Copper(II) complexes with barbiturate derivatives: Synthesis, characterization and catalytic applications

Article abstract of DOI:10.1016/j.poly.2019.02.044

Barbiturate derivatives were prepared by the reaction of 5-formylbarbiturates and primary aryl amines. The corresponding copper(II) complexes were synthesized by treatment of copper(II) acetate monohydrate with the prepared barbiturate derivatives. All th

Full text of DOI:10.1016/j.poly.2019.02.044

Accepted Manuscript
Copper(II) complexes with barbiturate derivatives: Synthesis, characterization
and catalytic applications
Erkan Fırıncı
PII:
S0277-5387(19)30145-7
https://doi.org/10.1016/j.poly.2019.02.044
POLY 13788
DOI:
Reference:
To appear in:
Polyhedron
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Revised Date:
Accepted Date:
4 January 2019
18 February 2019
19 February 2019
Please cite this article as: E. Fırıncı, Copper(II) complexes with barbiturate derivatives: Synthesis, characterization
and catalytic applications, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.02.044
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Copper(II) complexes with barbiturate derivatives:
Synthesis, characterization and catalytic applications
Erkan Fırıncı
Department of Chemistry, Faculty of Arts and Sciences, Aydın Adnan Menderes University,
0
9100-Efeler, Aydın, Turkey
E-mail Address: efirinci@adu.edu.tr
Abstract
Barbiturate derivatives were prepared by the reaction of 5-formylbarbiturates and primary aryl
amines. The corresponding copper(II) complexes were synthesized by treatment of copper(II)
acetate monohydrate with the prepared barbiturate derivatives. All the novel compounds were
characterized by elemental analysis and spectroscopic methods. The structures of 3b and 3j
were determined with X-ray crystallography. The X-ray crystallographic studies revealed that
complexes 3b and 3j are 2-D coordination polymers. Also, the catalytic activities of the
copper(II) complexes were examined for the peroxidative oxidation of cyclohexane under
mild conditions. The results showed that the yields of the oxidation products were in the range
1
1.3-22.1%, depending upon the complex used and the promoter.

KEYWORDS: Barbiturates, Copper(II) complexes, Peroxidative oxidation of cyclohexane, X-ray
diffraction.
1
. Introduction
Adipic acid is a valuable material for industrial processes due to its use in the manufacture of
polyamides. It is prepared from a mixture of cylohexanone and cyclohexanol, which is known
as KA oil [1]. Cyclohexane is a most important resource for KA oil because KA oil is the
oxidation product of cyclohexane. Traditional oxidation processes of cyclohexane in industry
suffer from harsh conditions, low yields, economic and environmental problems [2]. These
disadvantages can be overcome by using suitable catalytic systems. Copper catalysts are more
effective and inexpensive than other transition metals for the oxidation of cyclohexane, but
their catalytic activities have still to be advanced [3]. The first peroxidative oxidation of
cyclohexane with a copper catalyst was performed by Barton, with a low yield [4]. In 2005,
Pomberio and co-workers achieved an increased yield of the oxidation products of
cyclohexane, up to 38%, by using a copper cluster as the catalyst [2c]. Recently, mononuclear
copper complexes, with the ligands tris(2-pyridylmethyl)amine and 1,1-bis(pyridylmethyl)-
2
,2-bipyrrolodine, showed high catalytic activity in the peroxidative oxidation of cyclohexane
(yields up to 56%) [2e]. The electronic and steric properties of the ligands play a crucial role
in the catalytic activities of metal complexes, so the preparation of metal complexes including
new ligand derivatives and the investigation of their catalytic performances have attracted
great interest [5]. Pyrimidine-2,4,6-trione derivatives, which are known as barbituric acids,
can be seen as versatile ligands due to having three carbonyl groups, two amide groups and an
active methylene group in the same ring. Especially, the active methylene group in barbiturate
offers the opportunity to prepare different barbiturate derivatives, such as formyl [7], acetyl
[7], hydrazone [2d, 7, 8] and enamine [9], which are substituted at the 5-position of the ring.
Cu(II) complexes with 5-position substituted barbiturates have been extensively used in
biological activity studies [6, 7, 8b,e, 9a-c]. Two studies have been hitherto reported that
examine the catalytic activity of Cu(II) barbiturate complexes for the peroxidative oxidation
of cyclohexane [2j, 8c]. In this context, copper(II) complexes derived from 5-arylamino
substituted barbiturates were prepared and examined for their catalytic activities in the
reaction of cyclohexane oxidation.

