Mechanism of reduction of the nitrite ion by CuI complexes

Article abstract of DOI:10.1002/ejic.200400255

The reaction of Cu_aq⁺ with NO₂^- was studied. The results are consistent with the mechanism: Cu _aq⁺ NO₂^- ? (Cu ⁺NO₂^-), K = 140 ± 30 M

Full text of DOI:10.1002/ejic.200400255

FULL PAPER
I
Mechanism of Reduction of the Nitrite Ion by Cu Complexes
[a]
[a]
[a,b]
and Dan Meyerstein*[a,c]
Ariela Burg, Evgenia Lozinsky, Haim Cohen,
Keywords: Nitrite / Copper() / Reduction / NO
The reaction of Cu +a q with NO
−
was studied. The results are
show that the ligands reduce the value of K and that their
2
+
− ǟ
+
−
II/I i
consistent with the mechanism: Cuaq + NO
2
Ǟ
(Cu NO
2
),
effect on k depends on the redox potential of the (Cu L )
couple. This reaction is the simplest model for the CuNIR
(NIR = Nitric reductase) enzymes.
−
1
+
−
+
2+
−
K = 140 ± 30
M
; (Cu NO
2
) + H Ǟ Cu + NO + OH , k =
−1
4
.6 × 105
M
s−1. It is proposed that the copper is bound to
+
−
the nitrogen in the transient complex (Cu NO
the reaction was studied with (Cu L ) complexes. The results
2
). In addition, ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
I
i +
Germany, 2004)
Introduction
Results and Discussion
ϩ
When Cu is introduced into a solution containing ex-
The one-electron reduction of nitrite to NO is of major
aq
Ϫ3
[
1Ϫ3]
cess NaNO2 (2.5Ϫ100) ϫ 10
 in the pH range
importance in a variety of biological processes
and in
waste water treatment.[
4Ϫ6]
4.15Ϫ5.45 (buffered by 0.10  acetate, the results are inde-
pendent of the acetate concentration in the range 0.10Ϫ0.50
), a decrease in the absorption at 355 nm due to nitrite is
observed. The kinetics of this process, followed using the
stopped-flow technique, obey a first order rate law on Cu .
The observed rate increases with increasing [NO2 ]. How-
The reducing agent, or the elec-
tro-catalytic active agent,[
7Ϫ9]
is usually a low valent tran-
II [9Ϫ13]
I [14,15]
II [8,16]
sition metal complex, e.g. Fe ,
Co ,
Ru ,
I [17Ϫ20]
Cu .
For example, one of the steps in the important
I
[
21]
nitrogen cycle is the reduction of nitrite to nitric oxide.
Dinitrogen is released into the atmosphere by the activity
Ϫ
[
22Ϫ25]
ever, the results indicate that a limiting rate is observed at
of denitrifying bacteria found in soils and sediments.
The enzyme nitrite reductase (NIR) is essential for these
Ϫ
high [NO2 ] (see Figure 1).
[
1,2]
Analysis of the results assuming a mechanism involving
the fast formation of an intermediate complex fits well with
the experimental data [Figure 2, reaction (2)]. The value of
bacteria.
Several mechanistic studies on the reduction of
Ϫ
I
NO2 by Cu complexes in organic solvents were performed
in order to elucidate the reduction mechanism, and as
plausible models for the enzymatic process.[
26Ϫ35]
k (Table 1) derived from the intercepts of plots analogous
In the
2
ϩ
to Figure 2 is pH-dependent. A plot of k vs. [H O ] (Fig-
ure 3) yields a straight line (k ϭ k [H O ]). From the
present work, we decided to study the mechanism and kine-
2
3
ϩ
ϩ
aq
I
ϩ
tics of the reaction between Cu /(Cu L) complexes and
2 *2 3
Ϫ
slope of the line in this figure one obtains k ϭ (4.6 Ϯ 0.5)
*2
NO2 . The results indicate that a transient complex,
5
Ϫ1 Ϫ1
ϩ
Ϫ
ϫ 10 
s .
(
Cu NO ), is formed initially, and that this complex re-
2
ϩ
acts with H O to yield the final products.
3
III
It is of interest to note that the oxidation of NO by Fe L
complexes proceeds via the reverse mechanism[ [see reac-
tion (1)].
35]
(2)
(
1)
Ϫ
Ϫ
ϩ
Ϫ
Ϫd[NO
2
]/dt ϭ (k
2
K
2 2
2app[NO ][Cuaq])/(1 ϩ K2app[NO ])
Ϫ
1
/kobsd. ϭ 1/(k K2app[NO ]) ϩ 1/k
2 2 2
[
[
[
a]
b]
c]
Chemistry Department, Ben-Gurion University of the Negev,
Beer-Sheva, Israel
The results indicate that a mechanism in which an unre-
Nuclear Research Centre Negev,
Beer-Sheva, Israel
I
Ϫ
active Cu ϪNO
complex is formed [reactions (3) and (4)]
2
Department of Biological Chemistry, The College of Judea
and Samaria,
Ariel, Israel
E-mail: danmeyer@bgumail.bgu.ac.il
could yield the same rate law. However, this would require
that the reaction according to reaction (4) is a third order
process. Thus, this mechanism seems unlikely.
Eur. J. Inorg. Chem. 2004, 3675Ϫ3680
DOI: 10.1002/ejic.200400255
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3675

