Catalytic oxidation of benzyl alcohols by new Cu(II) complexes of 1,3-oxazolidine based ligand obtained from a solvent free reaction

Article abstract of DOI:10.1016/j.ica.2018.03.038

A new 1,3-oxazolidine based ligand, (2-pyridin-2-yl-1,3-oxazolidine-4,4-diyl)dimethanol (H₃L), has been synthesized from the reaction of 2-amino-2-(hydroxymethyl)-1,3-propanediol and 2-pyridinecarboxaldehyde at 110 °C. Two mononuclear Cu(II) complexes, [Cu(H₃L)Cl₂]·CH₃OH (1) and [Cu(H₃L)₂]·2Cl (2), have been synthesized with a similar procedure by the reaction of H₃L and CuCl₂·H₂O in 1:1 and 2:1 M ratios in methanol. The ligand and complexes have been characterized by elemental analysis and spectroscopic methods. The structure of complexes has been characterized by single-crystal X-ray diffraction analysis which showed the copper ion has a distorted square pyramidal geometry in 1 and distorted octahedral geometry in 2. The 1,3-oxazolidine ligand acts as a neutral tridentate N₂O-donor ligand in both 1 and 2. Two chloride anions remain coordinated to Cu(II) ion in complex 1, but they act as counter ions in complex 2. The catalytic activity of these complexes has been investigated in the oxidation of benzyl alcohol by using H₂O₂ or TBHP as oxidant. The effects of some influential parameters in catalytic reactions like molar ratio of oxidant-to-substrate, temperature, nature of oxidant and solvent have been studied to obtain the optimum condition. Moreover, the effects of chlorine substituent on the phenyl group (ortho-, meta- and para-positions) has also been explored. The results of these studies show that both 1 and 2 catalyze the oxidation of benzyl alcohol derivatives to the corresponding benzaldehydes and a little amount of the related benzoic acids, produced by over oxidation of benzaldehyde, are also detected in the reaction mixtures. The results indicate that the selectivity and the activity of these catalytic systems depend on the reaction conditions and the steric and electronic properties of the reagents.

Full text of DOI:10.1016/j.ica.2018.03.038

Inorganica Chimica Acta 478 (2018) 77–87
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Research paper
Catalytic oxidation of benzyl alcohols by new Cu(II) complexes of
1,3-oxazolidine based ligand obtained from a solvent free reaction
Rahman Bikas a, , Fatemeh Ajormal b, Marzieh Emami b, Nader Noshiranzadeh b, Anna Kozakiewicz c
⇑
a Department of Chemistry, Faculty of Science, Imam Khomeini International University, 34148-96818 Qazvin, Iran
b Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran
c Department of Crystallochemistry and Biocrystallography, Faculty of Chemistry, Nicolaus Copernicus University in Torun, 87-100 Torun, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A new 1,3-oxazolidine based ligand, (2-pyridin-2-yl-1,3-oxazolidine-4,4-diyl)dimethanol (H3L), has been
synthesized from the reaction of 2-amino-2-(hydroxymethyl)-1,3-propanediol and 2-pyridinecarbox-
aldehyde at 110 °C. Two mononuclear Cu(II) complexes, [Cu(H3L)Cl2]ꢀCH3OH (1) and [Cu(H3L)2]ꢀ2Cl (2),
have been synthesized with a similar procedure by the reaction of H3L and CuCl2ꢀH2O in 1:1 and 2:1 M
ratios in methanol. The ligand and complexes have been characterized by elemental analysis and spec-
troscopic methods. The structure of complexes has been characterized by single-crystal X-ray diffraction
analysis which showed the copper ion has a distorted square pyramidal geometry in 1 and distorted octa-
hedral geometry in 2. The 1,3-oxazolidine ligand acts as a neutral tridentate N2O-donor ligand in both 1
and 2. Two chloride anions remain coordinated to Cu(II) ion in complex 1, but they act as counter ions in
complex 2. The catalytic activity of these complexes has been investigated in the oxidation of benzyl alco-
hol by using H2O2 or TBHP as oxidant. The effects of some influential parameters in catalytic reactions like
molar ratio of oxidant-to-substrate, temperature, nature of oxidant and solvent have been studied to
obtain the optimum condition. Moreover, the effects of chlorine substituent on the phenyl group
(ortho-, meta- and para-positions) has also been explored. The results of these studies show that both
1 and 2 catalyze the oxidation of benzyl alcohol derivatives to the corresponding benzaldehydes and a
little amount of the related benzoic acids, produced by over oxidation of benzaldehyde, are also detected
in the reaction mixtures. The results indicate that the selectivity and the activity of these catalytic sys-
tems depend on the reaction conditions and the steric and electronic properties of the reagents.
Ó 2018 Elsevier B.V. All rights reserved.
Received 25 February 2018
Received in revised form 21 March 2018
Accepted 27 March 2018
Available online 4 April 2018
Keywords:
1,3-Oxazolidine
Copper(II) complex
Crystal structure
Catalytic oxidation
Spectroscopy
1. Introduction
used as chiral ligand in some catalytic reactions like in aza-Michael
reaction [5], organozinc reactions [6], Friedel–Crafts reaction [7], or
Oxazolidines are a class of heterocyclic compounds consisting
of a five-membered ring containing one nitrogen, one oxygen
and three carbon atoms [1]. Similar to well known oxazoles [2]
and oxazolines [3], the oxygen and nitrogen atoms in oxazolidines
are in the 1 and 3 positions, respectively, and there is always a car-
bon atom between them. In contrast, all of the carbon atoms in
oxazolidines are reduced and the five-membered ring is fully satu-
rated. Therefore, they have completely different steric structures
and electronic properties. Unlike oxazolines, which have been
extensively used as chiral ligands in asymmetric catalysis and
supramolecular chemistry [4], the oxazolidines are less studied.
