Copper(ii) complexes as turn on fluorescent sensors for nitric oxide

Article abstract of DOI:10.1039/c2dt31068f

Two copper(ii) complexes, 1 and 2, of two tridentate N-donor ligands, L₁ and L₂ [L₁ = dansyl derivative of bis-[3-(dimethylamino)-propyl]amine; L₂ = dansyl derivative of dipropylenetriamine] were synthesized and characterized. The quenched fluorescence intensity of complexes 1 and 2, in degassed methanol or aqueous (buffered at pH 7.2) solution, was found to reappear on exposure to nitric oxide. This is attributed to the reduction of paramagnetic Cu(ii) center by nitric oxide to diamagnetic Cu(i).

Full text of DOI:10.1039/c2dt31068f

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PAPER
Copper(II) complexes as turn on fluorescent sensors for nitric oxide†
Pankaj Kumar, Apurba Kalita and Biplab Mondal*
Received 13th March 2012, Accepted 6th July 2012
DOI: 10.1039/c2dt31068f
Two copper(II) complexes, 1 and 2, of two tridentate N-donor ligands, L and L [L = dansyl derivative
1
2
1
of bis-[3-(dimethylamino)-propyl]amine; L = dansyl derivative of dipropylenetriamine] were synthesized
2
and characterized. The quenched fluorescence intensity of complexes 1 and 2, in degassed methanol or
aqueous (buffered at pH 7.2) solution, was found to reappear on exposure to nitric oxide. This is
attributed to the reduction of paramagnetic Cu(II) center by nitric oxide to diamagnetic Cu(I).
has been adopted. The fluorescence intensity of fluorescent
Introduction
ligand is known to be quenched on coordination to Cu(II) centre.
The reduction of Cu(II) by nitric oxide to diamagnetic Cu(I) is
expected to restore the quenched fluorescence of a ligand fluoro-
The detection of nitric oxide (NO) directly from the living cell
has attracted an enormous attention from chemists since its dis-
covery to play the key roles in a number of physiological
13
phore. The reduction of Cu(II) center to Cu(I) by nitric oxide
has been reported earlier in copper(II) dithiocarbamate, Cu(II)-
phen or dmp (phen = 1,10-phenanthroline; dmp = 2,9-dimethyl-
1
–3
processes. Fluorescence-based detection techniques are found
4
,5
to be the ones satisfying almost all the requirements. Fluor-
escence sensors for nitric oxide based on organic molecules did
not find extensive application in biological systems as their fluor-
escence response depends on their oxidation products. On the
other hand, a fluorophore-displacement strategy has been used
for transition-metal complexes based nitric oxide sensors;
however, the low sensitivity and water insolubility of these com-
plexes precluded their application. A number of iron complexes
were reported to sense nitric oxide in aqueous medium; but
some of them exhibit diminished fluorescence, which is not
desirable for biological applications. In addition, iron com-
plexes which show fluorescence enhancement are air sensitive7
and exhibit only modest turn-on emission with nitric oxide.
These difficulties essentially prevent the applications of iron
complexes as nitric oxide sensor. On the other hand, the
reduction of Cu(II) to Cu(I) by nitric oxide can offer a methodo-
logy for nitric oxide detection.
14
1
,10-phenanthroline) complexes. Recently, Cu(II) complexes of
various amine ligands have also been reported to undergo
4
,5
15c
reduction by nitric oxide.
This strategy has been adopted by Ford et al. in their report of
copper(II) complexes of bis(2-(3,5-dimethyl-1-pyrazolyl)ethyl)-
ammine) (pza) with appended Ru(II) and Re(I) luminophores as
a possible luminescent sensor for nitric oxide; though these were
4
16
found to be too labile to be a practical sensor. On the other
hand, this strategy indeed found to work in the reaction of nitric
6
II
2+
oxide with [Cu (DAC)] [DAC is the N-derivatized cyclam-
17
1
,8-bis(anthracen-9-ylmethyl)-1,4,8,11-tetraazacyclotetradecane].
However, the reaction was found to be too slow under near
neutral condition to use for sensor. In addition, in this case, the
reduction of copper(II) centre by nitric oxide was found to result
in the simultaneous formation of N-nitrosamine due to the nitro-
sation of the coordinated amine.
