Kinetics and mechanism of the iron phthalocyanine catalyzed reduction of nitrite by dithionite and sulfoxylate in aqueous solution

Article abstract of DOI:10.1021/ic0505955

The reactions of sodium nitrite with sodium dithionite and sulfoxylate ion were studied in the presence of iron(III) tetrasulfophthalocyanine, Fe ^III(TSPc)^3-, in aqueous alkaline solution. Kinetic parameters for the different reaction steps in the catalytic reduction by dithionite were determined. The final product of the reaction was found to be nitrous oxide. Contrary to this, the product of the catalytic reduction of nitrite by sulfoxylate was found to be ammonia. The striking difference in the reaction products is accounted for in terms of different structures of the intermediate complexes formed during the reduction by dithionite and sulfoxylate, in which nitrite is suggested to coordinate to the iron complex via nitrogen and oxygen, respectively. Sulfoxylate is shown to be a convenient reductant for the synthesis of the highly reduced iron phthalocyanine species Fe^I(TSPc^?)^6- in aqueous solution. The kinetics of the reduction of Fe^I(TSPc)^5- to Fe ^I(TSPc^?)^6-, as well as the oxidation of the latter species by nitrite, was studied in detail.

Full text of DOI:10.1021/ic0505955

Inorg. Chem. 2005, 44, 6470−6475
Kinetics and Mechanism of the Iron Phthalocyanine Catalyzed
Reduction of Nitrite by Dithionite and Sulfoxylate in Aqueous Solution
Evgeny V. Kudrik,*,†,‡ Sergei V. Makarov,‡ Achim Zahl,† and Rudi van Eldik*,†
Institute for Inorganic Chemistry, UniVersity of Erlangen-Nu¨rnberg, Egerlandstrasse 1,
91058 Erlangen, Germany, and IVanoVo State UniVersity of Chemistry and Technology,
aV. F. Engels 7, 153460 IVanoVo, Russia
Received April 17, 2005
The reactions of sodium nitrite with sodium dithionite and sulfoxylate ion were studied in the presence of iron(III)
tetrasulfophthalocyanine, FeIII(TSPc)3-, in aqueous alkaline solution. Kinetic parameters for the different reaction
steps in the catalytic reduction by dithionite were determined. The final product of the reaction was found to be
nitrous oxide. Contrary to this, the product of the catalytic reduction of nitrite by sulfoxylate was found to be
ammonia. The striking difference in the reaction products is accounted for in terms of different structures of the
intermediate complexes formed during the reduction by dithionite and sulfoxylate, in which nitrite is suggested to
coordinate to the iron complex via nitrogen and oxygen, respectively. Sulfoxylate is shown to be a convenient
-
reductant for the synthesis of the highly reduced iron phthalocyanine species FeI(TSPc•)6 in aqueous solution.
-
The kinetics of the reduction of FeI(TSPc)5 to FeI(TSPc•)6-, as well as the oxidation of the latter species by nitrite,
was studied in detail.
Introduction
recently shown to be the first step in the catalytic reaction
between sodium dithionite and sodium nitrite.13 The final
product of the reaction was found to be ammonia. Contrary
to the reduction of nitrite, the products of the catalytic
reduction of nitrate were found to be dinitrogen and nitrous
oxide. The striking differences in the reduction products of
nitrite and nitrate are explained in terms of different structures
of the intermediate complex formed between CoI phthalo-
cyanine and the substrate, in which nitrite and nitrate are
suggested to coordinate via nitrogen and oxygen, respec-
tively.
Phthalocyanine complexes are effective catalysts for a
variety of reactions.1-9 The chemical, electrochemical,
photochemical, and radiolytic reductions of metallophtha-
locyanines strongly depend on the nature of the central metal
atom.10,11 For the Cu, Ni, and metal-free compounds, the data
show that the reduction involves only the ligand.10 For the
Co and Fe complexes, the first step of the reduction yields
CoI and FeI, respectively.10-12 The reduction of CoII tetra-
sulfophthalocyanine, CoII(TSPc)4-, to the CoI complex was
The possibility of different coordination modes of nitrite
in cytochrome cd1 reductase has recently been supported by
DFT calculations.14 Although the N-isomer is energetically
favored over the O-isomer, a single strong hydrogen bond
may provide the energy required to place the two isomers
on the same energy level. Important results have also recently
been obtained for Cu-containing reductase.15 The crystal
structure of a type two copper-nitrosyl complex of nitrite
reductase reveals an unprecedented side-on binding mode
in which the nitrogen and oxygen atoms are nearly equidis-
* To whom correspondence should be addressed. E-mail: vaneldik@
chemie.uni-erlangen.de (R.E.), ttoc@isuct.ru (E.V.K.).
