Mononuclear copper(ll) aspirinate or salicylate complexes with methylimidazoles as biomimetic catalysts for oxidative dealkylation of a hindered phenol, oxidation of catechol and their superoxide scavenging activities

Article abstract of DOI:10.1016/j.inoche.2011.02.029

Mononuclear complex bis(aspirinato) bis(2-methylimidazole) copper(II), Cu(asp)₂ (2-MeIm)₂ (1), has been synthesized and spectroscopically characterized. The biomimetic catalytic activities of this complex and our previously characterized complexes, Cu(asp)₂ (1,2-MeIm)₂ (2), Cu(Hsal)₂ (1,2-MeIm)₂ (3), and Cu(sal)(2-MeIm)₃, (4) [H₂sal = salicylic acid and MeIm = methylimidazole], for the oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone and the oxidative dealkylation of 2,4,6-tri-tert- butylphenol to 2,6-di-tert-butyl-1,4-benzoquinone and 4,6-di-tert-butyl-1,2- benzoquinone as the main products are reported. Complexes 1 and 2 are found to be potent SOD mimics and their SOD activities are compared with those obtained previously for complexes 3 and 4.

Full text of DOI:10.1016/j.inoche.2011.02.029

Inorganic Chemistry Communications 14 (2011) 759–762
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Inorganic Chemistry Communications
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Mononuclear copper(ll) aspirinate or salicylate complexes with methylimidazoles as
biomimetic catalysts for oxidative dealkylation of a hindered phenol, oxidation of
catechol and their superoxide scavenging activities
⁎
A. Latif Abuhijleh
Chemistry Department, Birzeit University, P.O. Box 14, West Bank, Palestine
a r t i c l e i n f o
a b s t r a c t
Article history:
Mononuclear complex bis(aspirinato) bis(2-methylimidazole) copper(II), Cu(asp)2 (2-MeIm)2 (1), has been
synthesized and spectroscopically characterized. The biomimetic catalytic activities of this complex and our
previously characterized complexes, Cu(asp)2 (1,2-MeIm)2 (2), Cu(Hsal)2 (1,2-MeIm)2 (3), and Cu(sal)(2-
MeIm)3, (4) [H2sal=salicylic acid and MeIm=methylimidazole], for the oxidation of 3,5-di-tert-
butylcatechol to the corresponding o-quinone and the oxidative dealkylation of 2,4,6-tri-tert-butylphenol
to 2,6-di-tert-butyl-1,4-benzoquinone and 4,6-di-tert-butyl-1,2-benzoquinone as the main products are
reported. Complexes 1 and 2 are found to be potent SOD mimics and their SOD activities are compared with
those obtained previously for complexes 3 and 4.
Received 21 July 2010
Accepted 26 February 2011
Available online 5 March 2011
Keywords:
Mononuclear copper(II) aspirinate
Catecholase
Oxidative dealkylation of phenol
SOD mimics
© 2011 Elsevier B.V. All rights reserved.
Salicylic acid and its derivatives including aspirin (acetylsalicylic
acid) are non-steroidal anti-inflammatory, anti-pyretic and analgesic
drugs. Copper complexes of anti-inflammatory drugs have been found
to be more potent and desirable drugs than their parent ligands
themselves [1,2]. In addition, these complexes are found to have several
pharmacological effects, which include anti-inflammatory, anti-convul-
sant, anti-ulcer and anti-cancer activities [1–5]. Some of these
pharmacological activities are enhanced in the presence of ancillary
nitrogen donor ligands such asimidazoles, diimines and pyridines [6–9].
Copper is one of the most ubiquitous transition metal (in addition
to Iron and Zinc) present in several metalloenzymes and is involved in
a large number of functions and processes [10]. Copper containing
enzymes are utilized for electron transfer (galactose oxidase, azurins,
and laccases), for oxidation and oxygenation reactions (polyphenol
oxidase, ascorbate oxidase, and hemocyanin) and for superoxide
dismutation (Cu,Zn-superoxide dismutase). Binary and ternary
copper complexes of anti-inflammatory drugs, such as aspirin and
other salicylate derivatives, have been used as biomimetics of some
copper containing enzymes [3,6–9,11–13]. In continuation of our
work on copper complexes biomimetics, we report here the synthesis
and spectral characterization of bis(aspirinato)bis(2-methylimida-
zole)copper(II),Cu(asp)2(2-MeIm)2(1) and studied the biomimetic
activities of this complex and our previously characterized complexes
(Scheme I), Cu(asp)2(1,2-MeIm)2 (2) [11a], Cu(Hsal)2(1,2-MeIm)2
(3) and Cu(sal)(2-MeIm)3 (4) [12a] (H2sal=salicylic acid), as
catalysts for oxidation of 3,5-di-tert-butylcatechol (DTBCH2) to
corresponding o-benzoquinone (DTBQ) and for oxidative dealkylation
of a hindered 2,4,6-tri-tert-butylphenol (TTBP). In addition, superox-
ide dismutase (SOD) mimetic activities of copper(II) aspirinate
complexes 1 and 2 were measured and compared to those activities
obtained previously for copper(II) salicylate complexes 3, 4 and for Cu
(II) complexes with other anti-inflammatory drugs.
