Nitric oxide reactivity of copper(II) complexes of bidentate amine ligands

Article abstract of DOI:10.1016/j.ica.2015.02.013

Two copper(II) complexes, 1 and 2 with L₁ and L₂ [L₁ = propane-1,3-diamine; L₂ = N-isopropylpropane-1,3-diamine], respectively, were synthesized and characterized structurally. In acetonitrile solution of the complexes, the Cu(II) centre was found to reduce in presence of nitric oxide gas. The formation of [Cu^II-NO] intermediate prior to the reduction of Cu(II) center was evidenced by UV-Vis, solution FT-IR, X-band EPR studies. This reduction led to the ligand transformation through diazotization at primary amine site in complex 1; whereas, N-nitrosation at the secondary amine site of the ligand was observed in 2. The final organic products were isolated and characterized by spectroscopic studies.

Full text of DOI:10.1016/j.ica.2015.02.013

Accepted Manuscript
Nitric oxide reactivity of copper(II) complexes of bidentate amine ligands
Aswini Kalita, Vikash Kumar, Biplab Mondal
PII:
S0020-1693(15)00095-X
http://dx.doi.org/10.1016/j.ica.2015.02.013
ICA 16431
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
31 December 2014
12 February 2015
13 February 2015
Please cite this article as: A. Kalita, V. Kumar, B. Mondal, Nitric oxide reactivity of copper(II) complexes of
bidentate amine ligands, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.02.013
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Nitric oxide reactivity of copper(II) complexes of bidentate amine ligands
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Aswini Kalita, Vikash Kumar and Biplab Mondal*
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India
E-mail: biplab@iitg.ernet.in; Phone: 91-361-258-2317; Fax: 91-361-258-2339
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Abstract
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Two copper(II) complexes, 1 and 2 with L1 and L2 [ L1 = propane-1,3-diamine ; L2 = N-
isopropylpropane-1,3-diamine], respectively, were synthesized and characterized structurally.
In acetonitrile solution of the complexes, the Cu(II) centre was found to reduce in presence of
nitric oxide gas. The formation of [CuII-NO] intermediate prior to the reduction of Cu(II)
center was evidenced by UV-visible, solution FT-IR, X-band EPR studies. This reduction led
to the ligand transformation through diazotization at primary amine site in complex 1;
whereas, N-nitrosation at the secondary amine site of the ligand was observed in 2. The final
organic products were isolated and characterized by spectroscopic studies.
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Keywords: Copper(II) complexes, nitric oxide, Cu(II)-nitrosyl, N-nitrosation

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Introduction
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Interaction of nitric oxide (NO) with transition metal ions has been a subject of interest since
the discovery of bioregulatory roles of NO in mammalian biology [1-16]. Much of these
activities are attributed to the interactions of NO with metal ions leading to the formation of
nitrosyl complexes of metallo-proteins [1-6]. In this context, iron-nitrosyls have been studied
extensively [17-33]. The reduction of Cu(II) centres in cytochrome c oxidase and laccase by
NO is known for a long time [34-41]. A number of Cu(II) complexes have been utilized
recently to exemplify the reduction of Cu(II) by NO. For example, in [Cu(dmp)2(X)]2+ (dmp
= 2,9-dimethyl-1,10-phenanthroline, X = solvent) and analogous complexes, Cu(II)
undergoes reduction by NO. Detailed mechanistic study revealed that the reduction proceeds
through a inner-sphere pathway [42-43]. The reduction was resulted in the nitrosation of the
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solvent resulting into methylnitrite or NO2 in case of methanol or water, respectively [42-
43]. In contrast, in methanol solution, [CuII(DAC)]2+ {DAC = 1,8-bis(9-anthracylmethyl)
derivative of the macrocyclic tetraamine cyclam (1,4,8,11-tetraazacyclotetradecane)} in
presence of NaOEt reacts with NO leading to the reduction of Cu(II) center with a
simultaneous nitrosation of the ligand [44-45]. From detail quantitative and theoretical
studies, a pathway analogous to the inner-sphere mechanism for electron transfer was
suggested where NO is the reductant, Cu(II) is the oxidant and the coordinated amido anion
behaves as the bridging ligand (Scheme 1).

