Reduction of copper(II) complexes of tridentate ligands by nitric oxide and fluorescent detection of NO in methanol and water media

Article abstract of DOI:10.1039/c1dt10773a

Two copper complexes, 1 and 2, with tridentate N-donor ligands, L ₁ and L₂ [L₁= (1-methyl-1H-imidazol-2-ylmethyl) -(2-pyridin-2-yl-ethyl)amine, L₂ = (2-pyridin-2-yl-ethyl)-pyridin-2 yl-methylamine] respectively, have been synthesized and characterized. On exposure to nitric oxide, the copper(ii) centers in complexes 1 and 2 were found to undergo reduction in various solvents. In acetonitrile solvent the reduction was accompanied by a simultaneous N-nitrosation on the secondary amine center on the ligand frameworks. Complexes 3 and 4 were prepared with ligands L ₃ and L₄, respectively. L₃ and L₄ [L₃ = 5-dimethylamino-naphthalene-1-sulfonic acid (1-methyl-1H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)-amide; L₄ = 5-dimethylamino-naphthalene-1-sulfonic acid(2-pyridin-2-yl-ethyl)-pyridin-2- ylmethyl-amide] are the dansyl derivatives of L₁ and L₂, respectively. Complex 4, due to paramagnetic quenching, does not display any fluorescence; however, on addition of nitric oxide to a methanol or water solution of complex 4, the fluorescence intensity of the fluorophore has been found to be restored. This is attributed to the reduction of the Cu(ii) center by nitric oxide to diamagnetic Cu(i). The turn-on of quenched fluorescence intensity has been observed both in methanol and water media. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10773a

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PAPER
Reduction of copper(II) complexes of tridentate ligands by nitric oxide and
fluorescent detection of NO in methanol and water media†
Pankaj Kumar, Apurba Kalita and Biplab Mondal*
Received 28th April 2011, Accepted 13th June 2011
DOI: 10.1039/c1dt10773a
Two copper complexes, 1 and 2, with tridentate N-donor ligands, L
1
and L
2
[L
1
= (1-methyl-1H-
imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)amine, L = (2-pyridin-2-yl-ethyl)-pyridin-2 yl-methylamine]
2
respectively, have been synthesized and characterized. On exposure to nitric oxide, the copper(II)
centers in complexes 1 and 2 were found to undergo reduction in various solvents. In acetonitrile
solvent the reduction was accompanied by a simultaneous N-nitrosation on the secondary amine center
on the ligand frameworks. Complexes 3 and 4 were prepared with ligands L
and L [L = 5-dimethylamino-naphthalene-1-sulfonic acid (1-methyl-1H-imidazol-2-ylmethyl)-
2-pyridin-2-yl-ethyl)-amide; L = 5-dimethylamino-naphthalene-1-sulfonic acid(2-pyridin-2-yl-
ethyl)-pyridin-2-ylmethyl-amide] are the dansyl derivatives of L and L , respectively. Complex 4, due to
3
and L
4
, respectively. L
3
4
3
(
4
1
2
paramagnetic quenching, does not display any fluorescence; however, on addition of nitric oxide to a
methanol or water solution of complex 4, the fluorescence intensity of the fluorophore has been found to
be restored. This is attributed to the reduction of the Cu(II) center by nitric oxide to diamagnetic Cu(I).
The turn-on of quenched fluorescence intensity has been observed both in methanol and water media.
Introduction
related bio-events would not be detected in real time. Recently, a
highly selective fluorescent imaging agent, NO550, for nitric oxide
has been reported which displays a rapid and linear response with
Nitric oxide (NO) has attracted enormous interest from chemists
and biochemists since it has been discovered as a signalling agent
37
a red-shifted turn-on signal. A number of metal complex-based
1–13
in humans.
It is also known to play roles in various phys-
fluorescence sensors for nitric oxide have been reported recently
iological processes like vascular regulation, neurotransmission
and cytotoxicity.
38–43
based on a fluorophore displacement strategy.
However, the
14–18
These essentially inspired a wide range of
low sensitivity and water insolubility of most of the cobalt,
research to identify the precise roles of nitric oxide in biology.
To study the nitric oxide induced reactions in cellular systems,
the most challenging aim is to detect the location of the nitric
oxide formation. Thus, a selective probe to detect the formation
and migration of nitric oxide with spatiotemporal resolution
directly from living cells is highly desirable. In this aspect, the
fluorescence-based detection technique is found to satisfy almost
all the requirements.
Starting from the early examples of fluorescence-based sensors
such as o-diaminonaphthalene (DAN) and o-diaminofluoresceins
ruthenium and di-rhodium complexes, precluded their further
21–36
application in biological systems.
