Influence of CH3 group of μ-N-C-S ligand on the properties of [Fe2(C4H5N2S)2(NO) 4] complex

Article abstract of DOI:10.1016/j.ica.2005.10.005

Bi-nuclear neutral sulfur-nitrosyl iron complex [Fe₂(SR) ₂(NO)₄] (I) has been obtained by replacement of thiosulfate ligands in dianion [Fe₂(S₂O₃) ₂(NO)₄]^2- by 1-methyl-imidazole-2-yl. From X-ray analysis data, the complex has centrosymmetrical dimeric structure, with the iron atoms being linked via μ-N-C-S bridge. From Mossbauer spectroscopy, isomeric shift δ_Fe is 0.180(1) mm/s and quadrupole splitting ΔE_Q is 0.928(2) mm/s at T = 290 K. By comparative studying the mass-spectra in the gaseous phase of solid samples decomposition and kinetics of NO release in 1% aqueous solutions of dimethylsulfoxide, using of the ligand with CH₃ substituent in position 1 of imidazole-2-thiol was shown to yield a more stable donor of nitrogen monoxide than earlier obtained analog with imidazole-2-thiol, [Fe₂(C₃H₃N ₂S)₂(NO)₄].

Full text of DOI:10.1016/j.ica.2005.10.005

Inorganica Chimica Acta 359 (2006) 570–576
www.elsevier.com/locate/ica
Influence of CH3 group of l-N–C–S ligand on the properties
of [Fe2(C4H5N2S)2(NO)4] complex
*
N.A. Sanina, T.N. Rudneva, S.M. Aldoshin , G.V. Shilov, D.V. Kortchagin,
Yu.M. Shulꢀga, V.M. Martynenko, N.S. Ovanesyan
Institute of Problems of Chemical Physics, 1, Acad. Semenov av., 142432 Chernogolovka, Moscow Region, Russia
Received 1 July 2005; received in revised form 6 October 2005; accepted 6 October 2005
Available online 23 November 2005
Abstract
Bi-nuclear neutral sulfur–nitrosyl iron complex [Fe2(SR)2(NO)4] (I) has been obtained by replacement of thiosulfate ligands in dianion
[Fe2(S2O3)2(NO)4]2ꢀ by 1-methyl-imidazole-2-yl. From X-ray analysis data, the complex has centrosymmetrical dimeric structure, with
the iron atoms being linked via l-N–C–S bridge. From Mossbauer spectroscopy, isomeric shift dFe is 0.180(1) mm/s and quadrupole
splitting DEQ is 0.928(2) mm/s at T = 290 K. By comparative studying the mass-spectra in the gaseous phase of solid samples decom-
position and kinetics of NO release in 1% aqueous solutions of dimethylsulfoxide, using of the ligand with CH3 substituent in position
1 of imidazole-2-thiol was shown to yield a more stable donor of nitrogen monoxide than earlier obtained analog with imidazole-2-thiol,
[Fe2(C3H3N2S)2(NO)4].
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: NO donors; Sulfur–nitrosyl iron complexes; 1-Methyl-imidazole-2-yl; X-ray analysis; Mossbauer spectroscopy; Kinetics of NO release
1. Introduction
anti-neoplastic agents, i.e., structural analogs of natural
aromatic thiols, in particular, mercaptohistidines used for
tumor chemotherapy [26] be employed as SR ligands in
[Fe2(SR)2(NO)4] complexes.
