## 2 Cu, Cu2O, CuO, and CuX Nanoparticles and Nanocomposites
### Insoluble Synthetic Organic Polymers  
CuAAC catalytic systems on organic polymers, **12** and **13** (Scheme 6), were obtained recently by depositing nanoparticles of cuprous halides on a poly(styrene-\(\omega\)-maleic anhydride) imide (SMI) or polyaniline (PANI). The former support material, studied by [PERSON] et al., was obtained by treating styrene-maleic anhydride copolymer with 4-aminopyridine under dehydrating conditions [123]. Subsequent refluxing with CuI nanoparticles, prepared by DMF precipitation of the cuprous salt from a MeCN solution, led **12**, that was shown by SEM to contain an uniform dispersion of copper halide crystals within the fiber-like polymeric matrix. In a similar approach, **13** was obtained by [PERSON] et al. by refluxing PANI and CuI in EtOH, under an inert atmosphere [124]. The catalytic activity of the materials was evaluated in three-component CuAAC reactions of activated halides or methyl iodide (Scheme 2a). The reactions were typically carried out in boiling water, with 1 mol % of **12** or, respectively, 5 mol % of **13**. Despite the potential solvolytic instability of the halide reactants under these conditions, fair to good yields were reported in both studies (71%-92% in 15-145 min). The scarce solubility of the respective support material in water, as well as in most organic solvents, allowed the recovery and reuse of the catalytic systems. Also from this point of view, the findings of the two investigations were quite similar, with some decrease of product yield (approximate 92% \(\rightarrow\) 83% and 92% \(\rightarrow\) 88%, respectively) and a moderate reduction of metal loading on the support (5% and 3%, respectively) after five consecutive runs. [PERSON] et al. investigated the use of electrospinningning as a mean for obtaining Cu\({}_{2}\)O nanoparticles embedded into membranes made of cross-linked poly(\(N\)-vinyl pyrrolone) (PVP) [125]. The preparation route (Scheme 7a) involved hydrazine reduction of a mixture of linear PVP and Cu(OAc)\({}_{2}\), followed by electrospinningning of the resulting colloidal solution on a target collector and final curing at 180 \({}^{\circ}\)C. With this procedure, initially formed Cu(0) clusters were probably oxidized to Cu\({}_{2}\)O and a material **14** was obtained that, contrarily to linear PVP, resulted insoluble in water. Interestingly, the curing stage did not cause dramatic variations of the morphology of the membrane, in particular for what it concerns the occurrence of cylindrical polymer fibrils supporting a uniform dispersion of metal oxide particles. The electrospun composite membrane **14** was effective in promoting the CuAAC addition of benzyl azide to phenylacetylene in 1,4-dioxane and in the presence of an equivalent amount of Et\({}_{3}\)N. However, a rather high metal loading (5 mol % Cu) was used in these experiments in order to obtain the triazole product in 87% yield after 2 h at 60 \({}^{\circ}\)C. Moreover, reuse of the material in three consecutive runs indicated a gradual loss of catalytic activity (87% \(\rightarrow\) 77% yield), accompanied by aggregation phenomena of the polymer fibers. The use of poly(vinyl alcohol) (PVA) for stabilizing Cu\({}_{2}\)O nanoparticles was explored by [PERSON] et al. [97]. In this case, the preparation of the catalytic system (Scheme 7b) involved the initial formation of a gel from PVA and CuSO\({}_{4}\) in alkaline NaOH solution, followed by reduction with sodium ascorbate, aging, filtration, and washing. Drying in air afforded a reddish product **15**, characterized by a broad UV band centered around 491 nm and an XRD pattern matching that of Cu\({}_{2}\)O. Interestingly, the choice of basic conditions (pH = 12) proved essential for the reduction to Cu(I) because the same step, carried out without the addition of NaOH, led to a material featuring the XRD diffractions of metallic copper. Screening of PVA-Cu\({}_{2}\)O in CuAAC involved reactions of preformed  
Figure 6: Preparation of CuI nanocrystals on SMI and PANI polymeric supports, **12** and **13**.