Synthesis, characterization and catalytic oxidation property of copper(I) complexes containing monodentate acylthiourea ligands and triphenylphosphine

Article abstract of DOI:10.1016/j.poly.2016.10.038

The reactions between [CuCl₂(PPh₃)₂] and 3,3-diphenyl-1-(2,4-dichlorobenzoyl)thiourea (HL1), 3,3-diisobutyl-1-(2,4-dichlorobenzoyl)thiourea (HL2) or 3,3-diethyl-1-(2,4-dichlorobenzoyl)thiourea (HL3) in benzene gave new four-coordinated tetrahedral copper(I) complexes of the type [CuCl(HL)(PPh₃)₂] (1–3) [HL = 3,3-dialkyl/aryl-1-(2,4-dichlorobenzoyl)thiourea derivatives]. These complexes were then characterized by analytical and spectroscopic (IR, UV/Vis,¹H,¹³C and³¹P NMR) techniques. The molecular structure of a representative complex, [CuCl(HL1)(PPh₃)₂] (1), was determined by the single crystal X-ray diffraction method which reveals a distorted tetrahedral geometry around the Cu(I) ion. The complexes 1–3 (0.007 mmol) were found to be active catalysts for the oxidation of primary and secondary alcohols 5.0 mmol to their corresponding acids and ketones, respectively, in the presence of hydrogen peroxide (6.0 mmol) in 1-butyl-3-methylimidazolium hexafluorophosphate {[bmim][PF₆]} (0.1 mL) without any additive and base, at 70 °C.

Full text of DOI:10.1016/j.poly.2016.10.038

Accepted Manuscript
Synthesis, characterization and catalytic oxidation property of copper(I) com-
plexes containing monodentate acylthiourea ligands and triphenylphosphine
N. Gunasekaran, N.S.P. Bhuvanesh, R. Karvembu
PII:
S0277-5387(16)30535-6
DOI:
http://dx.doi.org/10.1016/j.poly.2016.10.038
POLY 12291
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
11 August 2016
11 October 2016
13 October 2016
Please cite this article as: N. Gunasekaran, N.S.P. Bhuvanesh, R. Karvembu, Synthesis, characterization and catalytic
oxidation property of copper(I) complexes containing monodentate acylthiourea ligands and triphenylphosphine,
Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.10.038
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Synthesis, characterization and catalytic oxidation property of copper(I)
complexes
containing
monodentate
acylthiourea
ligands
and
triphenylphosphine
a,
b
c
N. Gunasekaran *, N.S.P. Bhuvanesh , R. Karvembu
a
Centre for Nanotechnology & Advanced Biomaterials (CeNTAB), SASTRA University, Thanjavur
13 401, India
6
b
c
Department of Chemistry, Texas A&M University, College Station, TX 77842, USA
Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India
ABSTRACT
The reactions between [CuCl
2
(PPh
3
)
2
] and 3,3-diphenyl-1-(2,4-dichlorobenzoyl)thiourea (HL1),
3
,3-diisobutyl-1-(2,4-dichlorobenzoyl)thiourea
dichlorobenzoyl)thiourea (HL3) in benzene gave new four-coordinated tetrahedral copper(I)
complexes of the type [CuCl(HL)(PPh (1–3) [HL 3,3-dialkyl/aryl-1-(2,4-
dichlorobenzoyl)thiourea derivatives]. These complexes were then characterized by analytical
(HL2)
or
3,3-diethyl-1-(2,4-
3
)
2
]
=
1
13
31
and spectroscopic (IR, UV/Vis, H, C and P NMR) techniques. The molecular structure of a
representative complex, [CuCl(HL1)(PPh ] (1), was determined by the single crystal X-ray
3 2
)
diffraction method which reveals a distorted tetrahedral geometry around the Cu(I) ion. The
complexes 1-3 (0.007 mmol) were found to be active catalysts for the oxidation of primary and
secondary alcohols (5 mmol) to their corresponding acids and ketones, respectively, in the
presence of hydrogen peroxide (6.0 mmol) in 1-butyl-3-methylimidazolium hexafluorophosphate
{
[bmim][PF
6
]} (0.1 mL) without any additive and base, at 70 °C.
Keywords: Copper(I), Thiourea derivatives, Catalytic oxidation, H
2
2
O , Ionic liquids
_
*
_____________
Corresponding author. Tel.: +914362304346; fax: +91 4362264120.
E-mail address: sciguna@scbt.sastra.edu (N. Gunasekaran).
1