Though, a number of iron
complexes were reported to sense nitric oxide in aqueous medium,
some of them either display diminished fluorescence or are air
sensitive and exhibit only modest turn-on emission with nitric
44–46
oxide, which again precluded their biological applications.
On the other hand, it has been observed that the fluorophore
19,20
displacement technique to sense nitric oxide works mostly in
38–43
organic solvents.
In aqueous medium, since the replacement
of the fluorophore ligand from the metal can also be achieved
by water, the turn-on fluorescence may also be possible in the
absence of nitric oxide. Hence, the reduction of the metal center
by nitric oxide has been found to be a more effective strategy.
The fluorescence intensity of a ligand comprising a fluorophore is
expected to be quenched on its complexation with a paramagnetic
Cu(II) center and the reduction of copper(II) center to diamagnetic
copper(I) by nitric oxide will restore the quenched fluorescence
(
DAFs), a number of fluorescent probes have been reported to
21–36
date.
However, they are unable to detect or monitor NO itself
as their fluorescent response depends on the formation of a triazole
species by oxidized NO products such as N . Thus, the NO-
2
O
3
Department of Chemistry, Indian Institute of Technology Guwahati, Assam
7
81039, India
47–53
intensity of the fluorescent ligand.
†
Electronic supplementary information (ESI) available. Characterization
//
//
data of L
2
1
, L
2
, L
4
, 1, 2, 4, L
1
and L
2
and cif files for complexes 1 and
In the continuation of our study of the reduction of copper(II)
are included. The UV-vis, EPR and fluorescence spectral studies for the
54–57
complexes by nitric oxide and ligand nitrosation,
we report
reactions of complexes 1, 2, 3 and 4 with nitric oxide are also incorporated.
CCDC reference numbers 816461–816462. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt10773a
here the nitric oxide reactivity of copper(II) complexes of two
tridentate ligands; and the use of the copper(II) complexes of those
8
656 | Dalton Trans., 2011, 40, 8656–8663
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-
1
ligands with dansyl fluorophore to sense nitric oxide in aqueous
and methanol media (Fig. 1).
speed of 3 s frame . The SMART software was used for data
acquisition. Data integration and reduction were undertaken with
58
SAINT and XPREP software. Multi-scan empirical absorption
corrections were applied to the data using the program SADABS.
59
Structures were solved by direct methods using SHELXS-97 and
2
60
refined with full-matrix least squares on F using SHELXL-97.
All non-hydrogen atoms were refined anisotropically. Structural
61
illustrations have been drawn with ORTEP-3 for Windows.
Syntheses
Synthesis of the ligands L
1
and L
2
.
The ligands L
1
and L
2
were
62–67
reported earlier
and have been prepared by the reaction of
pyridine 2-ethylamine with the appropriate aldehyde followed by
reduction of the imine with sodium borohydride (Scheme 1). The
details are given in the supporting information†.
Fig. 1 Ligands used for the present study.
A portion of the nitric oxide sensor study has been reported as
56
the preliminary findings of the present work.
Experimental
Materials and methods
1
Scheme 1 Synthesis of ligand, L .
All reagents and solvents were purchased from commercial sources
and were of reagent 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
For L
1
: Yield, 60%, 0.834 g. Elemental analyses: Calcd.(%) for
: C, 66.21; H, 7.22; N, 25.11. Found (%): C, 66.26; H,
7.23; N, 25.04. FT-IR in KBr: 3504, 2923, 1596, 1437 and 749
C
12
H
16
N
4
-
1
1
3
0 min. NO gas was purified by passing through KOH and P
2
O
5
cm . H-NMR: (400 MHz, CDCl
3
): dppm: 2.86 (1H), 2.95 (2H),
column. UV-visible spectra were recorded on a Perkin Elmer
Lambda 25 UV-visible spectrophotometer. FT-IR spectra of the
solid samples were taken on a Perkin Elmer spectrophotometer
with samples prepared as KBr pellets and for solutions, Varian
3.01 (2H), 3.58 (3H), 3.83 (2H), 6.76 (1H), 6.86 (1H) 7.0–7.1 (2H),
1
3
7.52 (1H), 8.47 (1H). C-NMR: (100 MHz, CDCl ) dppm: 32.47,
3
38.09, 45.47, 48.77, 121.06, 121.31, 126.64, 126.73, 136.66, 146.30,
+
149.04, 160.02. Mass, (M + H ): calculated, 217.13; found: 217.48.
6
60-IR FT-IR spectrometer and NaCl cell of 2 mm path length
Synthesis of the ligand L
2
.
