Nitric oxide reactivity of copper(ii) complexes of bidentate amine ligands: Effect of substitution on ligand nitrosation

Article abstract of DOI:10.1039/c2dt11082b

Three copper(ii) complexes with bidentate ligands L₁, L ₂ and L₃ [L₁, N,N^/- dimethylethylenediamine; L₂, N,N^/-diethylethylenediamine and L₃, N,N^/-diisobutylethylenediamine], respectively, were synthesized as their perchlorate salts. The single crystal structures for all the complexes were determined. The nitric oxide reactivity of the complexes was studied in acetonitrile solvent. The formation of thermally unstable [Cu^II-NO] intermediate on reaction of the complexes with nitric oxide in acetonitrile solution was observed prior to the reduction of copper(ii) centres to copper(i). The reduction was found to result with a simultaneous mono- and di-nitrosation at the secondary amine sites of the ligand. All the nitrosation products were isolated and characterized. The ratio of the yield of mono- and di-nitrosation product was found to be dependent on the N-substitution present in the ligand framework.

Full text of DOI:10.1039/c2dt11082b

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PAPER
Nitric oxide reactivity of copper(II) complexes of bidentate amine ligands:
effect of substitution on ligand nitrosation†
Moushumi Sarma and Biplab Mondal*
Received 9th June 2011, Accepted 1st December 2011
DOI: 10.1039/c2dt11082b
Three copper(II) complexes with bidentate ligands L1, L2 and L3 [L1, N,N/-dimethylethylenediamine; L2,
N,N/-diethylethylenediamine and L3, N,N/-diisobutylethylenediamine], respectively, were synthesized as
their perchlorate salts. The single crystal structures for all the complexes were determined. The nitric
oxide reactivity of the complexes was studied in acetonitrile solvent. The formation of thermally unstable
[CuII–NO] intermediate on reaction of the complexes with nitric oxide in acetonitrile solution was
observed prior to the reduction of copper(II) centres to copper(I). The reduction was found to result with a
simultaneous mono- and di-nitrosation at the secondary amine sites of the ligand. All the nitrosation
products were isolated and characterized. The ratio of the yield of mono- and di-nitrosation product was
found to be dependent on the N-substitution present in the ligand framework.
Introduction
Experimental
The interaction of nitric oxide with metallo-proteins leading to
the formation of their nitrosyls are believed to be the key step for
most of the biochemical activities of nitric oxide in mammalian
biology.1–6 Thus, the nitric oxide reactivity of the metal ions,
specially iron and copper, are of immense interest for the che-
mists and biochemists. Nitric oxide induced reduction of
copper(II) centers to copper(I) in some metallo-proteins has been
reported long back.7 This reduction is known to facilitate the
nitrosation of various thiolates to form S-nitrosothiols, e.g. S-
nitroso bovine serum albumin and S-nitroso glutathione.8–10
Hence, in small molecules, the interaction of copper(II) center
with nitric oxide is also an emerging field of research.11–14 In
this direction, Ford’s group have reported the detail studies of N-
nitrosation during the reduction of copper(II) by nitric oxide.15
Recently, in our laboratory, we have observed tri-nitrosation of
the ligand during the reduction of Cu(II) to Cu(I) by nitric oxide
in the cases of [Cu(tiaea)(CH3CN)]2+ and [Cu(teaea)(CH3CN)]2+
[tiaea = tris(2-isopropylaminoethyl)amine and teaea = tris(2-
ethylaminoethyl)amine].16 In continuation, to study the role of
the ligand framework and denticity to control the degree of N-
nitrosation of the ligand, we have chosen the following three
bidentate secondary amine ligands (Fig. 1) to prepare their
copper(II) complexes.
Materials and methods
All reagents and solvents were purchased from commercial
sources and were of reagent grade. Ligand L1 was purchased
from Sigma–Aldrich and used as received. Acetonitrile was dis-
tilled from calcium hydride. Deoxygenation of the solvent and
solutions were effected by repeated vacuum/purge cycles or bub-
bling with nitrogen for 30 min. NO gas was purified by passing
through KOH and P2O5 column. UV-visible spectra were
recorded on a Perkin Elmer Lambda 25 UV-visible spectropho-
tometer. FT-IR spectra of the solid samples were taken on a
Perkin Elmer spectrophotometer with samples prepared as KBr
pellets and for solutions, Varian 660-IR FT-IR spectrometer and
NaCl cell of 2 mm path length were used and the spectra shown
are the solvent subtracted ones. Solution electrical conductivity
was checked using a Systronic 305 conductivity bridge. 1H
NMR spectra were obtained with a 400 MHz Varian FT spec-
trometer. Chemical shifts (ppm) were referenced either with an
internal standard (Me4Si) or to the residual solvent peaks. The
X-band Electron Paramagnetic Resonance (EPR) spectra were
recorded on a JES-FA200 ESR spectrometer, at room tempera-
ture and 77 K with microwave power, 0.998 mW; microwave fre-
quency, 9.14 GHz and modulation amplitude, 2. For the
electrochemical studies, a Pt working electrode, Pt wire auxiliary
electrode and an SCE reference electrode were used in a three-
electrode configuration. All electrochemical measurements were
Department of Chemistry, Indian Institute of Technology Guwahati,
Assam, 781039, India. E-mail: biplab@iitg.ernet.in
†Electronic supplementary information (ESI) available: FT-IR, X-band
1
/
EPR, H NMR, 13C NMR, ESI-Mass spectra of complexes 1, 2, 3, L1 ,
L1 , L2 , L2 , L3/ and L3// included. Cyclic voltammograms of complexes
1, 2 and 3 are also given. CCDC reference numbers 829126–829129
and 854586. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt11082b
//
/
//
Fig. 1 List of the ligands used for the present study.
