Reductive nitrosylation of nickel(II) complex by nitric oxide followed by nitrous oxide release

Article abstract of DOI:10.1039/c6dt00826g

Ni(ii) complex of ligand L (L = bis(2-ethyl-4-methylimidazol-5-yl)methane) in methanol solution reacts with an equivalent amount of NO resulting in a corresponding Ni(i) complex. Adding further NO equivalent affords a Ni(i)-nitrosyl intermediate with the

Full text of DOI:10.1039/c6dt00826g

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Reductive nitrosylation of nickel(II) complex by
nitric oxide followed by nitrous oxide release†
Cite this: DOI: 10.1039/c6dt00826g
a
a
b
a
Somnath Ghosh, Hemanta Deka, Yuvraj B. Dangat, Soumen Saha,
a
b
a
Kuldeep Gogoi, Kumar Vanka and Biplab Mondal*
Ni(II) complex of ligand L (L = bis(2-ethyl-4-methylimidazol-5-yl)methane) in methanol solution reacts
with an equivalent amount of NO resulting in a corresponding Ni(I) complex. Adding further NO equi-
10
valent affords a Ni(I)-nitrosyl intermediate with the {NiNO} configuration. This nitrosyl intermediate upon
Received 1st March 2016,
Accepted 6th May 2016
subsequent reaction with additional NO results in the release of N O and formation of a Ni(II)-nitrito
2
2
complex. Crystallographic characterization of the nitrito complex revealed a symmetric η -O,O-nitrito
DOI: 10.1039/c6dt00826g
bonding to the metal ion. This study demonstrates the reductive nitrosylation of a Ni(II) center followed by
www.rsc.org/dalton
2
N O release in the presence of excess NO.
2
6–28
reducing agents.
However, reduction of Ni(II) center by NO
Introduction
10
leading to subsequent nitrosylation to form {NiNO} complexes
Transition metal ion induced activation of nitric oxide (NO) have not been explored extensively. Hayton’s group reported
10
has attracted enormous research interest as various biological examples of {NiNO} and their reactivity; however, in all the
2
9,30
reactivities are attributed for forming nitrosyl complexes of examples nickel(I)-nitrosyl complexes were utilized.
metallo-proteins and mostly iron or copper-proteins. In this
1
,2
Herein, we report the NO reactivity of a Ni(II) complex, 1 of
regard, the iron-nitrosyls have been studied extensively, both ligand L (L = bis(2-ethyl-4-methylimidazol-5-yl)methane). The
3
in protein and model systems. Ferriheme proteins are ligand framework has been chosen as it offers structural flexi-
reported to undergo reduction in aqueous media in the pres- bility and an accessible reduction potential for the transition
4
,5
ence of NO. The corresponding ferrous protein then reacts metal ions due to its π-acceptor property. In methanol solution
6
–8
with excess nitric oxide to form a stable ferroheme nitrosyl.
of complex 1, addition of NO resulted in the reductive nitro-
1
0
The reduction of Cu(II) centers in cytochrome c oxidase and sylation to yield the {NiNO} complex. This in the presence of
laccase to Cu(I) by nitric oxide has been known for a long excess NO released N
2
O with the formation of corresponding
9
–12
time.
In model systems, it has been exemplified by a Ni(II)-nitrito complex, 2.
1
3–23
number of Cu(II) complexes in recent years.
Mn(III) complex [Mn(PaPy Q)(OH)]ClO (PaPy
2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide)
2
4
2
QH = N,N-bis
(
was found to react with excess NO to afford the nitrosyl Results and discussion
2
4
2 4
complex [Mn(PaPy Q)(NO)]ClO via reductive nitrosylation.
