Structure and properties of μ2-S-[bis(benzenethiolato) tetranitrosyldiiron] in solution

Article abstract of DOI:10.1007/s11172-010-0215-z

Dinuclear iron tetranitrosyl complex with the composition [Fe ₂(SPh)₂(NO)₄] (1) was synthesized and its single crystals and polycrystals were studied by X-ray diffraction, IR spectroscopy, and elemental analysis. The decomposition products of complex 1 were investigated by electrochemical method and mass spectrometry. The mass spectrum of a solution of complex 1 shows two groups of ions: the primary decomposition products of 1 in solution (the complex ions [Fe(SPh)(NO) ₂(NO₂)]^-, [Fe(SPh)₂(NO)] ^-, and [Fe(SPh)₂(NO)₂]^-) and a series of the ions [FeO₂ + n(NO)]^- and [FeO₃ + n(NO)]^- (n = 0-4), which are formed in secondary reactions. The structures of the complexes, which were formed through the Fe-NO bond dissociation and the replacement of the NO ligand by aqua and oxygen ligands in complex 1, and the structure of the complex [FeO₃]^- were studied by quantum chemical modeling.

Full text of DOI:10.1007/s11172-010-0215-z

Russian Chemical Bulletin, International Edition, Vol. 59, No. 6, pp. 1126—1136, June, 2010
1126
Structure and properties
of μ2ꢀSꢀ[bis(benzenethiolato)tetranitrosyldiiron] in solution*
N. A. Sanina,a N. S. Emel´yanova,a A. N. Chekhlov,a A. F. Shestakov,a I. V. Sulimenkov,b and S. M. Aldoshina
aInstitute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 522 3507. Eꢀmail: sanina@icp.ac.ru
bInstitute of Energy Problems for Chemical Physics (Branch), Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (916) 680 3573
Dinuclear iron tetranitrosyl complex with the composition [Fe2(SPh)2(NO)4] (1) was synꢀ
thesized and its single crystals and polycrystals were studied by Xꢀray diffraction, IR spectroꢀ
scopy, and elemental analysis. The decomposition products of complex 1 were investigated by
electrochemical method and mass spectrometry. The mass spectrum of a solution of complex 1
shows two groups of ions: the primary decomposition products of 1 in solution (the complex
ions [Fe(SPh)(NO)2(NO2)]–, [Fe(SPh)2(NO)]–, and [Fe(SPh)2(NO)2]–) and a series of the
ions [FeO2 + n(NO)]– and [FeO3 + n(NO)]– (n = 0—4), which are formed in secondary
reactions. The structures of the complexes, which were formed through the Fe—NO bond
dissociation and the replacement of the NO ligand by aqua and oxygen ligands in complex 1,
and the structure of the complex [FeO3]– were studied by quantum chemical modeling.
Key words: NO donors, sulfur nitrosyl iron complexes, thiophenolates, Xꢀray diffraction
study, mass spectrometry.
In recent years, nitrogen monoxide donors have atꢀ
low toxicity. It is known that Roussin's red salt esters
[Fe2(SR)2(NO)4] containing alkyl substituents (R = Et,
But, or nꢀC5H11)11—14 are stable in solution and efficiently
generate NO upon thermal activation or photoactivation.15
Nitrosyl complexes with aryl substituents generate NO in
physiological solutions without additional activation and
exhibit high antitumor activity and low toxicity. Our inꢀ
vestigations confirmed that the iron benzenethiolate niꢀ
trosyl complex [Fe2(SPh)2(NO)4] (1) has the direct cytoꢀ
toxic effect on human tumor cells of different genesis.16,17
The toxicity of this complex (LD100 = 60 mg kg–1) is
substantially lower than that of the known clinical antituꢀ
mor agent cisplatin (LD100 = 16 mg kg–1).
In this connection, it is of interest to study the strucꢀ
tures and properties of complex 1 in the solid state and
solution with the aim of designing new drugs and investiꢀ
gating the mechanisms of the direct (not only adjuvant)
antitumor activity of iron nitrosyl complexes as a new
class of chemotherapeutic agents. It is known that the
majority of NO donors, including NO donors of the class
under study,18 have adjuvant activity, i.e., the ability to
enhance the antitumor effect of clinical antitumor agents,
due to which the latter can be used in lower doses.
tracted considerable attention as a new class of antitumor
agents due to the specific role of NO in the malignant
tumor growth.1 Nitrogen monoxide changes the level of
apoptosis (the evolutionary selfꢀdestruction) of tumor
cells, the p53 gene activity, and the neoangiogenesis (the
neoplasms of tumorꢀfeeding vessels)2 and suppresses the
activity of the key mammalian repair protein O6ꢀmethꢀ
ylguanine DNA methyltransferase.3 However, different
classes of the known synthetic NO donors are not used as
therapeutic drugs for the treatment of malignant diseases,
and have found application only as enhancers (to a differꢀ
ent degree depending on their chemical nature) of the
efficacy of chemotherapeutic agents or radiotherapy.1—8
Iron nitrosyl complexes with thiolꢀcontaining ligands
belong to one of the forms of natural carriers (depots) of
NO.9 The direct cytotoxic action of the iron nitrosyl comꢀ
plex Na[Fe4S3(NO)7] on human and mouse melanoma
cells was investigated.10 However, this complex generates
NO upon photoactivation and cannot be used as an anꢀ
titumor agent because of the high toxicity toward norꢀ
mal cells. Therefore, there is a need in new antituꢀ
mor compounds, NO donors, having high efficacy and
In the present study, complex 1 was synthesized acꢀ
cording to a new procedure, which is more efficient than
the methods described previously.13,14,18 This method alꢀ
* Dedicated to Academician I. L. Eremenko on the occasion of
his 60th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1104—1114, June, 2010.
1066ꢀ5285/10/5906ꢀ1126 © 2010 Springer Science+Business Media, Inc.

