A new Cu Schiff base complex with histidine and glutaraldehyde immobilized on modified iron oxide nanoparticles as a recyclable catalyst for the oxidative homocoupling of terminal alkynes

Article abstract of DOI:10.1007/s13738-016-0829-7

The novel functionalized Fe₃O₄@SiO₂@APTMS@Glu-His@Cu complex was prepared from modification of iron oxide nanomagnet particles with (3-aminopropyl) trimethoxysilane (APTMS) and glutaraldehyde–histidine Schiff base followed

Full text of DOI:10.1007/s13738-016-0829-7

J IRAN CHEM SOC
DOI 10.1007/s13738-016-0829-7
ORIGINAL PAPER
A new Cu Schiff base complex with histidine and glutaraldehyde
immobilized on modified iron oxide nanoparticles as a recyclable
catalyst for the oxidative homocoupling of terminal alkynes
Faezeh Farzaneh1 · Elnaz Rashtizadeh1
Received: 25 July 2015 / Accepted: 19 February 2016
© Iranian Chemical Society 2016
Abstract The novel functionalized Fe3O4@SiO2@
APTMS@Glu-His@Cu complex was prepared from modi-
fication of iron oxide nanomagnet particles with (3-ami-
nopropyl) trimethoxysilane (APTMS) and glutaralde-
hyde–histidine Schiff base followed by complexation with
Cu(I) salt. Characterization of this complex was carried
out by means of FTIR, XRD, SEM, TEM and VSM tech-
niques. The complex was found to successfully catalyze
the oxidative homocoupling of phenylacetylene, 4-tert-
butylphenylacetylene, 1-ethynyl-4-fluorobenzene and pent-
1-yn-3-ol with 67–100 % conversions and 95–100 % selec-
tivities. Magnetic recovery and recycling of the catalyst
without significant decrease in activity is described in this
presentation.
[1–3], pharmaceuticals [4, 5], and bioactive compounds [6]
with anti-inflammatory [7], antifungal [8], anti-HIV, anti-
bacterial, or antitumor activities [9]. Acetylenic couplings
cover a large area of scientific interests for the synthesis
of key structural motifs in linearly π-conjugated acety-
lenic oligomers and polymers [10], macrocyclic annulene
[11], agrochemicals, organic conductors, supramolecu-
lar switches, carbon-rich materials [12, 13], liquid crys-
tals, molecular wires and non-linear optic materials [14].
Therefore, much attention has been made to the develop-
ment of efficient synthetic procedures for the diyne deriva-
tives. The Cu-catalysis systems have widely been used
for homocoupling reactions (first discovered by Glaser in
1869) [15]. After the seminal paper published by Glaser,
many efforts have been devoted toward the development
of milder and more environmentally friendly protocols
for the homocoupling of terminal alkynes. In this regard,
a number of catalysis systems have been developed. Pal-
ladium and copper salts are efficient and mild catalysis
systems for the homocoupling of terminal alkynes [16–
24]. However, palladium catalysts are expensive and air-
sensitive and either phosphine or amine ligands are usually
required.
Whereas most homocoupling reactions were initially
carried out by homogeneous catalysis systems, the easy
separation and privilege of recycling of the heterogeneous
catalysts after the reaction workup increases the feasibility
of the reaction even in large scale fine-chemical applica-
tions. Utilization of heterogeneous catalysis system gener-
ally minimizes the contamination of organic reaction mix-
tures by the catalyst with along with significant advantage
in terms of time demanding purification procedures. Sup-
ported copper on inorganic supports such as silica [25],
Al-MCM-41 [26], zeolites [27], titanium oxide [28, 29]
and SBA-15 [30], are some representative examples of
Keywords Modified magnetic nanoparticles · Oxidative
homocoupling · Terminal alkynes · Schiff base copper
complex
Introduction
The homocoupling of terminal alkynes to 1,3-diynes
is an important carbon–carbon bond-formation reac-
tion. 1,3-Diynes are widespread in many natural products
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-016-0829-7) contains supplementary
material, which is available to authorized users.
