Cobalt(II) and Nickel(II) Compounds with 2-Thiobarbituric Acid

Article abstract of DOI:10.1134/S0036023620080082

Abstract: Interaction between the systems containing cobalt(II) or nickel(II) chlorides and 2-thiobarbituric acid (C₄H₄N₂O₂S, H₂L) was examined by spectrophotometry and pH-metry at pH < 4, I = 0.1 (NaCl) and T = 20°C. The compositions and the overall stability constants (log β₁₁) of protonated complex species were determined: [CoHL]⁺ (3.69 ± 0.04), [NiHL]⁺ (3.68 ± 0.08), and [M(HL)₂]. The compositions of the Co(C₄H₃N₂O₂S)₂ · 4H₂O and Ni(C₄H₃N₂O₂S)₂ · 5H₂O salts isolated from the studied systems were confirmed by different physicochemical methods; their solubility constants (log K_S) were calculated to be –12.82 ± 0.22 and –12.30 ± 0.10, respectively. A spectrophotometric technique was developed for determining thiobarbiturate ion in aqueous solutions. This technique is based on the absorbance of iron(III) thiobarbiturate complex [FeL]⁺ and can be recommended for analytical chemists and pharmacists.

Full text of DOI:10.1134/S0036023620080082

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2020, Vol. 65, No. 8, pp. 1232–1239. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Neorganicheskoi Khimii, 2020, Vol. 65, No. 8, pp. 1105–1113.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Cobalt(II) and Nickel(II) Compounds with 2-Thiobarbituric Acid
a,
a
A. P. Lakeev * and N. M. Korotchenko
a
National Research Tomsk State University, Tomsk, 634050 Russia
*e-mail: lakeevs@mail.ru
Received February 3, 2020; revised April 5, 2020; accepted April 6, 2020
Abstract—Interaction between the systems containing cobalt(II) or nickel(II) chlorides and 2-thiobarbituric
acid (C H N O S, H L) was examined by spectrophotometry and pH-metry at pH < 4, I = 0.1 (NaCl) and
4
4
2
2
2
T = 20°C. The compositions and the overall stability constants (logβ ) of protonated complex species were
1
1
+
+
determined: [CoHL] (3.69 ± 0.04), [NiHL] (3.68 ± 0.08), and [M(HL) ]. The compositions of the
2
Co(C H N O S) · 4H O and Ni(C H N O S) · 5H O salts isolated from the studied systems were con-
4
3
2
2
2
2
4
3
2
2
2
2
firmed by different physicochemical methods; their solubility constants (logK ) were calculated to be –12.82 ±
S
0
.22 and –12.30 ± 0.10, respectively. A spectrophotometric technique was developed for determining thio-
barbiturate ion in aqueous solutions. This technique is based on the absorbance of iron(III) thiobarbiturate
+
complex [FeL] and can be recommended for analytical chemists and pharmacists.
Keywords: thiobarbituric acid, cobalt, nickel, complex compounds
DOI: 10.1134/S0036023620080082
INTRODUCTION
derivatives of arylidene-anthranilic acid possess anti-
microbial activity [10].
Thiobarbituric acid (C H N O S, H thioBar, H L,
4
4
2
2
2
2
Malonylthiourea can occur in a large number of
keto-enol or lactam-lactim tautomeric forms. The iso-
meric forms are so diverse, since a thiobarbituric acid
molecule contains three mobile hydrogen atoms (one
2
-thiobarbituric acid, or malonylthiourea, a derivative
of barbituric acid) is a parent compound giving rise to
the thiobarbiturate class of drugs, which exhibit pro-
nounced sedating, anesthetic, anticonvulsant, antiscle-
rotic, and bacteriostatic properties. The best-known of
them are sodium thiopental, thiobarbital, thiobut-
abarbital, thialbarbital, and methitural [1, 2]. Thio-
barbituric acid is widely used for determining malonic
dialdehyde [3], a product of lipid peroxidation, in
experimental and clinical pharmacology. The
researchers from the Laboratory of Biochemistry and
Immunology of the Pitié-Salpêtrière Hospital (Paris,
France) proposed a procedure for the colorimetric
determination of phosphohexoisomerase with thio-
barbituric acid to quantify the amount of fructose
formed in enzyme-catalyzed processes [4]. Due to
atom in the methylene group >CH and two atoms in
2
the imide groups >NH) and three potentially enoliz-
able groups (two carbonyl >C=O and one thiocar-
bonyl group >C=S). However, equilibrium is prefer-
entially attained between the thiocarbonyl-dicarbonyl
(
I) and thiocarbonyl-monocarbonyl (II) tautomers in
aqueous solutions [5]:
H
O
H
OH
N C
N C
1 6
H
H
1 6
3 4
S
C2
5C
S
C2
5CH
3
4
N C
N C
specific colorimetric reactions of H thioBar, it can be
H
O
H
O
2
used as an analytical reagent to detect p- and d-metal
I
II
3+
2+
2+
3+
3+
ions (Bi , Cu , Pd , Fe , Ru , etc.) [5]. Thiobar-
biturate complexes of rare-earth elements are also
Due to the presence of electron-donating nitrogen,
used as precursors to synthesize the respective oxides, oxygen, and sulfur atoms within the aforelisted func-
sulfides, oxysulfides, and oxysulfates [6–8], which are tional groups, the acid can act as a mono- or bidentate
broadly employed in electrical engineering and mate- ligand during complexation. The acidic properties,
rials science. In addition, the complexes containing which are stronger than those of barbituric and violu-
thiobarbiturate anions as ligands possess biological ric acids (for the first dissociation step) [11–13], are
and pharmacological properties. The tin(IV) complex ensured by hydrogen atoms in the hydroxyl –OH (with
was found to exhibit antibacterial and anticancer allowance for the potential enolization of the >C=O
activities [9], while mixed-ligand salts of cobalt(II), group) and imide >NH groups [5, 14] (species I and
nickel(II), and copper(II) with malonylthiourea and II, positions 1 and 6).
