Nitric Oxide Dioxygenase Activity of a Nitrosyl Complex of Cobalt(II) Porphyrinate in the Presence of Hydrogen Peroxide via Putative Peroxynitrite Intermediate

Article abstract of DOI:10.1021/acs.inorgchem.8b02722

The reaction of a cobalt porphyrin complex, [(F₈TPP)Co], 1 {F₈TPP = 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrinate dianion} in dichloromethane with nitric oxide (NO) led to the nitrosyl complex, [(F₈TPP)Co(NO)], 2. Spectroscopic studies and structural characterization revealed it as a bent nitrosyl of {CoNO}⁸ description. It was stable in the presence of dioxygen. However, it reacts with H₂O₂ in acetonitrile (or THF) solution at -40 °C (or -80 °C) to result in the corresponding Co(III)-nitrate complex, [(F₈TPP)Co(NO₃)], 3. The reaction presumably proceeds via the formation of a Co-peroxynitrite intermediate. X-Band electron paramagnetic resonance and electrospray ionization-mass spectroscopic studies suggest the intermediate formation of the [(porphyrin)Co(III)-O^?] radical, which in turn supports the generation of the corresponding Co(IV)-oxo species during the reaction. This is in accord with the homolytic cleavage of the O-O bond in heme-peroxynitrite proposed in the nitric oxide dioxygenases activity. In addition, the characteristic peroxynitrite-induced phenol ring reaction was also observed.

Full text of DOI:10.1021/acs.inorgchem.8b02722

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Nitric Oxide Dioxygenase Activity of a Nitrosyl Complex of Cobalt(II)
Porphyrinate in the Presence of Hydrogen Peroxide via Putative
Peroxynitrite Intermediate
Baishakhi Mondal, Soumen Saha, Dibyajyoti Borah, Rakesh Mazumdar, and Biplab Mondal*
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India
S
* Supporting Information
ABSTRACT: The reaction of a cobalt porphyrin complex, [(F8TPP)Co], 1 {F8TPP =
5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrinate dianion} in dichloromethane with
nitric oxide (NO) led to the nitrosyl complex, [(F8TPP)Co(NO)], 2. Spectroscopic
studies and structural characterization revealed it as a bent nitrosyl of {CoNO}8
description. It was stable in the presence of dioxygen. However, it reacts with H2O2 in
acetonitrile (or THF) solution at −40 °C (or −80 °C) to result in the corresponding
Co(III)-nitrate complex, [(F8TPP)Co(NO3)], 3. The reaction presumably proceeds via
the formation of a Co-peroxynitrite intermediate. X-Band electron paramagnetic
resonance and electrospray ionization−mass spectroscopic studies suggest the
intermediate formation of the [(porphyrin)Co(III)−O•] radical, which in turn supports
the generation of the corresponding Co(IV)-oxo species during the reaction. This is in
accord with the homolytic cleavage of the O−O bond in heme-peroxynitrite proposed in
the nitric oxide dioxygenases activity. In addition, the characteristic peroxynitrite-induced
phenol ring reaction was also observed.
the corresponding nitrate complex.12 However, Cu(I)-nitrosyl
complex was exemplified to result in nitrite product in the
INTRODUCTION
■
Nitric oxide (NO) plays important roles in mammalian biology
including neurotransmission and in immune response.1,2 It is
presence of O2 through a presumed peroxynitrite intermedi-
ate.13 In addition, [Cu(NO)]10 complex in the presence of
believed that only a submicromolar concentration of NO is
H2O2 was also found to result in the copper-nitrato complex
required for its functions. However, when produced at high
through a presumed CuI-peroxynitrite intermediate forma-
concentrations, it induces cytotoxicity via the formation of
reactive nitrogen species (RNS) such as peroxynitrite
tion.14
(ONOO−) or nitrogen dioxide (NO2).3,4 These RNS are
Hence, the reaction of the nitrosyl complexes of transition
metal ions with O2 and the reduced O2 species is of great
proposed to form in the reaction of NO with superoxide
radical (O2•−), hydrogen peroxide (H2O2), and/or in the
interest. In this direction, the examples involving the reaction
presence of transition metal ions.3 The nitric oxide
of metal-nitrosyls with H2O2 are limited in the literature.
