Synthesis, characterization, and cytotoxicity of mixed-ligand complexes of palladium(II) with 1,10-phenanthroline and N-carbonyl-L-isoleucine dianion 1

Article abstract of DOI:10.1134/S1070328414020122

Three novel palladium(II) complexes [Pd(Phen)(Bzile)] (I), [Pd(Phen)(p-mBzile)] (II), and [Pd(Phen)(p-NBzile)]·H₂O (III), where Bzile = N-benzoyl-L-isolecine), p-MBzile = N-(p-methylbenzoyl)-L- isolecine), p-NBzile = N-(p-nitrobenzoyl)-L-isolec

Full text of DOI:10.1134/S1070328414020122

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2014, Vol. 40, No. 2, pp. 115–119. © Pleiades Publishing, Ltd., 2014.
Synthesis, Characterization, and Cytotoxicity of MixedꢀLigand
Complexes of Palladium(II) with 1,10ꢀPhenanthroline
and NꢀCarbonylꢀL
ꢀIsoleucine Dianion1
J. C. Zhang*, L. W. Wang, S. Y. Liu, F. F. Zhang, J. L. Du, L. W. Li, S. X. Wang**,
S. H. Li, and G. Q. Zhou
College of Chemistry & Environmental Science, Chemical Biology Key Laboratory of Hebei Province,
Hebei University, Baoding, 071002 P.R. China
Key Laboratory of Medicinal Chemistry and Molecular Diagnosis of the Ministry of Education,
Hebei University, Baoding, 071002 P.R. China
eꢀmail: * jczhang6970@yahoo.com.cn; ** wsx@hbu.edu.cn
Received September 25, 2012
Abstract
[Pd(Phen)(
zoyl)ꢀ ꢀisolecine),
—
Three novel palladium(II) complexes [Pd(Phen)(Bzile)] (
ꢀNBzile)] H2O (III), where Bzile = ꢀbenzoylꢀ ꢀisolecine),
ꢀNBzile = ꢀ( ꢀnitrobenzoyl)ꢀ ꢀisolecine), have been synthesized and characterized
I
), [Pd(Phen)(
p
ꢀmBzile)] (II), and
p
⋅
N
L
L
pꢀMBzile =
Nꢀ(pꢀmethylbenꢀ
L
p
N
p
by elemental analysis, ESꢀMS, and IR. The crystal and molecular structure of the complex II has been deterꢀ
mined by single crystal Xꢀray diffraction. The cytotoxicity was tested against carcinoma cell lines: KB,
BGCꢀ823, Belꢀ7402 and HLꢀ60 by MTT assay. The results indicated that these complexes exerted cytotoxꢀ
icity, but none of them showed higher cytotoxicity than cisplatin. The cytotoxicity against HLꢀ60, BGCꢀ823,
Belꢀ7402 and KB cell lines decreases in the sequence: pꢀMBzile > Bzile > pꢀNBzile. It suggests that the acyꢀ
lated groups have important impact on the cytotoxicity of complexes.
DOI: 10.1134/S1070328414020122
1
INTRODUCTION
used to synthesize palladium anticancer complexes.
These palladium complexes with amino acid ligands
are expected to be useful for the treatment of tumors of
the gastrointestinal region because of their little interꢀ
action with chloride ions compared with cisplatin [7].
Besides, less kidney toxic than cisplatin is expected beꢀ
cause of the difficulty of replacing the tightly bound
bidentate amino acids in these complexes by protein
bound sulphydryl groups of kidney tubule cells [8]. So
Phen and amino acid have been widely used in pallaꢀ
dium anticancer drugs as ligands. Mital’s group reꢀ
ported the synthesis and cytotoxicity of nine palladiꢀ
um(II) complexes of type [Pd(Phen)(AA)]+ (where
Nowadays cisplatin and its derivatives are the most
widely used clinical anticancer drugs. Unfortunately,
they have several major drawbacks. Common probꢀ
lems include cumulative toxicities of nephrotoxicity
and ototoxicity [1, 2]. In addition to the serious side
effects, the therapeutic efficacy is also limited by
inherent or treatmentꢀinduced resistant tumor cells.
These drawbacks have provided the motivation for
alternative chemotherapeutic strategies.
