Copper(II) azide complexes with NNO donor ligands: Syntheses, structure, catalysis and biological studies

Article abstract of DOI:10.1016/j.poly.2013.08.019

Three copper(II) tridentate (NNO) Schiff base azido complexes, [Cu(L ¹)(N₃)] (1), [Cu₂(L²) ₂(μ₂-1,1-N₃)₂] [Cu(L ²)(N₃)] (2) and [Cu(L³

Full text of DOI:10.1016/j.poly.2013.08.019

Polyhedron 65 (2013) 48–53
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Copper(II) azide complexes with NNO donor ligands: Syntheses,
structure, catalysis and biological studies
a
b
c
Chandan Adhikary a, , Sourajit Banerjee , Jishnunil Chakraborty , Sandra Ianelli
⇑
a Department of Chemistry, Government College of Education, Burdwan 713102, India
b Department of Chemistry, St. Paul’s Cathedral Mission College, Kolkata 700009, India
c Departimento di Chimica, Vialedelle Scienze, 17/A, 43100 Parma, Italy
a r t i c l e i n f o
a b s t r a c t
Three copper(II) tridentate (NNO) Schiff base azido complexes, [Cu(L1)(N3)] (1), [Cu2(L2)2(
l2-1,1-N3)2]
Article history:
[Cu(L2)(N3)] (2) and [Cu(L3)(N3)] (3) have been synthesized and characterized [where HL1 = 1-(N-5-meth-
oxy-ortho-hydroxyacetophenimino)-2,2-dimethyl-aminoethane], HL2 = 1-(N-ortho-hydroxyacetopheni-
mine)-2,2-diethyl-aminoethane and HL3 = 1-(N-salicylideneimino)-2-(N,N-diethyl)-aminoethane]. The
structure of complex (1) has been determined by single crystal X-ray diffraction analysis. In 1, out of four
coordination sites of copper(II) ion, three are occupied by the NNO donor atoms of a tridentate Schiff base
ligand, HL1 and the remaining site is occupied by terminal nitrogen of azido moiety. All the complexes
exhibit high catalytic activity in epoxidation reactions of a variety of olefins with tert-butyl-hydroperox-
ide in acetonitrile. The catalytic efficacy of the copper(II) complexes has been studied in different solvent
media. The antimicrobial activity of the complexes and their Schiff base ligand has also been studied.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 28 May 2013
Accepted 4 August 2013
Available online 15 August 2013
Keywords:
Copper(II)–azido complex
Schiff base ligand
X-ray crystal structure
Olefin oxidation
Antimicrobial activity
1. Introduction
has seldom been used as an oxidant in the investigations of cata-
lytic efficiency of copper(II) complexes towards epoxidation/oxida-
Schiff bases and their transition metal complexes continue to be
of interest even after over a hundred years of study [1–4]. Schiff
bases have a chelating structure and are in demand because they
are straightforward to prepare and are moderate electron donors
with easily-tunable electronic and steric effects thus being versa-
tile [5,6]. Schiff base metal complexes are still widely used in catal-
ysis but increasingly with a slightly modified concept [5,7–19].
Complexes have been immobilised on solid supports such as alu-
mina, silica, or polystyrene [20–24] or assembled inside a DNA du-
plex [25]. However, the number of studies where Schiff base
complexes especially copper(II) complexes have been used for ole-
fin oxidation in homogeneous condition has been scarcely reported
[26–29] and the interesting matter is that olefinic oxidation involv-
ing Schiff base copper(II) complexes have yet not been well docu-
mented in literature [30,31].
There are few reports of molecular oxygen catalyzed epoxida-
tion [32], and a 30% hydrogen peroxide is usually considered an
environmental friendly oxidant, nevertheless, due to explosive nat-
ure of hydrogen peroxide, the industrial processes still mainly rely
on tert-BuOOH [33]. Alkyl-hydroperoxides are used on a large scale
in industrial epoxidation, for example in Halcon-Arco and Sumito-
mo processes [34–36]. The recycling of co-product e.g. tert-BuOH
has been realized in the Sumitomo process. Notably, tert-BuOOH
tion reactions in homogeneous medium.