The ligand, L was synthesized by following an earlier reported
Another example demonstrated that adding NO to the aceto-
3
1
procedure. The Ni(II) complex, 1 was prepared by stirring a
mixture of nickel(II) chloride hexahydrate with two equivalents
of ligand, L in methanol. The microanalytical data of complex
1 shows good agreement with the calculated values (Experi-
mental section). It was characterized by various spectroscopic
analyses as well (Experimental section). The single crystal X-ray
structure of complex 1 was determined. The crystallographic
data, important bond angles and distances are listed in Tables
1–3, respectively. The ORTEP view of the complex 1 is shown
in Fig. 1. The crystal structure revealed that the Ni(II) centre
was coordinated by two ligand units in a distorted square
planar geometry. The average Ni–N distance was found to be
nitrile solution of [(Mn(PaPy )) (μ-O)](ClO ) (PaPy3 = the
3
2
4 2
anion of the designed ligand N,N-bis(2-pyridylmethyl)amine-
6
N-ethyl-2-pyridine-2-carboxamide) resulted in the {Mn–NO}
2
5
nitrosyl [Mn(PaPy )(NO)](ClO ) via reductive nitrosylation.
3
4
Ni(II) in different ligand environments are known to undergo
reduction to Ni(I) either by an electrochemical procedure or by
a
Department of Chemsitry, Indian Institute of Technology Guwahati, Assam 781039,
India. E-mail: biplab@iitg.ernet.in
b
Academy of Scientific and Innovative Research, National Chemical Laboratory,
Pune 411008, Maharashtra, India
†
Electronic supplementary information (ESI) available: All spectroscopic data
1
.893 Å, which is within the range of analogous reported com-
and related DFT studies. CCDC 985705, 1456771 and 1456772. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c6dt00826g
3
2
plexes. The average N–Ni–N bite angle was ∼90.0°.
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Table 1 Crystallographic data for complexes 1, 2 and 3
1
2
3
Formulae
Mol. wt
C
26
H
44Cl
2
N
8
NiO
2
C
26
H
44ClN
9
NiO
4
C
52 2 2 7
H82Cl N18Ni O
630.30
640.86
1259.68
Crystal system
Space group
Triclinic
Pˉ1
Monoclinic
P2/c
Monoclinic
P21
Temperature/K
Wavelength/Å
296(2)
0.71073
296(2)
296(2)
0.71073
9.4536(3)
9.3089(3)
18.4419(7)
90.00
91.025(2)
90.00
1622.68(10)
2
1.312
0.71073
9.7243(4)
33.0698(17)
9.7619(5)
90.00
97.492(3)
90.00
3112.4(3)
2
1.342
a/Å
b/Å
c/Å
α/°
β/°
γ/°
10.8400(7)
11.2885(10)
14.2205(7)
85.293(5)
77.072(5)
73.587(7)
1626.57(19)
2
1.287
0.796
Multi-scan
668
3
V/Å
Z
−
3
Density/Mg m
−
1
Abs. Coeff. /mm
Abs. correction
F(000)
0.725
Multi-scan
680
0.753
Multi-scan
1328
Total no. of reflections
Reflections, I > 2σ(I)
Max. 2θ/°
6030
4214
25.50
−13 ≤ h ≤ 12
2828
2406
25.00
9833
6257
25.00
Ranges (h, k, l)
−11 ≤ h ≤ 10
−11 ≤ k ≤ 10
−21 ≤ l ≤ 21
99.0
Full-matrix least-squares on F
1.069
−11 ≤ h ≤ 11
−37 ≤ k ≤ 39
−11 ≤ l ≤ 11
98.3
Full-matrix least-squares on F
1.181
−
13 ≤ k ≤ 8
17 ≤ l ≤ 17
−
Complete to 2θ (%)
99.7
2
2
2
Refinement method
Full-matrix least-squares on F
1.032
2
Goof (F )
R indices [I > 2σ(I)]
0.0655
0.0929
1.153/−1.075
0.0599
0.0668
1.050/−0.423
0.0760
0.1297
0.837/−1.034
R indices (all data)
Largest diff. peak/hole/e Å−
3
Table 2 Selected bond lengths (Å) for complexes 1, 2 and 3
636 nm is assigned to the d–d transition for the Ni(II) center.
The cyclic voltammogram of complex 1 was recorded in metha-
nol using TBAP supporting electrolyte in a three electrode con-
1
2
3
+
figuration with a saturated Ag/Ag reference, glassy carbon
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–O(1)
O(1)–N(9)
N(1)–C(3)
N(1)–C(6)
N(2)–C(3)
N(2)–C(4)
1.895(3)
1.890(3)
—
2.077(4)
2.079(4)
2.214(4)
1.268(5)
1.326(6)
1.392(6)
1.343(7)
1.374(7)
2.050(1)
2.070(1) working and Pt auxiliary electrodes.
