Stability, relaxometric and computational studies on Mn complexes with ligands containing a cyclobutane scaffold.

The stability constants of Mn complexes with ligands containing a trans-1,2-cyclobutanediamine spacer functionalized with picolinate and/or carboxylate functions were determined using potentiometric titrations (25 °C, 0.1 M KCl). The stability constant of the complex with a hexadentate ligand containing four acetate groups (L1, log K = 10.26) is improved upon replacing one (L2, log K = 14.71) or two (L3, log K = 15.81) carboxylate groups with picolinates. The [Mn(L1)] complex contains a water molecule coordinated to the metal ion in aqueous solutions, as evidenced by H NMRD studies and O chemical shifts and transverse relaxation rates. The H relaxivities determined at 60 MHz (3.3 and 2.4 mM s at 25 and 37 °C, respectively) are comparable to those of monohydrated complexes such as [Mn(edta)]. The exchange rate of the inner-sphere water molecule (k = 248 × 10 s) is slightly lower than that of the edta analogue. DFT calculations (M11/def2-TZVP) suggest that the water exchange reaction follows a dissociatively activated mechanism, providing activation parameters in reasonably good agreement with the experimental data. DFT calculations also show that the O hyperfine coupling constant A/ℏ is affected slightly by changes in the Mn-O distance and the orientation of the water molecule with respect to the Mn-O vector.