2
. Experimental
2
.1. General methods
All chemical reagents were obtained commercially and used without further purification. The
ligands were prepared by slightly modified literature procedures [7, 9d].
Melting points were measured with a Stuart SMP30 melting point apparatus in open capillary
tubes. Infrared spectra were recorded on an ATR Spectrum-II, PerkinElmer spectrometer.
Elemental analyses were carried out on a LECO CHNS-932 by METU Central Laboratory
1
13
(
Ankara, Turkey). H and C NMR spectra were recorded on a Varian 400 spectrometer in
1
CDCl or DMSO-d . Residual signals of the solvent were used as references for the H and
3
6
13C NMR spectra. The catalytic studies were performed on an Agilent 6890N GC system by
GC-flame ionization detection with an HP-5 column of 30 m length, 0.32 mm diameter and
0
.25 µm film thickness. Single crystal X-ray diffraction data of the compounds were collected
at room temperature on an Rigaku-Oxford Xcalibur diffractometer with an Eos-CCD detector,
operated at 50 kV and 40 mA using graphite monochromated Mo-Kα (λ = 0.71073 Å)
radiation. Data collection and reduction, along with absorption corrections, were performed
using the CrysAlisPro software package [10]. The structure solutions were performed using
SHELXT [11] embedded in Olex2 [12]. Refinement of the coordinates and anisotropic
thermal parameters of non-hydrogen atoms were carried out by the full-matrix least-squares
method in SHELXL [13]. All non-hydrogen atoms were refined anisotropically. The positions
of the hydrogen atoms of the title compounds on their respective parent carbon atoms were
generated geometrically.
2
.2. Synthesis of the ligands
The 5-formylbarbituric acid derivatives (1) were prepared according to the literature [7]. 1
5.0 mmol), the selected aryl amine derivative (5.0 mmol) and a catalytic amount of acetic
(
acid (one drop) were combined in methanol (30 mL) and refluxed for 4 h (Scheme 1). The
ligand (2) was isolated by vacuum filtration and rinsed with diethyl ether after the mixture
was cooled to room temperature. 2a, 2b, 2i and 2j were synthesized as described in literature
[9b, 14, 15].

a: R: H, Ar: phenyl
R
R
O
O
O
O
b: R: CH3, Ar: phenyl
c: R: H, Ar: 4-isopropylphenyl
N
N
O
H
N
N
NH Ar d: R: CH , Ar: 4-isopropylphenyl
3
Δ, 4h/ Methanol
e: R: H, Ar: 2,4-dimethylphenyl
f: R: CH3, Ar: 2,4-dimethylphenyl
g: R: H, Ar: 2,4,6-trimethylphenyl
h: R: CH3, Ar: 2,4,6-trimethylphenyl
i: R: H, Ar: 2-naphthyl
O
+
2
H N Ar
O
R
R
j: R: CH3, Ar: 2-naphthyl
1
2
Scheme 1. Synthesis of ligands
.2.1. 5-[(4-Isopropylphenylamino)-methylene]pyrimidine-2,4,6-trione (2c)
2
-1
White powder, 1.06 g, Yield: 77.1%, m.p.: 320-322 °C (dec.). IR (ν cm ): 3220, 3135, 2962,
737, 1683, 1637. Anal. Calc. for C H N O : C, 61.53; H, 5.53; N, 15.38; found: C, 61.28;
1
1
4
15
3
3
1
H, 5.43; N, 15.42 %. H NMR (400 MHz, DMSO-d , δ ppm): 1.17-1.19 (d, 6H, CH(CH ) , J =
6
3 2
6
.6 Hz), 2.83-2.94 (m, 1H, CH(CH ) ), 7.27-7.29 (d, 2H, C H -o-H), J = 8.2 Hz), 7.40-7.42
3 2 6 4
(
d, 2H, C H -m-H), J = 8.2 Hz), 8.48-8.50 (d, 1H, NHCH=C, J = 14.1 Hz), 10.82 (br. s, 1H,
6 4
1
3
O=CNH), 10.96 (br.s, 1H, O=CNH), 11.83-11.86 (d, 1H, NHCH=C, J = 13.7 Hz). C NMR
(
(
(
100 MHz, DMSO-d , δ ppm): 23.8 (CH(CH ) ), 32.9 (CH(CH ) ), 92.2 (NHCH=C), 118.4
6 3 2 3 2
C H ), 127.6 (C H ), 136.4 (C H ), 146.4 (C H ), 150.7 (NHCH=C), 151.2 (C=O), 163.6
6
4
6
4
6
4
6
4
C=O), 166.1 (C=O).
.2.2. 1,3-Dimethyl-5-[(4-isopropylphenylamino)-methylene]pyrimidine-2,4,6-trione (2d)
2
-1
White fluffy solid, 1.0 g, Yield: 65.8%, m.p.: 126-128 °C. IR (ν cm ): 3238, 2961, 1717,
622, 1575. Anal. Calc. for C H N O : C, 63.77; H, 6.36; N, 13.94; found: 63.74; H, 6.37;
1
1
6
19
3
3
1
N, 13.96 %. H NMR (400 MHz, CDCl , δ ppm): 1.50-1.51 (d, 6H, CH(CH ) , J = 6.3 Hz),
3
3 2
3
.16-3.20 (m, 1H, CH(CH ) ), 3.60 (s, 3H, NCH ), 3.61 (s, 3H, NCH ), 7.43-7.45 (d, 2H,
3 2 3 3
C H -o-H), J = 8.2 Hz), 7.53-7.55 (d, 2H, C H -m-H), J = 8.2 Hz), 8.91-8.94 (d, 1H,
6
4
6
4
1
3
NHCH=C, J = 14.1 Hz), 12.29-12.32 (d, 1H, NHCH=C, J = 13.7 Hz). C NMR (100 MHz,
CDCl , δ ppm): 23.8 (CH(CH ) ), 27.2 (NCH ), 27.9 (NCH ), 33.6 (CH(CH ) ), 92.6
3
3 2
3
3
3 2
(
NHCH=C), 117.9 (C H CH-p-CH(CH ) ), 128.0 (C H CH-p-CH(CH ) ), 135.8 (C H CH-p-
6 4 3 2 6 4 3 2 6 4
CH(CH ) ), 147.7 (C H CH-p-CH(CH ) ), 151.9 (NHCH=C), 152.0 (C=O), 162.7 (C=O),
3
2
6
4
3 2
1
65.0 (C=O).
.2.3. 5-[(2,4-Dimethylphenylamino)-methylene]pyrimidine-2,4,6-trione (2e)
2
This compound is commercially available with the CAS registry number 142336-33-2, but
there no spectroscopic data is available, therefore it is provided below.

