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, [Fe V (O)(TAML •+ )], effectively oxidizes water to evolve dioxygen (O 2 ) via O–O bond formation. The precursor [Fe III (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: [Fe III (TAML)] − and DDQ first generate [Fe V (O)(TAML)] − via a μ-oxo dimer intermediate, [(TAML)Fe IV (O)Fe IV (TAML)] 2– . Subsequent electron transfer (ET) to the triplet excited state of DDQ ( 3 DDQ*) yields the [Fe V (O)(TAML •+ )] intermediate and DDQ •– . In the presence of water, this complex reacts to produce a presumed [Fe IV (OOH)(TAML)] − intermediate. The rate constants of the reaction of [Fe V (O)(TAML •+ )] with H 2 O and D 2 O at 298 K are determined to be 1.4 × 10 6 and 7.1 × 10 5 M –1 s –1, respectively, giving a deuterium kinetic isotope effect (KIE) of 2.0. In ET reactions, [Fe V (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 [Fe V (O)(TAML •+ )]. To the best of our knowledge, this iron(V)-oxo TAML radical cation complex, [Fe V (O)(TAML •+ )], represents one of the most powerful high-valent iron-oxo oxidants identified to date in water oxidation and electron transfer reactions.