Kinetics and mechanism of the Co(II)-assisted oxidation of thioureas by dioxygen

Article abstract of DOI:10.1039/b417775d

Catalytic oxidation of N,N′-dimethylthiourea and thiourea by dioxygen in water using a new cobalt(II) complex of octasulfophenyltetrapyrazinoporphyrazine was performed under mild conditions. The reaction is shown to include the formation of an intermediate anionic five-coordinate complex followed by an unusual two-electron oxidation to produce the corresponding urea and elemental sulfur (S₈). Kinetic and thermodynamic parameters for the different reaction steps of the process were determined. Drastic differences in catalytic activity of cobalt and iron octasulfophenyltetrapyrazinoporphyrazines were observed. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417775d

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F U L L P A P E R
Kinetics and mechanism of the Co(II)-assisted oxidation of thioureas
by dioxygen†
Evgeny V. Kudrik,*a,b Alexander Theodoridis,a Rudi van Eldik*a and Sergei V. Makarovb
a Institute for Inorganic Chemistry, University of Erlangen-Nu¨rnberg, Egerlandstrasse 1, 91058,
Erlangen, Germany
b Ivanovo State University of Chemistry and Technology, ave. F. Engels 7, 153460, Ivanovo,
Russia
Received 24th November 2004, Accepted 27th January 2005
First published as an Advance Article on the web 14th February 2005
Catalytic oxidation of N,Nꢀ-dimethylthiourea and thiourea by dioxygen in water using a new cobalt(II) complex of
octasulfophenyltetrapyrazinoporphyrazine was performed under mild conditions. The reaction is shown to include
the formation of an intermediate anionic five-coordinate complex followed by an unusual two-electron oxidation to
produce the corresponding urea and elemental sulfur (S8). Kinetic and thermodynamic parameters for the different
reaction steps of the process were determined. Drastic differences in catalytic activity of cobalt and iron
octasulfophenyltetrapyrazinoporphyrazines were observed.
therapy of cancers,19 as molecular scaffolds20 and as highly
Introduction
conducting films.21
Thiourea (TU) and N,Nꢀ-dimethylthiourea (DMTU) are widely
used in chemistry, biochemistry and technology.1,2 Thus,
thiourea is an extremely versatile chemical with applications
involving melamine resins, catalysis, stabilization of textiles,
pharmaceuticals, flame retardants, and complex reactions
involved in electrolytic processes.1 N,Nꢀ-dimethylthiourea, a
highly permeate molecule, decreases injury in a wide variety of
biological systems.2,3 The protective effect of DMTU has been
Published data show that the properties of tetrapyrazinopor-
phyrazines and their metal complexes are quite different from
their phthalocyanine analogues. The observed redox potentials
of metallotetrapyrazinoporphyrazines appear to be markedly
shifted to the cathodic region when compared with those
of metallophthalocyanines. This behaviour is consistent with
the enhanced p-acceptor properties of the pyrazine annulated
porphyrazines.22 These peculiarities may be manifested in other
properties of the mentioned macrocyclic compounds, for ex-
ample, the mechanism of catalytic oxidations by dioxygen in
the presence of these metallocomplexes. Such reactions have
received significant attention, since several critical processes in
biology involve dioxygen,23,24 and most of these processes are
promoted by transition metal ions and complexes.
In this paper we report kinetic and mechanistic information
for the oxidation of N,Nꢀ-dimethylthiourea by dioxygen in the
presence of Co(II)-octasulfophenyltetrapyrazinoporphyrazine,
[CoII{PyzPz(PhSO3)8}(H2O)2]8−, that exhibits distinct differ-
ences compared to the other catalytic oxidations of thioureas.
Some experiments were also performed with thiourea.
ascribed to its capacity to inactivate reactive oxygen species.2
–
6
Both TU7 and DMTU8 are known to be toxic compounds.
It has been argued that the degree of desulfurization during
the metabolic processes may determine the level of toxicity of
each thiourea.7 Desulfurization of thioureas in vivo probably
occurs via oxygenation, followed by nucleophilic substitution
or elimination reactions.7 Oxidative desulfurization of thiourea
also plays an important role in industry.9,10 High concentrations
of TU in industrial wastes are not acceptable due to the
toxicity of this compound.9 Thus environmental concerns have
promoted studies on the destruction of TU.