Ligand L
from pyridine-2- ethylamine (1.22 g,
0 mmol) and pyridine-2-carbaldehyde (1.07 g, 10 mmol) (yield,
0%, 1.11 g). Elemental analyses: Calcd.(%) for C13 : C,
3.55; H, 7.13; N, 19.82. Found (%): C, 73.49; H, 7.13; N, 19.74.
2
was prepared following
were used and the spectra shown are the solvent subtracted ones.
The fluorescence spectra were recorded in solution in VARIAN
Cary Eclipse Fluorescence Spectrophotometer at room tempera-
ture. Quinine sulfate in acidic medium was used as the reference
compound for the determination of fluorescence quantum yield.
Solution electrical conductivity was checked using a Systronic
3
a 400 MHz Varian FT spectrometer. Chemical shifts (ppm)
were referenced either with an internal standard (Me Si) or to
the 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
moment of complexes is measured on a Cambridge Magnetic
Balance.
the same procedure as L
1
1
7
7
H
14
N
3
-
1
1
FT-IR in KBr: 2928, 2852, 1594, 1476, 1435 and 765 cm . H-
NMR: (400 MHz, CDCl ): dppm: 2.64–2.99 (4H), 3.86 (2H), 7.00–
3
1
05 conductivity bridge. H-NMR spectra were obtained with
1
3
7
.05 (2H), 7.08–7.10 (1H), 7.19–7.21 (1H), 7.47–7.54 (2H). C-
NMR: (100 MHz, CDCl ) dppm: 37.98, 48.58, 54.57, 120.82, 121.46,
3
4
1
21.77, 122.86, 135.93, 136.00, 148.72, 148.76, 159.23, 159.68.
+
Mass, (M + H ): calculated, 214.12; found: 214.13.
Synthesis of the ligands L and L Synthesis of L
3
4
.
3
. The
4
has been prepared
56
synthesis of L has been reported earlier.
3
L
following the same procedure (Scheme 2) and the details are given
in the supporting information†.
Single crystals were grown by a slow diffusion followed by slow
evaporation technique. The intensity data were collected using a
Bruker SMART APEX-II CCD diffractometer, equipped with a
Yield: ~70%, 0.625 g. Elemental analyses: Calcd.(%) for
C
25
H
26
N
4
O
2
S: C, 67.24; H, 5.87; N, 12.55. Found (%): C, 67.25;
H, 5.89; N, 12.44. FT-IR: 2921, 2850, 1590, 1436, 1323, 1142 and
-
1
1
˚
fine focus 1.75 kW sealed tube Mo-Ka radiation (l = 0.71073 A)
791 cm . H-NMR: (400 MHz, CDCl ) dppm: 2.82–2.89 (8H, m),
3
◦
at 273(3) K, with increasing w (width of 0.3 per frame) at a scan
3.70 (2H, t), 4.71 (2H, s), 6.81 (1H, d), 7.88 (1H, t), 7.12(2H,
This journal is © The Royal Society of Chemistry 2011
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4
Scheme 2 Synthesis of ligand L .
/
/
m), 7.30–7.37 (2H, m), 7.43–7.47 (2H, m), 7.53 (1H, t) 8.19 (3H,
Isolation of L
1
1
3
m), 8.45 (2H, m). C-NMR: (100 MHz, CDCl
3
) dppm: 36.29,
Complex 1 (0.510 g, 1.0 mmol) was dissolved in 10 ml of distilled
and degassed acetonitrile. To this solution an excess of NO gas was
purged for 1 min. and the resulting colorless solution was stirred
for 1 h at room temperature. The solvent was then removed under
reduced pressure using a rotavapor. Water (5 ml) was added to
4
1
1
4
5.32, 47.47, 52.82, 115.08, 119.42, 121.24, 122.42, 122.47, 123.08,
23.13, 128.00, 129.74, 129.96, 130.32, 134.95, 136.03, 136.67,
+
48.88, 148.95, 151.62, 156.73, 158.03. Mass, (M + H ): calculated,
47.17; found: 447.11.
the dried mass followed by the addition of 5 ml of saturated Na S
solution. The black precipitate of CuS was filtered out. The crude
organic part was then extracted from the aqueous layer using
2
Synthesis of complexes 1 and 2. Complexes 1 and 2 were
synthesized following the same procedure. The details are given
for complex 1.
CHCl (25 ml ¥ 4 portions). The crude product, obtained after
3
II
[
Cu (H
2
O)
6
](ClO
4
)
2
(0.370 g, 1.0 mmol) was dissolved in 10 ml
(0.216 g, 1.0 mmol) was
removal of solvent, was then purified by column chromatography
using a neutral alumina column and a hexane/ethyl acetate solvent
distilled acetonitrile. To this solution, L
1
added slowly with constant stirring. The color of the solution
turned into deep blue from light blue. The stirring was continued
for 1 h at room temperature. The volume of the solution was
then reduced to ~2 ml. To this, benzene (5 ml) was added to
layer on it and kept it overnight on freezer. This resulted into
blue color microcrystals of complex 1. Yield: 0.423 g (83%).
