Dicopper(II) complexes of a tridentate pyrimidine derived Schiff base ligand: Syntheses, crystal structures and catalytic reactions

Article abstract of DOI:10.1016/j.poly.2012.07.075

Three new dinuclear Cu(II) complexes, [Cu₂(L)₂(L ₁)]PF₆ (1), [Cu₂(L)₂(L ₂)]PF₆ (2) and [Cu₂(L)₂(L ₃)]PF₆ (3), of a potentially tridentate NNO-donor Schiff base ligand [HL = 2-(2-(4,6-dimethylpyrimidin-2-yl)hydrazono)methyl)phenol] and the ancillary ligands [HL₁ = benzoic acid, HL₂ = salicylic acid, HL₃ = 4-amino benzoic acid] have been synthesized and characterized by elemental analyses, IR, single crystal X-ray crystallography, spectroscopic, electrochemical and catalytic studies. The ligand HL is the [1+1] condensation product of salicylaldehyde with 2-hydrazino-4,6-dimethyl pyrimidine. In the complexes 1, 2 and 3, two Cu(II) centers are bridged by the carboxylic oxygen atoms of the respective acid moiety. All the metal centres have square-based pyramidal (SP) geometries (Γ = 0.120, 0.124, 0.109 for 1, 2 and 3, respectively). The dinuclear Cu(II) complexes show high catalytic activity and selectivity for the epoxidation of styrene, cyclohexene and cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant.

Full text of DOI:10.1016/j.poly.2012.07.075

Polyhedron 46 (2012) 74–80
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Polyhedron
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Dicopper(II) complexes of a tridentate pyrimidine derived Schiff base ligand:
Syntheses, crystal structures and catalytic reactions
Sangita Ray a, Sreyashi Jana a, Atanu Jana a, Saugata Konar a, Kinsuk Das b, Sudipta Chatterjee c,
Ray J. Butcher d, Susanta Kumar Kar a,
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
b Department of Chemistry, Haldia Government College, Debhog, Purba Midnapur 721657, India
c Department of Chemistry, Serampore College, Serampore, Hooghly 712 201, India
d Department of Chemistry, Howard University, 2400 Sixth Street, N.W., Washington, DC 200 59, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new dinuclear Cu(II) complexes, [Cu2(L)2(L1)]PF6 (1), [Cu2(L)2(L2)]PF6 (2) and [Cu2(L)2(L3)]PF6 (3), of
a potentially tridentate NNO-donor Schiff base ligand [HL = 2-(2-(4,6-dimethylpyrimidin-2-yl)hydrazon-
o)methyl)phenol] and the ancillary ligands [HL1 = benzoic acid, HL2 = salicylic acid, HL3 = 4-amino ben-
zoic acid] have been synthesized and characterized by elemental analyses, IR, single crystal X-ray
crystallography, spectroscopic, electrochemical and catalytic studies. The ligand HL is the [1+1] conden-
sation product of salicylaldehyde with 2-hydrazino-4,6-dimethyl pyrimidine. In the complexes 1, 2 and 3,
two Cu(II) centers are bridged by the carboxylic oxygen atoms of the respective acid moiety. All the metal
Received 6 June 2012
Accepted 10 July 2012
Available online 10 August 2012
Keywords:
Binuclear copper(II) complexes
Pyrimidine derived ligand
Spectral electrochemical properties
X-ray crystal structures
Catalytic epoxidation
centres have square-based pyramidal (SP) geometries (
C = 0.120, 0.124, 0.109 for 1, 2 and 3, respec-
tively). The dinuclear Cu(II) complexes show high catalytic activity and selectivity for the epoxidation
of styrene, cyclohexene and cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
systems provide potential binding sites for metal ions. The higher p
acidity and presence of more than one hetero atom in pyrimidine
play an important role in its coordination chemistry compared to
that of pyridine bases, and it serves as a better model in biological
systems [10–13]. Epoxides are very useful and versatile intermedi-
ates for the synthesis of many commodity and fine chemicals.