Metals, in particularly, transition metals offer poꢀ
tential advantages over the more common organicꢀ
based drugs. On the basis of the structural and thermoꢀ
dynamic analogy between platinum(II) and palladiꢀ
um(II) complexes, there is also much interest in the
study of palladium(II) derivatives as potential anticanꢀ
cer drugs [3–6]. It was reported that 1,10ꢀphenanthroꢀ
line (Phen) derivatives had the ability to participate as
DNA intercalators. Amino acids are the fundamental
matters of life and material base of metabolism. Introꢀ
ducing amino acids into the antitumor drug molecules
can improve their selectivity to tumor cells, enhance
their liposolubility, and remit their toxicities to normal
cells. In addition, owing to higher lability of palladium
versus platinum analogs, amino acid ligands, which do
not dissociate easily in aqueous solution, have been
AA is an anion of
ꢀleucine (Leu),
(Tyr), ꢀtryptophan (Try),
(Pro) or ꢀserine (Ser)). The palladium(II) complexes
L
L
ꢀglycine (Gly),
ꢀphenylalanine (Phe),
ꢀvaline (Val),
L
ꢀalanine (Ala),
ꢀtyrosine
ꢀproline
L
L
L
L
L
L
are found to exhibit growth inhibition of P388 lymꢀ
phocytic leukemic cells. The IC50 values for the pallaꢀ
dium(II) complexes with Gly and Val are comparable
to cisplatin, whereas the other palladium(II) complexꢀ
es show higher IC50 values [9].
Because the coordination behavior of Nꢀcarbonylꢀ
amino acids show some similarities to that of
the Oꢀterminal end of peptides and proteins, great atꢀ
tention has been paid to the coordination properties
of these ligands. The deprotonated Pt(II)/Pd(II)
1
The article is published in the original.
complexes, in which Nꢀacetylglycine acts as an N,Oꢀ
115

116
ZHANG et al.
chelating ligand, are obtained in [10]. Several palladiꢀ
um(II) complexes with deprotonated sulfonamides
nitrogen have been synthesized in [11]. Four new
mixedꢀligand complexes of palladium(II) with
Synthesis of complexes I–III. NꢀcarbonylꢀLꢀisoleꢀ
cine and [Pd(Phen)Cl2] were prepared by the reported
methods [13, 14]. Reaction of [Pd(Phen)Cl2] with two
equivalents of
title complexes (
N
ꢀcarbonylꢀ
Lꢀisolecine produced the
N
ꢀbenzoylꢀ
2,2 ꢀbipyridine (Bipy) or Phen were synthesized [12].
Until now, the cytotoxicity of mixedꢀligand complexes
of palladium(II) with ꢀcarbonyl amino acid dianion
α
ꢀamino acid dianion and ethyldiamine,
I–III) [11]. Complex II (15 mg,
'
0.028 mmol) was dissolved in 3 mL mixture CH3OH–
CHCl3 (v/v = 2 : 1). After several days, yellow crystals
suitable for Xꢀray studies were obtained by the slow
evaporation. The synthetic routines of complexes I–
III are given below:
N
and diamine has not been reported. In the present
work, we present the synthesis, characterization and
cytotoxicity of three mixedꢀligand palladium(II) comꢀ
plexes with NꢀcarbonylꢀLꢀisoleucine dianion and
Phen for the first time.
Step 1:
EXPERIMENTAL
Materials and methods. Benzoyl chloride,
ylbenzoyl chloride, ꢀnitrobenzoyl chloride and
K2PdCl4 were of chemical grade, Phen and ꢀisoleꢀ
pꢀmethꢀ
Cl
N
N
N
N
p
EtOH : H O = 1 : 1
2
2 h
Pd
K2PdCl4 +
L
Cl
cine (Ile) were of analytical grade. RPMIꢀ1640 mediꢀ
um, trypsin and fetal bovine serum were purchased from
Gibco. MTT [3ꢀ(4,5ꢀdimethylthiazolꢀ2ꢀyl)ꢀ2,5ꢀdipheꢀ
nyl tetrazolium bromide], SRB (sulforhodamine B),
benzyl penicillin and streptomycin were from Sigma.
