An efficient oxidation of alcohols using a new trinuclear copper complex as a reusable catalyst under solvent free conditions

Article abstract of DOI:10.1039/c4ra01275e

A new trinuclear copper(ii) complex [Cu₃(L)(μ₂-Cl) ₂(H₂O)₆] was synthesized and characterized by various spectroscopic techniques. The trinuclear complex was demonstrated as an efficient catalyst for the selective oxidation of primary, secondary, aliphatic, heteroatomic and conjugated allyl alcohols to the corresponding aldehydes/ketones in good to excellent yields under solvent free conditions using H₂O₂ as an oxidant. The catalyst is easily synthesizable, easy to handle and reusable up to eight runs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01275e

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An efficient oxidation of alcohols using a new
trinuclear copper complex as a reusable catalyst
under solvent free conditions†
a
Cite this: RSC Adv., 2014, 4, 21638
Rosmita Borthakur, Mrityunjaya Asthana,b Mithu Saha,a Arvind Kumarc
*
Received 13th February 2014
Accepted 30th April 2014
and Amarta Kumar Pala
DOI: 10.1039/c4ra01275e
www.rsc.org/advances
A
new trinuclear copper(II) complex [Cu3(L)(m2-Cl)2(H2O)6] was hazardous nature.22 Heterogeneous catalysts in the liquid phase
synthesized and characterized by various spectroscopic techniques. offer several advantages as compared to those of homogeneous
The trinuclear complex was demonstrated as an efficient catalyst for ones in respect of their recovery, recycling, atom utility and
the selective oxidation of primary, secondary, aliphatic, heteroatomic enhanced stability. Literature survey revealed that only a few
and conjugated allyl alcohols to the corresponding aldehydes/ketones examples using cheap and “green” copper catalyst and molec-
in good to excellent yields under solvent free conditions using H2O2 as ular oxygen23 or hydrogen peroxide24 are known so far.
an oxidant. The catalyst is easily synthesizable, easy to handle and
Continuing our interest in exploring the catalytic properties
of the synthesized complexes,25 we report herein, a new catalyst-
support system for the oxidation of alcohols, which is less
expensive than the currently used systems and leads to high
yield with high selectivity. The system consists of a copper
dihydrazone complex as catalyst, which can be easily prepared
and showed excellent tolerance for a broad range of alcoholic
substrates and was notably not deactivated by N/S-containing
heteroatom systems (Scheme 1) under solvent free condition.
The precursor complex [Cu(H2L)(H2O)] was synthesized by
reusable up to eight runs.
The oxidation of alcohols to aldehydes and ketones is a chem-
ical transformation1 of primary industrial importance because
carbonyl compounds are precursors of a variety of valuable ne
chemicals including fragrances, ame retardants and in phar-
maceuticals such as vitamins and drugs.2 Several methods have
been developed to accomplish this particular reaction using
catalysts from a variety of metals and supports, including
chromium reagents,3 manganese(IV) oxides,4 activated DMSO,5
hypervalent iodine reagents,6 ruthenium reagents,7 osmium(VI)
oxide,8 metals9–16 transition metal nitrates on solid supports17 or
NaOCl/TEMPO,8,18 as it was reported in some recent reviews.19
Although most of these processes offer distinct advantages, but
at the same time they suffer from certain drawbacks such as
longer reaction time, use of organic solvents, unsatisfactory
yields, high costs, use of stoichiometric amounts as well as
environmentally toxic catalysts. Thus the development of
simple, highly efficient methodologies remains desired.
template
method
by
treating
succinoyldihydrazine,
Cu(OAc)2$H2O and salicyldehyde in 1 : 1 : 3 molar ratio in
methanol by literature method.26 The Cu(II) complex on reaction
with copper chloride dihydrate in MeOH medium resulted into
a dark green precipitate which on the basis of elemental anal-
yses, molecular weight and mass spectral studies was found to
have the composition [Cu3(L)(m2-Cl)2(H2O)6] (Scheme 2).
The complex is brown in colour and air stable. It is soluble in
DMSO and DMF only. The compound is non-ionic in nature, as
determined by molar conductance Lm ¼ 1.2 U cm2 molꢀ1
.
27 The
complex has been further fully characterized by spectroscopic
techniques viz. IR, UV, EPR and CV (ESI, Fig. S1–S5†). The ESI-
mass spectrum of the complex show signals at m/z 719.48 [M+]
and 741.61 [M + Na ꢀ H]+ (Fig. 1).
