Electrochemical generation of free nitric oxide from nitrite catalyzed by iron meso-tetrakis(4-N-methylpyridiniumyl)porphyrin

Article abstract of DOI:10.1021/ic049323c

Electrochemical generation of free nitric oxide (NO) from nitrite (NO ₂^-) catalyzed by iron meso-tetrakis(4-N- methylpyridiniumyl)porphyrin, [Fe^III(TMPyP)]⁵⁺, has been developed in this study. To obtain free NO, a cathodic electrolysis and an anodic electrolysis were performed in two connected flow electrolytic cells in sequence. The flow electrolytic cell upstream was used for cathodic electrolysis, where the solution of [Fe^III(TMPyP)]⁵⁺ and NO₂^- was reduced at -0.25 V (vs Ag/AgCl) into [Fe ^II(NO₂^-)₂(TMPyP)]²⁺ and [Fe^II(NO)(TMPyP)]⁴⁺ in sequence. The flow electrolytic cell downstream was utilized for anodic electrolysis, where [Fe ^II(NO)(TMPyP)]⁴⁺ formed from the upstream cell was oxidized at +0.40 V (vs Ag/AgCl) into [Fe^III(TMPyP)]⁵⁺ and free NO. Finally, NO was bubbled out from anodic electrolyte by argon gas. The mechanism and the optimum conditions for electrochemical generation of NO from NO₂^- catalyzed by [Fe^IIITMPyPJ]⁵⁺ were studied in detail by voltammetric and spectroelectrochemical methods.

Full text of DOI:10.1021/ic049323c

Inorg. Chem. 2004, 43, 8437−8446
Electrochemical Generation of Free Nitric Oxide from Nitrite Catalyzed
by Iron meso-Tetrakis(4-N-methylpyridiniumyl)porphyrin
Yuwu Chi,* Jingyuan Chen, and Koichi Aoki
Department of Applied Physics, Faculty of Engineering, UniVersity of Fukui,
3-9-1 Bunkyo, Fukui-shi 910-8507, Japan
Received May 25, 2004
Electrochemical generation of free nitric oxide (NO) from nitrite (NO2-) catalyzed by iron meso-tetrakis(4-N-
methylpyridiniumyl)porphyrin, [FeIII(TMPyP)]5+, has been developed in this study. To obtain free NO, a cathodic
electrolysis and an anodic electrolysis were performed in two connected flow electrolytic cells in sequence. The
+
-
flow electrolytic cell upstream was used for cathodic electrolysis, where the solution of [FeIII(TMPyP)]5 and NO2
was reduced at
−
0.25 V (vs Ag/AgCl) into [FeII(NO2-)2(TMPyP)]2 and [FeII(NO)(TMPyP)]4 in sequence. The flow
+
+
electrolytic cell downstream was utilized for anodic electrolysis, where [FeII(NO)(TMPyP)]4+ formed from the upstream
+
cell was oxidized at
+
0.40 V (vs Ag/AgCl) into [FeIII(TMPyP)]5 and free NO. Finally, NO was bubbled out from
anodic electrolyte by argon gas. The mechanism and the optimum conditions for electrochemical generation of NO
from NO2- catalyzed by [FeIII(TMPyP)]5+ were studied in detail by voltammetric and spectroelectrochemical methods.
Introduction
generated from the decomposition of vasodilator drugs such
as nitroglycerin.10
NO has been recognized recently as one of the most
important physiological regulators.1 It plays an important role
as a biological second messenger in different functions of
the human body, for vascular smooth muscle relaxation for
blood pressure lowering and inhibition of platelet aggrega-
tion2-5 and for cellular communication,6 and even acts as a
major defense molecule in the immune system, for killing
bacteria and tumor cells.7 In the body, NO can be produced
by heme-containing nitric oxide synthetase from L-arginine
in the endothelial as well as other cells upon interaction with
inducing agents such as acetylcholine, bradykinin, adenine
nucleotides, thrombin, and serotonin,8,9 or NO can be
However, it is difficult to control the release of NO and
its distribution in the body by using either inducing agents
or vasodilator drugs. Thus, a controllable method for
generation of NO in a local target part of the body is of
great clinical interest in terms of improving efficiency of
therapy and avoiding undesirable side effects, for example,
release of NO locally to prevent the formation of thrombus
without causing a decrease of blood pressure, or generation
of NO in tumor tissue to kill tumor cells without killing
normal cells of the body. Recently, polymeric films capable
of continuously releasing low levels of NO have been studied
for improving the thromboresistivity and antibacterial ability
of biomedical devices.11-22 However, application of these NO
release polymeric films is limited by their short lifetime
* Author to whom correspondence should be addressed. E-mail:
y.w.chi@asura.apphy.fukui-u.ac.jp. Phone: (81)-90-82676873.
(1) Koshland, D. E., Jr. Science 1992, 258, 1861.
(2) Furchgott, R. F.; Zawadski, J. V. Nature 1980, 288, 373-376.
(3) Ignarro, L. I.; Buga, C. M.; Wood, K. S.; Byrns, R. E. Proc. Natl.
Acad. Sci. U.S.A. 1987, 84, 9265-9269.
(10) Katsuki, S.; Arnold, W.; Mittal, C.; Murad, F. J. Cyclic Nucleotide
Res. 1977, 3 (1), 23-35.
(11) Smith, D. J.; Chakravarthy, D.; Pulfer, S.; Simmons, M. L.; Hrabie,
J. A.; Citro, M. L.; Saavedra, J. E.; Davies, K. M.; Hustsell, T. C.;
Mooradian, D. L.; Hanson, S. R.; Keefer, L. K. J. Med. Chem. 1996,
39, 1148-1156.
(12) Mowery, K. A.; Schoenfisch, M. H.; Baliga, N.; Wahr, J. A.;
Meyerhoff, M. E. Electroanalysis 1999, 11, 681-686.
(13) Ramamurthi, A.; Lewis, R. S. Ann. Biomed. Eng. 2000, 28, 174-
181.
(14) Bohl, K. S.; West, J. L. Biomaterials 2000, 21, 2273-2278.
(15) Mowery, K. A.; Schoenfisch, M. H.; Savedra, J. E.; Keefer, L. K.;
Meyerhoff, M. E. Biomaterials 2000, 21, 9-21.
(4) Palmer, R. M. J.; Ferrige, A. G.; Moncada, S. Nature 1987, 327, 524-
526.
(5) Synder, S. H. Science 1992, 257, 494-496.
(6) Knowles, R. G.; Palacios, M.; Palmer, R. M. J.; Moncada, S. Proc.
Natl. Acad. Sci. U.S.A. 1989, 86, 5159-5162.
