Enhanced Redox Reactivity of a Nonheme Iron(V)–Oxo Complex Binding Proton

Acid effects on the chemical properties of metal–oxygen intermediates have attracted much attention recently, such as the enhanced reactivity of high-valent metal­(IV)–oxo species by binding proton(s) or Lewis acidic metal ion(s) in redox reactions. Herein, we report for the first time the proton effects of an iron­(V)–oxo complex bearing a negatively charged tetraamido macrocyclic ligand (TAML) in oxygen atom transfer (OAT) and electron-transfer (ET) reactions. First, we synthesized and characterized a mononuclear nonheme Fe­(V)–oxo TAML complex (1) and its protonated iron­(V)–oxo complexes binding two and three protons, which are denoted as 2 and 3, respectively. The protons were found to bind to the TAML ligand of the Fe­(V)–oxo species based on spectroscopic characterization, such as resonance Raman, extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) measurements, along with density functional theory (DFT) calculations. The two-protons binding constant of 1 to produce 2 and the third protonation constant of 2 to produce 3 were determined to be 8.0(7) × 108 M–2 and 10(1) M–1, respectively. The reactivities of the proton-bound iron­(V)–oxo complexes were investigated in OAT and ET reactions, showing a dramatic increase in the rate of sulfoxidation of thioanisole derivatives, such as 107 times increase in reactivity when the oxidation of p-CN-thioanisole by 1 was performed in the presence of HOTf (i.e., 200 mM). The one-electron reduction potential of 2 (E red vs SCE = 0.97 V) was significantly shifted to the positive direction, compared to that of 1 (E red vs SCE = 0.33 V). Upon further addition of a proton to a solution of 2, a more positive shift of the E red value was observed with a slope of 47 mV/log­([HOTf]). The sulfoxidation of thioanisole derivatives by 2 was shown to proceed via ET from thioanisoles to 2 or direct OAT from 2 to thioanisoles, depending on the ET driving force.