A
quasi-reversible
reduction at −1.05 V was observed in the voltammogram
ESI†). These are attributed to the Ni(II)/Ni(I) reduction. In the
2.290(1)
1.220(2)
1.360(2)
—
(
1.337(5)
1.397(6)
1.351(7)
1.378(7)
+
1.410(2) same setup, the NO/NO couple appeared at 1.15 V (ESI†).
1.340(2)
1.430(2)
Adding an equivalent amount of NO in the degassed
methanol solution of complex 1 at −80 °C displayed a shift in
the d–d band from 636 nm to 578 nm (Fig. 2). The shift in λmax
and change in intensity of the d–d band was attributed for
forming the corresponding Ni(I)-complex (Scheme 1).
Reduction of the Ni(II) center to Ni(I) by NO was also suggested
from the X-band EPR studies. The diamagnetic Ni(II) center,
Table 3 Selected bond angles (°) for complexes 1, 2 and 3
1
2
3
N(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(1A)
N(1)–Ni(1)–N(3A)
N(3)–Ni(1)–N(3A)
N(1)–Ni(1)–N(7)
O(1)–Ni(1)–N(1)
O(1)–Ni(1)–N(3)
Ni(1)–O(1)–N(9)
O(1)–N(9)–O(1A)
86.70 (1)
180.00(1)
93.30(1)
180.00(1)
—
—
—
—
—
89.10(1)
90.90(4)
upon adding an equivalent amount of NO became EPR active.
171.30(2)
96.80(2)
94.20(2)
—
86.20(1)
104.50(1)
95.40(4)
112.40(5)
The axial spectrum nature suggests the existence of Ni(I)
9
33
(d configuration; g , 2.2997; g , 2.1768) (Fig. 3). The EPR
∥ ⊥
98.50(4)
spectra of electrochemically generated Ni(I) complexes have
been reported earlier. For instance, Ni(I) complex of
158.70(4)
86.60(4)
89.40(8) 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane
33b
also displayed an axial EPR spectrum.
Double integration
9
using Cu(II) sulphate, pentahydrate as a standard for d elec-
tronic configuration revealed the presence of ∼80% Ni(I) (ESI†)
The complex 1 in dry methanol solution displayed absorp- in solution. On the other hand, the appearance of methyl
−
1
−1
tion bands at λmax(ε/mol cm ), 636 (25) and 398 (240) nm in nitrite in the GC-Mass spectrum of the reaction mixture also
+
the UV-Visible spectrum along with other intra-ligand tran- suggests NO ion was formed during the reaction (ESI†). This
sitions at lower wavelengths (Fig. 2; ESI†). The band centred at was found to be thermally unstable and highly sensitive
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Fig. 1 ORTEP diagram of complex 1 (50% thermal ellipsoid plot, H-atoms are omitted for clarity).
Fig. 2 UV-Visible spectra of complex 1 (black), after adding a stoichio-
metric amount of NO (red), {NiNO} intermediate (green) and complex
10
Fig. 3 X-Band EPR spectra of complex 1 (black), Ni(I) species (red) and
complex 2 (green) in methanol (concentration, 3.0 mmol) at 77 K.
2
(blue) in methanol (4.0 mmol concentration) at −80 °C.
towards air and moisture, which precluded isolation and
further characterization. Addition of one more equivalent of
NO resulted in the corresponding Ni(I)-nitrosyl intermediate
1
0
with the {NiNO} configuration (Scheme 1). This has been
monitored by UV-Visible, solution FT-IR, NMR and EPR spec-
troscopic studies. The intermediate was found to be thermally
unstable and highly sensitive towards air and moisture.