Four different human carcinoma cell lines: HLꢀ60
(immature granulocyte leukemia), Belꢀ7402 (liver
carcinoma), BGCꢀ823 (gastro carcinoma) and KB
(nasopharyngeal carcinoma) were obtained from
American Type Culture Collection.
1,10ꢀPhenanthroline
Pd(Phen)Cl2
Step 2:
R
Elemental analysis was determined on an Elemenꢀ
tar Vario EL III elemental analyzer. The IR spectra
were recorded using KBr pellets and a PerkinElmer
Modelꢀ683 spectrophotometer. The mass spectra were
measured by LCꢀMS apparatus Agilent 1200ꢀ6310.
Xꢀray single crystal structure was performed on a
Bruker SMART APEX II CCD diffractometer. The
OD was measured on a microplate spectrophotometer
(BioꢀRad Model 680, USA).
R
C O
H2N
HN
NaOH
5 h
+
C O
HO
O
HO
O
Cl
Carbonyl agent
LꢀIsolecine
NꢀCarbonylꢀLꢀisolecine
Step 3:
R
R
C O
N
C O
HN
Cl
Cl
N
N
N
N
⋅ nH2O
MeOH : H O = 1 : 1
2
2 h
Pd
+
Pd
O
O
HO
O
Pd(Phen)Cl2
I.
II
III
n
n
= 0, R = H;
= 0, R = CH3;
NꢀCarbonylꢀLꢀisolecine
.
. n = 1, R = NO2.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 40
No. 2
2014

SYNTHESIS, CHARACTERIZATION, AND CYTOTOXICITY OF MIXEDꢀLIGAND
117
IR for BzileH2,
cm–1): 3352, 3356, 3348
1642 (amide ), 1536, 1543, 1545
(OCO)as, 1208, 1206, 1346
p
ꢀMBzileH2,
(NHꢀamide), 1639, 1630,
(amide ΙΙ), 1726,
p
ꢀNBzileH2 (KBr;
ν
,
(no. 771187; deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
ν
ν
Ι
ν
In vitro cytotoxicity study. The complexes were disꢀ
solved in DMSO at a concentration of 5 mmol/L as
stock solutions and diluted in culture medium at conꢀ
1724, 1711
pectively.
ν
ν(OCO)s, resꢀ
centrations of 1.0, 10, 100, and 500 mol/L as workꢀ
μ
ingꢀsolution. To avoid DMSO toxicity, the concentraꢀ
tion of DMSO was less than 0.1% (v/v) in all experiꢀ
ments.
For C25H23N3O3Pd (
anal. calcd., %: C, 57.76;
Found, %: C, 57.76;
I, M = 519.89)
N, 8.08;
N, 8.16;
H, 4.46.
H, 4.36.
The cells harvested from exponential phase were
seeded equivalently into a 96ꢀwell plate, and then the
complexes were added to the wells to achieve final
concentrations. Control wells were prepared by addiꢀ
tion of culture medium. Wells containing culture meꢀ
dium without cells were used as blanks. All experiꢀ
ments were performed in quintuplicate. The MTT assay
was performed as described by Mosmann for HLꢀ60
[17]. Upon completion of the incubation for 44 h,
stock MTT dye solution (20 mL, 5 mg/mL) was added
to each well. After 4 h incubation, 2ꢀpropanol
(100 mL) was added to solubilize the MTT formazan.
The OD of each well was measured on a microplate
spectrophotometer at a wavelength of 570 nm. The
SRB assay was performed as previously described for
Belꢀ7402, BGCꢀ823, and KB [18]. Upon completion
of the incubation for 44 h, the cells were fixed in 10%
ESꢀMS (
m
/
z
): 542.89 [M + Na]+.
, cm–1): 1535
(amide
(Pd–N), 469
IR (KBr;
1394
ν
ν
Ι
), 1642
ν
(Pd–O).
ν
(OCO)as,
ν(OCO)s, 547
ν
1H NMR (600 MHz; CDCl3; δ, ppm): 0.84–0.78 (m.,
3H, CH3), 1.20 (t., J = 5.26 Hz, 3H, CH3), 1.48 (d.,
J = 6.81 Hz, 2H, CH2), 2.17 (s., 1H, CH), 4.00–3.97
(m., 1H, CH), 7.33–8.60 (13H, Ar–H).
