Cooperative catalysis by polymetallic copper-zinc complexes in the efficient oxidation of alcohols under solvent free condition

Article abstract of DOI:10.1016/j.inoche.2014.05.018

The oxidation of alcohols to corresponding aldehydes and ketones has been achieved using polymetallic complexes as catalyst with hydrogen peroxide as the terminal oxidant under a solvent free condition. These polymetallic complexes linked to a single ligand system exhibited remarkable cooperative effect in the oxidation process. This ecologically friendly procedure releases water as the only by-product.

Full text of DOI:10.1016/j.inoche.2014.05.018

Inorganic Chemistry Communications 46 (2014) 198–201
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Cooperative catalysis by polymetallic copper–zinc complexes in the
efficient oxidation of alcohols under solvent free condition
a
b
c
a,
Rosmita Borthakur , Mrityunjaya Asthana , Arvind Kumar , Ram A. Lal ⁎
a
Department of Chemistry, Centre for Advance Studies in Chemistry, North Eastern Hill University, Shillong 793022, Meghalaya, India
Departement of Chemistry, Centre of Advanced Study, Banaras Hindu University, Varanasi 221005, India
Department of Chemistry, Faculty of Science and Agriculture, The University of West-Indies, St. Augustine, Trinidad and Tobago
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of alcohols to corresponding aldehydes and ketones has been achieved using polymetallic com-
plexes as catalyst with hydrogen peroxide as the terminal oxidant under a solvent free condition. These
polymetallic complexes linked to a single ligand system exhibited remarkable cooperative effect in the oxidation
process. This ecologically friendly procedure releases water as the only by-product.
Received 24 March 2014
Accepted 8 May 2014
Available online 25 May 2014
©
2014 Elsevier B.V. All rights reserved.
Keywords:
Cooperative catalysis
Polymetallic complexes
Oxidation
Alcohols
Carbonyl compounds
Catalytic oxidation is a key technology to transform alcohols into
carbonyl compounds. In the current chemical industries noble metal
catalysts are used on a large scale. Due to limited resources of noble
metals chemists are prompted to develop new-generation catalysts in
which noble metal contents are effectively reduced [1]. In this regard,
the first transition metal polymetallic complexes often exhibit an en-
hancement of catalytic activity and selectivity compared with their
monometallic counterparts [2–4] and have the potential to be new-
generation catalysts by partially or even completely replacing noble
metals with non-noble metals. Mononuclear metal complexes have re-
ceived great attention but the adventure of hetero-metal complexes
comes from reactivity hitherto unseen in their mono/homo dinuclear
counterparts.
Heterogeneous catalysts are desirable from both economic and in-
dustrial viewpoints because of the reusability of the catalysts and the
easy purification of the products as compared to homogeneous
catalysts.
From both an economic and environmental point of view, the
quest for effective catalytic oxidation processes that use clean, inex-
pensive primary oxidants, such as molecular oxygen or hydrogen
peroxide, i.e., a “green method” for converting alcohols to carbonyl
compounds on an industrial scale, remains an important challenge
[7]. In this communication, we report heterogeneous catalytic sys-
tems of poly-metallic complexes linked to a single ligand scaffold
(Fig. 1) [8–10], exhibiting remarkable catalytic cooperative effects
in the selective oxidation of alcohols to carbonyl compounds using
Combining multiple catalytic sites into a single ligand structure can
induce cooperative reaction pathways, improving the activities and se-
lectivities. Research groups are developing polymetallic catalytic sys-
tems to find synergy and cooperative effects between different metal
centers for activation of reagents and better results [5]. Hydrazones
are a special kind of polyfunctional Schiff base ligands which have the
potential for yielding homo- and hetero-metallic complexes among
which succinoyldihydrazones are unique because of their greater
flexibility [6].
H
2 2
O as a terminal oxidant. The contribution is organized according
to the donor centers of the ligand to which the metal centers are
bonded [2]. These polymetallic complexes have been selected keeping
in view the fact that bis(5-bromosalicyaldimine)succinoyldihydrazone
is bulkier than bis(2-hydroxynaphthaldimine)succinoyldihydrazone.
Such special features associated with the structures of the ligand
are expected to play significant roles in the catalytic activity of the
catalyst.
Continuing our interest in exploring the catalytic properties of our
synthesized complexes [11] we report, herein a very efficient and selec-
2 2
tive oxidation of alcohols with H O using reusable heterogeneous cat-
alysts under solvent-free condition. The present protocol is very simple,
mild and environment friendly with water being the only by-product in
the reaction. No over oxidation of the alcohols to the acids occurred
(Scheme 1).
⁎
Corresponding author. Tel.: +91 0364 2722616; fax: +91 364 2550486.
E-mail address: profralal@gmail.com (R.A. Lal).
http://dx.doi.org/10.1016/j.inoche.2014.05.018
387-7003/© 2014 Elsevier B.V. All rights reserved.
1