The desulfurization of thioureas has been achieved by use of
a number of different reagents, including hydrogen peroxide,11
chlorine dioxide,12 ferrate(VI)10 and other substances. Frost
tested the oxidative degradation of TU in an alkaline medium
using air on an activated charcoal surface;13 major products
were dicyandiamide and thiosulfate, whereas urea and sulfur
were the minor products. Cyanamide and dicyandiamide were
found to be the major products of photo-catalysed destruction
of TU over TiO2 powder.9 Unfortunately, since dicyandiamide
is resistant to biodegradation,14 high concentrations are not
acceptable in waste water. Much more promising would be
oxidation by air (O2) leading to urea. It is known that catalytic
oxidation of the reduced sulfur-containing compounds can be
achieved by use of metallophthalocyanines as catalysts.15 Due
to their high stability, specificity, and strong catalytic activity,
metallophthalocyanines have been engaged in many studies
Experimental
Materials
All chemicals were of p.a. grade and used as received. CAPS,
TRISINE and BIS-TRIS buffers were purchased from Sigma.
Pure thiourea and N,Nꢀ-dimethylthiourea (Aldrich) were used.
Ultrapure water was used in all measurements.
(Octaphenyltetrapyrazinoporphyrazine)cobalt(II) (1).
2
g
g
(7.1 mmol) 2,3-dicyano-5,6-diphenylpyrazine25 and 0.8
(3.2 mmol) cobalt(II) acetate tetrahydrate were added to a quartz
tube. This mixture was stirred at 250 ◦C under inert atmosphere
for 20 min. After cooling the crude product was dissolved in
50 ml of conc. H2SO4 and precipitated by addition of 200 ml
of distilled water. The green precipitate was filtered and washed
with water. Complex 1 was purified by washing with hot acetone.
Yield 1.40 g (66%). Anal. Calcd. for [CoII{PyzPzPh8}(H2O)2],
C72H44CoN16O2; C, 70.64; H, 3.62; N, 18.31%. Found: C, 70.13;
H, 3.62; N, 18.04%. M 1224 (FD-MS).
in recent years.16
aza analogues, tetrapyrazinoporphyrazines, remain much less
studied although they are receiving rapidly increasing attention
due to their effectiveness as photo-sensitizers in photodynamic
In contrast to phthalocyanines (Pc), their
–
18
(Octasulfophenyltetrapyrazinoporphyrazine)cobalt(II) (2).
Complex 1 (1.0 g, 0.82 mmol) was dissolved in HSO3Cl (5 ml),
and the mixture was refluxed for 1 h under stirring. After cooling,
† Electronic supplementary information (ESI) available: Five figures
including Eyring plots and plots of kobs as a function of pressure. See
http://www.rsc.org/suppdata/dt/b4/b417775d/
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 1 1 7 – 1 1 2 2
1 1 1 7
©

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the mixture was added to an ice cold saturated NaCl solution
in water and the precipitate was filtered off and washed with
cold distilled water to neutrality. After filtration, the green
paste was heated at 100 ◦C. The final solid material was
washed with conc. HCl. After drying, this sample was used
as catalyst. For analysis the complex can be purified using
column chromatography. Yield 0.89 g. 0.1 g of the complex
was dissolved in 2 ml of water and chromatographed twice,
firstly on a column with ‘Dowex’ (eluent: water) and secondly
using gel permeation chromatography (Molselect G9, eluent:
water). The final product was precipitated from solution in water
by addition of ethanol, filtered off, washed with ethanol and
dried under vacuum for 2 h (0.073 g). UV-vis, k/nm (H2O):
NaOH. Kinetic pKa was determined from the dependence of
kobs on pH in the reaction of 2 with KOH.
