Marked Stabilization of Redox States and Enhanced Catalytic Activity in Galactose Oxidase Models Based on Transition MetalS-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.

Tetranuclear Copper(II) Complexes with Macrocyclic and Open‐Chain Disiloxane Ligands as Catalyst Precursors for Hydrocarboxylation and Oxidation of Alkanes and 1‐Phenylethanol

“…Complexes 1 and 2 catalyse efficiently this reaction and, under such conditions, the reactions led to 62 and 82 % acetophenone, respectively (Table 3, entries 1 and 9), for a catalyst/substrate molar ratio of 0.2 %. The yield was comparable with those obtained previously in the solvent‐free oxidation of 1‐phenylethanol catalysed by copper(II) complexes containing tetradentate N 2 O 2 ligands45 or copper(II) Schiff base complexes with O , N , O ‐donors 46…”

“…The addition of Ph 2 NH, a known O‐centred radical trap,52 almost completely suppressed the catalytic activity, thus suggesting41,45,46,53 the involvement of a radical mechanism. The mechanism may involve the metal‐assisted generation of t BuOO · and t BuO · radicals (upon oxidation and reduction of t BuOOH by a Cu II or a Cu I centre, respectively),54,55 with the latter behaving as an H‐atom abstractor from the alcohol 54–57…”

Novel Cage-Like Hexanuclear Nickel(II) Silsesquioxane. Synthesis, Structure, and Catalytic Activity in Oxidations with Peroxides

“…Nickel complexes are known to catalyze certain oxidation reactions of hydrocarbons [25,26,27,28,29,30,31,32,33,34,35,36,37] and alcohols [38,39,40,41,42,43,44] by peroxides. We have tested the catalytic effect of compound 1 in oxidations with various oxidants.…”

Synthesis and Structure of Copper Complexes of a N6O4 Macrocyclic Ligand and Catalytic Application in Alcohol Oxidation

“…As mentioned above, the aerobic oxidation of 1-phenylethanol, used as a model substrate for secondary alcohols, leads, as expected, to a lower aldehyde yield than in the case of the primary benzyl alcohols. However, the catalytic oxidation of 1-phenylethanol can be highly promoted by using microwave irradiation following a previously developed procedure [55] with tert-butyl hydroperoxide (t-BuOOH) as the oxidizing agent. Typical reaction conditions concern 2 equivalent of t-BuOOH at 80 or 120 • C, under low power (5 or 15 W) microwave (MW) irradiation, 0.5 s 6 h reaction time and in the absence of any added solvent (Scheme 4, Table 6).…”