Nitrous-acid-mediated synthesis of iron-nitrosyl-porphyrin: PH-dependent release of nitric oxide

Article abstract of DOI:10.1002/asia.201200518

Two iron-nitrosyl-porphyrins, nitrosyl[meso-tetrakis(3,4,5- trimethoxyphenylporphyrin]iron(II) acetic acid solvate (3) and nitrosyl[meso-tetrakis(4-methoxyphenylporphyrin]iron(II) CH₂Cl ₂ solvate (4), were synthesized in quantitative yield by using a modified procedure with nitrous acid, followed by oxygen-atom abstraction by triphenylphosphine under an argon atmosphere. These nitrosyl porphyrins are in the {FeNO}⁷ class. Under an argon atmosphere, these compounds are relatively stable over a broad range of pH values (4-8) but, under aerobic conditions, they release nitric oxide faster at high pH values than that at low pH values. The generated nitric-oxide-free iron(III)-porphyrin can be re-nitrosylated by using nitrous acid and triphenylphosphine. The rapid release of NO from these Fe^II complexes at high pH values seems to be similar to that in nitrophorin, a nitric-oxide-transport protein, which formally possesses Fe^III. However, because the release of NO occurs from ferrous-nitrosyl-porphyrin under aerobic conditions, these compounds are more closely related to nitrobindin, a recently discovered heme protein. Copyright

Full text of DOI:10.1002/asia.201200518

DOI: 10.1002/asia.201200518
Nitrous-Acid-Mediated Synthesis of Iron–Nitrosyl–Porphyrin: pH-
Dependent Release of Nitric Oxide
Jagannath Bhuyan[a] and Sabyasachi Sarkar*[b]
Abstract: Two iron–nitrosyl–porphyr-
are relatively stable over a broad range
of pH values (4–8) but, under aerobic
conditions, they release nitric oxide
faster at high pH values than that at
low pH values. The generated nitric-
oxide-free ironACTHNUTRGNE(NUG III)–porphyrin can be
re-nitrosylated by using nitrous acid
and triphenylphosphine. The rapid re-
lease of NO from these FeII complexes
ins, nitrosyl[meso-tetrakis(3,4,5-trime-
thoxyphenylporphyrin]iron(II) acetic
acid solvate (3) and nitrosyl[meso-tet-
rakis(4-methoxyphenylporphyrin]iro-
n(II) CH2Cl2 solvate (4), were synthe-
sized in quantitative yield by using
a modified procedure with nitrous acid,
followed by oxygen-atom abstraction
by triphenylphosphine under an argon
atmosphere. These nitrosyl porphyrins
are in the {FeNO}7 class. Under an
argon atmosphere, these compounds
at high pH values seems to be similar
to that in nitrophorin, a nitric-oxide-
transport protein, which formally pos-
sesses FeIII. However, because the re-
lease of NO occurs from ferrous–nitro-
syl–porphyrin under aerobic conditions,
these compounds are more closely re-
lated to nitrobindin, a recently discov-
ered heme protein.
