Nitric oxide reactivity of Cu(ii) complexes of tetra- and pentadentate ligands: Structural influence in deciding the reduction pathway

Article abstract of DOI:10.1039/c3dt32580f

Four Cu(ii) complexes, 1, 2, 3 and 4, are synthesized with ligands, L ₁, L₂, L₃ and L₄ [L₁ = N1,N2-bis((pyridin-2-yl)methyl)ethane-1,2-diamine; L₂ = N1,N3-bis((pyridin-2-yl)methyl)propane-1,3-diamine; L₃ = N1,N1,N2-tris((pyridin-2-yl)methyl)ethane-1,2-diamine; L₄ = N1-((1-methyl-1H-imidazol-2-yl)methyl)-N1,N2-bis((pyridin-2-yl)methyl)ethane-1, 2-diamine], respectively, as their perchlorate salts. The complexes were characterized by various spectroscopic techniques as well as single crystal X-ray structure determination. Nitric oxide reactivities of the complexes were studied in acetonitrile as well as methanol solvent. It has been found that the ligand frameworks have a considerable effect in controlling the mechanism of the reduction of a Cu(ii) center by nitric oxide. The flexibility of the ligand/s for a Cu(ii) complex to attain a trigonal bipyramidal geometry after NO coordination is found to be the most important parameter in dictating the pathway for their interaction. In the present study, all the four compounds, because of structural constraints, were found to follow a deprotonation pathway for the reduction of a Cu(ii) center by nitric oxide rather than [Cu ^II-NO] intermediate formation. All the ligands were found to yield an N-nitrosoamine product along with the reduction of Cu(ii) centers by nitric oxide.

Full text of DOI:10.1039/c3dt32580f

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PAPER
Nitric oxide reactivity of Cu(II) complexes of tetra-
and pentadentate ligands: structural influence in
deciding the reduction pathway†
Cite this: Dalton Trans., 2013, 42, 5731
Pankaj Kumar, Apurba Kalita and Biplab Mondal*
Four Cu(II) complexes, 1, 2, 3 and 4, are synthesized with ligands, L
(pyridin-2-yl)methyl)ethane-1,2-diamine; L = N1,N3-bis((pyridin-2-yl)methyl)propane-1,3-diamine; L
N1,N1,N2-tris((pyridin-2-yl)methyl)ethane-1,2-diamine; L = N1-((1-methyl-1H-imidazol-2-yl)methyl)-N1,
1 2 3 4 1
, L , L and L [L = N1,N2-bis-
(
2
3
=
4
N2-bis((pyridin-2-yl)methyl)ethane-1,2-diamine], respectively, as their perchlorate salts. The complexes
were characterized by various spectroscopic techniques as well as single crystal X-ray structure determi-
nation. Nitric oxide reactivities of the complexes were studied in acetonitrile as well as methanol solvent.
It has been found that the ligand frameworks have a considerable effect in controlling the mechanism of
the reduction of a Cu(II) center by nitric oxide. The flexibility of the ligand/s for a Cu(II) complex to attain
a trigonal bipyramidal geometry after NO coordination is found to be the most important parameter in
dictating the pathway for their interaction. In the present study, all the four compounds, because of
structural constraints, were found to follow a deprotonation pathway for the reduction of a Cu(II) center
Received 11th September 2012,
Accepted 4th February 2013
DOI: 10.1039/c3dt32580f
II
by nitric oxide rather than [Cu –NO] intermediate formation. All the ligands were found to yield an
www.rsc.org/dalton
N-nitrosoamine product along with the reduction of Cu(II) centers by nitric oxide.
number of examples of the interaction of nitric oxide with
copper(II) complexes leading to the reduction of the Cu(II)
Introduction
Activation of nitric oxide through its coordination to transition center have been reported in the literature. Nitric oxide is
2+
metal ions has been a subject of interest for chemists and bio- shown to reduce the Cu(II) center in [Cu(dmp) (X)] (dmp =
2
chemists since its discovery as it is found to play various roles 2,9-dimethyl-1,10-phenanthroline, X = solvent) and analogous
1
–6
14,15
in mammalian biology. In ferriheme proteins, nitric oxide is complexes through an inner-sphere pathway.
In different
II
2+
2+
known to coordinate to form an iron(III)-nitrosyl intermediate examples, the Cu(II) center of [Cu (DAC)] and [Cu(mtad)]
prior to the pH dependent reduction of the Fe(III) center. In {DAC = 1,8-bis(9-anthracylmethyl) derivative of the macrocyclic
subsequent steps, the hydroxide ion attacks the activated nitro- tetraamine cyclam (1,4,8,11-tetraazacyclotetradecane) and
4
,5
4
sonium group to afford a nitrite ion. The ferrous protein thus mtad = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetra-
formed reacts with the excess of nitric oxide to form stable decane} in methanol is reported to be reduced by nitric oxide
6
–8
16
ferroheme nitrosyl.
In copper-proteins, for instance, cyto- with concomitant nitrosation of the ligands. Quantitative
chrome c oxidase and laccase, the reduction of Cu(II) to Cu(I) and theoretical studies suggested that the reaction in these
9
–12
by nitric oxide has been known for a long time.
Wayland cases proceeds through a pathway analogous to the inner-
and others suggested a mechanism closer to that of ferriheme sphere mechanism for electron transfer between two metal
reduction involving the initial nitric oxide coordination to the centers through a bridging ligand where NO is the reductant,
II
I
+ 13
Cu(II) center to form [Cu –NO ↔ Cu –NO ]. Recently, a Cu(II) the oxidant and the coordinated amido anion behaves as
the bridging ligand (Scheme 1).
