Reaction of a Co(III)-Peroxo Complex and NO: Formation of a
Putative Peroxynitrite Intermediate
Soumen Saha,
†
Somnath Ghosh,
†
Kuldeep Gogoi,
†
Hemanta Deka,
†
Baishakhi Mondal,
†
and Biplab Mondal*
,†
†
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India
* S Supporting Information
ABSTRACT: A Co(II) complex, [Co(L)
2
]Cl
2
, 1 of the ligand
L (L = bis(2-ethyl-4-methylimidazol-5-yl)methane) upon
reaction with H
2
O
2
in methanol solution at -40 °C resulted
in the formation of the corresponding Co(III)-peroxo complex
[Co(L)
2
(O
2
)]
+
(2). The addition of NO gas to the freshly
generated solution of the complex 2 led to the formation of the
Co(II)-nitrato complex 3 through the putative formation of a
Co(II)-peroxynitrite intermediate, 2a. The intermediate 2a was
found to mediate the nitration of the externally added phenol
resembling the nitration of tyrosine in biological systems.
■
INTRODUCTION
Nitric oxide (NO) is an important small molecule that is
involved in various physiological processes such as neuro-
transmission, vasodilation, immune response, etc.
1-4
Under
normal conditions, NO is produced by the nitric oxide
synthases (NOS) at low concentration.
5
However, an excess
production of NO can have detrimental effects via the
formation of secondary reactive nitrogen species (RNS) such
as nitrogen dioxide (NO
2
) and peroxynitrite (ONOO
-
).
6
Both
of these RNS are known to play important roles in biomolecule
oxidation leading to the oxidative and nitrosative stress.
7
The
nitric oxide deoxygenases (NODs) are known to control the in
vivo level of NO by converting it into the biologically benign
nitrate (NO
3
-
) and thereby removing the excess of it.
8
The
conversion of NO to NO
3
-
by NODs is believed to proceed
through the formation of a peroxynitrite intermediate.
8,9
Peroxynitrite is proposed to form in vivo in a diffusion-
controlled reaction of NO and O
2
-
anion or H
2
O
2
and nitrite
(NO
2
-
) in the presence of the peroxidase enzymes.
10,11
An
aqueous peroxynitrite is known to isomerize to the nitrate
(NO
3
-
) very easily.
8
In the literature, peroxynitrites are
exemplified to form either in the reaction of oxy-heme
(formally, Fe
III
-O
2
·
-
) proteins with NO or metal-nitrosyls
with O
2
or O
2
-
.
9,12
Recently, the peroxynitrite intermediates
have been shown to form in the reaction of NO with Cr-
superoxo or peroxo species. A Cr(IV)-peroxo complex ([Cr
IV
-
(12-TMC)(O
2
)(Cl)]
+
) {12-TMC = 1,4,8,11-tetramethyl-
1,4,8,11-tetraazacyclotetradecane} is reported to react with
NO to result in a Cr(III)-nitrato complex, ([Cr
III
(12-TMC)-
(NO
3
)(Cl)]
+
), whereas the reaction of Cr(III)-superoxo
complex [Cr
III
(14-TMC)(O
2
)(Cl)]
+
and NO resulted in the
corresponding Cr(IV)-oxo complex ([Cr
IV
-(14-TMC)(O)-
(Cl)]
+
) and NO
2
presumably via the formation of a Cr(III)-
peroxynitrite intermediate, [Cr
III
(14-TMC)(OONO)-
(Cl)]
+
.
13-15
On the one hand, Nam et al. reported the
reactivity of an Fe(III)-peroxo complex, [Fe
III
(14-TMC)-
(O
2
)]
+
, with NO
+
as an approach for generating the
peroxynitrite intermediate, which led to the formation of an
Fe(III)-nitrato complex, [Fe
III
(14-TMC)(NO
3
)(F)]
+
.
16
On the
other hand, Basalo et al. reported the reaction of a Co-nitrosyl
with O
2
to result in the Co-nitrite, where the intermediate
formation of a peroxynitrite is presumed.
17
Similarly, a
nonheme dinitrosyliron complex has been shown to result in
the corresponding nitrite in its reaction with O
2
.
18
Recently,
Karlin and co-workers reported the reaction of a mixed-valent
nitrosyl complex of Cu(I)Cu(II), namely, [Cu
I,II
2
(UN-O
-
)-
(NO)]
2+
with O
2
, to result in the superoxide and nitrosyl
adduct, [Cu
II
2
(UN-O
-
)(NO)(O
2
-
)]
2+
, which was subsequently
converted to the corresponding peroxynitrite complex.
19
In our laboratory, we have been working on the reactivity of
the metal nitrosyls with O
2
-
anion and H
2
O
2
with an aim to
generate a peroxynitrite species. For example, {CuNO}
10
complex of the bis(2-ethyl-4-methylimidazol-5-yl)methane
ligand was found to react with H
2
O
2
to afford the
corresponding Cu(I)-nitrite, and the formation of a Cu(I)-
peroxynitrite intermediate is presumed.
20
The same nitrosyl
complex, upon reaction with O
2
-
anion, was observed to result
in Cu(II)-nitrite.
20
In an another example, the {CuNO}
10
complex in the presence of H
2
O
2
was shown to form Cu(II)-
nitrite via a presumed peroxynitrite intermediate.
21
Cobalt
nitrosyls, both with porphyrin and nonporphyrin ligands, were
reported to react with O
2
and O
2
-
to afford cobalt-nitrate/
nitrite complexes.
17,22
The involvement of a peroxynitrite
Received: May 3, 2017
Article
pubs.acs.org/IC
© XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.7b01110
Inorg. Chem. XXXX, XXX, XXX-XXX