Studies on the epoxidation of the C@C bond have become an
important research area in both organic synthesis and bioinorganic
modelling of oxygen-transfer metalloenzymes [14,15]. Various
metal complexes, including some metal enzymes, play an impor-
tant role in oxygen atom transfer reactions [16]. Dinuclear copper
complexes play an important role in biological metalloenzymes
[17–26]. The best investigated dinuclear copper(II) enzymes are
tyrosinase [22–24] and catecholase [25,26]. These complexes are
also of interest because of their structural, magnetic, catalytic
and electron transfer properties [27–30].
As a sequel to our long standing interest in pyrimidine derived
ligands [31–34] we have prepared copper(II) complexes of the tri-
dentate NNO donor Schiff base ligands synthesized from 2-hydra-
zino-4,6-dimethyl pyrimidine and salicylaldehyde in the presence
of the ancillary ligands HL1, HL2 and HL3.The catalytic epoxidation
reactions of various alkenes have been performed using these
dinuclear copper(II) complexes. In the present study, using copper
perchlorate as the metal precursor, three new dinuclear copper(II)
The catalysis of alkene oxidation by soluble transition metal
complexes is an area of contemporary research interest in both
biomimetic and synthetic chemistry [1,2]. Schiff base ligands and
their metal complexes can be easily synthesized. They are efficient
catalysts under homogeneous and heterogeneous conditions. The
catalytic activity of these complexes depends on the nature of
the ligands as well as on the metal centers [3,4]. Therefore, these
compounds are extensively used as catalysts in oxidation [5,6].
Copper(II) Schiff base complexes are amongst the most versatile
catalysts known for oxygenation reactions. The role played by
the copper ions in the active sites of a large number of metallopro-
teins has stimulated various workers to design and characterize
copper complexes as models for a better understanding of biolog-
ical systems [7]. Metal complexes of pyrimidine derived Schiff base
ligands have been extensively studied in recent years owing to
their great variety of biological activities, including antimalarial,
antibacterial, antitumoral, antiviral activities etc.[8,9], which have
often been related to their chelating ability with trace metal ions.
Nucleic acids, vitamins and coenzymes containing pyrimidine ring
⇑
Corresponding author. Fax: +44 1223 336 033.
E-mail address: skkar_cu@yahoo.co.in (S.K. Kar).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.075

S. Ray et al. / Polyhedron 46 (2012) 74–80
75
complexes have been synthesized, which have actually been crys-
tallized as the corresponding hexafluorophosphate salts (1, 2 and
3) using KPF6. We have reported the synthetic details, spectral
characterizations, catalytic epoxidation of alkenes, X-ray crystal
structures and electrochemical properties of 1, 2 and 3, where car-
boxylic acids act as bridgers in the presence of the ligand.
chemical measurements were made in DMF on a BAS (Epsilon
model) having a three-electrode setup consisting of a glassy carbon
(polished with alumina before measurement) working, platinum
wire auxiliary and Ag/AgCl reference electrodes. Oxygen was rigor-
ously removed from the DMF solutions of the samples by purging
with dry argon gas of high purity. The samples were identified
and quantified by a Perkin Elmer Gas Chromatograph (Clarus 600)
equipped with an FID detector and using a CP-Sil 8 CB capillary
column.
2. Experimental
2.1. Materials
2.3. Synthesis of the ligand (HL)
All chemicals were of reagent grade, purchased from commer-
cial sources and used without further purification. Salicylaldehyde,
styrene, cyclooctene, cyclohexene and tert-butylhydroperoxide
(TBHP) (70% aq.) were purchased from the Aldrich Chemical
Company, USA and were used without further purification.
Caution! Although we have not encountered any problems, it
should be kept in mind that perchlorate compounds of metal ions
are potentially explosive, especially in the presence of organic
ligands. Only a small amount of the material should be prepared
and it should be handled with care.
The ligand HL was synthesized following the literature method
[35].