* Faezeh Farzaneh
faezeh_farzaneh@yahoo.com; farzaneh@alzahra.ac.ir
1
Department of Chemistry, Faculty of Physics and Chemistry,
Alzahra University, P. O. Box 1993891176, Vanak, Tehran,
Iran
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J IRAN CHEM SOC
heterogeneous catalytic systems effectively applied to the
homocoupling of terminal alkynes.
Magnetic measurements were carried out using a Quantum
Design vibrating sample magnetometer (VSM) at room
temperature in an applied magnetic field sweeping from
−15 to 15 kOe. Structural examination of the catalyst by
scanning electron microscope (SEM) was performed on a
LEO 1455VP operating at an accelerated voltage of 20 kV
and Transmission electron microscopy (TEM) images were
taken by Zeiss-EM10C-100 KV. FTIR spectra of the sam-
ples were collected on a Bruker (Tensor 27) instrument in
the range of 4000–400 cm−1 (5 mg sample with 100 mg
KBr) under the atmospheric condition. The chemical analy-
sis was carried out with an atomic absorption Chermo dou-
ble beam instrument.
Another set of heterogeneous catalytic systems are nano-
magnetic catalysts due to easily separation from the reaction
medium using an external magnetic field, which avoids loss
of catalyst and increases its reusability and improve its recov-
ery rate during separation process in comparison to filtration
or centrifugation. Additionally, nano-catalyst with large spe-
cific surface area and high catalytic activity also exhibits good
catalytic property [31]. Whereas magnetic or nano-magnetic
particles have been widely applied as catalyst in many reac-
tions such as hydrogenation [32], Suzuki–Miyaura reaction
[33], olefin metathesis [34] and oxidation reactions [35], but
there are only a few reports on using these materials as cata-
lyst for oxidative homocoupling of terminal alkynes [36].
In this work, attempts has been made to functionalize
the magnetite nanoparticles with 3-aminopropyl trimethox-
ysilane and then incorporate a copper Schiff base complex
to obtain a heterogeneous copper catalyst for the homocou-
pling of terminal alkynes.
Preparation of amine‑functionalized magnetic silica
nanoparticles
Fe3O4 magnetic nanoparticles (MNPs), prepared according
to the previously reported method were coated with silica
[37]. A solution of Fe3O4@SiO2 (0.2 g) was then dispersed
in aqueous ethanol (20 %, 50 mL). APTMS (0.3 mL) was
added dropwise by ultrasonic treatment and the mixture
stirred for several hours. The brown modified MNPs solid
was subsequently separated and neutralized by washing
with HCl (0.1 M) for several times.
Experimental
Materials
All materials were of commercial reagent grade and used
without further purification. Copper(I) iodide, Copper
(II) acetate, phenylacetylene, sodium carbonate, 1-ethy-
nyl-4-fluorobenzene, 4-tertbutylphenylacetylene, sodium
chloride, ethylenediamine, 3-aminopropyltrimethoxysi-
lane (APTMS), glycerol, iron(II) chloride (FeCl2·4H2O),
iron(III) chloride (FeCl3·6H2O), ammonium hydroxide
[25 % (w/w)], ethyl propiolate, 1-pentyn-3-ol, 1,4-diazabi-
cyclo[2.2.2]octane (DABCO), sodium silicate, hydrochlo-
ric acid, methanol, ethanol, benzonitrile, glutaraldehyde,
l-histidine, acetonitrile and dimethylformamide were pur-
chased from Merck Chemical Company.
Preparation of Schiff base (Glu‑His)
An aqueous solution of l-histidine (0.155 g, 0.1 M in
deionized water) was added to glutaraldehyde (0.188 mL,
0.1 M in 10 mL deionized water). The solution was then
heated at 45 °C for 2 h. Observation of the solution color
change from transparent to light yellow after 30 min indi-
cated the completion of the reaction and formation of the
product (denoted as Glu-His) [38].