Copper compounds are well known as very important
reagent and catalyst20 in many organic reactions because of
their stability, ease of handling,21 economical and relatively less
aDepartment of Chemistry, Centre for Advanced Studies in Chemistry, North-Eastern
Hill University, Shillong-793022, Meghalaya, India. E-mail: roschem07@gmail.com
bDepartment of Chemistry, Centre of Advanced Study, Banaras Hindu University,
Varanasi-221005, India
cDepartment of Chemistry, Faculty of Science and Agriculture, The University of West-
Indies, St Augustine, Trinidad and Tobago, West-Indies
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra01275e
Scheme 1 Oxidation of alcohols.
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Scheme 2 Synthesis of [Cu3(L)(m2-Cl)2(H2O)6].
Fig. 2 Optimized structure of the complex [Cu3(L)(m2-Cl)2(H2O)6](I).
used.30 Selected bond distances and angles are listed in ESI,
TS1.† Frequency calculation was performed to verify that the
optimized structure is a minimum energy structure. In the
optimized structure of I, all the Cu(II) coordinates six atoms to
form a distorted octahedral structure. The central Cu(II) is
˚
bonded to two oxygen atoms at distances 2.031 and 1.999 A and
˚
two bridging chlorides at distances 2.395 and 2.434 A. The
˚
terminal Cu–N distances are 1.963 and 1.972 A while the Cu–O
˚
distances are 1.910, 2.011, 1.919 and 2.043 A. The Cu–Cl
˚
distances are 2.420 and 2.415 A which are quite close to the
distances reported for chloride bridged copper complex with
crystal structure.31 All the three copper atoms are coordinated to
two water molecules each. The Cu/Cu distances are 3.169 and
˚
˚
3.179 A. The terminal Cu/Cu separation is 6.223 A and Cu–Cu–
Cu angle is 155.2ꢁ.
Natural bond orbital (NBO) analysis32 provide details about
the type of hybridization and nature of bonding.33 According to
NBO results, the electronic conguration of Cu1 in [Cu3(L1)-
(m2-Cl)2(H2O)6] is: [core]4S0.323d9.424p0.035p0.01, 17.99733 core
electrons, 9.73941 valence electrons and 0.04217 Rydberg elec-
trons with 27.77892 electrons as a total electrons and +0.464 as
natural charge. Similarly, the electronic conguration for Cu2 is
[core]4S0.293d9.374p0.025p0.01, 17.99838 core electrons, 9.66412
valence electrons, 0.03821 Rydberg electrons with 27.70071
electrons as a total electrons and +0.273 as natural charge, Cu3
is [core]4S0.333d9.434p0.035p0.01, 17.99728 core electrons, 9.76501
valence electrons and 0.04360 Rydberg electrons with 27.80588
electrons as a total electrons and +0.467 as natural charge.
Now, the catalytic properties of copper(II) complex was
explored towards the oxidation of alcohols. The oxidation of 4-
methoxybenzyl alcohol was considered as a model reaction to
optimize the reaction conditions (Scheme 3). Initially, the
reaction was carried in presence of 5 mol% catalyst, 30% H2O2
at room temperature for 24 h, but yield of product was low
(40%) identied by spectral methods as 4-methoxy benzalde-
Fig. 1 Mass spectrum of the complex [Cu3(L)(m2-Cl)2(H2O)6] in DMSO
solution.
The room temperature magnetic susceptibility measurement
shows that the complex is paramagnetic with magnetic moment
per empirical formula [Cu(L1/3)(m2-Cl2/3)(H2O)2] containing one
copper(II) atom is 1.13 mB. This value is much less than the spin
– only value of 1.73 BM per copper atom on no-interaction basis.
This suggests that there is considerable spin–spin antiferro-
magnetic interaction between the metal centres.28 Electron
transfer reactions were also studied (Fig. S5†) and the electrode
reactions may be shown to be as follows:
1
hyde 2. The presence of singlet at d 9.20 ppm in H NMR and
absence of peaks at 4.50 ppm and 2.0 ppm, respectively and
We also performed geometry optimization of I (Fig. 2) using
density functional theory (DFT) based (B3LYP)29 method with
the 6-31G** basis set for all atoms except for Cu, for which
LANL2DZ basis set along with effective core potential was Scheme 3 Oxidation of 4-methoxy benzyl alcohol.
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bands at 2720, 2820 and 1715 cmꢀ1 in IR spectra clearly indi-
cated the formation of 2.