2.4. Preparation of the complexes
2.4.1. Preparation of complex 1
To an aqueous methanolic solution (30 ml) of Cu(ClO4)2Á6H2O
(0.740 g, 2 mmol), a solution of the Schiff base HL (0.484 g,
2 mmol) was added, followed by a solution of benzoic acid
(0.122 g, 1 mmol) in a minimum volume of methanol with con-
stant stirring, and the stirring was continued for 1 h. This mixture
was kept in a refrigerator at 10 °C for several days. No crystals ap-
peared. To the solution KPF6 (0.184 g, 1 mmol) in methanol was
added, and the resulting mixture was kept in refrigerator. Green
hexagonal crystals suitable for X-ray diffraction appeared after
10 days. The crystals were isolated by filtration and air-dried.
Yield: 0.548 g, 65.5%. Anal. Calc. for C33H43Cu2N8O10F6P: C, 40.25;
H, 4.37; N, 11.38; O, 16.26; Found: C, 40.11; H, 4.00; N, 8.01; O,
2.2. Physical measurement
Elemental analyses (carbon, hydrogen and nitrogen) of the
ligand and the metal complexes were determined with a Perkin–
Elmer CHN analyzer 2400 at the Indian Association for the Cultiva-
tion of Science, Kolkata. The electronic spectra of the complexes in
methanolic solutions were recorded on a Hitachi model U-3501
spectrophotometer. IR spectra (KBr pellet, 400–4000 cmÀ1) were
recorded on a Perkin–Elmer model 883 infrared spectrophotome-
ter. 1H NMR spectra of the ligands were recorded in CDCl3 with a
Bruker AM 300L (300 MHz) superconducting FTNMR. All electro-
16.10%. IR (KBr;
m
/cmÀ1): 1540 (vs)
m
as,COOÀ, 1611 (s) mc = Npym
.
Table 1
Experimental data for crystallographic analysis of 1, 2 and 3.
Compound
1
2
3
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C33 H43 Cu2 N8 O10 F6
983.80
295
1.54184
Monoclinic
C2/c
P
C33 H35 Cu2 N8 O8 F6
943.74
150
0.71073
Monoclinic
C2/c
P
C34 H40 Cu2 N9 O7 F6
958.78
150
0.71073
Monoclinic
C2/c
P
18.3159(6)
16.5195(4)
14.0459(4)
90
18.136(2)
16.445(1)
14.283(1)
90
18.1353(15)
16.8441(14)
14.1224(12)
90
b (Å)
c (Å)
a
(°)
b (°)
109.279(3)
90
4011.5(2)
4
1.629
2.515
107.74(2)
90
4057.3(7)
4
1.515
1.174
109.834(2)
90
4058.1(6)
4
1.517
1.369
c
(°)
Volume (Å3)
z
Densitycal (mg mÀ3
)
Absorption coefficient (mmÀ1
F(000)
)
2016
1880
1888
Crystal size (mm3)
0.18 Â 0.22 Â 0.49
5.1–77.4
À20 6 h 6 23
À20 6 k 6 20
À17 6 l 6 13
1.065
0.11 Â 0.12 Â 0.14
1.7–27.1
À23 6 h 6 22
0 6 k 6 21
0 6 l 6 18
1.088
0.12 Â 0.13 Â 0.15
1.7–25.9
À22 6 h 6 20
0 6 k 6 20
0 6 l 6 17
1.084
h Range (°) for data collection
Index ranges
Goodness-of-fit on F2
Completeness to theta = 25.00ꢀ(%)
99.9
98.4
98.6
Independent reflections [Rint
Absorption correction
Refinement method
Data/restraints/parameters
Reflections collected
]
4194 [R(int) = 0.054]
Multi-scan
4411[R(int) = 0.000]
Multi-scan
3913[R(int) = 0.001]
Multi-scan
Full-matrix least squares on F2
4194,17,295
Full-matrix least squares on F2
4411,0267
Full-matrix least squares on F2
3913,0269
8861
4411
3913
Final R indices [I > 2
r
(I)]
R1 = 0.0802 wR2 = 0.2229
R1 = À0.77 wR2 = 1.18
R1 = 0.0508, wR2 = 0.1573
R1 = À0.74 wR2 = 0.44
R1 = .0418 wR2 = .1269
R1 = À0.54 wR2 = 0.50
Largest difference peak and hole (eÅÀ3
)

76
S. Ray et al. / Polyhedron 46 (2012) 74–80
2.4.2. Preparation of complexes 2 and 3
Complexes 2 and 3 were prepared by following the same
procedure used in the case of 1, except that salicylic acid and
4-aminobenzoic acid were added, respectively for 2 and 3, instead
of benzoic acid.
