Zinc complexes with 3-Pyridinyl-5-(2-salicylideneiminophenyl)-1H-1,2,4- triazoles

Article abstract of DOI:10.1134/S0036023611010116

New coordination compounds of zinc with 3-(pyridine-2-yl)-5-(2- salicylideneiminophenyl)-1H-1,2,4-triazole (H₂L¹) and 3-(pyridine-4-yl)-5-(2-salicylideneiminophenyl)-1H-1,2,4-triazole (H ₂L²) are obtained. According to X-ray diffraction data, binuclear zinc complexes with L¹, namely, [Zn₂L ₂ ¹ ]. 0.5EtOH and [Zn₂L ₂ ¹ ] ? 2C₄H₈O₂ ? 2H ₂O obtained in different solvents, are structurally related molecular complexes. The product of the reaction with H₂L² is the {[ZnL²(Py)] ? CHCl₃} _n coordination polymer. The 1,2,4-triazoles under study and the complexes on their basis luminesce in solutions with emission maxima ranging from 412 to 503 nm. These coordination compounds in the solid state emit in the green range of the spectrum (λ_max = 496 and 485 nm).

Full text of DOI:10.1134/S0036023611010116

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 1, pp. 32–38. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.N. Gusev, V.F. Shul’gin, S.B. Meshkova, Z.M. Topilova, M.A. Kiskin, G.G. Aleksandrov, I.L. Eremenko, 2011, published in Zhurnal Neorganicheskoi
Khimii, 2011, Vol. 56, No. 1, pp. 35–42.
COORDINATION COMPOUNDS
Zinc Complexes with 3ꢀPyridinylꢀ5ꢀ(2ꢀsalicylideneiminophenyl)ꢀ
1Hꢀ1,2,4ꢀtriazoles
A. N. Guseva, V. F. Shul’gina, S. B. Meshkovab, Z. M. Topilovab,
M. A. Kiskinc, G. G. Aleksandrovc, and I. L. Eremenkoc
a Taurida National Vernadsky University, pr. Vernadsokogo 4, Simferopol, 95007 Ukraine
b Bogatsky Physicochemical Institute of the National Academy of Sciences of Ukraine,
Lustdorfskaya doroga 86, Odessa, 65080 Ukraine
c Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
Received June 19, 2009
Abstract—New coordination compounds of zinc with 3ꢀ(pyridineꢀ2ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneiminophenyl)ꢀ
1Hꢀ1,2,4ꢀtriazole (H2L1) and 3ꢀ(pyridineꢀ4ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneiminophenyl)ꢀ1Hꢀ1,2,4ꢀtriazole (H2L2)
1
1
⎡
⎤
·
are obtained. According to Xꢀray diffraction data, binuclear zinc complexes with L , namely,
Zn2L2
⎣
⎦
1
⎡
⎤
· 2C4H8O2
0.5EtOH and
·
2
H2O obtained in different solvents, are structurally related molecular
Zn2L2
⎣
⎦
complexes. The product of the reaction with H2L2 is the {[ZnL2(Py)]
·
CHCl3}n coordination polymer. The
1,2,4ꢀtriazoles under study and the complexes on their basis luminesce in solutions with emission maxima
ranging from 412 to 503 nm. These coordination compounds in the solid state emit in the green range of the
spectrum (λmax = 496 and 485 nm).
DOI: 10.1134/S0036023611010116
Coordination compounds of 1,2,4ꢀtriazoles are the ways to solve this problem is to change an ionic
known to be useful as magnetic and optical materials structure to a molecular one by introducing additional
[1–5]. The majority of the complexes with this type of chelate groups with mobile hydrogen atoms to the
ligand described in the literature are ionogenic, which ligand. In this work, we present the results of the study
considerably decreases their solubility in lowꢀpolarity of molecular zinc complexes with 3ꢀpyridinylꢀ5ꢀ(2ꢀ
solvents and restricts their utility in practice. One of salicylideneiminophenyl)ꢀ1Hꢀ1,2,4ꢀtriazoles.