Results and discussion
Our previous studies on octasulfophenyltetrapyrazinoporphy-
razine–iron(II) showed27 that the introduction of four electron
accepting pyrazine units into the macrocycle stabilizes the
FeII cation to oxidation and makes the complex a promising
electron scavenger. The reaction of the complex with either weak
(hydrochloric acid, bromine) or strong (SO52−, Ce4+) oxidants did
not lead to the formation of the Fe(III) complex. Contrary to the
oxidation, the Fe(II) complex can easily be reduced chemically
with weak reductants such as TU or Na2SO3. It was assumed
that octasulfophenyltetrapyrazinoporphyrazine–iron(II) will be
an effective catalyst for oxidation of TU. Unfortunately, the
catalytic oxidation of TU did not proceed with measurable rates
at room temperature and pH 5–9. This prompted us to synthe-
size octasulfophenyltetrapyrazinoporphyrazine–cobalt(II) and
to study its properties.
1
351, 636. H NMR, d (D2O): 8.40–7.42 (q). Anal. Calcd. for
[CoII{PyzPz(PhSO3H)8}(H2O)2], C72H44CoN16S8O24; C, 47.19;
H, 2.42; N, 12.23; S, 14.00%. Found: C, 46.76; H, 2.42; N,
12.11%.
Separation and identification of the products of catalytic
oxidation
Sulfur S8 was separated from solution by filtration. Analytical
data: EI-MS (70 eV) 256 (100%); elemental analysis (%) calcu-
lated for S8: S 100%; found: S 99.33%. N,Nꢀ-Dimethylurea was
separated by evaporation of the filtrate and purified by double
crystallization from aqueous ethanol. 1H NMR (300 MHz,
CDCl3): d = 5.80–5.91 (s, 2H), 2.62–2.80 (t, 6H); 13C NMR
(75 MHz) d = 163.4, 27.8. Urea was separated by evaporation of
the filtrate. The precipitate was dissolved in a minimum quantity
of water and urea precipitated as nitrate after addition of nitric
acid. Mp = 151–152 ◦C (dec.) following crystallization from
water.
Properties of complex 2
Fig. 1 reports cyclic voltammograms of complex 2 at pH 5.0.
Only one reduction at −0.819 mV and two successive oxidations
at +951 and 1091 mV were observed for this complex. The
reduction is an irreversible multi-electron process and assigned
to the reduction of the porphyrazine macrocycle, accompanied
by its destruction. The first oxidation is a pseudo-reversible
process with a corresponding anodic maximum at +667 mV
assigned to the CoIII/CoII couple. The second is an irreversible
one-electron oxidation of the ligand. By comparison, the cyclic
voltammogram of octasulfophenyltetrapyrazinoporphyrazine–
iron(II) consists of two reductions at +545 and −890 mV,
and an oxidation at +1019 mV.27 For both iron and cobalt
complexes, no peaks were observed for the FeII/FeI or CoII/CoI
couples, probably as a result of slow heterogeneous electron
Instrumentation
pH measurements were performed on a Mettler Delta 350 pH
meter. The pH meter was calibrated with standard buffer
solutions at pH 4.0 and 10.0. The UV-vis spectra were recorded
on a Cary 1 spectrophotometer. IR spectra were recorded on an
AT1 Mattson FTIRInfinity spectrophotometer. NMRmeasure-
ments were performed on a Bruker Avance DPX 300 NB spectro-
meter operating at 300.13 MHz for 1H NMR. EPR spectra were
recorded on a Bruker ESR 300 E spectrometer (9.96 GHz)
in H2O and DMSO at 177 K. MS spectra were recorded on
a Jeol M700 instrument. Cyclic voltammetric measurements on
complex 2 were carried out on an AUTOLAB (Eco Chemie B.V.)
instrument at 25 ◦C in a 0.1 M aqueous solution of NaBF4.
Cyclic voltammograms were recorded by potential scanning
rates of 50, 100, 300, 500 and 1000 mV s−1. The working electrode
was gold (Metrohm), the counter electrode was platinum wire
(Metrohm), and the reference electrode was Ag/AgCl, KCl
(sat.). The system was calibrated with K3[Fe(CN)6] as standard
before the measurements.
Stopped-flow kinetics
Kinetic measurements were performed on a thermostated
(
0.1 ◦C) stopped-flow spectrophotometer (SX-17MV,
Applied Photophysics) under pseudo-first order conditions. The
changes in absorbance were monitored at 515 nm. High-pressure
stopped-flow experiments were performed at pressures up to
130 MPa on a custom-built instrument described previously.26
Kinetic traces were recorded on an IBM-compatible computer
and analysed with the OLIS KINFIT (Bogart, GA, 1989) and
Origin 7.0 set of programs.
