1 Introduction

Applications of copper in the forms of Cu(I) and Cu(II) as efficient catalysts in organic transformations are well-studied and realized [1,2,3,4,5,6]. Based on the significant surface, size and quantum effects, nowadays applying nanoparticles (NPs), with the known unique properties are more preferred in comparison with their ordinary counter parts [7,8,9,10]. Copper NPs can be prepared via various methods such as thermal decomposition of precursors, reduction from a solution by mild reducing agents and electrochemical methods. It should be noticed that other strategies for their preparation are rather miscellaneous such as photoreduction, MWI-assisted syntheses, micro emulsion techniques and laser ablation [11,12,13,14,15,16]. Among these synthetic methods, using ultrasonic irradiation has recently attracted much attention, being frequently applied in the preparation of nanomaterials [17,18,19]. Since, ultrasonic irradiation is environmentally source of energy in comparison with the conventional heating, thus, nowadays, it is frequently being used in the homogenous dispersion of NPs in organic polymers [20]. In the recent decade, different polymers have attracted much interest for being employed as useful supports for immobilization of various metals. Particularly, immobilized copper NPs [21, 22] have stirred up the interest of synthetic organic chemists for being used as heterogeneous nanocatalysts due to the well-known superiority over homogeneous catalysts [23,24,25].

Among the polymeric supports, hydrogels are polymers capable of retentive a large amount of water when they swell in an aqueous media. One of the most important and commonly used synthetic hydrogels is poly(vinyl alcohol) (PVA) due to its small trailing hydroxyl group present in PVA, its crystalline structure [26] as well as capable of forming intra- and inter-chain hydrogen bonds when dissolved in water [27,28,29,30,31,32]. Insoluble PVA hydrogel by repeated freezing and thawing can be obtained due to the generation of inter-chain hydrogen bonding and the presence of crystalline domains.Worthy to mention that PVA has a higher mechanical strength in comparison with other hydrogels. This is particularly sound after it has been processed to increase its crystallinity [33, 34].

PVA is one of the most imperative vinyl linear polymers [35] with wide applications such as papermaking, textiles and a variety of coatings and also is supplied as beads or as solutions in water [36]. The excellent physical properties and chemical resistance have made PVA a resin of choice for many industrial applications especially in production of polyvinyl butyral fibers and paper sizing [37, 38]. PVA is known as atactic material but also shows crystallinity since the hydroxyl functional groups are small enough being fit into the lattice [39]. Moreover, PVA is one of the famous membrane materials due to its ability for good film-forming and toughness. Nevertheless, when PVA smeared as a stable material for its high hydrophilicity, it commonly has to be transformed into an insoluble material with good mechanical properties [37, 38].

There are two common routes for preparation cross-linked PVA: (a) the grafting of hydrophobic monomers to PVA [28,29,30,31,32] and (b) functionalization of hydroxyl groups in PVA [37, 38]. The properties of modified polymer are dependent on the choice of crosslinking or grafting [35] process. Grafting has been widely employed for modification of PVOH properties [39]. For instance, grafting with methacrylic acid or acrylamide [40,41,42] improves the mechanical properties of sized sample and increases their desirability. On the other hand, grafting with acrylic acid (AA) [43, 44] or methyl methacrylate [45] reformes the surface possessions and well-adjusted the hydrophilicity of PVOH hydrogel involving methyl-ethyl ketone (MEK) peroxide. Mechanisms of grafting of AA onto polyvinyl alcohol have been clearly disclosed by using a Ce(1V)-glucose redox system [46,47,48,49,50,51,52,53]. It is important to mention that chemical initiation methodology allows surface modification of the matrix polymer. Since polyvinyl alcohol-grafted-acrylic acid (PVA-g-AA) contains acrylate side groups, it may be able to substitute straight groups as the cross-linker.

It is worthwhile to mention that in many reports, complexation of Cu NPs and poly(acrylic acid) (PAA) have been assessed [54,55,56]. PVA based Cu NPs has been also synthesized and applied as a NPs stabilizer [18, 57, 58]. Nevertheless, their applications as support for immobilization of metal NPs have been largely overlooked [59].