A. Burg, E. Lozinsky, H. Cohen, D. Meyerstein
FULL PAPER
ϩ
3 2
Table 1. The effect of [H O ] on k and on the observed stability
Ϫ
ϩ
constant of the (Cu NO
2
) complex (the experiments were per-
Ϫ4
formed on helium-saturated solutions containing 5.0 ϫ 10
Cuaq, 0.10  NaClO , 0.10  acetic acid, 0.023  NH )
4 4

ϩ
ϩ
2app (Ϫ1
)
k
2
(
Ϫ1 Ϫ1
s
)
[H
3
O ] ()
ϩ
pH
K
Ϫ6
7
6
6
52
20
32
10
54
7
2
2
2.9
3.4
5.0
8.2
12
12.7
23
3.5 ϫ 10
4.0 ϫ 10
1.3 ϫ 10
2.0 ϫ 10
2.2 ϫ 10
4.0 ϫ 10
5.0 ϫ 10
7.1 ϫ 10
5.45
5.40
4.90
4.70
4.60
4.40
4.30
4.15
Ϫ6
Ϫ5
Ϫ5
Ϫ5
Ϫ5
Ϫ5
Ϫ5
1
1
1
1
1
33
The value of k (Table 1), derived from the intercepts of
2
plots analogous to Figure 2, is pH dependent.
ϩ
5 3
Figure 3. Dependence of k on [H O ]. The experiments were per-
Ϫ4 ϩ
4
formed in solutions containing 5.0 ϫ 10  Cuaq, 0.10  NaClO ,
ϩ
0
4
.10  acetic acid, 0.023  NH , helium saturated.
Ϫ
The apparent values of K2app, derived from the slopes of
Figure 1. Dependence of kobsd. on the [NO
2
]. The experiments
Ϫ4 ϩ
were performed in solutions containing 5.0 ϫ 10  Cuaq, 0.10  the straight lines, e.g. from Figure 2 (at several pH’s in this
ϩ
4
NaClO
4
, 0.10  acetic acid, 0.023  NH
, helium saturated at a
pH range) are presented in Table 1. The average of K2 in
app
final pH of: a. 4.3, b. 4.65, c. 5.25.
Ϫ1
the pH range 4.15Ϫ4.70 is 130 Ϯ25  (Table 1). When
the pH is increased, pH Ն 4.7, the formation of CuOH
Ϫ14 Ϫ2 [36]
[
K (CuOH) ϭ 1 ϫ 10

