A novel dinuclear schiff base copper complex as an efficient and cost effective catalyst for oxidation of alcohol: Synthesis, crystal structure and theoretical studies

Article abstract of DOI:10.1007/s12039-015-0896-9

An environmentally friendly protocol is described for an economic, practical laboratory-scale oxidation of primary and secondary alcohols to aldehydes and ketones, using a bis-chloro-bridged binuclear Cu(II) complex [(HL)Cu(μ ₂-Cl)₂Cu(HL)] 1.5 CH₃OH as catalyst. The catalyst was prepared in situ from commercially available reagents and is characterized by single crystal X-ray analysis, FT-IR, UV-visible spectra, mass spectrometry, and powder x-ray diffraction (PXRD). The geometry of the complex has been optimized using the B3LYP level of theory confirming the experimental data. Our results demonstrated well the efficiency, selectivity and stability of this new catalyst in the oxidation of alcohols in ethanol and tert-butyl hydroperoxide (tBuOOH) as a green solvent and oxidant, respectively. Turnover number and reusability have proven the high efficiency and relative stability of the catalyst. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0896-9

c
J. Chem. Sci. Vol. 127, No. 7, July 2015, pp. 1321–1328. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0896-9
A novel dinuclear schiff base copper complex as an efficient and cost
effective catalyst for oxidation of alcohol: Synthesis, crystal structure
and theoretical studies
ATENA NAEIMIa,∗, SAMIRA SAEEDNIAb, MEHDI YOOSEFIANc, HADI AMIRI RUDBARId
and VIVIANA MOLLICA NARDOe
aFaculty of science, Department of Chemistry, University of Jiroft, Jiroft, Iran
bDepartment of Chemistry, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran
cDepartment of Chemistry, Graduate University of Advanced Technology, Kerman, Iran
dFaculty of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
eUniversità degli Studi di Messina, dip. Scienze Chimiche. Viale Ferdinando S. d’Alcontres,
98166 Messina, Italy
e-mail: naeimi.atena@imdea.org
MS received 22 November 2014; revised 17 April 2015; accepted 21 April 2015
Abstract. An environmentally friendly protocol is described for an economic, practical laboratory-scale oxi-
dation of primary and secondary alcohols to aldehydes and ketones, using a bis-chloro-bridged binuclear Cu(II)
complex [(HL)Cu(μ2-Cl)2Cu(HL)]*1.5 CH3OH as catalyst. The catalyst was prepared in situ from commer-
cially available reagents and is characterized by single crystal X-ray analysis, FT-IR, UV-visible spectra, mass
spectrometry, and powder x-ray diffraction (PXRD). The geometry of the complex has been optimized using
the B3LYP level of theory confirming the experimental data. Our results demonstrated well the efficiency,
selectivity and stability of this new catalyst in the oxidation of alcohols in ethanol and tert-butyl hydroperoxide
(tBuOOH) as a green solvent and oxidant, respectively. Turnover number and reusability have proven the high
efficiency and relative stability of the catalyst.
Keywords. Schiff base; Oxidation; DFT calculations; Peroxide; Green chemistry
1. Introduction
as tBuOOH in this work. The present catalyst system
shows considerably high catalytic activity, enabling effi-
cient oxidation of primary and secondary alcohols with
tBuOOH, without overoxidation to the carboxylic acid.
All the reactions were carried out in ethanol, a green
solvent. This work deals with the synthesis and char-
acterization of a new Schiff base ligand obtained by
the condensation reaction of 2-hydroxynaphthalene-1-
carbaldehyde and 1-amino-2-propanol and its Cu(II)
complexascatalyst(scheme1). The coordination behav-
ior of the Schiff base towards transition metal ions was
investigated via FT-IR, UV–Vis and single crystal X-ray
diffraction analysis. The geometry of the complex has
been optimized using the B3LYP level of theory.
