Copper(II) mediated phenol ring nitration by nitrogen dioxide

Article abstract of DOI:10.1039/c5dt02318a

Cu(ii) complexes of N₂O₂ type ligands, L¹H₂ and L²H₂ [L¹H₂ = 6,6′-(((pyridin-2-ylmethyl)azanediyl)bis(methylene))bis(2,4-di-tert-butylphenol); L²H₂ = 2,4-di-tert-butyl-6-(((3-(tert-butyl)-2-hydroxy-5-methylbenzyl)(pyridin-2-yl-methyl)amino)methyl)phenol], have been synthesized. Addition of nitrogen dioxide (NO₂) in THF solutions of the complexes resulted in the nitration at the 4-position of a coordinated equatorial phenolate ring of the ligand frameworks. This nitration did not occur at the phenol ring which is axially coordinated to the metal center. Spectroscopic evidence suggests that the reaction proceeds through a phenoxyl radical complex formation.

Full text of DOI:10.1039/c5dt02318a

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Copper(II) mediated phenol ring nitration by
nitrogen dioxide†
Cite this: DOI: 10.1039/c5dt02318a
a
a
b
b
Vikash Kumar, Somnath Ghosh, Anoop Kumar Saini, Shaikh M. Mobin and
a
Biplab Mondal*
1
2
1
Cu(II) complexes of N
2
O
2
type ligands, L H
2
and L H
2
[L H
2
= 6,6’-(((pyridin-2-ylmethyl)azanediyl)-
2
bis(methylene))bis(2,4-di-tert-butylphenol); L H
2
= 2,4-di-tert-butyl-6-(((3-(tert-butyl)-2-hydroxy-5-
methylbenzyl)(pyridin-2-yl-methyl)amino)methyl)phenol], have been synthesized. Addition of nitrogen
Received 18th June 2015,
Accepted 19th October 2015
dioxide (NO ) in THF solutions of the complexes resulted in the nitration at the 4-position of a co-
2
ordinated equatorial phenolate ring of the ligand frameworks. This nitration did not occur at the phenol
DOI: 10.1039/c5dt02318a
ring which is axially coordinated to the metal center. Spectroscopic evidence suggests that the reaction
proceeds through a phenoxyl radical complex formation.
www.rsc.org/dalton
−
the decomposition of ONOO to oxidize substrates or induce
Introduction
2
3
nitration.
Tyrosine nitration by NO − ions was reported to be induced
In human physiology, tyrosine nitration has attracted consider-
2
able interest as it can alter the protein functions and thereby by Cu–Zn SOD through
a
Fenton like mechanism
2
4,25
can be used as a biomarker for various disease states caused (Scheme 1).
Initially, the Cu(II) ion forms the key Cu(II)–
1
–7
−
by oxidative stress. Tyrosine nitration is believed to occur in hydroperoxo complex in the presence of H O /HO . The
2
2
2
−
the presence of either peroxynitrite (ONOO ) ions or nitrogen hydroperoxo complex then decomposes to generate Cu(I),
8
–10
•
II •
dioxide (NO2).
NO2 generation can take place via several which in the presence of H O produces OH and/or Cu – OH
2 2
•
II •
mechanisms such as reaction of NO with O , oxidation of via a Fenton like reaction (Scheme 1). The OH and Cu – OH
2
−
nitrite (NO
2
) by H
2
O
2
and peroxidases or by decomposition of radicals then induce the formation of NO
It is widely accepted that the tyrosine from NO2− and tyrosine, respectively, and lead to the nitration
nitration by NO occurs through an initial formation of tyro- of tyrosine.
sine radicals. The roles of peroxidases, heme and heme-
containing proteins in catalyzing the nitration are well esta- of N O type ligands, L H and L H [L H = 6,6′-(((pyridin-2-
blished. A number of examples of tyrosine/phenol nitration ylmethyl)azanediyl)bis(methylene))bis(2,4-di-tert-butylphenol);
assisted by various metal complexes have been reported in L H
recent years. Though the copper ion is known to favor the benzyl)(pyridin-2-yl-methyl)amino)methyl)phenol] (Fig. 1),
generation of free radicals which cause oxidative damage to leading to the nitration of the phenol ring.
proteins and lipids, the role of Cu(II) ions in nitration has not
2
and tyrosyl radicals
ONOO− ions.
10–15
2
1
0
2
Here we report the interaction of NO with Cu(II) complexes
1
2
1
2
2
2
2
2
1
0
2
2
=
2,4-di-tert-butyl-6-(((3-(tert-butyl)-2-hydroxy-5-methyl-
1
6
been studied extensively, especially with respect to mechanistic
1
7,18
studies.
As such, most of the copper mediated tyrosine
nitration studies have focused on copper–zinc superoxide dis-
mutase (Cu–Zn SOD), which, apart from scavenging super-
−
oxide anions, has been shown to catalyze the oxidation of NO
2
1
9–22
to NO
2
.
