## 3 Results and Discussion
### Results at Near-Neutral pH
The present work also highlights the fact that Phen remains bound to Cu when DMPO and Cu-Phen are mixed, since the spectra of Cu-DMPO adducts with or without Phen display different EPR parameters (see Table S24 in the Supplementary Materials). Besides, the absence of a decrease of the signal integration when DMPO is added demonstrates that Cu(II) is not reduced by DMPO into Cu(I) under these experimental conditions. The H\({}_{2}\)O\({}_{2}\)/Cu-Phen system (exp 3) was tested in the presence of DMPO (Figure 5), leading to an intense DMPO-OH spectrum, a multiplet assigned to DMPO-R, and a triplet splitting. The simulations confirm that the DMPO-R and the radical that shows triplet splitting lines are the same as without H\({}_{2}\)O\({}_{2}\). The DMPO-OH signal is 4.8 times more intense than with Cu-Phen alone (38% DMPO-OH for a total area of 1.29 (entry 3 in Table 1) vs. 7% for a total area of 1.46 (entry 2)). This already indicates that hydroxyl radicals are the major species responsible for the formation of DMPO-OH with H\({}_{2}\)O\({}_{2}\)/Cu-Phen. Note that no hydroxyl radicals are present at room temperature with hydrogen peroxide alone. In the presence of DMSO (exp 4, see Figure 5 and entry 4 in Table 1a), a drop of the DMPO-OH signal is accompanied with the detection of a new carbon-centered DMPO adduct, assigned to DMPO-CH\({}_{3}\) based on its EPR parameters [33; 36]. DMPO-CH\({}_{3}\) is highly predominant, representing 73% of the detected radicals. This was confirmed when sodium formate was used as scavenger (exp 5, Figure S9), with 49% of DMPO-COO\({}^{-}\) among the detected radicals (entry 5 in Table 1a). These experiments demonstrate that hydroxyl radicals are produced in aqueous solution in the presence of both H\({}_{2}\)O\({}_{2}\) and Cu-Phen. The apparent disappearance of the DMPO-OH signal in the presence of both radical scavengers is consistent with the absence of nucleophilic water addition in this case. This can be explained by the low constant rate of the DMPO/H\({}_{2}\)O reaction, or by the fact that copper activates H\({}_{2}\)O\({}_{2}\)'s decomposition rather than induces nucleophilic addition of water onto DMPO. When analyzing the solution after 10 min at room temperature (Figure S10), the DMPO-COO\({}^{-}\) signal is more intense than after direct acquisition, meaning that the system is still active after a few minutes. Figure 4: X-band EPR spectra at 100 K of (**a**) CuSO\({}_{4}\) with and without DMPO and (**b**) Cu–Phen with and without DMPO, [Cu] = 0.15 mM, [DMPO] = 60 mM. For a solution of copper with an equimolar amount of DMPO, i.e., 0.15 mM, the structuration is no longer visible (results not shown), indicating a low affinity of DMPO for copper. For comparison, the H\({}_{2}\)O\({}_{2}\)/CuSO\({}_{4}\) system (exp 8, Figure 5b) does not exhibit much difference with CuSO\({}_{4}\) alone (only an increase of 7.5% of DMPO-OH) (entry 8 in Table 1a). The DMPO-OH signal is very weak compared to that observed earlier with H\({}_{2}\)O\({}_{2}\)/Cu-Phen. Since DMSO does not seem to scavenge hydroxyl radicals (exp 9), sodium formate was also tested (exp 5, Figure 6). In its presence, the DMPO-OH signal almost disappears with the concomitant appearance of the signal typical of DMPO-COO\({}^{-}\). This shows that only a low amount of hydroxyl radicals is generated under such conditions. Consistently, the group of [PERSON] [37] also observed DMPO-OH at physiological pH with DMPO, H\({}_{2}\)O\({}_{2}\), and CuCl\({}_{2}\). However, they did not identify the origin of the DMPO-OH adduct because they did not use a radical scavenger. As illustrated in Figure 6, the DMPO-COO\({}^{-}\) signal with H\({}_{2}\)O\({}_{2}\) + Cu-Phen is more intense than with H\({}_{2}\)O\({}_{2}\) + CuSO\({}_{4}\), approximately 85% higher (total area of 0.65 including 49% DMPO-COO\({}^{-}\) vs. 0.21 including 23% DMPO-COO\({}^{-}\), see entries 5 and 10 respectively in Table 1a). This unambiguously confirms the stronger H\({}_{2}\)O\({}_{2}\) decomposition in the presence of Cu-Phen.