It is worth noting that though there is not much change in
the d–d absorption band position of Ni(I) species and
1
0
{
NiNO} and the shoulder observed near ∼460 nm in the case
of the Ni(I) intermediate was found to disappear after adding
NO (Fig. 2). In the FT-IR spectrum of the complex 1 methanol
solution, adding two equivalents of NO results in the appear-
ance of a new band at 1668 cm− (Fig. 4), assignable to the υNO
1
Scheme 1
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ESI mass spectral studies showed a peak at 584.28
+
suggesting formation of [Ni(L) (NO)(MeOH)] (Fig. 5). The iso-
2
tropic distribution pattern matched very well with the simu-
lated spectrum suggesting a correct assignment (Fig. 5). In
1
0
X-band EPR studies, the {NiNO} intermediate appeared to be
silent due to the anti-ferromagnetic coupling of spins of the
paramagnetic Ni(I) and NO, resulting in an S = 0 spin state
(
Fig. 3).
In the H-NMR study, the broad signals of Ni(I) species
became sharp and well resolved suggesting formation of dia-
1
1
0
10
magnetic {NiNO} (Fig. 6 and ESI†). The proposed {NiNO}
intermediate shown in Scheme 1 has been investigated
through quantum chemical calculations employing density
functional theory (DFT). The calculations were carried out
3
7
using the Turbomole 6.0 suite of programs.
Geometry
optimizations were performed using the Perdew, Burke, and
3
8
Fig. 4 FT-IR spectra of complex 1 (black), after immediate NO addition Ernzerhof density functional (PBE). The electronic configur-
(
blue); and after 30 minutes of excess NO addition (pink; N
2
O formation
ation of the atoms was described by a triple-ζ basis set aug-
−1
is indicated by the peak at 2230 cm ) in methanol at room temperature.
mented by a polarization function (Turbomole basis set
3
9
40
TZVP). The resolution of identity (ri), along with the multi-
pole accelerated resolution of identity (marij) approximations
1
0
15
corresponding to the {NiNO} intermediate. NO labelling were employed for an accurate and efficient treatment of the
shows a shift of this frequency to 1636 cm (ESI†). This fre- electronic Coulomb term in the density functional calcu-
quency is actually at the low end of values reported for nickel lations. Furthermore, the solvent effects were incorporated
nitrosyl complexes (ranging from 1568 to 1915 cm ).
−
1
4
1
−
1
34,35
42
The into the COSMO model, with ε = 32.7 for methanol. The DFT
nitrosyl stretching frequency was reported to appear at 1869 optimized structure of the proposed intermediate (Scheme 1)
and 1868 cm− in cases of [Ni(NO)(bpy)](PF
μ-S Ph )](PF ), respectively, in dichloromethane solution.
It is known that the geometry around the central metal ion based pyramidal geometry with NO coordinated to nickel. It
also dictates the IR stretching frequency of metal nitrosyls. For was observed that methanol does not bind to the nickel centre
1
) and [Ni(NO)(bpy) is shown in Fig. 7 below.
6
2
9
10
(
2
2
6
As shown in Fig. 6, the {NiNO} intermediate has a square
instance, for [Ni(NO)(bpy)](PF
6
), the nitrosyl stretching but remains in close proximity by forming a hydrogen bond
appears at 1869 cm in CH Cl , whereas at 1567 cm in case with the nitrogen of NO, as shown in Fig. 6. The NO stretching
−1
−1
2
2
of [Ni(NO)(bpy)
2 6
](PF
); on the other hand, for [Ni(NO)(bpy)] frequency calculated with PBE/TZVP was found to be
−
1
−1
−1
(PF
6
) in acetonitrile solution, it appears at 1828 cm due to 1534.3 cm , which was lower by 135 cm with respect to the
3
0
solvent coordination to the metal ion. In addition, linear experimentally observed frequency for the NO stretch. It is to
nitrosyls are known to have a higher stretching frequency com- be noted that the stretching frequencies obtained from DFT
pared to that in the bent geometry.
3
6
calculations
normally
differ
with
experiments
by
+
2 3
Fig. 5 ESI mass spectra of [Ni(L) (NO)(CH OH)] with isotropic distribution (a) experimental and (b) simulated.