Determination of pKa values
Two methods were used for determination of pKa values:
spectrophotometric titration and kinetic determination. (1–4) ×
10−5 M solutions of 2 in water (pH = 6.1–6.3) were titrated with
Fig. 1 Cyclic voltammograms for [CoII{PyzPz(PhSO3)8}(H2O)2]8−
(10−3 M) at 25.0 ◦C, pH = 5.0. Scan rate 0.1 V s−1
.
1 1 1 8
D a l t o n T r a n s . , 2 0 0 5 , 1 1 1 7 – 1 1 2 2

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transfer kinetics for these couples. A comparison of the
voltammograms for the cobalt and iron complexes shows that
[CoII{PyzPz(PhSO3)8}(H2O)2]8− can be easier reduced and oxi-
dized than the iron complex, but differences in electrochemical
behaviour are relatively small. However, they lead to drastic
differences in catalytic activity of the complexes (see below).
Complex 2 is soluble in water and hence the UV-vis spectra
were recorded in water. Fig. 2 shows that spectra of 2 under
aerobic and anaerobic conditions differ slightly. The electronic
spectrum of complex 2 under anaerobic conditions is typical for
metal porphyrazines and comprises of intense p–p* transition
bands in the Soret (351 nm) and Q (636 nm) regions (Fig. 2).
takes place. Obviously, similar complexes are formed also in
the case of thioureas. It should be noted that the final complex
is stable under anaerobic conditions for at least a few hours
independently of the nature of the reductant.
Attempts to obtain the UV-vis spectra of TU or DMTU
solutions in the presence of complex 2 under aerobic conditions
were unsuccessful due to rapid formation of a precipitate. Data
from EI-MS analysis clearly showed that this precipitate is ele-
mental sulfur (S8). The desulfurization of thioureas proceeds in
acidic and neutral media (pH 1–8), but not in alkaline solutions
presumably due to deactivation of the catalyst (see below). The
rate of desulfurization increases with decreasing pH and does not
depend on the type of acid (HCl, H2SO4, CH3COOH). Besides
sulfur, other products of the catalytic oxidative desulfurization
are urea and N,Nꢀ-dimethylurea (DMU), respectively. It is
known that one-electron oxidation of thiourea in acidic media
leads to formamidinedisulfide.29 The carbamidinothiyl radical,
•
=
NH2( NH)CS , is the first product of a one-electron oxidation
of TU.30 The use of a spin trap 5,5-dimethyl-1-pyrroline-N-oxide
(DMPO), allows the detection of this radical. The stability of
the carbamidinothiyl radical was found to depend substantially
on pH; the most intense EPR signal being observed at pH 2.5–
3.0.30 However, in the present study we were unable to detect any
formation of this radical upon oxidation of TU with dioxygen in
acidic solutions in the presence of complex 2 and DMPO. Ad-
ditional evidence for the absence of formamidinedisulfide in
the intermediate steps of TU oxidation comes from the pH
dependence of the rate of sulfur formation (due to difficulties
with kinetic measurements of sulfur formation, Fig. 3 shows the
pH dependence of the quantity of sulfur formed in the same
time from thiourea under aerobic conditions). It is known that
formamidinedisulfide is unstable at high pH,30 and the rate of its
decomposition to sulfur increases in more alkaline solutions.
In contrast to this, in the present study the rate of sulfur
formation increases in more acidic solutions (Fig. 3). Thus,
all available data confirm a two-electron mechanism for the
oxidative desulfurization of thioureas catalysed by complex 2.
Fig. 2 UV-Vis spectra of [CoII{PyzPz(PhSO3)8}(H2O)2]8− (8 × 10−6 M)
in H2O in air saturated solution (dotted line), after bubbling with argon
(solid line), after addition of an excess of DMTU (broken line); last
spectrum was recorded for [CoII{PyzPz(PhSO3)8}(H2O)2]8− = 1.1 ×
10−5 M.