Herein, we report the NOD activity of a cobalt-nitrosyl,
dioxygenases (NODs) control the level of NO by converting
it into the biologically benign nitrate.5,6 The oxy-heme species
[(F8TPP)Co(NO)] (2), in the presence of hydrogen peroxide
−
(H2O2) where the intermediate formation of peroxynitrite is
implicated. Spectroscopic evidence suggests the formation of
the [(F8TPP)Co(III)-O•] radical and NO2 in the reaction.
of the NODs upon reaction with NO results in nitrate (NO3 )
through the intermediate formation of a peroxynitrite ion. A
number of examples are reported where the involvement of a
metal-peroxynitrite intermediate is proposed in the reaction of
NO with metal−oxygen species. The superoxo complexes of
heme and nonheme iron, cobalt, and copper are exemplified to
result in the metal-peroxynitrite intermediates in the presence
of NO.7−9 Recently, a nonheme CrIV-peroxo complex was
reported to react with NO to result in CrIII-nitrate; whereas the
CrIII-superoxo analogue resulted in the corresponding CrIV-oxo
and NO2 presumably via a CrIII-peroxynitrite intermediate.10
The examples related to the formation of a presumed metal-
peroxynitrite in the reaction of a metal-nitrosyl and dioxygen
(O2) are known in the literature. The formation of the nitrite
EXPERIMENTAL SECTION
■
Materials and Methods. All the reagents and solvents were
purchased from commercial sources and used as received unless
specified. All the reactions were carried out under Ar or N2
atmosphere. Repeated vacuum/purge cycles or bubbling with Ar
was used to remove the oxygen from the solvents and solutions.
Acetonitrile was distilled over calcium hydride. UV−visible spectra
were recorded on Agilent Cary 8454 UV−visible spectrophotometer
using Chemstation software. FT-IR spectra were taken on a
PerkinElmer spectrophotometer with samples prepared as KBr pellets
−
(NO2 ) product in the reaction of cobalt-nitrosyl with O2 was
reported long back.11 In recent time, the reaction of a
Received: September 24, 2018
nonheme dinitrosyliron complex with O2 was found to afford
© XXXX American Chemical Society
A
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1
or in solutions using NaCl cell having 1 cm path length. H NMR
spectra were recorded on a 600 MHz Varian FT spectrometer.
Chemical shifts (ppm) were referenced with respect to Me4Si as
internal standard for organic compounds or to the residual solvent
peaks. Elemental analyses were done on a PerkinElmer Series II
Analyzer. Mass spectra were recorded on a Waters, Model Q-Tof
Premier instrument with ESI mode of ionization. The X-band electron
paramagnetic resonance (EPR) spectra were recorded on a JES-
FA200 ESR spectrometer, at room temperature or at 77 K with
microwave power, 0.998 mW; microwave frequency, 9.14 GHz; and
modulation amplitude, 2.
2.30; N, 7.98. Found (%): C, 60.30; H, 2.31; N, 8.06. FT-IR (in KBr):
1622, 1466, 1385, 1238, 1024, 880, 805, and 791 cm−1. UV−visible
(CH3CN): 430 nm (ε/M−1 cm−1, 2.18 × 105), 542 nm (ε/M−1 cm−1,
1.77 × 104). 1H NMR (400 MHz, CDCl3): δppm, 9.10 (s, 8H), 7.83−
7.76 (m, 4H), 7.43−7.35 (m, 8H). ESI-Mass (m/z): calcd 877.07,
found 877.05 {molecular ion peak for [Co(F8TPP)(NO3)]}.
