CuI nanoparticles on modified poly(styrene-co-maleic anhydride) as an effective catalyst in regioselective synthesis of 1,2,3-triazoles via click reaction: a joint experimental and computational study

Article abstract of DOI:10.1080/00958972.2017.1321742

In situ immobilization of CuI nanoparticles (NPs) on modified poly(styrene-co-maleic anhydride) [SMA] was achieved. Proper immobilization of CuI on the prepared support was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray analysis

Full text of DOI:10.1080/00958972.2017.1321742

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://tandfonline.com/loi/gcoo20
CuI nanoparticles on modified poly(styrene-
co-maleic anhydride) as an effective catalyst in
regioselective synthesis of 1,2,3-triazoles via click
reaction: A joint experimental and computational
study
Tahmineh Baie Lashaki, Hossein A. Oskooie, Tayebeh Hosseinnejad & Majid
M. Heravi
To cite this article: Tahmineh Baie Lashaki, Hossein A. Oskooie, Tayebeh Hosseinnejad & Majid
M. Heravi (2017): CuI nanoparticles on modified poly(styrene-co-maleic anhydride) as an effective
catalyst in regioselective synthesis of 1,2,3-triazoles via click reaction: A joint experimental and
computational study, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2017.1321742
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1321742
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Date: 21 April 2017, At: 13:53

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2017.1321742
CuI nanoparticles on modified poly(styrene-co-maleic anhydride) as an
effective catalyst in regioselective synthesis of 1,2,3-triazoles via click reaction:
A joint experimental and computational study
TAHMINEH BAIE LASHAKI, HOSSEIN A. OSKOOIE, TAYEBEH HOSSEINNEJAD* and
MAJID M. HERAVI*
Department of Chemistry, Alzahra University, Vanak, Tehran, Iran
In situ immobilization of CuI nanoparticles (NPs) on modified poly(styrene-co-maleic
anhydride) [SMA] was achieved. Proper immobilization of CuI on the prepared support was
confirmed by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX)
and inductively coupled plasma (ICP). In order to present a quantitative description for
experimental features of CuI/SMI nanocatalyst, the coordination behavior and the nature of
interactions between Cu(I) ions and modified SMI ligand in gas and solution phases was
computationally assessed. In this line, the mathematical properties of electron density functions
were calculated and analyzed topologically via density functional theory (DFT) and quantum
theory of atoms in molecules (QTAIM) approaches for different coordination modes.
Encouraged by our computational results, this new catalyst was employed in a one-pot, three-
component reaction involving terminal alkynes, alkyl halides and sodium azide in water resulting
in highly regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles in high to excellent yields.
The catalyst was reused without pre-activation and recycled for at least five runs without
significant decrease in its activity being observed.
Keywords: Modified poly(styrene-co-maleic anhydride); CuI nanoparticles; Metal-ligand
interactions; Click chemistry; DFT and QTAIM computations
*Corresponding authors. Email: mmh1331@yahoo.com (M.M. Heravi); tayebeh.hosseinnejad@alzahra.ac.ir (T. Hosseinnejad)
1

1. Introduction
The 1,2,3-triazole derivatives exhibit several diverse biological activities. They have also been
widely employed as precursors in the synthesis of biologically active systems such as anticancer,
antimicrobial, antifungal and other cytotoxic active agents [1]. These compounds have also
shown high chemical stability towards severe hydrolytic, oxidizing and reducing conditions,
even at high temperatures [2].
1,2,3-Triazoles were synthesized via Huisgen 1,3-dipolar cycloaddition [3]. A developed
regioselective Huisgen 1,3-dipolarcycloaddition being performed under mild reaction conditions
which has a great impact on biological, industrial and synthetic applications was in much
demand. In 2002, Sharpless et al. [4] and Meldal and co-workers [5] independently reported that
the use of catalytic amounts of Cu(I) resulted in the rapid and regioselective azide-alkyne
cycloadditions at ambient temperature in organic medium, so-called click reaction [6-9]. The role
of Cu(I) as a coordinating agent in the mechanistical pathway of click reactions has been well
established via the coordination of Cu(I) with terminal alkynes and the activation of azide
species to produce a copper-azide-acetylide complex [10, 11].
It is known that the ligand protects the Cu ion from interactions leading to degradation
and generation of side products and more importantly prevents the oxidation of the Cu(I) species
to the Cu(II) [12, 13]. Moreover, immobilization of Cu(I) nanoparticles on different kinds of
supports is a useful method to make them heterogeneous, thus easily recoverable and reusable
[14-16]. Applications of various forms of polymer-supported CuI as recoverable catalyst for the
click synthesis of 1,2,3-triazoles between azides and alkynes have been reported [17].
Recently, the coordination of metal NPs on the polymer surfaces has attracted much
attention by rational design of organic ligands on the polymer surface as coordinating agent
[18, 19]. In another words, to synthesize the desired polymers with predictable structure and
coordination properties, it is substantial to apply appropriate multidentate ligands with strong
metal-ligand interactions.
To achieve higher stability, better dispersion of the particles, as well as sufficient
recyclability of the catalyst, CuI NPs have been immobilized on high surface area supports
[20-23]. For the purpose of effective immobilization of CuI NPs, the support surface is modified
in the multiple-step synthetic route so that it can be coordinatively bound to CuI NPs with a
considerable metal-ligand interaction energy. Significantly metal polymer-supported catalysts
2