For complex 2. Yield: 0.548 g, 65.5%. Anal. Calc. For C33H35Cu2-
N8O8F6P: C, 41.96; H, 3.70; N, 11.86; O, 13.56. Found: C, 41.51;
H, 3.61; N, 11.76; O, 13.46%. IR (KBr;
m m
/cmÀ1): 1541 (vs) as,COOÀ,
1612 (s) mc = Npym
.
For complex 3. Yield: 0.548 g, 65.5%. Anal. Calc. for C34H40Cu2-
N9O7F6P: C, 42.55; H, 4.17; N, 13.14; O, 11.68. Found: C, 42.50;
H, 4.13; N, 12.00; O, 11.58%. IR (KBr;
1613 (s) mc = Npym
m m
/cmÀ1): 1542 (vs) as, COOÀ,
.
3. Single crystal X-ray crystallography
The crystallographic data for two complexes are given in
Table 1. Diffraction data were collected on an Oxford Xcalibur,
Ruby, Gemini diffractometer at 295 K (for 1) and a Bruker-APEX
II SMART CCD diffractometer at 150 K (for 2 and 3) using CuK
radiation (k = 1.54184 Å) and graphite-monochromated MoK
a
a
radiation (k = 0.71073 Å). Cell parameter refinement and data
reduction were carried out using Bruker SMART [37] and CRYSALIS
programs for 1 [38] and Bruker SAINT software for 2 and 3. The
structures were solved by direct and Fourier methods and refined
by full-matrix least-squares based on F2 using the SHELXS-97 and
SHELXL-97 programs [39]. An empirical absorption correction for 1
and 2, 3 was carried out using the ABSPACK program [40] and SAD-
ABS [41]. X-ray Crystallography, in part, was performed at the DST
funded National Single Crystal Diffractometer facility at the
Department of Chemistry, University of Calcutta.
Fig. 2. Structural representation and atom numbering scheme of 2 (H-atoms are
omitted for clarity).
carried out in a two neck round-bottom flask equipped with con-
denser septum cap and stirrer. In a typical run, 0.5 mmol of the al-
kene was taken in 10 mL solvent and into this mixture 2.0 mg of
the catalyst were added. Then the mixture was equilibrated to
the required temperature in an oil bath. After addition of the oxi-
dant, tert-BuOOH (1 mL, 9 mmol), the reaction mixture was stirred
continuously. Samples were withdrawn by syringe at different
4. Catalytic epoxidation reactions
Epoxidation of alkenes over [Cu2(L)2(L1)]PF6 (1), [Cu2(L)2(L2)]PF6
(2) and [Cu2(L)2(L3)]PF6 (3) catalysts with tert-BuOOH oxidant were
Fig. 1. Structural representation and atom numbering scheme of 1 (H-atoms are
Fig. 3. Structural representation and atom numbering scheme of 3 (H-atoms are
omitted for clarity).
omitted for clarity).

S. Ray et al. / Polyhedron 46 (2012) 74–80
77
time intervals and were identified and quantified by a Perkin Elmer
Gas Chromatograph (model no. Clarus 600) equipped with an FID
detector and using a CP-Sil 8 CB capillary column.