† University of Erlangen-Nu¨rnberg.
‡ Ivanovo State University of Chemistry and Technology.
(1) Jain, S. L.; Sain, B. J. Mol. Catal. A: Chem. 2003, 195, 283.
(2) Sharma, V. B.; Jain, S. L.; Sain, B. Catal. Lett. 2004, 94, 57.
(3) Perollier, C.; Sorokin, A. B. Chem. Commun. 2002, 1548.
(4) Schutten, J. H.; Zwart, J. J. Mol. Catal. 1979, 5, 109.
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(6) Raja, R.; Jacob, C. R.; Ratnasamy, P. Catal. Today 1999, 49, 171.
(7) Stuchinskaya, T.; Kundo, N.; Gogina, L.; Schubert, U.; Lorenz, A.;
Maizlish, V. J. Mol. Catal. A: Chem. 1999, 140, 235.
(8) Hoffmann, M. R.; Lim, B. C. EnViron. Sci. Technol. 1979, 13, 1406.
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(10) Rollmann, L. D.; Iwamoto, R. T. J. Am. Chem. Soc. 1968, 90, 1455.
(11) Grodkowski, J.; Dhanasekaran, T.; Neta, P.; Hambright, P.; Brun-
schwig, B. S.; Shinozaki, K.; Fujita, E. J. Phys. Chem. A 2000, 104,
11332.
(13) Kudrik, E. V.; Makarov, S. V.; Zahl, A.; van Eldik, R. Inorg. Chem.
2003, 42, 618.
(14) Silaghi-Dumitrescu, R. Inorg. Chem. 2004, 43, 3715.
(15) Tocheva, E. I.; Rosell, F. I.; Mauk, A. G.; Murphy, M. E. P. Science
2004, 304, 867.
(12) Liao, M.-S.; Scheiner, S. J. Chem. Phys. 2001, 114, 9780.
6470 Inorganic Chemistry, Vol. 44, No. 18, 2005
10.1021/ic0505955 CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/04/2005

Catalyzed Reduction of Nitrite by Dithionite and Sulfoxylate
tant from the copper cofactor. Comparison of this structure
with a refined nitrite-bound crystal structure indicates how
coordination can change between copper-oxygen and
copper-nitrogen during catalysis.
Despite the significant progress in the understanding of
the mechanisms of catalytic nitrite reductions, several
problems remain unresolved. One of them is how the type
of reductant influences the rate of the overall reduction of
nitrite and the composition of the products. In continuation
of our studies on the reactions catalyzed by metallophtha-
locyanines, here we report kinetic data and their mechanistic
interpretation for the reaction between sodium nitrite and
sodium dithionite or sulfoxylate in the presence of iron(III)
phthalocyanine. Thiourea dioxide (TDO) served as a precur-
sor of sulfoxylate.16 The reported data demonstrate the
distinctive differences in the reactivity of sulfoxylate and
dithionite and enable a comparison of the role of iron and
cobalt phthalocyanines in the catalytic reduction of nitrite.
Figure 1. UV-vis spectra of FeIII(TSPc)3- (8.6 × 10-6 M, solid line),
FeII(TSPc)4- (8.6 × 10-6 M, dashed line), FeI(TSPc)5- (6.2 × 10-6 M,
dotted line), and FeI(TSPc•)6- (4.3 × 10-6 M, thick solid line) at pH 13
(0.1 M KOH).
Experimental Section
recorded were limited by the dead time of the stopped-flow
instrument at higher dithionite concentrations and tempera-
tures), i.e.,
Materials. Sodium nitrite, sodium dithionite (85% grade), and
thiourea dioxide (TDO) were obtained from Aldrich and used as
received. Iron(III) phthalocyanine was prepared and purified using
a literature method.17 BIS-TRIS, TRISINE, and CHES buffers were
used to control the pH. Na15NO2 (95% 15N grade) was used for
15N NMR analysis. Oxygen-free nitrogen was used to deoxygenate
solutions. Ultrapure water was used in all measurements.