Recently, intensive search has been performed for metal-
based antitumor medicines of new generation with
improved activity and reduced side effects as compared to
available clinical medications (cisplatin, sodium nitroprus-
side) for therapy of oncology diseases [1–13]. The results of
the latest investigations [14–21] show that medicines with
antitumor activity should be sought among nitrogen mon-
oxide donors. For example, nitrogen monoxide was shown
to affect apoptosis of tumor cells, activity of gene p53, neo-
plasm of vessels supplying a tumor [22,23]. Binuclear para-
magnetic sulfur–nitrosyl iron complexes with ligands of
l-N–C–S type ([Fe2(SR)2(NO)4] [24]) are synthetic analogs
of natural NO donors – dinitrosyl iron complexes (DNIC),
which can be detected in solutions by EPR method from a
typical signal with g = 2.03 [25]. We suggest that different
In this work, binuclear sulfur–nitrosyl iron complex
[Fe2(SC4H5N2)2(NO)4] (I) has been synthesized. 1-CH3-
imidazole-2-yl related to a class of pharmacologically
active bidentate N,S-ligands [27] was used as a ligand for
synthesis of the new NO donor of ‘‘g = 2.03 family’’. The
structure and kinetics of decomposition of I were studied
in the solid phase and solutions by X-ray analysis, IR-,
Mossbauer and mass-spectroscopy, and were compared
with those of earlier synthesized isostructural complex with
1-H-imidazole-2-yl, [Fe2(C3H3N2S)2(NO)4] (II) [28].
2. Experimental
2.1. Synthesis of I
*
Corresponding author. Tel.: +7 095 7857048; fax: +7 096 5249676.
For synthesis, the following chemicals were used:
E-mail addresses: ruta@icp.ac.ru (T.N. Rudneva), sma@icp.ac.ru
(S.M. Aldoshin).
Na2S2O3 Æ 5H2O (Aldrich), NaOH (Aldrich), 1-methyl-2-
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.10.005

N.A. Sanina et al. / Inorganica Chimica Acta 359 (2006) 570–576
571
Table 1
mercaptoimidazole (Aldrich). Complex Na2[Fe2(l2-S2O3)2
Crystal structural data and experimental parameters for complex I
(NO)4] Æ 4H2O was synthesized according to technique
[29]. The solutions preparation and mixing and the com-
plex isolation were performed in an atmosphere of nitro-
gen. Solvents CH2Cl2 (Aldrich), dimethyl ether and
ethanol were distilled using technique [30].
Parameter
Composition
M (g/mol)
T (K)
I
Fe2S2C8H10N8O4
458.06
200
Space group
a (A)
C2/c
˚
An alkali solution of thiol (21 ml, 10 mmol) was added
to an aqueous solution of Na2[Fe2(l2-S2O3)2(NO)4] Æ 2H2O
(2 mmol) and Na2S2O3 Æ 5H2O (4 mmol) (15 ml). Immedi-
ately deposited dark-brown precipitate was filtered and
washed with absolute dimethyl ether. Recrystallization
from C2H5OH:CH2Cl2 (1:7) mixture yielded single crystals
of (I) as black shining thin needle-shaped plates (Yield:
63.50%) (Anal. Calc. for Fe2S2N8C8H10O4: Fe, 24.45; S,
13.97; N, 24.45; C, 20.96; O, 13.97; H, 2.18. Found: Fe,
24.15; S, 13.79; N, 24.33; C, 21.01; H, 1.48%). IR-spectra
were recorded on SPECTRUM BX-II Fourier spectrome-
ter. The samples were used as pellets with KBr (1 mg of
the substance per 300 mg of KBr): mmax/cmꢀ1, 3469 (m),
3149 (m), 3127 (m), 2926 (m), 2854 (m), 2362 (m), 2342
(m), 1782 (vs.), 1748 (s), 1716 (vs), 1636 (m), 1578 (m),
1534 (m), 1456 (s), 1410 (m), 1399 (m), 1367 (m), 1315
(m), 1282 (m), 1145 (s), 1087 (m), 1032 (m), 958 (m), 741
(s), 694 (s), 620 (m), 590 (m), 532 (m); 1782, 1748, 1716
(NO).
14.455(3)
9.848(2)
13.108(3)
116.36(3)
1671.9(6)
4
1.820
2.017
56.12
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
Dcalc (g/cm3)
l (mmꢀ1
2hmax
)
N[I > 2r(I)]
Ntotal
765
1813
Goodness-of-fit
R1, wR2
0.864
0.0408, 0.0884
Table 2
Coordinates of the non-hydrogen atoms (104·) and equivalent thermal
parameters (103·) in I (200 K)
2
˚
Atom
X
Y
z
Uequiv (A )
Fe(1)
S(1)
3330(1)
1608(1)
3942(4)
4245(4)
3655(4)
3859(4)
1426(3)
1218(3)
1401(4)
1091(5)
1216(4)
1135(5)
798(1)
553(1)
611(1)
29(1)
32(1)
73(2)
60(1)
42(1)
33(1)
30(1)
28(1)
25(1)
36(1)
32(1)
39(1)
ꢀ554(1)
Complex II was synthesized using the procedure in [28].