Here we report the synthesis, characterization, X-ray single
crystal structure analyses of azido adduct of NNO donor tridentate
Schiff base (Scheme 1) copper(II) complexes and their catalytic effi-
cacy towards epoxidation of a variety of olefins using tert-BuOOH
as an oxidant in different solvent media. The antimicrobial activity
of the complexes and their Schiff base ligands has also been
investigated.
2. Experimental
2.1. Materials
All solvents used are of AR grade and were distilled and dried
before use. 5-Methoxy-2-hydroxy-acetophenone, 2-hydroxy-ace-
tophenone, salicylaldehyde, and copper(II) nitrate trihydrate were
purchased from Merck (India) and used as received. Styrene,
a-
methylstyrene, cyclooctene, cyclohexene, tert-BuOOH (70% aq.),
N,N-diethylethylenediamine were purchased form Aldrich and
used as received.
2.2. Physical measurements
⇑
Microanalysis (CHN) was performed in a Perkin Elmer 240 ele-
mental analyzer. IR spectra were recorded on a Bruker Alpha T
Corresponding author. Tel.: +91 0342 2656853.
E-mail address: cadhikary123@gmail.com (C. Adhikary).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.019

C. Adhikary et al. / Polyhedron 65 (2013) 48–53
49
200140 FT–IR spectrometer. Absorption spectra were studied on
Shimadzu UV2100 UV–Vis recording spectrophotometer. GC anal-
ysis was carried out with an Agilent Technologies 6890N network
were measured at 293 K. All calculations for data reduction, struc-
ture solution and refinement were done by standard procedures
(SHELXS-97) [39], (SHELXL-97) [39]. All non-hydrogen atoms were re-
GC system equipped with
(30 m  0.32 mm) and a FID detector.
a
fused silica capillary column
fined anisotropically by full-matrix least-squares techniques on
F2. All the hydrogen atoms of 1 were included at calculated posi-
tions and refined isotropically. Neutral atom scattering factors
were taken from Cromer and Weber [40] and anomalous disper-
sion effects were included in Fcalc [41]. Information concerning
crystallographic data collection and refinement of the structure
are summarized in Section 2.4.1.
2.3. Synthesis
Caution! Although our samples never exploded during handling,
azide metal complexes are potentially explosive: only a small
amount of material should be prepared and it should be handled
with care.
2.5. Catalytic reactions
2.3.1. Synthesis of the Schiff base ligands HL1
The homogeneous oxidation reactions were carried out under
stirring in a two-neck round-bottom flask fitted with a water con-
denser and placed in an oil bath at 333 K. The proportions used are:
Substrate (10 mmol), solvent (10 ml) and catalyst (0.005 mmol).
The tert-BuOOH (20 mmol) was added immediately before the
start of the reaction and the mixture was stirred continuously for
24 h. At different time intervals, the products were collected from
the reaction mixture and analyzed by gas chromatography. The
products were identified against known standards.
The ligand HL1 (Scheme 1) was prepared by refluxing a metha-
nolic solution (10 cm3) of 5-methoxy-2-hydroxy-acetophenone
(0.166 g, 1 mmol) and 2-(diethylamino)-ethylamine (0.106 g,
1 mmol) for 30 min. Methanol was removed completely from the
resultant orange solution under vacuum to obtain a highly viscous
orange liquid. Upon keeping in a refrigerator (ꢀ0 °C), yellow crys-
tals separated out from the orange liquid. The crystals were iso-
lated, washed with a minimum volume of methanol, dried over
CaCl2 in a refrigerator and characterized. The crystals melt down
into the previously mentioned viscous orange liquid at room tem-
perature. Yield 0.210 g (80%). Anal. Calc. for C15H24N2O2 (264): C,
68.2; H, 9.1; N, 10.6. Found: C, 67.9; H, 9.0; N, 10.3%. IR (Nujol,
2.6. Biological activity
The biological activities of synthesized Schiff bases (HL1, HL2
and HL3) and their copper azido complexes 1, 2 and 3 have been
studied by the standard disc diffusion method [42]. The Schiff
bases and their complexes were screened for their antifungal activ-
ity against fungi viz. Aspergillus niger and Fusarium oxysporum.