Dalton Trans., 2012, 41, 2927–2934 | 2927
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done at 298 K under nitrogen atmosphere in acetonitrile solvent
containing tetra-butylammonium perchlorate (TBAP) as support-
ing electrolyte. The scan rate used was 50 mV s−1. The half-
2.282; g⊥ = 2.098; A , 92 × 10−4 cm−1. Molar conductance: ΛM
∥
(Ω−1cm2mol−1), 241 in acetonitrile. μeff: 1.65 BM.
[CuI(CH3CN)4]ClO4. The
isolation
of
wave potential E0 was set equal to 0.5(Epa + Epc), where Epa
Isolation
of
298
[Cu(CH3CN)4]ClO4 from the reaction of the respective com-
plexes with NO has been done using the same general procedure.
The detail is given for complex 1.
and Epc are anodic and cathodic cyclic voltammetric peak poten-
tials, respectively. All the electrochemical data are uncorrected
for junction potential.
In a 50 ml Schlenk flask, complex 1 (219 mg, 0.5 mmol) was
dissolved in 10 ml degassed acetonitrile. To this deep blue sol-
ution, nitric oxide gas was purged through a needle for one
minute and allowed to stand for 10 min. To the resulting colour-
less solution, 15 ml of degassed benzene was added through a
syringe to make a layer. The layered solution was then kept over-
night in freezer. The white crystals of [Cu(CH3CN)4]ClO4 were
filtered out from the colourless solution under nitrogen atmos-
phere using a Schlenk frit. Yield, 110 mg, ∼65%.
The same procedure was followed to isolate the
[Cu(CH3CN)4]ClO4 from the reaction of complex 2 (247 mg,
0.5 mmol) and complex 3 (303 mg, 0.5 mmol) with nitric oxide
in acetonitrile solution. Yield: 118 mg, ∼72% (complex 2) and
105 mg ∼ 64% (complex 3).
Elemental analyses were obtained from a Perkin Elmer Series
II Analyzer. The magnetic moment of complexes is measured on
a Cambridge Magnetic Balance.
Single crystals were grown by slow diffusion followed by
slow evaporation technique. The intensity data were collected
using a Bruker SMART APEX-II CCD diffractometer, equipped
with a fine focus 1.75 kW sealed tube Mo-Kα radiation (λ =
0.71073 Å) at 273(3) K, with increasing ω (width of 0.3° per
frame) at a scan speed of 3 s per frame. The SMART software
was used for data acquisition. Data integration and reduction
were undertaken with SAINT and XPREP software and struc-
tures were solved by direct methods using SHELXS-97 and
refined with full-matrix least squares on F2 using
SHELXL-97.17–19 All non-hydrogen atoms were refined aniso-
tropically. Structural illustrations have been drawn with
ORTEP-3 for Windows.20
/
//
Isolation of L1 and L1 . In a 50 ml Schlenk flask, complex 1
(438 mg, 1 mmol) was dissolved in 30 ml degassed acetonitrile.
To this, nitric oxide gas was purged through a needle for one
minute and the mixture was then allowed to stand for 10 min. To
the colorless solution, thus obtained, 20 ml of degassed benzene
was added through a syringe to make a layer. The layered sol-
ution was then kept overnight in freezer. The white crystals of
[Cu(CH3CN)4]ClO4 were filtered out from the colorless solution
under nitrogen atmosphere using a Schlenk frit. The volume of
the filtrate was then reduced to 5 ml and stirred for 1 h in open
air in order to allow the residual CuI center to oxidise to CuII. To
this, 5 ml saturated aqueous solution of Na2S was added and
stirred for ½ h. The black precipitate thus obtained was filtered
off and the filtrate was diluted with 50 ml of distilled water. The
organic part was then extracted from the mixture using CHCl3 (3
portions × 25 ml). The collected organic layer was then dried
under reduced pressure and the residual oil was subjected to
Synthesis of the complexes
All the complexes have been synthesized using a general pro-
cedure of the reaction of hexa-aquacopper(II) perchlorate with
respective ligands. The details are given for complex 1.