Demetallation of the macrocyclic ring after the reduction of
Cu(II) takes place owing to the preference of Cu(I) for tetra-
hedral coordination and the decreased donor ability of the
nitrosated ligand. The formation of Ru(II)–dinitrogen complex,
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039,
India. E-mail: biplab@iitg.ernet.in
†
Electronic supplementary information (ESI) available: All the characterization
data for the ligands and the complexes; UV-visible, EPR spectra of all the com-
plexes before and reaction with nitric oxide are included. CCDC 910909–910912.
For ESI and crystallographic data in CIF or other electronic format see DOI:
2+
3+
[Ru(NH
3
)
5
(N
2
)] , from the reaction of [Ru(NH
3
)
6
]
with NO in
alkaline solution was proposed to follow a similar mechanistic
1
7
10.1039/c3dt32580f
pathway. Nitrosation of a coordinated amide ligand with the
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Fig. 1 Ligands used for the present study.
Scheme 1
simultaneous reduction of Ru(III) to Ru(II) results in a co-
ordinated nitroso amine, which on subsequent dehydration
affords the coordinated dinitrogen complex.
oxide. This has been exemplified by a comparative study of
2
+
2+
nitric oxide reactivity of [Cu(mtad)] and [Cu(tmd)2] [mtad =
,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane,
5
2
4
On the other hand, a series of Cu(II) complexes of N-donor
tmd = 5,5,7-trimethyl-[1,4]-diazepane]. It was observed that
in the case of [Cu(mtad)] , though the reduction takes place
through a deprotonation pathway, in [Cu(tmd)
through a [Cu –NO] intermediate formation.
II
2
+
ligands have been reported to form unstable [Cu –NO] inter-
2
+
mediates on reaction with nitric oxide prior to the reduction of
Cu(II). For example, in Cu(II) complexes of tripodal tetradentate
2
] , it proceeds
II
II
2+
II
2+
II
ligands, [Cu (tren)(CH CN)] , [Cu (taea)(CH CN)] , [Cu -
3
3
In this direction, the nitric oxide reactivities of Cu(II) com-
plexes of the following tetra- and pentadentate ligands (Fig. 1)
have been studied to have some insight into the factors which
control the reduction mechanism.
2
+
(tiaea)(CH
3
CN)] [tren = tris-(2-aminoethyl)amine; taea = tris-
(2-ethylaminoethyl)amine; tiaea = tris-(2-isopropylaminoethyl)-
amine], the reduction was found to proceed through the for-
II
18
mation of a thermally unstable [Cu –NO] intermediate. In
2
+
cases of [Cu(baea)(CH CN)] [baea = bis-(2-aminoethyl)amine,
3
a tridentate amine donor ligand], and complexes of various
bidentate N-donor ligands, such as pymea, pyeta, dmeta,
deaeta [pymea = pyridine-2-methylamine; pieta = pyridine-2-
Experimental
Materials and methods
ethylamine; dmeta = N,N′-dimethylethylenediamine; deteta = All reagents and solvents of reagent grade were purchased
N,N′-diethylethylenediamine, etc.], the formation of an from commercial sources and used as received except when
II
unstable [Cu –NO] intermediate was evidenced prior to the specified. Acetonitrile was distilled from calcium hydride.
1
9,20
reduction of Cu(II) to Cu(I) by nitric oxide.
The stability of Deoxygenation of the solvent and solutions was effected by
II
the [Cu –NO] intermediate was found to depend on the denti- repeated vacuum/purge cycles or bubbling with nitrogen for
city, chelate ring size and nature of the N-donor atom for these 30 minutes. NO gas was purified by passing through KOH and
II
cases. Recently, the example of formation of a stable [Cu –NO]
2 5
P O columns. UV-visible spectra were recorded on a Perkin
complex from the reaction of a Cu(II) complex with nitric oxide Elmer Lambda 25 UV-visible spectrophotometer. FT-IR spectra
2
1
has been reported. It would be worth mentioning here that of the solid samples were taken on a Perkin Elmer spectro-
Hayton et al. have recently reported the structurally charac- photometer with samples prepared as KBr pellets. Solution
II
terized [Cu –NO] complex, though prepared in a different electrical conductivity was measured using a Systronic 305 con-
2
2
1
pathway. Interestingly, in cases of Cu(II) complexes having ductivity bridge. H-NMR spectra were recorded in a 400 MHz
ppmea and mimpea [ppmea, 2-(pyridin-2-yl)-N-((pyridin-2-yl)- Varian FT spectrometer. Chemical shifts (ppm) were refer-
methyl)ethaneamine; mimpea, N-((methyl-1H-imidazol-2-yl)- enced either with an internal standard (Me Si) or to the
4
methyl)-2-(pyridine-2-yl)ethanamine] ligands, no indication of residual solvent peaks. The X-band Electron Paramagnetic
II
the formation of an [Cu –NO] inner-sphere complex has been Resonance (EPR) spectra were recorded on a JES-FA200
2
3
observed prior to the reduction. This is attributed to the ESR spectrometer, at room temperature and 77 K with micro-
much lower values of the equilibrium constants, K (eqn (1)), wave power, 0.998 mW; microwave frequency, 9.14 GHz and
NO
2
+ 15
as reported earlier in the case of [Cu(dmp)
2
(X)] .
modulation amplitude, 2. Elemental analyses were obtained
from a Perkin Elmer Series II Analyzer. The magnetic moment
of complexes was measured on a Cambridge Magnetic
Balance.
