## 2 Cu, Cu2O, CuO, and CuX Nanoparticles and Nanocomposites
### Oxides and Other Inorganic Supports  
Although details of the preparation and structure are not entirely clear, [PERSON] et al. reported the use of CuI on bare neutral alumina as a cost-effective supported CuAAC catalyst [131]. The material was used in the reactions of activated halides (Scheme 2a) or a few preformed azides (Scheme 1), in water or DMF-water (4:1) with microwave (MW) or ultrasound (US) irradiation. The former conditions (MW) led to the fastest conversion rate and furnished good yields in 6-13 min, even in the case of triazole products with relatively complex structure (70%-98%). Reuse of the recovered catalyst in the course of eight reaction cycles displayed a progressive decrease of efficiency (approximate 98% \(\rightarrow\) 89% yield), accompanied by some reduction of the copper content of the material (4.2 wt % \(\rightarrow\) 3.8 wt %). KIT-5 mesoporous silica modified with aminopropyl groups was employed by [PERSON] et al. for supporting CuI nanoclusters [83]. In spite of the fact that the aminated support was simply stirred at r.t. in a solution of CuI in MeCN or DMF, XRD measurements identified the presence of crystalline CuI (Marshite structure, estimated size 12-40 nm) in the resulting materials. The usefulness of the supported catalyst in CuAAC was assessed in the three-component process with activated alkyl halides (Scheme 2a). Good yields were generally obtained in refluxing water, in some cases after very short reaction time (69%-95% in 15-150 min). In addition, the recovered material could be used in 6 consecutive cycles with just a minor decrease of product yield (approximate 96% \(\rightarrow\) 91%) and an unspecified small variation of the copper content. Reduction of Fehling solution with glucose in boiling water, in the presence of various insoluble materials (silica gel, hydroxyapatite, cellulose, basic alumina), was used by [PERSON] et al. for preparing supported Cu\({}_{2}\)O nanoparticles [132]. Amongst the systems examined, that based on silica gel showed maximum activity in the three-component reaction (Scheme 2a) between phenylacetylene, benzyl chloride, and sodium azide. The reaction was carried out with a relatively high catalyst loading (5 mol %), preferentially in water at r.t. Under these conditions very good yields (91%-97%) were obtained in reasonable time (2-8 h), also on switching to the use of less reactive alkyl halides (e.g., \(n\)-butyl chloride). Reuse of the catalyst resulted in only slight reduction of product yield in the course of 5 cycles (96% \(\rightarrow\) 92%). [PERSON] et al. employed a CuO/CeO\({}_{2}\) nanocomposite (approximate 5 wt % Cu) for performing three-components reactions with activated bromides (Scheme 2a) and Amberlite-supported azide [133]. The latter was selected with the intent of reducing the environmental impact of the process and to avoid handling toxic NaN\({}_{3}\), at least in the CuAAC step. Optimization of the conditions revealed that the rate of CuAAC was maximum in refluxing ethanol, where different triazole products were obtained in good yield within 45-120 min (88%-92%). Interestingly, the CuO-CeO\({}_{2}\) nanocomposite and the resin were recovered together by filtration and the latter was restored in the azide form by stirring with aqueous NaN\({}_{3}\). With this procedure, the attainment of nearly unchanged yields in the course of five reaction cycles (91% \(\rightarrow\) 88%) required a slight increase of the reaction time, from 65 min to 75 min. The same group prepared CuO/ZnO nanocomposites by co-precipitation of the metal nitrates with aqueous NaHCO\({}_{3}\)[134]. The use of the material in three-component CuAAC reactions of benzyl halides (Scheme 2a) provided best results in boiling water and with a high loading of the catalyst (89%-92% yield in 15-65 min with approximate 17 mol % Cu). Also in this case, reuse of the recovered solid led to an evident decrease of catalytic activity after just five consecutive reaction cycles (92% yield in 20 min \(\rightarrow\) 88% yield in 32 min). Very recently, [PERSON] et al.