Further, effect of various solvent viz., DCM, CH3CN, water
and toluene on the model reaction was investigated using H2O2
(30%) and 5 mol% catalyst. Signicant improvement was ach-
ieved in acetonitrile, but the best results were obtained when
the reaction was carried out under solvent free condition
(Fig. 3).
We turned our attention towards screening of appropriate
concentration of catalyst loading. The catalyst loading was
varied from 2–15 mol%. The result revealed that when the Fig. 4 Effect of catalyst loading on the model reaction.
reaction was carried out at 2 mol% of catalyst it gave lower yield
of the product. Best yields were obtained in presence of just
5 mol% of catalyst. On increasing the catalyst loading to 10,
Table 1 Oxidation of alcohols to aldehydes and ketonesa
15 mol% the product yield decreased with longer reaction time.
The reaction was also carried out in absence of catalyst, but the
reaction did not proceed at all and only the starting material
was recovered. Thus, the catalyst plays an important role in the
success of reaction and product yield (Fig. 4).
Next, the effect of temperature was also evaluated. It was
observed that fast reaction occurred on raising the temperature
from 25 ꢁC to 100 ꢁC and the yield of desired product increased
considerably. We were pleased to nd that the reaction pro-
ceeded smoothly and almost complete conversion of reactants
ꢁ
was observed at 100 C to afford the desired product 2 in 92%
yield (Table 1, entry 2). However, at room temperature a lower
yield of product was obtained even aer longer reaction time.
Thus, it was concluded that 5 mol% catalyst, H2O2 (30%)
without solvent as the optimized reaction condition for this
conversion (Fig. 5).
The above optimized protocol was further extended for
oxidation of different primary, secondary and hetero-aromatic
a
Reaction condition: catalyst (5 mol%), alcohol (5 mmol), H2O2 (30%)
b
c
were stirred at 100 ꢁC for appropriate time. Isolated yield. Benzoin
alcohols. The feasibility of employing aliphatic alcohols instead used as substrate.
of aryl alcohols in the reaction was also investigated (Table 1).
Benzyl alcohol with substituents of varying electronic properties
smoothly converted to the corresponding aldehydes (Table 1, 1–
7). Electron donating substituents gave higher yields (2–4)
whereas electron withdrawing substituents slightly lowered the
yields (5–7). Benzyl alcohol and phenyl ethanol also gave good
yields (1 and 10). Electron donating and withdrawing substitu-
ents on phenyl ethanol also oxidized to the corresponding
ketones in good to excellent yields (10–13). It follows from Table
Fig. 5 Effect of temperature on the model reaction.
1 that both aromatic primary and secondary alcohols worked
well, giving excellent yields of products with high purity. It was
found that benzil was obtained in good yields from benzoin
(8) and cinnamaldehyde from cinnamyl alcohol giving yield up
to 90% (9). The conversion proceeded smoothly even in the
presence of heteroatoms such as N and S in the substrates (14
and 15). Oxidation of primary aliphatic alcohols to the
Fig. 3 Optimization of the solvent effect.
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measurement was made on a Sherwood Magnetic Suscepti-
bility Balance MSB-Auto. Diamagnetic correction was carried
out using Pascals Constants.36 The standard used in the
magnetic moment studies was a sealed sample of Manganese
Chloride solution in a standard sample tube. The tube is
labelled with the reading Cstd ¼ 1192, Ro ¼ ꢀ35 and T ꢁC ¼ 21,
where Cstd ¼ reading taken in a calibrated instrument using a
tared standard sample tube, Ro ¼ reading of the empty tube, T
ꢁC ¼ temperature at which the measurement was taken.
Firstly, an empty sample tube of known weight (g) was placed
into the tube guide and the reading Ro was taken. A small
amount of solid was introduced into the weighing sample
tube, and the bottom of the tube was gently tapped on a
wooden bench a number of times to settle the solid particles.
This procedure was repeated until a sufficient amount of
sample was added, corresponding to a sample length, l, in the
range 1.5–2 cm. Then the sample mass (m) in grams and the
sample length, (l), in cm was noted. Packed sample tube was
then placed into the tube guide and the reading R was taken.
The Mass susceptibility, cg in cgs, was calculated using cg ¼ C
ꢂ l in cm ꢂ (R ꢀ Ro)/109 ꢂ mass of sample in tube in g; cg ꢂ
Fig. 6 Reusability of catalyst.
corresponding aliphatic aldehydes is difficult using most
heterogeneous catalysts, thus the low conversion of these
alcohols (16 and 17) is understandable because of its low
reactivity.34 Hence, our catalyst oxidized various alcohols in
good to excellent yields. Further, scope of the catalyst is
currently underway in our laboratory.