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+
2+
R
R
N
N
KOH
H
H
CuII
+
H2O
+
OH-
CuII
N
N
N
N
H
N
N
NO
R
R
R
ka
R
+
N
N
H
+
CuI
N
H
N
CuI
N
NO
N
N
NO
N
R
R
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Scheme 1
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On the other hand, for the reduction of Cu(II) by NO, another mechanism involving the initial
NO coordination to the Cu(II) center to form [CuII-NO ↔ CuI-NO+] was suggested by
Wayland [46-47]. Subsequent amine deprotonation and migration of NO+ to the coordinated
amide would result into the nitrosoamine.
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Recently, with [CuII(tren)(CH3CN)]2+, [CuII(taea)(CH3CN)]2+, [CuII(tiaea)(CH3CN)]2+,
[Cu(pymea)2]2+ and [Cu(baea)(CH3CN)]2+ [tren = tris-(2-aminoethyl)amine; taea = tris-(2-
ethylaminoethyl)amine; tiaea = tris-(2-isopropylaminoethyl)amine; pymea = pyridine-2-
methylamine and baea = bis-(2-aminoethyl)amine], the reduction of Cu(II) center by NO was
found to proceed through the formation of a thermally unstable [CuII-NO] intermediate [48-
50]. However, in the reaction of Cu(II) complexes of ppmea and mimpea [ppmea, 2-(pyridin-
2-yl)-N-((pyridin-2-yl)methyl)ethaneamine; mimpea, N-((methyl-1H-imidazol-2-yl)methyl)-
2-(pyridine-2-yl)ethanamine], with NO, the formation of an [CuII–NO] complex has not been
observed prior to the reduction [51]. This is attributed to the much lower values of the
equilibrium constants, KNO (equation 2) as reported earlier in case of [Cu(dmp)2(X)]2+ [43].

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KNO
[(L)2CuII(NO)]2+
[(L)2CuII]2+
NO
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--------- (eq.1)
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In addition, in case of [Cu(mtad)]2+ [mtad = 5,5,7,12,12,14-hexamethyl-1,4,8,11-
tetraazacyclotetradecane], the reduction takes place in methanol in presence of sodium
methoxide through a deprotonation pathway as reported earlier in case of [Cu(DAC)]2+ [52].
However, [Cu(tmd)2]2+, tmd = 5,5,7-trimethyl-[1,4]-diazepane], facile reduction was
observed in dry acetonitrile through a [CuII-NO] intermediate. Hence, the ligand frameworks
definitely have a significant role in controlling the mechanistic pathway for the reduction of
copper(II) by NO.
In this context, two copper(II) complexes with ligands L1 and L2 [L1 = propane-1,3-diamine
; L2 = N-isopropylpropane-1,3-diamine] (Figure 1) have been prepared and their interactions
with NO have been studied.
N
H
H2N
NH2
NH2
L2
L1
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Figure 1: Ligands used for the present study.
Experimental
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Materials and methods
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All reagents and solvents of reagent grade were purchased from commercial sources and used
as received except specified. Ligand L1 was procured from Sigma Aldrich and used as
received. L2 was synthesized from the reaction of L1 with acetone followed by the reduction