In recent times, a number of metal complexes have been
exemplified as direct fluorescent nitric oxide sensors. These
exhibited fluorescence enhancement due to the fluorophore dis-
Lippard’s group used the same strategy to develop copper
complex-based NO sensors. They reported the examples of
copper(II) complexes of anthracenyl and dansyl fluorophore
8
–12
11,18
placement by nitric oxide from the metal center.
However,
ligands.
The quenched fluorescence intensity of the ligand
this strategy for the direct detection of nitric oxide is mostly
found to be compatible only with organic solvents. In aqueous
environments, fluorescence turn-on/enhancement may arise from
the replacement of the fluorophore ligand from the metal centre
by water itself. Thus, the strategy of reduction of a metal centre
by nitric oxide with a concomitant fluorescence enhancement
fluorophore was observed to be restored in presence of NO in
methanol/dichloromethane solutions of the complexes. In
addition, [Cu(Ds-en) ] and [Cu(Ds-AMP) ], where Ds-en and
2
2
Ds-AMP are the conjugate bases of dansylethylenediamine (Ds-
Hen) and dansyl aminomethylpyridine (Ds-HAMP), respectively,
11b
have found to detect NO in aqueous solution, also. However,
these compounds were unable to sense NO at a physiologically
11b
more relevant pH. Xiang et al. also reported an example uti-
19
Department of Chemistry, Indian Institute of Technology Guwahati,
Assam 781039, India. E-mail: biplab@iitg.ernet.in;
Fax: +913612582349; Tel: +913612582317
lizing this approach. In all these cases except [Cu(Ds-en) ] and
2
[
Cu(Ds-AMP) ], it has been observed that demetallation of the
2
18
fluorophore ligands takes place after the reduction.
†
Electronic supplementary information (ESI) available: The detail
An important report from the Lippard’s group described the
direct imaging of nitric oxide in cell using copper(II) complexes
experimental processes and all characterization data are included. See
DOI: 10.1039/c2dt31068f
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Dalton Trans., 2012, 41, 10543–10548 | 10543

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moment of complexes is measured on a Cambridge Magnetic
Balance.
Syntheses
Ligand L . The fluorescent ligand L was prepared by the
1
1
incorporation of the dansyl group in bis-[3-(dimethylamino)-
propyl]amine (Scheme 2). This has been effected by stirring an
equimolar mixture of bis-[3-(dimethylamino)-propyl]amine
Scheme 1
(
1.87 g, 10 mmol) and dansyl chloride (2.69 g, 10 mmol) in
1
2b
presence of 1.5 equivalent of triethylamine in distilled chloro-
form (20 ml) for 5 h at room temperature. The resulting solution
was then dried under reduced pressure in rotavapour and the
greenish yellow fluorescent mass thus obtained was subjected to
column chromatographic purification to result into the pure
of fluoresceine-based ligands. In the reaction of NO with the
+
copper(II) complex generates NO to react with the fluoresceine-
based ligands to result in the nitrosated ligand which then is
released from the Cu center with a substantial turn-on fluor-
escence. Another example of similar mechanism based NO
imaging was also reported from the Lippard’s group.
From our laboratory, recently we have reported two examples
of copper(II) complex-based fluorescent NO sensor for methanol
and water medium buffered at pH ∼ 7.2. It is important to note
here that in these cases no ligand nitrosation was observed.
However, these have not been found to behave as turn on/off
sensors; perhaps, because of the demetallation of the ligand after
reduction followed by undefined product formation during air
+
1
2c
greenish yellow fluorescent ligand, L . Yield: 3.25 g (∼78%).
