An efficient heterogeneous Cu(I) complex for the catalytic oxidation of alcohols and sulfides: synthesis, characterization, and investigation of the catalyst activity

Article abstract of DOI:10.1080/00958972.2021.1950698

A heterogeneous Cu(I) complex was synthesized by reaction of C₅H₄NC(O)NHP(O)[NHC(CH₃)₃]₂ (L) with CuCl and used as a catalyst for green oxidation of aromatic alcohols and sulfides. According to the characterization results obtained from ³¹P NMR, UV-Vis, FT-IR, mass, XRD, elemental analysis, and molar conductivity techniques, the chemical formula of the complex (1) was suggested as [CuClL₂]. Parameters on the catalytic activity (reaction temperature, the substrate to oxidant molar ratio, catalyst amount, and solvent type) were optimized, using H₂O₂ as a green oxidant, and excellent conversion amounts (100%) were achieved under optimal conditions. Moreover, the generality and recyclability of the catalyst were tested and confirmed. A comparison between the oxidation results of the alcohol and sulfide in the presence of 1 with other catalysts indicated that in most cases 1 showed better efficiency, in a more reasonable reaction time and conditions.

Full text of DOI:10.1080/00958972.2021.1950698

Journal of Coordination Chemistry
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An efficient heterogeneous Cu(I) complex for
the catalytic oxidation of alcohols and sulfides:
synthesis, characterization, and investigation of
the catalyst activity
Nasrin Oroujzadeh, Zahra Baradaran & Alireza Sedrpoushan
To cite this article: Nasrin Oroujzadeh, Zahra Baradaran & Alireza Sedrpoushan (2021): An
efficient heterogeneous Cu(I) complex for the catalytic oxidation of alcohols and sulfides: synthesis,
characterization, and investigation of the catalyst activity, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2021.1950698
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2021.1950698
An efficient heterogeneous Cu(I) complex for the catalytic
oxidation of alcohols and sulfides: synthesis,
characterization, and investigation of the catalyst activity
Nasrin Oroujzadeh, Zahra Baradaran and Alireza Sedrpoushan
Department of Chemical Technologies, Iranian Research Organization for Science and Technology
(IROST), Tehran, Iran
ABSTRACT
ARTICLE HISTORY
Received 21 February 2021
Accepted 8 June 2021
A heterogeneous Cu(I) complex was synthesized by reaction of
C5H4NC(O)NHP(O)[NHC(CH3)3]2 (L) with CuCl and used as a cata-
lyst for green oxidation of aromatic alcohols and sulfides.
According to the characterization results obtained from 31P NMR,
UV-Vis, FT-IR, mass, XRD, elemental analysis, and molar conductiv-
ity techniques, the chemical formula of the complex (1) was sug-
gested as [CuClL2]. Parameters on the catalytic activity (reaction
temperature, the substrate to oxidant molar ratio, catalyst
amount, and solvent type) were optimized, using H2O2 as a green
oxidant, and excellent conversion amounts (100%) were achieved
under optimal conditions. Moreover, the generality and recyclabil-
ity of the catalyst were tested and confirmed. A comparison
between the oxidation results of the alcohol and sulfide in the
presence of 1 with other catalysts indicated that in most cases 1
showed better efficiency, in a more reasonable reaction time and
conditions.
KEYWORDS
Cu(I) complex; N-nicotinyl;
N0,N00-bis(tert-butyl) phos-
phoric triamide;
heterogeneous catalyst;
alcohol oxidation;
sulfide oxidation
1. Introduction
Selective oxidation of alcohol to corresponding carbonyl compounds [1–6] and sulfides
to sulfoxide [7–11] are important in organic chemistry. Especially, the production of
aldehydes from alcohol reagents is crucial for food additive or fragrance industries.
CONTACT Nasrin Oroujzadeh
n_oroujzadeh@irost.ir
Department of Chemical Technologies, Iranian Research
Organization for Science and Technology (IROST), Tehran, P.O. Box 33535-111, Iran
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