1
. Introduction
The selective oxidation of alcohols is among the most important reactions in synthetic
chemistry and offers a significant methodology for the introduction and alteration of functional
groups [1-4]. Several catalytic oxidation reactions occurring with stoichiometric amounts of
hazardous inorganic oxidants (CrO
3
, KMnO
4
, MnO , etc.) bring considerable amounts of
2
environmentally harmful waste [5]. Hence the discovery of new environmental friendly methods
for the selective oxidation of alcohols is an important goal in modern chemistry. In recent years,
there has been a growing interest in the development of new transition metal catalytic systems
for the oxidation of alcohols that work with hydrogen peroxide [6,7] or dioxygen [8,9] as the
ultimate stoichiometric oxidant, due to their obvious cost-effectiveness and conservation
advantages.
Ionic liquids (ILs) are now gaining appreciation as greener solvents in organic synthesis
and catalysis because of their unique properties, such as high thermal stability, non-volatility,
good ionic conductivity and high polarity [10,11]. Though ILs are considered relatively recent
supernatural chemicals, due to their diverse applications such as sensors [12],
fuel cells [13], batteries [14], capacitors [15], thermalfluids [16], plasticizers [17],
lubricants [18], ionogels [19], extractants [20] and solvents in analysis [21],
synthesis [22], catalysis [22] and separation [23] etc., today they are centenarian [24]. Structural
modifications of the anionic and cationic parts of the ILs provide an opportunity to tune their
physical and chemical properties. So ILs can be utilized as good alternatives to the traditional
organic solvents for inorganic, organic and polymeric materials [25-27].
N-(Alkyl/aryl)-N'-acylthiourea and N-di(alkyl/aryl)-N'-acylthiourea derivatives act as
versatile ligands with various coordination modes towards transition metal ions as they have
2

oxygen, nitrogen and sulfur donor atoms. Although these ligands exhibit common monoanionic
O,S-bidentate coordination towards a variety of transition metal ions [28,29], the unusual neutral
monodentate coordination through the sulfur atom is reported in some Au(I) [30],
Ag(I) [31], Hg(II) [32], Pt(II) [33], Cu(I) [34] and Ru(II) [35] complexes. In addition,
exceptional coordination modes, such as neutral bidentate [36] and monobasic bridging [37], are
also showed by the acylthiourea ligands in some complexes. These types of ligands and their
transition metal complexes have long been known not only for their biological applications but
also for their role in analytical sciences [38-40]. For instance, acylthiourea derivatives and their
Co(III) complexes have shown antibacterial [41] and antifungal [42] activities. Ni(II) complexes
containing 3-dialkyl/aryl-1-benzoylthiourea ligands exhibit cytotoxicity against T47D cell lines
[
43]. Water soluble (2,2′-bipyridyl)- and (1,10-phenanthroline)-platinum(II) benzoylthiourea
complexes have shown significant antimalarial activity [44]. Further, acylthiourea derivatives are
utilized as effective ligands for the extraction of some transition metal and post transition metal
ions [45,46].
Though the use of transition metal complexes bearing acylthiourea ligands have been
explored in a range of fields, their utility in catalysis is relatively less studied. In recent years, we
have tested N-[di(alkyl/aryl)carbamothioyl]benzamide complexes of Ru(II) [47], Ru(III) [48],
Cu(I) [49] and Co(III) [50] as catalysts for the oxidation of alcohols in the presence of oxidants
such as N-methylmorpholine-N-oxide(NMO)/hydrogen peroxide/tert-butyl hydroperoxide
(
TBHP) in organic solvents such as CH
2
Cl
2
/CH CN. As all these catalytic systems exhibited
3
effective yields under mild reaction conditions, we are interested to investigate the catalytic
oxidation property of transition-metal complexes containing acylthiourea ligands in an IL
solvent. As a part of our research, we report herein the synthesis and characterization of
3

copper(I) complexes containing 3,3-dialkyl/aryl-1-(2,4-dichlorobenzoyl)thiourea and their
enhanced catalytic activity for the oxidation of primary and secondary alcohols to the
corresponding carboxylic acids and ketones, respectively, in the presence of H
2
O
2
in 1-butyl-3-
methylimidazolium hexafluorophosphate ([bmim][PF ]) solvent. Our report describes the first
6
catalytic oxidation property of Cu(I) complexes incorporating acylthiourea ligands in an ionic
liquid solvent. The structures of the ligands used in this study are given in Fig. 1.
Fig. 1. Chemical structure of the ligands investigated herein.
2
. Experimental section
2
.1. Materials and reagents
All chemicals, including the ILs, used in this work were purchased from commercial
sources and utilized without further purification. The organic solvents were distilled and purified
according to the literature procedures. The precursor [CuCl (PPh ] was prepared in accordance
2
)
3 2
with the literature method [51]. For the preparation of the ligands HL1, HL2 and HL3, a
previously described procedure was followed [52].
2
.2. Physical measurements
Elemental analyses were performed on a Vario EL AMX-400 elemental analyzer.
Melting points are uncorrected and were obtained in open capillary tubes on a Sigma melting
-1
point apparatus. FT-IR spectra in the mid-IR region (4000–600 cm ) were recorded on a Nicolet
iS5 FT-IR spectrophotometer with KBr pellets. UV-vis spectra of the complexes in the 800 to
2
00 nm range were recorded in ethanol solutions using a PG Instruments double-beam UV-vis
4