Kinetic Measurements and Instrumentation. Conventional
kinetic experiments were performed on a Cary 1 or Cary 5 UV-
vis spectrophotometer under anaerobic conditions. No salt was
added to control the ionic strength. The data were analyzed using
Origin 6.1 and SPECFIT software. A thermostated ((0.1 °C)
Applied Photophysics SX 18MV stopped-flow spectrophotometer
with an optical path length of 1.0 cm was used to follow the faster
reactions. High-pressure stopped-flow experiments were performed
at pressures up to 130 MPa on a custom-built instrument described
previously18 and analyzed with the OLIS KINFIT (Bogart, GA,
1989) set of programs. 15N NMR measurements were performed
using a Bruker Avance DRX 400 WB spectrometer equipped with
a superconducting BS-94789 magnet system at 40.56 MHz. 15N
chemical shifts were referenced externally to neat nitromethane.
D2O (99%) was used as a solvent for all NMR measurements.
2-
kobsd1 ) k1[S2O4
]
(1)
The value of k1 at pH 8.06 and 25 °C is (2.63 ( 0.09) ×
10-3 M-1 s-1. The first-order dependence on dithionite
2-
concentration indicates that direct reduction of - by S2O4
-
occurs and does not involve the monomeric species SO2
formed in reaction 2. Further mechanistic information can
-
S2O42- a 2SO2
(2)
be obtained from the thermal activation parameters which
were determined from the Eyring plot over the temperature
range 5-30 °C. The values of ∆H* and ∆S* were found to
be 46 ( 3 kJ mol-1 and +23 ( 11 J K-1 mol-1, respectively.
2-
-
It is known that both S2O4 and SO2 , in general, act as
outer-sphere reductants.19,20 Our data do not contradict this
2-
conclusion. Although such a reduction by S2O4 is not
common21 (more frequently, the actual reductant is the sulfur
-
dioxide anion radical SO2 ), it occurs, for example, in
Results and Discussion
3- 23
ferricytochrome c,22 Fe(CN)6
,
and some CoIII com-
plexes.19
Reaction of Sodium Dithionite with Iron Tetrasulfo-
phthalocyanine. The reaction of an excess of sodium
dithionite with iron tetrasulfophthalocyanine, FeIII(TSPc)3-,
is accompanied by two consecutive color changes. The first
is a very fast change from green to blue. An intense
absorption maximum appears at 671 nm (Figure 1). This
spectrum is essentially identical to that reported11 for
FeII(TSPc)4-. The rate of reduction of the FeIII complex was
found to show a linear dependence on the dithionite
concentration (Supporting Information Figure S1; the data
The second reduction step is accompanied by a color
change of the solution from blue to brown and is much
slower than the first. Absorbance maxima at 480 and 712
nm appear with simultaneous disappearance of the maximum
at 671 nm (Figure 1). This spectrum is very close to that
reported for FeI tetrasulfophthalocyanine.11 The rate of
reduction of the FeII complex determined from the increase
2- 0.5
in the absorbance at 480 nm depends linearly on [S2O4
]
( Supporting Information Figure S2). This observation shows
(16) Makarov, S. V.; Kudrik, E. V.; van Eldik, R.; Naidenko, E. V. J. Chem.
Soc., Dalton Trans. 2002, 4074.
(17) Weber, J. N.; Busch, D. H. Inorg. Chem. 1965, 4, 469.
(18) (a) van Eldik, R.; Palmer, D. A.; Schmidt, R.; Kelm, H. Inorg. Chim.
Acta 1981, 50, 131. (b) van Eldik, R.; Gaede, W.; Wieland, S.; Kraft,
J.; Spitzer, M.; Palmer, D. A. ReV. Sci. Instrum. 1993, 64, 1355.
(19) Mehrotra, R. N.; Wilkins, R. G. Inorg. Chem. 1980, 19, 2177.
(20) Tobe, M. L.; Burgess, J. Inorganic Reaction Mechanisms; Longman
Inc.: New York, 1999; p 475.