1232

COBALT(II) AND NICKEL(II) COMPOUNDS WITH 2-THIOBARBITURIC ACID
1233
Singh et al. [15] studied the thermodynamics of The aim of the present study is to determine the
stepwise complexation of thiobarbituric acid with conditions under which protonated complex species
2
+
2+
2+
2+
2+
2+
Cu , Fe , Zn , Co , and Ni ions giving rise to are formed between 2-thiobarbituric acid and Co
0
2–
7
2+
8
the [ML] and [ML ] species by pH-metry at con- (d , 0.089 nm) or Ni cations (d , 0.083 nm) in an
2
stant ionic strength I = 0.1 (NaClO ) at 18, 31 and aqueous solution, to determine their compositions
4
and stability constants, to isolate salts from the respec-
tive systems and perform their physicochemical analy-
sis. These study objects were selected due to the bio-
logical importance of both the cations (biometals
belonging to essential micronutrient elements) and
thiobarbituric acid, which is an exogenous bioligand.
Furthermore, the electrostatic parameters of these
ions are close, so they form compounds with similar
chemical properties.
4
(
2°C. An analysis of the thermodynamic parameters
ΔG°, ΔH°, and ΔS°; 31°C) shows that enthalpy makes
a major contribution in these reactions. The negative
ΔG° values indicate that the reactions are sponta-
neous, while the relatively low ΔS° values along with
further reasoning have led to a conclusion that in this
2+
case M and H thioBar act as soft electron acceptors
2
and donors, respectively. This is indirectly confirmed
by participation of sulfur, a “soft” electron-donating
atom, in ligand coordination [5]. The logarithms of
2+
2+
stepwise stability constants (æ ) for Co and Ni
i
EXPERIMENTAL
(
18°C) are 6.82 and 5.30; 6.22 and 4.45, respectively.
Reagents and equipment. 2-Thiobarbituric acid of
pure for analysis grade (manufactured in Russia) and
cobalt(II) and nickel(II) chloride hexahydrates of
chemically pure grade were used as initial reagents
without additional purification. Constant ionic
strength I = 0.1 (ensured by sodium chloride of chem-
ically pure grade) and temperature of 20°C were main-
tained in the working solutions. Potentiometric titra-
tion with an alkali solution was used to refine acid con-
centration; concentrations of respective metal salts
were refined by direct trilonometric titration using the
known procedures [22].
Spectrophotometric measurements in the UV and
visible regions were conducted on a PE-5400UV spec-
trophotometer (Ecros) in 10-mm thick quartz cells.
pH was measured using a pH meter 673 in a concen-
tration scale; for this purpose, a glass electrode was
pre-calibrated with respect to (H,Na)Cl solutions
having different concentrations of hydrochloric acid at
a constant ionic strength I = 0.1. Simultaneous ther-
Ahmadi et al. [16] reported on the overall stability
2+
2+
constants (β ) of the complexes formed by Cu , Zn ,
2
2+
2+
Co , and Ni ions with malonylthiourea, which were
measured by spectrophotometric titration at pH 6,
ionic strength of 0.2% w/v (NaCl), and temperature of
2+
2+
2
5°C. For Co and Ni , the logarithms of the overall
stability constants are 4.69 and 4.47, respectively. Nev-
ertheless, Golovnev and Molokeev [5] attributed these
constants to effective ones, since they do not take into
account the possible protonation of the ligand. Data
2+
on composition of the complex species in the M –
H thioBar (M = Cu, Ni) systems at pH 3–10 were
2
obtained by polarography, amperometric titration,
and spectrophotometry [17]. It was found that (M : L)
1
: 1, 1 : 2, 1 : 6 and 2 : 1 complexes are formed for
nickel(II) (the values of the stability constants were
not presented). Polarographic studies detected no
2+
2+
complexation between H thioBar and Co , Zn ,
2
2+
2+
Cd , and Pb .
