Cobalt tetrasulfophthalocyaninate as a catalyst of the reduction of nitrite with thiourea dioxide

Article abstract of DOI:10.1134/S0036024409120085

The kinetics of reduction of nitrite with thiourea dioxide in the presence of cobalt tetrasulfophth-alocyaninate was studied. The process was shown to involve the Co^II ? Co^I catalytic cycle. The kinetic parameters of the reduction of

Full text of DOI:10.1134/S0036024409120085

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2009, Vol. 83, No. 12, pp. 2050–2053. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © A.S. Pogorelova, S.V. Makarov, E.S. Ageeva, R. SilagiꢀDumitresku, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 12, pp. 2250–2254.
CHEMICAL KINETICS
AND CATALYSIS
Cobalt Tetrasulfophthalocyaninate as a Catalyst of the Reduction
of Nitrite with Thiourea Dioxide
A. S. Pogorelovaa, S. V. Makarovb, E. S. Ageevab, and R. SilagiꢀDumitreskuc
a Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
b Ivanovo State University of Chemical Technology, Ivanovo, Russia
c BabesꢀBolyai University, ClujꢀNapoca, Romania
eꢀmail: makarov@isuct.ru
Received December 24, 2008
Abstract—The kinetics of reduction of nitrite with thiourea dioxide in the presence of cobalt tetrasulfophthꢀ
alocyaninate was studied. The process was shown to involve the CoII
CoI catalytic cycle. The kinetic
parameters of the reduction of CoII and CoI tetrasulfophthalocyaninates were determined
DOI: 10.1134/S0036024409120085
INTRODUCTION
with the data on the kinetics of catalytic nitrite reducꢀ
tion with dithionite.
In recent years, views on the importance of and
role played by nitrite in biological processes have
changed substantially. Earlier, nitrite was treated only
as a harmful substance. Studies of the past decade
showed that nitrite played the most important role in
controlling blood pressure [1, 2]. It is the main reserꢀ
voir of nitrogen(II) oxide in organism, which is espeꢀ
cially important for hypoxia [1, 2]. The use of nitrite
for curing such a serious illness as cystic fibrosis offers
much promise [3]. The earlier unknown function of
hemoglobin and myoglobin as nitrite reductases was
revealed [4–6].
It was found that cobalt tetrasulfophthalocyaninate
had high catalytic activity in the reduction of nitrite
and nitrate with dithionite in alkaline solutions [7].
The end reaction products were ammonia and nitroꢀ
gen(I) oxide depending on the nature of the oxidizer.
These differences were likely caused by the formation
of intermediate complexes in the reaction between the
reduced form of phthalocyaninate and nitrite or
nitrate. The complexes had different structures: the
substrate was coordinated at the nitrogen atom with
nitrite and oxygen atom with nitrate [7]. The compoꢀ
sition of nitrite reduction products was shown to be
influenced substantially by the nature of the reducing
agent and metal in its complex with tetrasulfophthaloꢀ
cyanine. For instance, the use of iron tetrasulfophthaꢀ
locyaninate as a catalyst of the reduction of nitrite with
dithionite and sulfoxylate (thiourea dioxide was used
as a precursor of sulfoxylate) resulted in the formation
of nitrogen(I) oxide and ammonia, respectively [8].
EXPERIMENTAL
Cobalt tetrasulfophthalocyaninate was prepared
and purified following the known procedure [9, 10].
Sodium nitrite and thiourea dioxide (Aldrich) were
used without additional purification. The products of
the reduction of nitrite were determined by the 15N
NMR method using N15NO2 (95% 15N). Solution pH
was maintained at the required level with the use of the
universal Britton–Robinson buffer mixture. Kinetic
experiments were performed under temperature conꢀ
trolled conditions on a Specord Mꢀ40 spectrophoꢀ
tometer in the absence of oxygen (argon was prelimiꢀ
narily bubbled through solutions). The results were
analyzed using the Origin 7.5 program. The NMR
spectra were recorded on a Bruker Avance DRX400
WB spectrometer. The metal complex was reduced
with both freshly prepared and aged solutions of
TUDO. “Aged” means held in 0.1 N NaOH under
anaerobic conditions for 4 h [8], which caused the forꢀ
mation of SO22– (SO2H–)–sulfoxylate reducing partiꢀ
cles (the only nitrogenꢀcontaining product of the
decomposition of thiourea dioxide in strongly alkaline
media; urea did not influence the rate of redox proꢀ
cesses). The use of sulfoxylate allows the influence of
the decomposition of TUDO to be excluded and the
In this work, we present the results of a kinetic
study of the reduction of nitrite with thiourea dioxide
(TUDO) in the presence of cobalt tetrasulfophthaloꢀ
reaction between SO22– (SO2H–) and CoI(TSPc)5– to
cyaninate (CoII(TSPc)4–). The results are compared be studied.
2050