2
N. OROUJZADEH ET AL.
This conversion usually needs stoichiometric amounts of toxic metal salts, or catalysts
containing expensive transition metals such as gold [12], palladium [13–15], ruthe-
nium, platinum, tungsten [16, 17], or titanium [18]. Manufacturing more efficient cata-
lysts for these reactions are of interest [19–26] and catalytic activities of several metal
complexes have been investigated in these oxidation reactions [27–32].
Oxidation of sulfide compounds also has attracted attention. Oxo-vanadium(IV)
Schiff base complex (VO(acac)2) heterogenized through supporting on mesoporous
MCM-41 was utilized in the oxidation of various sulfides and thiols in the presence of
urea hydrogen peroxide as the oxidant [33]. Although these catalysts exhibited excel-
lent catalytic activities in solvent-free conditions, their active site still consisted of an
expensive transition metal [34]. Among transition metals, copper due to its availability,
nontoxic and green aspects has been used as the active site in the oxidation of
organic compounds [35–37], but needed ionic liquids to perform its catalytic activity
[35] or non-green acids such as periodic acid (H5IO6) as oxidant [36].
The need for environmentally friendly and cost-effective methodologies for oxida-
tions of alcohols continues to attract attention [16]. Phosphoramide compounds have
been used as catalysts in many reactions. Using the first polymer-supported phosphor-
amide as a catalyst, the aldol reaction between a trichlorosilyl enol ether and various
aldehydes was carried out with significant rate enhancements at low temperature [38].
Phosphoramide compounds have been utilized as catalysts for the reduction of aceto-
phenone, obtaining products of modest enantiomeric excess at dramatically increased
reaction rates [39]. A series of new tris (b-hydroxy phosphoramide) compounds have
been synthesized and used as catalysts of asymmetric borane reduction of ketones
[40]. Phosphoric triamide compounds containing -C(O)N(H)P(O)- skeleton are an
important group of phosphoramides. This class of compounds has been used for drug,
insecticide, growth regulator for plants, fungicide, herbicide, anti-corrosive additive,
and antioxidant applications [41]. They have been utilized as N, O-donor ligands in
reaction with several metal ions [42–44]. There are few reports about these metal com-
plexes as catalysts in chemical reactions [45, 46]. [Zn(l-Vp)Ln(L)2(l-OAc)] and [Ni(l-
Vp)(MeOH)Ln(L)2(l-OAc)] (L ¼ N-[bis(diethylamino)phosphoryl]-2,2,2-trichloroacetamide,
Vp ¼ a Schiff base ligand) have been applied for catalyzing intramolecular transesterifi-
cation of 2-(hydroxypropyl)-p-nitrophenyl phosphate (HPNP). Kinetic studies exhibited
that the rate of hydrolysis is linearly dependent on complex concentration [45].
MnL2Cl2 (L ¼ N-isonicotinyl, N0,Nʺ -bis(pyrrolidinyl) phosphoric triamide) has been
reported as an efficient catalyst for olefin epoxidation with H2O2 oxidant. The best
yields have been obtained using acetic acid as a co-catalyst at 0 ꢀC [46].
Here, we synthesized a new Cu(I) complex (1) with N-nicotinyl, N0,N00-bis(tert-butyl)
phosphoric triamide (L) ligand. Cu(I) as a green catalytic active site in oxidation reac-
tions and L as a phosphor amide ligand bearing catalytic activity created a heteroge-
neous green catalyst. Characterization of the complex proved that P ¼ O and Npyridine
participated in coordination. According to CHN, mass analysis, and conductivity meas-
urement, CuClL2 was proposed as the molecular formula of 1. The complex was
applied as a catalyst in the oxidation of aromatic alcohols and sulfides with hydrogen
peroxide as a green oxidant. Very good catalytic activities in the oxidation of aromatic
alcohols and sulfides were achieved and the catalyst showed good recyclability in the