N
N
N
N
N
N
N N
H
N
N
H
H
H
HO
HO
3ꢀ(pyridineꢀ2ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneꢀ
iminophenyl)ꢀ1Hꢀ1,2,4ꢀtriazole
3ꢀ(pyridineꢀ4ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneꢀ
iminophenyl)ꢀ1Hꢀ1,2,4ꢀtriazole
H2L1
)
(
H2L2
)
(
EXPERIMENTAL
Synthesis of the coordination compounds under
study was performed as follows. A solution of 250 mg
(2.1 mmol) of salicylaldehyde in 10 mL of ethanol was
added to a solution of 472 mg (2 mmol) of the approꢀ
priate 3ꢀ(pyridinyl)ꢀ5ꢀ(2ꢀaminophenyl)ꢀ1Hꢀ1,2,4ꢀ
The starting reagents, 3ꢀ(pyridinyl)ꢀ5ꢀ(2ꢀamiꢀ
nophenyl)ꢀ1Hꢀ1,2,4ꢀtriazoles, were prepared by
reacting nitriles of suitable pyridinecarboxylic acids
with 2ꢀaminobenzoic acid hydrazide [6].
32

ZINC COMPLEXES
33
triazole in 10 mL of 96% ethanol. The resulting reacꢀ selected crystallographic data are shown in Table 1.
tion mixture was stirred under heating with a magnetic Semiꢀempirical correction for absorption was applied
stirrer for 1 h. To the suspension formed, 2 mmol of (0.8646/0.7723 for
I, 0.8982/0.8527 for II, and
zinc acetate dihydrate was added, and the mixture was 0.8827/0.7132 for IV) [9]. The structures were solved
stirred for 2 h. The precipitate was kept under the by the direct method and refined by fullꢀmatrix leastꢀ
mother liquor for 12 h, then filtered off, washed with squares in the anisotropic approximation (SHELXꢀ
ethanol, and dried in air. The product was obtained as 97) [10]. Crystallographic data for compounds I, II,
520–660 mg of a yellow crystalline substance; triazole and IV have been deposited with the Cambridge Crysꢀ
yield, 65–75%.
tal Database, nos. 733658, 733659, and 7333660,
respectively, and can be free downloaded from the site
http://www.ccdc.cam.ac.uk/data_request/cif.
The zinc content was calculated from complexoꢀ
metric titration data [7] after thermal decomposition
of the test sample; the nitrogen content was deterꢀ
mined by the Dumas method [8].
IR spectra of the complexes were recorded as KBr
pellets in the range of 400–4000 cm–1 on a Nicolet
Nexus 470 FTIR spectrometer.
RESULTS AND DISCUSSION
Our studies show that the products of condensation
of salicylaldehyde and 3ꢀ(pyridine)ꢀ5ꢀ(2ꢀaminopheꢀ
nyl)ꢀ1Hꢀ1,2,4ꢀtriazoles react with zinc acetate to
form molecular complexes with the metal : ligand ratio
of 1 : 1. The complexes are stable up to 40–50°С, and
a further increase in temperature leads to desolvation
of the complexes. A crystallization ethanol molecule
1
⎡
⎤
·
For
0.5EtOH (C41H29N10O2.5Zn2) (I),
Zn2L2
⎣
⎦
anal. calcd. (wt %): Zn, 15.59; N, 16.66.
Found (wt %): Zn, 15.62; N, 16.36.
IR (cm–1) for
I
: 1610 (
1457, 1444, 1330, 1147, 800, 752.
For [ZnL2]
EtOH (C22H21N5O2Zn) (III), anal.
ν(C=NSchiff)), 1593, 1533,
entering the composition of complex
40 100 without noticeable thermal features. The
ethanol molecule of compound III is eliminated at a
higher temperature (50–180 ). The process is
accompanied by a small exotherm on the DTA curve
with a maximum at 70 . At 235 , complex III melts.
Thermal oxidative destruction of triazoles is observed
(at 230 and 300 for complexes and III, respecꢀ
tively, followed by combustion of the organic residue.
The process is completed at 650–700
The IR spectra of complexes and III do not conꢀ
I is removed at
–
°С
·
calcd. (wt %): Zn, 14.47; N, 15.48.
