) by circulating thermostated water. A buffered aqueous so-
lution of potassium hexacyanoferrate(III) was added to the Fe(II)
solution and the change in absorbance at 525 nm was recorded.
ꢁ
anol, a methanol solution of Been (18.1 g, 120 mmol in 20 mL) was
added, followed by an aqueous solution of NaCN (7.8 g, 160
mmol, 12 mL) at room temperature under argon. Yellow crystals
were separated upon the addition of ethanol (80 mL). The mixture
was kept at 0 ꢂC for 1 h, and yellow crystals were collected by fil-
teration and washed with acetone (30 mL) and ether (20 mL) suc-
cessively. A crude product (4.0 g) was recrystallized by dissolving
it in 85 mL of a mixture of water–methanol (2:8 v/v) at 50 ꢂC, fol-
lowed by filtration and the addition of 180 mL of acetonitrile. Yel-
low crystals. Yield, 1.74 g (37.6%). 1H NMR (D2O) ꢀ 2.00 (m, 1H,
CH2CH2), 2.34 (m, 2H, CH2CH2), 2.73 (m, 1H, CH2CH2), 3.70 (d,
1H, CHH), 4.30 (d, 1H, CHH), 7.30–7.40 (5H, Ph). 13C NMR
(D2O, int. ref. dioxane = 67.4 ppm) ꢀ 45.0 (C(1)), 52.0 (C(2)),
60.5 (C(3)), 130.6, 131.6, 132.1, 141.5. Anal. Calcd for
Na2[Fe(CN)4(Been)]: C, 43.85; H, 3.96; N, 23.60%. Found: C,
43.08; H, 3.81; N, 23.25%.
The initial velocity was obtained by v0 = (1/"525) (ꢀAbs/
ꢁ
ꢀt)t¼0, where "525 is the molar absorption coefficient of 3.
Electrochemical Measurements. Cyclic voltammetry meas-
urements were made with a BAS Model CV-27 voltammograph
and a Riken Denshi F-3EH XY recorder using a saturated Ag–
AgCl electrode as a reference electrode and a glassy carbon and
platinum-wire as the working and auxiliary electrodes. Measure-
ments were made on a solution containing 10ꢄ3 M iron complex,
using 0.1 M NaCl as a supporting electrolyte; argon was passed
for 10 min prior to the measurements.
Results
Preparation of Iron(II) and Iron(III) Complexes. Com-
plex (1) was prepared under argon by the successive addition of
2 equivalents of Been to form a bis(Been) complex and 4 equiv-
alents of sodium cyanide. The dehydrogenated products of a
monoimine complex (2) and a diimine complex (3) were ob-
tained by oxidation with 1 equivalent and 2 equivalent amounts
of hydrogen peroxide, respectively.
Na2 [Fe(CN)4(2-benzylimino-ethylamine
=
Been-2H)]
Á
1.5H2O (2). 1 (557 mg, 1.6 mmol) was suspended in a mixture
of water–methanol (5 mL, 2:8 in volume) and 0.78 mL of 2 M
aqueous hydrogen peroxide (1.6 mmol) was added at 50 ꢂC, result-
ing in a yellow solution. Yellow crystals were separated upon the
addition of 15 mL of acetone and chilling in an ice bath. The yel-
low crystals were collected on a filter. The crude product was re-
crystallized by dissolving in water (1 mL) and adding acetone.
Spectroscopic Property of Iron(II) and Iron(III)
Complexes. Visible spectra of 1, 2, and 3 in 1 mM hydrochlo-
ric acid (Fig. 1) showed weak absorptions due to d–d transitions
at 395 nm and 320 nm for 1, but intensive absorption at 354 nm
with " of 1620 for 2. A low-spin Fe(II) complex having
monoimine bonds in a macrocyclic ligand, bis(acetonitrile)-
(5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,
1
Yield, 40 mg (6.6%). H NMR (D2O) ꢀ 3.62 (s, methylene), 4.94
(s, benzyl), 7.18 (s, azomethine), 7.37–7.42 (Ph). 13C NMR
(D2O, int. ref. dioxane = 67.4 ppm) ꢀ 52.5 (C(1)), 67.1 (C(3)),
130.7, 131.5, 132.8, 139.6 (Ph), 171.8 (C(2)). Anal. Calcd for
Na2[Fe(CN)4(Been-2H)] 1.5H2O: C, 40.97; H, 3.97; N, 22.05%.