Anyway, in recent years oxazolidines have also attracted consider-
able attention in chemical science and recently they have been
in palladium-catalyzed Diels–Alder [8] and allylation [9] reactions.
Some of the oxazolidine derivatives are used as anticonvulsants
and have application as performance modifiers in polyurethane
coatings and paints [10]. Due to this, it is expected that the devel-
opment of new oxazolidines and their coordination compounds
can lead to new systems for several fields in medicinal, chemical
and material sciences. Therefore, introducing efficient synthetic
methods for selective synthesis and isolation of oxazolidines
remains an interesting open research field in synthetic and hetero-
cyclic chemistry.
Although oxazolidines can show interesting properties, search-
ing literature including Cambridge Structural Database (CSD) [11]
indicates that only a few number of oxazolidine compounds have
been structurally reported until now. Surprisingly, most of them
have been prepared by in situ metal template techniques
from the reaction of aldehydes/ketones, 1,2-amino-alcohols
⇑
Corresponding author.
E-mail addresses: bikas_r@yahoo.com, bikas@sci.ikiu.ac.ir (R. Bikas).
https://doi.org/10.1016/j.ica.2018.03.038
0020-1693/Ó 2018 Elsevier B.V. All rights reserved.

78
R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
(H2NACR2ACR2OH, R@H or C) and metal salts like Fe [12], Zn [13],
Pd [14], Nb, Gd, Sm [15], Cu [16] and Ni [17]. In these reports the
ligand has not been isolated or characterized and in most of them
it is suggested that the metal ion has catalytic effect on the forma-
tion of oxazolidine ring. Li and co-workers [18] have suggested that
maybe beside the metal ion some other materials in the reaction
mixture (like thiocyanate) have also effect on the formation of oxa-
zolidine ring. Nevertheless, recently some oxazolidine compounds
are produced by various methods in the presence or in the absence
of metal ions [19]. Therefore, there is a lack of information about
efficient synthetic methods for selective production of 1,3-oxazo-
lidines and also their structural and physical properties.
temperature can facilitate water elimination by OH functionality
instead of NH group in obtaining 1,3-oxazolidines from hemiami-
nal intermediate.
On the other hand, the coordination chemistry of copper(II) has
attracted considerable attention during the last decades which is
mainly due to the versatile applications of Cu(II) complexes [22]
and their high structural diversities. Copper(II) complexes have
been widely used in the fields like magnetism [23], biology [24],
biomimetic studies [25], medicine [26] and also in several catalytic
reactions (like cycloaddition [27], polymerization [28], CAC cou-
pling [29] and oxidation reactions). In oxidation reactions Cu(II)
complexes show catalytic activities in the epoxidation of olefins
[30], oxidation of alkanes [31], sulfoxidation [32] and oxidation
of alcohols [33].
In this paper we report a green reaction methodology under
solvent free condition for one-pot and selective synthesis of
1,3-oxazolidine based compound, (2-pyridin-2-yl-1,3-oxazolidine-
4,4-diyl)dimethanol (H3L). The coordination potential of this
compound was studied in preparing Cu(II) complexes. By consider-
ing this fact that the oxidation of alcohols to their corresponding
carbonyl compounds is one of the attractive and fundamental
reactions in organic synthesis and catalytic studies [33,34], and
also in continuous to our studies on the catalytic oxidation of
benzyl alcohols [35], the catalytic activity of the obtained Cu(II)
complexes were studied in the catalytic oxidation of benzyl
alcohols. The effect of various effective parameters in the catalytic
oxidation reactions were also studied in order to obtain optimum
condition for the catalytic reaction.
According to the previous reports it is obvious that the reaction
of aldehydes/ketones with 1,2-amino alcohols is the simplest
method for production of 1,3-oxazolidines. Although a Schiff base
is the usual product of the reaction between primary amines and
aldehydes/ketones [20], 1,3-oxazolidines can also be obtained if
the alcoholic functionality of 1,2-amino alcohol involve in the
water elimination reaction from hemiaminal intermediate (see
Scheme 1a). Recent reports on the crystal structure of hemiaminal
compounds [21] confirm that the hemiaminal intermediates can be
stabilized by controlling electronic parameters. So, it is predictable
that the elimination of water molecule from this intermediate can
be a controllable reaction, too. It is obvious that the elimination of
water molecule by hydrogen atom of alcoholic group (AOH) has
higher energy barrier than hydrogen atom of NH moiety. Undoubt-
edly the metal ions have catalytic effect in obtaining a 1,3-oxazo-
lidine instead of a Schiff base, but it is obvious that the other
parameters like electronic character of the substituents can also
control the water elimination reaction from hemiaminal interme-
diate. Considering literature indicates that in most of 1,3-oxazo-
lidine complexes [12–18] a pyridine ring is connected to 1,3-
oxazolidine ring while there is only one report on compounds con-
taining phenolic moiety [15] (which is formed in the presence of
lanthanide ions and also contains electron withdrawing aldehyde
moiety). This matter indicates that electron withdrawing groups
like pyridine ring facilitate the formation of 1,3-oxazolidine ring
while electron donating groups like phenol derivatives facilitate
the formation of Schiff base compounds. So, by employing strong
electron withdrawing groups connected to aldehyde/ketone func-
tionality and using 1,2-amino alcohols, selective production of
1,3-oxazolidines is not unreachable. Temperature is also the next
effective parameter in this reaction. It is obvious that higher
2. Experimental
2.1. Materials and instrumentations
Copper(II) chloride dihydrate, 2-amino-2-(hydroxymethyl)-
1,3-propanediol, 2-pyridinecarboxaldehyde and other starting
materials were purchased from Merck and used as received
without further purification. 1H and 13C NMR spectra of ligand in
CDCl3 solution were measured on a Bruker 250 and 62.9 MHz
spectrometer, respectively, and the chemical shifts were indicated
in ppm relative to tetramethylsilane (TMS). FT–IR spectra were
recorded as KBr disks with a Bruker FT–IR spectrophotometer.