Elemental analyses for C H N O S: Calcd (%): C, 62.82; H,
1
22 36 4 2
8
.63; N, 13.23. Found (%): C, 62.80, H 8.63, N, 13.18. FT-IR
1 1
in KBr): 2944, 2829, 1464, 1310, 1140, 789 cm− . H-NMR
2
0
(
(
400 MHz, CDCl ): δppm: 1.69–1.76 (4H, m), 2.18 (12H, s),
3
2
(
.23–2.27 (4H, t), 2.83 (6H, s), 3.31–3.34 (4H, t), 7.12–1.14
1H, d), 7.45–7.53 (2H, m), 8.12–8.13 (1H, d), 8.19–8.21 (1H,
13
d), 8.48–8.50 (1H, d). C-NMR (100 MHz, CDCl ): δppm: 26.2,
3
2
1
4
1
4
5.1, 45.2, 45.3, 56.6, 115.0, 119.5, 123.0, 127.8, 129.5, 129.9,
30.1, 135.1, 151.6. ESI-Mass (m + 1): calcd 421.26; found,
oxidation of the reduced copper(I).
In this direction we have studied the NO reactivity of two
copper(II) complexes of ligands L1 and L2 having pendant
dansyl fluorophore, respectively. (Scheme 1)
Karlin’s group reported earlier that the parent tridentate tri-
amine ligand, L forms a copper(I) complex with a distorted tri-
gonal planar geometry. Thus, it is assumed that after the
reduction of copper(II) by nitric oxide, demetallation of the
ligands will not occur.
21.13, quantum yield of L is 0.1845 at 298 K in methanol.
1
Synthesis of L . The ligand L has been synthesized from the
2
2
parent dipropylenetriamine in a three-step process described
below (Scheme 3).
2
2
2
Step I: synthesis of 1,7-phthalimido-4-azaheptane (L ′). 1,7-
Phthalimido-4-azaheptane was prepared from the reaction of
dipropylenetriamine (2.62 g, 0.02 mol) and phthalic anhydride
(
23
6.0 g, 0.04 mol) following the reported procedure. Yield:
6
.24 g (80%). Elemental analyses for C H N O : Calcd (%):
22 21 3 4
Experimental section
C, 67.51; H, 5.41; N, 10.73. Found (%): C, 67.55; H, 5.40; N,
−1 1
1
0.80. FT-IR: 2932, 1710, 1404, 1039, 730 cm . H-NMR:
General
(400 MHz, CDCl ): δppm: 1.89–1.95 (4H, m), 2.70–2.73 (4H, t),
3
All reagents and solvents were purchased from commercial
sources and the solvents were of HPLC grade. Acetonitrile was
distilled from calcium hydride. Deoxygenation of the solvent
and solutions were effected by repeated vacuum/purge cycles or
bubbling with nitrogen for 30 minutes. NO gas was purified by
passing through KOH and P O column. UV-visible spectra
3.73–3.76 (4H, t), 7.67–7.70 (4H, m) and 7.77–7.81 (4H, m).
C-NMR: (100 MHz, CDCl ) δppm: 31.05, 35.55, 46.08,
3
123.43, 132.19, 134.13, 168.56. ESI-Mass (m + 1): calcd
392.13; found, 392.15.
Step II: synthesis of 4-dansyl-1,7-diphthalimido-4-azaheptane
13
(L ′′). A mixture of 1,7-phthalimido-4-azaheptane (0.782 g,
2
2
5
were recorded on a Perkin Elmer Lambda 25 spectrophotometer.
FT-IR spectra of the solid samples 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 sulfate in acidic medium was used as the reference com-
pound for the determination of fluorescence quantum yield.
Solution electrical conductivity was checked using a Systronic 305
2 mmol), dansyl chloride (0.624 g, 2.3 mmol) and K CO3
2
(0.313 g, 2.4 mmol) was dissolved in 50 ml of dry acetonitrile.
The mixture was refluxed for 24 h under a nitrogen atmosphere.
After filtration, the solvent was removed on a rotary evaporator
under reduced pressure. The resulting residue was mixed with
15 ml of water and extracted with chloroform (20 ml × 3 por-
tions). The organic phase was dried with anhydrous sodium
sulfate and evaporation of the solvent on rotary evaporator
resulted in light yellow oil. 4-Dansyl-1,7-diphthalimido-4-aza-
heptane was obtained as precipitate on stirring the light yellow
oil with 10 ml cold ethanol. The precipitated product was filtered
and washed with cold ethanol (10 ml). Yield: 0.562 g (45%).
