Stability of nonplanar N-methylporphyrins and their zinc complexes

Article abstract of DOI:10.1134/S1070363206030200

Resistance of N-methyl-substituted porphyrins and their Zn(II) complexes to thermooxidative degradation and to the action of a DMSO-AcOH proton-donor solvent was studied by thermogravimetry and chemical kinetics. The fairly low stability of the complexes is caused by decreased planarity and, as a consequence, aromaticity of the macrorings and also by the tendency of the complexes to demethylation. The stability of the zinc complexes in AcOH varies in parallel with the degree of nonplanarity and resistance to thermooxidative degradation of their ligands. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070363206030200

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 3, pp. 482 487.
Pleiades Publishing, Inc., 2006.
Original Russian Text
pp. 506 512.
D.B. Berezin, E.N. Mis’ko, E.V. Antina, M.B. Berezin, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 3,
Stability of Nonplanar N-Methylporphyrins
and Their Zinc Complexes
,
D. B. Berezin* **, E. N. Mis’ko**, E. V. Antina**, and M. B. Berezin**
* Ivanovo State University of Chemical Technology, pr. F. Engel’sa 7, Ivanovo, 152000 Russia
** Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received April 21, 2005
Abstract Resistance of N-methyl-substituted porphyrins and their Zn(II) complexes to thermooxidative
degradation and to the action of a DMSO AcOH proton-donor solvent was studied by thermogravimetry and
chemical kinetics. The fairly low stability of the complexes is caused by decreased planarity and, as a
consequence, aromaticity of the macrorings and also by the tendency of the complexes to demethylation. The
stability of the zinc complexes in AcOH varies in parallel with the degree of nonplanarity and resistance to
thermooxidative degradation of their ligands.
DOI: 10.1134/S1070363206030200
Porphyrins (H2P), such as compounds I V, are
most commonly applied in engineering, technology,
and medicine in the form of metal complexes VI IX,
many of which are used for constructing supramole-
cular nanostructures, are perspective catalysts and
photosensitizers, pigments, and materials for nonlinear
optics [1]. Therefore, to assess resistance porphyrin
metal complexes (MP) to various external factors
(thermal, photochemical, and chemical reagents and
solvents) is an important criterion of their applicability.
R2
R2
R
R
X
X
X
R
R
R1
R
R1
X
R
X
R
R
R
X
R
N R3 N
N Me
Zn
N
N
N
N
N
X
X
X
R
Zn
N
N
N
HN
R1
R1
R
Ac
X
R2
R2
R
R
R
R
X
X
I V
VI, VII
VIII, IX
R1 = Me, R2 = Bu, R3 = X = H (I); R1 = R2 = Et, R3 = X = H (II); R1 = R2 = R3 = H, X = Ph (III); R1 = R2 = Et, R3 =
Me, X = H (IV); R1 = R2 = H, R3 = Me, X = Ph (V); R = Et, X = H (VI); R = H, X = Ph (IX).
Porphyrin-like molecules are divided into por-
phyrins and their analogs [2]. N-Substituted porphy-
rins, for example, monosubstituted compounds
H(NR)P IV and V can only arbitrarily be placed into
the first group. Characteristic features of H2P are the
presence of an N4H2 reaction center and the porphin-
like structure of the main -conjugation contour [3].
Replacement of the hydrogen atom of one endocyclic
NH group in H2P by a hydrocarbon fragment, such as
methyl, changes the structure of the reaction center of
the ligands from N2(NH)2 (compounds I III) to
N2(NH)(NR) (compounds IV and V) and of com-
plexes with divalent metal ions, from MN4 (com-
pounds VI and VII) to (X)MN3(NMe) (compounds
VIII and IX), where X is covalently bound counterion.
The conjugated porphyrin
system in N-methyl-
substituted ligands H(NMe)P is preserved (compounds
I V), but it changes strongly in going to N-substituted
complexes (X)M(NMe)P [4, 5] (compounds VI IX).
482

STABILITY OF NONPLANAR N-METHYLPORPHYRINS
483
N-Substitution in H2P much redistributes the elec-
tron density in the parent molecule both due to elec-
tronic effects of the additional functional group (R3)
and due to disturbance of the planar macroring struc-
ture with the N-substituted fragment drawn strongly
out of the plain. The degree of nonplanarity of the
molecule depends on the size of the N-substituent and
on the rigidity (aromaticity) of the starting unsub-
stituted porphyrin [4].