KNO
II 2þ
II
2þ
½
ðLÞ2Cu ꢀ þ NO Ð ½ðLÞ2Cu ðNOÞꢀ
ð1Þ
The difference in ligand environments, perhaps, leads to a
Single crystals were grown by slow diffusion followed by a
different pathway for the reduction of Cu(II) to Cu(I) by nitric slow evaporation technique. The intensity data were collected
5
732 | Dalton Trans., 2013, 42, 5731–5739
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using a Bruker SMART APEX-II CCD diffractometer, equipped (5 mmol) was dissolved in 50 ml methanol and to this 0.315 g
with a fine focus 1.75 kW sealed tube MoKα radiation (λ = (5 mmol) of NaBH CN dissolved in 4 ml methanol and 0.77 ml
3
0
3 2
.71073 Å) at 273(3) K, with increasing ω (width of 0.3° per (10 mmol) of CF CO H were added. The solution was stirred at
frame) at a scan speed of 3 s per frame. The SMART software room temperature for 8 h. To this NaOH solution (15%, 50 ml)
was used for data acquisition. Data integration and reduction was added and then extracted with CH Cl (4 × 100 ml por-
2
2
2
5
were undertaken with SAINT and XPREP software. Structures tions). Removal of the solvent under vacuum affords L
3
as a
were solved by direct methods using SHELXS-97 and refined yellow oil. Yield, 1.26 g, 76%. Elemental analyses for C20
with full-matrix least squares on F using SHELXL-97. All calcd (%): C, 72.04; H, 6.95; N, 21.00. Found (%): C, 72.11; H,
23 5
H N :
2
26
non-hydrogen atoms were refined anisotropically. Structural 6.95; N, 21.10. FT-IR in KBr: 2916, 2786, 1583, 1477, 1432,
2
7
−1
1
illustrations have been drawn with ORTEP-3 for Windows.
765 cm
.
3
H-NMR (400 MHz, CDCl ) δppm: 2.16 (s, 1H),
We removed the contribution of smeared electron density 2.78 (s, 4H), 3.82 (s, 6H), 7.12–7.15 (m, 4H), 7.27 (d, 1H),
1
3
from the crystal structure, which we presumed to be due to the 7.51 (d, 1H), 7.59–7.65 (m, 4H), 7.49 (d, 1H).
disordered water molecules by applying the SQUEEZE of (100 MHz, CDCl ) δppm: 48.8, 54.0, 54.8, 54.9, 60.5, 121.8,
PLATON suite. During squeeze the highest Q peak was 121.8, 121.9, 122.2, 122.2, 123.1, 136.3, 136.4, 148.8,
C-NMR
3
changed from 1.14 to 0.64.
149.1, 159.4 and 159.6. ESI-Mass (m + 1): calcd 334.43; found:
334.78.
Synthesis of the L . Ligand L was prepared following the
4
4
Syntheses
same procedure used for L
3
from the reaction of 1-methyl-
2
8
Synthesis of L . The ligand L1 was reported earlier. To
1
imidazole-2-carboxaldehyde with L . Yield 1.21 g, 72%.
1
a solution of pyridine-2-carboxaldehyde (2.14 g, 20 mmol)
in 20 ml methanol was added ethylenediamine (0.60 g,
Elemental analyses for C H N : calcd (%): C, 67.83; H, 7.19;
1
9
24 6
N, 24.98. Found (%): C, 63.77; H, 7.21; N, 24.89. FT-IR in KBr:
1
1
1
0 mmol) into a 50 ml round bottom flask equipped with a
stirring bar. The solution was refluxed for 5 h. The result-
ing reddish-yellow solution was then reduced by NaBH
1.52 g, 40 mmol). Removal of the solvent under reduced
pressure affords a crude mass. It was dissolved in water
50 ml) and extracted with chloroform (50 ml × 4 portions).
2
922, 2803, 1593, 1471, 1432, 1141, 767 cm−
.
H-NMR
(
400 MHz, CDCl ), δppm: 2.71–2.82 (m, 4H), 3.58 (s, 3H),
3
4
3
.75–3.79 (t, 6H), 6.75 (s, 1H), 6.88 (s, 1H), 7.12–7.16 (m, 2H),
.25–7.36 (m, 2H), 7.58–7.63 (m, 2H), 8.49–8.53 (m, 2H).
(
7
1
3
C-NMR (100 MHz, CDCl ), δppm: 32.6, 46.4, 48.8, 53.8, 54.6,
3
(
59.9, 121.3, 121.6, 121.8, 121.9, 123.2, 126.7, 136.1, 136.1,
148.7, 148.8, 148.9, 158.8 and 159.5. ESI-Mass (m + 1): calcd
337.43; found: 337.22.
The organic part was dried under reduced pressure and
the reddish yellow oil thus obtained was subjected to chroma-
tographic purification using a silica gel column to yield
Synthesis of complexes
the pure ligand, L as a yellow oil. Yield: 80%, 1.96 g. Elemen-
1
tal analyses for C14
N, 23.12. Found (%): C, 69.33; H, 7.50; N, 23.01. FT-IR in
18 4
H N : calcd (%): C, 69.39; H, 7.49;
The complexes have been synthesized following a general
experimental procedure for the reaction of copper(II) perchlor-
ate, hexahydrate with an equivalent quantity of the respective
−
1
1
KBr: 2791, 1591, 1475, 1431, 767 cm
CDCl ) δppm: 2.81 (s, 4H), 3.91 (s, 4H), 7.12–7.15 (t, 2H),
.30–7.32 (d, 4H) 7.60–7.64 (t, 2H), 8.52–8.53 (d, 2H).
. H-NMR (400 MHz,
3
ligand. The details are given for complex 1.
