Clay-supported copper(II) nitrate (claycop) has been used as an efficient catalyst for azide-alkyne cycloaddition reactions leading to 1,4-disubstituted 1,2,3-triazoles. The highly efficient claycop/hydrazine hydrate catalytic system affords triazoles in a few minutes (1-20 min) at room temperature, under mild and solvent-free conditions. High regioselectivity, excellent yields, ease of claycop synthesis and recyclability/reusability of the catalyst are considered as practical merits of the protocol. The 1,3-dipolar cycloaddition reaction between azide and alkyne leading to the formation of disubstituted 1,2,3-triazoles has attracted tremendous interest during the last few decades. Compounds containing the 1,2,3-triazole unit find various applications in medicinal chemistry, organic synthesis, biology and material sciences. [1][2][3][4] Some of the important properties that the 1,2,3-triazole unit possesses are high chemical stability, generally inert to severe hydrolytic, oxidizing and reducing conditions, even at high temperature, strong dipole moment (4.8-5.6 Debye), aromatic character and hydrogen bond accepting ability.[5]Various synthetic protocols have been developed for the synthesis of 1,2,3-triazole derivatives. Among these methods, the Cu-catalysed azide-alkyne cycloaddition (CuAAC) reaction, discovered independently by the groups of Meldal and Sharpless, is undoubtedly one of the most prominent strategies. [6,7] There have been many literature reports of using copper(I) source in regioselective azidealkyne cycloaddition. [8] The copper(I) species is either generated from copper(II) salts with reducing agents (e.g. sodium ascorbate) or added directly as cuprous salts, usually with stabilizing ligands.[9] Metallic copper or clusters are also employed, but these catalytic systems sometimes require oxidative agents. [10] The disadvantages of such systems are that the reaction kinetics is relatively slow and a significant amount of catalyst is required. There are also reports indicating the formation of diacetylenes and bistriazoles. [11] In order to overcome the drawbacks associated with homogeneous systems, heterogeneous catalysts have been developed. Along with many other advantages, recyclability and reusability are the key features that heterogeneous catalysts have for application in synthetic chemistry.[12] Heterogeneous catalyst systems with Cu catalysts have been successfully used in click reactions. Cu(I) catalysts immobilized on polymers, silica or zeolite, Cu(0) on charcoal, CuO nanostructures, etc., are some examples of heterogeneous systems used in azidealkyne cycloaddition.[13]To continue our study of 1,2,3-triazole synthesis, [14] in the work reported herein we developed a very efficient heterogeneous catalytic system for azide-alkyne cycloaddition reaction. Copper(II) nitrate immobilized on montmorillonite clay (claycop) was used as catalyst and for some substrates the reactions proceeded within 1-5 min affording almost quantitative yields of the desired 1,2,3-triazoles. The highlighting f...