These fungal species were isolated from potato dextrose agar.
The cultures of the fungi were purified by single spore isolation
technique. A concentration of 1.0 mg/ml of each compound in
DMSO solution was prepared for testing against spore germination
of each fungus. Filter paper discs of 5 mm in size, prepared by using
Whatman filter paper No. 1, sterilized in an autoclave were satu-
cm–1):
m
(azomethine) 1616. 1H NMR (300 MHz, CDCl3): (d 2.25
(m, 9H, CH3); 2.6 (t, 2H, CH2); 3.6 (t, 2H, CH2); 7.2 (m, 4H,
aromatic).
The ligands HL2 and HL3 (Scheme 1) were synthesized by apply-
ing exactly same procedure as stated above, refluxing methanolic
solution (20 cm3) of 2-hydroxy-acetophenone (0.136 g,1 mmol)
and salicylaldehyde (0.122 g, 1 mmol) with 2-(diethylamino)-eth-
ylamine (0.106 g, 1 mmol), respectively.
2.3.2. Synthesis of the complex [Cu(L1)(N3)] (1), [Cu2(L2)2(2-1,1-N3)2]
[Cu(L2)(N3)] (2) and [Cu(L3)(N3)] (3)
rated with 10 ll of the compounds dissolved in DMSO solution.
To prepare 1, methanolic solution of HL1 (1 mmol, 0.264 g) was
added to a methanolic solution of copper nitrate trihydrate
(1 mmol, 0.241 g) and stirred for 45 min. The resulting solution
upon slow evaporation afforded deep green crystals (Yield 60%).
Anal. Calc. for C15H23N5O2Cu: C, 48.8; H, 6.2; N, 19.0. Found: C,
The fungal culture plates were inoculated and incubated at 25 °C
for 48 h. The plates were then observed and the diameters of the
inhibition zones (in mm) were measured and tabulated.
The antibacterial activity of the complexes was studied against
gram-positive bacteria Staphylococcus aureus (MTCC 96) and gram-
negative bacteria Escherichia coli (MTCC 443). Each of the com-
pounds dissolved in DMSO at a concentration of 1.0 mg/ml was
prepared. Paper disc of Whatman filter paper No. 1 were cut and
sterilized in an autoclave. The paper discs were saturated with
49.4; H, 6.0; N, 19.3%. FTIR (cmÀ1): mas (N3), 2030;
and 1345; d(N3), 620;
620, 370, 297. eff. = 1.83 B.M.
m
s(N3), 1332
m
(C@N), 1620 cmÀ1. kmax/nm (methanol),
l
2 and 3 were synthesized by following the reported procedures
[37,38]. Analysis of complex (2), Anal. Calc. for C14H21N5OCu: C,
10 ll of the compounds dissolved in DMSO solution and was
49.6; H, 6.2; N, 20.7. Found: C, 49.9; H, 7.0; N, 21.3%. IR (KBr):
N3), 2055, 2031; d(N3), 640;
(C@N), 1598 cmÀ1. kmax/nm (metha-
nol), 268, 296, 385. Analysis of complex (3), Anal. Calc. for
13H19N5OCu: C, 48.02; H, 5.84; N, 21.54. Found: C, 48.2; H, 5.9;
mas(-
placed in the petridishes containing Nutrient agar media inocu-
lated with the above mentioned two bacteria separately. The pet-
ridishes were incubated at 37 °C and the inhibition zones were
recorded after 24 h of incubation.
m
C
N, 22.0%. IR (KBr):
m
as(N3), 2020 cmÀ1
;
m
.