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with NaBH4. Acetonitrile was distilled from calcium hydride. Deoxygenation of the solvent
and solutions was effected by repeated vacuum/purge cycles or bubbling with nitrogen for 30
minutes. NO gas was purified by passing it through a KOH and P2O5 column. UV-visible
spectra were recorded on a Perkin Elmer Lambda 25 UV-visible spectrophotometer. FT-IR
spectra were taken on a Perkin Elmer spectrophotometer with samples either prepared as KBr
pellets or in solution in sodium chloride cell. Solution electrical conductivity was measured
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using a Systronic 305 conductivity bridge. H-NMR spectra were recorded in a 400 MHz
Varian FT spectrometer. 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
temperature and 77 K with microwave power, 0.998 mW; microwave frequency, 9.14 GHz
and 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.
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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 MoKα 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/frame. The SMART software
was used for data acquisition. Data integration and reduction were undertaken with SAINT
and XPREP software [53]. Multi-scan empirical absorption corrections were applied to the
data using the program SADABS [54]. Structures were solved by direct methods using
SHELXS-97 and refined with full-matrix least squares on F2 using SHELXL-97 [55]. All
non-hydrogen atoms were refined anisotropically. Structural illustrations have been drawn
with ORTEP-3 for Windows [56-57].
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Synthesis
Ligand L1: Propane-1, 3-diamine (L1) was purchased from commercial source.
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Ligand L2: Propane-1, 3-diamine (740 mg) was allowed to react with acetone at - 40 °C for 5
h and then temperature was elevated to room temperature to result in the formation of
corresponding imine. It was then reduced by using 2.1 equivalent of NaBH4 in methanol
(scheme 2).
(1) stirred, 5h, - 40oC
O
N
H
NH2
H2N
NH2
+
(2) NaBH4 (2.1 eqv.)
1 eqv.
1 eqv.
L1
L2
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Scheme 2
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L2: Yield: 986 mg (~85%). Elemental analyses: calcd. (%): C, 62.01; H, 13.88; N, 24.11;
found (%): C, 62.11; H, 13.88; N, 24.19. FT-IR (in KBr): 3420, 2829, 1648, 1568, 1474,
1
1325, 1037 cm-1. H-NMR: (400 MHz, CDCl3): δppm: 3.19 (1H, N-H), 2.59-2.52(3H, m),
2.45 (2H, t), 1.81(2H, s), 1.44 (2H, m), 0.85 (6H, d). 13C-NMR: (100 MHz, CDCl3) δppm:
48.6, 45.0, 40.2, 33.7 and 22.6. Mass (M+H+)/z: calcd: 117.1313; found: 117.1445.
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Synthesis of complexes:
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Both the complexes were prepared by using similar protocol. The details are given for
complex 1.
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Complex 1:
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To a stirring solution of copper(II) perchlorate hexahydrate, Cu(ClO4)2·6H2O (0.741 g, 2
mmol) in 15 ml methanol, solution of ligand L1 (0.296 g, 4 mmol) in 10 ml methanol was

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added dropwise. The reaction mixture was allowed to stirr for 2 h at room temperature. The
blue solid pracipate of complex 1 was filtered and dried in vaccuo. Yield: 0.660 g (~80%).
UV-vis. (acetonitrile): λmax, 570 nm. FT-IR (KBr pellet): 3211, 1587, 1141, 1112, 1087, 496
cm-1. EPR data: gǁ, 2.334, g⊥, 2.013 and Aǁ, 203×10-4 cm-1. The complex 1 behaves as 1:2
electrolyte in methanol solution [Λ (S cm-1), 270]. The observed magnetic moment is found
M
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to be 1.64 BM. τ, 0.17.
Complex 2:
Yield: 0.690 g (~70%). UV-vis. (acetonitrile): λmax, 579 nm. FT-IR (KBr pellet): 3307, 3263,
1590, 1092, 1045, 1014, 623 cm-1. EPR data: gǁ, 2.302, g⊥, 2.010 and Aǁ, 198×10-4 cm-1 The
complex 2 behaves as 1:2 electrolyte in methanol solution [Λ (S cm-1), 265]. The observed
M
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magnetic moment is found to be 1.60 BM. τ, 0.23.
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Isolation of modified ligands:
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Both the modified ligands are isolated following same experimental procedures. The details
/
are given for L1 .
/
L1 :
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To 30 ml of degassed acetonitrile solution of complex 1 (410 mg), freshly prepared NO was
bubbled. The blue color of the solution turned green and finally becomes colorless. After
removal of excess NO the reaction mixture was opened to air and stirred for 2 h. The volume
was reduced to 5 ml and saturated solution of Na2S was added to ensure complete
precipitation of copper ions. The precipitate was filtered off and 50 ml of water was added.
The organic part was extracted using Chloroform (25 ml × 3 portions). The collected organic