(21) Tsukahara, K.; Wilkins, R. G. J. Am. Chem. Soc. 1985, 107, 2632.
(22) Lambeth, D. O.; Palmer, G. J. Biol. Chem. 1973, 248, 6095.
(23) Scaife, C. W. J.; Wilkins, R. G. Inorg. Chem. 1980, 19, 3244.
Inorganic Chemistry, Vol. 44, No. 18, 2005 6471

Kudrik et al.
that the reduction process is consistent with eq 3, where k2
-
is the second-order rate constant for the reaction of SO2
2- 0.5
kobs2 ) k2K0.5[S2O4
]
(3)
with the FeII complex and K is the equilibrium constant for
dissociation of S2O42- (reaction 2, viz., 1.4 × 10-9 M at 25
°C).21 Using the values of kobs2 and K, a rate constant, k2,
2- 0.5
can be estimated from the dependence of kobs2 on [S2O4
]
(Supporting Information Figure S2): k2 ) 225 ( 18 M-1
s-1 at 25 °C. To the best of our knowledge, this is the first
time the rate constant for the reduction of the FeII complex
by the SO2- anion radical to the corresponding FeI complex
has been determined.
Reaction of Sulfoxylate with Iron Tetrasulfophthalo-
cyanine. The use of sulfoxylate as a reductant leads to
substantial changes in the reaction. It should be noted that
“aged” alkaline solutions of TDO were used to produce
sulfoxylate (herein, the term aged means solutions stored for
4-24 h in 0.1 M NaOH under anaerobic conditions).16 This
provides a possibility to prevent the influence of the
decomposition of TDO to sulfoxylate (HSO2- or SO22-) and
urea and to study the reaction of sulfoxylate with FeIII(TSPc)3-.
The N-containing product of TDO decomposition, viz.,
urea,24 does not affect the redox process. Importantly, in the
case of sulfoxylate, the reaction does not stop with the FeI
complex as the final product. After the appearance of the
absorption maxima at 480 and 712 nm (FeI complex), the
red shift of the first and the gradual disappearance of the
second maxima are observed (the final spectrum is shown
in Figure 1). The color of the solution turns red. Because
the rates of formation and reduction of the FeI complex differ
substantially (Supporting Information Figure S3), it is
possible to study them independently using the absorption
maximum at 712 nm.
Figure 2. Plots of ln kobs1 and ln kobs2 vs pressure for the reaction between
FeII(TSPc)4- and sulfoxylate: triangles represent the first stage, and squares
represent the second stage at pH ) 13 (0.1 M KOH). Best fits of the data
(lines) give ∆V* ) -12 ( 2 and ∆V* ) -5.5 ( 0.6 cm3 mol-1
.
1
2
determining step. For reactions between FeII complexes with
porphyrin-like ligands and weak nucleophiles, for example,
water, pyridine, and pyrazine, a dissociative mechanism was
observed.25,26 Our data show that, in the case of a strong
nucleophile such as sulfoxylate, an associatively induced
redox mechanism operates.
For reduction of the FeI complex, both ∆H* and ∆S* have
substantially higher values (104 ( 2 kJ mol-1 and -139 (
5 J K-1 mol-1, respectively; see Supporting Information
Figure S7) than those for the reduction of FeII. The value of
∆V* is -5.5 ( 0.6 cm3 mol-1 (Figure 2). The small negative
value of ∆V* can seemingly be explained by solvation effects
because the transition state has a larger negative charge and,
as a consequence, is more effectively solvated. These data
show that the change in the iron redox state leads to drastic
changes in the mechanism of reduction. Indeed, the increase
in the overall negative charge on the iron complex leads to
stronger electrostatic repulsion with negatively charged
sulfoxylate. FeI, itself, has a large negative charge; therefore,
the attack by nucleophilic sulfoxylate will be difficult. In
addition, FeI(TSPc)5-, being a low-spin complex,11 has a low
reactivity in ligand substitution reactions. Thus, the data
mentioned above show that the most plausible mechanism
for the reaction between the FeI complex and sulfoxylate is
an outer-sphere electron transfer from sulfoxylate to the
tetrasulfophthalocyanine ligand.
