Synthesis and EPR studies of soluble vanadyl octakis (4-tert- butylbenzylthio) porphyrazine

Article abstract of DOI:10.1080/00958972.2013.811496

Magnesium porphyrazinate substituted with eight 4-tert-butylphenylthio- groups on peripheral positions has been synthesized by cyclotetramerization of 1,2-bis(4-t-butylphenylthio)maleonitrile in the presence of magnesium butanolate. The metal-free derivat

Full text of DOI:10.1080/00958972.2013.811496

This article was downloaded by: [University of Sussex Library]
On: 25 March 2015, At: 13:12
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Coordination Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gcoo20
Synthesis and EPR studies of soluble
vanadyl octakis(4-tert-butylbenzylthio)
porphyrazine
Bahadir Keskin a , Yusuf Yerli b & Ulvi Avciata a
a Department of Chemistry , Yildiz Technical University , Istanbul ,
Turkey
b Department of Physics , Gebze Institute of Technology , Kocaeli ,
Turkey
Accepted author version posted online: 04 Jun 2013.Published
online: 08 Jul 2013.
To cite this article: Bahadir Keskin , Yusuf Yerli & Ulvi Avciata (2013) Synthesis and EPR studies of
soluble vanadyl octakis(4-tert-butylbenzylthio) porphyrazine, Journal of Coordination Chemistry,
66:15, 2615-2622, DOI: 10.1080/00958972.2013.811496
To link to this article: http://dx.doi.org/10.1080/00958972.2013.811496
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Coordination Chemistry, 2013
Vol. 66, No. 15, 2615–2622, http://dx.doi.org/10.1080/00958972.2013.811496
Synthesis and EPR studies of soluble vanadyl octakis
(4-tert-butylbenzylthio) porphyrazine
BAHADIR KESKIN*†, YUSUF YERLI‡ and ULVI AVCIATA†
†Department of Chemistry, Yildiz Technical University, Istanbul, Turkey
‡Department of Physics, Gebze Institute of Technology, Kocaeli, Turkey
(Received 7 January 2013; in final form 18 April 2013)
Magnesium porphyrazinate substituted with eight 4-tert-butylphenylthio-groups on peripheral posi-
tions has been synthesized by cyclotetramerization of 1,2-bis(4-t-butylphenylthio)maleonitrile in the
presence of magnesium butanolate. The metal-free derivative was obtained by treatment with triflu-
oroacetic acid and further reaction of this product with vanadyl(IV) sulfate led to the vanadyl por-
phyrazinate. These new compounds have been characterized by elemental analysis, together with
FTIR, and UV–vis spectral data. The magnetic properties of the complex have been investigated
by electron paramagnetic resonance spectroscopy.
Keywords: Porphyrazine; Vanadyl(IV) sulfate; EPR
1. Introduction
Porphyrazines (Pz) belong to a broad class of macroheterocyclic tetrapyrrolic systems, con-
stitutionally tetraaza analogs of porphyrins in which meso nitrogens replace the methine
groups and phthalocyanines (Pc) which contain benzenoid rings fused to the macrocyclic
periphery. Porphyrazines have dominant blue and blue-green pigments due to their intense
absorption at long wavelengths in the visible spectrum, excellent durability, and unique
electronic and optical properties [1–3]. Strong correlation between the nature of the substi-
tuent and the presence of soft S-donors in affecting solid-state interactions of the macrocy-
clic ring system, coupled with relative ease of synthesis, has led to a large series of
derivatives with physical and chemical properties of the porphyrazines comparable to those
of phthalocyanines [4, 5]. In comparison to other family members, porphyrazines maintain
a unique class due to much increased solubility in organic solvents compared with their
phthalocyanine analogs and interactions with alkali or transition metal ions incorporated
into the macrocycle core provides candidates for a series of applications [6–13]. Although
porphyrazines possess numerous potential applications in technology including chemical
sensors, molecular electronics, organic solar cells, photodynamic therapy, organic field
effect transistors, biological agents, and metal-insulator-semiconductor-based devices, the
*Corresponding author. Emails: bahadirkeskin@gmail.com; bkeskin@yildiz.edu.tr
Ó 2013 Taylor & Francis