-1
Yellow powder, 0.77 g, Yield: 69.5%, m.p.: 327-329 °C (dec.). IR (ν cm ): 3203, 3188, 2946,
733, 1621, 1595. Anal. Calc. for C H N O : C, 60.22; H, 5.05; N, 16.21; found: C, 59.98;
1
1
3
13
3
3
1
H, 4.92; N, 16.29%. H NMR (400 MHz, DMSO-d , δ ppm): 2.26 (s, 3H, C H (CH ) -o-
6
6
3
3 2
CH ), 2.27 (s, 3H, C H (CH ) -p-CH ), 7.07 (s, 1H, C H -m-H), 7.08-7.10 (d, 1H, C H -m-H,
3
6
3
3 2
3
6
3
6
3
J = 8.2 Hz), 7.46-7.48 (d, 1H, C H -o-H, J = 8.2 Hz), 8.51 (br. s, 1H, NHCH=C), 10.83 (br. s,
6
3
1
3
1
H, O=CNH), 10.98 (br. s, 1H, O=CNH), 12.10 (br. s, 1H, NHCH=C). C NMR (100 MHz,
DMSO-d , δ ppm): 16.8 (C H (CH ) -o-CH ), 20.4 (C H (CH ) -p-CH ), 92.3 (NHCH=C),
6
6
3
3 2
3
6
3
3 2
3
1
16.5 (C H ), 127.4 (C H ), 128.0 (C H ), 131.5 (C H ), 134.4 (C H ), 135.4 (C H ), 150.7
6 3 6 3 6 3 6 3 6 3 6 3
(NHCH=C), 151.3 (C=O), 163.4 (C=O), 166.7 (C=O).
2
.2.4. 1,3-Dimethyl-5-[(2,4-dimethylphenylamino)-methylene]pyrimidine-2,4,6-trione (2f)
-1
Pale yellow fluffy solid, 0.96 g, Yield: 70.3%, m.p.: 207-209 °C. IR (ν cm ): 3214, 2916,
733, 1628, 1581. Anal. Calc. for C H N O : C, 62.71; H, 5.96; N, 14.63; found: C, 62.80;
1
1
5
17
3
3
1
H, 6.03; N, 14.78 %. H NMR (400 MHz, CDCl , δ ppm): 2.32 (s, 3H, C H (CH ) -o-CH ),
3
6
3
3 2
3
2
.37 (s, 3H, C H (CH ) -p-CH ), 3.34 (s, 3H, NCH ), 3.35 (s, 3H, NCH ), 7.06 (s, 1H, C H -
6 3 3 2 3 3 3 6 3
m-H), 7.07-7.09 (d, 1H, C H -m-H, J = 8.0 Hz), 7.21-7.23 (d, 1H, C H -o-H, J = 8.0 Hz),
6
3
6
3
1
3
8
.64-8.67 (d, 1H, NHCH=C, J = 13.8 Hz), 12.18-12.21 (d, 1H, NHCH=C, J = 13.6 Hz). C
NMR (100 MHz, CDCl , δ ppm): 17.4 (C H (CH ) -o-CH ), 20.9 (C H (CH ) -p-CH ), 27.3
3
6
3
3 2
3
6
3
3 2
3
(
(
(
NCH ), 27.9 (NCH ), 92.8 (NHCH=C), 116.3 (C H ), 127.9 (C H ), 128.2 (C H ), 132.0
3 3 6 3 6 3 6 3
C H ), 134.4 (C H ), 136.6 (C H ), 151.9 (NHCH=C), 152.2 (C=O), 162.7 (C=O), 165.2
6
3
6
3
6
3
C=O).
.2.5. 5-[(2,4,6-Trimethylphenylamino)-methylene]pyrimidine-2,4,6-trione (2g)
2
This compound is commercially available with the CAS registry number 414908-29-5, but
there no spectroscopic data is available, therefore it is provided below.
-1
Pale yellow powder, 1.22 g, Yield: 89.0%, m.p.: 270-272 °C. IR (ν cm ): 3213, 3182, 2977,
739, 1635, 1601. Anal. Calc. for C H N O : C, 61.53; H, 5.53; N, 15.38; found: C, 61.57;
1
1
4
15
3
3
1
H, 5.47; N, 15.24 %. H NMR (400 MHz, DMSO-d , δ ppm): 2.18 (s, 6 H, C H (CH ) -o-
6
6
2
3 3
CH ), 2.24 (s, 3 H, C H (CH ) -p-CH ), 6.97 (s, 2 H, C H -m-H), 7.91-7.95 (d, 1H,
3
6
2
3 3
3
6
2
NHCH=C, J = 14.5 Hz), 10.73 (br. s, 1H, O=CNH), 10.88 (br. s, 1H, O=CNH), 11.30-11.33
1
3
(
d, 1H, NHCH=C, J = 14.5 Hz). C NMR (100 MHz, DMSO-d , δ ppm): 17.9 (C H (CH ) -
6 6 2 3 3
o-CH ), 20.5 (C H (CH ) -p-CH ), 91.2 (NHCH=C), 129.2 (C H ), 132.3 (C H ), 135.1
3
6
2
3 3
3
6
2
6
2
(
C H ), 136.6 (C H ), 150.8 (NHCH=C), 157.8 (C=O), 163.7 (C=O), 166.1 (C=O).
6 2 6 2

