Copper-azide nanoparticle: a ‘catalyst-cum-reagent’ for the designing of 5-alkynyl 1,4-disubstituted triazoles

A single pot, wet chemical route has been applied for the synthesis of polymer supported copper azide, CuN3, nanoparticles (CANP). The hybrid system was used as ‘catalyst-cum-reagent’ for the azide-alkyne cyclo-addition reaction to construct triazole molecules using substituted benzyl bromide and terminal alkyne. The electron donating group containing terminal alkyne produced 5-alkynyl 1,4-disubstituded triazole product whereas for alkyne molecule with terminal electron withdrawing group facilitate the formation of 1,4-disubstituted triazole molecule.


Scientific RepoRtS
| (2020) 10:16720 | https://doi.org/10.1038/s41598-020-74018-8 www.nature.com/scientificreports/ catalyst 30 and photonic effect of carbon nitride on the catalytically active Cu center 31 for 1,2,3-triazole formation have also been reported recently by our group. In this current communication, we have describe a step-wise route for the synthesis of copper azide nanoparticles (CANP) and applied this material as 'catalyst cum reagent' where Cu(I) was performed as a catalytic role and azide counterpart acted as nucleophilic role for azide-alkyne cyclo-addition reaction to construct triazole molecules. experimental Synthesis of copper azide nanoparticles (CANP). In a typical experiment, 0.093 g of aniline (10 −2 mol/ dm 3 ) was diluted in 10 mL methanol in a conical flask. To this solution, 10 mL of aqueous CuSO 4 , 5H 2 O solution (10 −2 mol/dm 3 ) was added slowly under continuous stirring conditions. During the addition, the solution turned to green, while at the end, a green coloured precipitation was formed at the bottom of the conical flask. To the above precipitation, 5 mL aqueous solution of sodium azide (0.065 g) was added drop-wise and allowed to stir for another 2 h. A change in colour from light green to dark green was noticed. Little amount of the precipitated material was collected from the bottom of the conical flask and pipetted onto lacey, carbon-coated, nickel mesh grids for transmission electron microscopy (TEM) study, before and after the addition of sodium azide. The remaining portion of the compound was dried under vacuum at 60 °C and used as a catalyst for the title reaction.

Result and discussion
Mechanism for the formation of copper azide nanoparticles (CANP). Metal salt of gold, silver and palladium mediated synthesis of polyaniline, from aniline monomer, involve the release of electrons during the reaction, where the metal salts act as an oxidizing agents. The released electrons reduce the metal ions with the formation of the corresponding nanoparticles and the oxidation of aniline forms polyaniline, act as a stabilizer for the particles [32][33][34] . In the current experiment, during the reaction between cupric sulphate and aniline evidenced the formation of Cu(I)-polyaniline, due to the partial reduction of Cu(II) 27,28 . Polyaniline have several amine and imine moieties which can act as a macro ligand 35 , that coordinate with the Cu(I) species. The Cu(I)- www.nature.com/scientificreports/ polyaniline subsequently forms polyaniline stabilized CuN 3 nanoparticles by the addition of sodium azide. Figure 1 (A and B) display the TEM images of the organic-inorganic composite system before and after the addition of sodium azide, respectively. The selected area electron diffraction (SAED) image ( Fig. 1, inset) indicates highly crystalline nature of CuN 3 -polyaniline system. Figure 1, insets, show the optical images of Cu(I)-polyaniline and CuN 3 -polyaniline hybrid system. The TEM image (Fig. 1B) shows the formation of copper-azide nanoparticles (dark spots) with the size distribution ranging from 5 to 12 nm.
We also have performed the following experiments to find out the exclusivity of CANP for the title reaction. A Cu(I)-supported polyaniline (Cu-PANI), as reported earlier 27 , has been prepared and applied in the present reaction system under optimized condition, where, we observed only alkyne 2b and 2c yielded 5-alkynyl 1,4-disubstituded triazoles products (3) in minute amount, 4% and 7%, respectively, and 1,4-disubstituted triazole molecule (3′) formed as the major product ( Table 3).
In this communication, a facile route was adopted for the preparation of polyaniline stabilized copper-azide nanoparticle. The copper azide nanoparticle was performed as 'catalyst-cum-reagent' for synthesizing the triazole molecules through azide-alkyne cyclo-addition reaction using substituted benzyl bromide and terminal alkyne molecules, where copper was performed as a role of catalyst for the cyclo-addition reaction and azide was the source of triazole unit. The alkyne with electron donating group was produced 5-alkynyl 1,4-disubstituded triazoles as the sole product, whereas, alkyne molecule with terminal electron withdrawing group facilitate the formation of 1,4-disubstituted triazole molecule. The recovered catalyst (without azide counterpart) showed the catalytic performance for the reaction between sugar azide and terminal alkyne with the formation of triazole glycosides.

Methods
General procedure for the click reaction. In a 25 mL round bottom flask, substituted benzyl bromide 1 (1.0 equivalent), terminal alkyne 2 (2.0 equivalent or 1.0 equivalent for EWG attached alkyne), were mixed in 4 mL of solvent (methanol). To this reaction mixture, CANP (200 mg, 1.0 equivalent) and Et 3 N (240 µL, 2.0 equivalent or 140 µL, 1.0 equivalent for EWG attached alkyne) was added and allowed to stir for 3 h. The reaction mixture was monitored using a thin layer chromatography technique. After complete disappearance of starting materials, a previously reported technique 23 was followed to purify the the triazole products.
General procedure for click reaction with recovered catalyst. In a 25 mL round bottom flask, sugar azide 5 (1 equivalent), terminal alkyne 2 (1 equivalent), were charged in 4 mL methanol. To this reaction mixture the dried recovered catalyst (10 mg) and Et 3 N (1 equivalent) was added and allowed to stir for 3 h. The reaction mixture was monitored using thin layer chromatography technique. After completion, the reaction mixture was filtered and dried under residue pressure and followed the similar procedure as above.
Single crystal analysis. Computing details. Data collection: APEX2 2014-11; cell refinement: SAINT v8.38A; data reduction: SAINT v8.38A; program used to solve structure: SHELXT 2014/5; program used to refine structure: SHELXL 2018/3; molecular graphics: Olex2; software used to prepare material for publication: Olex2, PLATON.  www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.