The reusability of catalyst is important for catalysis reac-
tions. Therefore, the reusability of our prepared catalyst was
also examined. It was found that the catalyst can be reused for
another eight consecutive runs under similar reaction condi-
tions. Aer each run, the catalyst was ltered and washed with
DCM and dried in oven for 30 minutes and reused for the next
run. The yields of product decreased slightly with reuse of
catalyst over eight consecutive runs (Fig. 6).
molecular mass ¼ molar susceptibility (cM); meff ¼ 2.84OcM
ꢂ
T, T ¼ temperature at which the measurement was taken. The
magnetic moment for the complex per copper(II) atom was
calculated using the empirical formulation for the complex i.e.
[Cu(L1/3)(m2-Cl2/3)(H2O)2]. The empirical formula for the complex
is taken as one third of the molecular formula [Cu3(L)(m2-
Cl)2(H2O)6]. The FT-IR spectrum of the complex was recorded on
a Perkin-Elmer FT-IR system, Spectrum BX, infrared spectro-
photometer using KBr pellets. UV-visible Spectra of the complex
was recorded in DMSO solution at ꢃ10ꢀ3 M concentration on a
Perkin-Elmer Lambda-25 spectrophotometer. Electron para-
magnetic resonance spectrum of the complex was recorded at X-
band frequency on a Varian E-112 E-Line Century Service EPR
spectrometer using TCNE (g ¼ 2.0027) as an internal eld
marker. Variable temperature experiments were carried out with
a Varian Variable Temperature accessory. 1H and 13C NMR
spectra were recorded in CDCl3 on Bruker Avance – 400 spec-
trometer and Jeol – 300 and 500 spectrometers.
Conclusion
In conclusion, a new trinuclear copper(II) complex [Cu3(L)-
(m2-Cl)2(H2O)6] was synthesized and used as a general catalyst
for the efficient oxidation of a broad range of alcoholic
substrates including heteroatom systems with hydrogen
peroxide as an oxidant to give the corresponding aldehydes/
ketones in good to excellent yields under solvent free condition.
The main advantage of our protocol is that expensive noble
metals are not required for this transformation and the catalyst
could be easily synthesized from simple starting materials with
reusability upto eight runs without any signicant loss of
catalytic activity.
Typical procedure for one pot conversion of alcohols to
aldehydes catalyzed by [Cu3(L)(m2-Cl)2(H2O)6]
All of the reactions were carried out at 100 ꢁC under reux in a
25 ml ask equipped with a magnetic stirrer. 30% H2O2
solution was added to a mixture of alcohol (5 mmol) and the
Experimental section
Melting points were recorded in open capillaries and are catalyst (5 mol%). The reaction solutions in all cases were
uncorrected. A Perkin-Elmer 2400 series II CHN analyzer was vigorously stirred using magnetic stirrer and an oil bath was
used to collect micro analytical data (C, H and N). The APCI used to achieve the desired reaction temperature. Aer
mass spectrum of the complex was recorded on a Water ZQ completion (TLC), the reaction mixture was cooled to room
4000 Micro mass spectrometer in DMSO solution. The copper temperature and ltered. Filtrate was extracted with ethyl
content of the complex was determined by standard literature acetate (3 ꢂ 10 ml) and the combined organic extract was
procedure.35 Conductance measurement was made at 1 KHz washed with water (3 ꢂ 10 ml), brine (10 ml) and dried over
using Wayne Kerr B905 Automatic Precision Bridge. A dip-type anhydrous Na2SO4. Aer removing the solvent the crude
conductivity cell having a platinized platinum electrode was product was puried by column chromatography over silica
used. The cell constant was determined using a standard KCl gel (60–120 mesh) using ethyl acetate and hexane as eluent to
solution. Room temperature magnetic susceptibility afford the pure products 1–17.
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Acknowledgements
Authors are thankful to Head, SAIF, North Eastern Hill
University, Shillong for elemental analyses, recording spectra,
the Head, SAIF, IIT Bombay, India for recording EPR spectra.
Further, they would like to thank Prof. A. K. Chandra,
Department of Chemistry, NEHU for fruitful discussion
regarding DFT results. We are also thankful to the learned
referees for their valuable and constructive suggestions on our
manuscript. RB thanks UGC, New Delhi for award of RFSMS
and MA to CSIR, New Delhi for award of Senior Research
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