°С
Found (wt %): Zn, 14.09; N, 15.71.
IR (cm–1) for III: 1614 (
ν(C=NSchiff)), 1577, 1535,
°С
°С
1467, 1444, 1321, 1153, 754.
Thermogravimetric experiments were performed on
a PaulikꢀPaulikꢀErdey Qꢀderivatograph in an open
ceramic crucible under a static air atmosphere with
heating at a rate of 10 K/min; the standard was a calꢀ
cined aluminum oxide sample.
Absorption spectra were recorded on a Perkinꢀ
Elmer Lambdaꢀ9 UV/VIS/NIR spectrophotometer.
Luminescence spectra of solid samples were obtained
with a LOMO SDLꢀ1 spectrometer equipped with a
FEUꢀ79 photomultiplier.
Excitation and luminescence spectra of solutions
were recorded on a Horiba JobinꢀYvon FluorologꢀFL
3ꢀ22 spectrophotometer equipped with a 450 W Xe
lamp.
Xꢀray crystallography measurements were perꢀ
formed using a Bruker ApexꢀII CCD diffractometer
°С
I
°С.
I
tain bands corresponding to the stretching vibrations
of N–H and O–H groups, which are observed in the
IR spectra of the ligands at 3380 and 3270 cm–1,
respectively. The coordination of the phenoxyl oxygen
atom is accompanied by a shift of the band of the stretchꢀ
ing vibrations of the Cphen–O bond from 1290 cm–1 in
the free triazoles to 1330–1321 cm–1 in the complexes.
A shift of the band of the stretching vibrations of the
⎯
HC=N– bond by 15–20 cm–1 to the shortwave
range is indicative of the coordination of the imine
nitrogen atom.
According to Xꢀray crystallography, complex
I is
binuclear (Fig. 1). Each zinc ion is coordinated by two
pentadentate bridging ligands in the doubly deprotoꢀ
nated form. Each central atom has a distorted tetragoꢀ
nalꢀbipyramidal environment formed by four nitrogen
atoms in the base of the pyramid and an oxygen atom
in the axial position. The Zn2N4 central sixꢀmembered
metal ring has a boat conformation. The Zn(1) and
Zn(2) atoms deviate by 0.48 Å and 0.65 Å, respectively,
to the apical oxygen atoms from the plane of the four
nitrogen atoms. The intramolecular Zn···Zn distance is
4.038Å, which is characteristic of binuclear complexes
of 1,2,4ꢀtriazoles [1]. The planar configuration and
deprotonation of the 1,2,4ꢀtriazole moiety facilitate
delocalization of C=N double bonds in the N3С2 fiveꢀ
membered ring. Because of this, the N(2)–N(3) bond
(
МоКα radiation, graphite monochromator, λ =
0.71073 Å). Single crystals of the zinc complex with
3ꢀ(pyridineꢀ2ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneiminophenyl)ꢀ1Hꢀ
1,2,4ꢀtriazole were obtained by recrystallization from
a DMSO–ethanol mixture (2 : 1 vol/vol), as well as
from dioxane. The compositions of single crystals
grown from DMSO–ethanol and dioxane were
1
1
⎡
⎤
⎡
⎤
· 2C4H8O2 · 2H2O
Zn2L2
·
0.5EtOH (
I) and
Zn2L2
⎣
⎦
⎣
⎦
(
II), respectively. Single crystals of the zinc complex
with 3ꢀ(pyridineꢀ4ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneiminopheꢀ
nyl)ꢀ1Hꢀ1,2,4ꢀtriazole were obtained by slow diffuꢀ
sion of chloroform vapors into a pyridine solution of
complex III. The crystal composition is formulated as
{[ZnL2(Ру)]
· CHCl3}n (IV). Experimental details and
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 1 2011

34
GUSEV et al.