maintain some important benefits of homogenous catalysis such as giving short reaction times
and obtaining satisfactory yields in aqueous media [24]. SMA is an inexpensive commercially
available copolymer of styrene and N-aryl succinimide. It contains reactive anhydride groups,
which can be modified with different nucleophilic reagents [25, 26], providing useful support for
the design of numerous reagents and also catalysts for effective coordination with metal NPs
[27]. However, since then, the modification and application of SMA have been largely
overlooked.
We are interested in Cu(I)-catalyzed 1,3-dipolar cycloaddition via click reactions
considering the coordination behavior of Cu(I) as a catalyst in regioselective reaction pathway
[28-36]. We have recently synthesized and characterized copper-aminated KIT-5 and silica-
bonded propyl-N-sulfamic acid nanocatalysts and investigated metal-ligand interactions and
coordination modes from the structural and electronic viewpoints and also applied them in click
and other organic transformations [32, 33, 37].
We have also achieved the reaction of SMA with 4-aminopyridine and 2-aminothiazole
and used it as modified co-polymer as support for immobilization of CuI and PdCl2NPs to
prepare nanocatalysts and applied them successfully in click and coupling reactions. Moreover,
we analyzed and interpreted information on the coordination of metal NPs, based on observed IR
stretching frequency of coordinating residues, in conjunction with our obtained computational
results [34, 35].
Due to our interest in design and preparation of nanocatalysts, and also employing the
computational methods for the coordination chemistry of metal complexes with modified SMA
ligand in the nanocatalysts [32, 33, 35, 37], herein, we report in situ preparation and
characterization Cu(I) NPs immobilized on modified SMA with 2-aminopyridine to obtain
poly(styrene-co-maleimide) (SMI) copper catalyst. The composite was then characterized by
different methods such as SEM, EDAX and ICP. More importantly, to find a satisfactory
correlation between experiment and theory, we focused on theoretical interpretation of
immobilization behavior of CuI on SMI support surface using density functional theory (DFT)
[38] and quantum theory of atoms in molecules (QTAIM) [39] approaches. We have also
considered the solvent effect through the polarized continuum model (PCM) computations [40].
It is worthwhile to state that molecular information such as structural and electronic
properties of metal complexes give deeper insights into modulation of the coordination chemistry
3

of metal complexes and so the rational design of new catalysts. The recent advances in
computational chemistry approaches have led to present a theoretical description for catalytic
reactions with detail and accuracy required for reliable comparison with experiments [41, 42].
In the next step, the composite was used as an efficient heterogeneous catalyst in a one-
pot, three-component regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles via click
reaction in water as a green solvent. This heterogeneous catalyst exhibited an excellent catalytic
performance for click synthesis of various 1,2,3-triazole derivatives and was recovered easily
and re-used several times with no considerable loss of activity.
It should be mentioned that in our previous research, we just reported the synthesis and
characterization of CuI NPs immobilized on SMA modified by 4-aminopyridine and used it in
the regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles [34]. We actually overlooked an
important issue which could have been helpful in interpretation of the experimental observations.
That issue was the computational analysis of metal-ligand interactions in CuI/SMI complex. As
compensation, in this research, we focused on the employment of 2-aminopyridine instead of
4-aminopyridine, reacting it with SMA as a modifier. Since, in 2-aminopyridine-modified SMA,
the pyridine nitrogen is nearer to carbonyl functional group of maleimide, we assumed that the
former acts as a stronger coordinating agent towards metal ions in comparison with SMA
modified by 4-aminopyridine. This structural feature is expected to influence the coordination
behavior of CuI/SMI complex leading to more effective interaction between copper and modified
SMA. This phenomenon has been observed and reported previously in the case of a similar
system [43-45].
Thus we investigated these two different CuI/SMI complex models via quantum
chemistry computations. Strictly speaking, we made a comparative assessment on the
coordination behavior of CuI/SMI nanocatalyst modified with 2- and 4-aminopyridines to predict
the immobilization behavior of copper NPs on SMI polymeric surface. In this line, we also
compared our experimental results when SMA was modified with 2-aminopyridine and used as
support for immobilization of Cu(I) NPs catalyst, with those obtained when 4-aminopyridine was
used under similar conditions, reported previously [34].
It should be noticed that the lower cost of commercially available 2-aminopyridine and
also easily accessible via Chichibabin reaction [46] relative to 4-aminopyridine is motivating to
4