JOURNAL OF COORDINATION CHEMISTRY
3
Scheme 1. Synthesis pathway of 1.
optimum conditions (90% conversion of alcohols in the fourth run and 85% conver-
sion of sulfides in the fifth run). The effects of temperature, catalyst amount, solvent,
and substrate to oxidant ratio on the catalytic activity were investigated.
2. Materials and methods
2.1. Materials
The raw materials and solvents were purchased from Merck and Sigma-
Aldrich companies.
2.2. Synthesis of Cu(I) complex
N-nicotinyl, N0,N00-bis(tert-butyl) phosphoric triamide (L) was synthesized according to
the method previously reported [47].
Yield: 72%; m.p. ¼ 273.5 ꢀC. Elemental analysis (%) Calcd: C, 53.85; H, 8.01; N, 17.95.
Found: C, 53.86; H, 8.0; N, 17.93. IR (KBr, cmꢁ1): m ¼ 3320 (m, NH), 3120 (m), 2950 (m,
CH2), 1636 (vs, C ¼ O), 1584 (m, mring), 1441 (s, dCH), 1405 (s), 1287 (m, mring), 1237 (s,
P ¼ O), 1205 (s), 1117 (w), 1014 (s, mring), 886 (m), 850 (w), 803 (m), 730 (m), 584 (m),
1
530 (m), 488 (w). H NMR (500.13 MHz, D6-DMSO, 25 ꢀC, TMS): d ¼ 1.21 (m, 18H), 4.06
2
3
3
(d, J (PNH) ¼ 7.1 Hz, 2H, NHamine), 7.47 (dd, J(H,H) ¼ 7.8 Hz, J (H,H) ¼ 4.8 Hz, 1H),
3
3
5
8.29 (d, J (H,H) ¼ 7.9 Hz, 1H), 8.70 (d, J (H,H) ¼ 4.7 Hz, 1H), 9.07 (d, J (P,H) ¼ 2.1 Hz,
13
3
1H), 9.72 (s, 1H, NHamide). C NMR (125.76 MHz, D6-DMSO, 25 ꢀC, TMS): d ¼ 31.16 (d, J
3
(P,C) ¼ 4.9 Hz), 50.35 (s), 123.25 (s), 129.70 (d, J(P,C) ¼ 7.9 Hz, Cipso), 135.49 (s), 149.02
31
(s), 152.23 (s), 166.77 (s, C ¼ O). P NMR (202.46 MHz, D6-DMSO, 25 ꢀC, H3PO4 external):
d ¼ 3.66 (m) [47].
For the synthesis of 1, 0.312 g (1 mmol) of L was dissolved in 2 ml methanol and
0.495 g (0.5 mmol) copper chloride was added to the solution. This mixture was stirred
for two weeks at room temperature. The resulting precipitate was separated, washed
with methanol, and dried in a vacuum desiccator. The synthesis pathway is illustrated
in scheme 1.
Yield: 65%; m.p. ¼ 242 ꢀC. Elemental analysis (%) Calcd: C, 46.51; H, 6.91; N, 15.50.
Found: C, 46.64; H, 6.85; N, 15.43. IR (KBr, cmꢁ1): m ¼ 3365 (w, NH), 3137 (w), 2973 (m,
CH2), 1656 (m, C ¼ O), 1604 (w, mring), 1461 (vs, dCH), 1419 (s), 1409 (s), 1380 (w), 1292
(w, mring), 1220 (vs, P ¼ O), 1203 (vs), 1124 (w), 1022 (s, mring), 892 (w), 852 (w), 813 (w),

4
N. OROUJZADEH ET AL.
Table 1. Main FT-IR frequencies of L (cmꢁ1).
Vibrations of
L (this work)
L [47]
masNH,msNH
mCH2
3345, 3131
2973
3320, 3120
2950
mC¼O
mC¼N
dCH
1643
1592
1463
1636
1584
1441
mP¼O
mring
1243
1021
1237
1041
740 (m), 692 (w), 567 (m, Cu-N), 541 (w), 492 (w), 420 (m, Cu-O). 31P NMR (202.46 MHz,
DMSO-d6, 25 ꢀC, H3PO4 external): d ¼ 2.50 (m). UV-Vis in methanol: kmax ¼ 258 nm.
2.3. Physical measurements
Infrared spectra (4000–400 cmꢁ1) were recorded on a Philips 9624 FT-IR spectropho-
tometer with a KBr disc. Powder XRD pattern was obtained on an EQUINOX 3000
Advance instrument. Mass spectrometric measurement was done with an Agilent
5975 C spectrometer. Elemental analysis (C, H, N) was performed using a Perkin Elmer
2400 elemental analyzer. Hach14d digital conductivity meter was used for measuring
the molar conductance of a 10ꢁ3 M solution in DMSO at room temperature. The melt-
ing point was recorded on an Electrothermal type 9200 melting point apparatus.
Electronic absorption spectra were done using the Jasco V-570 UV-visible spectropho-
tometer. 31P-NMR spectrum was recorded on an Avance 300 (300 MHz) spectrometer
in DMSO-d6. A Young Lin 6500 gas chromatograph (YL-6500) was used to perform gas
chromatography (GC).
2.4. Catalytic reaction
Catalytic oxidation of alcohols and sulfides to corresponding acids and sulfoxides by
the synthesized copper complex was investigated in the presence of H2O2 (28 w%) as
oxidant. For each run, a suitable amount of 1 as catalyst (5–15 mol%), the substrate
(1 mmol), and H2O2 (28 wt%) were reacted in solvent (2 ml). The reactions were per-
formed with different solvents, temperatures, and molar ratio of substrate to oxidant
to get to optimal conditions. The oxidation reactions of sulfides were done at ambient
temperature. The reactions were monitored by thin layer chromatography. At the end
of the reactions, the catalyst was separated, washed with acetone, and dried in a vac-
uum desiccator to reuse and test the recycling capability.
3. Results and discussion
3.1. Characterization
3.1.1. FT-IR spectroscopy
FT-IR spectra of the ligand were in agreement with those previously reported [47]. The
small dissimilarity between the positions of IR bands (Table 1) is due to the differences
between FT-IR (here) and IR [47] instruments. FT-IR spectra of L and 1 are given in
and summarized in Table 2.

