Aerobic oxidation of primary alcohols (including methanol) by copper(II)- and zinc(II)-phenoxyl radical catalysts

The tetradentate ligand N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-phe nylenediamine, H4L(1), has been prepared, and its square planar complexes [ Cu-II(L-3)] and [Zn-II(L3)] have been synthesized from the reaction of H4L( 1) with [Cu-1(NCCH3)4](ClO4) or Zn(BF4)(2). 2H(2)O in methanol in the prese nce 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- diim inoquinone. Complexes [Cu-II(L-3)]. CH3CN and [Zn(L-3)] CH3CN have been cha racterized by X-ray crystallography, EPR spectroscopy, and magnetochemistry ; [CUII(L-3)] has an S = 1/2 ground state, and [Zn(L-3)] is diamagnetic. Cy clic voltammetry established that both complexes undergo two successive rev ersible one-electron oxidations and two successive reversible one-electron reductions. Thus, the coordinated ligand exists in five oxidation levels. T he species [M-II(L-4)]PF6 (M = Cu-II, Zn-II) and [M-II(L-5)](ClO4)(2) (M = Cu-II, Zn-II) have been isolated and characterized by UV/vis, EPR, and H-1 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 (L5) is the neutral ligand N ,N'-bis(3,5-di-tert-butyl-2-phenoxyl)-1,2-diiminoquinone. Similarly, two co mplexes of the type [M-II((LH2)-H-1)] (M = Cu-II Zn-II) have been isolated from the reaction of (LH4)-H-1 with CUII(ClO4)2 6H(2)O Or Zn(ClO4)2 6H20 un der anaerobic conditions in the presence of NEt3. Complexes [Cu-II(L-4)]PF6 and [Zn(L4)1PF(6)]selectively oxidize primary alcohols (including methanol and ethanol) in a stoichiometric fashion under anaerobic conditions, yield ing the corresponding aldehydes and [M-II((LH2)-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-hy droxyphenyl)-1,2-diiminosemiquinone. Since the latter reduced forms react r apidly with dioxygen with formation of [M-II(L-4)](+) (M = CU, Zn) and 1 eq uiv of H2O2 these oxidized species are catalysts for the air oxidation of p rimary 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 isot ope effects show that H-abstraction from the cr-carbon atom of a coordinate d alcoholato ligand is the rate-determining step in all cases.