Earlier studies have shown that Cu(II) can catalyze
a
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039,
India. E-mail: biplab@iitg.ernet.in
b
Department of Chemistry, Indian Institute of Technology Indore, Indore 452017,
India
†
Electronic supplementary information (ESI) available: Spectral data and crystal-
lographic data. CCDC 1058729–1058732. For ESI and crystallographic data in
Scheme 1 Tyrosine nitration through a Fenton like mechanism (L rep-
CIF or other electronic format see DOI: 10.1039/c5dt02318a
resents a ligand framework).
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and the phenolate ion. The acetate ion is coordinated to the
metal center from an equatorial position in a monodentate
fashion. The other two corners of the square plane are occu-
pied by two N atoms from the ligand framework and thereby
the square pyramidal geometry around the central Cu(II) ion is
completed. The discrete Cu(II) unit with the coordinated
ligand resembles the structural models of the active site of the
2
7–39
galactose oxidase enzyme.
It should be noted that the
average distances for other reported structural models of active
sites of galactose oxidase are in good agreement with the
27–39
Fig. 1 Ligands used in the present study.
observed metric parameters in the case of complex 1.
In the UV-visible spectroscopy, complex 1 display an
intense absorption band centered at ∼470 nm with a weaker
d–d transition near 675 nm in acetonitrile solution. The band
centered at 470 nm is assigned to the phenolato → Cu(II)
Results and discussion
charge transfer, while the bands at lower wavelengths are
1
The ligand L H
2
was prepared following a previously reported
26,40
attributed to the intra-ligand transitions.
The λmax of the
2
6
2
procedure. L H was prepared by a Mannich reaction invol-
2
phenolato → Cu(II) charge transfer was shifted to 458 nm in
methanol (Fig. 2). In the case of complex 2, the transitions
appeared at 480 and 680 nm in acetonitrile. In methanol,
the phenolato → Cu(II) charge transfer band of complex 2
appeared at 470 nm (ESI†).
ving the corresponding amine, phenol and formaldehyde in
ethanol (Experimental section). Both the ligands displayed
satisfactory elemental analyses (Experimental section). They
1
13
were characterized by FT-IR, H-NMR, C-NMR, and mass
spectroscopy. Complexes 1 and 2 were prepared by the reaction
of copper(II) acetate dihydrate with an equivalent amount of
the respective ligands in an acetonitrile : dichloromethane
(1 : 1) mixture (Scheme 2).
The formation of complexes 1 and 2 was established by
elemental analysis, UV-visible, FT-IR, and X-band EPR spectro-
scopic studies (Experimental section). Further, the single
crystal X-ray structure of complex 1 was determined (ESI†). The
2
6
structure of complex 1 has been reported previously. In the
mononuclear complex, the Cu(II) ion is coordinated to the
1
−
mono-deprotonated ligand (i.e. L H ) in a distorted square
1
2
pyramidal geometry. Out of the two phenol arms of L H , one
is deprotonated to the corresponding phenolate ion and the
other remains in the phenol form. The phenol is coordinated
to the Cu(II) center from an axial position, whereas the phenol-
2
6
ate ion is coordinated equatorially. The axial Cu–Ophenol dis-
tance is 2.404 Å, whereas the equatorial Cu–Ophenolate distance
is 1.900 Å. It is to be noted that the Cu–Ophenol distance is
more than the average Cu–Ophenolate distances owing to the (i)
Fig. 2 UV-visible spectra of complex 1 in different solvents {THF (black
axial elongation and (ii) electrostatic interaction between Cu(II) trace), acetonitrile (red trace) and methanol (green trace)}.
Scheme 2 Synthesis of complex 1.
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The X-band EPR spectra of the complexes were recorded solution in two steps: the abstraction of a hydrogen atom from
both in the solid state and in solution at 77 K. The crystalline the phenolic OH group by NO to form a phenoxyl radical fol-
2
1
6
complexes 1 and 2 were dissolved in methanol and the EPR lowed by the reaction of the phenoxyl radical with NO
spectra were recorded. The calculated g , g and A are 2.329, However, the free ligands under the reaction conditions are
.072 and 177.255 × 10 cm , respectively. For complex 2, not found to afford corresponding nitrophenols significantly
2
.
k
⊥
k
−
4
−1
2
−
4
−1
2
these values are 2.316, 2.065 and 172.236 × 10 cm , respecti- (<5%). On the other hand, the reactions of NO with Cu(II)
vely. The gzz/Azz ratio is found to be characteristic of a copper complexes of [2,4-di-tert-butyl-6-(((2-dimethylamino)ethyl)(iso-
center(II) within a square-pyramidal geometry with axial propyl)amino)methyl)phenol and 6,6′-(((2-(dimethylamino)-
2
6,40
coordination.
It also reveals the presence of only one ethyl)azanediyl)bis(methylene))bis(2,4-di-tert-butylphenol)],
respectively, were reported recently to result in the reduction
species in the solution.