Keywords: EPR spectroscopy
·
iron · nitric oxide · porphyrinoids ·
X-ray diffraction
Introduction
though the iron center is believed to be in its ferric state.[4]
Most interestingly, nitrobindin, which is a recently discov-
ered protein from the plant Arabidopsis thaliana that is
structurally similar to nitrophorins, stores NO in its ferrous
state in an anaerobic environment and then releases it in an
aerobic environment.[5] In addition, in cd1 nitrite reductase,
which reduces nitrite into nitric oxide, it was believed that
NO was released from ferric–d1-heme. However, the en-
hanced rate of NO-dissociation from the ferrous–heme of
cd1 nitrite reductase has been recently reported.[6] A study
of the spatial- and electron-distribution in the ferrous–d1-
heme–NO complex from Pseudomonas aeruginosa cd1 NIR
has also been performed to understand the unique proper-
ties that are responsible for the relatively fast release from
ferrous–d1-heme–NO.[7] There are also reports of a ferrous–
heme protein that reversibly binds nitric oxide.[8] In addi-
tion, the physiological actions of the nitroxy anion (NOꢀ),
which is the one-electron-reduction product of nitric oxide
(NO), is not properly understood. Little is known about the
release of the nitroxy anion from heme protein.[9] To under-
stand more about the interactions between NO and the
heme group, we wanted to investigate the reaction between
nitrous acid and iron–porphyrin. Herein, we report the syn-
thesis of iron–nitrosyl–porphyrin by using nitrous acid with
in situ oxygen-atom abstraction by triphenylphosphine. We
have used iron–porphyrins, such as meso-tetrakis(3,4,5-
Nitric oxide, a bioregulatory signaling molecule, can react
with FeII- and FeIII–heme to form {(heme)FeNO} moieties of
{FeNO}7- and {FeNO}6 type.[1] Nitrophorin, a nitric-oxide-
transport protein that is found in the saliva of blood-feeding
insects, is an FeIII–heme protein that reversibly binds nitric
oxide.[2] This protein binds with nitric oxide at low pH
values (in the saliva of blood-feeding insects) and releases it
at high pH values (about pH 7.5) in the blood stream of the
host to improve the flow of blood by inflating the blood-
flowing channels. The binding of the released histamine to
the nitric-oxide-free iron atom of nitrophorins prevents its
role in immune response that help to prolong the process of
blood-feeding, thereby leading to the transmission of T.
cruzi, which is the cause of Chagas’ disease. The release of
NO into the blood stream at high pH value and the binding
of free histamine at the site of released NO on the heme
moiety of nitrophorin results in an immune response to pre-
vent the detection of the insect bite.[3] There is a lot of ambi-
guity regarding the Fe-N-O angle in nitrophorin, even
[a] J. Bhuyan
Department of Chemistry
Indian Institute of Technology Kanpur
Kanpur-208016, U.P. (India)
[b] Prof. S. Sarkar
trimethoxyphenyl)porphyriniron
tetrakis(4-methoxyphenyl)porphyrinironACHTUNGTRENNUNG
for such nitrosylation reactions with nitrous acid. These
iron–porphyrins have been easily oxidized to generate their
corresponding isoporphyrins or higher-valent iron–porphyr-
ins.[10] These iron–nitrosyl–porphyrins release NO at a rela-
ACHTUNGTRENNUNG
Department of Chemistry
Bengal Engineering and Science University
Shibpur, Howrah-711103 (West Bengal)
E-mail: abya@iitk.ac.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200518.
Chem. Asian J. 2012, 00, 0 – 0
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FULL PAPER
tively faster rate at high pH values than at low pH values in
the presence of air. Such a release of NO from ferrous–ni-
trosyl–porphyrin under an aerobic atmosphere mimics the
reactivity of nitrobindin, which is a nitrophorin-type heme
protein.
Results and Discussion
Synthesis and Characterization
We have recently reported the meso-hydroxylation of iron–
porphyrin through the formation of a p-cation radical with
NO2 (a mixture of NO and O2) as an oxidizing agent for
compound 1.[10] The meso-hydroxylation of compound 2 also
readily occurs in the same manner. However, the meso-hy-
droxylation of the simple iron–tetraphenylporphyrin
(FeTPP) does not occur and, instead, nitration at the b-posi-
tion has been observed.[11] The meso-hydroxylation of these
two porphyrins is due to low oxidation potential for the gen-
eration of p-cation radicals. In the light of this result, we
were also able to obtain the first crystal structure of a zinc–
Scheme 1. Synthesis of iron–nitrosyl–porphyrins and their pH-dependent
release of nitric oxide.