//
mixture to get the pure L
analyses: Calcd.(%) for C12
Found (%): C, 58.66; H, 6.16; N, 28.53. FT-IR (KBr pellet): 3399,
1
. Yield: 0.137 g, 64%. Elemental
H
15
N
5
O: C, 58.76; H, 6.16; N, 28.55.
-1
1
2
926, 1437, 1116 and 752 cm . H-NMR (400 MHz, CDCl
3
) dppm:
2
.8 (2H, t), 3.22 (2H, t), 3.50 (3H, s), 3.95 (2H, s), 6.8–7.15 (4H,
+
m), 7.55 (1H, s), 8.22 (1H, s). Mass, (M + H ): calculated, 246.12;
found: 246.11.
Elemental analyses: Calcd.(%) for CuC12
H
19
N
4
O
10Cl : C, 28.05;
2
H, 3.70; N, 10.91. Found (%): C, 28.11; H, 3.69; N, 10.86. UV-vis.
-
1
-1
(
2
acetonitrile): lmax, 616 nm (e = 114 M cm ). X-Band EPR: gav
=
-
1
//
.060. FT-IR (KBr pellet): 2927, 1083, 1119 and 626 cm . Molar
Isolation of L2
-
1
conductivity in acetonitrile, K
M
(S cm ), 244. mobs, 1.56 BM.
/
/
II
L2 was isolated after the reaction of complex 2 (0.513 g,
Complex 2 was synthesized from [Cu (H
2
O)
6
](ClO
4
2
) and
1
.0 mmol) with nitric oxide in degassed acetonitrile following the
L
2
. Yield: 78%, 0.405 g. Elemental analyses: Calcd.(%) for
Cl : C, 34.90; H, 3.29; N, 10.86. Found (%): C,
4.95; H, 3.27; N, 10.81.UV-vis. (acetonitrile): lmax, 606 nm (e =
/
/
same procedure for L
1
. Yield: 62%, 0.134 g. Elemental analyses:
O: C, 64.45; H, 5.82; N, 23.13. Found (%):
C, 64.48; H, 5.83; N, 23.15. FT-IR (KBr pellet): 2924, 2853, 1593,
CuC15
H
17
N
4
O
8
2
Calcd.(%) for C13
H
14
N
4
3
1
1
K
-
1
-1
11 M cm ). X-Band EPR: gav = 2.042. FT-IR (KBr pellet): 2938,
-
1 1
-
1
1436, 1454, 1436, 1128 and 760 cm . H-NMR (400 MHz, CDCl )
086, 1116, 1145 and 626 cm . Molar conductivity in acetonitrile,
3
-
1
dppm: 2.94 (2H, t), 3.32 (2H, t), 4.89 (2H, s), 6.91–7.03 (3H, m), 7.13
M
(S cm ), 236. mobs, 1.51 BM.
+
(
1H, t), 7.42 (2H, s), 8.36 (2H, m). Mass, (M + H ): calculated,
2
43.27; found: 243.29.
Synthesis of complex 3. The synthesis of complex 3 was
reported elsewhere.
56
Results and discussion
Synthesis of complex 4. Copper(II)chloride dihydrate,
Cu(H O) ]Cl (0.170 g, 1.0 mmol) was dissolved in freshly distilled
methanol (20 ml) and to this, L (0.449 g, 1.0 mmol) was added.
The color of the solution changed from blue to light green. The
resulting solution was stirred at room temperature for 3 h. Then
the volume of the solution was reduced to 5 ml and diethyl ether
[
2
2
2
Ligands L
1
2
and L were prepared from the reaction of pyridine
4
2-ethylamine with the appropriate aldehyde followed by reduction
of the imine (experimental section). The formation of the ligands
were authenticated by elemental analysis and other spectroscopic
studies (experimental section). The ligands L
synthesized by incorporating the dansyl group into the secondary
amine site of L and L , respectively, following the reported
procedure (experimental section).
3
and L
4
were
(
15 ml) was added. Storage in a freezer for overnight afforded
a light green precipitate of complex 4. Yield: ~85%, 0.522 g.
Elemental analyses: Calcd.(%) for CuC26 : C, 42.14;
H, 4.05; N, 7.56. Found (%): C, 42.10; H, 4.05; N, 7.52. UV-
1
2
62–67
H
30
N
4
O
11SCl
2
4
L was found to display
satisfactory elemental analyses and was characterized further
by various spectral analysis (experimental section). All of the
complexes were prepared using a general method where hexa-
aquacopper(II) perchlorate was made to react with equivalent
-
1
-1
vis. (methanol): lmax, 744 nm (e, 70 M cm ). FT-IR (KBr
pellet): 2925, 1143 and 794 cm . X-band EPR: gav = 2.03. Molar
conductivity: K
-
1
-
1
M
(S cm ), 205. mobs, 1.52 BM.