4
1
Addition of NO in methanol solutions of complexes 1 and of the Cu(II) center. In these cases, UV-visible, EPR and
resulted in green complexes, 3 and 4, respectively. Complexes
and 4 were isolated as pure solids and characterized structu- reduction of the Cu(II) center by NO
2
1
2
3
H-NMR spectroscopic studies unambiguously confirmed the
. The reduction resulted
2
rally. The ORTEP diagrams are shown in Fig. 4. The crystallo- in the nitration of the phenol ring of the ligand frameworks in
+
graphic data and the metric parameters are given in Tables 1, a mechanism where the nitronium ion (NO
2
) is generated,
2
and 3, respectively. Characterization reveals that in com- which attacks at the 4-position of the phenol ring.
plexes 3 and 4, the equatorial phenolate ring of the ligands
However, in the present case, UV-visible and EPR spectro-
has undergone nitration at 4-position (Scheme 3). Phenols are scopic studies did not show any indication of the reduction of
known to afford nitrophenols in their reaction with NO
2
in the Cu(II) center in the reaction of complexes 1 and 2 in metha-
nol with NO2 (ESI†). Thus, the possibility of a mechanism
2
involving the reduction of Cu(II) by NO followed by the for-
mation of nitronium ions can be ruled out. In that case, one
could think of the possibility of a NO2 induced phenoxyl
radical formation in the reaction medium. In methanol solu-
tion, no intermediate was trapped in UV-visible spectroscopy
even at −80 °C or in the X-band EPR spectral study of the
frozen reaction mixture even at the very early stage of the reac-
tion. But in dry and degassed THF solution of complex 1 at
Table 1 Crystallographic table for complexes 3, 4 and 5
Complex 3
Complex 4
Complex 5
Formulae
Mol. wt.
Crystal
C
34
H
45
N
3
O
6
Cu
C
31
H
39
N
3
O
6
Cu
C
34
48 4 9
H N O Cu
655.27
Monoclinic
613.19
Monoclinic
720.30
Monoclinic
system
Space group
P2(1)/n
P2(1)/n
P2(1)/n
−
80 °C temperature, addition of NO resulted in a transient
2
Temperature/ 293(2)
K
Wavelength/Å 0.71073
150(2)
293(2)
pale brown intermediate. The phenolate → Cu(II) charge trans-
fer transition band centered at ∼470 nm gradually disappears
with concomitant appearance of bands at 365, 380 and
1.5418
0.71073
15.7058(9)
10.0428(6)
23.1982(13)
90.00
92.868(5)
90.00
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å
Z
14.4864(7)
13.3750(2)
14.3438(3)
17.9158(4)
90.00
95.880(2)
90.00
13.9322(5)
17.6786(6)
90.00
100.459(4)
90.00
3508.7(2)
4
3
95 nm. These have been assigned as the charge transfer
4
0
bands for the 2,4-ditertiarybutyl phenoxyl radical. The inten-
sity of the band centered at ∼675 nm decreases, but does not
disappear, indicating the existence of the Cu(II) state in the
intermediate.
3
3419.03(12)
4
3654.5(4)
4
Density/Mg
m
1.240
1.191
1.309
It would be worth mentioning here that addition of NO2
gas in THF at −80 °C does not show similar absorption bands
(ESI†). The intermediate complex decomposes to complex 3
with a strong absorption band at ∼385 nm which is attributed
−
3
Abs. co-eff./
mm
Abs.
0.667
1.243
0.654
−
1
Multi-scan
Multi-scan
Multi-scan
correction
F(000)
Total no. of
reflections
Reflections, I
40
to the π → π* of nitrophenol moiety (Fig. 3). On the other
1388
6181
1292
6580
1532
6420
2
hand, upon the addition of NO in the THF solution of
complex 1 at −80 °C followed by immediate freezing of the
reaction mixture at 77 K, the solution became silent in X-band
EPR spectroscopy, suggesting the formation of a diamagnetic
intermediate (Fig. 5). This has been attributed to the signifi-
4840
5488
4434
>
2σ(I)
Max. 2θ/°
Ranges (h, k,
l)
25.00
71.39
25.00
−17 ≤ h ≤ 17,
−11 ≤ h ≤ 16,
−17 ≤ k ≤ 16,
−21 ≤ l ≤ 20
98.9
−18 ≤ h ≤ 16,
−11 ≤ k ≤ 11,
−27 ≤ l ≤ 22
99.8
−11 ≤ k ≤ 16,
cant overlap between the d
x
2
−y
2
magnetic orbital of Cu(II) with
−
21 ≤ l ≤ 19
Complete to
99.8
the half occupied π-orbital of the phenoxyl radical, leading to
4
0
2
θ (%)
the S = 0 ground state. The intermediate was found to
become decomposed instantaneously at room temperature to
result in complex 3. It is to be noted that in methanol solution
even at −80 °C, the intermediate was not observed; reaction of
Refinement
method
Full-matrix
Full-matrix
Full-matrix
least-squares on least-squares on least-squares on
2
2
2
F
F
F
2
Goof (F )
1.837
1.103
1.085
R indices [I >
0.0460
0.0652
0.0648
2
complex 1 with NO resulted in complex 3.
