## 3 Discrete Cu(I/II) Complexes
### Cross-Linked Synthetic Organic Polymer Supports
Given also the structure of some of the halides used in the screening (e.g., cyclohexyl bromide and 1-adamantly bromide), these variations are likely to reflect more the difficulty of the latter to be converted into the corresponding azide intermediate than actual differences in catalyst performance under alternative reaction conditions. Recovery of the supported system by filtration, washing, and drying with an air stream made it possible four consecutive reaction cycles, with a drop of product yield more evident under three-component conditions (99% \(\rightarrow\) 71%) than under two-components ones (approximate 99% \(\rightarrow\) 87%). In a multi-step solid-phase synthesis approach, [PERSON] et al. pursued the preparation on Merrifield resin of a dicationic ligand provided with 1,2-dithioether chelating site [162]. Stirring of the material with Cu(OTf)\({}_{2}\) in MeOH led to the corresponding Cu(II) complex **30**, characterized _inter alia_ by cyclic voltammetry on glassy carbon electrode. Notwithstanding the presence of iodide in the material, addition of sodium ascorbate turned out to be essential for obtaining a catalytically active system for CuAAC. In the presence of the reducing agent and with 0.2 mol % of **30** in PEG\({}_{400}\) at 65 \({}^{\circ}\)C, quite fast reaction kinetics (87%-99% yield in 10-25 min) were observed in three-components CuAAC runs with mono-functional activated bromides (Scheme 2a). By contrast, somewhat reduced yields were recorded in the reaction of bis(bromomethyl)benzene starting materials, even after longer reaction time (68%-88% in 45-60 min). Despite the observation of some copper leaching in the first run (<2%), the supported system displayed a reasonable recyclability, with just a modest drop of product yield after seven consecutive cycles (approximate 99% \(\rightarrow\) 94%). A 1,10-phenanthroline-CuI catalytic system on Merrifield resin, **31**, was reported by [PERSON] et al. for the click-polymerization of di-functional azide and alkyne reagents (Scheme 15) [163]. Similarly to previous work from the same group [79], the solubility in THF of the resulting linear polymers allowed the separation of the supported catalyst and the obtainment of polyttriazole materials containing relatively low amounts of copper (190-410 ppm). At variance with the precedent study, however, in this case the process was found to proceed only in the presence of air and no recycling of the supported catalyst was attempted. In addition to the systems above, obtained from commercially available resins, examples were described also of materials prepared by radical copolymerization of functional monomers. For the purpose of increasing the loading capacity of GO and its effectiveness in retaining copper ions, [PERSON] et al. resorted to the entrapment of the carbonaceous support into a cross-linked poly(vinyl imidazole) matrix (Pim) [164]. With this aim, \(N\)-vinylimidazole and the corresponding 1,4-bis-(imidazol)butane di-functional monomer were radically polymerized in solution, in the presence of GO as a filler (Scheme 16a). Interestingly, while SEM examination of Pim devoid of GO showed a smooth surface, the nanocomposite was found to possess a corrugated morphology, attributed to the embedded GO sheets. Soaking the material in aqueous CuSO4 led to load a considerable amount of metal salt, corresponding to about one half of the imidazole units content estimated for the material. A preliminary optimization stage for the three-component CuAAC reaction of phenylacetylene, benzyl bromide, and NaN3 suggested the use of 1 mol % of the supported Cu(II) complex **32** and 10 mol % of sodium ascorbate, in water at 50 \({}^{\circ}\)C.  
**Scheme 16. ** Preparation of supported Cu(I/II) complex **32** (**a**) and **33** (**b**) onto resins obtained by radical copolymerization and plausible structures of Pim and P(im/IL)(X).