Iron(II) complexes containing octadentate tetraazamacrocycles as paraCEST magnetic resonance imaging contrast agents

Iron(II) complexes of the macrocyclic ligands 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (TCMC) and (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (STHP) contain a highly stabilized Fe(II) center in the high-spin state, which is encapsulated by an octadentate macrocycle. The complexes are resistant to acid, metal cations, phosphate, carbonate, and oxygen in aqueous solution. [Fe(TCMC)](2+) contains exchangeable amide protons, and [Fe(STHP)](2+) contains exchangeable protons attributed to alcohol OH donors, which give chemical exchange saturation transfer (CEST) peaks at physiological pH and 37 °C at 50 and 54 ppm from bulk water, respectively. The distinct pH dependence of the CEST peak of the two complexes over the range of pH 6-8 shows that these two groups may be useful in the development of ratiometric pH sensors based on iron(II).