## 4 Discussion  
The conversion rates obtained for electrochemical oxidation of VA to VAH are particularly high considering the mild conditions used in our experiments as compared to the experimental setups reported by other authors, where high temperatures and pressures were required to achieve  
Figure 5: Conversion rates of veratrylalcohol (VA) to veratrylaldehyde (VAH) at 23 and 40 \({}^{\circ}\)C after 2 h without catalyst (a) and with catalysts **(1-7)**; pH = 8.25. Figure 6: pH-dependence of conversion ratio of VA to VAH catalysed by **1** (\(\bullet\)), and without catalyst (\(\blacksquare\)) (at 40 \({}^{\circ}\)C). similar yields with different catalysts [46; 47]. Moreover, most of the oxygen-activating homogeneous transition-metal catalysts were investigated in organic solvents and only a few are known to be active in water [48]. A better electrocatalytic performance displayed by **1-4** in comparison to **5-7** may be explained by the difference in their Schiff base ligand structures. We have already proposed that improved catalytic performance, for different processes (peroxidase, catalase activities), may be associated with the ability of substrate (VA) molecules to coordinate directly to manganese(III). The coordination is favored if the catalyst has either a vacancy in the coordination sphere or a labile ligand [17]. The short two-carbon link between imine groups in **1-4** results in tighter confinement of the nitrogen atoms coordinated to the metal and to stronger tetragonal elongation distortion to the octahedral geometry of the cluster. An axial water molecule in this class of distorted geometries is quite a labile ligand, the loss of which would generate a vacant position in the coordination sphere to accommodate the substrate molecule. On the other hand, the flexible three-membered alkyl chain between the imine groups in the manganese compounds **5-7** favours more relaxed confinement and thus better stabilization of more symmetric octahedral geometry, which consequently makes the generation of an axial vacant site more difficult. The structural difference between **1-4** and **5-7** is evident from our parallel-mode EPR data (Figure 2, Table 3). Complexes **1** and **3** demonstrate lower \(g_{\mathit{eff}}\) = 8.08 (smaller orthorhombic distortion in the ligand field) and smaller \(A_{\parallel}\) = 42-43 G (higher metal-ligand covalencies for the equatorially coordinated N\({}_{2}\)O\({}_{2}\) atoms) as compared to the complexes **5** and **6** with \(g_{\mathit{eff}}\) = 8.16 and \(A_{\parallel}\) = 49 G [49]. The increased Mn-ligand covalency (a larger fraction of unpaired spin density being delocalized from the metal orbitals to the ligand orbitals [39]) indicates a smaller net positive charge on the center Mn(III) ion in case of complexes **1-4**. This immediately results in a weaker binding of the axial ligands to Mn(III) which become more mobile and readily exchangeable for the VA substrate molecules. The higher covalency (smaller positive charge) also causes a shift in Mn(III) redox potentials in **1-4** to more negative which may further contribute to their improved overall performance as the catalyst. The catalytic performance of the complexes is definitely influenced by their redox potentials. Negative E\({}_{1/2}\) versus SCE and reversible electrochemical behaviors usually lead to better performances. Nevertheless, comparison between redox potentials (see Table 3) in **1-4** and **5-7** reveals that the redox potentials are not the determining factor in the obtained catalytic rates for this case. For instance, the active compound **4** shows the lowest E\({}_{1/2}\) value (\(-\)196 mV), but the second lowest value (\(-\)186 mV) corresponds to the inactive complex **5**. The clear correlation between the peroxidase activity and the conversion rates for the electrochemical oxidation of VA reinforces the idea that both processes can follow similar mechanisms.