A Highly Water- and Air-Stable Iron-Containing MRI Contrast Agent Sensor for H 2 O 2 .

A highly water- and air-stable Fe(II) complex with the quinol-containing macrocyclic ligand H 4 qp4 reacts with H 2 O 2 to yield Fe(III) complexes with less highly chelating forms of the ligand that have either one or two para-quinones. The reaction increases the T 1 -weighted relaxivity over four-fold, enabling the complex to detect H 2 O 2 using clinical MRI technology. The iron-containing sensor differs from its recently characterized manganese analog, which also detects H 2 O 2 , in that it is the oxidation of the metal center, rather than the ligand, that primarily enhances the relaxivity.