Imaging contrast agents and uses thereof

A macrocyclic compound having a macrocyclic core having the following structure:

• wherein
• each R3 is independently selected from H and CH3, and
• R1 is a pendant group independently selected from the group consisting of:
• —H, —CH3,
• wherein each R4 is independently selected from the group consisting of H, C1 to C12 alkyl groups, PEG groups, thioether groups, and CH2CO2R8, R6 is H or OCH3, R7 is H, OCH3, or CO2H, each R8 is independently selected from the group consisting of C1 to C12alkyl groups, PEG groups group, and thioether groups, and each R9 is independently selected from the group consisting of H, C1 to C12 alkyl groups, PEG groups group, and thioether groups;
• the macrocyclic compound has at least one exchangeable proton;
• and
• a Fe(II) or Ni(II) cation is complexed to the macrocyclic core and/or at least one pendant group of the macrocyclic compound, wherein the complexed Fe(II) is high spin Fe(II),
• with the proviso:
• i) that when a high-spin Fe(II) cation is complexed with the macrocyclic core, the macrocyclic core does not have the following structure:
• wherein each R3 is H and each R1 is
• and
• ii) that when a Ni(II) cation is complexed with the macrocyclic core, the macrocyclic core does not have the following structure:
• wherein each R3 is H and each R1 is
• wherein each R4 is;
• and iii) that when a Ni(II) cation is complexed with the macrocyclic core, the macrocyclic core does not have the following structure
• wherein each R1 is