μ2ꢀSꢀ[Bis(benzenethiolato)tetranitrosyldiiron]
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1127
gram package23 and refined by the fullꢀmatrix leastꢀsquares
method based on F2 with anisotropic displacement parameꢀ
ters for nonhydrogen atoms using the SHELXLꢀ97 program
package24.
lowed us to obtain the previously unavailable single crysꢀ
tals of 1. The structures of complex 1 and its derivatives
that are formed in aerobic solutions were studied by Xꢀray
diffraction, quantum chemistry, and mass spectrometry.
All nonhydrogen atoms of molecules 1 were located by diꢀ
rect methods followed by difference Fourier maps. However,
the isotropic and subsequent anisotropic leastꢀsquares refineꢀ
ment of the parameters of all these atoms was unsuccessful: the
R and wR2 factors remained very high, and the difference Fouriꢀ
er maps had high residual electron density peaks. This was
a consequence of the unusual twinning in the exposed crystal of 1.
It was impossible to take into account the twinning in monoclinꢀ
ic crystals with the monoclinic angle β substantially different
from 90° with the use of a standard procedure implemented in
the SHELXLꢀ97 program package. It appeared that this crystal
with another equivalent (centered) unit cell has the monoclinic
angle β´ ≈ 90°, and its twinning was taken into account in this
unit cell with the use of the SHELXLꢀ97 program package; the
ratio of the volumes of two scattering components was approxiꢀ
mately 0.514 : 0.486(2). After taking into account the twinning
in the crystal of 1, the structure refinement by the leastꢀsquares
method in the initial unit cell met with success.
Experimental
The IR spectra were recorded on a Spectrum BXꢀII Fourierꢀ
transform spectrometer in KBr pellets (1 mg of the compound
per 300 mg of KBr). The mass spectra were obtained on a highꢀ
resolution timeꢀofꢀflight mass spectrometer with orthogonal ion
injection.19 The solutions for experiments were prepared as folꢀ
lows: complex 1 (0.0039 g) was dissolved in DMSO (10 mL),
which was purified according to the procedure described previꢀ
ously,20 0.2 mL of the solution was taken, and MeOH (0.8 mL),
which was purified according to a known procedure,20 was addꢀ
ed. The resulting solution (2•10–5 mol L–1) was analyzed by
mass spectrometry after 3—4 min. The ions were extracted from
the samples with the use of an atmospheric electrospray ionizaꢀ
tion source without forced supply of the solution (the flow rate
was 0.1 μL min–1, the inner diameter of the quartz capillary was
50 μm, the voltage between the capillary and the inlet of the
mass spectrometer was ~3 kV). Dry argon at ~20 °C was used as
the curtain gas and the buffer gas. The working resolution of the
timeꢀofꢀflight mass spectrometer was ~10 000. The accuracy of
the determination of the ion mass was not lower than 10 ppm.
All independent H atoms of molecule 1 were located in difꢀ
ference Fourier maps in the intermediate step of the anisotropic
refinement. The coordinates and individual isotropic thermal
parameters of the H atoms were refined by the leastꢀsquares
method. In the last cycle of the fullꢀmatrix refinement, the absoꢀ
lute shifts of all 129 variable parameters in the structure of 1 were
smaller than 0.001σ. The final R factors were R = 0.030 and
wR2 = 0.075 based on 1770 observed reflections with I ≥ 2σ(I);
R = 0.039 and wR2 = 0.085 based on all 1900 independent obꢀ
served reflections; the goodnessꢀofꢀfit S = 1.05 (the values of
wR2 and S have been determined previously23). In the final difꢀ
μ ꢀSꢀ[Bis(benzenethiolato)tetranitrosyldiiron] (1). Distilled
water was saturated with argon for 30 min. A mixture of
2
Na2S2O3•5H2O
(Aldrich)
(0.508
g,
3.2
mmol),
Na2[Fe2(S2O3)2(NO)4]•4H2O (0.588 g, 1.0 mmol), which was
synthesized according to a procedure described previously,21
granulated NaOH (Aldrich) (0.2 g, 5.1 mmol), and PhSH (Aldꢀ
rich) (0.52 mL, 5.1 mmol) were placed in a flask. Then water
(40 mL) was added, the reaction mixture was vigorously stirred
at ~20 °C for 2 h, and the product was extracted with dichloroꢀ
methane (Aldrich) in air (3×15 mL). The combined red extract
was concentrated to 1/5 of the initial volume, filtered through
a porous filter No. 4, and kept in air for several hours until the
solvent was evaporated. The darkꢀblue crystals that formed
were dried in air. The yield of the product was 0.319 g (87%).
Found (%): Fe, 24.82; S, 14.20; N, 12.38; C, 32.03; H, 2.20.
Fe2S2C12H10N4O4. Calculated (%): Fe, 24.89; S, 14.22; N, 12.44;
C, 32.01; H, 2.22. IR, ν/cm–1: 1778, 1763, 1723, 1583, 1478,
1441, 1303, 1181, 1118, 1092, 1071, 1024, 999, 916, 834, 734,
694, 688.
ference Fourier map, — 0.34 < Δρ < 0.33 e Å–3
.
Electrochemical determination of NO. The concentration of
nitrogen monoxide generated by complex 1 in solution was meaꢀ
sured with the use of an amiNOꢀ700 sensor of the inNO Nitric
Oxide Measuring System (USA). The concentration of NO was
detected during 200 s (with a step of 0.2 s) in a solution of the NO
donor (0.4•10–5 mol L–1). The sensor was calibrated with the
use of a standard aqueous NaNO2 solution (100 μmol L–1; supꢀ
plied with an electrode), which was added to a mixture consistꢀ
ing of KI (Aldrich, 20 mg), 1 M H2SO4 (reagent grade, 2 mL),
and water (20 mL). Solutions of complex 1 were prepared in 1%
aqueous DMSO with the use of a phosphate buffer, pH 6.5; in
the course of the measurements, the temperature of the solutions
was maintained at 25 °C.
