Monodentate coordination of N-[di(phenyl/ethyl)carbamothioyl]benzamide ligands: Synthesis, crystal structure and catalytic oxidation property of Cu(i) complexes

Article abstract of DOI:10.1039/c1dt10628g

New four-coordinated tetrahedral copper(i) complexes have been synthesized from the reactions between [CuCl₂(PPh₃)₂] and N-(diphenylcarbamothioyl)benzamide (HL1) or N-(diethylcarbamothioyl)benzamide (HL2) in benzene. These complexes have been characterized by elemental analyses, IR, UV/Vis, ¹H, ¹³C and ³¹P NMR spectroscopy. The molecular structure of both the complexes, [CuCl(HL1)₂(PPh ₃)] (1) and [CuCl(HL2)(PPh₃)₂] (2) were determined by single-crystal X-ray diffraction, which reveals distorted tetrahedral geometry around each Cu(i) ion. The combination of 2 (0.005 mmol) with hydrogen peroxide (2.5 mmol) in acetonitrile is found to be an active catalyst for the oxidation of primary and secondary alcohols (0.5 mmol) to their corresponding acids and ketones, respectively, at room temperature. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt10628g

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PAPER
Monodentate coordination of N-[di(phenyl/ethyl)carbamothioyl]benzamide
ligands: synthesis, crystal structure and catalytic oxidation property of Cu(I)
complexes†
Nanjappan Gunasekaran,a Pandian Ramesh,b Mondikalipudur Nanjappa Gounder Ponnuswamyb and
Ramasamy Karvembu*a
Received 12th April 2011, Accepted 31st August 2011
DOI: 10.1039/c1dt10628g
New four-coordinated tetrahedral copper(I) complexes have been synthesized from the
reactions between [CuCl2(PPh3)2] and N-(diphenylcarbamothioyl)benzamide (HL1) or
N-(diethylcarbamothioyl)benzamide (HL2) in benzene. These complexes have been characterized by
elemental analyses, IR, UV/Vis, 1H, 13C and 31P NMR spectroscopy. The molecular structure of both
the complexes, [CuCl(HL1)2(PPh3)] (1) and [CuCl(HL2)(PPh3)2] (2) were determined by single-crystal
X-ray diffraction, which reveals distorted tetrahedral geometry around each Cu(I) ion. The
combination of 2 (0.005 mmol) with hydrogen peroxide (2.5 mmol) in acetonitrile is found to be an
active catalyst for the oxidation of primary and secondary alcohols (0.5 mmol) to their corresponding
acids and ketones, respectively, at room temperature.
of alcohols to the corresponding carbonyl compounds with N-
Introduction
methylmorpholine-N-oxide (NMO) as oxidant.8
N-Dialkyl/aryl-N¢-acylthiourea derivatives of the type Ar–
C(O)N¢HC(S)NR2 have received a great deal of attention over the
decades because of their coordination versatility and remarkable
applications in analytical and biological sciences.1 Thiourea
derivatives have extensively been used in enantioselective synthesis
such as nitro-Mannich reactions, Aza–Henry reaction and the
Michael addition.2–4 The coordination compounds of substi-
tuted acylthiourea derivatives exhibit some interesting biological
properties such as anti-HIV,1g non-nucleoside inhibitor action
of HIV-1 reverse transcriptase,5 anti-fungal, herbicidal, antithy-
roid, antihelmintic, antibacterial, rodenticidal, insecticidal, plant
growth regulator properties6 and potent anticancer property.1h,7 N-
Dialkyl/aryl-N¢-acyl thiourea derivatives are utilized as efficient
ligands for the separation of platinum group metals.1d,i Cad-
mium(II) complexes of N,N-diethyl-N¢-benzoylthio(seleno)urea
are used as single-source precursors for the preparation of
CdS and CdSe nanoparticles.1c Ru(III) and Ru(II) complexes
incorporating N-[di(alkyl/aryl)carbamothioyl]benzamide ligands
and PPh3/AsPh3 have been used as catalysts for the oxidation
With the simultaneous presence of O, N, N¢ and S electron
donors, the substituted acylthiourea ligands exhibit various coor-
dination modes (Scheme 1): (a) O and S bonded to M (monobasic
bidentate),9 (b) only S bonded to M (neutral monodentate),10 (c) O
and N bonded to M (neutral bidentate),11 and (d) O and S bonded
to M and N is bonded to M¢ (monobasic bridging ligand).12 It
has been shown in the literature that the substituted acylthiourea
ligands, which do not form an intramolecular hydrogen bond,
tend to coordinate predominately in a bidentate (S, O) fashion