Synthesis of complex 1, [Cu(L1)2](ClO4)2. Copper(II)perchlor-
ate hexahydrate, [Cu(H2O)6](ClO4)2 (370 mg, 1.0 mmol) was
dissolved in 10 ml of freshly distilled acetonitrile and to this
blue solution, the ligand L1, N, N/-dimethylethylenediamine
(176 mg, 2.0 mmol), was added drop wise. The color of the sol-
ution changed to violet. The resulting mixture was stirred for
1 h. Then the volume of the solution was reduced to ∼2 ml and
layered with benzene. Storage of this at approximately −20 °C
overnight resulted in the precipitation of blue crystalline com-
pound. Yield: 385 mg (∼87%). Elemental analyses: Calcd.(%)
for C8H24N4O8Cl2Cu: C, 21.90; H, 5.51; N, 12.77. Found (%)
C, 21.94; H, 5.52; N, 12.71. UV-vis. (acetonitrile): λmax, 563 nm
//
column chromatography using silica gel to yield L1/ and L1 .
/
For L1 . Yield, 20 mg (∼16%). Elemental analyses: Calcd.(%)
for C4H11N3O: C, 41.01; H, 9.46; N, 35.87. Found (%) C,
(ε = 100 M−1 cm−1) FT-IR (KBr pellet): υClO −, 1113, 1088,
1
4
41.07; H, 9.47; N, 35.82. H NMR (400 MHz, CDCl3), δppm
,
625 cm−1. X-band EPR data: g , 2.280; g⊥ = 2.096; A∥, 91 ×
∥
2.42(s, 3H), 2.70 (t, 2H, J = 6.4 Hz), 3.04(s, 3H), 4.20(t, 2H, J =
6.4 Hz). 13C NMR (100 MHz, CDCl3), δppm, 53.17, 49.37,
36.00, 31.98. FT-IR in KBr pellet: 1456(s), 1384 (s), 1329(s),
1049(m) cm-1. ESI-Mass: (m+1)/z, Calcd. 118.15; Found,
118.14.
10−4 cm−1. Molar conductance: ΛM (Ω−1cm2mol−1), 247 in
acetonitrile. μeff: 1.76 BM.
Complex 2, [Cu(L2)2](ClO4)2. Yield: 421 mg (∼85%).
Elemental analyses: Calcd.(%) for C12H32N4O8Cl2Cu: C, 29.12;
H, 6.51; N, 11.32. Found (%) C, 29.07; H, 6.50; N, 11.26. UV-
vis. (acetonitrile): λmax, 575 nm (ε = 120 M−1 cm−1) FT-IR
//
For L1 . Yield, 25 mg (∼17%). Elemental analyses: Calcd.
(%) for C4H10N4O2: C, 32.87; H, 6.90; N, 38.34. Found (%) C,
(KBr pellet): υClO −, 1111, 1089, 627 cm−1. X-band EPR data:
32.83; H, 6.92; N, 38.30.1H NMR (400 MHz, CDCl3), δppm
,
4
g , 2.262; g⊥ = 2.082; A , 90 × 10−4 cm−1. Molar conductance:
∥
∥
3.00(s, 3H), 3.03 (s, 6H), 3.67(s, 4H), 3.72(s, 6H), 3.76(s, 3H),
Λ
M (Ω−1cm2mol−1), 232 in acetonitrile. μeff: 1.69 BM.
3.93(t, 2H, J = 6.0 Hz), 4.24(t, 2H, J = 6.0 Hz), 4.51(s, 4H). 13
C
Complex 3, [Cu(L3)2](ClO4)2. Yield: 519 mg (∼86%).
NMR (100 MHz, CDCl3), δppm, 51.08, 49.31, 42.59, 41.27,
1
Elemental analyses: Calcd.(%) for C20H48N4O8Cl2Cu: C, 39.56;
H, 7.97; N, 9.22. Found (%) C, 39.50; H, 7.98; N, 9.14. UV-vis.
(acetonitrile): λmax, 582 nm (ε = 124 M−1 cm−1) FT-IR (KBr
39.78, 39.38, 32.06, 31.62. The presence of extra signals in H
and 13C NMR spectra are due to isomeric impurities (see ESI†).
FT-IR in KBr pellet: 1449(s), 1421 (s), 1316(s), 1050(s), 1027(s)
cm−1. ESI-Mass: (m+1)/z, Calcd. 147.15; Found, 147.20.
pellet): υClO −, 1111, 1089, 625 cm−1. X-band EPR data g∥,
4
2928 | Dalton Trans., 2012, 41, 2927–2934
This journal is © The Royal Society of Chemistry 2012

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/
//
Isolation of L2 and L2 . Same procedure (as in case of
single crystal X-ray structure determination. It should be noted
that the crystal structure of complex 1 was reported earlier with
slightly different parameters.22 The perspective ORTEP view of
complexes 1, 2 and 3 are shown in Fig. 2.
It should be noted that complex 2 has crystallographically
imposed inversion symmetry with Cu1 on an inversion centre.