spectrophotometer (Model T90+) with a quartz cell of 1 cm path length. Magnetic susceptibility
measurements were taken on a Sherwood scientific auto magnetic susceptibility balance. NMR
1
13
31
1
(
H, C and P) spectra were recorded on a Bruker Avance 400 MHz instrument in CDCl
3
. H
1
3
and C NMR spectra were referenced to TMS as an internal standard and H
3
PO
4
was used as an
31
internal standard for the P NMR spectra. The course of the reactions was analyzed with a
Shimadzu GC-2014 gas chromatograph equipped with a RTX-5 column (60 m length, 0.32 mm
inner diameter) and flame ionization detector.
2
.3. Synthesis of the Cu(I) complexes
The following general procedure was used to prepare all the new Cu(I) complexes. A
solution of the ligand (HL) (0.1373-0.1806 g, 0.45 mmol) in 10 mL of benzene was added to
CuCl (PPh ] (0.3 g, 0.45 mmol) dissolved in benzene (20 mL). The reaction mixture was
[
2
)
3 2
stirred for 48 h at room temperature and then the solution was concentrated to 3 mL under
vacuum. An orange precipitate was developed by the addition of 25 mL of ethyl acetate/n-
hexane mixture (1:9), filtered and recrystallized from dichloromethane/n-hexane and dried in
vacuo.
[
CuCl(HL1)(PPh
3
)
2
] (1) was prepared from [CuCl
2
(PPh
3
)
2
] (0.3 g, 0.45 mmol) and HL1
o
(
0.1806 g, 0.45 mmol). Yield: 90%. Orange solid. Decomposition point: 164 C.
C
56
H
44Cl
3
CuN
2
OP
2
S (1024.82): calcd. C 65.62, H 4.32, N 2.73, S 3.12; found C 65.58, H 4.27,
-1
N 2.67, S 3.06%. µeff = 0. FT–IR (KBr, cm ) ν: 3364 (N–H), 1703 (C=O), 1222 (C=S), 1434,
3
-1
-1
1
093, 743 (PPh
3
). UV [Ethanol, λ in nm (ε in dm mol cm )]: 240 (27,954), 252 (23,693).
) δ, ppm: 11.78 (s, 1H, N–H), 7.30-7.77 (m, 30H, phenyl from PPh ), 7.17-7.22
) δ, ppm: 183.59 (C=S), 161.41 (C=O), 128.07,
1
H NMR (CDCl
3
3
1
3
(
m, 13H, phenyl from HL1). C NMR (CDCl
3
5