5.1.1. Structural description of complexes 1, 2 and 3
Perspective views of complexes 1, 2 and 3, with the atom-
labelling schemes, are shown in Figs. 1–3 respectively. All the
complexes crystallize in the space group C2/c. In each structure
two CuL units are bridged by carboxylic oxygen atoms of the ancil-
lary ligands HL1, HL2 and HL3, respectively. The unit cells of 1, 2 and
3 comprise of six molecules. In the complexes, each Cu(II) centre is
in a five-coordinate environment with an N2O3 chromophore and
with a distorted square pyramidal geometry. The basal plane for
each Cu(II) centre is formed by the O atoms from the alkoxy group
and bridging carboxyl group, and N atoms from the pyrimidine
ring and azomethine group. The axial position is occupied by a
coordinated oxygen O atom from a water molecule, to complete
the five coordinate geometry. The average Cu-Npym distance falls
in the range 1.951(2)–2.039(3) Å, Cu-Nazomethine 1.944(3)–
1.951(2) Å, Cu-Ocarboxy 1.959(2)–1.940(2) Å, Cu-Ow 2.304(3)–
5. Result and discussion
5.1. Syntheses
The ligand HL was synthesized following the literature method
[35]. The complexation behavior of HL toward Cu(II) salts was inves-
tigated. Complexes 1, 2 and 3 were obtained by reacting the ligand
HL, Cu(II) perchlorate salt and different carboxylic acids, taken in a
1:1:2 molar ratio in methanol. X-ray quality crystals of 1, 2 and 3
were obtained upon slow evaporation of the respective reaction
mixtures in the presence of KPF6 (1 mol) at 10 °C in a refrigerator.
Fig. 4. Hydrogen bonding interactions in complex 1.
Fig. 5. Hydrogen bonding interactions in complex 2.

78
S. Ray et al. / Polyhedron 46 (2012) 74–80
Fig. 6. Hydrogen bonding interactions in complex 3.
2.384(3) Å and Cu-Oalkoxy 1.905(3)–1.911(2) Å. The Addison
parameter ( ) values of the pentacoordinate Cu(II) centre are
by a network of intermolecular hydrogen bonding. (O1 of the
salicyl hydroxy oxygen and H1W2 of a water molecule, O2W and
H1W1 of water molecules, O3W and H2W2 of water molecules,
O3W of a water molecule and H3B of the imine, N4 of the pyrim-
idine and H2W1 of a water molecule) (Fig. 4), (O1 of the salicyl hy-
droxy oxygen and H4A of a water molecule) (Fig. 5) and (O1 of the
hydroxy oxygen and H3AW of a water molecule) (Fig. 6). Details of
the hydrogen bonding are given in Table 3. In all the structures 1, 2
s
0.120, 0.124, 0.109 for 1, 2 and 3, confirming their square-based
pyramidal (SP) geometry. The two Cu(II) centres are separated by
3.553, 3.288 and 3.262 Å, and the upward deviations of the cop-
per(II) centres from the basal plane are 0.139, 0.146, 0.120 Å for
complexes 1, 2 and 3, respectively. The structures are stabilized
and 3, intramolecular
features that impart extra stability to the structures [the distances
between the centroids of the -stacked rings are almost same
p–p stacking serves as one of the important
Table 2
Selected bond distances (Å) and angles (o) in 1, 2 and 3.
p
Selected bonds
Value (Å)
Selected angles
(o)
for the complexes, 3.520, 3.521 and 3.542 Å for 1, 2 and 3,
respectively].
Complex 1
Cu-O1
Cu-O1W
Cu-O2
Cu-N1
Cu-N2
1.911(2)
2.327(2)
1.9526(17)
1.951(2)
2.040(2)
O1-Cu-O1 W
Cu-O1 W-H1W2
O1-Cu-O2
O1-Cu-N1
O1-Cu-N2
O1 W -Cu-O2
O1 W-Cu-N1
O1 W-Cu-N2
O2-Cu-N1
91.18(9)
115.9(16)
87.52(8)
91.11(8)
172.18(8)
94.62(8)
100.37(8)
90.53(8)
164.97(8)
5.2. Catalytic epoxidation of alkenes
The catalytic activity of the immobilized catalysts, [Cu2(L)2(L1)]
PF6 (1), [Cu2(L)2(L2)]PF6 (2) and [Cu2(L)2(L3)]PF6 (3) for the epoxida-
tion of different alkenes were studied using tert-BuOOH as an oxi-
dant. The resultant conversions, selectivities and turnover
frequencies (TOFs) are given in Table 4. The epoxidation of styrene
shows 98% conversion with epoxide selectivity of ꢀ59% for
[Cu2(L)2(L1)]PF6 (1). For [Cu2(L)2(L2)]PF6 (2) and [Cu2(L)2(L3)]PF6
(3) the styrene conversion is 40% and 30%, respectively. Cycloalk-
enes, such as cyclooctene and cyclohexene, (Table 2) have also
been effectively converted by [Cu2(L)2(L1)]PF6 (1) selectively to
cyclooctene oxide (conversion 95%) and cyclohexene oxide (con-
version 80%), respectively.