to transition metal ions through the sulfur and acyl oxygen
atoms.12 Examples for this type of coordination are mononuclear
complexes of the type [ML2], where M = Ni(II), Cu(II), Co(II),
Zn(II), Cd(II), Pt(II) and Pd(II) and L = substituted acylthiourea
derivatives.13 Coordination solely through sulfur is rare and so
far only known for complexes with Au(I),14 Ag(I),15 Hg(II),16
and Cu(I)17. It has been suggested that intramolecular hydrogen
bonding exists between the thiourea N–H moiety (where R/R¢ =
H) and the amidic O-donor atom to form a six-membered ring,
aDepartment of Chemistry, National Institute of Technology, Tiruchirappalli,
620 015, India. E-mail: kar@nitt.edu; Fax: +91 431 2500133
bCentre of Advanced Study in Crystallography and Biophysics, University of
Madras, Guindy Campus, Chennai, 600025, India
† Electronic supplementary information (ESI) available: Electronic spec-
tra, representative 1H, 13C and 31P-NMR spectra, GC chromatogram, GC
conditions for catalytic studies and crystallographic data. CCDC reference
numbers 821178 and 821179. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10628g
Scheme 1 Four different coordination modes of the N-dialkyl/aryl-
N¢-acylthiourea ligands in various metal complexes.
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with the consequence that the ligands coordinate to metal centres
in a monodentate fashion through the sulfur donor atom.12 In
our case, no such hydrogen bonding exists in ligands as R/R¢
π H.18 However, since monodentate coordination through the
sulfur atom is observed with Cu(I) as shown below, this shows
that hydrogen bonding between N–H and amidic O is not the
only factor that decides the coordination mode of the ligands. The
other factors responsible for such coordination may be revealed
from experiments with other metal ions and ligands.
Oxidation of primary and secondary alcohols to carbonyl
compounds is a very fundamental process in organic synthesis.
The transition metal-catalyzed oxidation of alcohols is of current
interest, and various metal complexes and oxidant systems have
been reported.19 Further, a large number of transition-metal
phosphine complexes have been used as catalysts in oxidation
of alcohols to carbonyl compounds,20 due to the characteristic
steric and electronic properties of tertiary phosphine ligands.21
Although many studies have been performed on transition metal-
catalyzed oxidation of alcohols using various peroxide substances
as oxidant, only a few have used H2O2 as a source of oxygen for
the oxidation of alcohols.22 Some publications have been devoted
to the oxidation of alcohols catalyzed by Cu(II)/H2O2 catalytic
systems.23 To the best of our knowledge there is no report available
with Cu(I)/H2O2 catalytic system for oxidation of alcohols to
carbonyl compounds.
We report herein the synthesis, characterization and catalytic
oxidation property of the complexes formed by the reaction of
[CuCl2(PPh3)2] with N-(diphenylcarbamothioyl)benzamide or N-
(diethylcarbamothioyl)benzamide. The crystal structures of both
the complexes show a rare monodentate sulfur coordination
mode of N-[di(alkyl/aryl)carbamothioyl]benzamide ligands to-
wards Cu(I). It has been found that Cu(I) complexes are effective
catalysts for the oxidation of primary and secondary alcohols to
the corresponding carboxylic acids and ketones, respectively, in the
presence of H2O2 as oxidant at room temperature. The structure
of the ligands used in this study is given in Scheme 2.
of HL1 and complex (2) contains only one molecule of HL2.
The same complexes were obtained even when reactions were
performed with lower amount of HL1 or excess of HL2 so that the
stoichiometry of reactants did not alter the product. Interestingly,
Cu(II) is reduced to Cu(I) during the course of the reaction.
The complexes obtained are orange in colour, air stable, non-
hygroscopic in nature, soluble in acetone, acetonitrile, benzene,
dichloromethane, chloroform, DMSO and DMF, and insoluble
in n-hexane and water. The analytical data obtained are in good
agreement with the proposed molecular formulae.