AHCTUNGTREG(NNNU III)–nitrite–porphyrin has not been reported. First, the
iron
iron
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
phosphine, it transfers one oxygen atom, thereby concomi-
tantly forming iron–nitrosyl–porphyrin and triphenylphos-
phine oxide. UV/Vis spectroscopy of the dark-red solution
confirmed the complete conversion into iron(II)–nitrosyl–
porphyrin. This iron–nitrosyl–porphyrin could be easily pre-
cipitated out of solution by adding petroleum ether (60–
808C). The use of NaNO2 and acid is known for the conver-
isoporphyrin
(aqua-meso-hydroxytetraphenylporphyrin-
zinc(II) nitrate) of a tetraaryl derivative.[12] Owing to the
close relationship between nitrogen dioxide and nitrous
acid, we wanted to study the reaction between nitrous acid
and compounds 1 and 2. Under anaerobic conditions, no sig-
nificant change was observed in such a reaction. This result
sion of iron(IV)–corroles into ironACTHNUTRGNEUNG
(III)–nitrosyl–corroles.[16]
The FTIR spectra of nitrosyl[meso-tetrakis(3,4,5-trimethoxy-
phenylporphyrin]iron(II) acetic acid solvate (3) and nitro-
syl[meso-tetrakis(4-methoxyphenylporphyrin]iron(II)
CH2Cl2 solvate (4) showed the appearance of a n(NO) peak
at 1670 and 1687 cmꢀ1, respectively (see the Supporting In-
formation, Figure S4).
may be due to the instability of iron
rin.[13] Iron
(III)–nitrite–porphyrin may be present in equilib-
rium with iron(III)–chloride–porphyrin. Upon the addition
ACHTUNGERTN(NUNG III)–nitrite–porphy-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
of triphenylphosphine, the coordinated nitrite responds to
oxo-abstraction, thereby resulting in the formation of iron–
nitrosyl–porphyrin, which leads to the coordination of nitrite
Structure Analysis
from the corresponding chloride-coordinated ironACTHNUTRGNEUG(N III)–por-
phyrin, thereby leading to quantitative conversion into iron–
nitrosyl–porphyrin.
The respective molecular structures of complexes 3 and 4
are shown in Figure 1 and the crystallographic data are pre-
sented in Table 1. In nitrosyl[(meso-tetrakis(3,4,5-trimethox-
yphenylporphyrin]iron acetic acid solvate (3), the oxygen
atom of NO is disordered over two positions. Similarly, in
compound 4, the oxygen atom of NO is also disordered over
two positions. Such disorder indicates that the unit cells of
The synthesis of these iron–nitrosyl–porphyrins is shown
in Scheme 1 and the details are available in the Supporting
Information, Scheme S1. There have been reports of
oxygen-atom transfer from nitrite mediated by FeIII–por-
phyrins in aqueous solution.[14] The reaction of octaethylpor-
ꢀ
phyrin–iron
(III)–chloride with potassium crown ether
these complexes contain two types of Fe NO complex, only
ꢀ
([18]crown-6) nitrite in N-methylpyrrolidone also forms an
with respect to the Fe NO unit, which is common for
iron
N
almost all nitrosyl–porphyrin complexes of tetraaryl deriva-
tives.[17] Thus, the X-ray data represent disorder at only the
ꢀ
Fe NO moiety. Such disorder has been taken into consider-
ation by using free-variable instruction in SHELX 97. Be-
ꢀ
cause the N O bond is important in understanding this
chemistry, the refinement was performed with the electron-
density data from the X-ray reflections without any restraint
of the bond lengths or thermal parameters. One interesting
feature about complex 3 is that one molecule of acetic acid
is present in the asymmetric unit. The packing in the unit
cell of compound 3 shows the presence of acetic acid in
a dimer through strong H-bonding interactions (Figure 2a).
There is a weak interaction between the carbonyl oxo
groups of acetic acid and the 6th position of the iron atom,
Abstract in Bengali:
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Nitrous-Acid-Mediated Synthesis of Iron–Nitrosyl–Porphyrin
Figure 1. a) ORTEP of complex 3 with partial atom-labeling; thermal el-
lipsoids are set at 50% probability, hydrogen atoms are omitted for clari-
ty. The oxygen atom of the NO group is disordered over two positions.
b) ORTEP of complex 4 with partial atom-labeling; thermal ellipsoids
are set at 50% probability, hydrogen atoms are omitted for clarity.
