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A Co(II) complex with the polydentate quinol-containing ligand H 2 qp1 acts as an efficient electrocatalyst for oxygen reduction. Without any additional electron-proton transfer mediators, the electrocatalysis is selective for H 2 O; a related complex that substitutes a phenol for the quinol, conversely, instead produces mostly H 2 O 2 under the same conditions. We propose that the ability of the redox-active quinol to donate two electrons impacts the product-determining step.

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.)