Spectroscopy
In the IR spectra of free carbamothioyl ligands, a medium intensity
band is observed around 1255–1285 cm-1 for the C S group. In
complexes 1 and 2, this band appeared in lower frequency region
around 1178–1184 cm-1 which suggests that the sulfur atom of the
thiocarbonyl group is involved in coordination. The free ligands
exhibit a strong band corresponding to the C O group in the
region 1681–1692 cm-1 and N–H group in the region 3222–3267
cm-1. These bands also appeared in the complexes, which reveals
that C O and N–H groups are not involved in coordination. In
addition, these complexes show new bands around 1435, 1100 and
750 cm-1, which are due to the triphenylphosphine ligand.
Both copper complexes 1 and 2 are diamagnetic (meff = 0),
indicating the presence of copper in the +1 oxidation state. The
electronic spectra of copper(I) complexes 1 and 2 in ethanol
were recorded at room temperature. The electronic spectra of
the complexes are noticeably different from the spectra of the
corresponding ligands, indicating copper–ligand binding in the
complexes. In the spectra of complexes, no d–d transition is
expected as they possess a d10 configuration. The UV-Vis bands
observed in the region 224–260 nm (e = 17314–66285 dm3 mol-1
cm-1) are assigned to metal to ligand charge transfer (MLCT) or
ligand centered p–p* transitions.24
The 1H NMR spectra of complexes 1 and 2 in CDCl3 solution
show a multiplet around 7.1–8.1 ppm, which has been assigned
to the aromatic protons present in triphenylphosphine and the N-
[di(phenyl/ethyl)carbamothioyl]benzamide ligand. The character-
1
istic signal in the range 10.8–11.8 ppm for N–H appeared in H
NMR spectra of complexes indicating non-involvement of the N–
H group in coordination. Complex 2 exhibits two quartets in the
region 3.4–3.8 ppm corresponding to the methylene protons and a
multiplet (merging of two triplets) around 1.2 ppm corresponding
1
to methyl protons in the H NMR spectrum. 13C NMR spectra
Scheme 2 Structure of ligands.
of complexes 1 and 2 have been recorded in CDCl3 solution. The
resonances observed in the range 121.9–127.7 ppm in both the
complexes have been assigned to the aromatic carbons of triph-
enylphosphine and N-[di(phenyl/ethyl)carbamothioyl]benzamide
ligand. The signals appeared in the region 172.9–174.7 and 155.0–
Results and discussion
The bidentate ligands (HL1 and HL2) were synthesized
from benzoyl chloride, potassium thiocyanate and diphenyl
amine or diethyl amine in dry acetone.13a Two new four-
coordinated copper(I) tetrahedral complexes, [CuCl(HL1)2(PPh3)]
(1) and [CuCl(HL2)(PPh3)2] (2) were synthesized by react-
ing [CuCl2(PPh3)2] with N-(diphenylcarbamothioyl)benzamides
(HL1) or N-(diethylcarbamothioyl)benzamide (HL2), respec-
tively, in benzene. Analytical, spectral and single-crystal X-ray
diffraction studies reveal that complex (1) contains two molecules
158.0 ppm in both the complexes may be attributed to the C
S
and C O carbons, respectively. Complex 2 exhibits signals around
41.2–41.6 ppm due to the methylene carbons and 5.2–6.9 ppm due
to the methyl carbons in its 13C NMR spectrum. 31P NMR spectra
have also been recorded in CDCl3 solution to confirm the presence
of triphenylphosphine in the Cu(I) complexes. Complexes 1 and 2
showed only one signal, at 25.5 and 28.4 ppm respectively, in the
31P NMR spectrum.
12520 | Dalton Trans., 2011, 40, 12519–12526
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Scheme 3 Synthesis of Cu(I) complexes.
˚
The average Cu–S bond distance of 2.3383(8) A in complex 1
Crystal structures
is comparable with literature values (2.381–2.418 A).25 The Cu–Cl
˚
Complexes 1 and 2 crystallize in the monoclinic space groups
P21/n and P21/c, respectively (Table 1). The molecular structures
of 1 and 2 are depicted in Fig. 1 and Fig. 2, respectively. In
complex 1, the copper ion is bonded to one phosphorus atom
of PPh3, two sulfur atoms of HL1 and one chloride ion, with bond
angles around copper in the range ca. 100–126◦ (Table 2) and in
complex 2, the copper ion is bonded to one sulfur atom of HL2,
two phosphorus atoms of PPh3 and one chloride ion, with bond
angles around copper in the range ca. 100–128◦ (Table 3). This
reveals a distorted tetrahedral geometry around each copper ion
in mononuclear complexes.