R. Borthakur et al. / Inorganic Chemistry Communications 46 (2014) 198–201
199
N
N
N
Cu
N
Cu
H
2
O
OH
2
OH2
C
O
R
R
C
O
O
O
OH
2
OH
2
Zn
NO3
NO3
H O
2
OH
2
OH2
2 4 3 2 2 8 2 4 4 4
Fig. 1. The structure of [ZnCu (H ) (H O) ]·2H O (H
Ln)(NO Ln = H L1(1), H L2(2)). (R = 2-hydroxy-1-naphthaldehyde and Br, respectively).
The detail of preparation and characterization of catalysts [12]
different solvents for the oxidation of 1a using catalyst 1 under the re-
flux condition and also under the solvent free condition. Results are
n
n
1
2
[
ZnCu
2 3 2 2 8 2 4 4 4
(L )(NO ) (H O) ]·2H O where (H L = H L (1), H L (2))
has been reported in our earlier publication [8].
3
summarized in (Fig. 2). Except for CH CN all other solvents gave very
The catalytic efficiency of the synthesized trinuclear complexes 1–2
were explored for the oxidation of alcohols to corresponding carbonyl
compounds [13]. Our initial experiments were performed on benzyl
low yields of 2a. The oxidation of 1a under the solvent free condition
gave the best yield of 2a.
2 2
Thus, our optimized reaction conditions are 1a (5 mmol), H O and
alcohol with H
2
O
2
using complex 1 in acetonitrile under reflux to find
8 mol% of catalyst under a solvent free condition. The oxidation of other
primary, secondary, allylic and aliphatic alcohols were then examined
using the optimized protocol. Complex 2 was also found to catalyze
the oxidation of alcohols to corresponding carbonyl compounds under
the solvent free condition in 60–82%. Catalyst 1 showed better results
than catalyst 2. This may be due to the fact that the catalytic activity of
complexes varies with the size of the substituent. It was observed that
the activity decreases with an increase in the bulkiness of the substitu-
ent. This may be due to the steric hindrance caused by the substituent,
which can affect the planarity of the ligand in the complexes and
hence the approach of the incoming substrate to the center. The results
for the oxidation of a variety of alcohols are summarized in Table 2. All
the reactions occurred in open atmosphere with complete selectivity
for ketones or aldehydes and no other product was detected in the reac-
tion mixture. So, it is clear that air is not a significant co-oxidant in the
oxidation process and copper (II) complexes are air stable.
It is also important to verify the viability of reusing catalytic system
for different substrates. The basic advantage of heterogeneous catalyst is
their reuse for consecutive catalytic cycles. To evaluate the catalyst reus-
ability, it was recovered by filtration, washed with DCM and dried in the
oven for 30 min. Catalyst was reused under the optimized condition
with benzyl alcohol to give the corresponding benzaldehyde; the data
obtained are shown in (Fig. 3).
the best reaction conditions. The results are shown in (Table 1).
It was found that high conversion and good yield of oxidation prod-
uct were obtained by employing 8 mol% of catalyst 1 (Table 1, entry 3).
The synthetic transformation did not proceed in the absence of catalyst
(
entry 6). This observation revealed the catalytic role of the complex. It
was observed that the oxidation of benzyl alcohol to the corresponding
benzaldehyde did not proceed at all when the monometallic copper [14]
and zinc [15] complexes were used as catalyst (entries 7–10). This result
showed that two metal centers are better than one metal center and
work in better harmony with one another, thus exhibiting cooperative
effect of one another. Encouraged by these results, we screened
8
1
mol % catalyst,
o
00 C, H O
2
2
Solvent free
R
R
HO
O
2a-l
1a-l
Scheme 1. Oxidation of alcohol using catalyst.
In conclusion, we have developed an efficient heterogeneous cata-
lytic system based on Cu(II)–Zn(II), showing cooperative effect for the
oxidation of benzylic, allylic and aliphatic alcohols to the corresponding
carbonyl compounds using hydrogen peroxide as an oxidant under sol-
vent free condition. Furthermore, the catalyst can be recovered and
Table 1
Amount of catalyst variation of 1 for oxidation of benzyl alcohol.
9
8
7
6
50
40
30
0
0
0
0
Yield (%)
Time in hrs
Entry
Catalyst 1 (in mol%)
Time (h)
Yield (%)a
b
1
2%
5%
8%
10%
15%
–
36
16
12
16
20
24
24
24
24
24
15
35
40
30
25
–
c
2
d
3
e
4
2
0
0
0
f
5
1
g
6
h
7
5%
5%
8%
8%
–
i
8
–
h
9
–
1
0i
–
a Isolated yields.
b
c
d
e
b Reaction conditions: substrate (5 mmol), catalyst (2 mol% , 5 mol% , 8 mol% , 10 mol% ,
f
g
h
i
1
8
5 mol% , no catalyst , [Cu(H
0 °C (oil bath temperature).
2 2 2 2 2 2 2 3
L)(H O)] and [Zn(H L)(H O) ] ), H O , CH CN (3 mL),
Fig. 2. Optimization of solvent effect.