Table 1. Crystal data and experimental details for complexes I, II, and IV
Parameter
I
II
IV
Formula
Crystal dimension, mm
, K
C41H29N10O2,5Zn2
C48H46N10O8Zn2
C26H19Cl3N6OZn
0.20
×
0.14
298(2)
Monoclinic
2/
×
0.11
0.15
×
0.13
298(2)
Monoclinic
2/
×
0.10
0.34
×
0.23
296(2)
Monoclinic
21/
×
0.12
T
Crystal system
Space group
C
c
C
c
P
n
Unit cell parameters
a
, Å
, Å
25.405(18)
14.938(11)
19.430(14)
96.006(10)
7333.9(9)
8
23.074(12)
13.869(7)
16.675(9)
119.664(8)
4637(4)
4
8.839(14)
30.31(4)
11.760(11)
95.782(2)
3134.9(7)
4
b
c
, Å
deg
, Å3
β
,
V
Z
ρcalcd
,
g/cm–3
1.508
1.464
1.278
μMo, mm–1
(000)
θmax deg
1.362
1.098
1.066
F
3400
2072
1224
,
30.07
31.39
25.03
Index ranges
–29
–21
–27
≤
≤
≤
h
k
l
≤
≤
≤
35
20
25
–32
–20
–21
≤
≤
≤
h
k
l
≤
≤
≤
32
16
24
–10
–36
–13
≤
≤
≤
h
k
l
≤
≤
≤
10
36
13
Reflections measured/reflecꢀ
tions independent
28343/10636
18414/7217
20422/5360
Rint
0.0328
1.048
0.0294
1.044
GOOF
1.001
R
R
(
all data
> 2
)
R
1 = 0.0842
wR2 = 0.1278
1 = 0.0424,
R
1 = 0.1148
wR2 = 0.2548
1 = 0.0682,
R
1 = 0.1582
wR2 = 0.1964
1 = 0.0648,
(
I
σ
(I
))
R
R
R
wR2 = 0.1095
wR2 = 0.2146
wR2 = 0.1580
Residual electron density
(max/min), e/Å3
0.654/–0.254
1.482/–0.653
0.907/–0.403
is considerably shorter (1.353(3) Å) than a standard between 2ꢀaminobenzene rings of the ligands of adjaꢀ
nitrogen–nitrogen single bond (1.451 Å [11]). In both cent molecules, whose planes are at a distance of
L1 ligands, triazole and phenyl moieties are nearly 3.56 Å from each other (Fig. 3). The solvation water
coplanar with the pyridyl plane, and the planar 2ꢀ molecule is disordered and occupies two positions, in
imino(methylphenol) moiety is turned around the one of which it forms hydrogen bonds with the oxygen
benzene ring plane by 37.9° in the first ligand and by atoms of the coordinated ligands (O(1w)···O(1) 2.76 Å;
52.4° in the second. The O(2) and H atoms of the solꢀ O(1)O(1w)O(1a), 114.5
°) (Fig. 2).
vated ethanol molecule are bound by a strong hydroꢀ
gen bond (O(2)···O(1S) 2.82 Å).
Complex IV, which crystallizes as a solvate with
chloroform, has a polymeric structure. The chloroꢀ
Crystallization of I from dioxane leaves the molecꢀ form molecule is disordered over two positions with
ular structure of the complex unchanged, but changes the site occupancies of 0.35 and 0.65. A fragment of
the packing of complex molecules in a crystal of II the polymeric chain of complex IV is shown in Fig. 4.
(Fig. 2). As in I, the coordination polyhedron of the The coordination sphere of the zinc atom is a distorted
central atom in II, too, is a distorted square pyramid, trigonal bipyramid with nitrogen atoms of the azomeꢀ
and the central metal ring has the boat conformation. thine moiety and the pyridine ring of the adjacent
The intramolecular Zn···Zn distance is 4.030 Å. The molecule in the axial positions. The equatorial plane is
crystal structure has an extensive net of short intermoꢀ formed by the oxygen and nitrogen atoms of the douꢀ
lecular contacts. An interesting structural feature of bly deprotonated chelatophore group of the triazole
compound II is bonding due to stacking interaction and the nitrogen atom of the coordinated pyridine
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 1 2011

ZINC COMPLEXES
35
N(6)
O(1)
N(8)
N(10)
O(2)
N(7)
Zn(2)
Zn(1)
N(2)
N(3)
N(5)
N(1)
Fig. 1. Structure of complex I (hydrogen atoms are omitted). Selected bond lengths (Å) and bond angles (deg) are Zn(1)–N(1)
2.166(2), Zn(1)–N(2) 2.097(2), Zn(1)–N(8) 2.043(2), Zn(1)–N(10) 2.102(2), Zn(1)–O(2) 1.9270(19), O(2)Zn(1)N(8)
129.76(9), O(2)Zn(1)N(2) 100.17(9), N(8)Zn(1)N(2) 89.49(8), O(2)Zn(1)N(10) 93.75(8), N(8)Zn(1)N(10) 82.87(8),
N(2)Zn(1)N(10) 165.98(9), O(2)Zn(1)N(1) 100.56(9), N(8)Zn(1)N(1) 129.51(9), N(2)Zn(1)N(1) 76.17(8), N(10)Zn(1)N(1)
99.69(8).