Interaction of complex 2 with TU and DMTU
On addition of TU or DMTU to a solution of 2, the colour
changes from green to red. Fig. 2 shows that in the presence of
reductant a new intense band at 515 nm appears, whereas the
Q-band completely disappears. Similar spectral changes were
observed in the reaction with either strong (sodium dithionite)
or weak reductants (diethylamine in water with complex 2 or in
DMSO with complex 1). Therefore, it is reasonable to assume
that in all cases the reduction of complex 2 leads to the formation
of the same product. It is important to note that addition
of Na2S2O4 to a solution of Co(II) tetrasulfophthalocyanine
[CoII(TSPc)4−] leads to very different changes than observed
for octasulfophenyltetrapyrazinoporphyrazine–cobalt(II). In the
case of CoII(TSPc)4−, an intense absorption maximum at 450 nm
appears,18 and simultaneously the Q-band is red shifted. The
observed spectrum is identical to that reported for CoI(TSPc)5−.
If the reduction of complex 2 would also lead to formation
of a Co(I) complex, the appearance of a blue shifted band
compared to 450 nm is expected, since metalloporphyrazines
always absorb at lower wavelengths than the corresponding
metallophthalocyanines.28 Obviously, the dramatic changes in
UV-vis spectrum of complex 2 after reduction indicate strong
changes in the p-system of the ligand. Additional data on the
Fig. 3 pH dependence of quantity of S8 formed from thiourea under
aerobic conditions, [CoII{PyzPz(PhSO3)8}(H2O)2]8− = 1.2 × 10−5 M,
[TU] = 0.48 M, 25.0 ◦C, reaction time 24 h, reaction volume 5 ml.
Kinetic and mechanistic studies
1
structure of the reaction products can be obtained from H
The kinetic study of the catalytic desulfurization of DMTU
involved two series of experiments. In the first series the reaction
of an excess of DMTU with complex 2 under anaerobic
conditions and pH 7 was studied. A straight line plot of 1/kobs1 vs
1/[DMTU]0 (Fig. 4) shows that the reaction follows Michaelis-
Menten kinetics which is characteristic for intramolecular
electron transfer involving a rapid pre-equilibrium. The cor-
responding rate law is given in (3).
NMR. The overlap of signals from CH3 groups in free and
coordinated DMTU does not allow the determination of the
structure of reduced complex 2. Fortunately, no such overlap has
been observed for free and coordinated diethylamine (at least 10-
fold excess of (C2H5)2NH was used in the mixture of complex
1 and diethylamine in DMSOd6). It was therefore possible to
determine the correlation between integral intensities of the
signals of aromatic protons in eight phenyl groups of complex
1 and CH2 groups in coordinated diethylamine which was equal
to 10 : 1. This result shows that the formation of a 1 : 1 complex
[CoII{PyzPz(PhSO3)8}(H2O)2]8− + DMTU ꢀ
[CoII{PyzPz(PhSO3)8}(DMTU)(H2O)]8− + H2O; K1
(1)
D a l t o n T r a n s . , 2 0 0 5 , 1 1 1 7 – 1 1 2 2
1 1 1 9

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[CoII{PyzPz(PhSO3)8}(DMTU)(H2O)]8−
→
[CoII{PyzPz−(PhSO3)8}(DMTU+)]8− + H2O; k2
(2)
(3)
3
kobs1 = k2K1[DMTU]/{1 + K1[DMTU]}
The values of K1 and k2 from Fig. 4 are 2.1 0.2 M−1 and
0.21 0.01 s−1 at 298 K, respectively. It should be noted that
no sulfur formation was observed during the experiments under
anaerobic conditions. This indicates that no disulfide is formed
from DMTU and an oxidized form of DMTU does not leave
the coordination sphere of the complex. The results for TU were
comparable to the results obtained with DMTU, but in this case
rates of reduction were approx. 5 times slower than with DMTU
under the same conditions.
Fig. 5 Plotsoflnkobs2 vs.[O2]atdifferenttemperatures.Experimentalcon-
ditions: 0.1 M TRIS buffer, pH = 7, [CoII{PyzPz(PhSO3)8}(H2O)2]8−
(white triangles), 25.0 ◦C (black triangles), 30.0 ◦C (white circles).