Preparation of Fe3O4@SiO2@APTMS
Immobilized Glu-His was obtained by stirring the Fe3O4@
SiO2@APTMS (1.0 g) with Glu-His (10 mL) at 80 °C for
2 h. It was assumed that the Schiff base between Fe3O4@
SiO2@APTMS amino group and free aldehyde of the Glu-
His has been generated in this reaction [38]. The brown
solid product separated by means of an external magnet
was washed with deionized water for several times.
Characterization
C–C homocoupling products were analyzed by GC and
GC-Mass using Agilent 6890 series with a FID detector,
HP-5, 5 % phenylmethylsiloxane capillary and helium as
carrier gas and Agilent 5973 network, mass selective detec-
tor, HP-5 MS 6989 network GC system. The X-ray diffrac-
tions (XRD) patterns of samples were recorded on a Philips
PW1800 diffractometer using monochromatic nickel-fil-
tered Cu Kα radiation (k = 0.15405 nm). The X-ray gen-
erator was run at 40 kV and 30 mA and the diffractograms
were recorded in the 2 h range of 4°–90°. The phases were
identified using the powder diffraction file (PDF) data-
base (JCPDS, International Centre for Diffraction Data).
Preparation of Fe3O4@SiO2@APTMS@Glu‑His@Cu
complex
Fe3O4@SiO2@APTMS@Glu-His (1.0 g) was added to a
solution of CuI (0.06 M in water) and the resultant mixture
stirred over 8 h at 80 °C. The solid product was then sepa-
rated, washed with water and acetone and dried in air.
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Catalytic reactions
Results and discussion
Catalyst characterization
The catalytic reactions were carried out in a round bot-
tom flask equipped with a magnetic stirrer and a water-
cooled condenser under O2 atmosphere. Typically, a desired
amount of catalyst, substrate (1.0 mmol), solvent (5 mL)
and Na2CO3 (0.211 g, 2.0 mmol) were added to the reac-
tion flask and the mixture heated at reflux within the desired
time. The progress of reaction was monitored by TLC and
GC, until the complete conversion of starting material.
Separation of the catalyst was carried out by means of an
external magnet and the solution subjected to GC and GC
mass for analysis. After the conventional work up, the resi-
due was purified using column chromatography (petroleum
ether/ethylacetate). The products were confirmed by the
comparison of their GC retention time, GC mass, with those
of authentic data (Figure S1-5, see supplementary) [22, 24].
Fe3O4@SiO2@APTMS@Glu-His@Cu complex des-
ignated as compound 1 was prepared according to the
procedure presented in Scheme 1. MNPs were initially
prepared by precipitation of iron(II) and iron(III) ions
in a basic solution. Subsequently, silica was coated on
Fe3O4 to form Fe3O4@SiO2 core–shell particles using
sodium silicate in acidic solution to prevent the aggre-
gation and increasing the stability of Fe3O4 nanoparti-
cles in a harsh liquid environment. The modification of
magnetic nanoparticles surface is generally carried out
by functionalization with desired organic groups [38,
39]. Therefore, after the surface functionalization with
APTMS, Glu-His Schiff base was immobilized followed
Scheme 1 Preparation steps of Fe3O4@SiO2@APTMS@Glu-His@Cu complex magnetic nanoparticles
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J IRAN CHEM SOC
Fig. 1 The FTIR spectra of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@
SiO2@APTMS
Fig. 3 The SEM images of a Fe3O4, b Fe3O4@SiO2@APTMS and c
Fe3O4@SiO2@APTMS@Glu-His@Cu complex
by complexation with CuI to afford the desired Fe3O4@
SiO2@APTMS@Glu-His@Cu complex nanoparticles
(see, Scheme 1).