The vast majority of published papers are devoted mal analysis (STA) of malonylthiourea and its salts
to the synthesis, determination of the structure and was conducted on a NETZSCH STA 449 F1 instru-
study of the properties of complexes with malonylth- ment in an air flow. A typical measurement procedure
iourea; it is noted that these compounds possess included a non-isothermal heating from 30 to 600°C at
supramolecular structure. For instance, [Co(HL) ] a rate of 10°C/min in precalcined alumina crucibles.
2
Sample weight was ~ 5 mg. The transmittance IR
was isolated from aqueous solution [18];
–1
[
Ni(H L) (HL)]Cl and [Co(H L) (HL)]Cl · H O [19] spectra over the range 4000–400 cm were recorded
2 2 2 2 2
for solid samples in KBr pellets on a Nicolet 6700 FT-IR
spectrometer with a diamond accessory. The solubility
along with [Co(H O) (H L) ](CH COO) · 2H O
2
2
2
4
3
2
2
[
20] were isolated from ethanol solutions. The structures
of inner-sphere complex salts of cobalt(II), nickel(II),
iron(II), and cadmium with composition
constants (K ) of the synthesized salts and the stability
S
constants of the complexes (β ) were calculated with
11
M(H O) (HL) , which have a polymeric structure allowance for a possible protonation of ligand anions
2
2
2
2+
and first-step hydrolysis of metal cations using the pro-
cedures described in [23]. The detailed procedures of
experiments and calculations were presented in [24].
with octahedral coordination geometry of M ion,
were thoroughly described using the X-ray diffraction
–
(
XRD) and IR spectroscopy data in [5, 21]. HL is
coordinated via donor oxygen and sulfur atoms. The
outer-sphere nickel(II)
Determination of the composition and stability of
thiobarbiturate complex species. Spectrophotometric method and
[
Ni(H O) ](HL) · 2H O is also known; right after potentiometric titration of thiobarbituric acid with
2 6 2 2
being removed from the solution, it is rapidly con- sodium hydroxide solution made it possible to deter-
verted to an inner-sphere complex salt with the com- mine the protonation constant (I = 0.1, 20°C) of its
position specified above [21].
single-charged anion (æ ): logæ = 2.37 ± 0.06 and
2 2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 8 2020

1
234
ΔA
.125
.120
LAKEEV, KOROTCHENKO
α2
α
α1
α0
0.9
0.8
0.7
0
0
0
0
0.4
.6
.5
0
.115
.110
0
0
0
.3
.2
0.105
0
.1
0.100
0.5
1.0
1.5
2.0 C /C
L M
0
2
4
6
8
10
12 pH
Fig. 1. Changes in absorbance ΔA (ΔA = Acomplex – A ) as
a function of the CL CNi2+ ratio (λeff = 395 nm, pH 2.7,
I = 0.1 (NaCl), T = 20°C).
M
Fig. 2. Distribution diagram of equilibrium malonylth-
iourea species depending on pH of the aqueous solution:
–
2–
(α ) H thioBar, (α ) HthioBar , and (α ) thioBar (c =
2
2
1
0
L
0
.01 М; logB = 10.55; logB = 12.85; “ Yield of the Acid”
1 2
software [23]).
logæ = 2.30 ± 0.01, respectively [14]. Further calcu-
2
lations were conducted using the value determined
from the pH-metric data. The overall protonation
constants of the double-charged thiobarbiturate anion
were taken from [15] for 18°С (logB = 10.55; logB =
experimental data and the calculated logβ values are
11
presented in Tables 1 and 2.
Synthesis and analysis of the isolated
1
2
Co(C H N O S) · 4H O and Ni(C H N O S) · 5H O
4
3
2
2
2
2
4
3
2
2
2
2
1
2.85). The temperature difference was not taken into
salts. Cobalt(II) and nickel(II) thiobarbiturates were
synthesized at pH 3.5–4.0 by interaction between the
cooled down saturated aqueous solutions of the
respective metal chlorides and malonylthiourea,
account, since it was negligible. The B value involves
2
the value found experimentally æ (logB = logB +
2
2
1
logæ ).
2
The compositions of complex species formed in the whose solution was partially neutralized by sodium
2+
binary systems under study were identified spectro-
photometrically using the molar-ratio method, as well
as by analyzing the results of calculations using the
pH-metric data. The dependence of absorbance А on
the c /c ratio (Fig. 1, the saturation curve in the
hydroxide, at a M : H thioBar : NaOH (1 : 2 : 1.2)
2
molar ratio. The precipitated crystals were filtered off,
washed with ethanol, and dried to constant weight in a
desiccator cabinet at 45°C.
L
M
The isolated thiobarbiturates were pink and yellow
fine-grained substances. Metal contents in the synthe-
sized salts were determined gravimetrically according
2
+
Ni –H thioBar system) demonstrates that the solu-
tion (pH 2.7) contains complexes with 1 : 2 composi-
2
0
2+
tion: [M(HthioBar) ] (identically to the Co – to the weights of the respective oxides Co O and NiO
2
3
4
H thioBar system). It is most likely that the inner obtained after the samples had been calcined in a muf-
2
coordination sphere of these complexes contains pro- fle furnace at 800°C for 3 h and by direct trilonometric
tonated ligand species, since the distribution diagram titration [22] (with the salts being pre-dissolved in (H,
of equilibrium acid species as a function of pH (Fig. 2) Na)Cl solutions). Trilon B concentration was deter-
indicates that HthioBarˉ is the dominant species at mined using the solutions with the known concentra-
a given pH.