O(1)
O(2)
N(1)
N(2)
N(3)
N(4)
C(1)
C(2)
C(3)
C(4)
ꢀ1858(5)
1344(5)
ꢀ759(6)
1236(4)
2826(4)
902(5)
1479(5)
1928(6)
3114(6)
ꢀ562(7)
1435(5)
ꢀ851(4)
1219(4)
2.2. X-ray analysis
ꢀ242(4)
ꢀ1840(3)
ꢀ2768(3)
ꢀ1757(4)
ꢀ3522(5)
ꢀ2963(5)
ꢀ3010(4)
Experimental data (765 reflexes with I > 2r(I)) were
obtained from the black rhomb-shaped single crystal of
poor quality with the maximum linear size 0.2 mm on the
automatic four-circle diffractometer KM-4 (KUMA DIF-
FRACTION, Poland) using monochromatic Mo Ka irra-
˚
diation (k = 0.71073 A). An array of structural
amplitudes was collected by x/2h-scanning in the angle
range 2.59ꢁ < h < 26.12ꢁ. Parameters of the unit cell were
obtained and refined over 25 reflections. The crystalline
structure was solved by the direct method and refined by
the full-matrix least-square method in the anisotropic
approximation (for the non-hydrogen atoms) using SHELX
97 Program [31]. Positions of the hydrogen atoms were cal-
culated geometrically, their coordinates and thermal
parameters were not refined. Main crystallographic data
and experimental parameters are presented in Table 1.
Atomic coordinates and their equivalent isotropic thermal
parameters are shown in Table 2. In Table 3, inter-atomic
distances and valence angles are shown.
assuming
components.
a
lorentz form for individual spectral
2.4. Mass-spectroscopy
Mass-spectra of the gases released by the sample under
study were recorded using mass-spectrometer MI 1201 V.
The gas in the spectrometer ionic source was ionized by
the electron beam of 70 eV. To obtain the gaseous phase,
an 80-mg sample was placed in a quartz ampoule of the
pyrolysis reactor connected to the ionization chamber
through a fine control valve. The quartz ampoule with
the sample was pumped out to the pressure of about
2 · 10ꢀ5 Pa during an hour to remove surface and weakly
bound admixtures. After pumping out, the ampoule was
detached from the vacuum system and left at room temper-
ature (ꢁ20 ꢁC) for about a day. Then the fine control valve
was opened, and mass-spectrometric analysis of the gas in
the ampoule was performed. Then the fine control valve
was closed, the sample was heated to 70 ꢁC and kept at this
temperature during 1 h. The gathered gas was analyzed at
this temperature. Then the quartz ampoule was again
2.3. Mossbauer spectroscopy
Mossbauer absorption spectra were recorded on WissEl
in the mode of permanent acceleration. Co57 in Rh matrix
was used as a source. The spectra at low temperatures were
measured using a flow-through helium cryostat CF-506
(Oxford Instruments) with controlled temperature. Moss-
bauer spectra were processed by the least-square method

572
N.A. Sanina et al. / Inorganica Chimica Acta 359 (2006) 570–576
Table 3
(HANNA Instruments, Germany). Processing of experi-
mental spectra and calculation of decomposition constants
for complexes I and II have been performed using program
complex Origin 6.1.