The kinetics of reduction of iron tetrasulfophthalocyanine
by sulfoxylate was studied in 0.1 M NaOH under pseudo-
first-order conditions with an excess of sulfoxylate. The
reduction of the FeIII complex proceeds very quickly and
could not be studied using stopped-flow techniques. The rates
of both subsequent reactions (FeII(TSPc)4- to FeI(TSPc)5-
and FeI(TSPc)5- to FeI(TSPc•)6-) depend linearly on the
concentration of sulfoxylate (Supporting Information Figures
S4 and S5) because sulfoxylate is the only sulfur-containing
product of the decomposition of TDO in deoxygenated,
strongly alkaline solutions.16 Because it is stable in these
solutions, its concentration is assumed to be equal to the
initial concentration of thiourea dioxide. For reduction of
the FeII complex, the following thermal activation parameters
were found: ∆H* ) 62 ( 3 kJ mol-1 and ∆S* ) -21 ( 8
J K-1 mol-1 (Supporting Information Figure S6). The volume
of activation was determined from the pressure dependence
of the rate constant (∆V* ) -12 ( 2 cm3 mol-1, Figure 2).
The small negative value of ∆S* and the large negative value
of ∆V* suggest an associative process (A or Ia mechanism)
presumably due to inner sphere reduction of the FeII complex,
Reaction of Sodium Dithionite with Sodium Nitrite in
the Presence of Iron Tetrasulfophthalocyanine. The
catalytic reaction was studied at 315 nm for [NaNO2] .
[Na2S2O4] (direct reaction between dithionite and nitrite does
not proceed with measurable rates in alkaline solutions at
15-50 °C).16 Although nitrite also absorbs at 315 nm, its
(24) Svarovsky, S. A.; Simoyi, R. H.; Makarov, S. V. J. Chem. Soc., Dalton
Trans. 2000, 511.
(25) Kudrik, E. V.; van Eldik, R.; Makarov, S. V. Dalton Trans. 2004,
429.
(26) (a) Pennesi, G.; Ercolani, C.; Ascenzi, P.; Brunori, M.; Monacelli, F.
J. Chem. Soc., Dalton Trans. 1985, 1107. (b) Brunori, M.; Pennesi,
G.; Ercolani, C.; Monacelli, F. J. Chem. Soc., Dalton Trans. 1990,
105.
2-
in which the associative attack of the strong SO2 nucleo-
phile on the electron-deficient FeII center is the rate-
6472 Inorganic Chemistry, Vol. 44, No. 18, 2005

Catalyzed Reduction of Nitrite by Dithionite and Sulfoxylate
Figure 4. pH dependence of the rate of nitrite (0.056 M) reduction by
dithionite (3 × 10-4 M) in the presence of FeII(TSPc)4- (4.3 × 10-6 M).
Experimental conditions: 25 °C, 0.1 M BIS-TRIS buffer (pH 7-7.5), 0.1
M TRISINE buffer (pH 7.6-8.7), and 0.1 M KOH (pH 13).
-
2
Figure 3. Plot of kobs4 vs [NO2
] for the reaction between sodium
dithionite and sodium nitrite in the presence of FeII(TSPc)4- (4.3 × 10-6
M). Experimental conditions: 0.1 M TRISINE buffer, pH ) 8.06, 25 °C,
[S2O42-] ) 2.8 × 10-4 M.
molar absorbance coefficient (11 M-1 cm-1) is much lower
than that for dithionite27 (8043 M-1 cm-1). Because NaNO2
was used in excess, changes in its absorbance at 315 nm
during the reaction are negligible. Therefore, the observed
decrease in absorbance at 315 nm is associated with a
decrease in the dithionite concentration (Supporting Informa-
tion Figure S8). The rate of the reaction depends linearly on
[NaNO2]2 (Figure 3). Therefore, the overall reaction for nitrite
reduction by dithionite may be written as
S2O42- + 2NO2- f 2SO32- + 2NO
k4
(4)
Importantly, under the same conditions, the catalytic
reduction of nitrite proceeds much faster than the reduction
of the FeII complex by dithionite (Supporting Information
Figure S8). Thus, reduction of FeII to FeI tetrasulfophthalo-
cyanine does not play an important role in the catalytic
reduction of nitrite.