II
7
Complex 1. [Cu (H
2
O)
6
](ClO
4
)
2
(1.85 g, 5 mmol) was dis-
1
3
C-NMR (100 MHz, CDCl ) δppm: 46.9, 53.0, 120.7, 121.0,
3
solved in 10 ml of distilled acetonitrile. To this solution, L1
1.21 g, 5 mmol) was added slowly with constant stirring. The
color of the solution turned into greenish-blue from light blue.
was prepared following the same The stirring was continued for 1 h at room temperature. The
from the reaction of pyridine-2-carbox- volume of the solution was then reduced to ∼2 ml. To this,
aldehyde and propylenediamine. Yield, 85%, 2.18 g. Elemental benzene (5 ml) was added to layer on it and kept overnight in
analyses for C15 : calcd (%): C, 70.28; H, 7.86; N, 21.86. a freezer. This resulted in dark green color crystalline complex
Found (%): C, 70.23; H, 7.85; N, 21.94. FT-IR in KBr: 2791, 1. Yield: 2.14 g (85%) and UV-vis. (acetonitrile): λmax, 605 nm
1
2
35.1, 147.6 and 157.6. ESI-Mass (m + 1), calcd 243.32; found:
43.04.
(
Synthesis of L
2 2
. Ligand L
procedure used for L
1
20 4
H N
−
1
1
−1
−1
1
591, 1475, 1430, 1167, 767 cm . H-NMR (400 MHz, CDCl ) (ε = 245 M cm ). X-band EPR (in methanol at 77 K): g ,
3
||
−
4
−1
δ
ppm: 1.75–1.79 (m, 2H), 2.70–2.77 (m, 4H), 3.90 (s, 4H), 2.226; g , 2.012; A , 160 × 10 cm . FT-IR (KBr pellet): 3140,
⊥ ||
−
1
7
.13–7.16 (t, 4H), 7.28–7.30 (d, 2H), 7.60–7.64 (t, 2H), 7.52–7.54 2921, 1608, 1467, 1120, 1083, 625 cm . Molar conductivity in
1
3
−1
(d, 2H). C-NMR (100 MHz, CDCl ) δppm: 29.8, 47.5, 54.8, acetonitrile, ΛM (S cm ), 244. μobs, 1.56 BM.
3
II
1
21.5, 121.9, 136.0, 148.8 and 159.4. ESI-Mass (m + 1): calcd
2 6
Complex 2. Complex 2 was synthesized from [Cu (H O) ]-
2
57.35; found, 257.04.
(ClO ) and L . Yield: 2.12 g (82%). UV-vis (acetonitrile): λmax,
4 2 2
3 3
Synthesis of L . L
was synthesized by following the 618 nm (ε = 166 M− cm ). X-Band EPR (in methanol at 77 K):
1
−1
2
9
−4
−1
reported procedure. A solution of 1.21 g (5 mmol) of L
1
and
⊥
g||, 2.270; g , 2.061; A||, 164 × 10 cm . FT-IR (KBr pellet):
−
1
0
.54 g (5 mmol) of pyridine-2-carboxaldehyde in 20 ml of 3166, 2866, 1608, 1120, 1082, 765, 625 cm . Molar conduc-
(S cm− ) 236. μobs, 1.51 BM.
1
diethyl ether was stirred at room temperature for 3 h to afford tivity in acetonitrile, Λ
a white precipitate. The solid obtained after filtration was
M
II
2 6
Complex 3. Complex 3 was synthesized from [Cu (H O) ]-
washed with diethyl ether. In a 250 ml flask, 1.66 g of the solid (ClO ) and L . Yield: 2.62 g (∼88%). UV-vis (acetonitrile): λmax,
4
2
3
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1
6
1
72 nm (ε = 208 M− cm−1). FT-IR (KBr pellet): 3238, 3077,
−
1
610, 1484, 1144, 1108, 1088, 771, 624 cm . X-band EPR (in
methanol at 77 K): g||, 2.207; g
, 2.013; A||, 158 × 10− cm
4
−1
.
⊥
−
1
Molar conductivity: Λ
M
(S cm ), 205. μobs, 1.56 BM.
II
Complex 4. Complex 4 was synthesized from [Cu (H O) ]-
2
6
(
6
7
ClO
4
)
2
and L
4
. Yield: 2.54 g (85%). UV-vis (acetonitrile): λmax,
−
1
−1
40 nm (ε = 173 M cm ). The X-band EPR (in methanol at
7 K): g , 2.230; g , 2.035; A , 138 × 10 cm . FT-IR (KBr
−
4
−1
|
|
⊥
||
−
1
pellet): 3255, 1611, 1447, 1144, 1108, 1089, 767, 626 cm
Molar conductivity in acetonitrile, Λ
.
−
1
M
(S cm ), 236. μobs, 1.54
BM.
1
Isolation of L ′. Complex 1 (0.252 g, 0.5 mmol) was dis-
solved in 10 ml of distilled and degassed methanol. To this
solution one equivalent of the sodium ethoxide was added
and then the solution was purged with an excess of NO gas.
And the resulting colourless solution was stirred for 1 h at
room temperature. After removing the excess NO by several
cycles of vacuum/purge, 10 ml of degassed benzene was added
to this under a dinitrogen atmosphere. The reaction mixture
Scheme 2
Isolation of L ′. L ′ was isolated from the reaction of
complex 4 (0.308 g, 0.5 mmol) with nitric oxide following the
procedure used for the isolation of L ′. Yield: 155 mg (67%).
4
4
3
Elemental analyses for C H N ClO : calcd (%): C, 48.67; H,
19 23
7
5
4
(
(
(
7
3
1
.94; N, 20.91. Found (%): C, 48.73; H, 4.94; N, 20.83. FT-IR
was kept in a freezer overnight. The L ′-perchlorate was
1
KBr pellet): 1598, 1448, 1123, 756, 630 cm−
1
.