2
.2.6. 1,3-Dimethyl-5-[(2,4,6-trimethylphenylamino)-methylene]pyrimidine-2,4,6-trione (2h)
-1
White fluffy solid 1.16 g, Yield: 77.0%, m.p.: 248-250 °C. IR (ν cm ): 3204, 2966, 1755,
1
6
625, 1578. Anal. Calc. for C H N O : C, 63.77; H, 6.36; N, 13.94; found: C, 63.79; H,
16 19 3 3
1
.28; N, 13.92 %. H NMR (400 MHz, CDCl , δ ppm): 2.25 (s, 6H, C H (CH ) -o-CH ), 2.28
3
6
2
3 3
3
(
s, 3H, C H (CH ) -p-CH ), 3.33 (s, 3H, NCH ), 3.34 (s, 3H, NCH ), 6.93 (s, 2 H, C H -m-
6 2 3 3 3 3 3 6 2
H), 8.20-8.23 (d, 1H, NHCH=C, J = 14.3 Hz), 11.46-11.49 (d, 1H, NHCH=C, J = 14.1 Hz).
1
3
C NMR (100 MHz, CDCl , δ ppm): 18.3 (C H (CH ) -o-CH ), 20.9 (C H (CH ) -p-CH ),
3
6
2
3 3
3
6
2
3 3
3
2
1
7.2 (NCH ), 27.9 (NCH ), 91.8 (NHCH=C), 129.7 (C H ), 132.3 (C H ), 134.3 (C H ),
3 3 6 2 6 2 6 2
37.8 (C H ), 152.0 (NHCH=C), 159.3 (C=O), 162.8 (C=O), 165.2 (C=O).
6
2
2
.3. Synthesis of the copper(II) complexes
Cu(OAc) .H O (0.5 mmol) and 2 (1.0 mmol) were dissolved in methanol (10 mL). The
2
2
mixture was refluxed for 4 h (Scheme 2). The formed complex (3) was isolated by filtration
after the resulting suspension was cooled to room temperature. The complex was then washed
with diethyl ether and dried by vacuum.
R
O
N
O
N
R
a: R: H,
Ar: phenyl
b: R: CH3, Ar: phenyl
R
N
O
O
c: R: H, Ar: 4-isopropylphenyl
d: R: CH3, Ar: 4-isopropylphenyl
e: R: H, Ar: 2,4-dimethylphenyl
f: R: CH3, Ar: 2,4-dimethylphenyl
g: R: H, Ar: 2,4,6-trimethylphenyl
h: R: CH3, Ar: 2,4,6-trimethylphenyl
i: R: H, Ar: 2-naphthyl
Ar
O
N
N
N
NH Ar
Δ, 4h / Methanol
2 HOAc
2
O
+
1 Cu(OAc) .H O
Cu
2
2
-
O
Ar
R
j: R: CH3, Ar: 2-naphthyl
R
N
O
N
R
O
2
3
Scheme 2. Synthesis of the Cu(II) Complexes
2
.3.1 Complex 3a
-1
Bright green crystalline solid, 160 mg, Yield: 61.1%, m.p. > 380 °C. IR (ν cm ): 3231, 3100,
1
5
745, 1705, 1644. Anal. Calc. for C H CuN O : C, 50.43; H, 3.08; N, 16.04; found: C,
22 16 6 6
0.27; H, 3.12; N, 15.98 %.
2
.3.2 Complex 3b
-1
Green powder, 132 mg, Yield: 44.1%, m.p.: 318-320 °C. IR (ν cm ): 3001, 1706, 1622, 1602.
Anal. Calc. for C H CuN O : 53.84; H, 4.17; N, 14.49; found: C, 53.80; H, 4.23; N, 14.39
2
6
24
6
6
%.