The mononuclear complex (1) [14] was prepared by interacting of 2-
methylimidazole with binuclear Cu2(aspirinate)4 [15]. Its magnetic and
spectral measurements results are summarized as follows: Magnetic
moment=1.89 BM, Uv–visible spectrum in methanol solution showed
the d–d transitions at 684 nm (ε=95 L mol−1cm−1), IR spectrum
showed stretching frequencies of the acetoxy carbonyl moiety of the
aspirinate ligand at about 1765 cm−1 and those of the anti-symmetric
ν
asy (COO) and symmetric νsy (COO) of carboxylate group at 1595 and at
1390 cm−1, respectively. The ESR spectral data for methanol–toluene
solution at 77 K are g//=2.27, g⊥=2.05 and A//=158×10−4 cm−1
.
These magnetic and spectral data are in the range expected for
mononuclear copper(II) carboxylate complexes that contain the
CuN2O2…O2 chromophore in a tetragonally distorted geometry
[8,9,11–13]. In complex 1 two nitrogen atoms from two 2-methylimi-
dazole molecules and two caboxylate oxygen atoms from two aspirinato
molecules coordinated in Cu(II) plane. The second aspirinato carbox-
ylate oxygen atoms interact weakly in the axial positions to form
CuN2O2…O2 chromophore. These spectral data are also comparable
with those obtained for the bis (salicylato) bis (1,2-dimethylimidazole)
copper(II) complex (3) [12a] and bis(valproato) bis(2-methylimida-
zole) [11c] whom X-ray structures were determined and have
CuN2O2…O2 chromophore.
The catalytic activities of the complexes for the air oxidation of 3,5-
di-tert-butylcatechol (DTBCH2) to corresponding o-quinone (DTBQ)
⁎
Tel.: +972 97022982146; fax: +972 9702982084.
E-mail address: latif@birzeit.edu.
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.02.029

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A.L. Abuhijleh / Inorganic Chemistry Communications 14 (2011) 759–762
Scheme I. Chemical structures of mononuclear copper(II) complexes.
were followed spectrophotometrically by monitoring the absorbance
increase of DTBQ formation at 400 nm (ε=1800 M−1 cm−1) as
function of time. Methanol solution of copper complex (0.3 mL of
0.001 M) previously saturated with oxygen and 2.0 mL of a methanol
solution (0.1 M) of DTBCH2 were combined in a 1 cm quartz cell at
room temperature and the absorbance changes at 400 nm were
recorded for the first 15 min of the reaction. Complexes 1–4 catalyze
the oxidation of DTBCH2 to corresponding o-benzoquinone(DTBQ).
The oxidation activities of the complexes were measured as micro-
moles of substrate (DBPQ) produced per mg catalyst per min. They
are: 0.58 for 1, 0.57 for 2, 0.58 for 3 and 0.69 for 4.
It has been accepted now that the catecholase activity of
mononuclear copper(II) complexes follows the mononuclear pathway
as we and other researchers showed previously [12b,13,16,17].
DTBCH2 binds to Cu(II) after its dehydrogenation to form Cu(II)–
DTBC complex followed by an internal electron transfer to form Cu(I)-
o-semiquinone intermediate species. Oxidation by aerobic oxygen
occurs to produce `o-benzoquinone (DTBQ) and Cu(II) complex as
shown in the following equations:
rates would require equatorial coordination to copper(II) in order to
maximize overlap between the catecholate donor and the half empty
dx2–y2 copper(II) orbital to form a [Cu(II)(DTBC)−2] complex. An
intramolecular electron transfer from the coordinated ligand DTBC−2
to copper(II) resulting in Cu(I)-o-semiquinone intermediate species
[Cu(I) (DTBSQ)−] which is in equilibrium with [Cu(II) (DTBC)−2
]
complex (Eq. (2)). The copper (I) semiquinone species is oxidized by
air oxygen to produce o-benzoquinone (DTBQ) and Cu(II)complex
(Eq. (3)). The identification of the oxidation product (DTBQ) and the
formation of copper(I)- 3,5-di-t-butyl o-semiquinone intermediate
during the oxidation process was demonstrated in this study by
following the Uv–visible spectral changes of the catalytic reaction
mixture as we described previously [13].