] causes an apparent de-
sp
crease in K2app
,
(Table 1). The contribution of
[
37]
reaction (6)
to K2app was examined, K2 ϭ K2app(1 ϩ
ϩ
K [H ]).
a
ǟ
Ϫ
ϩ
HNO2 Ǟ NO
2
ϩ H ; pK
a
ϭ 3.25
(6)
This analysis indicates that reaction (6) has a minor effect
Ϫ1
on K , K ϭ 140Ϯ30  . From K , it can be calculated
2
2
2
Ϫ
Ϫ
Figure 2. Dependence of 1/kobsd. on 1/[NO
2
]. The experiments that when [NO
2
] ϭ 7 m, the observed rate of reaction is
ca. 50% of the maximum rate. Therefore, the binding of
Ϫ4
ϩ
were performed in solutions containing 5.0 ϫ 10  Cuaq, 0.10 
ϩ
NaClO
4 4
, 0.10  acetic acid, 0.023  NH , helium saturated, the
Ϫ
ϩ
aq
NO2 to Cu is ca. 100 times weaker than that to CuNIR,
final pH: 4.3 (-----), 4.65 (—)
[
38]
in which KM ϭ 0.074 m.
Solutions of NaNO were mixed with Cu at a final pH
ϩ
aq
2
of 2.20 (at this pH the nitrite is present mostly as HNO2).
Figure 4 describes kobsd. as a function of [NaNO ] at this
ϩ
Ϫ
Ϫ
Ϫ
ϩ
Ϫ
2
Cuaq ϩ NO
2
2
2
Ǟ (Cu NO
2
)
(3)
acidic pH. It is apparent that there is no limiting rate (at
ϩ
Cuaq ϩ NO
Ǟ P
(4)
Ϫ
higher pH’s and the same NO2 concentrations a deviation
ϩ
ϩ
2ϩ
Ϫ
Cuaq ϩ NO
ϩ H
3
O
Ǟ Cuaq ϩ NO ϩ OH
(5) from linearity is observed, as shown in Figure 1). Though
3676
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 3675Ϫ3680