Complex 4 and 2,4-Di-tert-butyl-6-nitrophenol. To a dry and
degassed acetonitrile solution of complex 2 (0.84 g, 1 mmol),
precooled solution of 2,4-ditertiarybutylphenol (1.03 g, 5 mmol) was
added followed by precooled H2O2 (37% v/v, 2.2 mmol) in
acetonitrile at −40 °C and stirred for 1/2 h at the same temperature.
The reaction mixture was then brought to room temperature and
dried in a rotavapor. Purification of the crude mass through column
chromatography resulted in pure 2,4-ditertiarybutyl-6-nitrophenol.
Yield: 0.125 g (ca. 50%). Complex 4 yield: 0.558 g (ca. 67%).
2,4-Di-tert-butyl-6-nitrophenol. Elemental analyses for
C14H21NO3, calcd (%): C, 66.91; H, 8.42; N, 5.57. Found (%): C,
Single crystals were grown from the respective dichloromethane
and chloroform solutions using slow evaporation technique. The
intensity data were collected using a Bruker SMART APEX-II CCD
diffractometer, equipped with a fine focus 1.75 kW sealed tube MoKα
radiation (λ = 0.71073 Å) at 293(3) K, with increasing ω (width of
0.3° per frame) at a scan speed of 3 s/frame. The SMART software
was used for data acquisition. Data integration and reduction were
undertaken with SAINT and XPREP software.15 Multiscan empirical
absorption corrections were applied to the data using the program
SADABS.16 Structures were solved by direct methods using SHELXS-
2016 and refined with full-matrix least-squares on F2 using SHELXL-
2016/6.17a Structural illustrations have been drawn with ORTEP-3 for
Windows.17b
Syntheses. Synthesis of Ligand F8TPPH2, [5,10,15,20-tetra-
kis(2,6-Difluorophenyl)porphyrin)]. The ligand F8TPPH2 was
prepared by general procedure of porphyrin synthesis with some
modification. 2,6-Difluorobenzaldehyde (2.84 g, 20 mmol) and
pyrrole (1.34 g, 20 mmol) were added to 37 mL of propionic acid.
The solution was refluxed for 2 h and then brought to room
temperature. The reaction mixture was neutralized using aqueous
Na2CO3. The precipitated mass was filtered off, washed with water,
and dried under vacuum. The crude mass was subjected to column
chromatography to result in the pure ligand as a purple crystalline
solid (0.61 g, ca.16% yield). Elemental analyses for C44H22N4F8, calcd
(%): C, 69.66; H, 2.92; N, 7.38. Found (%): C, 69.80; H, 2.90; N,
7.46. FT-IR (in KBr): 3334, 2925, 2853, 2852, 1621, 1588, 1462,
1273, 1234, 1001, 962, 780, 573 cm−1. 1H NMR (600 MHz, CDCl3):
δppm, 8.88 (s, 8H), 7.82−7.77 (m, 4H), 7.40−7.37 (m, 8H). Mass (m/
z): calcd 758.17, found 758.51 (molecular ion peak).
Syntheses of the Complexes. Complex 1. The ligand F8TPPH2
(0.15 g, 0.20 mmol) and Co(OAc)2·4H2O (0.50 g, 2.00 mmol) were
taken in 200 mL of acetonitrile and refluxed for 12 h. The solid mass
was filtered off and the filtrate was collected. After removal of the
solvent, the crude product was subjected to column chromatography
using neutral alumina column, and complex 1 was isolated as red solid
(0.11 g, ca. 66% yield). Elemental analyses for C44H20N4F8Co, calcd
(%): C, 64.80; H, 2.47; N, 6.87. Found (%): C, 64.90; H, 2.45; N,
6.99. FT-IR (in KBr): 1626, 1585, 1463, 1234, 1003, 783, 708, 586,
and 508 cm−1. UV−visible (CH3CN): 406 nm (ε/M−1 cm−1, 1.79 ×
105), 525 nm (ε/M−1 cm−1, 1.75 × 104). EPR in acetonitrile at 77 K:
gav, 2.37. ESI-Mass (m/z): calcd 815.08, found 815.09 (Molecular ion
peak).