COBALT TETRASULFOPHTHALOCYANINATE AS A CATALYST OF THE REDUCTION
2051
A450
3
A
3
2
1
2.5
1.2
1.0
0.8
0.6
0.4
0.2
0
2.0
1.5
1.0
0.5
0
1
2
300
450
600
750
0
1
2
3
λ
, nm
−
τ × 10 , s
2
II
5–
4–
Fig. 1. Absorption spectra of (
1
)
Co (TSPc) , (
2
) the
I
II
4–
product of its reduction, Co (TSPc) , and (
3) the prodꢀ
Fig. 2. Kinetic curves of the reduction of Co (TSPc)
with thiourea dioxide at pH ( ) 9.4, ( ) 10.9, and (
(0.1 mol/l NaOH); [TUDO]
uct of the oxidation of the reduced form with oxygen,
Co (TSPc) ; [Co (TSPc) ] = 4.96
= 298 K.
1
2
1
3
) 13
mol/l,
III
3–
II
4–
5
–3
×
10⎯ mol/l,
=
×
10
II
4–
–5
T
[Co (TSPc) ] = 4.96 × 10 mol/l, Т = 298 K.
RESULTS AND DISCUSSION
The absorption spectrum of the reduced complex
did not change for 2–3 min. Optical density then
slowly decreased over the whole wavelength range.
The solution gradually underwent discoloration. It
follows that CoI(TSPc)5– is not the final product of the
reaction under consideration. Similar effects were
observed during the radiolytic reduction of
CoI(TSPc)5– [11]. According to [11], the reduction of
the macroring occurred in that reaction.
The reduction of CoII(TSPc)4– with thiourea dioxꢀ
ide is accompanied by a change in solution color from
blue to olive. An intense band at 450 nm appears in the
absorption spectrum (Fig. 1); simultaneously, a bathoꢀ
chromic shift of the
the literature data [7, 11, 12], the resulting spectrum
corresponds to CoI(TSPc)5–
Q band is observed. According to
.
The kinetics of reduction of CoII(TSPc)4– was
studied under pseudofirstꢀorder reaction conditions in
the presence of excess TUDO (in this series of experiꢀ
ments, we used freshly prepared solutions of thiourea
dioxide, because reduction with sulfoxylate occurred
almost instantaneously). The kinetic curves obtained
by measuring optical density at 450 nm (this waveꢀ
length corresponds to the reduced form) and various
pH are shown in Fig. 2. We found that the rate of
reduction was independent of the concentration of the
metal complex; that is, the reaction was described by
The kinetic curves of the reduction of
CoI(TSPc)5– with thiourea dioxide at various temꢀ
peratures are shown in Fig. 3. The kinetics of reducꢀ
tion was studied under pseudofirstꢀorder reaction
conditions in the presence of excess TUDO. The
reaction was found to be described by the equation
r
= k
[CoI(TSPc)5–][TUDO]. The activation paramꢀ
≠
eters found from the Eyring dependence were H =
50
Δ
≠
2
kJ/mol and
Δ
S = –160 10 J/(mol K).
Importantly, the kinetic characteristics were almost
independent of whether freshly prepared or aged soluꢀ
tions of thiourea dioxide were used in experiments. We
can therefore assume that the rateꢀdetermining stage
is the formation of the [CoI(TSPc)SO2H]6– complex,
the kinetic equation r = k[TUDO]. The rate constants
calculated from the experimental data were commenꢀ
surate with the rate constants for the decomposition of
TUDO in alkaline solutions. For instance, at pH 11
CoI(TSPc)5– + SO2H–
and
T
= 298 K, the rate constant for the decomposiꢀ
tion of TUDO in solution was
6
×
10–4 s–1 [13], and the
(2)
5–
[CoI(TSPc) ⋅ SO2H–]
products.
rate constant for the reduction of cobalt phthalocyanꢀ
inate determined by dividing the observed zeroꢀorder
rate constant by the concentration of TUDO was 4.4
×
Note that the introduction of oxygen into the sysꢀ
tem immediately after the highest optical density at
450 nm was reached, that is, into a solution of
CoI(TSPc)5–, caused the formation of the initial comꢀ
plex CoII(TSPc)4–. At the same time, the oxidation of
the product of CoI(TSPc)5– reduction with oxygen
10–4 s–1 under equal conditions. It can therefore be
assumed that the decomposition of TUDO in solution
according to the reaction
NH2NHCSO–2 + H2O
SO2H– + (NH2)2CO (1)
resulted in the formation of CoIII(TSPc)3–
(λmax = 605,
is the limiting stage of the process.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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2009