2616
B. Keskin et al.
electronic and paramagnetic properties of porphyrazine have not been widely studied
[14–19].
Vanadyl phthalocyanine (VOPc), a non-planar metallo-phthalocyanine [20, 21] with
hydrophilic V=O core, high-valence vanadium ions (denoted as V(IV)), like the parent Pc
and many of its metal derivatives, exhibits photoconductive and semiconductive properties
[22]. The V–O axis in VOPc is located perpendicular to the plane of the molecule [23],
leading the vanadyl oxygen to “stick out” of the overall molecular plane [24]. VOPc is
known to be a dye with high thermal and chemical stabilities, large third-order nonlinear
optical susceptibility [25], ultra-fast optical response, and good stability against visible
light irradiation [26]. The coordination chemistry of vanadium originates from its relevance
in several biological systems, with applications of its complexes in medicinal chemistry,
organic synthesis, and catalysis [27]. They are promising candidates for realizing optical
and electronic functional devices; transparent organic thin-film transistors [28]; hole trans-
port layers with increased carrier density and conductivity [29]; and important biological
functions in the case of diabetes with oxovanadium (VO2+) ions [30]. Therefore, electronic
and magnetic properties of these compounds should be sensitive to the nature of substitu-
ent ions and peripheral complexes [15, 31].
The limited solubility of the parent VOPc molecule causes difficulties in purification
and interconversion among the morphological forms. This has been resolved by the addi-
tion of solubilizing groups such as tert-butyl on the periphery [31, 32]. The tert-butyl
substituted phthalocyanine and analogues provide high solubility in organic solvents [32]
and extends the potential applications of these macromolecules, such as good volatility in
vacuum and they were shown to be good candidates for the preparation of thin films by
vacuum thermal evaporation [33]. Thus, in the present article, we report a porphyrazine
core with eight 4-tert-butylbenzyl units attached to the peripheral positions through
methylthio bridges and vanadyl (VO2+) insertion into the inner core resulting in soluble
octakis(4-tert-butylbenzylthio)porphyrazinato vanadyl complex (VOPz). We also reported
an electron paramagnetic resonance (EPR) investigation of the paramagnetic properties of
the compound.
2. Experimental
2.1. Materials and methods
The 1,2-bis-(4-t-butylbenzylthio)maleonitrile (TBBTMNT), octakis(4-tert-butylbenzylthio)
porphyrazinato magnesium(II) (MgPz), and metal-free octakis(4-tert-butylbenzylthio)
21H,23H porphyrazine (H2Pz) were prepared according to previously reported procedures
with minor changes and characterized by comparing their spectral data to those reported
earlier [13]. Mg(II) complex and metal-free porphyrazine were stable at room temperature
and they were non-hygroscopic and insoluble in water but soluble in many common
organic solvents. Reagents: chemicals employed were of the highest grade available.
Unless specified otherwise, reagent grade reactants and solvents were used as received
from chemical suppliers.
FTIR spectra were recorded from 4000 to 400 cmꢀ1 on a Perkin-Elmer Spectrum One
spectrometer using KBr pellets. The electronic spectra and absorbance measurements were
recorded on an Agile 8453 UV–visible spectroscopic system. Proton NMR spectra were
recorded on Bruker 250 and 500 MHz Varian Inova spectrometers. Elemental analyses