ꢁ
ꢀ
11-diene)iron(II), has been reported to have a metal ! ꢂ tran-
Found: C, 40.17; H, 3.86; N, 22.07%.
Na [Fe(CN) (Been-4H)] H O (3). To a mixture of water–
sition of C=N–Fe at 361 nm with " ¼ 2440.17 The character-
istic Fe(II) to 1,2-diimine charge-transfer band was observed
for 3 at 525 nm with " of 4800 as well as a band at 370 nm.
The 1H NMR spectra of 1, 2, and 3 showed characteristic sig-
nals: for 1, all C–H protons of the 1,2-ethanediamine moiety
appeared as separated signals and benzyl protons appeared as
an AB quartet at 3.70 and 4.30 ppm: for 2, both NH2–CH2–
and benzyl CH2 appeared as singlets at 3.62 and 4.94 ppm,
and a new azomethine signal appeared at 7.18 ppm; for 3, a
benzyl proton appeared at 5.38 ppm and two azomethine pro-
Á
2
4
2
methanol (6 mL, 2:8 in volume) of Na2[Fe(CN)4(Been)] (535
mg, 1.5 mmol), 0.75 mL of an aqueous 4 M hydrogen peroxide
was added at 50 ꢂC. The resultant deep-red solution was stirred
for 5 min and concentrated under reduced pressure. A red residue
was dissolved in 1 mL of water and applied to the top of a Sepha-
dex G-15 column (2.6 cm ꢁ ꢃ 80 cm) and eluted with water. The
fractions that showed absorption maxima at 530 nm were com-
bined and concentrated under reduced pressure, yielding red crys-
tals. These crystals were stored in vacuo. Yield, 167 mg (30.1%).
1H NMR (D2O) ꢀ 5.38 (s, benzyl), 7.43 (Ph), 8.09 (s, azomethine
H(2)), 8.43 (s, azomethine H(3)). 13C NMR (D2O, int. ref. dioxane
= 67.4 ppm) ꢀ 68.4 (C(3)), 130.8, 131.6, 132.6, 139.8 (Ph), 164.4
(C(1)), 168.6 (C(2)). Anal. Calcd for Na2[Fe(CN)4(Been-
4H)] H2O: C, 42.19; H, 3.27; N, 22.70%. Found: C, 42.08; H,
3.26; N, 22.34%.
ꢁ
Physical Measurements. Electronic spectra of the aqueous so-
lutions of the Fe(II) complexes were recorded on a Shimadzu UV-
2200 spectrophotometer. Infrared spectra were recorded on a JEOL
1
JIR-6500 FT-IR spectrophotometer using KBr disks. The H and
13C NMR spectra of the Fe(II) complexes were measured by dis-
solving a weighed sample (15–20 mg) into D2O (0.35 mL) contain-
ing sodium 3-(trimethylsilyl)propionate-2,2,3,3-d4 as an internal
1
standard (ꢄ0:02 ppm for H NMR and ꢄ1:91 ppm for 13C NMR)
degassed by several freeze–pump–thaw cycles, followed by seal-
1
ing a tube of 5 mm-diameter. The H and 13C NMR spectra were
recorded on a JEOL JNM-GX-400 and a JEOL JNM-EX270 spec-
trometer.
Kinetic Measurements. An aqueous solution of 1 in a buffered
solution (50 mM phosphate buffer, pH 5.0, 5.5, 6.0) was placed in
Fig. 1. Visible spectra of (1) (——), (2) (- - - -), and (3)
(- - -).