˚
bond distance [2.3649(7) A] in complex 1 is less than sum the of
ionic radii of Cu and Cl (Cu Cl , 2.58 A).26 The sum of the bond
angles around the atoms N20 (353.7◦), N28 (358.8◦), N42 (356.8◦)
and N50 (359.4◦) are in accordance with sp2 hybridization.27
Complex 1 is stabilized by intermolecular and intramolecular
hydrogen bonds. Hydrogen bonding details are listed in the Table
4. The N–H ◊ ◊ ◊ Cl type intramolecular hydrogen bonds exhibited
by complex 1 are of note (thiourea N–H moieties are hydrogen
bonded to the chloride ion).
+
-
+
-
˚
In complex 2, the Cu–P distances lie in the range 2.2748(9)–
2.2811(9) A, which are similar to analogous complexes. The
28
˚
Table 1 Summary of crystallographic data for 1 and 2a
1
2
b
Empirical formula
C58H49ClCuN4O3PS2
C48H46ClCuN2OP2S
Mr
1044.09
293(2)
0.71073
Monoclinic
P21/n
21.1426(15)
10.4879(8)
25.0570(17)
108.708(3)
5262.6(7)
4
859.90
293(2)
0.71073
Monoclinic
P21/c
10.1804(6)
19.6686(12)
22.4500(14)
101.886(3)
4398.9(5)
4
T/K
˚
l/A
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Dc/Mg m-3
1.318
0.624
2168
1.298
0.715
1792
m/mm-1
F(000)
Crystal size/mm
0.20 ¥ 0.18 ¥ 0.17
1.53–28.37
-28 to 28, -13 to 13, -33 to 33
48742
0.21 ¥ 0.19 ¥ 0.17
1.39–28.34
-13 to 13, -20 to 26, -29 to 29
40854
q range/◦
Limiting indices, hkl
Refl. collected
Refl. unique
13110
10900
Rint
0.048
13110/0/640
1.000
R1 = 0.0478, wR2 = 0.1126
R1 = 0.1043, wR2 = 0.1390
0.51, -0.29
0.125
10900/0/510
0.842
R1 = 0.0522, wR2 = 0.1382
R1 = 0.1208, wR2 = 0.1608
0.59, -0.43
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
-3
˚
Drmax/min/e A
a Details in common: Refinement: full-matrix least-squares on F2. b Chemical formula and derived quantities idealized for a H2O content of one molecule
per formula unit. Occupation factor of this water molecule refined to 0.46(1).
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Fig. 1 ORTEP plot of 1 with ellipsoids at the 20% probability level. Hydrogen atoms are omitted for clarity.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1
Cu1–P1
Cu1–S2
Cu1–S1
Cu1–Cl1
S1–C19
S2–C41
O1–C43
O2–C21
2.2416(7)
2.3076(7)
2.3383(8)
2.3649(7)
1.673(3)
1.667(3)
1.215(4)
1.208(3)
C19–N28
C19–N20
N20–C21
C41–N50
N50–C51
N50–C57
N28–C35
N28–C29
1.357(3)
1.367(3)
1.393(3)
1.356(3)
1.432(3)
1.442(3)
1.440(3)
1.441(3)
P1–Cu1–S2
P1–Cu1–S1
S2–Cu1–S1
P1–Cu1–Cl1
S2–Cu1–Cl1
S1–Cu1–Cl1
C19–S1–Cu1
C41–S2–Cu1
C7–P1–C1
114.97(3)
117.44(3)
100.63(3)
107.22(3)
108.04(3)
108.05(3)
114.56(9)
109.59(9)
104.21(12)
101.97(12)
104.13(12)
120.34(9)
C7–P1–Cu1
112.19(9)
112.31(9)
116.3(2)
121.6(2)
121.94(19)
124.7(2)
121.7(3)
115.4(2)
122.1(2)
122.32(19)
125.8(3)
121.9(3)
C13–P1–Cu1
N28–C19–N20
N28–C19–S1
N20–C19–S1
C19–N20–C21
O2–C21–N20
N50–C41–N42
N50–C41–S2
N42–C41–S2
C41–N42–C43
O1–C43–N42
C1–P1–C13
C7–P1–C13
C1–P1–Cu1
Fig. 2 ORTEP plot of 2 with ellipsoids at the 20% probability level.