In 2015 Hajipour and co-workers claimed the synthesis of PVA/Cu(I) nanocomposite [60]. However, the literature survey showed no reports on the preparation of PVA-g-AA/Cu(I) NPs. PAA is a weak polyelectrolyte and its ionization degree is pH sensitive. As the pH value elevated, the ionization of PAA and AA will increase. Thus, due to this fact that COO– shows stronger interaction with copper ions than COOH, in pH high up to 6, COO– functional group will competite to interact with copper ions [61].

1,2,3-Triazoles are important class of heterocyclic compounds with wide range of applications. Some of them are pharmaceutical agents, agrochemicals, and photographic materials [62]. One of the most general method for the synthesis of 1,2,3-triazole derivatives, is the well-known 1,3-dipolar Huisgen cycloaddition which involves the Cu catalyzed cycloaddition reaction of azides with alkynes (CuAAC) via click reaction [63, 64]. Click chemistry demonstrates a most rapidly growing organic reaction with a wide range of application in synthesis of 5-membered heterocyclic compounds containing nitrogen atom, that proceed under room temperature giving high yields and showing high selectivity [65,66,67,68,69]. CuAAC click reaction is a powerful tool for preparation of regioselective 1,4-disubstituted 1,2,3-triazoles that are covalently linking functionalized molecules [70,71,72,73]. To control the production of 1,4- or 1,5-disubstituted regioselective 1,2,3-triazoles, several efforts on the catalytic reaction pathways have been described [74,75,76,77,78] (while this manuscript was in preparation, a microwave-assisted click chemistry synthesis of 1,4-disubstituted 1,2,3-triazoles via a three component reaction was reported: see [79]).

Due to our interests in heterogeneous catalysis, applied in a variety of organic transformations [80,81,82,83,84,85,86,87,88], recent successful attempts for the synthesis of 1,2,3-triazoles, via click reaction under heterogeneous catalysis [89,90,91,92,93,94] and using quantum chemistry approaches for calculation of the strength of metal–ligand interactions [1,2,3,4,5,6, 95,96,97,98,99,100], In this paper, we wish to study the preparation of copper nanocatalyst immobilized onto PVA–AA matrix from the experimental and computational viewpoints, and then assessed its application as efficient catalyst in the 1,4-disubstituted regioselective synthesis of 1,2,3-triazoles via click reaction.

2 Experimental

2.1 Materials and Solvents

All starting chemicals and reagents were commercially available and were utilized without further purification. The reagents and solvents were purchased from Merck, Fluka or Aldrich with high-grade quality and were used as received (except PVA, which was obtained from Sigma-Aldrich). The general formula of PVA, used in this study, is partially hydrolyzed.

2.2 Techniques

Melting points were determined with an X6 digital microscope melting point apparatus. FT-IR spectra were recorded by film without KBr with a Bruker FT-IR spectrophotometer. Thermal gravimetric analysis (TGA) data were gained with a Setaram Labsys TG (STA) in the temperature range 0–800 °C and heating rate was 20 °C/min which from 10 to 600 was under nitrogen atmosphere and then in the temperature 600 up to 800 °C under atmosphere switch to air. Scanning electron micrographs were recorded using a Lecia Cambridge S 360 scanning electron microscopy (SEM) instrument. Energy-dispersive X-ray spectroscopy (EDX; Genesis, with an SUTW detector equipped with SEM equipment) was used in order to confirm the presence of Cu. Copper content was measured using inductively coupled plasma-optical emission spectroscopy (ICP–OES, Germany- SPECTRO, Model ICP ARCOS) analysis with a Varian Vistapro analyzer. UV–Vis -Perkin Elmer lambda 35.

2.3 Preparation of PVA-g-AA/Cu(I)

A suspension of CuI (3.8 g) and acrylic acid (10 ml) in water/ammonia (20 ml) in pH, 9 was homogenized by continuous stirring at room temperature for 24 h, then solution was added into PVA (1 g) and finally, water (10 ml) was added for mixing improvement at 60 °C. After swelling and solving of polymer a homogenious mixture was created. In final stage initiators consisting of MEK-peroxide monomers (0.1 g) were added to homogenous mixture. The grafting reaction was carried out at 90 °C for 10 h under nitrogen atmospheres at sonicating bath.