ꢁ ꢁ

M. Goto et al.
Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
989
tons appeared at 8.09 and 8.43 ppm. The 13C NMR spectra of 1,
2, and 3 also showed the characteristic signals for 1; three sig-
nals appeared at methylene carbon region, but the number de-
creased by 1 and 2 for 2 and 3 and one and two azomethine car-
bon(s) appeared in the region between 160 and 172 ppm for 2
Dehydrogenation of Been Coordinated to Tetracyanofer-
rate(II) and (III). The oxidative dehydrogenation of diamines
of tetracyano(diamine)ferrate(II) is dependent on the pH of the
medium as well as the concentration and nature of the oxidant.
In this study, we used N-benzyl-1,2-ethanediamine, which is a
1,2-ethanediamine substituted at one end of amines. The visible
absorption spectral change of 1 upon oxidation with ammoni-
um peroxodisulfate at pH 10.66 borate buffer (12.5 mM, I ¼
0:3 M (NaCl)) is shown in Fig. 3. When the concentration of
ammonium peroxodisulfate was two-times that of 1, an inter-
mediate having an absorption maximum at 354 nm and a
shoulder at 320 nm appeared, and the absorbance of this species
reached a maximum at 15 min. The absorbance at 525 nm was
rapidly increased along with a decrease in the absorbance at
354 nm (Fig. 3A). When one equivalent amount of ammonium
peroxodisulfate was added, the increase in the absorbance at
525 nm was suppressed, and the absorbance at 354 nm increas-
ed, and no decrease was observed (Fig. 3B). These observations
show that one equivalent oxidant was consumed in the dehy-
drogenation of 1 to yield 2, according to
1
and 3. The H NMR spectrum of [Co(CN)4(Been)]ꢄ, which is
isoelectronic to 1, has been reported.18 Each signal of 1 was
found to appearance upper field than that of the corresponding
Co(III) complex. Based on the NMR data, we identified 2 as be-
ing an isolated product among the possible three compounds for
the monoimine stage shown in Chart 1. The azomethine signal
of 2 appeared at 7.18 ppm and shifted upfield compared to those
of [Fe(CN)4(CH3–N=CH–CH=NH)]2ꢄ (7.53, 7.66 ppm). This
upfield shift reflects that this proton locates above the intramo-
lecular phenyl ring.
Electrochemical Property. Cyclic voltammograms of 1, 2,
and 3 in 1 mM hydrochloric acid (I ¼ 0:1 M NaCl) and 1 and 2
in pH 8 (borate buffer) are shown in Fig. 2. One Fe2þ=3þ quasi-
reversible wave was found for each 1 through 3 under acidic
conditions with increasing the FeII/III redox potential in the or-
der of 1 (0.13 V vs Ag/AgCl), 2 (0.19 V), and 3 (0.41 V), while
two-step processes were found for 1 and 2 under basic condi-
tions, and their redox potentials were 0.19 and 0.41 V, which
is in agreement with the redox potentials of 2 and 3 under acidic
conditions. This shows that the Fe(III) complex for 1 generated
by one-electron oxidation converts rapidly to 2 and 3 under the
conditions of pH 8. As expected from the kinetic measurement
described below, the FeIII form of 1 is not stable under basic
conditions.
Fig. 2. Cyclic voltammograms of 1, 2, and 3 under acidic
conditions (1 mM HCl) (A) and 1 and 2 at pH 8 (B).
Chart 1.
Fig. 3. (A) Spectral change for the reaction of (1) (0.6 mM) with (NH4)2(S2O8) (1.2 mM) at pH 10.66 borate buffer, repetition pe-
riod, 5 min; (B) Spectral change for the reaction of (1) (0.6 mM) with (NH4)2(S2O8) (0.6 mM) at pH 10.66 borate buffer, repetition
period, 5 min.