Elemental analyses for C H N SO : Calcd (%): C, 65.37; H,
1
conductivity bridge. H-NMR spectra were obtained with a
4
00 MHz Varian FT spectrometer. Chemical shifts (ppm) were
referenced either with an internal standard (Me Si) or to the
4
residual solvent peaks. The X-band Electron Paramagnetic
Resonance (EPR) spectra were recorded on a JES-FA200 ESR
spectrometer, at room temperature. Elemental analyses were
obtained from a Perkin Elmer Series II Analyzer. The magnetic
34
32
4
6
5.16; N, 8.97. Found (%): C, 65.33; H, 5.16; N, 9.03. FT-IR:
2938, 1712, 1398, 1137, 720 cm−
1
.
1
H-NMR: (400 MHz,
10544 | Dalton Trans., 2012, 41, 10543–10548
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Scheme 2
CDCl ): δppm: 1.82–1.89 (4H, m), 2.83 (6H, s), 3.33–3.37 (4H,
t), 3.53–3.57 (4H, t), 7.10–1.12 (1H, d), 7.32–7.36 (1H, t),
212 mg (∼85%). Elemental analyses for [(L )Cu(CH OH)]
2 2 19 34 4 4 2
3
2
3
Cl ·H O, CuC H N SO Cl : Calcd (%): C, 41.56; H, 6.24; N,
7
.48–7.52 (1H, t), 7.65–7.69 (4H, m), 7.74–7.79 (4H, m),
.07–8.09 (1H, d), 8.21–8.23 (1H, d), 8.42–8.44 (1H, d).
10.20. Found (%): C, 41.52; H, 6.24; N, 10.24. UV-vis. (metha-
nol): λmax, 758 nm (ε = 47 M− cm ). FT-IR (KBr pellet):
1
−1
8
1
3
−1
C-NMR: (100 MHz, CDCl ) δ
27.4, 35.5, 44.9, 45.4,
1323, 1137, 781, 3110, 2961 cm . X-band EPR data: g
=
ave
3
ppm:
2.18. Molar conductance: 223 S cm− . μobs., 1.49 BM.
1
115.3, 119.4, 123.0, 123.3, 128.3, 129.8, 130.0, 130.5, 132.1,
1
34.0, 134.5, 151.8, 168.1. ESI-Mass (m + 1): calcd 625.20;
found, 625.04.
Results and discussion
Step III: hydrolysis of 4-dansyl-1,7-diphthalimido-4-azaheptane
to yield ligand L
azaheptane (0.5 g, 0.8 mmol) and hydrazine monohydrate
0.8 g, 16 mmol) was dissolved in 40 ml of ethanol. The mixture
was refluxed for 24 h under a nitrogen atmosphere. The mixture
was filtered and the filtrate was dried to crude mass. This was
dissolved in 50 ml of water and extracted with chloroform (3 ×
2
. A mixture of 4-dansyl-1,7-diphthalimido-4-
The ligands L and L were prepared by inserting the dansyl
1 2
fluorophore on the secondary amine position of bis-[3-(dimethyl-
amino)-propyl]amine and dipropylenetriamine, respectively
(Schemes 2 and 3, respectively; Experimental section). They
were characterized by elemental analyses as well as various
spectroscopic methods (Experimental section). The complexes
were prepared by stoichiometric reaction of copper(II)
chloride dihydrate with the respective ligands in methanol and
characterized by UV-visible, FT-IR, Mass, EPR spectroscopic
analyses. The microanalysis are also in well agreement with the
calculated values (Experimental section). Even after several
attempts to grow X-ray quality crystals of the complexes, we
have failed.
(
5
0 ml portions). The collected organic layer was dried to get the
crude product which upon purification by column chromato-
graphy yields L . Yield: 0.160 g, 55%. Elemental analyses for
2
C H N SO : Calcd (%): C, 59.31; H, 7.74; N, 15.37. Found
1
8
28
4
2
(
1
%): C, 59.36; H, 7.73; N, 15.44. FT-IR (in KBr): 2931, 1572,
315, 1138, 791, 623, 570 cm . H-NMR (400 MHz, CD OD):
3
−1 1
δppm: 1.55–1.62 (4H, m) 2.58–2.62 (4H, t), 2.82 (6H, s),
3
.27–3.31 (4H, t), 7.17–7.19 (1H, d), 7.48–7.52 (2H, m),
.06–8.08 (1H, d) , 8.18–8.20 (1H, d), 8.48–8.50 (1H, d).
