Aromatic C-nitrosation by a copper(ii)-nitrosyl complex

Article abstract of DOI:10.1039/c4dt02689f

Copper(ii) complex of 4-amino-3-hydroxy-1-sulphonic acid was synthesized and characterized. Upon addition of nitric oxide, the copper(ii) center of the complex in methanol was found to undergo reduction through an unstable copper(ii)-nitrosyl intermediate. The formation of the intermediate was confirmed by UV-visible and FT-IR spectroscopy. The reduction of the copper(ii) center was accompanied with a simultaneous C-nitrosation of the aromatic ring of the ligand. The C-nitrosation product was isolated and characterized by various spectroscopic analyses. This journal is

Full text of DOI:10.1039/c4dt02689f

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Aromatic C-nitrosation by a Copper(II)-nitrosyl complex
Kanhu Charan Rout, Biplab Mondal∗
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India
E-mail: biplab@iitg.ernet.in

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Abstract
Copper(II) complex of 4ꢀaminoꢀ3ꢀhydroxyꢀ1ꢀsulphonic acid was synthesized and characterized.
Upon addition of nitric oxide, the copper(II) center of the complex in methanol was found to
undergo reduction through unstable copper(II)ꢀnitrosyl intermediate. The formation of the
intermediate was evidenced by UVꢀvisible and FTꢀIR spectroscopy. The reduction of the
copper(II) center was accompanied with simultaneous Cꢀnitrosation of the aromatic ring of the
ligand. The Cꢀnitrosation product was isolated and characterized by various spectroscopic
analyses.

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Introduction
Nitrosoalkanes and nitroso arenes have attracted the attention of chemists since the discovery of
1
important roles of Cꢀnitroso compounds in various biological metabolic processes. For instance,
nitrobenzene poisoning leads to the formation of nitrosobenzene adduct of hemoglobin or some
2
ꢀ5
of the amine drugs are known to metabolize to corresponding nitroso compounds. These
actually resulted in an increased interest to study the fundamental properties, such as stabilization
of Cꢀnitroso group or its reactivity towards isomerization or with substrates, of Cꢀnitroso
compounds. However, the major difficulty of Cꢀnitroso compounds is their highly reactive
nature. For example, they can be oxidized very easily to the corresponding nitro compounds; or
1
isomerized to oximes. The nitrosation of the arenes have been achieved by their reactions either
with nitosonium salt in acetonitrile under inert atmosphere or with degassed tetrafluoroacetic
6
,7
acid under nitric oxide atmosphere. The formation of the nitrosoarene products in the latter
7
condition is proposed to proceed through a nitrosonium ion, also. Direct nitrosation of tertiary
aromatic amines by nitrous oxide is also known for the preparation of paraꢀnitrosoꢀN,Nꢀdi
8
ꢀ10
substituted anilines.
In comparison, Nꢀnitrosation has been studied well. A number of examples of Nꢀnitrosation via
the reaction of copper(II) complexes of amine ligands with nitric oxide have been reported
recently from our laboratory. In the course of reaction, copper(II) center undergoes reduction to
copper(I) by nitric oxide and nitrosonium ion thus generated leads to the Nꢀnitrosation at the
1
1ꢀ14
secondary amine site of the ligand frameworks.

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In continuation of Nꢀnitrosation, we wanted to utilize the same chemistry for Cꢀnitrosation. As
Cꢀnitrosation of the arenes needs substantial activation of the ring, the following ligand has been
chosen (Figure 1):
SO H
3
OH
NH2
Figure 1. Ligand (
L
) used for the present study.
Results and discussion
The ligand, , 4ꢀaminoꢀ3ꢀhydroxynaphthaleneꢀ1ꢀsulphonic acid was purchased from commercial
source. Copper(II) complex, , was synthesized by refluxing a mixture of copper(II) perchlorate
hexahydrate and two equivalent of the ligand, in methanol (Scheme 1; experimental section).
L
1
L
Scheme 1. Synthesis of complex
1