(7) Hibbs, J. B.; Taintor, R. R.; Vavrin, Z.; Rachlin, E. M. Biochem.
Biophys. Res. Commun. 1988, 157, 87-94.
(8) Palmer, R. M. J.; Ashton, D. S.; Moncada, S. Nature 1988, 333, 664-
666.
(9) Moncada, S.; Palmer, R. M. J.; Higgs, E. A. Hypertension 1988, 12,
365-372.
10.1021/ic049323c CCC: $27.50
Published on Web 12/02/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 26, 2004 8437

Chi et al.
because only a small amount of NO adducts can be loaded
within thin coatings of the polymeric materials. It has been
revealed that nitrite (NO2 ), a common metabolic product
mainly resulted from further reduction of coordinated NO
to ammonia and other species under high negative potential
(<-0.65 V vs SCE) conditions employed in these studies.
Therefore, to improve the yield of NO by using these water-
soluble iron porphyrins, the potential condition as well as
reaction kinetics should be studied in detail. Another problem
to be solved is how to obtain free NO. During the reduction
-
in the biological system, can be reduced to NO by iron
porphyrin-containing nitrite reductase23 and various model
enzymes (usually iron porphyrins).24-32 Further studies in
this field showed that NO could also be produced as an
intermediate or a main product during the electrochemical
-
of NO2 based on the catalysis of iron porphyrin, NO is
-
reduction of NO2 in the presence of iron porphyrins.33-39
usually produced in coordinated form, i.e., complexes of NO
with iron porphyrin, the stability of which varies with the
type of ligands and the valence of the central metal.41 So
this problem should also be considered and solved to obtain
free NO.
Apparently, these studies imply that, upon catalysis of iron
porphyrins, a low level of NO can be electrochemically
generated in the body from NO2-, the concentration of which
ranges from 0.5 to 10 µM in plasma, erythrocytes, and
tissues,40 and thus, an electrochemically controllable method
for a low level of NO release at a local site in the body can
be developed. However, to control the electrochemical
generation of NO, there are still several problems to solve,
among which the conversion rate of nitrite to NO is most
important. A high yield of coordinated NO was obtained
when using water-insoluble octaethylporphrin iron(III) chlo-
ride, Fe(OEP)Cl;39 however, the reaction can only be carried
out in nonaqueous media, which is unfit for generating NO
under the physiological condition. On the contrary, when
using water-soluble iron porphyrins, such as [FeIII(TPPS)]3- 34
and [FeIII(TMPyP)]5+,35 the yield of NO was very low, which
In this study, we examined the electrocatalysis of the
water-soluble porphyrin [FeIII(TMPyP)]5+ for the reduction
of nitrite to nitric oxide under the physiological pH condition
(pH7.4) and, on this basis, developed an electrochemical
generation method for free NO.
Experimental Section
Chemicals. 5,10,15,20-Tetrakis(N-methylpyridinium-4-yl)-21H,-
23H-porphine, tetrakis(p-toluenesulfonate), or [H2(TMPyP)]‚4Ts,
obtained from Dojindo (Japan) was used for preparation of
[FeII(TMPyP)]Cl4 and [FeIII(TMPyP)]Cl5 without further purifica-
tion. Sodium nitrite (from Wako, Japan) and other chemicals used
were of analytical reagent purity. All solutions were prepared by
using twice-distilled water.
(16) Schoenfisch, M. H.; Mowery, K. A.; Rader, M. V.; Baliga, N.; Wahr,
J. A.; Meyerhoff, M. E. Anal. Chem. 2000, 72, 1119-1126.
(17) Robbins, M. E.; Chen, T. Y.; Schoenfishch, M. H. J. Am. Chem. Soc.
2001, 123, 9712-9713.
(18) Zhang, H.; Annich, G. M.; Miskulin, J.; Osterholzer, K.; Merz. S. I.;
Bartlett, R. H.; Meyerhoff, M. E. Biomaterials 2002, 23, 1485-1494.
(19) Frost, M. C.; Rudich, S. M.; Zhang, H.; Maraschio, M. A.; Meyerhoff,
M. E. Anal. Chem. 2002, 74, 5942-5947.
Preparation of [FeII(TMPyP)]Cl4 was carried out in the following
-1
way: 100 mL of 1.0 × 10-3 mol‚L
[H2(TMPyP)]Cl4 solution
produced by ion exchange of 0.1364 g of [H2(TMPyP)]‚4Ts was
heated at reflux and simultaneously degassed by ultrapure argon
gas to remove oxygen from the system completely. Five hours later,
the solution was added with equivalent moles of FeCl2‚4H2O
(0.0199 g) and continuously heated at reflux for 15 h, during which
2 equiv of NaOH (8 mg) was added to neutralize HCl formed during
the reaction. Finally, 100 mL of 1.0 × 10-3 mol‚L -1 [FeII(TMPyP)]-
Cl4 solution was obtained and cooled to room temperature, with a
yield of higher than 95%. [FeII(TMPyP)]Cl4 synthesized by this
method showed electronic absorption spectral data of λmax (log ꢀ)
) 445 (5.05), 560 (3.98), and 610 (3.63), in pH 7.4 PBS, which
were consistent with those of [FeII(TMPyP)]Cl4 chemically reduced
from [FeIII(TMPyP)]Cl5 by ascorbate.42 Since NaCl formed from
the neutralization of HCl by NaOH did not interfere with the spectral
and electrochemical measurements, the synthesized [FeII(TMPyP)]-
Cl4 solution was then used without further purification. Because
[FeII(TMPyP)]Cl4 was found to be very readily oxidized by oxygen
when exposed to the air, it was protected rigidly by high-purity
argon during the whole process of its synthesis, storage, and
measurement.
(20) Marxer, S. M.; Rothrock, A. R.; Nablo, B. J.; Robbins, M. E.;
Schoenfisch, M. H. Chem. Mater. 2003, 15, 4193-4199.
(21) Robbins, M. E.; Schoenfishch, M. H. J. Am. Chem. Soc. 2003, 125,
6068-6069.
(22) Frost, M. C.; Meyerhoff, M. E. J. Am. Chem. Soc. 2004, 126, 1348-
1349.
(23) Silvestrini, M. C.; Falcinelli, S.; Ciabatti, I.; Cutruzzola, F.; Brunori,
M. Biochimie 1994, 76, 641-654.
(24) Shimada, H.; Orii, Y. FEBS Lett. 1975, 54, 237-240.
(25) Stolzenberg, A. M.; Strauss, S. H.; Holm, R. H. J. Am. Chem. Soc.
1981, 103, 4763-4778.
(26) Fujita, E.; Fajer, J. J. Am. Chem. Soc. 1983, 105, 6743-6745.