8
1
3
Nitric oxide reactivity studies
C-NMR (100 MHz, CDCl ): δ
31.4, 38.8, 44.7, 45.3,
ppm:
3
1
1
15.1, 119.3, 123.0, 127.9 129.1, 129.9, 130.0, 130.1, 135.1,
51.6. ESI-Mass (m + 1): Calcd 365.19; found, 365.10.
Ligand L was found to display modest fluorescence emission
1
upon excitation at 340 nm in methanol solution at room tempera-
ture. The quantum yield was calculated to be 0.1845 with
respect to quinine sulphate in methanol at room temperature.
Addition of equivalent amount of Cu(II) ion into the methanol
Quantum yield of L is 0.1731 in methanol at 298 K.
2
Synthesis of the complexes. Complexes 1 and 2 were syn-
thesized using same experimental protocol. The details are given
for complex 1.
solution of L has been found to display significant (>80%)
1
quenching in fluorescence spectroscopic studies at 298 K
(Fig. 1a). On the other hand, addition of 10 equivalent of nitric
oxide to the degassed methanol solution of complex 1 was found
to restore the emission intensity significantly (Fig. 1b). The
Complex 1. Copper(II) chloride dihydrate, [Cu(H O) ]Cl
2
2
2
(65 mg, 0.5 mmol) was dissolved in 10 ml of freshly distilled
methanol and to this light blue solution, the ligand L (210 mg,
0
1
.5 mmol) was added. The color of the solution was changed to
quenching of fluorescence intensity of ligand L in methanol
2
light green. The resulting mixture was stirred for 3 h. The
volume of the solution was reduced to 5 ml under reduced
pressure and diethyl ether (10 ml) was added and kept at −20 °C
for 12 h to afford complex 1 as light green precipitate. Yield:
was observed upon addition of Cu(II) ion (ESI†). However,
addition of nitric oxide to the methanol solution of complex 2
restored the quenched intensity of L2 in fluorescence spec-
troscopy at room temperature (ESI†). The restored intensity in
the complexes 1 and 2 starts appearing within 20 seconds of NO
addition and becomes 5.61 (±0.2) and 6.30 (±0.3) fold, respect-
ively within 10 minutes of addition of nitric oxide. In cases of
complexes [Cu(Ds-en) ] and [Cu(Ds-AMP) ], addition of 100
2
25 mg (∼80%). Elemental analyses for {[(L )Cu(CH OH)]
1
3
Cl ·2H O}, CuC H N SO Cl : Calcd (%): C, 44.33; H, 7.11;
2
2
23 44
4
5
2
N, 8.99. Found (%): C, 44.37, H 7.10, N, 9.06. UV-vis. (metha-
−
1
−1
nol): λmax, 720 nm (ε = 40 M cm ). FT-IR (KBr pellet):
2
2
−1
2
961, 1319, 1150, 714 cm . X-band EPR data: g = 2.21.
ave
Molar conductance: 212 S cm . μobs., 1.66 BM.
equivalent of nitric oxide to the degassed buffered aqueous or
methanol solution was reported to restore the emission intensity
and the respective enhancements in integrated fluorescence were
−1
Complex 2. It has been synthesised from copper(II) chloride
11b
dihydrate (85 mg, 0.5 mmol) and L (183 mg, 0.5 mmol). Yield:
6.1 (±0.2)- and 8.8 (±0.1)-fold, respectively.
In our earlier
2
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Scheme 3
Fig. 1 Fluorescence responses (λex, 340 nm) of 20 μM deoxygenated methanol solution of (a) free ligand, L
1
(dashed line) and after addition of one
equivalent of CuCl
2
(solid line); (b) complex 1 (solid) before and after purging of 10 equivalent of nitric oxide (dashed lines) at 1, 2, 3, 4, 5, 7 and
8
minutes at 298 K.