1
1
29.05, 129.70, 129.79, 130.45, 130.99, 131.61, 132.11, 132.80, 133.31, 133.37, 133.57, 133.91,
3
1
34.77, 135.01, 135.15, 139.00, 146.43 (aromatic). P NMR (CDCl ) δ, ppm: 30.1 (s).
3
[
CuCl(HL2)(PPh
3
)
2
] (2) was prepared from [CuCl
2
(PPh
3
)
2
] (0.3 g, 0.45 mmol) and HL2
o
(
0.1626 g, 0.45 mmol). Yield: 85%. Orange solid. Decomposition point: 115 C.
C
52
H
52Cl
3
CuN
2
OP S (984.90): calcd. C 63.41, H 5.32, N 2.84, S 3.26; found C 63.33, H 5.27,
2
-1
N 2.78, S 3.17%. µeff = 0. FT–IR (KBr, cm ) ν: 3172 (N–H), 1696 (C=O), 1138 (C=S), 1432,
3
-1
-1
1
102, 745 (PPh
3
). UV [Ethanol, λ in nm (ε in dm mol cm )]: 241 (18,230), 252 (16,700).
) δ, ppm: 11.44 (s, 1H, N–H), 7.31-7.67 (m, 30H, phenyl from PPh ), 7.16-7.25
m, 3H, phenyl from HL2), 3.71 (d, 2H, CH ), 3.39 (d, 2H, CH ), 2.21-2.31 (m, 1H, CH), 2.01-
.11 (m, 1H, CH), 0.96 (d, 6H, CH ), 0.83 (d, 6H, CH ). C NMR (CDCl ) δ, ppm: 180.62
C=S), 163.64 (C=O), 128.38, 129.48, 129.72, 129.81, 131.01, 131.37, 132.42, 133.32, 133.51,
1
H NMR (CDCl
3
3
(
2
2
1
3
2
3
3
3
(
1
2
33.83, 134.18, 135.00, 135.15, 138.72 (aromatic), 61.57, 63.63 (CH), 27.92, 28.90 (CH ),
2
31
1.17, 21.32 (CH
3
). P NMR (CDCl
3
) δ, ppm: 32.1 (s).
[
CuCl(HL3)(PPh
3
)
2
] (3) was prepared from [CuCl
2
(PPh
3
)
2
] (0.3 g, 0.45 mmol) and HL3
o
(
0.1373 g, 0.45 mmol). Yield: 82%. Orange solid. Decomposition point: 196 C.
C
48
H
44Cl
3
CuN
2
OP S (931.82): calcd. C 61.87, H 5.08, N 3.01, S 3.44; found C 61.80, H 5.02,
2
-1
N 2.93, S 3.39%. µeff = 0. FT–IR (KBr, cm ) ν: 3168 (N–H), 1686 (C=O), 1132 (C=S), 1434,
3
-1
-1
1
095, 744 (PPh
3
). UV [Ethanol, λ in nm (ε in dm mol cm )]: 241 (13,641), 252 (9,926).
) δ, ppm: 10.81 (s, 1H, N–H), 7.39-7.71 (m, 30H, phenyl from PPh ), 7.16-7.25
), 0.78 (t, 3H, CH ).
1
H NMR (CDCl
3
3
(
m, 3H, phenyl from HL3), 2.95-4.04 (m, 4H, CH
2
), 1.19 (t, 3H, CH
3
3
1
3
C NMR (CDCl
3
) δ, ppm: 179.08 (C=S), 164.32 (C=O), 128.30, 129.62, 129.74, 129.82,
),
1
1
31.18, 131.44, 132.50, 133.14, 133.28, 133.53, 135.12, 138.85 (aromatic), 49.04, 50.11 (CH
2
31
2.76, 14.16 (CH
3
). P NMR (CDCl
3
) δ, ppm: 29.4 (s).
6

2
.4. Single crystal X-ray diffraction study
Recrystallization of [CuCl(HL1)(PPh
4:1) mixture yielded yellow crystals at room temperature, and a plate having dimensions 0.57 ×
.441 × 0.308 mm was chosen for the study. Intensity data were measured at 110 K on a
3
) ] (1) in an ethyl acetate and dichloromethane
2
(
0
BRUKER APEX 2 X-ray (three-circle) diffractometer with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). For each reflection, integrated intensity information was obtained by
reduction of data frames with APEX2 [53]. SADABS was employed to correct the data for
absorption effects [54]. The structure was solved by direct methods and refined using SHELXT,
2
with full-matrix least squares on F and using all the unique data [55,56]. Non-hydrogen atoms
were refined with anisotropic thermal parameters. The absence of additional symmetry and/or
voids was verified with PLATON [57]. The crystallographic data and the results of the
refinements are summarized in Table 1.
2
.5. Procedure for catalytic oxidation
To a mixture of alcohol (5 mmol) and complex 1 (7 mg, 0.007 mmol) in [bmim][PF
0.1 mL), hydrogen peroxide (6 mmol, 0.678 mL) was added. The reaction mixture was stirred
6
]
(
for 48 h at 70 °C. After the appropriate time, the resulting mixture was extracted with n-hexane
and then the extract was analyzed by GC. To identify the reaction products and determine their
percentage yields, the retention times and corresponding peak areas were compared with
authentic commercial samples under identical experimental conditions. To calculate the
percentage yields of the formed products the area normalization method was used. To determine
the isolated yields of acetophenone and benzoic acid, their crude mixture was purified by column
chromatography. To investigate the recyclability of the IL and the catalyst, [bmim][PF ] with
6
complex 1 was recovered by overnight drying in vacuo. For the five consecutive cycles, the
7

recovered complex 1 in [bmim][PF
6
] was used by adding fresh 1-phenylethanol (5 mmol) and
hydrogen peroxide (6 mmol) under the same experimental conditions.
3
. Results and discussion
3
.1. Synthesis
The reactions of equimolar quantities of [CuCl
2
(PPh
3
2
) ] with 3,3-diphenyl-1-(2,4-
dichlorobenzoyl)thiourea (HL1), 3,3-diisobutyl-1-(2,4-dichlorobenzoyl)thiourea (HL2) or 3,3-
diethyl-1-(2,4-dichlorobenzoyl)thiourea (HL3), respectively, in benzene at room temperature
afforded the new Cu(I) complexes [CuCl(HL1)(PPh
3
)
2
] (1), [CuCl(HL2)(PPh
3
2
) ] (2) or
[
CuCl(HL3)(PPh ] (3) (Scheme 1). While the reactions were carried out, the Cu(II) ion was
3 2
)
reduced to the Cu(I) ion with the oxidation of the ligand [58]. According to analytical, spectral
and single crystal X-ray diffraction studies, the new copper(I) complexes (1, 2 and 3) have a four
coordinated distorted tetrahedral geometry with one molecule of HL1, HL2 and HL3,
Scheme 1. Synthesis of the Cu(I) complexes.
8