The crystallographic data and important bond angles and dis-
tances are listed in Table 1, 2 and 3, respectively. The central
metal ion, Cu(II), in all the complexes are coordinated with four
N-donor atoms in a distorted square planar geometry. The
average Cu–N distances are within the range observed in other
reported analogous complexes.23
The d-d transition for complexes 1, 2 and 3 appears at 563,
575 and 582 nm, respectively, in acetonitrile solvent. This shift
in λmax attributed to the increasing order of covalent character of
the σ-bond on moving from methyl to ethyl to isobutyl group at
the N-substitution.24 All the complexes displayed axial EPR
spectra at 77 K characteristic for the square planar copper(II)
complexes having dx2−y2 ground state (see ESI†).25 The calcu-
lated values gk, > g⊥; Ak were in the range of other reported ana-
logous copper(II) complexes.25 The complexes were found to
exhibit one electron paramagnetism at room temperature. They
behaved as 1 : 2 electrolyte in acetonitrile solvent (experimental
section).
The cyclic voltammetric studies of the pure complexes were
carried out in acetonitrile solvent. Complex 1 exhibited one irre-
versible couple at −0.84 v versus SCE and this has been attribu-
ted to the CuII/CuI couple (see ESI†). The CuII/CuI couple was
also observed to appear in this range for analogous reported
compounds.26 This irreversible couple was found at −0.76 and
−0.66 V versus SCE in cases of complexes 2 and 3, respectively
(see ESI†). The difference in reduction potential for the three
complexes is attributed to the difference of N-substituent in
ligand framework.27 In the positive side of SCE, in the potential
complex 1) was adopted for the isolation of L2/ and L2 .
//
/
For L2 . Yield, 24 mg (∼8%). Elemental analyses: Calcd.(%)
for C6H15N3O: C, 49.63; H, 10.41; N, 28.94. Found (%) C,
49.60; H, 10.41; N, 28.89. H NMR (400 MHz, CDCl3), δppm
1.03(m, 6H), 1.34 (t, 2H, J = 7.2 Hz), 3.59(q, 2H, J = 7.2 Hz),
3.63(t, 2H, J = 7.2 Hz), 4.14(m, 2H).13C NMR(100 MHz,
CDCl3), δppm, 51.85, 47.61, 43.94, 39.49, 14.13, 11.31. FT-IR in
KBr pellet: 1456(s), 1382 (m), 1233(m) cm−1. ESI-Mass:
(m+1)/z, Calcd. 146.20; Found, 146.12.
1
,
//
For L2 . Yield, 88 mg (∼24%). Elemental analyses: Calcd.
(%) for C6H14N4O2: C, 41.37; H, 8.10; N, 32.16. Found (%) C,
41.33; H, 8.11; N, 32.10. 1H NMR(400 MHz, CDCl3), δppm
,
1.06 (m, 6H), 1.22(s, 3H), 1.39(m, 8H), 2.14(s, 2H), 3.59(m,
8H), 3.89(t, 4H, J = 7.2 Hz), 4.13(m, 8H), 4.46(s, 2H). 13C
NMR(100 MHz, CDCl3), δppm, 49.78, 48.41, 48.10, 47.35,
42.53, 40.44, 39.67, 39.31, 29.87, 14.19, 14.13, 11.34. The pres-
ence of extra signals in H and 13C NMR spectra are due to iso-
1
meric impurities (see ESI†). FT-IR in KBr pellet: 1456(s), 1375
(s), 1309(s), 1071(s), 1027(s) cm−1. ESI-Mass: (m+Na+)/z,
Calcd. 197.20; Found, 197.19.
/
//
Isolation of L3 and L3 . Same procedure (as in case of
complex 1) was adopted for the isolation of L3/ and L3 .
//
/
For L3 . Yield, 21 mg (∼5%). Elemental analyses: Calcd.(%)
for C10H23N3O: C, 59.66; H, 11.52; N, 20.87. Found (%) C,
59.63; H, 11.51; N, 20.84. H NMR (400 MHz, CDCl3), δppm
1
,
4.15(t), 3.92 (d), 3.63 (t), 3.43(d), 2.97(t), 2.68(t), 2.42(d), 2.36
(d), 2.36(d), 2.09(m), 1.99(m), 1.65(m), 1.22(s), 0.94(d), 0.87
(d),0.86(d), 0.83(d). 13C NMR (100 MHz, CDCl3), δppm, 60.45,
57.85, 57.72, 52.71, 51.29, 49.72, 48.12, 45.99, 44.60, 29.84,
28.52, 27.35, 26.58. The presence of extra signals in 1H and 13
C
NMR spectra are due to isomeric impurities (see ESI†). FT-IR in
KBr pellet: 1960, 2962, 2858, 1465, 1375, 1096, 669 cm−1
ESI-Mass: (m+1)/z, Calcd. 202.31; Found, 202.16.
.
//
For L3 . Yield, 140 mg (∼30%). Elemental analyses: Calcd.