The overall stability constants of the thiobarbitu-
tion of metal(II) in the presence of thiobarbituric acid.
The content of thiobarbiturate ion in salts was deter-
mined using a spectrophotometric technique devel-
oped by us; this technique is similar to the one
reported earlier for the barbiturate ion [11] and is
rate complexes were determined by pH-metric titra-
tion of a mixture of cobalt(II) or nickel(II) salt (with
the known free acidity) and thiobarbituric acid with a
solution of prestandardized carbonate-free alkali con-
taining the background electrolyte under bubbling of
based on absorbance of the thiobarbiturate complex of
+
iron(III) [FethioBar] (λ = 490 or 540 nm, C
Fe3+
=
eff
purified nitrogen gas. The stability constants of pro- 0.01 М, c = 0.1 М, V = 25.0 mL, pH 2.3) under
HCl
total
+
tonated complexes [MHthioBar] were calculated at the conditions close to its maximum yield (~ 76%,
pH 2.3–3.3 according to the pH-metric data using the Fig. 3). The Beer–Lambert law is fulfilled within the
Bjerrum 1 software [23]. The β values expressed in range of thiobarbiturate anion concentrations of (0.5–
1
1
–3
logarithmic form for cobalt(II) and nickel(II) are 3.69 2.5) × 10 М. The resulting calibration curves at 490
0.04 and 3.68 ± 0.08, respectively. The required and 540 nm are characterized by good coefficients of
±
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
Vol. 65
No. 8
2020

COBALT(II) AND NICKEL(II) COMPOUNDS WITH 2-THIOBARBITURIC ACID
1235
+
°
°
°
L
Table 1. Stability constants of the [CoHthioBar] complex calculated using pH-metric data (
V
= VL = 5.00 mL; C =
M
°
3
2
.92 × 10–3 M;
С
= 5.67 × 10–3 М; cНCl = 0.01 М; cNaOH = 1.07 × 10–2 М; logKh1 = –9.76; logæ = 2.30; I = 0.1; T =
M
2
0°C); logβ11 = 3.69 ± 0.04
V , mL
pH
logβ11
V , mL
pH
logβ11
V , mL
рH
logβ11
alk
alk
alk
0
0
.60
.80
2.30
2.33
2.36
2.38
2.41
2.43
2.45
3.68
3.60
3.52
3.58
3.52
3.60
3.68
2.00
2.20
2.40
2.60
2.80
3.00
3.40
2.47
2.50
2.52
2.55
2.58
2.61
2.68
3.75
3.72
3.81
3.70
3.70
3.82
3.78
3.80
4.00
4.40
4.60
4.80
5.00
5.40
2.76
2.81
2.89
2.94
3.00
3.06
3.18
3.72
3.64
3.72
3.72
3.68
3.68
3.90
1
1
1
1
.00
.20
.40
.60
.80
1
+
°
°
°
L
Table 2. Stability constants of the [NiHthioBar] complex calculated using pH-metric data (V = VL = 5.00 mL; C =
M
°
3
2
.92 × 10–3 М;
С
= 6.03 × 10–3 М; cНCl = 0.01 М; cNaOH = 1.07 × 10–2 М; logKh1 = –9.70; logæ = 2.30; I = 0.1; T =
M
2
0°C); logβ11 = 3.68 ± 0.08
V , mL
pH
logβ11
V , mL
pH
logβ11
V , mL
рH
logβ11
alk
alk
alk
2.00
2.20
2.40
2.60
2.80
2.46
2.50
2.52
2.55
2.58
3.87
3.72
3.81
3.80
3.80
3.00
3.20
3.40
3.60
3.80
2.61
2.65
2.69
2.73
2.77
3.81
3.74
3.69
3.71
3.65
4.00
4.20
4.40
4.60
4.80
2.81
2.86
2.91
2.98
3.04
3.64
3.59
3.56
3.43
3.40
–
determination (0.9885 and 0.9919, respectively). The
Found (%): Ni, 13.53; C H N O S , 62.70; H O,
4 3 2 2 2
least squares fitting of these curves (t = 2.57) yields
α
18.62.
the following parameters of straight-line equation:
The presence of crystallization water in salts was
also qualitatively confirmed by IR spectra; the fre-
quencies of selected bands are presented below:
A = a + bСL;
(
in this case, coefficient a is non-significant, so we
–1
IR spectrum of Co(C H N O S) · 4H O (ν, cm ):
4
3
2
2
2
2
assume it to be zero):
3
1
353 w, 3106 w, 3055 w, 2662 w, 1585 vs, 1551vs, 1474 w,
392 vs, 1287 vs, 1230 s, 1190 vs, 1010 s, 944 m, 886 vs,
811vs, 731 w, 697 m, 627 m, 606 s, 538 vs, 488 vs, 414 w.
b = (130.0 ± 18.0) at λ = 490 nm;
eff
b = (100.9 ± 12.9) at λ = 540 nm.
eff
+
–1
The composition and stability of the [FethioBar]
IR spectrum of Ni(C H N O S) · 5H O (ν, cm ):
4
3
2
2
2
2
complex were determined preliminarily by spectro-
3
1
392 w, 3104 w, 3065 w, 2890 w, 2668 w, 1605 vs, 1540 vs,
photometry and potentiometry.