use former for the modification of SMA support being used as heterogeneous catalyst in the
regioselective click synthesis of 1,4-disubstitued 1,2,3-triazoles.
2. Experimental
2.1. Materials and methods
N,N-Dimethylformamide (DMF) and triethylamine (TEA) were distilled and stored over 4 Å
molecular sieve before use. Other reagents were purchased from Aldrich and Merck and used as
received. SMA was obtained from KARABOND. The general formula of SMA used in this
study is [(C8H8)0.6(C4H2O3)0.4]n with Mn = 86,666, Mw = 182,000, and Mw/Mn = 2.1.
1H NMR spectra were recorded using a Bruker Ultrashield 500 with DMSO-d6 as solvent. Proton
resonances are designated as singlet (s), doublet (d), triplet (t) and multiplet (m). FTIR spectra
were recorded using KBr disks on a FTIR Bruker Tensor 27 instrument from 500-4,000 cm-1.
Band intensities are assigned as weak (w), medium (m), and strong (s). Cu content was measured
by ICP on a Varian Vista pro analyzer. Scanning electron micrographs were recorded using a
Lecia Cambridge S 360 SEM instrument. All yields refer to isolated products.
2.1.1. Preparation of SMI. SMA (1) was modified in two steps in accord with the procedure
previously reported by Lee et al. [47]. In a 100 mL glass reactor SMA (1.00 g), 2-aminopyridine
(1.54 g, 16 mmol) and dry DMF (15 mL) were placed. Then gas N2 was charged into the reactor
and sealed. The reactor was located into an oil bath, and the reaction mixture was oscillated for
3 h at 35 C. During this period of time, reagents were dissolved in DMF. Then, acetic anhydride
(0.6 mL, 6 mmol), sodium acetate (0.33 g, 4 mmol) and triethylamine (0.3 mL, 2 mmol) were
consecutively added into this mixture by syringe (figure 1). The reaction temperature was raised
to 75 C, while oscillation was sustained for 3 h. The mixture was then cooled to ambient
temperature and poured into 300 mL of methanol under stirring. The fiber-like caused polymer
was separated by filtration, washed with methanol thoroughly and dried under reduced pressure
at 70 C. The SMI polymer (3) was further purified by re-precipitation in DMF.
The amine content of SMI was estimated according to previously reported procedures
[48, 49]. Accordingly, 10 mL of HCl (0.2 N) was added to 0.05 g of the SMI, and the obtained
mixture was stirred for 30 min at room temperature. The obtained SMI was washed with
deionized water. Excess HCl was then titrated with NaOH (0.2 N) in the presence of
5

phenolphthalein as an indicator. Amine site content of the synthesized SMI was determined to be
3.42 mmol/g.
2.1.2. Preparation of polymer-supported catalyst. CuI (0.247 mg, 0.001 mmol) was dissolved
in acetonitrile (2 mL) under ultrasonic irradiation. A transparent pale yellow solution was
formed. To this solution, DMF (20 mL) was added at ambient temperature, generating CuI NPs.
To this mixture, dry SMI (1.00 g) was added. This mixture was refluxed under nitrogen
atmosphere for 5 h. Then, the generated CuI/SMI (4) was washed, filtered off and washed with
acetonitrile and dried under vacuum at 60 C (figure 1).
2.1.3. Determination of the copper content in SMI–CuI. A sample of CuI/SMI (100 mg) was
extracted with concentrated HCl and HNO3 (1:1, 10 mL) for digestion of the metal complex
using a screw-capped vessel. Then, the mixture was relocated into a volumetric flask (100 mL),
diluted and made ready for ICP analysis. The Cu concentration was determined by conventional
atomic emission (324.754 nm) by reference to a linear (R = 0.99) calibration curve of (1-4 ppm),
similar to other CuI sample preparation. The Cu content was 6.92% w/w. The same procedure
was employed to measure the leaching accounts of the Cu catalyst after five cycles.
2.1.4. Synthesis of 1,4-disubstituted 1,2,3-triazoles: General procedure. A mixture involving
appropriate α-haloketones (1 mmol) or alkyl halide (1 mmol), a suitable alkyne (1 mmol),
sodium azide (1.1 mmol) and CuI/SMI (0.03 g) was refluxed for the indicated time. Progress of
the reaction was monitored by TLC using hexane and ethyl acetate as eluent. After completion of
the reaction, the supported catalyst was filtered off and washed with hot ethanol. The recovered
catalyst was then washed with acetone and dried under reduced pressure at 70 C for 3 h. This
recovered catalyst was stored for being recycled. The filtrate was evaporated under reduced
pressure and the residue was recrystallized from EtOH/H2O (3:1v/v) to give the pure crystalline
products. All the products were known and their physical data were compared with those of
authentic samples and found being identical [28, 50-54].
6