2
σ(I)]
R indices (all
0.0633
0.0732
0.0983
Since the formation of Cu(I) species in the reaction via
data)
reduction by NO has already been ruled out, it is more logical
2
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Table 2 Selected bond angles (°) for complexes 3, 4 and 5
Complex 3
Complex 4
Complex 5
Atoms
Angles (°)
Atoms
Angles (°)
Atoms
Angles (°)
O1–Cu1–O5
O1–Cu1–O4
O5–Cu1–N2
O5–Cu1–N1
N2–Cu1–N1
Cu1–O1–C13
Cu1–N2–C6
Cu1–N2–C18
Cu1–O4–C24
Cu1–N1–C1
O1–Cu1–N2
O1–Cu1–N1
O5–Cu1–O4
N2–Cu1–O4
O4–Cu1–N1
Cu1–O5–C33
Cu1–N2–C7
Cu1–N1–C5
87.95(9)
101.75(8)
172.48(9)
92.98(9)
83.62(9)
126.4(2)
106.6(2)
112.4(2)
113.9(2)
126.4(2)
94.04(9)
168.30(9)
99.91(8)
86.80(8)
89.58(8)
124.0(2)
108.9(2)
113.6(2)
O1–Cu1–O2
O1–Cu1–O3
O1–Cu1–N1
O1–Cu1–N2
O2–Cu1–O3
O2–Cu1–N1
O2–Cu1–N2
O3–Cu1–N1
O3–Cu1–N2
N1–Cu1–N2
Cu1–O1–C7
Cu1–O2–C14
Cu1–O3–C21
Cu1–N1–C1
Cu1–N1–C5
Cu1–N2–C6
Cu1–N2–C13
Cu1–N2–C20
99.04(8)
87.54(9)
169.20(9)
94.06(9)
98.84(8)
91.40(8)
87.69(8)
93.62(9)
172.96(9)
83.54(9)
127.0(2)
114.8(2)
124.0(2)
126.2(2)
114.1(2)
106.5(2)
108.9(2)
112.3(2)
O1–Cu1–N1
O1–Cu1–O4
N1–Cu1–N2
N1–Cu1–O5
N2–Cu1–O5
Cu1–O1–C13
Cu1–N1–C1
Cu1–N2–C18
Cu1–N2–C6
Cu1–O4–C24
O1–Cu1–N2
O1–Cu1–O5
N1–Cu1–O4
N2–Cu1–O4
O4–Cu1–O5
Cu1–N1–C5
Cu1–N2–C7
—
174.6(1)
95.4(1)
84.6(1)
90.0(1)
167.4(1)
130.3(2)
127.7(3)
112.4(3)
106.5(2)
110.0(3)
95.0(1)
89.3(1)
90.0(1)
89.1(1)
102.3(1)
113.6(3)
108.2(2)
—
Table 3 Selected bond distances (Å) for complexes 3, 4 and 5
Complex 3
Complex 4
Complex 5
Atoms
Distances (Å)
Atoms
Distances (Å)
Atoms
Distances (Å)
Cu1–O1
Cu1–N2
Cu1–N1
O5–C33
N2–C7
O4–C24
N1–C1
O6–C33
Cu1–O5
Cu1–O4
O1–C13
N2–C6
N2–C18
N1–C5
1.907(2)
2.018(2)
2.000(2)
1.272(4)
1.498(3)
1.379(4)
1.347(4)
1.224(5)
1.935(2)
2.481(2)
1.304(4)
1.485(4)
1.504(4)
1.323(4)
1.230(5)
1.228(6)
Cu1–O1
Cu1–O2
Cu1–O3
Cu1–N1
Cu1–N2
O1–C7
O2–C14
O2–H111
O3–C21
O4–C21
O5–N3
1.908(2)
2.393(2)
1.943(2)
1.995(2)
2.022(2)
1.307(3)
1.378(3)
0.80(3)
1.269(3)
1.232(3)
1.221(6)
1.230(5)
1.345(4)
1.333(4)
1.492(4)
1.506(3)
Cu1–O1
Cu1–N2
Cu1–O5
N1–C5
N1–C1
N2–C6
O2–N3
C24–O4
O7–N4
N4–O6
Cu1–N1
Cu1–O4
O1–C13
N2–C18
—
1.885(3)
2.010(3)
2.007(3)
1.349(5)
1.341(5)
1.488(5)
1.226(7)
1.393(5)
1.215(6)
1.219(7)
1.978(3)
2.432(3)
1.303(5)
1.486(5)
—
O6–N3
N1–C1
N1–C5
N2–C6
O2–N3
O3–N3
N2–C13
—
—
1
6
2 2
to believe that NO is promoting the formation of a phenoxyl tyrosine nitration. This is because the initial reaction of NO
radical in one of the phenol rings of the complex. Moreover, with tyrosine leading to the formation of tyrosyl radicals is
analysis of the reaction mixture reveals the presence of tert- slower than the other processes that NO undergoes.