I
Mechanism of Reduction of the Nitrite Ion by Cu Complexes
FULL PAPER
Ϫ [37]
HNO is a stronger oxidant than NO
,
2
there is no rea-
1. A copper() complex which is a stronger reducing agent
2
ϩ
I
1 ϩ
1
son to suggest that reaction (7) occurs.
than Cu . For this purpose, (Cu L ) (L ϭ 2,5,8,11-tetra-
aq
methyl-2,5,8,11-tetraazadodecane, Figure 5) was chosen,
0
II
I
1
[39]
E (Cu L/Cu L ) ϭ 0.116 V (vs. NHE), at pH 5.0. At this
Ϫ
I
1 ϩ
Ϫ1 Ϫ1
pH k[NO2 ϩ (Cu L ) ] ϭ 35Ϯ10 
observed rate is nearly independent of [H O ] in the pH
s
is obtained. The
ϩ
3
range 5.0Ϫ6.0. This rate is considerably higher than that
ϩ
obtained for Cu over this pH range. The lack of depen-
aq
ϩ
dence on [H O ] suggests that, in this case, H O acts as the
3
2
proton donor, i.e. the general mechanism for the decompo-
I
i ϩ
Ϫ
2
sition of the complex [(Cu L ) NO ] is according to reac-
tion (8).
2
Figure 4. Dependence of kobsd. on [NaNO ]. The experiments were
Ϫ4 ϩ
performed in solutions containing 5.0 ϫ 10  Cuaq, 0.10  Na-
ϩ
ClO
4
4
, 0.023  NH , the final pH was 2.20, helium saturated
ϩ
ǟ
ϩ
ϩ
Cuaq ϩ HNO2 Ǟ (Cu HNO
2
)
(7)
ϩ
ϩ
Moreover, if (Cu HNO ) were formed its pK is ex-
2
a
1
Figure 5. L ϭ 2,5,8,11-tetramethyl-2,5,8,11-tetraazadodecane
pected to be lower than the pK of HNO . Therefore, when
a
2
I
Ϫ
the pH Ն 4.15, (Cu NO ) is clearly the complex formed.
2
Ϫ
2
At pH values lower than the pK of HNO , the NO con-
a
2
centration is low so that the apparent value for k K is
expected to be lower at pH 2.20 than that observed at pH [(Cu L ) NO
*5 5
I
i ϩ
Ϫ
ϩ
2 3 2
] ϩ H O /H O Ǟ
(
8)
II i 2ϩ
Ϫ
Ϫ
(
2
Cu L ) ϩ NO ϩ (OH ϩ H O)/2OH
4
.15Ϫ5.45. The slope of the line in Figure 4 equals the ap-
ϩ
4
parent value of k K [H O ] ϭ 4.0 ϫ 10 , i.e. k K (at pH
*
2
2
3
*2
2
From the results, which indicate that the observed rate
6
Ϫ2
Ϫ1
2.20) ϭ 5.5 ϫ 10  sec as compared to k K ϭ 4.6 ϫ
Ϫ
2
*
2
2
increases linearly with [NO ], it can be estimated that K ϭ
5
7
Ϫ1
1
0 ϫ 140 ϭ 6.4 ϫ 10 Ϫ2sec at pH ϭ 4.15Ϫ4.70. Thus,
I
1 ϩ
Ϫ
I
1 ϩ
Ϫ
Ϫ1
[
(Cu L ) NO )]/([(Cu L ) ][NO ]) Յ 7  , i.e. the lig-
2 2
Ϫ
at pH 2.20 the apparent value of k K equals ca. 10% of
*
2
2
ation of NO2 to the complex is considerably weaker than
to Cu . This is probably due to steric hindrance caused by
the ligand L , which is a very bulky ligand, or to the compe-
tition for a ligation site between a terminal tertiary amine
that observed at pH 4.15Ϫ4.80. The nitrite concentration
at pH 2.20 is ca. 10% of its concentration at pH Ն 4.15.
This result is in agreement with the suggested mechanism:
ϩ
aq
1
It should be pointed out that if HNO were the oxidant,
1
Ϫ
2
group of L and NO2 .
a lower apparent rate is expected also at pH 2.20, as the
2
. Copper() complexes which are weaker reducing ag-
ϩ
concentration of HNO is not proportional to [H O ] be-
[40,41]
ϩ
aq
2
3
ents
than Cu . For this purpose, the complexes
low the pK of HNO . However, the nonlinear dependence
I
ϩ
a
2
[Cu (CH CN) ] were chosen. These solutions contain a
3 n
of the observed rate at pH Ն 4.1 and the linear dependence
ϩ
aq
I
ϩ
mixture of several species: [Cu , Cu (CH CN)] ,
3
Ϫ
at pH 2.20 on [NO ] ϩ [HNO ], proves that the active
I
ϩ
I
ϩ
2
2
[Cu (CH CN) ] , [Cu (CH CN) ] , and the detailed com-
3 2 3 3
oxidant is NO2Ϫ
.
position of each solution can be calculated from the known
The yield of NO from the reaction was measured by two
techniques: 1. EPR, by measuring the effect of the addition
of NO to 4,4,5,5-tetramethyl-2-(1-pyryl)-2-imidazoline-1-
oxyl 3-oxide (PN) on the EPR spectrum. 2. Mass spec-
[40,41]
stability constants of these complexes.
Solutions of
 were mixed with NO2
I
ϩ
Ϫ4
Ϫ
[
[
Cu (CH CN) ] 5.0 ϫ 10
3
n
Ϫ3
(2.5Ϫ100) ϫ 10 ) at different pH’s (4.0Ϫ4.8) and differ-
ent CH CN concentrations (0.048Ϫ0.96) . The results
demonstrate that:
3
trometry. Both techniques revealed that [NO] Ն 75%
ϩ
aq
[
Cu ]. This result demonstrates that the yield of NO is
quantitative as some dioxygen always enters the samples
during the handling of the solutions.
The mechanism observed here is in agreement with that
I
observed for several other Cu complexes in methanol/
[
26,28,29]
acetonitrile.
However, the rate constant is consider-
ϩ
aq
ably higher for Cu than for those complexes. This obser-
vation is attributed to the stabilization of Cu by the ligands
I
used in those studies, caused by an anodic shift of the redox
II/I
potential of the Cu couple. Therefore, it seemed of inter-
Ϫ
Figure 6. Dependence of kobsd. on [NO
2
]. The experiments were
Ϫ4 ϩ
est to study the mechanism and kinetics of reduction of
performed in solutions containing 5.0 ϫ 10  Cuaq, 0.10  Na-
ClO , 0.10  acetic acid, 0.048  CH CN, the final pH was 4.40,
Ϫ
I
ϩ
NO2 by (Cu L) complexes in aqueous solutions. Two
complexes were studied:
4
3
helium saturated.
Eur. J. Inorg. Chem. 2004, 3675Ϫ3680
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A. Burg, E. Lozinsky, H. Cohen, D. Meyerstein
FULL PAPER
Ϫ
ϩ
a) The observed rate depends linearly on [NO2 ] at all
If Cu were the only reacting species in solutions con-
aq
the CH CN concentrations studied (for example, see Fig- taining CH CN, i.e. if the rate of reaction of
3
3
I
ϩ
ure 6). From these results, it can be calculated that K
(Cu (CH CN) NO )]/([Cu (CH CN) ] [NO ])
ϭ
[Cu (CH CN) ] were negligible, then the expected ob-
10
3 n
I
Ϫ
2
I
ϩ
Ϫ
Ϫ3
[