1
67.07; H, 8.40; N, 5.68. H NMR (600 MHz, CDCl3): δppm, 1.32 (s,
9H), 1.45 (s, 9H), 5.22 (s, 1H), 7.11 (s, 1H), 7.40 (s, 1H). 13C NMR
(150 MHz, CDCl3): δppm, 29.6, 31.6, 34.4, 35.2, 122.3, 124.8, 125.2,
136.1, 142.9, 149.7. ESI-Mass (m/z): calcd 251.10, found 250.53 (M
− 1).
Complex 4. Elemental analyses for C44H21N4OF8Co, calcd (%): C,
63.47; H, 2.54; N, 6.73. Found (%): C, 63.56; H, 2.55; N, 6.88. FT-IR
(in KBr): 3449, 2925, 2855, 1624, 1574, 1464, 1428, 1372, 1348,
1276, 1234, 1074, 999, and 761 cm−1. UV−visible (CH2Cl2/
CH3CN): 426 nm (ε/M−1 cm−1, 1.91 × 105), 548 nm (ε/M−1
cm−1, 1.96 × 104).
RESULTS AND DISCUSSION
■
The ligand, F8TPPH2, and its cobalt(II) complex (1) were
prepared by following the literature methods.18 The single
crystal structure of the ligand was reported earlier.18 Complex
1 was characterized spectroscopically as well as by its X-ray
single crystal structure determination (Experimental Section
and Figures S5−S8). The nitrosyl complex of cobalt(II)
porphyrin (2) was prepared by bubbling NO gas into the
degassed dichloromethane solution of the cobalt(II) precursor.
Complex 2 was characterized by UV−visible, Fourier-trans-
form infrared (FT-IR) spectroscopy, mass spectrometry, and
X-ray structure determination (Experimental Section and
Figures S9−S10). The crystallographic data and the metric
parameters are listed in Tables S1−S3. The ORTEP views of
complexes 1 and 2 are shown in Figures 1 and 2, respectively.
In complex 1, the central metal ion Co(II) is coordinated by
four N atoms from the porphyrinate dianion in a square planar
geometry. The Co−Np distances are in the limit of reported
distances in analogous complexes.19 In complex 2, the Co(II)
center is coordinated by four N atoms from the porphyrinate
moiety. The NO group is coordinated to the metal ion from
the axial position resulting in a square pyramidal geometry
around the Co(II) center. The Co−NNO and N−O distances
are 1.894(10) and 1.158(13) Å, respectively, which are within
the range in other reported analogues.20 The observed Co−
N−O angle is 127.40(14)°. The metric parameters of the
nitrosyl complex are in accord with the bent nitrosyls having
{CoNO}8 configuration.20 It would be worth mentioning here
that the Spiro’s group reported the same compound in situ
generated for 1H NMR and FT-IR studies; however, it was not
isolated in solid state and structurally characterized.21 The
Co−NNO and N−O bond lengths were calculated to 1.810 and
1.186 Å, respectively.21 The calculated Co−N−O bond angle
is 121°.21 These are also in good agreement with the
structurally characterized parameters.
Complex 2. Complex 1 (0.81 g, 1 mmol) was dissolved in dry and
degassed CH2Cl2. NO gas was bubbled for ca. 10 min to this solution.
The color of the solution changes to deep orange red. After removal
of the excess NO, the product was isolated by continuous argon flash
with ca. 85% yield (0.72 g). Elemental analyses for C44H20N5OF8Co,
calcd (%): C, 62.50; H, 2.38; N, 8.28. Found (%): C, 62.58; H, 2.35;
N, 8.39. FT-IR (in KBr): 1692, 1461, 1000, 783, 710, 588, 509 cm−1
.