Complexes 1–4 were used to catalyze the oxidative dealkylation
reaction of 2,4,6- tri-tert-butylphenol (TTBP) to 4,6-di-tert-butyl-1,2-
benzoquinone (o-TTBQ) (A) and 2,6-di-tert-butyl-1,4-benzoquinone
(p-TTBQ) (B) as major products (Scheme II).
It has been found that the oxidative dealkylation of TTBP by
transition metal complexes proceed through the formation of TTBP
radical followed by formation of metal-superoxo intermediate species
which is used to oxygenate the TTBP radical [18–20]. For copper(II)
complexes (1–4) under investigations this is presented by Eqs. (4)–
(6) described below. The oxygenation reaction of TTBP through the
initial formation of the 2,4,6-tri-tert-butylphenoxyl radical was
followed spectrophotometrically. An oxygen saturated methanol or
CH2Cl2 solution of copper(II) complex (0.3 mL of 1×10−3 M) was
mixed with 2 mL (0.05 M) of TTBP dissolved in methanol or CH2Cl2
and the formation of phenoxyl radical was followed at 400 nm, since
TTB phenoxyl radical showed bands in visible region at about 634 nm,
400 nm and 382 nm [18,21].
h
i
CuðIIÞcomplex + DTBCH2→ CuðIIÞ ðDTBCÞ−2 complex
ð1Þ
ð2Þ
h
h
i
CuðIIÞ ðDTBCÞ−2 electron transfer ½CuðIÞ ðDTBSQÞ−
ꢀ
i
CuðIÞ ðDTBSQÞ oxidation CuðIIÞcomplex + DTBQ
ð3Þ
In complexes under investigation the aspirinate or salicylate
ligands dissociate and provide sites on the copper(II) plane for
catecholate bonding . Catecholate is a stronger ligand than carbox-
ylate and replacement occurs by dehydrogenating the DTBCH2
hydroxyl groups (Eq. (1)). Optimal intramolecular electron transfer
The first step of the reaction may involve deprotonation of
relatively weak O―H phenolic group of TTBP by the aspirinate or
salicylate groups of the Cu(II) complex and the coordination of the
deprotonated TTBP− to Cu (II) complex (Eq. (4)). An internal electron
Scheme II. Oxidative dealkylation reaction of TTBP catalyzed by copper(II) complex.

A.L. Abuhijleh / Inorganic Chemistry Communications 14 (2011) 759–762
761
transfer from TTBP− to Cu(II) occurs and Cu(I)-phenoxyl radical
Table 1
Superoxide dismutase mimetic activity of copper(II) complexes.
species is formed (Eq. (5)). Oxidation of Cu(I)-species with oxygen
produces Cu(II)-superoxo intermediate species (Cu(II)―O−2 •) which
is used to oxygenate the rearranged TTB phenoxyl radical at ortho and
at para-positions along with a rapidly elimination of t-BuOH
molecules to form 4,6-di-tert-butyl-1,2-benzoquinone (o-TTBQ) (A)
and 2,6-di-tert-butyl-1,4-benzoquinone (p-TTBQ) (B) as major
products (Eq. (6)). The formation of Cu(II)-superoxo species has
been recently demonstrated which is able to effect oxygenation,
including TTBP and hydroperoxylation of phenols, by incorporation
oxygen atom derived from the Cu(II)―O−2 • moiety[20].