Catalytic oxidation
Hydrogen atoms are omitted for clarity.
To evaluate the catalytic activity of copper(I) complexes, complexes
1 and 2 were examined as catalysts for the oxidation of 1-
phenylethanol (Table 5, entries 1 and 2) in presence of H2O2
as oxidant and acetonitrile as solvent at 27 ◦C. Though both
the complexes exhibited good catalytic activity, the yield of
acetophenone (Table 5, entry 2) was higher when 2 was used as
a catalyst. Hence, complex 2 was chosen as a catalyst for the
optimization and extending the scope of substrates. The catalytic
oxidation of 1-phenylethanol (Table 5, entries 2 and 3) was carried
˚
Cu–S bond distance of 2.4226(9) A in complex 2 is com-
25
˚
parable with literature values (2.381–2.418 A). The Cu–Cl
˚
bond distance of 2.3203(9) A in complex 2 is less than the
sum of ionic radii of Cu+ and Cl- (Cu Cl , 2.58 A). The
sum of the bond angles around the atoms N38 (360.0◦)
and N43 (354.2◦) in the ligand is in accordance with sp2
hybridization.27
+
-
26
˚
12522 | Dalton Trans., 2011, 40, 12519–12526
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◦
˚
Table 3 Selected bond lengths (A) and angles ( ) for 2
oxidation of 1-phenylethanol (Table 5, entries 2, 4 and 5) was
carried out in acetonitrile, dichloromethane and benzene in order
to select the most suitable solvent. Among these, acetonitrile (Table
5, entry 2) was found to be the best solvent as the yield of carbonyl
compound was 98%. It was observed that when no catalyst was
added, the blank reaction with solvent and oxidant exhibited
extremely low reactivity towards the yield of acetophenone from
1-phenylethanol. Control experiments were carried out under
identical conditions with copper salts such as CuCl2 (Table 5,
entry 9) and CuCl (Table 5, entry 10) as catalyst in the place of
complex 2 and the yield of acetophenone was found to be zero.
Further, the oxidation of 1-phenylethanol (Table 5, entries 11 and
12) was carried out with the use of ligands HL1 and HL2 in the
place of complex 2; no oxidation was observed. The oxidation of
1-phenylethanol (Table 5, entry 13) with complex 2 under nitrogen
atmosphere gave 98% of acetophenone, which indicates that only
H2O2 acts as oxidant in these reactions, and not air.
Cu1–P2
Cu1–P1
Cu1–Cl1
Cu1–S1
P1–C13
P1–C7
2.2748(9)
2.2811(9)
2.3203(9)
2.4226(9)
1.826(4)
1.832(3)
P1–C1
1.839(3)
1.827(3)
1.828(3)
1.829(4)
1.669(3)
1.227(4)
P2–C31
P2–C19
P2–C25
S1–C37
O1–C44
P2–Cu1–P1
P2–Cu1–Cl1
P1–Cu1–Cl1
P2–Cu1–S1
P1–Cu1–S1
Cl1–Cu1–S1
C13–P1–C7
C13–P1–C1
C7–P1–C1
128.19(3)
109.91(4)
104.78(4)
100.04(4)
103.10(4)
109.68(3)
103.18(18)
103.56(15)
102.89(14)
115.12(14)
C7–P1–Cu1
C1–P1–Cu1
C31–P2–C19
C31–P2–C25
C19–P2–C25
C31–P2–Cu1
C19–P2–Cu1
C25–P2–Cu1
C37–S1–Cu1
116.78(11)
113.62(10)
102.32(15)
104.80(18)
103.36(15)
112.69(12)
118.07(11)
114.05(11)
111.40(11)
C13–P1–Cu1
◦
˚
Table 4 Hydrogen-bond lengths (A) and angles ( ) for 1
In order to explore the scope of this protocol, the oxidation
of various primary and secondary alcohols was examined with
complex 2. Interestingly benzylic primary alcohols (Table 6,
entries 1, 2 and 3) were oxidized to form the corresponding
carboxylic acids at 27 ◦C in good yields by using 5 equivalents
of H2O2, whereas Cu(II) catalysed oxidation of primary alcohols
to carboxylic acids requires 10 equivalents of H2O2 and high
D–H ◊ ◊ ◊ A
d(D–H)
d(H ◊ ◊ ◊ A)
d(D ◊ ◊ ◊ A)
∠(DHA)
N20–H20 ◊ ◊ ◊ Cl1a
N42–H42 ◊ ◊ ◊ Cl1a
C52–H52 ◊ ◊ ◊ O3a
C3–H3 ◊ ◊ ◊ Cl1b
0.79(3)
0.84(3)
0.93
0.93
0.93
2.35(3)
2.30(3)
2.56
2.75
2.58
3.136(2)
3.132(3)
3.434(10)
3.566(4)
3.452(9)
173(3)
176(3)
157.5
147.0
156.0
C45–H45 ◊ ◊ ◊ O3c
◦
temperature (80 C) as reported by Punniyamurthy et al.23c
Symmetry transformations used to generate equivalent atoms:a x, y, z; b x,
y - 1, z; c -x + 1, -y + 1, -z.