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 1. The FT-IR spectrum of (a) 1 and (b) L.
Table 2. Main functional groups for FT-IR vibration bands of 1 and L.
Vibrations of
C5H4NC(O)NHP(O)[NHC(CH3)3]2 (L)
[CuClL2] (1)
masNH,msNH
3345, 3131
1643
1592
1243
—
3365, 3137
1656
1604
1220
576
mC¼O
mC¼N
mP¼O
mM-O
mM-N
—
420
The FT-IR spectrum of 1 confirms the presence of L in the complex. A comparison
between the FT-IR results of L and 1 shows that m (PO) has shifted 23 cmꢁ1 to lower
frequencies by forming the complex. The frequency of the carbonyl group in 1 repre-
sented a positive shift by complexation. Such a positive shift of m(CO) and a negative
shift of m (PO) are observed when a phosphoramide coordinates to a metal ion
through the oxygen of its P ¼ O site [48, 49]. The vibration of the pyridine ring in 1
showed a shift to higher frequency relative to L, confirming coordination of L from
Npyridine. According to the literature, when nitrogen of pyridine coordinates to a metal
center, the frequency of the specific vibrational modes increases, due to coupling with
M-N (pyridine) bond vibrations [50, 51]. The two bands at 420 and 576 cmꢁ1 could
also be assigned to Cu-N and Cu-O, respectively [36].
3.1.2. 31P NMR spectroscopy
31P NMR spectra of 1 and L are shown in Figure 1. Chemical shifts of 31P are 2.5 ppm
and 3.66 ppm, respectively. In both spectra just one peak is observed, so it can be
concluded that both materials are pure [52]. There is a significant difference between
d (31P) of the complex and ligand; this confirms the formation of a new molecule.
According to the literature, this difference approves the formation of a metal com-
plex [43].
3.1.3. UV-Vis spectroscopy
The UV spectra obtained from L and 1 in methanol at room temperature are shown in
Figure 2. Two absorption bands observed at 216 and 256 nm in the spectrum of L are

6
N. OROUJZADEH ET AL.
Figure 2. The 31P NMR spectrum of (a) 1 and (b) L.
Figure 3. The FT-IR spectrum of (a) 1 and (b) L.
ꢂ
ꢂ
in agreement with n!p and p!p transitions of pyridine derivatives [53–55]. Since
ꢂ
the p!p transition bond is more intense in polar solvents [56], the more intense
ꢂ
ꢂ
band at 216 nm could be assigned to p!p and the other one to n! p transitions.
Such absorption bands were also observed in the spectrum of 1 which confirms the
existence of L in the complex structure. The shifts of these bands from 216 and 256
to 218 and 258 in 1 can be due to the coordination of L to copper [57, 58].

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 4. The 31P NMR spectrum of (a) 1 and (b) L.
To observe the complex color (green) absorption band, the visible spectra of L and
1, obtained from their 10ꢁ2 M solutions in DMSO, are presented in Figure 3. The
shoulder in the visible spectrum of 1 (761 nm) is not observed in the spectrum of L.
Since copper(I) ion with a d10 configuration is diamagnetic, no d-d electronic transition
ꢂ
was expected for this complex. Since unsaturated ligands with empty p orbitals
which are coordinated to copper(I) center can act as electron acceptors [59], the
observed shoulder could be assigned to MLCT (metal to ligand charge transfer) transi-
tion [60].
3.1.4. Powder XRD
X-ray diffraction patterns of L and 1 are displayed in Figure 4 recorded in the 2h range
of 0–110ꢀ.
The XRD pattern of 1 is very similar to the one for Cu2P4Cl2N4O2C54H52, with COD
card no. 4300484, and is different from the ligand pattern. This difference between
the XRD patterns of the ligand and complex confirm the formation of the complex
[61]. Both compounds illustrate sharp crystalline peaks, pointing to the crystalline
nature of both the ligand and the complex. The diffraction data of the complex, angle
(2h), interplanar spacing (d), and the relative intensity (%), are given in Table 3.
Using Debye–Scherrer’s method, given in equation 1, the average crystallite size of
1 was 34.6 nm approximately.
D ¼ Kk=b cos h
(1)
K is the shape factor or the average crystallite (usually 0.9), k is the X-ray wave-
length of the instrument (0.154 nm), b is full width at half maximum of the diffraction
line (FWHM) in radians and Ɵ is Bragg’s diffraction angle.