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solution was dried over vacuum and subjected to column chromatography using silica gel to
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get the pure L1 . Yield: 70 mg (~55%). Elemental analyses: calcd. (%) C, 54.92; H, 13.06; N,
32.02; found (%): C, 54.98; H, 13.07; N, 32.13. FT-IR (in KBr): 3427, 2943, 1641, 1493,
1492, 694 cm-1. 1H-NMR: (400 MHz, CDCl3): δppm: 4.26 (4H, b s), 2.54-2.51 (4H, t), 2.45-
2.42 (4H, t), 1.45-1.38 (4H, m), 0.914 (1H, b s). 13C-NMR: (100 MHz, CDCl3) δppm: 47.6,
40.1 and 33.4. Mass (M+H+)/z: calcd: 132.1422, found: 132.0756.
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Isolation of modified ligand L2 :
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Yield: 95 mg (~80%). Elemental analyses: calcd. (%) C, 49.63; H, 10.41; N, 28.94; found
(%): C, 49.68; H, 10.42; N, 29.03. FT-IR (in KBr): 3418, 2930, 1564, 1386, 910, 734 cm-1.
1H-NMR: (400 MHz, CDCl3): δppm: 4.63-4.58 (1H, m), 3.56-3.53 (2H, t), 2.67-2.64 (2H, t),
2.12 (2H, b s), 1.66-1.59 (2H, m), 1.44-1.42 (6H, d). 13C-NMR: (100 MHz, CDCl3) δppm:
55.5, 40.3, 39.6, 30.3 and 21.7. Mass (M+H+)/z: calcd: 146.1215, found: 146.1279.
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Results and discussion
Complexes 1 and 2 were synthesized by the reaction of copper(II)perchlorate hexahydrate
with the respective ligands in 1:2 ratio (experimental section and scheme 2). Both the
complexes were isolated as solid. They displayed satisfactory elemental analyses
(experimental section). Further characterization of the complexes has been done using
various spectroscopic methods (experimental section and supporting information). The
formation of the complexes is authenticated by their X-ray single crystal structure
determination. Perspective ORTEP views are shown in figure 2. Crystallographic data,
important bond angles and distances are listed in tables 1, 2 and 3, respectively.

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NH2
Cu
Methanol
Stirred 2h
H2N
NH2
+
[Cu(H2O)6 ](ClO4)2
1 eqv.
(ClO4)2
H2N
H2N
NH2
2 eqv.
Complex 1
NH2
Methanol
Stirred 2h
+
[Cu(H2O)6 ](ClO4)2
1 eqv.
N
H
NH2
(ClO4)2
Cu
NH
NH
H2N
2 eqv.
Complex 2
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Scheme 3
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(a)
(b)
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Figure 2: ORTEP diagram of complexes (a) 1 and (b) 2 (50% thermal ellipsoid plot,
hydrogen atoms, perchlorate molecules and solvent molecules are omitted for clarity). For
complex 1: Bond distances/Å: Cu(1)-N(1), 1.995(6); Cu(1)-N(2), 2.020(5); Cu(1)-N(3),
2.020(6); Cu(1)-N(4), 2.005(7); Bond angles/°: N(3)-Cu(1)-N(4), 92.5(3); N(3)-Cu(1)-N(1),
88.4(3); N(1)-Cu(1)-N(2), 92.7(2); N(2)-Cu(1)-N(4), 90.4(2). For complex 2: Bond