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The complex was characterized by various spectroscopic analyses as well as by elemental
analyses (experimental section). Complex in methanol shows an absorption at 480 nm ( , 1016
M cm ) in the UVꢀvisible spectroscopy with strong intraꢀligand transitions in the UV region
1
ε
ꢀ1
ꢀ1
II
II
(
Figure 2). This is attributed to the phenolato → Cu charge transfer transition. Phenolato → Cu
15
charge transfer transition is reported to appear in this range in other examples. Solution
conductivity measurement in methanol indicates the neutral nature of the complex. The absence
of perchlorate vibration in the FTꢀIR spectrum of complex
1, is also in accord with the phenolato
binding of the ligand to copper center (supporting information).
Figure 2. UVꢀvisible spectrum of complex
1 in methanol.
The Xꢀband EPR spectrum of the complex was recorded in methanol at 77 K and was found to
ꢀ4
ꢀ1
be anisotropic with g , 2.382, g , 2.101 and A = 123 × 10 cm . The spectrum is axial in nature
║
⊥
║

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2
2
having g > g > 2.0023, indicating d ground state which is characteristic of square planar
x -y
║
⊥
1
6
geometry (Figure 3).
Figure 3. XꢀBand EPR spectrum of complex
1
1
in methanol at 77K.
The cyclic voltammetirc studies of complex
carried out in methanol solvent. It exhibited one
+
II
I
quasiꢀreversible couple at ꢀ0.17 v versus Ag/Ag and this has been attributed to the Cu /Cu
II
I
couple (Supporting Information). The Cu /Cu couple was observed to appear varyingly in the
/
range of ꢀ0.84 to ꢀ0.66 v against SCE in cases of Cu(II) complexes of alkyl substitutedꢀ N,N ꢀ
1
7
ethylenediamine ligands and analogous reported compounds. The difference in potential is
attributed to the presence of electron rich ligand framework in the present case. In the positive
+
side of Ag/Ag , in the potential range up to 1.8 v, no characteristic couple was observed.

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Addition of nitric oxide (NO) gas to a degassed methanol solution of complex
1 resulted in the
shift of the band at 480 nm to 440 nm with the concomitant formation of other band at 617 nm,
Figure 4). The intensity of the band at 617 nm was found to decrease with time indicating
presence of an unstable intermediate (Figure 5). Presumably, the intermediate is the
(
1
0
corresponding {CuNO} species which decomposes to result in Cu(I) with simultaneous
10
nitrosation of a substrate (e.g. solvent or ligand). In other reported examples, the {CuNO}
1
1ꢀ12
species absorbs in the range of 610 ꢀ 640 nm in acetonitrile solvent.
Figure 4. UVꢀVisible spectra of complex
1 before (blue line) and after (black line) purging nitric
oxide in methanol.

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(
a)
1
0
Figure 5. UVꢀVisible spectra of intermediate {CuNO} species (solid line) and it’s decay
dotted line) in methanol
(
.
Thermal instability and moisture sensitivity precluded the isolation and further characterization
1
0
10
of the {CuNO} intermediate. Though, a number of examples of {CuNO} intermediate have
been reported recently in the reaction of Cu(II) complexes with NO, the examples of stable
1
0
2+
{
CuNO} are still very limited. In case of [Cu(bemim)2] {bemim = bis-(2ꢀethylꢀ4ꢀ
methylimidazoleꢀ5ꢀyl)methane}, in acetonitrile solution, addition of NO resulted in the
1
0
18
corresponding {CuNO} complex which has been isolated as solid. The only structurally
1
0
I
characterized {CuNO} complex was prepared by the reaction of corresponding Cu complex
1
9
with NOBF4.
1
0
The formation of the {CuNO} intermediate was further supported from the solution FTꢀIR
spectroscopic studies. In FTꢀIR spectroscopy, upon addition of NO to the methanol solution of
ꢀ1
complex
1 a new strong frequency at 1650 cm appeared which is attributed to the coordinated

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1
0
NO stretching in the corresponding {CuNO} species (Figure 6). This band was shifted to 1619
ꢀ1
15
cm while the reaction was carried out with NO. The intensity of the band was found to decay
Figure 6. Solution FTꢀIR spectra of complex
1 before (magenta) and immediately after (red)
purging NO gas in methanol.
with time (supporting information) and finally disappeared indicating the unstable nature of the
intermediate complex. In case of the isolated copper(II)ꢀnitrosyl from the reaction of
2
+
ꢀ1 18
[
Cu(bemim) ] and NO, it was observed at 1662 cm .
2
ꢀ
For airꢀstable solid copperꢀnitrosyl of copper(II)ꢀdithiocarbamate, the νNO appears at 1682 cm
1
20
ꢀ1
.
Hayton et al reported the appearance of νNO band at 1933 cm for structurally characterized
1
9
copper(II)ꢀnitrosyl. The stretching frequency of the nitrosyl group in metal nitrosyls are known
to vary depending upon various factors like (i) charge on the metal ion, (ii) coordination
geometry around the central metal, (iii) nature of the ancillary ligands present and (iv) mode of
binding of the NO to the metal. In case of Hayton’s compound, the ancillary ligands were methyl
nitrite and the complex was distorted octahedral with bent NO at an equatorial position. In the

