## 3 Discrete Cu(I/II) Complexes
### Cross-Linked Synthetic Organic Polymer Supports
When the same experimental set-up was applied to the CuAAC of other terminal alkynes with benzyl or alkyl chlorides or bromides (Scheme 2a), good yields were uniformly recorded in 0.5-3.5 h (80%-96%). Remarkably, the reaction was found to proceed even with as little as 0.002 mol % of catalysts, but in this case a significantly longer reaction time was required for completion (20 h). Examination of leaching-related issues in the study under exam entailed a standard filtration test and the quantification of copper in the filtrate by ICP-OES. As desirable for a highly stable supported system, no evidence for catalytic activity in solution was gained by the former experiment, while in the latter the metal concentration was found to be below the detection limit. Therefore, the moderate reduction in product yield and TOF, observed in the course of eight recycle runs (approximate 99% \(\rightarrow\) 92% and 195 \(\rightarrow\) 140 h\({}^{-1}\), respectively), was tentatively ascribed to catalyst loss during washing and recovery procedures. The radical copolymerization of \(N\)-vinylimidazole with 3-methacrylamidopropyl trimethyl ammonium chloride and \(N\),\(N^{\prime}\)-methylenebisacrylamide (Scheme 16b) was employed by [PERSON] et al. for preparing the cross-linked P[im/IL][X] material, provided with imidazole units for copper complexation and ionic trimethylammonium groups for tuning the catalytic properties by variation of the counter-ion X [165]. In fact, before charging with CuSO\({}_{4}\), the resin was subjected to ion-exchange for the purpose of replacing chloride with other anions. Even though the result might look counterintuitive, because of the presence of a large excess of azide ions, when the different Cu(II)-loaded resins **33** were employed in three-component CuAAC runs (Scheme 2a) the activity of the supported systems was found to depend on the nature of the counter-ion in the order: Cl\({}^{-}\) > AcO\({}^{-}\) > Br\({}^{-}\) > BF\({}_{4}\)\({}^{-}\) > NO\({}_{3}\)\({}^{-}\). Therefore, the chloride form **33** (X = Cl) was selected for being examined in more detail. This resulted in an optimized procedure that, with the use of 0.1 mol % of the supported system and 10 mol % of sodium ascorbate in \(t\)-BuOH-H\({}_{2}\)O (1:3) at 55 \({}^{\circ}\)C, provided high yields of triazole products in 1.5-6 h (85%-99%). Remarkably, when the catalyst amount was lowered to 0.0013 mol % (130 mol ppm) a respectable 91% yield was still obtained after 18 h (TOF = 3889 h\({}^{-1}\)). Moreover, under standard conditions the reuse of the material recovered by centrifugation allowed to perform 12 reaction cycles, without any appreciable reduction of product yield and turnover frequency. A few resins and other insoluble polymers for the immobilization of CuAAC catalytic systems were obtained also by condensation or oxidative routes. For instance, an original approach to the preparation of a polymer-supported Cu(I) systems led [PERSON] et al. to realize polymer formation and copper immobilization at once, by addition of CuSO\({}_{4}\) in DMF to 2-aminobenzoic acid in water, under open air (Scheme 17a) [166]. At SEM magnification, the resulting green precipitate **34** appeared to consist of robust fibers, devoid of metal particles down to the TEM resolution level. Therefore, the inference was made that the material was a complex between poly(2-aminobenzoic acid) (pABA), from the oxidative polymerization of the amino acid precursor, and Cu(I), from the partial reduction of the copper salt. The conclusion was strengthened by Raman, XPS and XRD analyses, with the latter additionally showing the crystalline nature of the organic polymer. Despite the low surface area (7.8 m\({}^{2}\cdot\)g\({}^{-1}\)), the solid proved active in the benchmark addition of benzyl azide to phenylacetylene, reportedly carried out with 0.01 mol % Cu in solvents of different polarity. As revealed by kinetic measurements, the addition of Et\({}_{3}\)N (1 equiv.