1
H-NMR
found to be precipitated out. Yield: 133 mg (72%). Elemental
400 MHz, D O) δppm: 2.20 (2H), 3.78 (2H), 4.26 (3H, s), 4.62
4H, s), 5.02 (2H), 6.61 (1H), 6.80 (1H), 7.14 (2H), 7.65 (2H),
.83 (2H), 8.46 (2H). C-NMR (100 MHz, D O + CD CN) δppm:
2 3
3.6, 49.0, 51.3, 51.5, 51.9, 61.0, 123.5, 123.8, 123.9, 124.4,
27.5, 137.8, 136.1, 135.4, 150.1, 150.6, 150.9, 155.6, 160.0.
analyses for C14
H
22
N
5
ClO
7
: calcd (%): C, 41.23; H, 5.43; N,
2
1
7.17. Found (%): C, 41.15; H, 5.44; N, 17.26. FT-IR (KBr
1
3
−
1 1
pellet): 3282, 1589, 1437, 1358, 1117, 755, 623 cm . H-NMR
(400 MHz, D
2
O) δppm: 4.08–4.13 (2H, t), 4.27–4.34 (2H, t), 4.90
(
2H, s), 5.45 (2H, s), 7.22–7.32 (4H, m), 7.69–7.76 (2H, m),
.57–8.59 (2H, t). C-NMR (100 MHz, D O + CD CN) δppm:
2 3
3.4, 50.2, 51.8, 58.6, 123.8, 123.9, 123.9, 124.0, 138.4, 138.5,
50.6, 150.9, 155.3, 156.2. ESI-Mass (m + H)/z: calcd 272.32;
1
3
ESI-Mass (m + H)/z: calcd 366.43; found: 366.37.
7
4
1
found: 272.12.
Results and discussion
2 2
Isolation of L ′. L ′ was isolated as its perchlorate salt from
the reaction of complex 2 (0.260 g, 0.5 mmol) with nitric oxide Four Cu(II) complexes, 1, 2, 3 and 4, are synthesized with
following the procedure used for the isolation of the L
1
′-per- ligands, L , L , L
1 2 3
and L
4
[L
1
= N1,N2-bis((pyridin-2-yl)-methyl)
chlorate. Yield: 132 mg (69%). Elemental analyses for ethane-1,2-diamine,
L
2
= N1,N3-bis((pyridin-2-yl)-methyl)-
C H N ClO : calcd (%): C, 44.62; H, 5.49; N, 17.84. Found propane-1,3-diamine; L3 = N1,N1,N2-tris((pyridin-2-yl)methyl)-
1
5
22
5
6
(
1
%): C, 42.65; H, 5.49; N, 17.91. FT-IR (KBr pellet): 3067, 2927, ethane-1,2-diamine; L
4
= N1-((1-methyl-1H-imidazol-2-yl)methyl)-
−
1
1
451, 1437, 1358, 1083, 1123, 755, 629 cm .
H-NMR N1,N2-bis((pyridin-2-yl)methyl)ethane-1,2-diamine], respectively,
(400 MHz, D O) δppm: 2.36–2.39 (2H, m), 3.60–3.64 (2H, t), as their perchlorate salts. The complexes are characterized by
2
4
7
+
1
.32–4.35 (2H, t), 4.87 (2H, s), 5.43 (2H, s), 7.25–7.29 (4H, m), various analytical techniques (Experimental section). The single
1
3
.70–7.73 (2H, t), 7.58–8.59 (2H, m). C-NMR (100 MHz, D
2
O
crystal structures of all the complexes are determined. The per-
CD CN) δppm: 30.2, 42.8, 50.5, 51.9, 58.4, 123.6, 123.8, 123.9, spective ORTEP views for the complexes 1, 3 and 4 are shown in
3
24.5, 138.1, 138.5, 150.7, 150.9, 155.6, 155.7. ESI-Mass Fig. 2–4, respectively. Even after several attempts, the crystal
(
m + H)/z: calcd 286.34; found: 286.12.
quality of complex 2 was not good. The ORTEP view of complex 2
Isolation of L ′. L ′ was isolated as L ′-perchlorate from the is given in the ESI.† The crystallographic data, important bond
3
3
3
reaction of complex 3 (0.305 g, 0.5 mmol) and nitric oxide fol- distances and angles are listed in Tables 1–3, respectively. In 1
lowing the protocol used for L ′ (Scheme 2). Yield: 148 mg and 2, Cu(II) is found to be surrounded by four nitrogen donor
65%). Elemental analyses for C H N ClO : calcd (%): C, atoms from the respective ligands and two oxygen atoms from
1
(
2
0
22
6
5
5
2.01; H, 4.80; N, 18.19. Found (%): C, 52.08; H, 4.81; N, 18.12. two perchlorate ions resulting in an overall distorted octahedral
−
1
FT-IR (KBr pellet): 2936, 1594, 1438, 1118, 1087, 630 cm
.
coordination geometry around the Cu center. The Cu–O(perchlorate)
1
H-NMR (400 MHz, D O) δppm: 3.81 (2H, t), 4.46 (2H, t), 4.93 distances (2.682/2.592 and 2.697/2.581 Å for 1 and 2, respectively)
2
(
7
4H, s), 5.27 (2H, s), 7.14–7.19 (3H, m), 7.36–7.37 (2H), are within the range of reported Cu–O(perchlorate) dis-
1
3
19,20,23,24
.53–7.62 (3H, m), 7.86 (2H), 8.48 (2H). C-NMR (100 MHz, tances.