443 w, 1406 s, 1295 s, 1250 w, 1203 vs, 967 w, 869 w,
The contents of crystallization water and metal 814 m, 792 vs, 632 m, 604 m, 526 vs, 471 w, 416 w.
(
Co O and NiO) were estimated from the thermo-
3 4
Determination of solubility of the synthesized salts.
The solubility constants of the synthesized salts K =
gravimetric analysis data. The results obtained by
chemical analysis of cobalt(II) and nickel(II) thiobar-
biturates are presented below (the values averaged over
the gravimetric, thermogravimetric, and titrimetric
data are reported for metal contents).
S
2+
2
[
M ][HthioBarˉ] were determined at pH 2.5–5.3 for
Co(C H N O S) · 4H O and pH 2.5–2.8 for
4
3
2
2
2
2
Ni(C H N O S) · 5H O at 20°C. Small samples of
4
3
2
2
2
2
cobalt(II) and nickel(II) thiobarbiturates were placed
into 0.1 M (H, Na)Cl solutions. The suspensions were
periodically shaken in vessels with ground-glass stop-
pers and allowed to stand for several days until a het-
erogeneous equilibrium (constant pH) was attained.
The saturated solutions were filtered; their pH was
For Co(C H N O S) · 4H O anal. calcd. (%): Co,
4
3
2
–
2
2
2
1
4.12; C H N O S , 68.61; H O, 17.27.
4
3
2
2
2
–
Found (%): Co, 14.74; C H N O S , 65.13; H O,
4
3
2
2
2
19.59.
2+
For Ni(C H N O S) · 5H O anal. calcd. (%): Ni,
4
3
2
2
2
2
measured; concentrations of M ions were deter-
mined by trilonometric titration. The equilibria
1
3.49; C H N O S, 65.81; H O, 20.70.
4 3 2 2 2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 8 2020

1236
LAKEEV, KOROTCHENKO
attained in saturated aqueous solutions of salts without
allowance for complexation and the respective con-
stants can be written as:
α
1
.0
α0
α1
M(HthioBar)2(s) ↔ M(HthioBar)2(aq)
0
.8
(
1)
2
+
−
↔
M( + 2HthioBar
,
aq
)
(aq)
– 2
2+
0
0
0
.6
.4
.2
K = [M ][HthioBar ] ;
S
–
+
HthioBar + H ↔ H thioBar,
(2)
(3)
2
–
+
æ = [H thioBar]/([HthioBar ][H ]);
2
2
2
+
+
+
M
+ H O ↔ MOH + H ,
2
+
+
2+
K = ([MOH ][H ])/[M ].
h1
One can see from Eq. (1) that c = c (c is the
M
salt
salt
0
1
2
3
4
5
pH
solubility of salt, mol/L) in a saturated solution. Then,
the material-balance equation with respect to metal
ion will be written as:
3
+
+
Fig. 3. Distribution diagram of Fe (α ) and FethioBar
0
2
+
+
(
α ) species depending on pH of the aqueous solution
1
L
C = C = [M ] + [MOH ] =
M
salt
(
c = 0.01 M; logβ = 11.05; logB = 10.55; logB = 12.85;
(4)
1
1
2
2
+
+
2+
“
Yield of the complex” software [23]).
=
[M ](1 + K /[H ]) = [M ]ω.
h1
+
2+
+
The value [MOH ] = [M ]K /[H ] was found
h1
+
6 [15, 16]. Protonated complex species are generally
typical of nitrogen-containing ligands [25]. Therefore,
one should expect that various protonated thiobarbi-
turates will be formed in the acidic region, since
from Eq. (3) for K ; ω = 1 + K /[H ] is a function of
hydrolysis; K is the constant for the first-step hydro-
h1
h1
h1
2+
2+
lysis of M ions. From Eq. (4) we obtain that [M ] =
c /ω. With allowance for equilibrium (1), we obtain
salt
2+
because of the electrostatic characteristics of Co and
from the material-balance equation (5):
2+
Ni ions they cannot replace the second proton from
–
the single-charged thiobarbiturate anion (pK
=
CHthioBar = [HthioBar ] + [H thioBar]
a2
2
1
0.55), which is the predominant species within a
–
+
=
[HthioBar ] 1 + æ [H ]
(5)
(
2
)
broad pH range.