Catalytic oxidation at 27 ◦C of benzylic secondary alcohols, 1-
phenylethanol and benzhydrol (Table 6, entries 4 and 5) gave the
corresponding ketones in 98 and 95% yield, respectively, whereas
oxidation of the same substrates by Mn(III) complexes with H2O2
gav◦e the products in only 80 and 82% yield after stirring at
50 C.22a It has been observed that the present catalytic system
exhibits better efficiency in the case of oxidation of cyclic alcohols
(Table 6, entries 7 and 8) at 27 ◦C when compared with earlier
out with various amounts of catalyst and oxidant (Table 5, entries
2, 6, 7 and 8) in order to find the optimum amount of catalyst and
oxidant. With 0.005 mmol of catalyst and 2.5 mmol of oxidant
(Table 5, entry 2), the yield of acetophenone was highest (98%).
Generally 0.005 mmol of catalyst and 2.5 mmol of oxidant were
sufficient for oxidation (Table 5, entries 1–8 and 13). The catalytic
reports on FeBr3/H2O2 and Cu(II)/H2O2 catalytic systems.23c
22c
Table 5 Optimization of catalytic oxidation
Entry
Catalyst
Amount/mmol
Solvent
Oxidant
Amount/mmol
Yielda (%)
1
2
3
4
5
6
7
8
1
0.005
0.005
0.010
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
CH3CN
CH3CN
CH3CN
CH2Cl2
C6H6
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
2.5
2.5
2.5
2.5
2.5
1.0
1.5
2.0
2.5
2.5
2.5
2.5
2.5
95
98
97
97
22
93
90
92
2
2
2
2
2
2
2
9
CuCl2
CuCl
HL1
HL2
2
No reaction
No reaction
No reaction
No reaction
98b
10
11
12
13
a Yield is determined by GC with area normalization; GC conditions: RTX-5 column, 60 m ¥ 0.32 mm, 250 ◦C; FID detector, 280 ◦C; injector, 250 ◦C;
carrier gas: N2; rate: 1.39 mL/min. b Reaction conditions: 1-phenylethanol (0.5 mmol), oxidant (2.5 mmol), catalyst (0.005 mmol), solvent (3 mL), stirring
for 7 h at 27 ◦C under nitrogen atmosphere.
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Table 6 Oxidation of alcoholsa by 2
Entry
1
Substrate
Product
Yieldb (%)
88 (85)c
2
3
92 (89)c
87
4
5
6
98 (95)c
95
37
7
8
99
93
9
60d
84
40
10
11
a Reaction conditions: alcohol (0.5 mmol), H2O2 (2.5 mmol), catalyst (0.005 mmol), acetonitrile (3 mL), stirring for 7 h, 27 ◦C. b Yield is determined by
GC with area normalization. c Isolated yield is given in parenthesis. d Stirred for 7 h at 70 ◦C.
12524 | Dalton Trans., 2011, 40, 12519–12526
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a-Methyl-2-naphthalenemethanol (Table 6, entry 9) was smoothly
oxidized to the corresponding ketone in 60% yield after stirring for
7 h at 70 ◦C. Catalytic oxidation of 1-phenoxy-2-propanol (Table
6, entry 10) gave the corresponding ketone in 84% yield at 27 ◦C.