Complex 2
Cu1-O1
Cu1-O2
Cu1-O4
Cu1-N1
Cu1-N4
1.905(3)
1.959(2)
2.304(3)
2.039(3)
1.944(3)
O2-Cu -N2
N1-Cu-N2
99.95(9)
81.08(9)
O1-Cu1-O2
O1-Cu1-O4
O1-Cu1-N1
O1-Cu1-N4
O2-Cu1-O4
O2-Cu1-N1
O2-Cu1-N4
O4-Cu1-N1
O4-Cu1-N4
N1-Cu1-N4
Cu1-N4-N3
87.54(10)
93.29(12)
172.04(11)
91.10(11)
92.43(11)
99.91(11)
164.59(11)
89.22(11)
102.98(11)
80.97(12)
113.3(2)
Thus a comparison of the results of the catalytic activities of 1, 2
and 3 summarizes that 1 shows the highest styrene epoxidation
activity (98%) compared to that of 40% for 2 and 30% for 3. In case
87.37(10
Table 3
Details of hydrogen bond distances (Å) and angles (°) for 1, 2 and 3.
Complex 3
Cu1-O1
Cu1-O2
Cu1-O3
Cu1-N1
Cu1-N4
1.906(3)
1.940(2)
2.384(3)
2.034(3)
1.948(2)
O1-Cu1-O2
O1-Cu1-O3
O1-Cu1-N1
O1-Cu1-N4
O2-Cu1 -O3
O2-Cu1-N1
O2-Cu1-N4
O3-Cu1-N1
O3-Cu1-N4
N1-Cu1-N4
Cu1-N4-N3
Cu1-O3-H3AW
87.37(10)
90.16(10)
172.03(10)
91.16(10)
93.07(9)
100.57(11)
165.48(10)
90.16(10)
101.38(9)
80.97(11)
113.56(19)
118.00
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
Complex 1
O1W–H1W1. . . O2W
O1W–H1W2. . .O1
O2W–H2W1. . .N4
N3–H3B. . .O3W
O2W–H2W2. . .O3W
Complex 2
0.770(19)
0.75(3)
0.83(2)
0.8600
2.11(2)
2.13(3)
2.06(2)
2.0500
2.11(4)
2.876(5)
2.800(3)
2.889(4)
2.882(4)
2.840(9)
176(2)
149(3)
173(7)
163.00
148(6)
0.82(6)
O4–H4A. . .O1
Complex 3
O3–H3AW. . .O1
0.9600
0.7400
1.8600
2.0500
2.817(4)
2.788(3)
175.00
173.00

S. Ray et al. / Polyhedron 46 (2012) 74–80
79
Table 4
Epoxidation reaction of alkenes catalysed by [Cu2(L)2(L1)]PF6 (1), [Cu2(L)2(L2)]PF6 (2), [Cu2(L)2(L3)]PF6 (3).a
Catalyst
Substrate
Reaction time (h)
Reaction temp. (°C)
Conversionb (wt %)
Yield of products (%)
TOFe (hÀ1
)
Epoxide
Others
[Cu2(L)2(L1)]PF6 (1)
Styrene
Cyclooctene
Cyclohexene
24
24
20
80
80
75
98
95
90
60
95
80
38c
–
48
44
67
10d
[Cu2(L)2(L2)]PF6 (2)
[Cu2(L)2(L3)]PF6 (3)
Styrene
Cyclooctene
Cyclohexene
24
22
22
80
80
75
40
75
56
26
75
47
14c
–
19
36
37
9d
Styrene
Cyclooctene
Cyclohexene
24
22
20
80
80
75
30
50
35
25
50
25
5c
14
24
25
–
10d
a
b
c
Reaction condition: solvent: CH3CN, oxidant: tert-BuOOH, substrate 0.50 mmol.
