Stability, relaxometric and computational studies on Mn2+ complexes with ligands containing a cyclobutane scaffold. Issue 3 (24th December 2020)

• Record Type: Journal Article
• Authors: Porcar-Tost, Oriol; Pallier, Agnès; Esteban-Gómez, David; Illa, Ona; Platas-Iglesias, Carlos; Tóth, Éva; Ortuño, Rosa M.
• Is Part Of: Dalton transactions. Volume 50:Issue 3(2020)
• Page Start: 1076
• Page End: 1085
• Publication Date: 2020-12-24
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/d0dt03402a
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : The incorporation of a rigid cyclobutane spacer into the EDTA scaffold impacts the relaxivity, water exchange kinetics and stability of the Mn 2+ complex. Abstract : The stability constants of Mn 2+ 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 4−, log K MnL = 10.26) is improved upon replacing one (L2 4−, log K MnL = 14.71) or two (L3 4−, log K MnL = 15.81) carboxylate groups with picolinates. The [Mn(L1 )] 2− complex contains a water molecule coordinated to the metal ion in aqueous solutions, as evidenced by 1 H NMRD studies and 17 O chemical shifts and transverse relaxation rates. The 1 H relaxivities determined at 60 MHz (3.3 and 2.4 mM −1 s −1 at 25 and 37 °C, respectively) are comparable to those of monohydrated complexes such as [Mn(edta)] 2− . The exchange rate of the inner-sphere water molecule ( k 298ex = 248 × 10 6 s −1 ) is slightly lower than that of the edta 4− 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 17 O hyperfine coupling constant A /ℏ is affected slightly by changes in the Mn–Owater distance and the orientation of the water molecule with respect to the Mn–O vector.