Յ
30 served rate in solutions containing 0.96 mol·dm CH CN
3
3
n
3
n
2
Ϫ1
ϩ
ϩ
I
[reaction (9)].
at pH 4.4 would be k K ϫ [H O ] ϫ ([Cu ]/[Cu ] ) ϭ
40 ϫ 4.6 ϫ10 ϫ 4 ϫ 10 ϫ(2.1 ϫ 10 /5.0 ϫ 10 ) ϭ
s . (Cu concentration in this equation was cal-
culated using the stability constants
*2 2 3 aq T
5
Ϫ5
Ϫ9
Ϫ4
1
0
Ϫ1 Ϫ1
ϩ
aq
.011 
[
40,41]
of the
I
ϩ
Ϫ1 Ϫ1
(
9) [Cu (CH CN) ] complexes). However, kobsd. ϭ 24 
s
3 n
is obtained under these conditions. These results clearly
I
ϩ
demonstrate that the [Cu (CH CN) ] complexes react with
3
n
Ϫ
NO2 and contribute to the observed rates of reaction. The
The observation that K Ͻ K is probably due to one or results are not accurate enough to evaluate the rate constant
9
2
I
ϩ
more of the following facts:
of the reaction of each of the [Cu (CH CN) ] species accu-
3 n
I. Competition between one of the CH CN ligands and rately. However, the results (Figure 8) clearly demonstrate
3
Ϫ
NO2 for the same ligation site. II. A change of hybridiz- that the rate constants decrease with an increase in n.
ation from sp to sp is required for the binding of NO
to [Cu (CH CN) ] , which is the major complex present in
3
Ϫ
2
I
ϩ
3
2
the solution.
Conclusions
ϩ
b) The observed rate increases linearly with [H O ] at all
3
ϩ
ϩ
Ϫ
the CH CN concentrations studied, e.g. Figure 7. The slope
The value of K ϭ [(Cu NO )]/([Cu ][NO ]) ϭ 140Ϯ25
2 2 aq 2
3
5
Ϫ2
Ϫ1
Ϫ1
of the graph equals k K ϭ 4.0 ϫ 10  sec . k* K for

is similar to values reported in the literature for
9
9
2 2
the Cu system equals 140 ϫ 4.6 ϫ 105 ϭ 6.4 ϫ 107 Cu complexes with other ligands in organic solvents,
ϩ
aq
Ϫ1
I
[28,29]
Ϫ2

sec , i.e. at this CH CN concentration, acetonitrile re- but higher than the values measured in this work
3
I
1 ϩ
Ϫ
Ϫ1
duces the rate of reaction by a factor of 150.
for
[(Cu L ) NO
]
K
Յ
7

and
for
2
I
ϩ
Ϫ
Ϫ1
[
(Cu (CH CN) ) NO ], K Յ 30  . The stability con-
stant of the (Cu NO2 ) complex suggests that nitrite binds
to the copper via the nitrogen atom, as the stability con-
3
n
2
Ϫ
ϩ
I
stants of Cu complexes with oxo-anions are expected to be
[
42,43]
considerably lower.
Experimental Section
2 4 3
Materials and Methods: NaNO , CuSO , Cu , NH ϭ
, L1 (L1
0
ϩ
Figure 7. Dependence of k on [H
3
O ]. The experiments were per-
2,5,8,11-tetramethyl-2,5,8,11-tetraazadodecane), acetonitrile, acetic
acid, NaClO , and HClO were purchased from Aldrich. PN
Ϫ4
I
ϩ
3 n
formed in solutions containing 5.0 ϫ 10  [Cu (CH CN) ] , 0.10
4
4