Development of efficient and selective catalyst for oxi-
dation of alcohols to their corresponding carbonyl com-
pounds is a vital prerequisite in the pharmaceutical,
fragrance, flavoring and fine chemicals industries.1,2
These reactions are performed at present using harmful
oxidants and they have many drawbacks and limitations
Over the past decade, it was found that catalytic meth-
ods could potentially alleviate these problems.3 In this
regard, Schiff base complexes are ideal catalysts having
long-term shelf-stability and be readily prepared in situ
from commercially available reagents.4–8
We recently reported the several complex based cata-
lyst systems that have a broad substrate scope and are
capable of using tetrabutylammonium peroxomonosul-
fate (TBAOX) as the oxidant.9–15 To have practical
laboratory scale and green protocol, instead of using
TBAOX oxidant which is commercially unavailable,
prepared from expensive reagents, very short-term
shelf-stability (3 days), we use the green oxidants such
2. Experimental
2.1 Materials and measurements
CuCl2.2H2O (Merck India Ltd), 2-hydroxynaphthalene-
1-carbaldehyde and 1-amino-2-propanol (Sigma–Aldrich,
USA) were purchased. All the chemicals and solvents
employed for the synthesis were of analytical grade
∗For correspondence
1321

1322
Atena Naeimi et al.
δppm: 1.38 (d, J = 6.3, 3H), 3.51 (dd, J1 = 13, J2 = 7.5,
1H), 3.75 (dd, J1 = 13.2, J2 = 3.4, 1H), 4.17 (m, 1H),
6.86 (d, J = 9.2, 1H), 7.21 (t, J = 7, 1H), 7.43 (t, J =
9.1, 1H), 7.46 (d, J = 7.8, 1H), 7.53 (d, J = 9.2, 1H),
7.82 (d, J = 8.35, 1H), 8.74 (s, 1H), 14.1 (s, 1H); 13C
NMR (500 MHz, DMSO) δppm: 21.15, 67.17, 118.34,
123.17, 124.71, 126,60, 128.41, 129.63, 138.06; MS:
m/z 229 (M+).
2.2b Synthesisof the[(HL)Cu(μ2-Cl)2Cu(HL)]*1.5CH3
OHcomplex: To 0.01 mol H2L ligand (2.29 g) was added
0.01moldihydratecopper(II)chloride(1.704g)in10mL
methanol. The reaction mixture was stirred under reflux
condition for 2 h and a green precipitate was removed
by filtration. The resulting deep green solution was then
left undisturbed. The complex was recrystallized from
methanol and is quite air stable as solids and also in
solution. After 7 days, dark green, block-shaped, X-ray
diffraction quality single crystals appeared. The product
was secured in 62% yield. M.p.: >250◦C. IR (KBr disc,
cm−1):1623(νC=N),1543(νC=C), 560(νCu−O) and 480(νCu−N).
Scheme 1. Schematic representation of the synthesis of
[(HL)Cu(μ2-Cl)2Cu(HL)] *1.5CH3OH.
and used as received without further purification. FT-
IR spectra of the ligand and the complex in the 4000–
400 cm−1 region as KBr disks were recorded on a
Thermo SCIENTIFIC model NICOLET iS10 spec-
trophotometer. The electronic spectra were recorded on
a PG instruments Ltd, T70/T80 series (UV–Vis) spec-
trometer using HPLC grade methanol as solvent in the
range 800–200 nm. NMR spectra were recorded on a
Bruker Avance DPX 500 MHz instrument with TMS
as the internal standard, using DMSO as the solvent.
Mass spectra were recorded on a Shimadzu GC–MS-
QP5050A. Purity determinations of the products were
accomplished by GC-FID on a Agilent 6890 instrument
by using a 30 m, 0.32 mm ID, and 1 micrometer coating
capillary column and also on YL instrument by using a
60m, 0.32mm ID, and 0.5 micrometer coating capillary
column.
2.3 Crystallographic Data Collection Structure
Refinement and Computational methods
Single-crystal X-ray diffraction data for the complex
were collected on a Bruker APEX II equipped with
a CCD area detector and utilizing Mo-Kα radiation
(λ = 0.71073 Å) at room temperature. Data were col-
lected and reduced by SMART and SAINT software
in the Bruker [SHELXTL.97] and then solved.17 The
structures were solved by direct methods16 packages.