990 Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
Dehydrogenation of [Fe(CN)4(1,2-diamine)]2ꢄ
Table 1. Results of Kinetics of Dehydrogenation of Tetracyano(N-benzyl-1,2-ethanediamine)ferrate(II) 1
pH
1/10ꢄ4
M
[Fe(CN)63ꢄ]/10ꢄ3
M
ꢀAbs/ꢀt (sꢄ1
)
v0/10ꢄ9 M sꢄ1
kobs/103 Mꢄ1 sꢄ1
k/106 Mꢄ2 sꢄ1
5.02
5.02
4.98
5.55
5.52
5.52
5.98
5.99
5.98
2.33
2.34
2.33
2.33
2.34
2.33
2.33
2.33
2.33
5.0
10.0
20.0
5.03
9.98
20.0
4.99
9.96
20.0
8.52
17.0
22.7
22.7
37.6
83.8
54.2
123
1.84
3.67
4.90
4.90
8.13
18.1
11.7
26.6
64.1
1.58
1.58
1.05
4.20
3.49
3.89
10.0
11.4
13.8
1.58
1.58
1.05
1.40
1.16
1.16
1.00
1.14
1.38
297
ferrate(II).4 The value of the redox potential of 2 was nearer
to 1 than 3. This shows that the conjugated 1,2-diimine greatly
stabilized Fe(II) state, but the stabilization by monoimine group
is not large.
½Fe(CN)4(Been)ꢅ2ꢄð1Þ þ S2O8
2ꢄ
2ꢄ
! ½Fe(CN)4(Been-2H)ꢅ ð2Þ þ 2Hþ þ 2SO42ꢄ: ð1Þ
If the oxidant is present in two equivalents, a further oxida-
tion of 2 to 3 proceeds smoothly under basic conditions,
When 1 was oxidized with 1 equivalent of oxidant, new spe-
cies were detected in the visible spectra, and this was deter-
mined to be 2. The effect of benzyl group seems to be its strong
electron-withdrawing effect. Thus, deprotonation from the
CH2-group next to the N-benzyl group was sufficiently en-
hanced to overcome an acceleration effect of the monoimine
group to dehydrogenate, yielding the diimine complex. Thus,
the isolation of 2 was attained by kinetic control of the reaction.
The rate of dehydrogenation of 1 to 3 was proportional to the
concentration of the oxidant, and when the logarithm of the ob-
served rate constant was plotted against pH, a straight line with
a slope of about 1 was obtained. The third-order rate constant
(k) was 1:3 ꢃ 106 Mꢄ2 sꢄ1. At a pH lower than 5.0, the spectral
change showed that both 1 and 2 were oxidized at the iron cen-
ter, but were not dehydrogenated. From this, the rate of oxida-
tion of the central atom was not dependent on the pH, and base-
assisted disproportionation occurred at pH values higher than 5.
½Fe(CN)4(Been-2H)ꢅ2ꢄð2Þ þ S2O8
2ꢄ
2ꢄ
! ½Fe(CN)4(Been-4H)ꢅ ð3Þ þ 2Hþ þ 2SO42ꢄ: ð2Þ
When the oxidation was carried out at pH 3.71 acetate buffer
(0.2 M, I ¼ 0:3 M (NaCl)), no formation of 2 and 3 was ob-
served, and the corresponding ferrate(III) complex of 1, which
has an absorption maximum at 375 nm and a shoulder at 305
nm, were formed (data not shown).
The dehydrogenation of 2 with ammonium peroxodisulfate
occurred rapidly at pH 8 borate buffer with an isosbestic point.
However, under acidic conditions (1 mM hydrochloric acid),
only a small amount of 3 was formed, but new species, presum-
ably [FeIII(CN)4(Been-2H)]ꢄ, which had absorptions at 405,
375, and 315 nm, were formed.
The kinetics of the formation of 3 from 1 was evaluated in
the presence of excess potassium hexacyanoferrate(III) as an
oxidant at pH 5.0, 5.5, and 6.0. The initial increase in the ab-
sorption at 525 nm was converted to the rate. The data of the
kinetics are tabulated in Table 1. Since the initial rate is propor-
tional to the concentration of the oxidant, the second-order rate
constant (kobs) was obtained using v0 ¼ kobs[1][Fe(CN)63ꢄ]0.