8
658 | Dalton Trans., 2011, 40, 8656–8663
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Table 1 Crystallographic data for complexes 1 and 2
1) (2)
Table 2 Selected bond length ( A˚ ) for complexes 1, 2
(1)
(
(2)
Formulae
Mol. wt.
Crystal system
Space group
C
509.73
Triclinic
P1
293(2)
0.71073
8.3087(2)
10.3152(3)
11.6673(3)
85.8490(10)
83.5950(10)
85.1690(10)
988.18(5)
2
1.713
1.434
None
516.0
12
H
15Cl
2
CuN
4
O
10
C
515.78
Monoclinic
P21/c
296(2)
15
H
17Cl
2
CuN
4
O
8
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(1)–O(1)
C(2)–C(1)
C(2)–C(3)
C(1)–N(1)
C(5)–N(1)
C(10)–C(11)
N(2)–C(7)
C(7)–C(8)
N(2)–C(8)
C(9)–C(10)
C(4)–C(5)
C(5)–C(6)
C(8)–C(9)
C(9)–N(3)
2.000(3)
2.024(4)
1.955(3)
—
2.002(3)
1.369(6)
1.362(8)
1.354(6)
1.356(4)
1.334(6)
1.485(5)
—
1.439(5)
—
1.395(6)
1.494(6)
1.492(5)
1.315(5)
2.008(8)
2.00(1)
2.007(8)
1.97(1)
—
1.38(2)
1.36(2)
1.34(1)
1.33(1)
1.33(2)
1.50(2)
1.48(2)
—
¯
T /K
Wavelength /A˚
0.71073
8.4963(9)
9.0426(10)
26.440(3)
90.00
90.769(7)
90.00
2031.1(4)
4
1.687
1.383
None
1048.0
2355
a /A˚
b /A˚
c /A˚
◦
a ( )
◦
b ( )
◦
g ( )
V /A˚
3
Z
1.40(2)
1.43(2)
1.49(2)
1.54(2)
1.32(1)
-3
Density /Mg m
Abs. Coeff. /mm-
Abs. correction
F(000)
1
Total no. of reflections
4211
Reflections, I > 2s(I)
3287
27.50
1670
21.70
-8 £ h £ 8
-9 £ k £ 9
-26 £ l £ 27
98.3
Full-matrix
least-squares on F
1.056
◦
◦
Table 3 Selected bond angles ( ) for complexes 1, 2
Max. 2q /
Ranges (h, k, l)
-10 £ h £ 10
-
9 £ k £ 13
15 £ l £ 15
(1)
(2)
-
Complete to 2q (%)
Refinement method
92.8
N(1)–Cu(1)–N(2)
N(2)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
C(1)–N(1)–C(5)
C(6)–C(5)–N(1)
C(4)–C(5)–C(6)
C(7)–N(2)–C(8)
C(6)–N(2)–C(7)
C(7)–C(6)–C(5)
C(7)–C(8)–C(9)
N(4)–C(14)–C(15)
N(4)–C(10)–C(11)
C(9)–N(3)–C(11)
C(9)–N(3)–C(13)
Cu(1)–N(1)–C(1)
Cu(1)–N(1)–C(5)
Cu(1)–N(2)–C(7)
Cu(1)–N(2)–C(8)
Cu(1)–N(2)–C(6)
Cu(1)–N(3)–C(9)
Cu(1)–N(3)–C(11)
Cu(1)–N(3)–C(13)
C(1)–C(2)–C(3)
95.3(1)
81.8(1)
170.1(1)
117.7(3)
118.3(3)
121.3(4)
114.8(3)
—
79.7(4)
96.9(4)
170.9(3)
117.7(8)
114.6(9)
126(1)
—
114(1)
—
111(1)
178(1)
—
Full-matrix
least-squares on F
1.277
0.0486
0.0623
2
2
2
Goof (F )
R indices [I > 2s(I)]
0.0847
0.1045
R indices (all data)
quantity of the respective ligands (experimental section). All the
complexes showed satisfactory elemental analyses.