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present case, the FTꢀIR of the nitrosyl complex was recorded in methanol solution. The
copper(II) ion is coordinated to the chelating ligand, through electron rich phenolate moiety.
L
These might have caused such a large variation of the nitrosyl stretching frequency. The
intermediate species, as expected was EPR silent (supporting information).
The decay of the 617 nm band in UVꢀvisible spectroscopy is attributed to the reduction of the
+
copper(II) center with concomitant formation of NO . The reduction was further supported by
the EPR silent nature of the colorless reaction mixture.
Removal of copper ion from the reaction mixture followed by isolation and characterization of
the modified ligand suggests the formation of 4ꢀaminoꢀ3ꢀhydroxyꢀ1ꢀnitrosonaphthalene
(experimental section) (scheme 2).

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Scheme 2. Suggested mechanism of the reaction
Nitrosation of arenes was achieved mostly by using nitrosonium salt such as nitrosonium
6
ꢀ7
tetrafluoroborate in acetonitrile under inert atmosphere. Degassed trifluoroacetic acid under
NO atmosphere has also been used for successful nitrosation of some arenes like toluene, 1,2ꢀ
6
,7
dimethyl benzene and 1,3 dimethyl benzene etc. In these cases also the reaction was reported
to proceed through the formation of nitrosonium ion. In case of phenols and naphthols, nitrous
acid, sodium nitrite/trifluoroacetic acid, nitrosonium tetrafluoroborate, nitrosyl sulphuric acid,
sulphuric acid under NO atmosphere etc. are common and successful agent for nitrosation
8
reactions. On the other hand, aromatic amines undergo direct nitrosation by nitrous acid or

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2
8
sodium nitrite and HCl mixture. For aromatic secondary amines, nitrosation initially occurs at
nitrogen resulting in Nꢀnitrosoamine which then undergoes FischerꢀHepp rearrangement in
9
,10
acidic medium to yield paraꢀnitrosoamine product.
In majority of the cases, the nitrosation
+
occurs through nitrosonium ion. In case of primary aromatic amines, the presence of NO may
1
2
lead to the diazotization rather than nitrosation. Cꢀnitroso arenes are also reported to form
+
21
through
nitrosativedecarboxylation
in
presence
of
NO .
Nitrosomesitylenium
1
8/19
hexafluorophosphate, [1,3,5ꢀMe C H NO]PF , is also reported to induce arene nitrosation.
3
6
3
6
On the other hand, Cꢀnitrosation at the active methylene position of
βꢀdiketiminate ligand
2
2
framework by NO is also known though not explored much.
+
In the present study, though there is no direct evidence of formation of NO , it is now established
+
+
that the reduction Cu(II) by NO leads to the formation of NO . The NO formed during the
reduction of Cu(II) by NO presumably attacks the naphthalene ring at sulphonic acid position to
yield the modified ligand through an intramolecular pathway. To check this, the same reaction
was carried out in presence of pꢀtoluene sulphonic acid. The formation of pꢀnitrotoluene was not
observed. This suggests the involvement of an intramolecular mechanism in the present case
ꢀ
(
scheme 2). The presence of SO stretching frequency in the FTꢀIR spectrum of the intermediate
3
complex indicates the release of sulphonate group after the decomposition of the copper(II)ꢀ
nitrosyl complex. The liberated SO was trapped by passing through an 80% isopropanol/water
3
mixture. Quantification has been done by titrating SO trapped in 80% isopropanol/water
3
solution using BaCl in presence of thorin indicator (experimental section). This confirms the
2
release of ~70% SO from the reaction. The desulphonation of aromatic rings are known with
3
+
+
+ 23
H , Br or NO . In all the cases, it takes place through the initial attack by the electrophile at
2
+
the Cꢀcenter attached with the –SO H group. However, desulphonation of aromatic ring by NO
3

