## 2 Cu, Cu2O, CuO, and CuX Nanoparticles and Nanocomposites
### Polymer-Stabilized Soluble Systems and Quasi-Homogeneous CuNPs  
Leaf extracts from _Otostegia persica_ and _Ginkgo biloba_ were recently examined by [PERSON] et al. as green reducing agents in the preparation of unsupported CuNPs [147; 148]. These systems showed reasonable catalytic activity and possibility of reuse in three-component CuAAC reactions. Similarly to other quasi-homogeneous nanostructured materials reported in the last two years, i.e., lanthanum-loaded CuO nanoparticles [149], porous copper [99], copper salts reduced in ethylene glycol [101], and Cu\({}_{2}\)O nanocubes [96], these systems do not appear however to involve any additional polymeric component. Therefore, according to the scope of this review they will not be discussed in further detail herein.  
On the contrary, [PERSON] et al. prepared polymer-stabilized Cu\({}_{2}\)O nanoparticles, by dihydrogen reduction of copper acetate in glycerol in the presence of PVP (Scheme 12a) [95]. As confirmed by TEM, XRD and IR analyses, the use of glycerol as the solvent, H\({}_{2}\) as the reducing agent, and PVP as the stabilizer turned out to be all important for obtaining oxidation-stable Cu\({}_{2}\)O clusters devoid of Cu(0) or Cu(II) phases.  
The synthesized nanoparticles **23** proved moderately active in two- and three-components CuAAC reactions (Schemes 1 and 2a). The catalytic runs were carried out with 2.5 mol % Cu\({}_{2}\)O at 100 \({}^{\circ}\)C for 2 h to give an array of different triazoles in high yield (91%-98%) and with complete 1,4-regioselectivity. In this regard it is worth noting that, even if the thermal reaction proceeded appreciably in neat glycerol at 100 \({}^{\circ}\)C (40% yield in 2 h), as expected both regioisomeric products **1** and **2** (see Scheme 1) formed in substantial amounts (4:1 ratio) in the absence of the copper catalyst. Combination with Ullmann chemistry was also feasible and provided a tandem process for preparing arylaminoethyl-functionalized triazoles (Scheme 13). The separation of the products in standard CuAAC runs was effected by extraction with CH\({}_{2}\)Cl\({}_{2}\) or pentane of the catalyst-containing glycerol layer. Afterwards, the glycerol phase was heated under vacuum for removing the volatiles and employed in a new experiment. With this procedure ten cycles of the reaction between benzyl azide and phenylacetylene were carried out, obtaining the corresponding triazole product in nearly constant yield (94%-97%).  
Polyoxometalates (POMs) and heteropolyacids (HPAs) are ionic compounds containing cations and large polyanion clusters made, e.g., by the condensation of MOx polyhedra [150; 151]. Besides having catalytic activity on their own, these materials find use as supports for other catalytic systems and for the stabilization of metal nanoparticles. An example in this latter sense was reported recently by [PERSON] et al. (Scheme 12b), who prepared Cu\({}_{2}\)O/CuO nanoparticles on a molybdenum POM by treating a suspension of (NH\({}_{4}\))\({}_{12}\)(Mo\({}_{36}\)(NO)\({}_{4}\)O\({}_{108}\)(H\({}_{2}\)O)\({}_{16}\)) in AcOEt with Cu(NO\({}_{3}\))\({}_{2}\)[152]. With this procedure, the occurrence of the Cu(I) compound in the resulting material **24** was supposed to derive from direct reduction of the Cu(II) salt at the POM surface. Three-components CuAAC runs of activate halides (Scheme 2a) were typically performed in water, where the resulting nanocomposite was soluble. In the event, this permitted the recovery of the catalytic system in the aqueous phase, by filtering off the insoluble triazole product at the end of the experiment. Despite the rather high catalyst loading and reaction temperature adopted in these runs (5 mol % and 70 \({}^{\circ}\)C, respectively), recycling of the POM solution led, however, to rapid degradation of catalytic performance already after a few reaction cycles (approximate 88% \(\rightarrow\) 44% yield in three runs).