Table 1. Crystallographic data[a] for complexes 3 and 4.
Complex
3
4
formula
formula weight
crystal system
space group
T [K]
Z
a [ꢁ]
b [ꢁ]
c [ꢁ]
C67H65FeN5O15
1236.35
triclinic
P-1
100
2
14.415(5)
14.795(5)
15.108(5)
88.008(5)
71.017(5)
80.461(5)
3004(18)
1.367
C49H38N5O5FeCl2
903.59
monoclinic
P21/c
100
4
16.065(5)
18.374(5)
14.856(5)
90
109.294(5)
90
4139(2)
1.450
0.550
Figure 2. a) View of compound 3 that shows weak interactions between
the carbonyl oxo groups of acetic acid and the 6th position of the iron
ꢀ
atom. b) View of compound 3 that shows a weak O···H C intermolecular
ꢀ
H-bonding interaction between the oxygen atom of NO and the H C
moiety of a benzene solvent molecule.
a [8]
b [8]
scribed by Scheidt and Ellison,[18] which is an important fea-
ture of {FeNO}7 porphyrins, is very low (38) for compound
3. The iron atom is displaced toward nitric oxide by 0.25 ꢁ,
which is a typical value for low-spin iron(II) complexes.
However, complex 4 is highly saddled. Nitrosyl–porphyrins
3 and 4 are not very stable in the solid state and they re-
spond slowly to the trace amounts of air by releasing NO
with the oxidation of iron.
g [8]
V [ꢁ3]
dcalcd [gcmꢀ3
]
m [mmꢀ1
]
0.325
2.22–26.00
0.870
q range [8]
2.54–26.00
1.069
GOF (F2)
[c]
R1[b] (wR2
)
0.0719 (0.1787)
0.0602
ACHTUNGTRENN(UNG 0.1424)
[a] MoKa
G
radiation.
[b] R1 =ꢀj jF0 jꢀjFc j j/ꢀjF0 j.
[c] wR2 ={ꢀ[w-
2
2
(F0 ꢀFc2)2]/ꢀ[w
ACHTUNGTRENNUNG .
(F0 )2]}1/2
Electrochemistry
as shown in Figure 2a. In general, hexa-coordinated iron–ni-
trosyl–porphyrins are less stable, owing to the trans effect.
The electrochemistry of (TPP)Fe(NO) and (OEP)Fe(NO)
were reported[19] in CH2Cl2; both of these complexes are
stable in CH2Cl2. The oxidation of compounds 3 and 4 to
form {Fe(NO)}+ appeared at +0.64 and +0.61 V, respective-
ly (versus Ag/AgCl, Figure 3). Similarly, the one-electron re-
duction of compounds 3 and 4 took place at ꢀ0.93 and
ꢀ0.99 V (versus Ag/AgCl), respectively, to form {Fe(NO)}ꢀ.
The chemical oxidation of compound 3 with nitrogen diox-
ide also formed {Fe(NO)}+ instead of nitrosyl–isoporphyrin.
ꢀ
Interestingly, a weak O···H C intermolecular H-bonding in-
teraction is present in compound 3 between the oxygen
ꢀ
atom of NO and the H C atom of one benzene solvent mol-
ecule (Figure 2b). In complex 3, the porphyrin core is ruf-
fled but this distortion does not cause any significant
ꢀ
changes in the geometric parameters of the Fe NO moiety,
ꢀ
ꢀ
except for the long N O distance (1.3 ꢁ). This long N O
distance may be due to the disorder on the oxygen atom.
ꢀ
ꢀ
The other distances are typical: Fe Np =2.000 ꢁ, Fe
ꢀ
N(NO)=1.705(2) ꢁ. Tilting of the Fe NNO vector, as de-
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Figure 5. Changes in the absorbance spectrum of compound 3 at pH 5;
time interval: 3 min, (total time: 3 h).