8
N. OROUJZADEH ET AL.
Figure 5. UV absorption spectrum of (a) L and (b) 1 in methanol.
Table 3. XRD data of 1.
Peak No.
2h (ꢀ)
FWHM (ꢀ)
d (Å)
Intensity (%)
1
2
3
4
5
6
7
8
2.0342
6.5894
9.1589
1.0752
0.3456
0.3072
0.2304
0.9216
0.3840
0.5376
0.3072
0.4608
0.4608
0.3840
0.3456
0.7680
0.9216
43.43252
13.41442
9.65604
9.09825
7.89906
7.25556
5.39645
5.10329
4.83969
4.59065
4.11562
3.65644
2.26700
2.03066
13.31
60.89
58.23
100.00
59.46
85.05
54.84
87.63
26.21
28.08
40.54
27.61
26.23
3.60
9.7217
11.2020
12.1991
16.4270
17.3778
18.3322
19.3360
21.5932
24.3440
39.7628
44.6247
9
10
11
12
13
14
2h: the diffraction angle in degrees; d: the interplanar spacing in angstroms; FWHM: full-width at half-maximum
in degrees.

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 6. Visible absorption spectrum of (a) 1 and (b) L in DMSO.
Using Bragg’s law (k ¼ 2d sinh), the d-spacing (distance between crystalline layers
or polymeric chains) was calculated. For L and 1, the d-spacing values are 5.25 and
9.09 Å, respectively. By the formation of the complex, the d-spacing increased, which
can be related to the coordination of the ligand to the copper center.
3.1.5. Mass spectroscopy and molar conductivity
The mass spectrum of the copper(I) complex (1) is demonstrated in Figure 5. The
molecular ion peak at m/z ¼ 723.5 amu, which is the molecular weight of 1, confirms
the existence of two L molecules and one Cl in a molecule of 1. The most intense peak
at m/z¼ 57amu is related to stable tert-butyl cation [62]. The species [C5H4N]þ (pyridine
ring) and [C6H5N2]þ (C5H4NC(O)NH) are indicated with peaks at 78 and 105 amu,
respectively [63]. The peak at 297 amu is due to the presence of [C13N4PO2H22)]þ,
C5H4NC(O)NHP(O)[NHC(CH3)3][NHC(CH3)2], fragments. In addition, peaks at m/
z ¼ 433 amu and m/z ¼ 340 amu could correspond to [CuCl(C12N4P2O4H10)]þ and
[CuCl(C12N4O2H10)]þ fragments, respectively.
Conductivity studies were performed to determine whether the chloride is ion-
paired or coordinated. Due to the complete solubility of 1 in DMSO, the measurement
was done in this solvent. The molar conductivity value of the synthesized complex in
DMSO solution was in the range of 7–15 S cm2 molꢁ1, which indicates that the Cu(I)
complex is a non-electrolyte [64], confirming the coordination of the chloride to
Cu(I) [65].
Concerning the molar conductivity and mass spectroscopy results, it seems that the
structure of 1 is [CuClL2].
Based on all characterization analysis results, the molecular structure for 1 is sug-
gested in Figure 6(b). The structure of L [47] is demonstrated in Figure 6(a).
3.2. Investigating catalytic activity of the complex
To investigate the catalytic activity of 1, it was applied as a catalyst in the oxidation of
some aromatic alcohols and sulfides. For this purpose, benzyl alcohol and methyl

