## 3 Results and Discussion
### Results at Near-Neutral pH  
Before investigating the decomposition of H\({}_{2}\)O\({}_{2}\) by Cu-Phen, the stability of DMPO in the presence of Cu-Phen (exp 2) was evaluated (Figure 3). In the presence of DMPO, the EPR spectrum of a solution of Cu-Phen displays features corresponding to three different radical adducts: DMPO-OH, DMPO-R, and a radical showing triplet splitting lines (Figure 3a). The addition of DMSO (exp 13) does not reduce the DMPO-OH signal, indicating that DMPO-OH does not originate from the trapping of HO\({}^{\bullet}\), but from nucleophilic addition of water. This was confirmed using the other radical scavenger, HCOO\({}^{-}\) (exp 14), as illustrated in Figure S6. Note that the hyperfine splitting constants for DMPO-OH in the presence of DMSO (and water) are different from those in water only, as described in the literature [31; 32]. The DMPO-R and the species that shows triplet splitting lines (we will name it \"triplet radical\" in figure legends for easier reading) are most likely due to the decomposition of DMPO in the presence of copper. Controls have been carried out to understand the origin of these species. While DMPO does not exhibit any signal in the absence or presence of phenanthroline, an EPR signal is observed in the presence of CuSO\({}_{4}\) (exp 1), similar to that of DMPO + Cu-Phen, but less intense (about 50%; see Table 1a entries 1 and 2 and Figure 3b). The higher intensity of the DMPO-R and triplet signals observed with Cu-Phen compared to CuSO\({}_{4}\) demonstrates that DMPO is degraded faster in the presence of Cu-Phen. This is consistent with the redox potential of Cu\({}^{\text{II}}\)(Phen)\({}_{2}\)/Cu\({}^{\text{I}}\)(Phen)\({}_{2}\) that is slightly higher than that of Cu\({}^{\text{II}}\)/Cu\({}^{\text{I}}\) (0.17 and 0.16 V, respectively) [15]. In the present work, the coordination of DMPO to the Cu(II) ion was evidenced by EPR spectra recorded at 100 K with DMPO 60 mM and Cu(II) 0.15 mM (Figure 4). Under such conditions (low temperature and Cu(II) concentration 10 times larger than in the spin-trapping experiments), the EPR signal corresponding to the Cu(II) ion (\(S=1/2\), d\({}^{9}\)) is observed with a typical axial spectrum characterized by a quartet in the parallel component arising from hyperfine interaction (\(I_{Cu}=3/2\)). Figure 3: X-band EPR spectra of (**a**) DMPO + Cu–Phen and DMSO + DMPO + Cu–Phen; (**b**) DMPO + CuSO\({}_{4}\) and DMSO + DMPO + CuSO\({}_{4}\).The simulations revealed the presence of at least three radical species in each case. With DMPO + Cu–Phen (exp 2): DMPO–OH, \(\text{a}_{\text{N}}=15.088\) G, \(\text{a}_{\text{H}}=14.943\) G; DMPO–R, \(\text{a}_{\text{N}}=15.781\) G, \(\text{a}_{\text{H}}=23.27\) G; “triplet radical”, \(\text{a}_{\text{N}}=14.354\) G. With DMSO + DMPO + Cu–Phen (exp 13): DMPO–OH, \(\text{a}_{\text{N}}=14.697\) G, \(\text{a}_{\text{H}}=13.82\) G; DMPO–R, \(\text{a}_{\text{N}}=15.488\) G, \(\text{a}_{\text{H}}=22.635\) G; “triplet radical”, \(\text{a}_{\text{N}}=14.305\) G. With DMPO + CuSO\({}_{4}\) (exp 1); DMPO–OH, \(\text{a}_{\text{N}}=15.798\) G, \(\text{a}_{\text{H}}=14.928\) G [33; 34]; DMPO–R, \(\text{a}_{\text{N}}=15.798\) G, \(\text{a}_{\text{H}}=23.237\) G [35]; “triplet radical”, \(\text{a}_{\text{N}}=14.609\) G; other, \(\text{a}_{\text{N}}=16.323\) G, \(\text{a}_{\text{H}}=26.653\) G (uncertain). With DMSO + DMPO + CuSO\({}_{4}\) (exp 12): DMPO–OH, \(\text{a}_{\text{N}}=14.697\) G, \(\text{a}_{\text{H}}=14.015\) G; DMPO–R, \(\text{a}_{\text{N}}=15.439\) G, \(\text{a}_{\text{H}}=22.586\) G; “triplet radical”, \(\text{a}_{\text{N}}=14.354\) G; Assignments: o = DMPO–OH, \(\text{l}=\text{DMPO–R}\), + = “triplet radical”. The addition of DMPO notably modifies the EPR spectra of both Cu-Phen and CuSO\({}_{4}\), as demonstrated by changes in the g-value anisotropy. Such changes evidence the difference in coordination geometry around the Cu ion.