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

Reactions of 5-tert-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde S-methylisothiosemicairbazone and 5-tert-butyl-2-hydroxy-3-phenyl-sulfanyIbenzaldehyde S-methylisothiosemicarbazone pentane-2,4-dione (Hacac) and Methyl orthoformate in the presence of M(acac)(2) as template source at 107 degrees C afforded metal complexes of the type (ML1)-L-II and (ML2)-L-II, where M = Ni and Cu, with a new Schiff base ligand with thiomethyl (H2L1) and/or thiophenyl (H2L2) group in the ortho position of the phenolic moiety. Demetalation of NiL1 in CHCl3 with HCl(g) afforded H2L1. The latter reacts with Zn(OAc)(2)center dot 2H(2)O with formation of ZnL1. 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 [ML2](+) (M = Ni, Cu) at room temperature, as demonstrated by cyclic voltammetry, EPR spectroscopy, and UV-vis-NIR measurements. In solution, the oxidized CuL2 and NiL2 display intense low-energy NIR transitions consistent with their classification as metal-delocalized phenoxyl radical species. While the CuL2 complex shows reversible reduction, reduction of NiL2, CuL1), and NiL1 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 reclox states. The experimental room temperature spectroelectrochemical data can be reliably interpreted with the 3[CuL2] and 2[NiL2] 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 CuL2/air/TEMPO and the CuL2/TBHP/MW(rnicrowave-assisted) oxidation systems.