(27) Kim, C. H.; Hollocher, T. C. J. Biol. Chem. 1984, 259, 2092-2099.
(28) Finnegan, M. G.; Lappin, A. G.; Scheidt, W. R. Inorg. Chem. 1990,
29, 181-185.
(29) Silvestrini, M. C.; Tordi, M. G.; Musci, G.; Brunori, M. J. Biol. Chem.
1990, 265, 11783-11787.
(30) Ye, R. W.; Toro-suarez, I.; Tiedje, J. M.; Averill, B. A. J. Biol. Chem.
1991, 266, 12848-12851.
(31) Ozawa, S.; Fujii, H.; Morishima, I. J. Am. Chem. Soc. 1992, 114,
1548-1554.
(32) Ozawa, S.; Sakamoto, E.; Ichikawa, T.; Watanabe, Y.; Morishima, I.
Inorg. Chem. 1995, 34, 6363-6370
[FeIII(TMPyP)]Cl5 solution was obtained after oxygen gas was
bubbled into [FeII(TMPyP)]Cl4 solution for 12 h. It showed
electronic absorption spectral data of λmax (log ꢀ) ) 424 (4.98),
605 (3.75), and 640 (3.58), in pH 7.4 PBS, which were consistent
with those of [FeIII(TMPyP)]Cl5 prepared by the Pasternack
method.43
(33) Fernandes, J. B.; Feng, D.; Chang, A.; Keyser, A.; Ryan, M. D. Inorg.
Chem. 1986, 25, 2606-2610.
(34) Barley, M. H.; Takeuchi, K. J.; Meyer, T. J. J. Am. Chem. Soc. 1986,
108, 5876-5885.
(35) Barley, M. H.; Rhodes, M. R.; Meyer, T. J. Inorg. Chem. 1987, 26,
1746-1750. Younathan, J. N.; Wood, K. S.; Meyer, T. J. Inorg. Chem.
1992, 31, 3280-3285.
(36) Cheng, S. H.; Su Y. O. Inorg. Chem. 1994, 33, 5847-5854.
(37) Bedioui, F.; Trevin, S., Albin, V.; Villegas, M. G. G.; Devynck, J.
Anal. Chim. Acta 1997, 341, 177-185.
(41) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
1993, 115, 9568-9575.
(42) Tondreau, G. A.; Wilkins, R. G. Inorg. Chem. 1986, 25, 2745-2750.
(43) Pasternack, R. F.; Lee, H.; Malek, P.; Spencer, C. J. Inorg. Nucl. Chem.
1977, 39, 1865.
(38) Chen, J. Y.; Ikeda, O. Electroanalysis 2001, 13 (13), 1076-1081.
(39) Wei, Z. C.; Ryan, M. D. Inorg. Chim. Acta 2001, 314, 49-57.
(40) Rodriguez, J.; Maloney, R. E.; Rassaf, T.; Bryan, N. S.; Feelisch, M.
Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 336-341.
8438 Inorganic Chemistry, Vol. 43, No. 26, 2004

Electrochemical Generation of Free Nitric Oxide from Nitrite
Figure 2. Electronic absorption spectra obtained for (a) 2.5 × 10-4
mol‚L-1 [FeIII(TMPyP)]5+ and (b) 2.5 × 10-4 mol‚L-1 [FeII(TMPyP)]4+
in pH 7.4 PBS buffer.
Results
Figure 1. Schematic diagram of the system for electrochemical generation
of NO: (A, B) flow electrolytic cells; (C) solution reservoir; (D) micropump.
Interaction of NO and NO2- with [FeIII(TMPyP)]5+ and
[FeII(TMPyP)]4+. The electronic absorption spectra of [FeIII-
(TMPyP)]5+ and [FeII(TMPyP)]4+ at pH 7.4 are shown in
Figure 2. [FeIII(TMPyP)]5+ exhibits a strong Soret band at
424 nm and weak Q-bands at 605 and 640 nm (curve a in
Figure 2), whereas [FeII(TMPyP)]4+ has a Soret band at 445
nm and Q-bands at 560 and 610 nm (curve b in Figure 2).
It was found that the absorption spectrum of [FeIII(TMPyP)]5+
Solutions saturated with NO were prepared by bubbling 1% NO
gas (Nippon Sanso, Japan) into deoxygenated solutions for 30 min,
and their concentrations were evaluated from Ostwald’s solubility
coefficient for a given partial pressure of NO.44 All solutions were
deoxygenated by ultrapure argon gas for 15 min before electro-
chemical and spectral measurements, and protected by an argon
atmosphere during the measurements.
-
did not change in the presence of NO or NO2 , indicating
Unless mentioned otherwise, the aqueous medium used was a
0.1 mol‚L-1, pH 7.4 sodium phosphate buffer solution (PBS).
Apparatus. A potentiostat (Huso electrochemical system, model
1112, Japan) combined with a three-electrode system was used for
all electrochemical experiments. A Pt disk working electrode (0.071
cm2 area) was used for voltammetric measurements; a Pt mesh (The
Nilaco Co., Japan) working electrode with 50% transmittance was
placed in a spectrophotometric cell (0.4 mm light path length) and
used for spectroelectrochemical measurements. The other two
electrodes for the three-electrode system were a Ag/AgCl reference
electrode and a Pt coil auxiliary electrode.
that neither NO2- nor NO complexes with [FeIII(TMPyP)]5+.
Recently, Trofimova and co-workers reported that a high
concentration of NO (>5%) could react with [FeIII(T-
MPyP)]5+ via the reaction46
[FeIII(TMPyP)]5+ + 2NO + H2O f
[FeII(NO)(TMPyP)]4+ + NO2- + 2H+ (1)
Apparently, this equation includes a redox reaction rather
than a complex reaction between [FeIII(TMPyP)]5+ and NO,
which indicates that [FeIII(TMPyP)]5+ cannot complex with
NO to produce such a complex as [FeIII(NO)(TMPyP)]5+.
A Cray50 (Varian Australia Pty Ltd.) spectrophotometer was used
for measurements of electronic absorption spectra.