Table 1. Thermooxidative degradation of porphyrins I and
III V and their zinc(II) complexes VII IXa
Comp. no. Tb, C Tm,
C
Tt,
C
t, %
e, %
Ib
400
407
310
260
340
212
230
530
478
478
625
520
534
476
620
IIIb
IV
530
600
682
540
592
598
V
VIIb
VIII
IX
Such changes in molecular structure essentially
affect resistance of N-substituted ligands and their
complexes to various physical factors and chemical
reagents. Thus it is known that complexes with
N-substituted ligands are unstable, and even the most
stable of them undergo dissociation [Eq. (1)] even at
pH > 3 [4].
12.11 11.92
10.82 10.5
a
Processes studied: H P
N, C, and H oxides and MPC
2
ZnO + N, C, and H oxides. T is the initial process tempera-
b
ture; T , temperature of the maximal exo effect; T , final
m
t
process temperature; and
mental weights of ZnO.
and , theoretical and experi-
Data of [7].
t
t
b
(AcO)Zn(NMe)P + AcOH
H(NMe)P + Zn(OAc)2. (1)
Table 2. Kinetic parameters of reaction (1) of dissociation
of zinc(II) N-methylporphyrin complexes VIII and IX and
zinc tetraphenylporphine (VII) in DMSO AcOH (cMP
In the medium of organic bases (amines), com-
plexes of N-substituted porphyrins enter nucleophilic
substitution reactions [Eq. (2)] and lose the N-sub-
stituent, whereas their ligands are more stable in
dealkylation (dearylation) reactions [6]. Quantitative
data on the resistance of these compounds to various
factors are absent from the literature, and the me-
chanisms of their dissociation (degradation) are also
unknown.
5
6.7 10 M, 298 K)
cAcOH
M
,
kapp 103,
Ea,
kJmol
S
1
1
1
s
Jmol 1 K
Compound VIIa
0.26 0.02 46.1 2.0
Compound VIII
17.5
155 8
13 4
40 16
18 7
(X)M(NR)P + RNH2
MP + [R2NH2]+X . (2)
17.5
16.2
12.8
1.87 0.09
85.8 7.4
102.8 7.6
92.5 17.7
0.97 0.06
0.11 0.01
The promise N-substituted porphyrins capable of
stabilizing unusual oxidation states of metal ions
[Fe(II), Co(II), etc.] hold in catalysis [4] makes it
urgent to study their behavior in aggressive media and
in polythermal conditions. To reveal reasons for the
low stability of N-substituted porphyrins and their
complexes and to establish certain regularities of the
stability decrease, we studied resistance of zinc(II)
complexes VIII and IX with N-substituted ligands,
specifically N-methyloctaethylporphine (IV) and N-
methyltetraphenylporphine (V), to thermooxidative
degradation in the solid phase and to dissociation in
solutions containing a proton-donor component, as
well as resistance of ligands IV and V to thermooxi-
dation. The study was carried out by thermogravi-
metry and chemical kinetics.
Compound IX
17.5
16.2
14.1
10.9
26.65 2.03
34.18 0.32
47.36 3.72
30.88 3.11
9.19 0.97
27.9 6.3
47.8 11.2
66.0 15.1
72.8 14.3
38.4 7.0
189 30
120 35
57 10
38 9
9.43
163 24
a
Data of [3].
oxidative degradation of N-substituted compounds IV
and VIII are given in Fig. 1. The kinetic parameters
of reaction (1) involving zinc complexes of N-sub-
stituted porphyrins VIII and IX in the DMSO AcOH
medium are shown in Table 2. The concentration
dependences of the apparent rate constants of reaction
(1) are presented in Fig. 2.
Table 1 presents the results of studying thermo-
oxidative degradation of compounds IV, V, VIII, and
IX together with published data [7] for their structural
analogs, tetraphenylporphine (III), its complex with
Zn(II) (VII), and 2,3,6,7-tetrabutyl-1,4,5,8-tetrame-
thylporphine (I) as a model -octaalkyl-substituted
compound. The thermoanalytical curves for thermo-
One more goal of the present work was to correlate
the degree of nonplanarity of metal porphyrins or their
ligands and resistance of the complexes to dissocia-
tion in acid medium and to thermooxidative degrada-
tion.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 3 2006

484
BEREZIN et al.