3. Results and discussion
In 2005, initially SMA polymer was reacted with di(2-pyridyl)methylamine to obtain the
modified SMA [27]. Subsequently, palladium was immobilized on this modified SMA. This
system was used as an efficient and recyclable heterogeneous catalyst in different cross coupling
reactions including Heck, Suzuki and Sonogashira coupling reactions [55]. The high reactivity of
cyclic anhydride is obvious and known [56]. The combination of this high reactivity with high
concentration makes SMA a seamless and an ideal polymeric support for the immobilization of
metal nanoparticles being used as heterogeneous catalysts in different organic transformation
[27, 57].
In this light, we recently used 4-aminopyridine as a simpler amine for the successful
modification of SMA, prepared and characterized Cu(I) NPs immobilized on modified SMA
[34]. In continuation of this work, we thought that it is worthwhile to study the use of a less
expensive and more readily accessible isomer, 2-aminopyridine instead of 4-aminopyridine, for
the modification of SMA and convert it to SMI to give our previous work a broader overview
such as substrate scope.
3.1. Preparation and characterization of the modified SMA
In order to modify SMA (1) initially, it was reacted with 2-aminopyridine (2), following the
procedure previously reported by Lee etal. for the reaction of SMA with substituted anilines
[47]. FTIR spectroscopy confirmed the conversion of the maleic anhydride moiety in SMA to the
corresponding maleimide. FTIR spectra of 2-aminopyridine, SMA, SMI, and CuI/SMI are
illustrated in figure 2. The distinctive doublet of anhydride moieties is vividly observed at 1847
and 1794 cm-1 in figure 2b. A new imide band at 1716-1786 cm-1 is observed while no band at
3300-3500 cm-1 for amino-groups of 2-aminopyridine was observed in the product. These
results, evidences the total imidization of SMA. Furthermore, the 1H NMR spectrum of this SMI
(figure 3) showed an array similar to SMA [47], plus the desirable aromatic protons for the
pyridine moiety present in the amine, as follows; [1.15 (m, 2H), 2.64 (s, 1H), 2.89 (s, 1H), 3.17
(m, 1H), 6.02 (b, 1H, Ar-H), 6.47 (m, 1H, Ar-H), 6.50-7.36 (b, 5H, Ar-H), 7.38 (t, J = 5.6 Hz,
1H, Ar-H), 7.88 (d, J = 3.2 Hz, 1H, Ar-H)].
7