-
In contrast, both NO and NO2 complexed with [FeII-
A system (see Figure 1) consisting of two flow electrolytic cells
(A, B), a micropump (D), and a solution reservoir (C) was used
(TMPyP)]4+, which was confirmed by the obvious change
of the spectra. Figure 3 shows that, when NO was bubbled
into [FeII(TMPyP)]4+ solution, the band at 610 nm disap-
peared, the band at 560 nm shifted to 550 nm, and the Soret
band at 445 nm shifted to 426 nm. The final spectrum was
the same as that of [FeII(NO)(TMPyP)]4+,38,46 so this complex
reaction can be proposed as the following:
-
for electrochemical generation of NO from NO2 based on the
catalysis of [FeIII(TMPyP)]5+. The structure of the flow electrolytic
cell was similar to that used by Fujinaga et al.45 except that the
working electrode used in our study was Pt coils and was separated
from the other two electrodes (reference electrode and auxiliary
electrode) by a molecular porous membrane (Spectrum Laboratories
Inc.) with an MWCO of 1000. The micropump was a Masterflex
C/L pump system (Cole-Parmer Instrument Co.), which could
control the flow rate of solution from several microliters per minute
to several milliliters per minute. The solution reservoir was used
to contain [FeIII(TMPyP)]5+ and NaNO2 and bubbled with argon
gas to transfer NO (electrochemically produced) from the solution
phase to the gas phase. A NO gas sensor (Gastec Co., Japan) was
used to detect the concentration of NO in the gas phase in the
solution reservoir.
[FeII(TMPyP)]4+ + NO f [FeII(NO)(TMPyP)]4+ (2)
In the case of the complex reaction between [FeII-
-
(TMPyP)]4+ and NO2 (see Figure 4), with the decrease of
the band at 560 nm, a new band was first observed to appear
quickly at 540 nm (curve b in Figure 4); this new absorption
band has been suggested to come from the Q-band of
-
[FeII(NO2 )(TMPyP)]3+ formed in the solution.38 However,
(44) Lide, D. R. CRC Handbook of Chemistry Physics, 76th ed.; CRC
Press: Boca Raton, FL, 1995; Section 6-3.
(45) Fujinaga, T.; Kihara, S. CRC Crit. ReV. Anal. Chem. 1977, 6, 223.
(46) Trofimova, N. S.; Safronov, A. Y.; Ikeda, O. Inorg. Chem. 2003, 42,
1945-1951.
Inorganic Chemistry, Vol. 43, No. 26, 2004 8439

Chi et al.
Figure 5. Cyclic voltammograms obtained for 2.5 × 10-4 mol‚L-1 [FeIII-
(TMPyP)]5+ at the Pt electrode in the presence of NaNO2 of various
concentrations: (a) 0, (b) 2.5 × 10-3, (c) 2.5 × 10-2 mol‚L-1. The scan
Figure 3. Electronic absorption spectral changes obtained for a 2.5 ×
10-4 mol‚L-1 [FeII(TMPyP)]4+ solution: (a) before and (b) after being
bubbled with 1% NO gas.
rate was 10 mV‚s-1
.
peaks shift to more positive potentials with increasing
-
concentration of NO2 . This appears to result from the
formation of a nitrite-ferrous porphyrin complex in the
-
solution. The coordination number of NO2 in the nitrite-
ferrous porphyrin complex can be obtained by using Nernst
equation:
- n
-
E1/2 ) EK + 0.059 log [NO2 ] ) EK + 0.059n log [NO2 ]
(4a)
where E1/2 is the half-wave potential of ferric/ferrous
oxidation, EK is a constant, and n is the coordination number
of NO2- in the iron(II)-nitrite complex. When the mole ratio
-
of nitrite to iron porphyrin (CNO /CFeP) is high enough (>10),
2
-
Figure 4. Electronic absorption spectral changes obtained for the solution
containing 2.5 × 10-4 mol‚L-1 [FeII(TMPyP)]4+ and 2.5 × 10-2 mol‚L-1
NaNO2. The spectra were recorded at times (a) 0, (b) 1, (c) 5, and (d) 20
min after the solution was prepared.
-
i.e., [NO2 ] ≈ CNO , we can obtain the following equation:
2
E1/2 ≈ EK + 0.059n log cNO2- ) (EK + 0.059n log cFeP) +
c
-
NO2
0.059n log
our experimental results (described below) showed that in
(
)
cFeP
-
-
the presence of a high concentration of NO2 , [FeII(NO2 )2-
-
(TMPyP)]2+ rather than [FeII(NO2 )(TMPyP)]3+ was formed.
c
-
NO2
Five minutes after the reaction began (see curves c and d in
Figure 4), the band at 540 nm shifted slowly to 550 nm
(assigned to [FeII(NO)(TMPyP)]4+ by curve b in Figure 3),
) EK′ + 0.059n log
(4b)
(
)
cFeP
According to eq 4b, a plot of E1/2 vs the mole ratio of log-
-
which means that [FeII(NO2 )2(TMPyP)]2+ is unstable and
-
(CNO /CFeP) was made and is shown in Figure 6. It was
evident that there was a good linear relationship between
E1/2 and log(CNO /CFeP) at a high mole ratio of nitrite to
porphyrin (>10). A slope of 110 mV was obtained, indicating
2
can spontaneously change to [FeII(NO)(TMPyP)]4+. This
result supports the viewpoint that the instability of iron
porphyrin-nitrite complexes results from the extreme ther-
modynamic stability of the iorn(II)-nitrosyl.47
-
2
that the coordination number, n, was 2, i.e., bisnitrite-ferrous
Electrochemical Generation of [FeII(NO2-)2(TMPyP)]2+.
Cyclic voltammetric measurements were carried out for [FeIII-
(TMPyP)]5+ in the presence of various concentrations of
NaNO2. Figure 5a shows that, in the absence of NO2- at pH
7.4, [FeIII(TMPyP)]5+ exhibits a couple of well-defined redox
peaks at -0.110 and -0.175 V, which has been confirmed
to involve a reversible one-electron-transfer process:35
porphyrin was produced (reaction 5).
[FeIII(TMPyP)]5+ + e- + 2NO2- T
[FeII(NO2 )2(TMPyP)]2+ (5)
-
In this study, because [FeIII(TMPyP)]5+ was used as a
catalyst, its concentration was about 100 times less than that
-
of NO2 , i.e., nitrite concentration . 10 equiv of porphyrin;
[FeIII(TMPyP)]5+ + e- T [FeII(TMPyP)]4+
(3)
-
it was evident that [FeII(NO2 )2(TMPyP)]2+ was dominant
during the reactions. According to the cyclic voltammogram
shown in Figure 5, a suitable potential for electrochemical
-
However, in the presence of NO2 (Figure 5b,c) the redox
-
(47) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067-1089.
8440 Inorganic Chemistry, Vol. 43, No. 26, 2004
generation of [FeII(NO2 )2(TMPyP)]2+ was about -0.25 V

Electrochemical Generation of Free Nitric Oxide from Nitrite
Figure 6. Variation of the oxidation half-wave potential (E1/2) of ferrous
iron porphyrin (2.5 × 10-4 mol‚L-1) with the mole ratio of CNO /CFeP
.
2-
The scan rate was 10 mV‚s-1
.
Figure 8. Electronic absorption spectral changes of [FeII(NO2-)2(TMPyP)]2+
with time at open circuit potential. The spectra (solid lines) were recorded
within 6 min and compared with that of [FeIII(TMPyP)]5+ (dotted line).