10
N. OROUJZADEH ET AL.
Figure 7. X-ray diffraction patterns of (a) 1 and (b) L.
Table 4. Optimization of the solvent type in the oxidation reaction of benzyl alcohol.
Entry
Solvent
Time (h)
Maximum conversion (%)
1
2
3
4
5
6
CHCl3
n-hexane
CH3CN
17
22
10
12
18
15
30
25
95
60
50
40
CH3CN:H2O (2:1)
Acetone
H2O
Reaction conditions: 1 mmol benzyl alcohol, 15 mol% catalyst, 1.5 mmol 28% H2O2, 2 mL solvent at r.t.
phenyl sulfide were selected as substrates and their oxidation was carried out using
hydrogen peroxide (28% H2O2) as a green and effective oxidant. Parameters such as
solvent, catalyst amount, the substrate to oxidant ratio, and temperature were opti-
mized utilizing 1 as the catalyst (given in the following sections). Recyclabilities of 1 in
benzyl alcohol and methyl phenyl sulfide oxidations were tested. Different alcohols
and sulfides were chosen and their oxidations were done using optimum conditions
to study the generality of the catalyst.
3.2.1. Optimization of solvent type in oxidation reactions
Benzyl alcohol and methyl phenyl sulfide oxidation reactions were carried out in sev-
eral solvents. The solvent, the conversion percentage, and the time, in which the reac-
tions reach their maximum, are given in Tables 4 and 5. For both oxidation reactions
the conversion percentage of the substrates is higher in polar solvents (entries 3–6 in
Tables 4 and 5) than non-polar ones (entries 1 and 2 in Tables 4 and 5) [66]. The best
efficiency of the catalyst was obtained when the solvent was acetonitrile.
The higher catalytic activity of 1 in the oxidation reaction is attributed to the better
interaction of the heterogeneous complex in the non-protic polar medium. This find-
ing is confirmed by decreasing conversion upon changing reaction solvent to aceto-
nitrile/water (Tables 4 and 5, entries 4 and 6). Due to the high reaction efficiency in
acetonitrile, it was selected as an oxidation solvent and was applied in the rest of the
reactions that were done to optimize other parameters.

JOURNAL OF COORDINATION CHEMISTRY
11
Table 5. Optimization of the solvent type in the oxidation reaction of methyl phenyl sulphide.
Entry
Solvent
Time (h)
Maximum conversion (%)
1
2
3
4
5
6
CHCl3
n-hexane
CH3CN
4.5
6
3
3
3.5
80
60
100
80
95
85
CH3CN:H2O (2:1)
Acetone
H2O
4
Reaction conditions: 1 mmol methyl phenyl sulfide, 15 mol% catalyst, 2 mmol 28% H2O2, 2 mL solvent at r.t.
Table 6. Optimization of the catalyst amount in the oxidation reaction of benzyl alcohol.
Entry
Catalyst amount (mol%)
Time (h)
Maximum conversion (%)
1
2
3
5
10
15
14
10
10
50
95
95
Reaction conditions: 1 mmol benzyl alcohol, 1.5 mmol 28% H2O2, 2 mL CH3CN at r.t.
3.2.2. Optimization of the catalyst amount in the oxidation reactions
The use of larger amounts of catalysts usually imposes high costs [67]. In addition,
although the rate and efficiency of the reaction increase with increased amount of
catalyst, chemo, and stereoselectivity of conversion may be affected [68]. In some
cases, the use of high amounts of catalysts causes agglomeration of catalyst, reducing
available active sites for the reaction [69]. Therefore, it is necessary to optimize the
amount of used catalyst to select the lowest amount that maximizes the conversion
percentage and at the same time does not have the mentioned disadvantages. Here
we have optimized the catalyst amount for the oxidation reactions of benzyl alcohol
and methyl phenyl sulfide. The optimization results are summarized in Tables 6 and 7,
respectively. The effect of catalyst amount in both oxidation reactions was tested
using 5, 10 and 15 mol% catalysts (values usually used in oxidation reactions [70]),
while the reaction solvent was acetonitrile and other conditions stayed unchanged.
When 5 mol% of 1 was applied the conversion values were obtained, 50% and 70%
for benzyl alcohol and methyl phenyl sulfide oxidation reactions, respectively. The
best conversion values, 95% (for benzyl alcohol oxidation) and 97% (for methyl phenyl
sulfide oxidation) were achieved using 10 mol% of catalyst. By increasing the amount
of the catalyst to 15 mol%, conversion percentage and selectivity were not affected
significantly in both alcohol and sulfide oxidation reactions. Hence, all other studies
were performed with 10 mol% of catalyst.