Xꢀray diffraction study. The unit cell parameters and the Xꢀray
reflection intensities were obtained on an automated EnrafꢀNonꢀ
ius CADꢀ4 Xꢀray diffractometer (MoꢀKα radiation, graphite
monochromator). Crystals of 1 are monoclinic, space group
P21/c, C12H10Fe2N4O4S2, M = 450.06, a = 10.730(3) Å, b =
= 11.018(2) Å, c = 7.509(3) Å, β = 110.40(3)°, V = 832.1(4) Å3,
Z = 2, dcalc = 1.796 g cm–3, μ(MoꢀKα) = 20.18 cm–1. The
intensities of 2154 reflections were measured in a quadrant of
reciprocal space (2θ ≤ 55°) by the ω/2θꢀscanning technique from
a crystal of dimensions 0.08×0.09×1.05 mm. The intensities of
all reflections were corrected for absorption by the semiempiriꢀ
cal method.22 After the exclusion of systematic absences and
Results and Discussion
Singleꢀcrystalline and polycrystalline samples of comꢀ
plex 1 were synthesized according to a procedure that has
advantages over those described previously13,14,18 in its
simplicity and cheapness, based on the exchange of the
thiosulfate ligands for the benzenethiolate ligands in an
alkaline medium (Scheme 1).
Water (instead of acetonitrile or THF) was used as the
solvent. The reaction was performed in one step and gave
complex 1 in 87% yield.
merging of equivalent reflections,
F σ(F
2(hkl) and 2) for 1900
independent reflections were obtained. The structure of 1 was
solved by direct methods with the use of the SHELXSꢀ97 proꢀ

1128
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
Sanina et al.
Scheme 1
Table 1. Experimental and theoretical bond lengths (d) and bond
angles (ϕ) in the structure of 1
Parameter
Experiment
Calculations
B3LYP/6ꢀ31G* PBE/SBK
Bond
d/Å
Fe—Fe´
Fe—S
Fe—S´
Fe—N(1)
Fe—N(2)
S—C(1)
O(1)=N(1)
O(2)=N(2)
2.699(1)
2.260(1)
2.259(1)
1.675(2)
1.666(2)
1.785(2)
1.182(3)
1.167(3)
2.549
2.246
2.246
1.630
1.630
1.803
1.171
1.171
2.655
2.253
2.253
1.656
1.656
1.817
1.175
1.172
The structure of dinuclear molecule 1 in the crystal is
shown in Fig. 1. Selected bond lengths and bond angles
are given in Table 1. Molecule 1 is centrosymmetric; its
center coincides with the crystallographic center of inverꢀ
sion i(000). Molecule 1 has the approximate symmetry
C2h, i.e., has an approximate twofold axis passing through
the centers of the Fe and Fe´ atoms and an approximate
plane of symmetry passing through the centers of the S
and S´ atoms and all atoms of two Ph substituents.
In molecule 1, the Fe atom forms five single covalent
bonds with the adjacent Fe´, S, S´, N(1), and N(2) atoms.
The coordination environment of the Fe atom can be deꢀ
scribed as a distorted tetrahedron with one additional verꢀ
tex occupied by the Fe´ atom or a strongly distorted trigoꢀ
nal bipyramid with the base formed by the Fe´, N(1), and
N(2) atoms and with two opposite vertices occupied by
the S and S´ atoms. In molecule 1, the Fe—Fe, Fe—S,
and Fe—N covalent bond lengths and the corresponding
bond angles are very similar to those found previously in
nitrosyl complexes containing other aromatic substituents
(C6H4F, C5H4N, and C4H3N2) at the sulfur atoms.25—27
In molecule 1, individual atoms deviate from the plane
formed by the Fe, S, Fe´, and S´ atoms by the following
distances: O(1), –2.304(3) Å; N(1), –1.437(2) Å; O(2),
Angle
ϕ/deg
Fe´—Fe—S
Fe´—Fe—S´
53.32(3)
53.33(3)
120.75(8)
121.56(8)
106.65(3)
105.56(8)
110.61(7)
110.03(8)
105.86(9)
117.7(1)
73.35(3)
111.68(7)
112.39(8)
170.2(2)
168.1(2)
124.3(2)
115.4(2)
55.0
55.0
53.7
53.7
Fe´—Fe—N(1)
Fe´—Fe—N(2)
S—Fe—S´
S—Fe—N(1)
S—Fe—N(2)
S´—Fe—N(1)
S´—Fe—N(2)
N(1)—Fe—N(2)
Fe—S—Fe´
Fe—S—C(1)
Fe´—S—C(1)
Fe—N(1)—O(1)
Fe—N(2)—O(2)
S—C(1)—C(2)
S—C(1)—C(6)
121.6
121.7
110.1
105.6
105.6
109.4
109.5
116.5
69.9
112.5
112.5
171.0
171.0
119.9
119.9
120.8
121.0
107.4
105.6
105.6
109.8
109.7
118.2
72.6
111.4
111.4
169.9
169.9
119.7
119.7
Note. The primed atoms are related to the unprimed atoms by
a center of symmetry (–x, –y, –z).
2.263(3) Å; N(2), 1.417(2) Å; C(1), 1.578(3) Å. The
S atom has a pyramidal configuration and deviates from
the plane passing through three atoms (Fe, Fe´, and C(1))
bound to S by 0.928(1) Å. The independent Ph ring in
molecule 1 is planar within 0.008(2) Å for six C atoms.
The average C...C bond length is 1.383(9) Å and it is alꢀ
most equal to the average (for C,Hꢀsubstituted benzene
rings)28 bond length (1.384(13) Å). It should be noted that
the mean plane of this Ph ring passes almost along the
bisector of the Fe—S—Fe´ bond angle and is almost perꢀ
pendicular to the plane passing through the Fe, S, Fe´,
and S´ atoms; the dihedral angle between these two planes
is 89.15(7)°.
In the crystal structure of 1 (Fig. 2), all short intermoꢀ
lecular contacts are close to, or somewhat smaller than,
the sums of the corresponding van der Waals radii.
In protic solvents, complex 1 eliminates NO only withꢀ
in a particular time after the dissolution (Fig. 3); the inꢀ
tense elimination of NO was observed after the induction
period of 10—12 s. The kinetic curves are nonꢀmonotonic
C(4)
C(3)
C(5)
O(1´)
N(1´)
C(2)
O(2)
C(6)
C(1)
Fe´
N(2)
S´
N(2´)
O(2´)
S
Fe
C(1´)
N(1)
O(1)
Fig. 1. Structure of centrosymmetric molecule 1 in the crystal.
