Aerobic Oxidation of Primary Alcohols (Including Methanol) by Copper(II)− and Zinc(II)−Phenoxyl Radical Catalysts

Abstract

The tetradentate ligand N, N ‘-bis(3,5-di- tert -butyl-2-hydroxyphenyl)-1,2-phenylenediamine, H 4 L 1, has been prepared, and its square planar complexes [Cu II (L 3 )] and [Zn II (L 3 )] have been synthesized from the reaction of H 4 L 1 with [Cu I (NCCH 3 ) 4 ](ClO 4 ) or Zn(BF 4 ) 2 ·2H 2 O in methanol in the presence of air. The dianion (L 3 ) 2 - represents the two-electron oxidized form of (L 1 ) 4 -, namely N, N ‘-bis(3,5-di- tert -butyl-2-hydroxyphenyl)-1,2-diiminoquinone. Complexes [Cu II (L 3 )]·CH 3 CN and [Zn(L 3 )]·CH 3 CN have been characterized by X-ray crystallography, EPR spectroscopy, and magnetochemistry; [Cu II (L 3 )] has an S = 1 / 2 ground state, and [Zn(L 3 )] is diamagnetic. Cyclic voltammetry established that both complexes undergo two successive reversible one-electron oxidations and two successive reversible one-electron reductions. Thus, the coordinated ligand exists in five oxidation levels. The species [M II (L 4 )]PF 6 (M = Cu II, Zn II ) and [M II (L 5 )](ClO 4 ) 2 (M = Cu II, Zn II ) have been isolated and characterized by UV/vis, EPR, and 1 H NMR spectroscopy and by magnetic susceptibility measurements, where (L 4 ) - represents the monoanion N -(3,5-di- tert -butyl-2-hydroxyphenyl)- N ‘-(3,5-di- tert -butyl-2-phenoxyl)-1,2-diiminoquinone and (L 5 ) is the neutral ligand N, N ‘-bis(3,5-di- tert -butyl-2-phenoxyl)-1,2-diiminoquinone. Similarly, two complexes of the type [M II (L 1 H 2 )] (M = Cu II, Zn II ) have been isolated from the reaction of L 1 H 4 with Cu II (ClO 4 ) 2 ·6H 2 O or Zn(ClO 4 ) 2 ·6H 2 O under anaerobic conditions in the presence of NEt 3 . Complexes [Cu II (L 4 )]PF 6 and [Zn(L 4 )]PF 6 selectively oxidize primary alcohols (including methanol and ethanol) in a stoichiometric fashion under anaerobic conditions, yielding the corresponding aldehydes and [M II (L 2 H 2 )] + (M = Cu II, Zn II ), where (L 2 ) 3 - is the trianionic form of N, N ‘-bis(3,5-di- tert -butyl-2-hydroxyphenyl)-1,2-diiminosemiquinone. Since the latter reduced forms react rapidly with dioxygen with formation of [M II (L 4 )] + (M = Cu, Zn) and 1 equiv of H 2 O 2, these oxidized species are catalysts for the air oxidation of primary alcohols, including ethanol and methanol, with concomitant formation of H 2 O 2 and aldehydes. The kinetics of the stoichiometric reactions and of the catalyses (initial rate method) have been measured. Large kinetic isotope effects show that H-abstraction from the α-carbon atom of a coordinated alcoholato ligand is the rate-determining step in all cases.