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A highly water‐ and air‐stable Fe(II) complex with the quinol‐containing macrocyclic ligand H4qp4 reacts with H2O2 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 T1‐weighted relaxivity over four‐fold, enabling the complex to detect H2O2 using clinical MRI technology. The iron‐containing sensor differs from its recently characterized manganese analog, which also detects H2O2, in that it is the oxidation of the metal center, rather than the ligand, that primarily enhances the relaxivity. [ABSTRACT FROM AUTHOR]

A highly water‐ and air‐stable Fe(II) complex with the quinol‐containing macrocyclic ligand H4qp4 reacts with H2O2 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 T1‐weighted relaxivity over four‐fold, enabling the complex to detect H2O2 using clinical MRI technology. The iron‐containing sensor differs from its recently characterized manganese analog, which also detects H2O2, in that it is the oxidation of the metal center, rather than the ligand, that primarily enhances the relaxivity. [ABSTRACT FROM AUTHOR]

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., (© 2022 Wiley-VCH GmbH.)

Previously prepared Mn(II)- and quinol-containing magnetic resonance imaging (MRI) contrast agent sensors for H 2 O 2 relied on linear polydentate ligands to keep the redox-activatable quinols in close proximity to the manganese. Although these provide positive T 1 -weighted relaxivity responses to H 2 O 2 that result from oxidation of the quinol groups to p -quinones, these reactions weaken the binding affinity of the ligands, promoting dissociation of Mn(II) from the contrast agent in aqueous solution. Here, we report a new ligand, 1,8-bis(2,5-dihydroxybenzyl)-1,4,8,11-tetraazacyclotetradecane, that consists of two quinols covalently tethered to a cyclam macrocycle. The macrocycle provides stronger thermodynamic and kinetic barriers for metal-ion dissociation in both the reduced and oxidized forms of the ligand. The Mn(II) complex reacts with H 2 O 2 to produce a more highly aquated Mn(II) species that exhibits a 130% greater r 1 , quadrupling the percentile response of our next best sensor. With a large excess of H 2 O 2 , there is a noticeable induction period before quinol oxidation and r 1 enhancement occurs. Further investigation reveals that, under such conditions, catalase activity initially outcompetes ligand oxidation, with the latter occurring only after most of the H 2 O 2 has been depleted.