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3
is not known. On the other hand, nitrosativedecarboxylation of arenes are well known in
+
literature. In this case also, initially NO attacks the Cꢀcenter attached to the –CO H group prior
2
2
1
to decarboxylation.
+
The role of NO in the reaction was further confirmed by the addition of NOClO in the free
4
/
ligand (experimental section). This results in the formation of
L
.
It should be noted that [Cuꢀnitrosyl] complexes are known to afford methyl nitrite upon reaction
24
with methanol. In the present reaction case, we have observed the formation of methyl nitrite in
a negligible (~5 %) quantity. This is also in accord with the intramolecualr/concerted mechanism
of the reaction.
NO itself, under degassed condition was not found to react with the free ligand,
L
. However,
//
under aerobic condition, it reacts with the ligand and results in the formation of
L . This reaction
perhaps takes place through a cascade of reactions that starts with the air oxidation of NO to
9
NO . It somewhat similar to that observed in cases of the reaction of NO with oꢀdiamines.
2
SO Na
SO Na
3
3
i) MeOH
ii) NO/O2
+
H O
2
OH
O
NH2
N
N
/
/
(L)
(L )
Scheme 2.

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4
Though, it resembles to the synthesis of azo compounds via diazaotization or coupling reactions,
a significant difference was observed. Diazotization are known to take place in acidic medium;
however, here the reaction occurs readily in neutral (pH, ~ 7.2) and even in basic condition (pH,
~9). Analogous example was reported in case of NO550 where electrophilic aromatic substitution
on the electron deficient nitroꢀamine was observed yielding a hydroxyl hydrazine derivative;
which on subsequent water elimination results in the formation of AZO550 (supporting
2
5
information).
Fluorescence study
Since the free ligand is not highly soluble in methanol, for fluorescence study the mono sodium
ꢀ
ꢀ1
ꢀ1
salt of the ligand (NaL ) was used. It shows absorption band with λ at 350 nm (
ε
/lit mol cm ,
max
1
3
020) in methanol solution in the visible spectrum (supporting information). Upon excitation at
50 nm, significant fluorescence emission was observed centered at 435 nm in methanol or water
at room temperature. The quantum yield was found to be 0.207 in methanol.
ꢀ
The fluorescence emission band of NaL centered at 435 nm was found to disappear upon
addition of equivalent amount of Cu(II) ion (supporting information). Addition of NO gas to this
/
solution restores the emission; however, much less intense. This is because of the formation of L

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Figure 7. Fluorescence responses upon excitation at 350 nm {for 20µM solution of free ligand
(
black line) and after addition of equivalent amount of [Cu(H O) ](ClO ) (blue line) in
2 6 4 2
methanol}.
Figure 8. Fluorescence enhancement upon excitation at 440 nm of 20µM solution of complex
(solid line) and after addition of one equivalent of nitric oxide (dashed line) in methanol with
time
1
.

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during the reaction which is evident from the shift of UVꢀvisible absorption band from 480 nm
in case of complex 1) to 440 nm (after purging NO to the solution of complex 1). The change in
(
excitation wavelength from 350 nm to 440 nm results in a strong emission centered at 610 nm
/
from the dark background. This is attributed to the emission of L and has been confirmed with
/
free L , also. Similar result was observed upon addition of equivalent amount of NOClO as a
4
+
ꢀ
+
source of NO in methanol solution of NaL which in turn confirms the involvement of NO .
Experimental Section
All reagents and solvents were purchased from commercial sources and were of reagent grade.
Deoxygenation, wherever necessary, of the solvents and solutions were done by repeated
vacuum/purge cycles or bubbling with argon for 30 minutes. NO gas was purified by passing
through KOH and P O column. UVꢀvisible spectra were recorded on a Perkin Elmer Lamda 25
2
5
UVꢀvisible spectrophotometer. FTꢀIR spectra were taken on a Perkin Elmer spectrophotometer
with samples prepared as KBr pellets. The fluorescence spectra were recorded in solution in
VARIAN Cary Eclipse Fluorescence Spectrophotometer at room temperature. Quinine sulphate
in acidic medium was used as the reference compound for the determination of fluorescence
1
quantum yield. Hꢀ NMR spectra were obtained with a 400 MHz Varian FT spectrometer.
Chemical shifts (ppm) were referenced either with an internal standard (Me Si) for organic
4
compounds or to the residual solvent peaks. The Xꢀband Electron Paramagnetic Resonance
(
EPR) spectra were recorded on a JESꢀFA200 ESR spectrometer, at room temperature and 77 K
with microwave power, 0.998 mW; microwave frequency, 9.14 GHz and modulation amplitude,
. Elemental analyses were obtained from a Perkin Elmer Series II Analyzer. The magnetic
2