The absence of N-H peaks of the amino group reconfirms the presence of amido moieties
in the polymer side-chain. In addition, the free amine site content of the resulted SMI was found
3.23 mmol/g, as calculated by back titration (for details see the Experimental section) [48, 49].
3.2. Preparation and characterization of CuI/SMI
As discussed earlier, a general applicable strategy that prevents the presence of metal NPs of
catalyst in the reaction products is the immobilization of metal NPs onto a support. In this line, in
the previous step, we modified SMA support surface for the purpose of effective immobilization
of CuI onto the polymeric surface. So, in this step, CuI NPs were coordinatively attached to the
modified SMI polymer surface. After proper immobilization of CuI NPs, structure, coordination
properties, spectroscopic data, amount of coordinatively bonded CuI NPs and surface
morphology of the prepared CuI/SMI were discussed via SEM, EDAX and ICP analysis.
In order to avoid the agglomeration of the NPs, CuI NPs were prepared in situ and were
refluxed with the obtained SMI under N2 atmosphere in DMF as solvent (the molar ratio of
amine to Cu was 2.4:1.0) [58] to obtain the Cu(I) NPs immobilized on SMI. To find the optimal
reaction conditions, DMF and ethanol were used as solvent. Although the immobilization also
occurred in ethanol, better immobilization was observed for the product obtained in DMF. The
size of the immobilized copper particles on the surface of the catalysts determined by SEM
images of the two obtained catalysts from two different solvents, were documented. As shown in
figure 4, the superior immobilization and smaller size of CuI NPs on SMI is vividly observed for
the product obtained in DMF. Due to insolubility of the CuI/SMI catalyst in most conventional
organic solvents, we have to limit our structural elucidation to SEM, EDAX, IR and ICP
analysis. The scanning electron micrographs of the CuI/SMI obtained from DMF (figure 4)
vividly illustrate that the CuI NPs were well homogeneously immobilized on the SMI surface
with average size of 70 nm. The other obtained data such as EDAX for the CuI NPs immobilized
on SMI is illustrated in figure 5, which clearly approves the attachment of CuI onto the surface
of the polymer matrix. Concise comparative analysis of IR spectrum of SMI with CuI/SMI
shows a slight negative shift in stretching vibration frequency of C=N of pyridine ring and C=O
of maleimide for free ligand and its corresponding complex which can be attributed to the
coordination of copper ion with pyridine nitrogen and oxygen of carbonyl group and so
8

coordination is quantitatively distinguished (figure 2). In the following computational section,
this behaviour will be discussed and confirmed by the computational analysis.
Moreover, to estimate the Cu content, CuI NPs immobilized on SMI were initially treated
with concentrated HCl for digestion of the Cu species for Cu content estimation. The digested
prepared extract was ready analyzed by inductively coupled plasma (ICP) analysis. In this way,
6.92% w/w Cu content was determined.
Comparative study of ICP results between 2-aminopyridine and 4-aminopyridine
modified SMI catalyst shows slightly higher value for the copper content of 2-aminopyridine
modified SMI which is about 0.3% more than 4-aminopyridine modified polymeric support [34].
This increase in copper content and the lower cost of 2-aminopyridine (about half) in comparison
with 4-aminopyridine make it more economical and affordable and more useful compound for
the modification of SMA.
The immobilization technique on SMI as a polymeric support showed several merits. The
observed advantages are facile procedure and non-required pre-activation. Bearing in mind that
the Cu(I) catalyzed 1,3-dipolar Huisgen cycloaddition reaction via click reaction, we decided to
examine the catalytic potency of this in situ CuI NPs immobilized on SMI which was fully
characterized in the aforementioned reaction. The reaction of phenylacetylene as an alkyne and
sodium azide was chosen as model reaction. The model reaction proceeded smoothly in the
presence of 0.03 g of catalyst in water as the greenest and most abundant solvent at reflux
temperature. The progress of reaction was monitored by TLC using (n-hexane: ethyl acetate; 3:1)
as eluent. Upon completion of the reaction (indicated by TLC), the mixture was filtered off and
the filtrate was washed with hot ethanol to separate the products. The filtrated catalyst was
washed several times by acetone and then was dried and stored for use in subsequent reactions.
The residue was then crystallized in ethanol to give the corresponding pure crystalline
1,4-disubstituted 1,2,3-triazole as sole product.
To prove the heterogeneous nature of catalyst, the recovered solid was washed with
acetone dried and weighted. No appreciable loss in weight was observed. The recovered catalyst
was used for further five runs in the same reaction conditions to give the products. The yield for
each run is shown in figure 6. To establish the generality of method, various alkynes reacted with
a wide range of organic azides under optimized conditions which provided the corresponding
1,2,3-triazoles in excellent regioselectivity and yields (table 3).
9

