# Mn(II) Complexes of Enlarged Scorpiand-Type Azamacrocycles as Mimetics of MnSOD Enzyme
## 3 Results  
### Basicity of the Ligands and Mn(II) Coordination  
The compounds prepared are polyprotic systems and it is therefore necessary to determine their protonation constants in water before attempting to calculate the stability constants with metal cations, because the proton transfer reaction will be in competition with the metal coordination. For this reason, acid-base potentiometric titrations of the ligands were performed, and the data were adjusted to a given model by a non-linear fitting procedure to obtain the global basicity constants, from which the stepwise protonation constants of L1-L2 were calculated (Table 1). The protonation constants of L3 had already been published by the authors elsewhere (see [20]). As can be observed, for L1 and L3, the overall basicity, as well as the values for the stepwise protonation constants, are very close, as one would expect for structurally similar compounds. For these two ligands, four protonation constants were determined, corresponding to the protonation of the four secondary amines. In the case of the 2-quinoline substituted ligand L2, a fifth protonation step was detected, with a value of 2.25 log units, which can be ascribed to the protonation of the quinoline nitrogen atom. The overall basicity of this ligand is higher than for the other two and, as we will demonstrate later, this influences the Mn\({}^{2+}\) coordination properties. It was not possible to measure a fifth protonation constant for L1 or L3. This last protonation step should involve either a pyridine nitrogen atom or the central tertiary amino nitrogen of the macrocyclic ring. It is expected that the value of these protonation constants falls outside the pH interval of the technique, on account of the known lower basicity of tertiary amines in water and the electrostatic repulsion of the surrounding ammonium groups [29]. The molar distribution diagrams derived from these constants are available in the Supplementary Material (Figure S1). With this information, we can conclude that, for the free ligands and under the conditions at which the biological experiments are going to be performed (pH 7.4), the main species present in the solution, for the three ligands, is [H\({}_{3}\)L]\({}^{3+}\), with a prevalence of 65% for L1, 70% for L2, and 60% for L3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reaction (a)** & & **L1** & **L2** & **L3 (a)** \\ \hline L + H \(\rightleftarrows\) HL (b) & \(K_{\text{HIL}}\) & 9.49 (1) & 9.71 (2) & 9.43 (2) \\ LH + H \(\rightleftarrows\) H\({}_{2}\)L & \(K_{\text{HEL}}\) & 8.91 (1) & 9.03 (1) & 9.18 (2) \\ H\({}_{2}\)L + H \(\rightleftarrows\) H\({}_{3}\)L & \(K_{\text{HEL}}\) & 7.85 (1) & 8.00 (2) & 7.88 (3) \\ H\({}_{3}\)L + H \(\rightleftarrows\) H\({}_{4}\)L & \(K_{\text{HAL}}\) & 6.66 (2) & 6.70 (2) & 6.92 (3) \\ H\({}_{4}\)L + H \(\rightleftarrows\) H\({}_{5}\)L & \(K_{\text{HEL}}\) & & 2.25 (8) & \\ \hline
**log \(\beta\)** & & **32.92 (2)** & **35.69 (2)** & **33.41 (3)** \\ \hline \hline \end{tabular} \({}^{(a)}\) Logarithms of the stepwise (\(K_{\text{HEL}}\)) and cumulative (\(\beta\)) protonation constants, calculated as log \(\beta=\sum_{i}\log K_{\text{HEL}}\). Values were determined at 298 K in 0.15 M NaCl. Numbers in parentheses are standard deviations in the last significant figure. (b) Charges have been omitted. (c) Values taken from [20].  
\end{table} 
Table 1: Stepwise and cumulative protonation constants of L1–L3. The interaction of these macrocyclic ligands with Mn\({}^{2+}\) was then studied. The potentiometric data could only be obtained for L1 and L2, since the low solubility of [MnL3]\({}^{2+}\) at the concentrations required for the potentiometric technique (\(\approx\)1 mM) hindered the determination of the stability constant of this complex. Table 2 gathers the stability constants calculated by the best fitting of the data to a model taking into account the protonation of the ligands, as well as the hydrolysis equilibria of the metal cation.