Porphyrazine
2617
were recorded on Thermo Flashea 1112 series equipment. The EPR powder spectrum was
recorded with a Bruker EMX X-band spectrometer (9.512 GHz) with about 100.8 mW
microwave power and 100 kHz magnetic field modulation.
2.2. Synthesis of H2Pz
MgPz complex (985 mg, 0.559 mM) was dissolved in minimum CF3COOH and stirred for
6 h at room temperature. The reaction mixture was added into ammonia solution (25%,
7 mL) to neutralize and then cooled at 0 °C. It was treated with chloroform and then the
organic phase was washed first with water, then with a solution of 10% Na2CO3, and then
with water again. The organic layer was dried over anhydrous Na2SO4 and the solvent
was evaporated in vacuum. The oily product was treated with a small amount of diethyl
ether by cooling in an ice-bath to enhance precipitation and was filtered cold. Finally, pure
violet-colored metal-free porphyrazine was obtained after it was dried in vacuum. Product
2 was soluble in chloroform, dichloromethane, dimethylformamide (DMF) and THF.
Yield: 0.341 g (%35). IR (KBr), νmax/cmꢀ1: 3289w (m, N–H), 3024w (m, H–Ar), 2956 m
(C–H in C–(CH3)3), 1513, 1461, 1201, 834, 741, 703. 1H NMR (CDCl3 250 MHz):
d = 7.33–7.17 (m, 4H, Ar–H), 5.18 (s, 2H, CH2–S), 1.17 (s, 9H, (CH3)–C), ꢀ1.55 (br, m,
2H, N–H). UV–vis (in CHCl3) λmax (nm) (log ɛ): 336 (3.95) 652 (3.74) 711 (3.87).
2.3. Synthesis of VOPz
A mixture of H2Pz (0.12 g, 0.07 mM) and VOSO4∙5H2O (0.107 g, 0.8 mM) was refluxed
for 18 h in 15 mL of DMF. After cooling to room temperature, the vanadyl derivative of
porphyrazine was precipitated by the addition of methanol and the solid compound was
isolated by centrifugation. The complex was filtered, washed with methanol, and dried to
get constant weight under vacuum (0.05 g, yield ca. 61%). Product 3 was very soluble
in chloroform and THF. IR (KBr), νmax/cmꢀ1: 3048w (m, H–Ar), 2957 m (C–H in
C–(CH3)3), 1631, 1446, 990 m (V=O). UV–vis (in CHCl3) λmax (nm) (log ɛ): 354 (3.82),
692 (3.51). Calculated for C104H120N8OS8V: C 69.18%, H 6.70%, N 6.21%, S 14.21%.
Anal. Calcd Found: C 68.23%, H 6.52%, N 5.86%, S 12.34%.
3. Results and discussion
3.1. Synthesis
The starting point of this new VOPz structure with eight (4-tert-butylbenzylthio) groups
bound to the periphery is 1,2-bis-(4-t-butylbenzylthio)maleonitrile (TBBTMnt), which was
synthesized as a solid product in relatively high yield according to the previously reported
procedure (scheme 1) [13]. The presence of electron-donating sulfur is expected to shift
the absorption range of the porphyrazine Q-band to higher wavelength and tert-butyl
groups to enhance solubility [6, 12, 13, 34].
The cyclotetramerization process of dinitrile derivative (TBBTMnt) by the template
effect of magnesium butanolate in butanol resulted in blue-green octakis(4-tert-butyl-ben-
zylthio)porphyrazinato magnesium 1 in very good yield (figure 1). The metal-free deriva-
tives of 2 were obtained in a reasonable yield of 30–40% by treatment of 2 with
trifluoroacetic acid at room temperature for 6 h. Change of color from dark blue-green to

2618
B. Keskin et al.
Scheme 1. Synthesis of VOPz; (i) Mg turnings, I2, n-BuOH; (ii) CF3CO2H; (iii) VOSO4·5H2O, reflux.
Figure 1. Schematic representation of porphyrazines.
purplish blue and lower solubility are differences between magnesium and metal-free
derivatives. Porphyrazines having eight 4-tert-butyl-benzylthio groups, VOPz, are similar
to phthalocyanine macrocycles in terms of structure and other important properties like
low solubility, high stability, and characteristic Soret and Q-band absorptions [30]. Inser-
tion of vanadyl into 2 with vanadyl(IV) sulfate was performed in DMF at reflux for 18 h
affording 3 in good yield. Elemental analyses follow the values calculated for 3. Octakis
(4-tert-butyl-benzylthio)porphyrazines and t-Bu8VOPz are very poorly soluble in some
organic solvents, such as hydrocarbons, ethanol, and methanol, but have higher solubility
in chloroform and donor solvents (DMSO, DMF) [33, 35, 36].
3.2. Spectroscopic properties
Spectroscopic investigations on the newly synthesized intermediates and porphyrazines are
in accord with the proposed structures. In the FTIR spectrum of TBBTMNT, the stretch of
C≡N is observed at 2213 cmꢀ1, t-butyl peak at 2978 cmꢀ1, S–CH2 peak at 680 cmꢀ1, and
aromatic C–H peaks at 3028 cmꢀ1. These values comply with those reported in the litera-
ture for similar compounds and with the previous report for TBBTMNT [13]. The sharp
C≡N vibration at 2213 cmꢀ1 disappeared after the formation of porphyrazine 1. The N–H
stretch of the inner core of metal-free porphyrazine 2 was observed at 3289 cmꢀ1 by
demetalation [13, 37]. The FTIR spectrum of VOPz (3) showed a stretch of the t-butyl
peak at 2863–2956 cmꢀ1 and aromatic C–H peaks at 3028 cmꢀ1, which are very similar