For C26H25N3O3Pd (II
anal. calcd., %: C, 58.49;
Found, %: C, 58.35;
, M = 533.91)
N, 7.87;
N, 7.83;
H, 4.72.
H, 4.70.
ESꢀMS (
m
/
z
): 556.89 [M + Na]+.
, cm–1): 1545
(Amide
(Pd–N), 479
1H NMR (600 MHz; CDCl3; δ, ppm): 0.82 (q.,
IR (KBr;
ν
ν
Ι
), 1636
ν(OCO)as,
trichloroacetic acid (100 mL) for 30 min at 4°C,
1384
ν(OCO)s, 552
ν
ν
(Pd–O).
washed five times and stained with 0.1% SRB in 1%
acetic acid (100 mL) for 15 min. The cells were washed
four times in 1% acetic acid and airꢀdried. The stain was
solubilized in 10 mM unbuffered Tris base (100 mL)
and OD was measured at 540 nm as above. The IC50
value was determined from plot of % viability against
dose of compounds added.
J
=
7.38, 6.98 Hz, 3H, CH3), 1.25 (d., J = 7.06 Hz, 3H,
CH3), 1.46 (d., J = 6.75 Hz, 2H, CH2), 2.35 (s., 3H,
Ar–CH3), 3.74–3.69 (m., 1H, CH), 4.01–3.98 (m.,
1H, CH), 7.30–8.63 (12H, Ar–H).
For C25H22N4O5Pd (III
,
M
= 564.89)
N, 9.92;
C, 51.51; N, 9.67;
anal. calcd., %: C, 53.16;
H, 3.93.
H, 4.04.
RESULTS AND DISCUSSION
Found,%:
ESꢀMS (
The elemental analysis and mass showed that all
the complexes, isolated as crystalline solids, were of
high purity. There is good agreement between calcuꢀ
m
/
z
): 565.89 [M + H]+.
, cm–1): 3420
(water), 1554
(OCO)as, 1382 (OCO)s, 562
(Pd–O).
IR (KBr;
1638
474
ν
ν
ν(amide Ι),
ν
(Pd–N),
lated and found values for complexes
I
–
III
ꢀMBzileH2 and
(NH) at about
III
showing that the amide group has been deprotonated.
This is also confirmed by the amide shifting from
1640 to 1535–1554 cm–1 and the disappearance of
the amide II from
540 cm–1 region. New bands apꢀ
peared at 547–562 cm–1 are assigned to
(Pd–N). The
carboxylate group of the complexes III shows two
bands, an intense antisymmetric carboxylate stretching
.
ν
ν
The amide groups of BzileH2,
p
ν
p
ꢀNBzileH2 have a strong and sharp
ν
Xꢀray crystallography. The data collection of
the complex II was performed on a Bruker SMART
APEX II CCD diffractometer equipped with a graphꢀ
3350 cm–1. This peak disappears in complexes
I–
,
I
ite monochromatized Mo
K
radiation ( = 0.71073 Å)
λ
α
∼
at 296(2) K. The program SAINT was used for inteꢀ
gration of the diffraction profiles [15]. Multiscan abꢀ
sorption corrections were applied using the SADABS
program. The structures were solved by the direct
method using the SHELXSꢀ97 program. Refinements
∼
ν
I–
ν
(COO–)as and a symmetric carboxylate stretching
on F
2 were performed using SHELXLꢀ97 by the fullꢀ
ν
(COO–)s at about 1640 and 1380 cm–1, respectively.
matrix leastꢀsquares method with anisotropic thermal
parameters for all nonꢀhydrogen atoms [16]. The hyꢀ
drogen atoms were located from different Fourier maps.
Supplementary material for structure II has deposited
The values of
complexes I–
ν
(COO–) (
ν
(COO–)as –
III are in the range 248–256 cm–1,
which is greater than (COO–) of the corresponding
ν
(COO–)s) of
Δ
ν
Δ
with the Cambridge Crystallographic Data Centre sodium carboxylates, so the carboxylate group may be
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 40
No. 2
2014

118
ZHANG et al.
y
x
z
N(2)
N(1)
Pd(1)
N(3)
O(1)
The molecular structure of complex II
.
Table 1. Crystallographic data and refinement details for
complex II
monodentate coordinated through oxygen atoms
[19, 20]. This is further confirmed by the appearance
of the peaks of ν(Pd–O). These results are in good
agreement with the results revealed by Xꢀray crystal analꢀ
ysis.