(%) for C10H22N4O2: C, 52.15; H, 9.63; N, 24.33. Found (%) C,
1
52.12; H, 9.63; N, 24.29. H NMR (400 MHz, CDCl3), δppm
,
4.46(s), 4.14(t), 3.90(d), 3.87(d), 3.57(s), 3.38(t), 2.07(m), 1.95
(m), 1.22(s), 0.95(s), 0.93(d), 0.84(s), 0.82(d). 13C NMR
(100 MHz, CDCl3), δppm, 60.61, 40.87, 27.44, 20.05. FT-IR in
KBr pellet: 2962, 2927, 1458, 1374, 1290, 1025, 657 cm−1
ESI-Mass: (m+Na+)/z, Calcd. 253.31; Found, 253.17.
.
Results and discussions
Ligands L2 and L3 were synthesized following general procedure
for the mono-alkylation of primary amines (experimental
section).21 The elemental analyses of the synthesized ligands
were found to be in good agreement with the expected values
(experimental section). They were characterized by FT-IR, 1H
NMR, 13C NMR and mass spectroscopy (experimental section).
All the complexes were synthesized by the reaction of copper(II)
perchlorate, hexahydrate with the respective ligands and isolated
as solid. All the complexes displayed satisfactory elemental ana-
lyses (experimental section). They were characterized by various
spectroscopic methods (experimental section) as well as by
Fig. 2 ORTEP diagram of complexes (a) 1, (b) 2 [a (2−x, 2−y, 2−z)
symmetry transformation is implied by each additional atom level] and
(c) 3. (50% thermal ellipsoid plot; Hydrogen and perchlorate are
removed for clarity).
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 2927–2934 | 2929

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Table 1 Crystallographic data for complexes 1, 2 and 3
Table 2 Selected bond lengths (Å) for complexes 1, 2 and 3
Complex 1
Complex 2
Complex 3
Complex 1
Complex 2
Complex 3
Formulae
C8 H24Cl2 Cu
N4 O8
438.75
C12H36Cl2Cu
N4O10
530.89
C20 H48 Cl2 Cu
N4 O8
607.06
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
N1–C1
N1–C2
N1–C3
N1–C5
N2–C3
N2–C4
N2–C6
N3–C5
N3–C6
N3–C15
N4–C7
N4–C8
N4–C16
C2–C3
C3–C4
C5–C6
C6–C7
C15–C16
2.042(4)
2.045(4)
2.043(4)
2.036(3)
1.468(9)
1.505(8)
—
2.069(2)
2.026(2)
—
—
—
2.046(3)
2.055(3)
2.058(3)
2.056(3)
—
—
—
1.479(5)
—
—
1.479(4)
—
—
1.490(5)
—
—
1.471(4)
—
—
1.493(6)
—
Mol. wt.
Crystal
system
Space group
T/K
Wavelength/Å 0.71073
a/Å
b/Å
c/Å
α (°)
β (°)
Monoclinic
Monoclinic
Orthorhombic
Cc
296(2)
P21/c
296(2)
Pbca
—
296(2)
0.71073
14.8346(5)
16.8936(6)
24.3560(8)
90.00
90.00
90.00
6103.9(4)
8
1.321
1.485(3)
—
—
1.489(3)
—
—
—
—
—
—
—
—
1.501(4)
—
—
0.71073
7.3704(5)
9.9384(7)
16.4246(10)
90.00
107.915(4)
90.00
1144.77(13)
2
1.540
—
8.6311(3)
13.7336(3)
15.1029(5)
90.00
95.7460(10)
90.00
1781.24(9)
4
1.636
1.469(7)
1.483(8)
—
1.475(7)
1.501(6)
—
1.477(6)
1.466(6)
—
1.49(1)
—
—
γ (°)
V/Å3
Z
Density/
Mgm−3
μ/mm−1
Abs.
correction
F(000)
Total no. of
reflections
Reflections,
I > 2σ(I)
Max. 2θ/°
Ranges
(h, k, l)
1.568
None
1.240
None
0.935
None
1.479(7)
—
908
2786
558
2846
1992
7590
—
1.480(6)
2734
2491
3567
Table 3 Selected bond angles for complexes 1, 2 and 3
25.49
28.36
28.31
−19 < = h < =
19
−10 < = h < = ×9 < = h < = 9
8
Complex 1
Complex 2
Complex 3
−16< = k < =
−13< = k < = 13 −22 < = k < =
N1–Cu1–N2
N1–Cu1–N4
N1–Cu1–N3
N2–Cu1–N3
N2–Cu1–N4
N3–Cu1–N4
Cu1–N1–C2
Cu1–N1–C3
Cu1–N1–C5
Cu1–N2–C3
Cu1–N2–C4
Cu1–N2–C6
Cu1–N3–C6
Cu1–N3–C15
Cu1–N4–C7
Cu1–N4–C16
N1–C2–C3
N1–C3–C4
N1–C5–C6
N2–C3–C2
N2–C4–C3
N2–C6–C5
N3–C6–C7
N3–C15–C16
N4–C7–C6
N4–C16–C15
84.9(2)
95.1(1)
—
94.3(1)
—
85.7(1)
105.5(3)
—
85.6(7)
—
—
—
—
—
—
105.6(2)
—
—
107.0(1)
—
—
—
—
—
—
108.6(2)
—
—
85.1(1)
—
97.5(1)
—
93.4(1)
85.3(1)
—
16
22
−17 < = l < =
18
97.2%
−21 < = l < = 21 −31 < = l < = 32
Complete to
2θ (%)
Refinement
method
99.4%
99.9%
Full-matrix
least-squares
on F2
0.935
0.0381
Full-matrix least- Full-matrix least-
squares on F2
squares on F2
—
Goof (F2)
Rint
R indices
[I > 2σ(I)]
R indices
(all data)
1.064
0.0225
0.0397
1.006
0.0686
0.0560
—
105.9(2)
—
—
107.2(2)
—
104.8(2)
—
107.2(2)
—
—
108.9(3)
—
—
109.4(3)
—
109.9(3)
—
109.2(3)
107.8(3)
—
—
106.3(3)
—
105.8(3)
—
108.2(5)
—
—
108.1(5)
—
—
108.9(4)
—
0.0329
0.0335
0.0446
0.1379
range up to 1.8 V, no characteristic couple was observed in cases
of complexes 1, 2 and 3, respectively (see ESI†).
