## 3 Discrete Cu(I/II) Complexes
### Magnetically Recoverable Systems  
Similarly to materials containing copper and copper oxide clusters on super-paramagnetic supports (Section 2.6), the last few years saw progresses also for what it concerns magnetically recoverable systems containing Cu(I/II) species on a polymeric organic layer (Scheme 21). Advances in this direction were reported, for instance, by [PERSON] et al. [186]. With an approach in some respects  
Figure 4: Cu(I/II) complexes with organic ligands covalently immobilized on insoluble inorganic supports. analogous to that already discussed for the GO-based material **32** (Section 3.1), \(N\)-vinylimidazole, \(N\)-ethyl-\(N^{\prime}\)-vinylimidazolium tetrafluoroborate, and a dicationic cross-linking agent were subjected to radical copolymerization. In this case the process was carried out in the presence of core-shell Fe\({}_{3}\)O\({}_{4}\)-silica gel nanoparticles bearing surface methacrylate groups, in order to endow the polymeric material (55 wt % by TGA) with a covalently embedded magnetic core. Charging of the resin with CuSO\({}_{4}\) in the final step led to the supported catalyst precursor **46**. The activity of the material was tested in the three-components CuAAC reaction of benzyl bromide, with sodium ascorbate as the reducing agent. Compromise conditions for minimizing both catalyst loading and reaction time were determined to be the use of 0.75 mol % of supported copper complex in water at 50 \({}^{\circ}\)C. With this choice, different chlorides or bromides and terminal alkynes, either aliphatic or aromatic (Scheme 2a), could be effectively coupled in 2-4 h (80%-97% yield). Filtration of the mixture was confirmed to halt completely the reaction progress and ICP-AES assay of the filtrate did not detect any significant concentration of copper. Still, a moderate reduction of turnover frequency was recorded in the course of 6 recycle runs of the benchmark reaction, which forced to increase the time from 150 min to 170 min in order to maintain the product yield in the range 90%-95%. As in other cases, the trend was related to mechanical losses of the supported system during the separation and washing steps. A similar approach was followed by [PERSON] et al. for the preparation of magnetically recoverable nanoparticles consisting of a Fe\({}_{3}\)O\({}_{4}\) core embed into silica gel and cross-linked poly(methyl acrylate) (PMA) shell layers [187]. Aminolysis of the ester groups of the latter with \(N\),\(N\)-dimethylethylenediamine allowed the installation of chelating units, exploited in the last step for adsorbing CuSO\({}_{4}\) from an aqueous solution. When tested in three-components CuAAC reactions (Scheme 2a), the resulting supported complex **47** afforded best results in water at 50 \({}^{\circ}\)C, at 0.3 mol % loading and in the presence of an excess of sodium ascorbate. Under these conditions, different triazole products were smoothly formed from the corresponding alkyne and alkyl chloride, bromide or tosylate (75%-97% yield in 1.5-4 h). While no ICP-OES detectable metal was found in the liquid phases from seven consecutive reaction runs, analysis of the magnetic material recovered in the last cycle indicated a slight reduction of the copper content with respect to the original system (0.54 vs. 0.57 mmol-g\({}^{-1}\)). In addition, some mass loss of the nanoparticles as a whole was recorded. As suggested, this fact may represent the likely explanation of the progressively longer reaction time required for attaining constant conversion in consecutive CuAAC runs. Still, it cannot be excluded that metal leaching might play a role in the problem, as seemingly supported by the fact that TOF values corrected for mass loss underwent also some decrease on passing from the first to the last reaction cycle (160 h\({}^{-1}\)\(\rightarrow\) 142 h\({}^{-1}\)). Coating of Fe\({}_{3}\)O\({}_{4}\)-silica gel core-shell magnetic nanoparticles with a cross-linked organic polymeric layer was tackled also by [PERSON] et al. (188).