Parameter
Formula weight
Value
533.89
Crystal system, space group
Unitcell dimensions:
Monoclinic, P21
The crystal structures and numbering schemes of
the complex II are shown in figure. The crystal data,
data collection, and structural solution refinement
parameters for complex II are summarized in Table 1.
Selected bond distances and angles are listed in
Table 2. The crystal data reveal that the geometry
around Pd(II) of two complexes is approximately
square planar, composed of two nitrogen atoms from
Phen and one carbonyl amide nitrogen atom and one
a
b
c, Å
β
, Å
, Å
11.8184(12)
11.1624(11)
18.2371(19)
102.489(2)
2348.9(4)
1510
, deg
, Å3
V
ρcalcd, mg/cm3
Z
4
Crystal size, mm
μ
0.19
×
0.16
0.823
1088
×
0.07
, mm–1
F(000)
Table 2. Selected bond lengths (Å) and angles (deg) for
complex II
θ
Range for data collection, deg
1.14–28.28
Limiting indices
–13
–13
–23
≤
h
k
l
≤
≤
≤
15,
14,
24
≤
Bond
d
, Å
Bond
d, Å
≤
Pd(1)–O(1)
Pd(1)–N(1)
1.989(5) Pd(1)–N(2)
1.985(6) Pd(1)–N(3)
2.039(6)
2.032(6)
Reflections collected/unique 14497/9090 (Rint = 0.0312)
Completeness, %
Parameters
Goodnessꢀofꢀfit on F 2
97.2
602
1.462
Angle
ω
, deg
Angle
ω
, deg
O(1)Pd(1)N(1)
O(1)Pd(1)N(3)
82.1(2) O(1)Pd(1)N(2) 170.5(2)
92.6(2) N(1)Pd(1)N(2) 104.2(3)
Final
indices, all data
Largest diff. peak and hole,
R
indices,
I
> 2
σ
(
I
)
R
R
1 = 0.0438, wR2 = 0.0609
1 = 0.0787, wR2 = 0.0677
1.105 and –0.807
R
/Å3
N(1)Pd(1)N(3) 173.1(4) N(3)Pd(1)N(2) 81.5(3)
e
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 40
No. 2
2014

SYNTHESIS, CHARACTERIZATION, AND CYTOTOXICITY OF MIXEDꢀLIGAND
119
Table 3. Cytotoxity against four cancer cell lines: KB, BGCꢀ823, Belꢀ7402 and HLꢀ60
IC50, mmol/L
Compound
[Pd(Phen)(BzileNO)] (
HLꢀ60
BGCꢀ823
Belꢀ7402
KB
I)
16.99
9.54
37.90
34.68
46.32
38.32
27.35
42.78
23.58
19.52
31.46
[Pd(Phen)(
p
p
ꢀMBzileNO)] (II
)
[Pd(Phen)(
ꢀNBzileNO)] H2O (III
⋅
)
23.22
cisꢀDDP
2.89
6.48
8.12
2.65
carboxylic oxygen of the amino acid molecule. Thus dation of China—Shijiazhuang Pharmaceutical
Pd has planar square coordination geometry.
Group (CSPC) Foundation (B2011201174).
The angle between planar N(2)–Pd(1)–N(3) and
planar O(1)–Pd(1)–N(1) is 8.624(246)
that the Pd(1)–O(1)–N(1)–N(2)–N(3) plane is slightly
distorted. The Pd–N (deprotonated amide) bond length
(1.985(6) ) is shorter than the Pd–N(Phen) bond
lengths (2.032(6) and 2.0392(6) ), while it is similar
to Pd–O (carboxylic oxygen) bond length (1.989(5) ).
° which indicates
REFERENCES
1. Wong, E. and Giandomenico, C.M., Chem. Rev., 1999,
vol. 99, p. 2451.
2. Ho, Y.P., AuꢀYeung, S.C.F., and To, K.K.W., Med. Res.
Rev., 2003, vol. 23, p. 633.