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1
Fig. 6 H-NMR spectra of complex 1 (a), after purging one (b) and two (c) NO equivalents in complex 1 in CD
3
OD.
calculations at the PBE/TZVP level of theory, with solvent cor-
rections included. The intermediate complex showed three
absorption bands: (i) at 428 nm with oscillator strength of
0
5
.0035, (ii) at 521 nm with oscillator strength of 0.0036 and at
80 nm with oscillator strength of 0.0055. Therefore, the calcu-
lations indicated that the strong absorption band is at 580 nm,
which is in agreement with the experimentally observed λmax
of 578 nm (Fig. 2). Thus, the current computational investi-
gations suggest that the intermediate formed is indeed the
square based pyramidal geometry wherein the NO is co-
ordinated to the Ni(I) center, with a methanol solvent molecule
hydrogen bonded to the nitrosyl nitrogen, as shown in the
optimized structure in Fig. 6.
Fig. 7 The optimized structure, corresponding to the proposed inter-
mediate; the color scheme is as follows: nickel: green, carbon: black,
oxygen: red, nitrogen: blue and hydrogen: white; only the methanol
In the presence of excess NO, the intensity of 578 nm band
1
0
of {NiNO} was found to decay, indicating decomposition of
hydrogen is shown in the structure. The rest of the hydrogen atoms the intermediate (Fig. 2). This finally lead to formation of the
were removed for clarity.
corresponding Ni(II)-nitrito complex (2) and nitrous oxide
O) (Scheme 1). The final nitrito complex, 2 was isolated and
characterized by various spectroscopic techniques (Experi-
(N
2
50–200 cm− , as has been observed in previous reports.43–46 mental section). It behaves as a 1 : 1 electrolyte in solution
1
1
Therefore, the value can be deemed to be acceptable.
(Experimental section). The single crystal X-ray structure
In this regard, it is also noted that several other possible revealed that a Ni(II) centre is coordinated to two ligand moi-
2
structures for the intermediate have been looked into compu- eties and a (η -O,O)-nitrito group (Fig. 8). The crystallographic
tationally. This includes octahedral complexes having chlorine data, selected bond distances and angles are listed in Tables
coordinated to the nickel centre, square planar complexes 1–3, respectively.
where one of the ligands is mono coordinated, and a trigonal
The average Ni–N distances (2.078 Å) were found to bit
geometry where both the ligands are mono coordinated. These longer compared that in complex 1, perhaps because of a geo-
possible structures are shown in the ESI.† It was found that metry difference. The average N–O distance of the coordinated
none of the other structures were (a) able to form stabilized nitrito group was 1.268 Å and the Ni–O distance was 2.124 Å.
optimized minima or (b) comparable in stability (i.e. they were
2
N O formation in the reaction was confirmed by GC-mass
found to be energetically less stable) to the proposed inter- spectral analysis of the head space gas of the reaction vessel
mediate structure. This provides further indication of the val- (yield, ∼60%; ESI†). This was further confirmed by the FT-IR
idity of the proposed intermediate complex.
monitoring of the reaction mixture, which showed the appear-
Furthermore, the UV-Visible spectrum for the intermediate ance of a stretching frequency at 2230 cm− that was assign-
1
1
0
{
NiNO} complex in methanol was calculated with TD-DFT able to N O (Fig. 4). Recently, Hayton et al. reported the
2
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Fig. 8 ORTEP diagram of complex 2 (50% thermal ellipsoid plot,
H-atoms are omitted for clarity).
Fig. 9 ORTEP diagram of complex 3 (50% thermal ellipsoid plot,
H-atoms omitted for clarity).
isolation of a nickel nitrosyl, [Ni(NO)(bipy)
2
](PF
6
), which forms
2
−
II
−) complexes.54 It can be noted that the X-ray
N
2
O upon standing via a cis-[N
2 2
O ]
intermediate. In this
ponding Cu (NO
2
−
1 29,30
case, the N O stretching frequency appeared at 2228 cm
.
2
crystallography characterization of the corresponding nitrito
complexes revealed a symmetric η -O,O-nitrite binding to the
metal ions. Mechanistic studies suggested an electrophilic
2
The formation of N
2
O from NO can occur either by HNO
4
7
disproportionation or by hyponitrite intermediate formation.