Inter-atomic distances and valence angles in I (200 K)
˚
d (A)
Bond length
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)#1
Fe(1)–S(1)
S(1)–C(1)
N(1)–O(1)
N(2)–O(2)
N(3)–C(1)
N(3)–C(3)
N(3)–Fe(1)#1
N(4)–C(1)
N(4)–C(2)
N(4)–C(4)
C(2)–C(3)
1.696(5)
1.667(5)
2.013(4)
2.283(2)
1.729(5)
1.149(6)
1.164(6)
1.333(7)
1.394(6)
2.013(4)
1.356(6)
1.367(7)
1.470(8)
1.347(8)
3. Results and discussion
3.1. Scheme of [Fe2(SC4H5N2)2(NO)4] synthesis
Complex (I) was obtained by replacement of thiosulfate
ligands by 1-methyl-imidazole-2-yl according to
2ꢀ
3ꢀ
S½Fe2ðS2O3Þ ðNOÞ ꢃ þ 2S2O2ꢀ ! 2½FeðS2O3Þ ðNOÞ ꢃ
2
4
3
2
2
þ2C4H5N2Sꢀ;OHꢀ
½Fe ðSC H N Þ ðNOÞ ꢃ
ð1Þ
ƒƒƒƒƒƒƒƒ!
2
4
5
2
2
4
ꢀ4S2O2ꢀ
3
Bond angle
X (ꢁ)
I is well soluble in DNSO, DMFA, acetone, THF, acetoni-
trile and o-xylene, partially soluble in methanol, dimethyl
ether and carbon tetrachloride, poorly soluble in ethanol.
In acetonitrile, acetone, methylene chloride, o-xylol, CCl4
and THF it decomposes in a few minutes after dissolving
to yield a beige (in acetonitrile) or dirty-green (o-xylene)
amorphous precipitate or brown suspension. I is insoluble
in n-heptane and isopropyl alcohol.
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–N(3)#1
N(2)–Fe(1)–N(3)#1
N(1)–Fe(1)–S(1)
N(2)–Fe(1)–S(1)
N(3)#1–Fe(1)–S(1)
C(1)–S(1)–Fe(1)
O(1)–N(1)–Fe(1)
O(2)–N(2)–Fe(1)
C(1)–N(3)–C(3)
C(1)–N(3)–Fe(1)#1
C(3)–N(3)–Fe(1)#1
C(1)–N(4)–C(2)
C(1)–N(4)–C(4)
C(2)–N(4)–C(4)
N(3)–C(1)–N(4)
N(3)–C(1)–S(1)
N(4)–C(1)–S(1)
C(3)–C(2)–N(4)
C(2)–C(3)–N(3)
115.8(2)
108.6(2)
113.2(2)
102.2(2)
105.5(2)
111.0(1)
104.5(2)
164.3(5)
170.2(4)
106.7(5)
127.3(4)
126.3(4)
107.6(4)
125.7(4)
126.6(4)
109.7(5)
126.9(4)
123.4(4)
107.8(4)
108.1(5)
3.2. Structure of [Fe2(SC4H5N2)2(NO)4] complex
By incorporating substituents, physical–chemical prop-
erties of nitrosyl products, such as solubility, stability in
the solid phase and solutions, coordination opportunities
of a ligand, can be varied [32], this being important for
application of such systems in biologic and medical studies.
Sulfur–nitrosyl iron complex I forms according to
scheme (1), and it contains the ligand differing from that
involved in complex II by the presence of –CH3 group in
position 1 of 2-mercaptoimidazole. As follows from the
analysis of IR-spectrum of complex I, there is no absorp-
tion band m(N–H) at 3495 cmꢀ1, this being typical for II,
and two bands da(CH3) at 1456 cmꢀ1 and ds(CH3) at
1368 cmꢀ1 can be observed. A low-intensity absorption
band ds(CH3) is nonmonotonic background and might be
shifted to lower wave numbers. It is more difficult to iden-
tify stretching vibrations of CH3 group because of the pres-
ence of many low-intensity absorption bands in the region
Operations of symmetry used for generation of equivalent atoms: #1
ꢀx + 1/2, ꢀy + 1/2, ꢀz.
pumped out to a high vacuum, the valve was closed and the
sample was heated to the next temperature (120 ꢁC). The
measurements were performed in the range 1 6 m/z 6
120, m – atomic weight, z – the ion charge.