(a)
(b)
TG
TG
800
600
DTG
75
50
25
800
600
DTG
75
50
25
400
200
400
200
DTA
DTA
T
T
20 40 60 80 100
20 40 60 80 100
, min
Fig. 1. Thermoanalytical curves of (a) N-methyloctaethylporphine (IV) and (b) its zinc complex (VIII).
1
1
kapp 103, s
kapp 103, s
(a)
(b)
5
9
40
4
3
6
3
0
2
1
20
0
3
2
12 13 14 15 16 17
9
11
13
15
17
cAcOH, M
Fig. 2. Dependences of apparent dissociation rate constants (k ) of complexes (a) VIII and (b) IX on AcOH concentration
app
in DMSO. T, K: (1) 293, (2) 298, (3) 303, (4) 308, and (5) 313.
According to the general regularities of thermo-
by a loss of weight ( , %) and small exothermic peaks
on the DTA curve (Fig. 1). Within the experimental
error, the weight loss in the first stage numerically
corresponds to the weight loss due to cleavage and
removal of CH3 group from the sample [theoretical
oxidative degradation of porphyrins and their com-
plexes, established in [7], the process is characterized
by several stages including oxidation and macroring
opening, burning, and complete weight loss due to
the formation of gaseous nitrogen, carbon, and hydro-
gen oxides (Table 1 and Fig. 1).
( ) and experimental ( ) values fit each other and
e
eqtual 7.85, 8.78 and 7.6, 8.9% for complexes VIII
and IX, respectively]. The weight loss in the second
stage results from removal of the axial acido ligand
As compared to classical MP [7], the degradation
scheme of N-substituted metal porphyrins VIII and
IX have certain specific features. The thermooxidative
degradation of Zn(II) complexes of N-methylocta-
ethylporphine (VIII) and N-methyltetraphenylporphin
(IX) begins with two consecutive stages accompanied
AcO- ( and
are 2.0, 2.4 and 2.1, 2.31% for com-
t
e
pounds VIII and IX, respectively). Thus, the degrada-
tion of the zinc porphyrins studied begins with
cleavage of the N-methyl group from the macroring
and oxidation of this group by air oxygen, which is
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 3 2006

STABILITY OF NONPLANAR N-METHYLPORPHYRINS
485
favored by the more pronounced out-of-plane location
of this group in complexes as compared with starting
ligands [4] and also by the unstable hybrid state of
the substituted nitrogen atom [5, 8], leading to a
strained conformation of compounds VIII and IX.
The d metal in the N-substituted molecule catalyzes
cleavage of the N-substituent both in solution [6] and
in solid. The subsequent intramolecular rearrangement
in thermally demethylated compounds results in
formation of practically planar complexes of Zn octa-
ethylporphine (VI) and Zn tetraphenylporphine (VII).
In the second stage of the process, the acetate residue
is removed from the complex immediately after
methyl removal (Fig. 1). Thus, the chemical trans-
formations that initiate thermooxidation of complexes
with N-substituted porphyrins can schematically
presented by the overall reaction scheme (3).
more stable than compounds VIII and IX and un-
dergo oxidative degradation without preliminary
cleavage of the CH3 group (Fig. 1). The lower re-
sistance of MP to thermooxidative degradation results
from the presence of active oxidation, viz. metal
atoms, in the complexes. In the case of nonplanar MP,
the steric structure of the ligand makes metal free
valences difficult to saturate, thus ensuring an ap-
preciable effective charge on this atom. Furthermore,
the tendency of N-methylated complexes to demethyla-
tion reactions (2) at elevated temperatures, too, makes
them less stable thermally.