s(N3),1320 and
1340 cmÀ1; d(N3), 610 cmÀ1
;
m
(C@N), 1630 cmÀ1
3. Results and discussion
2.4. X-ray crystallography
3.1. Description of the structure of 1
2.4.1. X-ray crystal data of [CuL(N3)] (1)
C15H23N5O2Cu, M = 368.93, monoclinic, space group P21/c,
a = 18.319 (5), b = 9.252 (2), c = 10.113 (3) Å, b = 92.53(2)°,
The principal structural features of 1 are given in Fig. 1. The
structure consists of a discrete molecule of [Cu(L1)(N3)]. The copper
ion occupies the central position of a regular square planar
arrangement. Out of its four coordination sites three are occupied
by the tridentate Schiff base through two nitrogen (amino and imi-
no) atoms and de-protonated phenoxo group. The remaining
fourth coordination site is satisfied by an azido group which is
linked to copper(II) ion through terminal nitrogen atom. The Cu–
N bond lengths are found to be Cu(1)–N(1) = 1.914(5); Cu(1)–
V = 1712.4(8) Å3, Z = 4, R1(I > 2 (I)) = 0.0709, Dcalc = 1.431 g cmÀ3
r ,
F000 = 772,
l
= 1.29 mmÀ1, T = 293 K.
2.4.2. X-ray single crystal structure determination of [CuL(N3)] (1)
Crystals of 1 were mounted on a Enraf Nonius CAD4 diffractom-
eter which was equipped with graphite monochromated Mo-K
radiation (k = 0.71073 Å). Unit cell dimensions and intensity data
a

50
C. Adhikary et al. / Polyhedron 65 (2013) 48–53
[43,44]. In most of the cases the coordination geometry around
copper ions are either pseudo octahedral or distorted trigonal bipy-
ramidal while square planar is not so common. The present com-
plex acquires a square planar environment around the copper ion
probably due to the presence of two ethyl groups attached to the
N atom in 1,1 position of ethylenediamine backbone of the Schiff
base ligand.
3.2. Catalytic activities
The catalytic activity of 1, 2 and 3 in the epoxidation of various
olefins in homogeneous medium is summarized in Table 2. The
graphical representation of various alkene conversions has been
shown in Fig. 2 for 1 (for 2 and 3, see Figs. S1 and S2 respectively
in supporting information). It is evident that all the complexes fol-
low the alkene conversion order as
rene > cyclooctene > cyclohexene while
a
-methyl styrene > sty-
epoxide selectivity
Fig. 1. Molecular structure of complex 1. All hydrogen atoms have been omitted for
clarity.
follows the
sequence cyclooctene > a-methylstyrene > sty-
rene > cyclohexene. The results show that for all complexes epox-
ide selectivity is highest in cylcooctene due to its active double
bond and the relative stability of 1,2-epoxycyclooctane. For cyclo-
hexene, epoxide selectivity is least for all complexes since its active
allylic site could also be activated in the process of oxidation. For 1
styrene has been converted 98% to its oxidized products, giving
60% yield of epoxide, while the remaining 38% products were benz-
aldehyde and benzoic acid. When 2 and 3 are considered 96% and
90% styrene conversion occurred with 54% and 49% epoxide selec-
tivity, respectively. Thus in styrene the catalytic efficacy follows
the trend 1 > 2 > 3. Similar trend is also observed for other alkenes.
The yield of epoxide increased to 65% for1 and 62% for 2 and 60%
for 3 when an electron-donating methyl group was introduced at
the ortho-position of styrene; along with the epoxide, 4-methyl-
benzaldehyde and 4-methylbenzoic acid were also formed. The dif-
R1
R
N
OH
N
ference in the conversion between styrene and
a-methylstyrene
may be attributed to the electronic effect of the substituents
[45]. From the above results it is further noticed that 1 is more effi-
cient catalyst than 2 and 3, respectively. This might be due to the
slight variation in structural morphologies of the complexes. Thus
absence of electron donating group decreases the efficiency of the
catalyst (Scheme 1). It is known that copper(II) can bind peroxo
group on treatment with peroxides [46] and thepre-catalyst spe-
cies containing LxCu-OOR (where L = ligand) type moieties are
seemed to be capable of transferring oxo functionality to the or-
ganic substrates to give the corresponding oxidized products
[47,48]. We speculate that, in our case, a similar kind of mecha-
nism is operative. The probable mechanism for the catalytic cycle
is depicted in Fig. 3.
The effect of various reaction media on epoxidation of cyclooc-
tene catalyzed by 1, 2 and 3 has also been studied. The results of
this study have been collected in Table 3 and the graphical repre-
sentation of catalytic efficiency in different solvent media for 1 has
been shown in Fig. 4 (for 2 and 3, see Figures S3 and S4 respectively
in supporting information). The best performance of the catalyst
was observed in acetonitrile medium for all complexes (Table 3).