UV−visible (CH3CN): 404 nm (ε/M−1 cm−1, 1.40 × 105), 531 nm
(ε/M−1 cm−1, 1.53 × 104). H NMR (400 MHz, CDCl3): δppm, 8.95
1
(s, 8H), 7.80−7.75 (m, 4H), 7.39−7.36 (m, 8H). ESI-Mass (m/z):
calcd 815.08, found 815.27 {molecular ion peak for [Co(F8TPP)]}.
Complex 3. Complex 2 (0.42 g, 0.5 mmol) was taken in 20 mL of
dry and degassed CH3CN and cooled at −40 °C. Precooled hydrogen
peroxide (37% v/v, 2.2 mmol) in acetonitrile was added to this cold
solution and stirred for 2 h. The solution was brought to room
temperature and dried in a rotavapor (0.26 g, ca. 60% yield).
Elemental analyses for C44H20N5O3F8Co, calcd (%): C, 60.22; H,
B
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Figure 3. FT-IR spectrum of complex 2 in KBr.
Complex 2 in CH2Cl2 or CH3CN solution did not react with
dioxygen; however, it reacts with H2O2. Upon addition of 2.2
equiv of H2O2, complex 2 in CH3CN solution at −40 °C
resulted in the formation of corresponding nitrate complex, 3
(Scheme 1).
Figure 1. ORTEP diagram of complex 1 (35% thermal ellipsoid plot;
H atoms and solvent molecules are not shown for clarity).
Scheme 1. Reaction of Complex 2 with H2O2
The complex 2 in acetonitrile solution at −40 °C shows
absorption at 531 and 404 nm, respectively. Addition of 2.2
equiv of precooled H2O2 resulted in the gradual decay of these
absorption maxima along with a simultaneous formation of
new absorption bands at 542 and 430 nm, respectively (Figure
4). These are assignable to the complex 3. The reaction was
also carried out in THF solution at −80 °C (Figure S11), and
no intermediate formation was observed. Isolation as solid and
spectroscopic characterization revealed complex 3 as the
nitrate complex of the corresponding Co(III) porphyrinate,
[(F8TPP)Co(NO3)] (Experimental Section). Even after
Figure 2. ORTEP diagram of complex 2 (35% thermal ellipsoid plot;
H atoms are not shown for clarity).
In the FT-IR spectrum, the strong stretching frequency at
1692 cm−1 in KBr is assignable to the coordinated NO moiety
(Figure 3). Spiro and co-workers reported the νNO stretching
to appear at 1699 cm−1 in CH2Cl2.21 In earlier reported
examples like [(TPP)Co(NO)] and [(OEP)Co(NO)], the
NO stretching was observed at 1689 cm−1 (in KBr) and 1675
cm−1 (in CH2Cl2), respectively.22 In the cases of cobalt
porphyrin nitrosyls having general formula [{T(p/m-X)PP}-
Co(NO)] [p/m-X = p-OCH3, p-CH3, m-CH3, p-H, m-OCH3,
p-OCF3, p-CF3, p-CN, etc.] in CH2Cl2, the NO stretching
frequency appeared in the range of 1681−1695 cm−1.22 It
appeared at 1701 cm−1 in KBr disc for complex [(Cl4TPP)-
Co(NO)].23 In the ESI mass spectrum, complex 2 displayed a
peak at m/z 815.27 assignable to the [Co(F8TPP)] (calcd m/z
= 815.08). This is attributed to the facile loss of the axial NO
group, which has been observed earlier in other nitrosyl
complexes of Co(II)-porphyrinates.23,24
Figure 4. UV−visible spectral monitoring of complex 2 (11.50 μM)
(black) and after addition of 2.2 equiv of H2O2 to result in complex 3
(red) at −40 °C in acetonitrile.