NaClO
4
, 0.10  acetic acid, 0.96  CH
3
CN, helium saturated.
[4,4,5,5-tetramethyl-2-(1-pyryl)-2-imidazoline-1-oxyl 3-oxide] was
prepared according to modified procedures, along with small
[
44]
Ϫ
amounts of 4,4,5,5-tetramethyl-2-(1-pyryl)-2-imidazoline.
All
c) The observed rate at constant pH and [NO2 ] de-
solutions were prepared using A.R. grade chemicals and from dis-
tilled water further purified by passing through a Milli Q Millipore
creases with [CH CN], (e.g. Figure 8).
3
setup, final resistivity Ͼ10 mΩ/cm. The experiments were carried
Ϫ3
out at [NaClO ] ϭ 0.1 mol·dm , at room temperature, 22Ϯ2 °C.
4
I
ϩ
I i ϩ
Solutions of [Cu (NH
3 2
) ] and (Cu L ) were prepared via the con-
proportionation process, reaction (10). Solid copper metal was ad-
ded to deaerated solutions containing CuSO
NH , or L at the appropriate pH values.
3
4
and an excess of
i
0
II
a2 qϩ ϩ (2m Ϫ n)L ǟ
I
ϩ
aq
Cu
s
ϩ (Cu L
n
)
Ǟ
2(Cu L
m
)
(10)
ϩ
I
ϩ
Cuaq was obtained by injecting [Cu (NH
3
)
2
]
into an acetate buffer
4
solution containing
solution (acidic solution), or into an HClO
the nitrite. Under these conditions the following reactions (11) and
(12) occur.
Figure 8. Dependence of k on [CH
3
CN]. The experiments were
Ϫ4 I ϩ
performed in solutions containing 5.0 ϫ 10  [Cu (CH
3
CN)
, helium saturated.
CN, solid circles: results
n
] ,
Ϫ
0
.10  NaClO
solid triangle: result in the absence of CH
in presence of CH CN.
4 2
, 0.10  acetic acid, 0.10  NO
3
(
11)
3
3678
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Eur. J. Inorg. Chem. 2004, 3675Ϫ3680

I
Mechanism of Reduction of the Nitrite Ion by Cu Complexes
FULL PAPER
Acknowledgments
(
12)
all
This study was supported in part by a grant from the Budgeting
and Planning Committee of The Council of Higher Education. D.
M. wishes to express his thanks to Mrs. Irene Evens for her ongo-
ing interest and support.
ϩ
4
Thus, at pHs considerably lower than the pK
a
of NH
/NH
3
I
ϩ
ϩ
the [Cu (NH
3
[45]
)
2
]
is transformed into Cuaq (The ligand exchange
I
of Cu is fast ).
Kinetic Measurements: Stopped-flow experiments were carried out
using an SX.18 Applied PhotoPhysics stopped-flow instrument
that enables kinetic measurements of reactions with lifetimes Ն 2
msec. A 05Ϫ109 Spectra Kinetics Monochromator, which enables
measurements in the range of 200Ϫ700 nm, was used. The optical
path was 10 mm. All the kinetic runs are the average of at least a
triplicate. The error limit for the rate constants and for the equilib-
ria constants is Ϯ10%.
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Ϫ
This wavelength is the maximum of the absorption band of NO
2
ϩ
I
ϩ
for the reactions of Cuaq and Cu (CH
3
CN)
n
. Alternatively, the
absorbance of the band at 635 nm, the maximum of the absorption
[9]
II 1 2ϩ
I 1 ϩ
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[10]
Electron Paramagnetic Resonance (EPR) spectra were recorded
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with the following parameters: 20.12 mW nonsaturating microwave
[11]
[
[
12]
13]
power, 100 kHz field modulation of 0.1 mT amplitude. EPR spectra
[14]
TM
were processed using the Bruker WIN-EPR and Microcalc
Ori-
TM
gin
software.
[15]
[16]
I
ϩ
Solutions of (Cu L) , nitrite and PN were mixed producing 4,4,5,5-
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pletely different EPR spectrum (Figure 9) from PN (PN reacts only
with NO to produce PPN).
3
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Figure 9. The EPR spectrum of PPN (bold line), formed by mixing
Ϫ3
I
ϩ
Ϫ2
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1
:1 1.0 ϫ 10  [Cu (NH
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)
2
]
2
with 5.0 ϫ 10  NaNO ; this
Ϫ3
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was carried out in solutions containing also 0.08  NaClO
4
and
[33]
0
.08  acetic acid. The EPR spectrum of PN (thin line) which does
Ϫ4
not change after mixing 1:1 2.0 ϫ 10  PN with 0.03  NaNO
2
,
[34]
[35]
0
.10  NaClO , 0.20  acetic acid. The final pH was 4.7, the solu-
4
tions were helium-saturated, ν ϭ 9.424 GHz.
Eur. J. Inorg. Chem. 2004, 3675Ϫ3680
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3679

A. Burg, E. Lozinsky, H. Cohen, D. Meyerstein
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Received March 30, 2004
Early View Article
Published Online July 15, 2004
3680
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 3675Ϫ3680