2
.3.3 Complex 3c
-1
Pale green powder, 121 mg, Yield: 39.7%, m.p. > 380 °C. IR (ν cm ): 3190, 2995, 1743,
1
4
641, 1602. Anal. Calc. for C H CuN O : C, 55.30; H, 4.64; N, 13.82; found: C, 55.33; H,
28 28 6 6
.61; N, 13.89 %.
2
.3.4 Complex 3d
-1
Bright brown crystalline powder, 263 mg, Yield: 79.1%, m.p.: 314-316 °C. IR (ν cm ): 3003,
1
5
717, 1652, 1622. Anal. Calc. for C H CuN O : C, 57.86; H, 5.46; N, 12.65; found: C,
32 36 6 6
7.78; H, 5.41; N, 12.69 %.
2
.3.5 Complex 3e
-1
Pale green powder, 204 mg, Yield: 70.2%, m.p. > 380 °C, IR (ν cm ): 3156, 3004, 1717,
1
4
647, 1598. Anal. Calc. for C H CuN O : C, 53.84; H, 4.17; N, 14.49; found: C, 53.76; H,
26 24 6 6
.22; N, 14.42 %.
2
.3.6 Complex 3f
-1
Bright brown crystalline powder, 165 mg, Yield: 51.7%, m.p.: 280-287 °C. IR (ν cm ): 3001,
1
5
707, 1628, 1592. Anal. Calc. for C H CuN O : C, 56.64; H, 5.07; N, 13.21; found: C,
30 32 6 6
6.68; H, 5.11; N, 13.19 %.
2
.3.7 Complex 3g
-1
Dark brown powder, 210 mg, Yield: 69.2%, m.p.: 363-365 °C. IR (ν cm ): 3160, 2995, 1719,
1
4
648, 1598. Anal. Calc. for C H CuN O : C, 55.30; H, 4.64; N, 13.82; found: C, 55.26; H,
28 28 6 6
.72; N, 13.72 %.
2
.3.8 Complex 3h
-1
Green crystalline powder, 256 mg, Yield: 77.6%, m.p.: 325-327 °C. IR (ν cm ): 3012, 1705,
1
5
630, 1582. Anal. Calc. for C H CuN O : C, 57.86; H, 5.46; N, 12.65; found: C, 57.78; H,
32 36 6 6
.41; N, 12.59 %.
2
.3.9 Complex 3i
-1
Pale green powder, 172 mg, Yield: 55.2%, m.p. > 380 °C, IR (ν cm ): 3202, 3067, 1715,
1
3
647, 1604. Anal. Calc. for C H CuN O : C, 57.74; H, 3.23; N, 13.47; found: C, 57.68; H,
30 20 6 6
.18; N, 13.40 %.

