An Iron(V)-Oxo TAML Radical Cation Complex in Photocatalytic Water Oxidation and Electron-Transfer Reactions

Iron(V)-oxo complexes have been proposed as critical reactive intermediates in catalytic water oxidation. However, iron-oxo complexes bearing redox-active (noninnocent) ligands with a formal oxidation state exceeding +5 have remained elusive. Herein, we report that an iron(V)-oxo complex bearing a one-electron oxidized tetraamido macrocyclic ligand, [FeV(O)(TAML•+)], effectively oxidizes water to evolve dioxygen (O2) via O-O bond formation. The precursor [FeIII(TAML)]- complex acts as an effective catalyst for the photochemical oxidation of water by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Transient absorption measurements reveal a stepwise, photodriven pathway: [FeIII(TAML)]- and DDQ first generate [FeV(O)(TAML)]- via a μ-oxo dimer intermediate, [(TAML)FeIV(O)FeIV(TAML)]2-. Subsequent electron transfer (ET) to the triplet excited state of DDQ (3DDQ*) yields the [FeV(O)(TAML•+)] intermediate and DDQ•-. In the presence of water, this complex reacts to produce a presumed [FeIV(OOH)(TAML)]- intermediate. The rate constants of the reaction of [FeV(O)(TAML•+)] with H2O and D2O at 298 K are determined to be 1.4 × 106 and 7.1 × 105 M-1 s-1, respectively, giving a deuterium kinetic isotope effect (KIE) of 2.0. In ET reactions, [FeV(O)(TAML•+)] is a highly reactive oxidant with an extremely small reorganization energy (λ = 0.80 eV). Such a small reorganization energy is ascribed to the TAML ligand-centered ET reduction of [FeV(O)(TAML•+)]. To the best of our knowledge, this iron(V)-oxo TAML radical cation complex, [FeV(O)(TAML•+)], represents one of the most powerful high-valent iron-oxo oxidants identified to date in water oxidation and electron transfer reactions.