2.5. Electrochemical determination of NO
To measure concentration of NO produced by sulfur–
nitrosyl iron complexes I and II, sensor electrode ‘‘amiNO-
700’’ of ‘‘inNO Nitric Oxide Measuring System’’ (Innovative
Instruments, Inc., Tampa, FL, USA) was used. NO concen-
trations were recorded during ꢂ1 h (with 0.2-s pace) in 1%
aqueous solution of DMSO with NO donor concentration
0.1 lM. DMSO was purified using method [30]. For calibra-
tion of the electrochemical sensor, a standard aqueous solu-
tion of NaNO2 (100 lM) was used, which was added to the
mixture of 0.12 M of KI (Aldrich) and 2 ml of 1 M H2SO4
(chemical grade) in 18 ml of water. All experiments were per-
formed in aerobic solutions at 25 ꢁC. pH of the solutions
were measured using a membrane pH-meter HI 8314
of 2950–2800 cmꢀ1
.
From X-ray data, I has a centrosymmetric dimeric binu-
clear structure (Fig. 1), similar to that of earlier studied
binuclear sulfur–nitrosyl complex II with imidazole-2-yl
[28]. In the dimer, two tetrahedral-coordinated iron atoms
are linked by Fe–S–C–N–Fe0 bridge. The methyl-imidazole
ligand is bidentate and is able to coordinate according to a
thiol (Fig. 2(a)) and thion (Fig. 2(b)) type. A little short-
ened bond lengths in the structure of I, N(3)–C(1)
˚
˚
1.333(7) A, C(2)–C(3) 1.347(8) A, as compared to the other
˚
bonds in the cycle, N(4)–C(1) 1.356(6) A, N(3)–C(3)
˚
˚
˚
1.394(6) A, N(4)–C(2) 1.367(7) A and S(1)–C(1)
˚
1.729(5) A, and the bond length C(1)–S(1) 1.729(5) A

N.A. Sanina et al. / Inorganica Chimica Acta 359 (2006) 570–576
573
Mossbauer spectroscopy data, in I the value of isomeric
shift (d = 0.180(1) mm/s) is less than in isostructural II
(d = 0.196(1) mm/s). The decrease of isomeric shift value
points to the increase of s-electron density on the iron atom
(we have noted this in [33]) and, accordingly, to the reduc-
tion of Fe–S bond length in I. The Fe–Fe distance in I is
˚
˚
shorter (4.02 A) than in II (4.10 A).
Three planes can be marked out in the molecular struc-
ture of I: S(1)C(1)N(4)C(2)C(3)N(3)Fe(1A) {1}, S(1)Fe(1)
S(1A)Fe(1A) {2}, S(1A)C(1A)N(4A)C(2A)C(3A)N(3A)
Fe(1) {3}. The molecule has a ‘‘chair’’ shape, with planes
{1} and {2}, {2} and {3} intersecting at 68.1ꢁ. The values
of these dihedral angles are somewhat less than those in
II (70.6ꢁ [28]). Lability of the bond lengths distribution in
the Fe–N–O fragment was found out in other complexes,
too [34]. It is assumed [35] that in the Fe(NO)2 fragment
even internal disproportionation is possible to the point
of complete oxidation–reduction to yield (ON)+–Fe–
(NO)ꢀ. A partial oxidation–reduction in Fe–NO fragment
is possible because of easy direct dFe ! pꢄNO and reverse
donation, which is accompanied by shortening/elongation
of Fe–N and elongation/shortening of N–O bonds with
simultaneous change of the Fe–N–O group valence. Struc-
tural lability of the Fe–N–O fragment also appears in the
difference of stretching N–O vibrations in IR-spectra [36].
The difference between the frequencies of two absorption
bands in I (DmNO) is 68 cmꢀ1, whereas this value in the
spectrum of complex II is 55 cmꢀ1. The increase of DmNO
value suggests that NO groups in I are less equivalent than
those in II, this being confirmed by X-ray data for complex
I.
Fig. 1. Structure of [Fe2(l-SC4H5N2)2(NO)4] and atoms numeration.
Fig. 2. Prototropic tautomery and dissociation of 1-methyl-2-
mercaptoimidazole.