respectively, and two molecules of PPh . In all the new complexes, the ligands HL1, HL2 and
3
HL3 show an unusual neutral monodentate coordination through the sulfur atom to the Cu(I) ion.
As the new Cu(I) complexes are air stable in nature, they are fit for catalytic oxidation reactions.
The analytical data obtained were in excellent agreement with the proposed molecular formulae.
3
.2. Spectroscopy
A medium intensity band was observed around 1210–1371 cm in the FT-IR spectra of
the free acylthiourea ligands, which is characteristic of the C=S group [59]. The bands at 1132,
-1
-1
1
138 and 1222 cm in the IR spectra of 1, 2 and 3, respectively could be assigned to the C=S
stretching mode of the thiocarbonyl group. In the IR spectra of the complexes, the band due to
the C=S group appeared in the lower frequency region, indicating that the sulfur atom of the
thiocarbonyl group is involved in coordination. The ligands and their copper complexes
-1
exhibited a strong band in the region 1703–1686 cm , which is characteristic of the C=O group.
This indicates that the C=O group is in a similar environment in the ligands and the complexes,
and thus is free from coordination. A characteristic broad band corresponding to the N–H group
-1
was observed in the region 3168–3364 cm in the spectra of the copper complexes. In addition,
-1
bands (around 1435, 1100 and 750 cm ) of the triphenylphosphine ligand also appeared in the IR
spectra of the complexes [49].
Magnetic susceptibility measurements (µeff = 0) of the Cu(I) complexes (1, 2 and 3)
showed that they are diamagnetic in nature and the copper center is in the +1 oxidation state. The
room temperature electronic spectra of complexes 1, 2 and 3 in ethanol solution clearly revealed
copper–ligand binding in the complexes as they are different from those of the corresponding
1
0
free ligands. The d configuration of the copper center was also confirmed by the electronic
spectra of the Cu(I) complexes, which had no d–d transitions. UV–vis bands appeared in the
9

-1
-1
3
region 240-252 nm (ε = 9929-27954 mol cm dm ) due to metal to ligand charge transfer
MLCT) or ligand centered π–π* transitions [60].
For the new Cu(I) complexes (1, 2 and 3) the H, C and P NMR spectra were recorded
(
1
13
31
in CDCl
3
solution. All the complexes exhibited multiplets around δ 7.13-7.25 and 7.30-7.77 ppm
1
in their H NMR spectra, which are attributed to the phenyl protons present in the
triphenylphosphine and 3,3-diphenyl/isobutyl/ethyl-1-(2,4-dichlorobenzoyl)thiourea ligands. In
1
the H NMR spectra of the complexes the characteristic signal for the N–H group appeared in the
region δ 10.81–11.78 ppm, which was about 0.5 ppm less than that found in the spectra of the
ligands. This is due to the intramolecular hydrogen bonding between the N–H group and the
coordinated chloride ligand [58]. Complex 2 exhibited two doublets at δ 3.39 and 3.71 ppm,
corresponding to the –CH
2
– protons, and two more doublets at δ 0.83 and 0.96 ppm,
1
corresponding to the –CH
3
protons, in its H NMR spectrum. Complex 2 also showed two
multiplets at δ 2.01-2.11 and 2.21-2.31 ppm which were assigned to the methine protons in the
1
aliphatic chain. In the H NMR spectrum of complex 3, a multiplet (merging of two quartets) in
the region δ 2.95-4.05 ppm and two triplets at δ 0.78 and 1.19 ppm were observed, which were
attributed to the methylene protons proximate to the nitrogen atom and terminal methyl protons,
1
3
respectively. In the C NMR spectra of complexes 1, 2 and 3, the signals observed around δ
1
28.3-146.4 ppm were assigned to the aryl carbon atoms of the triphenylphosphine and
1
3
coordinated acythiourea ligands. The C NMR spectra of the complexes showed signals in the
regions δ 179.0-183.5 and 161.4-164.3 ppm due to the C=S and C=O carbon atoms, respectively.
Complexes 2 and 3 exhibited resonances for the aliphatic carbons in the expected regions. Only
3
1
one signal, obtained at δ 30.1, 32.1 and 29.4 ppm, in P NMR spectra of complexes 1, 2 and 3
10