2
butyl nitrite (Experimental section). During the reaction, a tert-
The thermal instability did not allow further characteriz-
butyl radical is formed and this resulted in the formation of ation of the intermediate complex. Complex 2 was also found
tert-butyl nitrite upon combination with NO (ESI†). The pres- to behave similarly. In dry and degassed THF solution, the
2
2
ence of tert-butyl nitrite also supports the involvement of the addition of NO resulted in the appearance of absorption
radical mechanism. It should be noted that in biological bands at ∼365, 380 and 395 nm with the decay of phenolato →
systems, peroxynitrite and heme-peroxidase pathways are the Cu(II) charge transfer (ESI†). The intermediate was found to be
two most widely accepted mechanisms leading to the simul- silent in X-band EPR spectroscopy at 77 K (ESI†). Upon
2
taneous formation of the tyrosyl radical and NO ; these then warming, the radical complex led to the formation of complex
combine at a diffusion controlled rate to afford 3-nitrotyrosine. 4 (Experimental section).
Other oxidants such as carbonate radicals, oxo-metal com-
plexes and hydroxyl radicals can lead to tyrosyl radical for- a phenoxyl radical, NO is expected to be reduced to NO . In
2 2
It is to be noted that while oxidizing a phenolate moiety to
−
mation; NO2 alone is found to be insufficient to promote the FT-IR spectrum of the crude reaction mixtures obtained
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Scheme 3
Fig. 4 ORTEP diagram of complexes (a) 3 and (b) 4 (50% thermal ellip-
soid plot, hydrogen atoms are omitted for clarity).
Fig. 3 UV-visible spectra of complex 1 (red trace) and after the addition
of NO in THF solution at −80 °C.
2
from complexes 1 and 2, respectively, a stretching frequency at
384 cm− was observed (ESI†). This is attributed to NO
1
−
1
3
−
stretching. However, no frequency corresponding to NO2 was
observed in any case. On the other hand, while the reaction
mixture from complex 1 was allowed to stand for a few days, a
small crop (∼10%) of the Cu(II) complex of the corresponding
−
nitrated ligand crystallized out as a nitrate (NO
3
) salt,
complex 5. The ORTEP diagram is shown in Fig. 6. The crystal-
lographic data and metric parameters are listed in Tables 1, 2
−
and 3, respectively. Presumably, the NO
the reaction resulted in HNO
2
ion formed during
+
2
after taking up one H from the
solvent methanol. HNO is known to decompose to NO at
room temperature. Thus, instead of NO , the stretching radical intermediate (blue trace) at 77 K in THF.
2
−
2
3
Fig. 5 X-Band EPR spectrum of complex 1 (black trace) and a phenoxyl
−
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In the present case, the nitration was observed only at one
phenol ring of the ligands. The crystal structures of the pro-
ducts i.e. complexes 3 and 4 revealed that in both the cases
nitration occurred at the 4-position of the phenolate ring
which is coordinated to the Cu(II) center at an equatorial posi-
+
tion. Interestingly, the nitration through the NO
2
ion for-
mation mechanism was reported to occur at the 4-position of
all the available phenol rings of the ligand frameworks. Since
the free ligands were not observed to afford appreciable
2
nitration in the presence of NO , this indirectly supports the
involvement of the Cu(II) center in the radical formation mech-
anism, though the exact role is not yet known. The selective
nitration is perhaps because of the higher probability of Cu(II)
induced formation of a phenoxyl radical at the equatorial
phenolate ring. In the case of [Cu(LH)(OAc)], the radical is
Fig. 6 ORTEP diagram of complex 5 (50% thermal ellipsoid plot, hydro-
gen atoms are omitted for clarity).
40
reported to form at the equatorial phenolate moiety.
It would be worth mentioning here that the tyrosine
nitration promoted by peroxynitrites in the presence of tran-
+
sition metal centers may undergo nitronium (NO
2
) ion for-
−
frequency corresponding to NO
3
was observed in the FT-IR
mation. However, it has not yet been confirmed that the
spectrum of the crude reaction mixture.
+
electrophilic aromatic nitration pathway involving the NO
2
The phenolate–Cu(II) charge transfer band at 470 nm of
complex 1 was reported previously to disappear on the oxi-
dation with Ce(IV) in dichloromethane/acetonitrile solution at
ion is a biologically relevant mechanism though reported
recently in model systems.
2
6,40
−40 °C though the phenoxyl radical could not be detected.
But copper(II) perchlorate hexahydrate was found to undergo a
1
rapid reaction with L H
2 2 2 3
in CH Cl /CH CN leading to the for-
Experimental
Materials and methods
mation of the Cu(II)–phenoxyl radical intermediate. However,
2
6,40
no such reactions are observed in methanol.