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moment of complexes was measured on a Cambridge Magnetic Balance. The electrical
conductivity of the complex in solution was measured using a Sytronic 305 conductivity bridge.
+
For the electrochemical studies, a Pt working electrode, Pt wire auxiliary electrode and a Ag/Ag
reference electrode were used in a threeꢀelectrode configuration. All electrochemical
measurements were done at 298 K under nitrogen atmosphere in acetonitrile solvent containing
tetraꢀbutylammonium perchlorate (TBAP) as supporting electrolyte. The scan rate used was 50
0
mV/s. The halfꢀ wave potential
E
was set equal to 0.5(Epa + Epc), where Epa and Epc are anodic
298
and cathodic cyclic voltammetric peak potentials, respectively. All the electrochemical data are
uncorrected for junction potential.
Synthesis of complex 1.
Copper(II) perchlorate hexahydrate (0.370 g, 1 mmol) was dissolved in 10 ml distilled methanol.
To this solution, 0.478 g (2 mmol) of the ligand L, was added slowly with constant stirring. The
colour of the solution turned into reddish brown from light blue. The reaction mixture was stirred
and refluxed for 2 hr. The volume of the solution then reduced to ~2 ml. To this, 5 ml of
diethylether was added to make a layer on it and kept it overnight on freezer. This resulted into
the precipitation of complex 1 as deep brown solid. Yield: 0.428 g (80%). Elemental analyses for
C H N O S Cu [Complex 1. CH OH]: Calcd (%). C, 44.09; H, 3.52; N, 4.90. Found (%). C,
21
20
2
9
2
3
ꢀ
1
ꢀ1
4
2
4.13; H, 3.51; N, 4.99. UVꢀvis. (methanol): λmax, 480 nm (
ε
, 1016 M cm ). XꢀBand EPR: g ,
ǁ
ꢀ4
ꢀ1
ꢀ1
.382; g , 2.101; A , 123 × 10 cm . FTꢀIR (KBr pellet): 1621, 1541, 1183, 1049, 766, 656 cm .
⊥
ǁ
ꢀeff, 1.53 BM.

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/
Isolation of L .
Complex 1(0.538 g, 1 mmol) was dissolved in 20 ml of dry methanol. The solution was degassed
by bubbling Ar for 30 mints. To this solution NO(g) was purged for 1 mint and the mixture was
then kept at room temperature for 4 h while the color of the solution turned yellowish. The
excess NO was removed then by applying several cycles of vacuum and Ar purge. The reaction
mixture was then opened to air and stirred for 2 h to ensure complete oxidation of the Cu(I) to
Cu(II). The volume of the solution was then reduced to 5 ml and 25 ml of water was added. To
this 5ml saturated aqueous solution of Na S was added and stirred for ½ hr while black
2
precipitate of CuS appeared. The precipitate was filtered off. The organic part was extracted
from the solution using CHCl (30 ml × 3 portions). After removal of the solvent under reduced
3
pressure, the crude organic mass was subjected to column chromatography on neutral alumina to
/
/
get the unreacted ligand, L and L as orange yellow oil. Yield of L : 150 mg (~40%). Elemental
/
analyses for L , C H N O : Calcd.(%): C, 63.82; H, 4.28; N, 14.89; found(%): C, 63.85; H, 4.27;
1
0
8
2
2
ꢀ1 1
N, 14.80. FTꢀIR(in KBr pellet): 2924, 1685, 1607, 1243, 1043, 722 cm . HꢀNMR (400 MHz,
1
3
CDCl ) δ : 6.16 (s, 1H), 7.67ꢀ7.75 (m, 2H), 8.05ꢀ8.12 (m, 2H). CꢀNMR: (100 MHz, CDCl )
3
ppm
3
+
δppm: 110.1, 126.4, 126.9, 131.2, 132.2, 133.5, 134.5, 160.6, 180.3, 185.0. ESIꢀMass (m+Na ):
Calcd. 211.06.; found, 211.02.
Yield of unreacted L, ~40%.
/
Isolation of L from the reaction of L with NOClO .
4
-
Sodium 4ꢀaminoꢀ3ꢀhydroxynaphthaleneꢀ1ꢀsulfonate (NaL ) (0.65 g, 2.5 mmol) was dissolved in
2
0 ml dried methanol under Ar atmosphere in a 50 ml round bottom flask fitted with a stirring