UV–Vis spectra of solutions were recorded on a thermo spectronic,
Helios Alpha spectrometer. The products of the oxidation reactions
were determined and analyzed by gas chromatography (GC) using
a HP Agilent 6890 gas chromatograph equipped with an HP-5 cap-
illary column (phenyl methyl siloxane 30 m ꢁ 320 mm ꢁ 0.25 mm).
Elemental analyses were carried out by using a Perkin–Elmer 240
elemental analyzer. The atomic absorption analyses were carried
out by using a Varian Spectra AA-220 equipment.
2.2. Synthesis of (2-pyridin-2-yl-1,3-oxazolidine-4,4-diyl)dimethanol
(H3L)
A
mixture of 2-Amino-2-(hydroxymethyl)-1,3-propanediol
(0.242 g, 2.00 mmol) and 2-pyridinecarboxaldehyde (0.188 ml,
2.00 mmol) were added to a closed glass and the mixture was
heated up to 110 °C for 4 h in solvent free condition. The resulting
oily product was dissolved in methanol and then, was purified by
column chromatography on silica gel. A mixture of ethyl acetate:
petroleum ether (60: 40 v/v) was used as washing solvent in chro-
matography. The pure ligand, (2-pyridin-2-yl-1,3-oxazolidine-4,4-
diyl)dimethanol (H3L), was obtained as white solid after evaporat-
ing the solvent at room temperature. Yield 74.9% (0.315 g). M.p.
145–148 °C. Anal. Calc. for C10H14N2O3 (MW = 210.23): C, 57.13;
H, 6.71; N, 13.33%. Found: C, 57.18; H, 6.67; N, 13.36%. FT-IR
Scheme 1. a) Mechanism of the formation and the synthesis pathway of H3L; b)
synthesis pathways of complex 1 and c) complex 2.

R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
79
(KBr, cmꢂ1): 3385 (br,s), 3013 (w), 2924 (m), 2874 (m), 1645 (w),
1596 (vs), 1574 (s), 1464 (s), 1437 (s), 1385 (m), 1300 (w), 1221
(w), 1151 (m), 1047 (vs), 1024 (vs), 1006 (w), 982 (w), 892 (w),
775 (s), 754 (s), 696 (w), 665 (w), 645 (w), 623 (m), 598 (w), 579
(w), 529 (w), 419 (w). 1H NMR (250 MHz, DMSO d6, 25 °C, TMS):
d 8.43 (d, 1H, CH = N, pyridine); 7.71–6.77 (m, 3H, CH, pyridine
ring); 5.66 (s, 1H, CH); 5.23 (s, 1H); 4.54 (s, 1H, NH); 4.94 (s, 1H,
OH); 4.52 and 4.13 (d-d, 2H, CH2, 1,2J = 8 and 8.75), 3.96–3.67 (m,
4H, 2CH2) ppm. 13C NMR (62.90 MHz, DMSO d6): d = 154.4, 149.2,
136.6, 123.4, 122.0, 94.5, 93.1, 74.0, 72.1, 65.0 ppm. UV–Vis (in
2942 (w), 2891 (w), 1606 (s), 1480 (m), 1470 (m), 1452 (m),
1420 (w), 1406 (s), 1384 (w), 1339 (m), 1304 (w), 1268 (w),
1238 (m), 1180 (w), 1166 (w), 1154 (w), 1113 (m), 1095 (vs),
1074 (s), 1054 (vs), 1027 (vs), 975 (m), 950 (m), 905 (w), 871
(w), 779 (s), 771 (s), 732 (m), 675 (w), 650 (m), 634 (m), 621
(m), 605 (s), 528 (w), 479 (w), 455 (w), 424 (s), UV–Vis (in CH3OH,
c = 5 ꢁ 10ꢂ5 M, kmax [nm] with
e
[Mꢂ1 cmꢂ1]): 218 (30800), 259
(29500), 720 nm (280).
2.5. X-ray crystallography
CH3OH, c = 5 ꢁ 10ꢂ5 M, kmax [nm] with
e
[Mꢂ1 cmꢂ1]): 213
(19900), 260 (24400).
The green needle crystals of complex 1 and blue needle crystals
of complex 2 were obtained from CH3OH solvent by thermal gradi-
ent method. The crystals of complex 1 and 2 are crystallized in
orthorhombic space group Pbca and monoclinic space group P2
(1)/c, respectively. The X-ray data were collected at 293(2 ) K with
2.3. Synthesis of [Cu(H3L)Cl2]ꢀCH3OH (1)
The ligand H3L (0.210 g, 1.00 mmol) and CuCl2ꢀ2H2O (0.170 g,
1.00 mmol) were placed in the main arm of a branched tube.
Methanol was carefully added to fill the arms. The tube was sealed
and the reagents containing arm immersed in an oil bath at 60 °C
while the other arm was kept at ambient temperature. After four
days, green crystals were appeared in the cooler arm, which were
separated and washed by methanol. Yield 77.8% (0.293 g). Anal.