3.2.1. Regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles. To evaluate the potency of
the copper-supported catalyst, it was applied in the click reaction. The Cu(I)-catalyzed reaction
of benzyl chloride (1 mmol), sodium azide (1.1 mmol), and phenylacetylene (1 mmol) was
selected as a model reaction and was performed in different solvents (table 1). As shown in
table 1, the best results were obtained for entries 4 and 5 (solvent: EtOH/H2O and water).
Naturally, neat water was selected as the solvent of choice (table 1, entry 5). To optimize the
amount of catalyst, the model reaction was conducted in water using different amounts of the
catalyst (table 2). The best result was obtained when the model reaction was performed in the
presence of 0.03 g of CuI/SMI in refluxing water (table 2, entry 3). Increasing the catalyst
concentration did not show any significant effect regarding the yield and reaction time. To
establish the generality of the method, the reaction of various α-haloketones or alkyl halides
(5a-j), different alkynes (6), and sodium azide in water was performed in the presence of the
optimized amount of CuI/SMI catalyst.
The progress of the reaction reactions were monitored by TLC. After completion of the
reactions and cooling the reaction mixtures to room temperature, all products were isolated by
simple filtration, and recrystallized for further purification from ethanol. In this way a wide range
of 1,4-disubstituted 1,2,3-triazoles were synthesized in excellent yields, with short reaction times
and high regioselectivity. The physical and spectral data of all products were compared with
1
those of authentic samples, which were identical (table 3). We have listed H NMR and IR
spectroscopic data for some of our synthesized 1,4-disubstituted 1,2,3-triazoles (Supplementary
Material).
From the mechanistical viewpoint, in the first step of Cu(I) catalyzed azide-alkyne
cycloaddition reaction, a Π-complex is formed between Cu(I) species with the triple bond of
terminal alkyne. Then, Cu(I) coordinates to alkyne by deprotonation of the terminal hydrogen of
alkyne to give Cu(I)-acetylide intermediate, in the presence of a base. So, in the consequent
transition state structure, one Cu atom is coordinated to acetylide species while the other Cu
atom activates the azide via the coordination with electrons on the nitrogen atom. Then, a weakly
coordinating ligand is displaced on azide to produce a copper-azide-acetylide complex. Finally,
cyclization occurs, is followed by protonation and the product is generated by dissociation. So,
Cu(I)-ligand complex is reproduced as catalyst for further reaction cycles [10, 11].
10

Comparative survey on the catalytic activity of 2-aminopyridine modified SMA copper
catalyst with previously reported modification of SMA polymer with 4-aminopyridine [26]
shows lower required reaction times and higher yields in the presence of CuI/SMI catalyst
modified with 2-aminopyridine in the regioselective synthesis of 1,4-disubstituted-1,2,3-triazoles
via click reaction which in turn proves more effectiveness of this new catalyst. Furthermore,
obtaining better yields for regioselective synthesis of 1,4-disubstituted-1,2,3-triazoles somehow
shows more regioselective potential of the new modified polymer catalyst. These results can be
mainly attributed to the higher copper NPs content immobilized on the modified SMI support.
It is noteworthy to mention that the catalyst can be reused several times without loss of its
catalytic activity. The recyclability of the catalyst was studied by performing the reaction of
2-bromoacetophenone, phenylacetylene, and sodium azide as model reaction (figure 7).
Consistent activity of the catalyst was observed over five consecutive cycles, without any need to
reload and activate the catalyst (figure 6). Moreover, we studied the morphology of CuI NPs for
fresh and reused catalyst, after five runs via the comparative analysis of SEM images (presented
in figure 8). It is worthwhile to note that we did not observe any considerable morphological
changes of CuI NPs and the average diameter of CuI particles remain at the nano size even after
five consecutive cycles.
The heterogeneous nature of the catalyst and possible leaching of CuI from the SMI was
tested for by using ICP-AES on the reaction mixture. The difference in the Cu content for the
fresh and reused catalyst, after the fifth run, was only 6%, approving a low rate of leaching.
3.3. Computational section
In the last decades computational chemistry and parallel synthesis are significant, chronically
and recurrently applied tools for prime identification and prediction of reactivity, selectivity and
optimization of reaction conditions before running the actual experiments, as well as the
development of therapeutically important small molecules using ligand docking, ligand-based
pharmacophores, molecular descriptors and quantitative structure-activity relationships [60].
In this content, due to our overgrowing theoretical interest in Cu(I) catalyzed Huisgen
1,3-dipolar cycloaddition via click reaction [32, 61, 62], we expanded our undertakings to
computational methods especially on its outstanding power in predicting regioselectivity which
led to publication of a comprehensive review [29].
11