One proposed mechanism involves the initial formation of a
cis-dinitrosyl complex followed by coupling of the two nitrosyl
groups and subsequent reduction to form a hyponitrite,
attack of a second NO molecule to the initially formed Cu-NO
complex resulting in Cu(NO) species, which in subsequent N–
2
N coupling and O-atom transfer obtains N
nitrito complex.
2
O and a Cu(II) ion
2
−
47,48
+
(
N O ) intermediate.
Reaction with H results in H O
2
2
2
1
0
and N
metal-nitrosyls reveals that the [M(NO)
an important role in the homogeneous solution phase
reduction of NO to N O for various transition metal ions such
as Rh, Ir, Ru, Fe and Pt.
2
O. A solid state thermal decomposition study of various
The {NiNO} intermediate was found to decompose to the
2
] motif actually plays
corresponding nitrato complex (3) in the presence of oxygen
(
Scheme 1). Iron-nitrosyls in heme models and copper-nitro-
2
syls are known to react with oxygen to afford peroxynitrite
4
9,50
−
(
ONOO ), which then isomerises to the corresponding nitrato
5
5
On the other hand, the bent nitrosyl ligands are known to
be the subject of electrophilic attack at the nitrogen lone
complexes. The decomposition product, complex 3 was iso-
lated as solid and characterized structurally. The solution con-
ductance measurement suggested a 1 : 1 electrolytic nature of
the complex. The perspective ORTEP view of complex 3 is
shown in Fig. 9.
4
7,51
pair.
There are several examples reported on the electro-
4
7,51
philic attack of free NO on a metal bound nitrosyl ligand.
For instance, [Co(NO)(NH
to generate an asymmetric hyponitrite bridged dimer. It has
also been suggested that electrophilic attack of the free NO on
that coordinated to the transition metal will result in a hyponi-
2
+
3
)
5
]
is known to react with free NO
5
1b
The single crystal X-ray structure suggests that the Ni(II)
2
centre is coordinated to two ligand moieties and a (η -O,O)-
nitrato group (Fig. 9). The crystallographic data, selected bond
distances and angles are listed in Tables 1–3, respectively. The
average Ni–N distances (2.06 Å) are shorter in comparison to
complex 2 but longer with respect to complex 1. The average
N–O distance of the coordinated nitrato group was 1.22 Å and
the Ni–O distance was 2.29 Å.
4
7
2
trite complex. This decomposes to yield N O and water. The
proposed mechanistic cycle of nitric oxide reductase activity of
cytochrome c oxidase, it is believed that initial binding of NO
to the reduced heme centre resulted in a bent nitrosyl (νNO
1
,
610 cm− ). In subsequent steps, nitrogen–nitrogen bond
1 52
formation was suggested through reaction of a second nitrosyl
5
2
group bound to the copper centre of CcO.
It would be worth mentioning that mononuclear copper(I)-
nitrosyl complexes were found to promote NO disproportiona-
tion with the Cu(II)-nitrito complex formation. For instance,
Experimental section
Materials and methods
5
3
Cu(I) complexes of bis-(phenyl) substituted tris-(pyrazole)- All reagents and solvents were purchased from commercial
hydroborate and 1,4-diisopropyl-7,2-pyridylmethyl-1,4,7-triaza- sources and were of reagent grade. HPLC grade methanol was
cyclononane were reported to catalyze the disproportionation used and dried using standard protocol. Deoxygenation of the
under an excess NO condition resulting in N O and corres- solvent and solutions were affected by repeated vacuum and
2
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purge cycles or bubbling with nitrogen or argon for this, a solution of 4.64 g of ligand L in methanol (5 mL) was
0 minutes. NO was used from a cylinder after purification by added and stirred for 1 h. The light green solution turned a
passing through KOH and P columns. Further purification deep green and after drying with a rotary evaporator and a
3
2 5
O
was carried out following reported fractional distillation green solid was obtained, which upon crystallization gave
methods. A stoichiometric amount of NO was added to the yellow crystals. Yield: 5.26 g (84%). CCDC no. 985705. Elemen-
sample using a Hamilton gas tight syringe after diluting with tal analyses: Calcd For C26
2 8
H40Cl N Ni: C, 52.55; H, 6.78; N,
argon gas. The required NO dilution was affected with argon 18.86. Found (%): C, 52.61; H, 6.77; N, 18.95. FT-IR (in KBr):
−
1
gas using an Environics Series 4040 computerized gas dilution 3435, 2980, 1643, 1468, 1432, 1319, 1287, 1089, 816, 591 cm
.
