## 2 Cu, Cu2O, CuO, and CuX Nanoparticles and Nanocomposites
### Carbon Supports  
Copper-in-charcoal (Cu/C) has been repeatedly employed as a cheap and relatively effective catalyst in CuAAC reactions [88; 89]. While the occurrence of CuO and Cu\({}_{2}\)O particles is generally acknowledged in Cu/C obtained by calcination [103; 104; 105], recently [PERSON] et al. [106] identified unsupported gerhardite (Cu\({}_{2}\)(OH)\({}_{3}\)NO\({}_{3}\)) as the likely catalyst precursor in materials prepared by the [PERSON]' low-temperature carbon impregnation method [107; 108]. Upon exposure to CuAAC conditions, the Cu(II) salt was found to transform into the actual active form, deemed to be a polymeric Cu(I) acetylide. The latter could be recovered by filtration, along with unreacted gerhardite, but no recycling of the material was reported. In this context it is worth noting also that [PERSON] and [PERSON] provided a detailed picture of commercial Cu/C catalyst under _in operando_ conditions [109]. The study, based on single molecule fluorescence microscopy for mapping CuAAC events, suggested that 90% of the charcoal particles in the sample were devoid of any active site. Moreover, for the remaining 10% ones the active sites were estimated to represent just a minute fraction (approximate 0.003%) of the total surface. These results might help to explain the scarce catalytic activity, reported in some investigations for Cu/C samples from commercial sources [106; 110]. An on-purpose prepared Pd-Cu bimetallic catalytic system on charcoal was studied by [PERSON] et al. for one-pot sequential Sonogashira-click and click-Heck reactions [111]. Unfortunately, an attempt to recycle the material in the former reaction evidenced essentially complete loss of its catalytic activity. [PERSON] et al. prepared ultra-small copper oxide nanoparticles on graphite by stirring a suspension of the powdered support in a solution of Cu(OAc)\({}_{2}\) in MeOH, kept under H\({}_{2}\)[112]. The catalysts, which consisted of CuO, Cu(OH)\({}_{2}\) and minor amounts of Cu\({}_{2}\)O, displayed fair activity in three-component CuAAC reactions of halides and styrene oxide (Scheme 2a,b). Although no recycling of the material was apparently attempted, filtration of the reaction mixture through a Nylon membrane permitted the prompt removal of the insoluble system and left only trace amounts of copper in the filtrate (6 ppb). Cu\({}_{2}\)O nanoparticles on graphene oxide (GO, **6**), were obtained by [PERSON] et al. through high-temperature treatment in an Ar stream of GO impregnated with Cu(OAc)\({}_{2}\)[110]. The material proved more active than commercial Cu/C and Cu\({}_{2}\)O in two-components CuAAC reactions (Scheme 1), but a relatively large catalyst loading (2 mol %) and extended reaction time (48 h at 40 \({}^{\circ}\)C) were still necessary for attaining good results. Moreover, recycling experiments in the benchmark reaction between benzyl azide and phenylacetylene in THF showed a marked decrease of the product yield already in the fourth run (99% \(\rightarrow\) 55%). Interestingly, **6** was effective also in the bulk cross-linking of a mixture of alkyne and azide trifunctional macromolecular reactants **7** and **8**, based on star poly(isobutylene) (PIB) chains (Scheme 3). Although in these cases trapping of the copper nanocomposite inside the resulting resin **9** prevents its reuse, it is worth mentioning that a kinetic study of click-polymerizations promoted by **6** was published also [113]. More recently, the same group reported highly dispersed copper clusters on carbon nano-tubes (CNT) and chemically-reduced graphene oxide (CRGO) [114]. These systems **10**, supposed to embed Cu(carbene)X or Cu-Cu fragments on the basis of literature precedents for Ag(I) analogs [115], were obtained by a stepwise procedure culminating (Scheme 4) in the decoration of the supports with triazole-imidazol units and formation of Cu(I)-carbene clusters.