respectively confirmed the presence of the coordinated triphenylphosphine ligand in the Cu(I)
complexes.
3
.3. Crystallography
A view of the molecular structure for complex 1 is illustrated in Fig. 2, and selected
geometric parameters are listed in Table 2. In the monomeric complex, the Cu(I) ion consists of
two triphenylphosphine ligands coordinated by phosphorus atom, a coordinated chloride ion and
a 3,3-diphenyl-1-(2,4-dichlorobenzoyl)thiourea ligand (HL1) coordinated solely through the
sulfur atom, with the bond angles around the copper ion in the range ca. 99-124º. The bond
angles around the copper ion are slightly distorted from those of an ideal tetrahedral geometry,
Fig. 2. Molecular structure of complex 1. Only selected atoms are labelled in the thermal
ellipsoid plot for clarity.
11

noticeably a narrow 99.054(17)° for P(2)-Cu(1)-S(1) to a wide 124.150(18)° for P(1)-Cu(1)-P(2),
due to the presence of two bulky triphenylphosphine ligands in the coordination sphere. The
distortion might also be associated with the presence of an intramolecular hydrogen bond
between the thiourea N–H group and the coordinated chloride ion. In complex 1, the Cu–S bond
length of 2.3859(5) Å was analogous to that (2.2668-2.4226 Å) of Cu(I) tetrahedral complexes
containing monodentate acylthiourea ligands [61,62]. The Cu–Cl bond length [2.3181(4) Å] in
+
–
complex 1 was less than the sum of the ideal ionic radii of the Cu and Cl ions (2.58 Å) [63].
.4. Catalytic oxidation
Investigations were initially conducted on the oxidation of 1-phenylethanol (Table 3,
entries 1-3) with the new Cu(I) complexes 1-3 in [bmim][PF ] in the presence of H oxidant at
0 °C for 48 h. Though the yields of acetophenone were good with all the new Cu(I) complexes,
3
6
2
O
2
7
the catalytic activity of complex 1 (Table 3, entry 1) was slightly higher (90%) and hence it has
been chosen as a catalyst for optimizing the reaction conditions and extending the scope of the
substrates. To select a suitable ionic liquid as the solvent, the oxidation of 1-phenylethanol was
studied in 0.1 mL of [bmim][PF
6
] and [bmim][BF
4
] (Table 3, entry 4). Among these ILs,
[
bmim][PF ] was chosen as the best solvent as the yield of acetophenone was higher when it was
6
used for the oxidation reaction (Table 3, entry 1). In order to find the optimum amount of
catalyst, oxidant and temperature, the catalytic oxidation of 1-phenylethanol (Table 3,
entries 5-10) was carried out with various amounts of catalyst and oxidant in a range of
temperatures. The results of the reaction confirmed that a better yield of acetophenone was
obtained with 0.007 mmol of catalyst and 6 mmol of oxidant at 70 °C (Table 3, entry 1). The
oxidation did not occur in the absence of complex 1 under identical conditions and an extremely
low yield (8-9%) of acetophenone was observed when copper salts such as CuCl (Table 3, entry
2
12

1
1) and CuCl (Table 3, entry 12) were added as the catalyst in the place of complex 1. In a
control experiment under identical conditions, no reaction was observed in the oxidation of
-phenylethanol when complex 1 was replaced by the ligand HL1 (Table 3, entry 13). To
confirm that H acted solely as the oxidant, and not added O , the oxidation of
-phenylethanol (Table 3, entry 14) was conducted with complex 1 under a nitrogen atmosphere,
1
2
O
2
2
1
which also gave 90% of acetophenone.
The results of the test reactions established 0.1 mL of [bmim][PF
6
], 6 mmol of H
2
2
O and
0
.007 mmol of complex 1 were sufficient for the oxidation of 5 mmol of 1-phenylethanol and
these optimal conditions were utilized for the scope extension to various primary and secondary
alcohols. Further oxidation did not occur after 48 h. The results obtained are summarized in
Table 4. Interestingly, benzylic primary alcohols (Table 4, entries 1-3) were oxidized to form the
corresponding carboxylic acids at 70 °C in fair to good yields by using
1
.2 equivalents of H
acids in acetonitrile solvent required 10 equivalents of H
our previous Cu(I)-acylthiourea/H catalytic system in CH
of H for the oxidation of alcohol substrates (1 mmol) at room temperature [49]. Our present
catalytic system is very active towards the oxidation of allylic alcohol viz cinnamyl alcohol
Table 4, entry 4) to cinnamic acid (92% yield), whereas oxidation of cinnamyl alcohol was not
catalyzed by our earlier Cu(I)-thiourea/H catalytic [49] and Cu(II)/H catalytic [64] systems.
2
O
2
, whereas Cu(II) catalyzed oxidation of primary alcohols to carboxylic
at the same temperature [64]. Even
CN solvent required 5 equivalents
2
O
2
2
O
2
3
2
O
2
(
2
O
2
2
O
2
The benzylic secondary alcohols 1-phenylethanol, α-methyl-2-naphthalenemethanol and 1-
phenoxy-2-propanol (Table 4, entries 5, 7 and 8) on oxidation gave the corresponding ketones in
9
0, 57 and 97% yield, respectively. Even though these results are comparable with the
Cu(I)-thiourea/H catalytic system in CH CN solvent, the present catalytic system worked
2
O
2
3
13

