¹H and ¹⁷O NMR relaxometric and computational study on macrocyclic Mn(II) complexes

Herein we report a detailed ¹H and ¹⁷O relaxometric investigation of Mn(II) complexes with cyclen-based ligands such as 2-(1,4,7,10-tetraazacyclododecan-1-yl)acetic acid (DO1A), 2,2′-(1,4,7,10- tetraazacyclododecane-1,4-diyl)diacetic acid (1,4-DO2A), 2,2′-(1,4,7,10- tetraazacyclododecane-1,7-diyl)diacetic acid (1,7-DO2A), and 2,2′,2″-(1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (DO3A). The Mn(II) complex with the heptadentate ligand DO3A does not have inner sphere water molecules (q = 0), and therefore, the metal ion is most likely seven-coordinate. The hexadentate DO2A ligand has two isomeric forms: 1,7-DO2A and 1,4-DO2A. The Mn(II) complex with 1,7-DO2A is predominantly six-coordinate (q = 0). In aqueous solutions of [Mn(1,4-DO2A)], a species with one coordinated water molecule (q = 1) prevails largely, whereas a q = 0 form represents only about 10% of the overall population. The Mn(II) complex of the pentadentate ligand DO1A also contains a coordinated water molecule. DFT calculations (B3LYP model) are used to obtain information about the structure of this family of closely related complexes in solution, as well as to determine theoretically the ¹⁷O and ¹H hyperfine coupling constants responsible for the scalar contribution to ¹⁷O and ¹H NMR relaxation rates and ¹⁷O NMR chemical shifts. These calculations provide ¹⁷O A/ values of ca. 40 × 10⁶ rad s^-1, in good agreement with experimental data. The [Mn(1,4-DO2A)(H₂O)] complex is endowed with a relatively fast water exchange rate (kex²⁹⁸ = 11.3 × 10⁸ s^-1) in comparison to the [Mn(EDTA)(H₂O)]^2- analogue (kex²⁹⁸ = 4.7 × 10⁸ s^-1), but about 5 times lower than that of the [Mn(DO1A)(H₂O)]⁺ complex (kex²⁹⁸ = 60 × 10⁸ s^-1). The water exchange rate measured for the latter complex represents the highest water exchange rate ever measured for a Mn(II) complex.