2
.3.10 Complex 3j
-1
Green powder, 256 mg, Yield: 75.3%, m.p.: 322-324 °C. IR (ν cm ): 3002, 1707, 1668, 1612.
Anal. Calc. for C H CuN O : C, 60.04; H, 4.15; N, 12.36; found: C, 59.95; H, 4.19; N,
3
4
28
6
6
1
2.39 %.
2
.4. General procedure for the peroxidative oxidation of cyclohexane
The typical procedure for the oxidation of cyclohexane with hydrogen peroxide was
performed in a round bottom flask under air atmosphere, as follows: the precatalyst (3a-j, 5
µmol), co-catalyst (125 µmol, trifluoroacetic acid (TFA) or nitric acid as the acid or pyridine
as the base), cyclohexane (5 mmol) and hydrogen peroxide (30% in H O, 10 mmol) were
2
added to the flask. The mixture was stirred for 6 h at 50 °C, and then it was cooled to room
temperature. 50 µL of diethylene glycol di-n-butyl ether as an internal standard and 10 mL of
diethyl ether were added to the flask at room temperature. The resulting mixture was stirred
for 10 min and excess PPh added to reduce the cyclohexyl hydroperoxide to cyclohexanol,
3
which was developed by Shul’pin [16]. 2 µL of the sample was taken from the organic phase
of the mixture and analyzed by GC. A blank experiment without addition of any catalyst was
carried out under the given conditions and no oxidation products were observed. The
synthesized ligands (2a-j) were tested as catalysts, but no oxidation products were found.
Also, Cu(OAc) .H O, which was used to prepare 3a-j, showed poor catalytic activity (ca. 4%)
2
2
under the optimum catalytic conditions. This value is consistent with the previously reported
studies [17, 18].
3
. Results and discussion
3
.1. Synthesis and characterization of the compounds
The barbiturate ligands (2a-j) were prepared by the reaction of 5-formylbarbiturate
derivatives with selected aryl amines in methanol [7, 9d]. The chemical structures of the
1
13
synthesized ligands (except 2a, 2b, 2i and 2j) were elucidated by ATR FT-IR, H and C
NMR spectroscopy. The sharp and strong bands in the ranges 1750-1600 and 1600-1550 cm-1
in the IR spectra of the ligands and the complexes were attributed to ν(C=O) and ν(C=N),
-1
respectively. The medium intensity broad bands at 3200-3050 cm can be assigned to the
overlapped ν(N-H) stretching vibrations of the ligands and complexes. The CH and NH
protons of the enamine moiety (NHCH=C) of the ligands were observed as doublet peaks at

1
around δ 7.90-8.95 and 11.30-12.30 ppm in the H NMR spectra, respectively. The NH
protons of 2c, e and g gave two broad singlets at δ 10.82 and 10.96, 10.83 and 10.98, 10.73
and 10.88 ppm, respectively. The observed signals in the range δ 3.20-3.60 ppm were
assigned as the N-CH protons of 2d, f and h. The NHCH=C carbon atoms of 2c-h were
3
1
3
observed at δ 91-93 and 150-152 ppm in the C NMR spectra. The carbonyl carbon atoms of
c-h appeared between δ 151 and 166 ppm, as expected. The obtained FT-IR and NMR data
2
for 2 and 3 comply with reported similar compounds [2d, 7, 8c, 9, 14, 15].
Suitable crystals of 3b and 3j for X-ray diffraction measuruments were obtained in an
CH Cl /Et O mixture at room temparature. A summary of the crystallographic data,
2
2
2
experimental details and refinement results for 3b and 3j are given in Table 1. X-ray
diffraction analysis revealed that 3b and 3j have a crystallographic center of symmetry that is
right in the middle point of the Cu(II) atom in a six coordinate structure (Figure 1). The
equatorial plane consists of two barbituric oxygen atoms (O1 and O1A) and two enamine
nitrogen atoms (N1 and N1A) where the maximum deviation distance of the N1 and Cu atoms
from the least squares plane determined from the four coordination atoms is 0 Å, which
means that the atoms N1, O1, N1A, O1A and Cu(II) are coplanar.
Figure 1. The molecular structures of 3b (left) and 3j (right) with the selected atom
numbering scheme and 50% probability displacement ellipsoids. H atoms are omitted for
clarity.
The N1-Cu1 and O1-Cu1 bond lengths are 1.972(3) and 1.952(2) Å for 3b, and 1.990(7) and
1
.937(5) Å for 3j, respectively. The axial positions are occupied by two barbiturate oxygen
atoms (O2, O2A for 3b and O3, O3A for 3j) which come from other barbiturate moieties.
Although the (O2–Cu1–O2A) angle in 3b and (O3–Cu1–O3A) angle in 3j are 180.0°, the O2–
Cu1 (2.631(3) Å) and O3–Cu1 (2.719(5) Å) bond distances are notably longer than the O1–
Cu1 bond distances in the equatorial positions. Due to the Jahn-Teller distortion at the axial