108.7(2)
—
—
—
—
Nitric oxide reactivity
In acetonitrile solvent and in presence of nitric oxide, the
copper(II) centers of all the complexes were found to undergo
reduction to copper(I) and it was studied by UV-visible, EPR
and solution IR spectroscopy. In case of complex 1, the d-d tran-
sition band at 563 nm shifted to 605 nm immediately after
purging nitric oxide owing to the formation of thermally unstable
green [CuII-NO] intermediate complex (see ESI†). [CuII–NO]
intermediates were observed to form in the reaction of [Cu(tiaea)
(CH3CN)]2+ and [Cu(teaea)(CH3CN)]2+ [tiaea = tris(2-isopropy-
laminoethyl)amine and teaea = tris(2-ethylaminoethyl)amine],
with NO and were reported to exhibit the d-d transition at
640 nm and 605 nm, respectively.16 The d-d transitions for the
[CuII–NO] intermediate was found to appear at 660 nm and
595 nm in cases of [Cu(amepy)2]2+ and [Cu(aeta)2]2+ [amepy =
108.8(4)
—
—
2-aminomethyl pyridine; aeta
= bis-(2-aminoethyl)amine],
respectively.28 The intensity of the band corresponding to the
[CuII–NO] intermediate decreases with time indicating the for-
mation of the Cu(I) species following pseudo-first order kinetics
(see ESI†). The rate constant was calculated to be 4.48 × 10−1
min−1 at 298 K. Similarly, the d-d band for complexes 2 and 3
were observed to be shifted from 575 nm and 582 nm to 622 nm
and 652 nm, respectively, on purging nitric oxide in acetonitrile
solvent indicating the formation of [CuII–NO] intermediates
(Fig. 3 and ESI†). The decomposition rate constants were found
2930 | Dalton Trans., 2012, 41, 2927–2934
This journal is © The Royal Society of Chemistry 2012

View Article Online
complexes clearly indicates the existence of [CuII–NO] rather
than Cu(I). Similar observations were reported for [Cu(tiaea)
(CH3CN)]2+, [Cu(teaea)(CH3CN)]2+, [Cu(amepy)2]2+ and
[Cu(aeta)2]2+, respectively.16,28 It would be worth mentioning
here that the structurally characterized [CuII–NO] complex, [Cu
(CH3NO2)5(NO)][PF6]2, was also found to be EPR silent owing
to the antiferromagnetic coupling between the paramagnetic
centers present in the molecule.29
In solution FT-IR spectroscopic studies of the acetonitrile sol-
utions of complexes after purging NO, a new band was found to
appear at 1632, 1630 and 1634 cm−1 in cases of complexes 1, 2
and 3, respectively, corresponding to the vibration of NO coordi-
nated to the Cu(II) center (Fig. 5 and ESI†). These νNO of [CuII–
NO] were found to disappear with time indicating the unstable
nature of the intermediate (Fig. 5 and ESI†). Thus, the appear-
ance of the band at ∼1632 cm−1, supports the formation of the
[CuII– NO] prior to the reduction of Cu(II) centers in the present
cases. In case of [Cu(tren)(CH3CN)]2+ [tren = N,N-bis(2-ami-
noethyl)ethane-1,2-diamine] complex, the νNO of [CuII–NO]
was found to appear at 1650 cm−1 in acetonitrile.16 In cases of
[Cu(amepy)2]2+ and [Cu(aeta)2]2+, the νNO frequency appears at
1642 and 1635 cm−1, respectively, in acetonitrile.28 It should be
noted that for the air-stable solid copper-nitrosyl of copper(II)-
dithiocarbamate, the νNO for the NO coordinated to copper
Fig. 3 UV-visible spectra of the reaction of complex 3 with nitric
oxide in acetonitrile solvent at room temperature. (Blue trace represents
the CuII-species, black represents that of the [CuII–NO] intermediate
which gradually reduced to CuI-species represented by down headed
arrow). Inset: Time-scan plot for the decomposition of [CuII–NO] at
room temperature.