12
N. OROUJZADEH ET AL.
Table 7. Optimization of the catalyst amount in the oxidation reaction of methyl phenyl sulphide.
Entry
Catalyst amount (mol%)
Time (h)
Maximum conversion (%)
1
2
3
5
10
15
7
3
3
70
97
100
Reaction conditions: 1 mmol methyl phenyl sulfide, 1.5 mmol 28% H2O2, 2 mL CH3CN at r.t.
3.2.3. Optimization of the molar ratio of substrate to oxidant in the oxida-
tion reactions
Although increasing hydrogen peroxide increases the conversion percentage, the oxi-
dant amount needs to be optimized to avoid formation of by-products in the pres-
ence of excess H2O2 [71]. In some cases, large amounts of hydrogen peroxide can
reduce the reaction efficiency, which can be due to more chemisorption of H2O2 and
less chemisorption of the substrate on the active site of the catalyst [72]. The oxida-
tion reaction of benzyl alcohol was carried out using three ratios of substrate to oxi-
dant (1:1, 1:1.5 and 1:2) commonly used in similar reactions [73]. When the ratio of the
substrate to oxidant was 1:1, the conversion was 70%. Applying the substrate to oxi-
dant molar ratio of 1:1.5, the catalyst amount of 10 mol%, and acetonitrile as a solvent
the conversion percentage was 95%, which was the best-obtained result. Increasing
the substrate to oxidant ratio from 1:1.5 to 1:2 had no significant effect on the reac-
tion time and conversion percentage.
In the case of the methyl phenyl sulfide oxidation, four ratios of the substrate to
oxidant (1:1, 1:1.5, 1:2 and 1:2.5) were applied. Utilizing 1:1 and 1:1.5 ratios of the sub-
strate to oxidant, the attained conversion percentages were 50% and 75%, respect-
ively. The best conversion value (100%) was acquired using a substrate to oxidant
molar ratio of 1:2. With increasing the ratio from 1:2 to 1:2.5, the conversion percent-
age was unaffected. Therefore, other studies in the oxidation of sulfides were done
with the substrate to oxidant molar ratio of 1:2.
3.2.4. Optimization of the temperature in the oxidation reactions
For temperature optimization of benzyl alcohol oxidation, the reaction was carried out
at 25 ꢀC, 50 ꢀC and reflux temperature (78 ꢀC) in optimized conditions (acetonitrile solv-
ent, catalyst amount of 10 mol% and alcohol to H2O2 ratio of 1.0:1.5). Results showed
that at room temperature, the conversion percentage of substrate oxidation was 95%,
and the reaction had no progress after 10 h (Table 8). Completion of the benzyl alco-
hol oxidation reaction (conversion values ¼ 100%) was achieved after 3 h at 50 ꢀC and
after 50 min at the reflux temperature (78 ꢀC). So, the reflux condition (78 ꢀC) (entry 3)
was selected as an optimum point because of the shorter reaction time.
Oxidation of methyl phenyl sulfide was carried out at ambient temperature with
excellent conversion value and in a reasonable time of 3 h, so no more temperature
optimization studies were done for this substrate.

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13
Table 8. Effect of temperature in the oxidation reaction of benzyl alcohol.
Entry
Temperature (ꢀC)
Time (h)
10
Maximum conversion (%)
95
1
2
3
25
50
3
100
100
78 (reflux)
50 (min)
Reaction conditions: 1 mmol benzyl alcohol, 10 mol% catalyst, 1.5 mmol 28% H2O2, and 2 mL acetonitrile.
Figure 8. Mass spectrum of 1.
3.2.5. Investigating catalyst recycling in the oxidation reactions
Recycling the heterogeneous catalyst after the reaction performance is essential [74].
To investigate the recycling ability of 1 the benzyl alcohol and methyl phenyl sulfide
oxidation reactions were performed under optimum conditions. The catalyst was sepa-
rated, washed, and dried after completion of the reactions. Then it was used further in
the oxidation reactions. Results indicated that for benzyl alcohol oxidation the catalyst
was recyclable and could be used four times with no significant loss of activity. For
methyl phenyl sulfide oxidation the catalyst maintained its catalytic activity up to five
runs (Figure 7).
3.2.6. The generality test of 1
The generality of 1 was tested using various substrates. Various alcohols and sulfides
were chosen as substrates, and their oxidations were carried out under the optimized
reaction conditions. Results are given in Tables 9 and 10. For all of the different alco-
hols and sulfides 1 catalyzed the reactions with high selectivity to produce the acids
and sulfoxides, respectively.
As Table 9 exhibits, different conversion percentages (50%–100%) were obtained
using various alcohols as substrates. These differences can be due to the steric and
electronic effects [75]. Comparing the oxidation results of benzyl alcohol (entry 2) with
4-methoxybenzyl alcohol (entry 3), in Table 9, indicates that with substituting an elec-
tron-donating group (-OMe) in the para-position of benzyl alcohol, the completion
time of oxidation (conversion amount ¼ 100%) was reduced from one hour to

14
N. OROUJZADEH ET AL.
Table 9. Oxidation results of various alcohols to acids using 1 as the catalyst.
Scheme 9.
Maximum
conversion (%)
Entry
1
Alcohol
Product
Time (h)
5
65
2
3
100
100
1
0.5
4
5
60
65
5
3.5
6
70
6.5
7
8
50
70
4
4
Reaction conditions: 1 mmol alcohol, 10 mol% catalyst, 1.5 mmol 28% H2O2, 2 mL acetonitrile and reflux condition.
30 minutes. On the contrary, the presence of an electron-withdrawing group (-NO2 in
entry 1) in this position reduced maximum conversion to 65% and the time of oxida-
tion to 5 h. In the case of alcohols 6–8, it seems that the steric factor plays a greater
role than the electronic effect. A comparison between the oxidation results of benzyl
alcohol (entry 2) and benzoin (entry 6) shows that with increasing the steric hindrance
(entry 6) the maximum conversion decreased to 70% and the time of oxidation was
prolonged to 6.5 h.