3. Giovagnini, L., Ronconi, L., Aldinucci, D., et al.,
Å
Å
Å
In [21], it was reported that the coordinating qualities
of the deprotonated amide nitrogen atoms were
“Oꢀlike” as the deprotonated amide group is isoelecꢀ
tronic with the carboxylate group, and this has been
conrmed by stability constants of some complexes. In
[20], it was also reported that the deprotonated amide
nitrogen atom was exactly different from the ordinary
amino nitrogen atom and its coordinating property
maybe “Oꢀlike”.
J. Med. Chem., 2005, vol. 48, p. 1588.
4. Quiroga, A.G. and Ranninger, C.N., Coord. Chem.
Rev., 2004, vol. 248, p. 119.
5. Ray, S., Mohan, R., Singh, J.K., et al., J. Am. Chem.
Soc., 2007, vol. 129, p. 15042.
6. Rocha, F.V., Barra, C.V., Netto, A.V.G., et al., Eur. J.
Med. Chem., 2010, vol. 45, p. 1698.
7. Metal Ions in Biological Systems, Cleare, M.J., Hyde, P.C.,
and Sigel, H., Eds., vol. 11, New York: Marcel Dekker,
1980, p. 1.
8. Borch, R.F. and Pleasants, M.E., Proc. Natl. Acad. Sci.
USA, 1979, vol. 76, p. 6611.
9. Mital, R., Srivastava, T.S., Parekh, H.K., and
Chitnis, M.P., J. Inorg. Biochem., 1991, vol. 41, p. 93.
10. Appleton, T.G., Hall, J.R., and Prenzler, P.D., Inorg.
Chem., 1989, vol. 28, p. 815.
As listed in Table 3, the complexes
cytotoxic effects against tested carcinoma cell lines
with a lower IC50 value (<50 mol/L), the complex II
I–III exerted
μ
has best activity against the four cancer cell lines, but
none of them show higher cytotoxicity than cisplatin.
The acylated groups have important impact on the cytoꢀ
toxicity of complexes. The cytotoxicity against HLꢀ60,
BGCꢀ823, Belꢀ7402 and KB cell lines decrease in the
11. Corradi, A.B., Gozzoli, E., Menabue, L., et al., Dalton
Trans., 1994, p. 273.
sequence: pꢀMBzile > Bzile > pꢀNBzile. In summary,
12. Gong, Y.Q., Chen, Y.F., Gu, J.M., and Hu, X.R., Sci.
China, B, 1998, vol. 28, p. 8.
13. Steiger, R.E., J. Org. Chem., 1944, vol. 9, p. 396.
14. Burmeister, J.L. and Basolo, F., Inorg. Chem., 1964,
vol. 3, p. 1587.
this study can enrich inorganic medicine area, these
results will be very helpful for designing new anticanꢀ
cer drugs.
15. SAINT Software Reference Manual, Madison (WI,
USF): Bruker AXS, 1998.
ACKNOWLEDGMENTS
16. Sheldrick, G.M., SHELXTL NT, Version 5.1, Program
This work was supported by the Special Foundation
for State Major New Drug Research Program of
China (grant no. 2009ZX09103ꢀ139), the National
for Solution and Refinement of Crystal Structures,
tingen (Germany): Univ. of G ttingen, 1997.
17. Fricker, S.P., Toxicol. in Vitro, 1994, vol. 8, p. 821.
18. Skehan, P., Storeng, R., Scudiero, D., et al., J. Natl.
Cancer Inst., 1990, vol. 82, p. 1107.
19. Deacon, G.B. and Phillips, R.J., Coord. Chem. Rev.
1980, vol. 33, p. 227.
Götꢀ
ö
Basic
Research
973
Program
(grant
no. 2010CB534913), Hebei Province Nature Science
Fund for Distinguished Young Scholars (grant
no. B2011201164), the Nature Science Fund of Hebei
Province (grant no. B2011201135), the Key Basic
Research Special Foundation of Science Technology
Ministry of Hebei Province (grants nos. 11966412D,
12966418D), Hebei Provincial Natural Science Founꢀ
,
20. Gong, Y.Q., Chen, Y.F., Gu, J.M., and Hu, X.R., Polyꢀ
hedron, 1997, vol. 16, p. 3743.
21. Sigel, H., Fischer, B.E., and Prijs, B., J. Am. Chem.
Soc., 1977, vol. 99, p. 4489.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 40
No. 2
2014