15-membered rigid macrocyclic ligand for Mn2+ complexation: synthesis and coordination chemical characterization

Abstract

The investigation of Mn²⁺ chelates as alternatives to Gd³⁺-based MRI contrast agents has gained significant attention due to their promising safety and performance profiles. My research focused on the synthesis and coordination chemistry of a novel 15-membered bipyridine-based macrocyclic ligand, 15-BPyN5. The ligand demonstrated excellent thermodynamic stability (log K_MnL = 13.44), high kinetic inertness, and favorable relaxivity properties (r1p/r2p = 3.60/6.60 mM⁻¹s⁻¹). Dissociation studies revealed slow exchange kinetics with Zn²⁺ ions, with half-lives of 79.0 and 28.7 hours at 25°C and 37°C, respectively, attributed to the rigid coordination environment provided by the bipyridine moiety. These properties highlight the potential of [Mn(15-BPyN5)] as a safe and effective MRI contrast agent. This work paves the way for developing non-toxic alternatives to gadolinium-based agents, offering improved safety and imaging performance.