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15
Table 10. Oxidation results of various sulfides to sulfoxides using 1.
Maximum
Entry
1
Sulfide
Product
conversion (%)
Time (h)
2
100
2
3
4
90
3
80
3.5
4
67
Reaction conditions: 1 mmol sulfides, 10 mol% catalyst, 2 mmol 28% H2O2, 2 mL acetonitrile.
Results of oxidation reactions of various sulfides (Table 10) indicated that 1 could
catalyze the oxidation reaction of various sulfides to produce their corresponding sulf-
oxides, without a trace amount of sulfones. The main reason for the difference
between the resulted conversion amounts for these reactions is the steric effects. A
comparison between the results of 1–4 (Table 10) shows that as the sulfides become
bulkier, the amount of maximum conversion becomes less (1 > 2>3 > 4).
Table 11 illustrates some examples from previous studies on benzyl alcohol oxida-
tion reactions with various catalysts. Comparison between conversion and time of
reaction of 1 and other listed catalysts indicates that the activity of 1 is in a moderate
to good range, confirming 1 as an efficient catalyst. As can be seen from Table 11, in
the presence of 1, the oxidation of benzyl alcohol is carried out at a lower reaction
time and with a higher conversion percentage relative to other reactions. In the case
of [RuH(CO)(py-NP)(PPh3)2]Cl catalyst (entry 9), with conversion of 100%, higher tem-
perature and reaction time have been applied relative to the mild conditions of 1.
Another advantage of 1, besides the mild conditions, is its good selectivity to one
product (see the results of entries 1 and 2).
Table 12 shows a comparison between our results for methyl phenyl sulfide oxida-
tion, using 1, with other catalysts reported. The conversion percentage of 100% was
obtained in a short time in the presence of 1 using mild conditions, while in most of
the other reactions (entries 1–9) such conversion has not been achieved even in lon-
ger reaction times. However, there also are cases in which the oxidation reaction of
methyl phenyl sulfide has been performed with a good conversion percentage in a
shorter time than ours (entries 11 and 12). Concerning the high cost of the Ti deriva-
tives, it seems that our catalyst could be more economical than these two catalysts.

16
N. OROUJZADEH ET AL.

JOURNAL OF COORDINATION CHEMISTRY
17

18
N. OROUJZADEH ET AL.
4. Conclusion
A Cu(I) complex (1) with a phosphoric triamide ligand was synthesized and character-
ized by various methods. Results indicated that 1 is in a crystalline phase, the ligand is
coordinated to copper via P ¼ O and Npyridine, one molecule of 1 contains one Cu, two
molecules of L, and one Clꢁ. According to the characterizations, the proposed formula
of 1 is [CuClL2].
Additionally, 1 was used as a heterogeneous catalyst for aromatic alcohols and sul-
fide oxidations, with H2O2 as a green oxidant. Solvent, catalyst amount, the substrate
to oxidant ratio, and temperature were optimized 1 as the catalyst and benzyl alcohol
and methyl phenyl sulfide as substrates. For oxidation of benzyl alcohol the optimum
conditions were catalyst amount of 10 mol%, substrate to oxidant ratio of 1:1.5, reflux
temperature (78 ꢀC), and acetonitrile as the solvent. For the oxidation of methyl phenyl
sulfide to its corresponding sulfoxide, the best results were achieved by 10 mol% of
the catalyst, substrate to oxidant ratio of 1:2, ambient temperature, and acetonitrile as
the solvent. Recycling experiments showed that 1 could catalyze the oxidation reac-
tions of benzyl alcohol and methyl phenyl sulfide for 4 and 5 runs, respectively.
Moreover, using the optimum conditions, the oxidation of various alcohols and sul-
fides were investigated. Comparison between the oxidation results of benzyl alcohol
and methyl phenyl sulfide in the presence of 1 with other catalysts (under the other
previously published conditions) showed that in most cases, 1 had better and more
efficient results, in terms of conversion percentage, reaction time, and reac-
tion conditions.
Acknowledgement
Supports by the Iranian Research Organization for Science and Technology (IROST) are gratefully
acknowledged.
Disclosure statement
No potential conflict of interest was reported by the authors.
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