Methylation of the NH group of the N4H2 reaction
center essentially (by 90 and 147 C, respectively)
decreases the initial thermooxidation temperature of
the N-methyloctaethylporphine (IV) and N-methyl-
tetraphenylporphine (V) ligands compared to their
unsubstituted analogs I and III (Table 1) [7]. This is
obviously connected with the less effective conjuga-
tion in and decreased aromaticity of nonplanar macro-
rings. The most distorted ligand V is least resistant to
to thermooxidative degradation. Earlier we showed,
by fluorescence spectroscopy and molecular me-
chanics [9], that compound IV is less planar than
unsubstituted compound II but much more planar than
ligand V. By resistance to thermooxidative degrada-
tion the compounds studied can be arranged in the
(AcO)Zn(NMe)P + 1/2O2
ZnP + CH2O + AcOH. (3)
Oxidation and degradation of the chromophore
molecule begin immediately after the oxidized
N-substituent and the axial ligand have been removed,
and these processes are characterized by exothermic
peaks in the DTA curve (Fig. 1), the number of peaks
varying from one to two depending on the nature of
the porphyrin ligand. It is necessary to note that under
these conditions MP bearing no endocyclic substi-
tuents, such as compound VII, do not undergo
thermooxidation (Table 1). Apparently, the sub-
sequent oxidation of N-substituted complexes
(AcO)Zn(NMe)P is connected with the fact that the
demethylated nitrogen atom has no time to form the
stable sp2 state.
analogous series I
III > IV > V.
To assess the effect of the nonplanar structure of
metal porphyrins on their resistance to protondonor
reagents, we studied the dissociation kinetics of
zinc(II) complexes with N-substituted compounds
VIII and IX in the DMSO AcOH medium and in
glacial AcOH.
Oxidation of Zn(II) metal porphyrins is completed
by formation of zinc(II) oxide. As seen from Table 1,
the experimental and calculated weights of the ZnO
formed nicely fit each other within the experimental
error, which confirms the composition of the
complexes once more.
It was shown that the complexes with nonplanar
ligands are less resistant or dissociation in an ionizing
medium than analogous complexes of tetraphenyl-
porphine (VII) and other planar classical H2P [3].
This suggests a negative effect of the nonplanar
structure of the macroring and factors favoring distor-
tion on the stability of its complex compounds. Thus,
the apparent dissociation rate constant of complex IX
in glacial acetic acid at 298 K (Table 2) is higher by a
factor of 15 than that of compound VIII and by two
orders of magnitude than that of unsubstituted com-
plex VII.
Comparison of the thermooxidation data for com-
pounds VII and IX (Table 1) shows that methylation
of the central nitrogen atom essentially (by 110 C)
decreases the initial degradation temperature (Ti) of
the porphyrin complexes. Compound VIII containing
ligand IV that is less inclined to deformations than
ligand V loses the N-substituent easier than complex
IX, since this process is accompanied by transition of
the molecule from the strained nonplanar conforma-
tion into a planar one.
Thus, the kinetic stability of the complexes in
glacial AcOH decreases with increasing conforma-
tional mobility of their ligands [4, 9] and follows the
series VII > VIII > IX. This series parallels the series
of resistance of the corresponding ligands to thermo-
oxidative degradation (Table 1).
The thermal stability series for porphyrin ligands
III > IV > V somewhat differs from the correspond-
ing series for the complexes VII > IX > VIII.
Furthermore, compounds IV and V are thermally
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 3 2006

486
BEREZIN et al.
The dissociation of the metal porphyrins in the
such as AcOH. However, it is not excluded that reac-
tion (1) in DMSO AcOH takes several pathways,
including that under the action of undissociated AcOH,
which is responsible for the complicated nature of the
concentration dependence of kapp (Fig. 2b).
DMSO AcOH medium is described by two types of
dependences of the apparent rate constants of reaction
(1) (k2ap9p8) on AcOH concentration (cAcOH) (Fig. 2,
Table 2). The dissociation rate of compound VIII
steadily decreases with decreasing AcOH concentra-
According to [13], the dependence of the autopro-
tolysis constant Kap on the composition of the
DMSO AcOH solvent is also complicated. The Kap
value decreases at low concentrations (up to 4.5 M) of
each of the solvent components and reaches its
maximum in the range 9 15 M AcOH. High Kap
values favor formation of species like AcOH H+ and
DMSO H+ participating in solvoprotolytic dissocia-
tion [3] of porphyrin complexes [reaction (5)].
tion in DMSO (Fig. 2a), whereas the kapp = f(cAcOH
)
dependence for the less stable complex IX is more
complicated and has a maximum in the region of
14 M AcOH (Fig. 2b). Dependences of the latter
type have never been observed for dissociation of
metal porphyrins.