The FT-IR spectra of Fe3O4, Fe3O4@SiO2 and Fe3O4@
SiO2@APTMS are presented in Fig. 1a–c, respectively.
As indicated in Fig. 1a, the bands appearing at 448 and
579 cm−1 are attributed to the Fe–O vibrations. In addition,
observation of the bands displaying at 571 and 1090 cm−1
Fig. 2 The FTIR spectra of (a) Glu-His, (b) Fe3O4@SiO2 @APTMS,
(c) Fe3O4@SiO2 @APTMS@Glu-His, (d) Fe3O4@SiO2@APTMS@
Glu-His @Cu complex before using and (e) after using as catalyst
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Fig. 4 The TEM image of Fe3O4@SiO2@APTMS@Glu-His @Cu
complex
Fig. 6 XRD pattern of (a) Fe3O4@SiO2@APTMS, (b) Fe3O4@
SiO2@APTMS@Glu-His@Cu complex before using and (c) after
using as catalyst
of aldehyde group. Compared to the FTIR spectrum of
Fe3O4@SiO2@APTMS (Fig. 2b), several changes can be
observed in all the frequency ranges in the spectrum of the
immobilized Schiff base (Fig. 2c). For example, increasing
the intensity of C=N and C–H stretching.
Fig. 5 The EDS of Fe3O4@SiO2@APTMS@Glu-His @Cu complex
Vibrations observed respectively at 1637 and 2865 and
2940 cm−1 indicates the Glu-His group bonded to Fe3O4@
SiO2@APTMS. Moreover, whereas the C=N stretching
vibrations of the Glu-His in Fe3O4@SiO2@APTMS@Glu-
His is traditionally appeared at 1637 cm−1, shifting to a
lower wave number at 1630 cm−1 by approximately 7 cm−1
confirms the coordination of the Schiff base nitrogen atom
to the metal ion (Fig. 2d). A broad band observed in the
range of 2400–3400 cm−1 due to the histidine OH stretch-
ing of carboxyl group indicates that no coordination to the
metal ion has occurred [38, 41].
Based on the SEM results (Fig. 3a–c), the change in the
particle size after immobilization of Glu-His and Cu com-
plex on modified MNPs is considerable (Fig. 3b, c). More-
over, the particles size was found to be about 15–35 nm
based on the TEM results of Fe3O4@SiO2@APTMS@
Glu-His @Cu complex indicated in Fig. 4. The EDS of
due to the Fe–O and Si–O vibrations demonstrate the exist-
ence of Fe3O4 and SiO2 components. After the functional-
izing with APTMS, two obvious bands appearing at 2855
and 2925 cm−1 are associated with C–H stretching vibra-
tions. This is in accordance with the predicted structure of
Fe3O4@SiO2@APTMS (Fig. 1c) [40].
The FTIR spectra of Schiff base Glu-His, Fe3O4@SiO2
@APTMS, Fe3O4@SiO2 @APTMS@Glu-His, Fe3O4@
SiO2@APTMS@Glu-His @Cu complex before and after
using as catalyst are shown in Fig. 2a–c, respectively. The
spectrum of the Schiff base Glu-His (Fig. 2a) shows a band
at the region 1360 cm−1 due to the C–N vibrations. Moreo-
ver, whereas the rather strong band displaying at 1635 cm−1
is assigned to the C=N stretching vibration, that appearing
at 1728 cm−1 is attributed to the C=O stretching vibration
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J IRAN CHEM SOC
Fig. 9 Effect of the amount of compound 1 as catalyst and time on
conversion at 100 °C under O2 atmosphere, in the presence of sodium
carbonate as base and DMF as solvent
Fig. 7 The TGA of Fe3O4@SiO2@APTMS@Glu-His@Cu complex
characteristic peaks with cubic structure. The similar set
of characteristic peaks also observed for Fe3O4@SiO2@
APTMS@Glu-His@Cu complex indicate the stability of
the Fe3O4 nanoparticle crystalline phase during silica coat-
ing and surface amino-functionalization.