2
+
2+
II
report with [Cu(Ds-mpea)(X)] and [Cu(Ds-pepm)(X)] [Ds-
mpea = 5-dimethylamino-naphthalene-1-sulfonic acid (1-methyl-
1
through the formation of [Cu –NO] intermediate in various
15,20b
examples.
However, no indication of formation of the
[Cu –NO] intermediate in the present case was observed in the
UV-visible study. Even in solution FT-IR studies, no stretching
band corresponding to νNO was observed.
With [Cu(dmp) (H O)] and [Cu(phen) (H O)] , even at
the early stage of mixing no spectral change for the inner-sphere
II
H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)-amide; Ds-pepm
5-dimethylamino-naphthalene-1-sulfonicacid(2-pyridin-2-yl-
ethyl)-pyridin-2-yl-methyl-amide; X = solvent] the restored
emission intensities are found to be 12.5 (±0.2)- and 6.7 (±0.2)-
fold in methanol solutions, respectively. In the present study,
the complexes were found to display fluorescence enhancement
for as low as 1 μM of NO.
The fluorescence enhancement in the present cases are mostly
attributed to the formation of Cu(I) species after reduction of
Cu(II) center by nitric oxide (Scheme 4). This has been
=
15
2+
2+
2
2
2
2
2
0
24
complex formation was reported. In cases of [Cu(Ds-mpea)-
2+
2+
(X)] and [Cu(Ds-pepm)(X)] also, the inner-sphere complex
20
formation was not found. These are attributed to the lower
equilibrium constant, KNO, values.
The complete reduction of Cu(II) in presence of nitric oxide
has been further supported by the EPR silent nature of the reac-
tion mixture in the X-band EPR study at room temperature
(Fig. 3). The appearance of a sharp signal in the H-NMR
spectrum after the reaction of complex 1 with nitric oxide is also
in agreement with the reduction of copper(II) centre to copper(I)
(ESI†).
1
confirmed by the UV-visible, X-band EPR and H-NMR spec-
troscopic studies. The d–d band at 720 nm of complex 1 in the
electronic spectrum in methanol was found to disappear after
addition of NO into the solution (Fig. 2). This is attributed to the
complete reduction of Cu(II) centre to Cu(I). The reduction of
Cu(II) center by nitric oxide has been reported to proceed
15
1
1
5
10546 | Dalton Trans., 2012, 41, 10543–10548
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Scheme 4
formation of Cu(I) species and dissociation of the sulphonamide
+
functionality following protonation by H formed in the reaction
1
1b
(Scheme 4).
They observed the presence of sulphonamide
group (νN–H ≈ 3083 cm− in KBr) in the FT-IR spectrum of the
reaction product which indicates that ligand protonation also
occurs. It has been anticipated that the protonated sulfonamide
1
11b
group may be contributing to the fluorescence intensity. In the
2+
2+
cases of [Cu(Ds-mpea)(X)] and [Cu(Ds-pepm)(X)] , we have
not observed the dissociation of sulphonamide group. In the
present case also, there was no indication of the dissociation of
sulphonamide group in the FT-IR spectrum of the reaction
mixture (ESI†). Hence, the reappearance of the fluorescence
intensity is attributed only to the reduction of the Cu(II) center to
the diamagnetic Cu(I) by nitric oxide. It is also important to note
that no N-nitrosamine or copper-nitrosyl frequency was observed
in the FT-IR spectrum of the reaction mixture which can be
+
attributed to the reaction of NO , formed during the reaction of
Fig. 2 UV-visible spectra of complex 1 in methanol (a) before and (b)
Cu(II) with nitric oxide, with methanol solvent to form CH ONO
after purging nitric oxide indicating reduction of Cu(II) to Cu(I).
3
+
11b,24
and H .
In fact, the formation of CH ONO has been
3
observed in GC-Mass spectral studies.
1
H-NMR spectral studies revealed that after reduction the
Cu(I) remains coordinated to the ligand (ESI†). However, in
earlier examples because of the geometrical constrain in most of
the cases, the Cu(I) centres were found to be separated from the
ligand frameworks.