Oxidation of 1-indanol to 1-indanone (Table 6, entry 11) occurred
in 40% yield at 27 ◦C, whereas oxidation of 4-chlorophenylethanol
(Table 6, entry 6) did not give satisfactory results even at elevated
temperature. The oxidation of cinnamyl alcohol was also tested;
however, this reaction mainly gave benzoic acid due to cleavage
of the carbon–carbon bonds. Unfortunately, and despite extensive
efforts, aliphatic alcohols did not give satisfactory results.
Preparation of the complexes
Synthesis of [CuCl(HL1)2(PPh3)] (1). N-(Diphenylcarbamo-
thioyl)benzamide (HL1) (0.2485 g, 0.75 mmol) dissolved in
benzene (15 mL) was added to [CuCl2(PPh3)2] (0.5 g, 0.75 mmol)
in benzene (25 mL). The mixture was stirred for 36 h at 27 ◦C. This
was concentrated to 3 mL and 25 mL of n-hexane–ethyl acetate
(9 : 1) was added. The formed orange oily precipitate was stirred,
filtered off and recrystallized from dichloromethane–n-hexane and
dried in vacuo. Single-crystals for X-ray diffraction were grown at
room temperature from ethyl acetate. Yield: 88%, decomp. 146 ◦C,
C58H47ClCuN4O3PS2 (1026.12): calc.: C 67.89, H 4.62, N 5.46, S
6.25; found: C 67.78, H 4.52, N 5.39, S 6.17%; meff = 0. FT-IR
(KBr): n = 3222 (N–H), 1680 (C O), 1178 (C S), 1435, 1095, 744
(PPh3) cm-1. UV (EtOH), l/nm (e/dm3 mol-1 cm-1): 224 (66 285),
Conclusions
In conclusion, we have prepared and characterized
copper(I) complexes with N-[di(phenyl/ethyl)carbamothioyl]-
benzamide (HL1 and HL2) and PPh3 ligands. N-
[Di(phenyl/ethyl)carbamothioyl]benzamide was coordinated
to the Cu(I) ion in a rare monodentate fashion through the S
atom. These complexes are employed as catalysts in combination
with H2O2 for the oxidation of various alcohols at room
temperature. The HL2 copper complex was found to be a better
catalyst for the oxidation of alcohols, without any additives,
and the scope of substrates was extended to various alcohols.
Interestingly, the ligands as well as complexes were synthesized
at room temperature. In addition, most of the catalytic reactions
were also carried out at room temperature.
1
260 (28 828). H NMR (CDCl3): d 7.2–7.9 (m, 45H, aromatic),
11.8 (s, 1H, N–H) ppm. 13C NMR (CDCl3): d 174.7 (C S), 155.0
(C O), 121.9, 122.2, 123.0, 123.4, 126.21, 126.51, 127.60, 127.72
(aromatic) ppm. 31P NMR (CDCl3): d 25.5 (s) ppm.
Synthesis of [CuCl(HL2)(PPh3)2] (2). N-(Diethylcarbamo-
thioyl)benzamide (HL2) (0.1764 g, 0.75 mmol) dissolved in
benzene (15 mL) was added to [CuCl2(PPh3)2] (0.5 g, 0.75 mmol)
in benzene (25 mL). The mixture was stirred for 36 h at 27 ◦C. This
was concentrated to 3 mL and 25 mL of n-hexane–ethyl acetate
(9 : 1) was added. The formed orange colour oily precipitate was
stirred, filtered off and recrystallized from dichloromethane–n-
hexane and dried in vacuo. Single-crystals for X-ray diffraction
were grown at room temperature from ethyl acetate solution by
the diffusion of diethyl ether vapour. Yield: 80%, decomp. 205 ◦C,
C48H46ClCuN2OP2S (859.90): calc.: C 67.04, H 5.39, N 3.26, S
3.73; found: C 66.95, H 5.30, N 3.21, S 3.69%; meff = 0. FT-
IR (KBr): n = 3267 (N–H), 1681 (C O), 1184 (C S), 1435,
1095, 744 (PPh3) cm-1. UV (EtOH), l/nm (e/dm3 mol-1 cm-1):
224 (43 742), 260 (17 314). 1H NMR (CDCl3): d 7.1–8.1 (m, 35H,
aromatic), 10.8 (s, 1H, N–H), 3.5–3.6 (q, 2H, CH2), 3.8–3.9 (q, 2H,
CH2), 1.21–1.28 (m, 6H, CH3) ppm. 13C NMR (CDCl3): d 172.9
(C S), 158.0 (C O), 122.0, 122.1, 122.8, 123.0, 126.4, 127.2,
Experimental section
Materials and reagents
All chemicals were obtained from commercial sources and
used as received. The solvents were purified and dried in ac-
cordance with the standard literature methods. The precursor
[CuCl2(PPh3)2] was prepared by a literature procedure29 while
the ligands N-(diphenylcarbamothioyl)benzamide (HL1) and
N-(diethylcarbamothioyl)benzamide (HL2) were prepared from
benzoyl chloride, potassium thiocyanate and diphenyl amine or
diethyl amine in dry acetone.13a
127.5, 127.7 (aromatic), 41.2, 41.6 (CH2), 5.2, 6.9 (CH3) ppm. 31
NMR (CDCl3): d 28.4 (s) ppm.