to be 9.30 × 10−1 min−1 and 36.67 × 10−1 min−1 at 298 K, in
cases of complexes 2 and 3, respectively. Thus, the order of the
rate of decomposition of the [CuII–NO] intermediate in cases of
complexes 1, 2 and 3 is 3 > 2 > 1. This is attributed to the effect
of bulk of alkyl substitution on the ligand framework. For
[Cu(tiaea)(CH3CN)]2+ and [Cu(teaea)(CH3CN)]2+, the order of
appears at 1682 cm−1 30
In contrast, for [Cu(CH3NO2)5(NO)]
.
[PF6]2, the νNO frequency was reported to appear at 1933 cm−1
in nujol mull. The higher νNO frequency in case of [Cu(CH3
NO2)5(NO)][PF6]2 might be resulted from the bent geometry
[Cu1–N1–O1) 121.0(3)°] of the nitrosyl ligand at an equatorial
site.29
rate constants was [Cu(tiaea)(CH3CN)]2+
>
[Cu(teaea)
(CH3CN)]2+ at 298 K indicating the effect of bulk of N-alkyl
group on the ligand framework.
The reduction of copper(II) centers in the present study has
been further authenticated by the isolation of the reduced
complex, [Cu(CH3CN)4]ClO4. Since the crystal structure of
[Cu(CH3CN)4]ClO4 has already been reported by Soregh et al.,
attempt were not made to determine the single crystal structure
of [Cu(CH3CN)4](ClO4).31
Though complexes 1, 2 and 3 were found to be EPR active in
acetonitrile solvent, the green intermediates obtained after their
reaction with nitric oxide were found to be EPR silent at 298 K
(Fig. 4 and ESI†). This is attributed to the formation of diamag-
netic [CuII–NO] intermediate in all the cases. It should be noted
that the complete reduction of Cu(II) center by nitric oxide will
also result into EPR silent Cu(I) species. However, in the present
cases, the presence of the d-d band of the intermediate
It would be worth mentioning here that Vagliasindi et al. has
reported a series of the interaction of nitric oxide with copper(II)
Fig. 4 X-band EPR spectrum of complex 3 in acetonitrile before (blue trace) and after (red trace) purging NO at room temperature.
This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 2927–2934 | 2931

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Scheme 1
Fig. 6 ORTEP diagram of ligand L1// [a (−x, y, 1/2−z) symmetry trans-
formation is implied by each additional atom level] (50% thermal ellip-
soid plot).
nitrosation at the ligand frameworks (Table 4). Ford et al.
reported earlier that the copper(II) center in [CuII(DAC)]2+ {DAC
= 1,8-bis(9-anthracylmethyl) derivative of the macrocyclic tetra-
amine cyclam (1,4,8,11-tetraazacyclotetradecane)} in methanol
solution in presence of a base by nitric oxide resulted with a con-
comitant mono-N-nitrosation of the ligand. Similar observations
were reported for Cu(Ds-AMP)2 and Cu(Ds-en)2 which gave
mono-N-nitrosation of the DS-en and DS-AMP ligands.34 On
the other hand, the reaction of [Cu(tiaea)(CH3CN)]2+ and
[Cu(teaea)(CH3CN)]2+ with nitric oxide were found to afford tri-
N-nitrosation of the ligand.16 From detail quantitative and theor-
etical studies of [CuII(DAC)]2+, it has been established that the
reaction proceeds through a pathway analogous to the inner-
sphere mechanism for electron transfer between two metal
centers through a bridging ligand. In this case, NO behaves as
the reductant, Cu(II), the oxidant and the coordinated amido
anion behaves as the bridging ligand. The preference of Cu(I) for
tetrahedral coordination and the decreased donor ability of the
nitrosated ligand resulted in the demetallation of the macrocyclic
ring after the reduction. An example of such a mechanism was
found in the reaction of [Ru(NH3)6]3+ with NO in alkaline
solution to yield the Ru(II)-dinitrogen complex, [Ru(NH3)5
(N2)]2+.35
Fig. 5 Solution FT-IR spectra complex 1 in acetonitrile (blue trace rep-
resents solution IR of complex 1, red represents that immediately after
purging nitric oxide which shows sharp peak at 1632 cm−1 and green
represents that after 5 min).
