## 2 Cu, Cu2O, CuO, and CuX Nanoparticles and Nanocomposites
### Magnetically Recoverable Systems  
The use of nano-sized super-paramagnetic supports offers the opportunity of catalyst recovery by the use of an external magnet. In principle, this technique can be more advantageous than filtration or centrifugation, even at scale, thus justifying the interest towards the approach in last decade [137; 138]. Hence, irrespective of the fact that many of these materials embed additional organic or inorganic components, recent advances with nano-structured copper catalysts that can benefit from such a specificity recovery mechanism are discusses separately in the present section. Magnetic Cu-Ni nano-alloy particles were obtained by [PERSON] et al. through reduction of nickel acetate and copper acetate with hydrazine [139]. The preparation (Scheme 11a) was carried out in the presence of PEG or poly(4-vinylphenol) (PVPh) as a polymer stabilizer, to give well dispersible organic-inorganic hybrid materials **19**. TEM and XPS analyses showed the presence of spherical alloy nanoparticles containing a Cu(I) surface layer and assembled in chain-like structures through the interaction of their magnetic dipoles. Benchmark runs (Scheme 1) between benzyl azide and phenylacetylene were carried out at r.t. in water or DMF, with an amount of nano-alloy corresponding to about 2 mol % of copper. Under these conditions, the PEG-containing catalyst **19** was found to be equally effective in both reaction media. By contrast, the PVPh-stabilized one proved significantly more active in DMF than in water, a difference attributed to the collapsed state of the less polar PVPh coating in the latter solvent. In any case, with the proper choice of reaction conditions either catalytic system provided acceptable results in CuAAC reactions involving other low-molecular weight reactants (56%-82% yield), a propargylated imidazolium ionic liquid, or azide-terminated substrates containing a tripeptide or a poly(methyl methacrylate) chain. Recovery of the catalyst by magnetic separation allowed the use of **19** (PEG) in three consecutive benchmark runs, albeit with reduction of the triazole yield (96% \(\rightarrow\) 87%). Because the XPS analysis of the recovered material showed some increase of Cu(I) peaks but not of those of Cu(II), this trend does not appear to be related to severe oxidation of the nano-alloy surface. On the contrary, it might be connected with particle aggregation phenomena, evidenced by TEM analysis of the spent catalyst. A tris-metallic system, entailing Cu(0) nanoparticles on glutamate-coated NiFe\({}_{2}\)O\({}_{4}\) (Scheme 11b), was prepared by [PERSON] et al. by sequentially loading the super-paramagnetic core nano-sheets with the amino acid ligand and Cu(OAc)\({}_{2}\), followed by reduction with N\({}_{2}\)H\({}_{4}\) and NaBH\({}_{4}\)[140]. The resulting material **20** was examined as a catalyst in multi-component CuAAC reactions in water, involving activated halides, epoxides, or aromatic boronic acids (Scheme 2a,b,d, respectively). In the presence of the composite catalyst (1 mol %-5 mol % Cu) the reaction proceeded in relatively short times at r.t. (1.5-8.5 h), allowing the preparation of a rather large library of triazole products in fair to excellent yields (50%-95%). Recovery with an external magnet and washing made it possible to reuse the supported catalyst in ten successive cycles, with moderate decrease of the product yield (95% \(\rightarrow\) 88%). CuAAC reactions promoted by Cu(0) nanoparticles on chitosan (CS)-magnetite have been described by [PERSON] et al. [141]. The nanocomposite was prepared by a three-step process (Scheme 11c), that began with the NaOH induced co-precipitation of CS, Fe(II), and Fe(III) from an AcOH solution. Characterization of the resulting material **21** by an array of techniques led to the conclusion that copper was present in the crystalline elemental form, as 10 nm spherical nanoparticles.