Calc. for C11H18Cl2CuN2O4 (MW = 376.72): C, 35.07; H, 4.82; N,
7.44; Cu, 16.87. Found: C, 35.09; H, 4.85; N, 7.41; Cu, 16.91%. FT-
IR (KBr, cmꢂ1): 3345 (s, br), 3306 (s), 3232 (s), 2951 (w), 2854
(w), 2839 (w), 1606 (s), 1573 (m), 1477 (m), 1445 (w), 1432 (w),
1384 (m), 1369 (m), 1270 (w), 1261 (m), 1248 (m), 1232 (m),
1170 (w), 1156 (s), 1106 (m), 1083 (s), 1072 (vs), 1048 (vs), 1027
(vs), 1008 (s), 959 (s), 936 (w), 905 (w), 861 (w), 799 (m), 779
(vs), 760 (m), 721 (m), 675 (w), 652 (s), 633 (s), 615 (m), 528
(m), 474 (w), 450 (w), 419 (m). UV–Vis (in CH3OH, c = 5 ꢁ 10ꢂ5
an Oxford Sapphire CCD diffractometer using MoKa radiation k =
0.71073 Å and -2h scans. The analytical absorption correction
x
was applied with the maximum and minimum transmissions of
0.919 and 0.512 (complex 1), 0.891 and 0.693 (complex 2), respec-
tively [36]. The structures were solved by direct methods and
refined with the full-matrix least-squares method on F2 with the
use of SHELX2014 program packages [37]. The C-bonded hydrogen
atoms were calculated form geometry, the OH and NH hydrogen
atoms were located from the difference electron density maps.
All hydrogen atoms were constrained during refinement. The
molecular structure plots were prepared using Diamond [38]. The
data collection and refinement processes are summarized in
Table 1, selected bond lengths and angels are presented in Table 2
and Table S1, respectively.
M, kmax [nm] with
e
[Mꢂ1 cmꢂ1]): 217 (32800), 258 (20900), 285
2.6. General procedure for catalytic studies
(17600), 650 nm (300).
Oxidation reactions were carried out under aerial condition
using 3 ml of solvent and aqueous H2O2 (30 wt%) or TBHP as oxi-
dant. In a typical experiment, the catalyst (complex 1 or 2), the sol-
vent (3 ml) and the substrate (1 mmol) were placed in a 25 ml
round bottomed glass flask. The mixture was heated to the desired
temperature and after addition of oxidant, the resulting solution
was kept stable at the selected temperature to continue the reac-
tion. At appropriate intervals, aliquots were taken and analyzed
by GC. The oxidation products were identified by comparison of
their retention times with those of authentic samples. Moreover,
the progress of the reaction and products were also checked by
TLC.
2.4. Synthesis of [Cu(H3L)2]ꢀ2Cl (2)
Complex 2 was synthesized by a similar procedure reported for
complex 1 with changing the molar ratios of the reagents. The blue
crystals of complex 2 were obtained from the reaction of H3L
(0.420 g, 2.00 mmol) and CuCl2ꢀ2H2O (0.170 g, 1.00 mmol) in
methanol during three days. The crystals were filtered off, washed
with cold methanol and dried at air. Yield 70% (0.38 g). Anal. Calc.
for C20H28Cl2CuN4O6 (MW = 554.91): C, 43.29; H, 5.09; N, 10.10;
Cu, 11.45. Found: C, 43.22; H, 5.05; N, 10.16; Cu, 11.38%. FT-IR
(KBr, cmꢂ1): 3380 (m, br), 3312 (s), 3258 (s), 3177 (s), 3143(s),
Table 1
Crystallographic data of complexes 1 and 2.
Compound
Complex 1
11H18Cl2CuN2O4
Complex 2
Chemical formula
MW
C
C40H56Cl4Cu2N8O12
1109.80
376.71
Crystal system, space group
Orthorhombic, Pbca
Monoclinic, P2(1)/c
a, b, c (Å)
b (°)
15.389(3), 8.939(2), 21.778(4)
90
2995.8(10)
1.670
14.8940(6), 8.6206(4), 18.2923(8)
99.387(4)
2317.20(18)
1.591
2
V (Å3)
D (g/cm3)
Z
8
F(000)
1544
1.827
1148
1.218
m (mmꢂ1
)
Crystal size (mm)
0.53 ꢁ 0.18 ꢁ 0.05
18792/3518 [R(int) = 0.0491]
25.00, 99.9%
3518/0/183
1.054
0.38 ꢁ 0.26 ꢁ 0.11
5238/5240 [R(int) = 0.0356]
25.00, 99.9%
5238/0/302
Reflections collected/unique
Completeness to theta
Data/restraints/parameters
Goodness-of-fit on F2
1.208
Final R indices [I > 2sigma(I)]
R indices (all data)
D
R1 = 0.0327, wR2 = 0.0788
R1 = 0.0491, wR2 = 0.0832
0.485, ꢂ0.430
R1 = 0.0553, wR2 = 0.1135
R1 = 0.0692, wR2 = 0.1162
0.490, ꢂ0.679
qmax, D )
qmin (e. Åꢂ3

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R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
about 3370 cmꢂ1 is due to the O–H groups of H3L involved in
hydrogen bonding interactions [41,42].
Table 2
Selected bond lengths [Å] for complexes 1 and 2.