115.0(3)
—
—
106.9(4)
106.3(3)
—
117.4(3)
124.9(2)
116.7(3)
111.9(3)
—
—
Crystal structure
117(1)
127.9(7)
114.4(6)
114.6(8)
—
109.8(7)
124.0(7)
—
Single crystal structures of complexes 1 and 2 have been deter-
mined. The perspective view of the ORTEP diagrams are shown
in Fig. 2. In both of the complexes, copper(II) is found to be
coordinated with the tridentate ligand and a solvent molecule in a
distorted square planar geometry. Weak interactions between the
perchlorate oxygens and copper(II) ion have been found in both
the complexes. In complex 1, the coordinated solvent is water
and in complex 2, it is acetonitrile. The crystallographic data,
important bond angles and distances were listed in Tables 1, 2
and 3 respectively. From the spectral analysis, presumably, the
complexes attain the distorted square planar geometry in solution
also.
114.1(2)
139.5(2)
—
119.0(7)
118(1)
119.4(5)
68
+
test. Generation of H (eqn (1)) was confirmed qualitatively in
unbuffered aqueous solution by monitoring the decrease in pH as
the reactions proceeded.
(
1)
Nitric oxide reactivity of complexes 1 and 2
2
+
Similar reduction was reported with [Cu(dmp)
2
(H
2
O)] and
2
+
Purging of excess nitric oxide to the degassed acetonitrile solution
of complexes 1 and 2 resulted in the rapid reduction of the
copper(II) center to copper(I). The reduction was monitored
by UV-visible spectroscopy and Fig. 3(a) and (b) represent the
observed spectral change during the reaction. The reduction of
Cu(II) in the presence of NO was noticed in water, methanol
and methanol/water mixture also. In a methanol and water
medium, the reduction led to the formation of a methyl nitrite
and a nitrite ion, respectively. The formation of methyl nitrite
was confirmed quantitatively by GC-Mass spectral studies and
nitrite ion in aqueous solution was authenticated by the Griess
[Cu(phen)
phen = 1,10-phenanthroline).
tween the these two cases and the present ones is, in the
2
(H
2
O)] (dmp = 2,9-dimethyl-1,10-phenanthroline;
50,51
The striking difference be-
2
+
2+
case of [Cu(dmp)
2
(H
2
O)] and [Cu(phen)
2
(H
2
O)] , the re-
duction was observed only in presence of a protic solvent,
5
0,51
but not in pure acetonitrile and dichloromethane.
the other hand, the Cu(II) centers in [Cu(tren)(CH
On
2
+
3
CN)] ,
2
+
2+
[Cu(tiaea)(CH
3
CN)] and [Cu(teaea)(CH CN)] [tren = tris(2-
aminoethyl)amine; tiaea = tris(2-isopropylaminoethyl)amine and
teaea = tris(2-ethylaminoethyl)amine] were found to undergo re-
3
54,55
duction in the presence of nitric oxide in pure acetonitrile also.
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Fig. 2 ORTEP diagrams of complexes (a) 1 and (b) 2 (50% thermal ellipsoid plot). (Hydrogen atoms, perchlorate anions and solvent of crystallization
were removed for clarity.)
Fig. 3 UV-visible spectral changes of complexes (a) 1 and (b) 2 after their reactions with nitric oxide in acetonitrile.
-
5
-1
2+
In these cases, the reduction was accompanied with either the N-
nitrosation or the diazotization of the primary amine center of the
and 7.35 ¥ 10 s , respectively. For [Cu(tiaea)(CH
3
CN)] and
2
+
[Cu(teaea)(CH
3
CN)] , the observed rates at 298 K were 5.64 ¥
54,55
-2
-3 -1
55
ligand followed by ligand transformation.
In the present cases,
10 and 6.55 ¥ 10 s , respectively. The pseudo first order rate
constants for [Cu(pymea)
pyridine-2-methylamine and baea = bis(2-aminoethyl)amine] are
also reported to be 3.10 ¥ 10 and 2.20 ¥ 10 s , respectively, at
2
+
2+
the reduction in acetonitrile medium, led to the N-nitrosation of
the ligand (Scheme 3). The N-nitrosation was reported in case of
2
]
and [Cu(baea)(CH
3
CN)] [pymea =
2
+
-3
-3 -1
[
Cu(DAC)] also (DAC = 1,8-bis(9-anthracylmethyl)-derivative
69,70
57
of the macrocyclic tetraamine cyclam).