Figure 9. ln(A0/(A0 - At)) versus time (s). A0 and At are the absorbance
at 540 nm at reaction time 0 and t, respectively. The data were obtained
from Figure 8.
Figure 7. Electronic absorption spectral changes obtained for the solution
containing 2.5 × 10-4 mol‚L-1 [FeIII(TMPyP)]5+ and 2.5 × 10-2 mol‚L-1
NaNO2 during electrolysis at -0.25 V. The spectra were recorded within
2 min.
-
associated with [FeII(NO2 )2(TMPyP)]2+ decreased and
shifted to 550 nm (the characteristic band of [FeII(NO)-
(TMPyP)]4+), and simultaneously, new absorptions at 605
and 640 nm corresponding to [FeIII(TMPyP)]5+ appeared;
(vs Ag/AgCl). Then, at this potential, the electrochemical
-
generation of [FeII(NO2 )2(TMPyP)]2+ was observed by
-
spectroelectrochemistry. [FeII(NO2 )2(TMPyP)]2+ formed
thus, it is confirmed that, at pH 7.4, [FeII(NO2 )2(TMPyP)]2+
-
-
rapidly from a mixed solution of [FeIII(TMPyP)]5+ and NO2
can change to [FeII(NO)(TMPyP)]4+ and [FeIII(TMPyP)]5+
via disproportionation. Furthermore, Figure 8 also shows that
when a -0.25 V potential was applied, which was confirmed
by the fact that the Q-bands at 605 and 640 nm decreased
while a new band at 540 nm increased gradually (see Figure
7). A reverse spectral change was observed (data not shown)
-
half the amount of [FeII(NO2 )2(TMPyP)]2+ was reduced to
[FeII(NO)(TMPyP)]4+ whereas the other half of the amount
-
of [FeII(NO2 )2(TMPyP)]2+ was oxidized to [FeIII(TMP-
-
upon applying a >+0.1 V potential, as soon as [FeII(NO2 )2-
yP)]5+. Thus, the disproportionation reaction can be proposed
as the following:
(TMPyP)]2+ was produced. This confirms that the electro-
-
chemical generation of [FeII(NO2 )2(TMPyP)]2+ (shown in
eq 5) is reversible.
2[FeII(NO2 )2(TMPyP)]2+ + H2O f
-
-
Disproportionation of [FeII(NO2 )2(TMPyP)]2+. As de-
scribed above, the study on the interaction of [FeII(T-
[FeII(NO)(TMPyP)]4+ + [FeIII(TMPyP)]5+ + 3NO2
+
-
-
MPyP)]4+ and NO2 (see Figure 4) has shown that
-
[FeII(NO2 )2(TMPyP)]2+ can spontaneously change to its
2OH- (6a)
reduced form, [FeII(NO)(TMPyP)]4+. In terms of redox
equilibrium, a corresponding oxidized species such as [FeIII-
(TMPyP)]5+ or NO3- should be produced at the same time.
To ensure this, the spectral change of [FeII(NO2-)2(TMPyP)]2+
with time was recorded at open circuit potential. Figure 8
shows that, upon standing, Q-band absorption at 540 nm
According to the change of absorptions at 540 nm (A0 -
At), we plotted ln(A0/(A0 - At)) vs time in Figure 9, and found
there was a good linear relationship (R2 ) 0.9981) between
ln(A0/(A0 - At)) and reaction time. Thus, the disproportion-
ation reaction was recognized to be a first-order reaction,
Inorganic Chemistry, Vol. 43, No. 26, 2004 8441

Chi et al.
with a rate constant of (7.30 ( 0.36) × 10-3 s-1 and half-
life time of 95 ( 5 s (n ) 7, confidence level 95%), when
pH 7.4 PBS buffer was used as the reaction medium. The
same conclusion can be drawn when the spectral data at 605
and 640 nm are analyzed. The recognition of a first-order
reaction means that reaction 6a is not a basic reaction but
an overall reaction, which includes a rate-determining
reaction step. This rate-determining step, i.e., the slowest step
should be a first-order reaction. On this basis, we propose
that the disproportionation of [FeII(NO2-)2(TMPyP)]2+ shown
in reaction 6a probably undergoes the following steps:
[FeII(NO2 )2(TMPyP)]2+ +H2O f
-
[FeIII(NO)(TMPyP)]5+ + NO2- + 2OH- (slow) (6b)
Figure 10. Electronic absorption spectral changes obtained during
electrochemical generation of [FeII(NO)(TMPyP)]4+ from the solution
containing 2.5 × 10-4 mol‚L-1 [FeIII(TMPyP)]5+ and 2.5 × 10-2 mol‚L-1
NaNO2 upon continuously applying -0.25 V. The spectra were recorded
within 6 min.
[FeIII(NO)(TMPyP)]5+ f [FeIII(TMPyP)]5+ + NO (fast)
(6c)
[FeIII(NO)(TMPyP)]5+ S [FeII(NO+)(TMPyP)]5+ (fast)
reaction 5). The second process (with isosbestic points at
544 and 576 nm) can be assigned to the further reduction of
(6d)
-
[FeII(NO2 )2(TMPyP)]2+ to [FeII(NO)(TMPyP)]4+ (see reac-
[FeII(NO+)(TMPyP)]5+ + NO f
-
tion 7). It is obvious that the formation of [FeII(NO2 )2-
(TMPyP)]2+ is faster than that of [FeII(NO)(TMPyP)]4+. This
[FeII(NO)TMPyP]4+ + NO+ (fast) (6e)
-
is because the formation rate of [FeII(NO2 )2(TMPyP)]2+ is
NO+ + H2O f NO2- + 2H+ (fast)
(6f)
determined by a relatively fast electrolysis process (reaction
5), whereas formation of [FeII(NO)(TMPyP)]4+ is controlled
by a relatively slow disproportionation process (see reactions
6a and 7). To confirm this, kinetic analysis was done on the
electroreductive process of [FeII(NO2-)2(TMPyP)]2+ (reaction
7) shown in Figure 10, as it had been done for the
Among these steps, the first step is the rate-determining step.
Rigidly, this is not a first-order reaction because, besides
-
[FeII(NO2 )2(TMPyP)]2+, H2O or H+ is also involved in the
reaction. However, when using buffer, the concentration of
H2O or H+ remains constant, and reaction 6b becomes a
pseudo-first-order reaction. Since the overall disproportion-
ation reaction (reaction 6a) is determined by the slowest step
(reaction 6b), it also appears to be a pseudo-first-order
reaction.
Electrochemical Generation of [FeII(NO)(TMPyP)]4+.