positions [21], the Cu(II) centers in 3b and 3j have an elongated octahedral geometry. On the
other hand, the O3–Cu1 bond distance of 3j is longer (ca. 0.09 Å) than the O2–Cu1 bond
distance of 3b because of steric crowding of the naphthyl group in the ligand. All the bond
lengths and angles in 3b and 3j are in good agreement with those reported for similar six
coordinated Cu(II)-barbiturate complexes [22]. Furthermore, complexes 3b and 3j are better
described as a net type 2-D coordination polymer, which contains one Cu(II) ion, two
bidentate barbiturate ligands and two other barbiturate ligands as linkers in the polymeric unit
(Figure 2).
Figure 2. 2-D structure of 3b (Left) and 3j (Right) with a wireframe presentation (Cu atoms
are ellipsoids)
3
.2. Catalytic studies
This study aimed to investigate the catalytic performances of copper(II) complexes derived
from barbiturates for the peroxidative oxidation of cyclohexane. All the Cu(II)-barbiturate
complexes indicated good catalytic activity under moderate conditions, compared to the
industrial process (yield ca. 4%, at 150 °C), according to the given results in Table 2 [2, 24].
The copper(II) complexes containing 5-formylbarbiturate derivatives (3a, c, e, g, i) showed
slightly higher activities than the other copper(II)complexes (3b, d, f, h, j ) in the present
study. This might be explained by the fact that the 5-formylbarbiturate derivatives have less
steric hindrance than 1,3-dimethyl-5-formylbarbiturates. A similar observation has been
reported by Kopylovich [25]. To our knowledge, only two studies have been reported where
the peroxidative oxidation of cyclohexane was catalyzed by barbiturate-copper complexes. A
tetrameric Cu(II)-arylhydrazonebarbiturate complex showed good catalytic activity and the
total oxidation products yield was recorded as 23% (nacid/ncatalyst = 25) at room

temperature [8c]. Complex 3a indicated similar performance with the same acid/catalyst ratio
at 50 °C (Entry 1, 22.1%). The total yield of the oxidation products was 20.6% by catalysis
with the mononuclear Cu(II)-arylhydrazonebarbiturate complex without any promoter at room
temperature [2d], but the amount of catalyst used was 50 µmol, which is ten times more than
the amount of catalyst used in this study [2d]. Complex 3a indicated better activity than the
mononuclear Cu(II)-barbiturate, but the catalytic study was performed at 50 °C with TFA as
the promoter. However, the turnover numbers of 3a and 3b are better than the TON of the
mononuclear Cu(II)-barbiturate, which is 10 [2d]. The catalytic performances of 3a and 3b
showed the best activity with TFA as an acid promoter. When HNO was used as the acid
3
promoter, the yields of the oxidation products were slightly lower than those using TFA
(Entries 2 and 4). The total yields in this study (ca. 20%, nacid/ncatalyst = 25) are similar to
the reported copper complexes [2d,8c,17, 18, 25, 26]. A study has reported that Cu(II)
complexes with pyridine as a base promoter could promote the peroxidative oxidation of
cyclohexane in good yield [27]. In order to see the effect of a base promoter, the catalytic
activities of 3a and 3b were examined with pyridine. However, in this case the total yields of
the oxidation products were very low (Entries 3 and 6).
Table 2. The yields for oxidation of cylohexane using 3 as a precatalyst[a]
Acid
Co-catalyst)
Yield (%)[b]
(
TON[c]
221
Entry
Precatalyst
3a
Cyclohexanol
16.3
Cyclohexanone
Total
22.1
1
TFA
4.9
10.8
1.0
8.7
10.5
0.5
7.0
5.0
6.6
8.5
5.9
4.8
3.2
3.8
2
3
4
5
6
7
8
9
1
1
1
1
1
3a
3a
3b
3b
3b
3c
3d
3e
3f
HNO
3
11.3
1.7
21.1
2.7
211
27
Pyridine
TFA
11.7
10.4
1.8
20.9
20.4
2.3
209
204
23
HNO
3
Pyridine
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
8.1
15.1
11.5
20.8
20.3
16.5
14.3
12.2
11.7
151
115
208
203
165
143
122
117
6.5
14.2
11.8
10.6
9.5
0
3
4
5
6
3g
3h
3i
8.9
3j
7.9