In structural fragment Fe(NO)2, bond lengths N(1)–
O(1) 1.149(6) A and N(2)–O(2) 1.164(6) A, Fe(1)–N(1)
˚
˚
˚
exceeding the length of the double S@C bond (1.684 A),
suggest that the ligand is present in the complex in the thiol
form (Fig. 2(a)). The bond lengths Fe(1)–S(1) and Fe(1)–
˚
˚
1.696(5) A and Fe(1)–N(2) 1.667(5) A differ, though they
are similar to those in II, and angles O(1)–N(1)–Fe(1)
(163.4(5)ꢁ) and O(2)–N(2)–Fe(1) (170.2(4)ꢁ) also differ (by
6.8ꢁ).
˚
˚
N(3) are 2.283(2) A and 2.013(4)(7) A, respectively.
Mossbauer spectrum of I looks like a single doublet thus
suggesting equivalence of iron atoms, this being consistent
with X-ray data. It should be also noted that in I bond
The crystalline structure of the studied complex (Fig. 3)
consists of the layers parallel to plane bc and linked by
weak Hꢅ ꢅ ꢅO and Hꢅ ꢅ ꢅS contacts. Each corrugated layer is
˚
Fe–S is a little shorter than that in II (2.299(1) A). From
Fig. 3. The fragment of the crystalline structure of I (projection on ac plane).

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N.A. Sanina et al. / Inorganica Chimica Acta 359 (2006) 570–576
displaced with regard to another by 1/2 of the translation
along axis c. Inter-molecular contacts of two types were
detected between molecules of the same layer: (1) C(3)–
3.3. Mass-spectrum investigation of complexes I and II
Figs. 4 and 5 show mass-spectra of I and II under heat-
ing. Both complexes are quite stable in vacuum at room
temperature. Pressure of the gases gathered while keeping
the samples in vacuum did not practically exceed the level
of residual gases in the mass-spectrometer. While heating
from 20 to 70 ꢁC, I appeared to be more stable than II.
The main component of the gas released by I is NO (the
ion with m/z = 30). In addition to the peak due to NO mol-
ecules, peaks with m/z = 14 ([N]+), 18 ([H2O]+), 28 ([CO]+
and/or [N2]+) and 44 ([CS]+ and/or [CO2]+) can be seen in
the mass-spectrum. Arising of [N]+ and [H2O]+ ions is
obvious (nitrogen is present in the nitrosyl group, and
water is adsorbed by crystals). Molecules CO, CO2 and
N2 may be captured during the sample contacts with air.
The captured and, hence, weakly bound molecules
should have been released into the gaseous phase upon
pumping. But they did not show in the spectra of gases
0
0
˚
H(2)ꢅ ꢅ ꢅO(2) 2.593 A, \C(3)–Hꢅ ꢅ ꢅO(2) 178.0ꢁ; (2) C(4)–
0
0
˚
H(5)ꢅ ꢅ ꢅS(1) 2.815 A, \C(4)–Hꢅ ꢅ ꢅS(1) 149.3ꢁ. Molecules
of different layers are directed to each other by NO groups,
0
˚
with a distance between them being O(1)ꢅ ꢅ ꢅO(2) 3.09 A.
Apparently, the layers are linked by electrostatic forces.
In solutions, as was found out earlier [37], activation of
the NO groups of sulfur–nitrosyl complex involves the ini-
tial stage of the Fe–S bond rupture and formation of a
mononuclear dinitrosyl complex. It can be assumed that
replacement of H by CH3 group in the ligand, which was
used for the present synthesis, results in the formation of
sulfur–nitrosyl iron complex I, which is more stable than
complex II (this being evidenced by shortening of the Fe–S
bond and intra-molecular Feꢅ ꢅ ꢅFe distance). This assump-
tion is confirmed by the properties of the complex in the
solid phase and in solutions.
Fig. 4. Mass-spectrum of gases generated by I under heating.
Fig. 5. Mass-spectrum of gases generated by II under heating.

