Mn(ii) complexes of novel hexadentate AAZTA-like chelators: A solution thermodynamics and relaxometric study

Three novel chelators based on the 6-amino-6-methylperhydro-1,4-diazepine scaffold and possessing three pendant N-acetic or N-α-methylacetic acid have been synthesised. The ligands contain six donor atoms for complexation of Mn^II ions and thus potentially leave an additional site for coordination of a water molecule. The protonation constants of the ligands and the stability constants of their complexes formed with Mn^II ion were determined by pH-potentiometric titrations in 0.15 M NaCl solution at 25 °C and compared to those of the parent AAZTA ligand (AAZTA = 6-amino-6- methylperhydro-1,4-diazepine tetraacetic acid). In spite of the similar value of the total basicity (Σlog K), the values of the stability constants of the Mn^IIAAZTA-like complexes are more than three orders of magnitude lower than that of MnAAZTA (log K_MnL = 14.19). A detailed ¹H and ¹⁷O NMR relaxometric study was carried out on the Mn^II complexes in aqueous solution as a function of pH, temperature and magnetic field strength. The ¹H NMRD profiles of all the complexes show a similar shape, typical of low-molecular weight systems, but amplitudes that markedly differ to indicate a different degree of hydration. A similar behaviour is shown by the ¹⁷O NMR transverse relaxation rates and chemical shift data as a function of temperature. The experimental data can be rationalised by considering the presence in solution of a mixture of two isomeric species differing in coordination number (7 and 6) and in the number (1 and 0) of bound water molecules. Whereas this type of coordination equilibrium has been previously reported for lanthanide(iii) complexes, it is observed for the first time on Mn^II chelates.