CCDC-968909 contains the supplementary18 by least
squares refinement on F2. Crystallographic data which
can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
The Gaussian 03W program suite19 was used for all
quantum chemistry computations. The geometry opti-
mizations were carried out at B3LYP method with
LANL2DZ basis set. Harmonic vibrational frequencies
were evaluated at the same level in order to compute
the zero point vibrational energy (ZPVE) correction.
The nature of the intramolecular interactions was stud-
ied by means of the Bader theory of atoms in molecules
(AIMs). The calculated electron density, ρ, and its
2.2 Synthesis
2.2a Synthesis of the Schiff base ligand (E)-1-(((2-
hydroxypropyl)imino)methyl)naphthalen-2-ol(H2L): The
asymmetric tridentate Schiff base H2L was obtained by
addition of a solution of 0.01 mol 1-amino-2-propanol
(0.75 g) in methanol (10 mL) to a solution of 0.01 mol
2-hydroxynaphthalene-1-carbaldehyde (1.72 g) in 10 mL
methanol and the reaction mixture was stirred and heat-
ed to reflux for 1 h. The yellow precipitate that formed
was filtered off and dried in air. The crude product was
recrystallized from CHCl3–hexane (1/4 v/v). Yield: 90%
2
second derivative, ∇ ρ were used for describing the
nature of the intramolecular interactions. The AIM2000
program was used to find BCPs and to analysis them.20
2.4 Oxidation Procedure
(1.52g). M.p.:46–48◦C. IR(KBr disc, cm−1):3224(υOH), To a solution of the alcohol (0.5 mmol) and complex
1636 (υC=N), 1545 (υC=C); 1H NMR (500 MHz, DMSO) (scheme 1; 0.01 mmol) in ethanol (5 mL) was added

Copper complex structure and catalytic application
1323
tBuOOH (0.5 mmol), and the reaction mixture was stir- absorption bands in the UV region 227 and 238 nm are
red in air at 80◦C for the required time. GC monitored the clearly charge transfer in origin attributed to the transi-
reaction progress and the yields of the products. Further tion from the coordinated unsaturated ligand to the metal
purification was achieved by silica chromatography.
ion (LMCT). Again, the intense high energy bands at
about 315 and 400 nm may be assigned to the intrali-
gand π → π* and n → π* transitions, respectively.
The broad low-intensity absorption band centered at
650 nm is a typical d–d band for the copper (II) ion.24
3. Results and Discussion
The ligand, (E)-1-(((2-hydroxypropyl)imino)methyl)naph-
thalen-2-ol (H2L), which is stable in air, was character-
izedbyIR, UV-Vis, 1H NMR,13CNMRspectraandmass
spectrometry. Copper complex[(HL)Cu(μ2-Cl)2Cu(HL)]
3.1 Crystal structure description of the complex and
theoretical studies
*1.5CH3OH was prepared by the reaction of H2L with The[(HL)Cu(μ2-Cl)2Cu(HL)]*1.5CH3OHcomplexcrys-
dihydrate copper(II)chloride.This new binuclear Cu(II) tallizes as dark green single crystals in the space group
complex, containing two the tridentate Schiff base lig- P-1 as seen from table S1 (Supporting Information). The
ands and chlorobridged Cu(μ - Cl)2Cu, was characterized unit cell contains two distinct entities of [Cu(HL)Cl]2∗
byUV-visible,IR spectroscopy and single-crystal X-ray 1.5CH3OH.An ORTEP view of the complex is shown in
analysis. The IR spectra of complex in KBr matrix con- figure 1. The geometry of metal complex as determined
firm that the strong absorption band at 1623 cm−1 can be by x-ray crystal structure analysis was fully optimized
assigned to the imine stretching frequency of the coor- at the above mentioned level of theory. The calculated
dinated ligand, whereas for the free ligand molecule the bond lengths and bonds are in good agreement with the
same band is observed at 1636 cm−1. The shift of this experimental values (table 1). The (E)-1-(((2-hydroxy-
band towards lower frequency on complexation with the propyl)imino)methyl)naphthalen-2-ol ligand is coordi-
metal suggests coordination to the metal ion through nated to the Cu(II) ion via its imine nitrogen, alkoxy
imine nitrogen atom.21,22 A very strong band of ν(C−O) oxygen and deprotonated phenolic oxygen atoms and
mode at about 1187 cm−1 and the C=C vibration band the coordination sphere of Cu(II) is completed by the
of the aromatic ring at 1476 cm−1 are observed. Ligand two bridging chloride anions. The crystal structure reveals
coordination to the metal center is substantiated by the that there are two complexes with three lattice methanol
single bands appearing at 560 and 480 cm−1 which cor- molecules (figure 2). The PXRD pattern of the complex
respond to the vibrations of Cu-O and Cu-N bonds, res- has shown sharp intense peaks throughout the spectrum.