The pH dependence of the observed rate constant is shown
in Figure 4. The data indicate that the reaction is substantially
accelerated in more acidic media. Unfortunately, we could
not determine the pKa value of coordinated nitrite nor the
rate constant for reduction of protonated nitrite because, in
acidic media, dithionite is unstable and decomposes auto-
catalytically.28 However, the data in Figure 4 show that, as
in the case of CoII(TSPc)4-,13 the most plausible role of the
catalyst is to shift the reaction into a more alkaline pH range,
i.e., to increase the pKa of the conjugated acid of the nitrite
coordinated to the metal center.
The analysis of the reaction products has been performed
using 15N NMR spectroscopy. In the 15N NMR spectra (see
Figure 5), four new signals were observed at 164, -156.3,
-217.1, and -236.5 ppm. The first signal corresponds to
coordinated nitrite, the second and fourth correspond to N2O
(nitrous oxide has signals at -147.3 and -237.0 ppm in the
Figure 5. 15N NMR spectrum of the products of the reaction between
sodium dithionite and an excess of Na15NO2 in 0.5 M NaOD in D2O in the
presence of FeIII(TSPc)3- (1 × 10-3 M).
gas phase),29 and the most intense, third signal seemingly
corresponds to the nitrosyl complex of FeII(TSPc)4-. The
presence of N2O signals and the absence of signals corre-
sponding to free or coordinated ammonia (they are located
in the -280 to -400 ppm region)13,30 show that the reaction
of nitrite with dithionite catalyzed by iron tetrasulfophtha-
locyanine proceeds with formation of nitrous oxide, which
differs drastically from the reaction catalyzed by CoIITSPc.13
(27) McKenna, C. E.; Gutheil, W. G.; Song, W. Biochim. Biophys. Acta
1991, 1075, 109.
(28) Cermak, V.; Smutek, M. Collect. Czech. Chem. Commun. 1975, 40,
(29) Mastickhin, V. M.; Mudrakovsky, I. L.; Filimonova, S. V. Chem. Phys.
Lett. 1988, 149, 175.
3241.
Inorganic Chemistry, Vol. 44, No. 18, 2005 6473

Kudrik et al.
Scheme 1
The most plausible explanation for this is the difference in
the structure of the intermediate complex formed between
FeII(TSPc)4- or CoI(TSPc)5- and the substrate, in which
nitrite is suggested to coordinate via oxygen and nitrogen,
respectively. Although, in iron porphyrin complexes, nitrite
always binds through the nitrogen atom to form nitro com-
plexes,31 O-coordination of nitrite is known for iron com-
plexes with a ligand such as tris(3,5-dimethylpyrazolyl)bor-
ate,32 as well as for ruthenium and manganese porphyrin sys-
tems.31 Unfortunately, so far no structural information on
metallophthalocyanine complexes with nitrite has been re-
ported. However, because phthalocyanines are known to be
stronger electron acceptors than porphyrins, it is reasonable
Figure 6. 15N NMR monitoring of the reduction of Na15NO2 with an
excess of sulfoxylate in 0.5 M NaOD in D2O in the presence of FeIII(TSPc)3-
(1 × 10-3 M) at pH 13.6. (a) After addition of thiourea dioxide to the
solution (1196 scans). (b) After 26 h (2500 scans). (c) After 65 h (986
scans).
below). It should be noted that, in the absence of the catalyst,
sulfoxylate very slowly reduces nitrite to N2, whereas in the
presence of the catalyst, nitrite is reduced to NH3. Further-
more, no spectral changes are observed during the catalytic
cycle. If all of the sulfoxylate reacts with the excess of nitrite,
then the Fe(II) complex is formed. However, the UV-vis
spectra are not that characteristic because of the interference
of other potential nucleophiles present in the solution. It can
therefore be concluded that the mechanism of catalytic
reduction by sulfoxylate in the presence of the iron complex
is very similar to the reduction by dithionite in the presence
of CoII(TSPc)4-. In both cases, the final product is ammonia.
Consequently, it is reasonable to assume that, in the
intermediate complexes, nitrite coordinates via nitrogen.