Another analogous tripodal ligand, LH
2
[LH
2
= 2-(tert- All reagents and solvents of reagent grade were purchased
butyl)-6-(((3-(tert-butyl)-2-hydroxy-5-methoxybenzyl)(pyridin-2- from commercial sources and used as received unless other-
ylmethyl)amino)methyl)-4-nitrophenol] with two phenolic wise specified. Acetonitrile was distilled from calcium hydride.
arms, was reported to allow the formation of a phenoxyl Methanol was dried by refluxing over iodine activated magne-
4
0
−1
radical in an equatorial phenolate moiety (Scheme 4). The sium with a magnesium loading of 5 g l . Then the dried
corresponding Cu(II) complex, [Cu(LH)(OAc)], was character- methanol was kept over 20% m/v 3 Å molecular sieves for 4–5
ized structurally. In a mononuclear complex the Cu(II) ion is days before use. THF was dried by refluxing over sodium metal
surrounded by a square pyramidal geometry like complex 1. in the presence of benzophenone. Deoxygenation of the
One-electron oxidation of [Cu(LH)(OAc)] results in the for- solvent and solutions was effected either by repeated vacuum/
•
+
mation of the [Cu(LH)(OAc)] radical complex which is EPR purge cycles or bubbling with argon for 30 minutes or using
4
0
silent. In the UV-visible spectra the formation of a sharp freeze–pump–thaw cycles. NO gas was prepared by the reac-
2
band at 410 nm was observed. This was attributed to the tion of purified NO with O followed by passage through an
2
4
0
absorption of the phenoxyl radical.
oxygen trap [Model 1000 oxygen trap; Z290246, Aldrich] to
Scheme 4
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remove extra oxygen, if any. UV-visible spectra were recorded mass was then dissolved in 50 ml of ethanol and made to react
on a PerkinElmer Lambda 25 UV-visible spectrophotometer. with 2-tert-butyl-4-methylphenol (1.64 g, 10 mmol) and formal-
FT-IR spectra of the solid samples were recorded on a Perkin- dehyde (1 g of 37% solution, 11 mmol); the reaction mixture
Elmer spectrophotometer with samples prepared as KBr was refluxed for 12 h to afford a white precipitate. The precipi-
pellets. The solution electrical conductivity was measured tate was isolated by filtration and washed with cold ethanol to
1
2
using a Systronic 305 conductivity bridge. H-NMR spectra obtain the ligand L H
were recorded on a 400 MHz Varian FT spectrometer. Chemi- from methanol to obtain pure L H
cal shifts (ppm) were referenced either to an internal standard Elemental analyses: calcd (%): C, 78.84; H, 9.22; N, 5.57;
Me Si) or to the residual solvent peaks. The X-band Electron found (%): C, 78.90; H, 9.24; N, 5.48. FT-IR (in KBr): 3417,
Paramagnetic Resonance (EPR) spectra were recorded on a 2958, 1599, 1479, 1232, 790 cm− . H-NMR: (400 MHz, CDCl
2
as white powder. It was recrystallized
2
2
. Yield: 3.50 g (∼70%).
(
4
1
1
3
):
JES-FA200 ESR spectrometer at room temperature and 77 K δppm: 10.56 (2H, s), 8.70 (1H, s), 7.70 (1H, t), 7.28 (2H, d), 7.13
with microwave power, 0.998 mW; microwave frequency, 9.14 (1H, d), 7.00 (1H, d), 6.91 (1H, s), 6.77 (1H, s), 3.82 (2H,s), 3.79
GHz and modulation amplitude, 2. Elemental analyses were (2H, s), 3.77 (2H, s), 2.24 (3H, s), 1.40 (18H, s), 1.28 (9H, s).
1
3
obtained using a PerkinElmer Series II Analyzer. The magnetic
moment of the complexes was measured using a Cambridge 140.5, 137.5, 137.3, 136.5, 129.2, 127.3, 127.2, 123.9, 123.5, 122.6,
Magnetic Balance. 122.3, 121.4, 57.0, 56.2, 55.4, 35.2, 34.9, 34.3, 31.9, 29.8, 29.8
Single crystals were grown by slow diffusion followed by the and 20.9. Mass (M + H )/z: calcd: 503.3559; found: 503.3706.
slow evaporation technique. The intensity data were collected Complex 1. To a stirred solution of copper(II) acetate di-
using a Bruker SMART APEX-II CCD diffractometer, equipped hydrate, Cu(OAc) ·H O (0.398 g, 2 mmol), in 50 ml acetonitrile
with a fine focus 1.75 kW sealed tube MoKα radiation (λ = was added a solution of the ligand L H (1.09 g, 2 mmol) in
C-NMR: (100 MHz, CDCl ) δ
156.3, 154.0, 154.0, 148.3,
3
ppm:
+
2
2
1
2
0
.71073 Å) at 273(3) K, with increasing w (width of 0.3° per 50 ml dichloromethane. The reaction mixture was stirred for
frame) at a scan speed of 3 s per frame. The SMART software 2 h. The volume of the solution was reduced under vacuum to
4
2
was used for data acquisition. Data integration and reduction ∼10 ml and kept in a freezer for 12 h to obtain complex 1 as a
4
3
were undertaken using SAINT and XPREP software. Struc- dark brown solid. Yield: 1.2 g (∼90%). UV-vis (methanol): λmax
,
tures were solved by direct methods using SHELXS-97 and 675 and 458 nm. FT-IR (KBr pellet): 3428, 2953, 1565, 1442,
2
−1
refined with full-matrix least squares on
F
using 1411, 1237, 757 cm . X-Band EPR data: g , 2.329; g , 2.072
k ⊥
4
4
−4
−1
k
SHELXL-97. Structural illustrations have been drawn with and A , 177.255 × 10 cm . The complex 1 behaves as a 1 : 1
4
5
−1
ORTEP-3 for Windows.
electrolyte in methanol solution [Λ
.57 BM. CCDC no. 1058729.