The primed atoms are related to the unprimed atoms by a center
of symmetry.

μ2ꢀSꢀ[Bis(benzenethiolato)tetranitrosyldiiron]
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1129
Fe
S
N
O
Fig. 2. Projection of the crystal packing of complex 1 along the b axis.
and have a maximum at ~100 s. In a 1% aqueous DMSO
solution under aerobic conditions (see Fig. 3, curve 1), the
maximum amount of nitrogen monoxide generated by
complex 1 is approximately five times larger than that
obtained under anaerobic conditions (see Fig. 3, curve 2).
It should be noted that the amount of NO generated by
complex 1 under aerobic conditions is approximately
20 times smaller than the maximum amount of NO geꢀ
nerated upon hydrolysis of the related iron pyrimidineꢀ
thiolate complex, which we have studied previously under
the same conditions.27 As in the case of hydrolysis of
the iron pyrimidinethiolate complex,27 it is noteworthy
that the maximum number of NO molecules per iron
nitrosyl complex is smaller than the minimum stoiꢀ
chiometric number (equal to unity). The question is
how to explain this fact. It is known that in solutions
of iron dinitrosyl complexes, N2O is generated due apꢀ
parently to the dimerization of intermediate nitroxyl
HNO. This transformation of NO leads to a decrease
in the observed concentration of NO. In our opinion,27
the ironꢀcontaining nitrosyl intermediate that is formed
as a result of the Fe—NO bond dissociation serves
as a reducing agent for the transformation of molecular
NO into the NO– anion. The stronger reducing properꢀ
ties of this intermediate compared to those of the starting
complex are attributed to the increase in the electron denꢀ
sity on the Fe atom due to the coordination of a water
molecule. The amount of NO eliminated as a result of the
decomposition of the complex under aerobic conditions is
several times larger than that eliminated under anaerobic
conditions, which we have previously attributed27 to the
faster electron transfer to the oxygen molecule from the
intermediate. This competitive redox reaction results in
an decrease in the percentage of reduced NO molecules,
which are transformed in subsequent reactions, and, as
a consequence, the detected concentration of nitrogen
monoxide increases.
NO/nmol
100
80
1
60
40
20
2
50 100 150 200 250 300 350 400 t/s
Fig. 3. Plot of the amount of nitrogen monoxide generated by
complex 1 under aerobic (1) and anaerobic (2) conditions vs
the time (0.4•10–5 М solutions in 1% aqueous DMSO, pH 6.50,
25 °C).

1130
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
Sanina et al.
Table 2. Mass spectrometric data for a solution of the
complex [Fe2(SPh)2(NO)4] (1) in methanol
The formation of other intermediates in the course of
decomposition of complex 1 in protic media was studied
by mass spectrometry. Figure 4 shows the mass spectrum
of a solution of complex 1 in methanol. All main detected
ions were identified based on the analysis of the isotoꢀ
pic distribution of the ion peaks and the measurements
of the precise ion masses. The mass spectrum shows
two groups of ions. The first group includes the comꢀ
plex ions [Fe(SPh)2(NO)2]–, [Fe(SPh)2(NO)]–, and
[Fe(SPh)(NO)2(NO2)]–, which are the primary decomꢀ
position products of the starting complex 1 in solution
under electrospray conditions. The second group includes
two series of ions, [FeO2 + n(NO)]– and [FeO3 + n(NO)]–
(n = 0—4), that are formed in the secondary reactions.
The measured differences in the m/z values for the ions in
the series, which are close to the mass of the main isotope
of NO (29.998), definitely indicate that the ions observed
in the mass spectrum belong to the series [FeO2 + n(NO)]–
and [FeO3 + n(NO)]–. The results of processing
of the mass spectrum are presented in Table 2, which
gives the experimental m/z values for the peaks corꢀ
responding to the main isotopes. The peak of the
[Fe(SPh)2(NO)2]– ion has the maximum intensity; the
intensities of the peaks of other ions in the spectrum were
normalized to the maximum intensity and are given in the
relative scale.
Ion
m/z
Irel
[NO3]–
61.993
87.930
0.12
0.03
0.33
0.01
0.08
0.03
0.02
0.14
0.20
0.91
0.69
0.13
0.25
1.00
[FeO2]–
[FeO3]–
103.926
109.018
117.929
133.923
147.926
163.921
177.925
193.917
207.921
270.941
303.962
333.959
[SPh]–
[FeNO3]–
[FeNO4]–
[FeN2O4]–
[FeN2O5]–
[FeN3O5]–
[FeN3O6]–
[FeN4O6]–
[Fe(SPh)N3O4]–
[Fe(SPh)2NO]–
[Fe(SPh)2(NO)2]–
PRIRODA program (see Ref. 32). The zeroꢀpoint enerꢀ
gy contribution was taken into account in the comparison
of the energies of the optimized structures. The total enerꢀ
gies were calculated at the B3LYP level of theory with the
use of the 6ꢀ311++G** basis set. The structures of the
calculated complexes in different spin states are shown in
Figs 5 and 6.
The energetics of the Fe—NO bond dissociation and
the replacement of the NO ligand by the aqua ligand and
the oxygen ligand in dinuclear complex 1 and the relatꢀ
ed mononuclear complex [Fe(SPh)(NO)2] was studied
by quantum chemical modeling. The calculations were
carried out by the density functional theory at the B3LYP
level of theory with the 6ꢀ31G* basis set using the
Gaussian 03 program29 and by the PBE method30 with the
extended basis set for the SBK pseudopotential31 using the
The geometry of the starting complex 1 is satisfactorily
described at both the B3LYP and PBE levels. Thus the
typical differences in the bond lengths are about 0.01 Å
(see Table 1). The largest difference (0.04 Å) is observed
for the Fe—N bond lengths determined at the B3LYP
level. The elimination of the NO ligand from the diaꢀ
magnetic dinuclear complex results in the doublet state of
the system, the energy of the quartet state being higher by
a few kcal mol–1 (Table 3). These states differ only in the
mutual orientation of the local spins of the moieties
Fe(NO) (S = 1) and Fe(NO)2 (S = 1/2) (see Table 3).