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distances/Å: Cu(1)-N(1), 2.036(3); Cu(1)-N(2), 2.050(2); Cu(1)-N(3), 2.047(2); Cu(1)-N(4),
2.052(2); Bond angles/°: N(3)-Cu(1)-N(4), 89.6(1); N(4)-Cu(1)-N(1), 90.4(1); N(1)-Cu(1)-
N(2), 89.3(1); N(2)-Cu(1)-N(3), 91.2(1).
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In both the complexes, Cu(II) center is surrounded by four N-atoms, two from each of the
ligand unit in an overall distorted square planar geometry. The Cu-Operchlorate distances are
2.642 Å and 2.398 Å, respectively for complexes 1 and 2; which are little more than the
bonding distances [48-50, 58-59]. Average Cu-N distances in complexes 1 and 2 are 2.020 Å
and 2.050 Å, respectively. These are within the range in other reported complexes [48-50, 58-
59].
In acetonitrile solution, complexes 1 and 2 displayed absorption at 570 nm and 579 nm in the
visible region which are attributed to the d-d transitions. The small shift in λmax in case of
complex 2 compared to 1 is attributed to the increasing covalent character of σ-bond on
moving from H to isopropyl group at N-substitution [60]. Both the complexes exhibited axial
EPR spectra at 77 K corresponding to the square planar copper(II) complexes having
dx2 − y2 ground state (experimental section and supporting information) [61-64].
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Nitric oxide reactivity
To a dry and degassed acetonitrile solution of complex 1, addition of NO gas resulted in a
green intermediate. In UV-visible study, a shift of λmax of the d-d transition from 570 nm to
615 nm was observed immediately after purging NO in acetonitrile solution of complex 1
(Figure 3). This new band is assigned as the d-d band of corresponding [CuII-NO]
intermediate complex. In acetonitrile solution, [CuII-NO] intermediate of
[Cu(tiaea)(CH3CN)]2+ and [Cu(teaea)(CH3CN)]2+ [tiaea = tris(2-isopropylaminoethyl)amine

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and teaea = tris(2-ethylaminoethyl)amine], displayed d-d transition at 640 nm and 605 nm,
respectively [48-50]. In cases of [Cu(amepy)2]2+ and [Cu(aeta)2]2+ [amepy = 2-aminomethyl
pyridine; aeta = bis-(2-aminoethyl)amine], the d-d band appeared at 660 nm and 595 nm,
respectively [48-50]. The intensity of this absorption band diminished with time suggesting
the reduction of Cu(II) center to Cu(I) following pseudo-first order kinetics (Figure 3, inset).
The rate constant was calculated to be 1.028 × 10-2 s-1 at 298 K. The λmax of the d-d transition
of complex 2 was shifted from 579 nm to 608 nm in presence of NO in acetonitrile solvent
suggesting the formation of [CuII-NO] intermediate (Figure 4). The rate constant for the
reduction of Cu(II) to Cu(I) in this case was 1.099 × 10-3 s-1 at 298 K (Figure 4, inset).
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Figure 3: UV-visible spectra of complex 1 before (black trace) and after purging NO in
acetonitrile.

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Figure 4: UV-visible spectra of complex 2 before (black trace) and after purging NO in
acetonitrile.
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The faster rate of decomposition of the intermediate in case of complex 2 compared to 1 is
attributed to the presence of electron donating isopropyl group in L2. In case of
[Cu(tiaea)(CH3CN)]2+ and [Cu(teaea)(CH3CN)]2+, the order of 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 [48-50].
The d-d transition for corresponding [CuII-NO] intermediate in case of [Cu(amepy)2]2+,
[Cu(aeta)2]2+, [Cu(tiaea)-(CH3CN)]2+ and [Cu(teaea)(CH3CN)]2+ appeared at 660, 595, 640
and 605 nm [48-50]. The difference in absorption band in the visible region of [CuII-NO] for
complexes 1 and 2 compared to [Cu(amepy)2]2+ is, presumably, because of the greater chelate
ring size.
Although complexes 1 and 2 exhibit characteristic EPR signals in acetonitrile solvent; the
respective intermediates formed in their reaction with NO were EPR silent at 298 K owing to

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the anti-ferromagnetic coupling of the paramagnetic Cu(II) center with NO (Figure 5 and
supporting information). Further, complete reduction of Cu(II) center by NO also results in to
EPR silent Cu(I) solution; however, the presence of the d-d band of the intermediate
complexes indicates the existence of [CuII-NO] rather than Cu(I). It should be noted that
structurally characterized [Cu(CH3NO2)5(NO)][PF6]2 complex was reported as EPR silent
[65].
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(a)
(b)
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Figure 5: X-Band EPR spectra of complexes 1 (a) and 2 (b) before (black trace) and after
purging NO (red trace) at room temperature in acetonitrile.
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Further support of the formation of the [CuII-NO] intermediate came from the solution FT-IR
studies. Complexes 1 and 2 in acetonitrile solution displayed the formation of new stretching
bands at ~1638 and 1635 cm-1, respectively, after their reaction with NO (Figure 6,
supporting information). These are attributed to the stretching frequencies of NO coordinated
to Cu(II) centers. The intensity of these bands diminished with time indicating unstable
nature of the intermediates. For [Cu(tren)(CH3CN)]2+ [tren = N,N-bis(2-aminoethyl)ethane-
1,2-diamine], νNO stretching appeared at 1650 cm-1 in acetonitrile [49-51]. On the other hand,