The presence of different kapp = f(cAcOH) depen-
dences points to different mechanisms of reaction (1)
for these MP, involving different attacking particles.
A diversity of attacking particles (DMSO H+, AcOH
H+, AcOH, etc.) is possible in DMSO AcOH, but the
absence of data on their equilibrium concentrations
does not allow us to write complete kinetic equations
and to calculate true rate constants of reaction (1) in
this medium. The complication of the concentration
dependence can be caused by a change of the attack-
ing particle on the change of solvent composition or
by simultaneous participation of several particles.
2X
MP + 2Hs+olv
H2P + MX2.
(5)
It should be noted that the maximum on the con-
centration dependence of the dissociation rate constant
of complex IX (Fig. 2) falls within the range of
DMSO AcOH compositions where Kap is maximal
[13]. Apparently, in this range of solvent composi-
tions a near-optimal combination of the activity of
attacking particles and the permittivity and ionizing
power of the medium is reached. Variation of the
DMSO AcOH solvent properties produces the stron-
gest effect with nonplanar complex IX and has almost
no effect with compound (VIII).
According to [10], complexes like (X)Zn(NMe)
TPP in the MeOH HCl system dissociate under the
action of one proton (participates in the limiting
stage), as opposed to two protons in the case of metal
porphyrins without endocyclic substituents. It was
shown in [10] that the reaction rate does not depend
on water additions up to the concentration of 0.2 M,
but it is sensitive to the presence of salt background
(NaCl and [(Et)4N]Cl) in the system. In the latter case,
the reaction rate increases by 20%. We can suggest
that the dissociation mechanism of complexes
(X)M(NMe)TPP in the MeOH HCl medium is acido-
protolytic in part, i.e. the complex dissociates under
the action of an undissociated acid molecule. Earlier
the acido-protolytic mechanism was suggested and
justified for the dissociation of Mg(II) azoporphine
complexes [Eq. (4)] in C6H6 AcOH and EtOH AcOH
[11, 12]. One of the necessary conditions for this
mechanism operate is the presence of a high effective
positive charge on the metal atom and of an effective
negative charge on any nucleophilic center in MP.
Thus, in our opinion, one of the major reasons for
the complication of the concentration dependence
kapp = f(cAcOH) for nonplanar compound IX is
admixing acidoprotolytic dissociation under the
action of undissociated AcOH. However, the acido-
protolytic dissociation mechanism (4) cannot fully
realize in the case of N-substituted MP. It is favored
by the availability of the basic N-substituted atom to
protonation, but conditions for further effective extra
coordination of the anion on the metal atom and
polarization of the particle are almost absent [11].
Thus, we showed that the disturbance of the planar
structure of the ligands in Zn(II) porphyrin complexes
has a negative effect both on the kinetic stability of
the complexes in proton-containing media and on
their resistance to thermooxidative degradation.
Removal of the N-substituent from N-substituted
porphyrins takes place not only in solution under
action of organic bases, but also in the solid phase at
elevated temperatures in the presence of air oxygen.
HX
HX
(4)
MP
(X)MPH+
H2P + MX2.
The fact that Zn(II) complexes with N-substituted
porphyrins partially meet the above conditions may
promote (in a certain ionizing medium) their acido-
protolytic dissociation. However, it is improbable that
the acidoprotolytic mechanism of reaction (1) is
realized in the medium of an electron-donor solvent,
EXPERIMENTAL
N-Substituted porphyrins and their zinc(II) com-
plexes were prepared, purified, and identified accord-
ing to the procedures [4, 14]. To remove traces of
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 3 2006

STABILITY OF NONPLANAR N-METHYLPORPHYRINS
487
moisture and organic solvents, the samples were ex-
posed to a vacuum at 80 C for 10 h before use. Pure
grade solvents (DMSO and AcOH) were dried and
additionally purified as recommended in [15].
3. Berezin, B.D., Koordinatsionnye soedineniya porfi-
rinov i ftalotsianinov (Coordination Compounds of
Porphyrins and Phthalocyanines), Moscow: Nauka,
1978.