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several attempts we could not grow the X-ray quality crystals of
complex 3. In the FT-IR spectroscopy, the nitrate stretching
frequency was observed at 1385 cm−1 (Figure S12). The
assignment has been done based on the observed stretching
frequencies for the nitrate groups in the earlier reported metal
porphyrin complexes.25 In H NMR spectrum, complex 3
1
displays well-resolved signals in CDCl3 suggesting the presence
of a low spin cobalt(III) ion in the complex (Figure S14). The
signal at 9.10 ppm corresponding to altogether eight protons is
assignable to the protons from pyrrole moiety. The signals
appearing at 7.83 and 7.43 ppm, respectively, are assigned to
the protons from the phenyl group (Experimental Section and
Supporting Information).
Complex 3 shows the molecular ion peak at m/z 877.05
(calcd. 877.07) corresponding to [(F8TPP)Co(NO3)] in the
ESI-mass spectrum (Figure 5). The isotopic distribution
pattern was found to be in accord with the simulated spectrum
(Figure 5).
Figure 6. X-band EPR spectra of complex 2 (24.5 mM) (black), the
intermediate (red) formed in the reaction of complex 2 and H2O2,
and complex 3 (green) in THF at 77 K.
bond cleaved homolytically to produce an oxo-ferryl (FeIV=O)
species and NO2 (Scheme 2).7−9 In the present case, the
appearance of the isotropic signal at g = 2.002 is actually
suggestive to the formation of a Co(IV)-oxo species and NO2
(Scheme 2). It has been reported by Winkler and Gray that for
oxo-metal complexes having d5 configuration, the [MIII−O•] is
more likely than [MIV=O] configuration.28 In the present case,
the Co(IV)-oxo species owing to its d5 configuration would
convert into (F8TPP)CoIII−O•. This accounts for the
appearance of sharp EPR signal at g = 2.002 and non-
observance of the quenching of Soret band in UV−visible
spectroscopy. NO2 at 77 K dimerizes to form diamagnetic
N2O4. The (F8TPP)CoIII−O• in the presence of NO2
undergoes fast decomposition to the final product, complex
3. In the ESI-mass spectrum of this reaction mixture, a
molecular ion peak (m/z) was observed at 831.09 (calculated:
831.08), which is assignable to the (F8TPP)CoIII−O• (Figure
7).
Figure 5. ESI-mass spectrum of complex 3 in acetonitrile [Inset: (a)
simulated and (b) experimental isotopic distribution pattern].
It is proposed that the reaction of complex 2 with H2O2
leading to the formation of the complex 3 proceeds through
the formation of a Co-peroxynitrite intermediate; however, no
intermediate was observed in UV−visible spectroscopy even
when the reaction was carried out in THF at −80 °C. It is to
be noted here that thermo-kinetic and theoretical studies
suggested that peroxynitrite intermediates should be too short-
lived to detect.26 Interestingly, when the reaction was carried
out in THF solution at −80 °C and followed by freezing at 77
K, the frozen (at 77 K) reaction mixture displayed a signal at g
= 2.002 in the X-band EPR spectrum (Figure 6). The sharp
and isotropic nature of the signal suggested the presence of a
radical in the mixture. The spectrum was contaminated with a
small fraction of Co(II) species. Even after several attempts we
could not remove this. Similar signal was reported earlier for
the Co(III) porphyrin radical cations generated by electro-
oxidation of the Co(II)-porphyrinates.27 It is to be noted that
the Co(III) porphyrin radical cations show characteristic
bleaching of their Soret bands due to loss of π-conjugation.27
However, in the present case no indication of such feature was
observed even at −80 °C temperature. The intensity of the
signal decreases gradually with warming it up and finally
disappeared suggesting the formation of final product, complex
3. It is proposed that in case of NODs the reaction of the
Fe(III)-oxy complex with NO resulted in the formation of the
peroxynitrite intermediate. In a subsequent step, the O−O
Comparison of the spectral characteristics with an authentic
sample generated in a separate reaction of Co(II) porphyrin
complex and m-CPBA also support the proposition (Figures
S17−S19). Highly unstable nature of the species precluded its
further characterization.