N.A. Sanina et al. / Inorganica Chimica Acta 359 (2006) 570–576
575
Fig. 6. Dependence of varying the concentration of NO generated upon decomposition of sulfur–nitrosyl iron complexes I and II (1 · 10ꢀ4 M/l) in 1%
DMSO water solutions at 25 ꢁC vs. time.
obtained after the sample was kept in vacuum without
heating. The assumption can be made that peaks with
m/z = 24 and 44 are due to [N2]+ and [CS]+ fragments of
imidazole C3H3N2S, respectively. The presence of the
peaks with m/z = 45 ([HCS]+) and 46 ([H2CS]+) in the
spectra is another confirmation of the assumption about
a possible transfer of the ligand or products of its decom-
position into the gaseous phase. However, no other ligand
fragments were detected in the spectrum of this sample.
Thus, heating to 70 ꢁC does not result in noticeable gas-
ification of sample I, while II releases NO and H2O
molecules into the gaseous phase upon such heating. Heat-
ing from 70ꢁ to 120ꢁ intensifies gasification of both
samples. For II, the composition of the gaseous phase
did not change, but the pressure of the released gas
increased essentially. For sample I the spectrum of the gas-
eous phase in this temperature range had the largest
number of peaks. In addition to NO and H2O molecules,
sumption of NO in proton medium start during a sample
preparation. In aerobic experiments, using a sensor elec-
trode, we register not the initial concentration of NO
generated by NO-donors, but the amount of NO con-
sumed in its reaction with oxygen in time. We assume
that decomposition of complexes I and II occurs accord-
ing to scheme:
k1
k2
A ! P ! B
where A is a complex; P is NO generated by the complex; B
is the products of NO and O2 interaction.
O2 concentration in the solution is 1.36 · 10ꢀ3 mol/l
[38], and this is higher than concentration of NO
generated by complexes I and II in the performed exper-
iments (6.4 · 10ꢀ7 mol/l). Therefore, we assume that
decomposition of complexes I and II is the first order
reaction. The rate constants of the complexes decomposi-
tion are shown in Table 4, and they are consistent with
the statement that complex I is more stable in solutions
than complex II.
there is
a noticeable amount of ligand molecules
C4H5N2S and the products of its decomposition in the gas-
eous phase.
Dependence of NO release by complexes I and II on
time in anaerobic solutions under similar conditions will
be studied in our further research.
3.4. Kinetics of NO release by complexes I and II
Fig. 6 and Table 4 present the data on kinetics of NO
release by complexes I and II. Kinetic curves in Fig. 6
describe the release of NO and its reactions with O2,
which is available in the solutions. The release and con-
4. Conclusion
It has been shown by the investigations performed that
using of aza-heterocyclic ligand, namely, the derivative of
imidazole-2-thiol with substituent CH3 in position 1 of
the imidazole cycle, results in the formation pf binuclear
sulfur–nitrosyl iron complex of l-N–C–S type, which is a
more stable donor of nitrogen monoxide in the solid phase
and in solutions than earlier obtained analog
[Fe2(C3H3N2S)2(NO)4].
Table 4
Rate constants for decomposition of complexes I and II in aerobic water
solutions (T = 25ꢁ)
NO-donor
pHini/pHfinal
k1 (sꢀ1
)
k2 (sꢀ1
)
[Fe2(SC4H5N2)2(NO)4] (I)
[Fe2(C3H3N2S)2(NO)4] (II)
5.50/5.77
5.55/5.57
1.10 · 10ꢀ3
1.72 · 10ꢀ3
343.2 · 10ꢀ3
373.2 · 10ꢀ3

576
N.A. Sanina et al. / Inorganica Chimica Acta 359 (2006) 570–576
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Crystallographic data have been deposited with the
Cambridge Crystallographic Data Centre, CCDC No.
273835. Copies of the data can be obtained free of charge
on application to CCDC, The Director, 12 Union Road,
Cambridge CB2 2EZ, UK (fax: +44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgements
The work has been performed with the financial support
of the Russian Foundation for Basic Research (Grant No.
04-03-08108) and the Program of the Russian Academy of
Sciences ‘‘Fundamental problems of medicine’’
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