When reaction 5 is combined with reaction 6a, two new
electrochemical reactions can be obtained:
-
disproportionation process of [FeII(NO2 )2(TMPyP)]2+ (reac-
tion 6a). From the variation of spectral absorption at 550
-
nm with time, it was found that the reduction of [FeII(NO2 )2-
(TMPyP)]2+ to [FeII(NO)(TMPyP)]4+ at -0.25 V (vs Ag/
AgCl) is also a first-order reaction with a rate constant of
(7.85 ( 0.44) × 10-3 s-1 (n ) 7, confidence level 95%).
This value is very near that of the disproportionation reaction
((7.30 ( 0.36) × 10-3 s-1) described above, which implies
that electrochemical reduction of [FeII(NO2 )2(TMPyP)]2+
-
-
[FeII(NO2 )2(TMPyP)]2+ + H2O + e- f
to [FeII(NO)(TMPyP)]4+ is controlled by disproportionation
reaction rather than electrochemical reaction. Thus, the
overall electrochemical reaction (reaction 7) maybe undergo
reaction steps similar to those shown in reactions 6b-6f and
an additional fast electrochemical reduction step similar to
that shown in reaction 5. In addition to the spectroelectro-
chemical results described above, the difference in formation
[FeII(NO)(TMPyP)]4+ + NO2- + 2OH- (7)
[FeIII(TMPyP)]5+ + NO2- + H2O + 2e- f
[FeII(NO)(TMPyP)]4+ + 2OH- (8)
-
Reaction 7 implies that [FeII(NO2 )2(TMPyP)]2+ can be
further reduced to [FeII(NO)(TMPyP)]4+ by one-electron
transfer, and reaction 8 suggests [FeII(NO)(TMPyP)]4+ can
be electrochemically generated from a mixed solution of
rate between [FeII(NO2 )2(TMPyP)]2+ and [FeII(NO)(T-
-
MPyP)]4+ can be observed more clearly by cyclic voltam-
metry (see Figure 11). The scanning potential without
preelectrolysis at -0.25 V gave rise to reversible redox peaks
of [FeIII(TMPyP)]5+/[FeII(NO2-)2(TMPyP)]2+, indicating elec-
-
[FeIII(TMPyP)]5+ and NO2 at a low negative potential
(-0.25 V).
trochemical formation of [FeII(NO2 )2(TMPyP)]2+ (reaction
-
Spectra were recorded during the electrochemical genera-
tion of [FeII(NO)(TMPyP)]4+ at -0.25 V (see Figure 10).
Two obvious electrolysis processes with different groups of
isosbestic points were found to happen in sequence. The first
process (with isosbestic points at 525 and 588 nm) corre-
sponds to the rapid electrochemical formation of [FeII(NO2-)2-
(TMPyP)]2+, which has been discussed previously (see
5) is fast. In contrast, no redox peaks corresponding to [FeII-
(NO)(TMPyP)]4+ in this case were observed. This means
that the formation of [FeII(NO)(TMPyP)]4+ is too slow at
the time scale of cyclic voltammetry (several seconds), and
thus, the amount of [FeII(NO)(TMPyP)]4+ formed is too small
to give a CV response. Until the preelectrolysis time was
8442 Inorganic Chemistry, Vol. 43, No. 26, 2004

Electrochemical Generation of Free Nitric Oxide from Nitrite
Figure 12. Electronic absorption spectral changes obtained for 2.5 × 10-4
mol‚L-1 [FeII(NO)(TMPyP)]4+ during anodic electrolysis at +0.40 V. The
spectra were recorded within 2 min.
Figure 11. Cyclic voltammograms obtained for the solution containing
2.5 × 10-4 mol‚L-1 [FeIII(TMPyP)]5+ and 2.5 × 10-2 mol‚L-1 NaNO2 at
the Pt electrode after preelectrolysis at -0.25 V for 0, 2, 5, and 10 min.
The scan rate was 10 mV‚s-1
.
more than 2 min, the anodic wave corresponding to [FeII-
(NO)(TMPyP)]4+ was observed to appear at 0.38 V,38
-
suggesting that reduction of [FeII(NO2 )2(TMPyP)]2+ to [FeII-
(NO)(TMPyP)]4+ was slow. According to Figure 10, whole
cathodic electrolysis (reaction 8) can be completed within 6
min and eventually gave >95% conversion of [FeIII-
(TMPyP)]5+ to [FeII(NO)(TMPyP)]4+.
The influence of applying a cathodic potential on the
stability of [FeII(NO)(TMPyP)]4+ was studied. It was found
that [FeII(NO)(TMPyP)]4+ was quite stable at the potential
range from -0.20 to -0.35 V; however, it began to
decompose when the potential was less than -0.40 V (vs
Ag/AgCl), giving rise to [FeII(TMPyP)]4+ as the final
product. This indicates that coordinated NO can be further
reduced to other nitrogen compounds such as nitrous oxide
or ammonia when a high negative potential is applied.34,35
Release of NO from [FeII(NO)(TMPyP)]4+. As shown
in Figure 11, the anodic peak at +0.38 V has been confirmed
to result from the oxidation of [FeII(NO)(TMPyP)]4+,35,38,46
the products of which were proposed to be [FeIII(TMPyP)]5+
and free NO.46 To release NO from [FeII(NO)(TMPyP)]4+,
the oxidation potential should be chosen between +0.35 and
+0.70 V, because when the potential is lower than +0.35
V, [FeII(NO)(TMPyP)]4+ is difficult to oxidize (see Figure
11), and when the potential is higher than +0.70 V, [FeII-
(NO)(TMPyP)]4+ will be oxidized to [FeIII(TMPyP)]5+ and
Figure 13. Electronic absorption spectral changes obtained for the final
anodic product in Figure 12 during its re-reduction at -0.25 V. The spectra
were recorded within 2 min.
with reproduced [FeII(TMPyP)]4+ to form [FeII(NO)(T-
MPyP)]4+, resulting in a gradual increase of the Q-band at
-
550 nm, while if NO2 was produced during oxidation at
+0.40 V, when -0.25 V was applied, reaction 5 would
happen, and an increase of the Q-band at 540 nm as shown
in Figure 7 would be observed. Actually, Figure 13 shows
that [FeII(NO)(TMPyP)]4+ (550 nm) is restored after this
redox cycle, which thus confirms that free NO is released
into solution when [FeII(NO)(TMPyP)]4+ is oxidized at
+0.40 V:
- 38
NO2 . In our study, the potential for oxidation of [FeII-
(NO)(TMPyP)]4+ was chosen to be +0.40 V, under which
a maximum oxidation rate of [FeII(NO)(TMPyP)]4+ was
obtained while undesirable side reactions were avoided.