pectively.23 On the other hand, the broad band at about It is indicating crystalline nature of the sample. The par-
2969 cm−1 is attributable to C-H stretching vibrations ticle size has been calculated using Scherrer’s formula
of the lattice methanol molecule. The electronic spectra t = 0.9λ/Bcos θ, where t is the particle size (same unit
of the H2L ligand and the complex in methanol are in as λ), B is half width (in radians) of diffraction line, θ
good agreement with their geometry. The two strong is the Bragg angle and λ is the wavelength. The particle
Figure 1. ORTEP view showing the molecular structure of [(HL)Cu(μ2-Cl)2Cu(HL)]
*1.5CH3OH with the atomic numbering scheme. The disordered CH3OH are omitted
for clarity. Atomic thermal ellipsoids are presented at the 50% probability level.

1324
Atena Naeimi et al.
Table 1. Selected bond lengths and bond angles for [(HL)Cu(μ2-Cl)2Cu(HL)] *1.5CH3OH complex.
Parameter
X-ray analysis
DFT
Parameter
X-ray analysis
DFT
Selected bond distances
Cu(1)-Cl(1)
O(1)-C(2)
O(1)-H(1)
O(2)-C(6)
O(2’)-C(6’)
O(1’)-C(2’)
O(1’)-H(1’)
N(1)-C(4)
2.763
1.443
0.792
1.324
1.312
1.453
0.783
1.284
2.640
1.470
1.057
1.339
1.341
1.474
1.015
1.307
Cu(1’) – O(2’)
1.902
1.916
2.047
2.258
1.913
1.927
1.984
2.260
1.954
1.996
2.044
2.475
1.953
1.985
1.994
2.433
Cu(1’) – N(1’)
Cu(1’) – O(1’)
Cu(1’) – Cl(1)
Cu(1)-O(2)
Cu(1)-N(1)
Cu(1)-O(1)
Cu(1)-Cl(1’)
Selected bond angles(◦)
N(1)- Cu(1)- Cl(1’)
O(1)- Cu(1)- Cl(1’)
O(2)- Cu(1)- Cl(1)
N(1)- Cu(1)- Cl(1)
O(1)- Cu(1)- Cl(1)
Cl(1’)- Cu(1)- Cl(1)
Cu(1’)- Cl(1)-Cu(1)
C(2)- O(1)- Cu(1)
161.5
91.4
91.0
103.9
88.4
93.2
156.7
89.4
O(2’)- Cu(1’)- O(1’)
174.8
82.6
94.4
167.2
90.7
92.5
172.0
82.8
95.4
152.4
87.5
91.6
N(1’)- Cu(1’)- O(1’)
O(2’)- Cu(1’)- Cl(1)
N(1’)- Cu(1’)- Cl(1)
O(1’)- Cu(1’)- Cl(1)
O(2)- Cu(1)- N(1)
O(2)- Cu(1)- O(1)
N(1)- Cu(1)- O(1)
92.9
103.9
91.4
98.6
82.3
86.4
110.2
174.4
82.2
174.5
83.2
107.7
0.12733(3), respectively) also confirmed the distorted
SP geometry for each copper ion. It should be noted
that the active Jahn–Teller distortion of the copper (II)
ion causes that equatorial Cu-Cl bonds are shorter than
axial bonds for each copper. For example, 2.7627(7) Å
and 2.2606(6) Å are related to the axial and equatorial
Cu– Cl bonds for Cu(1), respectively.