The average Cu–N distances in complexes 1 and
D O + CD CN) δppm: 42.8, 49.9, 50.9, 52.7, 61.0, 123.5, 123.8, 2 are 1.989 Å and 1.994 Å, respectively, which are in the range
2
3
19,20,23,24
1
1
23.9, 124.1, 124.3, 124.5, 137.8, 138.1, 135.2, 150.2, 150.6, observed in the reported complexes.
50.9, 155.7, 156.7, 160.5. ESI-Mass (m + H)/z: calcd 363.43;
In complexes 3 and 4, the Cu center is found to be sur-
found: 363.45.
rounded by five N-atoms from the respective ligands and one
5
734 | Dalton Trans., 2013, 42, 5731–5739
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Fig. 2 ORTEP diagram of complex 1 (50% thermal ellipsoid plot; H atoms
removed for clarity).
Fig. 4 ORTEP diagram of complex 4 (50% thermal ellipsoid plot; H atoms
removed for clarity).
⊥
calculated spectral parameters, g||, g and A||, are within the
observed range (Experimental section). All the complexes exhibit
one electron paramagnetism at room temperature, as expected.
The cyclic voltammetric studies of the pure complexes have
been carried out in methanol and acetonitrile solvents. The
+
quasi-reversible couple at −0.55 V versus Ag/Ag in the voltam-
II
I
mogram of complex 1 in methanol is attributed to the Cu /Cu
process (ESI†). This couple appeared at −0.53 and −0.59 V
+
Fig. 3 ORTEP diagram of complex 3 (50% thermal ellipsoid plot; H-atoms versus Ag/Ag electrode in methanol for complexes 2 and 3,
removed for clarity).
respectively. However, for complex 4, an irreversible couple at
+
2+
−
0.64 V versus Ag/Ag was appeared (ESI†). For [Cu(DAC)] ,
+
the couple was reported to appear at −0.61 V versus Fc /Fc in
1
6
oxygen atom from a perchlorate anion maintaining an overall DMF–MeOH (1 : 1) solution. The differences in potential are
distorted octahedral geometry (Fig. 3 and 4). In complex 3, the attributed to the change in ligand frameworks and denticity.
four equatorial distances are found to be 1.971, 2.002, 2.026 In addition, for complexes 3 and 4, the change in the axial
and 2.041 Å. The axial Cu–N distance (2.102 Å) is observed to donor atom from N(Py) to N(im) may also have an effect on the
II
I
be longer compared to the equatorial distances owing to the Cu /Cu potential. For Cu(II) complexes having bidentate N-
axial elongation. In complex 4, the equatorial Cu–N distances donor ligands like N,N-dimethylethylenediamine, N,N-ethyl-
are 1.968, 1.987, 2.053 and 2.019 Å and the axial one is ethylenediamine, N,N-diisobutylethylenediamine, tmd (tmd =
II
I
2
.137 Å. The Cu–O(perchlorate) distances are 3.967 and 3.900 Å in 5,5,7-trimethyl-[1,4]-diazepane), etc., the Cu /Cu couple was
2
0,24
complexes 3 and 4, respectively. These distances are a bit longer also observed to appear in a comparable range.
Thus, the
compared to those observed in complexes 1 and 2 and other ligand denticity and the nature of the donor atoms evidently
II
I
reported analogous examples. This suggests a weak interaction control the potential for the Cu /Cu couple. The cyclic voltam-
between the Cu and the perchlorate anion in these cases.
mograms of the complexes in methanol in the presence of one
The complexes 1, 2, 3 and 4, in an acetonitrile solvent, equivalent of sodium ethoxide were also recorded and it shows
−
1
−1
exhibit broad d–d bands at λmax(ε/M cm ) = 605 nm (245), the expected change in potential due to the formation of the
and 618 nm (170), 672 nm (208), and 640 nm (173), respect- amido anions of the corresponding ligands (ESI†).
ively, along with relatively strong intra-ligand absorptions in
Nitric oxide reactivity
the UV region (ESI†).
The acetonitrile solutions of the complexes displayed charac- Nitric oxide reactivities of all the complexes have been studied
30
teristic spectra in X-band EPR studies at 77 K (ESI†). The in dry acetonitrile and methanol solution. However, addition
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Table 1 Crystallographic data for complexes 1, 2, 3 and 4
Complex 1
Complex 2
Cu
Complex 3
CuN
Complex 4
Formulae
C
28
H
36Cl
4
Cu N O
2 8 16
C
30
H
36Cl
3
2
N
O
8 12
C
20
H
22Cl
2
O
5 9
21 2 7 8
C H27Cl CuN O
Mol. wt.