The electronic spectra of complex 1 (green), complex 2 (blue)
and H3L (colourless) in methanol are shown in Fig. 1. The ligand
(H3L) displays strong bands at 213 and 260 nm which can be
Complex 1
Complex 2
Bond
Lengths [Å]
Bond
Lengths [Å]
Cu1AN1
Cu1AN6
Cu1ACl2
Cu1ACl1
Cu1AO9
2.0099(18)
2.0482(17)
2.2523(7)
2.4332(17)
2.4332(17)
Cu1AAN6A
Cu1AAN1A
Cu1AAO9A
Cu1BAN6B
Cu1BAN1B
Cu1BAO9B
2.003(3)
2.078(3)
2.394(3)
2.012(3)
2.052(3)
2.417(3)
assigned to
p ?
p⁄ transitions and the shoulder at about 280 nm
can be attributed to n ? p⁄ transitions. The shifts of the bands in
complexes relative to the free ligand indicate the coordination of
H3L to Cu(II) ion. For complexes, the broad band at about
300–350 nm is due to the ligand to metal charge transfer (LMCT)
transitions. The other higher energy intense transitions at about
217 and 260 nm are due to intraligand transitions. Both 1 and 2
exhibit a weak broad band in the range of 600–700 nm which is
related to d-d transitions [43].
3. Results and discussion
3.1. Syntheses and spectroscopy
3.2. Crystal structure of complex 1
The reaction of 2-amino-2-hydroxymethyl-propane-1,3-diol
with 2-pyridinecarboxaldehyde with 1: 1 M ratio in solvent free
condition at 110 °C gave the 1,3-oxazolidine based ligand, H3L
(Scheme 1a). Although the reaction of primary amines with alde-
hydes usually generates Schiff base compounds, due to the pres-
ence of alcoholic functionalities in suitable position and the
presence of electron-withdrawing pyridine ring together with the
high temperature we could successfully obtain 1,3-oxazolidine
ring instead of the Schiff base compound. It is obvious that 1,3-oxa-
zolidine ring is produced by the attack of alcoholic OH group to the
hemiaminal intermediate which terminates to the 1,3-oxazolidine
ring by removing a H2O molecule (see Scheme 1a). Higher temper-
ature together with the formation of five membered ring facilitates
the formation of 1,3-oxazolidine instead of Schiff base. It is obvious
that the presence of pyridine ring as an electron-withdrawing
group has also considerable effect in the attack of OH group to
the hemiaminal intermediate. The formation of H3L was confirmed
by elemental analyses, FT-IR and NMR spectroscopy (Figs. S1–S3).
In the FT-IR spectrum of H3L, the broad absorption band at 3385
cmꢂ1 can be assigned to the NH and OH groups which the band
broadening can be attributed to the hydrogen bond interactions.
The characteristic signals for the vibration of CAO and CAN bonds
of oxazolidine ring were observed at 1000–1051 cmꢂ1. In the FT-IR
spectrum of H3L the absence of azomethine C@N vibration, which
usually observes at about 1610 cmꢂ1 [39], indicates the product of
the reaction between amine and aldehyde is not a Schiff base com-
pound. Both 1H NMR and 13C NMR spectra of the ligand (see
Figs. S2 and S3) indicate that there are two diastereomers in the
solution which can be attributed to the chiral carbon atom
between nitrogen and oxygen atoms of the 1,3-oxazolidine ring.
The copper(II) complexes were synthesized by a similar method
from the reaction of H3L and copper(II) chloride dihydrate with dif-
ferent molar ratios in methanol (see Scheme 1). During these syn-
thetic attempts two mononuclear Cu(II) complexes were isolated.
The reaction of H3L with CuCl2ꢀ2H2O in 1: 1 M ratios afforded com-
plex 1 with the general formula of [Cu(H3L)(Cl)2]ꢀCH3OH as the sole
product. In the same reaction system, another mononuclear copper
(II) complex with general formula of [Cu(H3L)2]2ꢀ2Cl (2) was
obtained by changing the molar ratios of materials into 2: 1. Con-
sequently, in this reaction the molar ratio of the reagents has con-
siderable effect on the structure of the final products. Comparing
the FT-IR spectra of complexes (Figs. S4 and S5) with free ligand
(Fig. S1) shows that some bands of the 1,3-oxazolidine ligand are
shifted to lower energies in the FT-IR spectra of complexes. This
matter confirms the coordination of 1,3-oxazolidine ring to the
metal ion. The bands in the range of 450–530 cmꢂ1 in the FT-IR
spectra of 1 and 2 can be assigned to the CuAN and CuAO vibra-
tions [40]. Also, FT-IR spectra of complexes show strong absorption
The molecular structure of complex [Cu(H3L)(Cl)2]ꢀCH3OH (1),
determined by X-ray analysis, is shown in Fig. 2a and selected bond
lengths and angles are presented in Table 2 and Table S1, respec-
tively. This complex crystallizes in the orthorhombic crystal sys-
tem (space group Pbca). X-ray analysis showed that 1 is a neutral
mononuclear complex of Cu(II) which is obtained by the coordina-
tion of one molecule of H3L ligand and two chloride anions to the
Cu(II) ion. A methanol molecule is also located beside the complex
molecule which is stabilized in the crystal structure by hydrogen
bond interactions. The ligand is coordinated to the metal ion as a
neutral tridentate ligand through nitrogen atom of pyridine ring,
nitrogen atom of oxazolidine ring and the oxygen atom of one of
the alcoholic groups. Both alcoholic oxygen and oxazolidine nitro-
gen are coordinated to the Cu(II) as neutral groups without loosing
the hydrogen atom. The Cu(II) ion in 1 has a distorted square
pyramidal geometry (CuN2OCl2 coordination environment) with
s
s
= 0.06 (where
= 1 for ideal trigonal–bipyramidal and
s
= difference between the two largest angles/60;
= 0 for ideal square-pyra-
s
midal). Two coordinated chloride anions and two nitrogen atoms
of H3L (Noxazolindine and Npyridine) form the equatorial plane of the
square pyramidal geometry and the axial position is occupied by
the oxygen atom of the coordinated alcoholic arm. The axial CuAO
bond length is longer than the other bonds which is consistent
with the Z-out Jahn–Teller effect usually found in Cu(II) complexes
with a d9 electronic configuration [44]. The bond lengths around
the atoms in oxazolidine ring is very close to the bond lengths in
previously reported 1,3-oxazolidines [12–18]. The bond lengths
and angels around the carbon atom connected to the N and O
atoms of the 1,3-oxazolidines ring (C6) are in agreement with sp3
hybridization. This matter shows that the hybridization of this
atom is changed from sp2 in the starting aldehyde to sp3 in the final
product.