DFT calculations at the B3LYP/LANL2DZ level of
theory using the GAUSSIAN 03 were carried out.
The Bader’s quantum theory27,28 of atoms in molecules
(QTAIM) was also applied to get more details about the
nature of the intramolecular interactions in this complex.
According to this theory, when two neighboring atoms
are chemically bonded, a critical point for bond forma-
tion appears between them. The sign of Laplacian for
2
electron density at a bond critical point, ∇ ρ, reveals
whether the charge is concentrated, as in a covalent
2
bond (∇ ρ < 0), or depleted, as in closed shell (electro-
2
Figure 2. Mercury view shows the unit cell of [Cu(HL)Cl]2
with ball and stick representation.
static) interactions (∇ ρ > 0). The calculated electron
2
density, ρ, and its second derivative, ∇ ρ were used
for describing the nature of the intramolecular inter-
actions.29–34 The molecular graph of the complex was
obtained from the B3LYP/LANL2DZ wave function
(figure S1, Supporting Information). The corresponding
size of Cu(II) complex was found to be around 27 nm.
The XRD pattern of the complex is shown in figure S2
(Supporting Information).
There are two common structures for a five coordi-
nated CuII center, square-pyramidal (SP) or trigonal-
bipyramidal(TBP)geometry, which can be evaluated by
the addison distortion index (τ). τ defined as τ = [|θ −
ꢀ|/60], where θ and ꢀ are the two largest coordination
angles and τ = 0 for perfect SP and 1 for ideal TBP.25,26
The calculated τ of Cu(1) and Cu(2) (0.21583(3) and
2
values of ρBCP and ∇ ρBCP were presented in table
S2 (Supporting Information). As shown in the molecu-
lar graph of the complex, all the critical bonds between
atoms have shown bonds and critical points. The com-
parison between the electron density of Cu-N, Cu-O
and Cu-Cl bonding have shown that ρBCP for the Cu-N
bonding is higher than the corresponding value for

Copper complex structure and catalytic application
1325
Cu-O and Cu-Cl. As a consequence, the strength of the temperature and all the reactions were conducted at
Cu-N bond is greater than the other. Furthermore, the reflux temperature (80◦C). The best yield of benzalde-
calculated electron densities indicate that Cu-N, Cu-O hyde was obtained during oxidation of benzyl alco-
and Cu-Cl bondings have low ρBCP (about 0.0851, hol (1 mmol) using 2 mol% of catalyst with 1 mmol of
2
0.0797 and 0.0379, respectively) and positive ∇ ρBCP tBuOOH in 1 mL ethanol for 1 h at 80◦C.
values. These properties are typical for close shell
interaction for them.
Primary benzylic alcohols bearing a variety of elec-
tron donating and electron withdrawing groups undergo
The analysis of the wave function indicates that the efficient and selective oxidation to the aldehyde, with-
electron adsorption corresponds to the transition from out overoxidation to the carboxylic acid. It represents a
the ground state to the first excited state and is mainly practical alternative to traditional reagents and methods
described by one- electron excitation from the highest for the oxidation of primary and secondary alcohols
occupied molecular orbital (HOMO) to the lowest unoc- Benzylic alcohols are generally excellent substrates for
cupied molecular orbital (LUMO). The HOMO and this catalyst (table 2, entries 1–7). Electronic and steric
LUMO orbitals for the investigated complex are shown effects do not seem to have a significant effect on the
in figure 3. The HOMO of the complex has shown anti- isolated yields for electronrich and electron-poor ben-
bonding character at Cu-Cl. There is no electronic projec- zylic alcohols. When we applied this catalytic system
tion over the methanol group. The LUMO has a larger fortheoxidationofsecondaryalcohols, good/highyields
electronic projection over Cu atom in this complex.
of ketones were secured (table 2, entries 8–11). The good
conversion of less reactive primary and secondary
aliphatic alcohols to their corresponding carbonyl com-
pound without overoxidation to carboxylic products is
a notable feature of this oxidation method.