with 1.2 equivalents of H
) did not give a satisfactory result. Very interestingly, the oxidation of 1-indanol to 1-indanone
Table 4, entry 9) proceeded to give 97% of the product, while our previous system exhibited
2
2
O . However, the oxidation of 4-chlorophenylethanol (Table 4, entry
6
(
only a 40% yield of 1-indanone [49]. The present catalytic system did not work for the oxidation
of cyclic alcohol, viz cyclohexanol (Table 4, entry 10). Unfortunately, and despite extensive
efforts, aliphatic alcohols could not be oxidized by the present catalytic system.
Additional experiments were conducted to analyze the recyclability and reusability of
both [bmim][PF
6
] and the catalyst (complex 1) for the same reaction. For the catalytic oxidation
]), recovered
of 1-phenylethanol to acetophenone, the catalytic phase (complex 1 in [bmim][PF
6
from overnight drying (in vacuo), maintained its good level catalytic activity even after being
recycled 5 times in consecutive runs. This indicated the viability of both the ionic liquid and
catalyst.
In order to understand the reactive species involved in the reaction, complex 1 in
−
5
ethanol (10 M) was treated with H
progress of the reaction was monitored by electronic absorption spectroscopy. The titration was
carried out by the addition of two drops of H for each addition to the solution of complex 1 in
2
O
2
(dissolving five drops in 5 mL of methanol) and the
2
O
2
ethanol. The results showed that the intensities of the bands at 240 and 252 nm increased
considerably, but no other new bands were formed. The data suggest the interaction of complex
1
with H
2
2
O and hence, presumably a Cu(I)-peroxo complex was formed and acted as an active
intermediate in the oxidation reaction [65].
4
. Conclusions
In summary, new four-coordinated tetrahedral copper(I) complexes incorporating
,3-dialkyl/aryl-1-(2,4-dichlorobenzoyl)thiourea derivatives (HL1, HL2 and HL3) and
3
14

triphenylphosphine have been prepared and characterized. An unusual neutral monodentate
coordination was exhibited by the 3,3-dialkyl/aryl-1-(2,4-dichlorobenzoyl)thiourea derivatives
through the S atom to the Cu(I) ion. Both the ligands and complexes were synthesized at room
temperature. The complexes were used as catalysts in 1-butyl-3-methylimidazolium
hexafluorophosphate medium in the presence of H
2
2
O for the oxidation of various alcohols at
7
0 °C. Interestingly, the present catalytic system worked for the oxidation of alcohols without
any additive and base.
Acknowledgements
N.G. gratefully acknowledges DST, Government of India, for financial assistance under
SERB Start-Up Research Grant scheme for Young Scientists (No.SB/FT/CS-189/2013) and
SASTRA University for the Prof. TRR fund and the infrastructural support.
Appendix A. Supplementary data
1
13
Supplementary data associated with this article (electronic spectra, representative H, C
3
1
and P-NMR spectra of the Cu(I) complexes, GC chromatogram, and GC conditions for
catalytic studies) can be found in the online version. Crystallographic data (excluding structure
factors) for the structure described in this paper have been deposited with the Cambridge
Crystallographic Data Centre, CCDC No. 1473182. Copies of the data can be obtained free of
charge on application to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax:
+44-1223-336033; e-mail deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
15

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19

Table 1
Summary of crystallographic data for 1
Empirical formula
Formula weight
Crystal color
C
56
H
44Cl
3
CuN
2
OP S
2
1024.82
Brown
3
Crystal dimensions (mm )
0.57 × 0.441 × 0.308
Triclinic
P-1
Crystal system
Space group
a (Å)
b (Å)
c (Å)
11.4077(8)
14.4196(11)
17.4337(13)
74.1040(10)
71.7640(10)
67.7780(10)
2481.6(3)
α (º)
β (º)
γ (º)
V (Å )
3
Z
2
-3
D
x
(g cm )
1.371
F(000)
µ(Mo Kα) (mm )
1056
0.750
-
1
Reflections collected
Independent reflections (Rint
Completeness to theta = 25.242°
Absorption correction
Max. and min. transmission
Refinement method
35827
11338 (0.0259)
99.6 %
Semi-empirical from equivalents
0.7456 and 0.6443
Full-matrix least-squares on F
)
2
Data / restraints / parameters
Goodness-of-fit on F
11338 / 0 / 595
1.041
2
Final R indices [I> 2σ(I)]
R indices (all data)
R
R
1
= 0.0337, wR
= 0.0404, wR
2
= 0.0866
= 0.0910
1
2
Extinction coefficient
Largest diff. peak and hole ( e Å ) 0.896 and -0.464
n/a
-3
20

