Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O to HO at High Rates and Low Overpotentials.

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 -(2,5-dihydroxybenzyl)-,','-tris(2-pyridinylmethyl)-1,2-ethanediamine (Hqp1) is inactive as an electrocatalyst, iron complexes with ,'-bis(2,5-dihydroxybenzyl)-,'-bis(2-pyridinylmethyl)-1,2-ethanediamine (Hqp2) and -(2,5-dihydroxybenzyl)-,'-bis(2-pyridinylmethyl)-1,2-ethanediamine (Hqp3) were found to be much more active and more selective for water production than a previously reported cobalt-Hqp1 electrocatalyst while operating at low overpotentials. The catalysts with Hqp3 can enter the catalytic cycle as either Fe(II) or Fe(III) species; entering the cycle through Fe(III) lowers the effective overpotential.

Diquinol Functionality Boosts the Superoxide Dismutase Mimicry of a Zn(II) Complex with a Redox-Active Ligand while Maintaining Catalyst Stability and Enhanced Activity in Phosphate Solution.

In the current work, we demonstrate ligand design concepts that significantly improve the superoxide dismutase (SOD) activity of a zinc complex; the catalysis is enhanced when two quinol groups are present in the polydentate ligand. We investigate the mechanism through which the quinols influence the catalysis and determine the impact of entirely removing a chelating group from the original hexadentate ligand. Our results suggest that SOD mimicry with these compounds requires a ligand that coordinates Zn(II) strongly in both its oxidized and reduced forms and that the activity proceeds through Zn(II)-semiquinone complexes.

A Macrocyclic Ligand Framework That Improves Both the Stability and -Weighted MRI Response of Quinol-Containing HO Sensors.

Previously prepared Mn(II)- and quinol-containing magnetic resonance imaging (MRI) contrast agent sensors for HO relied on linear polydentate ligands to keep the redox-activatable quinols in close proximity to the manganese. Although these provide positive -weighted relaxivity responses to HO that result from oxidation of the quinol groups to -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.