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the ν
frequency appears at 1642 and 1635 cm-1 in cases of [Cu(amepy)2]2+ and
NO
[Cu(aeta)2]2+, respectively, in acetonitrile [48-50].
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Figure 6: FT-IR spectra of complex 1 after purging NO (pink trace) and gradual decay of the
peak at 1638 cm-1 in acetonitrile at room temperature.
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For [Cu(CH3NO2)5(NO)][PF6]2, it appear at 1933 cm-1 in nujol mull. This higher ν
NO
frequency in case of [Cu(CH3NO2)5(NO)][PF6]2 can be attribute to the combined effect of
nature of ancillary ligands attached to the metal center and bent geometry [Cu1-N1-O1 )
121.0(3)°] of the nitrosyl ligand at an equatorial site [65].
The reduction of Cu(II) center by NO in complex 1 was associated with concomitant
diazotization of the primary amine center of the ligand which resulted into the formation of
/
L1 (scheme 3). The diazotization of primary amines by during the reduction of Cu(II) by
nitric oxide was observed earlier also [48-50].
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However, N-nitrosation at the ligand framework was observed in case of complex 2 under
similar reaction condition (scheme 4). This is attributed to the better nucleophile character of
secondary amine compared to the primary one.

16
2+
NH2
Cu
H
N
NO
H2N
NH2
Cu(I)
H2N
H2N
NH2
CH3CN
(1)
279
280 Scheme 4
2+
NH2
NO
NH Cu
H2N
NH
Cu(I)
N
NH2
CH3CN
NO
(2)
281
28 2 Scheme 5
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The organic products were isolated and characterized by the regular spectroscopic studies.
Conclusion
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In conclusion, the reactivity of NO with two Cu(II) complexes in acetonitrile has been
studied. The formation of unstable [CuII-NO] intermediate prior to the reduction of Cu(II)
was observed upon addition of NO in acetonitrile solution of the complexes. The stability of
the [CuII-NO] intermediate formed depends on the chelate ring size and substitution present
in the ligand frameworks. The reduction was resulted into modification of the ligand through
diazotization or N-nitrosation of the ligand frame works.
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Acknowledgement

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The authors would like to thank the Department of Science and Technology, India for
financial support; DST-FIST, India for X-ray diffraction facility. AK would like to thank
CSIR, India for providing the scholarship.
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Reference
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Highlights
ꢀ Cu(II) complexes of propane-1,3-diamine and N-isopropylpropane-1,3-diamine are
prepared.
389
390
391
392
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394
ꢀ Reduction of the Cu(II) centre of the complexes are observed by nitric oxide in
acetonitrile solution.
ꢀ Spectroscopic studies revealed the formation of [CuII-NO] intermediate prior to the
reduction of Cu(II).
ꢀ This reduction led to the ligand modification in case of both the complexes.
395

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Graphical abstract
2+
NH2
H
N
NO
H2N
NH2
Cu(I)
H2N Cu
H2N
NH2
CH3CN
(1)
401
2+
NH2
NO
NH Cu
H2N
NH
Cu(I)
N
NH2
CH3CN
NO
(2)
402
403
404
405
406
407
Addition of nitric oxide in acetonitrile solution of copper(II) complexes of ligands L1 and L2
[ L1 = propane-1,3-diamine ; L2 = N-isopropylpropane-1,3-diamine], resulted in the
formation of [CuII-NO] intermediate prior to the reduction of Cu(II). This reduction led to
the ligand transformation through diazotization at primary amine site in complex 1; whereas,
N-nitrosation at the secondary amine site of the ligand was observed in 2.
408