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bar. To this solid NOClO (~3 mmol) was added under Ar flow. The reaction mixture was stirred
4
for 1h at room temperature. The solvent was then dried under reduced pressure. To the solid
mass thus obtained, 30 ml of distilled water was added. The organic part was extracted with
chloroform (25 ml × 3 portions). Removal of solvent followed by chromatographic purification
/
afforded pure L . Yield: 155 mg (~82%).
//
Isolation of L .
-
Sodium 4ꢀaminoꢀ3ꢀhydroxynaphthaleneꢀ1ꢀsulfonate (NaL ) (1.31 g, 5 mmol) was dissolved in 20
ml distilled methanol. To this solution, nitric oxide gas was bubbled through a needle for 1 mint.
and then oxygen gas was bubbled for 1 mint. The dark red color of the solution turned pink.
Then the excess of gasses were removed by vacuum and Ar purging and the solution mixture
was dried under reduce pressure. The solid compound obtained was washed with diethyl ether to
/
/
1
13
get the pure compound L . It was characterized by microanalysis, FTꢀIR, Hꢀ NMR, Cꢀ NMR
and mass spectroscopy. Yield: 1.08 g (~80%). Elemental analyses for C H N O SNa:
1
0
5
2
4
Calcd.(%): C, 44.12; H, 1.85; N, 10.29; found (%): C, 44.17; H, 1.86; N, 10.22. FTꢀIR (in KBr
ꢀ1 1
pellet): 2086, 1609, 1571, 1217, 1060, 749, 648 cm . HꢀNMR (400 MHz, D O) δppm: 6.970ꢀ
2
6
.998 (m, 1H), 7.145ꢀ7.197 (m, 1H), 7.302ꢀ7.341 (t, 1H) 7.414ꢀ7.476(m, 1H) 8.251ꢀ8.292(t, 1H).
1
3
CꢀNMR: (100 MHz, DMSOꢀd ) δppm: 118.1, 126.3, 126.5, 127.4, 128.0, 130.9, 131.4, 132.1,
6
+
1
34.9. ESIꢀMass (m+2K ): Calcd. 327.15.; found, 327.26.
Estimation of SO released during reaction.
3
Complex 1 (500 mg) was dissolved in 20 ml dry methanol in a 50 ml Schlenk falsk connected
with another flask containing 20 ml of degassed isopropanol: water mixture (4:1) by a rubber

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tubing. The stopper of both the flask kept closed and nitric oxide gas was purged through the
solution of complex 1 for 30 sec. The excess of NO gas was removed by applying several cycles
of vacuum and Ar purge. The reaction was allowed to stir at room temperature. The stoppers of
both the flask were released and Ar gas was allowed to bubble though the reaction mixture by a
long needle. This ensures the flushing out of all SO produced from the reaction vessel through
3
the isopropanol solution. It was continued for 2 h. Then 5 drops of 10% thorin solution was
added to it and titrated against 0.005N BaCl till the pink end point. Observed yield of SO is
2
3
~70%.
Conclusion
Nꢀnitrosation at various secondary amine centers present in the ligand framework was observed
1
0
through {CuNO} species. Cꢀnitrosations reportedly requires either nitrosonium salts under inert
atmosphere or strong acid in nitric oxide stream. In the present study, complex 1 reacts with
1
0
nitric oxide in methanol solution to form {CuNO} intermediate species. This has been found to
induce aromatic Cꢀnitrosation reaction through desulphonation of the ligand. Though Nꢀ
1
0
nitrosation is reported in many examples through {CuNO} , but the examples of Cꢀnitrosation
1
0
by {CuNO} are not known.
Acknowledgement
KCR gratefully acknowledges IIT Guwahati for providing the fellowship. BM would like to
thank Department of Science and Technology (DST), India for financial support (SR/S1/ICꢀ
3
8/2010).

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2
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Graphical Abstract
SO H
3
NO
SO H
3
i) MeOH
ii) NO
+
+ SO + H
3
O
+
II
Cu
H N
2
OH
OH
NH2
NH2
L/)
NH2
O
(
(L)
Cu+
+
SO H
3
(1)
Copper(II) complex of 4-amino-3-hydroxy-1-sulphonic acid was synthesized and characterized.
Upon addition of nitric oxide, the copper(II) center of the complex in methanol was found to
undergo reduction through unstable copper(II)-nitrosyl intermediate. The formation of the
intermediate was evidenced by UV-visible and FT-IR spectroscopy. The reduction of the
copper(II) center was accompanied with simultaneous C-nitrosation of the aromatic ring of the
ligand. The C-nitrosation product was isolated and characterized by various spectroscopic
analyses.