molecule. The N(3) atom of the triazole ring is not bands at 413 (H2L1) and 479 nm (H2L2) in the photoꢀ
coordinated, which is not characteristic of 1,2,4ꢀtriaꢀ luminescence spectra of solutions of the Schiff bases
zoles [1]. This may be due to additional coordination studied (Table 2) are due to energy transfers between
of the N(1) atom of the salicylidene chelatophore the highest occupied and the lowest unoccupied
group, which stabilizes the monodentate coordination molecular orbitals. Deprotonation of organic ligands
of the triazole. The polymeric chain of complex IV has on formation of
d
10 metal complexes is known to conꢀ
a zigzag structure; the ZnZnZn angle is 87.17 . A charꢀ siderably decrease the energy gap between these orbitꢀ
°
acteristic structural feature of compound IV is the forꢀ als and to result in a bathochromic shift of maxima in
mation of channels in which disordered solvation photoluminescence spectra [12]. The data we
chloroform molecules are placed.
obtained show that this tendency is intrinsic to comꢀ
plexes and III: the complexes emit in the green range
(maxima at 503 and 493 nm, respectively) (Table 2).
I
The absorption spectra of DMSO solutions of the
compounds under study (Fig. 5) show that the maxiꢀ
mum absorption occurs in the range of 350–400 nm.
3ꢀ(Pyridineꢀ2ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneiminophenyl)ꢀ
This implies that the luminescence can be excited by 1Hꢀ1,2,4ꢀtriazole does not display any visible lumiꢀ
the intense mercury line at 365 nm. A broad emission nescence in the solid state (Fig. 6), whereas complex
I
Table 2. Spectralꢀluminescent characteristics of the H2L1, H2L2 ligands and complexes I, III (recorded at equal conditions)
Emission
Absorption
Excitation
solution
solid sample
Compound
I
,
Ilum
quantum/s
,
Ilum,
rel. units
λ, nm
A
λ
, nm
λ, nm
λmax, nm
quantum/s
H2L1
I
H2L2
348
399
354
395
0.242
0.531
0.348
0.272
390
682568
10612
67500
31000
412.7
503.5
479
853420
10534
65300
32000
–
–
930
76
359.1
406
496
540
485
III
437
490
82
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 1 2011

36
GUSEV et al.
O(1w)
O(1)
O(1A)
Zn(1A)
N(5)
Zn(1)
N(2)
N(1)
N(3)
Fig. 2. Structure of complex II (hydrogen atoms are omitted). Selected bond lengths (Å) and bond angles (deg) are Zn(1)–N(1)
2.096(3), Zn(1)–N(2) 2.033(3), Zn(1)–N(3A) 2.106(3), Zn(1)–N(5A) 2.161(3), Zn(1)–O(1) 1.932(3), O(1)Zn(1)N(2)
122.00(13), O(1)Zn(1)N(1) 93.54(13), N(2)Zn(1)N(1) 83.93(13), O(1)Zn(1)N(3A) 103.52(12), N(2)Zn(1)N(3A) 89.42(13),
N(1)Zn(1)N(3A) 162.69(13), O(1)Zn(1)N(5A) 105.55(13), N(2)Zn(1)N(5A) 132.34(13), N(1)Zn(1)N(5A) 96.98(12),
N(3A)Zn(1)N(5A) 75.81(12).
