title: Marked stabilization of redox states and enhanced catalytic activity in galactose oxidase models based on transition metal S-methylisothiosemicarbazonates with -SR group in ortho position to the phenolic oxygen.
abstract: Reactions of 5-tert-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde S-methylisothiosemicarbazone and 5-tert-butyl-2-hydroxy-3-phenylsulfanylbenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione (Hacac) and triethyl orthoformate in the presence of M(acac)2 as template source at 107 °C afforded metal complexes of the type M(II)L(1) and M(II)L(2), where M = Ni and Cu, with a new Schiff base ligand with thiomethyl (H2L(1)) and/or thiophenyl (H2L(2)) group in the ortho position of the phenolic moiety. Demetalation of NiL(1) in CHCl3 with HCl(g) afforded H2L(1). The latter reacts with Zn(OAc)2·2H2O with formation of ZnL(1). The effect of -SR groups and metal ion identity on stabilization of phenoxyl radicals generated electrochemically was studied in detail. A marked stabilization of phenoxyl radical was observed in one-electron-oxidized complexes [ML(2)](+) (M = Ni, Cu) at room temperature, as demonstrated by cyclic voltammetry, EPR spectroscopy, and UV-vis-NIR measurements. In solution, the oxidized CuL(2) and NiL(2) display intense low-energy NIR transitions consistent with their classification as metal-delocalized phenoxyl radical species. While the CuL(2) complex shows reversible reduction, reduction of NiL(2), CuL(1), and NiL(1) is irreversible. EPR measurements in conjunction with density functional theory calculations provided insights into the extent of electron delocalization as well as spin density in different redox states. The experimental room temperature spectroelectrochemical data can be reliably interpreted with the (3)[CuL(2)](+) and (2)[NiL(2)](+) oxidation ground states. The catalytic activity of synthesized complexes in the selective oxidations of alcohols has been studied as well. The remarkable efficiency is evident from the high yields of carbonyl products when employing both the CuL(2)/air/TEMPO and the CuL(2)/TBHP/MW(microwave-assisted) oxidation systems.
authors.affiliation: Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria. vladimir.arion@univie.ac.at
authors.forename: Vladimir B
authors.initials: VB
authors.lastname: Arion
authors.forename: Sonja
authors.initials: S
authors.lastname: Platzer
authors.forename: Peter
authors.initials: P
authors.lastname: Rapta
authors.forename: Peter
authors.initials: P
authors.lastname: Machata
authors.forename: Martin
authors.initials: M
authors.lastname: Breza
authors.forename: Daniel
authors.initials: D
authors.lastname: Vegh
authors.forename: Lothar
authors.initials: L
authors.lastname: Dunsch
authors.forename: Joshua
authors.initials: J
authors.lastname: Telser
authors.forename: Sergiu
authors.initials: S
authors.lastname: Shova
authors.forename: Tatiana C O
authors.initials: TC
authors.lastname: Mac Leod
authors.forename: Armando J L
authors.initials: AJ
authors.lastname: Pombeiro
mesh_terms: D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry*; D003300:Copper / Q000737:chemistry; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D005692:Galactose Oxidase / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D009532:Nickel / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D015032:Zinc / Q000737:chemistry
chemical_list: D056831:Coordination Complexes; D008024:Ligands; D010636:Phenols; D003300:Copper; D009532:Nickel; D005692:Galactose Oxidase; D015032:Zinc; D010100:Oxygen