Table 4
R
/
//
Me
Et
L1/, 16%
L1//, 17%
L2/, 8%
L3 , 5%
L2//, 24%
L3 , 30%
iBu
complexes of small peptides coming from the N-terminal prion
protein octa-repeat region. In aqueous solutions, Cu-Ac-
HGGG-NH2 and Cu-Ac-PHGGGWGQ-NH2 systems at ∼pH
7.5, the reduction of copper(II) centers were observed in presence
of NO source.32 Spectral studies suggested that these reductions
were probably mediated by the formation of a labile Cu(II)-NO
adduct.32 However, for an analogous dinuclear system, Cu2-Ac-
(PHGGGWGQ)2-NH2, the NO interaction study in aqueous sol-
utions at physiological pH suggested that the reduction of
copper(II) proceeds through a complicated pathway involving
two different intermediate species and a positive cooperativity
between two copper centers was observed.33
On the contrary, with [Cu(tiaea)(CH3CN)]2+ and [Cu(teaea)
(CH3CN)]2+, the reduction was found to proceed through the for-
mation of a thermally unstable [CuII–NO] intermediate and tri-
nitrosation at the ligand was observed.16 Similar observations
were reported for [Cu(amepy)2]2+ and [Cu(aeta)2]2+, also.28 This
mechanism, in fact, is more close to that of ferriheme reduction,
where the nitrosation would be the one involving the initial
nitric oxide coordination to the Cu(II) center to form [CuII–NO
↔ CuI–NO+] followed by amine deprotonation and migration of
NO+ to the coordinated amide to result into the
nitrosoamine.36–42 Subsequently, demetallation from the ligand
The reduction of the copper(II) centers by nitric oxide in the
present set of complexes were found to be accompanied with N-
2932 | Dalton Trans., 2012, 41, 2927–2934
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View Article Online
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cally prior to the reduction.
It is interesting to note that though in cases of [Cu(tiaea)
(CH3CN)]2+ and [Cu(teaea)(CH3CN)]2+, exclusively tri-nitrosa-
tion of the ligand was observed; in the present study, both the
mono- and di-nitrosation of the ligand were found (Scheme 1)
with almost 65% of un-reacted ligand in each cases. All the
nitrosation products were isolated and characterized completely
(experimental section). It should be noted that the free ligands
do not react with nitric oxide in the experimental condition to
afford the N-nitrosation products.
Z. Tyekllr, Chapman
& Hall, Inc., New York, 1993, pp 397;
The formation of di-nitrosation product in case of complex 1
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//
mination. The ORTEP diagram of L1 is shown in Fig. 6. It has
crystallographically imposed twofold symmetry.
It is interesting to note that the yield of mono and di-nitrosa-
tion products obtained in cases of complexes 1, 2 and 3 are in
the order of 1 > 2 ≈ 3 and 3 > 2 > 1, respectively. Hence, it is
evident that the yield of di-nitrosation product increases on
moving from methyl, ethyl to isobutyl group on N-substitution
of the ligand. Perhaps, the better electron donor ability of the
group at N-substitution, facilitates more than one nitrosation at
the secondary amine centers of the ligand. The formation of the
[CuII–NO] intermediate prior to the reduction of copper(II) to
copper(I) might also have some role to control the degree of
nitrosation.
9 (a) J. S. Stamler, Cell, 1994, 78, 931; (b) M. S. Feelisch, T. Rassaf,
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Conclusion
The nitric oxide reactivity of three copper(II) complexes of
bidentate amine ligands were studied in acetonitrile solvent. All
the complexes afforded thermally unstable [CuII–NO] intermedi-
ate on reaction with nitric oxide in acetonitrile solution followed
by the reduction of copper(II) centers to copper(I). The reduction
was resulted with a simultaneous mono- and di-nitrosation at the
secondary amine sites of the ligand. Similar N-nitrosation was
12 (a) J. Torres, C. E. Cooper and M. T. Wilson, J. Biol. Chem., 1998, 273,
8756; (b) J. Torres, D. Svistunenko, B. Karlsson, C. E. Cooper and M.
T. Wilson, J. Am. Chem. Soc., 2002, 124, 963.
observed
with
[Cu(tiaea)(CH3CN)]2+
and
[Cu(teaea)
(CH3CN)]2+; however, in these cases exclusive tri-nitrosation
was observed. On the other hand, in the cases of [CuII(DAC)]2+,
Cu(Ds-AMP)2 and Cu(Ds-en)2, where the reduction took place
through intermediate amide formation in presence of base, exclu-
sive mono-nitrosation was found. The ratio of the yield of mono-
and di-nitrosation product, in the present case, is found to be
dependent on the N-substitution present in the ligand
framework.
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and M. P. Verbeet, Biochemistry, 2004, 43, 10467.
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Acknowledgements
17 SMART, SAINT and XPREP, Siemens Analytical X-Ray Instruments Inc.,
Madison, Wisconsin, USA, 1995
18 G. M. Sheldrick, SADABS: software for Empirical Absorption
Correction, University of Gottingen, Institut fur Anorganische
Chemieder Universitat, Tammanstrasse 4, D-3400 Gottingen, Germany,
1999–2003
The authors sincerely thank the Department of Science and
Technology, India for financial support; DST-FIST for X-ray dif-
fraction facility. MS would like to thank UGC, India for provid-
ing the scholarship.
19 G. M. Sheldrick, SHELXS-97, University of Gottingen, Germany, 1997
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