Complex 2. Complex 2 was prepared using the same proto-
. 2-Picolylamine (1.08 g, 10 mmol), 2,4-di-tert- col used for complex 1. Yield: 1.05 g (∼85%). UV-vis (metha-
M
(S cm ), 151]. μobs,
1
Synthesis
1
Ligand L H
2
butylphenol (4.12 g, 20 mmol) and formaldehyde (2 g of 37% nol): λmax, 680 and 470 nm. FT-IR (KBr pellet): 3425, 2956,
−
1
k
solution, 21 mmol) were taken in 50 ml of ethanol and the 1569, 1438, 1413, 1237, 757 cm . X-Band EPR data: g , 2.316;
4
−1
mixture was refluxed for 12 h during which a white precipitate
g
⊥
, 2.065 and A
k
, 172.236 × 10− cm . Complex 2 behaves as a
was formed. The precipitate was isolated by filtration and 1 : 1 electrolyte in methanol solution [ΛM (S cm− ), 148]. μobs
as white 1.63 BM.
Complex 3. To a degassed solution of complex 1 (500 mg) in
L H . Yield: 4.35 g (∼80%). Elemental analyses: calcd (%): C, methanol (or in THF, 20 ml), excess NO gas was purged at
1
,
1
washed with cold ethanol to obtain the ligand L H
2
powder. It was recrystallized from methanol to obtain pure
1
2
2
7
9.36; H, 9.62; N, 5.14; found (%): C, 79.39; H, 9.65; N, 5.07. room temperature. The brown colored solution became green-
−
1
FT-IR (in KBr): 2958, 2866, 1597, 1232, 1201, 757 cm
.
ish. The volume of the solution was reduced to ∼5 ml and kept
1
H-NMR: (400 MHz, CDCl ): δppm: 10.60 (2H, s), 8.71(1H, d), in a freezer to afford crystals of complex 3. Yield: (60%). UV-vis
3
7
3
.71(1H, t), 7.29 (1H, t), 7.24 (2H, s), 7.14 (1H, d), 6.95 (2H, s), (methanol): λmax, 690 and 390 nm. FT-IR (KBr pellet): 3416,
13
−1
.86 (2H,s), 3.82 (4H, s), 1.42 (18H, s), 1.30 (18H, s). C-NMR: 2957, 1583, 1281, 1104, 728 cm . X-Band EPR data: g
k
, 2.332;
−
4
−1
(
1
100 MHz, CDCl ) δ
156.4, 154.0, 148.3, 140.6, 137.4, 136.5, g , 2.085 and A , 166.984 × 10 cm . Complex 3 behaves as a
3 ppm:
⊥
k
M
25.3, 123.8, 123.6, 122.6, 121.5, 57.1, 55.6, 35.3, 34.3, 31.9, 1 : 1 electrolyte in methanol solution [Λ ,
(S cm− ), 155]. μobs
1
+
and 29.8. Mass (M + H )/z: calcd: 545.4029; found: 545.4189.
1.58 BM. CCDC no. 1058730.
Complex 4. It was prepared from complex 2 (500 mg) using
2
Ligand L H . 2-Picolylamine (1.08 g, 10 mmol) and 3,5-di-
2
tert-butyl-2-hydroxybenzaldehyde (2.34, 10 mmol) were added the same protocol used for the preparation of complex 3 from
into 50 ml of methanol. The mixture was stirred for 1 h at complex 1. Yield: ∼65%. UV-vis (methanol): λmax, 688 and
room temperature to give the corresponding imine. The imine 390 nm. FT-IR (KBr pellet): 3405, 2965, 1607, 1434, 1092,
−
1
k ⊥ k
was then reduced by 2.1 equivalents of sodium borohydride to 708 cm . X-Band EPR data: g , 2.334; g , 2.086 and A ,
give the corresponding amine. The solvent was dried under 167.447 × 10− cm . Complex 4 behaves as a 1 : 1 electrolyte
vacuum. 50 ml water was added to the crude mass and it was in methanol solution [ΛM (S cm− ), 156]. μobs, 1.64 BM. CCDC
neutralized with acetic acid. The organic component was no. 1058731.
4
−1
1
extracted from the crude solution using chloroform (50 ml × 3
Isolation of tert-butyl nitrite. For both complexes 1 and 2,
portions). The solvent was dried under vacuum. The crude the same protocol was used to isolate tertiarybutyl nitrite. The
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details are given for complex 1. 500 mg of complex 1 was dis-
solved in dry and degassed THF (or methanol, 10 ml). To this,
excess NO gas was purged at room temperature. The brown
2
4 S. Kondo, S. Toyokuni, T. Tsuruyama, M. Ozeki,
T. Tachibana, M. Echizenya, H. Hiai, H. Onodera and
M. Imamura, Cancer Lett., 2002, 179, 87.
colored solution became greenish. The reaction mixture was
allowed to stand at room temperature for 1 h. Then after
5 R. V. Mendes, A. R. Martins, G. de Nucci, F. Murad and
F. A. Soares, Histopathology, 2001, 39, 172.
removing excess NO
2
, the reaction mixture was subjected to
6 N. Fukuyama, Y. Takebayashi, M. Hida, H. Ishida,
K. Ichimori and H. Nakazawa, Free Radicals Biol. Med.,
1997, 22, 771.