298 K. Thus, the rates of reduction of the Cu(II) center to Cu(I)
for the present set of complexes are much slower than the earlier
reported ones. This is, presumably, because of the difference in
ligand denticity and the nature of the N-donor atoms. From eqn (1)
+
and Scheme 3, generation of H is obvious during the reduction,
it is expected to be catalyzed by the presence of base. However, no
effect of pH was observed on the rates of the reactions indicating
that the conjugate base has very little role in the reaction. It would
be worth mentioning here that the rate of reductive nitrosylation
of ferriheme proteins in aqueous medium is demonstrated to be
strongly dependent on the hydroxide ion concentration in the pH
Scheme 3
71
The d-d bands at 616 and 606 nm in the case of complexes 1
and 2, respectively, were studied during the reduction of Cu(II)
centers in the presence of nitric oxide (Fig. 3). In the presence of
excess NO, the reduction of Cu(II) to Cu(I) was found to follow
simple first-order kinetics in water, methanol and acetonitrile
media and the rate behavior was observed to be independent
of the initial concentration of the complexes. The observed rate
range 6–9. This was presumably because of the rate limiting
III
attack of hydroxide ion on the Fe -coordinated NO.
The NO reduction of copper(II) centers in complexes 1 and
2 can be rationalized by an inner-sphere mechanism involving
three steps: (i) reversible displacement of the solvent by NO from
the coordination sphere of copper(II) leading to the formation
II
of a inner-sphere [Cu –NO] intermediate; (ii) nucleophilic at-
-
5
-1
constants at 298 K for complexes 1 and 2 are 15.55 ¥ 10
s
tack of H
2
O or CH OH (in the case of water and methanol
3
8
660 | Dalton Trans., 2011, 40, 8656–8663
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Though both the complexes in acetonitrile solvent displayed
characteristic EPR spectra, the colorless solutions are found to
be silent (Fig. 4). This can be attributed to the reduction of
paramagnetic Cu(II) to diamagnetic Cu(I).
The N-nitrosated ligands in both the cases were iso-
lated and characterized completely using various spectroscopic
techniques.
NO reactivity of complexes 3 and 4
Since the copper(II) centers in complexes 1 and 2 exhibit rapid
reduction in presence of nitric oxide, these in combination with a
pendant fluorophore might be good sensors for nitric oxide. Lig-
ands L
group into L
3
and L
4
are prepared by incorporating the pendant dansyl
and L . L and L display moderate fluorescence at
1
2
3
4
Scheme 4
room temperature in methanol, water and methanol/water solvent
systems. The fluorescence quantum yields were calculated to be
+
media) or the generation of highly electrophilic NO owing to
0.198 and 0.177 for L
temperature. Metallation with paramagnetic copper(II), resulted
in the quenching of the fluorescence intensity of free L and
(Fig. 5, supporting information†). The quenched fluorescence
3
4
and L , respectively, in methanol at room
II
I
+
[
Cu –NO ↔ Cu –NO ] charge distribution followed by the (iii)
-
release of NO
2
(or CH
3
ONO) or N-nitrosated ligand. Step
3
three, perhaps, became more facile owing to the geometrical
preference of Cu complexes for tetrahedral coordination. In our
L
4
I
intensity of the ligand fluorophore is expected to be restored on
2
+
2+
29
earlier studies with [Cu(tren)(CH
3
CN)] , [Cu(tiaea)(CH
3
CN)]
reduction of the copper(II) center by nitric oxide (Scheme 4). The
2
+
II
and [Cu(teaea)(CH CN)] the formation of the transient [Cu –
NO] intermediate complex was observed.
3
use of complex 3 as a fluorescent sensor of nitric oxide in water
54,55
56
Similar instances
and methanol media has already been reported. Complex 4 is
of nucleophilic attack at the coordinated NO were reported
in the reaction of hydroxide ion in [Ru –NO] to result in the
also found to detect nitric oxide by fluorescence turn-on in both
methanol and pH 7.2 buffered aqueous solutions. Except for a
few, all earlier examples of copper(II) complexes as fluorescent
sensor in aqueous methanol or in an aqueous pH 7.0 buffer
and in cells, were found to detect nitric oxide by a different
II
corresponding nitro complexes and also the in the reaction of
72,73
alcohols with Ir(III)–NO to yield alkyl nitrite complexes.
In the present cases, we have not observed any indication of
II
II
the formation of an [Cu –NO] inner-sphere complex (Fig. 3).
mechanism, and that involves the reduction of Cu by nitric oxide
2
+
2+
56,73
Similarly, with [Cu(dmp)
2
(H
2
O)] and [Cu(phen)
2
(H
2
O)] , even
followed by dissociation of the N-nitrosated ligand.
[Cu(Ds-
at the early stage of mixing no spectral change for the inner-sphere
en) ] and [Cu(Ds-AMP) ], where Ds-en and Ds-AMP are the
2
2
50,51
complex formation was reported.
This can be rationalized in
conjugate bases of dansylethylenediamine (Ds-Hen) and dansyl
aminomethylpyridine (Ds-HAMP), respectively, were reported
recentlyas aprobe for fluorescence-basedNO detection in aqueous
two ways: (i) either the spectral patterns of the complexes 1 and
II
2
are very similar to their respective [Cu –NO] intermediates or
48,49
(
ii) the values of the equilibrium constants, KNO are much lower.
solution.
to sense NO at a physiologically more relevant pH.