=
2.1 × 10−6 M, 6.0 ◦C (black squares), 15.0 ◦C (black circles), 20.0 ◦C
4 J K−1 mol−1, Fig. S2) and an unusually large negative activation
volume (−23.9
0.5 cm3 mol−1, Fig. 6). The activation data
show that oxidation of complex 3ꢀ is an associative process
accompanied by addition of dioxygen to cobalt, followed by fast
intramolecular electron transfer. The first step of this process
may be represented as follows:
[CoII{PyzPz−(PhSO3)8}NH2OH+]8− + O2 →
3ꢀ
[CoII{PyzPz(PhSO3)8}NH2OH+(O−)]8−; k3
(4)
2
Fig. 4 Plot of 1/kobs1 vs. 1/[DMTU]. Experimental conditions: 0.1 M
BIS-TRIS buffer, pH = 7.0, 25.0 ◦C, [CoII{PyzPz(PhSO3)8}(H2O)2]8−
1.46 × 10−5 M.
=
4
The determination of the activation volume for this reaction
was performed for a high excess of DMTU to prevent the
influence of the pre-equilibrium, i.e. where kobs1 = k2. The
obtained data for DV= = +14.1
0.6 cm3 mol−1 (Fig. S1†)
confirms the conclusion that the second step is a dissociation of
the six-coordinate complex to the five-coordinate product and
water (it is known that dissociation of coordinated water via a
dissociative mechanism has a DV= value of ca. +13 cm3 mol−1 for
an octahedral complex31). The formation of the five-coordinate
complex is accompanied by inner-sphere electron transfer from
DMTU to the macrocyclic ligand as shown in (2).
As mentioned above, the reaction of complex 2 with TU
or DMTU under aerobic conditions leads to formation of
sulfur that makes it difficult to study the second part of the
catalytic process, viz. oxidation of the five-coordinate complex
by dioxygen. On assuming that the mechanism of this reaction
does not depend on the type of reductant taken for preparing
of the five-coordinate complex, hydroxylamine sulfate was used
instead of TU or DMTU. A solution of the five-coordinate
complex in neutral buffer was prepared in a syringe. The
end of the reduction process (formation of the red complex
[CoII{PyzPz−(PhSO3)8}NH2OH+]8−, 3ꢀ) was determined on the
basis of spectral data. The second syringe contained a buffered
solution (pH 7) with different concentrations of dioxygen. In all
experiments an excess of dioxygen was used.
Fig. 6 Plot of kobs2 vs. pressure. Experimental conditions: 0.1 M TRIS
◦
buffer, pH = 7, 3, 25.0 C, [CoII{PyzPz(PhSO3)8}(H2O)2]8− = 2.1 ×
10−6 M, [O2] = 1.2 × 10−4 M.
In the case of hydroxylamine sulfate, the subsequent reduction
of the six-coordinate complex 4 proceeds slowly (s1/2 is approx.
2 h at pH 7). This creates the possibility of determining the rate
constant k3 = (3.07 0.15) × 105 M−1 s−1 at 25 ◦C.
Studies on the influence of pH on the catalytic oxidation of
thioureas show that the higher the pH, the slower the reaction,
and in strongly alkaline solutions the oxidation practically does
not proceed at all. It should be noted that this is valid for both
steps of the catalytic process, viz. reduction of complex 2 and
oxidation of complex 3 (Fig. 3). Therefore, it is reasonable to
assume that 2 forms a stable complex with OH−, which is inert
in the redox process. The interaction of complex 2 with OH− is
well described by pseudo-first-order kinetics and proceeds quite
slowly. This indicates that the rate-determining step is not an
The decrease in concentration of complex 3ꢀ with time is
satisfactorily described by an exponential law. A plot of kobs2
vs. [O2]0 is linear over the selected concentration range of
dioxygen (Fig. 5). The oxidation of complex 3ꢀ was found to be
irreversible. The overall reaction is characterized by a relatively
small activation enthalpy (29.3
0.9 kJ mol−1, Fig. S2†).