Due to the presence of chloride atoms, methanol molecule, oxa-
zolidine ring and alcoholic functionality in the structure of com-
plex 1, the presence of strong and directed hydrogen bond
interactions is predictable for this compound. Parameters of hydro-
gen bonding geometries in the structure of complex 1 are given in
Table S2. The coordinated alcoholic group and the NH group of oxa-
zolidine ring form hydrogen bonds with the coordinated chloride
atom of the neighboring complex. Uncoordinated alcoholic arm
of H3L also forms OAHꢀ ꢀ ꢀO hydrogen bond with uncoordinated
methanol molecule. As a result, a 1D polymeric chain is generated
by these interactions (see Fig. 2b).
3.3. Crystal structure of complex 2
The needle blue crystals of complex 2 crystallize in the mono-
clinic crystal system (P2(1)/c space group). X-ray analysis indicated
that the unit cell in complex 2 contains two crystallographically
bands at about 3170–3270 cmꢂ1 which can be ascribed to
stretching vibration of the oxazolidine ring [41]. The broad peak at
m(NAH)

R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
81
Fig. 1. The electronic spectra of H3L, complex 1 and complex 2 in methanol.
Fig. 2. a) Molecular structure of complex 1 with atom numbering scheme; b) 1D polymeric chain obtained by hydrogen bonds in the crystal structure of complex 1. Pink
dashed lines shows hydrogen bond interactions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
Fig. 3. a) Molecular structure of two independent molecules in the crystal of complex 2, and b) their overlay (indicated by green and blue colour); c) 2D polymeric network
obtained by O–Hꢀ ꢀ ꢀCl, C–Hꢀ ꢀ ꢀCl and N–Hꢀ ꢀ ꢀCl interactions in the crystal structure of complex 2. Pink dashed lines show hydrogen bond interactions. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
independent molecules (Fig. 3a). However, the overlay of these two
molecules shows that they are almost identical (Fig. 3b). The struc-
tures of these two molecules are shown in Fig. 3 and selected bond
lengths and angles around the metal centers are collected in Table 2
and Table S1, respectively. In complex 2 the Cu atoms are located
on the center of symmetry and two parts of molecules are related
to each other by symmetry operation (i = ꢂx+1,ꢂy,ꢂz + 1). In 2, two
molecules of ligand are coordinated to the metal ion as neutral tri-
dentate N2O-donor ligands. Therefore, this compound is a cationic
complex and two chloride anions are located beside it as counter
anions. The coordination environment around the Cu(II) centers
(trans-CuN4O2) is best described as distorted octahedral geometry
which is provided by coordination of one oxygen and two nitrogen
atoms of two H3L ligands. The equatorial position of octahedral
geometry is created by coordination of four nitrogen atoms (two
N-pyridine and two N-oxazolidine) and the axial positions are
occupied by the oxygen atoms of alcoholic groups. The axial CuAO
bonds are considerably longer than the equatorial CuAN bonds
which can be attributed to the Z-out Jahn-Teller distortion. The
CuAN and CuAO bonds are in the normal range reported for Cu
(II) complexes [45]. The chloride anions, located beside the com-
plex molecules, are stabilized in the crystal packing by strong
and directed OAHꢀ ꢀ ꢀCl, CAHꢀ ꢀ ꢀCl and NAHꢀ ꢀ ꢀCl interactions (see
Fig. 3c and Table S2). A 2D polymeric network is generated by
these interactions.
studies the catalytic oxidation of organic substrates in the presence
of transition metal complexes can be affected by some parameters
like temperature, the nature of oxidant, time, solvent, etc [35,46].