Copper complex
IC50
References
Cu(asp)2(2-MeIm)2 (1)
Cu(asp)2(1,2-MeIm)2 (2)
Cu(Hsal)2(1,2-MeIm)2 (3)
Cu(sal)(2-MeIm)3 (4)
Cu(Hsal)2(benzimidazole)2
Cu(sal)(phenanthroline)
Cu2 (Hsal)4
0.81
0.95
0.65
0.53
0.74
1.01
1.30
2.13
0.23
1.97
0.04
This work
This work
11a
11a
9
9
2
2
Cu2 (asp)4
Cu2(Indomethacin)4(DMF)2
Cu2(Tolfenamate)4(DMF)2
Cu,Zn-SOD enzyme
24
25
2
CuðllÞcomplex + TTBP→½CuðllÞ ðTTBP−Þ ꢀ
ð4Þ
ð5Þ
½CuðllÞ ðTTBP−Þꢀ electron transfer ½CuðlÞTTBP:ꢀradicalspecies
Superoxide dismutase activity was assayed, as described previ-
ously [7], using the xanthine–xanthine oxidase for the production of
superoxide and iodophenyl–nitrophenyl–phenyltrazolium salt (INT)
reduction at 510 nm for the superoxide detection. A 50% inhibition
reduction of INT (the IC50 value) for complexes 1 and 2 was obtained
from the plot of percentage of inhibition versus the complex
concentration as shown in Fig. 1 for complex 1. The IC50 values are
summarized in Table 1 along with other IC50 values obtained
previously for copper(II) complexes with other ant-inflammatory
drugs [2,9,11a, 23–25]. The SOD-like activity of copper(II) complexes
is a function of several factors which we recently discussed [7,12].
Among them is the presence of groups in the active site of Cu(II)
complex which are capable to have electrostatic interactions or to
form hydrogen bonding with the coordinated superoxide radical
which will result in an increase of SOD activity [12,23]. This may
explain the relatively higher SOD activities (lower IC50 values) of
copper(II) salicylate complexes 3 and 4 compare to those for the
aspirinate complexes 1 and 2. The presence of salicylate hydroxyl
group (in complexes 3 and 4) which is capable of forming hydrogen
bonding with the O2•− anion that may stabilize its coordination to
copper ion, which will result in SOD activity enhancement for these
complexes. This may also explain higher SOD activity exhibited by
binary Cu2(salicylate)4 complex compared with that of binary Cu2
(aspirinate)4 complex, Table 1. The little higher SOD activity exhibited
by 2-methylimidazole adduct 1 compared to corresponding activity
exhibited by 1,2-methylimidazole adduct 2 may be attributed to the
presence of free polar N―H group of 2-methylimidazol ligand in the
former adduct. The group is capable to have electrostatic interaction
with the O2•− anion that may assist in its induction to copper ion
active site which will result in SOD activity enhancement for this
complex. The IC50 values exhibited by complexes 1 and 2 (Table 1) fall
in the lower end of the range (0.17–29 μM) previously reported for
copper(II) complexes with salicylate derivatives [2,7,9]. This IC50
range was used therapeutically for anti-inflammatory agents in
human and veterinary medicine [2,9,24]. In addition, complexes 1
and 2 are potent SOD mimics considering their low molecular weight
when compared to that of the native Cu,Zn-SOD enzyme. They are
easily formed with biologically active aspirinate and methylimidazole
ligands.
½CuðlÞTTBP:ꢀ oxygenation; O2; Hþ CuðllÞcomplex + o−TTBQ
+ p−TTBQ + 2t−BuOH
ð6Þ
It has been recently shown that the ortho and para resonance
forms of the TTB phenoxyl radical are the predominate forms with
para resonance is the largest contributor [21b]. This may explain that
the main products of the oxidative dealkylation of TTBP by the copper
(II) complexes are the o-TTBQ (A) and the p-TTBQ (B) in about 20%
and 35%, respectively. These products [A and B] are obtained in larger
scale, separated and characterized by IR, Uv–visible and NMR
spectroscopy [22].
The catalytic activities of the mononuclear Cu(II) complexes 1–4
were determined as micromoles TTB phenoxyl radical produced per
mg catalyst per minute by monitoring the absorbance increase of
radical formation at 400 nm (ε in methanol=2000 M−1 cm−1) [21c]
of the above reaction mixture for the first 15 min. These values are:
0.879 for 1; 0.595 for 2; 0.601 for 3; and 1.134 for 4.
The catecholase oxidation of DTBC and the TTB phenoxyl radical
production activities for complex 4 are higher than those for
complexes 1–3 which showed comparable catecholase activities,
with an activity of complex 1 for TTBP phenoxyl radical production is
little higher than those for complexes 2 and 3. This may be attributed
to similarities in structures of complexes 1–3 having distorted
tetragonal CuN2O2…O2 chromophore while complex 4 has distorted
square pyramidal geometry with CuN3O+O chromophore. One of the
axial positions at this complex is free and can initially accommodate
the incoming catechol or phenol ligands faster than in complexes 1–3.
In addition, the salicylate dianion chelate in complex 4 is a stronger
base than the aspirinate or salicylate monoanions in complexes 1–3
and can dehydrogenate the incoming catechol or phenol faster and
this will result in higher catalytic activities for this complex.
100
90
80
70
60
50
40
30
20
10
0
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