Porphyrazine
2619
with the literature (M = Cu, Co, Zn) as expected [12, 13]. The absence of N–H peaks and
the presence of a V=O peak at 990 cmꢀ1 are evidences for the formation of VOPz [31].
In 1H NMR spectra of 1 and 2, chemical shifts corresponding to tert-butyl protons
are the expected singlet at 1.3 ppm in TBBTMNT, 1.14 ppm in 1, and 1.33 ppm in 3.
The inner core N–H protons of the metal-free porphyrazine 2 were also identified in the
1H NMR spectra with a broad peak at ꢀ1.55 ppm presenting the typical shielding of inner
1
core protons, common in H NMR spectra of metal-free porphyrazines [3, 13, 37].
To identify the structure of 1–3, electronic spectra are especially useful. The electronic
absorption spectra of metallo-porphyrazines 1 and 3 exhibit a strong absorption between
675 and 692 nm, which is due to a π → π⁄ transition from HOMO–LUMO energy levels
having D4h molecular symmetry and is commonly referred to as the Q-band. A second
intense and broad π → π⁄ transition in the near UV region (336–378 nm), called the soret
or B-band, is also a characteristic of tetrapyrrole derivatives [3, 12, 13]. UV–vis spectra of
metallo-porphyrazines 1 and 3 (4 ꢁ 10ꢀ5 M solutions in chloroform) prepared in the pres-
ent work exhibited intense single Q-band absorption of the π → π⁄ transition at 675 and
692 nm and B-bands at 378 and 352 nm, respectively (figure 2). The Q-band absorption of
VOPz in chloroform is a broad single peak which indicates axial ligation of chloroform at
the vacant site of vanadium with similar symmetry to the Mg(II) complex [31]. The
change of symmetry of porphyrazine core from D4h for metallo-species to D2h in metal-
free 2 shows a split Q-band at 652 and 711 nm, as expected [4, 13, 38]. UV–vis spectra
were measured at different concentrations to assess whether or not 3 reveals aggregation.
3.3. EPR spectroscopy of VOPz
The powder EPR spectrum of VOPz recorded at room temperature is given in figure 3,
with its computer simulation by using SimFonia program with the experimental values as
input data. Low and high intensity lines with hyperfine splitting have been seen in this
Figure 2. Electronic spectra of octakis(4-tert-butylbenzylthio) substituted porphyrazines 1, 2, and 3 as solutions
in CHCl3.

2620
B. Keskin et al.
Figure 3. The EPR powder spectrum of VOPz at room temperature and its computer simulation.
spectrum. The group with low intensity is the parallel component, because in this case the
external dc field is parallel to the symmetry axis of the crystal field around the paramag-
netic center. The group with high intensity represents a perpendicular component owing to
external dc field, perpendicular to the symmetry axis around the paramagnetic center.
These groups are denoted as g// and g\ in terms of spin-Hamiltonian parameters in figure
3. Each set of eight hyperfine lines can be explained in terms of an unpaired electron
(S = 1/2) interacting with 51V having a nuclear spin I = 7/2. The VO2+ ion consists of a
V4+ in 3d1 configuration. The intensities of the two sets of lines are a measure of popula-
tion of VO2+ ions in the parallel and perpendicular orientations.
The spin-Hamiltonian parameters obtained from the spectra are g// = 1.975 and
g\ = 1.994 with gav = 1.987, where gav = 1/3(g// + 2g\); A// = 171 G and A\ = 61 G with
Aav = 98 G, where Aav = 1/3(A// + 2A\). The g values suggest that local symmetry of the
paramagnetic center is axial. The order of ge > g\ > g// (free electron g value, ge = 2.0023)
reveals a VO2+ in a tetragonally compressed octahedron and the b2 (dxy) level is lowest
[39]. Small variations in the value of g and A indicate sensitivity of VO2+ in different
ligand field environments. Thus, the EPR spectrum of this ion is strongly dependent on
the donors bound to the metal, different for nitrogen and oxygen donors. With more cova-
lent bonding, the g values are higher and hyperfine splitting lowers [40].
VO2+ has very large A// principal value of the 5lV hyperfine splitting constant; A\ prin-
cipal value (= Axx, Ayy) is normally about three times smaller. This originates from a
strong tendency to form a very short V–O bond. Because of this, A// (= Azz) is approxi-
mately aligned along the V–O bond direction which defines the oxygen involved in the
V–O bond.
EPR data can be interpreted in terms of molecular orbitals by using g and A parameters.
The values of A// and A\ for VO2+ can be written as [40]:
2
2
4
7
3
7
A== ¼ ꢀPK ꢀ b P ꢀ ðge ꢀ g==ÞP ꢀ ðge ꢀ g ÞP
ð1Þ
?

