A Highly Reactive Chromium(V)-Oxo TAML Cation Radical Complex in Electron Transfer and Oxygen Atom Transfer Reactions

We report the synthesis, characterization, and electron-transfer (ET) oxidation reactivity of a chromium(V)-oxo TAML cation radical complex binding Sc3+ ion, {[CrV(O)(TAML•+)]-Sc3+}3+ (2-Sc). Its precursors, such as [CrV(O)(TAML)]- (1) and {[CrV(O)(TAML)]-Sc3+}2+ (1-Sc), were also characterized spectroscopically and/or structurally. In ET and oxygen atom transfer (OAT) reactions, while 1 and 1-Sc were sluggish oxidants, 2-Sc was a highly reactive oxidant with an extremely small reorganization energy. For example, in ET oxidation reactions, nanosecond laser-induced transient absorption measurements were performed to examine the fast ET from electron donors (e.g., ferrocene derivatives) to 2-Sc, affording a small reorganization energy (λ = 0.26 eV) of ET, which is even much smaller than the λ values reported in the ET reduction of heme Compound I (Cpd I) models and non-heme metal-oxo complexes. Such a small reorganization energy is ascribed to the TAML ligand centered ET reduction of 2-Sc. The λ value of 0.26 eV was also obtained in the electron self-exchange reaction between 2-Sc and 1-Sc. In OAT reactions, the rate constants of the sulfoxidation of thioanisole derivatives by 2-Sc at -40 °C were much greater than those reported in the oxidation of thioanisoles by heme Cpd I and non-heme metal-oxo complexes. The reactivity of 2-Sc in hydrogen atom transfer (HAT) reactions is also discussed briefly. To the best of our knowledge, this Cr(V)-oxo TAML cation radical complex binding Sc3+ ion, {[CrV(O)(TAML•+)]-Sc3+}3+, with an extremely small reorganization energy is one of the most powerful high-valent metal-oxo oxidants in ET and OAT reactions.