Going deep into the subject, in this work, we focused on theoretical interpretation of
immobilization of CuI NPs on modified SMI, by performing DFT [38] and QTAIM [39]
calculations. It should be emphasized that our computational modeling on the coordination
behavior of CuI NPs with a confined size model of SMI polymeric system will make accurate
simulation feasible, allowing detailed results to be produced about the immobilization of CuI
NPs on polymeric SMI, therefore, can be used to modify the properties of existing polymer-
supported catalyst and design the analogues of existing catalyst or propose designing similar new
catalysts.
At the first step, we designed effective computational models for SMI and CuI/SMI
compounds (figure 9) considering that this model has a reliable comprise between accuracy and
time saving efficiency of computational procedure. As illustrated in figure 9, we have modeled
three coordinating modes for description of metal-ligand interactions denoted as N-CuI/SMI
(Cu(I)coordinates to pyridine nitrogen), O1-CuI/SMI and O2-CuI/SMI (Cu(I) coordinates to
carbonyl oxygen), respectively. Thus, we determined initially the optimized structures of SMI,
N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI complexes via DFT calculations at M06/6-31G* level
of theory. It is noteworthy to state that M06 functional has been introduced recently as a top
performer within modern functional with a high performance and accuracy and was
recommended for application in organometallic thermochemistry, kinetic studies and non-
covalent interactions [63].
In the case of Cu(I) and iodine, the effective core potentials (ECP), LANL2DZ, were
used together with the accompanying basis set to describe the valence electron density [64]. All
DFT computations have been performed using GAMESS suite of programs [65]. In figure 10,
the energy minimized structures of SMI and N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI
complexes calculated at M06/6-31G* level of theory are displayed.
In order to assess the variation of bond orders via complexation process and interpret the
interaction strength between copper ion and SMI, the bond order of some selected key bonds in
SMI were compared with its corresponded bonds in N-CuI/SMI and O-CuI/SMI complexes. In
table 4, the bond-order of selected bonds in SMI ligand and N-CuI/SMI, O1-CuI/SMI and
O2-CuI/SMI complexes calculated at M06/6-311+G** level of theory are listed. The reported
results in table 4 indicate that the bond-order of carbonyl group and also C-N of pyridine ring
decreases through complexation in N-CuI/SMI and O2-CuI/SMI complexes, and this does not
12

happen in O1-CuI/SMI complex. This behavior can be mainly ascribed to the donation of shared
electrons from these chemical bonds to copper ions and is in agreement with the reported FT-IR
spectroscopic observations. Moreover, it has been demonstrated that Cu-N interactions in
N-CuI/SMI complexes are stronger than Cu-O interactions in O1-CuI/SMI and O2-CuI/SMI
complexes. On the basis of the obtained results, we can predict that copper NPs will bind more
effectively to pyridine nitrogen than carbonyl oxygen. On the other hand, we investigated
comparatively the interaction between copper ions and SMI modified with 2-aminopyridine and
4-aminopyridine (denoted as 2N-CuI/SMI and 4N-CuI/SMI complexes, respectively) via the
analysis of calculated bond-order values. This comparison demonstrates that the interaction
between pyridine nitrogen and copper ions in 4N-CuI/SMI complex is slightly weaker than
2N-CuI/SMI complex and so the copper NPs will coordinate more effectively to pyridinic
nitrogen of 2N-SMI in comparison to 4N-SMI ligand.
Since QTAIM provides a great deal of information about the nature of bonding
environment [66], in the next step we explored the topological properties of the electron charge
density and its Laplacian at various bond critical points (BCPs) in CuI/SMI complexes.
In this respect, resulting 6-311+G** and 6-31G* wavefunction files for the optimized
structures of SMI ligand, N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI complexes were employed
as inputs to AIM2000 program package [67]. In table 5, the calculated values of electron density,
, its Laplacian, , electronic kinetic energy density, , electronic potential energy density,
, total electronic energy density, and of selected bonds BCPs are presented for
N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI complexes.
Comparative assessment on the reported results of table 5 reveals the following: i) the
small values of electron density with the positive values of and small negative values of Hb
and also calculated values at Cu-N, Cu-O and Cu-I BCPs indicate the presence
of semi covalent-electrostatic interactions [68], ii) the decrease in the calculated values of at
C=O and C-N BCPs through complexation affirms the donation of shared electrons to the metal
center, iii) the comparison of the calculated electron density values of Cu-N and Cu-O BCPs in
N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI complexes approves that interaction of copper cation
with pyridine nitrogen is stronger than oxygen of carbonyl groups. Moreover, a more stringent
analysis of QTAIM molecular graphs of N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI complexes
demonstrate that in N-CuI/SMI, there are Cu-O, C-O and I-H BCPs that lead to some ring critical
13