Porphyrazine
2621
2
2
2
7
11
14
A ¼ ꢀPK þ b P ꢀ ðge ꢀ g ÞP
ð2Þ
?
?
where P is the direct dipole–dipole interaction constant of the magnetic moment of the
electron with the magnetic moment of vanadium (taken as 13.6 mT) [41], K is the Fermi
contact term and indicates the d-orbital population for unpaired electron, and b2 is in plane
π-bonding coefficient of VO2+ with the ligands. Using the A//, A\ (taking as negative) and
g//, g\ values given for VO2+, we obtain K ¼ 0:703 and b22 ¼ 0:915:
The largest value of b22 is 1 and deviation from this value is a measure of the bonding
of the dxy orbital of VO2+ with the π-orbitals of ligands. The lowering of b22 value arises
from delocalization of electron onto the ligand with increase in covalent bonding. The val-
ues obtained in this study are in agreement with the literature values of VO2+ [30, 42–48].
4. Conclusions
We have described the synthesis and spectral characterization of new vanadyl complex of
porphyrazine with eight tert-butylphenyl groups on the periphery. These compounds have
good solubility in many common organic solvents. The presence of bulky tert-butyl groups
hindered aggregation at higher concentrations (e.g. 10ꢀ3 M).
The g parameters and hyperfine splittings indicate the presence of one paramagnetic axi-
ally symmetric VO2+ coordinated by pyrrolic N with axial symmetry for 3. The values
obtained in this study are in agreement with the literature values for VO2+ complexes of
porphyrinic macrocycles.
Acknowledgment
This work was supported by the Research Fund of the Yildiz Technical University; Project
No. 2012-01-02-GEP08.
References
[1] H.S. Nalwa. Adv. Mater., 5, 341 (1993).
[2] C.C. Leznoff, A.B.P. Lever (Eds). Phthalocyanines: Properties and Applications, Vol. 4, p. 79, VCH Pub-
lishers, Weinheim (1996).
[3] A.E. Pullen, C. Faulman, P. Cassoux. Eur. J. Inorg. Chem., 2, 269 (1999).
[4] N. Kabay, Y. Baygu, H.K. Alpoguz, A. Kaya, Y. Gök. Dyes Pigm., 96, 372 (2013).
[5] D.P. Goldberg, S.L.J. Michel, A.J.P. White, D.J. Williams, A.G.M. Barrett, B.M. Hoffman. Inorg. Chem., 37,
2100 (1998).
[6] M.E. Anderson, A.G.M. Barrett, B.M. Hoffman. Inorg. Chem., 38, 6143 (1999).
[7] E.A. Lukyanets, V.N. Nemykin. J. Porphyrins Phthalocyanines, 14, 1 (2010).
[8] J.W. Buchler, J.R. Simon. Eur. J. Inorg. Chem., 12, 2615 (2000).
[9] E. Gonca, B. Keskin. J. Coord. Chem., 62, 2875 (2009).
[10] H.M. Neu, V.V. Zhdankin, V.N. Nemykin. Tetrahedron Lett., 51, 6545 (2010).
[11] T.F. Baumann, M.S. Nasir, J.W. Sibert, A.J.P. White, M.M. Olmstead, D.J. William, A.G.M. Barrett,
B.M. Hoffman. J. Am. Chem. Soc., 118, 10479 (1996).
[12] C.F. van Nostrum, R.J.M. Nolte. Chem. Commun., 21, 2385 (1996).
[13] B. Keskin, Y. Koseoglu, U. Avciata, A. Gul. Polyhedron, 27, 1155 (2008).
[14] M.S. Rodriguez-Morgade, P.A. Stuzhin. J. Porphyrins Phthalocyanines, 8, 1129 (2004).
[15] C. Zhong, M. Zhao, T. Goslinski, C. Stern, A.G.M. Barrett, B.M. Hoffman. Inorg. Chem., 45, 3983 (2006).