200
R. Borthakur et al. / Inorganic Chemistry Communications 46 (2014) 198–201
Table 2
a
Oxidation of alcohols catalyzed by Cu(II)–Zn(II) complexes 1 and 2 .
80
60
Entry Substrate
Product
1
2
yield (%)b yield (%)
b
40
time (h)
time (h)
1
84%
81%
20
12 h
14 h
1
a
2a
2b
2
3
86%
2 h
82%
15 h
0
1
1
2
3
4
No. of cycles
1b
82%
1 h
80%
14 h
1
Fig. 3. Reusability chart of catalyst.
to thank UGC, New Delhi for the award of RFSMS and MA to CSIR,
New Delhi for the award of Senior Research Fellowship.
1
c
2c
4
5
6
80%
1 h
77%
16 h
1
Appendix A. Supplementary material
1
d
e
2d
76%
2 h
72%
16 h
Spectral data of the compounds are provided in the SI file. This ma-
terial is available free of charge via the Internet. Supplementary data as-
sociated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.inoche.2014.05.018. These data include MOL files
and InChiKeys of the most important compounds described in this
article.
1
1
2
e
f
80%
4 h
76%
16 h
1
1f
2
7
8
81%
2 h
78%
14 h
References
1
[
[
[
[
1] J. Lin, J. Chen, W. Sua, Adv. Synth. Catal. 355 (2013) 41–46.
2] I. Bratko, M. Gómez, Dalton Trans. 42 (2013) 10664–10681.
3] J.M. Ready, E.N. Jacobsen, Angew. Chem. Int. Ed. 41 (2002) 1374–1377.
4] M.L. Kantama, R.S. Reddya, U. Pala, M. Sudhakara, A. Venugopala, K.J. Ratnama, F.
Figuerasb, V.R. Chintareddy, Y. Nishinad, J. Mol. Catal. A Chem. 359 (2012) 1–7.
5] (a) E.K. Beuken, B.L. Feringa, Tetrahedron 54 (1998) 12985–13011;
1
g
2g
2h
78%
4 h
75%
17 h
1
[
(b) M. Shibasaki, N. Yoshikawa, Chem. Rev. 102 (2002) 2187–2209;
1
h
i
(c) J.I. van der Vlugt, Eur. J. Inorg. Chem. (2012) 363–375;
(d) J. Park, S. Hong, Chem. Soc. Rev. 41 (2012) 6931–6943;
9
1
75%
7 h
70%
20 h
1
(e) K.-H. Wu, H.-M. Gau, J. Am. Chem. Soc. 128 (2006) 14808–14809.
6] (a) M. Sutradhar, T.R. Barman, J. Klanke, M.G.B. Drew, E. Rentschler, Polyhedron 53
[
(
2013) 48–55;
b) R.A. Lal, D. Basumatary, A. Adhikari, A. Kumar, Spectrochim. Acta A 69 (2008)
06–714;
(
1
2i
7
0
79%
4 h
75%
18 h
(c) A. Kumar, R. Borthakur, A. Koch, O.B. Chanu, S. Choudhury, A. Lemtur, R.A. Lal, J.
1
Mol. Struct. 999 (2011) 89–97;
(
(
d) M. Sutradhar, T.R. Barman, G. Mukherjee, M.G. Drew, S. Ghosh, Polyhedron 34
2012) 92–101;
e) R.A. Lal, S. Choudhury, A. Ahmed, R. Borthakur, M. Asthana, A. Kumar,
(
1
j
2j
Spectrochim. Acta A 75 (2010) 212–224.
1
1
1
2
70%
2 h
65%
0 h
67%
27 h
[
7] A. Dijksman, A. M.-Gonza'lez, A. M. i Payeras, I. W. C. E. Arends, R. A. Sheldon, J. Am.
Chem. Soc. 123 (2001) 6826–6833.
[8] R. Borthakur, A. Kumar, R.A. Lal, J. Ind. Chem. Soc. 9 (2014) 407–423.
[9] R. Borthakur, A. Kumar, A. Lemtur, R.A. Lal, RSC Adv. 3 (2013) 15139–15147.
