Stability, relaxometric and computational studies on Mn²⁺ complexes with ligands containing a cyclobutane scaffold

Area of Science:

• Coordination Chemistry
• Inorganic Chemistry
• Materials Science

Background:

• Manganese complexes are investigated for various applications, including as MRI contrast agents.
• Ligand design significantly influences the stability and properties of metal complexes.
• Understanding water exchange dynamics is critical for optimizing relaxivity in manganese-based agents.

Purpose of the Study:

• To synthesize and characterize novel manganese(II) complexes with functionalized cyclobutane diamine ligands.
• To evaluate the impact of picolinate and carboxylate functionalization on complex stability.
• To investigate the water coordination and exchange properties of these manganese complexes.

Main Methods:

• Potentiometric titrations were used to determine stability constants of Mn(II) complexes.
• Proton Nuclear Magnetic Resonance (1H NMR) Dynamic studies and 17O NMR spectroscopy were employed to study water coordination.
• Density Functional Theory (DFT) calculations were performed to elucidate water exchange mechanisms and electronic properties.

Main Results:

• Stability constants (log KMnL) increased from 10.26 for a tetra-acetate ligand (L1) to 14.71 (L2) and 15.81 (L3) upon incorporating picolinate groups.
• The [Mn(L1)]2- complex was found to be monohydrated, with 1H relaxivities comparable to [Mn(edta)]2-.
• Water exchange rate (k = 248 × 10^6 s^-1) was slightly lower than the edta4- analogue, and DFT confirmed a dissociative mechanism.

Conclusions:

• Replacing carboxylates with picolinates significantly enhances the stability of manganese complexes.
• The studied complexes exhibit favorable water coordination and exchange characteristics for potential applications.
• DFT calculations provide valuable insights into the factors governing water exchange and electronic structure.