1
system. UV-Visible spectra were obtained on an Agilent HP
453 UV-Visible spectrophotometer. FT-IR spectra were 1.31(6H). C-NMR (100 MHz, CDCl
obtained on a Perkin-Elmer spectrophotometer with either the 123.8, 23.7, 21.4, 13.5, 9.0. Molar conductance: 239 S cm
H-NMR (400 MHz, CDCl
3
): δppm, 2.22(6H), 2.06(4H), 1.39(2H),
1
3
8
3
): δppm, 151.8, 131.2,
2
−
1
sample prepared as KBr pellets or in solution in a potassium mol (in methanol).
bromide cell. Solution electrical conductivity was checked
1
using a Systronic 305 conductivity bridge. H-NMR spectra
Complex 2
were obtained with a 400 MHz Varian FT spectrometer. Chemi-
In 20 mL of dry and degassed methanol solution of complex 1
180 mg), freshly prepared nitric oxide was bubbled in excess
until the deep colour of the solution faded. The reaction
mixture was placed at room temperature for crystallization.
Yield: 138.5 mg (72%). CCDC no. 1456771. Elemental analyses:
Calcd For C H ClN O Ni: C, 49.04; H, 6.33; N, 19.79. Found
cal shifts (ppm) were referenced either with an internal stan-
(
dard (Me Si) for organic compounds or to the residual solvent
4
peak. The X-band Electron Paramagnetic Resonance (EPR)
spectra were obtained 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
6
40
9 4
(
3
%): C, 49.11; H, 6.36; N, 19.92. FT-IR (in KBr): 3390, 3174,
2
. The magnetic moment of the complexes were measured on
−
1
126, 1627, 1530, 1464, 1429, 1322, 1197, 1048, 776 cm .
a Cambridge magnetic balance. Mass spectra of the com-
pounds were obtained in a Waters Q-Tof Premier and Aquity
instrument. Elemental analyses were obtained with a Perkin
Elmer Series II Analyzer.
−
1
−1
UV: λ(ε/mol cm ), 621 (32), 979 (14). Molar conductance:
60 S cm mol (in methanol), observed magnetic moment:
.93 BM.
2
−1
1
1
Single crystals were grown by slow diffusion followed by a
slow evaporation technique. The intensity data were collected Complex 3
using a Bruker SMART APEX-II CCD diffractometer, equipped
To a degassed methanol solution of complex 1 (240 mg) at
80 °C, freshly prepared nitric oxide was bubbled through for
0 s. Then, the excess NO gas was removed using four cycles of
vacuum and Ar purging. To this, an excess of dioxygen was
added through a gas tight syringe. The reaction mixture was
warmed up to room temperature and kept for two days while
complex 3 crystallized out. Yield: 128 mg (∼52%). CCDC no.
with a fine focus 1.75 kW sealed tube MoKα radiation (λ =
.71073 Å) at 273(3) K, with increasing ω (width of 0.3° per
frame) at a 3 s per frame scan speed. The SMART software was
−
3
0
5
6
used for data acquisition. Data integration and reduction
5
7
was undertaken with SAINT and XPREP software. Structures
were solved by direct methods using SHELXS-97 and refined
2
58
with full-matrix least squares on F using SHELXL-97. Struc-
9 5
1456772. Elemental analyses: Calcd For C26H40ClN O Ni:
5
9
tural illustrations were drawn with ORTEP-3 for Windows.
C, 47.84; H, 6.17; N, 19.31. Found (%): C, 47.91; H, 6.16; N, 19.40.
FT-IR (in KBr): 3168, 3127, 3126, 1625, 1531, 1464, 1429, 1384,
Ligand L synthesis
1
−1
−1
1
6
333, 1222, 1053, 839, 752, 620 cm− . UV: λ(ε/mol cm ),
2
-Ethyl-4-methyl imidazole (2.2 g; 20 mmol) was taken in a
2
−1
00 (41), 987 (11). Molar conductance: 176 S cm mol
round bottomed flask with 50 mL of methanol; to this 0.45 g
of (15 mmol) formaldehyde and an aqueous 5.1 g (91 mmol)
of potassium hydroxide solution was added dropwise. The
reaction mixture was stirred for 24 h at room temperature. A
(
in methanol), observed magnetic moment: 1.84 BM.
