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We prepare iron(II) and iron(III) complexes with polydentate ligands that contain quinols, which can act as electron proton transfer mediators. Although the iron(II) complex with N -(2,5-dihydroxybenzyl)- N , N ', N '-tris(2-pyridinylmethyl)-1,2-ethanediamine (H 2 qp1) is inactive as an electrocatalyst, iron complexes with N , N '-bis(2,5-dihydroxybenzyl)- N , N '-bis(2-pyridinylmethyl)-1,2-ethanediamine (H 4 qp2) and N -(2,5-dihydroxybenzyl)- N , N '-bis(2-pyridinylmethyl)-1,2-ethanediamine (H 2 qp3) were found to be much more active and more selective for water production than a previously reported cobalt-H 2 qp1 electrocatalyst while operating at low overpotentials. The catalysts with H 2 qp3 can enter the catalytic cycle as either Fe(II) or Fe(III) species; entering the cycle through Fe(III) lowers the effective overpotential. On the basis of their TOF 0 values, the successful iron-quinol complexes are better electrocatalysts for oxygen reduction than previously reported iron-porphyrin compounds, with the Fe(III)-H 2 qp3 arguably being the best homogeneous electrocatalyst for this reaction. With iron, the quinol-for-phenol substitution shifts the product selectivity from H 2 O 2 to water with little impact on the overpotential, but unlike cobalt, this substitution also greatly improves the activity, as assessed by TOF max , by hastening the protonation and oxygen binding steps. The addition of a second quinol further enhances the activity and selectivity for water but modestly increases the effective overpotential.

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.