Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O 2 to H 2 O 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 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.