The quantitative calculations of the N—O bond energy
in complex 1 present certain difficulties due to the use of
the singleꢀdeterminant approximation in the density funcꢀ
tional theory. The absolute values of the spin density on
both the iron atoms and the nitrosyl ligands, which were
estimated for the starting complex 1 in the singlet state
based on the solution with disturbed symmetry, are signiꢀ
ficantly smaller than the idealized theoretical values (3 and 1,
respectively). For all other complexes under consideration,
the distribution of the spin densities on the Fe atom and
the NO ligands is in better agreement with the expected
distribution (Table 4, B3LYP data). Hence, the solution
found for complex 1 (the geometry with disturbed symmeꢀ
try) does not have the necessary spin structure, although it
N•10–4
N•10–4
40
30
20
10
[FeO3 + 3 (NO)]–
[Fe(SPh)2(NO)2]–
193.917
50
333.959 (exp)
[FeO2 + 3 (NO)]–
177.925
333.953 (calc)
40
30
20
10
[FeO3]–
103.926
332 336 m/z
[FeO3 +
[FeO2 + 4 (NO)]–
[FeO2]–
+ 2 (NO)]–
207.921
87.930
[Fe(SPh)2(NO)]–
303.962
163.921
[NO3]–
61.993
[SPh]–
109.018
[Fe(SPh)(NO)2 +
+ NO2]–
270.941
100 150
200
250
300
350 m/z
Fig. 4. Mass spectrum of
a solution of the complex
[Fe2(SPh)2(NO)4] in methanol (2•10–5 mol L–1). The inset
shows the mass spectrometric isotopic distribution of the ion
peak corresponding to the complex ions [Fe(SPh)2(NO)2]–; N is
the number of ions.
The calculations were carried out with the use of the PRIRODA
program at the Interdepartmental Supercomputer Center of the
Russian Academy of Sciences.

μ2ꢀSꢀ[Bis(benzenethiolato)tetranitrosyldiiron]
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1131
C
C
C
C
C
1.177
(1.175)
C
C
C
O
O
C
C
1.777
N
166.2
1.807
(1.820)
1.726
(1.663)
N
C
C
116.6
(119.7)
S
117.5
1.728
Fe
1.803
177.7
Fe
S
(178.7)
165.1
(172.0)
Fe
S
N
N
2.249
(2.236)
N
O
O
1.720
(1.665)
O
S
Fe
2.421
1.191
(1.182)
179.4
2.376
C
C
N
1.725
C
3a
C
C
C
O
1.190
C
3.079
(2.579)
C
C
Fe—Fe
C
C
C
3b
C
C
C
C
174.4
(172.3)
C
1.197
(1.187)
O
C
1.800
(1.816)
2.400
C
171.9
(163.9)
C
S
N
1.198
(1.180) 109.5
(116.3)
1.738
(1.665)
O
C
1.727
(1.660)
C
C
C
2.134
(2.191)
1.745
(1.698)
(2.256)Fe
114.8
(108.2)
N
S
1.177
(1.177)
2.143
1.730
108.0
(106.8)
1.178
Fe
H
(2.153)
(1.662)
O
H
(1.180)
O
H
N
S
Fe
O
107.4
166.1
(173.2)
O
164.6
(170.0)
N
1.730
(1.665)
(106.6)
Fe
C
S
H
C
2.420
(2.330)
115.9
(118.8)
N
C
N
C
C
1.182
(1.179)
115.7
164.2
C
O
(115.5)
O
(168.7)
1.794
(1.811)
3.100
(2.678)
C
C
Fe—Fe
C
C
3.278
(2.723)
Fe—Fe
C
C
4a
4b
C
C
C
C
C
C
3.284
(2.681)
C
C
C
2.531
(2.290)
1.399
(1.289)
C
1.825
(1.804)
C
3.369
C
1.176
166.4
(167.9)
1.798
(1.808)
1.308
(1.287)
(2.683)
1.780
(1.811)
O
(1.174)
1.178
O
66.2
(126.8)
O
(1.174)
N
O
1.725
(1.667)
1.922
(1.804)
S
1.727
(1.662)
O
S
O
164.8
(170.1)
116.5
(117.0)
149.1
(113.8)
N
116.3
Fe
68.6
(134.2)
172.5
(169.2)
Fe
Fe
Fe
164.3
(171.1)
1.731
(1.662)
148.5
(113.3)
(117.4)
165.8
(168.0)
N
N
S
1.166
(1.174)
1.676
(1.684)
1.715
(1.685)
178.0
(175.2)
O
S
O
N
N
1.178
(1.174)
1.801
(1.813)
O
O
1.797
1.175
(1.172)
2.410
(2.276)
(1.814) C
C
C
2.418
(2.269)
C
3.369
(2.683)
C
C
C
Fe—Fe
C
C
C
C
C
5a
5b
Fig. 5. Calculated structures of the dinuclear complexes in different spin states: [Fe2(μꢀSPh)2(NO)3] (3a,b: S = 1/2 (a) and 3/2 (b)),
[Fe2(μꢀSPh)2(NO)3(H2O)] (4a,b: S = 1/2 (a) and 3/2 (b)), and [Fe2(μꢀSPh)2(NO)3(O2)] (5a,b: S = 1/2 (a) and 3/2 (b)). Here and in Fig. 6,
the distances are given in Å and the angles are given in degrees; the results of the PBE/SBK calculations are given in parentheses.

1132
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
Sanina et al.
