## 3 Discrete Cu(I/II) Complexes
### Magnetically Recoverable Systems
Considering that cyclodext (CDs) have found use in CuAAC reactions, with probable roles ranging from phase-transfer agents [191], to ligands for Cu(II) or Cu(I) [192; 193], [PERSON] et al. used a surface silanization approach for binding \(\beta\)-CD units to super-paramagnetic iron nanoparticles [194]. Treatment of the resulting white material with CuCl under alkaline conditions afforded a dark powder **50** that, although hard to assess for what it concerns oxidation state, aggregation degree, and location of Cu centers, displayed satisfactory catalytic activity in a four-components reaction involving a CuAAC click step (Scheme 22). Reuse of the magnetically-recovered nanoparticles led to nearly constant yields in the course of 5 cycles and a cumulative TON of 8300 after 10 runs (average TOF = 347 h\({}^{-1}\)). A 'filtration experiment', in which the supported catalyst was stirred with EtOH-H\({}_{2}\)O for 24 h revealed a very low metal concentration in the liquid phase (<1 ppm by ICP-AES) that, expectedly, did not lead to any appreciable conversion when standard reactants of the process above were added to the solution. Nonetheless, the fact that in some documented cases copper leaching was significantly larger in the presence of CuAAC substrates and products, than in the reaction solvent alone [75], makes it difficult to evaluate the relevance of the experiment for the problem of metal release under actual four-component reaction conditions. Figure 5: Cu(I/II) complexes **49–53**, containing organic ligands immobilized on magnetically recoverable supports. Given the strong tendency of triazoles to bind copper, the same structural motif has been exploited in the past for retaining Cu(I/II) onto an insoluble support bearing tris-triazole units, e.g., of the TBTA type [195]. More recently, [PERSON] et al. examined the use for the same purpose of mono-triazoles provided with an ancillary coordination site [196; 197]. Grafting of ligand units was realized either by reaction of surface-anchored functional groups with a preformed triazole derivative, or by generating the heterocyclic nucleus in a three-components CuAAC step. In the latter case the resulting material **51** was obtained already charged with Cu(I). Instead, in the former case the supported Cu(II) complex **52** was prepared by stirring the magnetic nanoparticles with CuCl\({}_{2}\) in water. At 0.5 mol % loading and in the presence of sodium ascorbate, both systems were active in three-component CuAAC reactions in water with alkyl halides or tosylates (Scheme 2a) and afforded the corresponding triazoles in high yield, after 2-8 h at 55 \({}^{\circ}\)C (85%-98%). Recycling of the materials proved feasible, with slight reduction of product yield (approximate 5%) in the course of 10 consecutive runs. Similarly, 2,2\({}^{\prime}\)-bis(imidazole) on core-shell Fe\({}_{3}\)O\({}_{4}\)-SiO\({}_{2}\) nanomagnetite particles was obtained by [PERSON] et al. through reaction of the ligand with the chlorophyll-modified support [198]. Loading with different metal salts led to the corresponding complexes, among which that prepared from CuI (**53**) showed the highest activity in three-component CuAAC reactions (Scheme 2a). With the use of 0.85 mol % of this catalyst at r.t., in water or EtOH-H\({}_{2}\)O (1:1), various chlorides, bromides or tosylates were transformed effectively in the corresponding triazole within 0.5-3.5 h (88%-99% yield). Reuse of the magnetically recovered material demonstrated good preservation of its catalytic activity, with a measurable drop of product yield only in the 5 th cycle (99% \(\rightarrow\) 93%) (and the attainment of a still respectable 96% yield in the 6 th run, carried out with somewhat extended reaction time: 30 min \(\rightarrow\) 40 min). No textural changes in the recovered solid were noticed by electron microscopy, but copper was detected in minute amounts (0.08% of the initial quantity) in the product phase after the 5 th run.