Further, the pH profile of kobs showed a straight line having a
slope of 0.95. Therefore, the total rate equation is expressed as
This work was supported, in part, by a Grant-in-Aid for Sci-
entific Research No. 13470497 from the Ministry of Education,
Culture, Sports, Science and Technology.
References
1
2
E. R. Stadtman, Science, 257, 1220 (1992).
K. Pohl, K. Wieghardt, W. Kaim, and S. Streenken, Inorg.
v0 ¼ k½1ꢅ½Fe(CN)63ꢄꢅ0½OHꢄꢅ;
and the third-order rate constant is determined to be k ¼ 1:3 ꢃ
106 Mꢄ2 sꢄ1
ð3Þ
Chem., 27, 440 (1988).
3 Y. Kuroda, N. Tanaka, M. Goto, and T. Sakai, Inorg.
Chem., 28, 2163 (1989).
4 M. Goto, M. Takeshita, N. Kanda, T. Sakai, and V. L.
Goedken, Inorg. Chem., 24, 582 (1985).
.
Discussion
5
54, 2491 (1981).
M. Goto, M. Takeshita, and T. Sakai, Bull. Chem. Soc. Jpn.,
The expected intermediate of tetracyano(monoimine)-
ferrate(II) has not been isolated so far using 1,2-diamines by
oxidation. This is ascribed to the high susceptibility to dispro-
portionation of the monoimine complex compared to the di-
amine complex. In this study, based on the fact that the meth-
ylation of aminonitrogens of 1,2-diamine caused an increase
in the disproportionation by about 3000 times, benzyl substitu-
ent was chosen as the most efficient enhancer for disproportio-
nation.
6
7
V. L. Goedken, J. Chem. Soc., Chem. Commun., 1972, 207.
V. L. Goedken and D. H. Busch, J. Am. Chem. Soc., 94,
7355 (1972).
8
M. Yamaguchi, K. Machiguchi, T. Mori, K. Kikuchi, I.
Ikemoto, and T. Yamagishi, Inorg. Chem., 35, 143 (1996).
9 M. Yamaguchi and T. Yamagishi, Inorg. Chem., 32, 2981
(1993).
10 F. R. Keene, M. J. Ridd, and M. R. Snow, J. Am. Chem.
Soc., 105, 7075 (1983).
11 P. Bernhard and A. M. Sargeson, J. Am. Chem. Soc., 111,
The redox potentials of 1 and 3 have similar values with typi-
cal tetracyano(diamine)ferrate(II) and tetracyano(diimine)-

M. Goto et al.
Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
J. Am. Chem. Soc., 104, 6161 (1982).
991
597 (1989).
12 P. Bernhard, D. H. Bull, H.-B. Burgi, P. Osvath, A. Raselli,
¨
and A. M. Sargeson, Inorg. Chem., 36, 2804 (1997).
16 E. C. Constable, ‘‘Metals and Ligand Reactivity—An Intro-
duction to the Organic Chemistry of Metal Complexes,’’ VCH
Verlag, Weinheim, Germany (1996), Chap. 9.3.
17 V. L. Goedken, P. H. Murrell, and D. H. Busch, J. Am.
Chem. Soc., 94, 3397 (1972).
13 A. Cabort, B. Therrien, H. Stoeckli-Evans, K. Bernauer,
and G. Suss-Fink, Inorg. Chem. Commun., 5, 787 (2002).
¨
14 A. Patel, A. Ludi, H.-B. Burgi, A. Raselli, and A. P. Bigler,
¨
Inorg. Chem., 31, 3405 (1992).
15 P. A. Lay, A. M. Sargeson, B. W. Skelton, and A. H. White,
18 H. Kurosaki, Y. Koga, and M. Goto, Bull. Chem. Soc. Jpn.,
68, 843 (1995).