Figure 3. Electrochemistry of iron–nitrosyl–porphyrins in CH2Cl2 (298 K):
3 slowly releases NO to form FeIII–porphyrin, as observed
by the appearance of the characteristic band at 675 nm in
the electronic spectrum (Figure 5). At pH 8 with imidazole
or histamine (pH value maintained by adding acetic acid),
complete denitrosylation occurs within 1 h (see the Support-
ing Information, Figure S3), which is much faster than that
at pH 5.5 (3 h, Figure 5). The bulky trimethoxy group
cannot prevent the denitrosylation of compound 3. This
result may be due to the remarkably bent Fe-N-O (1298)
angle, which causes a less-effective orbital interaction that
facilitates the release of NO. The formation of six-coordi-
nate [(imidazole)(NO)FeP] was not observed. At neutral
pH, compound 3 is relatively stable and at a high pH it re-
leases nitric oxide faster than that at a low pH. Therefore,
the pH value has a unique role in dictating the reactivity of
nitrosyl–porphyrins, irrespective of the oxidation state of the
iron center. The release of NO from compound 3 is also visi-
ble by using Griess’s test. When Griess-reagent-dipped
tissue paper was placed above a solution of compound 3 in
CH2Cl2 or toluene, the color of the tissue paper changed to
a) Cyclic voltammogram of 1 mm of compound 3 (scan rate: 100 mVsꢀ1
)
with 0.2m TBAP as a supporting electrolyte. b) Cyclic voltammogram of
1 mm of compound 4 (scan rate: 100 mVsꢀ1) with 0.2m TBAP as a sup-
porting electrolyte.
EPR Spectroscopy
EPR spectroscopy of iron–nitrosyl–porphyrins is very useful
in determining the coordination geometry around the iron
center, as well as the oxidation state of the metal. The X-
band EPR spectra of compounds 3 and 4 in solution showed
the typical triplet pattern of ferrous–nitrosyl–porphyrin[20]
(as shown for compound 3 in Figure 4). Upon addition of
imidazole, no peaks for the six-coordinate nitrosyl complex,
{Fe(NO)(imidazole)}, were observed in the EPR spectrum.
II
III
ꢀ
pink. The conversion of Fe NO into Fe –porphyrin at low
II
ꢀ
pH values may be due to the oxidation of Fe NO into an
FeIII NO complex, which then releases NO. The release of
ꢀ
NO from this complex is similar to the recently discovered
nitrophorin-type protein nitobindin, which stores NO in its
FeII state in an anaerobic environment and releases it in an
aerobic atmosphere owing to the aerial oxidation of FeII
into FeIII.[5] Interestingly, after the release of NO from nitro-
syl–porphyrin, the as-formed FeIII–porphyrin, can be re-ni-
trosylated by using nitrous acid in the presence of triphenyl-
phosphine. In addition, it is known that, in heme-containing
bacterial NO reductase, FeIII–porphyrin is formed from fer-
rous–nitrosyl–porphyrin.[21]
Figure 4. X-band EPR spectrum of compound 3 in CH2Cl2 at room tem-
perature.
NO Release
Under an argon atmosphere, both compounds 3 and 4 are
relatively stable in the pH range 4–8. However, under aero-
bic conditions, at high pH values, compound 3 releases NO
with the concomitant formation of the m-oxo dimer
(Scheme 1; also see the Supporting Information, Figure S3).
Under aerobic conditions at low pH value (5.5), compound
Conclusions
Iron–nitrosyl–porphyrins have been synthesized by using
a modified method with nitrous acid, followed by oxygen-
atom abstraction with triphenylphosphine under an argon
atmosphere. These compounds were characterized by spec-
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Nitrous-Acid-Mediated Synthesis of Iron–Nitrosyl–Porphyrin
color of this solution turned bright red after 5 min. Stirring was continued
for a further 30 min under an argon atmosphere. The mixture was filtered
and the filtrate was layered three times with petroleum ether (60–808C).
The reaction mixture was allowed to stand for 5 days to yield dark-
purple block-shaped diffraction-quality crystals. Yield 104 mg (90%);
troscopic analysis and by X-ray crystallography. Under an
argon atmosphere, these compounds were relatively stable
over a broad range of pH values (4–8). However, under
aerobic conditions, these compounds released nitric oxide
more rapidly at higher pH values than at lower pH values.