-
to assume that, in the complex of FeII(TSPc)4- with NO2 ,
coordination via oxygen occurs. The overall scheme for cata-
lytic reduction of nitrite by dithionite may be formulated as
shown in Scheme 1. It should be noted that dithionite disso-
ciates into 2SO2- and therefore requires two catalytic cycles
with nitrite for each dithionite molecule, which results in
the square dependence observed for the nitrite concentration.
Reaction of Sulfoxylate with Nitrite in the Presence of
Iron Tetrasulfophthalocyanine. It is known that sulfoxylate
slowly reduces nitrite to nitrous oxide and dinitrogen in
strongly alkaline solutions.16 The presence of iron tetra-
sulfophthalocyanine leads to substantial changes in the
reaction mechanism, as manifested by significant changes
in the observed reaction products. The results of 15N NMR
studies show that the final product of the catalytic reduction
of nitrite is ammonia (signal at -362 ppm; Figure 6). The
15N NMR spectra in Figure 6 also indicate the formation of
an intermediate at -130 ppm, which could be due to
coordinated hydroxylamine, the most stable intermediate in
this reaction. Contrary to reduction by dithionite, the color
of the reaction mixture during the course of the catalytic
reaction of nitrite and sulfoxylate is brown (FeI complex)
and not blue (FeII complex). This observation indicates that,
because sulfoxylate reduces FeII to FeI rapidly (see above),
the latter species participates in the catalytic cycle (i.e.,
further reduction of the FeI complex proceeds slowly; see
From a comparison of the results for reduction by
dithionite and sulfoxylate, it should be noted that, because
-
protonated sulfoxylate (HSO2 ) can disproportionate in
weakly alkaline solutions,33 we could not study its reactions
at pH 10 as in the case of dithionite. Therefore, the influence
of the pH of the solutions used for reduction by dithionite
and sulfoxylate cannot be excluded completely. Because iron-
(II) phthalocyanine forms a relatively stable complex with
NO,34 we assume that nitric oxide undergoes an inner-sphere
reduction reaction. The overall scheme for the catalytic
reduction of nitrite by sulfoxylate may be written as shown
in Scheme 2.
Unfortunately, we could not study the detailed kinetics of
the catalytic reaction between sulfoxylate and nitrite because
of its complexity. Not only sulfoxylate but also the product
(30) (a) Martin, G. J.; Martin, M. L.; Gouesnard, J.-P. 15N NMR
Spectroscopy; Springer-Verlag: Berlin, Heidelberg, Germany, 1981;
382 pp. (b) Anderson, L.-O.; (Banus) Mason, J.; van Bronswijk, W.
J. Chem. Soc. A 1970, 296. (c) Chen, Y.; Lin, F.-T.; Shepherd, R. E.
Inorg. Chem. 1999, 38, 973.
-
of its one-electron oxidation, the anion radical SO2 , can
(31) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067.
(32) Arulsamy, N.; Bohle, D. S.; Hansert, B.; Powell, A. K.; Thomson, A.
J.; Wocaldo, S. Inorg. Chem. 1998, 37, 746.
(33) Makarov, S. V. Russ. Chem. ReV. 2001, 70, 885.
(34) Ascenzi, P.; Fruttero, R.; Ercolani, C.; Monacelli, F. Analysis 1996,
24, 316.
6474 Inorganic Chemistry, Vol. 44, No. 18, 2005

Catalyzed Reduction of Nitrite by Dithionite and Sulfoxylate
Scheme 2
participate in the reaction (see above). Besides this, at least
three possibly catalytically active species are simultaneously
present in solution.
Reaction of FeI(TSPc•)6- with Nitrite. A very interesting
aspect of the use of sulfoxylate is the possibility to study
the reaction between nitrite and FeI(TSPc•)6-. The latter
species is of special interest for the activation of small
molecules such as CO2.11 It should be noted, however, that
FeI(TSPc•)6- has been much less studied than FeI(TSPc)5-,
possibly because of difficulties encountered with its synthesis
via chemical methods in aqueous solution. In a known
chemical procedure, sodium in tetrahydrofuran is used as
the reductant.11 The only known method used for reduction
of FeI(TSPc)5- to FeI(TSPc•)6- in aqueous solution is
radiolysis.11 Therefore, the use of sulfoxylate (thiourea
dioxide) is a promising method for chemical synthesis of
highly reduced forms of metallophthalocyanines. To prepare
FeI(TSPc•)6-, a 3-4-fold excess of sulfoxylate was added
to iron tetrasulfophthalocyanine and stored for 3 h under
anaerobic conditions until the color changed from brown to
red. After that, the deoxygenated solution of nitrite was
mixed with the solution of FeI(TSPc•)6-. All experiments
were performed in 0.1 M NaOH. Rate constants were
determined from the decrease in absorbance at 500 nm.