Addition of an equivalent amount of copper(II) in the methanol
solution of L , displayed a significant (>85%) quenching of the
However, these Cu(II) dansyl compounds were unable
73
Since, in FT-IR studies also, no uNO frequency corresponding to
II
the formation of [Cu –NO] was observed, the second option is
more logical.
4
II
The complete reduction of the Cu centers in complexes 1 and 2
ligand fluorescence at 298 K (Fig. 5). This has been observed
in aqueous medium buffered at pH 7.2 using TRIS-HCl buffer
by nitric oxide was further confirmed by X-band EPR studies.
Fig. 4 X-band EPR spectra of the reaction of complexes 1 (a) and 2 (b) with nitric oxide in acetonitrile solvent at room temperature. (solid traces
correspond to the respective complexes and dashed traces represent the spectra of the colorless solutions obtained after reaction of the respective
complexes with nitric oxide).
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2+
4 2 6
Fig. 5 Fluorescence responses (lex, 350 nm) for (a) 25 mM solution of free ligand, L (dotted line) and after addition of one equivalent of [Cu(H O) ]
in methanol (solid line); (b) for a deoxygenated methanol solution of complex 4 before (solid line) and after (dashed lines) purging of 5 equivalent of NO
at 10, 20, 30, 40, 50 and 60 min at 298 K (lines I–VI, respectively).
also (supporting information†). In the case of complex 3, similar
Conclusion
56
quenching was observed in methanol and water solutions. It
has been found that the addition of 2–5 equivalents of nitric
oxide into a degassed methanol solution of complex 4 immediately
restored the emission intensity significantly. However, the restored
emission was observed to be less in aqueous medium buffered
at pH 7.2, compared to that in methanol. Similar behavior was
In conclusion, two Cu(II) complexes have been prepared with two
tridentate N-donor ligands and their nitric oxide reactivity were
studied. The copper(II) centers in complexes 1 and 2 were found
to undergo reduction in the presence of nitric oxide in acetonitrile,
methanol and water media. No [Cu –NO] intermediate was
observed in the present cases presumably because of very low
II
56
observed in the case of complex 3 also. In the case of [Cu(Ds-
en) ] and [Cu(Ds-AMP) ], the ligand fluorescence intensity was
value of KNO. The reduction in acetonitrile solvent was found to
2
2
be accompanied by N-nitrosation of the ligands. However, in water
found to quench to 31(± 2)- and 23(± 0.5)-fold relative to free
Ds-Hen and Ds-HAMP (40 mM), respectively, upon addition
-
and methanol solvents, the formation of NO
2
and CH ONO were
3
48,49
observed. Complex 3 has already been reported to behave as nitric
oxide sensor in methanol or water media. In the case of complex
of Cu(II).
In buffered aqueous or methanol solution of the
complexes [Cu(Ds-en)
2
] and [Cu(Ds-AMP) ], under anaerobic
2
4, addition of nitric oxide to methanol or aqueous (at pH 7.2)
conditions, the emission intensity was found to be restored
significantly upon addition of 100 equivalents of nitric oxide
and the enhancements in integrated fluorescence were reported
solutions resulted in a significant increase in fluorescence intensity.
Thus, complex 4 can function as a fluorescence-based nitric oxide
sensor. The fluorescence enhancement is attributed mostly to
the formation of diamagnetic Cu(I) species after reduction of
paramagnetic Cu(II) center by nitric oxide.
4
8,49
to be 6.1(±0.2)- and 8.8(±0.1)-fold, respectively.
complex 3, the restored emission intensities are found to be 12.5
±0.2)- and 8.3(±0.2)-fold in methanol and aqueous (at pH 7.2)
In the case of
(
56
medium, respectively. Addition of 5 equivalents of nitric oxide
to a degassed aqueous (buffered at pH 7.2) or methanol solution
of complex 4 restored the emission intensity to 6.7 (±0.2)- and
Acknowledgements
4
.6(±0.2)-fold, respectively. The fluorescence enhancement in the
The authors sincerely thank the Department of Science and
Technology, India for financial support; DST-FIST for X-ray
diffraction facility. PK and AK would like to thank CSIR, India
for providing the scholarship.
case of the Ds-HAMP ligand is attributed to both the formation of
Cu(I) species and dissociation of the sulphonamide functionality
following protonation by H formed in the reaction. The disso-
ciation was confirmed by the presence of a sulphonamide group
(nN–H ª 3083 cm in KBr) in the FT-IR spectrum of the reaction
+
48
-
1
product which indicates that ligand protonation also occurs. The
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