This result shows that the reaction is not accompanied by
the breakage of any strong bonds. On the other hand, the
reaction is characterized by a negative activation entropy (−40
1 1 2 0
D a l t o n T r a n s . , 2 0 0 5 , 1 1 1 7 – 1 1 2 2

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acid–base interaction. The UV-vis spectra show that the product
of this reaction is a complex similar to that in the case of the
thioureas (Fig. 2). The pH dependence of kobs3 is given in Fig. 7.
The activation parameters for this reaction at pH 13 are: DH= =
+14.1 0.6 cm3 mol−1 (Fig. S3 and S4, respectively†). A high
positive value of DV= and the saturation of kobs3 at high pH
indicate that the most plausible mechanism is rapid formation of
the aquahydroxo complex, followed by rate-determining disso-
ciation of a water molecule to form the five-coordinate hydroxo
complex. The observed volume of activation is very typical for
the dissociation of water from an octahedral complex.31
two consecutive steps without the intermediate formation of
disulfide. Seemingly, a rate-limiting step is the addition of water
to a product of a one-electron reduction of DMTU (TU).
Since the rate of the overall reaction significantly increases
in acidic solutions and does not depend on the type of
acid used, it is reasonably to conclude that the reaction is
catalysed by protons. Indeed, water is known to be a weak
77
4 kJ mol−1, DS= = −29
13 J K−1 mol−1 and DV=
=
=
nucleophile and for its addition to a C N bond a preliminary
activation of the substrate by specific catalysis is mandatory.
Furthermore, the product formed is oxidized to sulfur and
the corresponding urea. A regeneration of the catalyst in-
volves one-electron oxidation of the five-coordinate complex
by a second molecule of dioxygen. Importantly, in contrast
to a majority of catalytic oxidations by dioxygen, here the
catalyst does not activate a molecule of O2 but the substrate
itself. Due to its extremely strong electron accepting proper-
ties, (octasulfophenyltetrapyrazinoporphyrazine)–cobalt(II) eas-
ily accepts an electron from the substrate (DMTU or TU). After
that, the complex formed is also easily oxidized by dioxygen.
The product of dioxygen reduction, viz. superoxide, seems to
disproportionate to dioxygen and hydrogen peroxide. The latter
disproportionates to dioxygen and water (it is well known that
metallophthalocyanines and metalloporphyrazines catalyse this
reaction32). In the case of (octasulfophenyltetrapyrazinopor-
phyrazine)iron(II), dioxygen cannot oxidize a corresponding
five-coordinate complex. This explains the very weak catalytic
activity of the iron complex in the studied processes.
[CoII{PyzPz(PhSO3)8}(H2O)2]8−
ꢀ
[CoII{PyzPz(PhSO3)8}(H2O)(OH)]9− + H+; K4
(5)
(6)
[CoII{PyzPz(PhSO3)8}(H2O)(OH)]9−
→
[CoII{PyzPz(PhSO3)8}(OH)]9− + H2O; k5
Acknowledgements
The authors gratefully acknowledge financial support from a
NATO Collaborative Linkage Grant (PST.CLG.979442) and a
grant from the Russian Ministry of Education (E 02–5.0–345).
References
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Fig. 7 Plot of kobs3 vs. pH. Experimental conditions: 25.0 ◦C,
[CoII{PyzPz(PhSO3)8}(H2O)2]8− = 1.46 × 10−5 M.
2 W. E. Curtis, M. E. Muldrow, N. B. Parker, R. Barkley, S. L. Linas
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performed (Fig. S5†). The pK4 value (12.4
0.1) is in good
agreement with the observed kinetic results (see Fig. 7).
From a combination of the data for the reaction between
complex 2 and DMTU under anaerobic conditions and for the
reaction of the five-coordinate complex 3ꢀ with dioxygen at pH 7,
the overall scheme given above for the catalytic desulfurization
of DMTU can be postulated.
This scheme is also applicable for TU. The scheme shows that
oxidation of five-coordinate complex 3 by dioxygen involves
D a l t o n T r a n s . , 2 0 0 5 , 1 1 1 7 – 1 1 2 2
1 1 2 1

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1 1 2 2
D a l t o n T r a n s . , 2 0 0 5 , 1 1 1 7 – 1 1 2 2