Table 2
Selected bond lengths [Å] and angles [º] for 1
Cu(1)-Cl(1)
Cu(1)-S(1)
Cu(1)-P(1)
Cu(1)-P(2)
S(1)–C(1)
O(1)–C(2)
2.3181(4)
2.3859(5)
2.2675(5)
2.2771(5)
1.6779(17)
1.215(2)
N(1)–C(1)
1.398(2)
N(2)–C(1)
1.348(2)
Cl(2)-C(6)
Cl(3)-C(8)
1.7432(19)
1.7352(19)
108.956(17)
105.448(17)
113.238(17)
104.782(16)
99.054(17)
124.150(18)
119.40(16)
118.71(15)
120.61(14)
118.14(14)
117.4
Cl(1)-Cu(1)-S(1)
P(1)-Cu(1)-Cl(1)
P(2)-Cu(1)-Cl(1)
P(1)-Cu(1)-S(1)
P(2)-Cu(1)-S(1)
P(1)-Cu(1)-P(2)
C(5)-C(6)-Cl(2)
C(7)-C(6)-Cl(2)
C(3)-C(8)-Cl(3)
C(7)-C(8)-Cl(3)
C(1)-N(1)-H(1)
C(2)-N(1)-H(1)
117.4
21

Table 3
Optimization of catalytic oxidation.
Complex/
Compound
Complex/Compound Amount
Oxidant Amount
(mmol)
a
Entry
Yield (%)
(mmol)
1
2
3
4
5
1
2
3
1
1
0.007
0.007
0.007
0.007
0.010
3.0
3.0
3.0
3.0
3.0
90
88
85
b
78
79
80
80
83
6
7
8
9
1
0.005
0.007
0.007
0.007
0.007
0.007
3.0
2.5
3.5
3.0
3.0
3.0
1
1
c
1
1
82
d
1
1
0
1
81
CuCl2
9
8
1
1
1
2
3
4
CuCl
HL1
1
0.007
0.007
0.007
3.0
3.0
3.0
No
reaction
e
90
a
Yield is determined by GC with area normalization; GC conditions: RTX-5 Column, 60 m ×
.32 mm, initial column temperature was increased from 130 to 250 °C at a rate of
°C/min; FID detector, 290 °C; Injector, 260 °C; Carrier Gas: N ; Rate: 2.00 mL/min.
bmim][BF ] was used as the solvent
0
7
[
2
b
c
d
e
4
o
Reaction occurred at 75 C
o
Reaction occurred at 65 C
Reaction conditions: 1-phenylethanol (5.0 mmol), oxidant (6.0 mmol), catalyst (0.007 mmol),
o
solvent (0.1 mL), stirring for 48 h at 70 C under a nitrogen atmosphere.
22

Table 4
Oxidation of alcohols by 1.
a
c
Entry
Substrate
Product
Yield (%)
d
1
75 (72)
2
67
b
3
4
53
92
d
5
6
90 (86)
16
7
8
b
5
7
9
7
23

b
9
97
1
0
2
3
a
Reaction conditions: alcohol (5 mmol), H
0.007 mmol), [bmim][PF ] (0.1 mL), stirring for 48 h, 70 C.
Reaction conditions: alcohol (5 mmol), H (3.0 mmol), catalyst
0.007 mmol), [bmim][PF ] (0.4 mL), stirring for 48 h, 70 C.
Yield is determined by GC with area normalization.
Isolated yield is given in parenthesis.
2
O (6.0 mmol), catalyst
2
o
(
6
b
2
O
2
o
(
6
c
d
24

Synthesis, characterization and catalytic oxidation property of copper(I)
complexes containing monodentate acylthiourea ligands and
triphenylphosphine
N. Gunasekaran, N.S.P. Bhuvanesh, R. Karvembu
Synthesis, characterization and catalytic oxidation property of copper(I)
complexes containing monodentate acylthiourea ligands and
triphenylphosphine
N. Gunasekaran, N.S.P. Bhuvanesh, R. Karvembu
A rare monodentate coordination mode has been observed for 3,3-dialkyl/aryl-1-(2,4-
dichlorobenzoyl)thiourea ligands with the Cu(I) ion. The resulting four-coordinated tetrahedral
copper(I) complexes were found to be active catalysts in 1-butyl-3-methylimidazolium
hexafluorophosphate medium in the presence of H
0 °C. Interestingly, the present catalytic system worked for the oxidation of alcohols without
any additive and base.
2
O
2
for the oxidation of various alcohols at
7
25