2052
POGORELOVA et al.
A450
1.3
1.2
1.1
1.0
0.9
1
2
3
4
5
6
0
2.5
5.0
7.5
−
τ × 10 , s
3
I
5–
Fig. 3. Kinetic curves of the reduction of Co (TSPc) with thiourea dioxide at temperatures of (
1) 298, (2) 303, (3) 308, (4) 313,
–3
I
5–
–5
(5) 318, and (6) 323 K; pH 10.7, [TUDO] = 1 × 10 mol/l, [Co (TSPc) ] = 4.96 × 10 mol/l.
670 nm) (Fig. 1) [7, 12], which was accompanied by a induction period, the duration of which depends on
solution color change from olive to blueꢀgreen. Let us the concentrations of both nitrite and TUDO. Note
consider the reasons for this difference. It is known that the kinetic curves are similar in shape to those
that CoIII(TSPc)3– is formed in the reaction between
CoII(TSPc)4– and sulfite in the presence of oxygen [7].
Under these conditions, the SO–5 particle, which is a
obtained in the reduction of nitrite with sodium
dithionite in the presence of cobalt phthalocyaniꢀ
nate [7].
The presence of an induction period is explained by
the redox cycle of the metal complex. During this
cycle, CoI(TSPc)5– is oxidized to CoII(TSPc)4– with
nitrite and again reduced with excess dithionite in
solution. This cycle works until all reducing agent is
consumed. After this, only the oxidation of
CoI(TSPc)5– to CoII(TSPc)4– occurs, and a decrease
in the concentration of the reduced form with time is
well described by the exponential law. We found in this
work that, as with dithionite [7], the rate constant
observed at nitrite concentrations up to 0.025–
0.03 mol/l increased linearly as the concentration of
nitrite grew and reached a constant value (Fig. 5).
strong oxidizer, is formed. In our system, sulfite is
formed in the decomposition of TUDO. The reducꢀ
tion of CoII(TSPc)4– to CoI(TSPc)5– occurs at a high
rate, the concentration of sulfite is therefore low, and
it does not influence the oxidation of the complex.
Conversely, the reduction of CoI(TSPc)5– occurs at a
low rate, and a substantial amount of sulfite is accuꢀ
mulated during the reaction. Sulfite reacts with oxyꢀ
gen to produce SO–5 , which results in deeper oxidation
of the metal complex to CoIII(TSPc)3–
.
The kinetics of interaction between CoI(TSPc)5–
and nitrite was studied under anaerobic conditions at
pH 10 and 298 K. Changes in absorption at 450 nm
It is noteworthy that the observed rate constants
were used to monitor kinetics. Reduction was studied obtained in this work from a decrease in optical density
under pseudofirstꢀorder reaction conditions in the at 450 nm were much lower than when dithionite was
presence of excess nitrite, which was introduced into a used as a reducing agent [7]. For instance, the rate
solution of the reduced phthalocyaninate form immeꢀ constant corresponding to a plateau in Fig. 5 was 4.4
×
diately after the highest optical density at 450 nm was 10–4 s–1 against 1.21 10–2 s–1 for dithionite [7].
×
reached, that is, before the beginning of the second Clearly, this is related to different natures of the rateꢀ
stage of metal complex reduction. Freshly prepared determining stage when two different reducing agents
solutions of TUDO were used. A typical kinetic curve are used. With dithionite, the rate constant for the
is shown in Fig. 4. We see from this figure that the oxiꢀ reduction of the CoII complex is 2.2 s–1 [7], and, with
dation of CoI(TSPc)5– with nitrite develops with an TUDO, this rate constant is only 4.4 10–4 s–1. It folꢀ
×
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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COBALT TETRASULFOPHTHALOCYANINATE AS A CATALYST OF THE REDUCTION
2053
−1
104, s
A450
kobs
×
4
3
2
1
0
1.1
0.9
0.7
0
0.01
0.02
0.03
0
2
4
6
8
−
τ × 10 , s
[NaNO2], mol/l
3
Fig. 5. Dependence of the rate constant for the reduction
Fig. 4. Kinetic curve of the reduction of nitrite with thioꢀ
of nitrite with thiourea dioxide in the presence of
I
5–
I
I
5–
urea dioxide in the presence of Co (TSPc) ; [TUDO] =
Co (TSPc) on the concentration of nitrite; [TUDO] =
–3
5–
–3
I
5–
–5
1
× 10 mol/l, [NaNO ] = 0.03 mol/l, [Co (TSPc) ] =
1
×
10 mol/l, [Co (TSPc) ] = 4.96
× 10 mol/l, T =
2
–5
4.96 × 10 mol/l, T = 298 K, pH 10.0.
298 K, pH 10.0.
lows that, with dithionite, the rateꢀdetermining stage
of the reduction of nitrite is its interaction with
CoI(TSPc)5–, whereas, with TUDO, this is the reducꢀ
tion of CoII(TSPc)4–. For this reason, we were unable
to determine the kinetic characteristics of the interacꢀ
ACKNOWLEDGMENTS
This work was financially supported by the Russian
Foundation for Basic Research and Romanian Acadꢀ
emy, project no. 07ꢀ03ꢀ91687ꢀRAꢀa.
tion of nitrite with CoI(TSPc)5–
.
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,
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No. 12
2009