–
=
[HthioBar ]f = 2Csalt,
where f = 1 + æ [H ] is the function of single-charged
An analysis of saturation curves (Fig. 1) in the
2+
+
M –H thioBar systems shows that 1 : 2 complexes
2
2
thiobarbiturate anion protonation (æ is the protona- are formed at pH 2.7, thus indicating that coordina-
2
–
–
tion constant of HthioBar ions), that [HthioBar ] = tion the deprotonated ligand species does not take
–
+
2
c /f. The value [H thioBar] = [HthioBar ][H ]æ
place at this pH. The resulting titration curves (Fig. 4)
salt
2
2
was found from Eq. (2) for æ . By substituting the of thiobarbituric acid solutions and its mixtures con-
2
respective values into the expression
M ][HthioBar ] , we obtain that:
K
=
taining pre-acidified (c
= 0.01 M) cobalt(II) or
S
HCl
2+
– 2
[
nickel(II) chlorides with an alkali have different
shapes, thus demonstrating that cations of these met-
als replace the proton and providing indirect evidence
for complexation processes occurring in the systems
under study. The experimentally determined equiva-
2
3
salt
2
K = (c
ω)(2csalt f ) = 4c (ω f ).
(6)
S
salt
Table 3 summarizes the experimentally determined
solubilities of cobalt(II) and nickel(II) thiobarbitu-
rates in 0.1 M (H, Na)Cl solutions and their calculated
lent volumes of alkali (V ), which amount to 6.53
exp
2+
2+
solubility constants without allowance for complex- (Co ) and 6.41 mL (Ni ), agree fairly well with the
ation. Calculations of the solubility constants of salts theoretically expected value (Vtheor = 6.50 mL) calcu-
with allowance for possible complexation using the lated with allowance for neutralization of one proton
“
Solubility” software [23] yields the following data: released during the interaction. According to the spe-
pK = 12.82 ± 0.22 for Co(C H N O S) · 4H O and cies distribution diagram (Fig. 2), the yield of acid spe-
S
4
3
2
2
2
2
pK = 12.30 ± 0.10 for Ni(C H N O S) · 5H O.
cies within the pH range 2.3–3.3 varies from 46.6 to
S
4
3
2
2
2
2
–
9
2.0% for HthioBar and from 53.4 to 8.0% for
2
H thioBar. With allowance for this fact, the formation
of protonated thiobarbiturate complexes of cobalt(II)
and nickel(II) can be represented by ionic equilibria:
RESULTS AND DISCUSSION
For the complexes formed by divalent metals M
2+
0
2
with H thioBar, [MthioBar] and [M(thioBar) ] ‾ are
2
2
2
+
K
1
+
+
the predominant species in aqueous solutions at pH >
M
+ H
2
L ←⎯ ⎯→ [MHL] + H ,
(7)
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COBALT(II) AND NICKEL(II) COMPOUNDS WITH 2-THIOBARBITURIC ACID
1237
Table 3. Solubility constants of the synthesized salts calculated using their solubilities in (H,Na)Cl solutions:
2+
2+
(
logK (Co ) = –9.76; logK (Ni ) = –9.70; logæ = 2.30; I = 0.1; T = 20°C
h1
h1
2
3
3
No.*
pH
cCо × 10 , M
pKS
pH
cNi × 10 , M
pKS
Co(C H N O S) · 4H O, pK = 8.27 ± 0.31
Ni(C H N O S) · 5H O, pK = 7.58 ± 0.15
4
3
2
2
2
2
S
4
3
2
2
2
2
S
1
2
3
4
5
6
7
8
9
4.25
4.14
3.63
3.58
3.10
2.86
2.73
2.56
2.53
0.74
1.07
0.81
0.83
1.18
1.78
1.65
1.96
2.17
8.81
8.32
8.74
8.66
8.33
7.86
8.02
7.91
7.79
2.81
2.78
2.78
2.65
2.65
2.62
2.61
2.59
2.54
2.31
2.12
2.65
2.16
2.51
2.98
1.82
2.31
2.77
7.54
7.67
7.38
7.72
7.52
7.32
7.96
7.67
7.47
*
Number of the vessel containing a saturated solution of the salt.
2
+
K
0
+
suggest that the resulting complexes have similar sta-
bility, which is also evidenced by their stability con-
2
M
+ 2H L ←⎯ ⎯ → [M
(
HL)2] + 2H ,
(8)
(9)
2
stants (logβ ) calculated using the pH-metric data.
2
+
−
β
1
+
11
1
M
+ HL ←⎯ ⎯ → [MHL] ,
+
The stability constants for [CoHthioBar] and
[NiHthioBar] are 3.69 ± 0.04 and 3.68 ± 0.08,
+
2
+
−
β
1
0
2
M
+ 2HL ←⎯ ⎯ → [M
(
HL)2] .
(10)
respectively. Overall, the proposed interaction model
is experimentally confirmed by the titration curves,
the spectrophotometric data, and consistency
achieved in determining the stability constants. The
calculations using the “Bjerrum 2” software [23] show
The almost identical positions of the titration
2+
curves of the M –H thioBar systems due to the sim-
2
ilar electrostatic characteristics of the ions under study
+
that the simultaneous dominance of [MHthioBar]
0
and [M(HthioBar) ] complex species in the solution
pH
2
is not observed at pH 2.3–3.3, since the condition c > cM
1
L
is not met. Therefore, the presence of biligand proton-
ated species was not taken into account when calculat-
ing the β values.