The catalytic efficiency of all complexes follows the order: acetoni-
trile > dichloromethane > methanol > chloroform. An exactly simi-
lar trend was reported by Bera et al. [30] in the study of
cyclooctene catalyzed epoxidation of single end on azido bridged
Schiff base copper(II) complex.
C2H5
C2H5
1 1
R OCH HL
R=CH
,
=
:
3
3
R=CH3,R1=H:HL2
R=H,R1=H:HL3
Scheme 1. Schematic representation of the ligands.
N(2) = 2.020(5); Cu(1)–N(3) = 1.905(7) Å while the Cu(1)–O(1)
bond length is 1.844(4) Å. The azide group, which acts as a mono-
dentate ligand is slightly asymmetric [N(3)–N(4) = 1.193(8) and
N(4)–N(5) = 1.171(8) Å] and linear within the experimental error
[N(5)–N(4)–N(3) = 178.6(8)]. The other bond lengths and angles
are summarized in Table 1. The bond lengths and bond angles
determined from X-ray measurements are in agreement with the
other similar reported systems [37,38]. The structure of 1, how-
ever, differs from the other X-ray crystallographically investigated
copper(II) azide complexes containing terminal azido ligand
Table 1
Selected bond lengths (Å) and bond angles (°) of 1.
Bond distances (Å)
Cu1–N1
Cu1–N2
1.914(5)
2.020(5)
Cu1–N3
Cu1–O1
1.905(7)
1.844(4)
3.3. In-vitro antimicrobial assay
Bond angles (°)
N1–Cu1–N2
N2–Cu1–N3
N3–Cu1–O1
N4–N3–Cu1
84.0(2)
92.2(3)
88.3(2)
122.1(5)
O1–Cu1–N1
N1–Cu1–N3
N2–Cu1–O1
N5–N4–N3
95.2(2)
3.3.1. Antibacterial activity
175.7(3)
175.8(2)
178.6(8)
A comparative account of the growth inhibition zone values of
Schiff bases (HL1, HL2 and HL3) and their complexes 1, 2 and 3 re-
vealed that azido complexes exhibited higher antibacterial activity

C. Adhikary et al. / Polyhedron 65 (2013) 48–53
51
Table 2
Homogeneous alkene oxidation catalyzed by complex [Cu(L1)(N3)] (1), [Cu2(L2)2(
l
2-1,1-N3)2] [Cu(L2)(N3)] (2) and [Cu(L3)(N3)] (3)a in acetonitrile medium.
Catalyst
Substrate
Reaction time (h)
Conversion (wt%)
% Yield of products
TONb
TOFc (hÀ1
)
Epoxide
60
Others
[Cu(L1)(N3)] (1)
24
98
38d
14208
592
24
24
100
82
65
35e
15480
645
585
39
43f
14040
24
24
95
96
70
54
25g
42e
14160
14280
590
595
[Cu2(L2)2( 2-1,1-N3)2] [Cu(L2)(N3)] (2)
l
24
100
62
38e
15336
639
24
84
45
39f
14040
585
24
24
90
90
65
49
25g
41e
15360
13752
604
573
[Cu(L3)(N3)] (3)
24
100
60
40e
15240
635
24
24
70
85
45
65
25f
20g
13680
12912
570
538
a
b
c
d
e
f
Reaction conditions: alkenes (10 mmol); catalysts (0.005 mmol); tert-BuOOH (20 mmol); acetonitrile (10 mL); temperature 70 °C.
TON = turn over number = mol converted/mol of copper (active site) taken for reaction.
TOF = Turn over frequency = mol converted/(mol of copper (active site) taken for reaction  reaction time).
Benzaldehyde and benzoic acid.
4-Methylbenzaldehyde and 4-methylbenzoic acid.