The products of the epoxidation reactions were collected at different time intervals and were identified and qualified by gas chromatograph (GC).
Benzaldehyde.
2-Cyclohexen-1-Ol was formed.
d
e
TOF (Turnover Frequency) = moles converted/(mol of active site time).
of cyclooctene, 95% activity is recorded for 1 while 75% and 50% is
recorded for 2 and 3, respectively. Furthermore, when cyclohexene
is used, 1 shows 90% activity compared to 56% and 30% for 2 and 3,
respectively.
could be attributed to the pyrimidine to Cu(II) charge transfer
(LMCT) transition (d9 electronic configuration). The low-intensity
absorption bands at 602, 604 and 605 nm in the spectra of com-
plexes 1, 2 and 3, respectively, are associated with a d–d transition
of the copper centre.
6. Characterization of the complex species
6.3. Electrochemistry
6.1. Infrared spectroscopy
A study of the electrochemical behavior of complex 1 was car-
ried out in the positive range. Fig. 7 displays a representative cyclic
voltammogram of 1. Complex 1 exhibits an irreversible reduction
peak (E1/2) in DMF solution near +0.15 V versus SCE due to the
The infrared spectra of the complexes are consistent with the
X-ray crystal structure studies. The asymmetric stretching mode
for the carboxylate groups are observed at 1540, 1541 and
1542 cmÀ1 for complexes 1, 2 and 3, respectively. The strong
mC@N
Cu(II)/Cu(I) couple, and the
DEpa value of 145 mV suggests an irre-
bands occurring at 1611, 1612 and 1613 cmÀ1 for 1, 2, 3, respec-
tively are shifted considerably towards lower frequencies
compared to that of the free Schiff base ligand (1635 cmÀ1),
indicating the coordination of the imino nitrogen atom [36].
versible one electron process.
7. Concluding remarks
Three dimeric copper(II) complexes with an NNO donor ligand
and carboxylic acids have been synthesized and structurally char-
acterized by X-ray crystallography. The metal ions have a square-
pyramidal N2O3 chromophore in these complexes. In the dimers,
the carboxylate ions of the acids act as diatomic bidentate bridging
units. The dinuclear Cu(II) complexes have shown excellent cata-
lytic activities in epoxidation reactions towards various olefins.
Notably, [Cu2(L)2(L1)]PF6 (1) shows an impressive conversion
(98%) of styrene with tert-butylhydroperoxide (tert-BuOOH) as
the oxidant.
6.2. Electronic spectra
The electronic solution spectra of the complexes were recorded
in MeOH solution. The spectra of the Cu(II) complexes 1, 2 and 3
exhibit a strong band at 386, 387 and 390 nm, respectively, which
Acknowledgements
Financial support [Sanc. No. 01(2401)/10/EMR-II, dated 05.01.
2011] from the Council of Scientific and Industrial Research (CSIR),
New Delhi, India, is gratefully acknowledged. S.K. and A.J thank CSIR
for the award of a SRF. Sangita Ray acknowledges the University
Grants Commission (UGC), New Delhi, India [Sa. No. F. 5-1/
2007(BSR)/5-16/2007(BSR), dated 12.02.2006] for a JRF. Financial
support [Sanc. No. SR/S1/IC-27/2008, dated 13.03.2009] from the
Department of Science and Technology (DST), New Delhi, India, is
also gratefully acknowledged. X-ray Crystallography, in part, was
performed at the DST funded National Single Crystal Diffractometer
facility at the Department of Chemistry, University of Calcutta.
Appendix A. Supplementary data
CCDC 871301, 871302 and 871303 contain the supplementary
Fig. 7. Cyclic voltammogram of complex 1.
crystallographic data for 1, 2 and 3. These data can be obtained free

80
S. Ray et al. / Polyhedron 46 (2012) 74–80
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