8
7
6
5
4
3
2
2
3
11
The yield diagram of acid species (Fig. 2) shows
–
that HthioBar anions are dominant (94.1–98.3%) in
the range of pH 3.5–4.0 where cobalt(II) and
nickel(II) salts were synthesized. Since proton of the
‒
OH group is the first one to dissociate within an acid
molecule (pK = 2.30), O-coordination should be
a1
expected. The imide group is deprotonated in the
strongly alkaline medium (pK = 10.55). Therefore,
a2
0
2
4
6
8
N-coordination can be ruled out because the nitrogen
Valk, mL
–
atom in the –NH– group in HthioBar is blocked by
a proton. The ions under study cannot replace the sec-
ond proton, so thiobarbiturates containing the pro-
tonated ligand species are precipitated from saturated
solutions. According to the IR spectroscopy data and
studies [5, 21], O,S-coordination is observed in the
Fig. 4. Curves recorded for titration of 10.00 mL of acid
solution or its mixture with metal salts with alkali (c
=
NaOH
–2
–3
1
.07 × 10 М): (1) 1.96 × 10 М H thioBar; (2) 1.96 ×
2
–3
–3
1
0
М H thioBar and 3.02 × 10 М NiCl (c
= 0.01 М);
2
2
HCl
–3
–3
(
3) 1.96 × 10 М H thioBar and 2.84 × 10 М CoCl
2 2
synthesized Co(C H N O S)
·
4H O and
2
(c
= 0.01 М).
4
3
2
2
2
HCl
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 8 2020

1238
LAKEEV, KOROTCHENKO
Ni(C H N O S) · 5H O salts. The spectra of the ana- intra- and intermolecular hydrogen bonds (N−H···O
4
3
2
2
2
2
lyzed compounds contain low-intensity bands at and O−H···O), as well as π–π interactions between the
–
1
–
3
400–3300 cm corresponding to ν(O–H) vibrations HthioBar ligands [5, 21], which are also responsible
of bound water molecules. The characteristic bands for their low solubility.
ν(>C=O) are shifted towards lower frequencies com-
–1
pared to those in H thioBar (1720–1640 cm ), thus
2
REFERENCES
confirming that oxygen atoms are involved in ligand
coordination. The disappearance of the peak at 1165–
1
. M. D. Mashkovskii, Mediaments Novaya Volna, Mos-
cow, 2012) [in Russian].
. R. M. C. Dowson, D. C. Elliot, W. H. Elliot, and
K. M. Jones, in Data for Biochemical Research (Claren-
don, Oxford, 1986; Mir, Moscow, 1991).
–1
1
145 cm ascribed to ν(>C=S) is an indication of S-
2
coordination [5], which also agrees with the presence
–1
of low-intensity bands at 1551 and 1540 cm for the
cobalt(II) and nickel(II) salts, respectively, whose
stretching vibration frequencies are lower than those
3. K. M. Oldham and P. E. Bowen, J. Am. Diet. Assoc.
98, 1001 (1998).
–1
observed for the acid (at 1561 cm ). These signals are
ascribed to the –NH–(–C=S) cyclic thioamide struc-
ture and demonstrate that the coordinated ligand
exists in the thiocarbonyl form. The minor differ-
ences in the region where ν(>N–H) vibrations are
https://doi.org/10.1016/S0002-8223(98)00230-2
4. A. German, A. Galli, C. Maison, et al., Clin. Chim.
Acta 22, 551 (1968).
https://doi.org/10.1016/0009-8981(68)90103-4
–
1
detected (~10 cm ) are caused by the involvement of
these groups in hydrogen bonding. According to the
STA data, two water molecules are cleaved in salts in
the high-temperature region (T > 200°C), which may
indicate that they were located in the inner sphere (at
lower temperatures cobalt(II) thiobarbiturate tetrahy-
drate and nickel(II) thiobarbiturate pentahydrate lose
two and three water molecules, respectively). The
coincidence between the main frequencies in the IR
spectra, as well as the similar intervals of thermal
destruction for the salts synthesized in this study, and
the data reported in [5, 21] give grounds for claiming
that the complexes have the same composition.
5. N. N. Golovnev and M. S. Molokeev, 2-Thiobarbituric
Acid and Its Metal Complexes: Synthesis, Structure,
and Properties (Siberian Federal Univ., Krasnoyarsk,
2014) [in Russian].
