Stability and relaxometric characterization of a manganase(<scp>ii</scp>) based macrocyclic complex containing malonate pendant

In this study, we report the thermodynamic, kinetic, relaxation and structural features of the Mn(II) complex formed with a newly synthetized O-pyclen ligand bearing a malonate pendant. The thermodynamic stability of [Mn(OPMMA)] is lower (<i>p</i>Mn = 6.27) than that reported for the Mn(II) chelates of OPC2A^2- (<i>p</i>Mn = 8.69) and 3,9-PC2A^2- ligands (<i>p</i>Mn = 8.64), and it dissociates faster than those at physiological conditions (<i>t</i>_1/2 = 2.1 min). This behavior can be related to the assymetric structure of the ligand and the presence of an unsubstituted N donor in the macrocycle. DFT calculations revealed a capped trigonal prismatic coordination geometry for [Mn(OPMMA)], involving the coordination of the macrocyclic donor atoms, the two carboxylates of the malonate pendant arm as well as an inner sphere water molecule to Mn(II). The relaxivity values of [Mn(OPMMA)] (<i>r</i>_1p = 3.48 and <i>r</i>_2p = 5.85 mM^-1 s^-1 at 20 MHz and 298 K, respectively) are slightly higher than those found for [Mn(OPC2A)] and [Mn(3,9-PC2A)]. The water exchange rate (<i>k</i>298ex = (22.1 ± 0.1) × 10⁷ s^-1) is 2-4-times higher than that of the [Mn(OPC2A)] and [Mn(3,9-PC2A)] complexes. The slightly higher rotational correlation time as compared to [Mn(OPC2A)] (<i>τ</i>298rH = 55 ± 3 ps <i>vs.</i> 40 ps) can be related to the less compact structure of [Mn(OPMMA)].