Crystal system
Space group
1009.55
Orthorhombic
Pnma
934.12
Orthorhombic
Pnma
610.88
Monoclinic
Pn
639.95
Triclinic
P
ˉ1
Temperature/K
Wavelength/Å
296(2)
296(2)
296(2)
293(2)
0.71073
8.9766(2)
9.9809(2)
16.3225(4)
91.1700(10)
94.8440(10)
110.2890(10)
1364.84(5)
2
1.557
1.054
Multi-scan
658
0.71073
14.4349(13)
26.361(3)
10.5333(9)
90.00
90.00
90.00
4008.1(7)
4
1.673
1.406
Multi-scan
2056
0.71073
13.6904(5)
26.3804(12)
11.3183(5)
90.00
90.00
90.00
4089.3(3)
4
1.517
1.302
Multi-scan
1908.0
0.71073
8.7895(3)
12.8229(5)
11.1763(5)
90.00
94.308(2)
90.00
1256.09(9)
2
1.615
a/Å
b/Å
c/Å
α/°
β/°
γ/°
3
V/Å
Z
−
3
Density/Mg
−
1
Abs. coeff./mm
Abs. correction
F(000)
1.142
Multi-scan
624
Total no. of reflections
Reflections, I > 2σ(I)
Max. 2θ/°
4618
3059
27.67
−18 ≤ h ≤ 17
5258
2659
28.77
−18 ≤ h ≤ 16
−35 ≤ k ≤ 26
−14 ≤ l ≤ 15
96.9
5401
3406
28.58
−11 ≤ h ≤ 11
−17 ≤ k ≤ 14
−15 ≤ l ≤ 14
97.5
6639
3939
28.43
−12 ≤ h ≤ 11
−13 ≤ k ≤ 12
−21 ≤ l ≤ 21
96.7
Ranges (h, k, l)
−
30 ≤ k ≤ 34
13 ≤ l ≤ 13
−
Complete to 2θ (%)
96.5
Refinement method
Full-matrix least-squares
on F
1.073
Full-matrix least-squares
on F
1.016
Full-matrix least-squares
on F
0.906
Full-matrix least-squares
on F
0.992
2
2
2
2
2
Goof (F )
R indices [I > 2σ(I)]
R indices (all data)
0.0543
0.0913
0.0943
0.1477
0.0454
0.0719
0.0549
0.0747
Table 2 Selected bond lengths (Å) for complexes 1, 2, 3 and 4
of nitric oxide gas to the acetonitrile solution of the respective com-
plexes was found to be unreactive. In methanol solution, in the
presence of one equivalent of sodium ethoxide as a base, the com-
plexes are found to react with nitric oxide leading to the reduction
of the Cu(II) center to Cu(I). This has been monitored by UV-
visible, X-band EPR spectroscopy. For complex 1, the d–d band
appears with a λmax at 605 nm in degassed methanol at room
temperature. Addition of one equivalent of sodium ethoxide
shifted the λmax to 642 nm (Fig. 5). This is attributed to the for-
mation of the corresponding amido anion coordinated
complex after deprotonation at the secondary amine position
Complex 1
Complex 2
Complex 3
Complex 4
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(1)–N(5)
C(1)–C(2)
C(4)–C(5)
C(7)–C(8)
C(10)–C(11)
C(1)–N(1)
C(7)–N(2)
C(8)–N(3)
1.999(4)
2.003(4)
1.986(4)
1.991(3)
—
1.384(8)
1.374(7)
1.492(8)
1.388(6)
1.327(7)
1.471(7)
1.488(6)
1.990(2)
2.000(3)
2.020(2)
1.990(2)
—
1.40(5)
1.41(5)
1.42(5)
1.49(5)
1.35(4)
1.29(4)
—
2.102(4)
2.002(3)
2.026(4)
2.041(4)
1.971(4)
1.371(7)
1.383(8)
1.497(9)
1.39(1)
1.968(2)
2.019(4)
2.053(2)
2.137(3)
1.987(3)
1.358(5)
1.384(5)
1.512(5)
1.408(6)
1.349(5)
1.480(4)
1.487(5)
1.310(6)
1.509(7)
1.333(8)
1
6
(
Scheme 1). Addition of nitric oxide gas to this solution
resulted in immediate decay of the intensity of the absorption
band centered at 642 nm and finally disappeared indicating
complete reduction of the Cu(II) center to Cu(I) (Fig. 5). It
should be noted that similar behaviour was reported for the
Table 3 Selected bond angles (°) for complexes 1, 2, 3 and 4
II
2+
II
2+
nitric oxide reactivity of [Cu (DAC)] and [Cu (mtad)] com-
Complex 1 Complex 2 Complex 3 Complex 4 plexes. The spectral changes for the reduction of Cu(II) by
II
2+
nitric oxide in the case of [Cu (DAC)] were observed to be
condition dependent. In an unbuffered MeOH–water mixture,
the spectroscopic changes appeared to show an induction
period which was no longer apparent in the buffered medium.
This was, presumably, because of the shift in effective pH in
N(1)–Cu(1)–N(2) 82.7(2)
N(1)–Cu(1)–N(3) 167.7(2)
N(1)–Cu(1)–N(4) 110.4(1)
83.0(1)
168.0(1)
103.8(9)
—
93.0(1)
82.0(1)
—
83.2(1)
83.1(1)
124.0(2)
100.3(2)
100.3(2)
83.5(2)
132.3(2)
83.1(2)
166.8(1)
108.4(1)
102.9(1)
85.9(1)
N(1)–Cu(1)–N(5)
—
N(2)–Cu(1)–N(3) 85.1(2)
N(3)–Cu(1)–N(4) 81.9(2)
81.4(1)
1
6
II
2+
N(4)–Cu(1)–N(5)
C(2)–C(1)–N(1)
C(1)–C(2)–C(3)
C(1)–N(1)–C(5)
C(4)–C(5)–C(6)
—
106.7(1)
122.7(4)
118.5(4)
118.7(3)
122.6(3)
the course of the reaction. In [Cu (mtad)] , an induction
period was observed in a methanol–water (8 : 2, v/v) medium
under unbuffered conditions. The absorbance of a single wave-
length (at 523 nm) was plotted versus time, however, there was
122.6(5)
118.3(5)
118.5(4)
124.2(4)
122(3)
118(3)
119(2)
123(3)
122.9(5)
119.6(6)
118.8(4)
123.9(5)
5
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Fig. 5 UV-visible spectra of complex 1 in methanol before (solid line), after
Fig. 7 UV-visible spectra of complex 3 in methanol before (solid line), after
(dashed line) addition of one equivalent of sodium ethoxide and after (dotted
(dashed line) addition of one equivalent of sodium ethoxide and after (dotted
line) purging nitric oxide.
line) purging nitric oxide.