Therefore, we studied the effect of some important parameters in
order to find the optimum conditions for the catalytic oxidation
of benzyl alcohol in the presence of complexes 1 and 2. The results
of these studies are summarized in Table 3. In the control reactions
(in the absence of catalyst or oxidant) the oxidation of benzyl alco-
hol did not occur after 5 h. The results of these experiments
revealed that the presence of both catalyst and oxidant (H2O2) is
essential for the oxidation of benzyl alcohol. The effect of oxidant
to substrate ratio is one of the key parameters influencing on this
type of catalytic reactions. Thus, oxidations of benzyl alcohol with
different H2O2: bezylic alcohol molar ratios were carried out at
70 °C. The reactions were done by using 1:1, 2:1, 3:1 and 4:1 M
ratios of H2O2: benzyl alcohol in acetonitrile as solvent. The profile
for the oxidation of benzyl alcohol is depicted in Scheme 2, which
shows the oxidation reaction can proceed via two steps: (1) oxida-
tion of benzyl alcohol to benzaldehyde, and (2) oxidation of
benzaldehyde to benzoic acid. The analyses of the reactions
showed benzyl alcohol was oxidized to both benzaldehyde and
benzoic acid in the reaction condition. Benzaldehyde was the main
product of the catalytic reaction, but nevertheless, some amounts
of benzoic acid were also formed as the side product by over
oxidation of benzaldehyde. The results of these studies are shown
in Fig. 4 which indicates the conversion of benzyl alcohol increases
continuously as time increases and then remains relatively con-
stant after about 120 min. This can be attributed to the bleaching
of the catalyst or to the other deactivation reactions which cause
to decrease the catalytic acitivty. After 120 min the selectivity
was decreased by increasing the amount of benzoic acid. Higher
3.4. Catalytic oxidation of benzyl alcohol
The catalytic activity of complexes 1 and 2 was tested in the
oxidation of bezylic alcohols in the presence of aqueous H2O2
(30%) as a cheap, safe and green oxidant. According to previous

R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
83
Table 3
Comparison the catalytic activities of complexes 1 and 2 in the oxidation of benzyl alcohol under different conditions.
Entry
Catalyst
Oxidant (mmol)
Solvent
T (°C)
Yield (%)
Selectivity (%)a
Aldehyde/acid ratio
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
H2O2 (1)
H2O2 (2)
H2O2 (3)
H2O2 (4)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
TBHP (2)
TBHP (2)
TBHP (2)
H2O2 (2)
H2O2 (2)
H2O2 (2)
TBHP (2)
TBHP (2)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CHCl3
Acetone
CH2Cl2
THF
CH3OH
EtOAc
Toluene
DMF
70
70
70
70
r.t.
40
60
80
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
58
96
88
91
28
55
93
97
42
85
72
70
63
83
35
12
98
42
75
88
58
40
92
62
72.4
73.9
68.2
58.2
78.6
76.4
82.8
72.2
76.2
82.4
86.1
82.9
60.3
78.3
77.1
83.3
89.8
76.1
78.6
79.5
62.1
77.1
84.8
75.8
42/16
71/25
60/28
53/38
22/6
42/13
77/16
70/27
32/10
70/15
62/10
58/12
38/25
65/18
27/8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
10/2
CH3CN
CHCl3
CH3OH
CH3CN
CH3OH
CHCl3
88/10
32/10
59/16
70/18
36/22
31/9
CH3CN
CH3OH
78/14
47/15
Reaction conditions: solvent 3 ml; catalyst 2.65 lmol (0.0010 g complex 1, 0.0015 g complex 2); time: 120 min.
a
molAldehyde
Selectivity ¼ molAldehydeþmolAcid ꢁ 100.
amounts of H2O2 led to a growth in the conversions but the selec-
tivity toward the aldehyde was decreased. This matter shows that
the higher amount of oxidant facilitates over oxidation of
benzaldehyde to benzoic acid. The highest selectivity in aldehyde
formation with reasonably high conversion efficiency was obtained
with 2 equiv. of H2O2. Therefore, this ratio was selected as the
optimum molar ratio of oxidant: substrate in other experiments.
Scheme 2. Oxidation of benzyl alcohol catalyzed by complexes 1 and 2.
Fig. 4. Effect of H2O2 concentration on the oxidation of benzyl alcohol by complex 1. Reaction conditions: complex 1 (2.65
mmol); temperature (70 1 °C).
lmol, 0.001 g); CH3CN (3 ml); benzyl alcohol (1

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R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
Fig. 5. Effect of the reaction temperature on the oxidation of benzyl alcohol by complex 1. Reaction conditions: complex 1 (2.65 lmol, 0.001 g); CH3CN (3 ml); benzyl alcohol
(1 mmol); H2O2 (2 mmol).
In order to get the best reaction temperature, the oxidation of
benzyl alcohol was studied at different temperatures (room tem-
perature, 40, 60, 70 and 80 °C), and the results are shown in
Fig. 5. At room temperature, the oxidation of benzyl alcohol was
low. Increasing temperature from 25 to 70 °C increased the conver-
sion but the selectivity toward aldehyde was also decreased at
higher temperatures. At 80 °C, the oxidation of benzyl alcohol
was almost the same as 70 °C. This matter can be attributed to over
oxidation of benzaldehyde to benzoic acid since at 80 °C the selec-
tivity of aldehyde was significantly decreased. It is obvious that
there is a competition between oxidation of benzaldehyde and oxi-
dation of benzyl alcohol at higher temperatures. Although maxi-
mum conversion of benzyl alcohol was higher at 70 °C, since
selectivity toward aldehyde was better at 60 °C, this temperature
was considered as the optimum temperature for oxidation of ben-
zyl alcohol to benzaldehyde.
The solvent effect in the catalytic oxidation of benzyl alcohol
was also investigated (Table 3, Fig. 6). For this purpose, acetonitrile,
toluene, methanol, chloroform, dichloromethane, DMF, THF and
ethyl acetate were used as solvents in other catalytic reactions. It
Fig. 6. Effect of the solvent on the oxidation of benzyl alcohol with H2O2 catalyzed by complex 1. Reaction conditions: complex 1 (2.65
alcohol (1 mmol); H2O2 (2 mmol); temperature (60 1 °C).
lmol, 0.001 g); solvent (3 ml); benzyl

R. Bikas et al. / Inorganica Chimica Acta 478 (2018) 77–87
85
was observed that the catalytic activity of the catalyst decreased
with respect to acetonitrile > acetone > ethyl acetate > dichloro-
methane > THF > methanol > chloroform > toluene > DMF. The sol-
vent has a significant effect on the reaction even though the details
of the exact role of the solvent is not very clear. However, it is
known that acetonitrile also affect on the activation of H2O2 by
forming a perhydroxyl anion (OOHꢂ) or other oxygen transfer
intermediates. In addition, acetonitrile has a comparatively good
solubility, high dielectric constant and low donor number (coordi-
nation ability). Low catalytic activity in DMF can be attributed to
its high coordination ability.