O
1.724
(1.674)
S
N
1.177
(1.176)
171.4
(165.4)
2.214
(2.181)
1.721
C
C
Fe
123.8
C
(120.1)
(1.670)
C
170.8
N
C
C
(165.3)
1.177
(1.167)
O
6
C
C
C
1.184
(1.177)
161.0
(164.6)
O
153.4
(156.5)
1.879
C
C
O
N
1.190
(1.184)
1.745
(1.678)
C
C
N
115.8
(116.4)
160.1
(164.8)
C
C
S
S
108.0
(146.7)
163.8
O
(161.0)
(1.738)
C
Fe
2.286
(2.231) (2.191)
2.146
2.271
(2.230)
O
1.181
N
C
Fe
1.736
(1.675)
N
1.743
(1.716)
2.147
(2.255)
(1.180)
C
1.178
(1.185)
O
O
7a
7b
S
162.1
(170.0)
O
169.7
(170.7)
1.193
(1.186)
S
2.256
(2.239)
C
1.184
(1.186)
C
N
C
C
Fe
1.725
(1.679)
C
2.169
(2.152)
O
N
1.595
(1.609)
C
C
Fe
2.067
(2.087)
C
C
C
C
C
1.997
(2.037)
O
O
8a
8b
1.293
1.324
(1.288)
(1.278)
O
O
2.031
C
O
C
(2.070)
O
2.018
(3.060)
2.010
(1.966)
1.976
2.261
(2.185)
C
173.0
(165.1)
C
C
(1.876)
Fe
2.284
(2.210)
C
S
Fe
1.172
(1.171)
C
N
O
1.740
N
1.712
(1.664)
C
C
(1.676)
1.173
(1.171)
127.8
(164.7)
1.721
(1.673)
161.8
C
N
2.014
(1.678)
S
O
N
C
1.172
(1.170)
C
158.3
(163.0)
O
1.165
(1.171)
(162.7)
O
9a
9b
170.5
O
(172.1)
S
2.197
(2.191)
1.169
(1.169)
N
C
C
C
C
1.705
(1.664)
Fe
1.781
(1.822)
1.797
(1.827)
C
C
O
O
1.400
(1.384)
10
Fig. 6. Calculated structures of the mononuclear complexes in different spin states.

μ2ꢀSꢀ[Bis(benzenethiolato)tetranitrosyldiiron]
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1133
Table 3. Electronic characteristicsa of the complexes [Fe2(μꢀSPh)2(NO)3(X)] (X = NO, the free coordination site, H2O, and O2)
b
Compꢀ
lex
Erel
/kcal mol–1
qFe
ρ
qNO
ρ
qS
ρ
<S2>
<S2
>
A
Fe
NO
S
B3LYP/6ꢀ31G* calculations
1c
—
0
0.62
2.41
–0.23
0.77
–0.19
–0.35
0
2.62
4.49
10.82
14.82
3a
0.60,
0.89
0.66,
0.91
3.19,
–3.15
3.25,
3.24
–0.34, –0.24,
–0.25
–0.36, –0.25,
–0.25
–1.36, 1.16,
1.18
–1.37, 1.19,
1.19
–0.015
3b
4a
4b
5a
5b
3.2
0
–0.37
–0.39
–0.41
0.10
6.58
4.60
6.66
4.23
6.35
10.70
15.19
11.07
13.87
9.78
0.76,
0.85
3.30,
–3.19
–0.41, –0.24,
–0.28
–1.41, 1.17,
1.21
–0.04
0.08
1.6
0
0.81,
0.87
3.37,
3.22
–0.41, –0.25,
–0.28
–1.43, –1.18,
1.11
0.91,
0.86
–2.19,
3.20
–0.30, –0.26,
–0.27
1.08, –1.18,
–1.19
–0.37,
–0.34
0.03,
0.11
–7.0
1.01,
0.86
3.01,
3.22
–0.29, –0.24,
–0.26
–1.09, –1.17,
–1.19
–0.37,
–0.33
0.21,
0.06
6
—
0
0.85
0.90
0.91
0.48
0.69
0.93
1.01
0.93
3.25
3.29
3.42
—
–0.27
–1.20
–0.28
–0.31
–0.30
–0.23
–0.31
–0.17
–0.22
–0.20
0.12
0.12
0.10
0—
2.51
2.62
4.88
0—
4.91]
5.44
4.37
0—
7a
7b
8a
8b
9a
9b
10
–0.33, –0.29
–0.33, –0.28
–0.28
–1.25, –1.21
0.56, –1.14
0—
15.8
43.8
0
3.28
–2.51
3.46
2.81
–0.38
–1.39
0.05
0.09
0.28
0.10
3.12
2.48
5.40
2.82
2.61
4.97
5.36
2.37
2.7
0
–0.22, –0.22
–0.10, –0.25
–0.18
1.06, 1.07
–0.93, –1.18
–1.09
—
PBE/SBK calculations
1
0
0
0.50
0—
–0.23
0—
–0.02
–0.12
0—
—
—
—
—
3a
0.68,
0.52
1.99,
–0.79
–0.40, –0.25,
–0.25
–0.57, 0.21,
0.21
–0.03
3b
5.2
0.66,
0.64
2.63,
1.16
–0.33, –0.31,
–0.30
–0.57, –0.29,
–0.30
–0.17,
–0.12
0.17,
0.14
—
—
4a
4b
0
0.76,
0.48
0.70,
1.94,
–0.90
2.82,
–0.44, –0.27,
–0.27
–0.29, –0.30,
–0.53, 0.25,
0.26
–0.67, –0.21,
–0.12,
–0.08
–0.19,
–0.05,
–0.02
0.15,
—
—
—
—
2.3
0.65
0.73,
0.55
0.93
2.19,
–0.82
–0.36
–0.23, –0.27,
–0.25
–0.25
–0.42, 0.24,
0.23
–0.13
–0.05,
–0.08
0.15
0.03,
0.02
5a
5b
0
—
—
—
—
4.7
0.71,
0.55
2.42,
–1.00
–0.26, –0.25,
–0.24
–0.42, 0.29,
0.27
–0.06,
–0.09
0.01,
0.03
6
0
0
0.72
0.76
0.73
0.52
0.72
0.64
0.65
0.94
1.68
1.76
2.21
0—
–0.29, –0.26
–0.32, –0.28
–0.33, –0.26
–0.37
–0.42, –0.42
–0.51, –0.51
0.16, 0.27
0—
–0.14
–0.20
–0.16
–0.09
–0.26
–0.07
–0.04
–0.09
0.13
0.18
0.23
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
7a
7b
8a
8b
9a
9b
10
25.0
24.9
0
2.64
–0.56
1.23
1.90
–0.38
–0.78
0.09
–0.03
0.33
0.21
0
–0.16, –0.15
–0.19, –0.17
–0.26
0.19, 0.28
–0.14, –0.09
–0.57
10.5
0
a Erel is the relative energy; qFe, qNO, and qS are the charges on the Fe atom, the NO group, and the S atom, respectively; ρFe, ρNO, and
ρ are the spin density on the Fe atom, the NO group, and the S atom, respectively; <S2> is the square of the spin.