During the course of the reaction, the color solution from light blue turned to dark blue. The reaction mixture was then cooled to room temperature, water was evaporated under reduced pressure and dried in vacuum oven at 60 °C. The obtained solid film was cleanly isolated after evaporation in green color. This film was washed with MEK to remove residual of ammonia. Then the film was dried in the vacuumed oven at 30 °C.

2.4 Synthesis of 1,4-Disubstituted 1,2,3-Triazoles: General Procedure

In a round-bottom flask, an appropriate α-haloketone (1 mmol) or alkyl halide (1 mmol), alkyne (1 mmol) and sodium azide (1.1 mmol) were mixed in water (10 ml) (Scheme 1). To this mixture, freshly prepared PVA-g-AA/Cu(I) NPs (0.03 g) was added. The reaction mixture was magnetically stirred under reflux conditions for required time, as indicated in Table 1. The progress of the reaction was monitored by TLC (using n-hexane: ethyl acetate; 7:3 as eluent). Upon the completion of the reaction, the resin like mixture was filtered under reduced pressure and washed with hot ethanol, then acetone, dried under reduced pressure at 70 °C and stored for re-usage in another fresh reaction. The filtrate was evaporated to dryness under reduced pressure to afford the crude material. The residue was crystallized from a mixture of EtOH/H2O to afford the entitled products (5a–f). All products (5a–f) were known and their physical data were compared with those of authentic compounds and found to be identical [1]. It is worthwhile to mention that easily separated catalyst from the reaction mixture was reused at least three times in three consecutive runs without appreciable loss of activity.

Scheme 1
scheme 1

Regioselective synthesis of 1,4-disubstite 1,2,3-triazoles catalyzed by PVA-g-AA/Cu(I) NPs via click reaction

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Table 1 1,4-Disubstited 1,2,3-triazoles obtained from catalyzed by PVA-g-AA/Cu(I) NPs via click reaction
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3 Results and Discussion

3.1 Preparation and Characterization of Catalyst

After cooling the solution, the product was emerged as solid film. Solubility of the prepared film sample in water, toluene, ethanol, MEK was tested. In water, ethanol and toluene, the sample was insoluble and stable without any changing in color. Clean polymer was gained by multiple MEK washing. After this processing, the obtained solid film was dried under vacuum and analyzed.

3.2 Determination of the Copper Content in PVA-g-AA/Cu(I) NPs

In a screw-capped vessel, a sample of PVA-g-AA/Cu(I) NPs (93 mg) was extracted with concentrated HCl and HNO3 (1:1, 10 ml) for digestion of the metal complex. The mixture was transferred into a volumetric flask (100 ml), diluted and was analyzed by ICP–OES. The copper concentration was determined from the atomic emission (324.754 nm) by reference to a linear (R = 0.99) calibration curve of (1–4 ppm) of CuI prepared in a manner identical to the sample preparation. The copper content was determined to be 1.23% w/w (Ref. [100,101,102,103,104] for ICP–OES < 2). The same procedure was used to measure the amount of leaching from the supported catalyst after five consecutive runs.

3.3 Morphologic Characterization

The size of the NPs and the degree of dispersion in the matrix were detected by SEM microscopy (Hitachi S5500). The Film was analyzed by SEM, EDAX, FT-IR, and TGA. FT-IR spectrum (Fig. 1) of synthesized Cu NPs catalyst exhibited major peaks at around 1500 cm−1 indicating interaction of PVA matrix with Cu NPs or assigned to the asymmetric vibration absorption of the carboxylate (COO−) group. A complexation of copper ions with carboxyl containing polymers has been reported [105]. Figure 1 shown, grafting of AA/Cu(I) on PVA chain changed IR absobtion bands at 1000–2000 cm−1. A series of changes in shift bands positions was appeared when FT-IR spectrum of pure PVA with that of immobilized copper NPs catalyst film (the differences are within chosen resolution; Fig. 1). The hydroxyl group band of resulting catalyst is stronger than PVA; this showed that the hydroxyl units of PVA might undergo different H-bond interactions or coordination.