[a] Reaction conditions: Cyclohexane (5 mmol), Precatalyst 3 (5µmol), H
2 2
O (10 mmol), MeCN (up to 5 mL total volume), 6
h, 50 °C
[
[
b] Determined by GC after treatment with PPh
c] Turnover number (moles of oxidation products per moles of precatalyst)
3
Table 1.
and
refinement
compounds.
Crystal data
structure
data of the
3
b
3j
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C
26
H
24CuN
580.06
292(2)
Monoclinic
I2/a
6
O
6
C
34
H
6 6
28CuN O
680.18
N/A
Monoclinic
P2 /c
1
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
1
9
2
1.8961(8)
.4059(8)
2.1807(15)
12.0803(13)
10.3128(9)
11.8067(9)
90
90.197(7)
90
1470.9(2)
2
1.5356
0.802
9
0
9
0.500(7)
9
0
Volume (Å3)
2481.8(3)
4
Z
-3
D
calc (g/cm )
Absorption coefficient (mm )
1.5523
0.936
1197.9
-1
F (000)
703.0
0
.172 × 0.167 ×
.122
7 ≤ ℎ ≤ 14,
- 11 ≤ 푘 ≤ 1,
0.245 × 0.145 ×
0.062
14 ≤ ℎ ≤ 13,
- 12 ≤ 푘 ≤ 10,
- 14 ≤ 푙 ≤ 14
Crystal size (mm)
0
-
Index ranges
-
27 ≤ 푙 ≤ 25
4005/2341
0.990
Reflections collected/unique
Completeness
4624/2571
0.992
Data / restrains / parameters
2341/0/180
1.056
2571/0/162
1.040
2
Goodness of fit on F
푅1 = 0.0479,
푅1 = 0.0893,
Final R indices [I > 2σ(I)]
푤푅 = 0.0920
푤푅 = 0.1994
2
2
푅1 = 0.0813
푅1 = 0.1413,
R indices (all data)
푤푅 = 0.1086
푤푅 = 0.2397
2
2
Largest difference peak and hole (e
0
.59/0.56
1.49/-1.05
-3
Å )

4
. Conclusion
A series of barbiturate derivatives have been synthesized and characterized by elemental
analysis, ATR-FTIR, H and 13C NMR spectroscopy. The copper(II) complexes of the
synthesized barbiturates have been prepared and characterized by elemental analysis and
ATR-FTIR spectroscopy. The structures of 3b and 3j were confirmed by X-ray
crystallography, being 2-D coordination polymers with elongated octahedral geometry. The
barbiturate-Cu(II) complexes behaved as catalysts for the oxidation of cyclohexane with
hydrogen peroxide under mild conditions in acetonitrile. The yields of the oxidation products
were found to be moderate to good (up to 22.1%) when using the complexes with a promoter.
The best results were obtained in acidic medium, as expected. This study showed that the
catalytic activities of the synthesized complexes can be related to the steric environment
around of the copper center. Further studies are in progress for exploring the relationship
between the catalytic activity and the steric hindrance of copper complexes for the oxidation
of cyclohexane.
1
Acknowledgments
This work has been supported financially by the “Scientific Research Project Unit (BAP) of
Adnan Menderes University” (Project No: FEF-17020). The author is grateful to the X-ray
Laboratory, Faculty of Science, Dokuz Eylül University, for use of the Oxford-Rigaku
Xcalibur Eos diffractometer (purchased under University Research grant No: 2010.
KB.FEN.13).
Appendix A. Supplementary data
NMR spectra of the ligands and crystallographic details related to this article can be found at
https://doi.org/10.1016/j.poly.XXXX. Also, CCDC 1834401 and 1834413 contain the
supplementary crystallographic data for 3b and 3j respectively. 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-
3
36-033; or e-mail:deposit@ccdc.cam.ac.uk.

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The present study involved preparing babitutare derivatives and their copper(II) complexes
and examined their catalytic eficiency for the peroxidative oxidation of cyclohexane.