Zn
Zn
Zn
Zn
Fig. 3. Fragment of crystal packing of compound II
.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 1 2011

ZINC COMPLEXES
37
N(2)
Zn(1)
Zn(1)
N(5)
N(5)
Zn(1)
N(2)
N(6)
O(1)
N(1)
Fig. 4. Fragment of crystalline polymeric chain of compound IV (hydrogen atoms are omitted). Selected bond lengths (Å) and
bond angles (deg) are Zn(1) O(1) 1.937(6), Zn(1) N(2) 2.001(6), Zn(1) N(6) 2.091(7), Zn(1) N(5) 2.159(6), Zn(1) N(1)
–
–
–
–
–
2.208(6), O(1)Zn(1)N(2) 136.1(3), O(1)Zn(1)N(6) 111.1(3), N(2)Zn(1)N(6) 111.8(3), O(1)Zn(1)N(5) 89.8(2),
N(2)Zn(1)N(5) 99.2(2).
A
A
0.5
0.5
2
4
1
3
0
300
0
400
λ
, nm 300
400
λ
, nm
1
2
Fig. 5. Absorption spectra of solutions of (
1)
Н
L , (
2) complex
I, (3)
Н
L , and (
4
) complex III
.
2
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 1 2011

38
GUSEV et al.
496 nm
I
, rel. units
I
, rel. units
100
2
4
3
×
3
×
3
50
1
0
400
500
600
λ
, nm
400
500
600
λ
, nm
1
2
Fig. 6. Luminescence spectra of solid samples of (1) Н L , (2) complex I, (3) H L , and (4) complex III.
2 2
4. K. Sung and A. Lee, J. Heterocycl. Chem. 29, 1101
emits an intense green light with a maximum at 495 nm.
3ꢀ(Pyridineꢀ4ꢀyl)ꢀ5ꢀ(2ꢀsalicylideneiminophenyl)ꢀ1Hꢀ
1,2,4ꢀtriazole and complex III emit in the visible
range with maxima at 540 and 485 nm, respectively.
No considerable change in emission intensity was
detected when passing from the ligand to the complex.
A possible reason for this is the presence of the coordiꢀ
nated ethanol molecule. The stretching vibrations of
the ethanol OH group lead to energy losses without
(1992).
5. L. G. Lavrenova, N. G. Yudina, V. N. Ikorskii, et al.,
Polyhedron 14, 1333 (1995).
6. US Pat. 4,198,513 USA, Published April 15, 1980.
7. R. Pribil, Analytical Application of EDTA and Related
Compounds (Pergamon, Oxford (U.K.), 1972; Mir,
Moscow, 1975).
8. V. A. Klimova, in Basic Methods of Organic Microanaꢀ
laysis (Khimiya, Moscow, 1975), p. 224 [in Russian].
9. G. M. Sheldrick, SADABS. Program for Scanning and
Correction of Area Detector Data (Göttingen Univ.,
Gottingen, 1997).
emission. Intense luminescence of complexes
III is interesting in the view of use of these compounds
I and
to obtain thin films of electroluminescent materials.
10. G. M. Sheldrick, SHELX97. Program for the Solution
of Crystal Structures (Göttingen Univ., Gottingen,
1997).
11. A. J. Gordon and R. A. Ford, The Chemist’s Companion:
A Handbook of Practical Data, Techniques and Referꢀ
ences (Wiley, New York, 1972; Mir, Moscow, 1976).
12. O. V. Kotova, Extended Abstract of Candidate’s Disserꢀ
tation in Chemistry (Mosk. Gos. Univ., Moscow,
2008).
REFERENCES
1. J. G. Haasnoot, Coord. Chem. Rev. 200–202, 131
(2000).
2. D. Mulhern, S. Brooker, H. Georls, et al., J. Chem.
Soc. Dalton Trans. 51 (2006).
3. S. Ferrer, J. G. Haasnoot, J. Reedijk, et al. Inorg.
Chem. 39, 1859 (2000).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 1 2011