7 J. S. Liu, M. L. Zhao, C. F. Brosnan and S. C. Lee,
Am. J. Pathol., 2001, 158, 2057.
column chromatography using neutral alumina. Yield, ∼50%.
The formation of tert-butyl nitrite was confirmed by ESI-
mass spectroscopy as well as by comparing the other spectral
data with the commercially available ones. CCDC no. 1058732.
Elemental analyses: calcd (%): C, 46.59; H, 8.80; N, 13.58;
8 H. Gunaydin and K. N. Houk, Chem. Res. Toxicol., 2009, 22,
894.
+
found (%): C, 46.56; H, 8.81; N, 13.66. Mass (M + H )/z: calcd:
1
04.06; found: 103.95.
Complex 5. To a degassed solution of complex 1 in metha-
9 A. van der Vliet, J. P. Eiserich, B. Halliwell and C. E. Cross,
J. Biol. Chem., 1997, 272, 7617.
2
nol (20 ml), excess NO gas was purged at room temperature. 10 R. Radi, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 4003.
The brown colored solution became greenish. The volume of 11 R. Radi, Acc. Chem. Res., 2013, 46, 550.
the solution was reduced to ∼5 ml and kept at room tempera- 12 N. B. Surmeli, N. K. Litterman, A. F. Miller and J. T. Groves,
ture for 5–6 days to afford crystals of complex 5. Yield: (∼10%).
J. Am. Chem. Soc., 2010, 132, 17174.
1
1
3 J. Olbregts, Int. J. Chem. Kinet., 1985, 17, 835.
4 J. Su and J. T. Groves, J. Am. Chem. Soc., 2009, 131,
12979.
Conclusion
1
5 K. Bian, Z. H. Gao, N. Weisbrodt and F. Murad, Proc. Natl.
Acad. Sci. U. S. A., 2003, 100, 5712.
6 J. Su and J. T. Groves, Inorg. Chem., 2010, 49, 6317;
M. P. Schopfer, B. Mondal, D.-H. Lee, A. A. Narducci
Sarjeant and K. D. Karlin, J. Am. Chem. Soc., 2009, 131,
In conclusion, the present manuscript demonstrates the NO
reactivity of two Cu(II) complexes of N O type ligands. The
2
2
2
1
reaction leads to the nitration at the 4-position of a co-
ordinated equatorial phenolate ring of the ligand frameworks.
This nitration did not occur at the phenol ring which is axially
coordinated to the metal center. Spectroscopic evidence
suggests that the reaction proceeds through a phenoxyl radical
complex formation in the presence of NO . In contrast, NO
11304; G. Y. Park, S. Deepalatha, S. C. Puiu, D.-H. Lee,
B. Mondal, A. A. Narducci Sarjeant, D. del Rio,
M. Y. M. Pau, E. I. Solomon and K. D. Karlin, J. Biol. Inorg.
Chem., 2009, 14, 1301; D. Maiti, D.-H. Lee, A. A. Narducci
Sarjeant, M. Y. M. Pau, E. I. Solomon, K. Gaoutchenova,
J. Sundermeyer and K. D. Karlin, J. Am. Chem. Soc., 2008,
2
2
reactivity of Cu(II) complexes of [2,4-di-tert-butyl-6-(((2-di-
methylamino)ethyl) (isopropyl)amino)methyl)phenol and 6,6′-
(((2-(dimethylamino)ethyl)azanediyl)bis(methylene))bis(2,4-di-
130, 6700; A. Yokoyama, K.-B. Cho, K. D. Karlin and
tert-butylphenol)] was reported to induce the reduction of
Cu(II) centers leading to the nitration of the phenol ring of the
ligand through NO2 ions.
W. Nam, J. Am. Chem. Soc., 2013, 135, 14900.
7 P. V. B. Reddy, K. V. R. Rao and M. D. Norenberg, Lab.
Invest., 2008, 88, 816.
8 D. D. Thomas, M. G. Espey, M. P. Vitek, K. M. Miranda and
D. A. Wink, Proc. Natl. Acad. Sci. U. S. A., 2002, 99,
1
1
+
Acknowledgements
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1
2
2
2
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9 S. P. A. Goss, R. J. Singh and B. Kalyanaraman, J. Biol.
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The authors thank the Department of Science and Technology
(DST), India for financial support. VK thanks CSIR, India for
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a
fellowship. The authors express their gratitude to
Dr T. K. Paine, Indian Association for the Cultivation
of Sciences, Kolkata, India for extending his help to record
UV-visible spectra at his laboratory.
3 J. S. Beckman, M. Carson, C. D. Smith and
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