S
b The average square of the spin after the annihilation of the wave function term with the spin 1, 2, 3/2, and 5/2 for the singlet, triplet,
doublet, and quartet states, respectively.
c Characteristics of the singlet state with disturbed symmetry.

1134
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
Sanina et al.
has the intrinsic stability. This is also evident from the
average square of the spin, which, as can easily be shown,
can be calculated by the equation
tions at the B3LYP level and the data for the spin states
with the lowest energies for all systems.
For the coordinatively unsaturated dinuclear (3a) and
mononuclear (6) complexes, the binding of the water
molecule leads to a decrease in the energy by 8.9 and
11.2 kcal mol–1, respectively. In mononuclear aqua dinitrosyl
<S2> = (na + nb)/2 + (na – nb)2/4
for the singleꢀdeterminant function containing na unpaired
electrons with the spin α and nb unpaired electrons with
the spin β.
complex 7a, the N—O bond energy is 25.5 kcal mol–1
;
this energy decreases to 14.6 kcal mol–1 in complex 9b,
which is formed in the case when the O2 molecule rather
than H2O binds at the free coordination site in complex 6.
This process is accompanied by shortening of the Fe—O
bonds and elongation of the O—O bond by 0.09 Å. Conseꢀ
quently, the stronger binding of the oxygen molecule leads
to a decrease in the energy consumption for the Fe—NO
bond dissociation in the mononuclear complex. A similar
effect would be expected for the dinuclear complexes, but
it was beyond the scope of our study. These results suggest
a new mechanism of the influence of oxygen on the intenꢀ
sification of the elimination of NO due to the formation of
less stable mixed oxygen nitrosyl complexes of iron.
The calculations at the PBE level of theory for oxygen
complexes 5a,b showed that the O2 molecule has the anꢀ
gular coordination, as opposed to the π coordination obꢀ
tained in the calculations at the B3LYP level (see Fig. 5).
It is also noteworthy that the calculations at the PBE level
resulted in the transformation into the quartet state 5a → 5b
not due to a change in the mutual orientation of the local
spins on the atoms of the Fe units, like in all other cases,
but due to a change in the orientation of the spin on O2
(the spin density is 0.58) from antiparallel to parallel with
respect to the spin on the Fe atom. This accounts for the
qualitative difference in the mutual arrangement of the
quartet and doublet states for the terminal and lateral coꢀ
ordination of O2 (see Table 3). In addition, we failed to
find solutions with the similar local spin density distribuꢀ
tion in the calculations at the B3LYP level for the states
with the spins S = 1/2 and 3/2. In the doublet complex,
the O2 ligand bears the unpaired electron (the spin density
is 1.12), whereas the spin density on this ligand in the
quartet complex is small (–0.10).
The electronic configuration of the starting complex is
d1αd2αd3απ1βπ2βd1´βd2´βd3´βπ1´απ2´α, where the di and
πj orbitals are located predominantly on the Fe atoms and
the NO ligands, respectively; the orbitals belonging to anꢀ
other Fe(NO)2 unit are primed. Hence, na = nb = 5 for the
starting complex; after removal of one NO group, na = 5,
nb = 4 for the doublet states of the system (complexes 3a,
4a, and 5a), and na = 6, nb = 3 for the quartet states
(complexes 3b, 4b, and 5b). Therefore, <S2> = 5 would be
expected for complex 1 and <S2> = 4.75 would be expectꢀ
ed for complexes 3a, 4a, and 5a. The former value is inꢀ
consistent with the results of calculations (2.60), whereas
the latter value is in good agreement with the experimental
values (4.49, 4.60, and 4.23, respectively). The underestiꢀ
mated spin densities on the Fe atoms and the NO ligands
in complex 1 are indicative of a substantial admixture of
the singlet state with filled electron shells, whose energy is
higher than that of the singlet state with disturbed symmeꢀ
try by 21.0 kcal mol–1 (see Ref. 33). This leads to a conꢀ
siderable underestimation of the bond energy of the NO
ligand in the complex (up to 1.3 kcal mol–1), as well as of
the energy consumption for its decomposition into two
mononuclear complexes (up to 6.2 kcal mol–1). In the
mononuclear Fe dinitrosyl complex with the doublet
ground state, when there is no need to find the solution
with disturbed symmetry, this artifact is virtually absent,
and the Fe—NO bond dissociation energy estimated at
the B3LYP level34 is 29.2 kcal mol–1
.
For the quartet states of complexes 3b, 4b, and 5b, the
calculated values of <S2> (6.58, 6.66, and 6.35, respecꢀ
tively) are also in satisfactory agreement with the theoretꢀ
ical value of 6.75. The good agreement with the theoretiꢀ
cal values (2.75, 4.75, and 3) is observed also for the monoꢀ
nuclear complexes in the doublet (6, 7a, 9a, and 10), quarꢀ
tet (9b), and triplet (8b) states, respectively. Therefore, it
can be expected that the relative energies of these comꢀ
plexes with the spins S = 1/2 and 3/2, as well as the
energies of the addition of neutral H2O and O2 ligands to
coordinatively unsaturated complexes, would be obtained
with satisfactory accuracy.
The results of calculations for the openꢀshell complexꢀ
es at the PBE level of theory (see Table 3) show that the
spin density distribution substantially differs from the ideꢀ
alized distribution. Hence, these calculations are poorly
informative for the estimation of the energy changes in the
reactions. For this purpose, we used the results of calculaꢀ
In mononuclear complexes 9a,b, the O2 ligands have
the same spin density (1.3), i.e., these ligands are analoꢀ
gous to the superoxide anion with the weak O—O bond.