At low pH values, these compounds slowly release nitric
molecular formula: C67H65FeN5O15
; Mw =1236.09; UV/Vis (CH2Cl2):
lmax(e)=412 (120234), 474 (20069), 537 nm (9709 molꢀ1 m3 cmꢀ1); FTIR
(KBr): n˜ =1670 cmꢀ1 (NO); elemental analysis calcd (%) for
C56H65FeN5O15: C 65.10, H 5.30, N 5.67; found: C 65.01, H 5.14, N 5.54.
oxide, thereby concomitantly forming ironACHTNUTRGNEUNG(III)–porphyrin.
Thus, the reactivity pattern reported herein for simple nitro-
syl–porphyrins clearly shows that the binding of NO to
heme is tuned by the pH value of the solution. In addition
to the oxidation state of the metal atom, the possible role of
structural changes and of the synthetic conditions under
which the complex is formed may have a combined role in
controlling the reactivity of iron–nitrosyl–porphyrins. The
release of NO from iron(II)–nitrosyl–porphyrin under aero-
bic conditions is in accordance with the chemistry of nitro-
bindin, which is a recently characterized new nitrophorin-
type protein.
Synthesis of Nitrosyl[meso-tetrakis(4-methoxyphenylporphyrin]iron(II)
CH2Cl2 Solvate (4)
To a solution of meso-tetrakis (4-methoxyphenyl)porphyrinironACHTUNGTRENNUNG(III) chlo-
ride (100 mg, 0.121 mmol) in CH2Cl2 (8 mL) was added NaNO2 (7 mg,
0.101 mmol) and glacial acetic acid (0.1 mL) under an argon atmosphere
with constant stirring. After 10 min, triphenylphosphine (40 mg,
0.152 mmol) was added to this solution. The initial dark-brown color of
this solution turned bright red after 5 min. Stirring was continued for an-
other 30 min in argon atmosphere. The mixture was filtered and the fil-
trate was layered three times with petroleum ether (60–808C). The reac-
tion mixture was allowed to stand for 5 days to yield dark-purple block-
shaped diffraction-quality crystals. Yield 97.5 mg (89%); molecular for-
mula: C49H38Cl2FeN5O5; molecular weight: 903.61; UV/Vis (CH2Cl2):
lmax(e)=410 (125634), 484 (21069), 537 nm (7209 molꢀ1 m3 cmꢀ1); FTIR
(KBr): n˜ =1687 cmꢀ1(NO); elemental analysis calcd (%) for
C49H38·Cl2FeN5O5: C 65.13, H 4.24, N 7.75; found: C 65.01, H 4.16, N 7.59.
Experimental Section
Experimental Details
X-Ray Crystallography
All reactions were performed under a dry N2 atmosphere with standard
Schlenk techniques unless otherwise noted. Solvents, such as acetone,
CH2Cl2, n-hexane, petroleum ether, and pyrrole were obtained from S. D.
Fine-Chem Ltd. India, purified, and dried by using standard methods
before use. 3,4,5-Trimethoxybenzaldehyde and 4-methoxybenzaldehyde
were obtained from Sigma–Aldrich and DMF was obtained from
Thomas Baker. Histamine dihydrochloride was purchased from Sigma–
Aldrich and imidazole was purchased from Loba Chemie, India. Elec-
The single crystal was glued to
a glass fiber and mounted onto
a BRUKER SMART APEX diffractometer. The cell constant was ob-
tained from least-squares refinement of 3D centroids through CCD re-
cording of narrow w-rotation frames, thereby completing almost-all recip-
rocal space in the stated q range. The instrument was equipped with
a CCD area detector and the data were collected by using graphite-mon-
ochromated MoKa radiation (l=0.71073 ꢁ) at low temperature (100 K).