6. N. N. Golovnev and M. S. Molokeev, Russ. J. Coord.
Chem. 40, 648 (2014).
https://doi.org/10.7868/S0132344X14090035
7
8
9
. N. N. Golovnev, M. S. Molokeev, S. N. Vereshchagin,
et al., J. Coord. Chem. 68, 1865 (2015).
https://doi.org/10.1080/00958972.2015.1031119
. N. N. Golovnev, M. S. Molokeev, I. V. Sterkhova,
et al., Russ. J. Stuct. Chem. 58, 539 (2017).
https://doi.org/10.15372/JSC20170315
. V. I. Balas, I. I. Verginadis, G. D. Geromichalos, et al.,
Eur. J. Med. Chem. 46, 2835 (2011).
https://doi.org/10.1016/j.ejmech.2011.04.005
0. M. A. El-Gahami, S. A. Ibrahim, and H. M. A. Sal-
man, Synth. React. Inorg. Met.-Org. Chem. 21, 1497
1991).
https://doi.org/10.1080/15533179108020625
Zaki and Mohamed [20] showed that the spectra of
zinc and cadmium complex salts of similar composi-
tion feature no bands corresponding to ν(>N–H)
–1
1
vibrations (3200–3000 cm ), which may indicate that
2+
the nitrogen atom in the imide group binds to M ions
or the ligand undergoes tautomeric conversion to the
enol-thiol form prior to coordination. The latter vari-
ant is the most plausible one, since the researchers
believe that tautomerization of the keto form of the
(
1
1. N. M. Korotchenko and N. A. Skorik, Russ. J. Inorg.
Chem. 45, 1099 (2000).
acid to the enol-thiol form is a prerequisite for ligand 12. N. M. Korotchenko and N. A. Skorik, Russ. J. Inorg.
binding. In general, the O,S-coordination was also
Chem. 47, 790 (2002).
–1
proposed in [20]. Weak vibrations at 430–410 cm
13. N. M. Korotchenko and N. A. Skorik, Russ. J. Inorg.
Chem. 57, 133 (2012).
corresponding to ν(M–S) indicate that the M–S bond
is present. Signals are also observed in this region for
https://doi.org/10.1134/S0036023612010135
–1
the salts synthesized in the current study: at 414 cm
1
4. A. P. Lakeev, N. M. Korotchenko, and E. R. Sayfulin,
–1
for the cobalt(II) salt and at 416 cm for the nickel(II)
salt. It is clear that the bond between the metal and the
acid residue in the synthesized thiobarbiturates is sub-
stantially covalent due to the polarizing effect of
d-metal cations on the one hand and, on the other
hand, high polarizability of the acid residue as verified
J. Sib. Fed. Univ. Chem. 12, 6 (2019).
https://doi.org/10.17516/1998-2836-0104
1
5. B. R. Singh, R. K. Jain, M. K. Jain, et al., Thermo-
chim. Acta 78, 175 (1984).
https://doi.org/10.1016/0040-6031(84)87144-0
1
6. F. Ahmadi, A. H. M. Sarrafi, and M. M. Ghashghaee,
–9
by the low values of their K : 5.40 × 10 and 2.63 ×
S
Eur. J. Chem. 6, 47 (2009).
–8
–13
1
0 (without allowance for complexation); 1.51 × 10
https://doi.org/10.1155/2009/965619
–
13
and 5.01 × 10 (with allowance for complexation) for 17. R. J. Murphy and G. Svehla, Anal. Chim. Acta 99, 115
cobalt(II) and nickel(II) salts, respectively. Further-
more, these structures are additionally stabilized by
(1978).
https://doi.org/10.1016/S0003-2670(01)84503-7
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
Vol. 65
No. 8
2020

COBALT(II) AND NICKEL(II) COMPOUNDS WITH 2-THIOBARBITURIC ACID
1239
1
8. K. S. Siddiqi, P. Khan, S. Khan, et al., Synth. React. 22. G. Schwarzenbach and H. Flaschka, Die komplexome-
Inorg. Met.-Org. Chem. 12, 681 (1982).
trische Titration (Ferdinand Enke, Stuttgart, 1965;
https://doi.org/10.1080/00945718208082685
Khimiya, Moscow, 1970).
2
2
2
3. N. A. Skorik and E. B. Chernov, Personal Computers in
the Chemistry of Complex Compounds (Tomsk Univ.
Press, Tomsk Univ., 2009) [in Russian].
4. V. N. Kumok and N. A. Skorik, Laboratory Work in the
Chemistry of Complex Compounds (Tomsk Univ. Press,
Tomsk Univ., 1983) [in Russian].
1
9. M. S. Masoud, A. M. Heiba, and F. M. Ashmawy,
Trans. Met. Chem. 8, 124 (1983).
https://doi.org/10.1007/BF01036097
2
0. Z. M. Zaki and G. G. Mohamed, Spectrochim. Acta
A 56, 1245 (2000).
https://doi.org/10.1016/S1386-1425(99)00225-5
5. R. M. Smith and A. E. Martell, Critical Stability Con-
stants (Plenum, New York/London, 1976), Vol. 2.
21. N. N. Golovnev, M. S. Molokeev, S. N. Vereshchagin,
et al., Polyhedron 70, 71 (2014).
https://doi.org/10.1016/j.poly.2013.12.021
Translated by D. Terpilovskaya
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 8 2020