In situ generation of Ni(I) and Ni(I)-NO intermediates
white coloured solid was obtained. It was filtered off and washed Complex 1 (200 mg) was dissolved in 20 ml of dry methanol in
with water. After washing the solid was dried in air and then a 50 mL Schlenk flask and then the solution was degassed
placed in a desiccator overnight. Yield: 1.97 g (85%). Elemental with bubbling argon for 1 h. The solution was then cooled to
20 4
analyses: Calcd (%) for C13H N : C, 67.21; H, 8.68; N, 24.12. −80 °C and an equivalent amount of nitric oxide gas was
Found (%): C, 67.19; H, 8.68; N, 24.02. FT-IR (in KBr): 2964, purged into the solution using a gas tight Hamilton syringe.
−
1 1
2
δ
842, 1614, 1529, 1455, 1071 cm ; H-NMR (400 MHz, CDCl ): The color of the solution became light, indicating formation of
3
13
ppm, 3.53(2H), 2.44(4H), 1.87(6H), 1.08(6H). C-NMR (100 MHz, a Ni(I) intermediate. The reaction mixture was transferred into
CDCl
3
): δppm, 149.2, 129.1, 127.2, 23.3, 22.6, 13.7, 10.6.
EPR and NMR tubes using a gas tight Hamilton syringe and
the spectra were obtained.
Complex 1 synthesis
10
A similar procedure was followed to generate {NiNO}
Nickel(II) chloride hexahydrate (2.38 g) was taken in a 50 mL wherein two NO equivalents were added resulting in a blue
round bottomed flask and dissolved in 20 mL of MeOH. To solution.
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Dalton Trans.

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Paper
Dalton Transactions
G. R. A. Wyllie, M. K. Ellison and W. R. Scheidt, J. Am.
Chem. Soc., 2004, 126, 14136.
Conclusion
In conclusion, the Ni(II) complex of bis(2-ethyl-4-methyl-
imidazol-5-yl)methane, 1 in methanol solution reacted with
NO to result in reductive nitrosylation forming a corres-
ponding {NiNO} intermediate. This intermediate upon sub-
sequent reaction with an additional NO resulted in the release
8 (a) M. D. Lim, I. M. Lorkovic and P. C. Ford, J. Inorg.
Biochem., 2005, 99, 151; (b) D. Shamir, I. Zilbermann,
E. Maimon, G. Gellerman, H. Cohen and D. Meyerstein,
Eur. J. Inorg. Chem., 2007, 5029.
9 A. C. F. Gorren, E. de Boer and R. Wever, Biochim. Biophys.
Acta, 1987, 916, 38.
1
0
of N O and Ni(II)-nitrito complex formation. The crystallo-
2
graphic characterization of the nitrito complex revealed a sym- 10 J. Torres, C. E. Cooper and M. T. Wilson, J. Biol. Chem.,
2
metric η -O,O-nitrito bonding to the metal ion as observed in
1998, 273, 8756.
cases of iron, manganese and copper complexes.
11 (a) J. Torres, D. Svistunenko, B. Karlsson, C. E. Cooper and
M. T. Wilson, J. Am. Chem. Soc., 2002, 124, 963;
(b) G. C. Brown, Biochim. Biophys. Acta, 2001, 1504, 46.
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2 (a) J. Torres, M. A. Sharpe, A. Rosquist, C. E. Cooper and
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Acknowledgements
BM would like to thank Department of Science and Techno-
logy, India for financial support (SR/S1/IC-38/2010); DST-FIST
for the X-ray diffraction facility. SG gratefully acknowledges IIT
Guwahati for providing a research fellowship.
2004, 43, 10467.
13 K. M. Miranda, X. Bu, I. M. Lorkovic and P. C. Ford, Inorg.
Chem., 1997, 36, 4838.
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