In order to study the effect of the oxidant nature, the oxidation
of benzyl alcohol was investigated in the presence of TBHP instead
of H2O2. The results of these studies are summarized in Table 3
which indicates that the nature of the oxidant has also significant
effect on this catalytic system. In general, the results show that
both conversion and selectivity are better when TBHP was used
as oxidant. This matter can be attributed to weaker OAO bond in
TBHP respect to H2O2. Better selectivity in TBHP maybe is related
to the steric hindrance of TBHP which can affect on the oxidation
process.
The catalytic activity of complex 2 was examined under the
optimized conditions obtained for complex 1 (H2O2/benzyl alcohol
molar ratio = 2, acetonitrile, reaction temperature 60 °C). The cat-
alytic activity of complex 2 was lower than complex 1 which can
be attributed to its octahedral coordination geometry and higher
steric hindrance. The available space around the metal ion to inter-
act with the oxidant and substrate is limited in the case of complex
2. The metal core in complex 1 with square pyramidal geometry
has more available environment to interact with the reagents
and shows higher catalytic activity. However, the selectivity of
complex 2 toward aldehyde is higher than complex 1 which can
be also due to the higher steric hindrance around the metal ion
in this complex.
and the summary of results is collected in Table 4. Chloro-substi-
tuted benzyl alcohols were also oxidized to aldehyde by 1 in the
presence of H2O2. Generally, all these substrates showed lower
conversion than benzyl alcohol but their selectivity toward alde-
hyde was considerably better. This matter indicates that both
selectivity and efficiency of this catalytic system are dependent
on the electronic and steric demands of the chlorine group. The
substrate with chlorine substituent on the ortho- position showed
poor activity in comparison with the corresponding meta- and
para-substituted substrates. This matter can be attributed to the
higher steric hindrance in the former case. Low conversion of sub-
strate with the chlorine substituent on the meta-position relative
to the para- can be attributed to the higher electron-withdrawing
effect of the Cl group at this position.
Although the details of catalytic mechanism are not studied in
the present work and the total mechanism of the reaction is not
fully clear, on the basis of the electronic absorption spectroscopy
studies (Figs. S6 and S7) and also by considering the previous stud-
ies on catalytic oxidation reactions by copper(II) complexes [47], it
is predicted that the key step in this catalytic process is the oxida-
tion of substrates by a peroxido-copper species formed after addi-
tion of H2O2 to the solution of complex. Although such species are
not identified in this study, electronic absorption spectroscopic
studies (see Figs. S6 and S7) indicate that the solutions of com-
plexes 1 and 2 are sensitive towards addition of H2O2. By increas-
ing H2O2 to the methanolic solution of complexes 1 and 2, the
intensity of absorption band at about 200–300 nm increases. These
changes can be attributed to the interaction of Cu(II) complexes
with H2O2 and the formation of peroxido-copper species that oxi-
dize the substrate [47].
4. Conclusion
In summary, 1,3-oxazolidine based ligand, H3L, was selectively
synthesized in solvent free condition at 110 °C. Two new Cu(II)
complexes, [Cu(H3L)Cl2]ꢀCH3OH (1) and [Cu(H3L)2]ꢀ2Cl (2), were
synthesized by the reaction of H3L and CuCl2ꢀH2O in methanol.
The ligand and its Cu(II) complexes were characterized by elemen-
tal analysis and spectroscopic methods. The structure of complexes
was also studied by single crystal X-ray analysis which revealed
square pyramidal and octahedral geometry around the Cu(II) ions
3.5. Oxidation of chloro-substituted benzyl alcohols
To explore the potential of complex 1 in the oxidation of benzyl
alcohols, ortho-, meta- and para-chloro substituted benzyl alcohols
were selected as substrates. The catalytic reactions were per-
formed under the optimized condition obtained for benzyl alcohol
Table 4
Oxidation of various primary benzyl alcohols by complex 1.
Entry
1
Substrate
Yield (%) (Selectivity)
Products (%)
Aldehyde
Acid
93 (83)
(77)
(16)
2
3
82 (90)
75 (96)
4
72 (93)
Reaction conditions: complex 1 (2.65
lmol, 0.001 g); CH3CN (3 ml); H2O2 (2 mmol); temperature (60 1 °C).

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in 1 and 2, respectively. The catalytic activity of complexes was
studied in the oxidation of benzyl alcohol and its chloro-substi-
tuted derivatives with H2O2 or TBHP as oxidant. The results of cat-
alytic reactions showed that these complexes can catalyze the
oxidation of primary benzyl alcohols but the reaction conditions
such as the nature and molar ratio of oxidant, temperature, solvent
and the electronic and steric effects of the substrate are some
important parameters in the activity and selectivity of these cat-
alytic systems.
Acknowledgments
The authors are grateful to Imam Khomeini International
University, the University of Zanjan and Nicolaus Copernicus
University for support of this study.
Appendix A. Supplementary data
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(2018) 267–274;
CCDC 1557443 and 1557444 contain the supplementary crys-
tallographic data for 1 and 2, respectively. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/re-
trieving.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 https://doi.
org/10.1016/j.ica.2018.03.038.
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