10] R. Borthakur, A. Kumar, R.A. Lal, Spectrochim. Acta A 118 (2014) 94–101.
2
1k
2k
60%
35 h
3
1l
2l
[
a
Reaction condition: substrate (5 mmol), catalyst 1/2 (8 mol%), H
O
2 2
was heated at
[11] (a) R. Borthakur, M. Asthana, A. Kumar, A. Koch, R.A. Lal, RSC Adv. 3 (2013)
22957–22962;
1
00 °C for appropriate time.
b
(b) R. Borthakur, M. Asthana, M. Saha, A. Kumar, A.K. Pal, RSC Adv. 4 (2014)
Isolated yields.
2
1638–21643.
12] (a) All the reagents used were chemically pure and were of analytical reagent grade.
-nitro benzyl alcohol and 4-methoxy benzyl alcohol were purchased from Al-
[
4
drich and the rest of the chemicals along with solvent were procured from
local suppliers. The solvents were dried and distilled before use following the
reused without any significant loss of catalytic activity. The catalytic sys-
tem derived from more sterically crowded polyfunctional ligand is less
reactive than that derived from less sterically crowded ligand.
1
2
standard procedures. Melting points were recorded in open capillaries and
1
13
are uncorrected. H and C NMR spectra were recorded in CDCl3 on a Bruker
Avance-400 spectrometer and Jeol-300 and 500 spectrometer.
(
b)A.I. Vogel, Textbook of Practical Organic Chemistry, 5th ed. Longman, London,
1
989.
13] All of the reactions were carried out at 100 °C under reflux in a 25 mL flask equipped
with a magnetic stirrer. 30% H solution was added to a mixture of alcohol
5 mmol) and the catalyst (8 mol%). The reaction solutions in all cases were vigorous-
[
Acknowledgments
2 2
O
(
ly stirred using magnetic stirrers, and an oil bath was used to achieve the desired re-
action temperature. After completion (TLC), the reaction mixture was cooled to room
temperature and filtered. Filtrate was extracted with ethylacetate (3 × 10 mL) and the
Authors are thankful to Head, SAIF, North Eastern Hill University,
Shillong for elemental analyses and recording spectra. RB would like

R. Borthakur et al. / Inorganic Chemistry Communications 46 (2014) 198–201
201
combined organic extract was washed with water (3 × 10 mL), brine (10 mL) and
dried over anhydrous Na SO . After removing the solvent, the crude product was pu-
[14] R. Borthakur, A. Kumar, A.K. De, R.A. Lal, Arab. J. Chem. (2014) (Revision submitted
for publication).
2
4
rified by column chromatography over silica gel (60–120 mesh) using ethyl acetate
and hexane as eluent to afford the pure products 2a–l.
[15] R.A. Lal, M. Chakraborty, O.B. Chanu, S. Choudhury, R. Borthakur, S. Copperfield, A.
Kumar, J. Coord. Chem. 63 (2010) 1239–1251.