Cerium‐Complex‐Catalyzed Oxidation of Arylmethanols under Atmospheric Pressure of Dioxygen and Its Mechanism through a Side‐On μ‐Peroxo Dicerium(IV) Complex

A new Ce IV complex [Ce{NH(CH 2 CH 2 N=CHC 6 H 2 ‐3,5‐( t Bu) 2 ‐2‐O) 2 }(NO 3 ) 2 ] ( 1 ), bearing a dianionic pentadentate ligand with an N 3 O 2 donor set, has been prepared by treating (NH 4 ) 2 Ce(NO 3 ) 6 with the sodium salt of ligand L1 . Complex 1 in the presence of TEMPO and 4 Å molecular sieves (MS4 A) has been found to serve as a catalyst for the oxidation of arylmethanols using dioxygen as an oxidant. We propose an oxidation mechanism based on the isolation and reactivity study of a trivalent cerium complex [Ce{NH(CH 2 CH 2 N=CHC 6 H 2 ‐3,5‐( t Bu) 2 ‐2‐O) 2 }(NO 3 )(THF)] ( 2 ), its side‐on μ‐O 2 adduct [Ce{NH(CH 2 CH 2 N=CHC 6 H 2 ‐3,5‐( t Bu) 2 ‐2‐O) 2 }(NO 3 )] 2 (μ‐η 2 :η 2 ‐O 2 ) ( 3 ), and the hydroxo‐bridged Ce IV complex [Ce{NH(CH 2 CH 2 N=CHC 6 H 2 ‐3,5‐( t Bu) 2 ‐2‐O) 2 }(NO 3 )] 2 (μ‐OH) 2 ( 4 ) as key intermediates during the catalytic cycle. Complex 2 was synthesized by reduction of 1 with 2,5‐dimethyl‐1,4‐bis(trimethylsilyl)‐1,4‐diazacyclohexadiene. Bubbling O 2 into a solution of 2 resulted in formation of the peroxo complex 3 . This provides the first direct evidence for cerium‐catalyzed oxidation of alcohols under an O 2 atmosphere.