3.2 Oxidation of alcohols
Oxidation of benzyl alcohol was studied exclusively for
solvent effect, catalyst concentration, oxidant volume,
reaction time and the results of the study are as fol-
lows. The oxidation of benzyl alcohol in the presence
of tBuOOH without catalyst was slow (7% after 24 h).
By addition 2 mol% catalyst, the conversion in reaction
was enhanced markedly (50% after 24 h) and increasing
the mole percentage of catalyst up to 10% did not affect
the rate of reaction. The influence of axial nitrogenous
bases having dramatic effects on the rate and selectivity
in the oxidation reactions catalyzed by various transition
metal complexes such as porphyrin35 and salen36 cata-
lysts has also been studied Imidazole and pyridine as
commonly used axial base were added and no improve-
ment in the reaction rate was observed. Water and etha-
nol were investigated as green media for the reaction
(net reaction, 2%, 50% respectively after 24 h) and
ethanol was found to be the best solvent in terms of
conversion rate and benzaldehyde yield. The reaction
rates and products yields were found to be poor at room
To prove the high efficiency and relative stability of this
noble catalyst, the turnover number (TON) and turnover
frequency(TOF) in the oxidation of benzylic alcohol were
investigated. In this oxidation protocol, 5000:10000:1
molar ratio was used for substrate/oxidant/catalyst and
impressive turnover frequency (TOF = 1640/h) and
turnover number (TON = 5000 in 24 h) were obtained.
After completion of the oxidation of benzylic alcohol,
2 mol% of catalyst with 1 mmol of tBuOOH was added
again to reaction media and was stirred for another 1 h.
After repeating three times, the solvent was dried under
vacuum washed with diethyl ether and used directly for
the next round of reaction without further purification.
Comparison of the IR spectra of the used [(HL)Cu(μ2-
Cl)2Cu(HL)] *1.5CH3OH (figure 4) with fresh cata-
lyst showed that the structure and morphology of the
catalyst remained almost intact after four recoveries.
Thus, the methodology described herein is cost effec-
tive and environmentally benign due to the use of
Figure 3. HOMO and LUMO orbitals of complex [(HL)Cu(μ2-Cl)2Cu(HL)]
*1.5CH3OH obtained with B3LYP/LANL2DZ level of theory.

1326
Atena Naeimi et al.
ethanol and tBuOOH as a reaction medium and oxidant. the use of a semi scale-up procedure (10 mmol) for
Further, catalyst is stable and the reaction does not need the oxidation of benzylic alcohol in the presence of
nitrogen donors. These advantages of this high yielding [(HL)Cu(μ2-Cl)2Cu(HL)] *1.5CH3OH led to the isola-
oxidation method offered ready scalability. For example tion of the related aldehyde in 91% yield.
Table 2. Oxidation of alcohols with tBuOOH catalyzed by [(HL)Cu(μ2-Cl)2Cu(HL)] *1.5CH3OH complexa.
Entry
Alcohol
Productb
Time (h)
Yield (%)C
1
2
3
1
1
2
90
85
85
4
5
2
1
85
90
6
7
2
2
85
75
8
9
1
1
95
95
10
11
2
2
90
85

Copper complex structure and catalytic application
1327
Table 2.
(continued)
Alcohol
Entry
Productb
Time (h)
Yield (%)C
12
13
2
2
75
83
aThe reactions were run under air at 80◦C and the molar ratio of alcohol:
TBAOX:catalyst was 100:100:2.
bAll the products were identified by comparison with commercial and authentic samples.
cYields of isolated products. No over oxidation products were observed for all substrates.
Figure 4. FT-IR spectra of fresh [(HL)Cu(μ2-Cl)2Cu(HL)]*1.5CH3OH (A) and after 4 reuses (B).
4. Conclusions
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