On the other hand, addition of a base to a methanol solu-
tion of complexes 3 and 4 was not found to result in much
shifting of λmax like what was observed in cases of complexes 1
and 2; rather, the appearance of the new absorption bands at
∼
500 nm in both the cases was observed owing to the for-
mation of the corresponding Cu(II)-amido anion complexes
Fig. 7 and ESI†). Upon addition of nitric oxide, these bands
(
were found to diminish gradually along with the correspond-
ing d–d bands owing to the reduction of Cu(II) to Cu(I). The
observed rate constants for complexes 3 and 4 are also found
to be linearly dependent on [NO] (Fig. 6).
It should be noted that the reduction of the Cu(II) centers
was not observed even with four equivalents of NEt . On the
3
other hand, as expected, in acidic pH, addition of nitric oxide
is not found to reduce the Cu(II) center of the complexes.
In the X-band EPR spectroscopy studies, all the complexes
display characteristic signals. Addition of one equivalent of
sodium ethoxide to the methanol solution of the complexes
resulted in a difference in the g values (Fig. 8 and ESI†).
Fig. 6 Plot of kobs vs. equivalent of nitric oxide added to a methanol solution
for complexes 1, 2, 3 and 4 (black, red, green and blue, respectively) at 298 K.
no indication of the presence of an induction period in a Nitric oxide purging of these solutions immediately makes
2
4
neutral medium. In the present study, a shift in λmax from them EPR silent. This is because of the formation of diamag-
05 to 642 nm upon addition of one equivalent of sodium eth- netic Cu(I).
6
II
oxide suggests the formation of the proposed Cu -amido
Thus, from the present study it is evident that the reduction
anion complex, which then reacts with nitric oxide to afford mechanism of the Cu(II) center by nitric oxide is very much
I
II
Cu from Cu . This was found to follow a second order rate law dependent upon the coordinated ligands framework and den-
which depends on both the [Cu-amido] and [NO]. A plot of ticity. In earlier reports, it was shown that as the macro-cyclic
2
+
rate constants against [NO] at 298 K shows a linear relation- ligands offer extra inertness to the metal center in [Cu(DAC)]
2
+
ship of the rate constants with [NO] at 298 K (Fig. 6).
or [Cu(mtad)] , the reduction of Cu(II) by nitric oxide does not
A similar spectral change was observed in the case of proceed through the formation of the corresponding [Cu –NO]
complex 2, also. The λmax of the d–d band in methanol was intermediate; whereas in cases of analogous non-macrocyclic
II
II
shifted from 618 to 664 nm upon addition of one equivalent of ligands, it follows the [Cu –NO] intermediate pathway. In the
sodium ethoxide suggesting the formation of the correspond- present study, though the ligands used are non-macrocycles,
II
ing Cu -amido complex (ESI†). Addition of nitric oxide gas to they offer an environment to the metal center which is not sus-
II
this solution was found to diminish the intensity of the d–d ceptible towards the nitric oxide binding to form a [Cu –NO]
band indicating the reduction of the Cu(II) center to Cu(I) fol- intermediate. Now the question arises, which factor plays the
II
lowing a second order rate equation (Fig. 6).
most important role in dictating whether the [Cu –NO]
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particular Cu(II) complex will form the Cu(II)-nitrosyl inter-
mediate upon reaction with nitric oxide or not.
The N-nitrosated ligands in all the cases have been isolated
from the reaction mixture as they were demetallated owing to
the weaker donor ability and geometrical preference of Cu(I)
and characterized (Experimental section). In the case of ana-
2
+
2+
logous [Cu(DAC)] , [Cu(mtad)] complexes, similar results
were exemplified.
Conclusion
Thus, the present study demonstrated that the ligand frame-
works have a considerable effect in controlling the mechanism of
the reduction of a Cu(II) center by nitric oxide. The flexibility of
Fig. 8 (a) X-band EPR spectra of complex 1 in methanol before (solid line), the ligand/s for a particular Cu(II) complex to attain a trigonal
after (dashed line) addition of one equivalent of sodium ethoxide and after bipyramidal geometry after NO coordination is found to be the
(
dotted line) purging nitric oxide at room temperature.
most important parameter in dictating the pathway for their
interaction. In the present study, all the four compounds,
because of structural constraints, were found to follow a depro-
tonation pathway for the reduction of a Cu(II) center by nitric
complex will be formed or not in a particular case? Is it con-
trolled by the donor ability of the ligand (in other words, the
electrode potential)? Or, it is a structure dependent phenom-
enon? In a comparative study of the other reported examples,
it has been found that the electrode potential of the Cu /Cu
couple essentially does not play much role in the formation of
the [Cu –NO] intermediate. On the other hand, it has been
observed that complexes having structural flexibility to attain
trigonal-bipyramidal geometry after coordination to the nitric
II
oxide rather than [Cu –NO] intermediate formation.
II
I
Acknowledgements
II
The authors thank the Department of Science and Technology,
India for financial support (SR/S1/IC-38/2010); DST-FIST for
X-ray diffraction facility. PK and AK gratefully acknowledge
CSIR, India for the fellowship.
II
oxide only form the [Cu –NO] intermediate. For instance, the
tetradentate ligands, such as DAC or mtad, because of struc-
tural constraints do not allow the corresponding Cu(II)
complex to attain trigonal bipyramidal geometry after nitric
oxide coordination. On the other hand, an mtd ligand, which
is analogous to mtad, but bidentate offers the required flexi-
bility to the Cu(II)-nitrosyl complex to attain trigonal bipyrami-
dal geometry very easily. Theoretical studies also suggest a
trigonal bipyramidal geometry for this intermediate com-
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