Thermal decomposition of Fe3O4@SiO2@APTMS@
Glu-His@Cu complex was studied. As seen in this Fig. 7,
the first and second weight loosings up to 110 and 550 °C
should be due to the removal of water and organic phase
decompositions, respectively.
The magnetic properties of the prepared catalyst con-
taining a magnetite component were studied by a VSM
at 300 K. The magnetization curve of Fe3O4@SiO2@
APTMS@Glu-His @Cu complex before and after
using as catalyst are shown in Fig. 8a, b. Since both
remanence and coercivity are zero and no hysteresis is
observed, it can be concluded that such nanospheres
are superparamagnetic [37]. The saturation magnetiza-
tion values for these catalysts were determined to be
30 emu/g (Fig. 8a, b).
Fig. 8 Magnetization curves of (a) Fe3O4@SiO2@APTMS@Glu-
His@Cu complex before and (b) after using as catalyst
Fe3O4@SiO2@APTMS@Glu-His @Cu complex is shown
in Fig. 5. The results confirm the presence of Cu, Si, Fe, I
and C in the sample.
Catalytic activity
The XRD patterns of the prepared Fe3O4@SiO2@
APTMS, Fe3O4@SiO2@APTMS@Glu-His@Cu complex
before and after using as catalyst are shown in Fig. 6a–c.
The diffraction peaks with d values at 4.90, 2.95, 2.52,
2.08, 1.70, 1.60, 1.48, 1.27 and 1.11 observed for Fe3O4@
SiO2@APTMS nanoparticles are related to the (111),
(220), (311), (400), (422), (511), (440), (533), and (444)
planes (Fig. 6a). These results are similar to those of the
standard JCPDS card No. 1-1111 exhibiting the iron oxide
Fe3O4@SiO2@APTMS@Glu-His@Cu complex (1) was
used as catalyst for the alkyne–alkyne homocoupling reac-
tion and phenylacetylene was used as the representative
substrate. To see the effect of catalyst amount and time on
the product distribution, reaction times of 1, 2, 3 and 4 h
were examined in the presence of 0.1–0.5 g of catalyst. The
results indicated in Fig. 9 shows that by increasing the cata-
lyst amount from 0.1 to 0.5 g, reaction conversion increases
from 9 to 92 % during 4 h.
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J IRAN CHEM SOC
Fig. 11 Effect of different base with compound 1 as catalyst, at reac-
tion temperature 100 °C under O2 atmosphere
Fig. 10 Effect of solvent with compound 1 as catalyst, at reaction
temperature 100 °C under O2 atmosphere, in the presence of sodium
carbonate as base
studied by recycling the recovering catalyst and determi-
nation of the copper content using atomic absorption spec-
troscopy. The copper content of the fresh and used catalysts
were determined to be 2.41 and 2.38 %, respectively.
To further investigate the heterogeneous character of
catalyst, compound 1 was recovered by external magnet
from reaction mixture after 30 min. The reaction was then
allowed to continue in the absence of catalyst. The observa-
tion of no additional conversion clearly indicated the het-
erogeneous character of the catalyst. On the other hand, the
similarity of the FTIR, XRD and VSM patterns of Fe3O4@
SiO2@APTMS@Glu-His@Cu complex before and after
using as catalyst shown respectively in Figs. 2e, 4c, and 5b
also supported the catalyst stability.
The effect of time with different solvents such as eth-
anol, acetonitrile, benzonitrile, and DMF on homocou-
pling reaction of phenylacetylene catalyzed by Fe3O4@
SiO2@APTMS@Glu-His@Cu complex (1) is presented
in Fig. 10. As seen in Fig. 10, the best conversion was
achieved in benzonitrile within 2 h. Hence, benzoni-
trile was selected as the reaction solvent for subsequent
reactions.