Because the reaction of sulfoxylate with FeI(TSPc)5- pro-
ceeds approximately 6 times slower than the reaction of
FeI(TSPc•)6- with nitrite (rate constants at 25 °C are 255 (
18 and 1500 ( 23 M-1 s-1, respectively; see Supporting
Information Figures S4 and S5) and the ratio of [NaNO2]/
[SO22-] is ∼100:1, the influence of the first reaction on the
oxidation of FeI(TSPc•)6- is negligible. Control experiments
show that the direct reaction between sulfoxylate and nitrite
proceeds very slowly and does not influence the oxidation
of FeI(TSPc•)6-. It was found that rate constants depend
linearly on the nitrite concentration (Figure 7). The reaction
has relatively high positive activation enthalpy (87 ( 3 kJ
mol-1) and entropy (108 ( 11 J K-1 mol-1) values
(Supporting Information Figure S9). Unexpectedly, the
reaction rate does not depend on the applied pressure (∆V*
) 0.4 ( 0.6 cm3 mol-1) (Supporting Information Figure
S10). The absence of a kinetic saturation in the dependence
of the observed rate constant on the nitrite concentration
indicates that, if an intermediate complex between FeI(TSPc•)6-
Figure 7. Plot of kobs5 vs [NaNO2] for the reaction between FeI(TSPc•)6-
and nitrite as a function of temperature. Experimental conditions: pH )
13 (0.1 M KOH), [FeI(TSPc•)6-] ) 4.3 × 10-6 M.
reduced iron complex and -1 for nitrite). Obviously, the
observed kinetics may be explained in terms of an outer-
sphere electron-transfer mechanism.
Conclusions
Our results have shown that iron tetrasulfophthalocyanine
is an effective catalyst for the reduction of nitrite by dithionite
or sulfoxylate in aqueous solutions. In the case of dithionite,
the catalytic cycle includes a reversible reaction, FeIII a FeII,
and reduction of the coordinated substrate. Surprisingly,
reduction of nitrite by dithionite in the presence of iron and
cobalt tetrasulfophthalocyanines leads to the formation of
different products, viz., nitrous oxide and ammonia, respec-
tively. This finding suggests that differences in the structure
of the intermediate complex determine the composition of
the reaction products. In our opinion, in the case of
FeII(TSPc)4-, the unusual O-coordination of nitrite takes place
-
contrary to N-coordination in the CoI(TSPc)5- f NO2
intermediate complex. Because sulfoxylate reduces the com-
plex of FeII to FeI much faster than dithionite, the FeI complex
participates in the catalytic cycle. The reaction of nitrite with
sulfoxylate catalyzed by iron tetrasulfophthalocyanine pro-
ceeds very similarly to the reduction by dithionite in the
presence of cobalt tetrasulfophthalocyanine. In both cases,
the final product is ammonia, and in the intermediate com-
plex, nitrite coordinates presumably via nitrogen. Thus, the
valence state of the metal in the intermediate complex with
nitrite determines the composition of the reaction products.
Contrary to dithionite, sulfoxylate was shown to be able to
reduce FeI(TSPc)5- to FeI(TSPc•)6-. The reaction is proposed
to follow an outer-sphere electron-transfer process.
Acknowledgment. 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).
Supporting Information Available: Plots of reactions of
-
sodium dithionite, sulfoxylate, and nitrite with FeIII(TSPc)3-
,
and NO2 is formed, it is rather unstable. This is expected
because both reactants, especially FeI(TSPc•)6-, are strong
nucleophiles. Further difficulties are related to electrostatic
problems (an overall large negative charge of -6 for the
FeII(TSPc)4-, FeI(TSPc)5-, and FeI(TSPc•)6-. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC0505955
Inorganic Chemistry, Vol. 44, No. 18, 2005 6475