Macrocyclic ligands for Fe(II) paraCEST and chemical shift MRI contrast agents

► Fe(II) macrocyclic complexes are under development as contrast agents for paraCEST and chemical shift magnetic resonance imaging. ► Stabilization of high spin Fe(II) complexes with hexadentate or octadentate macrocyclic ligands. ► Fe(II) complexes containing heterocycles or amide donor groups have exchangeable protons that give paraCEST spectra. ► Non-bulky pendent groups give Fe(II) complexes that are fluxional on the NMR time scale.