2622
B. Keskin et al.
[16] P.A. Stuzhin, C. Ercolani, K.M. Kadish, K.M. Smith, R. Guilard (Eds.). The Porphyrin Handbook, Vol. 15,
pp. 263–365, Academic Press, New York (2003).
[17] B. Keskin, A. Peksel, U. Avcıata, A. Gul. J. Coord. Chem., 63, 3999 (2010).
[18] B. Keskin, C. Denktas, A. Altındal, U. Avcıata, A. Gul. Polyhedron, 38, 121 (2012).
[19] M. Altunkaya, E. Gonca. Polyhedron, 30, 1035 (2011).
[20] L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds. Adv. Mater., 12, 481 (2000).
[21] M. Berggren, O. Inganas, C. Gustafsson, J. Rasmusson, M.R. Andersson, T. Hjertberg, O. Wennerstrom.
Nature, 372, 444 (1994).
[22] K.-Y. Law. Inorg. Chem., 24, 1778 (1985).
[23] Ł. Łapok, M. Lener, O. Tsaryova, S. Nagel, C. Keil, R. Gerdes, D. Schlettwein, S.M. Gorun. Inorg. Chem.,
50, 4086 (2011).
[24] D.A. Duncan, W. Unterberger, K.A. Hogan, T.J. Lerotholi, C.L.A. Lamont, D.P. Woodruff. Surface Sci., 604,
47 (2010).
[25] T. Someya, A. Dodabalapur, A. Gelperin, H.E. Katz, Z. Bao. Langmuir, 18, 5299 (2002).
[26] L. Guo, G. Ma, Y. Liu, J. Mi, S. Qian, L. Qiu. Appl. Phys. B, 74, 253 (2002).
[27] K. Nagaraju, A. Sarkar, S. Pal. J. Coord. Chem., 66, 77 (2013).
[28] C. Li, F. Pan, F. Zhu, D. Song, H. Wang, D. Yan. Semicond. Sci. Technol., 24, 085009 (2009).
[29] R. Ozturk, S. Guner, B. Aktas, A. Gul. Supramol. Chem., 17, 233 (2005).
[30] H. Wang, D. Song, J. Yang, B. Yu, Y. Geng, D. Yan. Appl. Phys. Lett., 90, 253510 (2007).
[31] H. Onay, Y. Yerli, R. Ozturk. Transition Met. Chem., 34, 163 (2009).
[32] K.-Y. Law. Chem. Rev., 93, 449 (1993).
[33] V. Plyashkevich, T. Basova, P. Semyannikov, A. Hassan. Thermochim. Acta, 501, 108 (2010).
[34] X.L. Liang, D.E. Ellis, O.V. Gubanova, B.M. Hoffman, R.L. Musselman. Int. J. Quantum Chem., 52, 657
(1994).
[35] S. Angeloni, E.M. Bauer, C. Ercolani, I.A. Popkova, P.A. Stuzhin. J. Porphyrins Phthalocyanines, 5, 881
(2001).
[36] M.P. Donzello, C. Ercolani, P.A. Stuzhin. Coord. Chem. Rev., 250, 1530 (2006).
[37] D.F. Dye, K. Raghavachari, J.M. Zaleski. Inorg. Chim. Acta, 361, 1177 (2008).
[38] W.S. Szulbinski, J.R. Kincaid. Inorg. Chem., 37, 5014 (1998).
[39] E. Samuel, G. Labauze, D. Vivien. J. Chem. Soc., Dalton Trans., 956 (1979).
[40] L.J. Boucher, E.C. Tynan, T.F. Yen, In Electron Spin Resonance of Metal Complexes, T.F. Yen (Ed.), New
York, Plenum Press, 111 (1969).
[41] B.R. McGarvey. J. Phys. Chem., 71, 51 (1967).
[42] Y. Yerli, A. Zerentürk, K. Özdoğan. Spectrochim. Acta, Part A, 68, 147 (2007).
[43] Y. Yerli, F. Köksal, A. Karadag. Solid State Sci., 5, 1319 (2003).
[44] M. Narayana, S.G. Satyanarayana, G.S. Sastry. Mol. Phys., 31, 203 (1976).
[45] K.S. Mısra, J. Sun, U. Orhun. Phys. Status Solidi B, 162, 585 (1990).
[46] S.V.J. Lakshaman, A.S. Jacop. J. Solid State Commun., 45, 141 (1983).
[47] K.V.S. Rao, M.D. Sastry, P. Venkateswarlu. J. Chem. Phys., 49, 4984 (1968).
[48] B.G. Umesh, M.A. Satish, D.N. Anil, K.R. Vidyanad, B.M. Vinayak. J. Mol. Struct. Theochem., 572, 61
(2001).