An empirical absorption correction was applied by using the SADABS
program. All data were collected with SMART 5.628 (BRUKER, 2003)
and were integrated with the BRUKER SAINT program. The structure
was solved by using SIR97[23] and refined by using SHELXL-97.[24] All
non-hydrogen atoms were refined with anisotropic displacement parame-
ters. CCDC 872693 (4) and CCDC 872694 (3) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
tronic absorption spectroscopy was performed on
a Perkin–Elmer
(Lamda 35) spectrophotometer. IR spectra of pressed KBr disks were re-
corded on a Bruker Vertex 70 FTIR spectrophotometer. Elemental anal-
ysis was performed on a Perkin–Elmer 2400 microanalyzer. X-band EPR
measurements were carried out at room temperature on a Bruker EMX
spectrometer with a microwave power of around 0.188 mW and a frequen-
cy range of 9.398–9.455 GHz. Cyclic voltammetry was performed on
a BASi Epsilon-EC Bioanalytical systems Inc. Cyclic voltammograms
and differential pulse polarographs of 10ꢀ3 m solutions of the compounds
were obtained in CH2Cl2 with 0.2m Bu4NClO4 as a supporting electrolyte,
Ag/AgCl as the reference electrode, platinum as the auxiliary electrode,
and glassy carbon electrode as the working electrode. All electrochemical
experiments were performed under an argon atmosphere at 298 K unless
otherwise stated. Potentials are referenced against ferrocene (Fc) and are
reported relative to the Ag/AgCl electrode (E1/2 =0.459 V versus Ag/
AgCl (Fc+/Fc).
Acknowledgements
Meso-tetrakis(3,4,5-trimethoxyphenyl)porphyriniron
ACHTUNGTREN(NUNG III) chloride (1) and
J.B. acknowledges a Doctoral Research Fellowship from the CSIR, New
Delhi and S.S. thanks the DST, New Delhi for funding this project.
meso-tetrakis(4-methoxyphenyl)porphyriniron(III) chloride (2) were pre-
AHCTUNGTRENNUNG
pared by using a one-pot method[22] and their synthesis and characteriza-
tion (UV/Vis, IR spectroscopy) are provided in the Supporting Informa-
tion. The solutions of histamine and imidazole were adjusted to pH
values of about 7.8 and 5.5, by adding acetic acid. For the Griess-reagent
test, a solution of compound 3 in CH2Cl2 was added to a test tube and
Griess-reagent-dipped tissue paper was used as a cap to cover the test
tube.
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Synthesis of Nitrosyl[meso-tetrakis(3,4,5-
trimethoxyphenylporphyrin]iron(II) Acetic Acid Solvate (3)
To a solution of meso-tetrakis (3,4,5-trimethoxyphenyl)porphyrinironACTHNUTRGNE(UNG III)
chloride (100 mg, 0.094 mmol) in benzene (8 mL) was added NaNO2
(7 mg, 0.101 mmol) and glacial acetic acid (0.1 mL) under an argon at-
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(30 mg, 0.114 mmol) was added to this solution. The initial dark-brown
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Jagannath Bhuyan and Sabyasachi Sarkar
FULL PAPER
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petroleum ether to this solution and allowing it to stand for crystalli-
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Received: June 11, 2012
Revised: July 1, 2012
Published online: && &&, 0000
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FULL PAPER
Porphyrins
Jagannath Bhuyan,
Sabyasachi Sarkar*
&&&& — &&&&
Nitrous-Acid-Mediated Synthesis of
Iron–Nitrosyl–Porphyrin: pH-Depen-
dent Release of Nitric Oxide
FeNO-menal chemistry: Iron–nitrosyl–
porphyrins have been synthesized in
quantitative yield by using a modified
method with nitrous acid, followed by
oxygen-atom abstraction by triphenyl-
phosphine under an argon atmosphere.
These nitrosyl–porphyrins, which are
in the {FeNO}7 class, rapidly released
NO at high pH values. The release of
NO from iron(II)–nitrosyl–porphyrin
in aerobic environments is in accord-
ance with the chemistry of nitrobindin.
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