Cyclohex-2-en-1-ol and cyclohex-2-en-1-one.
g
Cyclooctane-1,2-diol.
than the free ligand as represented in Table 4. This is probably due
to the greater lipophilic nature of the complexes. Such increased
activity of the metal chelates can be explained on the basis of Over-
tone’s concept and Tweedy’s chelation theory [49]. According to
Overtone’s concept of cell permeability, the lipid membrane that
surrounds the cell favors the passage of only lipid soluble materials
due to which liposolubility is considered to be an important factor
that controls the anti microbial activity. On chelation, thepolarity
of the metal ion will be reduced to a greater extent due to the over-
lap of the ligand orbital and partial sharing of the positive charge of
the metal ion with donor groups [50,51]. Further, it increases the
delocalization of the p electrons over the whole chelate ring and
enhances the lipophilicity of the complex. This increased lipophil-
icity enhances the penetration of the complexes into lipid mem-
brane and thus blocks the metal binding sites on enzymes of
microorganisms [52]. These metal complexes also disturb the res-
piration process of the cell and thus block the synthesis of proteins,
which restricts further growth of the organism [53]. The variation
in the activity of different complexes against different organisms
depend either on the impermeability of the cells of the microbes
or difference in ribosomesof microbial cells. The inhibition zones
of antibacterial activity have been presented in the Table 4.

52
C. Adhikary et al. / Polyhedron 65 (2013) 48–53
Fig. 4. Effect of solvents on the conversion of cyclooctene to its epoxide catalyzed
by 1.
Fig. 2. Reaction profile for the epoxidation of olefins with tert-BuOOH in presence
of 1.
Table 4
Growth inhibition zone of microbes in mm.
Microbes
Ligand Complex Ligand Complex
Ligand Complex
HL1
[Cu(L1) HL2
[Cu2(L2)2
HL3
[Cu(L3)
(N3)] (1)
(l
2-1,1-N3)2]
(N3)] (3)
[Cu(L2)(N3)] (2)
E. coli
S. aureus
A. niger
7
5
15
12
12
22
15
6
5
12
11
9
13
18
16
7
6
12
12
10
10
18
14
F. oxysporum 11
shown a lesser activity as compared to the metal complexes. The
experimental results of the compounds are expressed as inhibition
zone diameter (in mm).
4. Conclusion
In summary, we have synthesized and characterized tridentate
Schiff base copper(II) complexes of their azido derivatives. All the
complexes exhibited excellent catalytic activity in homogeneous
alkene oxidation. A better selectivity was found for all the com-
plexes when acetonitrile was used as a solvent. Biological studies
indicate that better activity was found for the complexes when
compared to that against their corresponding Schiff base ligands.
Fig. 3. Catalytic cycle for the epoxidation reaction.
Table 3
Comparison of catalytic efficacy of complex [Cu(L1)(N3)] (1), [Cu2(L2)2(
l2-1,1-N3)2]
[Cu(L2)(N3)] (2) and [Cu(L3)(N3)] (3) in oxidation of cyclooctene with tert-BuOOH in
different solvent media.
Complex
% Of conversion (% of epoxide yield) in
different solvent
Acknowledgement
CH3CN
CH3OH CHCl3
CH2Cl2
The work is financially supported by the Department of Science
and Technology, Government of India, by a grant (SR/S1/IC-13/
2010) (to C.A.).
[Cu(L1)(N3)] (1)
95 (70) 80 (62) 70 (54) 85 (49)
90 (65) 75 (53) 70 (48) 85 (42)
[Cu2(L2)2( 2-1,1-N3)2] [Cu(L2)(N3)]
l
(2)
[Cu(L3)(N3)] (3)
85 (65) 70 (54) 65 (45) 78 (39)
Appendix A. Supplementary data
CCDC 937471 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK. Fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.08.019.
3.3.2. Antifungal activity
The antifungal activities [54] of the ligands and their metal
complexes were tested against seven-day old cultures of A. niger
and F. oxysporum using disc diffusion method. The results show
that the metal complexes are more active than the free ligands
and can be explained by Overtone’s concept and Tweedy’s chela-
tion theory. The mode of action of the compounds may involve for-
mation of a hydrogen bond through the azomethine group with the
active centres of cell constituents, resulting in an interference with
normal cell process [55]. The antifungal activity data (Table 4) indi-
cate that the complexes show an appreciable activity against A. ni-
ger and F. oxysporum at a concentration of 1.0 mg/ml. Ligand has
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