Figure 12 shows that, as soon as the +0.40 V potential is
applied, the amount of [FeII(NO)(TMPyP)]4+ (550 nm)
decreases whereas the amount of [FeIII(TMPyP)]5+ (605 and
640 nm) increases with time, indicating that [FeII(NO)-
(TMPyP)]4+ can be oxidized to [FeIII(TMPyP)]5+ at this
potential. To confirm that NO rather than NO2- was another
product during the oxidation of [FeII(NO)(TMPyP)]4+ at the
potential of +0.40 V, the potential was switched to -0.25
V, where [FeIII(TMPyP)]5+ could be reduced to [FeII-
(TMPyP)]4+. If NO existed in the solution, it would combine
[FeII(NO)(TMPyP)]4+ f [FeIII(TMPyP)]5+ + NO + e-
(9)
Continuous Generation of Free NO. The electrochemical
generation of free NO from nitrite requires the reduction at
-0.25 V (see reaction 8) and the oxidation at +0.40 V (see
reaction 9) in sequence. Thus, two electrolytic cells involved
in a flow system (Figure 1) were utilized to realize continu-
ous generation of free NO. Cell A was used for the generation
of [FeII(NO)(TMPyP)]4+. Cell B was used for the oxidation
of [FeII(NO)(TMPyP)]4+ to release free NO into the solution.
By bubbling argon gas, NO was transferred from the solution
phase to the gas phase in solution reservoir C, where the
Inorganic Chemistry, Vol. 43, No. 26, 2004 8443

Chi et al.
Figure 14. Chemical structure of [FeIII(TMPyP)]5+
.
concentration of NO gas was monitored by the NO gas
sensor. Because the cathodic electrolytic rate is controlled
-
by the disproportionation of [FeII(NO2 )2(TMPyP)]2+, to
Figure 15. Various [FeIII(TMPyP)]5+ species in aqueous solution.
ensure >90% conversion of [FeII(NO2-)2(TMPyP)]2+ to [FeII-
(NO)(TMPyP)]4+, 5 min is essential for cathodic electrolysis.
To meet the electrolytic time requirement, the electrolytic
solution was pumped through the flow electrolytic cells (with
a volume of 1.0 mL) at the rate of 200 µL/min. Under these
conditions, a electrolytic solution containing 2.5 × 10-4
mol‚L-1 [FeIII(TMPyP)]5+, 2.5 × 10-2 mol‚L -1 NaNO2, and
0.25 mol‚L-1 PBS (pH 7.4) was used for flow electrolysis.
Finally, a 75 ppm NO gas stream at a rate of 10 mL/min
was obtained.
means that the porphyrin is readily combined with H2O. And
it was reported that, in aqueous solution, [FeIII(TMPyP)]5+
interacted with H2O and OH- to form various five- or six-
coordinate species (see Figure 15, I-IX),42,49 depending on
the solution pH and the concentration of [FeIII(TMPyP)]5+.
At a low pH value (pH < 6), most [FeIII(TMPyP)]5+ existed
in a diaquo six-coordinated complex (species I), at a high
pH value (pH > 11), [FeIII(TMPyP)]5+ was mainly present
in the form of a dihydroxy complex (species IX),49 and at a
medium pH value, there was a concentration-determined
dimer equilibrium between a monohydroxy complex (species
II) and a µ-oxo dimer (species III). The monohydroxy
complex was proved to exist in diluted solution, whereas
the µ-oxo dimer was present in concentrated solution or in
the crystal.42 Probably, it was just this kind of strong
electrostatic interaction of [FeIII(TMPyP)]5+ with other
stronger ligands, especially OH- in aqueous solution, that
competitively blocked the complexation of [FeIII(TMPyP)]5+
with nitrite and NO. In contrast, in nonaqueous media, the
complexation of ferric porphyrins with nitrite or NO was
often observed. This was probably due to the different
complexation conditions in nonaqueous solution. Generally,
the strong axial ligand OH- was absent in nonaqueous media,
so ferric porphyrins could complex with nitrite via electro-
static interaction without being competitively blocked by this
strong ligand. The effect of other strong axial ligands on
the complexation of ferric porphyrins with nitrite has been
confirmed by Fernandes and co-workers.33 It was reported
that, in nonaqueous solution, if a stronger axial ligand, Cl-,
was present, nitrite did not complex with ferric porphyrin.
Only when the concentration of nitrite was high enough to
displace chloride could the complex reaction happen, giving
rise to bisnitrite complexes of ferric porphyrin. It is evident
that strong ligands tend to prevent the complex reaction of
ferric porphyrins with nitrite and NO.
Discussion
-
Electrochemical generation of NO from NO2 can be
catalyzed by [FeIII(TMPyP)]5+. First, at a low negative
-
potential (-0.250 V vs Ag/AgCl), NO2 complexes with
[FeII(TMPyP)]4+ to yield an unstable intermediate,
-
[FeII(NO2 )2(TMPyP)]2+, and then this complexes spontane-
ously but slowly changes to a stable coordinated NO,
[FeII(NO)(TMPyP)]4+, via a disproportionation reaction. At
a low positive potential (+0.400 mV vs Ag/AgCl), free NO
can be released into solution for the oxidation of [FeII(NO)-
(TMPyP)]4+ to [FeIII(TMPyP)]5+ and NO. Thus, [FeIII-
(TMPyP)]5+ acts as a catalyst during the electrochemical
-
generation of NO from NO2 .
Apparently, our experimental results showed that the
interactions of nitrite and NO with ferrous and ferric
porphyrins in aqueous solutions were quite different from
those observed in nonaqueous media.
As described above, nitrite and NO did not complex with
ferric porphyrin, [FeIII(TMPyP)]5+, at all in aqueous solution.
In contrast, many other studies showed that either nitrite28,33,39
or NO32,48 could complex with ferric porphyrins to form five-
or six-coordinate species in nonaqueous media. These
unusual complexation behaviors of [FeIII(TMPyP)]5+ with
nitrite and NO in aqueous solution are probably due to the
unique chemical structure of [FeIII(TMPyP)]5+ and the effect
of other strong axial ligands. As shown in Figure 14, [FeIII-
(TMPyP)]5+ carries five positive charges, which results in
the central iron(III) being readily attacked by anionic ligands
or even a water molecule. It is quite well-known that the
crystal of [FeIII(TMPyP)]Cl5 is highly hygroscopic, which
Unlike [FeIII(TMPyP)]5+, [FeII(TMPyP)]4+ was found to
readily complex with both nitrite and NO in aqueous solution.
These different behaviors can be explained by the difference
in electronic structures of ferric and ferrous porphyrins. When
(49) Bell, S. E. J.; Hester, R. E.; Hill, J. N.; Shawcross, D. R.; Linsay
(48) Settin, M. F.; Fanning, J. C. Inorg. Chem. 1988, 27, 1431-1435.