points and consequently stabilize complex electronically. It is noteworthy to mention that this
electronic feature has not been observed in QTAIM topological analysis of wavefunctions in
O1-CuI/SMI and O2-CuI/SMI complexes.
In the next step, the bond ellipticity of selected bonds was considered. As reported in
table 6, the calculated values of ellipticity of carbonyl BCP in SMI ligand are decreased during
the coordination with CuI, indicating that the π-character of the carbonyl bonds is decreased
while oxygen interacts with copper in O1-CuI/SMI and O2-CuI/SMI complexes.
Another QTAIM reliable indicator for quantifying covalent character of interatomic
interactions is the electron delocalization index (DI), which is the average number of electrons
delocalized (shared) between two bonded atoms. As reported in table 6, DI calculated values on
C=O and C-N BCPs have decreased via complexation. Strictly speaking, the coordination
between copper and oxygen or nitrogen in N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI complexes
reduce the bond strength of C=O and C−N bonds that completely confirm the FT-IR
spectroscopic observations.
In order to assess the solvent effect in the synthesis of CuI/SMI nanocatalyst, we focused
on comparative evaluation of bond-orders of Cu-N (pyridine nitrogen), Cu-O1 and Cu-O2
(carbonyl oxygen) in the presence of ethanol and DMF as solvents. It is noteworthy to mention
that better immobilization and smaller size of CuI NPs on SMI for the catalyst prepared in DMF
solution in comparison with ethanol were practically observed.
Comparison of the calculated bond orders of Cu-N, Cu-O1 and Cu-O2 chemical bonds in
DMF and ethanol solution phases confirmed that Cu-N bond in N-CuI/SMI complex is stronger
than Cu-O bond in O1-CuI/SMI and O2-CuI/SMI complexes. Moreover, Cu-N bond order in
N-CuI/SMI complex has higher calculated values in DMF solution in comparison to those
calculated in ethanol and so corroborates with the observed practical preference of DMF as
solvent rather than ethanol in the synthesis of catalyst.
4. Conclusion
We prepared and characterized a polymer-supported CuI nanocatalyst and investigated its
catalytic activity. The catalyst exhibited high activity and afforded a diverse range of products in
good to excellent yields. Furthermore, the catalyst which is stable in the reaction conditions can
14

be used with no pre-activation and can also be recycled for at least five consecutive cycles
without an appreciable loss of activity.
We assessed also computationally the nature of interactions between copper ions with
modified SMI ligand in several CuI/SMI complex models. In this respect, the mathematical
properties of electron density functions have been calculated and analyzed via DFT and QTAIM
in the gas and solution phases. Based on stringent analysis of calculated values for QTAIM
electronic indicators, we have demonstrated that interaction of copper cation with pyridine
nitrogen in N-CuI/SMI is stronger than with oxygen of carbonyl groups in O1-CuI/SMI and
O2-CuI/SMI models and subsequently predicted that CuI NPs immobilize mainly on nitrogen site
of polymer-supported catalysts. Furthermore, our calculated results are in reliable agreement
with IR spectroscopic data and solvent effect observations.
Finally, our comparative computational and experimental analysis on CuI/SMI catalyst
modified with 2-aminopyridine and 4-aminopyridine reveals the fact that 2-aminopyridine
modified polymer support can be considered as an effective species with more catalytic activity
and copper content value and making stronger metal-ligand interactions.
Acknowledgments
The authors gratefully acknowledge the financial support received from the research council of
Alzahra University. We are also thankful to Iran National Science Foundation (INSF) for partial
financial assistance.
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Figure 1. Preparation of CuI/SMI catalyst.
19

Figure 2. FT-IR spectra of (a) 2-aminopyridine, (b) SMA, (c) SMI, and (d) CuI/SMI.
20

Figure 3. 1H NMR spectrum of SMI in DMSO-d6.
21

Figure 4. SEM image of CuI/SMI prepared in a) DMF, b) EtOH solvent.
22

Figure 5. EDAX data of Cu/SMI.
23

Figure 6. The recyclability of the CuI/SMI in the preparation of 7a.
24

Figure 7. Synthesis of 1,4-disubstituted-1H-1,2,3-triazoles.
25

Figure 8. SEM image of CuI nanoparticles after five consecutive cycles.
26

CH2 CH2
O
CH2 CH2
CH2 CH2
O
O
O
O
O
N
N
N
CuI
CuI
CuI
N
N
N
N-CuI/SMI complex
O1-CuI/SMI complex
O2-CuI/SMI complex
Figure 9. A simple model for three coordinating modes in polymer-supported Cu(I) catalyst.
27

SMI ligand
N-CuI-SMI complex
28

O1-CuI-SMI complex
O2-CuI-SMI complex
Figure 10. Atomic numbering and optimized structures obtained at 6-31G* level of theory for
SMI, N-CuI/SMI, O1-CuI/SMI and O2-CuI/SMI complexes. Note that M06/6-311+G**
calculated bond order values of key bonds are also presented.
29