P
X-Ray structure determination
Physical measurements
The data for both compounds were collected at 293(2) K on a
Bruker Smart CCD diffractometer. The w-scan method was used
to measure the intensities up to a maximum of 2q = 28.4◦, with
Microanalyses were carried out with a Vario EL AMX-400
elemental analyzer. Melting points were recorded with a Veego
VMP-D melting point apparatus and were uncorrected. FT-IR
spectra were recorded as KBr pellets with a Perkin Elmer Spectrum
RX1 FT-IR spectrophotometer in the 4000–400 cm-1 range.
Electronic spectra of the complexes were recorded in ethanol
solutions using a PG Instruments Ltd T90+ spectrophotometer
in the 800–200 nm range. Magnetic susceptibility measurements
were made with a Sherwood Scientific auto magnetic susceptibility
˚
graphite-monochromatised Mo-Ka radiation (l = 0.71073 A).
Cell parameters were refined by using APEX2 (Bruker, 2008). The
data were corrected for Lorentz and polarization factors. A multi-
scan absorption correction was applied. The structure was solved
by the direct methods and refined by full-matrix least-squares
procedures based on F2. All non-hydrogen atoms were refined
anisotropically, and the hydrogens bonded to the carbon atoms
were positioned geometrically and allowed to ride on the parent
atoms, whereas the hydrogen(s) bonded to the nitrogen atom(s),
located in a difference Fourier map, were refined isotropically.
Data reduction (SAINT),31 structure solution (SHELXS97),31
refinement (SHELXL97)31 and molecular graphics (PLATON)32
were performed using WINGX software packages.33 Hydrogens
of the water molecule could not be located from the difference
1
balance. H, 13C and 31P NMR spectra were recorded in Bruker
Avance 400 MHz instrument in CDCl3. TMS was used as an
internal standard for 1H and 13C NMR spectra. H3PO4 was
used as an internal standard for 31P NMR spectra. Capillary
gas chromatography was performed on a Shimadzu GC-2010 gas
chromatography with a RTX-5 column (60 m length, 0.32 mm
inner diameter).
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Fourier map and have not been included in the model, however
the population parameter of this oxygen was allowed to vary. The
crystal data and refinement parameters are summarized in Table 1.
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Procedure for catalytic oxidation
To a solution of the alcohol (0.5 mmol) in acetonitrile solvent (3
mL), hydrogen peroxide (2.5 mmol, 0.565 mL) and the complex 2
(0.005 mmol, 0.0043 g) were added. The solution was stirred for 7
h at 27 ◦C. At the requisite time aliquots of the reaction mixture
were removed and the alcohol and acid/ketone were extracted
with n-hexane. The n-hexane extract was then analyzed by GC.
Acknowledgements
N. G. thanks NITT for a fellowship. R. K. acknowledges DST,
Government of India, for financial assistance under SERC Fast
Track scheme for Young Scientists (No. SR/FTP/CS-80/2006).
We thank Prof. Kurt Mereiter, Vienna University of Technology,
Austria for useful discussion on crystallography. P. R. thanks
Council of Scientific and Industrial Research, Government of
India for the financial support, in the form of Senior Research
Fellowship.
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