This provides prerequisites for the subsequent intramolecꢀ
ular or intermolecular oxidation accompanied by the transꢀ
formation of the iron oxygen complexes into the oxo comꢀ
plexes. The elucidation of their transformation pathways
are beyond the scope of the present study.
Based on the results of the present study, it would be
expected that the decomposition of dinuclear complex 1
into two mononuclear complexes is quite probable on the
condition of the coordination of the solvent molecule
(6 → 7a). A decrease in the calculated Fe—NO bond enꢀ
ergy in the mononuclear aqua dinitrosyl complex by
3.7 kcal mol–1 compared to the mononuclear dinitrosyl

μ2ꢀSꢀ[Bis(benzenethiolato)tetranitrosyldiiron]
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1135
complex with two sulfurꢀcontaining ligands34 also indiꢀ
cates that the mononuclear complex should be less stable
than the dinuclear complex. This result is in qualitative
agreement with the observed induction period for the elimꢀ
ination of NO.
3. L. Liu, M. ХuꢀWelliver, S. Kanagula, H. E. Pegg, Canser
Res., 2002, 62, 3037.
4. N. P. Konovalova, S. A. Goncharova, L. M. Volkova, T. A.
Raevskaya, L. T. Eremenko, A. M. Korolev, Nitric Oxide:
Biol. Chem., 2003, 8, 59.
5. W. Yang, P. A. Rogers, H. Ding, J. Biol. Chem., 2002, 277,
12868.
6. O. Siri, A. Tabard, P. Pullumbi, R. Guilard, Inorg. Chim.
Acta, 2003, 350, 633.
–
The mass spectra show an intense peak of the FeO3
ion. To elucidate the nature of this species, we carried out
–
calculations of the energy of the complexes FeO3 in
different spin states and with different geometry at the
PBE/SBK level of theory. It appeared that the quartet
complex with the symmetry D3h has the lowest energy.
The Fe—O bond length in this complex is 1.643 Å. Since
only 75% of the spin density is located on the Fe atom, it
should formally be described as the FeIV complex. The
7. J. L. Burgaud, E. Jngini, P. Del Soldato, N. Y. Ann. Acad.
Sci., 2002, 962, 360.
8. T. I. Karu, L. V. Pyatibrat, G. S. Kalendo, Toxicol. Lett.,
2001, 121, 57.
9. A. R. Butler, I. I. Megson, Chem. Rev., 2002, 102, 1155.
10. A. Janczyk, A. WolnichkaꢀGlubisz, A. Chmura, M. Elas,
Z. Matuszak, G. Stochel, K. Urbanska, Nitric Oxide: Biol.
Chem., 2004, 10, 42.
–
doublet FeO3 complex has a slightly higher energy (by
1.6 kcal mol–1). The symmetry of the latter complex reꢀ
duces to C2v due to the nonꢀequivalent Fe—O bonds; two
bond lengths are 1.626 Å, and one bond length is 1.606 Å.
The energy of the quartet FeIII complex containing the
peroxo group (O—O, 1.450 Å) is substantially higher
(48.6 kcal mol–1). In this complex, there is one short
Fe—O distance (1.653 Å) and two long Fe—O distances
(1.840 Å). As mentioned above, the formation of the iron
trioxo complex can be attributed to the decomposition of
the reaction products of the mononuclear nitrosyl comꢀ
plexes with oxygen.
11. T. Thomas, J. H. Robertson, E. G. Cox, Acta Crystallogr.,
1958, 11, 599.
12. C. Glidewell, M. E. Harman, M. B. Hursthouse, I. L.
Johnson, M. Motevalli, J. Chem. Res., 1998, 212, 1676.
13. Т. C. Harrop, D. Song, S. J. Lippard, J. Am. Chem. Soc.,
2006, 128, 3528.
14. R. E. Marsh, A. L. Spek, Acta Crystallogr., Sect. B, 2001,
57, 800.
15. J. L. Bourassa, P. C. Ford, Coord. Chem. Rev., 2000,
200—202, 887.
16. N. A. Sanina, O. S. Zhukova, S. M. Aldoshin, N. S.
Emel´yanova, G. K. Gerasimova, RF Pat. No. RST/
RU2007000285, May 30, 2007 (in Russian).
Therefore, we synthesized single crystals of the diꢀ
nuclear iron benzenethiolate tetranitrosyl complex 1 and
studied its structure and its decomposition products
in solution. In the mass spectrum of a solution of 1,
the primary decomposition products of the starting
complex 1 (the complex ions [Fe(SPh)(NO)2(NO2)]–,
[Fe(SPh)2(NO)]–, and [Fe(SPh)2(NO)2]–) and a seꢀ
ries of ions that are formed in the secondary reactions
([FeO2 + n(NO)]– and [FeO3 + n(NO)]–, n = 0—4) were
identified. The Fe—NO bond dissociation and the replaceꢀ
ment of the NO ligand by aqua and oxygen ligands in
complex 1 were studied by quantum chemical modeling.
Presumably, the stronger Fe—O bond and the fact that
the oxidation of NO by oxygen is a favorable reaction are
responsible for the energetically favorable destruction of
the primary oxygen complexes giving Fe—O and Fe=O
bonds and new N—O bonds under electrospray conditions
in aerobic solutions.
17. E. V. Rudakova, M. A. Lapshina, A. A. Sivova, K. I. Zvereꢀ
va, A. A. Terent´ev, V. D. Sen´, N. A. Sanina, S. A. Aldoꢀ
shin, Tez. dokl. VII Mezhdunar. molodezh. konf. "IBKhF
RAN — VUZy" [Abstrs. of Papers, VII Inernational Youth Conꢀ
ference "IBCP RAS — Universities"] (November 2—14, 2007,
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We thank Dr. T. N. Rudneva for performing the
electrochemical experiment.
This study was financially supported by the Presidium
of the Russian Academy of Sciences (Program No. 5 "Baꢀ
sic Sciences to Medicine").
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Received April 10, 2009;
in revised form February 3, 2010