## 3 Discrete Cu(I/II) Complexes
### Magnetically Recoverable Systems
In this case, the macromolecular material was obtained by copolymerization of acrylamide and N,_N_'-methylenebisacrylamide, in the presence of starch and support nanoparticles bearing methacrylate groups on the surface. Loading with CuCl\({}_{2}\) afforded the insoluble catalyst precursor **48**, which was examined in three-components CuAAC reactions of halides (Scheme 2a). With sodium ascorbate as the reducing agent, the use of 0.5 mol % of the system in water required to warm the reaction mixture to 50 \({}^{\circ}\)C for obtaining the triazole products in good yield after a short reaction time (82%-99% in 1-5 h). A standard 'filtration experiment' and ICP-OES determination of copper in the separated reaction mixture excluded significant amounts of metal in the solution. At the same time, the reuse of recovered magnetic nanoparticles revealed a reasonable durability of the catalyst. Indeed, while a step drop of average turnover frequency was observed on passing from the 5 th to the 6 th reaction cycle (TOF approximate 400 h\({}^{-1}\)\(\rightarrow\) 300 h\({}^{-1}\)), no further reduction of turnover rate was recorded in the next five runs. Because FT-IR and ICP-OES data for the recovered catalyst were not very dissimilar from those of the original material, the trend was attributed to mechanical losses during the isolation step. At the borderline between low molecular weight ligands and high-polymeric ones, functional dendrimers have proved to be very effective in the development of recoverable catalytic systems [189], including those for CuAAC [77]. Very recently, [PERSON] and [PERSON] described the preparation of a triazine-based, 2 nd generation dendrimer-Cu(II) complex on ferromagnetic nanoparticles **49** (Figure 5) and its use in three-component CuAAC reactions [190]. The material was obtained by initially coating the Fe\({}_{3}\)O\({}_{4}\) nanoparticles with an aminopropylsilica layer and then reacting the surface functional groups with trichlorotriazine and ethylenediamine, in a two-round iterative fashion. Although these solid-phase steps were found not to be complete, the final material showed an appreciable adsorbing capacity towards Cu(OAc)\({}_{2}\) and eventually provided the supported Cu(II) complex as a dark-green solid. Examination of the powder by various techniques (SEM, XRD, and TEM) revealed the presence of well dispersed particles, uniform in shape and size (35 nm average diameter) and consisting of a Fe\({}_{3}\)O\({}_{4}\) core coated by a thinner silica shell. The catalytic activity of **49** was optimized in the reaction between benzyl chloride, sodium azide, and phenylacetylene, rather unusually carried out in the presence of an equivalent amount of Na\({}_{2}\)CO\({}_{3}\). In spite of the relatively low catalyst loading (mostly 0.3 mol % of Cu), nearly quantitative yields were recorded in 10-15 min at r.t. in EtOH-H\({}_{2}\)O (1:1). The same conditions were therefore adopted for the CuAAC of other alkynes and halides (Scheme 2a), to give the corresponding triazoles with excellent yield in no more than 40 min (90%-99%). Surprisingly, this held true not only in the case of very reactive halides (benzyl, allyl, and \(\alpha\)-halocarbonyl chlorides and bromides) but even for a reactant, like hexyl bromide, which is expected to undergo the halogen/azide exchange at much slower rate. Recycling of the nanoparticles by magnetic decantation showed some slight reduction of activity along six consecutive runs in the aforementioned benchmark relation (99% \(\rightarrow\) 93% yield). Such a trend was tentatively assigned to \"the normal loss the catalyst during the work-up stage\", but no clue was provided about the possible relevance of missing nanoparticles for the problem of product contamination. Also for this class of recoverable CuAAC catalytic systems, examples were reported recently of materials embedding discrete organic ligands (Figure 5).