Investigation of the effect of base on the model reac-
tion revealed that whereas DABCO, sodium carbonate
and sodium acetate afforded excellent yields (97–100 %),
reaction was rather inefficient using triethylamine (32 %)
(Fig. 11).
In the next step, 4-tert-butylphenylacetylene, 4-fluoro-
phenylacetylene, ethyl propionate and pent-1-yn-3-ol were
then examined for homocoupling reactions using benzoni-
trile as solvent and O2 as oxidant in the presence of 0.5 g
of Fe3O4@SiO2@APTMS@Glu-His@Cu complex (1)
as catalyst and sodium carbonate as base (Table 1). The
products were characterized by comparison of the GC and
GC–MS spectra with those of the authentic samples (see
supplementary).
To test the reusability of the catalyst, compound 1
was recovered by external magnet from reaction mix-
ture after completion of the first run and the catalyst was
investigated in another run by addition of fresh phenyla-
cetylene and benzonitrile similar to the initial reaction. A
small decrease in catalyst activity from 98 % to the 91 %
clearly revealed the reusability of the prepared catalyst
(Fig. 12).
As outlined in Scheme 2, we have formulated a work-
ing mechanism for the copper-catalyzed homocoupling of
terminal alkynes based on the proposed mechanism of the
previous reports and our results [42].
The initially generated Cu(I)-acetylide intermediate I
from the base-catalyzed reaction of phenylacetylene with
copper site of 1 (path a) is dimerized, affording II (path
b). Subsequent intramolecular rearrangement with con-
comitant reduction of Cu(I) to Cu(0) together with homo-
coupling reaction affords intermediate III (path c). The
product 1,3-diyne and regeneration of catalyst 1 occur via
oxidation of III with O2 (path d). To obtain further insight
into the reaction mechanism, the homocoupling reac-
tion of phenylacetylene was carried out in the presence of
diphenylamine as a radical scavenger. The involvement of
a free-radical pathway and implication of alkynyl radical
in the reaction mechanism was ruled out since no consid-
erable change in reaction rate and product selectivity was
observed in the presence of diphenylamine as a radical
scavenger.
The stability of the magnetically recovered Fe3O4@
SiO2@APTMS@Glu-His@Cu complex (1) was also
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J IRAN CHEM SOC
Table 1 Results obtained for the homocoupling reaction of different monosubstituted terminal alkynes catalized by Fe3O4@SiO2@APTMS@
Glu-His@Cu complex
Reaction conditions: substrate (1 mmol), catalyst (0.5 g), sodium carbonate (2 mmol), benzonitrile (5 mL) at 100 °C under O2 atmosphere
Conclusions
Fe3O4@SiO2@APTMS@Glu-His@Cu complex (1) was
prepared from modification of iron oxide nanomagnet parti-
cles followed by immobilization of glutaraldehyde-histidine
Schiff base ligand and then complexation with Cu(I) salt.
Compound 1 was found to successfully catalyze the oxi-
dative homocoupling of phenylacetylene, 4-tert-butylphe-
nylacetylene, 4-fluorophenylacetylene and pent-1-yn-3-ol
with 98–100 % conversion and 97–100 % selectivity within
60 min. Recovery of the catalyst was easily achieved simply
by magnetic decantation using an external magnet. It was
also found that the catalyst can be used three times without
significant catalytic activity losing. Utilization of magnetic
Fig. 12 The effect of recyclability of the catalyst Fe3O4@SiO2@
nanoparticle catalyst with promising advantages in com-
APTMS@Glu-His@Cu complex for the homocoupling reaction of
parison to those of conventional catalyst supports is recom-
mended to be considered by chemical industry researchers.
phenylacetylene at optimum condition
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Scheme 2 Suggested mechanism for the C–C homocoupling of terminal alkynes using compound 1 as catalyst (reaction condition: solvent:
benzonitrile at 100 °C, under O2 atmosphere)
Acknowledgments The financial support from Alzahra University
is gratefully acknowledged.
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