Because of very short lifetime of the proposed Co-
peroxynitrite, no direct spectroscopic evidence has been
observed, and indirect chemical evidence for the postulated
species has been sought. It is to be noted that commonly a
phenol nitration reaction is used to provide evidence in
support of the presence of metal-peroxynitrite. It has been
observed that when the reaction of complex 2 with H2O2 was
done in the presence of 2,4-ditertiarybutylphenol (DTBP), it
resulted in an effective phenol ring nitration (ca. 50%).
Isolation and characterization of the products from the mixture
revealed the formation of the corresponding CoIII-hydroxo
product, [(F8TPP)Co(OH)] 4 (ca. 67%) and 2,4-ditertiar-
ybutyl-6-nitrophenol (ca. 50%) (Scheme 3; Experimental
Section and Figures S15−S16 and S20−S22). No product
corresponding to the oxidative coupled phenol is observed in
isolable scale. However, when reaction is followed by the
addition of 2,4-ditertiarybutylphenol (DTBP), exclusive
formation of complex 3 was observed along with unreacted
DTBP.
D
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Scheme 2
formation of a peroxynitrite intermediate was implicated.25
However, in that case no indication of the formation of
Co(IV)-oxo ↔ (porphyrin)CoIII−O• was evidenced.
CONCLUSION
■
Thus, the nitrosyl complex of cobalt porphyrin, [(F8TPP)Co-
(NO)], in acetonitrile (or THF) solution at −40 °C (or −80
°C) was found to react with H2O2 to result in the
corresponding Co-nitrate complex, [(F8TPP)Co(NO3)]. The
reaction proceeds via the formation of a presumed Co-
peroxynitrite intermediate. X-Band EPR and ESI-Mass
spectroscopic studies suggest the intermediate formation of
the (F8TPP)CoIII−O•, which in turn supports the generation
of the corresponding Co(IV)-oxo species during the reaction.
This is in accord with the homolytic cleavage of the O−O
bond in heme-peroxynitrite proposed in the NOD activity. In
addition, the nitration of phenol ring induced by the above
reaction also suggests the formation of a peroxynitrite
intermediate.
Figure 7. ESI-mass spectrum of the reaction mixture obtained from
the reaction of complex 2 and H2O2 (in CH3CN) in THF at −80 °C.
[Inset: (a) simulated and (b) experimental isotopic distribution
pattern].
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02722.
Scheme 3. Phenol Ring Nitration Reaction by Complex 2 in
the Presence of H2O2
Spectral analyses, crystallographic data, and metric
parameters of the complexes (PDF)
Accession Codes
CCDC 1869139−1869141 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
It is to be noted that the reaction of the nitrosyl of
copper(II) complexes of methyl 2-(2-hydroxybenzylamino)-3-
(1H-imidazol-5-yl)propanoate29 and bis(2-ethyl-4-methyl-imi-
dazol-5yl)methane14 ligands with H2O2 in CH3CN at −40 °C
also led to the corresponding nitrate products through a
presumed peroxynitrite intermediate. An appreciable amount
of phenol ring nitration was observed in those cases. Recently,
[(Cl4TPP)Co(NO)] complex in the presence of H2O2 in
acetonitrile at −40 °C was exemplified to result in the
corresponding nitrite complex, [(Cl4TPP)Co(NO2)], and the
AUTHOR INFORMATION
Corresponding Author
*E-mail: biplab@iitg.ernet.in. Phone: +91-361-258-2317. Fax:
+91-361-258-2339.
■
ORCID
Biplab Mondal: 0000-0002-0594-6749
E
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(9) (a) Goodwin, J. A.; Coor, J. L.; Kavanagh, D. F.; Sabbagh, M.;
Howard, J. W.; Adamec, J. R.; Parmley, D. J.; Tarsis, E. M.; Kurtikyan,
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Notes
The authors declare no competing financial interest.
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