Stability and relaxometric characterization of a manganase(II) 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 (pMn = 6.27) than that reported for the Mn(II) chelates of OPC2A (pMn = 8.69) and 3,9-PC2A ligands (pMn = 8.64), and it dissociates faster than those at physiological conditions (t1/2 = 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 atomes, the two carboxylates of the malonate pendant arm as well as an inner spehere water molecule to Mn(II). The relaxivity values of [Mn(OPMMA)] (r1p = 3.48 and r2p = 5.85 mM–1s–1 at 20 MHz and 298K, respectively) are slightly higher than those found for [Mn(OPC2A)] and [Mn(3,9-PC2A)]. The water exchange rate (kex298 = (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)] (τrH298 = 55±3 ps vs. 40 ps) can be related to the less compact structure of [Mn(OPMMA)].