Thermogravimatric studies were performed on an
MOM-1000D derivatograph (Hungary). A crystalline
sample of 40 50 mg was placed in a platinum
crucible and heated in a static oxygen atmosphere at a
4. Lavallee, D.K., Chemistry and Biochemistry of
N-Substituted Porphyrins, New York: VHC, 1987.
5. Berezin, D.B., Zharnikova, N.V., Andrianov, V.G.,
Shatunov, P.A., and Semeikin, A.S., Koord. Khim.,
2002, vol. 28, no. 5, p. 348.
1
rate of 5 deg min in the range of 15 1000 C. The
sensitivity of the galvanometer for DTG was 1 mV
and for DT, 250 mV, the sensitivity of the balances
was 100 or 50 mg per 250 mm of the instrument scale.
The error of measuring temperature was 0.5 C, and
the error of measuring sample weights was no less
than 0.2 mg.
6. Shauer, C.K., Anderson, O.P., Lavallee, D.K., Bat-
tioni, J.-P., and Mansuy, D., J. Am. Chem. Soc., 1987,
vol. 109, no. 13, p. 3922.
7. Antina E.V., Barannikov, V.P., Berezin, M.B., and
V’yugin, A.I., Problemy khimii rastvorov i tekhno-
logii zhidkofaznykh materialov (Problems of Che-
mistry of Solutions and Technology of Liquid-Phase
Materials), Ivanovo: Inst. Khimii Rastvorov Ross.
Akad. Nauk, 2001, p. 217.
Kinetic measurements in the course of dissociation
reaction (1) in DMSO AcOH or glacial AcOH were
carried out on an SF-46 spectrophotometer by follow-
ing the optical density at the experimental wavelength.
The ln(c0MP/cMP) = f( ) dependence for reaction (1) is
rectilinear [Eq. (6)], which suggests that the reaction
is first-order in metal complex. In Eq. (6), kapp is the
apparent rate constant of reaction (1). The kinetic
parameters (Ea and S#) of reaction (1) were
estimated using standard equations of formal kinetics.
8. Aizawa, S.-I., Tsuda, Y., Ito, Y., Hatano, K., and
Funahashi, S., Inorg. Chem., 1993, vol. 32, no. 7,
p. 1119.
9. Sazanovich, I.V., van Hoek, A., Panarin, A.Yu.,
Bolotin, V.L., Semeykin, A.S., Berezin, D.B., and
Chirvony, V.S., J. Porphyrins Phthalocyanines, 2005,
vol. 9, no. 1, p. 59.
dcMP/d = kapp MP
c
.
(6)
10. Shears, B., Shah, B., and Hambright, P., J. Am. Chem.
Soc., 1971, vol. 93, no. 3, p. 776.
ACKNOWLEDGMENTS
The authors express sincere gratitude to
A.S. Semeikin and P.A. Shatunov (Ivanovo State
University of Chemical Technology) for kindly
provided objects of the study. This work was sup-
ported by the Foundation for Support of Domestic
Science.
11. Berezin, B.D., Shukhto, O.V., and Berezin, D.B.,
Zh. Neorg. Khim., 2002, vol. 47, no. 11, p. 1914.
12. Berezin, D.B. and Shukhto, O.V., Izv. Vyssh. Uchebn.
Zaved., Khim. Khim. Tekhnol., 2003, vol. 46, no. 8,
p. 34.
13. Fialkov, Yu.A., Rastvoritel’ kak sredstvo upravleniya
khimicheskim protsessom (Solvent as a Tool for
Controlling Chemical Process), Leningrad: Khimiya,
1990.
REFERENCES
1. The Porphyrin Handbook, Kadish, K.M., Smith, K.M.,
and Guilard, R., Eds., San Diego: Academic, 2000,
vol. 6.
2. The Porphyrin Handbook, Kadish, K.M., Smith, K.M.,
and Guilard, R., Eds., San Diego: Academic, 2000,
vols. 1 10.
14. Latos-Grazynski, L., Inorg. Chem., 1985, vol. 24,
no. 20, p. 1681.
15. Gordon, A.J. and Ford, R.A., The Chemist’s Com-
panion, New York: Wiley, 1972.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 3 2006