Fig. 1
figure 1

FT-IR spectrum of PVA compared with prepared PVA-g-AA/Cu(I) NPs

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The better immobilization and smaller size of CuI NPs on PVA was clearly observed for the obtained film. The scanning electron micrographs of the PVA-g-AA/Cu(I) NPs catalyst (Fig. 2a) clearly demonstrated that the CuI NPs were homogeneously immobilized on the PVA surface. The SEM micrograph showed that the mean value of cupper particle size is under 100 nm. The dispersion and distribution of particles were satisfactory. Energy dispersive spectroscopy analysis of X-rays (EDAX) data for the PVA-g-AA/Cu(I) NPs catalyst are given in (Fig. 2b–d). The EDAX displays the attachment of copper onto the surface of the PVA matrix. Moreover, from these figures, it can be observed that PVA beads and CuI being well distributed throughout the matrix.

Fig. 2
figure 2

SEM image of PVA-g-AA/Cu(I) NPs (a), dispersion of catalyst (b, c) and EDAX data of PVA-g-AA/Cu(I) NPs (d)

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For estimation of the Cu content, the PVA-g-AA/Cu(I) catalyst was reacted with concentrated hydrochloric nitric acids to digest the copper species, and the resulting extract was analyzed by inductively coupled plasma (ICP–OES) analysis. The Cu content was determined to be 1.23% w/w (see Sect. 2 for details).

The thermal stability of pure PVA and PVA-g-AA/Cu(I) NPs catalyst was initially investigated using TGA. Thermal decomposition behavior of PVA and PVA-g-AA/CuNPs is demonstrated in Fig. 3. Thermal degradation of the polymers were changed slightly. Thermal decomposition confirmed the incorporation of copper iodide groups in PVA-g-AA/Cu(I) NPs matrix. As shown in Fig. 3. the first of mass loss accomplished at 100 °C, which can be due to the loss of a small quantity of absorbed water. At the second stage of mass loss, the loss of mass region at 400 °C which was correlated with further decomposition of polyene residues to yield the carbon and hydrocarbons. It is clear that the modified polymers (PVA-g-Aa/Cu(I) NPs) loose 10% weight around 300 °C while at 800 °C only 5% of PVA-g-AA/Cu(I) undecomposed is left but PVA loose 30% weight around 300 °C while at 800 °C and only 2% undecomposed. Thermal stability of the graft polymer appears to be decreased slightly. This enhancement of the formation was related to the high heat resistance applied by the Cu(I). In addition, it was resulted that the thermal stability of the prepared catalyst was better with dispersion of modified Cu(I) in the polymer matrix.

Fig. 3
figure 3

TGA curves of PVA and PVA-g-AA/Cu(I) NPs

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A spectrum acquired in the 550–700 nm region illustrate a small shoulder peak starting around 580 nm (Fig. 4), which changed with nanocopper band absorbance reported in the literature [7, 106].

Fig. 4
figure 4

UV–Vis spectrum of PVA-g-AA/Cu(I) NPs

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3.4 Mechanism

A plausible mechanism of the reaction is depicted in Scheme 1. It is assumed that reaction proceeds via tacking advantage of the complexation behavior of AA and copper (Cu) ions. The adsorption mechanism is related to the ionization capability of AA in pH, 9. This solution is then added to PVA, an initiator consisting of MEK peroxide in which grafting occurs and an insoluble polymer–metal complex is produced (Scheme 2) [54,55,56, 61].

Scheme 2
scheme 2

Mechanism of synthesis PVA-g-AA/Cu(I) NPs

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3.5 Computational Section

Regarding our joint experimental and computational assessments on the synthesis, characterization and properties of heterogeneous catalysts, we have recently investigated metal–ligand interactions in various modified poly (styrene-co-maleic anhydride) metal nanocatalysts, metal-aminated KIT-5 silica surfaces and metal-functionalized halloysite nanotubes porous supports [1,2,3,4,5,6, 97, 98] via quantum chemistry approaches. Armed with these experiences, in this work, we have concentrated on presenting a quantitative description for immobilization behavior of copper NPs on AA grafted PVA support via density functional theory (DFT) computations [107, 108] and frontier molecular orbital (FMO) analysis [109].