8444 Inorganic Chemistry, Vol. 43, No. 26, 2004
Smith, J. R. J. Chem. Soc., Faraday Trans. 1990, 86, 4017-4023.

Electrochemical Generation of Free Nitric Oxide from Nitrite
Scheme 1
[FeIII(TMPyP)]5+ is reduced to [FeII(TMPyP)]4+, the electron
density of the central iron increases, resulting in a decrease
of the electrostatic interaction between the iron(II) center
(II) to nitrite whereas, for ferric porphyrins, electrons were
donated to iron(III) by ligands (nitrite or NO).
Just because of the extreme stability of [FeII(NO)-
(TMPyP)]4+, it was impossible to decompose this complex
into NO and [FeII(TMPyP)]4+ by bubbling argon gas; i.e., it
was impossible to shift the complex equilibrium to the left
in eq 2. It means that we cannot obtain free NO by bubbling
argon gas into [FeII(NO)(TMPyP)]4+ solution. Fortunately,
according to our experimental results (see Figures 12 and
13), we found that [FeII(NO)(TMPyP)]4+ could be oxidized
into [FeIII(TMPyP)]5+ and free NO at a low positive potential
(ca. +400 mV vs Ag/AgCl). It was evident that oxidation
of [FeII(NO)(TMPyP)]4+ was an irreversible process, because
only an anodic peak for the oxidation of [FeII(NO)(T-
MPyP)]4+ was found at +0.38 V (see Figure 11); no
corresponding cathodic peak was observed. This implies that
[FeIII(NO)(TMPyP)]5+ does not exist stably in the aqueous
solution. Maybe [FeIII(NO)(TMPyP)]5+ was produced, but
attacked by some strong anodic ligands, mostly OH-, and
decomposed into [FeIII(TMPyP)]5+ (the actual molecular
structure is much more complex as shown in Figure 15) and
free NO rapidly. Thus, from our viewpoint, the effect of
ligands is also very important for the decomposition of
nitrosyl-ferric porphyrins just as it is important for the
formation of nitrite or nitrosyl-ferric porphyrins (already
discussed above). The effect of ligands or the displacement
of NO ligand bound to the iron(III) center of porphyrins by
other strong axial ligands can also be found in other nitrosyl-
ferric porphyrin systems, such as FeIII(NO)(OEP), FeIII(NO)-
(OEC), and FeIII(NO)(OEiBC), in nonaqueous media.32 When
a sufficient amount of N-methylimidazole was added to these
nitrosyl-ferric porphyrin solutions, NO was displaced and
released into solution. This is analogous to our discovery
that NO in [FeIII(NO)(TMPyP)]5+ was displaced by OH- and
released into aqueous solution.
-
and anionic ligands, such as Cl-, OH-, and NO2 . However,
ferrous porphyrin (d6) has one additional d-orbital electron
compared to ferric porphyrin (d5), which gives ferrous
porphyrins a much stronger orbital interaction (dπ f π*)
with strong π-accepting ligands, such as nitrite50 and NO.51
In fact, besides [FeII(TMPyP)]4+, many other ferrous por-
phyrins can complex with nitrite or NO.31,32,47,50
Although both nitrite and NO can complex with [FeII-
(TMPyP)]4+, the thermodynamic stabilities of nitrite- and
NO-coordinated ferrous porphyrins are quite different; the
latter is much more stable than the former. This difference
was confirmed by our experiments. First, the half-wave
potential of oxidation of [FeII(NO)(TMPyP)]4+ was more
-
positive than that of [FeII(NO2 )2(TMPyP)]3+ (see Figure 11),
which means that NO complexes with [FeII(TMPyP)]4+ more
strongly than nitrite does, and thus makes the oxidation of
ferrous iron to ferric iron more difficult. Second, it was
-
clearly evident that [FeII(NO2 )2(TMPyP)]3+ spontaneously
changed into [FeII(NO)(TMPyP)]4+ and [FeIII(TMPyP)]5+ via
disproportionation (see Figures 4, 8, and 10). This spontane-
ous reaction appears to result from the extreme thermody-
namic stability of the nitrosyl-ferrous porphyrins, [FeII-
(NO)(TMPyP)]4+. For the same reason, disproportionation
reactions were observed for other coordinated iron porphyrin
systems, such as iron(III)-nitrite,28,39 iron(III)-NO,48 and
iron(II)-nitrite.52 These disproportionation reactions com-
monly yielded a stable iron(II)-nitrosyl species, although
different reaction pathways were undergone; for example,
for ferrous porphyrins, electrons were transferred from iron-
(50) Nasri, H.; Ellison, M. K.; Krebs, G.; Huynh, B. H.; Scheidt, W. R. J.
Am. Chem. Soc. 2000, 122, 10795-10804.
(51) McCleverty, J. A. Chem. ReV. 2004, 104, 403-418.
(52) Lancaster, J. R.; Vega, M. J.; Kamin, H.; Orme-Johnson, N. R.; Orme-
Johson, W. H.; Krueger, R. J.; Siegel, L. M. J. Biol. Chem. 1979,
254, 1268.
In summary, our studies found that [FeIII(TMPyP)]5+ did
not complex with nitrite or NO, probably for its strong
Inorganic Chemistry, Vol. 43, No. 26, 2004 8445

Chi et al.
electrostatic interaction with OH- in the aqueous solution,
whereas [FeII(TMPyP)]4+ could complex with both nitrite
and NO maybe via a strong orbital interaction (dπ f π*),
tion. The irreversible oxidation possibly involved formation
and rapid decomposition of [FeIII(NO)(TMPyP)]5+ intermedi-
ate in the presence of a strong axial ligand, OH-, in the
aqueous solution. The overall mechanism for electrochemical
generation of NO from nitrite catalyzed by [FeIII(TMPyP)]5+
in aqueous solution is proposed in Scheme 1. On this basis,
a method for electrochemical generation of free NO with a
high yield has been developed.
-
giving rise to [FeII(NO2 )2(TMPyP)]2+ and [FeII(NO)(T-
-
MPyP)]4+, respectively. [FeII(NO2 )2(TMPyP)]2+ was not
stable, and could be reduced to a much more stable species,
[FeII(NO)(TMPyP)]4+, by a pseudo-first-order dispropor-
tionation reaction. At low potential (ca. +400 mV vs Ag/
AgCl), [FeII(NO)(TMPyP)]4+ was irreversibly oxidized,
yielding [FeIII(TMPyP)]5+ and releasing free NO into solu-
IC049323C
8446 Inorganic Chemistry, Vol. 43, No. 26, 2004