It would be worth mentioning here that the quenched fluor-
escence intensity of complexes 1 and 2 in an aqueous solution
buffered at pH ∼ 7.2 by TRIS-HCl buffer was found to restore
upon addition of 10 equivalent nitric oxide within 10 minutes
(Fig. 4 and ESI†). The intensity of the restored emission, in both
the cases, was found to be less compared to that in methanol. In
2
+
2+
cases of [Cu(Ds-mpea)(X)] and [Cu(Ds-pepm)(X)] , similar
20
results were reported. The reappearance of the fluorescence
intensity in the buffered aqueous solution is attributed to the
reduction of Cu(II) to Cu(I) as no dissociation of the sulphona-
+
mide group following protonation by H formed in the reactions
Fig. 3 X-band EPR spectra of complex 1 before (black) and after
was evidenced from the FT-IR spectrum of the reaction
mixture. The ESI-Mass of the reaction mixtures also suggest
20
(blue) purging nitric oxide in methanol at room temperature.
the presence of L and L in un-dissociated form (ESI†).
1
2
In case of complex 2, the d–d band at 758 nm was found to
diminished after addition of nitric oxide (ESI†) which is an indi-
Conclusion
cation of the reduction of Cu(II) center to Cu(I) by nitric oxide.
1
X-band EPR and H-NMR spectroscopic studies also suggest
In conclusion, two Cu(II) complexes, 1 and 2 have been prepared
with a tridentate N-donor ligands having a pendant dansyl fluo-
rophore. Addition of nitric oxide to methanol solution of the
complexes resulted in the reduction of copper(II) centre to
the reduction is occurring (ESI†).
Lippard’s group reported that in the case of Ds-HAMP ligand,
the fluorescence enhancement is essentially because of both the
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4
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escence Spectroscopy, ed. C. D. Geddes, J. R. Lakowicz, Springer,
Berlin, 2005, p. 163.
5
6
E. Sasaki, H. Kojima, H. Nishimatsu, Y. Urano, K. Kikuchi, Y. Hirata and
T. Nagano, J. Am. Chem. Soc., 2005, 127, 3684.
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(a) S. A. Hilderbrand and S. J. Lippard, Inorg. Chem., 2004, 43, 4674;
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2
2
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1
1
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Fig. 4 Fluorescence responses (λex, 340 nm) of 20 μM of complex 1
in aqueous (pH 7.2; tris-buffer) before (solid line) and after purging of
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1
0 equivalent of nitric oxide (dotted lines) at 1, 2, 3, 5 and 10 minutes,
respectively, at 298 K.
8525.
1
3 M. H. Lim, B. A. Wong, W. H. Pitcock Jr., D. Mokshagundam, M. Baik
and S. J. Lippard, J. Am. Chem. Soc., 2006, 128, 14364.
copper(I) with a substantial increase in fluorescence intensity.
Thus the complexes can act as a fluorescence-based nitric oxide
sensor in methanol. In aqueous solutions (buffered at pH ∼ 7.2)
of complexes 1 and 2, the quenched fluorescence intensity was
observed to be restored upon addition of nitric oxide. The
reappearance of the fluorescence intensity in both the cases was
attributed to the reduction of Cu(II) to diamagnetic Cu(I).
14 (a) A. Dıaz, M. Ortiz, I. Sanchez, R. Cao, A. Mederos, J. Sanchiz and
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1
5 (a) M. Sarma, A. Singh, S. G. Gupta, G. Das and B. Mondal, Inorg.
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Acknowledgements
16 M. Riklin, D. Tran, X. Bu, L. E. Laverman and P. C. Ford, J. Chem. Soc.,
Dalton Trans., 2001, 1813.
The authors sincerely thank the Department of Science and
Technology, India for financial support (SR/S1/IC-38/2010);
DST-FIST for the X-ray diffraction facility. PK and AK would
like to thank CSIR, India, for providing a scholarship.
17 (a) K. Tsuge, F. DeRosa, M. D. Lim and P. C. Ford, J. Am. Chem. Soc.,
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1
1
8 M. H. Lim and S. J. Lippard, Inorg. Chem., 2006, 45, 8980.
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2
011, 47, 2964; (b) P. Kumar, A. Kalita and B. Mondal, Dalton Trans.,
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