In the first step, we have designed an effective model for the acrylic acid functionalized PVA copper complex via two different coordination modes (as was illustrated in Fig. 5). In these two coordination modes, Cu coordinates to hydroxyl functional group of PVA (denoted as PVA–Cu complex) and in the second coordination mode, it coordinates to carbonyl functional group of acrylic acid moiety (denoted as AA–Cu complex). More importantly, we have reported in our previous computational investigations that this model has a credible comprise between accuracy and time saving efficiency of computational procedure [1,2,3,4,5,6, 97, 98].

Fig. 5
figure 5

A simple model for the coordination modes of copper with AA grafted PVA support surface

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In the next step, we determined the optimized structure of PVA–Cu and AA–Cu complexes at M08-HX/6-311+G** level of theory with no symmetry constrains in geometry optimization process. The harmonic frequency analysis was performed to confirm that the optimized structures correspond to minima. It should be noted that M08-HX functional has been recognized as a modern hybrid meta-generalized gradient approximation exchange–correlation functional combined with Hartree–Fock exchange contribution [110].

In the case of copper and iodine atoms, LANL2DZ effective core potentials were used in conjunction with the accompanying basis sets to describe the valence electron density [111, 112]. All DFT computations have been performed using GAMESS suite of programs [113].

In Fig. 6, we have displayed the optimized geometries of PVA–Cu and AA–Cu complexes calculated at M08-HX/6-311+G** level of theory with the atomic numbering. The M08-HX/6-311+G** calculated atomic charges on copper and oxygen atoms in PVA–Cu and AA–Cu complexes have been also presented in Fig. 6. Furthermore, the calculated values of bond order and bond length of some key bonds in PVA–Cu and AA–Cu complexes have been listed in Table 2. The comparative analysis of reported results in Table 2 show that the interaction of copper NPs with carbonyl oxygen in AA–Cu complex (with the larger bond order and smaller bond length values) is slightly stronger than the interaction of Cu with hydroxyl oxygen in PVA–Cu complex. This feature is in confirmation with more positive calculated charge of Cu atom and more negative calculated charge of oxygen atom in PVA–Cu complex in comparison with those calculated in AA–Cu complex.

Fig. 6
figure 6

The optimized structure of PVA–Cu and AA–Cu complexes together with the calculated atomic charges on copper and oxygen atoms obtained at M08-HX/6-311+G** level of theory

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Table 2 The calculated values of some selected bond orders (bond lengths) in PVA–Cu and AA–Cu complexes obtained at M08-HX/6-311+G** level of theory. Note that numbering of atoms is in accordance with Fig. 6
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In the other hand, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PVA–Cu and AA–Cu complexes were analyzed at M08-HX/6-311+G** level of theory (as were displayed in Fig. 7). Regarding the organometallic FMO theory [109], metal atom primarily coordinates with carbonyl oxygen and hydroxyl oxygen in PVA–Cu and AA–Cu complexes, respectively, through interactions between filled non-bonding orbitals (HOMO) on O atoms and an empty dσ orbital (LUMO) on copper atoms. Based on our electronic and structural analysis of PVA–Cu and AA–Cu complex models, it can be predicted that copper NPs will immobilized more effectively to the carbonyl oxygen than the hydroxyl oxygen of functionalized polymeric support.

Fig. 7
figure 7

FMOs of PVA–Cu and AA–Cu complexes obtained at M08-HX/6-311+G** level of theory

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4 Conclusions

The objective of this study is the efficient preparation of PVA-g-AA/Cu(I) NPs film with different Cu(I) NPs content by ultrasonic irradiation process. We presented evidence that Cu(I) NPs is dispersed in the PVA matrix. The prepared Cu(I) NPs are of mid-size of under 100 nm which could be useful for practical usages. The resulting film preserves the catalyst activity which is represented by its effective Cu(I) catalysis of 1,3-dipolar Huisgen cycloaddition so-called click reaction. The produced Cu(I) NPs catalyst was characterized by FT-IR, SEM–EDX, TGA, ICP–OES and UV–Vis methods. All the produced systems were stable long-time and demonstrated no sign of aggregation or degradation. Moreover, our computational results on metal–ligand interactions in two PVA–Cu and AA–Cu complex models demonstrated that copper NPs tend to bind more effectively with carbonyl oxygen than hydroxyl oxygen on functionalized polymeric support.