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, H4L1, has been prepared, and its square planar complexes [CuII(L3)] and [ZnII(L3)] have been synthesized from the reaction of H4L1 with [CuI(NCCH3)4](ClO4) or Zn(BF4)2·2H2O in methanol in the presence of air. The dianion (L3)2- represents the two-electron oxidized form of (L1)4-, namely N,N‘-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-diiminoquinone. Complexes [CuII(L3)]·CH3CN and [Zn(L3)]·CH3CN have been characterized by X-ray crystallography, EPR spectroscopy, and magnetochemistry; [CuII(L3)] has an S = 1/2 ground state, and [Zn(L3)] 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 [MII(L4)]PF6 (M = CuII, ZnII) and [MII(L5)](ClO4)2 (M = CuII, ZnII) have been isolated and characterized by UV/vis, EPR, and 1H NMR spectroscopy and by magnetic susceptibility measurements, where (L4)- represents the monoanion N-(3,5-di-tert-butyl-2-hydroxyphenyl)-N‘-(3,5-di-tert-butyl-2-phenoxyl)-1,2-diiminoquinone and (L5) is the neutral ligand N,N‘-bis(3,5-di-tert-butyl-2-phenoxyl)-1,2-diiminoquinone. Similarly, two complexes of the type [MII(L1H2)] (M = CuII, ZnII) have been isolated from the reaction of L1H4 with CuII(ClO4)2·6H2O or Zn(ClO4)2·6H2O under anaerobic conditions in the presence of NEt3. Complexes [CuII(L4)]PF6 and [Zn(L4)]PF6 selectively oxidize primary alcohols (including methanol and ethanol) in a stoichiometric fashion under anaerobic conditions, yielding the corresponding aldehydes and [MII(L2H2)]+ (M = CuII, ZnII), where (L2)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 [MII(L4)]+ (M = Cu, Zn) and 1 equiv of H2O2, these oxidized species are catalysts for the air oxidation of primary alcohols, including ethanol and methanol, with concomitant formation of H2O2 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.